Thermopower compounds

The interest of physicists in the conducting polymers, their properties and applications, has been focused on dry materials 93-94 Most of the discussions center on the conductivity of the polymers and the nature of the carriers. The current knowledge is not clear because the conducting polymers exhibit a number of metallic properties, i.e., temperature-independent behavior of a linear relation between thermopower and temperature, and a free carrier absorption typical of a metal. Nevertheless, the conductivity of these specimens is quite low (about 1 S cm"1), and increases when the temperature rises, as in semiconductors. However, polymers are not semiconductors because in inorganic semiconductors, the dopant substitutes for the host atomic sites. In conducting polymers, the dopants are not substitutional, they are part of a nonstoichiometric compound, the composition of which changes from zero up to 40-50% in... 

The electrical conductivity of CoOP as a function of temperature is shown in Figure 6. Above room temperature the compound exhibits metallic behaviour but coincidental with the development of the superstructure the conductivity falls rapidly with decreasing temperature. Below 250 K CoOp behaves as a semiconductor with an activation energy of meV.74 The conduction has been shown to be frequency dependent below 250 K.75 Thermopower studies have also clearly demonstrated the changeover from metallic behaviour above 300 K. to semiconductor behaviour below 250 K.72 The behaviour of ZnOP is very similar to that of CoOp, with the phase transition from the Cccm to Pccn space group occurring at 278 K. Superstructure formation is complete by about 260 K.77... 

The pressure-dependent electrical resistivity of the heavy-fermion compound YbNi2B2C (see also Section 4.12) could be explained by competing contributions from crystal-electric-field splitting and Kondo effect (Oomi et al., 2006). The pressure-dependent room-temperature thermoelectric power of YNi2B2C exhibits a peak around 2 GPa, which was explained by changes in the Fermi-surface topology (Meenakshi et al., 1998). A possible correlation with a small peak in the temperature-dependent thermopower around 200 K (Fisher et al., 1995 Section 3.4.3) needs further investigation. 

Dc, ac, impedance, and thermoelectric power of the compounds 33-38 in Fig. 9 have been investigated in detail. The measured temperature dependence of the thermoelectric power of 33-38 in thin film varied approximately exponentially with temperature. Compared to 38, the absolute value of the thermopower for the film of 34 is larger by nearly a factor of 3. The positive sign of Seebeck coefficient confirms that thin films of the compounds behave as a p-type semiconductor [46],... 

The electrical resistivity and thermopower of metallic calcium, strontium, and barium have been measured from room temperature to near their melting points. From discontinuities observed in these parameters as functions of temperature, the f.c.c.-b.c.c. phase transition was determined in calcium at 428 2 °C and in strontium at 542 2 C, both at ambient pressure. Four compounds have been identified in the Ca-Ni system by means of X-ray methods. The intermetallic... 

Rare earth intermetallic (RI) compounds have been the subject of many recent experimental investigations because of the nature and variety of their physical properties (Buschow, 1977, 1979 Kirchmayr and Poldy, 1979). In this review article we concentrate on one aspect of such investigations, namely the transport properties of these interesting compounds. We describe in particular experimental data for the resistivity, thermopower and the thermal conductivity. 

The temperature dependence of thermopower for several non-magnetic RI compounds is shown in fig. 8 (LaAy, fig. 9 (YAI2, LuAl ), and fig. 10 (YCu2, LuCu2). 

Experimental data presented in section 3.1.4 for the thermopower of magnetic RI compounds also suggest that magnon drag effects may indeed influence the thermopower quite strongly. For example, a pronounced peak is found in GdAl2 at 80 K whereas no such anomaly occurs in the thermopower of the isostructural YAI2. This is discussed further below. 

S" and S denote the diffusion thermopower of a non-magnetic RI compound and a magnetic RI compound, respectively. So, Sph, and Sspd are the contributions to the thermopower due to impurity scattering, phonon scattering, and spin-disorder scattering, respectively, p, po, Pph, and Pspd represent the total resistivity, and residual resistivity, the phonon resistivity, and the spin-disorder resistivity, respectively. 

