The goal of this Faculty Early Career Development (CAREER) project is to establish a comprehensive research and education program focusing on the processing and mechanical properties of nanostructured refractory ceramics in order to extend the basic and practical knowledge of sintering as well as mechanical behavior in these types of materials, while at the same time producing materials that have a direct application in ultra-high temperature environments. Graduate students, undergraduate students, high school teachers, and high school students will be incorporated into research and educational experiences possible with a project such as this one. Building on the principal investigator's strengths in the synthesis and characterization of ceramic materials, the specific objectives of the project are: (1) to synthesize nanostructured carbide powders using a solvothermal synthesis approach; (2) to characterize the process of solvothermal synthesis over a range of experimental conditions (i.e., reaction time, reaction temperature, and cleaning solvents) in order to optimize and scale-up the process; (3) to densify the nanostructured powders into compacts via spark-plasma sintering; (4) to characterize the process of densification over a range of experimental conditions (i.e., starting crystallite size of the powders, starting agglomerate size of the powders, pressure, temperature, heating rate, and time of densification); (5) to examine the microstructural development during the densification process; (6) to characterize the mechanical behavior of the densified compacts at room and high temperatures and to determine the changes in mechanical behavior with changes in SPS processing conditions; (7) to characterize the thermal shock resistance of the compacts, since in their ultimate expected application the materials will be undergoing cycles of very rapid heating and cooling; and (8) to enlist graduate students, undergraduate students, high school teachers, and high school students in the process of meeting the above research objectives.
Complementary to the research goals described above, a multi-objective education plan is outlined. The plan contains the following four objectives: (1) to involve graduate and undergraduate students from the USA and Mexico's National Polytechnic Instittue in research experiences and collaborative activities; (2) to involve undergraduate students of Hispanic descent from Southwestern College in research experiences and collaborative activities; (3) to continue outreach activities to northern Nevada high school students, and (4) to involve Hispanic high school students and teachers from Hug High School (Reno) in summer research experiences.
The goal of this project was to establish a comprehensive research and education program focusing on the low-cost manufacturing of carbide ceramics for aerospace applications. Our materials can potentially be used as coatings for turbine engines and as thermal protection barriers in space re-entry vehicles. Thus, commercial interest in these materials is very high. The project achieved all its goals and was particularly successful in (1) manufacturing nanopowders of tantalum carbide (TaC) and hafnium carbide (HfC), both of which have the highest melting temperatures of any known materials, (2) scaling the manufacturing process, and (3) consolidating the powders to obtain dense samples. These samples were used for characterization of mechanical behavior of the materials. At a fundamental level, we developed an understanding and model of the effect of powder crystallite size and particle size on the consolidation behavior of carbide nanopowders, which is of great importance in many different industrial applications in which nanostructured components are desired. We also found that the density of our materials does not change significantly with consolidation temperature, but the ratio of open and closed porosity varies, undergoing a transition at a critical temperature, a major discovery of our work. We link this transition to carbon volatility. Consolidation below this critical temperature, and with adequate carbon additions, can result in densities as high as 98% of theoretical, the highest values reported in the literature without the use of extreme pressures or sintering additives. TaC-ZrC and TaC-WC composites were also consolidated to evaluate the effect of densification and grain growth on these important high-temperature composites. Results from this study are useful for the design and selection of materials for industrial applications in which high temperature resistance is a requirement. This project prepared one post-doctoral researcher and eight graduate students for contributions in the field of materials manufacturing, an area of great importance for job development in the US. One MS thesis ("A New Approach for the Synthesis of Tungsten Nanopowders") and one PhD thesis ("The Scaled-Up Synthesis of Nanostructured Ultra-High-Temperature Ceramics and Resistance Sintering of Tantalum Carbide Nanopowders") were direct products of this project. Also, ten undergraduate students (five of them Hispanic), were exposed and contributed to a project that expanded their understanding of science and engineering. In addition, this project promoted a strong international collaboration between Mexican investigators at the Instituto Politecnico Nacional, the University of Nevada, Reno, and Alfred University.
