Evaluation of connectivity, flux pinning, and upper critical field contributions to the critical current density of bulk pure and SiC-alloyed MgB2

Abstract

Ќ transition is more appropriate for untextured samples than the bottom or 10% point on the small-current-density, resistive H c2 transition which corresponds to H c2 ʈ . However, the resistive H c2 transition is still useful for measuring the breadth of the parallel H c2 transition ⌬H, which may be indicative of inhomogeneity in composition in the sample. Hopes for expanding the useful range of MgB 2 are encouraged by earlier work that has shown that H c2 ʈ ͑0͒ can exceed 70 T in C-doped MgB 2 thin films, 2 but so far the highest H c2 ͑0͒ of C-or SiC-doped wires or bulks is ϳ35 T, 3,4,9,10 only half this value. Since H c2 and H irr enhancement is crucial for magnet applications, we have here systematically studied the H c2 transition and J c ͑H , T͒ behavior of pure and SiC-doped bulks. Irrespective of this high-field perspective on MgB 2 , we should also point out that J c ͑H͒ falls off only slowly in the 10-30 K range, making MgB 2 useful for lower field applications without liquid He.Our previous reports 6,7 showed that higher J c values were obtained in tapes using MgH 2 rather than Mg powder. Nano-SiC addition improved the high-field J c at low temperatures and produced a measured H c2 value of 23 T at 4.2 K. Here we present a more detailed study of MgB 2 samples cut from this same tape measured without any extraneous sheath material.MgB 2 bulk samples were prepared by conventional in situ powder-in-tube method with commercial MgH 2 and amorphous B powders which were mixed and packed into a pure Fe tube in air.7 5 or 10 mol % of ϳ30 nm SiC powder 5 was added for the doped samples. The filled tubes were groove rolled into 2 mm square rods and then flat rolled into 0.5 mm thick by 4 mm wide tapes. 50 mm long samples were heat treated at 600, 700, 800, and 900°C for 1 h under Ar atmosphere making the 12-sample set. 7 After peeling away the Fe sheath, resistivity curves were measured with 5 mA transport currents in a 9 T Quantum Design physical properties measurement system, the 33 T Bitter magnet at the National High Magnetic Field Laboratory ͑NHMFL͒ in Tallahassee, and the 60 T short pulse magnet at the NHMFL in Los Alamos National Laboratory. The 10% and 90% points on the resistive transition curves were used to define a transition breadth ⌬H and H c2 ʈ . Magnetization properties were measured in an Oxford Instruments vibrating sample magnetometer, from which the critical current density J c ͑H , T͒ was calculated assuming fully connected samples using the expression J c ͑H , T͒ = 0.5⌬M12b / ͑3bd − d 2 ͒, where b and d are the width and thickness of the rectangular section bar. Extrapolation of J c ͑H͒ to zero allowed extraction of H irr . However, following Rowell, 11 we believe that the connected cross section 1 / F of our samples is much less than unity, based on calculations of 1 / F using the relation ͑T͒ = F͓⌬ sc ͑T͒ + ͑0͔͒, where n is the measured normal state resistivity and ⌬ sc ͑300-50 K͒ = 7.3 ⍀ cm ͑Table I͒. Table I provides an overview of the properties of the four samples. SiC additions depres...

SiC and carbon nanotube distinctive effects on the superconducting properties of bulk MgB2

“…2͑b͔͒, in agreement with very recently reported data. 28 Matsumoto et al 26 reported a similar to our record H c2 ͑0͒ value for a SiC-doped MgB 2 tape prepared by the powder-in-tube ͑PIT͒ method. These results correspond to a PIT tape sample that probably has some texturing [31][32][33] and the reported H c2 data were measured with the applied field parallel to the tape.…”

“…Although the effect of carbon substitution is one of the most studied in MgB 2 , the results on C solubility and the effect of C doping on T c , J c , and H c2 reported so far vary significantly, due to precursor materials, fabrication techniques, and processing conditions used. [20][21][22][23][24][25] Recently, Matsumoto et al 26 reported that J c in SiC-alloyed MgB 2 tapes depends on a complex relation between grain connectivity, H c2 , and flux pinning induced by grain boundaries and precipitates. However, the distinct effect of C incorporation through different routes in J c and H c2 is still not entirely understood.…”

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

“…Matsumoto et al reported that enhanced grain connectivity degraded H c2 and flux pinning for nano-SiC doped MgB 2 sintered at high temperature. 16 Moreover, carbon doped MgB 2 has an improved homogeneity at higher processing temperature, as compared to SiC doping. 17 Monofilamentary wires of nominal atomic ratio MgB 2 and MgB 1.9 C 0.1 ͑carbon doped samples͒ were prepared by powder in tube via in situ reaction method, with powders of magnesium ͑99%, −325 mesh͒, amorphous boron ͑99%, 1-2 m͒, and amorphous carbon ͑Ͻ20 nm͒.…”

“…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.…”

Carbon Nanotubes Addition Effects on MgB2 Superconducting Properties

“…This is a good indication that CNTs could be aligned along the longitudinal axis of the MgB 2 wires by mechanical drawing in the PIT process. Matsumoto (Matsumoto et al, 2006) reported that J c in SiC-alloyed MgB 2 tapes depends on a complex relation between grain connectivity, H c2 , and flux pinning induced by grain boundaries and precipitates. However, the distinct effect of C incorporation through different routes in J c and H c2 is still not entirely understood.…”