Thermoelectric wire

Figure 2.6 A textile-based heat flux sensor comprising a thermoelectric wire woven into a textile substrate.

The basic ideas of thermoelectricity have been known for nearly two centuries, but until well after the Second World War the primary use was for temperature measurement (qv) using metallic wires. Then, upon improvements in semiconductor technology, thermoelectric power generation and refrigeration came under serious consideration. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

Kohlrausch s theory leaves quite unexplained the fact that no thermoelectric current is set up in a homogeneous wire along which a current of heat is flowing, whilst the theory of Lord Kelvin is difficult to reconcile with the fact that thermoelectric currents cannot be set up in a circuit of liquid metals, although these show the Thomson effect. The latter seems, therefore, to be to a certain extent independent of the Peltier effect. Theories intended to escape these difficulties have been proposed by Planck (1889), and Duhem, in which the conception of the entropy of electricity is introduced. 

Calvet and Persoz (29) have discussed at length the question of the sensitivity of the Calvet calorimeter in terms of the number of thermocouples used, the cross section and the length of the wires, and the thermoelectric power of the couples. On the basis of this analysis, the micro-calorimetric elements are designed to operate near maximum sensitivity. The present-day version of a Tian-Calvet microcalorimetric element, which has been presented in Fig. 2, contains approximately 500 chromel-to-constantan thermocouples. The microcalorimeter, now commercially available, in which two of these elements are placed (Fig. 3) may be used from room temperature up to 200°C. 

Figure 2 Configuration of reservoirs of energy and electrons used in the Onsager treatment of thermoelectric effects in wire A.

The determination of the heat flow relies on the so-called Seebeck effect. An electric potential, known as thermoelectric force and represented by E, is observed when two wires of different metals are joined at both ends and these junctions are subjected to dilferent temperatures, 7j and T2 (figure 9.1a). Several thermocouples can be associated, forming a thermopile (figure 9.1b). For small temperature differences, the thermoelectric force generated by the thermopile is proportional to 7j - T2 and to the number of thermocouples of the pile (>/) ... 

In order to protect the thermocouple against chemical or mechanical damage, it is normally enclosed in a sheath of mineral packing or within a thermowell (Fig. 6.24). Any material which contains the junction should be a good conductor of heat on the one hand, but an electrical insulator on the other. A potentiometric converter is frequently employed to convert the thermocouple signal to the standard 4-20 mA current range prior to further processing and control room presentation. The extension wires which connect the thermocouple element to the control room should have similar thermoelectric properties to those of the thermocouple junction wires. 

Extension wires having same thermoelectric properties as thermocouple elements... 

M. S. Dresselhaus, Y.-M. Lin, T. Koga, S. B. Cronin, O. Rabin, M. R. Black, and G. Dresselhaus, Quantum Wells and Quantum Wires for Potential Thermoelectric Applications D. A. Broido and T. L. Reinecke, Thermoelectric Transport in Quantum Well and Quantum Wire Superlattices G. D. Mahan, Thermionic Refrigeration... 

Figure 15 shows the calculated DOS for electrons in a 40-nm bismuth nanowire compared to that of bulk bismuth. The DOS in nanowires is a superposition of one-dimensional transport channels, each located at a quantized subband energy Snm- We note that the DOS in nanowires has sharp peaks at the subband edges, whereas that in a bulk material is a smooth monotonic function of energy. The enhanced DOS at the subband edges of nano wires has important implications for many applications, such as in optics (Black et al, 2000) and thermoelectrics (Hicks and Dresselhaus, 1993). 

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