THE ELECTRICAL RESISTIVITY AND THERMOPOWER OF NICKEL-COPPER ALLOYS

Abstract

Measurements are presented of the electrical resistivity, p, and thermoelectric power, S, of a number of Ni-Cu alloys in the mid-range of composition at temperatures between 200 K and 900 K. For alloys containing between 45 % and 60 % Cu, p falls with T between 200 K and 690 K in reasonable agreement with earlier work. For these same specimens hysteresis effects 1 % appear in p below 690 K every time the specimen is cycled above that temperature. By contrast, there are no anomalies in the S versus T curves of these alloys at any temperature. Rough values are given for the critical exponents of the decrease in dp/dT above the Curie points. We have recently been developing some semiautomatic equipment for obtaining resistivity and thermopower data over the temperature range 4.2 K to about 1 000 K. From this we have obtained a large number of measurements on Ni-Cu alloys across the complete range of composition. In this, the first report of this work, we shall concentrate on two points : [I] the temperature dependence of the resistivity and thermopower, above 200 K, and [2] critical point anomalies. The alloys were prepared by Mr. M. J. Walker from J M Spec-Pure nickel and copper. The copperrich specimens were pre-melted in an argon arc followed by a homogenisation melt in quartz tubes, while the nickel-rich alloys were melted in the argon arc only. The ingots were rolled and drawn into + mm diameter wires about 120 cm in length. Roughly 10 cm was then cut from each of the wires for resistivity measurements and annealed in vacuum for about 24 hours a t 950 0C. The remaining lengths were used for determining the thermopowers. Details of the specimens are shown in Table I. Resistivity data were obtained by the usual potentiometric technique with up to five specimens inves-