Browsing by Author "Thompson, Gregory B."
Now showing 1 - 20 of 81
Results Per Page
Sort Options
Item Advanced characterization of the oxidation behavior of grain refined NiAl(University of Alabama Libraries, 2019) White, Rachel Ellen; Weaver, Mark Lovell; University of Alabama TuscaloosaReactive element doped β-NiAl is one of the most oxidation resistant materials available for high temperature use. It has been extensively studied to create the most adherent, slow growing, and passive layer possible. One recent area of interest is grain refinement, whereby the reduced metal grain size improves mechanical properties, transports reacting elements rapidly to the oxidizing surface, and facilitates the growth of a more adherent scale. This research focused on the effect of substrate grain refinement on the microstructure of its thermally grown oxide, in comparison to the oxide grown on extruded and single crystal NiAl alloys. The oxidation behavior of grain refined materials produced by via sputter deposition, ball milling, and cryomilling was found to vary significantly. Sputter deposition was shown to significantly increase the parabolic steady state oxidation rate constant, while decreasing the length of transient oxidation. Ball milling did not result in an increase in oxidation rate, but did show increased interfacial void formation as a result of the Al2O3 dispersions incorporated during the milling process. Last, cryomilling resulted in an increase in steady state oxidation rate and increased interfacial void formation that was correlated to AlN dispersions incorporated during milling. All three grain refinement methods were found to decrease the oxide grain size approximately three-fold in comparison with the oxide grown on extruded NiAl, though a consistent relationship between oxide grain size and steady state oxidation rate was not observed. This suggests that microstructural features other than substrate and oxide grain size dominate the oxidation behavior.Item An analysis of the grain refinement of magnesium by zirconium(University of Alabama Libraries, 2010) Saha, Partha; Viswanathan, S.; University of Alabama TuscaloosaA Design of Experiments (DOE) approach was used to conduct a systematic study of the grain refinement of magnesium by zirconium; variables included the amount of zirconium, the pouring temperature, and the settling time prior to casting. Samples were poured into a special "hockey puck" mold designed to reproduce the conditions in permanent mold casting. Optical and scanning electron microscopy (SEM) was utilized to measure the grain size in the final microstructure. Sample dissolution followed by SEM was used to characterize zirconium particle size and morphology both in the master alloy and grain refined samples, while an AccuSizer 770 Photozone/Light Obscuration instrument was used to measure total particle size distributions in the master alloy and grain refined samples. Transmission Electron Microscopy (TEM) was used to identify particles that likely act as suitable heterogeneous nucleation sites for grain refinement. The TEM results show that a range of particle sizes are likely substrates and that only zirconium particles which are faceted are likely nucleation sites. It is apparent that only 1 to 3% of the total particles serve as nucleation sites, but a comparison of the grain density vs. faceted particle density shows close agreement. Equal Channel Angular Extrusion (ECAE) processing of the magnesium-15wt% zirconium master alloy to increase the number of faceted particles resulted in improved grain refinement efficacy. This work suggests that there is a tremendous potential to engineer a more efficient grain refiner.Item Bacterium E. coli- and phage P22-templated synthesis of semiconductor nanostructures(University of Alabama Libraries, 2010) Shen, Liming; Gupta, Arunava; University of Alabama TuscaloosaThe properties of inorganic materials in the nanoscale are found to be size- and shape-dependent due to quantum confinement effects, and thereby nanomaterials possess properties very different from those of single molecules as well as those of bulk materials. Assembling monodispersed nanoparticles into highly ordered hierarchical architectures is expected to generate novel collective properties for potential applications in catalysis, energy, biomedicine, etc. The major challenge in the assembly of nanoparticles lies in the development of controllable synthetic strategies that enable the growth and assembly of nanoparticles with high selectivity and good controllability. Biological matter possesses robust and precisely ordered structures that exist in a large variety of shapes and sizes, providing an ideal platform for synthesizing high-performance nanostructures. The primary goal of this thesis work has been to develop