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1
Vuong, Amanda. "Nanocarbon : defect architectures and properties." Thesis, University of Surrey, 2017. http://epubs.surrey.ac.uk/845194/.
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The allotropes of carbon make its solid phases amongst the most diverse of any element. It can occur naturally as graphite and diamond, which have very different properties that make them suitable for a wide range of technological and commercial purposes. Recent developments in synthetic carbon include Highly Oriented Pyrolytic Graphite (HOPG) and nano-carbons, such as fullerenes, nanotubes and graphene. The main industrial application of bulk graphite is as an electrode material in steel production, but in purified nuclear graphite form, it is also used as a moderator in Advanced Gas-cooled Reactors across the United Kingdom. Both graphene and graphite are damaged over time when subjected to bombardment by electrons, neutrons or ions, and these have a wide range of effects on their physical and electrical properties, depending on the radiation flux and temperature. This research focuses on intrinsic defects in graphene and dimensional change in nuclear graphite. The method used here is computational chemistry, which complements physical experiments. Techniques used comprise of density functional theory (DFT) and molecular dynamics (MD), which are discussed in chapter 2 and chapter 3, respectively. The succeeding chapters describe the results of simulations performed to model defects in graphene and graphite. Chapter 4 presents the results of ab initio DFT calculations performed to investigate vacancy complexes that are formed in AA stacked bilayer graphene. In AB stacking, carbon atoms surrounding the lattice vacancies can form interlayer structures with sp2 bonding that are lower in energy compared to in-plane reconstructions. From the investigation of AA stacking, sp2 interlayer bonding of adjacent multivacancy defects in registry creates a type of stable sp2 bonded wormhole between the layers. Also, a new class of mezzanine structure characterised by sp3 interlayer bonding, resembling a prismatic vacancy loop has also been identified. The mezzanine, which is a V6 hexavacancy variant, where six sp3 carbon atoms sit midway between two carbon layers and bond to both, is substantially more stable than any other vacancy aggregate in AA stacked layers. Chapter 5 presents the results of ab initio DFT calculations performed to investigate the wormhole and mezzanine defect that were identified in chapter 4 and the ramp defect discovered by Trevethan et al. DFT calculations were performed on these defects in twisted bilayer graphene. From the investigation of vacancy complexes in twisted bilayer graphene, it is found that vacancy complexes are unstable in the twisted region and are more favourable in formation energy when the stacking arrangement is close to AA or AB stacking. It has also been discovered that the ramp defect is more stable in the twisted bilayer graphene compared to the mezzanine defect. Chapter 6 presents the results of ab initio DFT calculations performed to investigate a form of extending defect, prismatic edge dislocation. Suarez-Martinez et al.’s research suggest the armchair core is disconnected from any other layer, whilst the zigzag core is connected. In the investigation here, the curvature of the mezzanine defect allows it to swing between the armchair, zigzag and Klein in the AA stacking. For the AB stacking configuration, the armchair and zigzag core are connected from any other layer. Chapter 7 present results of MD simulations using the adaptive intermolecular reactive empirical bond order (AIREBO) potential to investigate the dimensional change of graphite due to the formation of vacancies present in a single crystal. It has been identified that there is an expansion along the c-axis, whilst a contraction along the a- and b- axes due to the coalescence of vacancy forming in-plane and between the layers. The results here are in good agreement with experimental studies of low temperature irradiation. The final chapter gives conclusions to this work.
2
Weight, Braden Michael. "Inspection of Excited State Properties in Defected Carbon Nanotubes from Multiple Exciton Generation to Defect-Defect Interactions." Thesis, North Dakota State University, 2020. https://hdl.handle.net/10365/31784.
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Covalent SP3-hybridization defects in single-walled carbon nanotubes (CNTs) have been prevalent in recent experimental and theoretical studies for their interesting photophysical properties. These systems are able to act as excellent sources of single, infrared photons, even at room temperature, making them marketable for applications to sensing, telecommunications, and quantum information. This work was motivated by recent experimental studies on controllable defect placement and concentration as well as investigating carrier multiplication (CM) using DFT-based many-body perturbation theory (MBPT) methods to describe excitonic relaxation processes. We find that pristine CNTs do not yield appreciable MEG at the minimum threshold of twice the optical gap 2Eg, but covalent functionalization allows for improved MEG at the threshold. Finally, we see that defect-defect interactions within CNT systems can be modeled simply as HJ-aggregates in an effective Hamiltonian model, which is shown to be valid for certain, highly-redshifted defect configurations at low defect-defect separation lengths.
3
Dogo, Harun. "Point defect properties in iron chromium alloys." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2006. http://library.nps.navy.mil/uhtbin/hyperion/06Sep%5FDogo.pdf.
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Thesis (M.S. in Applied Physics)--Naval Postgraduate School, September 2006.Thesis Advisor(s): Craig Smith, Xavier Maruyama. "September 2006." Includes bibliographical references (p. 57-59). Also available in print.
4
Ashley, Nicholas J. "Defect Properties of Binary Non-Oxide Ceramics." Thesis, Imperial College London, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.520879.
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Smith, A. G. H. "Structural and defect properties of strontium titanate." Thesis, University College London (University of London), 2012. http://discovery.ucl.ac.uk/1344085/.
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Strontium titanate is a material of considerable interest, with many applications. Though it has been extensively studied experimentally and computationally, there are unresolved issues regarding the structure and defect properties. Experimental examination is challenging due to the propensity of the material to form small domains. Even when single crystals are available, the observed behaviour is an average of these local domains. In this thesis we undertook an extensive computational investigation of strontium titanate. The material is known to undergo a second order phase transition from cubic (Pm-3m) to tetragonal (I4/mcm) at ~105 K. We began the investigation by examining the tetragonal phase. Using plane-wave DFT with the LDA, PBE and PBEsol density functionals, we mapped out in three dimensions the I4/mcm space group and fitted the resultant potential energy surfaces to polynomials. Extensive analysis was performed and expectation values for the 0 K octahedral rotation angle were calculated as being between 4.6° and 6.1° for the three functionals. We continued by carrying out an extensive study, where we examined many different low symmetry saddle points, and minima using the PBEsol functional (which was found to have produced the best results). A total of 38 structures (many of which are unique) were examined in detail and the vibrational and imaginary modes involved in the transitions between higher and lower symmetry structures were identified and described. We identified the lowest energy structure as being monoclinic, with a rhombohedral structure at slightly higher energy. We also note that, as the energy differences are so small between these lower symmetry structures (~0.1 meV per formula unit), it is unlikely that these phases will appear in nature. In the final results chapter we present three new sets of classical pair potentials for modelling strontium titanate. We applied these potentials to model vacancy type defects and investigated the transition pathway for oxygen and strontium migration between adjacent sites. We calculated migration barriers of between 0.96 and 1.35 eV for oxygen and between 3.17 and 3.20 eV for strontium, which are improved estimates over previous pair potential results.
6
Wilson, Daniel John. "Defect and surface properties of the silver halides." Thesis, University College London (University of London), 2005. http://discovery.ucl.ac.uk/1446536/.
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In this thesis, state-of-the-art density functional theory calculations have been performed to study a number of properties of the rocksalt-structured silver halides which can broadly be defined as photographically-relevant. These involve point defects and their interaction with free electrons and holes, created upon excitation by actinic light. In our initial calculations, we studied the primary intrinsic point defects within the bulk of the material, in both charged and neutral forms, using the supercell technique. We have correctly predicted the dominance of the Frenkel defect in both AgCl and AgBr, and have found that the lowest energy configuration for the interstitial cation defect in both materials involved a second cation, forming a Ag22+ split-interstitial species, orientated in a 111 direction. We then extended this work by applying a hybrid QM/MM embedding technique to model two systems: the first represented the ideal (100) surface, while the second represented a 'realistic' finite cluster. With the first, we have calculated defect structures and formation energies on the flat surface, and have examined the well-known space-charge layer. With our finite cluster, we studied extended surface defects (steps, kinks etc.) and their interaction with individual point defects. We have also calculated trap depths, and the localisation of holes and electrons at each of the sites. We have determined that, regardless of their location, an electron diffusely localises around the interstitial cation and strongly localises on the anion vacancy, while the corresponding hole becomes trapped around the cation vacancy on a nearest-neighbour cation. However, overall, we consider the positive kink on the surface to be the most likely electron trap, due to its relative abundance and its position within the band gap. Finally, we have investigated the properties of the latent pre-image centre, which plays a key role in the photographic process.
7
Bruce, J. M. "Self-diffusion and point defect studies in plastic crystals." Thesis, University of Strathclyde, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382256.
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Shi, Hongting. "Defect structure and optical properties of alkaline earth fluorides." Doctoral thesis, [S.l.] : [s.n.], 2007. http://deposit.ddb.de/cgi-bin/dokserv?idn=984572015.
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Chirkov, Andrey S., and Andrei V. Nazarov. "N-body potentials in simulation of point defect properties." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-195230.
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Chirkov, Andrey, and Andrei Nazarov. "N-body potentials in simulation of point defect properties." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-194546.
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This work is devoted to simulation of the diffusion features of point defects in bcc metals. The properties of point defects have been investigated with the usage of many-body interatomic potentials. This approach, based on the density-functional theory, permitted us to derive more adequate diffusion features of solids. The investigation is carried out within the framework of the Finnis-Sinclair formalism, developed for an assembly of N atoms and represents the second-moment approximation of the tight-binding theory. We used a new model, based on the molecular static method for simulating the atomic structure near the defect and vacancy migration in pure metals. This approach gives the opportunity to simulate the formation and the migration volumes of the point defects, taking into consideration the influence of pressure on structure and consequently on energy. The diffusion characteristics of bcc α-Fe and anomalous β-Zr have been simulated. The preliminary results of this research were presented and taken for publishing on the international conference “Diffusion in solids” (DiSo-2005) in May 2005.
