Dissertations / Theses: 'Structural phase'

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Giddy, Andrew Peter. "Computational studies of structural phase transitions." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239582.

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Gibson, Robert Gould. "Phase variability of structural transfer functions." Thesis, Massachusetts Institute of Technology, 1986. http://hdl.handle.net/1721.1/15023.

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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1986. MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING Bibliography: leaf 84. by Robert Gould Gibson. M.S.

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Jia, Zhihong. "Structural modulation and phase transitions in melilites." [S.l.] : [s.n.], 2005. http://archiv.ub.uni-marburg.de/diss/z2005/0066/.

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Jackson, Andrew N. "Structural phase behaviour via Monte Carlo techniques." Thesis, University of Edinburgh, 2001. http://hdl.handle.net/1842/4850.

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There are few reliable computational techniques applicable to the problem of structural phase behaviour. This is starkly emphasised by the fact that there are still a number of unanswered questions concerning the solid state of some of the simplest models of matter. To determine the phase behaviour of a given system we invoke the machinery of statistical physics, which identifies the equilibrium phase as that which minimises the free-energy. This type of problem can only be dealt with fully via numerical simulation, as any less direct approach will involve making some uncontrolled approximation. In particular, a numerical simulation can be used to evaluate the free-energy difference between two phases if the simulation is free to visit them both. However, it has proven very difficult to find an algorithm which is capable of efficiently exploring two different phases, particularly when one or both of them is a crystalline solid. This thesis builds on previous work (Physical Review Letters 79 p.3002), exploring a new Monte Carlo approach to this class of problem. This new simulation technique uses a global coordinate transformation to switch between two different crystalline structures. Generally, this `lattice switch' is found to be extremely unlikely to succeed in a normal Monte Carlo simulation. To overcome this, extended-sampling techniques are used to encourage the simulation to visit `gateway' microstates where the switch will be successful. After compensating for this bias in the sampling, the free-energy difference between the two structures can be evaluated directly from their relative probabilities. As concrete examples on which to base the research, the lattice-switch Monte Carlo method is used to determine the free-energy difference between the face-centred cubic (fcc) and hexagonal close-packed (hcp) phases of two generic model systems --- the hard-sphere and Lennard-Jones potentials. The structural phase behaviour of the hard-sphere solid is determined at densities near melting and in the close-packed limit. The factors controlling the efficiency of the lattice-switch approach are explored, as is the character of the `gateway' microstates. The face-centred cubic structure is identified as the thermodynamically stable phase, and the free-energy difference between the two structures is determined with high precision. These results are shown to be in complete agreement with the results of other authors in the field (published during the course of this work), some of whom adopted the lattice-switch method for their calculations. Also, the results are favourably compared against the experimentally observed structural phase behaviour of sterically-stabilised colloidal dispersions, which are believed to behave like systems of hard spheres. The logical extension of the hard sphere work is to generalise the lattice-switch technique to deal with `softer' systems, such as the Lennard-Jones solid. The results in the literature for the structural phase behaviour of this relatively simple system are found to be completely inconsistent. A number of different approaches to this problem are explored, leading to the conclusion that these inconsistencies arise from the way in which the potential is truncated. Using results for the ground-state energies and from the harmonic approximation, we develop a new truncation scheme which allows this system to be simulated accurately and efficiently. Lattice-switch Monte Carlo is then used to determine the fcc-hcp phase boundary of the Lennard-Jones solid in its entirety. These results are compared against the experimental results for the Lennard-Jones potential's closest physical analogue, the rare-gas solids. While some of the published rare-gas observations are in approximate agreement with the lattice-switch results, these findings contradict the widely held belief that fcc is the equilibrium structure of the heavier rare-gas solids for all pressures and temperatures. The possible reasons for this disagreement are discussed. Finally, we examine the pros and cons of the lattice-switch technique, and explore ways in which it can be extended to cover an even wider range of structures and interactions.

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McLinko, Ryan (Ryan M. ). "Conceptual Phase Structural Design Tool for Microsatellites." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/67068.

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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2011. This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. Cataloged from student submitted PDF version of thesis. Includes bibliographical references (p. 122-124). Gaining traction or momentum in the conceptual design phase for a complex system can be an arduous and daunting process, whether the complex system being designed is a satellite, airplane, car, or one of countless other systems. The design of small satellites is particularly affected by the difficulties in gaining traction since most of the customized tools that exist are proprietary, a significant experience base is required to be able to perform system level design trades, and the issue that most satellites serve one-of-a-kind applications. Of the subsystems in a satellite, the structures subsystem (along with other "downstream" subsystems, such as power and thermal), tends to be less mature during the conceptual design phase since its design depends strongly on the particular designs and requirements of each of the other subsystems, which also take time to mature. The Conceptual Phase Structural Design Tool for Microsatellites (SDT) facilitates the development of potential small satellite structural architectures and the selection of an initial satellite architecture to use in the detailed design process. The tool is capable of evaluating the strength, stiffness, mass, and inertial properties of a satellite architecture and is customizable to a wide range of potential missions by allowing for a number of structural architectures and customizable component placement. Furthermore, the tool has been developed with two key niches in mind. First, it is available to students with little to no satellite design experience, thus enabling a greater number of people, including those who are unfamiliar with the process of structural design at the beginning of the program, to design higher quality spacecraft from the start. Second, it is open source and deployable in a state that is usable and customizable by members of the satellite design industry. by Ryan McLinko. S.M.

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Wilding, Nigel Blair. "Studies of structural patterns at phase transitions." Thesis, University of Edinburgh, 1991. http://hdl.handle.net/1842/13233.

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The work described in this thesis comprises two distinct components. In the first part, Monte-Carlo computer simulation methods are employed within a finite-size scaling framework to investigate both universal and non-universal behaviour in two scalar models, the 1-d φ4 model and the 2-d Lennard-Jones fluid. In both these models the properties of interest are obtained from studies of the large-length scale configurational patterns via measurements of the probability distribution function (PDF) of the coarse-grained (block) ordering variable. For the 1-d φ4 model, simulations are employed to obtain the block PDF of the spin variable. This function is shown to map onto an analytically-derived expression for the 1-d Ising chain, thus exposing the model's essentially Ising-like character. It is further demonstrated that the corrections to the limiting form of the block PDF reflect system-specific features of the 1-d φ4 model associated with its elementary excitations. In the 2-d Lennard-Jones fluid, the combined use of simulation and finite-size scaling is shown to provide a powerful method for accurately determining the location of the liquid-vapour coexistence curve and critical point. At the critical point, the limiting form of the coarse-grained density distribution is found to collapse onto a previously determined function characteristic of the 2-d Ising model, thereby confirming and clarifying fluid-magnet universality. Clear evidence is also presented for mixing of the temperature and chemical potential in the two relevant scaling fields-a phenomenon responsible for the failure of the law of rectilinear diameter. As an addendum, a discussion is given of the prospects for generalising to fluids, the cluster updating techniques recently developed to reduce critical slowing down in simulations of spin systems.

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Allen, Patryck Kevyn Kidd. "Structural studies of lead-free piezoelectrics with the fresnoite structure type." Thesis, The University of Sydney, 2012. http://hdl.handle.net/2123/11986.

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The lead free piezoelectric fresnoite A2M3O8 (A = Ba, Sr, K, Cs; M = Ti, V, Si, Ge) modulated structure type has been investigated owing to its potential to exhibit excellent piezoelectric response coefficients. Ba2TiSi2O8, Sr2TiSi2O8, and Ba2TiGe2O8 end members in addition to members of the Ba2xSr2-2xTiSi2O8, Ba2xSr2 2xTiGe2O8, Ba2TiGe2ySi2 2yO8, and BaSrTiGe2ySi2 2yO8 series were synthesised and characterised using a combination of variable temperature diffraction techniques. The Ba2TiSi2O8 and Sr2TiSi2O8 modulated structures at ambient temperature were characterised using neutron powder diffraction for the first time. Variable temperature synchrotron X ray diffraction data showed a new means of identifying the incommensurate to prototypic structural phase transition at 433 K. Resonant ultrasound spectroscopy has shown coupling between the elastic moduli and structural changes in fresnoite samples. Polycrystalline Sr2TiSi2O8 samples were shown to undergo a first order phase transition from a two phase mixture of incommensurately modulated tetragonal and orthorhombic phases to a single incommensurately modulated orthorhombic phase that is complete by 567 K. The proportion of the orthorhombic phase Sr2TiSi2O8 samples was shown to slowly decrease on cooling to 125 K. The Sr2TiSi2O8 structure was also shown to undergo an additional phase transition from the incommensurately modulated orthorhombic phase to a tetragonal phase at 1323 K for the first time. The inclusion of barium or germanium into the Sr2TiSi2O8 structure was shown to suppress the formation of the orthorhombic phase at ambient temperature and elevated temperatures. New phase diagrams for the Ba2xSr2-2xTiSi2O8 and Sr2TiGe2ySi2 2yO8 systems summarise the phase transitions investigated. The intrinsic piezoelectric coefficients were calculated to be approximately 5 pm V 1 and 27 pm V 1 for polycrystalline samples of Ba2TiSi2O8 and Sr2TiSi2O8 respectively and compared to common piezoelectric materials.

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Rabe, Karin M. (Karin Maria). "Ab initio statistical mechanics of structural phase transitions." Thesis, Massachusetts Institute of Technology, 1987. http://hdl.handle.net/1721.1/14630.

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Warren, Michele Carol. "Ab initio lattice dynamics and structural phase transitions." Thesis, University of Edinburgh, 1997. http://hdl.handle.net/1842/14635.

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Prediction of the conditions required for the transformation of one phase of a mineral into another has long been a goal of condensed matter physics. This is especially desirable for phase transitions which are believed to be involved in geological processes, but for which the conditions of temperature or pressure are hard to reproduce experimentally. This thesis examines a number of structural phase transitions including those of MgSiO3 perovskite, which is thought to form the largest part of the Earth's mantle and of ZrO2 which plays an important role in inhibiting crack formation in ceramics. These phase transitions, in which an alternative phase may be reached by continuous distortions of the structure on an atomic level, are examined primarily through the use of first principles electronic structure calculations. Existing first principles techniques were extended to facilitate determination of the equilibrium structure by relaxation of the unit cell and the calculation of the lattice dynamics of complex phases. The distortion involved in most of the phase transitions studied is found to reflect the normal vibrational modes of one or both phases. The phase transitions of MgSiO3 are found to be well described by only a few normal modes of the highest-symmetry cubic phase, dominated by two modes involving tilting of the SiO6 octahedra. These modes resemble rigid unit modes, in which SiO6 octahedra are assumed to remain perfectly rigid but may rotate with respect to other octahedra, whilst preserving linkages between them. The extent to which such simple modes are an accurate description of the dynamics of MgSiO3, BaZrO3 and SiO2 is investigated by way of structural analysis and lattice dynamics of both stable and metastable phases. Both simple models deduced from the lattice dynamical analysis and molecular dynamics using forces calculated from first principles are used to estimate transition temperatures for thermally induced phase transitions in MgSiO3.

