Effects of growth angle and solidification rate on crystal growth of precious metal prepared by electron beam floating zone method

Abstract

Precious metals exhibit fascinating properties and extensive applications in chemical engineering, high-temperature measurement, and electronic industry. The microstructures of these metals are generally polycrystalline and the precious metals like Ir and Ru with polycrystalline microstructures are difficult to deform at room temperature. However, the single crystal of precious metal can be well deformed to the final product, and it can be effectively used as a material. In this paper, electron beam floating zone method (EBFZM) is employed to prepare single crystals of precious metals, due to the fact that precious metals, e. g. Ir and Ru have high melting points of 2443 ℃ and 2310 ℃ respectively, and no crucible can be used for this processing. Considering the fact that the height of floating zone plays a key role in EBFZM, we deduce the expression for height of floating zone in EBFZM based on pedestal growth and zone melting techniques. The effects of crystal growth angle, interface growth mechanism, and solidification rate on the height of floating zone are discussed. The results show that the heights of floating zone for six precious metals are in a sequence order of Ru >Pd >Ir >Pt >Ag >Au. The crystal growth angles of these metals are calculated in a range of 8.4°-10.7°. For the same growth angle, the heights of floating zone, calculated by the Pedestal growth, zone melting and Czochralski-like growth techniques, are close to each other. But for different growth angles, the height of floating zone increases with increasing the growth angle for pedestal growth and Czochralski-like growth techniques, different from zone melting technique. Meanwhile, the height of floating zone changes with interface growth mechanism and solidification rate. For the pedestal growth technique, the height of floating zone is low for continuous growth mechanism, and for zone melting technique, its height of floating zone, calculated from continuous growth mechanism, is larger than those from the dislocation and faceting growth mechanisms. Furthermore, it reveals that the growth angle and height of floating zone vary slightly with continuous growth mechanism. In addition, a predicted solidification rate of 2.4 mm/min, available for single-crystal growth of precious metals, is in agreement with the previous experimental results of single crystals Ir and Ru prepared by EBFZM.