Intergranular diffusion of platinum by bismuth

Damage to precious-metal crucibles is a problem associated with the Czrochalski single-crystal growing technique. During the growth of Bi sub 12 SiO sub 20 single crystals, a failure of a pure Pt crucible occurred in the initial melting of the Bi sub 2 O sub 3 and SiO sub 2 component powders. This failure was examined using scanning electron microscopy and energy dispersive X-ray analysis. It is shown that bismuth diffuses rapidly along the Pt grain boundaries with a diffusion coefficient at 1000 deg C of approx 8 x 10 exp --8 cm exp 2 s exp --1 . This can be compared to a volume diffusion coefficient at 1000 deg C of 2 x 10 exp --11 cm exp 2 s exp --1 . The grain boundary diffusion embrittles the Pt crystallites. Furthermore, the crucibles are slowly damaged even in oxidizing atmospheres by the forcible removal of Pt crystallites by the solidifying melt. 6 ref.--D.O.N.