The composite material system of Iron (III) Oxide (Fe2O3) and Silica (SiO2) was explored due to the unique magnetic properties of Iron (III) Oxide along with the optical properties of Silica. Through a highly controllable and tunable chemical synthesis method utilizing reverse-micelle/sol-gel processes, phase and morphology of Iron (III) Oxide nanoparticles was controlled through variation of calcination temperature from 800ºC - 1100ºC. Four polymorphs of Iron (III) Oxide were observed. Furthermore, core-shell structures comprised of iron oxide cores and silica shells were created with relatively high initial iron oxide concentrations, and densified via Current Activated Pressure Assisted Densification (CAPAD). The densified core-shell were highly transparent despite mismatches in index of refraction of iron oxide and silica, and high initial concentrations of iron oxide.

Optical ceramics have attracted increasing interest because of their superior mechanical toughness, better homogeneity, and potential for scaling and microstructural design compared to state-of-the-art single crystals. Optical transparency in ceramics were traditionally only achievable using optically isotropic materials because of the birefringent scattering loss in optically anisotropic materials. However, commonly used optically isotropic materials have limited thermal conductivities that fundamentally restrain heat extraction in high power applications. Excessive temperature gradient caused by low thermal conductivity affects the light transmitting properties of these materials and can eventually lead to thermomechanical failure. One solution for this challenge is to use optically anisotropic materials such as aluminum oxide (alumina, Al2O3) and aluminum nitride (AlN) that have better thermal conductivity and superior mechanical toughness. By reducing the grain size of these optically anisotropic materials, scattering loss caused by birefringence can be minimized, thus recover the material transparency.Here, transparent nanocrystalline thulium doped Al2O3 ceramic is produced and characterized, with the highest in-line transparency among rare earth doped Al2O3 ceramics reported. Nanocrystalline AlN powder is synthesized and densified using current-activated, pressure-assisted densification (CAPAD). The resulting AlN nanocrystalline ceramic is the finest-grained fully dense AlN ceramic reported and showed significant transparency improvement compared to transparent AlN ceramics reported. Finally, thulium dopant incorporation into lab synthesized nanocrystalline AlN is studied. These results show the potential of nanocrystalline microstructure in enabling the use of optically anisotropic materials in transparent ceramic applications. The use of nanocrystalline Al2O3 and AlN ceramics may enable thermally and mechanically more robust light transmitting/emitting devices under challenging conditions. The significant improvement in transparency of nanocrystalline AlN ceramic by reducing its grain size to sub-micrometer range also broadens the material selection range for optical ceramics.

The thermal conductivity of liquids is an integral part of the thermal design for future clean energy sources that can provide higher temperature heat and higher thermodynamic efficiencies. Molten salts and molten metals are leading candidates for heat-transfer fluids in next-generation concentrated solar power and nuclear plants, which can provide both clean electricity and industrial heat. However, the thermal conductivity of liquids at high temperature is not well understood since there is no well-established model for liquid thermal conductivity and because errors from convection, radiation, and corrosion have created a large spread in experimental data at high temperatures. In Chapter 1, I review the various steady-state, time-domain, and frequency-domain experimental techniques used to measure liquid thermal conductivity at high temperature, as well as the various modified-gas and quasi-crystalline models of liquid thermal conductivity – rather than providing exhaustive lists of all previous methods, I provide frameworks for understanding the diverse approaches for measuring and modeling liquid thermal conductivity. In Chapter 2, I formulate a new phonon gas model for liquid thermal conductivity, which can accurately calculate the thermal conductivity of dense, strongly-interacting liquids like water and molten nitrate salts. The model is able to match reliable experimental results, explain the mechanism of heat conduction in certain liquids, and encapsulate previous quasi-crystalline models that took slightly different forms depending on which liquids they were formulated for. In Chapter 3, I present a newly developed frequency-domain hot-wire measurement technique – designed specifically to measure the thermal conductivity of high temperature liquids and minimize errors from convection, radiation, and corrosion. Using frequency-domain measurements, I show that the thermal conductivity of molten nitrate salts is ~15% higher than the current reference value. In Chapter 4, I use frequency-domain measurements and various models to better understand ionic liquid thermal conductivity, which have elements of both molten salts and long molecular chained liquids. And in Chapter 5, I provide a brief outlook on future research directions for enhancing the effective thermal conductivity of high-temperature liquids for clean energy applications.

