Ceramics, squared: J. Carlos Diez and colleagues discuss thermoelectric ceramics and the challenge of optimizing their properties

Abstract

Thermoelectric materials can directly convert a temperature gradient into an electric voltage thanks to the Seebeck effect. Since its discovery in 1823, this characteristic has allowed the use of metallic junctions to control temperatures. These thermocouples possess a low electrical resistivity and Seebeck coefficient with high thermal conductivity. The thermoelectric performances of these materials are evaluated by the dimensionless figure-of-merit (ZT = SsT/k, where S is the Seebeck coefficient, T is the absolute temperature, s is the electrical conductivity, and k is the thermal conductivity) which is very small, indicating that metallic materials possess poor thermoelectric behavior. The introduction of intermetallic semiconductor materials, with higher thermoelectric performances than their metallic counterparts, has resulted in their practical application in thermoelectric modules. These modules can be found as Peltier devices, when used for refrigeration purposes, or Seebeck devices when used for electrical energy generation. Current applications of this kind of material include the environmentally friendly recoveryof industrial and automobilewasteheat, and inthe production of electrical energy in radioisotope thermoelectric generators installed on spacecrafts, or in lighthouses located in isolated regions. Nevertheless, the application of semiconductor materials for energy generation has been limited due to their relatively low thermal stability under air, which can result in the release of heavy and/or toxic elements, and degradation and/or oxidation processes at high temperatures which can diminish their performances. Nowadays, research effort is being focused on ceramic materials with relatively high thermoelectric performances which can work at higher temperatures than the intermetallic ones. Some of these ceramics are based on Co-oxides, with a high Seebeck coefficient and low electrical resistivity and thermal conductivity. Moreover, they are mostly composed of abundant, less expensive and more environmental friendly elements than the intermetallic compounds. Their main drawback can be found in their relatively low (compared with the intermetallic materials) ZT values, meaning that raising the figure-or merit is the most pressing task to be performed, so that they may be used in practical power generation applications. One of these Co-oxides is [Bi0.87SrO2]2[CoO2]1.82, described as a misfit layer compound with monoclinic symmetry, composed – in turn – of two subsystems exhibiting incommensurate periodicities. The structure of the misfit layer crystal can be described as an alternation along the c-axis of distorted rock-salt-type slabs, formed from [BiO] and [SrO] layers (the first subsystem), and of [CoO2] layers (the second subsystem and the electrical conducting one) displaying a distorted CdI2-type structure [1]. In order to improve the performance of these layered materials texturing techniques can be applied.