To see the other types of publications on this topic, follow the link: Stability of grain structure.
Dissertations / Theses on the topic 'Stability of grain structure'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Stability of grain structure.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Bommareddy, Aravinda Reddy Materials Science & Engineering Faculty of Science UNSW. "Thermal stability of submicron grain structure in an Al-Sc alloy." Publisher:University of New South Wales. Materials Science & Engineering, 2008. http://handle.unsw.edu.au/1959.4/41492.
Full textAbstract:
Severe plastic deformation (SPD) has been used over the past few decades for producing submicron grain (SMG) structures in range of metals and alloys. Equal channel angular pressing (ECAP) is a useful process for producing these types of structures whereby the material is deformed to very high plastic strains by passing a billet several times through the ECAP die. This process has an added advantage maintaining the initial dimensions of the billet. SMG materials produced by ECAP and related routes are useful as they usually exhibit excellent properties including high strength and hardness, and excellent superplastic formability: these and other properties make SMG materials useful for industrial and aerospace applications. In this thesis, a binary aluminium alloy containing a very low concentration of scandium (0.1 wt. %) Sc alloy was investigated and compared with higher Sc-containing alloys. The material was deformed by ECAP in the solution treated condition to an equivalent von Mises strain of 9.2 then pre-aged at 250 0C to generate a submicron grained material containing a relatively uniform dispersion of nanosized Al3Sc dispersiods. The thermal stability of this pre-aged microstructure was investigated by annealing at temperatures up to 450 0C resulted in continuous grain coarsening by the process of continuous recrystallization whereby the initial microstructure evolves gradually with no marked change in the grain size distribution, texture and grain boundary character. However, extended annealing (> 1h) at 4500 C resulted in discontinuous grain coarsening (often termed recrystallization) whereby a few grains grow rapidly to eventually produce a coarse-grained final microstructure. Throughout annealing, there was a good correlation between the dispersion parameter, (f/d) where f and d is the volume fraction and the mean diameter of Al3Sc particles in the alloy, respectively, and both the mean grain size (D ) and D /D max where max D is the maximum grain diameter observed in the microstructure. The grain structure was found to undergo moderate coarsening at the high f/d-values but converted to a coarsegrained structure for f/d ~<0.5/μm, and this change occurred when the mean grain diameter was ~ 3-4μm. Hence, the critical value of the dispersion parameter for the transition from continuous to discontinuous coarsening falls between the theoretical value for submicron grain size alloys (f/d ~ 1.5/μm) and the value found for conventionally-deformed alloys (f/d ~ 0.1/μm). This behaviour is the result of the alloy no longer being ultra-fine grained at the onset of discontinuous coarsening.
2
Pan, L. (Longxiu). "Two methods for processing an ultrafine ferritic grain size in steels and the thermal stability of the structure." Doctoral thesis, University of Oulu, 2004. http://urn.fi/urn:isbn:951427489X.
Full textAbstract:
Abstract
In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route).
In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted.
When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work.
The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.
3
Klemm, Robert. "Zyklische Plastizität von mikro- und submikrokristallinem Nickel." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2004. http://nbn-resolving.de/urn:nbn:de:swb:14-1085403884093-17423.
Full textAbstract:
Im Rahmen der vorliegenden Arbeit wurde der Einfluss der Korngröße und der Gefügestabilität auf die zyklische Plastizität von mikro- und submikrokristallinem Nickel, hergestellt durch ECAP und PED, untersucht. Zur Gefügecharakterisierung kamen verschiedene elektronenmikroskopische und röntgenographische Methoden zum Einsatz. Die Untersuchungen zur Gefügestabilität zeigten, dass (i) die Stabilität der Korn- und Substruktur bei der zyklischen Verformung empfindlich vom Gefüge im Ausgangszustand abhängt, (ii) generell die Tendenz zur Umwandlung der vorhandenen Substruktur in eine universelle ermüdungstypische Substruktur besteht, diese Transformation jedoch durch die lokale Gefügebeschaffenheit be- bzw. verhindert sein kann und (iii) zur Erklärung des Entfestigungsverhaltens der ECAP-Materialien sowohl die Transformation der Substruktur als auch die Vergröberung der Kornstruktur berücksichtigt werden müssen. Auf der Basis der Ergebnisse der vorliegenden Arbeit und unter Hinzunahme von Resultaten aus der Literatur lassen sich Schlussfolgerungen zum Einfluss der Korngröße auf die zyklische Plastizität in einem vier Größenordnungen umfassenden Korngrößenbereich ziehen. In grob- und feinkörnigem Nickel bilden sich bei der zyklischen Verformung ermüdungstypische Versetzungsstrukturen, deren Abmessungen kaum von der Korngröße abhängen. Der Versetzungslaufweg in diesen Materialien ist wesentlich kleiner als die Kornabmessungen. Dementsprechend besteht höchstens ein schwacher Einfluss der Korngröße auf das sich bei der Wechselverformung einstellende Spannungsniveau. Bei mikro- und submikrokristallinem Nickel, wo der Versetzungslaufweg in der Größenordung der Kornabmessungen liegt, wird ein deutlicher Umschlag bei der Versetzungsmusterbildung und dem zyklischen Verformungsverhalten beobachtet. In diesem Korngrößenbereich entstehen entweder qualitativ andere (D<DS1=5µm) oder keine Versetzungsstrukturen (D<DS2=1µm) und das Spannungsniveau steigt mit sinkender Korngröße entsprechend einer HALL-PETCH-Beziehung.
4
Klemm, Robert. "Zyklische Plastizität von mikro- und submikrokristallinem Nickel." Doctoral thesis, Technische Universität Dresden, 2003. https://tud.qucosa.de/id/qucosa%3A23831.