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Zierenberg, Johannes. "From Particle Condensation to Polymer Aggregation: Phase Transitions and Structural Phases in Mesoscopic Systems." Doctoral thesis, Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-197255.

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Die vorliegende Arbeit befasst sich mit den Gleichgewichtseigenschaften und Phasenübergängen in verdünnten Teilchen- und Polymersystemen, mit einem Fokus auf Teilchenkondensation und Polymeraggregation. Dazu werden sowohl analytische Argumente als auch hochentwickelte Monte Carlo Simulationen verwendet. Um die in dieser Arbeit erreichten Systemgrößen zu simulieren, wurde eine parallele Version der multikanonischen Methode entwickelt. Die Leistungsfähigkeit dieser Erweiterung wird an mehreren relevanten Beispielen demonstriert. Um Teilchenkondensation und Polymeraggregation in finiten Systemen und in geometrisch beschränkten Strukturen besser zu verstehen, wird der Einfluss von verschiedenen Parametern auf die jeweiligen Übergange untersucht. Dies beinhaltet unter anderem die Systemgröße und Dichte, sowie im Speziellen für semiflexible Polymere deren Steifigkeit. Betrachtet werden sowohl kanonische Observablen (Energie, Tropfen- bzw. Aggregatgröße, etc.) mit der dazugehörigen Übergangstemperatur und -breite, als auch eine mikrokanonische Analyse sowie die Barrieren der Freien Energie. Für semiflexible Polymere wird insbesondere der Einfluss von Steifigkeit auf die resultierende Struktur der Aggregate untersucht, die von amorphen Kugeln für flexible Polymere bis hin zu verdrehten Bündeln für steifere Polymere reichen. Ein weiterer Fokus liegt auf der Untersuchung von Übereinstimmungen zwischen den generischen Mechanismen in Kondensation und Aggregation: dem Übergang zwischen einer homogenen Phase und einer inhomogenen (gemischten) Phase. Auf diesem Niveau kann man Polymeraggregation als Kondensation von ausgedehnten Objekten verstehen. Dies zeigt sich vor allem in dem Skalierungsverhalten von kanonischen und mikrokanonischen Observablen, insbesondere an einem unerwarteten aber konsistenten Bereich für mittelgroße (mesoskopische) Systemgrößen.

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Skellern, Matthew G. "Structural and electronic properties of barium lanthanum vanadates." Thesis, University of Aberdeen, 2003. http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU167989.

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A study of the subsolidus region of the BaO-La2O3-V 2O5 phase diagram has been carried out. Four ternary phases can be prepared, one of which has not been previously identified. The four ternary phases are a palmierite solid solution Ba3-3xLa2x V2O8, Ba2LaV3O11, BaLa10V4O26 (new phase) and Ba3La 40V12O93. Addition of BaLa10V 4O26 and the previously omitted phase La1.42V 0.58)3.58 to the ternary system has resulted n a re-determination of the complete phase diagram. A comprehensive study of the Ba3-3xLa 2xV2O8 solid solution has been carried out, employing a number of techniques including x-ray diffraction, infra-red spectroscopy, thermogravemetric analysis, impedance spectroscopy, electron paramagenetic resonance spectroscopy, inductively coupled plasma mass spectroscopy and x-ray absorption near edge structure spectroscopy. Conductivity measurements show the end-member Ba3V2O8 to be an oxide ion conductor, with the conduction mechanism being facilitated by the ease of transformation of the BaO3 layers to BaO2 and vice versa. The La-doped members show an increase in conductivity, with electrons becoming the dominant conducting species. The crystal structure of Ba2LaV 3O11 was confirmed to be isostructural with Ba2BiV 3O11. A full refinement of the structural parameters has been performed. The unit cell is primitive monoclinic, a = 12.44510(3)A, b = 7.78854(12)A, c = 11.26245(21)A and beta = 103.134(4)°, space group P21/c. Preliminary structural studies have begun on the new phase BaLa10V4O26. The x-ray powder pattern has been fully indexed and a possible monoclinic unit cell of a = 20.2939A, b = 5.886A, c = 12.6234A and beta = 118.05° is proposed. The serendipitous synthesis of a barium-deficient celsian phase, Ba0.8A11.6 Si2.4O8, is reported; the structure was solved using Patterson methods. The phase has a monoclinic unit cell with, a = 8.6090(8)A, b = 13.0858(12)A, c = 7.2047(7)A and beta = 115.418(2)°, space group C2/m.

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Hervoches, Charles H. "Structural studies of Aurivillius phase ferroelectrics and related materials." Thesis, University of St Andrews, 2002. http://hdl.handle.net/10023/12926.

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The research carried out for this thesis has concentrated on Aurivillius bismuth oxide materials and the closely related Sillen and Bipox bismuth oxyhalides. X-ray and neutron powder diffraction techniques have been used to characterise precisely their structure and revealed several important features. The unexpected presence of a double phase transition scheme has been discovered for the Aurivillius phases Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15, both materials present the particularity of having an even number of perovskite layers n (n = 2 and n = 4 for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). The first transition from orthorhombic space group A21am to Amam, occurs at Tc (375deg.C and 550deg.C for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). The second transition from orthorhombic Amam to tetragonal I4/mmm, occurs at higher temperature (550deg.C and 650deg.C for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). These phase transitions have been understood in terms of specific octahedral tilt and displacive modes. In contrast, the n = 2 phase SrBi2Nb2O9 and the n = 3 phase Bi4Ti3O12 are shown to undergo a single-step transition from orthorhombic space group A21am to tetragonal I4/mmm at Tc = 440deg.C for SrBi2Nb2O9, and from orthorhombic space group B2cb to tetragonal I4/mmm at Tc = 675deg.C for Bi4Ti3O12. The atomic disorder of the Bi and A cations in the fluorite and perovskite sites of the Aurivillius phases has been demonstrated and thoroughly studied for the solid solutions Bi4-xSrxTi3-xNbxO12 and Bi4-xLaxTi3O12, and "size-matching" between the fluorite and the perovskite site has been found to be the key to understanding this disorder phenomenon. In the Sillen family, we extended the members of the Bi2MO4Cl group, where M = Lanthanide to the three following compounds: Bi2ErO4Cl, Bi2YbO4Cl, and Bi2LuO4Cl. We have demonstrated that in the Bi2MO4Cl group, the ionic radius of M must be greater than that of Sc3+ (r (Sc3+) = 0.87 Å for CN = 8). Another new compound with a novel ordering scheme for this family has been studied, viz. Bi5TeO8.5I2, crystallising in the orthorhombic space group Cmm2. Te4+ is shown to adopt only one of the three available M sites, thus inducing a polar structure. In the Bipox family, the crystal structures of two materials exhibiting ferroelectric properties, Bi4NbO8Cl and Bi4TaO8Cl have been determined to be orthorhombic, space group P21cn.

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Schmidt, Marek Wojciech, and Marek Schmidt@rl ac uk. "Phase formation and structural transformation of strontium ferrite SrFeOx." The Australian National University. Research School of Physical Sciences and Engineering, 2001. http://thesis.anu.edu.au./public/adt-ANU20020708.190055.

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Non-stoichiometric strontium iron oxide is described by an abbreviated formula SrFeOx (2.5 ≤ x ≤ 3.0) exhibits a variety of interesting physical and chemical properties over a broad range of temperatures and in different gaseous environments. The oxide contains a mixture of iron in the trivalent and the rare tetravalent state. The material at elevated temperature is a mixed oxygen conductor and it, or its derivatives,can have practical applications in oxygen conducting devices such as pressure driven oxygen generators, partial oxidation reactors in electrodes for solid oxide fuel cells (SOFC). ¶ This thesis examines the behaviour of the material at ambient and elevated temperatures using a broad spectrum of solid state experimental techniques such as: x-ray and neutron powder diffraction,thermogravimetric and calorimetric methods,scanning electron microscopy and Mossbauer spectroscopy. Changes in the oxide were induced using conventional thermal treatment in various atmospheres as well as mechanical energy (ball milling). The first experimental chapter examines the formation of the ferrite from a mixture of reactants.It describes the chemical reactions and phase transitions that lead to the formation of the oxide. Ball milling of the reactants prior to annealing was found to eliminate transient phases from the reaction route and to increase the kinetics of the reaction at lower temperatures. Examination of the thermodynamics of iron oxide (hematite) used for the reactions led to a new route of synthesis of the ferrite frommagnetite and strontium carbonate.This chapter also explores the possibility of synthesis of the material at room temperature using ball milling. ¶ The ferrite strongly interacts with the gas phase so its behaviour was studied under different pressures of oxygen and in carbon dioxide.The changes in ferrite composition have an equilibrium character and depend on temperature and oxygen concentration in the atmosphere. Variations of the oxygen content x were described as a function of temperature and oxygen partial pressure, the results were used to plot an equilibrium composition diagram. The heat of oxidation was also measured as a function of temperature and oxygen partial pressure. ¶ Interaction of the ferrite with carbon dioxide below a critical temperature causes decomposition of the material to strontium carbonate and SrFe12O19 . The critical temperature depends on the partial pressure of CO2 and above the critical temperature the carbonate and SrFe12O19 are converted back into the ferrite.The resulting SrFe12O19 is very resistant towards carbonation and the thermal carbonation reaction does not lead to a complete decomposition of SrFeOx to hematite and strontium carbonate. ¶ The thermally induced oxidation and carbonation reactions cease at room temperature due to sluggish kinetics however,they can be carried out at ambient temperature using ball milling.The reaction routes for these processes are different from the thermal routes.The mechanical oxidation induces two or more concurrent reactions which lead to samples containing two or more phases. The mechanical carbonation on the other hand produces an unknown metastable iron carbonate and leads a complete decomposition of the ferrite to strontiumcarbonate and hematite. ¶ Thermally and mechanically oxidized samples were studied using Mossbauer spectroscopy. The author proposes a new interpretation of the Sr4Fe4O11 (x=2.75) and Sr8Fe8O23 (x=2.875)spectra.The interpretation is based on the chemistry of the compounds and provides a simpler explanation of the observed absorption lines.The Mossbauer results froma range of compositions revealed the roomtemperature phase behaviour of the ferrite also examined using x-ray diffraction. ¶ The high-temperature crystal structure of the ferrite was examined using neutron powder diffraction.The measurements were done at temperatures up to 1273K in argon and air atmospheres.The former atmosphere protects Sr2Fe2O5 (x=2.5) against oxidation and the measurements in air allowed variation of the composition of the oxide in the range 2.56 ≤ x ≤ 2.81. Sr2Fe2O5 is an antiferromagnet and undergoes phase transitions to the paramagnetic state at 692K and from the orthorhombic to the cubic structure around 1140K.The oxidized formof the ferrite also undergoes a transition to the high-temperature cubic form.The author proposes a new structural model for the cubic phase based on a unit cell with the Fm3c symmetry. The new model allows a description of the high-temperature cubic form of the ferrite as a solid solution of the composition end members.The results were used to draw a phase diagramfor the SrFeOx system. ¶ The last chapter summarizes the findings and suggests directions for further research.

