We describe the phase stability and suppressed → phase transformation in SiC/SiC fiber-reinforced composites prepared by spark plasma sintering (SPS), resulting in samples of >96% relative density and uniform -SiC microstructure. These samples are compared to ones produced by a hot pressing (HP) technique that results in samples with ~80% conversion to α-SiC hexagonal phases (4H and 6H). The phase transformation in the samples manufactured by SPS is shown to be significantly retarded due to the influence of lower sintering temperature, shorter processing time, and higher pressure. In addition, these samples exhibited hardness values in the range of 22 to 26 GPa, which were higher than those produced by HP, due to the higher density of the SPS samples, as well as relevant differences in phase composition. It was demonstrated that the → phase transformation is independent of fiber content in the composites. This study is of special interest for the production of SiC/SiC composite structures for electronic and structural applications.

The use of electrically conductive mesoporous ceramic powders is a new way for enzyme immobilization aiming for biocatalysts. In this study, mesoporous indium oxide, zinc oxide and titanium dioxide were synthesized by a hydrothermal reaction method. Soft-template and hard-template approaches were utilized to enable mesoporous materials with varying pore sizes. Pore sizes were evaluated by the Brunauer–Emmett–Teller (BET) method and were in the range of 7 to 10 nm. Additionally, indium oxide, zinc oxide and titanium dioxide were doped with tin, gallium, and silver/niobium, respectively, to obtain increased values of electrical conductivity. Afterwards, stable suspensions of mesoporous powders with varying powder volume fractions and pH values were produced through magnetic stirring and ultrasonication and further evaluated by dynamic light scattering (DLS).

Meanwhile, we describe an analytical model predictive of the electrical conductivity of stable ceramic suspensions. Specific model systems in this study include fluids containing mesoporous powders of TiO2 doped with silver or niobium, ZnO doped with gallium, and In2O3 doped with tin. The electrical conductivities for all four suspensions were found to be lower in acidic solutions (i.e., lower pH) compared to those in basic ones (i.e., higher pH). The behavior of these ceramic suspensions can be explained by considering surface charge and suspension stability. We have also determined that the particle size of the powders has a more pronounced effect on the electrical conductivity of the suspensions compared to powder volume fraction. Particularly, for the same material, larger particle sized suspensions were found to have smaller electrical conductivity due to smaller cumulative surface area of the particles. A theoretical model to describe the electrical conductivity of diluted powder suspensions by incorporating the effects of powder volume fraction, particle size and suspension stability was proposed. This new model predicts the final electrical conductivity of the ceramic suspension by incorporating the effects of powder volume fraction, particle size and suspension stability. The calculations from our model present excellent correlation with the experimental results.

The following work provides new insight into the behavior of the alkaline-earth hexaborides CaB6, SrB6 and BaB6. Both binary and ternary compounds were produced for the first time by solution combustion synthesis, resulting in cubic particles averaging 500 nm in size. Detailed X-ray diffraction analysis revealed phase separation and formation of nano-domains in ternary (Ba-Ca)B6 and (Ba-Sr)B6 compounds. High- resolution transmission electron microscopy confirms the presence of these phases in the form of homogenous nano-regimes in individual equilibrium within the hexaboride particles. Thermal treatments up to 2000 K enhance the overall homogeneity of the samples and merge the phases into one, indicating that the as-synthesized state is unstable as the collection of nano-regimes results in increased microstrain and lattice imperfections. Analysis of the chemical reactions that occur during synthesis suggests that the decomposition of the metal precursors to metal oxides introduces variance into the formation process of mixed-cation hexaboride compounds, producing nano-regimes within the crystal lattice. Spark plasma sintering was shown to effectively produce high- density hexaboride samples from powders in as little as 45 minutes. The effect of pulsed direct current on metal atom diffusion in CaB6-SrB6 couples was investigated. It was shown that the diffusivity of Ca and Sr atoms across the interface was towards the positive electrode when subjected to a current flow of 2.2 kA at a temperature of 2007 K. No new phases were formed at the interface between CaB6 and SrB6 as hexaboride compounds readily form solid solutions. Energy-dispersive spectroscopy was used to map the concentration of Ca and Sr surrounding the interface, and diffusion profiles were fitted with error functions. The concentration curves display concentration-dependent diffusivity. Total dopant values (Q) were used to numerically compare the differences between Ca diffusion in SrB6 and Sr diffusion in CaB6 under the influence of current. The results provide evidence for the role of pulsed direct current on the diffusion of metal ions in hexaboride compounds.