Thermoelectric materials are unique in their ability to directly convert thermal energy into electrical. Various applications include: energy scavenging, spot cooling and heating, and interstellar power generation. Current research is geared towards improving thermoelectric materials for greater energy conversion efficiency. Mg2Si has been a good candidate for study due to its low toxicity, low density, and availability of constituent elements. Recent efforts to improve efficiency of bulk thermoelectric materials have included, but are not limited to, changing the electronic band structure through doping or altering the microstructure. While the former has been well studied and verified, the latter remains relatively untouched experimentally. There is evidence that thermoelectric materials with low dimensionality show improvement in certain properties. Thin films and nanowires are known to have lower thermal conductivities than their bulk counterparts. The goal of this work is to provide evidence of microstructural effects on the thermoelectric properties of bulk Mg2Si.

The three relevant properties for thermoelectric materials are thermal conductivity, electrical conductivity, and the Seebeck coefficient. Because commercial measurement systems are costly and have limited measurement ranges and accuracies, the development of custom equipment is sometimes necessary. An electrical AC current source is developed and used to measure the electrical conductivity and, through the 3ω method, thermal conductivity of bulk samples. The measured values are comparable, with a 3% agreement, to a separate well-established system.

The bulk samples are consolidated from powder using the Current Activated Pressure Assisted Densification (CAPAD) process. It is known that different processing temperatures alter the amount of grain growth during the densification process. Samples processed at higher temperatures show increasing thermal and electrical conductivities while the Seebeck coefficient decreases. These trends lead to a maximum figure of merit value with the sample processed at an intermediate temperature.

Mg-Sn alloys have many uses due to their relative elemental abundance, low toxicity and low densities. Some applications include thermoelectric energy conversion, hydrogen storage, and enhancement of structural alloys. The thermoelectric application is of particular interest because Mg-Sn-Si based materials have shown promise in recent years.

It is well accepted that materials structure-property relationships are at the root of the improvement of a device’s performance. Due to the interdependence of the transport properties, research for efficient thermoelectric conversion has focused on synthesizing materials with novel microstructures and compositions. Metastable structures present a unique opportunity in this regard. The Mg-Sn material system has a relatively unknown metastable phase which remains largely unstudied.

My work represents one of the first studies of the metastable Mg-Sn phase in a polycrystalline bulk form. I present a new synthesis route using a combination of high energy ball milling (powder synthesis) and current activated pressure assisted densification (CAPAD) (powder densification). This method allowed for the synthesis of the metastable trigonal phase at 600 °C and 112 MPa, significantly lower pressures and temperatures than have been demonstrated previously. This method produces samples large enough for the first neutron diffraction study allowing for Rietveld structural refinements to a high degree of accuracy.

Through careful control of the synthesis process, I have also studied and characterized the densification and transformation kinetics for the trigonal phase. Analysis of the real time deformation data during CAPAD processing reveals that the transformation mechanisms can be isolated from the densification mechanisms. The transformation process is analyzed using the KJMA model modified for constant heating and shows an activation energy of 74 kJ/mol. Transformation under isothermal conditions follows a second order rate law with a higher activation energy (448 kJ/mol) caused by differing degrees of transformation completion.

The amount of metastable phase can be controlled by varying the CAPAD processing parameters. Incorporating the metastable phase into the microstructure changes the thermal, electrical and Seebeck coefficient behavior. The first measurements of this trigonal phase show bipolar diffusion behavior at much lower temperatures than the previously reported Mg-Sn based materials without the metastable trigonal phase.

