This seed project seeks to discover new materials exhibiting desirable combinations of low, isotropic coefficients of thermal expansion (CTE) with improved thermal stability, and compatibility with other engineering ceramics. These materials are critical to enable the next generation of durable thermo-structural ceramics by managing thermal stresses arising from spatial variations in local thermal expansion induced by thermal transients, thermal gradients, and bi-material interfaces. The desired CTE behavior is exhibited by crystal structures comprising frameworks of rigid polyhedra, e.g., SiO4, PO4, and WO4 tetrahedra or ReO6 and ZrO6 octahedra, that rotate in a coordinated manner upon thermal excitation. These rigid unit mode (RUM) rotations lead to minimal change in the unit cell volume even as individual bonds expand. However, many of these materials decompose, melt, or undergo polymorph phase transitions at intermediate temperatures that limit their utility in extreme environments. The approach applies first principles calculations and the Grüneisen theory of thermal expansion to understand the structural and RUM contributions to the anisotropy in known low-CTE materials. These insights are then applied to identify compounds based on thermally-stable oxides (e.g., aluminates and tantalates) that are likely to exhibit low-CTE with minimal anisotropy. Parallel experimental efforts will assess the phase stability as a function of composition, temperature, and annealing time to identify sources of metastability and the associated equilibrium structures. The temperature-dependent axial thermal expansion coefficients will then be measured by high temperature XRD.
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