Doping of MgB 2 by nano-SiC and its potential for improvement of flux pinning was studied for MgB 2-x (SiC) x/2 with x = 0, 0.2 and 0.3 and a 10wt% nano-SiC doped MgB 2 samples. Co-substitution of B by Si and C counterbalanced the effects of singleelement doping, decreasing T c by only 1.5K, introducing pinning centres effective at high fields and temperatures and enhancing J c and H irr significantly. Compared to the non-doped sample, J c for the 10wt% doped sample increased by a factor of 32 at 5K and 8T, 42 at 20K and 5T, and 14 at 30K and 2T. At 20K, which is considered to be a benchmark operating temperature for MgB 2 , the best J c for the doped sample was 2.4x10 5 A/cm 2 at 2T, which is comparable to J c of the best Ag/Bi-2223 tapes. At 20K and 4T, J c was 36,000A/cm 2 , which was twice as high as for the best MgB 2 thin films and an order of magnitude higher than for the best Fe/MgB 2 tapes. Because of such high performance, it is anticipated that the future MgB 2 conductors will be made using the formula of MgB x Si y C z instead of the pure MgB 2 .
The effect of carbohydrate doping on lattice parameters, microstructure, Tc, Jc, Hirr, and Hc2 of MgB2 has been studied. In this work the authors used malic acid as an example of carbohydrates as an additive to MgB2. The advantages of carbohydrate doping include homogeneous mixing of precursor powders, avoidance of expansive nanoadditives, production of highly reactive C, and significant enhancement in Jc, Hirr, and Hc2 of MgB2, compared to undoped samples. The Jc for MgB2+30wt% C4H6O5 sample was increased by a factor of 21 at 5K and 8T without degradation of self-field Jc.
By doping MgB2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB2 grains (intra-grain pinning). It became possible due to the combination of counter-balanced Si and C co-substitution for B, leading to a large number of intra-granular dislocations and the dispersed nano-size impurities induced by the substitution. The magnetic field dependence of the critical current density was significantly improved in a wide temperature range, whereas the transition temperature in the sample MgB2(SiC)x having x = 0.34, the highest doping level prepared, dropped only by 2.6 K.
We investigated the effect of SiC nano-particle doping on the crystal lattice structure, critical temperature Tc, critical current density Jc, and flux pinning in MgB2 superconductor. A series of MgB2−x(SiC) x/2 samples with x = 0 to 1.0 were fabricated using in-situ reaction process. The contraction of the lattice and depression of Tc with increasing SiC doping level remained rather small due to the counter-balanced effect of Si and C co-doping. The high level Si and C co-doping allowed the creation of intra-grain defects and highly dispersed nano-inclusions within the grains which can act as effective pinning centers for vortices, improving Jc behavior as a function of the applied magnetic field. The enhanced pinning is mainly attributable to the substitution-induced defects and a local structure fluctuations within grains. A pinning mechanism is proposed to account for different contributions of different defects in MgB2−x(SiC) x/2 superconductors.
Local magneto-optical imaging and global magnetization measurement techniques were used in order to visualize shielding effects in the superconducting core of MgB2 wires sheathed by ferromagnetic iron (Fe). The magnetic shielding can provide a Meissner-like state in the superconducting core in applied magnetic fields up to ∼ 1 T. The maximum shielding fields are shown to correlate with the saturation fields of magnetization in Fe-sheaths. The shielding has been found to facilitate the appearance of an overcritical state, which is capable of achieving a critical current density (Jc) in the core which is larger than Jc in the same wire without the sheath by a factor of ∼ 2. Other effects caused by the magnetic interaction between the sheath and the superconducting core are discussed.
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