High purity Cu is prepared by a floating zone refining method. The effect of the annealing atmosphere on the residual resistivity ratio of Cu is examined for purity evaluation of floating zone refined Cu. An advantage of hydrogen annealing as a heat treatment for residual resistivity measurement of Cu compared to ultra high vacuum annealing has been revealed. The residual resistivity ratio of the starting material, 99.9999% Cu, was 3200, and that of purified Cu was 36000. This RRR is the highest value reported on annealed samples under a strong reducting atmosphere. Floating zone refining under reduced hydrogen atmosphere is effective for the preparation of ultra high purity Cu.
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