Conductivity Thermopower

Dopant mol% Electric conductivity" Thermopower" Contribution from Curie-law susceptibility6 Contribution from Pauli susceptibility0... 

The photoconductivity measurements are usually performed after the dark conductivity-thermopower run. As a light source, a tungsten-iodine lamp is used with an intensity of — 50 mW cm of white light at the sample position. The results presented in this chapter refer to room temperature measurements of the photoconductivity only. The conductance under illumination (Jiu is determined 15 sec after switching on the light source and the photoconductivity Affp is calculated by subtracting the dark conductivity. 

From the results obtained thus far, we speculate that, in both the and X, j-phase, disorder may play an important role. Table 2 lists the room-temperature conductivity, thermopower, and average transfer integral of the two new phases, and the other known phases. The unusually large transfer integral tg of the X j-phase (estimated from the slope of the S-T curve) does not conflict with its high conductivity, as shown in Fig. 10. However, further work is needed, especially for the X j-phase. 

Figure 6.31 Electrical conductivity, thermopower, and thermal conductivity of CNT/ polymer composites at room temperature.

Correlated transport studies (including temperature dependent dc conductivity, audio-frequency (lOHz-105 Hz) conductivity and dielectric constant, electric field dependence of conductivity, thermopower, electron spin resonance, microwave conductivity and dielectric constant) have been carried out on oriented emeraldine hydrochloride films [15], the... 

Heremans J. and Beetz, C.P.,. Thermal conductivity and thermopower of vapor-grown graphite fibers J. Phys. Rev.B 32, 1985, p.l981... 

After briefly introducing the main electronic features of CNTs (Sec. 2) and some general aspects of electronic conduction and transmission (Sec.. 1), we will show how complex electrical measurements to perform on such tiny entities are (Sec. 4). Then we will present the main experimental results obtained on the electrical resistivity of MWCNT and SWCNT and the very recent data relative to the thermopower of SWCNT bundles (Sec. 5). We will also discuss the effect of intercalation on the electrical resistivity of SWCNT bundles (Sec. 6). Finally, we will present some potential applications (Sec. 7). 

In conclusion, wc have shown the interesting information which one can get from electrical resistivity measurements on SWCNT and MWCNT and the exciting applications which can be derived. MWCNTs behave as an ultimate carbon fibre revealing specific 2D quantum transport features at low temperatures weak localisation and universal conductance fluctuations. SWCNTs behave as pure quantum wires which, if limited in length, reduce to quantum dots. Thus, each type of CNT has its own features which are strongly dependent on the dimensionality of the electronic gas. We have also briefly discussed the very recent experimental results obtained on the thermopower of SWCNT bundles and the effect of intercalation on the electrical resistivity of these systems. 

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... 

Nagels and Krikor143 studied the effect of y-irradiation on the electrical properties of fraws-polyacetylene. They reported a marked decrease of the conductivity and a slight increase of the thermopower after y-irradiation of 10 kGy (1 Mrad). Their study showed that no essential structural changes occur during irradiation. 

The effect of fast neutron fluence on thermal conductivity and thermopower has been determined by Uher and Huang (70). For fluences to 3 x 1018 n/cm2 Tc decreases in Y-Ba-Cu-O to a temperature of 86 K, the thermal conductivity decreases and is without a peak above Tc and the thermopower starts from a negative value and approaches zero and becomes positive. As will be seen below the more usual value of thermopower is positive in the superconducting material but these authors note the variability dependent on sample preparation conditions. 

Figure 6.48 (a) Effect of doping on the electrical conductivity (solid line) and thermopower (broken line) of polyacetylene. (Following Etemad et al, 1982.) (b) solitons in trans-polyacetylene (i) neutral, (ii) positive and (iii) negative solitons. Arrow marks the boundary between the two symmetric configurations. A, acceptor D, donor. (Following Subramanyam Naik, 1985.)... 

In crystalline materials a characteristic of polaron motion is a difference between E the activation for conduction, and s, that for the thermopower written as S=(kB/e) (EsjkBT+ const). We expect that Ea=Ec—EF+WH and Es=Ec-Ef, where Ee is the extremity of the band in which the carriers move. 

This material, which has the corundum structure, is a semiconductor at low temperatures, the optical band gap being 0.2 eV (Lucovsky et al. 1979). We should probably consider it to be an intrinsic semiconductor, but the activation energy in the conductivity does not appear to be constant. The thermopower (Chandrasekhar et al. 1970) is about 900pVK 1 at 100K and 500pVK 1 at 200 K this would suggest an activation energy of about 0.06 eV, or less than half the band gap.f This makes it likely that one of the carriers is a small polaron the... 

Turning now to fluid mercury and caesium, a fairly recent review is given by Freyland and Hensel (1985). Figure 10.9 shows the classic results of Hensel and Franck (1968) on the conductivity of mercury as a function of volume V the metal-insulator transition occurs when V/V0xl.3. Schonherr et al (1979) measured the conductivity a and thermopower S of mercury for conductivities between 200 and 5 2 1 cm 1 for mercury. Their results for a as a function of S are shown in Fig. 10.10(a). The slope is exactly as predicted, and a0 is in the range... 

A second historical line which, is of paramount importance to the present understanding of solid state processes is concerned with electronic particles (defects) rather than with atomic particles (defects). Let us therefore sketch briefly the, history of semiconductors [see H. J, Welker (1979)]. Although, the term semiconductor was coined in 1911 [J. KOnigsberger, J, Weiss (1911)], the thermoelectric effect had already been discovered almost one century earlier [T. J. Seebeck (1822)], It was found that PbS and ZnSb exhibited temperature-dependent thermopowers, and from todays state of knowledge use had been made of n-type and p-type semiconductors. Faraday and Hittorf found negative temperature coefficients for the electrical conductivities of AgzS and Se. In 1873, the decrease in the resistance of Se when irradiated by visible light was reported [W. Smith (1873) L. Sale (1873)]. It was also... 

A large number of partially oxidized divalent cation salts of the bis(oxalato)platinates have been reported (see Table 2).63 Only the series Mx[Pt(C204)]-6H20 (MOP where M = Fe, Co, Ni, Zn, Mg and 0.80 < x < 0.85) has been extensively studied.68 83 For most of these salts detailed studies have been made of their crystal structures and optical reflectivity at room temperature, and the variation of superlattice reflections, diffuse X-ray scattering, thermopower and DC electrical conductivity with temperature. 

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... 

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