The effects of melt spinning process on microstructure and thermoelectric properties of Zn-doped type-I clathrates

Abstract

The single-phase Zn-doped type-I Ba8Ga12Zn2Ge32 clathrates have been prepared by a novel melt spinning (MS) technique combined with spark plasma sintering (SPS) method, and the effects of melt spinning process on their microstructure and thermoelectric transport properties have been investigated. The results show that the free surface of ribbon obtained by MS consists of cubic grains with the size of 0.3—1.0μm, and after SPS a highly dense bulk material with lots of fine-layered structure has been obtained. Compared with the bulk Ba8Ga12Zn2Ge32 material prepared by melting method combined with SPS (MM+SPS sample), the bulk Ba8Ga12Zn2Ge32 material obtained by melting method combined with MS and SPS (MM+MS+SPS sample) has higher carrier concentration and lower carrier mobility at room temperature. Furthermore, the MS process leads to a slight decrease of electrical conductivity, somewhat increase of Seebeck coefficient, as a well as a remarkable decrease of lattice thermal conductivity for bulk Ba8Ga12Zn2Ge32 clathrates in the testing temperature range of 300—900K. At 900K, the latticethermal conductivities of MM+SPS sample and MM+MS+SPS sample are 1.06 and 0.42W/mK respectively. As a result, the maximum dimensionless figure of merit ZT of 0.90 is obtained at 900K for the MM+MS+SPS sample. Compared with that of MM+SPS sample, it increases by 75% at the same temperature.