VTechWorks Repository :: Browsing by Author "Reynolds, William T. Jr."

Browsing by Author "Reynolds, William T. Jr."

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Advanced Characterization of Materials for Superconducting Radiofrequency Accelerator Cavities
Tuggle, James Robert Jr. (Virginia Tech, 2019-06-24)
Particle accelerators are a leading tool for frontier science. Pushing that frontier further demands more machines with higher performance, and more of a very expensive technology: superconducting radio-frequency (SRF) acceleration. From a materials perspective this means reducing residual surface resistance or raising the operating temperature (currently ~2 K) of SRF cavities. Both are pursued by materials modification: nitrogen doping/infusion in the first instance and coating with Nb3Sn in the second. Materials characterization is key to achieving understanding and directing RandD. However, very little has been done. This present work aims to fill the knowledge gap and to provide needed, validated tools to the accelerator science community. In this connection, SIMS, XPS and EBSD have proven especially valuable and represent the majority of discussion in this dissertation.
Advanced Chemical-Mechanical Dewatering of Fine Particles
Asmatulu, Ramazan (Virginia Tech, 2001-03-12)
In the present work, novel dewatering aids and a novel centrifuge configuration were developed and applied for the purpose of dewatering fine particles. Three different types dewatering reagents were tested in different filtration and centrifugation units. These chemicals included low-HLB surfactants, naturally occurring lipids, and modified lipids. Most of these reagents are insoluble in water; therefore, they were used in solutions of appropriate solvents, such as light hydrocarbon oils and short-chain alcohols. The role of these reagents was to increase the hydrophobicity of the coal and selected mineral particles (chalcopyrite, sphalerite, galena, talc, clay, phosphate, PCC and silica) for the dewatering. In the presence of these reagents, the water contact angles on the coal samples were increased up to 90o. According to the Laplace equation, an increase in contact angle with the surfactant addition should decrease the capillary pressure in a filter cake, which should in turn increase the rate of dewatering and help reduce the cake moisture. The use of the novel dewatering aids causes a decrease in the surface tension of water and an increase in the porosity of the cake, both of which also contribute to improved dewatering. A series of batch-scale dewatering tests were conducted on a variety of the coal and mineral samples using the novel dewatering aids. The results obtained with a Buchner funnel and air pressure filters showed that cake moistures could be reduced substantially, the extent of which depends on the particle size, cake thickness, drying time, reagent dosage, conditioning time, reagent type, sample aging, water chemistry, etc. It was determined that use of the novel dewatering aids could reduce the cake formation time by a significant degree due to the increased kinetics of dewatering. At the same time, the use of the dewatering aids reduced the cake moistures by allowing the water trapped in smaller capillaries of the filter cake. It was found that final cake moistures could be reduced by 50% of what can be normally achieved without using the reagents. However, the moisture reduction becomes difficult with increasing cake thickness. This problem can be minimized by applying a mechanical vibration to the cake, spraying a short-chain alcohol on the cake and by adding a small amount of an appropriate coagulant, such as alum and CaCl2 to the coal and mineral slurries. The novel dewatering aids were also tested using several different continuous filters, including a drum filter, disc filter and horizontal belt filter (HBF). The results obtained with these continuous filtration devices were consistent with those obtained from the batch filters. Depending on the coal and mineral samples and the type of the reagent, 40 to 60% reductions in moisture were readily achieved. When using vacuum disc filters, the cake thickness increased substantially in the presence of the novel dewatering aids, which could be attributed to the increased kinetics of dewatering. A dual vacuum system was developed in the present work in order to be able to control the cake thickness, which was necessary to achieve lower cake moistures. It was based on using a lower vacuum pressure during the cake formation time, while a full vacuum pressure was used during the drying cycle time. Thus, use of the dual vacuum system allowed the disc filter to be used in conjunction with the novel dewatering aids. Its performance was similar to that of HBF, which is designed to control cake thickness and cake formation time independently. The effectiveness of using the novel dewatering aids were also tested in a full-continuous pilot plant, in which coal samples were cleaned by a flotation column before the flotation product was subjected to the disc filter. The tests were conducted with and without using novel dewatering aids. These results were consistent with those obtained from the laboratory and batch-scale tests. The novel centrifuge developed in the present work was a unit, which combined a gravity force and air pressure. The new centrifuge was based on increasing the pressure drop across the filter cake formed on the surface of the medium (centrifuge wall). This provision made it possible to take advantage of Darcy s law and improve the removal of capillary water, which should help lower the cake moisture. A series of tests were conducted on several fine coal and mineral particles and obtained more than 50% moisture reduction even at very fine particle size (2 mm x 0). Based on the test results obtained in the present work, two proof-of-concept (POC) plants have been designed. The first was for the recovery of cyclone overflows that are currently being discarded in Virginia, and the other was for the recovery of fines from a pond in southern West Virginia. The former was designed based on the results of the plant tests conducted in the present work. Cost vs. benefit analyses were conducted on the two POC plants. The results showed very favorable internal rates of return when using the novel dewatering aids. Surface chemistry studies were conducted on the coal samples based on the results obtained in the present investigation. These consisted mainly of the surface characterization of the coal samples (surface mineral composition, surface area, zeta potential, x-ray photoelectron microscopy (XPS)), acid-base interactions of the solids and liquids, dewatering kinetic tests, contact angle measurements of the coal samples and surface force measurements using AFM. In addition, carbon coating on a silica plate using palsed laser deposition (PLD) and Langmuir-Blodgett (LB) film deposition tests were conducted on the sample to better understand the surfactant adsorption and dewatering processes. The test results showed that the moisture reductions on the fine particles agree well with the surface chemistry results.