Development of fine and sophisticated nanosized composite colloidal particles has been intensively studied due to their huge potential to solve a lot of problems in a variety of fields. To have higher degree of freedom in designing nano-architecture on colloidal particles and controlling their unique properties, a more detailed understanding of chemical and physical phenomena in colloidal suspension is required. This dissertation discusses new mathematical approaches to describe in detail the behaviors of silica nanoparticle deposition and gold coating on submicron polystyrene (PS) colloidal particles in terms of colloidal science including surface chemistry and surface physics. To consider realistic phenomena inside the colloidal suspension, the proposed models for these two topics are based on the non-linear relationship between surface charge density and surface potential which are regulated by free ions. Chapter 1 presents the numerical analysis for the deposition behavior of monodispersed silica nanoparticles onto PS particles by using modified pairwise DLVO (Derjaguin, Landau, Verwey, and Overbeek) interaction force at different pH. This model includes an improved nonlinear charge regulation model considering redistribution of ions. To vindicate the model, silanol-terminated silica nanoparticles and L-lysine covered silica nanoparticles were separately deposited on polyallylamine hydrochloride-covered polystyrene (PAH/PS) particles. The morphological analysis of the experimental results shows that this modified DLVO force well describe the deposition behavior of the silica nanoparticles onto the PAH/PS particles. Chapter 2 discusses the computational models to elucidate the gold coating behavior on the positively charged PAH/PS particles at different concentration of L-ascorbic acid in terms of the initial nucleation stage. This analysis focuses on the initial generation rate gradient of Au(I) complex ions and the Casimir-Lifshitz interaction force between particles under the potential gradient. To justify our models, a direct gold coating method was performed on the PAH/PS particles with a constant concentration of HAuCl4 and various concentrations of L-ascorbic acid. The morphological analysis of the variation of gold coating shows that the computational results properly describe the morphological difference in gold coating by the trend of nucleation of gold along with the distance away from the PAH/PS particles.

In order to improve the performance of solid oxide fuel cell (SOFC), both micro-scaled continuous linear pore (CLP) and nano-sized interparticle pore structures were created inside of the NiO-YSZ anode by thermal decomposition of unidirectional carbon fibers in green body during sintering under air atmosphere. A comprehensive analysis of the prepared sample was performed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) mapping, x-ray diffraction (XRD), and thermogravimetric analysis (TGA) to explain how the CLP structure forms during sintering. The CLP structure prepared by this low-cost sacrificial templating method is characterized by its controllable uniform size in micro scale and a tortuosity value of 1.003 (coefficient of variation, CV: 0.59). Further discussion proposes that the optimized combination of the micro-scaled CLP and the nano-sized interparticle pore structures in an anode would be an ideal candidate to maximize the performance of SOFC by maximizing active triple phase boundary sites, minimizing Knudsen diffusion, and improving gas transports.

This thesis mainly focused on investigating the suitable synthesis methods to get barium titanate (BaTiO3) and strontium titanate (SrTiO3) nanocubes without using any capping agents. Two synthesis methods (hydrothermal and molten-salt) were used to synthesize BaTiO3 and SrTiO3 nanoparticles. X-ray diffraction (XRD) was used to identify phases and determine the phase purity of synthesized BaTiO3 and SrTiO3 nanoparticles. The morphology and formation mechanisms were investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was verified that BaTiO3 nanocubes can be successfully synthesized by molten-salt method, while SrTiO3 nanocubes were obtained by hydrothermal method. The synthesized nanocubes for both materials have high crystallinity and high purity.The formation mechanism of BaTiO3 nanocubes was proposed due to the low surface energy of {100} crystal planes. Crystal planes with low surface energy are preferred in the crystal growth stage. The formation of SrTiO3 nanocubes was proposed due to the crystal growth of {100} crystal planes and face-to-face attachment. These mechanisms provided clear explanations for the formation of BaTiO3 and SrTiO3 nanocubes without using any capping agents, which can be applied to other metal oxide nanocubes. Future studies can be focused on the synthesis of other metal oxide nanocubes by using the formation mechanisms proposed in this thesis.

