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

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