High-energy milling of magnesium diboride ͑MgB 2 ͒ prereacted powder renders the material largely amorphous through extreme mechanical deformation and is suitable for mechanically alloying MgB 2 with dopants including carbon. Bulk samples of milled carbon and MgB 2 powders subjected to hot isostatic pressing and Mg vapor annealing have achieved upper critical fields in excess of 32 T and critical current density approaching 10 6 A/cm 2 . 6 The measured H c2 in this case is the higher parallel critical field H c2 ʈ ͑0 K͒, rather than the lower, perpendicular critical field that controls the bulk J C . One immediate question is how thin films and bulk differ. A partial answer was given by Braccini et al.1 who noted that the c-axis parameter of the highest H c2 films was expanded relative to C-doped bulk samples, which have lower H c2 values.Several groups 7-10 have also produced wires containing SiC, the latter finding that H c2 could attain values as high as the CVD filaments of Wilke et al. 6 In fact there is increasing suspicion that one of the beneficial effects of SiC addition occurs by C doping of the MgB 2 . In general there is good agreement that C doping of MgB 2 is a useful means of alloying MgB 2 , even if the best bulk H c2 values are only about half those of the best thin films. The present Letter discusses the ex situ synthesis of alloyed MgB 2 powder using high energy ball milling of MgB 2 with C. Since a major goal of MgB 2 technology is the fabrication of high critical current density, multifilament wire suitable for magnet applications, we need a scalable bulk process capable of producing carbon-doped precursor powder. One such method is provided by this paper.The composition of carbon-doped MgB 2 is commonly given using the format Mg͑B 1−X C X ͒ 2 . Alfa-Aesar prereacted MgB 2 powder was mixed with powdered graphite in the following proportions: 0.1C + 0.9MgB 2 ͑nominal X = 0.0525͒ and 0.3C + 0.7MgB 2 ͑nominal X = 0.17͒. The mixtures were high energy ball milled in a Spex 8000M mixer/mill with a WC milling container and milling media. Powders were milled until no graphite peak was discerned in the x-ray diffraction ͑XRD͒ pattern, which took 10 h for X = 0.0525 and 25 h for X = 0.17. The milled powders were then cold isostatic pressed to form pellets that were welded into evacuated stainless steel tubes and hot isostatic pressed ͑HIP͒ at 1000°C and Ͼ30 ksi for 200 min. These HIP-treated pellets were cut apart and examined. They were black, lusterless, and lacked mechanical strength. These features were attributed to incomplete reaction due to a deficiency of Mg with respect to ͑B+C͒ resulting from adding C to the ex situ powder without adding Mg. Accordingly, portions of the HIP-treated pellets were welded into evacuated stainless steel tubing with a quantity of Mg metal that was about half the volume of the MgB 2 pellet. MgB 2 pellets and Mg metal were arranged in such a way that the sample would be exposed to Mg vapor, while limiting exposure to liquid Mg, as described by Braccini et al. 11 ...

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...

A key issue affecting the critical current density of MgB 2 is the lack of full electrical connectivity. In situ MgB 2 wires can be easily fabricated by reacting Mg powder with amorphous B powder, but such powders normally contain impurities such as B 2 O 3 as a result of exposure to air. Here we introduce a practical method for removing B 2 O 3 prior to making MgB 2 . X-ray diffraction and microstructure analysis show that removing B 2 O 3 results in a decrease in MgO from 5.3 to 1.5 mol% in the final reacted MgB 2 . Normal state transport measurements using the Rowell analysis indicate that the active current-carrying area fraction (A F ) increased from ∼0.25 to ∼0.48 in MgB 2 made with purified B. These results demonstrate that intergranular MgO is an important current barrier in MgB 2 and that it can be significantly reduced by boron purification.

We have systematically doped pre-reacted MgB2 with C by high-energy ball milling, and obtained peak Jc (8 T, 4.2 K)>500 A mm−2 in a form compatible with powder-in-tube wire manufacture. Effects of carbon doping by this method were qualitatively similar to single crystals and CVD (chemical-vapour-deposited) filaments, but Tc, Hc2, and resistivity measurements indicate more electron scattering than in single crystals and CVD filaments of similar composition. We attribute both the high Jc(H) and the large degree of electron scattering to the high grain boundary density associated with our very fine ∼50 nm grain size. At high C concentrations (C/(B+C)>0.07) Jc fell sharply. We identified the cause of the Jc decline to be a sharp decline in connectivity. This work demonstrates how Jc(H) of pre-reacted MgB2 can be improved by simultaneously alloying with C and refining grains through cold work and further improvement of connectivity.