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Chirkov, Andrey, and Andrei Nazarov. "N-body potentials in simulation of point defect properties." Diffusion fundamentals 3 (2005) 1, S. 1-11, 2005. https://ul.qucosa.de/id/qucosa%3A14289.
Full textAbstract:
This work is devoted to simulation of the diffusion features of point defects in bcc metals. The properties of point defects have been investigated with the usage of many-body interatomic potentials. This approach, based on the density-functional theory, permitted us to derive more adequate diffusion features of solids. The investigation is carried out within the framework of the Finnis-Sinclair formalism, developed for an assembly of N atoms and represents the second-moment approximation of the tight-binding theory. We used a new model, based on the molecular static method for simulating the atomic structure near the defect and vacancy migration in pure metals. This approach gives the opportunity to simulate the formation and the migration volumes of the point defects, taking into consideration the influence of pressure on structure and consequently on energy. The diffusion characteristics of bcc α-Fe and anomalous β-Zr have been simulated. The preliminary results of this research were presented and taken for publishing on the international conference “Diffusion in solids” (DiSo-2005) in May 2005.
12
Chirkov, Andrey S., and Andrei V. Nazarov. "N-body potentials in simulation of point defect properties." Diffusion fundamentals 2 (2005) 12, S. 1-2, 2005. https://ul.qucosa.de/id/qucosa%3A13291.
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Lee, M. E. "Crystallographic, optical and electrical properties of some* ?? defect chalocpyrite semiconductors." Thesis, University of Nottingham, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.353559.
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14
Lu, Qiyang. "Controlling properties of functional oxides by tuning oxygen defect chemistry." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/115715.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 164-192).
Oxygen defects are essential building blocks for properties and functionalities of oxides, including electrical conductivity, magnetism, ferroelectricity as well as catalytic and electrocatalytic activity. Therefore, fundamental understanding of how to tune the oxygen defect chemistry is essential for advancing applications based on these defect sensitive properties. This thesis investigated pathways to controlling the concentration and structure of oxygen defects on selected case studies with model oxide systems. Three novel effects were assessed and shown to be operative for obtaining a large impact on the oxygen defect chemistry equilibria. These are heterogeneous chemical doping of the surface for improving surface electrocatalytic activity and stability, electrochemical bias to control phase with drastic changes obtained in electronic and phonon transport properties, as well as strain engineering to alter the oxygen interstitial capacity and oxygen exchange kinetics. Surface chemical modifications were applied to the near-surface regions of Lao.8Sro.2CoO 3 (LSC) by replacing the Co cations locally with less reducible cations, such as Hf and Ti. This strategy was shown to effectively stabilize the LSC surfaces and suppress surface segregation of Sr at elevated temperatures. This introduced surface stability by local chemical doping greatly enhanced the long-term electrochemical performance of LSC electrode, which provides a new route for improving the efficiency of solid oxide fuel and electrolysis cells. Applying electrical bias was investigated as another effective method to tune the oxygen stoichiometry, exemplified by the case studies on SrCoOx (SCO). In situ X-ray diffraction was used to investigate the topotactic phase transition between brownmillerite phase SrCoO2.5 (BM-SCO) and perovskite phase SrCoO 3 6 (P-SCO) triggered electrochemically at elevated temperatures. An electrical bias of merely 30 mV was shown sufficient to trigger the BM-->P phase transition. This is much more feasible than chemically induced phase transition, which requires high pressure (> 1 bar) and specialized pressurized apparatus. Moreover, the evolution of electronic structure during the BM4P phase transition was probed in operando by using ambient-pressure X-ray photoelectron and absorption spectroscopy (AP-XPS/XAS). The similar experimental scheme, which combines in operando surface characterizations and electrochemical controlling of oxygen stoichiometry, was extended to oxide systems beyond perovskites. This allows us to investigate the defect chemistry of oxides in a much broader range of effective oxygen partial pressure than what conventional methods can achieve. Firstly, we showed that the surface defect chemistry equilibrium of fluoritestructured Pro.iCeo.902-6 (PCO) strongly deviated from the bulk counterpart, due to the possibly enhanced defect-defect interactions or lattice strain effect at surfaces. Secondly, we found a novel metal-insulator transition triggered electrochemically in VO, by changing the phase between the metallic dioxide VO2 and the insulating pentoxide V2O5 Lastly, we lowered the operation temperature of this electrochemical control of oxygen stoichiometry down to room temperature by using ionic liquid or ion gels as the electrolyte. We achieved tuning of thermal conductivity in SrCoOx with a range of more than one order of magnitude, by using electrochemically triggered phase transitions at room temperature. We also investigated the effect of lattice strain on oxygen defect formation energy in Ruddlesden-Popper (RP) phase oxide Nd2NiO4+6 (NNO). We found that tensile strain along the c-axis of NNO lattice effectively reduced the formation enthalpy of oxygen interstitials, which can provide a new route for designing the defect chemistry of RP phase oxide for electrocatalytic applications..
by Qiyang Lu.
Ph. D.
15
Healy, Fiona. "Computer modelling of crystal and defect properties of silver halides." Thesis, Keele University, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.332367.
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Tsai, Ming-Jinn. "The defect structure and transport properties of some high Tc superconductors." Thesis, Massachusetts Institute of Technology, 1990. http://hdl.handle.net/1721.1/60122.
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Stefanik, Todd Stanley 1973. "Electrical properties and defect structures of praseodymium-cerium oxide solid solutions." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/16623.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, February 2004.Includes bibliographical references (p. 130-135).
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
A defect chemistry model consistent with observed trends in the pO2 and temperature dependence of electrical conductivity in praseodymium cerium oxide (PCO) was developed. Four point DC conductivity measurements were made from 1 atm to 1018 atm p02 over isotherms ranging from 600-1 000ʻC in materials containing 0-20% Pr. A pO02-dependent ionic conductivity was observed at high pO2 values in compositions containing 0.5% and 1% Pr. This behavior was attributed to oxidation of Pr3+ to Pr4+ under oxidizing conditions, thereby decreasing the concentration of acceptor dopants in the PCO material. In compositions containing 10% and 20% Pr, an electron hopping conductivity was observed at high pO02 values. This contribution was strongest at low temperatures and was attributed to the formation of a praseodymium impurity band within the CeO2 band gap. Defect association significantly altered the predicted pO2 dependence of the impurity band conductivity, especially at low temperatures. The temperature dependences of the thermodynamic parameters governing defect formation and transport in PCO were determined. The reduction enthalpy of cerium was significantly decreased with additions of Pr from approximately 4.7 eV (the value in pure CeO2) to 3.4 eV in 20% PCO. The energy between the Pr impurity band and the CeO2 conduction band was approximately 0.95 eV for 10% and 20% PCO samples. The measured trap depth was significantly higher (approximately 1.6 eV) in 0.5% and 1% PCO. The migration enthalpy for impurity band hopping conductivity was approximately 0.55 eV, slightly higher than the hopping enthalpy for intrinsic carriers in CeO2 (0.4 eV).
(cont.) The oxygen ion migration enthalpy measured for most samples was approximately 0.6- 0.7 eV, in agreement with values determined for other rare-earth doped systems. At 20% Pr, the total migration energy increased to approximately 0.9 eV. This increase was attributed to an association energy at high doping levels. Coulometric titration and points to the possible existence of uncharged oxygen vacancies, particularly at low temperatures. During the course of these experiments, it became evident that the mechanical stability of PCO needs to be addressed if the material is to be used in real applications. Oxygen uptake/evolution during reduction/oxidation cycles appears to result in development of significant stresses and cracking. While the material may be useful in powder form, this cracking issue must be addressed if it is to be used in bulk or thin film form.
by Todd Stanley Stefanik.
Ph.D.
18
Anindya, Khalid. "Interlayer Defect Effects on the Phonon Properties of Bilayer Graphene and its Nanoribbon." Thesis, Université d'Ottawa / University of Ottawa, 2020. http://hdl.handle.net/10393/40411.
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Phonon properties of AB (Bernal) stacked bilayer graphene (BLG) with various types of defects have been investigated theoretically. Forced Vibrational (FV) method has been employed to compute the phonon modes of disordered BLG. A downward linear shift of E2g mode frequencies has been observed with an increasing amount of defect concentration. Moreover, two identical E2g peaks have been observed in PDOS of the bilayer system where the individual layer contains 12C and 13C atoms respectively. From computed typical mode patterns of in-plane K-point optical mode phonons, it has been noticed that phonons become strongly localized around a few nanometers area at the presence of defects and localized modes increase with the increasing amount of defect concentration. The edge effect on the localized phonon modes has also been discussed for bilayer armchair graphene nanoribbons (BiAGNRs). The impact of defects on the phonon conduction properties has also been studied for BiAGNRs. My investigated results can be convenient to study the thermal conductivity and electron-phonon interaction of bilayer graphene-based nanodevices and to interpret the Raman and infrared spectra of disordered system.
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Kamani, Sandeep Kumar. "Influence of defects on thermal and mechanical properties of metals." [College Station, Tex. : Texas A&M University, 2008. http://hdl.handle.net/1969.1/ETD-TAMU-2901.
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Nowotny, Maria Materials Science & Engineering Faculty of Science UNSW. "Defect disorder, semiconducting properties and chemical diffusion of titanium dioxide single crystal." Awarded by:University of New South Wales. School of Materials Science and Engineering, 2006. http://handle.unsw.edu.au/1959.4/27223.
