Structure-sensitive Conductivity Processes

Conductivity data may be employed to demonstrate the properties of structure-sensitive dUfusion in crystals. It is often found that the conductivity-temperature curve of ionic crystals divides itself into two sections, a reversible high-temperature curve, obeying a law K = 

The positions of the low-temperature curves depend upon the treatment accorded the specimen and are therefore structure sensitive. The high-temperature curve is obtained for all specimens of a given crystal, and is structure insensitive (27) (Fig. 117). When crystals of sodium chloride were heated for periods of 10 hr. at a series of temperatures, the conductivity rose as the temperature of heating rose, corresponding to an increased number of faults and flaws resulting from the preheating (Fig. 118). Smekal (26) expressed the results of Figs. 117 and 118 for the conductivity (in ohm cm. ) by the expression K = X 10 c-23 / + 3-6 x io c-25.7oo/r 

Single crystals of sodium nitrate (2T), because of the smaller internal surface, show a lower conductivity than the poly-crystalline mass solidified from the melt If a powdered salt is put under pressure, the coherent block, composed of a mosaic 

100-fold smaller than that of a rock-salt polycrystalline mosaic prepared from a melt(30). These observations lead to the conclusion that internal surfaces are of great importance in conductivity measurements. 

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 

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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). 

Cathodoluminescence, CL, involves emission in the UV and visible region and as such is not element specific, since the valence/conduction band electrons are involved in the process. It is therefore sensitive to electronic structure effects and is sensitive to defects, dopants, etc., in electronic materials. Its major use is to map out such regions spatially, using a photomultiplier to detect all emitted light without... 

Infrared analyses are conducted on dispersive (scanning) and Fourier transform spectrometers. Non-dispersive industrial infrared analysers are also available. These are used to conduct specialised analyses on predetermined compounds (e.g. gases) and also for process control allowing continuous analysis on production lines. The use of Fourier transform has significantly enhanced the possibilities of conventional infrared by allowing spectral treatment and analysis of microsamples (infrared microanalysis). Although the near infrared does not contain any specific absorption that yields structural information on the compound studied, it is an important method for quantitative applications. One of the key factors in its present use is the sensitivity of the detectors. Use of the far infrared is still confined to the research laboratory. 

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