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Cao, Hu. "Phase transformations in highly electrostrictive and magnetostrictive crystals: structural heterogeneity and history dependent phase stability." Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/28549.

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Ferroelectric and ferromagnetic materials have been extensively studied for potential applications in sensors, actuators and transducers. Highly electrostrictive (1-x)Pb(Mg1/3Nb2/3)-xPbTiO₃ (PMN-xPT) and highly magnetostrictive Fe-xat.%Ga are two such novel materials. Both materials systems have chemical disorders and structural inhomogeneity on a microscale, giving rise to an interesting diversity of crystal structures and novel macroscopic physical properties, which are dependent on thermal and electrical histories of the crystals. In this thesis, I have to investigated phase transformations in these two systems under thermal and field (electric/magnetic) histories, using x-ray and neutron scattering techniques. In PMN-xPT crystals, x-ray and neutron diffractions were performed along the different crystallographic orientations and for different thermal and electrical histories. Various intermediate monoclinic (M) phases that structurally “bridge” the rhombohedral (R) and tetragonal (T) ones across a morphtropic phase boundary (MPB) have been observed. Systematic investigations of (001) and (110) electric (E) field-temperature phase diagrams of PMN-xPT crystals have demonstrated that the phase stability of PMN-xPT crystals is quite fragile: depending not only on modest changes in E (≤ 0.5kV/cm), but also on the direction along which E is applied. Structurally bridging monoclinic Mc or orthorhombic (O) phases were found to be associated with the T phase, whereas the monoclinic Ma or Mb phases bridged the Cubic (C) and R ones. In addition, neutron inelastic scattering was performed on PMN-0.32PT to study the dynamic origin of the MPB. Data were obtained between 100 and 600 K under various E applied along the cubic [001] direction. The lowest frequency zone-center, transverse optic phonon was strongly damped and softened over a wide temperature range, but started to recover on cooling into the T phase at the Curie temperature (TC). Comparisons of my findings with prior ones for PMN and PMN-0.60PT suggest that the temperature dependence and energy scales of the soft mode dynamics in PMN-xPT are independent of PT concentration below the MPB, and that the MPB may be defined in composition space x when TC matches the temperature at which the soft mode frequency begins to recover. High-resolution x-ray studies then showed that the C–T phase boundary shifted to higher temperatures under E by an expected amount within the MPB region: suggesting an unusual instability within the apparently cubic phase at the MPB. In Fe-xat.%Ga alloys, the addition of Ga atoms into the b.c.c. α-Fe phase also results in diversity of crystal structures and structural inhomogeneity, which are likely the source of its unusual magneto-elastic properties. I have carefully investigated decomposition of Fe-xat.%Ga alloys subjected to different thermal treatments by x-ray and neutron diffraction for 12 < x < 25. Quenching was found to suppress the formation of a DO₃ structure in favor of a high-temperature disordered bcc (A2) one. By contrast, annealing produced a two-phase mixture of A2 + DO₃ for 14 < x < 20 and a fully DO₃ phase for x = 25. A splitting of the (2 0 0) and (0 0 2) Bragg peaks observed along the respective transverse directions indicated that Fe-xat.%Ga –crystals' are composed of several crystal grain orientations (or texture structures), which are slightly tilted with respect to each other. In order to investigate the local structural distortions and heterogeneities, neutron diffuse scattering was performed on Fe-x%Ga alloys for different thermal conditions. Diffuse scattering around a (100) superlattice reflection was found for 14 < x < 22 in the furnace-cooled condition, indicative of short-range ordered DO₃ nanoprecipitates in an A2 matrix. This diffuse intensity had an asymmetric radial contour and an off-centering. Analysis (x=19) revealed two broad peaks with c/a–1.2: indicating that the DO₃-like nanoprecipitates are not cubic, but rather of lower symmetry with a large elastic strain. The strongest diffuse scattering was observed for x=19, which correspondingly had maximum magnetostriction: indicating a structural origin for enhanced magnetostriction. Ph. D.

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Gucinski, Ashley Christine. "Gas Phase Structural Studies of Peptide Fragment Ions: Structural Insights into Mass Spectrometry Fragmentation Mechanisms." Diss., The University of Arizona, 2011. http://hdl.handle.net/10150/202766.

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This dissertation presents extensive structural studies of gas-phase peptide fragment ions, with a specific focus on b₂⁺ ions. Fragment ion structures can provide important insights into peptide fragmentation mechanisms. Based on the structures formed, information about the preference of competing b ion formation pathways can be obtained. b₂⁺ ion structures are of interest because of their large relative abundances in MS/MS spectra, which are difficult to predict. Prior to this work, only a few b₂⁺ ion structures were determined; these systems featured only aliphatic residues and all formed oxazolones. The work presented herein examines the influence of basic, acidic, and backbone-attached sidechains on peptide fragmentation mechanisms, as revealed by the resulting b₂⁺ fragment ion structure(s) formed. Specifically, the structures of several histidine, aspartic acid, and proline-containing b₂⁺ ions are determined by using action IRMPD spectroscopy, fragment ion HDX, and DFT calculations. The structures of a series of histidine analogue-containing b₂⁺ ions reveal that the location and availability of the pi-nitrogen is essential for diketopiperazine formation. The histidine sidechain bulk or strain interferes with the complete trans-cis isomerization required for diketopiperazine formation, so the oxazolone structure is also present. Xxx- Pro b₂⁺ ions favor oxazolone formation with aliphatic N-terminal residues. HP favors the diketopiperazine, combining the histidine effect and the proline cis conformation propensity. For Xxx-Asp b₂⁺ ions, aspartic acid significantly influences b₂⁺ ion structure only with an N-terminal histidine or lysine; both HD and KD form a mixture of oxazolone, anhydride, and diketopiperazine structures, presenting the first spectroscopic evidence for the anhydride b₂⁺ion structure. The HA and AH b₂⁺ ions feature the same structures, but HP and PH do not, showing that residue position matters. Additionally, while relative intensities and HDX rates featured some fluctuation, peptide precursor composition differences did not alter the mixture of b₂⁺ ion structures formed for a given b₂⁺ ion. To complement existing gas-phase structural methods, the utility of a new technique, QCID-HDX-IRMPD, was applied to m/z 552.28 from YAGFL-OH. Both the standard b₅⁺ fragment ion and an isobaric non-C-terminal water loss ion are present. Without separation of these isomers, MS/MS spectral interpretation would be complicated.

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Clark, Rowan Elizabeth. "Structural studies of salt hydrates for heat-storage applications." Thesis, University of Edinburgh, 2018. http://hdl.handle.net/1842/31333.

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Salt hydrates have the potential to be used in heat storage as both phase-change materials (PCMs) and thermochemical materials (TCMs). These materials offer advantages over traditional heat storage methods due to their high energy densities. However, both domestic and industrial applications require thousands of thermal cycles and there are often many issues that need to be overcome before these materials can be used reliably for heat storage. One of the major issues with using salt hydrates as PCMs is incongruency - the formation of anhydrous phases during melting. In this research, the mechanisms of the action of polymers to prevent incongruency in sodium acetate trihydrate have been investigated. A new polymorph of anhydrous sodium acetate, Form IV, was obtained in the presence of the polymer. This polymorph crystallises as long, blade-shaped crystals, thereby increasing the surface area to volume ratio. Indexing of the crystal faces revealed that every face had Na+ or the oxygen atoms of the acetate ion near or on the surface, as opposed to hydrophobic methyl groups found on the faces of the anhydrous salt grown without polymer. These two factors are believed to significantly increase the dissolution kinetics. This technique has the potential to be used for screening polymers to reformulate other salt hydrates that display incongruent behaviour. Eutectic compositions of NaCl and KCl with strontium hydroxide octahydrate were investigated as a potential means to prevent the incongruency of this PCM. However, degradation was observed with thermal cycling. Variable temperature PXRD studies discovered a new Sr(OH)2 hydrate when heating above 75 °C - Sr(OH)2. ⅓H2O. The recrystallisation of the octahydrate from the new phase was slow with incomplete conversion, explaining the degradation with continuous cycling. The effect of addition of NaCl and KCl to congruent barium hydroxide octahydrate was also investigated. On heating, a phase transition was observed, but the samples remained solid. Variable temperature PXRD investigations discovered that this was due to the formation of the salt hydrate, Ba(OH)Cl.2H2O. This hydrate melted at 110 °C, showing its potential as a high temperature PCM. The dehydration pathways of magnesium sulfate heptahydrate were investigated. In-situ PXRD studies showed that changing the heating rate changed the intermediates present during the dehydration. The fast dehydration rate saw both the known phases of trihydrate and 2.5 hydrate form as the dehydration product of the tetrahydrate. These both then dehydrated to the known dihydrate. This differed when the slower heating rate was used, as the trihydrate was the only product of dehydration from the tetrahydrate. The trihydrate then proceeded to dehydrate to a new phase. This was found to be a new polymorph of the dihydrate, β-MgSO4.2H2O. Dehydration of MgSO4.7H2O with 50 mol% NaCl was also performed. Loeweite, Na12Mg7(SO4)13.15H2O, a dication sulfate hydrate, was formed as the major intermediate. This mixture showed advantages over the pure MgSO4.7H2O as dehydration to the monohydrate took less time and occurred at a lower temperature. There were also three fewer intermediate phases before dehydration to the monohydrate. Suspension and encapsulation materials were used in order to overcome the major issue of agglomeration with magnesium sulfate. Liquid water was ruled out as a viable hydration medium. Apparatus was developed to test humidity cycling, which allowed the effects of dehydration time and temperature to be investigated, as well testing a range of different formulations.

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17

Ryan, T. W. "High resolution X-ray scattering studies of structural phase transitions." Thesis, University of Edinburgh, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.371891.

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18

Davies, Z. "Application of representation theory to magnetic and structural phase transitions." Thesis, University College London (University of London), 2009. http://discovery.ucl.ac.uk/18908/.

