Heusler Compounds

**Chapter 4**

**Abstract**

Applications

Magnetic Full-Heusler

*Kei Hayashi, Hezhang Li, Mao Eguchi,*

*Yoshimi Nagashima and Yuzuru Miyazaki*

and order degree of full-Heusler compounds is elaborated.

thermoelectric properties, order degree

**1. Introduction**

electric current.

**65**

figure-of-merit, *zT*, expressed as:

Compounds for Thermoelectric

Full-Heusler compounds exhibit a variety of magnetic properties such as non-magnetism, ferromagnetism, ferrimagnetism and anti-ferromagnetism. In recent years, they have attracted significant attention as potential thermoelectric (TE) materials that convert thermal energy directly into electricity. This chapter reviews the theoretical and experimental studies on the TE properties of magnetic full-Heusler compounds. In Section 1, a brief outline of TE power generation is described. Section 2 introduces the crystal structures and magnetic properties of full-Heusler compounds. The TE properties of full-Heusler compounds are

presented in Sections 3 and 4. The relationship between magnetism, TE properties

**Keywords:** full-Heusler compounds, half-metal, spin-gapless semiconductor,

Thermoelectric (TE) power generation using TE devices is one of the key technologies to solve global energy problem, owing to its availability of direct conversion of thermal energy into electricity [1–3]. A schematic figure of a TE device is shown in **Figure 1**. It consists of n- and p-type TE materials connected in series electrically with metal electrodes and arranged thermally in parallel. The TE materials are wedged between ceramic plates. When one side of the device is heated and the other side is cooled, electrons and holes in the n- and p-type TE materials, respectively, diffuse from the hot side to the cold side, thus generating a flow of

To commercialise TE devices, there is a need to improve their TE efficiency. The

where *T*<sup>H</sup> and *T*<sup>C</sup> are the heating and cooling temperature, respectively. The dimensionless figure-of-merit, *zT*, is determined by TE properties (*S*: Seebeck

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi <sup>1</sup> <sup>þ</sup> *zT* <sup>p</sup> � <sup>1</sup> ffiffiffiffiffiffiffiffiffiffiffiffiffiffi <sup>1</sup> <sup>þ</sup> *zT* <sup>p</sup> <sup>þ</sup> *<sup>T</sup>*C*=T*<sup>H</sup>

, (1)

maximum TE efficiency, *η*max, is an increasing function of the dimensionless

*<sup>η</sup>*max <sup>¼</sup> *<sup>T</sup>*<sup>H</sup> � *<sup>T</sup>*<sup>C</sup> *T*<sup>H</sup>
