Synthesis of Nd123 superconducting powder via oxalate coprecipitation

The preparation of pure and fine Nd123 powder was investigated in five ways, Precursor powders were generated by the oxalate coprecipitation. Some of the powders were heated in vacuum (770 degrees C for 20 h at 10-15 Pa), followed by calcination at different temperatures and oxygen partial pressures. The vacuum step was omitted with the remaining powders. Calcination under pure oxygen requires excessively high temperatures (T>940 degrees C). Under these conditions the powder is badly fused, with grain sizes in thr 7-12 mu m range. In addition, the orthorhombicity of this powder is low due to the formation of Nd123 solid solution - implying the presence of residual BaCuO2. The optimal route to produce the desired powder is in a low partial pressure of 350-1000 ppm oxygen in nitrogen, without a vacuum preheating step. Similar powders can be obtained in 1% O-2 with a vacuum preheating step. At 60 ppm O-2 the reaction between Nd1+xBa2-xCu3O6.(5+x/2+delta) and BaCuO2 is incomplete. with about 5% of barium cuprate remaining. Finally, the T-c of the powder produced at the optimal conditions was found by DC magnetization to be similar to 99 K. (C) 1997 Elsevier Science B.V.