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Tao, LiangAtomic-scale calculations of interfacial structures and their properties in electronic materials
Doctor of Philosophy, The Ohio State University, 2005, Materials Science and Engineering
With the tremendous increase in computational power over the last two decades as well as the continuous shrinkage of Si-based Metal Oxide Semiconductor Field Effect Transistors (MOSFET), quantum mechanically based ab initio methods become an indispensable tools in nano-scale device engineering. The following atomistic simulations, including ab initio, nudged elastic band (NEB) and kinetic Monte Carlo methods, have been presented in this work. Using VASP, an ab initio simulation package, we calculated B segregation energy at different atomic sites in perfect and defected Si/SiO2 interfaces and arsenic segregation energy in Si/LaAlO3 structures. With the presence of O vacancies and H in B doped systems, the predicted segregation energy is 0.85 eV for neutral systems and 1.12 eV for negatively charged systems, which is consistent with experimental measurements (0.51 to 1.47 eV). Focussing on the La deficient Si/LaAlO3 interfacial structure, we find that the arsenic prefers energetically not to segregate into LaAlO3 nor does it pile up in front of the interface. In combation of atomic-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS), we theorectically calculated the band structure and EELS of a Ge/SiO2 interface. we actually found a chemically abrupt Ge/SiO2 interface, which has never been reported before and which is quite desirable for applications. Furthermore, we formulated a kinetic Monte Carlo model to simulate the oxidation process of Ge ion-implanted Si, which explained the formation of abrupt Ge/SiO2 interface. Using nudged elastic band (NEB) method, we systematically calculate the vacancy formation, diffusion activation energy and pre-exponential diffusion factor at pure and Cu doped Al grain boundaries. Though grain boundary diffusion is still much faster than that of bulk, adding small amounts of Cu can dramatically improve the electromigration reliability of Al interconnects. A comparison of the vacancy formation energy at Al, Al(Cu) and strained Al grain boundaries, in which all the Al atoms keep their positions as they are in the Al(Cu) structure, highly indicates that the increase of the vacancy formation energy at the Al(Cu) grain boundary is a combined result of electronic and strain effects from the impurity-atom segregation to the grain boundaries.

Committee:

WOLFGANG WINDL (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

ab initio; first principles; NEB; Monte Carlo; MOSFET; electronic devices; segregation; high-k; LaAlO3; oxidation; strained interface; diffusion; grain boundary; interconnects

Zhao, XinyanExfoliation corrosion kinetics of high strength aluminum alloys
Doctor of Philosophy, The Ohio State University, 2006, Materials Science and Engineering
The objective of this study was to quantitatively study localized corrosion, especially exfoliation corrosion (EFC) of high strength aluminum alloys and to investigate the mechanism of exfoliation corrosion with a focus on the effects of alloy temper, microstructure, relative humidity (RH) and mechanical stress. A new technique, Exfoliation of Slices in Humidity (ESH), was developed for the determination of exfoliation corrosion (EFC) susceptibility and quantification of EFC kinetics. This technique involves in exposing properly oriented and unconstrained samples to high humidity following an electrochemical pretreatment. The EFC kinetics was determined by measuring the width of the central unattacked region of the samples. The ESH results show the capability of the ESH test to discriminate between plates of varying susceptibility and to determine EFC rates quantitatively. Optical microscopy and analytical TEM were used to investigate the effects of microstructure and local chemistry at grain boundary on EFC susceptibility. Alloys with more elongated grain shape are more susceptible to EFC and a high Zn content in grain boundary precipitate free zone relates to a high susceptibility. The effects of RH, temper and applied stress on EFC kinetics of AA7178 were investigated by ESH tests. The critical RH for EFC propagation in AA7178 was found to be about 56% and the EFC kinetics increased with RH. ESH tests provide a quantitative description of the temper effect on EFC kinetics. The effects of applied compressive and tensile stresses on EFC kinetics were studied using a four-point bending jig. Compression accelerated EFC significantly and tension reduced kinetics. An equation describing the effects of RH, stress and time on EFC kinetics was developed based on the ESH results using Eyring model. In situ X-ray radiography was used to characterize intergranular and exfoliation corrosion in high strength Al alloys. The samples were either exposed to sodium chloride solution (NaCl) at a controlled potential or to high humidity after an electrochemical pretreatment in NaCl solution. In situ X-ray radiography of intergranular corrosion attack provides a wide range of IGC kinetics including the fastest growing sites. This method is a good approach for visualizing the EFC process.

Committee:

Gerald Frankel (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

exfoliation corrosion; kinetics; aluminum alloys; relative humidity; Exfoliation of Slices in Humidity (ESH); temper; microstructure; mechanical stress; grain boundary precipitate free zone

Shen, YaoA peierls model of dislocation transmission through coherent interfaces and embedded layers
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
The interfacial obstacle to dislocation transmission is an important factor controlling the strength of nanometer-scale multilayers. The critical shear stress (CSS) to transmit a screw dislocation across a coherent interface was studied using a Peierls framework. First a one-interface model was developed. A screw dislocation of Burgers vector b is represented by N model dislocations each of Burgers vector b/N, which can spread into a slipping interface upon energy relaxation during transmission. An envelope of E-X is constructed for the transmission with E being the line energy and X being the work conjugate to the Peach-Koehler force. CSS is taken as the maximum slope of the E-X curve. Several factors influencing CSS are studied. Interfacial spreading increases CSS with the effect more pronounced for smaller modulus mismatch. The slip plane inclination angle has little effect on CSS. Coherency stress increases CSS significantly for small modulus mismatch. Dislocation dissociation reduces CSS. Generally, CSS increases with DeltaE/DeltaX, where DeltaE is the change in dislocation line energy and DeltaX is the dislocation motion over which the energy transition occurs. There are three concepts to increase DeltaE/DeltaX: (1) increase the change in line energy for a dislocation on the outgoing versus incoming side; (2) decrease DeltaX; (3) create an energy trap by interfacial spreading. The elastic moduli, unstable stacking fault energies and antiphase boundary energies of the left phase, the interface, and the right phase are available to change DeltaE/DeltaX and therefore CSS. Later an embedded layer model was developed to study the effect of the layer thickness on CSS. The decrease of CSS is about 5-20% at a layer thickness of 4b relative to that at larger than 103b. Further a 3D model was developed to study the effect of thermal activation on CSS. The activation energy and corresponding temperature were estimated as functions of applied shear stress. The room temperature CSS for a full screw dislocation in Cu/Ni multilayer system is about 60% of that at 0K, while 75% for a partial dislocation. Finally the simulation is found comparable with experiments for Cu/Ni multilayers.

