THE ELECTRICAL RESISTIVITY AND THERMOPOWER OF NICKEL-COPPER ALLOYS

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1 THE ELECTRICAL RESISTIVITY AND THERMOPOWER OF NICKELCOPPER ALLOYS H. Ahmad, D. Greig To cite this version: H. Ahmad, D. Greig. THE ELECTRICAL RESISTIVITY AND THERMOPOWER OF NICKELCOPPER ALLOYS. Journal de Physique Colloques, 1974, 35 (C4), pp.c4223c4226. < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1974 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C4, suppl6ment au no 5, tome 35, Mai 1974, page C4223 THE ELECTRICAL RESISTIVITY AND THERMOPOWER OF NICKELCOPPER ALLOYS H. M. AHMAD and D. GREIG Physics Dept., The University of Leeds, Leeds LS2 9JT, U. K. RBsumB. Cette communication prksente des chiffres relatifs la rksistivitk electrique, p, et du pouvoir thermoelectrique, S, d'un certain nombre d'alliages a parts relativement Bgales de NiCu a des temperatures entre 200 K et 900 K. Lorsqu'un alliage contient entre 45 % et 60 % de Cu, p tombe au fur et mesure que T augmente entre 200 K et 690 K resultat qui confirme plus ou moins les travaux anterieurs. Chaque fois qu'un de ces Bchantillons a kt6 soumis a une temperature superieure a 690 K un effet d'hysterbsis de 1 % apparait dans p lorsque la temperature est ramenee audessous de cette valeur. Par contre, il n'y a aucune anomalie dans la courbe ST, quelle que soit la temperature a laquelle l'alliage est soumis. Nous prksentons kgalement des valeurs approximatives quant aux exposants critiques de la diminution dans dp/dt audessous du point de Curie pour chaque alliage. Abstract. Measurements are presented of the electrical resistivity, p, and thermoelectric power, S, of a number of NiCu alloys in the midrange 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 K. From this we have obtained a large number of measurements on NiCu 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 SpecPure nickel and copper. The copper rich specimens were premelted in an argon arc followed by a homogenisation melt in quartz tubes, while the nickelrich 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 at 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 TABLE I Details of specimens ; residual and ice point resistivities, resistance ratio, and Curie temperature Po (clq cm> P273 ( CL~ cm) RRR Tc (K) Ni + 1 %Cu E+ 2%Cu Ni + 5 % Cu Ni % CU %+28 %Cu Ni + 38 % Cu Ni 45 % Cu % + 50 %Cu %i 60 %CU % + 70 % Cu Pure Cu Article published online by EDP Sciences and available at

3 C4224 H. M. AHMAD AND D. GREIG tigated in each experimental run. The thermopower data were also obtained from about five specimens simultaneously as part of a programme for the automatic acquisition of data. They were obtained by reading the e. m. f. as a function of temperature as one junction of the alloy versus copper thermocouple drifted slowly upwards. The data were analysed by a computer technique designed by Mr. P. Clark. The measured resistivity, p,, of the pure metals and concentrated alloys is shown as a function of T in figure 1, with values at selected temperatures given in Tables I and 11. These.resistivity data are accurate to better than 0.1 %, with the temperatures known to better than 1 %. A specimen of pure Ni was measured in each experimental run, from which it could be checked that the apparent Curie temperature always lay within f 1 % of 631 K. TEMPERATURE K FIG. 2. Temperature dependence of the measured thermoelectric power. The symbols represent the same alloys as in figure TEMPERATURE K FIG. 1. Temperature dependence of the measured resistivity. The symbols represent ; A Ni + 28 % Cu ; A Nj+38%Cu; OE+45% Cu; ONi+50%Cu; q Ni + 60 % Cu ; x + 70 % Cu ; pure Cu. The absolute thermoelectric power, S, of the pure Ni and concentrated alloys is shown in figure 2, with selected values in Table IJI. These data are accurate to better than 3 %, and are in close agreement with the known values in pure Ni [I]. In figure 3 we show an example of an interesting hysteresis effect observed in all specimens containing between 40 % and 60 % copper. On repeated thermal 42.4 I I I I I I I I I TEMPERATURE 'C FIG. 3. Hysteresis effect in the resistivity of N~~oCU~O. cycling no change in p, for any given specimen was ever observed above 420 OC, but on cooling below that temperature the values of p, could vary from run to run by up to 1 %. These changes can be attributed to atomic segregation (clustering) as the temperature is lowered, although 420 OC is high compared to the critical temperature for phase separation calculated by Mozer et al. [2] from neutron scattering measurements. Their value was between 233 OC and 263 OC. Other evidence for this clustering effect has been