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The tools of representation theory offer us a powerful insight in those terms in a system’s Hamiltonian which cause it to become ordered. Such is its power that, in many fields, the vocabulary of representations has become conventional; crystallography remains a notable exception. This thesis develops the existing methods for applying representation theory to symmetry lowering phase transitions in crystalline systems, and presents examples of its use. The opening section reviews the foundations and previous applications of representation theory to magnetic and structural phase transitions. Complimentary to the mathematical framework is a discussion of the physical interpretation of irreducible representations and basis vectors, the building blocks of any system model constructed in this way. Symmetry arguments are used to qualitatively discuss the symmetry breaking in ferroelectric materials and the role of phase factors in the loss of centro-symmetry. The body of this work is concerned with developing fast, reliable and repeatable methods for applying representation theory to displacive transitions. Calculation of a system’s basis vectors requires both a reliable method, and suitable starting resources. In this section, the first verifiable validation of the tables of Kovalev is presented, along with a strategy for determining the appropriate set of trial functions for use with the method of projection operators. Further, a new module in SARAh-Refine has been written which performs basis vector refinement of powder diffraction data to facilitate quantitative analysis using these techniques. Finally, the techniques of representation theory are applied to two experimental investigations: iron oxyborate and potassium selenate. The use of a single symmetry framework to discuss the structural, magnetic and charge-ordering transitions in these systems demonstrate the power of this technique. Representation theory provides a bridge between structure and properties; this work aims to strengthen the foundations of that bridge.

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19

Amin, Shara Jalal. "Studies of competing interactions in hydrogen bonded systems." Thesis, University of Edinburgh, 1988. http://hdl.handle.net/1842/11976.

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20

Shakhovoy, Roman. "Structural properties and dynamics of alkali sulfates." Thesis, Orléans, 2015. http://www.theses.fr/2015ORLE2029/document.

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Le sujet de cette thèse est principalement ciblé sur l’étude du transport ionique dans les sulfates d’alcalins de la famille LIMSO₄, où M=Na, K, Rb, Cs. Une attention particulière est portée sur l’étude du système LiNaSO₄, le plus intéressant en termes de dynamique ionique, par RMN en fonction de la température. Dans le cadre de cette étude, nous avons effectué des mesures de temps de relaxation et de largeurs de raie pour toute la série de composés. Des mesures de coefficients d’autodiffusion du ₇li et du ²³Na, ont été menées depuis l’ambiante jusqu’à la température de fusion. Pour la première fois, nous avons pu mesurer la cinétique de transition de phase dans LiNaSO₄, à partir d’une nouvelle méthode basée sur la différence de temps de relaxation dans les deux phases de part et d’autre d e la transition, mais sans mesurer forcément le T₁. Cette technique élaborée dans le cadre de ce travail permet de mesurer l’évolution au cours du temps du volume de la phase qui apparait pour des vitesses de refroidissement contrôlées. Nous avons aussi mené une étude par RMN des réorientations des groupements sulfates dans la phase basse température. L’influence des réorientations des SO ₄² sur les couplages quadripolaires au noyau 7li, a été étudiée par un modèle de réorientation par sauts, qui n’avait encore jamais été utilisé pour les sulfates. La méthode proposée est une méthode à « bas couts » car elle permet d’atteindre l’information sur la dynamique des groupements sulfates sans enrichir l’échantillon en ¹⁷O ou de mener des expériences très longues pour le ³³S, ou sans passer par les mesures de temps de relaxation. Afin d’analyser le rétrécissement par le mouvement (motional narrowing) en fonction de la température dans les solides avec deux sous réseaux cationiques diffusant comme dans le cas du LiNaSO₄ , nous avons élaboré un modèle permettant de fitter l’évolution observée à deux marches de la largeur de raie RMN avec la température. La fonction analytique obtenue a été étendue au cas de distributions de temps de corrélation The main goal of a present research is a detailed study of ionic transfer in double sulfates belonging to the LIMSO₄ family, where M = Na, K, Rb, Cs. The most attention has been paid to LiNaSO₄ as to the most interesting (in terms of the ion dynamics) compound among other double sulfates. We have carried out magnetic relaxation measurements and line width analysis for all compounds under consideration. Moreover, PGF NMR measurements of ₇li and ²³Na self-diffusion coefficients in LiNaSO4 have been carried out. For the first time, we have measured the phase transition kinetics in LiNaSO₄. For this purpose, we developed a new technique, which is based on the difference of spin-lattice relaxation times in the two phases, but which does not involve the direct measurement of T₁. Elaborated technique allows measuring time evolution of the volume of the appearing phase at controlled cooling rates. We have carried out NMR study of the sulfate ion reorientations in the low-temperature modification of LiNaSO₄. The influence of the SO ₄² reorientational jumps on the quadrupolar interactions of 7Li nuclei was investigated b y a j ump reorientational model, which has not previously been app lied to sulfates. The proposed method is a “low-cost” technique, since it does not require an ¹⁷O enriched sample and dispenses with time-consuming ³³S NMR. Other advantage of a given method is a possibility to probe reorientational motions without NMR relaxation measurements. To analyze motional narrowing in solids with two diffusing spin sublattices (such case occurs, e.g., in LiNaSO₄) we deduced a formula, which can be used for fitting of the two-step temperature dependencies of the NMR line width. The obtained function has been al so ex tended to the case, when a distribution of correlation times takes place. The advantage of this approach is that even in the case of distribution of correlation times, the fitting function could be expressed in the analytical form

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21

Gonzalez, Vallejo Isabel. "Study of the structural dynamics of phase transitions using time resolved electron and X-ray diffraction." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS496.

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L'application d'une perturbation externe à certains matériaux (tels que la température, la pression ou la lumière) provoque souvent l'émergence de nouvelles propriétés macroscopiques dont l'origine se situe au niveau atomique. Par conséquent, une étude détaillée de la dynamique atomique devient essentielle pour la compréhension de processus tels que les transformations chimiques ou les transitions de phase. Dans le cas particulier des transitions de phase structurales, la symétrie du cristal est soumise à une transformation entre deux états différents à une valeur critique. Les échelles de temps typiques de la dynamique structurale sont de l'ordre de quelques centaines de femtosecondes à plusieurs picosecondes. Le développement des impulsions laser femtosecondes a permis aux chercheurs d'accéder aux échelles de temps nécessaires pour explorer la dynamique structurale ultra-rapide. Plus précisément, la diffraction résolue en temps s'est révélée être une technique idéale pour suivre et dévoiler les voies hors équilibre suivies par le réseau après une courte impulsion laser. Cette thèse présente des études expérimentales réalisées par des techniques de diffraction d'électrons et de rayons X résolue en temps sur deux transitions de phase structurelles différentes. Dans une première partie, nous démontrons les capacités du montage expérimental de diffraction d'électrons ultra-rapide (UED) développé au Laboratoire d'Optique Appliquée. Nos résultats expérimentaux réalisés sur des échantillons monocristallins de haute qualité sont présentés ainsi qu'une étude quantitative des implications des effets de diffraction dynamique dans les expériences UED. Dans une deuxième partie, nous présentons un ensemble d'expériences de diffraction d'électrons pompe-sonde réalisée sur GdTe₃, un composé appartenant à la famille des Tritellurides de terres rares qui présente un état d'onde de densité de charge. L'arrivée d'une excitation optique déclenche la transition de phase de manière non thermique. La dynamique de relaxation est caractérisée par un ralentissement qui augmente avec la fluence incidente ainsi qu'avec la température initiale de l'échantillon. Ces résultats éclairent les interprétations controversées actuelles concernant l’ émergence de défauts topologiques photo-induits.Dans une troisième partie, nous présentons des expériences de diffraction des rayons X en temps résolu réalisées à la ligne CRISTAL du synchrotron SOLEIL. Dans ce cas, nous avons étudié le composé A15 Nb₃Sn, qui présente une transition de phase d'une symétrie cubique à une symétrie tétragonale à l'équilibre thermique. Les résultats de notre expérience pompe-sonde mettent en évidence une réponse différente du réseau par rapport à la transition d’ équilibre thermique, ce qui encourage les futures recherches sur la dynamique structural de ce matériau The application of an external perturbation in certain materials (such as temperature, pressure or light) often gives rise to the emergence of new macroscopic properties with their origin at the atomic level. Therefore, a detailed study of the atomic dynamics becomes essential to the understanding of processes such as chemical transformations or phase transitions. In the particular case of structural phase transitions, the symmetry of the crystal undergoes a transformation between two different states at a given critical value. The typical time scales of the structural dynamics occur on the order of few hundreds of femtoseconds to several picoseconds. The development of femtosecond laser pulses has enabled scientists to access the required time scales to explore the ultrafast dynamics of the lattice structure in the relevant time scales. More precisely, time-resolved diffraction has proven to be an ideal technique to track and unveil the out-of-equilibrium pathways followed by the lattice after a short laser pulse.This thesis presents experimental studies performed by time-resolved electron and X-ray diffraction techniques on two different structural phase transitions. In a first part, we demonstrate the capabilities of the ultrafast electron diffraction (UED) experimental setup developed at Laboratoire d’Optique Apliquée. Our UED experimental results performed on high quality single crystal samples are presented along with a quantitative study of the implications of dynamical diffraction effects in UED experiments. In a second part, we present a set of pump-probe electron diffraction experiments performed on GdTe₃, a compound belonging to the rare-earth Tritellurides family which presents a charge density wave state. The arrival of an optical excitation triggers the phase transition non-thermally with the out-of-equilibrium relaxation dynamics of the charge density wave state characterized by slowing down that increases with incident fluence as well as with initial sample temperature. These results shed more light on current controversial interpretations involving the emergence of photoinduced topological defects. In a third part, we present time resolved X-ray diffraction experiments performed at CRISTAL beamline at SOLEIL synchrotron. In this case we have studied the A15 compound Nb₃Sn, which displays a displacive phase transition evolving from cubic to tetragonal symmetry at thermal equilibrium. Our pump-probe results present evidence of a different lattice response with respect to the thermal equilibrium transition, encouragingf uture investigations on the dynamics of this material

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22

Sengupta, Niladri. "Going beyond the Random Phase Approximation: A systematic assessment of structural phase transitions and interlayer binding energies." Diss., Temple University Libraries, 2018. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/513054.

