Monday, February 17, 2014

Intro to composite and nanocomposite thermoelectrics

One of the approaches to enhance the performance of thermoelectric (TE) materials (i. e.ZT), is reducing the thermal conductivity by scattering the phonons or in other words, decreasing the lattice thermal conductivity.
Addition of nano-inclusions to increase interfaces has shown a significant impact on lattice thermal conductivity reduction despite its simultaneous effect on electrical conductivity . Alternatively, composite thermoelectric materials containing several phases may enhance the performance of thermoelectric materials by conserving a relatively high electrical conductivity while introducing more interfaces leading to a decrease in the thermal conductivity.
The words, composite and nanocomposite, in the thermoelectric related literature are used with different meanings. In some reports, nanocomposite refers to adding nano-inclusions to the bulk thermoelectric materials, in some others, it pertains to mixing two or more different particle sizes ranging from nano to micro-scale of the same material, or mixing two different TE phases. In general, one can call a material, TE nanocomposites when it consists of nano-inclusions, nano-particles randomly dispersed, nano-grains, and secondary phases or precipitates. More recently several approaches have been developed to produce TE nanocomposites mainly based on the idea of the phonons scattering by introducing more interfaces in the material, such as utilizing phase decomposition to create embedded nanostructures. Several types of nanao-inclusions such as semimetals, semiconductors, metals, ceramics, and even carbon nanotubes were also used in thermoelectric materials to produce TE nanocomposites.
In TE materials with anisotropic properties, fabrication method is also an important issue. Specially, in case of powder metallurgy approach that nanostructures in the powders need to be conserved during the consolidation process. We have used hot-extrusion and microwave sintering for bismuth telluride based materials to obtain the bulk nanostructured TE materials. However, hot extrusion has shown more benefits in case of texturing.