Platinum Metals Rev., 1972, 16, (2), 49
The high affinities of both palladium and platinum for silicon are well known, for disastrous consequences result when these metals are heated under reducing conditions in contact with siliceous materials. Although low melting point phases are formed, the alloying processes which result in this eutectic formation have been incompletely understood and since 1957 (1) it has been known that the palladium-silicon diagram originally published in Hansen was inaccurate. Rao and Winterhagen confirmed the existence of the compound Pd3Si but concluded incorrectly that it reacted directly with pure palladium to form a eutectic melting at 760°C.
Dr E. Röschel and Dr C. J. Raub of the Forschungsinstitut für Edelmetalle at Schwäbisch Gmünd (2) have now re-examined the system, using silicon of semiconductor quality, and have revealed a fairly complex situation. As silicon is taken up by palladium, the compounds Pd5Si and Pd4Si are successively formed. Pd5Si and Pd3Si melt congruently at 835° and 1070°C and the intermediate compound Pd4Si forms peritectically at 822°C. The first eutectic is formed between pure palladium and Pd5Si and melts at 825°C. A second eutectic melting at 798°C then forms between Pd5Si and Pd4Si.
The diagram advanced by Röschel and Raub, although considerably more complicated than that previously accepted, is soundly based on the thermal analysis and metallographic evidence, both of which fix the eutectic compositions very precisely. Although the structures of Pd5Si and Pd4Si have not been completely elucidated, their diffraction lines are tabulated in considerable detail and the only remaining uncertainty in this system is the extent of the primary solid solution of silicon in palladium. As previously shown (3) this interstitially dissolved silicon causes discontinuous yielding effects.
- 1N. K. Rao and H. Winterhagen, Trans. Indian Inst. Metals, 1956 –57, 10, 139 – 148
- 2E. Röschel and C. J. Raub, Z. Metallkunde, 1971, 62, ( 11 ), 840 – 842
- 3R. G. Hollister and A. S. Darling, Platinum Metals Rev., 1967, 11, ( 3 ), 94 – 99