rational synthetic strategies for high-performance nanostructured materials using biological templates, which are difficult to achieve through traditional chemical synthetic methods. These approaches can serve as general bio-inspired approaches for synthesizing nanoparticle assemblies with desired components and architectures. CdS- and TiO_2 -binding peptides have been identified using phage display biopanning technique and the mechanism behind the specific affinity between the selected peptides and inorganic substrates are analyzed. The ZnS- and CdS-binding peptides, identified by the phage display biopanning, are utilized for the selective nucleation and growth of sulfides over self-assembled genetically engineered P22 coat proteins, resulting in ordered nanostructures of sulfide nanocrystal assemblies. The synthetic strategy can be extended to the fabrication of a variety of other nanostructures. A simple sonochemical route for the synthesis and assembly of CdS nanostructures with high yield under ambient conditions has been developed by exploiting the chemical characteristics and structure of permeabilized E. coli bacteria. The crystal phase, morphology, micro/nanostructure, optical absorption, and photocatalytic properties of the CdS nanostructures are tailored over a wide range by merely changing the synthetic conditions. Photoanodes fabricated using the nanoporous hollow CdS microrods exhibit excellent performance for the photocatalytic hydrogen production. This facile approach has been extended to the synthesis and assembly of other semiconducting sulfides, including PbS, ZnS, and HgS.Item Carbide Nanoparticle Dispersion Techniques for Metal Powder Metallurgy(MDPI, 2021) Rocky, Bahrum Prang; Weinberger, Christopher R.; Daniewicz, Steven R.; Thompson, Gregory B.; University of Alabama Tuscaloosa; Colorado State UniversityNanoparticles (NP) embedded into a matrix material have been shown to improve mechanical properties such as strength, hardness, and wear-resistance. However, the tendency of NPs to agglomerate in the powder mixing process is a major concern. This study investigates five different mechanochemical processing (MCP) routes to mitigate agglomeration to achieve a uniform dispersion of ZrC NPs in an Fe-based metal matrix composite. Our results suggest that MCP with only process controlling agents is ineffective in avoiding aggregation of these NPs. Instead, the uniformity of the carbide NP dispersion is achieved by pre-dispersing the NPs under ultrasonication using suitable surfactants followed by mechanically mixing of the NPs with iron powders in an alcohol solvent which is then dried. High-energy MCP is then used to embed the NPs within the powders. These collective steps resulted in a uniform dispersion of ZrC in the sintered (consolidated) Fe sample.Item Coating yttria stabilized zirconia powders by magnetron sputtering(University of Alabama Libraries, 2019) Togaru, Maanas; Thompson, Gregory B.; University of Alabama TuscaloosaThis thesis describes the application of Physical Vapor Deposition (PVD) for coating powders, with the work motivated by the need to provide conformal coatings for nuclear fuel for use in Nuclear Thermal Propulsion (NTP). The coated material was tungsten, because of its high melting point and low neutron cross-section, yttria-stabilized zirconia (YSZ) was used for the nuclear powder surrogate. The coating was done in a rotating drum that held and moved the powders under a cylindrical cathode. The sphericity of the powders, to improve their flow in the drum, was achieved using a gravity-based plasma Powder Alloying Spheroidization (PAS) process. The particles were coated between 5.5 kWh to 40 kWh resulting in a coating thickness between approximately 70 nm to 540 nm. The coatings were found to have powdery morphology spheres resulting from the particle-to-particle collisions. To further understand the stress state of the deposited film, a series of 100 nm tungsten films were deposited at two different rates (0.05 and 0.2 nm/s) and three pressures (2, 5 and 10 mTorr). At the lowest pressure, regardless of rate, the films had a compressive stress state. Upon increasing the pressure for both rates, the residual stress was near zero. X-ray diffraction revealed that the nominally body centered cubic tungsten film adopted the A15 phase referred to as beta-tungsten.Item A computational investigation into the microstructures and stability of the zeta phase in transition metal carbides and nitrides(Taylor & Francis, 2018) Weinberger, Christopher R.; Yu, Hang; Wang, Billie; Thompson, Gregory B.; Colorado State University; Drexel University; University of Alabama TuscaloosaA high-volume fraction of the zeta phase in multiphase group VB transition metal tantalum carbides has been shown to dramatically increase fracture toughness. This