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99.999% aluminum was zone-refined in vacuum better than 1×10−6 Torr, and various zone speeds, zone lengths and numbers of passes were tried to know the optimum refining conditions. The impurity content was estimated from the residual resistivity, and the highest resistance ratio in bulk R300⁄R4.2 obtained in the present work was 50000.
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Kino et al. (1976) studied this question.
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