High transport critical current density and largeH_c2andH_irrin nanoscale SiC doped MgB₂wires sintered at low temperature

Abstract

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

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

“…2,12 Powders of magnesium ͑99%͒ and amorphous boron ͑99%͒ were well mixed for the fabrication of a pure MgB 2 wire. SiC doped MgB 2 wires were prepared from powders with atomic ratios of Mg:2B plus 5, 10, or 15 wt % of SiC additions ͑SiC powder size of 10-100 nm͒.…”

“…The exceptional properties of SiC as a dopant have been verified by a number of groups over the past few years. [7][8][9][10][11][12][13][14][15][16][17][18] However SiC doping was found to have some negative effect on J c in the low field region. The J c for SiC doped MgB 2 was lower than that for undoped MgB 2 below 4 T at 5 K and below 2.5 T at 20 K. 2,13,17 There are many applications in the low field region such as in open magnetic resonance imaging ͑MRI͒ transformers and electric cables which normally operate at around 1-3 T. Thus it is important that the enhancement of J c by SiC doping can be extended to include all the field regions.…”

Effect of processing temperature on high field critical current density and upper critical field of nanocarbon doped MgB2

“…2͒. 25,26 The residual resistivity ratio ͑RRR͒ is derived as R͑300 K͒ / R͑40 K͒ and for nano-C doped samples it decreases with the sintering temperature ͑Table I͒. However, for the undoped and SiC doped MgB 2 , it follows the opposite trend.…”

“…However, for the undoped and SiC doped MgB 2 , it follows the opposite trend. 16,26 All of this can be understood in terms of carbon substitution being the dominant defect changing the H c2 for nanocarbon doped samples and hence increasing the charge carrier scattering rates. [27][28][29] On the other hand, the drop of H c2 and higher RRR for both undoped and SiC doped MgB 2 sintered at high temperature suggests a reduction of the impurity scattering of charge carriers due to improved crystallnity with high sintering temperature.…”

Superconducting Properties of MgB<SUB>2</SUB> Particle Impregnated with Mg-Based Alloys

“…Especially J c improvement in MgB 2 has been one of major issues of research; a well known problem is that J c drops rapidly with increasing magnetic field due to its poor flux pinning. Improvements in J c (enhancements of flux pinning) were achieved by various approaches; doping by chemical compounds, 7) substitution of Mg and/or B by other elements, 8,9) high-pressure application during process, 10) various heat-treatment, 11) etc. Theoretical studies for variations of the band structure by element substitution haven been also carried out.…”