has been attributed to its unique nanoscale lath-based microstructure. However, what governs the microstructure and how it forms is still not well understood. In this paper, we propose a precipitation model for the formation of these phases and demonstrate that the anisotropic surface energies govern the observed zeta-phase morphology. The energetics and zeta-phase microstructure for other group VB carbides were found to be similar. In contrast, multiphase hafnium nitrides can form both thin-lath-based microstructure as well as large, single zeta-phase grains. The difference between hafnium nitride and the group VB carbides is attributed to the relative bulk free energies and low-temperature stability between the phases.Item Containerless melting and characterization of cast magnesium AZ31-B alloy at low superheat in the magnetic suspension melting process(University of Alabama Libraries, 2011) Rimkus, Nathan Wayne; El-Kaddah, N.; University of Alabama TuscaloosaThis study deals with containerless induction melting and characterization of cast magnesium AZ31-B alloy at low superheat via the Magnetic Suspension Melting (MSM) process. The operating conditions for melting and confinement of a 63.5mm molten column of the alloy with respect to current and frequency were found to range from 770-830 Ampere and 3552-3600 Hz, respectively. The macro/micro-structure, oxide entrapment, segregation of alloying elements, and the formation of intermetallic precipitates in MSM cast AZ31-B alloy in a unidirectional bottom chilled ceramic mold were investigated. For a baseline comparison, the alloy was also cast using conventional methods at a superheat of 60°C. Analysis of cooling curves showed that the cooling rate during solidification is essentially constant at about 1°C/s. The growth velocity, V, for MSM casting produced at low superheat is almost constant during solidification, around 1 mm/s, while the thermal gradient, G, decreases with increasing solid fraction from 2.07°C/mm at f=0, to 1.10°C/mm at f=0.5. In contrast, V for castings produced at high superheat is four times smaller than that for low superheat castings--the conditions that favor equiaxed dendritic solidification morphology. Metallographic examination of the MSM alloy cast at low superheat shows no evidence of oxide formation. It was also found that casting at this low superheat produced a fine globular grain structure compared to the equiaxed dendritic structure in conventionally cast alloys at high superheat. The average grain sizes for 5 and 8°C MSM produced castings were 83.96 and 94.81μm, respectively. The 60°C superheat castings were found to have a much larger average grain size of 334.42μm. Elemental segregation analysis was performed and showed the presence of primary-α Mg and secondary-α Al rich Mg phases, along with γMg_17 Al_12 and Al_8 Mn_5 intermetallic phases. For the globular structures, the intermetallic phases were found to form in the secondary-α phase along the grain boundaries. Comparatively, the dendritic entrapment of the secondary-α phase was found to lead to intermetallic phase formation in the matrix of the grains.Item Crystallization Characteristics in Co-Based Magnetic Amorphous Nanocomposite Alloys(University of Alabama Libraries, 2022) Koenig, Alicia Grace; Thompson, Gregory B.; University of Alabama TuscaloosaThis document will describe analytical procedures for the microstructural characterization of Co-based soft magnetic amorphous nanocomposite materials and the evolution of that microstructure after heat treatment. Atom probe tomography analysis of these alloys reveals increased chemical diffusion with additional solute content, as well as a reduction in defects in the crystalline phase. It was confirmed that Co and Fe partition preferentially to the crystalline phase, and that other elements (B, Si, Nb, Mn) segregate to the amorphous matrix. It was found that a combination of FCC/HCP structures were the basis for the crystalline phase.Differential scanning calorimetry was used to evaluate the characteristics of phase transition as a function of solute content, revealing an increase in the necessary energy for the formation of the primary crystallization phase with higher solute concentrations. From this data, the primary crystallization temperature was estimated and anneals were performed at a temperature just below that to slow the kinetics of crystallite nucleation and growth. Postmortem atom probe tomography and transmission electron microscopy data revealed that the alloy undergoes a constant nucleation condition, and that the added solute content suppresses the nucleation and growth behavior of both the primary and secondary crystalline phases.Finally, as strain annealing has been shown to improve the desired magnetic properties in these alloys, but the