Full textAbstract:
Im Rahmen der vorliegenden Arbeit wurde der Einfluss der Korngröße und der Gefügestabilität auf die zyklische Plastizität von mikro- und submikrokristallinem Nickel, hergestellt durch ECAP und PED, untersucht. Zur Gefügecharakterisierung kamen verschiedene elektronenmikroskopische und röntgenographische Methoden zum Einsatz. Die Untersuchungen zur Gefügestabilität zeigten, dass (i) die Stabilität der Korn- und Substruktur bei der zyklischen Verformung empfindlich vom Gefüge im Ausgangszustand abhängt, (ii) generell die Tendenz zur Umwandlung der vorhandenen Substruktur in eine universelle ermüdungstypische Substruktur besteht, diese Transformation jedoch durch die lokale Gefügebeschaffenheit be- bzw. verhindert sein kann und (iii) zur Erklärung des Entfestigungsverhaltens der ECAP-Materialien sowohl die Transformation der Substruktur als auch die Vergröberung der Kornstruktur berücksichtigt werden müssen. Auf der Basis der Ergebnisse der vorliegenden Arbeit und unter Hinzunahme von Resultaten aus der Literatur lassen sich Schlussfolgerungen zum Einfluss der Korngröße auf die zyklische Plastizität in einem vier Größenordnungen umfassenden Korngrößenbereich ziehen. In grob- und feinkörnigem Nickel bilden sich bei der zyklischen Verformung ermüdungstypische Versetzungsstrukturen, deren Abmessungen kaum von der Korngröße abhängen. Der Versetzungslaufweg in diesen Materialien ist wesentlich kleiner als die Kornabmessungen. Dementsprechend besteht höchstens ein schwacher Einfluss der Korngröße auf das sich bei der Wechselverformung einstellende Spannungsniveau. Bei mikro- und submikrokristallinem Nickel, wo der Versetzungslaufweg in der Größenordung der Kornabmessungen liegt, wird ein deutlicher Umschlag bei der Versetzungsmusterbildung und dem zyklischen Verformungsverhalten beobachtet. In diesem Korngrößenbereich entstehen entweder qualitativ andere (D<DS1=5µm) oder keine Versetzungsstrukturen (D<DS2=1µm) und das Spannungsniveau steigt mit sinkender Korngröße entsprechend einer HALL-PETCH-Beziehung.
5
Figueroa, Oscar III. "Variations in grain boundary segregation for nanocrystalline stability and strength." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/76179.
Full textAbstract:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 39).
In the last few decades, nanocrystalline metals have been of increasing interest. Their ability to show increased yield strength and uniform structure show them to be potentially useful in many applications. Additionally, nanocrystalline metals have become more easily manufactured in recent years, allowing for more testing and more use within industrial settings. However, nanocrystalline metals are still highly unstable, mainly due to temperature related growth. Grain boundary segregation is one way in which materials can keep nano length-scale grains. This process involves metal alloys that preferentially segregate the alloying material to the grain boundaries, potentially leading to Grain Boundary Embrittlement (GBE). Using an ideal work of fracture equation, [gamma] = 2[sigma]s - [sigma]g, the energy required to fracture nanocrystalline metal alloys was obtained, and predicted grain stability. Fracture toughness data is also calculated and compared. A contrast between bulk and nanocrystalline alloys is then made, showing benefits to the use of either set of materials for specific alloy functions.
by Oscar Figueroa, III.
S.B.
6
Vazquez, Isaura 1960. "On aluminum grain boundary structure and segregation." Thesis, The University of Arizona, 1991. http://hdl.handle.net/10150/291693.
Full textAbstract:
The effect of an alloying agent, such as Mg, on Al grain boundary microstructure and chemistry was investigated. HREM measurements on an Al-5% Mg Σ = 5 (310) grain boundary showed Al grain boundary structure modification because of the presence of Mg. Chemical analysis, through TEM/STEM EDS of Al-2% Mg alloy, indicated the possibility of Mg segregation at Al grain boundaries, although this should be further investigated. An experiment is proposed to determine the change in grain boundary torque with impurity chemical potential. Molecular dynamic simulations of the effect of segregated vacancies was also studied. This study showed that grain boundaries act as sinks for vacancies. In addition, the presence of vacancies caused a relocation of the grain boundary plane, through a sliding-migration, or atomic restructuring of the boundary depending upon the vacancy distribution.
7
Flood, S. C. "Factors affecting the grain structure during solidification." Thesis, University of Oxford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.355749.
Full text
8
Sarrazit, Franck. "Structure of grain boundaries in hexagonal materials." Thesis, University of Liverpool, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367291.
Full text
9
Rezki, Samir. "Structuration, dynamique et réponse des communautés microbiennes associées aux graines lors de la transmission d'agents phytopathogènes Assembly of seed-associated microbial communities within and across successive plant generations Differences in stability of seed-associated microbial assemblages in response to invasion by phytopathogenic microorganisms." Thesis, Angers, 2017. http://www.theses.fr/2017ANGE0092.
Full textAbstract:
La graine constitue le point de départ du cycle d’une plante et abrite une diversité de micro-organismes qui peuvent impacter négativement ou positivement la fitness de la plante. De plus, la graine permet la dispersion et la survie des agents phytopathogènes entre deux cycles de culture de la plante hôte. Dans ce contexte, l’objectif de ce travail était de : (i) décrypter les processus écologiques impliqués dans l’acquisition du microbiote des graines, (ii) analyser sa réponse à l’invasion par des agents phytopathogènes et(iii) suivre sa dynamique durant la germination de la graine et l’émergence de la plantule. Premièrement, nous avons analysé la structure du microbiote de graines de radis(Raphanus sativus) produites dans un même site sur trois générations successives. Ces analyses ont révélé une faible héritabilité du microbiote des graines avec peu de taxons dominants transmis d’une génération à l’autre. Ceci pourrait être expliqué par l’importance des processus neutres dans l’assemblage du microbiote des graines.Ensuite, nous avons étudié la réponse de ce microbiote à une invasion par Xanthomonas campestris pv. campestris(Xcc) et Alternaria brassicicola (Ab), deux agents phytopathogènes transmis par les graines. La transmission de Xcc aux graines n’impacte pas la composition globale du microbiote. En revanche, la transmission d’Ab modifie la structure des communautés fongiques. Ces différences de réponse sont probablement dues aux compétitions pour l’espace et les nutriments entre l’agent phytopathogène et les autres membres du microbiote. Finalement, la composition et la structure du microbiote des graines germées et des plantules ont révélé une transmission de la majorité des taxons associés à la graine y compris Xcc etAb. Globalement, les résultats de ce travail de thèse permettront à terme d’élaborer des stratégies de biocontrôle basées sur la modulation du microbiote des grainesSeed represents the initial step of the plant life cycle and harbors diverse microorganisms that can have detrimental or beneficial impacts on plant fitness. Moreover, seed represents an important means of pathogen dispersion and survival during intercrop periods. For those reasons, the aims of this work were to (i) unveil the ecological processes involved in the acquisition of the seedmicrobiota, (ii) to analyze its response against plant pathogens invasion and (iii) to monitor its dynamics during the first plant developmental stages, namely germination and emergence. First, we assessed the structure of the radish seed microbiota (Raphanus sativus) in the same experimental site across three successive plant generations. These analyses revealed a low heritability of the seed microbiota with few dominant taxa transmitted across generations. Neutral-based processes seem to be important in assembly of the seed microbiota. Second, we monitored the response of the seed microbiota to invasions by Xanthomonas campestris pv. campestris (Xcc) and Alternaria brassicicola (Ab), two seed-transmitted pathogens. While Xcc seed transmission do not change the composition of microbial communities, Ab transmission modified the structure of seed-associated fungal communities. This differences in response could be partly explained by competition for space and nutrients between the pathogenic agents and the members of the seed microbiota. Finally, composition and structure of microbial communities associated to germinating seed and seedling revealed transmission of most seed-borne microorganisms including Xcc and Ab from seed to seedling. Altogether, the results of this thesis could be helpful for designing future biocontrol strategies based on seed microbiota modulation
10
Pankhurst, David Anthony. "Electronic structure at grain boundaries in nickel aluminide." Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.621407.
