(KCo)3<b>+</b> co-substitution and enhancement of negative thermal expansion in (KCo)<i>x</i>Sc2-<i>x</i>Mo3O12 ceramics

Negative thermal expansion (NTE) materials shrink in volume when heated, which show abnormal thermal expansion property. Sc2Mo3O12 is a typical NTE material, but its high synthesis cost, low density, and low coefficient of thermal expansion (CTE) need to be improved. In this work, a series of (KCo)xSc2-xMo3O12 ceramics has been prepared via solid-state reaction. Effects of (KCo)3+ doping on crystal structure, microstructure, and NTE behaviors of the (KCo)xSc2-xMo3O12 ceramics were studied. The results show that (KCo)xSc2-xMo3O12 experiences a crystal structure change with increasing (KCo)3+ substitution content. At x = 0.25, the (KCo)xSc2-xMo3O12 sample includes both hexagonal (KCo)xSc2-xMo3O12 and orthorhombic Sc2Mo3O12. A single-phase, hexagonal (KCo)xSc2-xMo3O12, was prepared for 0.25 &amp;lt; x ≤ 0.75. Meanwhile, the grain size of (KCo)xSc2-xMo3O12 ceramics increases gradually as x increases. Compared to Sc2Mo3O12 ceramics, hexagonal (KCo)xSc2-xMo3O12 ceramics present higher densification and better NTE performance. Thermal mechanical analyzer analysis indicates that the linear CTEs of hexagonal (KCo)xSc2-xMo3O12 ceramics decrease from −4.36 × 10−6 °C−1 to −7.87 × 10−6 °C−1 as x increases. High-temperature X-ray diffraction (XRD) results reveal that the hexagonal (KCo)xSc2-xMo3O12 samples exhibit anisotropic NTE. The linear CTEs of αa and αb (αa=αb) are −15.47 × 10−6 °C−1, while that of αc is 13.19 × 10−6 °C−1. The linear CTE of αl is −5.95 × 10−6 °C−1.