mechanisms are not yet understood, a method for applying digital image correlation techniques to tensile testing in an in situ tensile testing environment is described. This will establish precedent for applying these analyses to in situ tensile testing of sputter-deposited magnetic alloys, and eventually to in situ thermomechanical testing.Item Deformation and phase stability behavior in transition metal carbides(University of Alabama Libraries, 2018) Smith, Chase; Thompson, Gregory B.; University of Alabama TuscaloosaExtreme environment applications require materials that have melting temperatures in excess of 3000 °C and that can retain good mechanical properties at high temperatures. Transition metal carbides (TMCs) are excellent candidates due to their high melting temperatures, high hardness, and low chemical reactivity. These materials display a duality of mechanical responses dependent on structure and temperature. This research aims at understanding the phase stability and deformation behavior of TMCs at compositions and temperatures that have received little investigation. A series of HfxTa1-xC compositions were computationally predicted, fabricated, and verified by experimentally identifying their phase formation, hardness, and dislocation behavior. Hardness values obtained via nanoindentation verified computational trends which predicted a modest rise in the Hf-rich ternary compositions. The presence of small amounts of Ta in Hf-rich ternary compositions yielded a change in slip system from the reported <110>{110} in HfC to <110>{111} commonly observed in TaC. To gain insight into the deformation and slip behavior of TaC and HfC at ultra-high temperatures, a thermo-mechanical testing apparatus was built for deforming specimens between 2100 °C to 2900 °C. Samples were resistively heated in the presence of a magnetic field to produce a non-contact Lorentz force. Greater deflection was observed for HfC up to 2300 °C attributed to differences in grain size. TaC deflection increased with rising temperature whereas HfC deflection decreased. This unexpected observation was discovered to be an artifact of plastic deformation that occurred during the preload. Mass transport and diffusional creep were found dominant with a preference for <110>{110} slip behavior observed for both carbides. To understand phase stability in the Nb-C system, a series of NbCx compositions were fabricated to span between the single-phase NbC and single-phase Nb2C with several compositions residing in multi-phase regions of the phase diagrams. Equiaxed grains formed for all compositions with those between ~ 0.56 to ~ 0.63 C/Nb exhibiting a lath–like microstructure as well. Additionally, a diffusion couple was processed near the same conditions to establish the phase transformations that lead to the observed microstructures. Carbon was observed to deplete from NbC and react with the Nb metal to form β-Nb2C.Item The Density and Molecular Phase Field Methods(University of Alabama Libraries, 2022) Jacobson, David Wallace; Thompson, Gregory B.; University of Alabama TuscaloosaDensity phase field (DPF) methods have arisen as a means of more closely linking free energy functionals with grain boundary physics as opposed to using functionals that are purely phenomenological. In their current form, DPF methods exhibit a number of theoretical and computational problems that limit their applicability. These issues include the following:(1) being unable to simulate moving grain boundaries, (2) low computational performance due to high order gradient energy terms, and (3) failing to predict stable bulk equilibriums. We solve the mobility and performance issues mentioned above by coupling the density field of DPF simulations with traditional order parameters. The stable equilibrium problem is solved through the development of a criteria list that can be used to determine the set of DPF free energy functionals that correctly predict bulk equilibrium states. A subset of the free energy functionals that meet aforementioned criteria were identified and studied further because of their direct connection with atomistic physics. Termed the Molecular Phase Field (MoPF) method, these free energy functionals are constructed from interatomic potentials. Grain boundaries simulated using the MoPF method are material specific and their calculated excess energies are a natural consequence of the interatomic potential parameters used as model inputs. Lennard Jones, M-N, Mie, and Morse potential parameters have been calculated for over 30 different transition metals such that MoPF simulations of these metals may be carried out. Finally, MoPF models allow for the thermodynamic description of specific grain boundaries to be incorporated within phase field models such that the large variation in properties over the