May 28, 2009

Superconductor Improvements: Critical Current and Critical Temperature

1. Superconductor Meissner Transition found at 242 K reports efforts to increase the Tc of the 233K superconductive material, announced in March 2009, have been successful. Producing a new world record near 242K (-24F, -31C), the host compound has the formula (Tl4Ba)Ba4Ca2Cu10Oy and is believed to have a 9212/2212C intergrowth structure.

The chemical formula is written (Tl4Ba)Ba4 to indicate an extra barium atom is being substituted into a thallium atomic site. The purpose behind this 20% substitution is to hole-dope the insulating layer. Potential heavy metal dopants with oxidation states lower than +3 included mercury, cadmium and barium. However, both mercury and cadmium create a topological defect in the structure due to their affinity for other atomic sites. This left barium as the best candidate for doping.

2. Enhanced critical current densities in MgB2 by mixing relatively impure boron powders

S K Chen (Universiti Putra Malaysia) and J L MacManus-Driscoll (University of Cambridge)

Polycrystalline MgB2 samples were prepared from 99.98% purity Mg powder and different mixtures of relatively impure boron (99% pure crystalline boron and 95–97% amorphous boron) precursor powders. At both 6 and 20 K, for the mixed boron samples a doubling in Jc was observed over the highest values for single precursor samples. It is shown that the enhanced Jc results from the mixing effect of using different reaction rates of the different boron precursor powders. The work represents a cost-effective means of significantly improving current carrying performance in MgB2 conductors.

By use of two different boron precursor powders (99% pure crystalline B and 95–97% amorphous boron), MgB2 samples were fabricated with enhanced Jc(B) behaviors by more than a factor of two over samples made from the single precursors. Grain size bimodality and associated disorder were enhanced in the mixed powders, leading to enhanced pinning effective to at least 4.5 T at 6 K and to around 3 T at 20 K. Compared to expensive, high purity amorphous boron, only a small compromise in Jc is observed by use of the optimum powder mixture. The work represents a very practical, simple and cost-effective means of improving current carrying performance in MgB2. With further pinning additions to the powders (e.g. addition of nanoparticles) and optimization of heat treatments, yet further enhancement in Jc(B) is expected.

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