Optimization of sintering conditions in bulk MgB2 material for improvement of critical current density

The present investigation focuses on methods to further improve the J(c) values of disk-shaped bulk MgB2 superconductors by optimizing the sintering conditions. We prepared two sets of bulk MgB2 material from commercial high-purity powders of Mg metal and amorphous B using a single-step solid-state reaction process. To optimize the sintering time, a set of samples was sintered at 775 degrees C with sintering duration ranging between 1 and 10 h (pure Ar atmosphere). A second set of samples was produced similarly at 775, 780, 785, 795, 800 and 805 degrees C (3 h, pure argon atmosphere). X-ray diffraction analysis showed that both sets of samples were single phase MgB2. Magnetization measurements confirmed a sharp superconducting transition with T-c,T- onset approximate to 38.2 K-38.8 K. The critical current density (J(c)) values for MgB2 samples produced for 1 h were the highest in all processed materials, i.e., the high J(c) value of 270,000 A/cm(2) and 125,000 A/cm(2) (20 K, self-field and 1 T) were achieved in the sample produced at 775 degrees C, without any additional doping. In contrast, the second series of samples clearly indicated that at 805 degrees C (3h) the highest J(c) of 245,000 A/cm(2) and 110,000 A/cm(2) (20 K, self-field and 1 T) were achieved. AFM and EBSD observations indicated that largest amount of fine grains do exist in the sample sintered at 775 degrees C, but the narrowest distribution of grains does exist in the sample sintered at 800 degrees C. The present results clearly demonstrate a strong relation between the microstructure and the pinning performance. The optimization of the sintering conditions is crucial to improve the performance of bulk MgB2 samples. (C) 2015 Elsevier B.V. All rights reserved.