Committee:

Peter Anderson (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Dislocation transmission; Critical shear stress; Peierls Model; Coherent interface; Multilayer

WAGH, VIJAY HEMANTVAPOR PHASE SILANATION OF PLASMA-POLYMERIZED SILICA-LIKE FILMS BY 3-AMINOPROPYLTRIETHOXYSILANE
MS, University of Cincinnati, 2005, Engineering : Materials Science
Silica-like films were deposited onto aluminum substrates by plasma polymerization of hexamethyldisiloxane (HMDSO) in the presence of oxygen using a capacitively-coupled RF-powered plasma reactor.Two types of films, referred to as low-hydroxyl and high-hydroxyl, were obtained by varying parameters such as the power and the flow rates of oxygen and HMDSO. 3-Aminopropyltriethoxysilane (3-APS) was adsorbed onto the silica-like films from the vapor phase at a substrate temperature of 100 deg C. Results obtained from reflection-absorption infrared (RAIR) spectroscopy showed the presence of a band near 1100 cm-1. This band was not present in the spectra of neat silica-like films and was assigned to a vibrational mode characteristic of Si-O-Si bonds formed between the primer films and the chemisorbed silane. Results obtained from x-ray photoelectron spectroscopy (XPS) showed that some of the nitrogen atoms in the adsorbed 3-APS films were protonated, probably by strong hydrogen bonding with silanol groups on the surface of the silica-like films. However, most of the amino groups were not protonated, indicating that adsorption in that case involved condensation of ethoxy groups with surface silanol groups. The reactivity of silanated silica-like films with a typical epoxy compound was investigated. Upon reaction with epoxy, increments in the intensity of the C(1s) peak in the survey spectra and the peak attributed to C-O bonds in the high-resolution C(1s) curve fit were observed, meaning that some of the epoxy reacted with silica-like films that were silanated with (3-APS) in the vapor phase. Lap joints were prepared using a 2-part epoxy adhesive and aluminum substrates that were primed with silica-like films. Joints prepared from substrates that were primed with low-hydroxyl films had an average strength of about 23.4 MPa while those prepared from substrates primed with high-hydroxyl films had an average strength of about 14.3 MPa. Vapor phase silanation of the primer films did not have a significant effect on the initial strength of the joints. However, it did have an effect on the locus of failure as determined by XPS. When no silane was used, joints prepared using substrates primed with the low-hydroxyl films failed near the adhesive/primer interface. Joints prepared using substrates primed with the low hydroxyl films and coated with 3-APS failed cohesively, within the adhesive. Lap joints prepared from aluminum substrates primed with neat low-hydroxyl and high-hydroxyl silica-like films using the 2-part adhesive failed the durability test. Joints prepared from aluminum substrates that were primed with low-hydroxyl silica-like films and silanated with 3-APS passed the durability test; although the residual strength of such joints was a fraction of the initial strength. Analysis of failure surfaces of lap joints prepared from aluminum substrates primed with low-hydroxyl silica-like films that were silanated by 3-APS revealed that the mode of failure changed from mostly cohesive within the adhesive before the durability test, to mostly interfacial between the silica-like film and the 2-part adhesive after the durability test.

Committee:

Dr. James Boerio (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

plasma; aminosilane; epoxy; lap joints; vapor phase

MUKHERJEE, NILOYDEVELOPMENT OF HIGH SENSITIVITY BENDING MODE POLYMER PIEZOELECTRIC DEVICES FOR INNER EAR
PhD, University of Cincinnati, 2003, Engineering : Materials Science
The requirements of a piezoelectric device based inner ear cochlear implant are high sensitivity, millimeter dimensions, flexibility, acoustic impedance matching and biocompatibility. Two cantilever devices, based on cochlear structures, were designed for investigation as cochlear implants. The specific material chosen was poly(vinylidene fluoride)(PVF2), used in the bending/flexure mode of deformation. Fabrication of devices of approximately implantable size was demonstrated. The piezoelectric sensitivity of devices to both airborne and underwater acoustic waves was measured. Very high sensitivities have been obtained; the response in air is several Volts for conversational sounds at close range. Underwater sensitivity is approximately -200 dB in the 2 - 10 kHz range. Dependance of sensitivity on device dimensions and dielectric coating was investigated. Also, 2-element devices were found to be approximately 1.5 times more sensitive than 1-element devices, proving that amplification can be obtained by increasing the number of piezoelectric elements. A sub-kHz experimental modal analysis study revealed that cantilevers in the length range approximately 2 - 20 mm have three bending resonances < 1 kHz, all resonances are piezoelectrically active (keff = 0.2 - 0.35), and the resonance frequency depends upon material properties, film dimensions and electrode properties. The suitability of PVF2 material for use in the temperature and frequency conditions faced by an implant was also confirmed. Also investigated was the origin of bending piezoelectricity in single films of PVF2. Thermal current studies revealed the existence of single-polarity space charges, which can contribute to bending piezoelectricity. Small angle x-ray scattering studies revealed the the lamellae are not perpendicular to the machine direction, but make an angle with all the film axes. Lamellae are stacked in the form of fibers oriented in the direction of film drawing. Thus the mesoscale structure of the polymer is heterogeneous, and bending piezoelectricity is believed to originate as a result of the heterogeneous structure and from space charge effects. Lastly, some first generation devices have been specified.

Committee:

Dr. Rodney D. Roseman (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

piezoelectric; cochlear implant; polyvinylidene flouride; polymer; acoustic

Dutta, IndrajitElectro-Mechanical Behavior of Strontium Modified Lead Zirconate Titanate Ceramics
PhD, University of Cincinnati, 2007, Engineering : Materials Science

Since the discovery of perovskite ferroelectrics, the solid solution of PbZrO3-PbTiO3 (PZT) system is intensively studied due to its excellent electro-mechanical properties for transducers and actuators applications. The strontium modified PZT (PSZT) at the Zr rich end shows a long antiferroelectric-ferroelectric boundary suitable for field induced phase transition study and associated electro-mechanical behavior study.

A modified oxide route was developed to process PSZT ceramics with different Zr4+/Ti4+ ratios for repeatable ultra-high strain applications. Repeatable ultrahigh strain of ~ 0.8% was achieved for PSZT compositions near the AT-FR phase boundary. Other compositions showed strain (0.35-0.8%) which were still better than the commercially available materials of same class. The PSZT ceramics showed high polarization response as well. Polarization over 41µC/cm2 was observed for certain ceramics on the FR side.

A novel dynamic in-situ X-Ray diffraction was conducted on the different PSZT ceramics to understand the field induced electromechanical property changes. The ultrahigh strain in these materials, on the antiferroelectric tetragonal side of the phase diagram, was found to be due to unit cell volume change along with the movement of the non 180° domains. For the rhombohedral ferroelectric materials the strain was mostly contributed by the unit cell volume increase. Compositions farther away from the boundary in the AFT side did not show any structural change or domain movements.

