Characteristic and synthesis mechanism of MgB2 nanoparticles in solid-state reactive sintering

MgB₂ nanoparticles were synthesized by a one-step reactive sintering method. The sample was heated from room temperature to 994 K, and then directly cooled down at a rate of 40 K/min. The results of X-ray diffraction indicate that MgB₂ superconductors with high quality were successfully prepared, and only a few impurities were detected on it. The sample consisted of two distinguishable structures by the TEM observation: nanoparticles and single crystals, which is a result of nonequilibrium conversion under the direct cooling condition. The nanoparticles with a diameter of about 10–20 nm are considered to be at the onset for the formation of MgB₂ phase. Transition temperature (T_c) and critical current density (J_c) of the compound structures were determined to be 38.5 K and 1.8 × 10⁵ A cm⁻² by means of SQUID measurement, which indicate good superconducting properties. The inter-diffusion and dislocation incorporation mechanisms for the formation and growth of MgB₂ nanoparticles are also proposed, and the further growth will be accelerated due to an adequate holding time at 994 K.