Nanocrystalline oxides can have attractive properties such as high mechanical hardness, wear resistance, and high temperature plasticity. Fine (<500nm) grain and pore sizes are also beneficial for optical transparency and can help reduce dopant agglomeration beneficial for photoluminescence. However, most transparent polycrystalline oxides have large grains (>10μm) as a byproduct of their high-temperature fabrication conditions. Use of nanocrystalline powders as precursors and reactants seems an obvious choice for producing nanocrystalline oxides, but their use is complicated by high surface energies and reactivity with the environment. An improved understanding of the reaction and densification of nanocrystalline reactants is necessary to produce oxide ceramics of varying grain sizes, porosity ranges, and functionalities. Here, the reaction and densification kinetics of three important oxide ceramics, MgO, Al2O3¬, and Magnesium Aluminate Spinel (MgAl2O4) are studied. Investigations lead to novel method to produce polycrystalline MgAl2O4 via widely available metastable and nanocrystalline reactants using current-activated, pressure-assisted densification (CAPAD). First, a comprehensive processing and densification study was performed, exploring the effects of nanocrystallinity and moisture adsorption on nanocrystalline MgO - which may be used as an analogue for alternative hygroscopic oxide systems and is an integral part of synthesizing MgAl2O4 by reaction densification. Second, the effects of metastability and nanocrystallinity (MgO and Gamma-Al2O3, <30nm) on MgAl2O4 phase formation, densification, and transparency of CAPAD densified reactant powder was considered. Finally, transition metal doping of MgAl2O4 from reactant powder is explored. These results show metastable and nanocrystalline reactant powders combined with the flexibility of CAPAD allow for enhanced densification and reactivity. A wide array of microstructures, and new functionalities in polycrystalline MgAl2O4 are demonstrated. The metastable/nanocrystalline reactant powders facilitate comparably low temperatures for product formation and full density in MgAl2O4 – with up to 400°C reduction in the temperature for full phase formation and 300°C reduction for full density when compared to large-grained, stable reactant densification studies. The microstructural flexibility of this route is also highlighted with the production of both fully-dense transparent and robust nanoporous spinels, which conventionally arise from entirely different processing routes. Inclusion of transition metal dopants with reactant powders yields dense polycrystalline MgAl2O4, capable of tunability in the visible and previously unreported in published literature in polycrystalline form.

Electro-optic (EO) materials allow for the precise control of light using electrical signals, which has allowed for the advancement of an incredible array of photonic technologies such as laser systems and optical telecommunications. Most EO devices currently utilize single crystals, but high performance EO single crystals often have composition limitations since dopants can segregate and not all compositions can be grown using equilibrium restricted techniques. Bulk polycrystalline ceramic materials can potentially overcome such limitations and allow for the exploration of new EO systems. Due to the specific microstructures required for transparency, conventional processing techniques have difficulty in producing bulk polycrystalline EO ceramics. Reported here for the first time are the optical and EO properties of a new class of transparent lead free ceramic that outperforms EO materials in use today. This material is a barium titanate (BaTiO3) based solid solution, (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7 Ca0.3)TiO3 referred to here as BXT. The EO material was successfully processed using the Current Activated Pressure Assisted Densification (CAPAD) technique, commonly called Spark Plasma Sintering (SPS), which has been shown to be effective at consolidating optical materials. Using this technique along with a new powder synthesis method, it was possible to produce a transparent EO BXT ceramic with a highly dense and homogeneously reacted microstructure. Densified BXT shows a remarkable EO coefficient of 530 pm/V, which is superior not only to state of the art LiNbO3 crystals but also top-quality lead containing ferroelectric ceramics such as PLZT. This exceptional coefficient will allow for miniaturized EO systems with reduced operating voltages. The mechanisms behind the high EO performance in BXT were determined using additional EO and ferroelectric measurements. These measurements indicate that BXT undergoes a field induced structural evolution which heavily contributes to the EO effect. Along with efficient domain motion, this structural evolution includes a field induced phase change to a lower symmetry crystal structure. This work shows that lead free EO polycrystalline ceramics can have properties that are competitive or superior to state of the art EO materials. Due to its exceptional EO properties, BXT in particular has a promising future as an EO material.