Electrical fatigue behavior of PSZT ceramics was investigated by subjecting the as-sintered ceramics to low frequency (30 Hz) electric field up to 107 cycles. The fatigue free polarization behavior (<10% degradation) was attributed to the high density and low porosity of the PSZT ceramics. The high strain degradation nature (~50% degradation) was found to be partially due to internal stress development during the electrical cycles and partially due to the formation of fatigue induced mechanical damage layer under the electrode. Thermal annealing and removal of the damaged layer showed the complete recovery of the strain to its original as-sintered value. Some preliminary electric fatigue experiments with electron beam deposited aluminum (Al) electrode were conducted on PSZT ceramics. The results showed promising fatigue resistance behaviors.

Committee:

Raj Singh (Advisor)

Subjects:

Engineering, Materials Science

Chokalingam, KumarPoly (Allylamine Hydrochloride) and Poly (Acrylic Acid) Multilayers for Gas Separation
MS, University of Cincinnati, 2007, Engineering : Materials Science
This work focuses on the formation of multilayers by layer-by-layer deposition of weak polyelectrolytes, poly (allylamine hydrochloride) and poly (acrylic acid). These multilayers were built on smooth surfaces (silicon wafer) and rough surfaces (Millipore membrane, clay-coated paper). The effect of concentrations, pH of the depositing polyelectrolyte solutions and drying between the absorption steps were investigated in films deposited on Si wafers. These films were characterized using ellipsometry, contact angle and AFM. In the pH ranges investigated in the study (2.5-4.5), at concentrations of 10-2 M and 10-1 M, the layer-by-layer deposition proceeds in a linear manner, with pH influencing the thickness of the bilayers. Thickness of the films increased with increase in the solution pH. The films deposited at higher concentrations showed higher thickness and roughness. Drying in all cases, lead to thinner and rougher films. Contact angle studies showed that the wettability of the polyelectrolyte multilayers is likely dependent upon the interpenetration of the underlying layer and can change with the pH of the solution. Drying had a significant effect when films were deposited at lower concentration but no effect when films were deposited at higher concentrations. Polyelectrolytes were also deposited on Millipore membranes and clay- coated papers. USAXS data on coated Millipore membranes showed that coating densified the fibers and also reduced the pore and solid chords. Permeation studies in coated Millipore membranes showed that bilayers increase the selectivity for CO2. Permeation studies in coated clay-coated paper showed that bilayers reduce water vapor permeability.

Committee:

Dr. Dale Schaefer (Advisor)

Subjects:

Engineering, Materials Science

KANDELL, BRIAN MPIEZOELECTRIC POLYMER (PVDF) RIBBON FOR CHOCHLEAR IMPLANTATION - GUIDELINES AND COMPARISONS WITH TOOTHBRUSH STYLE PROTOTYPES
MS, University of Cincinnati, 2006, Engineering : Materials Science
This thesis describes the efforts undertaken in the development of a highly sensitive piezoelectric device to eventually be used as a stand-alone cochlear implant. Through previous studies the most important device requirements were found to be highly sensitive with small dimensions, large voltage response in the audible frequency range (~20 Hz – 20,000 Hz), highly impedance matched with water, and be biocompatible. Previous studies by Mukherjee, Dwividi, Mitra, and Kant, respectively, have found the ‘toothbrush’ design to be a highly effective candidate to meet the above requirements with the exception of size limitations. Thus a ‘ribbon’ style device was devised and tested to give a more readily implantable device. Testing of the ribbon style prototypes showed very good correlation to the previous toothbrush tests. Maximum responses given by a single element ribbon device was in the order of 3.5V to 5V. Double element ribbon devices showed much large coupling of individual voltages (~50% increase in toothbrush designs and ~65% to 95% increase in ribbon designs). In addition to the large voltage responses to semi-quantitative testing, the ribbon devices have shown response to frequency in the audible range. Thus, the ribbon devices has shown to be as effective (and in the cases of multi-array systems more effective) as the toothbrush designs in achieving large voltage responses while dramatically reducing the size of the overall device to a point where implantation can be realized in upcoming studies.

Committee:

Dr. Rodney Roseman (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

PVDF; Cochlear Implant; Sensorineural Hearing Loss; Piezoelectricity; Smart Material; Hydroacoustic; Acoustics

Levin, Victor D.Laves phase strengthening in a cold-worked iron-chromium-nickel-molybdenum austenitic stainless steel
Doctor of Philosophy, Case Western Reserve University, 1993, Materials Science and Engineering
The feasibility of using Laves phase precipitation as a major strengthening constituent in a work-hardenable austenitic Fe-Cr-Ni-Mo stainless steel has been demonstrated. Thermodynamic and kinetic analyses showed that among the three intermetallic phases: sigma (σ), chi (χ) and Laves (η) that may form in the system, increased Mo content promoted most extensively Laves phase formation in a cold-worked structure. By altering the AISI 316L base alloy composition by the addition of up to 4 wt.% Mo favorable precipitation reactions were induced to occur before the onset of recrystallization. This precipitation of Laves phase produced a fine, well distributed particles in an austenitic matrix. Overall three groups of materials were investigated and compared based upon their response to aging at 704°C (1300°F) after having been cold worked by extrusion to a true strain of 1.08.

Committee:

G. Michal (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Laves phase; Austenitic stainless steel

Rigney, Joseph DavidDuctile-phase toughening of in situ niobium silicide-niobium composites
Doctor of Philosophy, Case Western Reserve University, 1994, Materials Science and Engineering
In situ processing techniques were used to incorporate elongated primary niobium (Nb p) and secondary niobium (Nb s) phases in a brittle niobium silicide (Nb5Si3) intermetallic matrix. The presence of these phases increased the fracture resistance of the composite by the ductile-phase toughening mechanism Resistance-curve behavior and peak toughnesses of 28 Mpasurdm at 380 μm crack extension were determined in slow displacement rate tests monitored in real-time. Bridging and deformation of Nb p ligaments between the crack faces in the Nb5Si3, and plasticity in a process zone ahead of the crack tips, provided for the increased fracture resistance. The properties were compared to previously developed fracture models. The work of rupture (χ) of the ligaments were determined to be 1.6 and constrained flow stresses were predicted to be about six times the uniaxial tensile yield stress (σ y). Even under these high constraints, the Nb p ligaments plastically deformed and exhibited dimpled fracture. Experiments were performed over a range of strain rates at 298 K and 77 K to increase the yield stresses of the body-centered cubic Nb p ligaments and increase propensity for cleavage fracture behavior . On each fracture surface created at 298 K, the Nb p phases exhibited a consistent, smoothly varying fracture behavior with respect to the specimen dimensions. The increasing frequency of cleavage fracture was found to be related to global strain rates experienced by the samples, induced through the motion of the loading point in the notched three point bend test. The higher strain rates augmented yield stresses and subsequently produced a transition in fracture mode. The toughness values determined from experiments spanning six orders magnitude in loading rate at 298 K and 77 K, exhibited little change, even in situations when the majority of Nb p phases failed by cleavage. Although χ-values were calculated to drop to fractions of the slow, 298 K value, the constancy of the toughness values illustrated the importance in considering the toughness of the "ductile" phase (σ ycdotχ), or the area beneath the constrained stress-strain curve. The loss in ductility at high strain rates and/or low temperatures was compensated by high yield stresses in order to match the toughening provided by the ligaments when yield stresses were lower and strains to failure were greater