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Semiconducting properties and related defect disorder for well defined TiO2 single crystal were studies. Semiconducting properties have been determined using simultaneous measurements of two independent electrical properties, including electrical conductivity, ??, and thermoelectric power, S, at elevated temperatures (1073-1323 K) in the gas phase of controlled oxygen activity (10-10 Pa < p(O2) < 75 kPa). Measurements of s and S were conducted (i) in the gas/solid equilibrium and (ii) during equilibration. Oxygen vacancies have been identified as the predominant defects in TiO2 over a wide range of p(O2). Individual conductivity components related to electrons, electron holes and ions, were determined from the obtained ?? data. The effect of p(O2) on these individual components was considered in the form of a diagram. This work led to the discovery of the formation and diffusion of Ti vacancies. However, the obtained diffusion data indicate that, in the temperature ranges commonly used in studies of semiconducting properties (1000-1400K), the Ti vacancies concentration is quenched and may thus be assumed constant. In addition it was shown that Ti vacancies in appreciable concentrations form only during prolonged oxidation. It was determined that the discrepancies in the reported n-p transition point are related to the concentration and spectrum of impurities as well as the concentration of Ti vacancies. It has been shown that the n-p transition point in high-purity TiO2 is determined by the Ti vacancy concentration. A well defined chemical diffusion coefficient, Dchem, was determined using kinetic data obtained during equilibration. A complex relationship between p(O2) and Dchem was observed. These data showed a good agreement between the obtained diffusion data and defect disorder. Examination of the determined equilibration kinetics, led to the discovery of two kinetic regimes, the result of the transport of defects at different mobilities. The determined data are considered well defined due to the following reasons: 1. The studied specimen was of exceptionally high purity and free of grain boundaries (single crystal) 2. The specimen was studied in the gas phase of controlled and well defined oxygen activity which was continuously monitored. 3. Whenever the experimental data were measured in equilibrium, the gas/solid equilibrium has been verified experimentally. 4. A good agreement between the two, self-confirmatory, electrical properties, including ?? and S has been determined simultaneously and independently. The defect disorder model derived in the present work may be used for tailoring controlled semiconducting properties through the selection of annealing conditions involving the temperature and oxygen activity.
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Goodman, Stewart Alexander. "Influence of particle irradiation on the electrical and defect properties of GaAs." Thesis, University of Pretoria, 1994. http://hdl.handle.net/2263/37284.
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The beginning of the space-age in the 1950s led to interest in the effects of radiation on
semiconductors. The systematic investigation of defect centres in semiconductors began
in earnest over 30 years ago. In addition to defect identification, information was also
obtained on energy-level structures and defect migration properties. When designing
electronic systems for operation in a radiation environment, ~tis imperative to know the
effect of radiation on the properties of electronic components and materials comprising
these systems.
In some instances, the effects of irradiating electronic materials can be used to obtain
desired material properties (mesa isolation, implantation, etc.). However, when electronic
devices are exposed to radiation, defects may be introduced into the material. Depending
on the application, these defects may have a detrimental effect on the performance of such
a device. For this study, the semiconductor gallium arsenide (GaAs) was used and the
defects were introduced by electrons, alpha-particles, protons, neutrons and argon sputtering. These particles were generated using radio-nuclides, a high-energy neutron
source, a 2.5 MV Van de Graaff accelerator and a sputter gun.
The influence of particle irradiation on the device properties of Schottky barrier diodes
(SBDs) fabricated on GaAs is presented. These device properties were monitored using a
variable temperature current-voltage (I-V) and capacitance-voltage (C-V) apparatus. In
order to have an understanding of the change in electrical properties of these contacts after
irradiation, it is necessary to characterize the radiation-induced defects. Deep level
transient spectroscopy (DLTS) was used to characterise the defects in terms of their
DLTS "signature", defect concentration, field enhanced emission, and thermodynamic
properties.Thesis (PhD)--University of Pretoria, 1994.
gm2014
Physics
unrestricted
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Shi, Tingting. "Optoelectronic and Defect Properties in Earth Abundant Photovoltaic Materials: First-principle Calculations." University of Toledo / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1418391935.
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Wiktor, Julia. "Identification of equilibrium and irradiation-induced defects in nuclear ceramics : electronic structure calculations of defect properties and positron annihilation characteristics." Thesis, Aix-Marseille, 2015. http://www.theses.fr/2015AIXM4050.
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Durant l'irradiation en réacteur la fission des atomes d'actinides entraine la création de grandes quantités de défauts, qui affecte les propriétés physiques et chimiques des matériaux dans le réacteur, en particulier les matériaux combustibles ou de structure. Une des méthodes non destructives pouvant être utilisées pour caractériser les défauts induits par irradiation, vides ou contenant les produits de fission, est la spectroscopie d'annihilation de positons (SAP). Cette technique expérimentale consiste à détecter le rayonnement généré lors de l'annihilation du paire électron-positon dans un échantillon et en déduire les propriétés de la matière étudiée. Les positons peuvent être piégés dans les défauts de type lacunaire dans les solides, et en mesurant leur temps de vie et les distribution de moment du rayonnement d'annihilation, on peut obtenir des informations sur les volumes libres et les environnements chimiques des défauts. Dans ce travail, des calculs de structure électronique des caractéristiques d'annihilation de positons ont été effectués en utilisant la théorie de la fonctionnelle de la densité à deux composants (TCDFT). Pour calculer les distributions de moment rayonnement d'annihilation, nous avons implémenté les méthodes nécessaires dans le code de calcul libre ABINIT. Les résultats théoriques ont été utilités pour contribuer à l'identification des défauts d'irradiation dans deux céramiques nucléaires, le carbure de silicium (SiC) et le dioxyde d'uranium (UO2)During in-pile irradiation the fission of actinide nuclei causes the creation of large amounts of defects, which affect the physical and chemical properties of materials inside the reactor, in particular the fuel and structural materials. Positron annihilation spectroscopy (PAS) can be used to characterize irradiation induced defects, empty or containing fission products. This non-destructive experimental technique involves detecting the radiation generated during electron-positron annihilation in a sample and deducing the properties of the material studied. As positrons get trapped in open volume defects in solids, by measuring their lifetime and momentum distributions of the annihilation radiation, one can obtain information on the open and the chemical environments of the defects. In this work electronic structure calculations of positron annihilation characteristics were performed using two-component density functional theory (TCDFT). To calculate the momentum distributions of the annihilation radiation, we implemented the necessary methods in the open-source ABINIT program. The theoretical results have been used to contribute to the identification of the vacancy defects in two nuclear ceramics, silicon carbide (SiC) and uranium dioxide (UO2)
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Zhan, Xiaowen. "DEFECT CHEMISTRY AND TRANSPORT PROPERTIES OF SOLID STATE MATERIALS FOR ENERGY STORAGE APPLICATIONS." UKnowledge, 2018. https://uknowledge.uky.edu/cme_etds/88.
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Replacing organic liquid electrolytes with nonflammable solid electrolytes can improve safety, offer higher volumetric and gravimetric energy densities, and lower the cost of lithium-ion batteries. However, today’s all-solid-state batteries suffer from low Li-ion conductivity in the electrolyte, slow Li-ion transport across the electrolyte/electrode interface, and slow solid-state Li-ion diffusion within the electrode. Defect chemistry is critical to understanding ionic conductivity and predicting the charge transport through heterogeneous solid interfaces. The goal of this dissertation is to analyze and improve solid state materials for energy storage applications by understanding their defect structure and transport properties.
I have investigated defect chemistry of cubic Li7La3Zr2O12 (c-LLZO), one of the most promising candidate solid electrolytes for all-solid-state lithium batteries. By combining conductivity measurements with defect modeling, I constructed a defect diagram of c-LLZO featuring the intrinsic formation of lithium vacancy-hole pairs. The findings provided insights into tailoring single-phase mixed lithium-ion/electron conducting materials for emerging ionic devices, i.e., composite cathodes requiring both fast electronic and ionic paths in solid-state batteries. I suggested that oxygen vacancies could increase the Li-ion conductivity by reducing the amount of electron holes bound with lithium vacancies.
Using a simpler but also attractive solid electrolyte Li2ZrO3 (LZO) as an example, I significantly improved Li-ion conductivity by creating extra oxygen vacancies via cation doping. In particular, Fe-doped LZO shows the highest Li-ion conductivity reported for the family of LZO compounds, reaching 3.3 mS/cm at 300 °C. This study brought attentions to the long-neglected oxygen vacancy defects in lithium-ion conductors and revealed their critical role in promoting Li-ion transport. More importantly, it established a novel defect engineering strategy for designing Li-oxide based solid electrolytes for all-solid-state batteries.
I surface-modified LiNi0.6Co0.2Mn0.2O2 cathode material with a LZO coating prepared under dry air and oxygen, and systematically investigated the effect of coating atmosphere on their transport properties and electrochemical behaviors. The LZO coating prepared in oxygen is largely amorphous. It not only provided surface protection against the electrolyte corrosion but also enabled faster lithium-ion transport. Additionally, oxygen atmosphere facilitated Zr diffusion from the surface coating to the bulk of LiNi0.6Co0.2Mn0.2O2, which stabilized the crystal structure and enhanced lithium ion diffusion. Consequently, LiNi0.6Co0.2Mn0.2O2 cathodes coated with Li2ZrO3 in oxygen achieved a significant improvement in high-voltage cycling stability and high-rate performance.
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Fritze, Holger [Verfasser]. "Electromechanical properties and defect chemistry of High-Temperature piezoelectric materials / Dr. Holger Fritze." Clausthal-Zellerfeld : Universitätsbibliothek Clausthal, 2008. http://d-nb.info/1010653776/34.
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Khaliq, Jibran. "Effect of doping and defect structures on thermo physical properties of thermoelectric materials." Thesis, Queen Mary, University of London, 2014. http://qmro.qmul.ac.uk/xmlui/handle/123456789/8444.