Al-Ga Sacrificial Anodes: Understanding Performance via Simulation and Modification of Alloy Segregation
Kidd, Michael Scott Jr. (Virginia Tech, 2019-04-19)
Marine structures must withstand the corrosive effects of salt water in a way that is low cost, reliable, and environmentally friendly. Aluminum satisfies these conditions, and would be a good choice for a sacrificial anode to protect steel structures if it did not passivate. However, various elements can be added to aluminum to prevent this passivation. Currently, Al-Ga alloys are used commercially as sacrificial anodes but their performance is not consistent. In this research, Thermo-Calc software was used to simulate various aspects of the Al-Ga system in an attempt to understand and potentially correct this reliability issue. Simulations showed that gallium segregates to the grain boundaries during solidification and then diffuses back into the grains during cooling to room temperature. Simulations also suggest that faster cooling rates and larger grains cause the potential segregation of gallium at the grain boundaries to remain after cooling. A set of aluminum plus 0.1% weight percent gallium alloy plates were produced with varying cooling rates, along with a control set (cooled slowly in a sand mold). Some samples were later homogenized via annealing. Samples were subjected to a 168 hour long galvanostatic test to assess voltage response. The corrosion performance of samples was found to have both consistent and optimal voltage range when subjected to quick cooling rates followed by annealing. Testing samples at near freezing temperature seems to completely remove optimal corrosion behavior, suggesting that there are multiple causes for the voltage behavior.
Alkali/steam corrosion resistance of commercial SiC products coated with sol-gel deposited Mg-doped Al₂TiO₅ and CMZP
Kang, Min (Virginia Tech, 1994-05-05)
The corrosion resistance of two commercially available SiC filter materials coated with Mg-doped Al₂ TiO₅ and (Ca _0.6.6' Mg_0.52) Zr₄P₆O₂₄ (CMZP) was investigated in high-temperature high pressure (HTHP) alkali-steam environments. Coated specimen properties, including cold and hot compressive strengths, bulk density, apparent porosity, permeability, and weight change, detected after exposure to 92% air-S% steam 10 ppm Na at 8OO°C and 1.8 MPs for 500 h were compared with those of uncoated specimens. Procedures for applying homogeneous coatings of Mg-doped Al₂ TiO₅ and CMZP to porous SiC filters were established and coating of the materials was successfully accomplished. Efforts to stabilize the Al₂ TiO₅ coating composition at elevated temperature were successful. Coatings show promise for providing improved corrosion resistance of the materials in pressurized fluidized bed combustion (PFBC) environments as evidenced by higher compressive strengths exhibited by coated SiC specimens than by uncoated SiC specimens following HTHP alkali-steam exposure.
Application of Steepest-Entropy-Ascent Quantum Thermodynamics to Solid-State Phenomena
Yamada, Ryo (Virginia Tech, 2018-11-16)
Steepest-entropy-ascent quantum thermodynamics (SEAQT) is a mathematical and theoretical framework for intrinsic quantum thermodynamics (IQT), a unified theory of quantum mechanics and thermodynamics. In the theoretical framework, entropy is viewed as a measure of energy load sharing among available energy eigenlevels, and a unique relaxation path of a system from an initial non-equilibrium state to a stable equilibrium is determined from the greatest entropy generation viewpoint. The SEAQT modeling has seen a great development recently. However, the applications have mainly focused on gas phases, where a simple energy eigenstructure (a set of energy eigenlevels) can be constructed from appropriate quantum models by assuming that gas-particles behave independently. The focus of this research is to extend the applicability to solid phases, where interactions between constituent particles play a definitive role in their properties so that an energy eigenstructure becomes quite complicated and intractable from quantum models. To cope with the problem, a highly simplified energy eigenstructure (so-called ``pseudo-eigenstructure") of a condensed matter is constructed using a reduced-order method, where quantum models are replaced by typical solid-state models. The details of the approach are given and the method is applied to make kinetic predictions in various solid-state phenomena: the thermal expansion of silver, the magnetization of iron, and the continuous/discontinuous phase separation and ordering in binary alloys where a pseudo-eigenstructure is constructed using atomic/spin coupled oscillators or a mean-field approximation. In each application, the reliability of the approach is confirmed and the time-evolution processes are tracked from different initial states under varying conditions (including interactions with a heat reservoir and external magnetic field) using the SEAQT equation of motion derived for each specific application. Specifically, the SEAQT framework with a pseudo-eigenstructure successfully predicts: (i) lattice relaxations in any temperature range while accounting explicitly for anharmonic effects, (ii) low-temperature spin relaxations with fundamental descriptions of non-equilibrium temperature and magnetic field strength, and (iii) continuous and discontinuous mechanisms as well as concurrent ordering and phase separation mechanisms during the decomposition of solid-solutions.
