Synthesis pressure–temperature effect on pinning in MgB2-based superconductors

The volume pinning force, Fp(max), increases with increasing synthesis or sintering pressure (0.1 MPa–2 GPa) in materials prepared at high temperature (1050 °C) while it stays practically unchanged in those prepared at low temperature (800 °C). The position of Fp(max) can be shifted to higher magnetic fields by: (1) increasing the manufacturing pressure or decreasing the temperature (2) additions (Ti, SiC, or C, for example), and (3) in-situ preparation. Grain boundary pinning (GBP) dominates in materials prepared at low temperatures (600–800 °C), while high-temperature preparation induces strong point pinning (PP) or mixed pinning (MP) leading to outstanding properties. In materials produced by spark plasma sintering (SPS), the position of Fp(max) is higher than expected for both grain boundary and point pinning. The distribution of boron and oxygen in MgB2 based material, which can changed by additions or the preparation conditions, significantly affects the type and strength of pining. Materials prepared under a pressure of 2 GPa with a nominal composition of Mg:7B or Mg:12B consist of 88.5 wt % MgB₁₂, 2.5 wt% MgB₂, 9 wt % MgO or 53 wt % MgB₁₂, 31 wt %MgB₂₀ 16 wt %MgO, respectively. Their magnetic shielding fractions at low temperatures are 10% and 1.5%, with a transition temperature, T_c of 37.4–37.6 K. Although their magnetic critical current density at zero field and 20 K was 2–5 × 10² A/cm², they were found to be insulating on the macroscopic level.