grain boundary phase space might be more accurately represented in phase field simulations.Item Effect of nickel content on the crystallization behavior in nanocrystallin (Co_(1-x)Ni_x)_88Zr_7B_4Cu1 soft magnetic alloys(University of Alabama Libraries, 2012) Hornbuckle, Billy Chad; Thompson, Gregory B.; University of Alabama TuscaloosaA series of (Co_(1-x)Ni_x)_88Zr_7B_4Cu1 soft magnetic alloys, where X was varied from 0 to 1, were fabricated by a melt spinning process into thin ribbons of the material. This process was followed by an isothermal anneal to produce a nanocomposite alloy, i.e. nanocrystalline grains in a residual amorphous matrix. The alloy series was designed to investigate crystallization kinetics and limits to the compositional regime where a nanocomposite could be formed. The primary and secondary crystallization temperatures of each alloy were determined using Differential Scanning Calorimetry (DSC) from which the crystallization activation energies were calculated using the Kissinger Method. When X exceeded 0.75, the as-spun ribbons exhibited partial crystallization, resulting in reduced exothermic crystallization peaks. For lower Ni contents, the ribbons were amorphous in the as-spun state. The activation energy for crystallization decreased with increasing Ni content. Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) revealed fine nanocrystallite and boron segregation to the grain boundaries with increasing Ni content. The previously suspected use of Cu clustering, which can act as heterogeneous nucleation sites, showed no clear correlation with observed spatial location of the crystallites. Chemical partitioning between species in the as-spun and primary crystallization heat treatments were correlated to the resulting changes in magnetic properties. As Ni content increased, the saturation magnetization and normalized magnetization for these samples decreased accordingly.Item The Effect of Solutes on Interface Structure and Microstructural Stability in Binary, Nanocrystalline Alloys(University of Alabama Libraries, 2022) Priedeman, Jonathan; Thompson, Gregory B.; University of Alabama TuscaloosaStabilizing a nanocrystalline material against grain growth is necessary to preserve the nanostructure and associated strength while experiencing conditions experienced during manufacturing and/or service. This work examines the structures and stability provided by solutes in different alloys. First, a platinum-gold alloy thin film is examined during in-situ annealing where observations of a faceted grain boundary structure are made. Atomistic simulations reveal that segregation of gold to that grain boundary stabilize such a structure. Second, a bulk copper-niobium alloy is plastically deformed at elevated temperature. The nanostructure of the alloy is preserved, indicating good stability. The strength of the niobium-doped alloy, however, is more sensitive to temperature than a related copper-tantalum alloy. This sensitivity is found to be a result of the oxidation behavior of the solutes: theoretical calculations of the misfit strengthening provided by these oxide precipitates reveal greater temperature sensitivity than either the niobium or tantalum precipitates. Third, a bulk copper-hafnium alloy is also plastically deformed at elevated temperature, and the nanostructure is also retained. Here the hafnium was processed as a conformal coating over the copper powder prior to consolidation rather than use of an elemental powder-powder mixing. The copper-hafnium alloy has good low-temperature strength, but begins deforming by grain boundary mediated mechanisms at much lower temperatures compared to copper-tantalum and copper-niobium. Again, the precipitates contribute to this behavior, as the hafnium oxidizes to form a monoclinic hafnium dioxide. This oxide provides good resistance to dislocation propagation but does not reinforce grain boundaries due to lack of coherency. Thus, in all three alloys studied, the solute behavior as it seeks to achieve local equilibrium has profound effects on observed structural evolution (and mechanical response).Item Effects of Environmental Factors on Functional Properties of Particulate Matter(University of Alabama Libraries, 2022) Ranjit, Smriti; Hauser, Adam J.; University of Alabama TuscaloosaThe properties of the material are determined by its structure and the functional motif, understanding the structure of a material can explain its behavior under certain conditions. The effects of environmental factors on the functional properties of thin films of particulate matter due to fabrication method or exposure to the environment are investigated. Understanding the mechanism behind material defects/degradation provides insight to optimize the functional properties of the film, suitability for the application, or