Assessment of the Risks Associated with Thin Film Solar Panel Technology
Reynolds, William T. Jr.; Karmis, Michael E. (Virginia Tech. Virginia Center for Coal and Energy Research., 2019-03-08)
This report reviews the environmental risk profile of utility-scale cadmium telluride (CdTe) photovoltaic installations with relevant information from the scientific literature and an audit of the manufacturing and recycling facilities of a domestic manufacturer. Current photovoltaic technologies are described, and the environmental and health issues associated with CdTe are identified. Solubility measurements, bioavailability, acute aquatic toxicity, oral and inhalation toxicity, and mutagenicity studies all confirm CdTe has different physical, chemical, and toxicological properties than Cd. The CdTe compound is less leachable and less toxic than elemental Cd. The risks to the environment arising from broken solar panels during adverse events are considered by reviewing experimental results, theoretical worstcase modeling, and observational data from historical events. In each case considered, the potential negative health and safety impacts of utility-scale photovoltaic installations are low. The need for end-of-life management of solar panels is highlighted in the context of recycling to recover valuable and environmentally sensitive materials. Based upon the potential environmental health and safety impacts of CdTe photovoltaic installations across their life cycle, it is concluded they pose little to no risk under normal operating conditions and foreseeable accidents such as fire, breakage, and extreme weather events like tornadoes and hurricanes.
Atomistic Molecular Dynamics Studies of Grain Boundary Structure and Deformation Response in Metallic Nanostructures
Smith, Laura Anne Patrick (Virginia Tech, 2014-05-06)
The research reported in this dissertation focuses on the response of grain boundaries in polycrystalline metallic nanostructures to applied strain using molecular dynamics simulations and empirical interatomic force laws. The specific goals of the work include establishing how local grain boundary structure affects deformation behavior through the quantitative estimation of various plasticity mechanisms, such as dislocation emission and grain boundary sliding. The effects of strain rate and temperature on the plastic deformation process were also investigated. To achieve this, molecular dynamics simulations were performed on both thin-film and quasi-2D virtual samples constructed using a Voronoi tessellation technique. The samples were subjected to virtual mechanical testing using uniaxial strain at strain rates ranging from 105s-1 to 109s-1. Seven different interatomic embedded atom method potentials were used in this work. The model potentials describe different metals with fcc or bcc crystal structures. The model was validated against experimental results from studying the tensile deformation of irradiated austenitic stainless steels performed by collaborators at the University of Michigan. The results from the model validation include a novel technique for detecting strain localization through adherence of gold nanoparticles to the surface of an experimental sample prior to deformation. Similar trends with respect to intergranular crack initiation were observed between the model and the experiments. Simulations of deformation in the virtual samples revealed for the first time that equilibrium grain boundary structures can be non-planar for model potentials representing fcc materials with low stacking fault energy. Non-planar grain boundary features promote dislocation as deformation mechanisms, and hinder grain boundary sliding. This dissertation also reports the effects of temperature and strain rate on deformation behavior and correlates specific deformation mechanisms that originate from grain boundaries with controlling material properties, deformation temperature and strain rate.
Atomistic simulation of dislocation core structures in B2 NiAl
Xie, Zhao-Yang (Virginia Tech, 1993)
A systematic study of the core structures of (100), (110), and (111) dislocations in B2 NiAI has been conducted using atomistic simulations with an embedded atom method (EAM) potential. New flexible boundary conditions and a new method of graphic representation of dislocation core structure have been employed. The main findings are the following: Core structures: There are no planar core structures of the dislocations found in B2 NiAl. The core spreading of (100) dislocations in NiAl can occur along a variety of planes depending on dislocation slip plane and line orientation. Discrete lattice effects reduced the high strain levels from anisotropic elasticity solution at the dislocation core considerably and resulted in asymmetrical core structures. The core structure of the (110) dislocations is mutilayered with spreading on the {110} plane. The extent of the same strain level comparing with (100) and (111) dislocations is much larger. The complete (111) dislocations in NiAl are also highly non-planar and are stable with respect to splitting into exact 1/2(111) partials as well as to alternative splittings that correspond to the stable fault in the vicinity of the antiphase boundary (APB), in both {110} and {112} planes. Peierls stresses: Peierls stresses of the dislocations have been calculated and have been compared for their relative ease of motion. Local disordering effects: The local disordering effects on the core structure are found to be significant only in the immediate vicinity of the point defect. Compositional deviation from stoichiometry: The simulation results of (100), (110), and (111)dislocations in off stoichiometric NiAl show that the core structures became more extended than the ones in the stoichiometric NiAl. The core structures are not only dependent on the overall composition but also on their local atomic arrangement near the core region. When compositional deviation from stoichiometry is introduced, the response to the applied stress is different for the various slip systems. The Peierls stresses for the usually easiest moving (100){110} dislocations increased and for the (100){100} dislocations decreased, and the latter are expected to be more active in the deformation processes. The practical implications of these results are that it seems very difficult to modify the alloy behaviors through local changes in stoichiometry and ordering state. The best way to improve the ductility of B2 NiAl is to stabilize (111) slip through the addition of alloying elements that can lower the APB energy.