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Physics Ph.D. The Random Phase Approximation and beyond Random Phase Approximation methods based on Adiabatic Connection Fluctuation Dissipation Theorem (ACFD) are tested for structural phase transitions of different groups of materials, including metal to metal, metal to semiconductor, semiconductor to semiconductor transitions. Also the performance assessment of semilocal density functionals with or without empirical long range dispersion corrections has been explored for the same cases. We have investigated the structural phase transitions of three broad group of materials, semi- conductor to metal transitions involving two symmetric structures, semiconductor to metal and wide bandgap semiconductor to semiconductor transitions involving at least one lower symmetric structure and lastly special cases comprising metal to metal transitions and transitions between energetically very close structural phases. The first group contains Si (diamond → β-tin), Ge (diamond → β-tin) and SiC (zinc blende → rocksalt), second group contains GaAs (zinc blende → cmcm) and SiO 2 (quartz → stishovite) and third group contains Pb (fcc → hcp), C(graphite → diamond) and BN (cubic → hexagonal) respectively. We have found that the difference in behavior of exchange and correlation in semilocal functionals and ACFD methods is striking. For the former, the exchange potential and energy often comprise the majority of the binding described by density functional approximations, and the addition of the correlation energy and potential often induce only a (relatively) small shift from the exchange- only results. For the ACFD, however, non self-consistent EXX typically underbinds by a considerable degree resulting in wildly inaccurate results. Thus the addition of correlation leads to very large shifts in the exchange-only results, in direct contrast to semilocal correlation. This difference in behavior is directly linked to the non-local nature of the EXX, and even though the exchange-only starting point is often nowhere close to experiment, the non-local correlation from the ACFD corrects this deficiency and yields the missing binding needed to produce accurate results. Thus we find the ACFD approach to be vital in the validation of semilocal results and recommend its use in materials where experimental results cannot be straightforwardly compared to other approximate electronic structure calculations. Utilizing the second-order approximation to Random Phase Approximation renormalized (RPAr) many-body perturbation theory for the interacting density-density response function, we have used a so-called higher-order terms (HOT) approximation for the correlation energy. In combination with the first-order RPAr correction, the HOT method faithfully captures the infinite- order correlation for a given exchange-correlation kernel, yielding errors of the total correlation energy on the order of 1% or less for most systems. For exchange-like kernels, our new method has the further benefit that the coupling-strength integration can be completely eliminated resulting in a modest reduction in computational cost compared to the traditional approach. When the correlation energy is accurately reproduced by the HOT approximation, structural properties and energy differences are also accurately reproduced, as confirmed by finding interlayer binding energies of several periodic solids and compared that to some molecular systems along with some phase transition parameters of SiC. Energy differences involving fragmentation have proved to be challenging for the HOT method, however, due to errors that do not cancel between a composite system and its constituent pieces which has been verified in our work as well. Temple University--Theses

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23

Martinavičius, Andrius. "Structural and transport property changes in austenitic stainless steel induced by nitrogen incorporation." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-68719.

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The saturation of the near surface layers of metals with different elements is a powerful tool to change their surface properties. In this work, structure and transport changes induced by incorporation of large amounts of nitrogen at moderate temperatures (∼370−430°C) in austenitic stainless steel are investigated. The structural study of the plasma nitrided ASS has been carried out using a combination of global (X-ray diffraction, nuclear reaction analysis) and local probe techniques (Mossbauer, X-ray absorption near edge structure, extended X-ray absorption fine structure spectroscopies). It reveals that nitriding at moderate temperatures (∼400°C) results in the nitrided layer with Fe, Cr and Ni being in different local chemical environments: Cr in the CrN-like state, Fe in the Fe4N-like state, Ni in the metallic state. The results demonstrate that the incorporation of interstitial nitrogen destabilizes homogeneous distribution of the ASS constituents, which leads to the segregation of the elements into small zones rich in Cr and Ni and provide strong evidence that the decomposition is of a spinodal nature. These experimental findings contradict the widely accepted view that the phase formed during nitriding at moderate temperatures is a homogeneous supersaturated nitrogen solid solution. The nitrogen atomic transport study has been carried out by using ion beam nitriding of single-crystalline stainless steel, and the issues of the influence of the crystalline orientation, nitriding temperature, ion flux and ion energy are addressed. The diffusion coefficients have been extracted from the fitting of the nitrogen depth profiles by using the trapping-detrapping model. It is shown that the crystalline orientation plays a significant role in nitrogen diffusion: the penetration depth is largest, intermediate and lowest for the (001), (110) and (111) orientation, respectively. The pre-exponential factor D0 varies by two orders of magnitude depending on the orientation, while the activation energy E is similar (∼1.1 eV) for the (111) and (110) orientations and higher for the (001) orientation (∼1.4 eV). It is found that the nitrogen ion energy and the flux have the effect on the nitrogen transport in the bulk with higher energies (or fluxes) showing larger diffusion coefficients. The ion energy effect is more pronounced for the (001) than for the (111) orientation, while the flux effect is similar in both orientations. In addition, the diffusivity during post-nitriding thermal annealing without ion irradiation is found to be independent of the crystalline orientation. The observed radiation enhanced diffusion and anisotropy are discussed on the basis of nitrogen incorporation induced changes in the matrix structure (ASS decomposition and formation heterogeneous structure), ion bombardment induced effects (defects, localized vibrations) and correlated diffusion.

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24

Zheng, Pingping. "Bayesian analysis of structural change in trend." Thesis, Nottingham Trent University, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.391001.

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25

McCaffrey, Philip D. "Equilibrium structures from gas-phase electron-diffraction data." Thesis, University of Edinburgh, 2007. http://hdl.handle.net/1842/2601.

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For the past 75 years gas-phase electron diffraction (GED) has remained the most valuable technique for determining structures of small molecules, free from intermolecular interactions. Throughout this period many improvements have been made to both the experimental and theoretical aspects of this technique, leading to the determination of more accurate structures. As the uncertainties associated with many stages of the process have been greatly reduced, errors introduced by assumptions, which were previously neglected, now play an important role in the overall accuracy of the determined structure. This work is focused on two such areas, namely the treatment of vibrational corrections and the vibrational effects on the scattering of individual electrons by multiple atoms. A novel method has been developed which allows the extraction of equilibrium structures (re) from distances obtained directly from GED experiments (ra). In unfavourable cases (such as small molecules with large-amplitude and / or highly anharmonic modes of vibration) traditional methods can introduce errors of comparable size to those obtained from the experiment. The newly developed method, EXPRESS (EXPeriments Resulting in Equilibrium StructureS), overcomes the problems which have plagued previous attempts through exploring a more extensive region of the potential-energy surface (PES), specifically regions relating to the normal modes of vibration. The method has been applied, initially, to sodium chloride in the gas phase as this contains dimer molecules with very low-frequency large-amplitude modes of vibration. The experimentally determined re structure gives good agreement with high-level ab initio calculations. Following this success, the EXPRESS method was then applied to sodium fluoride, sodium bromide and sodium iodide, giving similarly good agreement with theoretical calculations. The regular mixed alkali halide dimers (D2h symmetry) cannot be studied by microwave spectroscopy as they do not have a permanent dipole moment. However, vi mixed dimers (C2v) and asymmetric dimers (Cs) do not suffer from this constraint. Using insights learned from the ab initio studies of the sodium halides, geometries and dipole moments have been calculated for a range of mixed and asymmetric alkali halide dimers to enable their study by microwave spectroscopy. A multi-dimensional version of the EXPRESS method has been applied to the lowfrequency modes of chlorofluoroacetylene and chlorodifluoronitrosomethane to assess the effects of coupling between these modes of vibration in these structurally challenging systems. To obtain re structures of larger molecules a second method, using molecular dynamics (MD), has been developed and has been implemented on two test cases: the sodium chloride dimer and octasilsesquioxane. Traditional scattering theory used in GED employs the first-order Born approximation (FBO). However, this ignores any multiple scattering events, which are important for heavier atoms. Using a method similar in nature to EXPRESS a full vibrational analysis of three-atom scattering has been conducted on tellurium dibromide and tellurium tetrabromide.

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26

Seyyidoglu, Semih. "High Temperature Chemistry Of Some Borophosphates, Phase Relations And Structural Studies." Master's thesis, METU, 2003. http://etd.lib.metu.edu.tr/upload/1045863/index.pdf.

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The solid state, hydrothermal and flux methods were used for the investigation of alkaline earth and transition metal borophosphate compounds. The products and the phase relations were investigated by XRD, IR, DTA, and EDX methods. The solid state reactions of several boron compounds with different phosphating agents have been studied in the temperature range of 400-1200 oC. Hydrothermal and flux techniques were performed at 150 oC and 1200 oC, respectively. On the other hand, an attempt has been made to prepare a novel borophosphate compound MIIMIV[BPO7] (where MIV= Zr4+, Si4+, and MII= Sr2+, Ca2+) by solid state reactions and to investigate intermediate and final products. (NH4)2HPO4 and NH4H2PO4 were used as a phosphating agent. For the synthesis of these new compounds, the following reaction was predicted using the stoichiometric amount of the reactants: 2MIVO2 + 2MIICO3 + B2O3 + 2(NH4)2HPO4 &amp #8594 2MIIO.MIVO2.B2O3.P2O5 + 4NH3 + 3H2O + 2CO2 (According to IUPAC formulation for the compounds composed of oxides) In the case of MIV=Zr4+ and MII=Sr2+, the formation of ZrSr[BPO7] was observed together with ZrO2 and SrBPO5. The formation of a new phase was proved by indexing the XRD pattern of the product after separating ZrO2 and SrBPO5 lines. Its crystal system was found to be orthorhombic and the unit cell parameters are a=11.85&Aring , b=12.99 &Aring , c=17.32 &Aring . IR analysis shows that there is [BPO7]6- bands in the spectrum. At higher temperatures, Sr7Zr(PO4)6 was obtained. In the case of MIV=Si4+, SrBPO5 was the main product together with unreacted SiO2. At 1100 oC, Si4+ entered SrBPO5 structure and the product was indexed in orthorhombic system with a=8.9243 &Aring , b=13.1548 &Aring , and c=5.4036 &Aring . Several other M:B:P ratios were tried for solid state systems. For compositions with different cations (such as Al3+, Ca2+, Na+), reactions generally pass through metal phosphates and BPO4. The X-ray diffraction powder pattern and infrared spectrum of several intermediate products obtained at different temperatures were presented and the several phase relations were investigated. The DTA and EDX analyses of some products were also reported.

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27

Nomura, Seiji. "Fundamental aspects of coal structural changes in thermoplastic phase during carbonisation." Thesis, University of Newcastle Upon Tyne, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.361542.

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28

Padlewski, Stephane. "The theory of structural phase transitions with particular reference to mullite." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239516.

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29

Johnston, Blair Fraser. "ed@ed : a new gas-phase electron diffraction structural refinement program." Thesis, University of Edinburgh, 2002. http://hdl.handle.net/1842/12307.

