Effects of nonstoichiometry on thermoelectric properties of CoSi-based materials

A series of Si-rich and Si-deficient materials with nominal composition CoSix (x = 1.05, 1.00, 0.95, 0.90) have been synthesized by arc melting and hot-pressing sintering. The effects of nonstoichiometry on the phase composition and thermoelectric properties were systematically investigated. All the materials are composed of main phase CoSi, while trace impurity phase Co2Si or CoSi2 was detected in the nonstoichiometric materials. Transport measurements revealed that the Seebeck coefficient was remarkably decreased and the electrical conductivity was increased due to the significant enhancement in carrier concentration, and the lattice thermal conductivity was almost unchanged. As a result, a serious reduction in ZT was found in the nonstoichiometric materials because of the deteriorated power factor. This work demonstrates that a slight deviation from stoichiometric composition in CoSi-based material would cause a seriously detrimental influence on thermoelectric performance due to the dramatical decrease in Seebeck coefficient.