Hall effect measurement apparatus

In Section 1 we will use the theory developed in Appendix A to discuss the various measurement and analysis techniques that have been applied to SI GaAs. We will also discuss the precautions that must be observed in apparatus design, and present an automated Hall-effect and photo-electronic system capable of measuring high resistivity samples. 

Carrier densities and carrier mobilities were obtained for polypyrrole via the Hall effect technique. The apparatus was also used to measure conductivities. The analysis was performed for polypyrrole containing three different counterions p-toluenesulfonate, perchlorate, and tetrafluoroborate. The carrier density was found to vary by one order of magnitude, from 1019 to 1020 cm-3. The carrier mobility, on the other hand, remained constant with respect to counterion, at 1 cm2V-lsec-l. In order to ensure that any solvent from preparation of the polymer was not affecting the results, one polymer, polypyrrole-p-toluenesulfonate, was subjected to vacuum dehydration for varying periods of time, followed by re-analysis. It was found that the carrier density changes very little with dehydration. There was no affect on the carrier mobility. 

Figure 8.10 Diagram of the Hall effect simple shear apparatus. C is the Hall plate between magnets Mi and M2. Samples Si and S2 are mounted between plates Pi and P2 and the moveable block B pulled by force F. (Redrawn from Lewis, E.L.V., Richardson, I.D. and Ward, I.M. (1979) Hall-effect apparatus for the measurement of simple shear In polymers, j. Phys. E, 12, 189 Copyright (1979).)...

High precision is especially hard to obtain in measurements on vapours. Pressure ranges in expansions are restricted by proximity to saturation conditions, and high-accuracy measurements of low pressures are difficult. Adsorption also complicates studies on vapours. For example, in measurements by the Burnett method on pure Ar and Kr near saturation, Weir et al. determined the effect of adsorption in their copper apparatus by performing experiments in vessels of different surface-to-volume ratios. Corrections for adsorption first became significant at temperatures at which the saturation pressure was just in excess of atmospheric, and rose rapidly at lower temperatures. For Ar at its triple point, where the saturation pressure is 70 kPa and B = — 280 cm mol, the correction due to adsorption was 8 cm mol. Hall and Eubank recommend that Burnett measurements be combined with isochoric data to avoid systematic errors due to adsorption. 

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