Full text
11
Atkinson, Helen Valerie. "Evolution of grain structure in nickel oxide scales." Thesis, Imperial College London, 1986. http://hdl.handle.net/10044/1/37929.
Full text
12
Paxton, A. T. "Atomic and electronic structure of grain boundaries in silicon." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379930.
Full text
13
Sagalowicz, Laurent. "Dislocation/grain boundary interactions in the diamond cubic structure /." The Ohio State University, 1992. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487780865408123.
Full text
14
Zhong, Yuan. "Sub-grain structure in additive manufactured stainless steel 316L." Doctoral thesis, Stockholms universitet, Institutionen för material- och miljökemi (MMK), 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-144519.
Full textAbstract:
The thesis focuses on exploring the sub-grain structure in stainless steel 316L prepared by additive manufacturing (AM). Two powder-bed based AM methods are involved: selective laser melting (SLM) and electron beam melting (EBM). It is already known that AM 316L has heterogeneous property and hierarchy structure: micro-sized melt pools, micro-sized grains, nano-sized sub-grain structure and nano-sized inclusions. Yet, the relation among these structures and their influence on mechanical properties have not been clearly revealed so far. Melt pool boundaries having lower amount of sub-grain segregated network structures (Cellular structure) are weaker compared to the base material. Compared with cell boundaries, grain boundaries have less influence on strength but are still important for ductility. Cell boundaries strengthen the material without losing ductility as revealed by mechanical tests. Cellular structure can be continuous across the melt pool boundaries, low angle sub-grain boundaries, but not grain boundaries. Based on the above understanding, AM process parameters were adjusted to achieve customized mechanical properties. Comprehensive characterization were carried out to investigate the density, composition, microstructure, phase, magnetic permeability, tensile property, Charpy impact property, and fatigue property of both SLM and EBM SS316L at room temperature and at elevated temperatures (250°C and 400°C). In general, SLM SS316L has better strength while EBM SS316L has better ductility due to the different process conditions. Improved cell connection between melt pools were achieved by rotating 45° scanning direction between each layer compared to rotating 90°. Superior mechanical properties (yield strength 552 MPa and elongation 83%) were achieved in SLM SS316L fabricated with 20 µm layer thickness and tested in the building direction. Y2O3 added oxide dispersed strengthening steel (ODSS) were also prepared by SLM to further improve its performance at elevated temperatures. Slightly improved strength and ductility (yield strength 574 MPa and elongation 90%) were obtained on 0.3%Y2O3-ODSS with evenly dispersed nanoparticles (20 nm). The strength drops slightly but ductility drops dramatically at elevated temperatures. Fractographic analysis results revealed that the coalescence of nano-voids is hindered at room temperature but not at elevated temperatures. The achieved promising properties in large AM specimens assure its potential application in nuclear fusion. For the first time, ITER first wall panel parts with complex inner pipe structure were successfully fabricated by both SLM and EBM which gives great confidence to application of AM in nuclear industry.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.
15
Zhang, Pan. "A virtual grain structure representation system for micromechanics simulations." Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/9656.
Full textAbstract:
Representing a grain structure within a combined finite element computer aided engineering environment is essential for micromechanics simulations. Methods are required to effectively generate high-fidelity virtual grain structures for accurate studies. A high-fidelity virtual grain structure means a statistically equivalent structure in conjunction with desired grain size distribution features, and must be represented with realistic grain morphology. A family of controlled Poisson Voronoi tessellation (CPVT) models have been developed in this work for systematically generating virtual grain structures with the aforementioned properties. Three tasks have been accomplished in the development of the CPVT models: (i) defining the grain structure’s regularity that specifies the uniformity of a tessellation as well as deriving a control parameter based on the regularity; (ii) modelling the mapping from a grain structure’s regularity to its grain size distribution; and (iii) establishing the relation between a set of physical parameters and a distribution function. A one-gamma distribution function is used to describe a grain size distribution characteristic and a group of four physical parameters are employed to represent the metallographic measurements of a grain size distribution property. Mathematical proofs of the uniqueness of the determination of the distribution parameter from the proposed set of physical parameters have been studied, and an efficient numerical procedure is provided for computing the distribution parameter. Based on the general scheme, two- and three-dimensional CPVT models have been formulated, which respectively define the quantities of regularity and control parameters, and model the mapping between regularity and grain size distribution. For the 2D-CPVT model, statistical tests have been carried out to validate the accuracy and robustness of regularity and grain size distribution control. In addition, micrographs with different grain size distribution features are employed to examine the capability of the 2D-CPVT model to generate virtual grain structures that meet physical measurements. A crystal plasticity finite element (CPFE) simulation of plane strain uniaxial tension has been performed to show the effect of grain size distribution on local strain distribution. For the 3D-CPVT model, a set of CPFE analyses of micro-pillar compression have been run and the effects of both regularity and grain size on deformation responses investigated. Further to this, a multi-zone scheme is proposed for the CPVT models to generate virtual gradient grain structures. In conjunction with the CPVT model that controls the seed generating process within individual zones, the multi-zone CPVT model has been developed by incorporating a novel mechanism of controlling the seed generation for grains spanning different zones. This model has the flexibility of generating various gradient grain structures and the natural morphology for interfacial grains between adjacent zones. Both of the 2D- and 3D-CPVT models are capable of generating a virtual grain structure with a mean grain size gradient for the grain structure domain and grain size distribution control for individual zones. A true gradient grain structure, two simulated gradient grain structure, and a true gradient grain structure with an elongated zone have been used to examine the capability of the multi-zone CPVT model. To facilitate the CPFE analyses of inter-granular crack initiation and evolution using the cohesive zone models, a Voronoi tessellation model with non-zero thickness cohesive zone representation was developed. A grain boundary offsetting algorithm is proposed to efficiently produce the cohesive boundaries for a Voronoi tessellation. The most challenging issue of automatically meshing multiple junctions with quadrilateral elements has been resolved and a rule-based method is presented to perform the automatically partitioning of cohesive zone junctions, including data representation, edge event processing and cut-trim operations. In order to demonstrate the novelty of the proposed cohesive zone modelling and junction partitioning schemes, the CPFE simulations of plane strain uniaxial tension and three point bending have been studied. A software system, VGRAIN, was developed to implement the proposed virtual grain structure modelling methods. Via user-friendly interfaces and the well-organised functional modules a virtual grain structure can be automatically generated to a very large-scale with the desired grain morphology and grain size properties. As a pre-processing grain structure representation system, VGRAIN is also capable of defining crystallographic orientations and mechanical constants for a generated grain structure. A set of additional functions has also been developed for users to study a generated grain structure and verify the feasibility of the generated case for their simulation requirements. A well-built grain structure model in VGRAIN can be easily exported into the commercial FE/CAE platform, e.g. ABAQUS and DEFORM, via script input, whereby the VGRAIN system is seamlessly integrated into CPFE modelling and simulation processing.
