Determination of the Purity of Metals by Temperature-Dependent Electrical Properties

Abstract

It is well known that the electrical properties of metals are strongly affected by the presence of very small amounts of impurities. The employment of the relatively large changes in their temperature coefficients of electrical resistance and in their thermoelectric properties are well established as measures of purity for scientific purposes. However, their utility and worth as means for the determination and control of the degree of purity of commercial metals are seldom recognized or exploited. The tests described here are applicable to any metal whose degree of purity is of importance in its technical or commercial applications. Very dilute silver-base alloys are used for illustrative purposes. The results of simple electrical resistance and thermoelectric voltage measurements obtained from these alloys, made at just two convenient and easily reproducible temperatures (0 and 100°C), are given. Special equations, derived from fundamental relationships, for the temperature coefficient of electrical resistance and for the thermoelectric voltage, as functions of small impurity contents are in excellent agreement with those obtained from the experimental data. Both greater utility and direct comparison of the effects of impurities are afforded by the use of equivalent composition, rather than atom percent, in these equations. The large degrees to which both the temperature coefficient of electrical resistance and the thermoelectric properties are affected by impurities are apparent. The sensitivities of these properties are compared and their applications are discussed. The thermoelectric properties demonstrate a greater change induced by the presence of a given, small, total amount of impurity present in the metal (sensitivity) than does the temperature coefficient of resistance. Thus, for many practical applications, the employment of simple thermoelectric voltage measurements can provide an accurate, sensitive, rapid, and inexpensive means for the determination of the purity of metals.