Effect of cubic and hexagonal boron nitride additions on the microstructure and properties of bulk MgB2 superconductors

Abstract MgB 2 is a promising material for intermediate temperature applications where conventional low temperature superconductors cannot be used, especially if the range of magnetic fields over which is has acceptable current carrying performance can be expanded. However, its applicability is limited by poor properties at elevated magnetic fields. Carbon-based dopants can be used to dramatically improve the high-field performance of MgB 2 , but at the cost of a reduction in the superconducting transition temperature ( T c ) that limits the operation temperature to 20 K or below. Here we report an enhancement of superconducting performance of MgB 2 with the addition of cubic and hexagonal boron nitride (BN), without any significant reduction in T c . Ex-situ bulk samples of MgB 2 with two forms of BN addition were manufactured by the field assisted sintering technique after high energy ball milling of powder mixtures. We find that hexagonal BN (hBN) nanoparticles mixed homogenously with MgB 2 powder react much more easily to produce Mg–N–B impurities than larger cubic BN (cBN) particles (∼10 µ m) under the same processing conditions. The addition of 1 wt% hBN or 5 wt% cBN combined with 6 h of milling has been demonstrated to improve the critical current density ( J c ) of MgB 2 over the entire magnetic field range. It is proposed that the nano-sized Mg–N–B impurities, that typically reside at MgB 2 grain boundaries, increase pinning strength by introducing additional flux pinning centres. In addition, excess Mg may benefit the low-field performance by improving the connectivity. This work shows the significance of microstructural characterization on inhomogeneous superconducting materials to analyse their performance.