The Influence of Preparation Temperature on the Different Facets of Bulk MgB₂ Superconductors

Two MgB₂ samples were prepared using the spark plasma sintering (SPS) technique at different temperatures-950 °C (S1) and 975 °C (S2)-for 2 h under 50 MPa pressure to study the influence of preparation temperature on different facets, namely those perpendicular (PeF) and parallel (PaF) to the compression direction of uniaxial pressure during the SPS of MgB₂ samples. We analyzed the superconducting properties of the PeF and PaF of two MgB₂ samples prepared at different temperatures from the curves of the critical temperature (T_C), the curves of critical current density (J_C), the microstructures of MgB₂ samples, and the crystal size from SEM. The values of the onset of the critical transition temperature, T_c,onset, were around 37.5 K and the transition widths were about 1 K, which indicates that the two samples exhibit good crystallinity and homogeneity. The PeF of the SPSed samples exhibited slightly higher J_C compared with that of the PaF of the SPSed samples over the whole magnetic field. The values of the pinning force related to parameters h₀ and K_n of the PeF were lower than those of the PaF, except for K_n of the PeF of S1, which means that the PeF has a stronger GBP than the PaF. In low field, the most outstanding performance was S1-PeF, whose critical current density (J_C) was 503 kA/cm² self-field at 10 K, and its crystal size was the smallest (0.24 µm) among all the tested samples, which is consistent with the theory that a smaller crystal size can improve the J_C of MgB₂. However, in high field, S2-PeF had the highest J_C value, which is related to the pinning mechanism and can be explained by grain boundary pinning (GBP). With an increase in preparation temperature, S2 showed a slightly stronger anisotropy of properties. In addition, with an increase in temperature, point pinning becomes stronger to form effective pinning centers, leading to a higher J_C.