Temperature dependence of electronic conductivity

FIGURE 3.3 Temperature dependence of electronic conductivity (cr) of La1 ISrIMn03+d (0 < x 0.7) at pure oxygen (Pq2 = lbar). (From Mizusaki, J. et al., Solid State Ionics,132 167-180, 2000. With permission.)... 

Figure 23.3 Temperature dependence of electronic conductivity of EMI - TCNQ (1 1).

So, despite the very small diameter of the MWCNT with respeet to the de Broglie wavelengths of the charge carriers, the cylindrical structure of the honeycomb lattice gives rise to a 2D electron gas for both weak localisation and UCF effects. Indeed, both the amplitude and the temperature dependence of the conductance fluctuations were found to be consistent with the universal conductance fluctuations models for mesoscopic 2D systems applied to the particular cylindrical structure of MWCNTs [10]. 

Metals and semiconductors are electronic conductors in which an electric current is carried by delocalized electrons. A metallic conductor is an electronic conductor in which the electrical conductivity decreases as the temperature is raised. A semiconductor is an electronic conductor in which the electrical conductivity increases as the temperature is raised. In most cases, a metallic conductor has a much higher electrical conductivity than a semiconductor, but it is the temperature dependence of the conductivity that distinguishes the two types of conductors. An insulator does not conduct electricity. A superconductor is a solid that has zero resistance to an electric current. Some metals become superconductors at very low temperatures, at about 20 K or less, and some compounds also show superconductivity (see Box 5.2). High-temperature superconductors have enormous technological potential because they offer the prospect of more efficient power transmission and the generation of high magnetic fields for use in transport systems (Fig. 3.42). 

Another semiconducting fulleride salt, [Ru(bpy)3](C5o)2 with bpy = 2,2 -bipyridine, crystallizes on the Pt electrode surface out of dichloromethane solutions saturated with [Ru(bpy)3]PF5 within a few minutes [79]. The NIR spectra of benzonitrile solutions of this salt demonstrate that the only fulleride anion present is 55 . The temperature dependence of the conductivity is typical for a semiconductor, with the room temperature conductivity being 0.01 S cm and the activation energy 0.1 kj mol (0.15 eV). It was postulated that there is an electronic overlap between the two ions of this salt leading to a donation of electron density from the 55 to the ligand orbitals in the [Ru(bpy)3] " AI 0.7) [79]. 

This competition between electrons and the heat carriers in the lattice (phonons) is the key factor in determining not only whether a material is a good heat conductor or not, but also the temperature dependence of thermal conductivity. In fact, Eq. (4.40) can be written for either thermal conduction via electrons, k, or thermal conduction via phonons, kp, where the mean free path corresponds to either electrons or phonons, respectively. For pure metals, kg/kp 30, so that electronic conduction dominates. This is because the mean free path for electrons is 10 to 100 times higher than that of phonons, which more than compensates for the fact that C <, is only 10% of the total heat capacity at normal temperatures. In disordered metallic mixtures, such as alloys, the disorder limits the mean free path of both the electrons and the phonons, such that the two modes of thermal conductivity are more similar, and kg/kp 3. Similarly, in semiconductors, the density of free electrons is so low that heat transport by phonon conduction dominates. 

The temperature dependence of the conductivity behavior of the nanoparticle films predicted that electronic conduction occurred via an electron hopping mechanism. 

In the corundum structure, the metal atoms are in crystallographic sites with variable coordinates. Because edge and face sharing of MOe octahedra can result in an abnormally close approach of the metal atoms to one another, such interactions can be minimized if the atoms move off center away from one another. However, in Ti203, the Ti atoms are only 2.58 A apart in face-shared octahedra and the d electrons are used to form a metal-metal bond. This compound is a semiconductor, but between 125 to 325 °C a broad structural transition takes place where this bond is broken, thus providing free electrons for electrical conduction and the compound now displays metallic conductivity. V2O3 also has this structure but has a more complex temperature dependence of its conductivity. 

Figure 21 shows temperature dependence of electrical conductivity and magnetic susceptibility of MEM(Af-methyl-iV-ethyl-morpholinium)-(TCNQ)2 [70]. At about 335 K it undergoes a metal-insulator transition accompanied by the onset of a two-fold superstructure and a temperature dependent magnetic susceptibility characteristic of localized moments. It is considered as depicted in Fig. 22(a) that a dimerized TCNQ accepts an electron localized by, for example, the Mott transition or the Wigner crystallization. The solid curve shown in Fig. 21(b) denotes the theoretical prediction for the magnetic susceptibility of a one-... 

Rice (1961) and Raleigh (1963) supposed that the concentration of electrons is proportional to the concentration of cations in the lower oxidation state. Such a condition is well fulfilled in metal-metal halide systems in the range of high concentrations of metal halide (when the metal is a minor component). However, in systems with comparable concentrations of both the cations, the situation is somewhat different. An electron can jump only when an electron donor has an electron acceptor in its neighborhood. The probability that such an acceptor is available is equal to the product x(Me +)-x(Me + " ). The exponential character of the temperature dependence of electrical conductivity is due to the fact that the concentration of cations in lower oxidation state increases with increasing temperature, which consequently increases the jump probability of the electron. 

It should be noted that in situ conductivity measurements of ECP films allow for an estimation of the absolute values ofconductivity, although high values of conductivity are not the only feature of a metallic state. Ex situ direct current electronic conductivity measurements of the films should be carried out in order to examine any temperature dependence of the conductivity and thermoelectric power over a wide range of temperatures, starting with very low values (of a few K) [30]. Typical metals have negative temperature coefficients for their electronic conductivity, and positive temperature coefficients for their thermopower, which contrasts with... 

Fig.2 Temperature dependence of electrical conductivity for 2-stage FGM of PbTe shown with those for component (1) and (2) with different electron concentration listed in Table 1...

Only a few conductivity measurements on organic glasses have been reported. Albrecht et ah (14, 15) studied the photoconductance of 3-methylpentane (3-MP) while Viseall and Willard (18) determined the conductivity of the same compound after y-irradiation. In an earlier paper (16) we reported the observations on the conductivity of irradiated 2-methyltetrahydrofuran. The presence of trapped electrons and ions that can be freed by illumination or thermally can be inferred from the current increase caused by their movements. The temperature dependence of the conductivity in these glasses suggested the occurrence of structural changes and indicated the existence of traps with different depths. 

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