Synthesis, processing and characterization of nanocrystalline aluminum nitride has been systematically studied. Non-carbon based gas nitridation was used to reduce nanocrystalline γ-alumina, having a grain size of ~80 nm. Single phase aluminum nitride powder was obtained at firing temperatures of 1200°C. Further processing of AlN powders was performed by CAPAD (Current Activated Pressure Assisted Densification) to obtain dense single phase aluminum nitride. Dense bulk aluminum nitride was obtained at temperatures of 1300°C. The reason we were able to obtain dense AlN at such temperatures is due to the nature of the nanocrystalline powders.

Current Activated Pressure Assisted Densification (CAPAD) has emerged as one of the most promising methods of materials processing in recent years. This process involves the use of large currents to generate heat coupled with applied pressure in what has proven to be a very powerful powder consolidation technique. Despite intensive research, there is still much to learn about the intricacies of how this process works. In the first part of this work, three material systems, yttria stabilized zirconia, Si, and Al, were processed using the CAPAD technique in order to see how processing conditions affect the densification rates in the CAPAD process. It was found that all materials show a peak densification rate at a particular homologous temperature, which is discussed in terms of material bond type. Pressure was found to directly affect the magnitude of the maximum densification rate in all samples. In the second part of this study, a model was developed to estimate the density-temperature relationship in the CAPAD process. This model is dependent on two parameters that are linked to physical quantities. Experimental data for oxide ceramic materials was used to demonstrate the fitting capability of the model. Oxides were chosen because of the abundance of experimental data available in the literature. Estimating the dominant parameter can be done by finding a material system's activation energy for diffusion and comparing it to other known materials. Guidelines were given on how future CAPAD workers can use the model to save time and resources.

The rare earth iron garnets (REIG) are a technologically important class of materials owing to their ferrimagnetism, low spin damping, large band gaps, high Curie points and chemical stability. Epitaxial REIG films are also capable of producing perpendicular magnetic anisotropy (PMA) which results from the interfacial strain due to lattice mismatch between film and substrate, and causes the easy axis of magnetization to point out-of-plane of the film. This unique set of properties makes REIGs attractive for spintronics device research in addition to fundamental spin transport studies. One obstacle that researchers face is that high-quality targets are not commercially available for many REIG compositions. Our work demonstrates a method of producing dense, fine structured REIG targets via wet chemical synthesis routes and subsequent consolidation of REIG powders using the current-activated, pressure-assisted densification technique. So far, PMA has been successfully tuned in REIG films by means of controlling the substrate material, substrate orientation, film thickness, and to a lesser extent by chemical substitution. Our method can easily produce chemically substituted REIG targets because we synthesize our own powders, which we demonstrated by the synthesis of a full compositional range of Y3(1-X)Tm3XFe5O12 solid solution powders. The lattice parameter and magnetic saturation of the Y3(1-X)Tm3XFe5O12 powders varied with composition, enabling further control over growth induced PMA. Most REIG films with PMA grown for research are produced using pulsed laser deposition (PLD), however, our work demonstrates that off-axis RF sputtering can also produce films with PMA. Sputtering is important for the large-scale manufacture of these films because it is more widely used in industry and has the added benefit of producing larger area films than PLD. Specifically, we grew epitaxial europium iron garnet (EuIG) films with robust PMA at a maximum thickness of 102 nm, the thickest EuIG films with PMA to date. Through the use of asymmetric reciprocal space mapping, we additionally showed that the lattice parameter and elastic properties of EuIG films grown by sputtering or PLD can deviate substantially from the literature values, which is likely explained through a combination of stoichiometric deviation and atomic point defects.