Phase Transition of Nb₃Sn during the Heat Treatment of Precursors after Mechanical Alloying

Abstract

The phase transition process of Nb3Sn during heat treatment exerts important influences on Nb3Sn formation and the superconducting characteristics of Nb3Sn superconductors. A simple method for quickly preparing Nb3Sn was studied. First, Nb, Sn, and Cu powders were mechanically alloyed to prepare the precursor. Then, the precursor was heat treated at different times to form Nb3Sn. During the first stage, the morphology and crystal structure of the products were analyzed after different milling times. The results of the transmission electron microscopy showed the poor crystallinity of the products compared with the original materials. During the second stage, heat treatment was performed at different temperatures ranging from room temperature to 1073 K. After treatment, the products were studied via X-ray diffraction analysis to determine how the structure changed with increasing temperature. Only the Nb diffraction peaks in the precursor were observed after high-energy ball milling for more than 3 h. When the heat treatment temperature was above 773 K and heat treatment time was 15 min, Nb3Sn began to form. When the temperature was above 973 K, some impurities, such as Nb2O5, appeared. After 5 h of ball milling, the precursor was heat treated at different times in a vacuum heat treatment furnace. The crystal structure of the product exhibited evident diffraction peaks of Nb3Sn. The critical temperatures of the samples that were heat treated at different times were between 17 K and 18 K. The magnetic critical current density of the sample versus the applied magnetic field at 4.2 K indicated that the magnetic Jc was approximately 30,000 A/cm2.

Keywords

• Nb₃Sn
• superconducting characteristics
• phase transition
• mechanical alloying