Yoshihisa Ohzono

Effect of platinum on the critical current density of fluorine-doped YBa2Cu3Ox superconductors

The critical current density, Jc, in fluorine-doped YBa2Cu3Ox bulk superconductors has been measured at several temperatures. The measured Jc value is 108 A m-2 at 77 K and 0.5 T. A sample with a larger critical current density can be produced by the addition of a small amount of platinum during the sintering process. The added platinum has the effect of evenly distributing Y2BaCuO5 particles and voids in the matrix in comparison with the undoped sample. These distributions of Y2BaCuO5 particles and voids are reflected in an increase in the critical current density. However, these distributions are not directly related to the observed peak effect in the critical current density. If the fluorine-doped YBa2Cu3Ox superconductor consists of the matrix and a phase with a Ginzburg–Landau parameter, that is slightly different from that of the matrix, then the occurrence of the peak effect can be explained.

Influence of Grain Size on Notch Sensitivities in Fatigue of Carbon Steel

In order to investigate the influence of grain size on notch sensitivities in fatigue of a fine-grained carbon steel, rotating bending fatigue tests were carried out using specimens with a V-grooved circumferential notch of commercial fine-grained carbon steel with grain size of 6.5µm. The results were compared with those of a larger grain sized carbon steel (grain size: 20 µm) and the notch sensitivities were evaluated based on Linear notch mechanics proposed by Nisitani. Notch sensitivities for both of fatigue limits for a crack initiation and its propagation of the fine grained steel were high. The results were discussed from the view points of the size of area related to crack initiation and the crack growth resistance.

Fine Y2BaCuO5 particles in fluorine-doped YBa2Cu3Ox superconductors synthesized at temperatures below 950°C

The size of Y2BaCuO5 particles in fluorine-doped YBa2Cu3O superconductors synthesized at a lower temperature than in the quench and melt growth processed materials are studied and compared with samples with a different amount of platinum doping. The size of Y2BaCuO5 particles reduces as platinum doping increases. The effect of the platinum doping on reducing Y2BaCuO5 particles in YBa2Cu3Ox superconductor was investigated for samples which were quenched at different temperatures. The Ba4CuPt2O9 compound, which reduces the Y2BaCuO5 particle size, has been detected directly in the quenched samples. Samples with high critical density include voids and cracks in the microstructure. Since the shapes of voids and cracks are larger than those of fluxoid, they may not act as effective pinning centers as the Y2BaCuO5 particles do. Therefore, the increase of the critical current density is believed to be associated with the size of the fine Y2BaCuO5 particles.