晶体生长角和凝固速率对贵金属电子束区域熔炼晶体生长的作用

基于电子束区域熔炼中熔区上力的平衡关系式, 计算获得了基座法、等径区熔法两种工艺下稳定成形熔区高度的表达式, 探讨了试样尺寸、晶体生长角和凝固速率等参数对六种贵金属稳定成形熔区高度的影响. 结果发现, 区熔相同尺寸试样时, 六种贵金属能够稳定成形熔区高度大小依次排序为 Ru> Pd> Ir> Pt> Ag> Au. 同时获得了这六种贵金属的晶体生长角在8.4°-10.7°之间, 而实际的晶体生长角与界面生长机制有关. 在基座法中, 连续生长机制所能支撑的熔区高度最小, 而等径区熔法中连续生长机制支撑的熔区高度大于位错生长机制和小面生长机制. 这三种晶体界面生长机制中连续生长方式对晶体生长角和区熔熔区高度影响较小, 有利于贵金属区熔单晶制备. 另外当凝固速率达到2.4 mm·min^-1, 位错和小面生长机制对区熔熔区高度的影响也变得很小, 预测的工艺参数与Ir和Ru单晶区熔实验报道结果基本符合.

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

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.