Powder Processing Effects on the Properties of Ultra-High Temperature Solid Solution Carbides as Fuel Surrogates for Nuclear Thermal Propulsion

Ultra-high temperature ceramics (UHTCs) are a promising class of materials for extreme environments including hypersonics, fusion energy, and nuclear thermal propulsion (NTP) applications. Refractory carbides (a subset of UHTCs) including zirconium carbide (ZrC), niobium carbide (NbC), and tantalum carbide (TaC) in solid solution with uranium carbide (UC) are a promising fuel candidate for NTP due to potential high melting temperature, hot hydrogen compatibility, and favorable neutronics. In this dissertation, ZrC, NbC, and TaC were fabricated by spark plasma sintering (SPS) into high density (> 90 % relative density) compacts with varying levels of microstructural homogeneity. Powder processes were assessed to achieve homogeneous solid solution. Impacts of microstructural homogeneity on mechanical and thermophysical properties were determined for (Nb_0.5, Zr_0.5)C and (Nb_0.5, Ta_0.5)C. Detailed characterization was performed on the powder, consolidated, and fractured specimens. Mechanical properties include room temperature (RT) fracture toughness, hardness, flexural strength, compressive strength, and elastic properties. Thermophysical properties include specific heat capacity, thermal diffusivity, and coefficient of thermal expansion from RT to 1000 ^oC. Electrical resistivity was measured from RT to 500 ^oC. Thermal conductivity and its mechanisms were determined by the combination of the resulting specific heat capacity, thermal diffusivity, and electrical resistivity measurements. The same properties were measured for the surrogate tri-carbide NTP fuel form, (Zr_0.8, Nb_0.1, Ta_0.1)C, creating a novel database for NTP applications.