Committee:

John Lewandowski (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Ductile-phase toughening; In situ processing; Niobium-silicide-niobium composites

Chung, SamwonThe effect of internal stress on diffusion
Doctor of Philosophy, Case Western Reserve University, 1995, Materials Science and Engineering
Free strains produced by diffusion occur from the difference in either (i) the partial molar volumes or (ii) the self-diffusion coefficients between the components. In this thesis, the above two kinds of free strain were treated separately for convenience and clarity. It was assumed that stress is produced by diffusion without making a new phase. All mechanical properties like Young's modulus, Poisson's ratio and viscosity were assumed constant and isotropic. The mixture with different partial molar volumes was assumed to follow elastic behavior. The viscoelastic (Maxwell) mixture with different partial molar volumes was also treated by extending the elastic model using the viscoelastic analogy (the formal similarity between the Laplace transformed viscoelastic and elastic equations). Difference in the components' self-diffusion coefficients causes a non equilibrium vacancy concentration to occur. This non-equilibrium vacancy concentration was regarded as a free strain. Following Darken's assumption, this free strain induced the strain that maintained the mixture density constant. With the above assumptions, the differential equations for concentration in elastic, Maxwell and Darken's mixtures were obtained. The concentration as a function of time and position in an elastic mixture was obtained for the case of an in finite diffusion couple while the concentrations as functions of time and position in Maxwell and Darken's mixtures were obtained for the case of a finite diffusion couple. The stress or strain was also calculated from the concentration of each mixture. In the mixture with different partial molar volumes (elastic and Maxwell mixtures), the concentration discontinuity at the interface decreased but did not disappear with the induced stress around the interface. In the mixture with different self-diffusion coefficients (Darken's mixture), the induced strain resulted in a deviation from the antisymmetric concentration profile about the interface.

Committee:

Alfred Cooper (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

internal stress; diffusion

Hughes, Paula JeanneDevelopment and breakdown of chromia scales on iron-25 chromium alloys in hydrogen/water/hydrogen sulfide atmospheres at 700 degrees C
Doctor of Philosophy, Case Western Reserve University, 1991, Materials Science and Engineering
The corrosion behavior of Fe25wt%Cr was studied in H2/H2O and in H2/H2O/H2S environments at 700°C. The scales which formed after exposure to these environments were characterized with scanning auger microscopy (SAM), scanning electron microscopy (SEM), transmission election microscopy (TEM), and X-ray diffraction techniques. The mechanism of scale formation and breakdown was observed to be very dependent on the p(H2O)/p(H2S) ratio of the corrosion environment. In all cases, the gas conditions studied were in the region where FeS and Cr2O3 are the thermodynamically stable phases. After exposure to environments with a very low p(H2O)/p(H2S) ratio (to the left of the kinetic boundary) chromium sulfides were observed to dominate scale growth and scale breakdown was characterized by the growth of chromium sulfide nodules protruding above the chromium oxide scale. These nodules eventually became enriched in iron leading to rapid growth of FeS and compete sulfidation of the alloy.At higher p(H2O)/p(H2S) ratios (to the right of the kinetic boundary) initally protective chromium oxide scales were formed. Significant amounts of sulfur present at the scale/alloy interface were characteristic of the scales formed under these conditions. The amount of sulfur at the scale/alloy interface was greatest over the alloy grain boundaries. In contrast to the scales formed at lower p(H2O)/p(H2S) ratios, no sulfide nodules were observed during the initial stages of oxidation. After extended exposure to these environments however nodule formation was sometimes observed. These nodules were most concentrated over the alloy grain boundaries. The amount of sulfur observed at the scale alloy interface of samples which exhibited nodule formation was significantly greater than the amount of sulfur observed at the scale/alloy interface of samples which did not exhibit nodule formation. Samples which were preoxidized under a range of p(O2) conditions were exposed to a mixed gas environment with a low p(H2O)/p(H2S) ratio. All scales exposed to the mixed gas environment were characterized by chromium sulfide nodule formation similar in morphology to that observed on bare alloy samples exposed to mixed gas conditions to the left of the kinetic boundary

Committee:

Krishna Vedula (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Chromia scales; development and breakdown

Rogers, Richard B.The Measurement of Solid-Liquid Interfacial Energy in Colloidal Suspensions Using Grain Boundary Grooves
Doctor of Philosophy, Case Western Reserve University, 2006, Materials Science and Engineering
Interfacial energy is a fundamental physiochemical property of any multi-phase system. Among the most direct approaches for determining solid-liquid interfacial energy are techniques based on measuring the shape of grain boundary grooves in specimens subjected to a linear temperature gradient. We have adapted several of these techniques to crystallizing colloids in a gravitational field. Such colloids exhibit an order-disorder phase transition and are important not only as self-assembling precursors to photonic crystals, but also as physical models of the melting-freezing phase transition in atomic and molecular systems. We have also developed and tested parallel-beam and divergent-beam techniques for determining the orientation and lattice parameters of randomly oriented single colloidal crystals in situ using optical microscopy. We tested our grain boundary groove and crystallography techniques using suspensions of sterically stabilized poly-(methyl methacrylate) (PMMA) spheres, which have been shown to closely approximate the hard sphere potential. Isotropic model fits to grain boundary groove data were inconsistent, but we obtained γ110 = (0.58 ± 0.05)kBT/σ2 using a capillary vector approach, which is suitable for both isotropic and anisotropic surface energies. Kinks observed in groove profiles suggest a minimum anisotropy parameter of ε6 = 0.029 for hard spheres. We also observed grain tips at the termination of the solid phase in each sample, resulting from a slight side-to-side tilt of our samples with respect to gravity. These grain tips offer significantly more interface height and accessible crystallographic orientations compared to typical grain boundary grooves, suggesting their potential use for interfacial energy measurements in both colloidal and atomic/molecular systems. Initial tests of our crystallography techniques suggest that they are suitable for extracting lattice parameters to within about 1 percent and orientation data to within about 2°. The value of our interfacial energy measurement using the capillary vector model is in close agreement with the majority of published experimental and computer simulation values for hard spheres, indicating the validity of our grain boundary groove technique adaptations to colloidal systems in a gravitational field.

