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 .

A comparative study of pure, SiC, and C doped MgB2 wires has revealed that the SiC doping allowed C substitution and MgB2 formation to take place simultaneously at low temperatures. C substitution enhances H_{c2}, while the defects, small grain size, and nanoinclusions induced by C incorporation and low-temperature processing are responsible for the improvement in J_{c}. The irreversibility field (H_{irr}) for the SiC doped sample reached the benchmarking value of 10 T at 20 K, exceeding that of NbTi at 4.2 K. This dual reaction model also enables us to predict desirable dopants for enhancing the performance properties of MgB2.

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The effect of nanoscale-SiC doping of MgB 2 was investigated in comparison with undoped, clean-limit, and Mg-vapor-exposed samples using transport and magnetic measurements. It was found that there are two distinguishable but related mechanisms that control the critical current-density-field J c ͑H͒ behavior: increase of upper critical field H c2 and improvement of flux pinning. There is a clear correlation between the critical temperature T c , the resistivity , the residual resistivity ratio RRR= R͑300 K͒ / R͑40 K͒, the irreversibility field H*, and the alloying state in the samples. The H c2 is about the same within the measured field range for both the Mg-vapor-treated and the SiC-doped samples. However, the J c ͑H͒ for the latter is higher than the former in a high-field regime by an order of magnitude. Mg vapor treatment induced intrinsic scattering and contributed to an increase in H c2 . SiC doping, on the other hand, introduced many nanoscale precipitates and disorder at B and Mg sites, provoking an increase of ͑40 K͒ from 1 ⍀ cm ͑RRR= 15͒ for the clean-limit sample to 300 ⍀ cm ͑RRR= 1.75͒ for the SiC-doped sample, leading to significant enhancement of both H c2 and H* with only a minor effect on T c . Electron energy-loss spectroscope and transmission electron microscope analysis revealed impurity phases: Mg 2 Si, MgO, MgB 4 , BO x , Si x B y O z , and BC at a scale below 10 nm and an extensive domain structure of 2 -4-nm domains in the doped sample, which serve as strong pinning centers.

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