robustness for a potential application. This dissertation explores the effect on the functional properties of the material due to aerosol deposition method, interaction with chemical warfare agents, and environmental exposure. Barium Hexaferrite (BaM) films deposited on a-plane sapphire substrate by aerosol deposition are investigated in a subtractive wedge series to determine the extent of energetic substrate damage and indentation. The Al2O3 particulates ejected from the substrate surface during growth and the estimate of indentation depth is ~600 nm. The magnetic moment of the deposited film is lower than the bulk and thickness dependence is consistent with the fractional increase of Al2O3 content in the film.Fe2O3 nanoparticles exhibit chemical changes due to contact with the four chemical warfare agents simulants. Due to the exposure, a redox reaction occurs and Fe2O3 nanoparticles show lowered magnetic moment. The differentiable, frequency-dependent responses to the simulants are observed due to changes in the sensor material that opens the possibilities for the use of Fe2O3 nanoparticles for frequency-dependent impedance fingerprinting. The effects of UV and visible light exposure in dry air and humid environments have been investigated using the Zr-based metal-organic framework, UiO-66-NH2. The metal-organic framework undergoes irreversible photochemical change due to prolonged UV light and blue light exposure. These changes happen more rapidly and grow larger in total cumulative magnitude as the atmospheric humidity increases. However, humidity introduced in dark or with lower energy photons than blue light results in no material change to the MOF. Impedance data modeling suggests that humidity increases the ionic conductivity of the material and that the degradation occurs at grain boundaries, to a depth that increases with humidity. Importantly, the result of the degradation is the loss of chemical sensitivity, defining the conditions for applications in both aqueous and airborne filtration and sensing applications.Item Enhancing Mechanical Properties and Design of Metallic Glasses Through Nanoscale Heterogeneities and Machine Learning Optimization(University of Alabama Libraries, 2023) Gu, Yucong; Daniewicz, Steven R.; Li, LinMetallic glasses (MGs) are a type of metal alloy that has a disordered atomic structure similar to glass. They possess unique properties such as high strength, elasticity, and corrosion resistance. However, their susceptibility to catastrophic failure through shear banding has limited their widespread use. Recently, the local ordering in MGs has been identified within the amorphous structure, which can influence the physical and mechanical properties of these materials. This dissertation investigates the effect of local ordering and nanoscale heterogeneities on shear band behaviors and optimizes MG designs accordingly. Firstly, a mesoscale shear transformation zone (STZ) dynamic model is employed to investigate the deformation behaviors of MGs. The presence of nanoscale heterogeneity results in a Hall-Petch-like relationship between yield stress and spatial correlation length of heterogeneity. Secondly, dynamic mechanical responses of MGs are studied via experiments and simulations on thin film MGs with various nanoscale heterogeneities. The strain rate sensitivity transition is attributed to a shift in deformation mechanisms from structure-dictated strain localization to stress-dictated strain percolation into a shear band. Finally, a data-driven design framework is developed using artificial neural networks (ANN) and a genetic algorithm (GA) to optimize dual-phase MG designs. The ANN models are trained using simulation data from the STZ dynamic model, and the GA is used to guide the development of new dual-phase MGs with improved mechanical properties. Additionally, the data-driven design framework is extended to optimize material performance under both mechanical and electrochemical processes, minimizing material degradation under the simultaneous action of wear and corrosion based on hierarchical ANN models trained on multiphysics simulations. This dissertation integrates machine learning with multiscale and multiphysics models to uncover the mechanisms that explain the emergence of mechanical behaviors of MGs and explores a massive design space for property optimization. These findings provide valuable insights into the material structure-property relationships, enabling informed decision-making for material design.Item Fabrication and characterization of graded magnetocrystalline anisotropy iron-nickel-platinum alloy thin films(University of Alabama Libraries, 2011) Fu, Bianzhu; Thompson, Gregory B.; University of Alabama TuscaloosaThe further increase of magnetic storage density is limited by superparamagnetism: