Mechanical Properties of Metallic Perovskite Mn₃Cu _0.5Ge_0.5N:High-Stiffness Isotropic Negative Thermal Expansion Material

The basic mechanical properties of antiperovskite manganese nitride, Mn₃Cu_1-xGe_xN, known as an isotropic negative thermal expansion (NTE), have been evaluated. The mother material, Mn ₃CuN, was a soft and ductile intermetallic compound with a positive linear thermal expansion coefficient (α=+20 × 10^-6 K ^-1) at room temperature. The thermal expansion properties of Mn ₃CuN were dramatically changed by Ge doping, resulting in an extremely large NTE material (α=-12.5 × 10^-6 K ^-1) by 50% substitution of Cu with Ge. Associate with this, the mechanical properties were found to be strongly modified by Ge doping, and both the stiffness and the hardness of Mn₃Cu_1-xGe_xN increased with increasing the Ge content, x. The NTE material, Mn ₃Cu_0.5Ge_0.5N, had an enhanced hardness of 570-620 H_v and an exceptionally large Young's modulus of 211-320 GPa. The giant, isotropic NTE compatible with the enhanced value of Young's modulus enables us to fabricate extremely stiff, stable, and lightweight zero thermal expansion composites for the use in many fields of industries.