Biaxial Mechanical Evaluation of Uterosacral and Cardinal Ligaments
Baah-Dwomoh, Adwoa Sarpong (Virginia Tech, 2018-03-06)
The uterosacral ligament (USL) and the cardinal ligament (CL) are two major suspensory tissues that provide structural support to the vagina/cervix/uterus complex. These ligaments have been studied mainly due for their role in the surgical repair for pelvic organ prolapse (POP). POP, which is the descent of a pelvic organ from its normal place towards the vaginal walls and into the vaginal cavity, affects an estimated 3.3 million women in the United States annually. Despite their important mechanical function, little is known about the elastic and viscoelastic properties of the USL and CL due to ethical concerns with in vivo testing of human tissues and the lack of accepted animal models. The goal of this first study is to help establish an appropriate animal model for studying the mechanics of these pelvic supportive ligaments. To achieve this, the first rigorous comparison of histological and planar equi-biaxial mechanical properties of the swine and human USLs was completed. Relative collagen, smooth muscle, and elastin contents were quantified from histological sections and the USL was found to have similar components in both species, with a comparable relative collagen content. Using the digital image correlation (DIC) method to calculate the in-plane Lagrangian strain, no differences in the peak strain during precon- ditioning/cyclic loading tests, secant modulus of the pre-creep/elastic response, and strain at the end of creep tests were detected in the USLs from the two species along both axial loading directions (the main in vivo loading direction and the direction that is perpendicular to it). Because these ligaments are subjected to repeated constant loads in vivo, the effect of re- peated biaxial loads at three different load levels (1 N, 2 N, or 3 N) on elastic and creep properties of the swine CL was investigated. The results showed that CL was elastically anisotropic, as statistical differences were found between the mean strains along the two axial loading directions for specimens at all three different load levels. The increase in strain over time by the end of the 3rd creep test was comparable along the axial loading direc- tions. The greatest mean normalized strain (or, equivalently, the largest increase in strain over time) was measured at the end of the 1st creep test, regardless of the equi-biaxial load magnitude or loading direction. Overall, these experimental findings validate the use of swine as an appropriate animal model and offer new knowledge of the mechanical properties of the USL and CL that can guide the development of better treatment methods such as surgical reconstruction for POP.
Characterization and Thermal Modeling of Laser Formed Ti-6Al-4V
Kelly, Shawn Michael (Virginia Tech, 2002-05-09)
The current work focuses on three aspects of laser formed Ti-6Al-4V: an evaluation of the as-deposited and heat treated macro and microstructures and preliminary results obtained from a model developed to calculate the temperature profile resultant of the laser forming process. A "solution treat and age" heat treatment with a variable cooling rate was performed on the Laser Formed Ti-6Al-4V single line builds. Increasing the cooling rate decreases the acicular alpha grain size in the basketweave WidmanstÃ¤tten alpha plus untransformed beta microstructure. Distinct features of the as-deposited macrostructure include: large columnar prior-beta grains that have grown epitaxially through multiple deposited layers; a well defined heat affected zone in the substrate; and the presence of "layer bands," a macroscopic banding present at the top of every layer except for the last three layers to be deposited. The nominal microstructure between the layer bands consists of acicular basketweave WidmanstÃ¤tten alpha outlined in untransformed beta. The alpha grain width is smaller just above a layer band and larger just below a layer band. The microstructure of the layer band consists of larger colonies of acicular alpha outlined in untransformed beta. The gradient in the alpha grain size and presence of the layer band is due to thermal cycling as opposed to segregation effects which were ruled out using quantitative compositional analyses. Through analysis of the microstructural results the gradient in the nominal microstructure and formation of the layer band in layer n was caused by the deposition of layer n+2, and n+3, respectively. A thermal model has been developed to assist in the prediction and interpretation of the as-processed microstructure. The model is used to explain that the microstructural evolution of the layer bands and gradient microstructure in layer n is due to the deposition of layer n+2. The difference in the two analyses of microstructural evolution based on microstructural observations and thermal model results are due to differences in the parameter sets used to build and model the deposit.