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As gas-phase electron diffraction (GED) is only used routinely for structure determination by a small number of groups world-wide, structural refinement software tends to be developed specifically for each particular group. Work in this thesis is concerned with the upgrading of the old MS-DOS based structural refinement software, Ed96, to a new MS-Windows application, ed@ed, complete with graphical user interface. The modifications made to the original Ed96 code in producing ed@ed warranted thorough testing to ensure that errors had not been introduced to the new program. Seven distinctive structural refinements were carried out using ed@ed in order to achieve this. The test cases presented in this thesis are 1,3,5-trisilylbenzene and hexasilylbenzene (in chapter 3), 1 -bromopentaborane(9) and 2-bromopentaborane(9) (in chapter 4), Ru(η-C5Me5) (η-C5F5) (in chapter 5) and Me3SnC4F9 and Me3SnO2CC2F5 (in chapter 6).

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30

Andreev, A. A., S. N. Grigorev, O. V. Sobol, M. A. Volosova, V. A. Stolbovoy, and V. Ye Filchikov. "Structural-phase and Strained State of Vacuum-ARC Mo-N Coatings." Thesis, Сумський державний університет, 2012. http://essuir.sumdu.edu.ua/handle/123456789/34806.

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The effect of substrate bias on the structural-phase and elastic stress-strained state during the formation of vacuum-arc deposited nanostructural coatings of the Mo–N system has been studied. An increase in the bias potential leads to (i) predominant [111] orientation of the growing molybdenum nitride crystals with a NaCl-type cubic lattice (γ-Mo2N phase) and (ii) the appearance of a second phase with a body-centered cubic crystal lattice that is characteristic of pure molybdenum. The elastically strained (stressed) state of the coating is determined not only by the conditions of deposition, but also by the mechanical properties of a substrate. In order to provide for formation of coatings in a high-elastic-strained (stressed) state, it is necessary to deposit coatings onto substrates with high elastic modulus, which prevent metal flow at the interface. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/34806

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31

Gründmayer, Jürgen. "TEM investigations on magnetic and structural phase transitions at low temperatures /." Berlin : Logos-Verl, 2009. http://d-nb.info/995946256/04.

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32

Chapman, Brandon D. "The role of disorder in structural phase transitions in perovskite ferroelectrics /." Thesis, Connect to this title online; UW restricted, 2003. http://hdl.handle.net/1773/9692.

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Qasim, Ilyas. "Structural and Electronic Phase Transitions in Mixed Transition Metal Perovskite Oxides." Thesis, The University of Sydney, 2013. http://hdl.handle.net/2123/10029.

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The reported multiferroic perovskite series Sr1-xAxTi1/2Mn1/2O3 has been the subject of numerous structural studies, without reaching consensus. In the current work, the cubic Pm3 ̅m is confirmed for end member SrTi1/2Mn1/2O3 in the Sr1-xAxTi1/2Mn1/2O3 ( A= Ca, La; 0 ≤ x ≤ 1) series. The Pm3 ̅m  I4/mcm  Pbnm structural evolution was observed with increased doping level of Ca. A cubic Pm3 ̅m  rhombohedral R3 ̅c transition occurred when La is substituted instead of Ca. Interesting magnetic behaviours were observed and the major contribution to this was concluded to be the mixed Mn4+/Mn3+ ratio. Ru and Ir have almost identical ionic radii and behave similarly in many ways. Remarkably the structure and properties of SrRuO3 and SrIrO3 are different. The current study revealed that the divalent transition metal doped materials of the type SrR1-xMxO3 (R = Ru, Ir, and M = 3d transition metals) are isostructural. This was achieved by the synthesis of a number of new materials of the type SrIr1-xMxO3. Therefore, these two series are comparatively described in the thesis. The structure and physical properties of the iron doped series SrIr1-xFexO3 are found to be different from those of the divalent doped ones, and this was even true for Ru analogues. Therefore, Fe-doped SrRuO3 and SrIrO3, based on the results of the same level doped materials are presented in a separate chapter. In the final chapter, the impact of Cu2+ doping on the structure and electronic properties of LaCrO3 is described. In order to understand structure property relationships, all the materials structurally characterised have had magnetic and resistivity measurements conducted. Special attention is given to realise the correlations between structure, magnetism, and conductivity.

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34

Winters, Ian Douglas. "The rheological and structural properties of blends of polyethylene with paraffin wax." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/45833.

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This research addresses and illuminates a little understood region of miscible polymer mixtures and demonstrates a new means of separating wax from such blends. The method, termed Deformation Induced Phase Segregation potentially eliminates need of toxic processing solvents for wax removal or recovery in these types of blends. Previous theories of polymer combinations address them exclusively as solutions or as blends, two independent classes having very different behaviors. This study provides bridge connecting these two classes by identifying crossover points between them and the behaviors exhibited therein. The blends of this form were found to be semi-miscible, forming a homogenous phase in the melt but a two-phase system in the solid, with the rheological behavior influenced by the polymer's molecular weight and architecture. It also demonstrates practical promise of this regime by introducing a mechanical compression process to separate the wax phase from such a type of blend. This process potentially permits production of ultra-high molecular weight polyethylene (UHMwPE) films and fibers by melt processing, thereby obviating need of otherwise essential but expensive and environmentally unfriendly toxic solvents.

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Helbley, Jennifer A. "Structural effects on perylene bis(dicarboximide) organization in aqueous and solid phase." abstract and full text PDF (free order & download UNR users only), 2006. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3221396.

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36

Danz, Thomas Christian [Verfasser]. "Ultrafast transmission electron microscopy of a structural phase transition / Thomas Christian Danz." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2021. http://d-nb.info/1239061234/34.

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37

Szilágyi, Anna Tünde. "Structural characterization of liver fibrosis in magnetic resonance images." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:1860b1d9-2b10-409f-9220-e12b002a9e32.

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The overall clinical motivation of this thesis is to differentiate between the different stages of liver disease stratifying into: no disease, mild disease, and severe fibrosis using Magnetic Resonance Imaging (MRI). As a related aim, we seek to differentiate as much as possible pericellular and nonpericellular fibrosis. This latter is clinically important, but currently no method exists that is able to perform this. Quickly, we realised that these aims push low level image analysis beyond their current bounds and so a great deal of the thesis is dedicated to extending such techniques before they can be applied. To work on the most fundamental low level image analysis concepts and algorithms we choose one of the most recent developments, namely continuous intrinsic dimensionality (ciD), which allows the continuous classification of homogeneous patches from 1D structures to intrinsically 2D structures. We show that the current formalism has several fundamental limitations and we propose a number of developments to improve on these. We re-evaluated feature energy statistics that were originally proposed in ciD, and additionally we examined the confidence one may have in stateof- the-art methods to estimate the orientation of features. We show that new statistical methods are required for feature energy, and that orientation predictability is more important than correctness of the estimation. This evaluation led us to the monogenic signal local orientation. Analysis of feature or texture energy is also a main contribution of this thesis. Within this framework we propose the Riesz-weighted phase congruency model. This is able to detect internal texture structures but it is not capable of delineating boundaries. Nevertheless, it proves an appropriate basis for texture quantification. Finally, we show that in contrast to using the standard established Kovesi approach, the developed texture measure leads to good results on the suboptimal T1w MRI liver image staging images. We show that we are able to differentiate automatically between the separate disease scores and between pericellular and non-pericellular fibrosis.

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38

Link, Jessica. "Stabilization and structural study of new nanocomposite materials." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSE1238/document.