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Development of thermoelectric materials to date has focused on materials that can operate at lower temperatures. However; there is now an increased need to develop materials for higher temperature applications. In this research, medium to high temperature oxide and non-oxide thermoelectric materials were fabricated and characterized. For oxide thermoelectric materials, La4Ti4O14 and Sr4Nb4O14 were chosen. These compounds are members of the homologous A4B4O14 series and possess perovskite-like layered structure (PLS). PLS compounds have low thermal conductivity due to a layered structure compared to the perovskite materials (e.g. SrTiO3). These atomic scale layers help to reduce the thermal conductivity of PLS compounds. Doping in PLS materials also creates atomic scale disorders. The effect of acceptor-donor doping and oxidation-reduction on the thermal conductivity of PLS ceramics were investigated in relation to mass contrast and compositional non-stoichiometry. High resolution TEM and XPS revealed that acceptor doping of La4Ti4O14 produced nanoscale intergrowth regions of n=5 layered phase inside n=4 layered phase, while donor doping produced nanoscale intergrowth regions of n=3 layered structure. As a result of these nanoscale intergrowths, the thermal conductivity value reduced by ~ 20% compared to the theoretical value. Pure La4Ti4O14 has a thermal conductivity value of ~ 1.1 W/m.K which dropped to a value of ~ 0.98 W/m.K in Sr doped La4Ti4O14 and ~ 0.93 W/m.K in Ta doped La4Ti4O14. Pure Sr4Nb4O14 has a thermal conductivity value of ~ 1.05 W/m.K which dropped to ~ 0.6 W/m.K after La doping. The factors influencing the thermal conductivity of PLS compounds were also discussed.For non-oxide ceramics, CoSb3 was chosen due to its cage-like structure and ideal for the application of Phonon Glass Electron Crystal Concept. The cage like structure gives room to engineer its electrical and thermal properties without affecting the other. For the first time, CoSb3 stuffed with Yb and substituted with Te (YbyCoSb3-xTex) was synthesized by mechanical alloying and spark plasma sintering. The electrical and thermal properties were characterized for pure and doped material. A Seebeck coefficient value of ~ 160 μV/K was obtained at ~ 600-800 K for Yb0.075CoSb2.85Te0.15. The electric resistivity dropped from ~ 1000 μΩm for pure CoSb3 to ~ 9 μΩm for Yb0.075CoSb2.85Te0.15. Lattice thermal conductivity was significantly reduced to a very low value of 1.17 W/m.K by the addition of Yb atoms into CoSb2.85Te0.15 without significantly affecting its Seebeck coefficient and electrical resistivity. This value is comparable to those produced by the costly processing of nanostructured materials. A zT value of ~ 0.70 was obtained at 600 K. This research has shown that by engineering the defect chemistry of thermoelectric materials, it is possible to significantly reduce their thermal conductivity without compromising their electrical properties.
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Spears, Marlene Ann. "Defect chemistry and electrical properties of ruthenium- and bismuth-substituted gadolinium titanate pyrochlore." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/11443.
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Fan, Yue Ph D. Massachusetts Institute of Technology. "Atomistic simulation of defect structure evolution and mechanical properties at long time scales." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/82865.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2013.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 127-146).
This thesis is a computational and theoretical investigation of the response of materials' mechanical properties to a wide range of environmental conditions, with a particular focus on the coupled effects of strain rate and temperature. The thesis provides original contributions to the fundamental understanding of how the materials mechanical properties change, as manifested by defect structure evolution, with temperature and strain rate conditions, as well as to the development of methodology used for enabling the investigation of dislocation-defect interactions over a much wider range of time scales than of reach to traditional techniques. This thesis advanced the capabilities of a recently developed activation-relaxation based atomistic method to enhance the accuracy of kinetic predictions, and to enable the investigation of dislocation-defect interactions dynamically at long time scales. We took the Autonomous Basin Climbing (ABC) method as a starting point, and incorporated the ability to sample multiple transition pathways associated with a given state. This new feature addresses the problem of overestimating the system evolution time due to the one-dimensional nature of the original ABC algorithm. The ABC method was further implemented in a dynamic framework, which makes it possible for the first time to directly simulate the dislocation-obstacle interactions at very low strain rates. This approach allows for a new way to connect the atomistic results to models at the meso-scale for simulating the plasticity of metals. We analytically derived how the applied strain rate couples with the thermal activation process, based on the framework of transition state theory informed by the atomistic approach described above. We demonstrated the coupling effect is a common mechanism behind many important phenomena, and provide three examples from the atomic level on the dislocation mobility and dislocation interactions with radiation induced defects. (i) A well-known universal flow stress upturn behavior in metals has been examined. We provide a simple physically based model to predict the flow stress at various strain rates, without invoking any assumed mechanisms or fitting parameters as in the traditional constitutional models. (ii) We implemented this new model in (i) to investigate the dislocation-obstacle interactions. The approach enabled us to map the interaction between an edge dislocation and a self interstitial atom (SIA) cluster in Zr in a two-parameter space consisting of temperature and strain rate. This approach allows the direct atomistic simulation of dislocation-obstacle interactions at experimental time scale, namely at low strain rates, which cannot be reached by traditional atomistic techniques. The dislocation is found to absorb the SIA cluster and climb at low strain rates and high temperatures, while it passes through the SIA cluster at high strain rates and low temperatures. The predicted mechanism map is able to reconcile the seeming controversy between previous experimental and computational findings. (iii) A dislocation-void interaction in bcc Fe at prescribed strain rate is also investigated. We demonstrated that different applied strain rates can affect the interaction mechanism and the defect microstructure, and eventually lead to a negative strain rate sensitivity (nSRS) of yield strength below a critical strain rate. This finding at the unit process level supplements the previous explanations of the nSRS with higher level constitutive relations. Beyond the specific cases analyzed in metals in this thesis, the insights gained on the coupling between strain rate and thermal activation can be used to explain the dependence on strain rate and temperature in other important classes of materials (e.g. colloids, cement) and phenomena (e.g. corrosion, creep).
by Yue Fan.
Ph.D.
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Mishra, Shantanu, Doreen Beyer, Reinhard Berger, Junzhi Liu, Oliver Gröning, José I. Urgel, Klaus Müllen, Pascal Ruffieux, Xinliang Feng, and Roman Fasel. "Topological defect-induced magnetism in a nanographene." American Chemical Society, 2019. https://tud.qucosa.de/id/qucosa%3A73172.
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The on-surface reactions of 10-bromo-10'-(2,6-dimethylphenyl)-9,9'-bianthracene on Au(111) surface have been investigated by a combination of bond-resolved scanning tunneling microscopy, scanning tunneling spectroscopy, and tightbinding and mean-field Hubbard calculations. The reactions afford the synthesis of two open-shell nanographenes (1a and 1b) exhibiting different scenarios of all-carbon magnetism. 1a, an allbenzenoid nanographene with previously unreported triangulenelike termini, contains a high proportion of zigzag edges, which endows it with an exceedingly low frontier gap of 110 meV and edge-localized states. The dominant reaction product (1b) is a non-benzenoid nanographene consisting of a single pentagonal ring in a benzenoid framework. The presence of this nonbenzenoid topological defect, which alters the bond connectivity in the hexagonal lattice, results in a non-Kekulé nanographene with a spin S = ½, which is detected as a Kondo resonance. Our work provides evidence of all-carbon magnetism, and motivates the use of topological defects as structural elements toward engineering agnetism in carbon-based nanomaterials for spintronics.
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Knapp, Meghan C. "Investigations into the structure and properties of ordered perovskites, layered perovskites, and defect pyrochlores." Columbus, Ohio : Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1149097068.
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Dragan, Mirela-Anca. "Defect chemistry, transport properties and thermodynamic stability of acceptor doped and undoped layered La2NiO4." [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=979096375.
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Read, Mark S. D. "Atomistic simulation studies of the defect and surface properties of perovskite-based oxide-catalysts." Thesis, University of Surrey, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298920.
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Willis, A. E. "Mammalian DNA ligases : characterization of their biochemical properties and a defect in Bloom's syndrome." Thesis, Imperial College London, 1988. http://hdl.handle.net/10044/1/47305.
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Hallander, Per. "Towards defect free forming of multi-stacked composite aerospace components using tailored interlayer properties." Doctoral thesis, KTH, Lättkonstruktioner, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-185694.
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Use of lightweight materials is an important part of reduction of fuel consumption by commercial aircraft. A considerable number of structural aircraft parts are therefore built of thin layers of epoxy pre-impregnated carbon fibres stacked to laminates. Manufacturing these by hand is costly and different methods of automation have therefore been developed. One cost-effective way of manufacturing is Automated Tape Lay-up of flat stacks followed by a Hot Drape Forming operation. A well-known problem in the industry within forming is fibre wrinkling, which can cause a serious strength knock down. The focus of this thesis has therefore been on understanding how and why wrinkles develop during forming of multi-layer stacks and, based on this, investigate different methods for process and material improvements. The work presented initially investigates the dependency between stacking sequence and wrinkle development. It is shown that wrinkle free forming can be obtained by changing the fibre stacking order. In the following investigation it is shown that the wrinkles cannot be entirely eliminated by local stiffening of the critical layers. In a, related study it is shown that different kinds of wrinkles develops during forming; wrinkles may be either due to global buckling of the entire lay-up or local compression of single layers. Global buckling is due to excessive material. Local compression occurs as the material shear during forming. The work presented leads to an understanding of the importance of making the beneficial neighbouring fibre layers interact during forming. One way to connect neighbouring layers is to tailor the interlayer properties. A study is presented that shows how local manipulation of interlayer properties may steer the multi-layered material into a different deformation mechanisms. The manipulation in this thesis is performed using Multi Wall Carbon Nano Tubes, thermoplastic veils or consolidation of thermoplastic toughener particle interlayers.QC 20160425
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Burg, Tristan Kevin Materials Science & Engineering Faculty of Science UNSW. "Semiconducting properties of polycrystalline titanium dioxide." Publisher:University of New South Wales. Materials Science & Engineering, 2008. http://handle.unsw.edu.au/1959.4/41262.
