A SHS process has been established for synthesis of AlN powder under low nitrogen pressures. Al and NaN3 powders were used as the metal and nitrogen sources, respectively. The compact of the mixture of the reactants was wrapped up with aluminum foil and then wrapped up with an igniting agent (i.e., Ti + C). The synthesis reaction was triggered by the combustion of the igniting agent. The wrappings were found necessary in achieving high product conversions under low nitrogen pressures (<1 MPa). The product conversion was also affected by the reactant composition and the nitrogen pressure. High conversions were obtained when the mass ratio of Al to NaN3 was 1/2 or lower and the nitrogen pressure was 0.5 MPa or higher. The AlN powders as synthesized were observed to have two major types of morphology, i.e., granular particles with 0.5–3 μm in diameter and fibers with aspect ratios of 10–800.

Iron-based superconductors (IBSs) have received extensive attention in superconductivity mechanisms and high-field applications based on their ultra-high upper critical fields and low anisotropy. A major goal in the application research of IBSs is the fabrication of superconducting wires and tapes with high critical current density (J c ) and low cost. The use of Cu/Ag composite sheath can reduce the use of expensive silver, thereby reducing the cost of the tapes. In this paper, (Ba,K)Fe 2 As 2 (Ba-122) tapes with Cu/Ag composite sheath were fabricated by a pre-composite process, which provides a feasible method for fabricating high-performance iron-based superconducting wires and tapes. Pre-composite tapes sintered at 750°C for 3 h achieved a high J c of 6.2 × 10 4 A cm −2 at 10 T and 4.2 K. It is found that the high-density superconducting cores, large-size grains and uniform element distribution are the major causes of the high J c . The results suggest that the low-cost Ba-122 tapes with pre-composite Cu/ Ag sheath have great practical application prospect in the future.

BaxK1-xFe2As2 (BaK-122) iron-based superconductors (IBSs) have been considered to be promising for high-field applications. The transport J c performance of BaK-122 wires and tapes is continuously enhanced by introducing advanced fabricating methods. The mass density of BaK-122 superconducting core in wires and tapes is important to the transport J c performance and related to the mechanical behavior during preparation. In this work, the mechanical property parameters including Poisson's ratio-density, yield strength-density, and elastic modulus-density of BaK-122 IBS powder were examined via uniaxial compression experiments. The density-dependent mechanical constitutive of BaK-122 was obtained for the first time. The relationship function between density and Vickers hardness of BaK-122 was established as HV0.05=0.0249ρ5.332 based on the numerical simulation of hardness testing, and a method for characterizing the BaK-122 core density was developed. It had been found the sheath materials and preparation method have great influences on the stress state of the BaK-122 core, and then affect the density. The densification mechanism and corresponding improvement method were revealed to provide guidance for preparing high-density BaK-122 wires and tapes. Finally, the generalized relationship between density and the superconducting transport J c was established according to lots of experimental data from multiple BaK-122 samples, which has confirmed the positive correlation of ρcore and J c. We comparatively discussed the various cold-work and heat-treatment processes used in our team for preparing the BaK-122 wires and tapes, and the critical factors affecting the transport performance were summarized.