Insulating behavior

In the broad range of ceramic materials that are used for electrical and electronic apphcations, each category of material exhibits unique property characteristics which directiy reflect composition, processing, and microstmcture. Detailed treatment is given primarily to those property characteristics relating to insulation behavior and electrical conduction processes. Further details concerning the more specialized electrical behavior in ceramic materials, eg, polarization, dielectric, ferroelectric, piezoelectric, electrooptic, and magnetic phenomena, are covered in References 1—9. 

The Au films formed at the interface show interesting electrical transport properties that are dependent on the reaction temperature (see Figure 4).24 Four-probe electrical resistance measurements on the nanocrystalline films show a metal to insulator transition, metallic behavior being shown by the films formed at high temperatures (>45 °C). The films formed at lower temperatures (<45 °C) show insulating behavior. 

IV. MATERIALS WITH INSULATING BEHAVIOR AT ROOM TEMPERATURE... 

Consider a J-electron system, such as a transition metal compound. The valence d atomic orbitals do not range far from the nucleus, so COs comprised of Bloch sums of d orbitals and, say, O 2p orbitals, tend to be narrow. As the interatomic distance increases, the bandwidth of the CO decreases because of poorer overlap between the d and p Bloch SUMS. In general, when the interatomic distance is greater than a critical value, the bandwidth is so small that the electron transfer energy becomes prohibitively large. Thus, the condition for metallic behavior is not met insulating behavior is observed. 

Koiller and Falicov used this behavior as a basis of a criterion for whether metallic or insulating behavior is to be expected. They estimated the promotion energy per ion as the atomic promotion energy from Table 19-1, weighted by the multiplicity of the parallel and antiparallel spin configurations for example, for NiO the promotion energy is... 

The term Ed, stands for charging contributions. This term is absent in spectra of samples exhibiting a finite density of states at the Eermi level. For all practical purposes this is correct for aU true metals and for many semiconductors with intrinsic states near zero binding energy. Many systems relevant in catalysis do, however, not fulfill this condition (all non-black samples, glasses, porous materials, supports) or even worse, are composites of metallic and non-metaUic systems, giving rise to mixed metalhc-insulating behavior of their surface under PES. Such... 

Figure 8 Temperature dependence of the magnetic spin susceptibility of members of (TMljX series. The arrows indicate the temperature scale for the onset of the insulating behavior in the resistivity for (TMTTF)2PFj and (TMTTFljBr (see Fig. 4).

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