The first two terms in eq. (43) are replaced by SJ" of eq. (42). This is equivalent now to the assumption that to the first approximation the impurity and the phonon contribution of corresponding magnetic and non-magnetic compounds are equal. These considerations lead to the following expression for the spin-disorder contribution for the thermopower of GdX (X = AI2, Cua, Ni)... 

The analysis of the thermopower of other magnetic R-compounds (RAI2, RCU2) using eqs. (42)-(44) shows that Sjpd is a linear function of the temperature and the slope behaves in a systematic manner as a function of the rare earth. Another analysis which uses the method of Foiles (1980, 1981) is in progress. The results will be published elsewhere. 

The thermopower S as a function of temperature for RAI2 compounds is shown in figs. 43-45 (Mikovits, 1981). The shape of the S vs. T curves for these... 

PrAl2 and NdAli are ferromagnetic at low temperatures, their Curie temperature being 33 K and 73 K, respectively. Fig. 43 shows that the thermopower of both compounds jumps almost discontinuously at the Curie temperature. The... 

All four order ferromagnetically at low enough temperatures. Fig. 44 shows that there is no longer a sharp jump in the thermopower obtained for the light RI compounds in this case. Instead, the following behaviour occurs ... 

These three compounds are again ferromagnetically ordered at low temperatures. However, no anomalous behaviour is seen in the thermopower of these three compounds below T. There is also no observable kink at the Curie temperature. The temperature coefficient at high temperatures continues to decrease up to the Er compound and increases in going from Er to Tm (see fig. 45). 

The temperature dependence of the thermopower S of RCU2 compounds is shown in figs. 47-49 (Mikovits, 1981). As mentioned above the light RCU2 compounds are paramagnetic and the rest are antiferromagnetic (see section 3.2.1.2). 

This compound is ferromagnetic (To = 74 K, cubic MgCu2 structure) and exhibits a kink in the resistivity (see section 3.2.1.10) and the absolute thermopower (fig. 53) near the Curie temperature Tj. Zorid et al. (1973) have shown that these are correlated discontinuities in the temperature dependence of the thermopower and the resistivity at in GdNi2. This is shown in fig. 54. These anomalies have been described theoretically by Zorid et al. (1973) and the agreement between the results shown in fig. 54 and the theoretical conclusion described in section 3.1.3.4 is good. 

As was already discussed in connection with the thermopower data, both of these ferromagnetic compounds show anomalous behaviour in the thermopower... 

At present there is no detailed understanding of the temperature dependence of the thermal conductivity of these RAI2 compounds. However, we believe that the systematic investigation of the temperature dependence of A across the rare earths of this section can give us an indication of the effect of local magnetic moments on the thermal conductivity. This is analogous to the situation for the thermopower. 

Gratz et al. (1980b) studied the effect of a first-order magnetic phase transition on the thermopower of RC02 compounds. The results are shown in fig. 80. The first-order phase transition causes a discontinuity in the thermopower. The magnitude of the discontinuity increases with decreasing T, i.e. as the lanthanide atoms change from Tb to Er. This is consistent with the resistivity data of section 4.2.1.1 and the thermal expansion of Lee and Pourarian (1976). 

The thermopower of these compounds was measured by Gratz et al. (1980c) and is shown in fig. 81. Fig. 81 shows that the thermopower depends in a complex manner on the temperature below T. There is clear evidence for the... 

The temperature dependence of the thermopower of (Gd, Y)4Co3 compounds is given in fig. 83 for several Gd concentrations. Gratz et al. (1980d) have shown that the Curie temperature of these compounds varies almost linearly with Gd... 

The thermopower of some Gd4(Co, NOs pseudobinary compounds is given in fig. 84 (Gratz et al., 1982b). Note that the substitution of only 5% for Co causes a pronounced change in the S vs. T curve. This could possibly imply that a variation of the conduction electron concentration has considerably more effect on the thermopower than the substitution of the isoelectronic Gd by Y ions (see section 4.2.2.4). Fig. 84 shows that the kink disappears with increasing Ni concentration. This is also seen in the behaviour of the resistivity of these compounds (see section 4.2.1.5). 

The variation of the thermopower of these compounds with Co concentration is given in fig. 85 (Hilscher et al., 1980). We see that the thermopower is negative for the whole temperature range and for all Co concentrations. It is interesting to... 

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