MS-12-P-2981 Effect of SiC doping on improvement of superconductive properties of MgB₂ wires studied by TEM

One of the problems of the promising superconductor materials of MgB₂ is that the critical current density (J_C) is not high enough for practical use. It has been established that doping with carbon or/and nano-sized inclusions can improve J_C of MgB₂. SiC is one the most effective dopes for improving the superconductive properties of MgB₂. Although many works have been carried out on the doping methods and the changes in physical properties of MgB₂, few concerned the effect of SiC doping on microstructure, and especially on the status of other existed impurities in detail. MgO is a kind of impurity in MgB₂ which is introduced in the fabrication process of the material and is difficult to be removed. The existence and distribution of MgO inclusions in MgB₂ should be also very important to the properties of the material. In the present work, we characterized the microstructure of MgB₂ wires, without and with SiC doping, fabricated with internal magnesium diffusion (IMD) process, especially focused on the status of MgO, by means of TEM including tomography, and correlated the microstructure and the distribution of MgO nano-inclusions to the superconductive properties of the wires.

TEM thin film specimens were prepared with a focused ion beam instrument, JEM-9310FIB. JEM-2100F and JEM-3000F were used for the observation and analysis. Two typical areas, called as area A and B respectively hereafter, were observed in both the specimens. Area A was composed of small crystal grains while area B was in amorphous status. The crystal grains were confirmed to be those of MgB₂, Mg₂Si and MgO in the SiC-doped specimen, and MgB₂ and MgO in the specimen without addition of SiC. Area B was confirmed to be amorphous boron with EELS and energy filtering imaging. In HAADF observation, bright contrast particles in size ~ 10 nm were identified which were considered as MgO particles, since MgO had a highest density of the constituents. The MgO nanoparticles distributed dispersively well in the crystalline area in the specimen doped with SiC, while concentrated in a layer-like volume surrounding area B in the specimen without doping of SiC. Figure 1 shows typical images of a MgB₂ specimen doped with SiC. TEM tomography revealed that MgO particles in the specimen without doping of SiC distributed mainly on a 3-dementional layer-network which was considered to be the surfaces of starting boron particles. It was considered that doping of SiC seemed to slow down the reaction of Mg with O, and resulted in the dispersion of MgO nanoparticles. Since MgO nanoparticles could act as magnetic pinning centers in superconductor MgB₂ wires, the dispersed distribution of MgO particles should be one of the main reasons for the improvement of J_C for the MgB₂ wires doped with SiC.