Effect of HPHT Processing on Structural and Thermoelectric Properties of Low-Cost Type-I Clathrate Ba8Cu6Si40

Si-based clathrate thermoelectric (TE) materials composed of low-cost, nontoxic, lightweight, and earth-abundant elements are typical representatives of cheaper thermoelectric materials. However, synthesis is difficult, and the relatively low ZT values are the barriers hindering the further development of the silicon clathrates. So, it is necessary to explore an alternative synthetic method and improve the ZT values simultaneously. In this work, Si-based clathrate Ba8Cu6Si40 samples have been synthesized by a simple, rapid, and feasible HPHT method with 25 min. We have investigated the changes of the TE properties and microstructures depending on the pressure in detail. The power factor has been increased due to the decreased carrier concentration, meanwhile, the thermal conductivity has been cut down on account of the strengthened full-spectrum-phonons scattering resulting from the multiple and multiscale microstructures by high-pressure processing. Ultimately, a relatively good ZT value 0.31 is achieved at 773 K for the sample synthesized at 4 GPa. To the best of our knowledge, it is higher than all of the results of pure Si-based Ba8Cu6Si40 compounds formed by other methods.