Zone refining has been tried on high-purity aluminum produced by a combination of the three-layer electrolytic refining process and the segregation process. The cropping procedure was effective in increasing the efficiency of zone refining. The highest residual resistance ratio R(300 K)/R(4.2 K) obtained was about 90000 in the bulk value, corresponding to 99.99999% aluminum. In the region of very high purities, it has been found that the residual electrical resistivity in single crystals of aluminum exhibits strong anisotropy, not yet predicted for a normal metal with cubic symmetry. The resistivity increases in the order of the 〈110〉, 〈111〉 and 〈100〉 directions.
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Hashimoto et al. (1994) studied this question.
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