Characterization of Laminated Magnetoelectric Vector Magnetometers to Assess Feasibility for Multi-Axis Gradiometer Configurations
Berry, David (Virginia Tech, 2010-11-19)
Wide arrays of applications exist for sensing systems capable of magnetic field detection. A broad range of sensors are already used in this capacity, but future sensors need to increase sensitivity while remaining economical. A promising sensor system to meet these requirements is that of magnetoelectric (ME) laminates. ME sensors produce an electric field when a magnetic field is applied. While this ME effect exists to a limited degree in single phase materials, it is more easily achieved by laminating a magnetostrictive material, which deforms when exposed to a magnetic field, to a piezoelectric material. The transfer of strain from the magnetostrictive material to the piezoelectric material results in an electric field proportional to the induced magnetic field. Other fabrication techniques may impart the directionality needed to classify the ME sensor as a vector magnetometer. ME laminate sensors are more affordable to fabricate than competing vector magnetometers and with recent increases in sensitivity, have potential for use in arrays and gradiometer configurations. However, little is known about their total field detection, the effects of multiple sensors in close proximity and the signal processing needed for target localization. The goal for this project is to closely examine the single axis ME sensor response in different orientations with a moving magnetic dipole to assess the field detection capabilities. Multiple sensors were tested together to determine if the response characteristics are altered by the DC magnetic bias of ME sensors in close proximity. And finally, the ME sensor characteristics were compared to alternate vector magnetometers.
Characterization Study of Plasma Spray Attachment of Intrinsic Fabry-Perot Interferometric Sensors in Power Generation Applications
Krause, Amanda Rochelle (Virginia Tech, 2012-06-11)
The purpose of this study is to characterize the plasma spray deposits used for attaching intrinsic Fabry-Perot interferometric fiber optic strain sensors. The deposits must maintain adhesion at elevated temperatures without distorting the sensors' signals. Two different material systems were tested and modeled, a nickel based alloy and yttria-stabilized zirconia. The material properties of the deposits and the thermal stresses in the system were evaluated to determine attachment lifetime of the sensors. The encapsulated sensors' signals were collected before and after plasma spraying and at elevated temperatures. The material properties of the deposits were evaluated by electron microscopy, energy dispersive x-ray spectroscopy, scratch testing, thermal fatigue testing, and nanoindentation. The thermal stresses were evaluated by Raman spectroscopy and from finite element analysis in COMSOL® Multiphysics®. Several of the sensors broke during encapsulation due to the plasma spray processing conditions and the signals experienced distortion at elevated temperatures. The sensors can be treated to remove this interference to allow for this deposit attachment. The nickel based alloy's ductility and lamellar microstructure allowed for non catastrophic relaxation mechanisms to relieve induced thermal stresses. The yttria stabilized zirconia failed catastrophically at elevated temperatures due its lack of compliance to mismatches in thermal expansion. A high melting point metallic deposit, similar to the nickel based alloy, is desirable for fiber optic sensor attachment due to its ductility, thermal expansion, and dominant relaxation mechanisms. The processing conditions may need to be optimized to allow for the sensors' protection during encapsulation.
Characterizing the Effects of Mechanical Alloying on Solid State Amorphization of Nanoscaled Multilayered Ni-Ti
Monsegue, Niven (Virginia Tech, 2010-07-09)
Equiatomic composition of Ni and Ti was cryomilled with varying milling times to create Ni-Ti lamella structures with average spacings of 50 nm, 470 nm, and 583 nm in powder particles to vary the interfacial surface area per volume. These surfaces form interfaces for diffusion that are essential for solid state amorphization during low temperature annealing. To compare solid state amorphization in a relatively defect free multilayer system, elemental Ni and Ti were deposited by electron beam physical vapor deposition on titanium plates with comparable spacing as above. Both milled and deposited multilayers were annealed between 225 and 350°C for up to 50 hours. X-ray diffraction characterization and in situ annealing was conducted on cryomilled and deposited multilayers of Ni-Ti. Based on this characterization, an amorphization model based on the Johnson-Mehl-Avrami nucleation and growth equation has been established to predict the amorphization of both cryomilled and deposited multilayers. Cryomilled powders experienced much larger amorphization rates during annealing than that of deposited multilayer structures, for all layer spacings. This superior amorphization is seen despite the formation of amorphous phase during the milling process; the amount of which increases with increasing milling time. The difference in amorphization rates between cryomilled and deposited multilayers is attributed to excess driving force due to the extensive preexisting defects in the powders caused by cryomilling. Serial 3D reconstruction of cryomilled Ni-Ti powders was done by scanning electron microscopy and focused ion beam. Through 3D reconstruction it was observed that a random and non-linear lamella structure has been formed in cryomilled powders. Furthermore, lamellar spacing was seen to become smaller with increased milling time while at the same time becoming more homogeneous through the material's volume. 3D reconstruction of cryomilled Ni-Ti offers a unique insight into the microstructures and surface areas of cryomilled powder particles that has never been accomplished.
