Structure-sensitive conductivity

Another kind of experiment which has thrown light upon the nature of structure-sensitive conductivity requires the addition of small quantities of impurity to the crystal lattice. As early as 1897 (3i) it was noted that the addition of sodium chloride to lead chloride caused an increase in the conductivity of the latter. One of the most remarkable examples of this phenomenon was given by Ketzer (32), who by adding 0 001 % of rock-salt to lead chloride raised the conductivity of the lead chloride 50-fold. Gyulai (33) repeated these experiments, adding... 

When a current is passed through a crystal, the resistance in the low-temperature structure-sensitive conductivity region rises rapidly, as a counter-electromotive force is set up. This... 

There are, of course, many more ceramics available than those listed here alumina is available in many densities, silicon carbide in many qualities. As before, the structure-insensitive properties (density, modulus and melting point) depend little on quality -they do not vary by more than 10%. But the structure-sensitive properties (fracture toughness, modulus of rupture and some thermal properties including expansion) are much more variable. For these, it is essential to consult manufacturers data sheets or conduct your own tests. 

The type of conductance exhibited by the oxide and its value are structure sensitive. The oxide is essentially an ionic conductor. One could maintain that it has a relatively high concentration of low-mobility ionic charge carriers. As far as electronic conductance is concerned although pure alumina is an insulator with a band gap of 8 to 9 eV, one has to bear in mind that when it is produced anodically as a thin film adhering firmly to the metal, an entirely different electronic situation may arise [cf. Section V(2)]. 

The linearity leads to another important conclusion a constant field for a constant current implies a constant overall conductivity throughout the film. Since the conductivity is very structure sensitive, this implies also that either (i) the film grows homogeneously... 

The macroscopic dielectric constant of PS is very sensitive to the frequency range investigated. For very high frequencies the dielectric constant can be obtained by optical measurements. In this regime the effective medium approximation has been found to be an adequate description [Th3]. For lower frequencies the dielectric constant e is commonly obtained by capacitance measurements of metal/PS/ Si structures. AC conductivity measurements of such structures indicate a hop-... 

Because electronic and ionic conduction are so structure-sensitive, the simple rule-of-mixtures approach to estimating the conductivity and resistivity of composites is not normally of use. As a result, the conductivity of specific composites for specific applications must be experimentally determined. In the next two sections, we examine two examples of how composites can be used in electrical applications, and we describe the influence of each component on the electrical properties. The first example involves the electrical insulating properties of polymers, and the second one involves enhancing the electrically conducting properties of polymers. 

The particle beam LC/FT-IR spectrometry interface can also be used for peptide and protein HPLC experiments to provide another degree of structural characterization that is not possible with other detection techniques. Infrared absorption is sensitive to both specific amino acid functionalities and secondary structure. (5, 6) Secondary structure information is contained in the amide I, II, and III absorption bands which arise from delocalized vibrations of the peptide backbone. (7) The amide I band is recognized as the most structurally sensitive of the amide bands. The amide I band in proteins is intrinsically broad as it is composed of multiple underlying absorption bands due to the presence of multiple secondary structure elements. Infrared analysis provides secondary structure details for proteins, while for peptides, residual secondary structure details and amino acid functionalities can be observed. The particle beam (PB) LC/FT-IR spectrometry interface is a low temperature and pressure solvent elimination apparatus which serves to restrict the conformational motions of a protein while in flight. (8,12) The desolvated protein is deposited on an infrared transparent substrate and analyzed with the use of an FT-IR microscope. The PB LC/FT-IR spectrometric technique is an off-line method in that the spectral analysis is conducted after chromatographic analysis. It has been demonstrated that desolvated proteins retain the conformation that they possessed prior to introduction into the PB interface. (8) The ability of the particle beam to determine the conformational state of chromatographically analyzed proteins has recently been demonstrated. (9, 10) As with the ESI interface, the low flow rates required with the use of narrow- or microbore HPLC columns are compatible with the PB interface. 

As a matter of convenience, it is useful to give a name to reactions that, on a given metal, exhibit a lack of sensitivity to details of surface structure. We have proposed to call these reactions facile. Another name for them would be structure-insensitive. The concept is probably as old as the concept of active centers and can be found in Taylor s 1925 paper in which he wrote There will be all extremes between the case in which all the atoms in the surface are active and that in which relatively few are so active and .. . the amount of surface which is catalytically active is determined by the reaction catalyzed (42). Similar ideas have been presented by Crawford et al. (43), who found that specific rates of ethylene hydrogenation on nickel evaporated films change only by a factor of 3 when, as a result of sintering, crystallite sizes change from 625 to 21,000 A. The authors concluded that studies of sintering should be conducted with more structure-sensitive mechanisms. ... 

Figure 2 demonstrates that the reaction rates did not decrease with time to any appreciable extent. For the samples of low Pd content, hydrogenation was completed in a remarkably short time. The Pd content of the samples of higher loadings formed aggregates which, as in the case of 1-octene, tend to block the interlamellar space and restrict conversion to the surface active sites. This particularly holds for 10.2% Pd-HDAM, which was the least effective sample in the reaction. Similarly to conventional Pd supported catalysts, styrene hydrogenation conducted on Pd-HDAM samples was found to be a moderately structure sensitive reaction. 

Pq and are the density and the shear modulus of quartz. is the Sauerbrey constant. A special cut of a disc from the quartz crystal is necessary to provide a useful quartz plate. The value of the Sauerbrey constant depends on the cut. For an AT-cut Cjg is 226.01 Hz cm pg L The plate is then coated with the metal on which the adsorption is investigated (Figure 4.16). To provide a good adhesion of the metal film to the quartz plate, a very thin chromium layer is evaporated. The hmitation to evaporated metal films weakens this method for investigating structure sensitive surface processes. Otherwise, the method is of extreme value for the investigation of surface films like conducting polymers (Chapter 11). 

As a result of a large number of researches it has been established that two types of feult system may exist in crystals which may be called reversible and irreversible fault systems. The former have reproducible properties in many respects, but properties of the latter depend upon the previous history of the specimen. In particular, irreversible fault systems give rise to the structure-sensitive diffusion and conductivity data described in the following chapter, while reversible fault systems give rise to reproducible conductivity and diffusion phenomena. Some of the reproducible and equilibrium types of fault systems may be briefiy discussed before the conductivity and diffusion data are given. 

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