Committee:

K. Peter Lagerlof (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

interfacial energy; colloidal crystal; hard sphere; grain boundary groove

Bhattacharyya, DhritiThe development of textures and microstructures in alpha/beta titanium alloys
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
The relationship between the texture and the microstructure of both beta processed and alpha/beta processed Ti alloys has been examined in this study. In the beta-processed microstructures, it has been shown that two sets of alpha colonies sharing a common {0001} plane and rotated by ~10.5° from each other may have growth directions which have very large angles of about ~80.7° between them. Moreover, it was observed that alpha laths growing from certain prior beta grain boundaries sometimes shared common basal planes. In some special cases, the alpha laths growing into two different prior beta grains from the grain boundary between them had almost exactly the same orientation, although they had vastly different growth directions. Additionally, there were some cases in which alpha laths growing into different prior beta grains not only had the same crystallographic orientation, but also had the same growth direction. Scanning Electron Microscopy (SEM), Orientation Imaging Microscopy (OIM) and Transmission Electron Microscopy (TEM) have been used to investigate these phenomena and the existing theories of growth directions have been used in conjunction with the results obtained to explain them. In alpha/beta-processed alloys, the phenomenon of globularization of alpha laths breaks down the beta-processed microstructure and modifies the texture of these alloys. Samples of Ti-6Al-4V having colony and basketweave microstructures were hot deformed in the high alpha/beta temperature range and their microstructure and microtexture were examined by the use of SEM and OIM. It is shown that the samples which had a colony microstructure had greater “clustering” of grains with similar orientations than those having a basketweave microstructure. The mode of transformation on heating from the alpha to the beta phase was investigated by measuring the texture of both phases at different temperatures, in situ, using the HIPPO instrument at the Los Alamos Neutron Science Center. A comparison of the pole figures for both phases has allowed an insight into the mode of transformation of the alpha to the beta phase, and it appears that the beta phase forms by the growth of the preexisting beta, and not by fresh nucleation of beta in the alpha phase.

Committee:

Hamish Fraser (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Titanium alloys; Alpha/beta; Texture; Lath morphology; Globularization

Cao, FeinaShape Memory Polyurethane Nanocomposites
Doctor of Philosophy, University of Akron, 2008, Polymer Engineering

Shape memory polymers are smart materials which can remember their original shapes. However, the low recovery stress and low mechanical strength limit the commercial applications of shape memory polymers. In this study, nanoclays were introduced to shape memory polyurethanes (SMPU) to augment these properties by enhance the network of SMPU. Several factors which influence the shape recovery stress were evaluated, including the nature of polymer chain by using different monomers, type of clay particles, extent of filler dispersion, clay content and deformation conditions.

It was found that only reactive clay particles were well dispersed into polyurethane matrix by the tethering between –CH 2CH 2OH functional groups in clay surfactants and polyurethane chains. Two different shape memory polyurethanes ( Systems I & II) prepared by bulk polymerization were compared. The shape memory effect of System Iwas triggered by melting of the soft segment crystals, while that of System IIwas by glass transition of the soft segments. It was seen that the reactive clay particles dispersed well in both polyurethane matrices and augmented the recovery stress, e.g., 20% increase with 1 wt % nanoclay in System Iand 40% increase with 5 wt % nanoclay in System IIwere observed.

In System I, clay particles interfered with soft segment crystallization, and promoted phase mixing between the hard and soft segments, thus affecting the fixity and recovery ratio. Nevertheless, the soft segment crystallinity was still enough and in some cases increased due to stretching to exhibit excellent shape fixity and shape recovery ratio. The higher loading of clay particles accelerated the stress relaxation, resulting in reduction of recovery stress.

In System II, no significant effect of clay particles in phase separation was observed, so there was no influence of clay on shape fixity and recovery ratio. The recovery stress increased with reactive nanoclay content. It was also found that the recovery stress could be tailored by the processing conditions. The recovery stress increased with decrease of stretching rate, and increase of stretching temperature and stretch ratio. The recovery stress of polyurethane/clay nanocomposites largely depended on the degree of clay exfoliation. Higher recovery stress was found in nanocomposites with better clay dispersion. The dependence of stress relaxation on stretching conditions, clay type, and clay content was also investigated and related to shape recovery stress. It was found that stress relaxation occurred more easily in the presence of nanoclay.

Committee:

Sadhan Jana (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Shape memory; Polyurethane; Nanoclay; Nanocomposite

Kanuga, Karnav D.A FUNDAMENTAL STUDY ON THE NON-LINEAR MECHANO-OPTICAL BEHAVIOR OF POLYETHYLENE NAPHTHALATE, ITS BLENDS WITH POLYETHERIMIDE AND ITS NANOCOMPOSITES
Doctor of Philosophy, University of Akron, 2006, Polymer Engineering
Non-Linear Mechano-Optical behavior and structural organizational process of Polyethylene Naphtlalate (PEN)/Polyetherimide (PEI) blends as well as Polyethylene Naphthalate nanocomposites are investigated under uniaxial deformation above their respective glass transition temperatures. A uniaxial stretching machine fitted with a spectral birefringence setup and a laser micrometer was used to monitor real-time the birefringence, true stress as well as true strains developed in the material during deformation. Offline characterization techniques such as WAXD, DSC and FT-IR were used to unravel and augment the understanding of the structural development mechanisms occurring in the material. The stress-optical behavior of PEN/PEI blends as well as PEN nanocomposites under large deformation revealed that there are three distinct stress-optical regimes with an additional glassy component appearing at low temperatures. The final structure and deformation behavior of the blends have been mapped out in a dynamic phase diagram showing that the material undergoes three critical structural transitions. At low temperatures near Tg the polymer remains in a nematic-like state and orientation induced crystallization occurs only above a certain stretching temperature. At intermediate temperatures Liquid-Liquid (Tll) transition occurs wherein the material transforms from a ‘structured liquid’ to a ‘true liquid’ state at (1.08 Tg(°K)) exhibited by the disappearance of the initial glassy component as the material becomes devoid of the segmental correlations. At higher temperatures, where the relaxation process dominates and where the thermal induced crystallization is still suppressed, the material was found to remain in amorphous state even after being stretched to large deformation levels. The Nanocomposites were found to undergo two critical structural transitions: i) Nematic-crystalline transition wherein the material stretched below a certain temperature does not undergo orientation-induced crystallization but develops a highly ordered nematic state. ii) Liquid-Liquid (Tll) transition wherein the material transforms from a ‘fixed liquid’ to a ‘true liquid’ state at 1.25 times Tg(°C) or 1.07 times Tg(°K), exhibited by the disappearance of the initial glassy component as the material becomes devoid of the inherent structure due to segmental correlations. While unfilled PEN and PEN/PEI blends remains amorphous at high temperatures due to high relaxation rates combined with suppressed thermal crystallizability, the presence of nanoplatelets was found to facilitate strain induced crystallization even at such high temperatures primarily as a result of its suppression of relaxation mechanism in its sphere of influence. The addition of nanoparticles in PEN did not change the stress-optical constant of PEN but with further deformation the birefringence developed decreased with the addition of nanoparticles due to lower crystallinity and amorphous orientation in the material. Subjecting these nanocomposites to further relaxation showed that distinct differences were observed above and below the liquid-liquid transition temperature (Tll). Relaxation below Tll exhibits an instant stress recovery as the segmental correlations remaining unbroken during stretching, elastically relax during stretching which is absent above Tll. Addition of nanoparticles suppresses the relaxation rate in PEN and even though addition of nanoparticles suppresses the development of crystallinity during stretching they no longer play that role during relaxation.

