The silicide Mo5Si3, with the W5Si3 structure, exhibits a high anisotropy of its coefficients of thermal expansion (CTEs) in the a and c directions, namely, CTE(c)/CTE(a)=2.2. In order to determine whether the CTE anisotropy can be controlled, molybdenum was partially substituted with Nb. CTEs were determined by high temperature x-ray powder diffraction. Partial substitution with 44 at. % Nb reduced the value of CTE(c)/CTE(a) to a value near 1. For higher Nb concentrations, CTE(c)/CTE(a) increased again. When nearly all the Mo was replaced by Nb, the crystal structure changed to the Cr5B3 structure type and the CTE anisotropy decreased to a value near 1. Our thermal expansion results are interpreted in terms of the site occupation of Nb, the Nb-induced increase in the interatomic spacing of the Mo atom chains along the c-direction, and the reduction in the anisotropy of the lattice anharmonicity in the a and c directions.
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This work was sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences; and the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory. ORNL is operated by UT-Battelle, LLC, for the U.S. DOE under contract DE-AC05-00OR22725.
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Schneibel, J.H., Rawn, C.J. & Fu, C.L. Tailoring the thermal expansion anisotropy of Mo5Si3. MRS Online Proceedings Library 646, 401–406 (2000). https://doi.org/10.1557/PROC-646-N5.40.1