Scalable Multi-nanostructured Silicon for Room-Temperature Thermoelectrics

Thermoelectric conversion is capable of converting heat directly to electricity. However, actual implementations of thermoelectric devices are still limited with the leading bottleneck being the costs of materials and device integration, which calls for thermoelectric materials based on standard semiconductors with sufficient figure of merit (ZT) at room temperature. Bulk silicon crystal comes on the top of the list; however, the ZT even with nanostructuring has been limited due to its high thermal conductivity. Here, we have realized nanostructured silicon material with ZT larger than 0.3 at room temperature by a scalable process consisting of metal-assisted chemical etching and plasma-activated sintering. The material structure is highly complex being composed of randomly distributed nanograins, nanopores, and metal nanoprecipitates with hierarchical sizes, which significantly reduces thermal conductivity without appreciably sacrificing electrical conductivity. It is further identified by detailed experimental and theoretical investigations that the key contribution to the reduction comes from the softening of grain boundaries significantly limiting the interfacial phonon transmission. The developed high-performance silicon nanocomposite is expected to greatly enhance the application of thermoelectrics by lowering the material and process costs.