Committee:

Mukerrem Cakmak (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Polyethylene Naphthalate; Birefringence; Polyetherimide; Stress-Optical Behavior;Nanocomposites

Jadhav, Amol D.Processing, characterization, and properties of some novel thermal barrier coatings
Doctor of Philosophy, The Ohio State University, 2007, Materials Science and Engineering
The efficacy of ceramic thermal barrier coatings (TBCs) used to protect and to insulate metal components in engines increases with the thickness of the TBCs. However, the durabilities of thick TBCs deposited using conventional ceramic-coating deposition methods have not been adequate. Here the feasibility of depositing highly durable thick TBCs (1.5 to 4 mm thickness) of ZrO2-7 wt.% Y2O3 (7YSZ) on bond-coated superalloy substrates using the solution-precursor plasma spray (SPPS) method has been demonstrated. Thermal cyclic durabilities of the thick SPPS TBCs have been shown to be much superior compared to TBCs deposited using the conventional air-plasma-spray (APS) process. To evaluate the performance of thick APS and SPPS TBCs, mechanical properties of free-standing coatings and coating/substrate interfaces have been determined experimentally. Additional evaluation of TBC performance has been obtained from studies of damage and development of thermally grown oxide (TGO) at the interface as a result of thermal cycling. The later results are used to suggest mechanisms of chemical failure of TGO in thick plasma-sprayed TBCs. Based on the experimental results and numerical analysis of the TBC residual stresses, the dramatic improvement in the thermal cycling life in the SPPS TBCs is attributed to superior mechanical properties of SPPS coatings. The presence of the strain tolerant vertical cracks in SPPS TBCs reduces the driving force for TBC spallation under mode-II loading. Additionally, high in-plane fracture toughness in the SPPS TBCs under mode-I loading delays the TBC spallation significantly. Finally, thermal conductivity of the SPPS TBCs has been reduced by microstructural tailoring. Analytical and object-oriented finite element (OOF) models have been used to analyze the experimental thermal conductivity data, and to predict thermal conductivities of engineered TBCs.

Committee:

Nitin Padture (Advisor); Sheikh Akbar (Other); John Morral (Other)

Subjects:

Engineering, Materials Science

Keywords:

Thick thermal barrier coatings; Solution-precursor plasma spray; Mechanical properties; Thermal conductivity; Failure analysis

Nam, JinElectrospun polycaprolactone scaffolds under strain and their application in cartilage tissue engineering
Doctor of Philosophy, The Ohio State University, 2006, Materials Science and Engineering
Electrospinning is a promising fabrication method for three dimensional tissue engineering scaffolds due to its ability to produce a nano-/micro-sized non-woven fibrous structure which resembles the natural extracellular matrix. We investigated the mechanical behavior of two different electrospun microstructures. Polycaprolactone (PCL) fibers with or without “point-bonding” exhibited different deformation behaviors having significant biomedical consequences. While fibers with point-bonded structure failed due to the generation of voids by the fracture of fiber interconnections under strain, fibers without point-bonds produced a ‘bamboo’ structure with fiber joining visible at higher levels of strain. In addition, gelatin and PCL were electrospun and the residual solvent contents were systematically investigated. A simple and effective means of reducing residual solvent content was developed. The interaction between these electrospun matrices and chondrocytic cells were compared to other topographies having the same chemistry. Electrospun polycaprolactone fibers supported better proliferation and extracellular matrix production than the corresponding semi-porous and dense surfaces and even, at some time points, glass surfaces. The intrinsic capability of electrospinning to produce high porosity appears to offset the relative hydrophobicity of polycaprolactone resulting in a more uniform cell seeding. Electrospun fibers induced a higher level of glycosaminoglycans (GAG) production by providing a ‘dynamic scaffold’ in which chondrocytes are able to maintain a morphology associated with the appropriate phenotype. Finally, based on this study, a method producing macro-pores within an electrospun scaffold was developed. With this method, not only can cellular infiltration into a thick electrospun scaffold be facilitated, but scaffolds having designed, anisotropic structures can be produced that better approximate the final tissue.

Committee:

John Lannutti (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Tissue Engineering; Electrospinning; Polycaprolactone; Articular cartilage

McClellan, Kenneth JamesStructure/property relations in yttrium oxide-stabilized cubic zirconium oxide single crystals
Doctor of Philosophy, Case Western Reserve University, 1994, Materials Science and Engineering
The goal of this research was to develop improved understanding of the structure/property relations in Y2O3-stabilized cubic ZrO2 (Y-CSZ) single crystals with emphasis on the potential use of these materials as a fibrous reinforcement for high temperature structural applications. In particular, the issues addressed include: (i) determination of the structure of as-grown Y-CSZ and its relation to the deformation behavior, (ii) the deformation characteristics and thermophysical properties of Y-CSZ, with emphasis on potential fiber applications and (iii) initial fiber growth and evaluation. In this study, the structure of as-grown 9.4 mol% (and probably (18 mol%) Y-CSZ was determined by CBED analysis to belong to the P43m space group at room temperature. The discrepancy between the determined primitive structure and high temperature deformation behavior characteristic of a face centered structure can be reconciled most easily by a cubic-to-cubic second-order phase transformation. Such a transformation was indicated by a peak in specific heat at ∼820°C. Additional support for this transformation is given by a distinct change in dislocation mobility around Knoop indents between 800°C and 850°C. An extensive data base exists for the deformat ion behavior of Y-CSZ as a function of temperature, strain rate, orientation and composition. Additional tests were performed to determine the composition (21 mol%) and orientation (< 001>) recommended for initial fiber growth. Localized deformation previously observed in Y-CSZ alloys was analyzed by linear stability analysis (LSA). The phenomenon responsible for the onset of an instability is avalanche multiplication, and the factor determining the subsequent growth or decay of this instability is the extent of slip (heat) localization. The thermal expansion of Y-CSZ is relatively high and is just one advantage in favor of ZrO2-fiber reinforced composites. Due to a number of difficulties identified in Y-CSZ fiber growth, the initial fibers grown can only be characterized as developmental. However, the strength of these fibers is substantial, and a higher fraction of strength is retained at high temperatures than for the more developed sapphire fibers. Classical stress corrosion is operational in this system

Committee:

Arthur Heuer (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Yttrium oxide-stabilized cubic zirconium oxide single crystals