Chemical vapor deposition of β-SiC thin films on Si(100) in a hot wall reactor
Chiu, Chienchia (Virginia Tech, 1994-01-12)
A systematic method was developed for the deposition of β-SiC thin films on Si(100) substrates in a hot wall reactor, using low pressure chemical vapor deposition (LPCVD). Due to poor adhesion resulting from lattice mismatch and difference in thermal expansion coefficients between the (SiC films and the Si(100) substrates, the feasibility of forming a SiC buffer layer on the Si(100) surface before beginning the chemical vapor deposition (CVD) process was investigated. The SiC buffer layers were formed with either a smooth or porous morphology. A nonporous Si(100) substrate with a 35Å thick SiC buffer layer was formed when the Si surface was heated at 1050°C in an atmosphere of C₂H₂ and H₂. A porous surface was obtained when the Si substrate was heated at 1000°C in C₂H₂ alone. The porous defects were correlated to the out—diffusion of Si in the carburizing process. On smooth Si(100) substrates, polycrystalline and stoichiometric β-SiC thin films with the (111) planes paralleling the Si(100) substrates were grown from a CH₃SiCl₃ (MTS)—H₂ mixture at 1050°C. At high H₂/MTS ratios and/or low deposition pressures, no etching on the Si substrates of the β-SiC films was observed, resulting in a smooth topography. Degradation in film morphology, changes in the preferred orientation, and etching of the Si substrates were observed at higher pressures, temperatures, and H₂/MTS ratios. The etching of the Si substrate was due to the out—diffusion of Si atoms from the substrate and the presence of Cl—containing radicals, which resulted from the decomposition of MTS molecules before arriving at the substrates. A model of the deposition mechanism is proposed which predicts the deposition rates in a hot wall CVD reactor and agrees very well with the experimental data. On the Si(100) substrate with a porous topography, epitaxial β-SiC(100) thin films were grown from MTS—H₂ at 1150°C. The crystallinity of the deposited films was influenced by the deposition time. With increasing deposition time, rotational β-SiC(100) crystals and polycrystalline β-SiC with a highly preferred orientation of (100) and/or (111) were obtained. At a lower temperature of 1100°C, poor morphology and polycrystalline β-SiC thin films were observed. Finally, a new approach to the calculation of the local equilibrium CVD phase diagrams, which represent the most stable phases above the substrates in a hot wall reactor, for SiC deposition from the MTS—H₂ gas mixture by coupling the depletion effects to the equilibrium thermodynamic computer code SOLGASMIX—PV. The calculated CVD phase diagrams were also compared with experimental and the literature data. Although the local equilibrium CVD phase diagrams predicted the deposition of single phase SiC better than established CVD phase diagrams, the experimental regions for depositing single phase SiC are larger than those calculated from local CVD phase diagrams. This may be because of the high linear velocity of the gas flux under low pressure and the polarity of the Si—containing intermediate species.
Codeformation Processing of Mechanically-Dissimilar Metal/Intermetallic Composites
Marte, Judson Sloan (Virginia Tech, 1999-01-12)
A systematic and scientific approach has been applied to the study of codeformation processing. A series of composites having mechanically-dissimilar phases were developed in which the high temperature flow behavior of the reinforcement material could be varied independent of the matrix. This was accomplished through the use of a series of intermetallic matrix composites (IMCs) as discontinuous reinforcements in an otherwise conventional metal matrix composite. The IMCs are produced using an in-situ reaction synthesis technique, called the XD™ process. The temperature of the exothermic synthesis reaction, called the adiabatic temperature, has been calculated and shown to increase with increasing volume percentage of TiB2 reinforcement. Further, this temperature has been shown to effect the size and spacing of the TiB2, microstructural features which are often used in discontinuous composite strength models. Study of the high temperature flow behavior of the components of the metal/IMC composite is critical to the development of an understanding of codeformation. A series of compression tests performed at 1000° to 1200°C and strain-rates of 10-3 and 10-4 sec-1. Peak flow stresses were used to evaluate the influence of material properties and process conditions. These data were incorporated into phenomenologically-based constitutive equations that have been used to predict the flow behavior. It has been determined that plastic deformation of the IMCs occurs readily, and is largely TiB2 independent, at temperatures approaching the melting point of the intermetallic matrices.
