Thermoelectric materials 203-4

After this, there was a long period of quiescence, broken by a new bout of innovation in the 1990s. Thermoelectric efficiency depends on physical parameters through a dimensionless of merit, ZT, where Z = S lKp. Here S is the Seebeck 

Another strategy reported by Sales links back to the superlattices discussed in Section 7.2.1.4. It was suggested by Mildred Dresselhaus s group at MIT (Hicks et al. 1993) that semiconductor quantum wells would have enhanced figures of merit compared with the same semiconductor in bulk form. PbTe quantum wells were confined by suitable intervening barrier layers. From the results, ZT values of 2 were estimated from single quantum wells. This piece of research shows the intimate links often found nowadays between apparently quite distinct functional features in materials. 

We showed above that wide-bandgap semiconductors are promising for the development of high temperature gas sensors such as FET, Schottky, and MOS-based devices aimed at operation in corrosive environments. However, there are fields of gas sensing where semiconductors with much smaller band gap and lower chemical stability can be applied. Thermoelectric gas sensors are applicable in this 

It should be noted, however, that in spite of the great number of thermoelectric materials, only SiGe and Si are applied in gas sensors (see Table 5.3). The typical configuration of thermoelectric gas sensors is shown in Fig. 5.16. The power factors of Si and SiGe are almost the same. Films of these materials can be deposited using the same technological methods. For example, they can be deposited 

Material Seebeck coefficient (pV/K) Detected gas Threshold sensitivity 

A sintered target of 1 at.% boron-doped Si Ge j (p-type) with the purity of 99.997% was used for the RF magnetron sputtering deposition of SiGe thin film (Reprinted with permission from Shin et al. (2006). Copyright 2006 Elsevier) 

As can be seen, II-VI semiconductor compounds can be used as sensing materials in all types of gas sensors, including chemiresistors, SAW, heterojunction based, and optical. They can be applied to surface functionalizing and composites forming as well. The application of II-VI saniconductor compounds in quantum dots-based gas sensors will be discussed in Chap. 5 (Vol. 2). Operating characteristics of several Il-VI-based gas sensors are shown in Figs. 5.18 and 5.19. 

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Table 1. Operating Modes and Temperatures of Thermoelectric Materials...

Further specific information and recent results of the electrodeposition of Bi and Sb binary and ternary systems as well as of other thermoelectric materials can be found in the review of Xiao et al. [232],... 

At present, the elements used in the formation of compounds by EC-ALE include the chalcogenides S, Se, and Te the pnictides As and Sb the group three metals Ga and In the group II metals Zn, Cd, and Hg as well as Cu, and Co. The range of compounds accessible by EC-ALE is not clear. The majority of work has been performed on II VI compounds (Table 1). The III-V compounds InAs and InSb have recently been formed, and the first deposits of a III-VI compound, InSe, have been made [151], In addition, Shannon et al. have begun studies of CoSb [152] with the intent of forming thermoelectric materials. 

C. Shannon, A. Gichuhi, P. A. Barnes, and M. J. Bozack, Electrosynthesis of thermoelectric materials by electrochemical atomic layer epitaxy a prelimnary investigation, Seattle, Washington, 1999, pp 282. 

The transition-metal monopnictides MPn with the MnP-type structure discussed above contain strong M-M and weak Pn-Pn bonds. Compounds richer in Pn can also be examined by XPS, such as the binary skutterudites MPn , (M = Co, Rh, Ir Pn = P, As, Sb), which contain strong Pn-Pn bonds but no M-M bonds [79,80], The cubic crystal structure consists of a network of comer-sharing M-centred octa-hedra, which are tilted to form nearly square Pnn rings creating large dodecahedral voids [81]. These voids can be filled with rare-earth atoms to form ternary variants REM Pnn (RE = rare earth M = Fe, Ru, Os Pn = P, As, Sb) (Fig. 26) [81,82], the antimonides being of interest as thermoelectric materials [83]. 

Figure 4.10 Direct thermoelectric generators will compete in efficiency with internal combustion engines when thermoelectric materials with figures of merit ZT of the order of 2 are available at temperatures above 900 K.

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