Shaw, Greg SamuelThe effect of processing variables on steel surface chemistry
Doctor of Philosophy, Case Western Reserve University, 1993, Materials Science and Engineering
Differences in processing at all stages in the steel making process can effect the surface chemistry of steel. The surface chemistry can dictate the performance of many different products and processes including electrogalvanized steel. If the steel chemistry is not controlled sufficiently, the zinc electrodeposit will be subject to hydrogen blistering. This dissertation examined the effect of many processing variables on both the organic and inorganic contaminants on steel surfaces. Processes both before the electrogalvanizing line and those after the electrogalvanizing line were examined. Mechanisms were determined for reactions of organic oils with the steel surface. Reaction between the carbonyl functionality, present as an acid or an ester, and iron hydroxide to form a tenaciously adherent iron soap was demonstrated using infrared spectroscopy and mass spectrometry. Thermodynamics and kinetics of segregation of metallic elements to the surface of the steel were determined. It was shown that the primary mechanism for segregation of elements such as aluminum, silicon, chromium, and titanium is oxidation during the anneal. Analysis of the kinetics indicated the feasibility of this mechanism. Differences in processing at all stages in the steel making process can effect the surface chemistry of steel. The surface chemistry can dictate the performance of many different products and processes including electrogalvanized steel. If the steel chemistry is not controlled sufficiently, the zinc electrodeposit will be subject to hydrogen blistering. This dissertation examined the effect of many processing variables on both the organic and inorganic contaminants on steel surfaces. Processes both before the electrogalvanizing line and those after the electrogalvanizing line were examined. Mechanisms were determined for reactions of organic oils with the steel surface. Reaction between the carbonyl functionality, present as an acid or an ester, and iron hydroxide to form a tenaciously adherent iron soap was demonstrated using infrared spectroscopy and mass spectrometry. Thermodynamics and kinetics of segregation of metallic elements to the surface of the steel were determined. It was shown that the primary mechanism for segregation of elements such as aluminum, silicon, chromium, and titanium is oxidation during the anneal. Analysis of the kinetics indicated the feasibility of this mechanism. The mechanisms of electrocleaning were elucidated, based on bot h electrochemical and physical effects. Improved methods of electrocleaning were described based upon optimization of the cleaning frequency. It was found that increasing the frequency of anodic and cathodic cycling increases the cleaning efficiency up to about 5 Hz. Above this frequency bubbles, which provide physical scrubbing and convection near the interface, can not form rapidly enough to be effective. Increased current density was shown to be effective in improving electrocleaning. Two evaluative techniques were developed to assist in the evaluation of the steel surface chemistry. The blister backcharging test entails electrochemically forcing atomic hydrogen through a steel membrane until it reaches the interface with an impermeable coating, such as zinc. The pressure of the hydrogen at the interface forces the coating to blister. The blister density is a measure of the interfacial adhesion strength. The second technique developed was based upon the use of cyclic voltammetry. It was demonstrated that the slope of the hydrogen evolution region of the voltammogram is inversely proportional to the amount of carbonaceous material on the steel surface as measured by x-ray photoelectron spectroscopy

Committee:

Joe Payer (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Processing variables; Steel, surface chemistry

Behrens, GesaThe martensitic transformation in zirconia
Doctor of Philosophy, Case Western Reserve University, 1993, Materials Science and Engineering
The metastable t-ZrO2 particles in both Y-TZP and Mg-PSZ can transform to m-ZrO2, and in the PSZ also to o-ZrO2, via a martensitic transformation. This research has demonstrated that the transformation is controlled by the thermally activated, stress-assisted nucleation of the martensite phase. The thermally activated character of the phase change was manifested as isothermal transformation kinetics in both materials: Residually stressed regions in indented Y-TZP's underwent isothermal t ⇒ m transformation during post-indentation anneals up to ~500°C. In Mg-PSZ's, slow isothermal t ⇒ m and t ⇒ o transformations occurred at room temperature after a short anneal at 1000°C, which reverses the room temperature aging. The phase transitions were accompanied by substantial transformation plasticity: Forward and reverse transformations during anneals in the residually stressed regions in the TZP caused large volume and shape changes, whose simulation with a finite element model was partially successful. In Mg-PSZ's the transformation was induced by internal stresses during room temperature aging as well as by external stresses applied during stress-relaxation tests; again, ample transformation plasticity ensued. Furthermore, the effect of the indentation-induced transformation in Mg-PSZ on the growth of indent cracks during four-point bending was investigated, and the crystallographically oriented and autocatalytic deformation zones around indents in Y-PSZ single crystals were analyzed.

Committee:

Arthur Heuer (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

martensitic transformation; zirconia

Hanes, Mark DavidDiffraction techniques and molecular modeling calculations in the determination of the configurational structures in poly(vinyl fluoride)
Doctor of Philosophy, Case Western Reserve University, 1991, Macromolecular Science
The present research has investigated the presence of configurational defects in poly(vinyl fluoride). Both tacticity variations and head-to-head/tail-to-tail defects have been considered. Focus has been placed on the use of X-ray analysis and has been complimented by molecular modeling calculations and differential scanning calorimetry measurements. Configurational variations can exist in the crystalline regions of PVF due to the isomorphous nature of hydrogen and fluorine. However, configurational variations are expected to influence the conformation of the chain. In addition, the large electronegativity of the fluorine should affect the packing of the polymer. Molecular modeling calculations indicate a syndiotactic sequence is favored in a planar zigzag conformation, whereas for the possible HH arrangements, an extended conformation is very close to the minimum energy. For an isotactic sequence, there is a large energy penalty for the trans conformation. However, if a syndiotactic or meso HH/TT unit is placed in the middle of an isotactic sequence the energy of the structure decreases. Packing calculations indicate that for all systems studied the energy was negative, thus favoring crystallization. The melting point, as determined by DSC, has been used as a relative measure of the HH/TT content in PVF. The melting temperature of several samples was measured and correlated to the polymerization temperature. It was found that the melting temperature increased as the polymerization temperature was decreased. Three X-ray techniques were used in the study: a refinement of fiber data using the LALS program, a Rietveld analysis of powder data, and an analysis of the meridional intensities. The LALS refinement used data reported by Natta and Allegra and data collected for a commercial sample of PVF. No improvement was achieved in refining the data reported by Natta. The refinement of the commercial sample resulted in a system where the chains were packed in a statistical manner. The weighted residual for refinement was 0.10. The Rietveld analysis was applied to a perfect HT sample and appears to confirm the statistical packing of the chains. An analysis of the meridional intensities indicates that the ratio of the 001 to the 002 structure factor is a sensitive measure of the HH/TT content. Values were determined for several samples and correlated to the polymerization temperature. The results agree with those determined by DSC.

Committee:

Jerome Lando (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

diffraction techniques molecular modeling configurational structures poly(vinyl fluoride)

Roy, TapanA TEM investigation of high Tc superconductors and related perovskites
Doctor of Philosophy, Case Western Reserve University, 1990, Materials Science and Engineering

The structure and microstructure of the new perovskite superconductors, MBa2Cu3O7 (where M = Y, Eu, Gd; known as 123) and La2-xSrxCuO4 (known as 214), and the related perovskites, Eu2CuO4, Gd2CuO4 and La2CuO4 have been investigated using transmission electron microscopy. Various experiments were performed in situ in the microscope on bulk ion-milled specimens using heating and cooling holders.