16
Petton, Guy J. "Computer simulation study of grain boundary structure in B2 NiAl." Thesis, This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-03122009-040708/.
Full text
17
Krishna, Sujata. "Atomic and electronic structure of grain boundaries in gallium arsenide." Thesis, University of Oxford, 1994. http://ora.ox.ac.uk/objects/uuid:0f7d1156-12e4-4a8d-b3a7-71ff7a8b65f1.
Full textAbstract:
HREM imaging was performed using the Jeol 4000ex microscope on specimens prepared from an as-grown ingot of semi-insulating Gallium Arsenide. Various low angle grain boundaries were imaged in the [110] orientation, misorientations varying between 4°-13°. Detailed study of a grain boundary of 11.5° misorientation about the [110] rotation axis has been carried out. Burgers vector analysis showed the presence of perfect 60° and [001] dislocations. Modelling of the [001] dislocation has been carried out using the Tersoff potential, Bond Order Potential and a tight binding Hamiltonian for GaAs, using Chadi (1984) parameters. The dislocation core was associated with an 8-membered and two 5-membered rings. Assum- ing there is a minimum of wrong bonds, we predict that the core has two wrong bonds, one being Ga-Ga, and the other As-As, both in equivalent positions where the two 5-membered rings were appended to the 8-membered ring. The Ga-Ga bond is considerably shorter and hence stronger than the As-As bond. Band structure calculations performed using a Vogl (1983) sp3s* Hamiltonian revealed deep states in the gap, which are associated with atoms in the core only. Using Stadelmann's (1987) EMS program, successful image matching of calculated images of the [001] dislocation has been achieved with the experimental image, using the atomic structure generated by tight binding relaxation. Ga and As being only two atomic numbers apart have similar scattering factors and cannot be easily distinguished in the experimental image. The equivalence of the position of the two wrong bonds greatly eases image matching as it is no longer necessary to know which is the Ga-Ga , and which is the As-As bond. This is the first suggested model of the [001] dislocation in GaAs, to the best of my knowledge. It is found to be similar to the atomic structure of the 90° partial dislocation in silicon (Bigger et al., 1992). No account of segregation of impurities to the grain boundary, or the [001] dislocation core is taken here, though it is very likely that an impurity atom would sit itself in this large space. The relaxed atomic structure for the 60° dislocation showed a doubling of periodicity along the dislocation line, similar to that found in the 30° partial in Si. The core consists of a 7-membered and a 5-membered ring with a minimum of two wrong bonds. In addition to this, quantitative comparisons of the [001] HREM image and simulated structures have been made and an iterative structure refinement carried out in order to achieve the best image matching. The resultant 'experimental-best-fit' structure was not found to be physically or chemically plausible.
18
Shaw, N. J. "Structure and grain coarsening during the processing of engineering ceramics." Thesis, University of Leeds, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.378400.
Full text
19
Kvam, Eric Peter. "TEM study of dislocation structure in grain boundaries in metals." Thesis, Massachusetts Institute of Technology, 1985. http://hdl.handle.net/1721.1/15325.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1985.MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE.
Vita.
Bibliography: leaves 150-154.
by Eric Peter Kvam.
Ph.D.
20
Venkataraman, Rangarajan. "A study of grain boundary structure in B doped Ni3Al." Thesis, Virginia Polytechnic Institute and State University, 1986. http://hdl.handle.net/10919/94486.
Full textAbstract:
A theoretical and experimental study of Grain boundary structure in B doped Ni₃Al was carried out. Geometrical modelling was done to obtain the grain boundary structure in Ll₂ compounds for different misorientations between two grains. The size of interstitial sites in these boundaries was calculated to understand the segregation of B to the boundaries. Ratios of the B-B equilibrium interatomic distance to the nearest neighbour distances in various (A₃B) Ll₂ compounds were calculated, in an attempt to predict the grain boundary fracture behaviour in these compounds. Selected area channeling patterns were obtained from a number of grains to solve for the orientation relationship between grains, in order to predict the structure of the boundaries between them. A distribution of grain boundary structure that is different from the distribution for an undoped Ni₃Al is obtained in this work and is compared with results from other investigators.M.S.
21
E, Xinyi. "Hemicellulose fiber gum from distillers grain: isolation, structure and properties." Thesis, Kansas State University, 2010. http://hdl.handle.net/2097/4618.
Full textAbstract:
Master of ScienceDepartment of Grain Science and Industry
Yong Cheng Shi
Isolation of hemicellulose from distillers dry grain with solubles (DDGS) was investigated. Hemicellulose fiber gum (HFG) is a mixture of hemicelluose, protein, ash and starch. It was extracted from a commercial DDGS by heating with or without alkali. Three extraction methods (water heating, alkaline heating and alkaline hydrogen peroxide heating) were evaluated. Yield of HFG and the recovery of hemicellulose were obtained. High heating temperature (100 and 120°C), alkali or hydrogen peroxide facilitated the release of hemicellulose from the cell wall matrix. However combining alkali with 2.5% H2O2 did not extract more hemicellulose out than did alkali alone. The highest hemicellulose recovery was 32% achieved by cooking at 120°C with 2% alkaline solution. Hemicellulose can function as an emulsifier in the oil-in-water emulsions, such as beverage, and potentially replace gum arabic. HFGs obtained by a series of extracting methods were applied in both the concentrated emulsion with the gum: oil: water ratio of 0.5:1: 8.5 and the diluted emulsion with the gum: oil: water ratio of 0.005: 0.01: 1. The emulsion stability was evaluated by turbidity and creaming test. HFG extracted by 2% NaOH solution at 120°C and HFG extracted by 2% NaOH and 2.5% H2O2 solution at 100°C showed the best emulsifying ability among 15 HFG samples. DDGS was produced from corn, sorghum, wheat in the lab. HFGs extracted from sorghum and wheat DDGS were compared with that from corn DDGS. The composition of the three DDGS varied in protein, fat and non-starch carbohydrate contents. Sorghum and wheat DDGS contained higher levels of protein and lower levels of fat and non-starch carbohydrate than corn DDGS. HFG was extracted by 2% NaOH solution at 100°C for one hour and purified by 100% ethanol. The yield of HFG from corn, sorghum and wheat DDGS was 21.08, 11.07, 11.64% respectively, while the hemicellulose recovery was 30.95, 29.74, 22.71% respectively. The water extractable hemicelluloses from all three DDGS had similar ratios of arabinose to xylose.