4 THE ELECTRICAL RESISTIVITY AND THERMOPOWER OF NICKELCOPPER ALLOYS C4225 TABLE I1 Measured resistivity (yq cm) at selected temperatures (deg K) Ni + 28 % Cu Ni + 38 % Cu Ni + 45 % Cu + 50 % Cu Ni + 60 % Cu Ni + 70 % Cu %re Cu Absolute thermopower (pv deg K l) at selected temperatures (deg K) Ni + 28 % Cu + 38 %Cu Ni+ 45 %Cu % + 50 % Cu %+ 60%Cu Ni + 70 % Cu obtained from magnetic measurements by Jach et al. [3] and Hicks et al Houghton et al. [5] recently showed that for NiCu alloys containing between 50 % and 60 % Cu, there is a linear decrease in p, between 200 K and 600 K, after which p, again rises with T in the normal manner. This fall they attributed to spin disorder scattering from giant polarization clouds that die away as the temperature approaches that of the Curie point of pure Ni. From figure 1 it is clear that we have found the same anomalous behaviour of p, in the same composition range, except that the temperature of the minimum resistivity is 690 K. As regards thermopower, we see from figure 2 that for the same composition range, the variation of S with T is remarkably ordinary. There is no distinguishing feature at either 200 K or 690 K in the midrange alloys, and indeed, both the temperature variation and the magnitude of S in all the nonferromagnetic alloys reported here are very much alike (I). At the same time it has been recognized that, as compared to the analogous AgPd systems, the magnitude of the thermopower of these alloys is surprisingly large (see Ziman [6]). We can speculate that this is due to (1) We recall that the alloy, Constantan, Ni45Cu55 lies in this range, and is frequently used as a thermocouple element because its variation of S with T is large but undistinguished. some sort of magnetic scattering although there is clearly no detailed correlation with the temperature range of the magnetic anomalies in p,. Finally, we can comment briefly on the behaviour of p and S at the critical points a subject on which there has been considerable recent interest [7]. It is now wellestablished that both dp/dt and ds/dt pass through cusplike anomalies at the Curie point T, of ferromagnetic metals and alloys, and that both are given empirically by a law of the form, (Up3 (dpldt) = (AIL) (E" 1 + B, where E = I (T TJT, 1, p, is the resistivity at the Curie point, and where A, B, and 1 are constants. In general, data are best for T > T,, and it has been shown that, for pure Ni, A' (i. e., 1 for T > Tc) is 0.1 for dp/dt, ds/dt, and for the decrease in specific heat with T [8]. With the present experiments, the values of A+ obtained from resistivity data are shown in Table IV, together with the range of E for which the above law is roughly valid. Although the values of A+ are subject to large uncertainties, and although Sousa et al. [9] have shown that the choice of T, is crucial, it is clear that there is a systematic decrease in A' with increasing alloy concentration. Our method of obtaining S unfortunately does not give the accuracy required to obtain A+ from thermopower data.

5 H. M. AHMAD AND D. GREIG dp Critical exponent for dt Ni+ 1 %Cu Ni+ 2%Cu Ni 5 %Cu Ni 9.4 %Cu Ni 28 % Cu Ni + 38 % Cu Ni + 45 % CU Range of E References [I] VEDERNIKOV, M. V., Adv. Phys. 18 (1969) 337. [2] MOZER, B., KEATING, D. T. and Moss, S. C., Phys. Rev. 175 (1968) 868. [3] JACH, J., BORG, R. J. and LAI, D. Y. F., J. Appl. Phys., 42 (1971) [4] HICKS, T. J., RAINFORD, B. D., KOUVEL, J. S., LOW, G. G. and COMLY, J. B., Phys. Rev. Lett. 22 (1969) 531. [5] HOUGHTON, R. W., SARACHIK, M. P. and KOUVEL, J. S., Phys. Rev. Lett. 24 (1970) 238. [6] ZIMAN, J. M., Electrons and Phonons (Clarendon Press) 1960, p [7] ZORIC, I., THOMAS, G. A. and PARKS, R. D., Phys. Rev. Lett. 30 (1973) 22. [8] TANG, S. H., KITCHENS, T. A., CADIEU, F. J. and CRAIG, P. P., Proc. of L. T. 13. to be published. [9] so us^, J. B., CHAVES, M. R., PINTO, R. S. and PINHEIRO, M. F., J. Phys. F 2 (1972) L 83.