Thermoelectrics borides

T. Mori, Boride Thermoelectrics High temperature thermoelectric materials in Modules, Systems and Applications , ed. D. M. Rowe, CRC Press, London, 2012, pp. 14-1-14-18. 

Room temperature resistivities of YB and GdB66 take values of 3 x 102 and 5 x 102 12 cm, respectively (Golikova, 1987) and the RB66 phases can be considered as insulators. The characteristic temperature of the VRH To for example for GdB66 was determined as 4 x 107 K. The conductivity is p-type. The thermal conductivity and thermopower of RB66, will be discussed in Section 12 on "Thermoelectrics in higher borides". 

Doping has been investigated extensively for compounds like /3-boron and boron carbide to try to modify their thermoelectric properties (e.g., Werheit et al., 1981 Slack et al., 1987 Aselage and Emin, 2003). There have not been as many attempts to dope the rare earth higher borides, we are only aware of transition metal doping into YB66 (Tanaka et al., 2000,2006 Mori and Tanaka, 2006). 

However, carbon doping was found to decrease the thermal conductivity of YB66 while not having a sizable detrimental effect on the other properties, and this could be a powerful method for improving the thermoelectric properties of higher borides in general (Mori and Tanaka, 2006). 

From Eq. (11), an obviously desirable characteristic for thermoelectric materials is to have low thermal conductivity k. The thermal diffusivity constant, Dt, of ErB44Si2 has been found to have small values of Dt < 1.1 x 10 2 cm 2/s (Mori, 2006c). These values are significantly smaller than what has been observed for boron carbide samples (Wood et ah, 1985). Although no data exists for the sound velocities of ErB44Si2, the velocities are probably high since borides are typically hard materials. Therefore, the small values of Dt indicate extremely short phonon... 

Although the presently obtained absolute values of power factors still need to be improved, these compounds may offer a n-type boride counterpart to the well known p-type boron carbide which is one of the few thermoelectric material viable for extremely high temperature use (1500 K), and further research on the R-B-C(N) compounds is merited. 

Interesting problems remain to be solved. Although the figure of merit is still quite low due to the poor density, can the homologous R-B-C(N) borides be den-sified and/or doped to become a viable n-type counterpart to boron carbide which is an exemplar p-type high temperature thermoelectric compound ... 

The authors have reported the synthesis and thermoelectric properties of arc-melted silicon borides [14]. As-melted silicon borides in the boron content range of 80 to 94mol% consisted... 

This chapter briefly reviews what is known of the rare earth borides, carbides, and nitrides. This review wonld particularly like to showcase interesting features of the crystal structure and intriguing physical properties ranging ft-om the fundamental to the very applicable, with an emphasis on recent emerging results in important functionalities such as magnetism, thermoelectricity, and superconductivity. 

Bellosi, A., Guicciardi, S., Medri, V., Monteverde, F., Sciti, D., Silvestroni, L. (2011). Processing and Properties of Ultra-Refractory Composites Based onZr- and Hf-Borides State of the Art and Perspectives. In Orlovskaya, N., Lugovy, M. (Eds.), Boron Rich Solids. Sensors, Ultra High Temperature Ceramics, Thermoelectrics, Armor Series NATO Science for Peace and Security Series B Physics and Biophysics (pp. 147-160). Springer. 

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