the size of the magnetic grains has reached a scale (several nanometers) at which thermal fluctuations can erase the stored information. Attempts to increase the thermal stability by making the grains `magnetically harder' have failed because they have also made them impossible to write with available write fields. One approach to overcome this perplexing dilemma is to grade the uniaxial magnetocrystalline anisotropy, Ku, such that one end is `magnetically soft' to switch while the other end is `magnetically hard' to anchor the switch from intrinsic thermal stability issues. This change in Ku can be accomplished by changing the composition of the magnetic material along the magnetic easy-axis direction. However, there has been a lack of experimental studies on the fabrication of the gradients in the [001] orientation in FePt based structures coupled with isolated magnetic pillars to verify domain wall switching advantages in gradients. In this dissertation, a highly ordered [001] oriented FexNi0.48-xPt0.52 (0Item Fabrication and properties of nanoscale multiferroic heterostructures for application in Magnetoelectric Random Access Memory (MeRAM) devices(University of Alabama Libraries, 2012) Kim, Gunwoo; Gupta, Arunava; University of Alabama TuscaloosaMagnetoelectric random access memory (MERAM) has emerged as a promising new class of non-volatile solid-state memory device. It offers nondestructive reading along with low power consumption during the write operation. A common implementation of MERAM involves use of multiferroic tunneling junctions (MFTJs), which besides offering non-volatility are both electrically and magnetically tunable. Fundamentally, a MFTJ consists of a heterostructure of an ultrathin multiferroic or ferroelectric material as the active tunneling barrier sandwiched between ferromagnetic electrodes. Thereby, the MFTJ exhibits both tunnel electroresistance (TER) and tunnel magnetoresistance (TMR) effects with application of an electric and magnetic field, respectively. In this thesis work, we have developed two-dimensional (2D) thin-film multiferroic heterostructure METJ prototypes consisting of ultrathin ferroelectric BaTiO3 (BTO) layer and a conducting ferromagnetic La0.67Sr0.33MnO3 (LSMO) electrode. The heteroepitaxial films are grown using the pulsed laser deposition (PLD) technique. This oxide heterostructure offers the opportunity to study the nano-scale details of the tunnel electroresistance (TER) effect using scanning probe microscopy techniques. We performed the measurements using the MFP-3D (Asylum Research) scanning probe microscope. The ultrathin BTO films (1.2 - 2.0 nm) grown on LSMO electrodes display both ferro- and piezo-electric properties and exhibit large tunnel resistance effect. We have explored the growth and properties of one-dimensional (1D) heterostructures, referred to as multiferoric nanowire (NW) heterostructures. The ferromagnetic/ferroelectric composite heterostructures are grown as sheath layers using PLD on lattice-matched template NWs, e.g. MgO, that are deposited by chemical vapor deposition utilizing the vapor-liquid-solid (VLS) mechanism. The one-dimensional geometry can substantially overcome the clamping effect of the substrate present in two-dimensional structures because of the reduced volume of the template. This leads to minimum constraint of displacements at the interface and thereby significantly enhances the magnetoelectric (ME) effect. We characterized the nanostructures using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results of our studies utilizing multiferroic 2-D thin films and 1-D NW architectures clearly demonstrate the potential of these heterostructures for future device applications, such as in MERAM, data storage, magneto-electric field sensors, etcItem Fabrication and reliability testing of copper-filled through-silicon vias for three-dimensional chip stacking applications(University of Alabama Libraries, 2010) Kamto Tegueu, Alphonse Marie; Burkett, Susan L.; University of Alabama TuscaloosaThrough-silicon vias (TSVs) have been extensively studied because of their ability to achieve chip stacking for enhanced system performance. The fabrication process is becoming somewhat mature. However, reliability issues need to be addressed in order for an eventual transition from laboratory to production. This dissertation discusses the TSV fabrication process, testing results for TSV reliability investigation of an integration of TSVs and capacitor devices. In our laboratory, vias with tapered sidewalls are formed through a modified Bosch process using deep reactive ion etching (DRIE). Cryogenic etching is also considered as a means to etch vias without sidewall scalloping that is observed for the Bosch process. Vias are lined with silicon dioxide using plasma enhanced chemical vapor