22
Moosavian, Mohammad Ali. "The structure and stability of liquid foam." Thesis, University of Manchester, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.332024.
Full text
23
Chan, Chau Ping. "Stability and structure of suspo-emulsion components." Thesis, University of Bristol, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.440050.
Full text
24
Pan, Zhenguo. "Simulation and analysis of coupled surface and grain boundary motion." Thesis, University of British Columbia, 2008. http://hdl.handle.net/2429/2733.
Full textAbstract:
At the microscopic level, many materials are made of smaller and randomly oriented grains. These grains are separated by grain boundaries which tend to decrease the electrical and thermal conductivity of the material. The motion of grain boundaries is an important phenomenon controlling the grain growth in materials processing and synthesis.
Mathematical modeling and simulation is a powerful tool for studying the motion of grain boundaries. The research reported in this thesis is focused on the numerical simulation and analysis of a coupled surface and grain boundary motion which models the evolution of grain boundary and the diffusion of the free surface during the process of grain growth.
The “quarter loop” geometry provides a convenient model for the study of this coupled motion. Two types of normal curve velocities are involved in this model: motion by mean curvature and motion by surface diffusion. They are coupled together at a triple junction. A front tracking method is used to simulate the migration. To describe the problem, different formulations are presented and discussed. A new formulation that comprises partial differential equations and algebraic equations is proposed. It preserves arc length parametrization up to scaling and exhibits good numerical performance. This formulation is shown to be well-posed in a reduced, linear setting. Numerical simulations are implemented and compared for all formulations. The new formulation is also applied to some other related problems.
We investigate numerically the linear stability of the travelling wave solutions for the quarter loop problem and a simple grain boundary motion problem for both curves in two dimensions and surfaces in three dimensions. The numerical results give evidence that they are convectively stable.
A class of high order three-phase boundary motion problems are also studied. We consider a region where three phase boundaries meet at a triple junction and evolve with specified normal velocities. A system of partial differential algebraic equations (PDAE) is proposed to describe this class of problems by extending the discussion for the coupled surface and grain boundary motion. The linear well-posedness of the system is analyzed and numerical simulations are performed.
25
RALANTOSON, NDRIANAINA. "Etude de la stabilite thermique de la structure atomique des joints de grains dans le silicium par la dynamique moleculaire." Caen, 1994. http://www.theses.fr/1994CAEN2057.
Full textAbstract:
L'etude de la stabilite thermique des structures atomiques des joints de grains dans le silicium necessite le calcul de leur energie libre. Nous avons utilise, d'une part, la dynamique moleculaire et la methode de nose pour calculer les valeurs moyennes et pour simuler un thermostat et, d'autre part, le potentiel de stillinger et weber pour decrire les interactions interatomiques. Les differents termes statique, harmonique et anharmoniques de l'energie sont calcules a l'aide de methodes adaptees. La methode d'integration indirecte le long d'un chemin a deformation constante permet en particulier de calculer la contribution des termes anharmoniques. Une deuxieme methode d'integration directe donne immediatement la difference d'energie libre de deux etats en imposant des contraintes sur les deplacements atomiques. Les methodes et les programmes informatiques ont ete mis au point et testes avec succes sur le silicium monocristallin. Les resultats de calcul par la methode d'integration indirecte montre que la structure a de la macle d'indice de coincidence 11 est plus stable que la structure b quelle que soit la temperature. La methode directe donne exactement la meme difference d'energie libre que celle calculee par la methode indirecte. Un avantage de cette methode directe est de fournir la barriere d'energie libre a franchir pour passer de la structure a a la structure b. A haute temperature, des sauts atomiques correles sont observes entre a et b mais le passage eventuel d'une structure a l'autre est trop peu probable pour etre observe en dynamique moleculaire parce que son echelle de temps est trop petite par rapport a notre echelle de temps macroscopique et que la barriere d'energie a franchir est trop importante
26
Picasso, Risso Valentín Daniel. "Diversity, productivity, and stability in perennial polycultures used for grain, forage, and biomass production." [Ames, Iowa : Iowa State University], 2008.
Find full text
27
Grawe, Rebecca Ellen. "Effects of Tethering Placement and Linker Variations on Antibody Stability on Surfaces." BYU ScholarsArchive, 2016. https://scholarsarchive.byu.edu/etd/6236.
Full textAbstract:
An antibody microarray consists of antibody bound to a surface. Antibody microarrays have great potential in many fields, particularly as a tool to detect antigens. Unfortunately, antibodies suffer from poor performance. A greater understanding of how antibodies interact with surfaces would improve microarray design and performance, but experimental methods fall short of being able to observe these interactions. Therefore molecular simulation has emerged as the primary method to study protein/surface interactions.The simulations here were coarse grain simulations performed using the model of Karanicolas and Brooks. Additionally, an advanced surface model was used that allows for different surface chemistries. PyMBAR analysis was used to find heat capacities and determine relative stabilities of different linkers and tethering sites for the antibody/surface system.The actual work looked at how 24 different tethering sites affect antibody stability on two different surfaces and examined nine linkers varying in length and rigidity. Ultimately the findings were that antibody stability is a function of tethering position when tethered to a hydrophobic surface, but not when tethered to a hydrophilic surface. Furthermore, the length and rigidity of the linkers do not have a significant impact on stability.
28
Marra, Joseph J. "Relationship of grain boundary structure and mechanical properties of Inconel 690." Thesis, Cambridge, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/10945/4304.
Full textAbstract:
CIVINSApproved for public release, distribution unlimited
Stress corrosion cracking (SCC), failure due to environmentally assisted crack nucleation and propagation, is an important metallurgical problem with impact on current and future designs of ship structural components and nuclear reactors. SCC mechanism is dependent on the microstructure, particularly the grain boundaries, for a given alloy composition. This thesis investigated the relationship between the structural nature and the nanoscale mechanical properties on and near the grain boundaries to identify their role in the resistance to stress corrosion cracking. Inconel 690, an austenitic Ni-alloy, was chosen as the material of interest for its relevance in current applications in the nuclear energy technology. Inherent hardness of the solution annealed Inconel 690 were found to be the same for both the grain boundary and bulk regions, and did not vary with grain boundary type, specifically the 3 and high angle boundaries. These results indicate comparable ease of dislocation mobility and absorption at the different grain boundaries of Inconel 690 when indented at room temperature.
29
Dorban, Andrew Michael. "Superplasticity of Quasi single phase alloys : the influence of grain structure." Thesis, Imperial College London, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.498228.
Full text
30
Ayed, Ayeda. "Stability and structure of Escherichia coli citrate synthase." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ31962.pdf.
Full text
31
Alhummiany, Haya A. "The structure and stability of colloidal nanoparticle assemblies." Thesis, University of Nottingham, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604308.
