Understanding the route to high critical current density in mechanically alloyed Mg(B_1−xC_x)₂

Abstract

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.

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.…”

“…We observe the same dependence reported in previous works. 22,25,28 The H c2 ͑T͒ dependence was obtained from four-probe transport measurements in a 200 ms midpulse magnet up to 50 T at NHMFL-LANL, performed at temperatures between 1.4 and 35 K. Figure 2͑a͒ displays the H c2 defined as the onset ͑extrapolation of maximum slope up to normal state resistivity͒ of the R versus H data as a function of the reduced temperature t = T / T c0 , where T c0 =39 K ͑Ref. 29͒ for all samples, as listed in Table II.…”

Carbon Nanotubes Addition Effects on MgB2 Superconducting Properties

“…The H c2 (0) of CNTdw10 is close to the maximum H c2 value as a function of x. A decrease in H c2 (0) was also observed for a larger x value (see Fig.8)), in agreement with other reported data (Senkowicz et al, 2007).…”

EFFECTS of DUAL DOPING OF C AND TiC NANOPARTICLES ON SUPERCONDUCTING PROPERTIES OF Fe-SHEATHED MgB[sub 2] TAPES

“…We believe that the MgO impurity phase was formed due to the trapped oxygen in the raw powder, while the formation of Fe 2 B should be due to the reaction between B and iron sheath. Because of the excess carbon in this system, Mg(B 1-x C x ) 2 phase was formed with 0 < x < 1 [17,18]. This may cause the excess B atoms to react with the Fe-sheath to form Fe 2 B.…”