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 .
Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg + 2B mixture was used as the central conductor core and reacted in-situ to form MgB 2 . The tapes were sintered in pure Ar at 800 o C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 µm. The Fe clad tape shows a sharp transition with transition width of ∆T c of 0.2 K and T c0 at 37.5 K. We have achieved the highest transport critical current reported so far at 1.6 × 10 4 A/cm 2 for both 29.5 K in 1 Tesla and 33 K in null field. R-T and critical current were also measured for fields perpendicular and parallel to the tape plane. The iron cladding shielded on the core from the applied external field, with the shielding being less effective for the field in the tape plane. Fe cladding may be advantageous for some applications as it could reduce the effects of both the self-field and external fields.
The transport properties of domain walls in oxygen deficient multiferroic YMnO 3 single crystals have been probed using conductive atomic force microscopy and piezoresponse force microscopy. Domain walls exhibit significantly enhanced conductance after being poled in electric fields, possibly induced by oxygen vacancy ordering at domain walls. The electronic conduction can be understood by the Schottky emission and Fowler-Nordheim tunnelling mechanisms. Our results show that the domain wall conductance can be modulated through band structure engineering by manipulating ordered oxygen vacancies in the poling fields.
The effects of single-wall carbon nanotube (SWCNT) doping in n-type Bi2Te3 bulk samples on the electrical and thermal transport properties have been studied. Bi2Te3 samples doped with 0–5 wt. % SWCNTs were fabricated using solid state reaction and investigated using x-ray diffraction, transmission electron microscopy, and magneto transport measurements. Results show that the 0.5% doping results in the significant enhancement of the Seebeck coefficience to as high as −231.8 μV/K, giant magneto resistance of up to 110%, reduction of thermal conductivity, and change of sign of the Seebeck coefficient from n to p type depending on the doping level and temperature. The figure of merit, ZT, of the optimum SWCNT doped Bi2Te3 was increased by 25%–40% over a wide temperature range compared to the undoped sample.
The flux pinning mechanisms of nano-Si-doped MgB 2 are reported in this work. The field dependence of the critical current density, J c (B), was analyzed within the collective pinning model. We found that the mechanisms for both δl pinning, i.e., pinning associated with charge-carrier mean free path fluctuations, and δT c pinning, which is associated with spatial fluctuations of the transition temperature, coexist in the nano-Si-doped MgB 2 samples, while H c2 increases greatly with increasing nano-Si doping level. However, their contributions are strongly temperature dependent. The δl pinning is dominant at low temperatures, decreases with increasing temperature, and is suppressed completely at temperatures close to the critical temperature, T c. However, the δT c pinning mechanism shows opposite trends.
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