Thermoelectric properties

Electrical conduction and heat transport are closely linked, the connection being described by three thermoelectric coefficients, the Seebeck coefficient (or thermopower), the Peltier coefficient and the Thomson coefficient, all of which have relevance to thermoelectric power generation and refrigeration. In perovskites, the most reported values are for the Seebeck coefficient. The magnitude and sign (+ or -) of the Seebeck coefficient are related to the concentration and type of mobile charge carriers present. For band-like perovskites, the magnitude of the Seebeck coefficient is proportional to the density of states, either in the conduction band, for electron transport, or the valence band for hole transport. 

For perovskites that can be treated as ionic with charge carriers jumping from one cation to another, the Seebeck coefficient can be discussed in terms of the defect chemistry of the phase. An approximate relationship between the number of defects and the Seebeck coefficient is frequently given by the simple relationship  

This equation is formally equivalent to the Heikes equation  

This type of behaviour is shown by many cobaltites. At lower temperatures, where complexities due to magnetic transitions between high, low or intermediate spin states can be ignored, the Seebeck coefficient is well described by the 

An analogous situation occurs for appropriate B-site substitution. For example, the B-site substitution of Tb for Co in the parent LaCoOj structure forces the transformation of two Co cations to Co to maintain charge balance in the substituted perovskite LaTi COj Oj. Each Co cation can be considered to be a Co cation ion plus a trapped electron Co. Electronic conductivity can then be considered to occur by the migration of electrons from one Co ion to a neighbouring Co ion  

A compilation of data for the Seebeck coefficient a in (iV/K is given in the following table (r.t. = room temperature)  

The temperature dependence of a up to 1400 K was measured in vacuum relative to Pt for nearly stoichiometric 1 2863 with M = Sc, Gd, Ho, Er, Yb, and Lu. With the exception of Yb2Se3, all compounds studied show anomalies which are connected with intrinsic defects. A curve a vs. 1/T is given for Gd2Se3 [13]. A linear increase in a negative sense for PrSeg.gse up to 1073 K (above 1073 K a levels off) and for Ce and Nd sesquiselenides is found by [12]. Nonlinear a vs. T curves up to 1273 K for M = La, Gd, Er, and Yb are given in [7]. 

The variation of a is dependent on the electron concentration to the power for Ce3 xSe4, Pr3 xSe4, and Nd3 xSe4, as expected from the nearly free electron model [12]. 

Phase Coefficient of thermo-emf (abs. values), V/deg Accu- racy ( ). fiV/deg Ref. Year Remarks 

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Antimony is also used as a dopant in n-ty e semiconductors. It is a common additive in dopants for siHcon crystals with impurities, to alter the electrical conductivity. Interesting semiconductor properties have been reported for cadmium antimonide [12050-27-0] CdSb, and zinc antimonide [12039-35-9] ZnSb. The latter has good thermoelectric properties. Antimony with a purity as low as 99.9+% is an important alloying ingredient in the bismuth teUuride [1304-82-17, Bi Te, class of alloys which are used for thermoelectric cooling. 

Bismuth trisulfide has been used as a high temperature lubricant and has been of interest for its photo- and thermoelectric properties. 

Katsuyama S, Tanaka Y, Hashimoto H, Majima K, Nagai H (1997) Effect of substitution of La by alkaline earth metal on the thermoelectric properties and the phase stability of y-La3S4.1 Appl Phys 82 5513-5519... 

Assoud A, Soherinia N, Kleinke H (2007) Thermoelectric properties of the new teUurides SrSc2Tc4and BaSc2Te4 in comparison to BaY2Tc4. IntermetaUics 15 371-376 Wu P, Ibers JA (1995) Quaternary chalcogenides containing a rare earth and an alkali- or alkaMne-earth metal. J Alloy Compd 229 206-215... 

The work in this group has focussed mainly in antimony and bismuth because of the thermoelectric properties of the chalcogenides186 and as low temperature single-source precursors to related semiconductor materials.187 The use of bismuth compounds in the treatment of gastrointestinal disorders has lead to the study of several thiolate compounds as models to understand the bioactivity. 

In the field of nonmetallic catalysts, particularly of oxides, Hauffe and co-workers (14a) used only semiconductors for which information concerning electronic and ion defects was available from measurements of electrical conductivity, thermoelectric properties, and Hall effect. These workers obtain a quantitative correlation between the reaction rate, the amount of chemisorption, and the number of electron defects of the catalysts. Since every catalyzed reaction is initiated by a chemisorption process involving one or several of the reacting gases, and because the nature of this chemisorption process determines the subsequent steps of the reaction, it seems appropriate to begin with a discussion of the mechanism of chemisorption. 

Important scientific and industrial applications for thulium and its compounds remain to be developed. In particular, the photoelectric, semiconductor, and thermoelectric properties of the element and compounds, particularly behavior in the near-infrared region of the spectrum, are being studied. Thulium has been used in phosphors, ferrite bubble devices, and catalysis. Irradiated thulium (169Tm) is used in a portable x-ray unit. 

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... 

LaFe4Sbn is a poor metal or heavily doped semiconductor with good thermoelectric properties above room temperature (700-1000 K) (Sales et al., 1996, 1997). Only polycrystalline samples have been investigated. The room temperature resistivity is about 0.5 m 2cm de-... 

Ce[ 64Asi2 is probably a narrow gap semiconductor, but little low temperature data are available for this compound. The resistivity of a polycrystalline sample indicates a small gap on the order of 0.01 eV (Grandjean et al., 1984). The high temperature thermoelectric properties of this compound were investigated by Watcharapasorn et al. (2002). They found semimetallic behavior with a room temperature resistivity of 0.49 m 2 cm, a Seebeck coefficient of 40 pV/K, and a thermal conductivity of 3.8 W/mK. The maximum value for ZT, the thermoelectric figure of merit, was estimated to be 0.4 at 850 K. 

The lead chapter reviews the remarkable physical behaviors of a distinctive family of inter-metallic compounds - the filled lanthanide skutterudites. These unique compounds, which have the RM4X12 stoichiometry (where R = lanthanides, M = Fe, Ru, and Os, and X = I1, As and Sb), are best known for their excellent high temperature (> 700 K) thermoelectric properties. But as Brian Sales points out, they also exhibit a rich variety of electronic and... 

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). 

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