Twins are the most prominent microstructural features in both the 123 and 214 type compounds. These twins are formed to accommodate the strains arising from the difference in the a and b parameters below the tetragonal to orthorhombic transformation temperature.

The b/a ratio of YCa2Cu3O7 and La2CuO4 has been determined over a range of temperatures. The b/a ratio in 123 materials reaches a peak at ∼150K below the transformation temperature and remains fairly constant below that temperature but in 214 compounds, the b/a ratio increases monotonically. In 123 materials, the orthorhombic distortion arises from the ordering of the O atoms to form Cu-O chains along the b axis whereas in the 214 materials the orthorhombic distortion occurs by the tilting of the CuO6 octahedra that make up the structure.

The electron beam when condensed is intense enough to displace oxygen atoms in 123 materials leaving behind vacancies. These oxygen vacancies order themselves and this is observed in the diffraction pattern as superlattice reflections. Two basic types of ordering were observed – the first type with extra reflections at the 1/3(100) and 2/3(100)* type positions and the second with extra reflections at the 1/4(110)*, 1/2(110)* and 3/4(110)* type positions. The two types of ordering that were observed were modeled in terms of superlattices with lattice parameters of 3aT x aT x cT and surd2aT x 2surd2aT x cT respectively. Sometimes the 123 specimen can be melted incongruently with an intense electron beam and the products of melting are found to be BaCuO2, Y2BaCuO5 and Cu2O. Using energy considerations, the twin boundary spacing (λ) is related to the colony size(L) by the expression λ = ε-1 (2.γ.L/μ)0.5. This expression has been verified from measurements of λ and L in bright field micrographs. From the slope of a λ vs L1/2 plot, γ can be calculated from the expression: γ = 0.5(slope)2.ε*μ and the values obtained are 2.2mJ/m2 for 123 materials and 4.8 mJ/m2 for La2CuO4

Committee:

T. Mitchell (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

TEM superconductors perovskites

JAYASEELAN, VIDHYA SAGARSTUDY OF POLYCRYSTALLINE DIAMOND THIN FILMS GROWN IN A CUSTOM BUILT ECR PE-CVD SYSTEM
MS, University of Cincinnati, 2000, Engineering : Materials Science
The increasing importance of high temperature electronics has necessitated a search for new materials. Silicon provides low reliability or fails to function altogether at elevated (>500F /260(C) temperatures. Diamond, being a wide-band-gap semiconductor, is a very promising candidate for these applications as well as others that function in adverse conditions. However, the present day diamond film technology, with respect to quality and consistency of properties of the films, has not seen the advancement required for its commercial application. This research is an effort to investigate the growth of diamond thin films to enable their application for advanced electronic devices. The first objective of the research was to construct a state of the art Electron Cyclotron Resonance Microwave Plasma Chemical Vapor Deposition (ECR-MPCVD) system for processing diamond and diamond like materials. Such a facility is the most advanced and powerful tool for CVD of poly-crystalline diamond and cubic Boron Nitride. Important factors like ease of operation and maintenance, stability and reproducibility of process conditions, reliability, safety, cost etc were considered while constructing the machine. The system was subjected to a series of tests to ensure that these factors were adequately satisfied. The second objective of this project was to grow polycrystalline diamond films under various conditions of chamber pressure, substrate temperature, hydrogen flow rate, composition and substrate pretreatment. The properties of the films grown under these conditions were evaluated by various modern characterization techniques such as optical microscopy, scanning electron microscopy, Raman spectroscopy and X-ray diffraction. Optical microscopy is an inexpensive, quick and effective method for initial evaluation of the uniformity and general morphology of the films obtained. Scanning electron microscopy gives information about the grain size, thickness, growth rate, uniformity, faceting, roughness, and continuous nature of the film. Raman spectroscopy is probably the most important characterization method for diamond films for electronic applications. It helps to identify the presence of diamond and the nature of the film. Raman spectrum is used to evaluate the quality of the diamond grains in the film, defect concentration and crystallinity. It also gives information about the presence of the non-diamond graphite phase and hence indirectly the intrinsic conductivity. XRD is also a positive identification tool. It was also used to derive information about the grain orientation with respect to the silicon substrate. The properties of the films obtained were correlated with the growth parameters and conclusions were drawn about the effect of these parameters on the film properties. This also helped in the characterization of the MPCVD reactor and in determining an effective range of parameters for future studies on the selective growth, long term growth, or growth of films with oxygen addition to the precursor. These efforts would be stepping stones for future studies by the group in this field of research. Based on these efforts, a versatile ECR plasma CVD system has been designed and created. High quality polycrystalline diamond films were prepared in a methane-hydrogen plasma environment and characterized. It was demonstrated that the morphology and crystallinity of the diamond deposit could be controlled by changing the growth parameters.

Committee:

Dr. Raj Singh (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

micro electronics; diamond devices; high temperature semiconductor; cubic boron nitride; functional coatings

ZHOU, HONGKINETICS, PROCESSING, AND PROPERTIES OF Si/SiC COMPOSITES FABRICATED BY REACTIVE-MELT INFILTRATION
PhD, University of Cincinnati, 2001, Engineering : Materials Science
The kinetics and mechanism of reaction of carbon with Si melt or Si+Mo melt were investigated. The growth of the reaction-formed SiC layer was found to follow a fourth power rate law. A kinetics model was developed, in which an internal electric field was assumed to be set up over the SiC layer through a negative space charge. The diffusion of carbon-ion vacancies was considered as the rate-limiting step for the growth of SiC layers. The effect of processing temperature and addition of Mo on the kinetics was also studied. Based on the kinetics study, a processing model was developed to relate the Si content in the Si/SiC composites with the porosity in the preforms for the reactive-melt infiltration processes. The minimum porosity was obtained for fabricating completely-reacted, near-net-shape, and fully dense materials. Using this model as guideline, the preforms consisting of α-SiC powder and graphite were designed and made by tape casting followed by hot lamination and binder burnout. The porous preforms were then infiltrated by liquid Si at 1450°C to produce the Si/SiC composites containing 10 to 45 vol.% Si. The composition of the composites were verified by quantitative analyses using X-ray diffraction and image analysis. The effects of Si content on the mechanical properties as well as creep behavior were studied using four-point bending tests. The elastic modulus and flexural strength were reduced with increasing Si content. The change in elastic modulus with Si content followed the rule of mixtures at room temperature. The elevated-temperature flexural strength decreased significantly when Si content was above 20 vol.%. The creep rate increased with Si content, especially when Si content was above 20 vol.%. Composites containing less than 20 vol.% Si exhibited a significant creep resistance up to 1550°C. The stress and temperature thresholds for creep were found for the composites containing more than 20 vol.% Si. Temperature or stress beyond their corresponding threshold value caused a significant acceleration in creep. The creep data were analyzed to identify the creep mechanisms.

Committee:

Dr. Raj Singh (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

kinetics; Si/SiC composites; processing; reactive-melt infiltration; strength

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