Full textAbstract:
Thiol-passivated gold nanoparticies deposited onto a silicon substrate from an organic solvent form a remarkable variety of self-organised nanostructured and microstructured patterns. The work in this thesis focuses on three key aspects of these systems: (i) their environmental stability, (ii) the formation of nanoparticie rings via condensation of water droplets, and (iii) the morphology of nanoparticie mono layers formed at the water-air interface and transferred to silicon substrates using the Langmuir-Blodgett technique. The nanoparticie structures were studied by combined atomic force microscopy (A FM) and contrast-enhanced optical microscopy. Striking changes in the morphology of self-organised nanoparticie patterns are observed during exposure to high humidity or high temperature environment. Annealing the sample at 1200 C leads to a high degree of nanoparticie sintering promoted by break-up of the initial assembly. Conversely, exposing the sample to a high relative humidity (80%) environment induces layer-by-layer assembly of 3D aggregates with no evidence of sintering. We focus also on the formation of rings of thiol-passivated Au nanoparticies, addressing the controversy in the literature regarding the formation of these structures. We provide compelling evidence that nanoparticie rings formed via the adsorption of droplets of condensed water. This so-called "breath figure" effect plays the dominant role in the dynamics of ring formation via iii evaporative dewetting. Different sizes and size distributions of nanoparticle rings have been created under controlled relative humidity environments ranging from ~50% to 80%. Fmthermore, the dynamic behaviour of water droplets on surface has been examined using a contrast-enhanced optical mIcroscope. Analysis of the observed droplet dynamics shows significant differences between the behaviour of droplets formed on top of a silicon substrate and the nanoparticle films. Finally, the formation of gold nanoparticle assemblies at the water-air interface using the Langmuir-Blodgett deposition technique has also been studied. The LB films were transferred to a solid substrate (silicon) and the nanoparticle assemblies fOlmed close to the contact line imaged by AFM and contrastenhanced optical microscopy. The experimental control parameters resulted in the emergence of variety of nanoparticie morphologies. We propose formation mechanisms for a number of these morphologies.
32
Plotka, Hanna. "The structure, stability and interaction of geophysical vortices." Thesis, University of St Andrews, 2013. http://hdl.handle.net/10023/3729.
Full textAbstract:
This thesis examines the structure, stability and interaction of geophysical vortices. We do so by restricting our attention to relative vortex equilibria, or states which appear stationary in a co-rotating frame of reference. We approach the problem from three different perspectives, namely by first studying the single-vortex, quasi-geostrophic shallow-water problem, next by generalising it to an (asymmetric) two-vortex problem, and finally by re-visiting the single-vortex problem, making use of the more realistic, although more complicated, shallow-water model. We find that in all of the systems studied, small vortices (compared to the Rossby deformation length) are more likely to be unstable than large ones. For the single-vortex problem, this means that large vortices can sustain much greater deformations before destabilising than small vortices, and for the two-vortex problem this means that vortices are able to come closer together before destabilising. Additionally, we find that for large vortices, the degree of asymmetry of a vortex pair does not affect its stability, although it does affect the underlying steady state into which an unstable state transitions. Lastly, by carefully defining the "equivalence" between cyclones and anticyclones which appear in the shallow-water system, we find that cyclones are more stable than anticyclones. This is contrary to what is generally reported in the literature.
33
Olsson, Mirja. "Chemical stability of grain boundariesin β-tricalcium phosphate ceramics : β-TCP as bone substitute material." Thesis, Uppsala universitet, Institutionen för kemi - Ångström, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-173285.
Full textAbstract:
β – Tricalcium phosphate (β-TCP, Ca3(PO4)2) is a commonly used bone substitute material due to its biocompatibility and resorption. This study focused on the production of almost fully dense β-TCP ceramics with varying degrees of impurities (Ca/PO4 ratio, addition of 5% Mg). Three methods were used to produce the β-TCP ceramics, uniaxial pressing, slip-casting and isostatic pressing. In this study the isostatic pressing and sintering at 1150ºC for 20h and 15min, resulted in the densest β-TCP ceramics (97.7-99.2%). No significant differences of grain size and density could be detected between the samples produced with various compositions. These isostatically pressed samples sintered at 1150ºC were then dissolved in 0.08M aceticacid solution to simulate the in vivo resorption. It was found that the samples containing extra Mg dissolved slower. Attempts to determine the chemical composition of the grain boundaries were made without success. However, SEM observations of partly dissolved β-TCP ceramics revealed that the grain boundaries dissolved faster than the grains. The study was performed at the RMS foundation in Switzerland.
34
Hu, Bin. "Stability analysis of linear thin shells." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/7360/.
Full textAbstract:
Shell structure is widely used in engineering area. The purpose of this dissertation is to show the behavior of a thin shell under external load, especially for long cylindrical shell under compressive load, I analyzed not only for linear elastic problem and also for buckling problem, and by using finite element analysis it shows that the imperfection of a cylinder could affect the critical load which means the buckling capability of this cylinder.
For linear elastic problem, I compared the theoretical results with the results got from Straus7 and Abaqus, and the results are really close. For the buckling problem I did the same: compared the theoretical and Abaqus results, the error is less than 1%, but in reality, it’s not possible to reach the theoretical buckling capability due to the imperfection of the cylinder, so I put different imperfection for the cylinder in Abaqus, and found out that with the increasing of the percentage of imperfection, the buckling capability decreases, for example 10% imperfection could decrease 40% of the buckling capability, and the outcome meet the buckling behavior in reality.
35
Wei, Shuai. "Protein-Surface Interactions with Coarse-Grain Simulation Methods." BYU ScholarsArchive, 2013. https://scholarsarchive.byu.edu/etd/3943.
Full textAbstract:
The interaction of proteins with surfaces is a major process involved in protein microarrays. Understanding protein-surface interactions is key to improving the performance of protein microarrays, but current understanding of the behavior of proteins on surfaces is lacking. Prevailing theories on the subject, which suggest that proteins should be stabilized when tethered to surfaces, do not explain the experimentally observed fact that proteins are often denatured on surfaces. This document outlines several studies done to develop a model which is capable of predicting the stabilization and destabilization of proteins tethered to surfaces. As the start point of the research, part of this research showed that the stability of five mainly-alpha, orthogonal-bundle proteins tethered to surfaces can be correlated to the shape of the loop region where the tether is placed and the free rotation ability of the part of proteins near surfaces. To test the expandability of the protein stability prediction pattern derived for mainly-alpha, orthogonal-bundle proteins, same analysis is performed for proteins from other structure motifs. Besides the study in these small two-state proteins, a further analysis of surface-induced change of folding mechanism is also studied with a multi-state lysozyme protein 7LZM. The result showed that by tethering a protein on a surface, the melting temperature of a part of the protein changed, which leads to an avoidance of the meta-stable state. Besides the change of folding mechanism, by tethering the lysozyme protein to a certain site, the protein could both keep a stable structure and a good orientation, allowing active sites to be available to other proteins in bulk solution. All the work described above are done with a purely repulsive surface model which was widely used to roughly simulate solid surfaces in protein microarrays. For a next-level understanding of protein-surface interactions, a novel coarse-grain surface model was developed, parameterized, and validated according to experimental results from different groups. A case study of interaction between lysozyme protein 7LZM and three types of surfaces with the novel model has been performed. The results showed that protein stabilities and structures are dependent on the types of surfaces and their different hydrophobicities. This result is consistent with previously published experimental work.
