Preparation of Ultra High Purity Niobium

The preparation of ultra high purity niobium (“UHP-Nb”) from commercial starting materials (niobium metal or Nb2O5) requires the elimination of all impurities down to the ppm, sub-ppm or even ppb level. This is possible only by application of a complex technology comprising the following essential processing steps:(1) Removal of critical metallic elements (especially Ta and W) which cannot be evaporated by electron beam melting through suitable chemical and/or physical purification methods as:— liquid-liquid extraction of Nb2O5 with MIBK, chlorination of the oxide, distillation of NbCl5 and thermal decomposition to metallic niobium,— molten salt electrolysis using LiF–NaF–KF melts containing K2NbF7,— iodination of Nb, distillation of NbI5/partial reduction/re-iodination/thermal decomposition to Nb,(2) Removal of volatile impurities by EBM or EBFZM in high or ultra high vacuum,(3) Decarburization by high temperature annealing under low O2 partial pressure,(4) Removal of H, O and N by UHV degassing at high temperature.The various purification methods developed for ultra purification of Nb are described with special emphasis on investigations carried out to optimize the conditions of liquid-liquid extraction and molten salt electrolysis. Both processing routes allowed to produce Nb with W and Ta impurity concentrations in the ppb range and residual resistivity ratios RRR>10000 (best value obtained was 90000). The superconductivity of Nb below the transition temperature Tc=9.288 K requires a special RRR measuring technique which will be described in the paper.