deposition (PECVD) followed by a sputter deposited titanium barrier and a copper seed layer before filling by a reverse pulse copper electroplating process. Following attachment of the process wafer to a carrier wafer, the process wafer is thinned from the backside by a combination of mechanical methods and reactive ion etching (RIE). Fabricated vias are subjected to thermal cycling with temperatures ranging from 25 °C to 125 °C. TSVs are shown to be stable with small increases in measured resistance for 200 cycles. In addition, small changes in resistance are observed when vias are held at elevated temperatures for extended periods of time. Integration of decoupling capacitors with TSVs represents a good alternative to conventional 2-D layouts to achieve miniaturization and increased density. Therefore, decoupling capacitors can be brought in close proximity to the active elements, thereby, reducing their parasitic inductance and allowing higher clock rates. In this study, capacitors with anodized tantalum as the dielectric are integrated with TSVs without negatively impacting their operation. The performance of these capacitors was evaluated by measuring resonant frequency, parasitic inductance, and parasitic resistance.Item The FePt L10 Phase Transformation in Thin Films using Multiple Laser Pulsing(2010-03-04) Inaba, Yuki; Thompson, Gregory B.; Harrell, J. W.; Klemmer, Tim; Kubota, Yukiko; University of Alabama TuscaloosaA series of ≈12 nm thick FePt thin films deposited onto glass substrates have been annealed with multiple 1064 nm wavelength laser pulses. The fluence was varied using pulse widths of 10.0, 5.0, and 2.5 ms. The peak temperature for each individual pulse was kept near 700 °C. The A1 to L10 phase transformation was confirmed by x-ray diffraction. A single pulse was not sufficient to obtain a fully ordered state. A maximum order parameter of 0.89 and coercivity of 10.6 kOe was obtained after 5×10 ms pulses. This particular annealed film showed the greatest amount of grain growth with a mean size of 55.1 nm. This grain size is 20% smaller than that of a furnace annealed sample which was annealed for 60 s and yielded an approximately equivalent order parameter. Similar order parameters, grain sizes, and coercivity values were observed for films that had equivalent total annealing times regardless of pulse widths.Item Formation Mechanism and Composition Distribution of FePt Nanoparticles(2007-11-27) Srivastava, Chandan; Balasubramanian, Jayendra; Turner, C. Heath; Wiest, John M.; Bagaria, Hitesh G.; Thompson, Gregory B.; University of Alabama TuscaloosaSelf-assembled FePt nanoparticle arrays are candidate structures for ultrahigh density magnetic storage media. One of the factors limiting their application to this technology is particle-to-particle compositional variation. This variation will affect the A1 to L10 transformation as well as the magnetic properties of the nanoparticles. In the present study, an analysis is provided for the formation mechanism of these nanoparticles when synthesized by the superhydride reduction method. Additionally, a comparison is provided of the composition distributions of nanoparticles synthesized by the thermal decomposition of Fe(CO)5 and the reduction of FeCl2 by superhydride. The latter process produced a much narrower composition distribution. A thermodynamic analysis of the mechanism is described in terms of free energy perturbation Monte Carlo simulations.Item Grain Boundary Specific Segregation in Nanocrystalline Fe(Cr)(2016-10-06) Zhou, Xuyang; Yu, Xiao-xiang; Kaub, Tyler; Martens, Richard L.; Thompson, Gregory B.; University of Alabama TuscaloosaA cross-correlative precession electron diffraction – atom probe tomography investigation of Cr segregation in a Fe(Cr) nanocrystalline alloy was undertaken. Solute segregation was found to be dependent on grain boundary type. The results of which were compared to a hybrid Molecular Dynamics and Monte Carlo simulation that predicted the segregation for special character, low angle, and high angle grain boundaries, as well as the angle of inclination of the grain boundary. It was found that the highest segregation concentration was for the high angle grain boundaries and is explained in terms of clustering driven by the onset of phase separation. For special character boundaries, the highest Gibbsain interfacial excess was predicted at the incoherent ∑3 followed by ∑9 and ∑11 boundaries with negligible segregation to the twin and ∑5 boundaries. In addition, the low angle grain boundaries predicted negligible segregation. All of these trends matched well with the experiment. This solute-boundary segregation dependency for the special character grain boundaries is explained in terms of excess volume and the energetic distribution of the solute in the boundary.