In-situ synthesis of carbon capsulated Ni nanoparticles and their cooperative doping effects on superconducting properties of MgB2

Carbon-encapsulated Ni nanoparticles (the average diameter of the particles is 7-8 nm) in B matrix were successfully in situ synthesized by chemical vapor deposition (CVD) in order to solve the agglomeration of nanoparticles by traditional mixing method. The influences of CVD technique on the carbon yield, morphology and structure were well investigated based on model system, and the (MgB2)0.96Ni0.04 bulks containing 3.8, 8.2 and 8.5 wt% carbon were prepared at 750 °C. The X-ray diffraction patterns showed no substitution of Ni for Mg and less substitution of C for B during the synthesis process, whereas the TEM analysis suggested such decorated insertions with core-shell structure were stable within the MgB2 matrix and hopefully acted as pinning center due to the small size. In our experiment, although the advantage of nanoparticles was suppressed by the large amount of MgO impurities during the deposition process, a small enhancement of Jc was observed in the low field range, which indicated improved grain connectivity because of the refinement of MgB2 grains by inducing homogeneous nanoparticles. In view of this, a desirable Jc performance at high field could be further obtained under the control of MgO content.