Electron Beam Float Zone Melting and Vacuum Degassing of Niobium Single Crystals

Niobium metal powder produced by electrodeposition from molten salts was electron beam float zone melted (EBFZM) for purification and growth of single crystals. It was necessary to EBFZM with a high oxygen partial pressure to reduce the carbon level. A high resistance ratio (R300 K/R4.2 K(15 kG)=6500) and low flow stress (τly=460 gm/mm2) was obtained after a high temperature anneal (2350 °C) in high vacuum (2×10−10 Torr). Extensive material characterization results are reported for the niobium at various stages in the purification process. The impurity content determined by chemical analyses did not correlate with the resistance ratios; however, the flow stress exhibited an excellent correlation with resistance ratio. These results imply that interstitial impurity in solid solution is the impurity distribution most affecting disslocation motion.