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Register a new userNanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride
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Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes. Full C incorporation required only 30-60 min of milling. Grain size of sintered samples decreased with increased milling time to less than 30 nm for 20-50 hrs of milling. Milling had a weak detrimental effect on connectivity. Strong irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to increased milling time was observed and correlated linearly with inverse grain size (1/d). As a result, high field Jc benefited greatly from lengthy powder milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional performance increase is attributed to grain refinement of the unsintered powder by milling, and to the probable suppression of grain growth by milling-induced MgO nano-dispersions.
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We investigated the effect of nanoscale-C doping on the critical current density Jc and irreversibility field Birr of Fe-sheathed MgB2 tapes prepared by the in-situ powder-in-tube method. The tapes were heat treated at 600-950C for 1 h. Higher values of Jc and Birr were seen for 5 at.%C-doped MgB2 tapes at higher sintering temperatures, where substantial substitution of boron for carbon occurred. The C-doped samples sintered at 950C showed the highest Birr, for example, at 4.2 K, the Birr reached 22.9 T. In particular, at 20 K, Birr for the C-doped tape achieved 9 T, which is comparable to the upper critical field of the commercial NbTi at 4.2 K. This role of nano-sized C particles can be very beneficial in the fabrication of MgB2 tapes for magnetic resonance imaging applications at 20 K.
The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. With twice the critical temperature of Nb_3Sn and four times that of Nb-Ti alloy, MgB_2 has the potential to reach much higher fields and current densities than either of these technological superconductors. A vital prerequisite, strongly linked current flow, has already been demonstrated even at this early stage. One possible drawback is the observation that the field at which superconductivity is destroyed is modest. Further, the field which limits the range of practical applications, the irreversibility field H*(T), is ~7 T at liquid helium temperature (4.2 K), significantly lower than ~10 T for Nb-Ti and ~20 T for Nb_3Sn. Here we show that MgB_2 thin films can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding H*(4.2 K) above 14 T. In addition, very high critical current densities at 4.2 K, 1 MA/cm_2 at 1 T and 10_5 A/cm_2 at 10 T, are possible. These data demonstrate that MgB_2 has credible potential for high-field superconducting applications.
Carbon-doped magnesium diboride was fabricated from pre-reacted pure MgB2 by mechanical alloying. The sample set had excellent critical current densities Jc(8T,4.2K) ranging from 15-60 kA/cm2, depending on composition. Magneto-optical imaging detected regions up to 0.5 mm in size which were nearly 100% dense with Jc 2-6 times that of the matrix. Evaluation of resistivity curves using the Rowell method predicts that only 10-50% of the cross sectional area carries the normal state measurement current, suggesting that considerable increases in Jc in these ~80% dense MgB2 samples would be possible with complete grain and particle connectivity.
MgB2 monofilamentary nickel-sheated tapes and wires were fabricated by means of the ex-situ powder-in-tube method using either high-energy ball milled and low temperature synthesized powders. All sample were sintered at 920 C in Ar flow. The milling time and the revolution speed were tuned in order to maximize the critical current density in field (Jc): the maximum Jc value of 6 x 10e4 A/cm2 at 5 K and 4 T was obtained corresponding to the tape prepared with powders milled for 144h at 180rpm. Vorious synthesis temperature were also investigated (730-900 C) finding a best Jc value for the wire prepared with powders synthesized at 745 C. We speculate that this optimal temperature is due to the fluidifying effect of unreacted magnesium content before the sintering process which could better connect the grains.
A relatively high critical temperature, Tc, approaching 40 K, places the recently-discovered superconductor magnesium diboride (MgB2) intermediate between the families of low- and copper-oxide-based high-temperature superconductors (HTS). Supercurrent flow in MgB2 is unhindered by grain boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB2 is that in bulk material the critical current density, Jc, appears to drop more rapidly with increasing magnetic field than it does in the HTS phases. The magnitude and field dependence of Jc are related to the presence of structural defects that can pin the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies suggest that it is the paucity of suitable defects in MgB2 that causes the rapid decay of Jc with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase Jc significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB2 by economically-viable routes.
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