Computer Modeling and Simulation of Morphotropic Phase Boundary Ferroelectrics
Rao, Weifeng (Virginia Tech, 2009-07-31)
Phase field modeling and simulation is employed to study the underlying mechanism of enhancing electromechanical properties in single crystals and polycrystals of perovskite-type ferroelectrics around the morphotropic phase boundary (MPB). The findings include: (I) Coherent phase decomposition near MPB in PZT is investigated. It reveals characteristic multidomain microstructures, where nanoscale lamellar domains of tetragonal and rhombohedral phases coexist with well-defined crystallographic orientation relationships and produce coherent diffraction effects. (II) A bridging domain mechanism for explaining the phase coexistence observed around MPBs is presented. It shows that minor domains of metastable phase spontaneously coexist with and bridge major domains of stable phase to reduce total system free energy, which explains the enhanced piezoelectric response around MPBs. (III) We demonstrate a grain size- and composition-dependent behavior of phase coexistence around the MPBs in polycrystals of ferroelectric solid solutions. It shows that grain boundaries impose internal mechanical and electric boundary conditions, which give rise to the grain size effect of phase coexistence, that is, the width of phase coexistence composition range increases with decreasing grain sizes. (IV) The domain size effect is explained by the domain wall broadening mechanism. It shows that, under electric field applied along the nonpolar axis, without domain wall motion, the domain wall broadens and serves as embryo of field-induced new phase, producing large reversible strain free from hysteresis. (V) The control mechanisms of domain configurations and sizes in crystallographically engineered ferroelectric single crystals are investigated. It reveals that highest domain wall densities are obtained with intermediate magnitude of electric field applied along non-polar axis of ferroelectric crystals. (VI) The domain-dependent internal electric field associated with the short-range ordering of charged point defects is demonstrated to stabilize engineered domain microstructure. The internal electric field strength is estimated, which is in agreement with the magnitude evaluated from available experimental data. (VII) The poling-induced piezoelectric anisotropy in untextured ferroelectric ceramics is investigated. It is found that the maximum piezoelectric response in the poled ceramics is obtained along a macroscopic nonpolar direction; and extrinsic contributions from preferred domain wall motions play a dominant role in piezoelectric anisotropy and enhancement in macroscopic nonpolar direction. (VIII) Stress effects on domain microstructure are investigated for the MPB-based ferroelectric polycrystals. It shows that stress alone cannot pole the sample, but can be utilized to reduce the strength of poling electric field. (IX) The effects of compressions on hysteresis loops and domain microstructures of MPB-based ferroelectric polycrystals are investigated. It shows that longitudinal piezoelectric coefficient can be enhanced by compressions, with the best value found when compression is about to initiate the depolarization process.
Constrained sintering of gold circuit films on rigid substrates
Choe, JoonWon (Virginia Tech, 1994-07-15)
The densification behavior of porous gold films made from commercial circuit paste used in microelectronic packaging applications was studied. Constrained gold circuit films of 60-65μm thick were formed by multiple screen printing of the gold paste on rigid alumina substrates, while freestanding films were obtained by carefully peeling off gold films from the substrates after binder burn-out. Optical techniques were developed to determine the densification kinetics of the constrained and freestanding films at temperatures below 1000°C. The densification kinetics of gold films constrained on rigid substrates were observed to be significantly retarded relative to the free films, at all sintering temperatures between 650°C and 900°C studied. SEM studies revealed the microstructure of the constrained films to be much more porous than its freestanding film counterpart. Considerably higher sintering temperatures were required to obtain densities comparable to those of freestanding films. SEM studies also showed no significant difference in grain size between the sintered freestanding and constrained gold films. Inplane tensile stresses generated during constrained-film sintering, was determined to have a maximum value of 460 KPa at the sintering temperature of 750°C. The negligible difference in grain size between the sintered freestanding and constrained gold films, and the small magnitude of the measured tensile stresses, were both determined to be insufficient to account for the observed retardation in the densification kinetics of the constrained gold films. The activation energies for densification of the porous gold films during isothermal sintering, were found to be 21.54±1.03 Kcal/mole and 45.12±1.6 Kcal/mole for freestanding and constrained gold films respectively. These values corresponded very well with the activation energies for grain-boundary diffusion and lattice diffusion respectively, for gold as found in literature. Hence from our results of the activation energies for densification of the constrained and freestanding gold films, coupled with our studies on grain growth and stress, we suggest that the observed retardation in the densification kinetics of the constrained gold films are due to a change in the dominant diffusion mechanism during sintering of the porous gold films constrained on rigid substrates.
