Application of the Transverse Thermoelectric Effects

Most thermoelectric applications make use of the longitudinal Seebeck and Peltier effects. However, it is possible to produce thermoelectric effects in which the electrical and thermal flows are perpendicular to one another. This has certain practical advantages; for example, it makes possible the realization of an infinite-staged cascade refrigerator using a single piece of material. The benefits of separating the directions of the electrical and thermal currents have been demonstrated convincingly at low temperatures using the transverse thermomagnetic effects, but at ordinary temperatures the transverse thermoelectric effects are of more interest. These effects are to be found in any material in which the Seebeck coefficient is anisotropic. However, for them to be of practical value, a large and strongly anisotropic Seebeck coefficient must be combined with a high ratio of electrical to thermal conductivity. There is no known homogeneous substance in which this combination of properties is to be found. On the other hand, it is possible to obtain the required parameters in a synthetic material consisting of two phases. The two components A and B should form a couple with a large conventional figure of merit Z (AB). The optimum value for the transverse figure of merit Z (I center dot) can approach Z (AB) only when the two components have very different electrical and thermal conductivities. Various workers have used semiconductor-metal or semiconductor-semimetal combinations in attempts to satisfy these requirements. It has also been proposed that the necessary combination of properties might be met by making one of the components porous. The work that has been done on synthetic transverse thermoelements will be reviewed, and the prospects for practical devices will be discussed.