Studies On The Preparation Method Of Superconducting Nb₃Al

Fusion energy has been considered to be a clean energy for solving the constantly growing demands of energy with almost no radioactive pollution. The ITER project is an important step to harness the fusion energy. Nb3Al is a promising candidate for fusion superconducting materials on account for its excellent strain tolerance and high field performance. However, the rather stringent phase-forming conditions with regard to the high hypoeutectic temperatures of Nb3Al phase (1940℃) and the very small phase domain make it difficult to prepare Nb3Al.The experimental ansatz of this work was inspired from the ultra-high reactivity of nano-particles.The niobium powder which was obtained by the reduction of NbCl5 is much finer than the commercial one, resulting in a dramatic increase of its reactivity due to the decrease of reaction activation energy between Nb and Al. It is found that the mean size of the obtained Nb particles decreases with the lengthening of the reduction reaction time and the increase of the temperature. One optimized condition for the reduction reaction is 50℃ and 2 hours, with which one can obtain significantly finer niobium powder than the commercial one. Although sometimes a negligibly side product, Nb6O, may be formed in the reduction process, it didn’t make any significant impacts on the preparation of Nb3Al sample.Niobium powders thus prepared were used as starting reagents to prepare the Nb3Al sample. Three different reaction temperatures,900℃,1100℃ and 1300℃, have been tested by using Nb powders of different particle sizes. The obtained Nb3Al samples were characterized by using X-ray powder diffraction method and superconducting transition temperature measurement by PPMS(Physical Property Measurement System). In all the cases, the best sample is obtained in the reaction of the finest Nb powder, while the higher temperature favors the formation of Nb3Al too. One typical sample obtained in our experiments has a measured superconducting transition temperature of 16.8K.It is well-known that the reactivity of a particle and/or its specific surface is tightly related to the corresponding surface energy. To give a theoretical explanation for our experimental approach, the first-principle method has been used to calculate the surface energy of Nb particles of different sizes and shapes. Our results give a quantitative theoretical explanation for the success of our experimental ansatz.