Correlation between structure, doping and performance of thermoelectric materials
Zhao, Yu (Virginia Tech, 2014-09-08)
Thermoelectric materials can convert thermal energy into electrical energy and vice-versa. They are widely used in energy harvesters, thermal sensors, and cooling systems. However, the low efficiency and high cost of the known material compositions limit their widespread utilization in electricity generation applications. Therefore, there is a strong interest in identifying new thermoelectric materials with high figure of merit. In response to this need, this dissertation works on the synthesis, structure, doping mechanism, and thermoelectric properties of zinc oxide (ZnO) and lead tellurium (PbTe). The main focus is on ZnO based materials and in improving their performance. The influences of micro- or nano-structures on thermal conductivity, as well as the correlation between the electrical property and synthesis conditions, have been systematically investigated. ZnO is a likely candidate for thermoelectric applications, because of its good Seebeck coefficient, high stability at high temperature, non-toxicity and abundance. Its main drawbacks are the high thermal conductivity (κ) and low electrical conductivity (σ). To decrease κ, two novel structures—namely, precipitate system and layered-and-correlated grain microstructure—have been proposed and synthesized in ZnO. The mechanisms iii governing the nature of thermal behavior in these structures have been explored and quantified. Due to strong phonon scattering, the nano-precipitates can reduce the thermal conductivity of ZnO by 73%. The ZnO with layered-and-correlated grains can further reduce κ by about 52%, which compares favorably with the dense ZnO with nanoprecipitates. The figure of merit of this ZnO based structure was 0.14×10⁻³ K⁻¹ at 573 K. In order to understand the electrical behavior in nanostructured ZnO, the impact of Al doping and chemical defects in ZnO under different synthesis conditions were studied. Under varying sintering temperatures, atmospheres and initial physical conditions, ZnO exhibited very distinct σ. High temperature, lack of oxygen, vacuum condition, and chemically synthesized powder can increase the carrier concentration and σ of ZnO. A promising alloy system, PbTe-PbS, undergoes natural phase separation by nucleation and growth, and spinodal decomposition depending on the thermal treatment. The correlation between the thermal treatment, structure, and the thermoelectric properties of Pb0.9S0.1Te has been studied. The nano-precipitates were incorporated in the annealed alloy resulting in a 40% decrease in κ. The PbS precipitation was shown to enhance the carrier concentration and improves the Seebeck coefficient. These concomitant effects result in a maximum ZT of 0.76 at 573 K. Throughout the thesis, the emphasis was on understanding the impact of the microstructures on thermal conductivity and the effect of the synthesis condition on thermal and electrical properties. The process and control variables identified in this study provide practical ways to optimize the figure of merit of ZnO and PbTe materials for thermoelectric applications.
Creep Behavior Of Thin Laminates Of Iron-Cobalt Alloys For Use In Switched Reluctance Motors And Generators
Fingers, Richard Todd (Virginia Tech, 1998-06-17)
The United States Air Force is in the process of developing magnetic bearings as well as an aircraft Integrated Power Unit and an Internal Starter/Generator for main propulsion engines. These developments are the driving force behind a new emphasis on high temperature, high strength magnetic materials for power applications. Analytical work, utilizing elasticity theory, in conjunction with design requirements, indicates a need for magnetic materials to have strengths in excess of 80 ksi up to about 1000 degrees F. It is this combination of desired material characteristics that is the motivation for this effort to measure, model, and predict the creep behavior of such advanced magnetic materials. HipercoÂ® Alloy 50HS, manufactured by Carpenter Technology Corporation, is one of the leading candidates for application and is studied in this effort by subjecting mechanical test specimens to a battery of tensile and creep tests. The tensile tests provide stress versus strain behaviors that clearly indicate: a yield point, a heterogeneous deformation described as LuÌ ders elongation, the Portevin-LeChatelier effect at elevated temperatures, and, most often, a section of homogeneous deformation that concluded with necking and fracture. Creep testing indicated two distinct types of behavior. The first was a traditional response with primary, secondary and tertiary stages, while the second type could be characterized by an abrupt increase in strain rate that acted as a transition from one steady state behavior to another. This second linear region was then followed by the tertiary stage. The relationship between the tensile response and the creep responses is discussed. Analyses of the mechanical behavior includes double linear regression of empirically modeled data, scanning electron microscopy for microstructural investigations, isochronous stress-strain relations, and constant strain rate testing to relate the tensile and creep test parameters. Also, elastic and creep deformation analyses are done, which incorporate material property data and material constants determined along with stress and displacement profiles for a specific Air Force design configuration.
Cryomilling of Aluminum-based and Magnesium-based Metal Powders
Maisano, Adam J. (Virginia Tech, 2006-01-13)
Ball milling has been shown to produce nanostructures in metal powders through severe repetitive deformation. Ball milling at cryogenic temperatures (cryomilling) is more effective in this capacity due to the low temperature by slowing recovery and minimizing diffusion distances between different components. Nanostructured metals are of interest because of their unique physical and mechanical properties. The result of cryomilling is powder consisting of crystallites on the order of 30 – 50 nm. In order to characterize the properties of this material, it is often necessary to consolidate the powder, which is often difficult without causing significant grain growth. In this work, aluminum-rich and magnesium-rich alloys of varying composition are produced by cryomilling and characterized by x-ray diffraction. A novel consolidation process called high shear powder consolidation (HSPC) is used to densify as-received and as-milled powders with minimal growth. The construction of a cryomill, along with a modification for improving process yield, has provided a platform for the study of nanocrystalline metals. It has been shown that bulk nanocrystalline materials are attainable and that alloy composition influences mechanical properties.

Browsing by Author "Reynolds, William T. Jr."

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