Transition Temperatures and Upper Critical Fields in Multifilamentary NbTi Wires
The development of multifilamentary superconductors is expected to realize electrical machinery and apparatus with high efficiency. The filament diameter df of superconductors being used for such AC applications is reduced to a level of submicrons or below to decrease AC loss. According to recent work on NbTi composite wires with ultrafine filaments1,2 transition temperature Tc and upper critical field B c2 decrease while critical current density Jc shows an anomalous maximum with decreasing df in the range of below 0.1 μ m. The increase in Jc may be qualitatively explained by surface pinning and one-dimensional flux line lattice model. On the other hand, the proximity effect3 is well-known as one of the origins yielding such a decrease in Tc for fine filament superconductors covered with normal metals and depends on the superconducting filament size, the spacing between filaments, and the matrix material. In order to produce NbTi composite wires with high performance for AC use, the decrease in Tc and B c2 for NbTi filaments of ~ 0.1 μm must be suppressed and further wide data on NbTi composite wires which are different in filament size and matrix material would be required.
KeywordsMatrix Material Critical Current Density Critical Field Filament Size Filament Diameter
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