Abstract
The freezing and melting behaviour of indium samples with nominal purities of 99,9999%, 99,999% and 99,99% has been studied to find the influence of conventional freezing techniques on the freezing temperature and to assess the effect of trace impurities on the quality of a fixed-point sample. Plateaus of constant temperature (± 0,1 mK) are readily obtained from both the induced and outside-chilled induced freezes, while freezing ranges of about 2 mK are observed in both the normal and outside-chilled freezes for all the samples. The maximum freezing temperatures of the conventional freezes are found to coincide very closely for each sample and to within ± 0,1 mK for the purest sample. The purest sample showed a narrow melting range on the melting curve after very fast freezing, in contrast to the other samples which showed the melting ranges expected of alloys. The average melting temperature of the purest sample closely agreed with the maximum freezing temperature. The difference between the average melting temperature after the very fast freeze and the maximum freezing temperature appears to be a useful criterion for assessing the quality of indium as a fixed-point sample.