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Titanium dioxide, TiO2, has potential applications as a photoelectrode for photoelectrochemical generation of hydrogen by splitting water using solar energy and as a photocatalyst for water purification. This study is part of the UNSW research program to process TiO2-based oxide semiconductors as high-performance photoelectrodes and photocatalysts. This study investigates the effect of defect disorder on semiconducting properties of polycrystalline TiO2. This study involved the processing of high-purity polycrystalline TiO2 and determination of its semiconducting properties through measurement of electrical conductivity and thermoelectric power at elevated temperatures (1073-1323K) in controlled oxygen activities [1x10-13 Pa < p(O2) < 75 kPa]. The study included two types of experiments: Determination of electrical properties under conditions of gas/solid equilibrium. The data obtained was used to derive defect disorder and related semiconducting properties Monitoring of electrical properties during equilibration. This data was used to determine the chemical diffusion coefficient. The data obtained under equilibrium conditions indicates that oxygen may be used as a dopant to impose controlled semiconducting properties. In reduced conditions TiO2 is an n-type semiconductor and under oxidizing conditions TiO2 is a p-type semiconductor. The n-type behaviour is associated with oxygen vacancies as the predominant defects and titanium interstitials as the minority defects. The p-type behaviour is closely related to titanium vacancies that are formed during prolonged oxidation. Charge transport at elevated temperature was shown to involve substantial contribution from ions. Analysis of electrical properties enabled determination of several defect-related quantities including the activation enthalpy for oxygen vacancy formation, and the activation energy of the electrical conductivity components related to electrons, holes and ions. The kinetic data obtained during gas/solid equilibration enabled determination of the chemical diffusion coefficient which exhibited a complex dependence on nonstoichiometry. In addition, prolonged oxidation showed that equilibration occurred in two kinetic regimes. One for highly mobile oxygen vacancies and titanium interstitials which quickly reached an ??operational equilibrium?? within hours and another slow kinetic regime for equilibration of titanium vacancies over many thousand hours. The determined chemical diffusion coefficient data may be used to select the processing conditions required to impose uniform concentration of defects within a TiO2.
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Neagu, Dragos. "Materials and microstructures for high temperature electrochemical devices through control of perovskite defect chemistry." Thesis, University of St Andrews, 2013. http://hdl.handle.net/10023/3606.
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The development of technologies that enable efficient and reliable energy inter-conversion and storage is of key importance for tempering the intermittent availability of renewable energy sources, and thus for developing an energy economy based on sustainable, clean energy production. Solid oxide electrolysis cells (SOECs) may be used to store excess electrical energy as hydrogen, while solid oxide fuel cells (SOFCs) could convert back hydrogen into electricity, thus balancing energy availability and demand. However, the current state-of-the-art hydrogen electrode used in both SOECs and SOFCs, the Ni-yttria-stabilised zirconia cermet (Ni-YSZ), is unreliable in conjunction with intermittent energy sources, in particular due to its innate redox instability. This thesis explores the fundamental properties of various inherently redox stable A-site deficient titanate perovskite systems (A1-αBO3, B = Ti), seeking to uncover the principles that enhance their properties so that they may be used to replace Ni-YSZ. In particular, this work demonstrates that the versatility of perovskites with respect to the introduction of lattice defects such as vacancies and cation substitutions enables considerable improvements in the extent of reduction, electronic conductivity and overall electrochemical activity. Most importantly, the defect chemistry context set by the presence of A-site vacancies was found to trigger the exsolution of electrocatalytically active nanoparticles from the parent perovskite, upon reduction. This is an entirely new phenomenon which was explored and exploited throughout this study to produce perovskite surfaces decorated with uniformly distributed catalytically active nanoparticles. As demonstrated in this study, the exsolution phenomenon excels in terms of producing nanoparticles with uniform size, distribution, diverse composition and ‘unconventional' surface anchorage. The resulting enhanced properties, and especially the exsolution phenomenon, contributed coherently towards improving the suitability of the perovskites developed here towards their application as hydrogen electrode materials. Consequently, when integrated into SOEC button cells as hydrogen electrodes, they exhibited a step-change increase in performance compared to other perovskites considered to date. Many of the principles and perovskite defect chemistry explored and exemplified in this study on perovskite titanates may be extended to other perovskites as well. In particular the advanced control and understanding achieved in this work over the exsolution phenomenon may inspire the formulation of new and sophisticated oxide materials with advanced functionality.
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Nakao, Takayuki. "Studies on Point-Defect Structure of Solid Oxide Fuel Cell Materials and Their Electrochemical Properties." Kyoto University, 2011. http://hdl.handle.net/2433/142306.
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Kyoto University (京都大学)0048
新制・課程博士
博士(人間・環境学)
甲第16178号
人博第561号
新制||人||135(附属図書館)
22||人博||561(吉田南総合図書館)
28757
京都大学大学院人間・環境学研究科相関環境学専攻
(主査)教授 内本 喜晴, 教授 杉山 雅人, 教授 田部 勢津久, 准教授 福塚 友和
学位規則第4条第1項該当
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Darvish, Shadi. "Thermodynamic Investigation of La0.8Sr0.2MnO3±δ Cathode, including the Prediction of Defect Chemistry, Electrical Conductivity and Thermo-Mechanical Properties." FIU Digital Commons, 2018. https://digitalcommons.fiu.edu/etd/3653.
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Fundamental thermodynamic investigations have been carried out regarding the phase equilibria of La0.8Sr0.2MnO3±δ (LSM), a cathode of a solid oxide fuel cell (SOFC), utilizing the CALculation of PHAse Diagram (CALPHAD) approach. The assessed thermodynamic databases developed for LSM perovskite in contact with YSZ fluorite and the other species have been discussed. The application of computational thermodynamics to the cathode is comprehensively explained in detail, including the defect chemistry analysis as well as the quantitative Brouwer diagrams, electronic conductivity, cathode/electrolyte interface stability, thermomechanical properties of the cathode and the impact of gas impurities, such as CO2 as well as humidity, on the phase stability of the cathode. The quantitative Brouwer diagrams for LSM at different temperatures are developed and the detailed analysis of the Mn3+ charge disproportionation behavior and the electronic conductivity in the temperature range of 1000-1200°C revealed a good agreement with the available experimental observations. The effects of temperature, CO2 partial pressure, O2 partial pressure, humidity level and the cathode composition on the formation of secondary phases have been investigated and correlated with the available experimental results found in the literature. It has been indicated that the CO2 exposure does not change the electronic/ionic carriers’ concentration in the perovskite phase. The observed electrical conductivity drop is predicted to occur due to the formation of secondary phases such as LaZr2O7, SrZrO3, SrCO3 and Mn oxides at the LSM/YSZ interface, resulting in the blocking of the electron/ion transfer paths. For the thermo-mechanical properties of LSM, a new weight loss Mechanism for (La0.8Sr0.2)0.98MnO3±δ using the La-Sr-Mn-O thermodynamic database is modeled with respect to the compound energy formalism model. This newly proposed mechanism comprehensively explains the defect formation as a result of volume/weight change during the thermal cycles. According to the proposed mechanism the impact of cation vacancies regarding the volume change of cathode was explained.
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White, Brandon M. "Characterizing the Association Between Mandible Mechanical Properties and Function in the Rabbit." Case Western Reserve University School of Graduate Studies / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=case1506426954676547.
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Nawaz, Ali. "Modification of charge transport properties in defect-free poly(3-hexylthiophene-2,5-diyl) field-effect transistors." reponame:Repositório Institucional da UFPR, 2017. http://hdl.handle.net/1884/53109.
