Phase Transformation and Processing of Poly crystalline Gadolinium Selenide, GdSei. 1.49

Polymorphic phase transformation in a thermoelectric material, GdSe 1.49 , from cubic to orthorhombic symmetry at temperatures from 800° to 1000°C causes a 1% volume expansion, which generates microcracks. Sintered polycrystalline cubic GdSe 1.49 preforms with at least 97% densities were vacuum annealed at 900°C for 300 h to fully convert to the orthorhombic lattice, encapsulated in nickel containers, and then isostatically hot‐pressed. Hot‐pressing at or below 1000°C resulted in 96%‐dense orthorhombic GdSe 1.49 polycrystals containing large pores, whereas hot‐pressing above 1200°C produced fully dense cubic polycrystals with a duplex microstructure. The electrical resistivity of the orthorhombic GdSe 1.49 (hot‐pressed below 1000°C) was about 5 times that of the as‐sintered cubic GdSe 1.49 ; the increase appears to be related to the presence of the orthorhombic phase, micro‐cracks, large pores, and a Se‐rich grain‐boundary phase in the hot‐pressed orthorhombic GdSei 1.49 .