Float zone purification in palladium
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x[o F i g . 3--Surface concentration of s i l v e r as a function of b u l k
concentration. The solid curve is based on the assumption of a monolayer model and the results of this investigation. The data points were obtained by Williams and Norris6 through a photoemission technique. 1 and 2 in the bulk. From Fig. 2, Eqs. [2] and [4], the mole fraction of s i l v e r at the surface, X.m Ag, is deduced and plotted in Fig. 3 as a function of the mole fraction of s i l v e r in the bulk. The resulting curve is in good qualitative agreement with the surface enrichment data of Williams and N o r r i s6 which were obtained by a photoemission technique. As the s i l v e r concentration is increased, the surface becomes rapidly saturated
Float Zone Purification in Palladium N. B. SANDESARA AND J. J. VUILLEMIN To fill a need for high quality crystals in fundamental studies of the electronic properties of metals, we have undertaken a program of extensive rf float zoning in palladium.l The results of such a p r o g r a m have not been previously reported, but the available information about the purification and crystal growth of palladium suggests that it is beneficial to zone refine this m e t a l in an i n e r t gas atmosphere,2 and to anneal it in oxygen at a reduced pressure. 3'4 Encouraged by this work, w e performed all of the zoning in air. This note describes the technique and reports the resulting residual resistivity r a t i o ( P 3 0 0 K) which exceeds 25,000, a four fold i n c r e a s e over the published values. The purification process is not completely understood, but it is thought t o be related to the presence of oxygen during melting. Float zoning of palladium was accomplished using a 450-kc 10-kW generator. A water cooled eddy current concentrator transfers power efficiently to the sam-
K/Po
J. J. VUILLEMIN, is Associate Professor of Physics, and N. B. SANDESARA, Research Assistant in the Department of Physics, University of Arizona, Tucson, Ariz. 8 5 7 2 1 . Manuscript submitted May 9, 1977. METALLURGICAL
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with s i l v e r until an atom fraction of about 0.8 is reached, a f t e r which the bulk and the surface monol a y e r have essentially the same compositions. In reality, it is probable that segregation at the surface extends over a few atomic layers. Indeed, some of the quantitative discrepancy between the two sets of results in Fig. 3 is likely to be caused by our use of the monolayer model. In summary, the surface tensions of liquid silvercopper alloys have been determined at 1108°C by the sessile drop technique. Over a substantial composition range, s i l v e r is strongly adsorbed at the surface. Surface concentrations calculated by the monolayer m o d e l are in satisfactory agreement with photoemission data. The authors wish t o acknowledge the support of the Materials R e s e a r c h Laboratory Section of the National Science Foundation, under grant DMR72-03297A03, for the p r o g r a m from which this note wa
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