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Orientador: Prof. Dr. Ivo Alexandre HümmelgenTese (doutorado) - Universidade Federal do Paraná, Setor de Ciências Exatas, Curso de Pós-Graduação em Física. Defesa: Curitiba, 15/12/2017
Inclui referências : f. 93-102
Resumo: O trabalho atual investiga a melhoria das propriedades de transporte de carga em transistores de efeito de campo de baixa tensão (FETs), que utilizam poli(3-hexiltiofeno-2,5-diil) (P3HT) não-100% e 100% regioregular como os semicondutores orgânicos, e poli(álcool vinílico) reticulado (cr-PVA) como isolante. O trabalho de pesquisa realizado durante o projeto pode ser dividido em duas partes. A primeira parte investiga a melhoria das propriedades de transporte de carga na interface cr-PVA/P3HT, e a influência de defeitos de regioregularidade de P3HT nas propriedades da interface. A segunda parte demonstra a preparação de filmes finos que consistem em moléculas alinhadas de P3HT 100% regioregular e, consequentemente, a aplicação desses filmes alinhados para o desenvolvimento de dispositivos de alto desempenho. No caso da primeira parte, o problema essencial é que o transporte de carga na interface de cr-PVA/P3HT está limitado pela presença de armadilhas na interface que correspondem aos dipolos de superfície de cr-PVA. Esses dipolos de superfície possuem a capacidade de modificar a distribuição de carga em moléculas adjacentes de P3HT, o que pode levar à localização e a captura de cargas. Isso representa um problema fisico complexo, sendo que a variação de energia potencial na interface depende da posição e orientação das armadilhas dipolares em relação às moléculas de P3HT. No entanto, a solução é conceitualmente simples, pois, em princípio, é apenas necessário passivar as armadilhas. Para conseguir isso, é apresentada uma técnica experimental econômica, na qual a superfície de cr-PVA é tratada com um surfactante catiônico, brometo de hexadeciltrimetilamónio (CTAB). As cabeças hidrofílicas carregadas positivamente de CTAB visam a passivação das armadilhas carregadas negativamente da superfície de cr-PVA. A deposição de CTAB sobre o cr-PVA, em relação ao cr-PVA somente, resulta em aumento significativo na capacitância do isolanate (Ci), e as imagens de microscopia de força atômica (AFM) mostram que a superfície de cr-PVA é coberta com grãos de surfactante bem conectados. Em caso de dispositivos baseados em P3HT não-100% regioregular, este tratamento resulta em uma melhora da mobilidade de efeito de campo (?FET) por um fator de ~3 (?FET médio de 0.44 cm2/V.s) quando comparado aos dispositivos não tratados. Para investigar como o tratamento do surfactante modifica o transporte de carga na interface, a variação de ?FET em função da espessura efetiva do gargalo do canal (l0) também é analisada e discutida detalhadamente. Curiosamente, ao contrário dos dispositivos baseado em P3HT não-100% regioregular, o tratamento com surfactante em dispositivos baseado em P3HT 100% regioregular resulta em degradação de ?FET e do desempenho geral dos dispositivos. Isso indica que a interação de defeitos de regioregularidade e armadilhas de superfície de cr-PVA é um fator crítico que afeta as propriedades de transporte de carga na interface cr-PVA/P3HT. Para investigar este assunto, a interação das moléculas de P3HT não-100% e 100% regioregular com dipolos de superfície de cr-PVA é investigada usando espectroscopia de absorbância, AFM e cálculos de química quântica. Observa-se que, dependendo da presença ou ausência de defeitos de regioregularidade de P3HT (e, portanto, da planaridade molecular), o contato entre as moléculas de P3HT e os dipolos de superfície de cr-PVA afeta a ordem molecular do P3HT de forma diferente. Por causa dos defeitos de regioregularidade, as moléculas de polímero não-100% regioregular produzem momentos de dipolo mais altos em comparação com moléculas 100% regioregular. Consequentemente, discute-se como a interação de moléculas de P3HT não-100% e 100% regioregular com dipolos de cr-PVA contribuem à desordem energética na interface cr-PVA/P3HT. Neste caso, o transporte de carga em dispositivos de FET é investigado para quatro espessuras diferentes de P3HT não-100% e 100% regioregular. Os resultados elétricos mostram que o comportamento de ?FET × l0 e a dependência de ?FET na espessura do canal são uma função forte da presença ou ausência de defeitos de regioregularidade de P3HT. Neste trabalho, os dispositivos (não tratados) baseados em P3HT 100% regioregular demonstram ?FET tão alto quanto 1.20 cm2/V.s. Esses valores tornam esses dispositivos reconhecíveis para a integração em várias aplicações comerciais. No entanto, um desenho de circuitos para muitas outras aplicações de alto desempenho impõem um requisito de ?FET mais rigoroso (> 5 cm2/V.s). Para alcançar este marco, na segunda parte do projeto de pesquisa, está apresentada uma técnica de deposição simples (chamado, floating-film transfer method, em inglês), que permite o alinhamento supra-molecular das moléculas de P3HT 100% regioregular. A aplicação de filmes de polímero alinhados em FETs resulta em valores de ?FET de até 8 cm2/V.s, que é o valor mais alto reportado até agora para os FETs baseados em P3HT. Palavras-chaves: Transistores orgânicos de efeito de campo, poli(3-hexiltiofeno-2,5-diil) livre de defeitos, regioregularidade, poli(álcool vinílico) reticulado, interface isolante/semicondutor, armadilhas dipolares.
Abstract: The current work investigates the improvement of charge transport properties in low-voltage organic field-effect transistors (OFETs) that utilize non-100% and 100% regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) as the organic semiconductors, and cross-linked poly(vinyl alcohol) (cr-PVA) as the gate insulator. The essential research work performed during the project can be divided into two parts. The first part investigates the improvement of charge transport properties at the cr-PVA/P3HT interface, and the influence of regioregularity defects of P3HT on interface properties. The second part demonstrates the development of high performance OFETs based on supra-molecularly aligned thin films of 100% regioregular P3HT. In the case of the first part, the essential problem in hand is that charge transport at the cr-PVA/P3HT interface is limited by the presence of charge traps at the interface corresponding to the surface dipoles of cr-PVA. These surface dipoles hold the ability to modify charge distribution on adjacent P3HT molecules, which can lead to localization and trapping of otherwise mobile charge carriers. This presents a physically complex problem, since the potential energy variations at the interface depends on the position and orientation of the dipolar traps with respect to P3HT molecules. However, the solution is conceptually simple since, in principle, it is only required to passivate the traps. In order to achieve this, a cost-effective experimental technique is presented, in which the cr-PVA surface is treated with a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB). The positively charged hydrophilic heads of CTAB are aimed at passivating the negatively charged traps of the cr-PVA surface. The deposition of CTAB over cr-PVA leads to significant enhancement in gate insulator capacitance (Ci), and the atomic force microscopy (AFM) images show that the cr-PVA surface is covered with well-connected surfactant grains. In the case of non-100% regioregular P3HT OFETs, this treatment results in an improvement of field-effect mobility (?FET) by a factor of ~3 (average ?FET of 0.44 cm2/V.s) when compared to untreated devices. In order to investigate how the surfactant treatment modifies charge transport at the interface, variation of ?FET as a function of the effective bottleneck thickness of the conducting channel (l0) is also analyzed and thoroughly discussed. Quite interestingly, contrary to non-100% regioregular P3HT devices, the surfactant treatment in 100% regioregular P3HT devices leads to degradation of ?FET and overall device performance. This indicates that the interaction of regioregularity defects and cr-PVA surface traps is a crucial factor affecting charge transport properties at the cr-PVA/P3HT interface. In order to address this issue, the interaction of non-100% and 100% regioregular P3HT molecules with cr-PVA surface dipoles is investigated using UV-vis absorbance spectroscopy, AFM and quantum chemical calculations. It is observed that, depending on the presence or absence of regioregularity defects of P3HT (and thus the molecular planarity); the intimate contact between P3HT molecules and cr-PVA surface dipoles affects the molecular order of P3HT differently. Because of the regioregularity defects, the non-100% regioregular polymer molecules produce higher dipole moments compared to 100% regioregular molecules. Consequently, it is discussed how the interaction of non-100% and 100% regioregular P3HT molecules with cr-PVA surface dipoles contribute differently to the potential energy variations at the cr-PVA/P3HT interface. In this case, the charge transport in FET devices is investigated for four different thicknesses of both non-100% and 100% regioregular P3HT. The electrical results reveal that the behavior of ?FET × l0 and the dependence of ?FET on channel thickness are a strong function of the presence/absence of the regioregularity defects of P3HT. In this project, the untreated 100% regioregular P3HT devices demonstrate ?FET as high as 1.20 cm2/V.s. Such high values make these devices recognizable for translation to various commercial applications. However, the circuit designs of many other high performance applications impose a more stringent ?FET requirement (> 5 cm2/V.s). In order to achieve this landmark, in the second part of the research project, a simple and cost-effective deposition technique (floating-film transfer method) is presented, which allows supra-molecular alignment of 100% regioregular P3HT molecules. The application of aligned polymer films in FET devices leads to the demonstration of ?FET values as high as 8 cm2/V.s, which is the highest value reported so far for P3HT based OFETs. Keywords: Organic field-effect transistors, defect-free poly(3-hexylthiophene-2,5-diyl), regioregularity, cross-linked poly(vinyl alcohol), insulator/semiconductor interface, dipolar charge traps.
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Upadhyay, Manas Vijay. "On the role of defect incompatibilities on mechanical properties of polycrystalline aggregates: a multi-scale study." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/53041.
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The main objective of this thesis is to obtain critical insight on the role of crystalline incompatibilities in strain and curvature, induced in presence of line defects i.e. dislocations and disclinations, on the energy and geometry of specific features of the local microstructure, and on the bulk mechanical response of nanocrystalline/ultra-fine grained
materials. To that end, studies are performed at the (1) inter-atomic and fine scale, and (2) at the mesoscale. The modelling approach is based on the field dislocation and disclination mechanics theory of continuously representated dislocations and disclinations. New, thermodynamically rigorous, multi-scale elastic constitutive laws based on the couple stress theory are developed to capture the effect of strain and curvature incompatibilities on the Cauchy and couple stresses. A new meso-scale elasto-viscoplastic constitutive model of defect incompatibilities based on a fast Fourier transform technique is developed. The desired scale transitioning is achieved via novel phenomenological defect density transport equations and the newly developed elastic constitutive laws.
At the fine scale, the model is applied to study energetic interactions between strain and curvature incompatibilities associated with grain boundaries and their influence on triple line energies. Results reveal that incompatible lattice strains have the most significant contribution to the energy. Incompatible lattice curvatures have negligible energetic contributions but are necessary to characterize the geometry of grain boundaries. Finally, both incompatible lattice strains and curvatures are necessary to capture the structure sensitive mechanical behavior of grain boundaries.
At the mesoscale, deformation of nanocrystalline aggregates characterized by residual curvatures is studied to identify the impact of the latter's presence on the local and bulk mechanical response of the aggregate. Relaxation of local stresses generated from residual curvatures reproduces the effect of GB dislocation emission. Uniaxial tensile loading of nanocrystalline microstructures containing residual curvatures reveals a softening in the yield stress which could explain the breakdown in Hall-Petch law in the nanocrystalline regime.