White light-emitting diode (WLED) has been widely used because of its excellent performance in solid-state lighting. Among all the white LEDs (WLEDs), the phosphor-converted WLED is the most popular one, and its performance depends on the characteristics of the phosphor to a great extent. For this reason, it is necessary to study new phosphors for their potential use in WLEDs. In this thesis, a new garnet CaEr2Mg2Si3O12 matrix for phosphors was prepared by a solution combustion synthesis. Parameters of the synthesis, such as fuel-to-oxidizer ratio, amounts of precursors, order of mixing, as well as combustion and calcination parameters were optimized during this process. The garnet structure was successfully obtained and confirmed by X-ray diffraction characterization technique.

We present a detailed study of the devitrification behavior of five Ni-based amorphous metal foils of different compositions, consolidated using spark plasma sintering (SPS) or pressureless sintering. Samples of approximately 5 mm thickness and 20 mm in diameter were prepared by stacking layers of amorphous foil on top of each other. The sintering temperatures were based on the crystallization temperatures (Tx) for each sample as determined from differential scanning calorimetry (DSC). Through detailed X-ray diffraction (XRD) analysis, full-width-half-maximum (FWHM) measurements and DSC analysis, we observed that the width of the 100% diffraction peak decreased with increasing temperature. The changes in FWHM with temperature correspond to similar exothermic crystallization reaction stages observed in DSC analysis. In addition, differences in crystallization kinetics were noticed for samples sintered by SPS versus samples sintered in ambient atmosphere using an external heat source (free sintering). Microstructure analysis of the samples by scanning electron microscopy shows the extent of sintering between the foil layers and bonding mechanisms with different sintering methods. Unique morphologies with positive effect on foil bonding were observed when consolidating foils using SPS.

Hydroxyapatite (HAp) is an essential biomedical material. HAp’s shaping is an important topic in the area of implantology and scaffolding. This project took a novel and unique approach in attempting to to control the morphology of materials without varying parameters such as pH levels, types of solvents, pressure and reaction time. Present work studied the feasibility of controlling the morphology of hydroxyapatite (HAp), with micrometer scale molding and casting. Emphasis was placed on HAp cylinders and cubes of micrometer size. First, cylindrically shaped cavities were patterned and etched with photolithography methods and reactive ion etching techniques on a Si substrate. Cavities were filled with slurry of deionized water, commercially purchased HAp powder, and Darvan 811 dispersant. Samples were successfully sintered at 1250°C. The obtained samples resembled the cylindrical cavities they were caste in. Scanning electron microscopy (SEM) was used to characterize the size and shape of sintered samples. This technique demonstrates great potential in forming large amounts of homogeneously shaped samples of HAp or similar ceramics for prospective biomedical applications

In the first part of this research, we developed a differential scanning calorimetry method for calculating the initial crystallinity, change of crystallinity and crystallinity percentage of amorphous metal alloys as a function of temperature. Using thermodynamic enthalpies of amorphous, crystalline and partially devitrified specimens, our methodology can determine crystallinity percentages as low as a few percent. This technique also eliminates the need for expensive in situ accessories, such as those required in electron microscopy. In the second part, we report the effect of crystallinity percentage on the compressive deformation response of partially devitrified in situ Fe-based bulk metallic glass matrix composites (SAM25-Dev), consisting of an amorphous matrix and evenly dispersed nanocrystals (BCC iron, carbides, and borides) of sizes smaller than 20 nm. The SAM25-Dev specimens were obtained by consolidating SAM25 powders using the spark plasma sintering technique. The crystallinity percentage of the sintered samples varied between 20% and 61% depending on the sintering temperature. Macroscopically, bulk samples tested under compression exhibited an ~8% elastic deformation, which is significantly higher than elastic strains of other Fe-based bulk metallic glass composites, and strengths of ~2 GPa at fracture. Indentation toughness of the samples was below 1.6 MPa•m0.5, which categorizes the material as a brittle composite. Compression tests on micro-pillars resulted in specimen failure from the propagation of one principle shear band. Here, yield strength was corrected for the effect of taper angle of the micro-pillars and values above 5 GPa were obtained, making the material an ultra-hard composite. Such a high strength is attributed to the extremely small Poisson’s ratio (< 0.1) and higher glass transition temperature (883 K) of this material Both the compressive strength and fracture toughness of the specimens were negligibly dependent on the crystallinity percentage, implying that the radii of the plastic zones at the crack tips are smaller than the distance between nanocrystals on the specimens. Moreover, we find that SAM25-Dev shows a very high sample size dependency of strength (above 200%), which indicates a significant defect sensitivity. As a result, the residual porosity despite being minimal, strongly deteriorates the strength of the materials.