36
Llewellyn, Kate S. "Genetic structure and dispersal in cereal aphid populations." Thesis, University of Nottingham, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.342025.
Full text
37
Kong, Xiangli. "Molecular structure and functional properties of amaranthus starch." Click to view the E-thesis via HKUTO, 2009. http://sunzi.lib.hku.hk/hkuto/record/B43224210.
Full text
38
Poteet, David, David Robinson, Steve Smith, and Albert Dobrenz. "Genetic Gains and Stability for Germination Salt Tolerance in Alfalfa." College of Agriculture, University of Arizona (Tucson, AZ), 1987. http://hdl.handle.net/10150/203789.
Full textAbstract:
Improving the germination salt tolerance of alfalfa will help this crop endure both increasingly saline irrigation water and salt build -up in the soil. Seven previous cycles of selection for salt tolerance at germination plus the parental line, Mesa-Sirsa, were evaluated for percentage germination under various levels of NaCI solutions. Germination improved linearly from the earliest to the latest cycle of selection. Germination salt tolerance can be improved through recurrent selection techniques.
39
Tongdang, Taewee. "Molecular structure of native and processed rices." Thesis, University of Nottingham, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.368246.
Full text
40
Konstantopoulos, G. P. "A unified treatment of semisubmersible stability." Thesis, University of Strathclyde, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382306.
Full text
41
Kundu, Amrita. "Grain structure development during casting, reheating and deformation of Nb-microalloyed steel." Thesis, University of Birmingham, 2011. http://etheses.bham.ac.uk//id/eprint/2917/.
Full textAbstract:
This thesis looks at the effect of micro segregation of Nb, generated from the continuous casting on grain structure development during reheating and deformation. The amount of segregated material present in the microstructure has been quantified in the as-cast condition and that has been used to model grain size distribution after reheating and deformation which is required for subsequent toughness prediction in 0.045 wt % Nb steel. The limits of validity of the original Dutta-Sellars model for predicting the amount of recrystallisation; investigating the influence of the variables such as starting grain size distribution, strain and Nb content has been examined. A modified Dutta-Sellars model considering the starting grain size distribution has been found to be more accurate in quantifying the amount of recrystallisation. This approach has been used to model grain size distribution after deformation both in the homogenised and in presence of Nb segregation. The effect of AlN, to replace Nb(C,N), on pinning the austenite grain boundaries during reheating has been investigated, as the segregation tendency of Al is much less compared with Nb.
42
Leib, Jeffrey Scott. "Relationships between grain structure and stress in thin Volmer-Weber metallic films." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/46682.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.Includes bibliographical references (leaves 150-154).
In Volmer-Weber growth, islands that nucleate on the substrate surface impinge and coalesce into grains of a continuous film. During deposition of these polycrystalline films, the intrinsic stress for materials of sufficiently high mobility often evolves through three distinct stages, switching from compressive to tensile and back to compressive. Many studies of stress evolution during this process have indicated that a tensile stress develops as the islands coalescence, with the peak stress occurring when the film become continuous. The magnitude of this tensile stress is strongly dependent on the grain structure. The grain structure is in turn strongly dependent on atomic processes at the substrate surface at the onset of film growth. In this study, Kinetic Monte Carlo simulations were used to investigate the relationship between characteristics of the amorphous surface, nucleation and growth of islands, and the tensile stresses observed as films form. It is demonstrated that island nucleation on amorphous substrates can be dominated by the spatial characteristics of the amorphous surface. The simulation parameters providing the best fit to experimental data from gold deposited on silicon nitride included a trapping energy of ET = 0.69 eV. The compressive stresses that develop have also been shown to reversibly change during interruptions of growth. One proposed model for this reversibility is that the compressive stress is related to adatom trapping and de-trapping at grain boundaries, while others attribute the stress to surface changes. In the current study, intrinsic stresses monitored in-situ using a capacitive curvature measurement system are studied with respect to the film grain structure, deposition rate, and substrate temperature.
(cont.) The kinetics of the post-deposition tensile rise receive special attention. The "reversible" compressive stress exhibited by polycrystalline, low melting point fcc metal films is found to be absent in epitaxial cases. The stress magnitude in polycrystalline films is shown to be inversely related to grain size and very weakly dependent on temperature. Densification stresses from abnormal grain growth are found to account for the post-deposition tensile rise. Finally, the compressive stresses observed during the deposition of polycrystalline, high mobility gold films is explained using a simple model of trapping of adatoms as grain boundary interstitials.
by Jeffrey S. Leib.
Ph.D.
43
Scott, Brian Lee. "Semi-conductor Core Optical Fibers and Fabrication Dependence of the Grain Structure." Diss., Virginia Tech, 2011. http://hdl.handle.net/10919/77206.
Full textAbstract:
The production and fabrication of semi-conductor core optical fibers was shown to be feasible and controllable. This was accomplished through the step sequence of fabrication and characterization of 4 fiber types, an experiment on controlling the grain length in the core and a simple model of the heat transfer during fabrication. Fibers were first made with a silicon core, followed by a phosphorous doped n-type silicon core, then a boron doped p-type silicon core, and a tellurium doped n-type gallium antimonide core. Characterization of the fibers was accomplished with energy dispersive spectroscopy (EDS) for compositional analysis, electron backscatter diffraction (EBSD) for crystal orientation and grain size, optical and electron microscopy for physical fiber quality and optical transmission for core optical quality. A model was developed to relate the heat transfer with the grain structure of the fiber core. All of the fibers fabricated had a polycrystalline core with either no detectable oxygen in the case of the silicon fibers or low amounts of oxygen diffusion into the core as in the case of the GaSb fibers. Fiber lengths ranged from 7 cm for the initial silicon fibers to 60 cm and outside diameters down to 100 µm for n and p type silicon fibers. Core diameters for all fiber types ranged from 10 – 200 µm depending on the fabrication parameters. Lengths of major grains in the core are dependent on the core diameter and the pulling speed. The grain lengths of the major grains in the core generally increase in length with an increase in core diameter. Grain lengths in all fibers are thought to be suitable for use in fabrication of electronic structures in the core region with even the smallest average grain length of around 300 µm. This grain structure satisfies the grain boundary requirements for fabrication of boundary free p-n junctions and other more complicated electronic structures. Small core diameter fibers had better physical quality with fewer cracks and longer continuous length than the larger core fibers.Ph. D.