Next, the possibility of characterizing incompatibilities using X-ray or neutron diffraction techniques is tested. Results reveal that only strains and their gradients contribute to the broadening of diffraction peaks; curvatures and their gradients have no contribution. This study leads to the development of a new multi-scale averaged strain based Fourier technique for generating virtual diffraction peaks.
42
Yan, Miaolei. "Defect Analysis and Microstructural Effects on the Surface Exchange Properties of La0.7Sr0.3MnO3(LSM) Epitaxial Thin Films." Research Showcase @ CMU, 2015. http://repository.cmu.edu/dissertations/650.
Full textAbstract:
La0.7Sr0.3MnO3 (LSM) is a perovskite oxide material that possesses many interesting electromagnetic and electrochemical properties, making it desirable as magnetic tunnel junction (MTJ) and solid oxide fuel cell (SOFC) electrodes.
43
Riahi, Fatma, Sofiane Laouar, and Djamel Eddine Mekki. "Contribution to the understanding of the point defect influence on some transport properties in UO 2+x." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-191474.
Full textAbstract:
A point defect model is developed in hyperstoichiometric uranium dioxide, UO2+x,in order to explain some experimental results of deviation from stoichiometry and electrical conductivity existing in the literature. This model takes into account both singly charged uranium vacancy, V'U , and two types of Willis clusters, characterized by two kinds of interstitial oxygen atoms that are located on two different sites and two oxygen vacancies, namely 2(Oi"Oi'Vȯ)4' and . The electron holes, ḣ , are also introcued to ensure the crystal global electroneutrality. An adjustment procedure of the most convincing experimental data of the literature, corresponding to the temperature and oxygen partial pressure dependencies of departure from stoichiometry,x, and electrical conductivity, [sigma] , has been undertaken. The fit model/experience obtained has been found fairly good, justifying, a posteriori, the relevance of the nature of the selected defects in the model.
44
Riahi, Fatma, Sofiane Laouar, and Djamel Eddine Mekki. "Contribution to the understanding of the point defect influence on some transport properties in UO 2+x." Diffusion fundamentals 11 (2009) 98, S. 1-15, 2009. https://ul.qucosa.de/id/qucosa%3A14071.
Full textAbstract:
A point defect model is developed in hyperstoichiometric uranium dioxide, UO2+x,in order to explain some experimental results of deviation from stoichiometry and electrical conductivity existing in the literature. This model takes into account both singly charged uranium vacancy, V''U , and two types of Willis clusters, characterized by two kinds of interstitial oxygen atoms that are located on two different sites and two oxygen vacancies, namely 2(Oi"Oi''Vȯ)4'' and . The electron holes, ḣ , are also introcued to ensure the crystal global electroneutrality. An adjustment procedure of the most convincing experimental data of the literature, corresponding to the temperature and oxygen partial pressure dependencies of departure from stoichiometry,x, and electrical conductivity, [sigma] , has been undertaken. The fit model/experience obtained has been found fairly good, justifying, a posteriori, the relevance of the nature of the selected defects in the model.
45
Kim, Sunho Ph D. Massachusetts Institute of Technology. "Defect and electrical properties of high-K̳ dielectric Gd₂O₃ for magneto-ionic and memristive memory devices." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/129007.
Full textAbstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2020Cataloged from student-submitted PDF of thesis. The "K̳̳" in title on title page appeared as subscript "K."
Includes bibliographical references (pages 127-134).
While high-[subscript K] dielectrics utilized in CMOS technology are noted for their highly insulating characteristics, they have demonstrated surprising electrolytic behavior as key components in a variety of thin film memory devices, including those based on magneto-ionic and memristive behavior. In this work, we focus on the rare earth sesquioxide, Gd₂O₃, a well-known high-κ dielectric that has exhibited a variety of electrolytic properties during the development and operation of the first magneto-ionic devices developed at MIT. Specifically, we focused our investigation on the defect chemistry and electrical properties of Gd₂O₃ in order to better understand the relationship between the structure, chemistry, processing conditions, and operating environment and the material's low-temperature ionic and electronic transport properties and the means for their optimization vis-à-vis memory device operation.
Phase (monoclinic and cubic) and dopant controlled (Ca, Ce, Sr, Zr) polycrystalline pellets of 8 different Gd₂O₃ systems were prepared to investigate various defect regimes in consideration of this material's polymorphism. We considered intrinsic anion-Frenkel disorder and electronic disorder, equilibration with the gas phase, water incorporation, and dopant incorporation in the defect modeling, taking into account the roles of crystallographic structure as well as oxygen ion defect and protonic generation. The primary method utilized to characterize the defect chemistry and transport properties of Gd₂O₃ was the analysis of the dopant, p0₂ and temperature dependencies of the electrical conductivity extracted from complex impedance spectra obtained over the p0₂ range of 1 to 10⁻¹⁵ atm, for 5 isotherms between 700 and 900 °C with 50 °C steps and for a range of acceptor and donor dopants.
Based on the p0₂ dependency of conductivities, in light of the defect modeling, the majority point defects in each system were identified. Electronic and ionic migration energies and thermodynamic parameters were extracted via the defect modeling and temperature dependencies of conductivities. In nearly all cases, the predominant charge carrier under oxidizing conditions at elevated temperatures was identified as the p-type electron-hole, largely due to oxygen excess non-stoichiometry in these systems. With decreasing p0₂, transport tended to switch from semiconducting towards ionic. Depending on phase, dopant type & concentration, temperature, and relative humidity, the predominant ionic conductivity was found to be via oxygen interstitials, oxygen vacancies, and/or protons, the latter given by the propensity of Gd₂O₃ to take up water in solid solution from the environment by the formation of OH[superscript .]species.
Unexpectedly, the ionic mobilities of defects in the denser and less symmetric monoclinic system exhibited higher ionic mobilities than the more open bixbyite structure. The hole electronic species in the investigated systems were found to migrate via the small polaron hopping mechanism with rather large hopping energies. This resulted in an inversion of hole and proton mobility magnitudes at reduced temperatures in the monoclinic system. Extrapolation of ionic and electronic defect conductivities to near room temperature, based on our derived defect and transport models, was not able to explain, on its own, the observed electrolytic properties of the Gd₂O₃ thin films utilized in magneto-ionic devices.
In an attempt to connect the transport properties obtained under equilibrium conditions at elevated temperatures with the behavior of Gd₂O₃ near room temperature, selected thin films Gd₂O₃, prepared by pulsed laser deposition or sputtering, were investigated by complex impedance spectroscopy over the temperature range of 20 - 170°C. While films prepared under dry conditions were indeed found to be highly electrically insulating, films exposed to water vapor exhibited dramatically higher proton conductivities (more than ~10⁸ x) than values extrapolated from high temperature. Parallel thermogravimetric analysis on Gd₂O₃ powder specimens, as a function of temperature, under high humidity conditions, demonstrated a correlation between uptake/loss of incorporated water and conductivity upon cooling and heating, respectively.
We can therefore conclude that the large disconnect between the electrical and electrolytic properties observed between high-κ dielectrics used in CMOS devices such as Gd₂O₃, and their much more highly conductive counterparts used in thin film memory devices, depends strategically on the thin film processing conditions. High-κ dielectrics are fabricated in carefully controlled environments with low relative humidity, while research on, for example, Gd₂O₃ - based magneto-ionic memory devices, is performed under ambient laboratory conditions, where significant water uptake becomes possible at surfaces and grain boundaries. The results and insights obtained in this study can be expected to be applied in achieving further progress in the understanding and optimization of magneto-ionic, memristive, and other devices that rely on proton gating.
by Sunho Kim.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineering
46
Alabaster, C. M. "The Microwave properties of tissue and other lossy dielectrics." Thesis, Cranfield University, 2004. http://hdl.handle.net/1826/251.
Full textAbstract:
This thesis describes work on the theoretical modelling and experimental measurement
of the complex permittivity of dielectrics. The main focus of research has been into the
characterisation of permittivity of planar and layered samples within the millimetre
wave band. The measurement method is based on the free-space measurement of the
transmission and reflection coefficients of samples. A novel analytical method of
determining the transmission and reflection coefficients as functions of frequency
arising from a generalised structure of planar dielectric layers is also described and
validated. The analytical method is based on signal flow techniques. The measurement
and analytical techniques have been applied in two main areas: firstly, the acquisition of
new data on human skin in the band 57 to 100GHz and secondly, the detection and
location of defects in composite materials for which a band of 90 to 100GHz was used.
Measurements have been made on the complex permittivity of a single sample of
excised human skin fixed in formaldehyde. The experimental results have been
corrected to account for the fixing process in formaldehyde and are projected to body
temperature. This data is, to the best of the author’s knowledge, the first of its kind to be
published. Predicted skin permittivity based on various relaxation models varies widely
and only partially fits the measured data. The experimental results have been used to
determine the parameters of a Cole-Cole function which gives the best fit to the
measured data. The measured skin data has also been used to calculate power deposition
in skin exposed to millimetre wave radiation. This work concludes that a skin surface
temperature rise of only 0.20C results from a thirty second exposure to signals of
100W/m2.
Experimental work with fibreglass composite samples has shown that defects such as
delaminations, voids, matrix cracks and improper cure result in resolvable differences in
the dielectric properties of the samples at 90 – 100GHz. The measurement technique is
particularly sensitive to the detection of cracks and its spatial resolution is 20mm or
better. Whilst confirming the general conclusions of previously published work, the
specific findings of this study are novel.
47
Pelini, Thomas. "Optical properties of point defects in hexagonal boron nitride." Thesis, Montpellier, 2019. http://www.theses.fr/2019MONTS139.