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Ce travail de thèse a été consacré au développement de nanocomposites polymère (PVDF-co-HFP) – silice hautement chargés en nanoparticules par voie solvant. La combinaison d’un polymère fluoré ; ayant des propriétés mécaniques, diélectriques, piézoélectriques et pyroélectriques très intéressantes ; avec des nanoparticules de silice pourrait nous permettre le développement de nouvelles solutions dans le domaine de l’énergie. Ainsi nous avons étudié l’impact de la chimie de surface des nanoparticules de silice, des paramètres expérimentaux et du procédé par voie solvant sur la structure et les propriétés finales des matériaux obtenus. Pour la réalisation de ces matériaux par voie solvant, une solution P(VDF-co-HFP) – silice dans un solvant commun est préparée puis séchée. Dans le cadre de ce projet nous avons travaillé avec des cétones aliphatiques présentant différentes longueurs de chaînes: la Methyl Ethyl Ketone (MEK) et la 2-heptanone. Ainsi pour la préparation des solutions nous avons développé une approche générique pour transférer des nanoparticules en solution dans l’eau dans un solvant organique par l’intermédiaire d’un agent de transfert. Cependant, avant l’étape de dépôt et séchage, nous avons constaté que le PVDF et ses copolymères (dont le P(VDF-co-HFP)) forment un gel thermoréversible dans ces deux solvants ; avec ou sans silice. Cette gélification impacte certainement l’étape de solvent casting or aucun consensus concernant les mécanismes responsables de la gélification de ce polymère n’a été trouvé dans la littérature. Il apparait primordial de comprendre les mécanismes de gélification du P(VDF-co-HFP) avant d’étudier les propriétés des nanocomposites. Pour cela nous avons étudié l’influence de différents paramètres : nature du solvant, concentration en copolymère, température, présence de charges (concentration et chimie de surface) sur les mécanismes de gélification (Chapitre 3). La cinétique de gélification a été étudiée par tube-tilting et rhéologie linéaire. Une combinaison de RMN 19F, DSC, SAXS, WAXS et rhéologie non-linéaire a été utilisée pour déterminer les mécanismes réponsables de la gélification du P(VDF-co-HFP) dans ces solvants. La 19F RMN a montré l'existence d'un réseau de polymère où des zones rigides agiraient comme des noeuds de réticulation. Ce comportement n'a été observé qu'à l'état gel: aucun réseau de polymère ou aucune zone rigide n'existent quand la solution de polymère est à l'état liquide. La nature et la fraction volumique de ces zones rigides présentes à l'état gel ont été étudiés par DSC et diffraction des rayons X. Une faible fraction de cristallites, qui correspondrait aux zones rigides, a été observée dans ces gels. Dans une troisième étape (Chapitre 4), l'impact de l'addition de silice sur la gélification du P(VDF-co-HFP) a été étudié. Pour cela, des gels polymère-silice ont été formulés dans la MEK et la 2-heptanone en utilisant deux sources de silice nanométrique: celle préparée par transfert de phase et une solution commerciale Nissan. Nous avons observé que la présence de silice impacte peu la gélification du polymère contrairement au procédé, et plus particulièrement à la présence d'eau résiduelle dans le matériau. La rhéologie non linéaire des gels dans la 2-heptanone, chargés ou non en silice, a ensuite été étudiée sous LAOS. Le comportement de ces matériaux varie avec la contrainte et passe d'un régime élastique à un régime visqueux. Cependant avant de passer dans le domaine visqueux, les gels non chargés montre un fort strain-hardening. Au contraire, l'incorporation de nanoparticules inhibe le strain-hardening. Dans une dernière étape, des films P(VDF-co-HFP)-silice ont été préparés par voie solvant. Des matériaux homogènes contenant jusqu'à 40wt% de nanoparticules ont été obtenus, avec des états de dispersions différents et finement controlés. L'impact de la concentration et de l'état de dispersion sur les propriétés mécaniques a été finalement étudiés The general context of this PhD thesis is the development of highly filled polymer– silica model nanocomposites by solvent route, based on copolymers of vinylidene fluoride and hexafluoropropylene, denoted as P(VDF-co-HFP). Due to their unique combination of mechanical, dielectric, piezoelectric and pyroelectric properties, PVDF and VDF-copolymers combined with silica are good candidates for new advanced applications like actuation or energy harvesting. In this context, the primary objective was to understand how the filler surface chemistry and formulation parameters, as well as the solvent casting process, affect the final structure and properties of the materials. To elaborate P(VDF-co-HFP) nanocomposites with silica by solvent casting, a first step was to prepare solutions of silica and the polymer in a common solvent which will be subsequently dried. Aliphatic ketone solvents of various chain length, namely methyl ethyl ketone (MEK) and 2-heptanone, were used. For this purpose a generic approach to transfer silica nanoparticles from water to organic solvent through a transfer agent was developed. VDF-based copolymer solutions (filled or not with nanoparticles) exhibit thermoreversible gelation in ketone solvents, which may subsequently impact the processing of materials by solvent casting. Studying the phenomenology of PVDF gelation in thus of major concern for controlling the processing of those nanocomposites. No general consensus on the gelation mechanisms is found in the literature. In this context, the gelation of a semi-crystalline P(VDF-co-HFP) in MEK or 2-heptanone was studied. The gelation kinetics was investigated by tube-tilting and linear rheology. A combination of 19F Nuclear Magnetic Resonance (NMR), DSC, SAXS, WAXS and nonlinear rheology was used to probe the structure of these systems and the gelation mechanisms. 19F NMR shows the occurrence of a polymer network-like structure with rigid zones which may act as cross-links. Such a behavior is only observed in the gel state: no polymer network or rigid zones are present when the polymer solution is in the liquid state. The nature and volume fraction of the rigid zones present in the gel state were investigated with DSC and X-ray diffraction. A small crystalline fraction, which may correspond to the rigid zones, is observed in gels. In a third step, the impact of adding silica nanoparticles on the gelation kinetics of P(VDF-co-HFP) was studied. To do so, copolymer – silica gels were formulated in MEK and 2-heptanone using two sources of organic silica solutions, the one prepared previously by phase transfer and a commercial one from Nissan. We found that the presence of silica nanoparticles hardly disturbs the gelation of P(VDF-co-HFP). Conversely, the process, and more particularly the presence of residual water within the material, has a strong impact on the structure of the gels and gelation kinetics.The nonlinear rheological properties of the P(VDF-co-HFP) physical gels filled or not with silica nanoparticles were investigated in 2-heptanone only (as MEK is too volatile), using Large Amplitude Oscillatory Shear (LAOS) experiments (Chapter 5). The behavior of all materials changes from elastic at small strain to viscous at high strain amplitudes. Before abruptly changing from elastic to viscous behavior, unfilled gels exhibit strong strain-hardening. Incorporating silica nanoparticles is found to inhibit strain-hardening. In a last step, P(VDF-co-HFP) – silica solid films were obtained by solvent casting the previously prepared systems. Homogeneous materials filled up to 40 wt% of silica nanoparticles were obtained, with different and finely controlled dispersion states of nanoparticles, related to the structure of the initial gels or solutions. The impact of the concentration and dispersion state of the fillers on the mechanical performances (particularly the mechanical toughness) was investigated

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39

Nicholls, Ursula Joy. "An investigation of a structural phase transition : the R-point instability in KMnF₃." Thesis, University of Edinburgh, 1987. http://hdl.handle.net/1842/15519.

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40

Nallapaneni, Asritha. "BIOMIMETIC NON-IRIDESCENT STRUCTURAL COLORATION VIA PHASE-SEPARATION OF COMPATIBILIZED POLYMER BLEND FILMS." University of Akron / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=akron1590749223865508.

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41

Blakey, David Mark. "Structural studies of vapour-deposited amorphous-ice and argon/amorphous-ice systems by neutron diffraction." Thesis, University of Kent, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240416.

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42

Magyari-Köpe, Blanka. "Structural stability of solids from first principles theory." Doctoral thesis, KTH, Physics, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3366.

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43

Shanmugam, Janaki. "Correlation of optical anisotropy with structural changes in Ge2Sb2Te5." Thesis, University of Oxford, 2018. http://ora.ox.ac.uk/objects/uuid:8cee7355-0ff6-4939-a606-a406c7a9823d.

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Ge2Sb2Te5 (GST) is an established phase-change material that undergoes fast reversible transitions between amorphous and crystalline states with a high electro-optical contrast, enabling applications in non-volatile optical and electronic memories and optically-switchable structured metamaterials. This work demonstrates that optical anisotropy can be induced and recorded in pure and doped GST thin films using circularly polarised light (CPL), opening up the possibility of controlled induction of anisotropic phase transition in these and related materials for optoelectronic and photonic applications. While the amorphous-to-crystalline phase transition in GST has generally been understood to proceed via a thermal mechanism, significant optical anisotropy (measured by circular dichroism (CD) spectroscopy in this work) strongly suggests that there is an electronic athermal component of the phase change induced by the handedness of circularly polarised nanosecond laser pulses and implies the existence of chiral structures or motifs. Optically active and inactive regions in the films have also been studied using X-ray and electron diffraction and spectroscopic techniques in order to obtain a structural picture that can be correlated to the optical changes observed and the findings offer surprising evidence of the nature of the phase transition. Regions exhibiting higher CD signal intensities were found to be mostly amorphous with elemental phase separation observed within modified surface features. Several mechanisms are proposed for the observed phenomena, including the retention of chiral crystalline fragments in laser- irradiated and melt-quenched amorphous regions, which could explain the results of CD spectroscopy. This may be extended to other material systems and harnessed in potential metamaterials, plasmonics, photonics or chiroptical applications.

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44

Baldo, Silvia. "Innovative steels for structural and corrosion resistance applications." Doctoral thesis, Università degli studi di Padova, 2011. http://hdl.handle.net/11577/3422747.

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The attention towards innovative steels at limited cost increased significantly in the last years. The research focused mainly in the development of new high strength steels where a combination of elevated mechanical properties, good formability and weldability is required and of duplex stainless steels if high corrosion resistance and mechanical properties are demanded. The possibility to design new light components thanks to the higher strength of such steels, the substitution of expensive raw elements and the new specific production processing have permitted to achieve a global costs saving. However a deep knowledge about the critical aspects of these two classes of steels is of fundamental importance to avoid problems in service and eventually catastrophic failures. The aim of this study was to analyze the effects of metallurgical features and phase transformations on the properties of duplex stainless steels (DSS), high strength low alloy (HSLA) steels and advanced high strength dual phase (DP) steels. A detailed review on the state of art of the innovative steels considered has been carried out. The experimental work has been organized into two sections dealing with the critical aspects of duplex stainless steels and high strength steels. In the section concerning DSS, an overall study about secondary phase precipitation occurring during heat treatments of different DSS grades was performed. Then a deeper investigation on lower alloyed DSS, the so called Lean DSS, and their behavior was analyzed. In particular a relation between the morphology of intermetallics precipitation and the fracture toughness was found and compared for two Lean DSS. To reduce the costs, strong austenite phase stabilizers such as Ni are substituted with less stabilizing element as Mn, leading to a certain austenite phase (γ) instability which eventually transforms into ferromagnetic lath martensite (α') during plastic deformation. This phase transformation can potentially affect the properties of the material. Therefore the possible γ→α' evolution during cold rolling was evaluated mainly through magnetic and X-ray diffraction techniques. The second section focused on the influence of microstructure on the mechanical properties and weldability of high strength and advanced high strength steels. Fatigue behavior and weldability are of extreme importance in these two types of steels, especially if designed for structural automotive applications. Hence the role of microllaoying elements and thermo-mechanical processing on fatigue properties and fracture was revealed for different micro-alloyied HSLA steels, whereas the influence of braze-welding parameters on microstructural and mechanical properties was highlighted in a DP steel. Nel corso degli ultimi anni la ricerca si è focalizzata sulla messa a punto di acciai innovativi a costo contenuto. Grande interesse è stato posto sullo sviluppo di nuovi acciai alto resistenziali in grado di avere una buona combinazione di elevate proprietà meccaniche, formabilità e saldabilità, e sullo sviluppo di acciai inossidabili bifasici nelle applicazioni richiedenti alta resistenza a corrosione e proprietà meccaniche. Inoltre la possibilità di progettare con materiali più leggeri, grazie all’elevata resistenza meccanica che presentano, il risparmio dovuto alla sostituzione di elementi costosi e all’utilizzo di nuovi processi produttivi hanno permesso una riduzione globale dei costi. Al fine di evitare problemi in esercizio con eventuali rotture catastrofiche si è resa necessaria una approfondita conoscenza degli aspetti critici di queste tipologie di acciai di ultima generazione, considerato il potenziale ampio utilizzo di tali materiali anche in applicazioni di uso comune. L’obiettivo di questa tesi è di analizzare gli effetti da un punto di vista metallurgico sulle proprietà principali caratterizzanti gli acciai inossidabili Duplex (DSS), gli acciai basso legati ad alto limite di snervamento (HSLA) e gli acciai alto resistenziali avanzati Dual Phase. Per una maggiore completezza della ricerca e una migliore interpretazione dei risultati ottenuti nello studio sperimentale è stata condotta una dettagliata ricerca bibliografica sullo stato dell’arte delle categorie di acciai considerati. Il lavoro sperimentale è stato diviso in due parti nelle quali sono stati messi in luce gli aspetti critici degli acciai inossidabili duplex e degli acciai alto resistenziali. La sezione riguardante gli acciai inossidabili duplex comprende uno studio completo sui fenomeni di precipitazione di fasi secondarie che hanno luogo durante trattamento termico di diverse tipologie di tali acciai. In una fase successiva lo studio si è concentrato sui cosiddetti “Lean” Duplex, caratterizzati da un minore contenuto di elementi in lega. In particolare è stata rilevata una certa relazione tra la presenza e distribuzione di fasi infragilenti e le proprietà di tenacità di due acciai inossidabili “Lean” Duplex. Negli acciai “Lean” Duplex il contenuto di elementi costosi e volatili quali il Ni è ridotto per mantenere contenuto il loro costo. Il Ni viene sostituito principalmente dal Mn, avente tuttavia un minore potere stabilizzante nei confronti della fase austenitica (γ), che potenzialmente può evolvere in “lath” martensite ferromagnetica (α') con la deformazione a freddo. L’introduzione di questa nuova fase nel materiale può indurre cambiamenti nelle proprietà del materiale stesso. Pertanto la possibile trasformazione γ→α' in seguito a laminazione a freddo è stata valutata mediante misure magnetica e diffrazione a raggi X. La seconda parte del lavoro è stata incentrata sull’influenza della microstruttura sulle proprietà meccaniche e di saldabilità di acciai alto resistenziali. Le proprietà a fatica e la saldabilità sono di estrema importanza in questa classe di acciai, specialmente se destinati ad applicazioni nel campo auto motive. È stato quindi analizzato il ruolo che gli elementi microalliganti e gli specifici trattamenti termo meccanici rivestono sulle proprietà a fatica e sul relativo meccanismo di frattura in diverse tipologie di acciai HSLA. Inoltre sono stati valutati gli effetti della variazione dei parametri di saldobrasatura sulle proprietà microstrutturali e meccaniche di un acciaio DP.

