Thermodynamic stability of palladium alloys: Part I: The palladium-niobium system
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I.
INTRODUCTION
R E C E N T L Y , several palladium alloys (Pd-M; M = another transition metal) have been considered as promising substitutes for gold in the electronics industry, tl-51 Some of these alloys are being considered as suitable lead materials for the newer high Tc oxide superconductors, t6,7] Any assessment of the materials compatibility and the economic viability of these alloys in the projected applications demands a set of accurate data on their thermodynamic properties. Recognizing the paucity of such data at present, a research program was started in our laboratory to determine the high-temperature thermodynamic properties of selected Pd-M alloys (M = second or third period transition metal). The results of the Pd-Nb system are reported here. There was an additional motivation for this work. The prediction of the thermodynamic stabilities of alloys is still a challenging proposition. This task becomes much more difficult with the additional contributions of d-electrons to the bonding. The transition metal alloys are typical examples of such systems. The "ab initio" calculations are yet unable to give quantitative results. ~8~ The semiempirical models of Brewer-Engel tg"~~ and of Miedema and co-workers ~1'''2] have better success in predicting the stabilities of several binary alloy systems. However, the empirical nature of these models necessitates further critical examination before their general acceptance. Accurate data for a series of Pd-M (M = transition metal) alloys form one such basis to evaluate these two models. As the palladium vapor pressure is many orders of magnitude higher than that of niobium t'3J (e.g., at 1500 K, PPd ---- 2 x 10 -7 atm a n d p N b -~ 6 • 10 -'s atm), the activities of palladium, apd, in a number of Pd-Nb alloys of compositions from 60Pd40Nb to 97Pd3Nb have been measured with a Knudsen cell mass spectrometer. The high sensitivity of the method minimizes errors in the activity data due to surface compositional changes M.J. STICKNEY and J.A. SPEED, Graduate Students, and K.A. GINGERICH, Professor, are with the Department of Chemistry, Texas A&M University, College Station, TX 77843-3255. M.S. CHANDRASEKHARAIAH, formerly Visiting Scientist, Department of Chemistry, Texas A&M University, is Senior Scientist with HARC Materials Science Research Center, The Woodlands, TX 77381. Manuscript submitted January 27, 1989. METALLURGICAL TRANSACTIONS A
of the solid alloys during the activity measurements. The accuracy of the activity data is improved further by the use of a twin-chamber Knudsen cell source. The details of this source for activity measurements have been published separately. |14,15,16] II.
EXPERIMENTAL
Alloy samples were prepared directly by melting mixtures of weighed amounts of palladium (99.95 pct pure, - 2 0 mesh sponge, Aesar Co.) and niobium (99.99 pct pure rod, Aesar Co,, Seabrook, NH) in a water-cooled copper hearth arc furnace under purified argon with tungsten electrodes. Each ingot (--2 g) was turned over and remelted 3 to 4 times to ensure compos
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