Thermopower measurements in some binary metallic alloys

Physica I07B (1981) 131-132 North.Holland Publishing Company

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THERMOPOWER MEASUREMENTS IN SOME BINARY METALLIC ALLOYS R.W. Cochrane, J. Destry and J.L. Brebner Universit~ de Montreal, Montreal, Quebec, Canada and M.N. Baiblch and W.B. Muir McGill University, Montreal, Ou~bec, Canada

We describe the results of measurements on several copper zirconium glasses, in a Pd80Si20 glass and in a Mg7oZn30 glass, in the temperature range from 80 to 300K. The copper zirconium results can, with a reservatlon, b e interpreted in terms of the Ziman liquid metal theory. It is shown that the thermal power data for the Pd80Si20 sample is at variance with what one would expect from results on the pressure dependence of its resistivity, i.e., a description in terms of liquid metal theory. The results for the Mg70Zn30 sample are completely at variance with the predictions of liquid metal theory.

i.

INTRODUCTION

The behavior of the absolute thermopower, Sab s as a function of temperature depends on scattering mechanism, and can glve information relative to this mechanism [I]. We give our results for Sab s for a number of vitreous copper zirconium alloys, as well as of a vitreous Pd80Si20 sample and one of Mg70Zn30, in the temperature range 80-300K. 2.

We show, in Fig. i, Sab s vs. T for a Cu65Zr35 sample, in the "as received" and etched condition. (The other compositions show very similar behavior). It is seen from this figure that Sab s is positive and a slowly varying linear function of T, of positive slope.

EXPERIMENTAL DETAILS

A differential technique was used for the measurements. Two copper constantan thermocouples were indium soldered to the sample near its midpoint, separated by ~ i cm. The copper arms of the couples served as potential probes. Control of the heating function, data accumulation, storage and analysis were accomplished automatically using a microprocessor and associated equipment. We used the absolute thermopower results for lead [23 to obtain Sab s for our samples. The CuZr was fabricated by a melt spinning in air technique, as was that of Pd80Si20 [3]; the MgTOZn30 sample was splat cooled [4]. Diffraction patterns for these samples showed no peaks. 3.

made one measurement on a melt spun sample produced in vacuum, of composition CuZr 2. This sample, a bright silver in appearance, gave the same results as the former group.

RESULTS AND DISCUSSION

We measured the following CuZr compositions: "as received": 70/30;65/35;60/40;55/45; and 40/60. The figures are in atomic percent and the first figure represents the copper component. At one point in these measurements, we became concerned that the "as received" samples, because of their appearance, a reddish sheen on a silver background, might be samples where there had been a non-unlform mixing of the components. We etched several samples which had already been measured, thereby cleaning the surface, but found no difference in Sab s between the "as received" samples and those etched, with the exception of 55/45. Additionally, we

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We also show, in Fig. i, Sabs(T) for the Pd80Si20 sample, together with the Mg70Zn30 sample. The thermopower of the Pd80Si20 sample is positive and considerably smaller than in those of CuZr, and non linear. Sabs(T) in Mg70Zn30 is small and negative, becoming increasingly negative with increase in temperature. Nagel [i] has discussed the correlation between a negative temperature coefficient of resistivity and a small positive Sabs, of positive temperature coefficient and linear. He bases his discussion on an elaboration of Ziman's theory of electronic transport in llquid metals [5]. Both CuZr and MgT0Zn30 show a negative do/dT, where p is the resistivity [4]. However, as our measurements show, while the CuZr samples have a small and positive Sabs, of positive temperature coefficient, Sab s for the Mg70Zn30 sample is small and negative, becoming more negative with increase in temperature. Also the Pd80Si20 sample, which does show an Sab s that is small and positive, shows also a non linear increase with temperature, Fig. i, and has a dp/dT > 0 [3]. On the basis of our measurements, it would seem that, for the CuZr alloys, the thermopower and resistivity results are consistent with the Ziman picture, whereas this cannot be the case for the samples of Mg70Zn30 and Pd80Si20. In measurements of the effect of pressure on the electrical resistance of Pd. V Si- metallic 6~-xx I glasses (x varying from 0 to atomic percent),

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Lazarus [6] found the pressure coefficient to be identically zero for all his samples. He concluded that this observation was consistent only with the Ziman liquid metal model [5], rather than with the Kondo model [7] or the Mott model [8]. However, our present measurements of thermopower in a Pd80Si20 sample indicate that the "thermopower parameter" $ ~ O. A plot of dln0/dlnV vs. ~ would give a straight llne of slope 2/3 for a Ziman liquid metal [9]. The coordinates, on such a plot, of ourPd80Si20 sample are ~ ~ 0 and dlnp/dlnV = 1/3, the latter figure a consequence of the zero pressure dependence of resistivity [6,9]. This point is way off the 2/3 line for the Ziman liquid metal. The BeTiZr glass, of all glasses studied, appears to enjoy a privileged positio~ in that its dp/dt behavior [I], the behavior of its thermopower [I] and that of dlno/dlnV vs. [9] all conform to the Ziman picture. On the other hand, a sample llke Pd$oSi20 , which seemed, from the results of Lazarus [6] to be a good example of a Ziman liquid metal, cannot, Judging from our present thermopower data and from the temperature variation of its resistiviry [37, be considered in so simple a fashion.

that of the pressure dependence of resistivity in the CuZr alloys, to see how close these alloys would come to the 2/31i~e. If the points were to lie close to this line, such an observation, coupled to the resistivity and thermopower behavior, would lend credence to an interpretation in these alloys in terms of the Ziman formalism. [i] [2] [3]

S.R. Nagel, Phys. Rev. BI6 (1977), 1649. R.B. Roberts, Phil. Mag. 36 (1977), 91. J. Kastner, H.-J. Schink, E.F. Wassermann Solid State Comm. 33 (1979), 527. We wish to thank these authors for providing the Pd8oSi20 sample. H.-J. GUntherodt, Private communication We thank Dr. GUntherodt for providing the Mg7oZn30 sample. J.M. Ziman, Phil. Mag. 6 (1961), 1013. D. Lazarus, Solid State Comm. 32 (1979), 175. R.W. Cochrane, R. Harris, J.O. Strom Olsen and M.J. Zuckermann, Phys. Rev. Lett. 35 (1975), 676. N.F. Mort, Phil. Mag. 26 (1972), 1249. R.W." Cochrane, J.O. Strom Olsen, J.-P. Rebouillat, A. Blanchard, Solid State Comm. 35 (1980), 199.

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[8] [9]

Accordingly, we would suggest that an interesting set of measurements to perform would be

Research supported by National Science and Engineering Council, Canada and Minist~re de l'Education du Qu6bec, Programme FCAC.

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