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Murphy, Helen M. "Investigation of the structural and magnetic phase transitions in CeSiₓ by neutron scattering". Thesis, Loughborough University, 1993. https://dspace.lboro.ac.uk/2134/27056.

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In an attempt to gain a greater understanding of the magnetic interactions within the rare-earth metallic alloy CeSix (1.60 ≤ x ≤ 2.00) and the validity of the various theories of magnetism, intermediate valency and the 'dense' Kondo effect that are commonly invoked to account for its magnetic behaviour, the structural and magnetic properties of CeSi1.80 and CeSi1.85 samples have been investigated using the technique of thermal neutron scattering. Spin polarized neutrons and neutron spin polarization analysis have been employed to unambiguously separate the paramagnetic and antiferromagnetic scattering of CeSix samples from all other scattering contributions.

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46

Dawson, Simon Jonathon. "Solid-Phase-based Synthesis and Biological Evaluation of Quinoxaline Antibiotics and Structural Analogues." Thesis, University College London (University of London), 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.509309.

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47

Khoshnevisan, Bahram. "Diffraction studies of the structural phase transition in the high temperature superconductor YBCO." Thesis, University of Salford, 2002. http://usir.salford.ac.uk/14732/.

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48

Szkoda, Iwona. "Phase relations, structural studies and physical properties of mixed metal oxides and sulphides." Thesis, Heriot-Watt University, 2008. http://hdl.handle.net/10399/2212.

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The phase relations in three oxide systems; ZnO–BiVO4, Pb2V2O7–BiVO4 and PbO– BiVO4, have been studied and their phase diagrams over the whole component concentration range up to 1273 K have been established. As a result of solid-state reaction between ZnO and BiVO4 mixed at a molar ratio of 2:1 or among ZnO, V2O5 and Bi2O3, mixed at a molar ratio of 4:1:1, a new double vanadate BiZn2VO6 has been obtained. Its crystallographic system was determined, its unit cell parameters were calculated and its incongruent melting temperature was established. A new compound is also formed in the Pb2V2O7–BiVO4 system. It has been shown that BiVO4 and Pb2V2O7 react with each other forming a compound of the formula Pb2BiV3O11, when their molar ratio is equal to 1:1, or between PbO, Bi2O3 and V2O5, mixed at a molar ratio of 4:1:3. This material melts congruently and it crystallises in the triclinic system. A new series of non-stoichiometric sulphides Ga1-xGexV4S8 (0 x 1) has been synthesised by standard solid-state reaction. The samples have been characterised by powder X-ray and neutron diffraction, SQUID magnetometry and electrical transportproperty measurements. Structural analysis reveals that a solid solution is formed throughout this composition range. Magnetic measurements suggest that the ferromagnetic behaviour of the end-member phase GaV4S8 is retained at x 0.7. By contrast Ga0.25Ge0.75V4S8 appears to undergo antiferromagnetic ordering at ca. 15 K. All materials with x ¹ 1 are n-type semiconductors whose resistivity falls by almost six orders of magnitude with decreasing gallium content, whilst the end-member phase GeV4S8 is a ptype semiconductor. Powder neutron diffraction studies show that the cubic unit cell is retained for non-stoichiometric materials to the lowest temperatures studied. Single crystals of five erbium-chromium sulphides have been grown by chemical vapour transport using iodine as the transporting agent. Single-crystal X-ray diffraction reveals that in Er3CrS6, octahedral sites are occupied exclusively by Cr3+ cations, leading to onedimensional CrS4 5- chains of edge-sharing octahedra, whilst in Er2CrS4, Er3+ and Cr2+ cations occupy the available octahedral sites in an ordered manner. By contrast, in Er6Cr2S11, Er4CrS7 and Er8Cr3S15, Er3+ and Cr2+ ions are disordered over the octahedral sites. In Er2CrS4, Er6Cr2S11, Er4CrS7 and Er8Cr3S15, the network of octahedra generates an anionic framework constructed from M2S5 slabs of varying thickness, linked by onedimensional octahedral chains. This suggests that these four phases belong to a series in which the anionic framework may be described by the general formula [M2n+1S4n+3]x-, with charge balancing provided by Er3+ cations located in sites of high-coordination number within one-dimensional channels defined by the framework. Er4CrS7, Er6Cr2S11, Er8Cr3S15 ii and Er2CrS4 may thus be considered as the n = 1, 2, 3 and members of this series. Whilst Er4CrS7 is paramagnetic, successive magnetic transitions associated with ordering of the chromium and erbium sub-lattices are observed on cooling Er3CrS6 (TC(Cr) = 30 K; TC(Er) = 11 K) and Er2CrS4 (TN(Cr) = 42 K, TN(Er) = 10 K) whereas Er6Cr2S11 exhibits ordering of the chromium sub-lattice only (TN = 11.4 K). These four materials have been studied using neutron diffraction which allowed magnetic ordering to be examined.

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Byakova, A. V., V. V. Cherednichenko, A. A. Scheretskiy, A. A. Vlasov, and A. I. Yurkova. "Structural Evolution and Phase Transformation in Nanoquasicrystalline Al-Fe-Cr Alloy: DSC Analysis." Thesis, Sumy State University, 2012. http://essuir.sumdu.edu.ua/handle/123456789/34854.

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Kinetic parameters for microstructural evolution and phase transformation in water atomized Al-Fe-Cr based alloy with nominal composition Al94Fe3Cr3 have been examined by means of developed precise method of DSC technique and confirmed by the results of XRD analysis. Two exothermic reactions including that with a maximum at 380 – 400 degrees Celsius has been ascribed to dislocation reorganisation and recrystallisation process within the Al matrix although the main exothermic reaction with a maximum around 540 degrees Celsius arose from decomposition of icosahedral quasicrystalline particles and simultaneous formation of the metastable Al6Fe phase and more stable crystalline particles compositionally corresponded to the O-Al13Cr2 and O-Al13Fe4 phases. Activation energy for the main exothermic reaction has been found to be roughly about 53.1 kJ mol-1 which is significantly smaller than that for the bulk diffusion of either iron or chromium atoms in aluminium and very close to that for the vacancy migration. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/34854

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50

De, Oliveira Dominic. "Phase and structural changes of nickel catalysts as a function of reaction conditions." Master's thesis, Faculty of Engineering and the Built Environment, 2019. http://hdl.handle.net/11427/31520.

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Abstract:

Carbon dioxide hydrogenation is a route for the production of methane from hydrogen and carbon dioxide, which has attracted increased attention in recent times. It provides a means for both energy storage through substitute natural gas (SNG) production and as a process for the conversion of carbon dioxide into valuable products. Ni is the most widely used metal for SNG production due to its high activity, high selectivity towards methane and low cost in comparison to the other active metals. Ni catalysts have been extensively studied due to their uses in steam reforming and CO methanation, and it is known that deactivation by sintering, sulphur poisoning and carbon formation are the most common deactivation mechanisms for Ni catalysts. Some deactivation of Ni by oxidation has been reported, despite the fact the oxidation of Ni to NiO is not thermodynamically feasible at reaction conditions relevant to Ni catalysts. This has also been observed with Co catalysts for Fischer-Tropsch synthesis applications, where it has been explained that the size dependent oxidation, by produced water, can occur due to the higher surface energy contributions of smaller crystallites. The aim of this project is to study the phase changes of nickel catalysts, specifically through the oxidation of Ni in the presence of steam using in situ magnetic techniques to identify the loss of metallic Ni. As this oxidation is thought to be a size dependent effect, a set of catalysts with narrow crystallite size distributions and tuneable size is required for the experimental testing. The use of organometallic precursor reduction (OPR) and homogeneous deposition precipitation (HDP) was investigated for the synthesis in this study. OPR produced unsupported nanoparticles with a suitable size, however the occurrence of sintering during the anchoring and supporting of these nanoparticles on silica spheres, due to the high temperature calcination step, made these catalysts unsuitable for use in the oxidation study. The catalysts synthesised by HDP produced supported nickel catalysts with high loadings and sizes of 3.6 and 7.5 nm, with minimal overlap of size distributions, making them suitable for oxidation testing. The size of these nanoparticles was controlled by varying the reduction temperature. The HDP catalysts were tested at model conditions (i.e. in the absence of CO2) where the partial pressure ratios of steam to hydrogen, simulating different conversion levels, were increased up to a steam to hydrogen ratio of 400, to determine at what ratio the catalysts would begin to oxidise. The smaller catalysts showed significant oxidation at lower partial pressure ratios and to a greater extent than the larger particle size. These results showed the size dependence of the oxidation, with the large particles showing greater resistance to oxidation. These results were compared to iii thermodynamic calculations made for the size dependent oxidation of Ni, and good agreement between the experimental and predicted results was observed. The use of magnetic characterisation of the particle size was conducted by application of the Langevin equation as well as by a dispersion measurement, carried out by the titration of the Ni surface with H2. These in situ characterisation techniques showed consistency with the conventional external characterisation techniques and also showed that no size changes occurred throughout the testing, indicating that the results are truly due to size effects. Upon re-reduction of the oxidised catalysts, the full recovery of oxidised Ni was achieved with the large sample, whereas the smaller sample only achieved 60 % recovery of oxidised material. This is thought to be due to the formation of a less reducible phase, specifically metal-support compounds such as nickel silicate.

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Dissertations / Theses on the topic 'Structural phase'

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