Synthesis and high temperature thermoelectric transport properties of Si-based type-I clathrates

N-type Si-based type-I clathrates with different Ga content were synthesized by combining the solid-state reaction method, melting method and spark plasma sintering (SPS) method. The effects of Ga composition on high temperature thermoelectric transport properties were investigated. The results show that at room temperature, the carrier concentration decreases, while the carrier mobility increases slightly with increasing Ga content. The Seebeck coefficient increases with increasing Ga content. Among all the samples, Ba$_{7.93}$Ga$_{17.13}$Si$_{28.72 }$exhibits higher Seebeck coefficient than the others and reaches $-135~\mu $V$\cdot$K$^{ - 1}$ at 1000 K. The sample prepared by this method exhibits very high electrical conductivity, and reaches 1.95$\times $10$^{5 }$~S$\cdot$m$^{ - 1}$ for Ba$_{8.01}$Ga$_{16.61}$Si$_{28.93}$ at room temperature. The thermal conductivity of all samples is almost temperature independent in the temperature range of 300--1000~K, indicating the behaviour of a typical metal. The maximum {ZT} value of 0.75 is obtained at 1000~K for the compound Ba$_{7.93}$Ga$_{17.13}$Si$_{28.72}$.