Full textAbstract:
L’objectif de cette thèse était d’explorer et de caractériser optiquement les défauts ponctuels dans le nitrure de bore hexagonal. L’étude des défauts dans ce semiconducteur revêt un intérêt fondamental à la fois pour la science des matériaux dans laquelle il joue un rôle clé de part sa nature lamellaire (matériau 2D) et sa stabilité thermique et chimique très élevées, et également dans le domaine des technologies quantiques où son grand gap (~ 6 eV) permet d’exploiter les défauts ponctuels profonds comme «atome artificiel» dans la matrice cristalline. Au cours de cette thèse, des défauts appartenant à deux gammes spectrales ont été étudiés: une première classe émettant dans le visible, et une seconde émettant dans la gamme ultraviolette.Ainsi, dans un premier temps, nous avons exploité un microscope confocal à balayage fonctionnant à l’ambiante et dans les longueurs d’onde visibles. La réalisation de cartes spatiales de photoluminescence a permis de mettre en évidence l’existence de points chauds de photoluminescence localisés, sous la limite de diffraction du microscope, et émettant autour de 600 nm (2 eV). Des mesures de corrélations temporelles de photons montre d’une part qu’il s’agit d’émetteurs quantiques uniques, et permet d’autre part de sonder la photo-dynamique de ces systèmes, en particulier aux très longues échelles de temps. Différents régimes de photo-stabilité sont observés et discutés. Enfin, l’étude en puissance a aussi été effectuée et montre qu’une part des émetteurs (~ 5%) sont photo-stables à haute puissance d’excitation optique et saturent à un taux d’émission de plusieurs millions de coups par seconde: ces défauts ponctuels constituent une source de photons uniques parmi les plus brillantes à température ambiante dans un système à l’état solide.Dans un second temps, nous avons exploré les défauts émettant dans la gamme ultraviolette. Un préalable à la maitrise et l’utilisation des défauts dans les semiconducteurs à des fins technologiques est la connaissance de leur origine chimique. Dans cette optique nous avons tout d’abord étudié les niveaux énergétiques superficiels et profonds d’échantillons de nitrure de bore hexagonal enrichis en carbone en combinant des mesures de macro-photoluminescence et de réflectivité. L’existence de nouvelles transitions optiquement actives est révélée (autour de 300 nm), et l’implication du carbone comme origine de ces transitions est discutée. L’étude approfondie de ces nouvelles émissions a requiert la réalisation d’un microscope confocal à balayage fonctionnant dans l’ultraviolet à 266 nm et à température cryogénique. Le design du microscope est détaillé, les difficultés de sa mise en oeuvre expliquées, et ses performances démontrées. Ce nouvel outil expérimental nous permet d’examiner avec précision les défauts profonds. En particulier, une étude est faite sur la corrélation spatiale de ces nouvelles raies avec celle du défaut ponctuel bien connu à 4.1 eV. Ensuite, nous avons utilisé des nouveaux échantillons dopés en carbone isotopiquement purifié comme stratégie pour déterminer la nature chimique du défaut à 4.1 eV. À travers cette tentative, nous avons mis en lumière l’inhomogénéité spatiale des caractéristiques optiques de cet émetteur. Enfin, dans la dernière partie, on tente d’isoler l’émission provenant d’un défaut unique à 4.1 eV. Pour cela, on utilise des flocons fins pré-caractérisés en microscopie électronique et contenant une faible densité d’émetteurs. Leur photostabilité est étudiéeThe purpose of this thesis was to explore and caracterize optically the point defects in hexagonal boron nitride. The study of defects in this semiconductor is of fundamental importance firstly for the material science in which it plays a key role thanks to its lamellar structure (2D material) and its high thermal and chemical stability, and secondly for the quantum nanotechnology domain where its large bandgap (~ 6 eV) allows for exploiting deep levels point imperfections as «artificial atom» in the crystal lattice. During this thesis, defects in two spectral ranges have been studied: a first family emitting in the visible wavelengths, and a second one emitting in the ultraviolet range.Firstly, we made use of a scanning confocal microscope working in ambient conditions and at visible wavelengths. The recording of photoluminescence spatial maps permited to show the existence of localised hot spot of light, under the diffraction limit of the miscroscope, and emitting around 600 nm (2 eV). Time photon-correlation measurements revealed on one hand that we were dealing with single quantum emitters, and on the other hand allowed for probing the photodynamics of those systems, in particular at very long time-scale. Various photostability regimes are observed and discussed. Last but not least, power resolved study was also performed and demonstrated that a number of the emitters (~ 5%) are photo-stable at high excitation power and saturate at few millions counts per second: those point defects are one of the brightest single-photon source at room temperature in solid-state systems.Secondly, we explored the defects in the ultraviolet spectral range. A prerequisite to the engineering of defects in semiconductors for technological applications is the knowledge of their chemical origin. With this in mind, we studied shallow and deep levels in carbon-doped hBN samples by combining macro-photoluminescence and reflectance measurements. We showed the existence of new optically-active transitions (around 300 nm) and discussed the implication of carbon in these levels. The in-depth study of these levels have required the development of a new scanning micro-photoluminescence confocal microscope operating at 266 nm under cryogenic environment. The design and performances of the optical system are described, and the experimental challenges explained in details. Using this new setup, we went further into the examination of the deep levels. In particular, a study was carried out regarding the spatial correlation between these new spectral lines and the well-known point defect at 4.1 eV. Then, we used new crystals with isotopically-purified carbon doping as a strategy to investigate the long-standing question concerning the chemical origin of the 4.1 eV defect. Through this attempt, we brought to light the spatial dependence of the optical features for this specific emitter. Last but not least, we present our work dedicated to isolate the emission of a single 4.1 eV defect. We studied the photoluminescence of thin undoped flakes, pre-characterized with an electron microscope, that contain a low density of emitters, and inspected in particular their photostability in these thin crystals
48
Katayama, Shota. "Electronic structures and optical properties of Sn(II) ternary oxides." 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/199306.
Full text
49
Guler, Ipek. "Optical And Electrical Transport Properties Of Some Quaternarythallium Dichalcogenides." Phd thesis, METU, 2011. http://etd.lib.metu.edu.tr/upload/12613362/index.pdf.
Full textAbstract:
In this thesis, in order to study the structural, optical and electrical transport properties of
Tl2In2S3Se, TlInSeS and Tl2In2SSe3 crystals, X-ray diffraction (XRD), energy dispersive
spectroscopic analysis (EDSA), transmission, reflection, photoluminescence (PL), thermally
stimulated current (TSC) and photoconductivity decay (PC) measurements were carried out.
Lattice parameters and atomic composition of these crystals were determined from XRD and
EDSA experiments, respectively. By the help of transmission and reflection experiments,
the room temperature absorption data were analyzed and it was revealed the coexistence of
indirect and direct band gap energies of the studied crystals. Moreover, the refractive index
dispersion parameters - oscillator energies, dispersion energies, oscillator strengths, oscillator
wavelengths and zero-frequency refractive indexes were determined. Temperature-dependent
transmission measurements made it possible to find the rate of change of indirect band gaps
with temperature, absolute zero values of the band gap energies and Debye temperatures of
these crystals. From the analysis of the transmission and reflection measurements, it was
established that, there is a decrease in the values of indirect and direct band gaps energies and
an increase in zero-frequency refractive indexes with increasing of selenium content.
PL measurements were carried out to obtain the detailed information about recombination
levels in crystals studied. The behavior of PL spectra were investigated as a function of
laser excitation intensity and temperature. The variation of the spectra with laser excitation
intensity and temperature suggested that the observed emission bands in these crystals were
due to the donor-acceptor pair recombination.
TSC measurements were carried out with various heating rates at different illumination temperatures
to obtain information about trap levels in these crystals. The mean activation energies,
attempt-to-escape frequencies, concentrations and capture cross sections of the traps
were determined as a result of TSC spectra analysis. The analysis of experimental TSC curves
registered at different light illumination temperatures revealed the exponential trap distribution
in the studied crystals. From the analysis of PC measurements, carrier lifetimes were
obtained.
50
Alabaster, Clive M. "The microwave properties of tissue and other lossy dielectrics." Thesis, Cranfield University, 2004. http://dspace.lib.cranfield.ac.uk/handle/1826/251.
Full textAbstract:
This thesis describes work on the theoretical modelling and experimental measurement of the complex permittivity of dielectrics. The main focus of research has been into the characterisation of permittivity of planar and layered samples within the millimetre wave band. The measurement method is based on the free-space measurement of the transmission and reflection coefficients of samples. A novel analytical method of determining the transmission and reflection coefficients as functions of frequency arising from a generalised structure of planar dielectric layers is also described and validated. The analytical method is based on signal flow techniques. The measurement and analytical techniques have been applied in two main areas: firstly, the acquisition of new data on human skin in the band 57 to 100GHz and secondly, the detection and location of defects in composite materials for which a band of 90 to 100GHz was used. Measurements have been made on the complex permittivity of a single sample of excised human skin fixed in formaldehyde. The experimental results have been corrected to account for the fixing process in formaldehyde and are projected to body temperature. This data is, to the best of the author’s knowledge, the first of its kind to be published. Predicted skin permittivity based on various relaxation models varies widely and only partially fits the measured data. The experimental results have been used to determine the parameters of a Cole-Cole function which gives the best fit to the measured data. The measured skin data has also been used to calculate power deposition in skin exposed to millimetre wave radiation. This work concludes that a skin surface temperature rise of only 0.20C results from a thirty second exposure to signals of 100W/m2. Experimental work with fibreglass composite samples has shown that defects such as delaminations, voids, matrix cracks and improper cure result in resolvable differences in the dielectric properties of the samples at 90 – 100GHz. The measurement technique is particularly sensitive to the detection of cracks and its spatial resolution is 20mm or better. Whilst confirming the general conclusions of previously published work, the specific findings of this study are novel.