44
Saadi, Yusron. "The influence of different time varying antecedent flows on the stability of mixed grain size deposits." Thesis, University of Sheffield, 2002. http://etheses.whiterose.ac.uk/12833/.
Full textAbstract:
The objective of this work was to examine the impact of unsteady flows on the erosion and movement of mixed grain size sediment. Time varying flows were examined as flowrates in natural rivers are rarely constant. There are very few reported studies on the movement of sediment in unsteady open channel flow and most of those used single sized sediment. River reach has its own sedimentological character and non-uniform beds exhibit very different behaviour from that of single sized material. Therefore it was thought important to examine the impact of time varying flow on the stability of water worked mixed grain size sediment beds. The thesis reports on a series of laboratory experiments in which a bimodal sediment bed was exposed to different flow hydrographs. The flow hydrographs consisted of constant flowrate with different duration and time varying flows with different rising and falling limb but had the same peak flowrate. Each experiment was followed by a stability test in which a standard "triangular shaped hydrograph" was used to assess the stability of each water worked deposit. The stability observation demonstrated that grain size fractions have different thresholds of motion when beds are formed by different antecedent flow patterns. The bed stability increased as the antecedent constant flow hydrograph progressed. The rising and falling limbs of the flowrate hydrographs were found to have a significant effect on the bed stabilisation process. It revealed that the shortest rising limb of flow hydrograph formed the weakest bed while the longest recession limb of flow hydro graph formed the most stable bed. It is believed that the short period of flowrate acceleration did not allow the coarse grains to stabilise with numerous exposed large grains spread on the bed. In a longer duration of recession limb of hydrograph, the coarse grains moved and eventually deposited over a length of time. As the flowrate declined the finer grains also rolled and then deposited forming a strong bond with the coarse grams. These experiments also provided important information on the flow structures and the changes in the bed topography as the tests progressed. There is strong evidence that only upward interactions (ejections) with high momentum magnitude were able to transport coarser grains. The lack of change in the distribution of downward looking-bed interactions (sweeps) in all tests indicated that these features are not important in determining transport. Changes in bed topography were also measured and characteristics of the distribution of bed surface elevation were linked to the observed changes in bed stability.
45
Acharya, Ananta R. "Indium Nitride Surface Structure, Desorption Kinetics and Thermal Stability." Digital Archive @ GSU, 2013. http://digitalarchive.gsu.edu/phy_astr_diss/62.
Full textAbstract:
Unique physical properties such as small effective mass, high electron drift velocities, high electron mobility and small band gap energy make InN a candidate for applications in high-speed microelectronic and optoelectronic devices. The aim of this research is to understand the surface properties, desorption kinetics and thermal stability of InN epilayers that affect the growth processes and determine film quality as well as device performance and life time. We have investigated the structural properties, the surface desorption kinetics, and the thermal stability using Auger electron spectroscopy (AES), x-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), high resolution electron energy loss spectroscopy (HREELS), and temperature programmed desorption (TPD). Investigations on high pressure chemical vapor deposition (HPCVD)-grown InN samples revealed the presence of tilted crystallites, which were attributed to high group V/III flux ratio and lattice mismatch. A study of the thermal stability of HPCVD-grown InN epilayers revealed that the activation energy for nitrogen desorption was 1.6±0.2 eV, independent of the group V/III flux ratio. Initial investigations on the ternary alloy In0.96Ga0.04N showed single-phase, N-polar epilayers using XRD and HREELS, while a thermal desorption study revealed an activation energy for nitrogen desorption of 1.14 ± 0.06 eV.
HREELS investigations of atomic layer epitaxy (ALE)-grown InN revealed vibrational modes assigned to N-N vibrations. The atomic hydrogen cleaned InN surface also exhibited modes assigned to surface N-H without showing In-H species, which indicated N-polar InN. Complete desorption of hydrogen from the InN surface was best described by the first-order desorption kinetics with an activation energy of 0.88 ± 0.06 eV and pre-exponential factor of (1.5 ± 0.5) ×105 s-1.
Overall, we have used a number of techniques to characterize the structure, surface bonding configuration, thermal stability and hydrogen desorption kinetics of InN and In0.96Ga0.04N epilayers grown by HPCVD and ALE. High group V/III precursors ratio and lattice mismatch have a crucial influence on the film orientation. The effects of hydrogen on the decomposition add to the wide variation in the activation energy of nitrogen desorption. Presence of surface defects lowers the activation energy for hydrogen desorption from the surface.
46
Shehan, Joe Michael. "Stability analysis of a 2-D acoustic/structure model." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1995. http://handle.dtic.mil/100.2/ADA303097.
Full text
47
Jokela, Slade Joseph. "Stability and structure of hydrogen defects in zinc oxide." Online access for everyone, 2006. http://www.dissertations.wsu.edu/Dissertations/Fall2006/s_jokela_122106.pdf.
Full text
48
Schleer, Matthias. "Flow structure and stability of a turbocharger centrifugal compressor /." Düsseldorf : VDI-Verl, 2006. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=015054634&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.
Full text
49
Söderberg, Karin. "Structure and stability of Bi₁₋xSbx and CaAl₂₋xZnx /." Stockholm : Department of Structural Chemistry, Stockholm University, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-624.
Full text
50
Radford, Matthew. "Purification structure and stability of voltage-gated sodium channels." Thesis, Birkbeck (University of London), 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.535845.
Full textAbstract:
Voltage gated sodium channels (VGSC) are integral membrane proteins that selectively transport sodium ions across cellular membranes in response to changes in membrane potential. In vertebrates sodium channels are composed of a pore forming alpha subunit and one or more auxiliary beta subunit. There are a total of nine different alpha subunit isoforms and four known beta subunits. In higher eukaryotes VGSCs are responsible for fast electrical signalling through the propagation of action potentials in excitable cells such as neurons and muscle. Voltage gated sodium channels due to their physiological importance are established drug targets for anticonvulsants, local anaesthetics and antiarrhythmics. They are also the target of a number of naturally produced neurotoxins that form a diverse group of compounds including polypeptides such as scorpion toxins, alkaloids such as veratridine and others such as heterocyclic guanidines. The work of this thesis has been to isolate and purify firstly a higher eukaryotic sodium channel sourced from the electroplax tissue of the eel Electrophorus electricus and secondly a prokaryotic channel (NaChBac) that was over expressed in an E. Coli cell system. Purified channels were biophysically characterised using circular dichroism spectroscopy to analyse secondary structure content. Purified NaChBac was chemically and thermally unfolded using SDS and monitored using CD. The drug mibefradil was able to bind NaChBac and increase its Tm by 4°C. Ligand binding to the Eel channel was also investigated using CD and tryptophan fluorescence to establish if secondary or tertiary structural changes could be observed upon ligand binding.