Nanoscale-SiC doping for enhancing Jc and Hc2 in superconducting MgB2

Abstract

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.

Enhancement of critical current density in low level Al-doped MgB2

“…Nevertheless, we find a good correlation between room temperature resistivity (Table 1) and self-field J c at 20K (Figure 4) indicative of an increased H c2 through alloying [6]. We observe an increase in resistivity up to 2.5 %, as would be expected, but then a decrease for the 5% sample.…”

Influence of the upper critical-field anisotropy on the transport properties of polycrystalline MgB2

“…It is well documented that B c2 can be enhanced significantly by various techniques [1,2,3,4,5,6,7,8,9,10,11]. Although the mechanisms involved in these changes are not yet fully understood, impurity scattering in both bands seems to play a major role.…”

“…In MgB 2 thermal effects should play only a minor role due to its comparatively low Ginzburg number. Nevertheless, the critical currents become too small for power applications at fields well below H c2 , even at 4.2 K. Fortunately, the upper critical field of MgB 2 can be rather easily enhanced by certain preparation conditions [1,2,3], doping [4,5,6,7,8] or irradiation [9,10,11], and exceeds 30 T (at 0 K). Even for such "high-H c2 " materials, the application range is limited to around 10 T in polycrystalline samples.…”

The effect of doping level and sintering temperature on Jc(H) performance in nano-SiC doped and pure MgB2 wires

“…[1][2][3][4][5][6][7][8][9] Among these, both SiC and C have become well established additives which can result in the enhancement of J c , H irr , and H c2 . The authors' group found that doping of MgB 2 with nanoparticle SiC can significantly enhance J c in high fields, with only slight reductions in T c up to a doping level as high as 30% of B.…”