Thermoelectric materials, which directly convert thermal energy into electricity, have become quite popular recently due to the development and world spreading of alternative power energy sources. Nowadays powder metallurgy methods are widely used for production of almost all types of thermoelectric (TE) materials, from low-temperature bismuth-telluride-based materials to high-temperature (operating above 1000 K) Cu2−xSe alloys. Historically, crystallization methods were widely used in production [1], for example, Czochralski method, Bridgman method and zone melting method are the most common. However, in case of crystallized materials, we face one disadvantage that is weak mechanical properties. In TE devices working bodies in the devices are bulk TE materials with dimensions about millimeters, so they can be exposed to different shifts or tensile stresses. Therefore, powder metallurgy methods were used to improve mechanical strength, in particular, various sintering methods. Later, it was found that sintering from powders allows to reduce one significant parameter for thermoelectrics – thermal conductivity, and in the 2000s the propagation of powder technologies started over.
Traditionally there were two stages of powder method production: first, synthesis and crystallization [1], and then, grounding and sintering by hot vacuum pressing, which is the simplest and the most cost-effective from all powder metallurgy technologies used. Then, with the invention of powerful ball mills, mechanoactivation was applied. It allows to eliminate the process of crystallization synthesis, and yet, due to the complexity of TE materials, the complete formation of solid solutions without any initial phases was not always possible to achieve [2]. Extrusion is one of the promising technologies. The samples prepared by this method are very dense and solid. The quality factor of the material (which will be described below) is very high too, but the structure seems rather isotropic, which is typical of many powder technologies, so the N-type conductivity material, which is sensitive to anisotropy, turned out to be markedly worse than P-type. However, with the usage of some improved technologies, for example, equal channel angular extrusion, it is possible to achieve certain texturing and high properties of n-type material [3]. The Spark Plasma Sintering and the melt spinning as the two most promising powder technologies for preparing thermoelectric materials will be described in detail below.
This article appeared in the July/Aug issue of Metal Powder Report.
