Tomoyuki Akasaka

Excellent Critical Current Properties of Dilute Sr-Doped Dy123 Melt-Solidified Bulks at Low Temperatures

We have previously reported that dilute impurity doping technique universally enhances the critical current density of RE123 superconductors. These doping effects on the J c properties depend on substituted cation sites. In this paper, we report the flux pinning properties of the dilute Sr- or Co-doped Dy123 melt-solidified bulks in a wide temperature range. It was found that the dilute Sr-doping effectively enhance the J c for Dy123 melt-solidified bulks with low level Dy/Ba substitution. For the dilute Sr-doped Dy123 samples, the temperature coefficient of J c in 30 kOe was found to be approximately two times larger than for undoped materials above 40 K, meaning that it is an effective method for developing high-J c RE123 materials operating at low temperatures.

Cation Nonstoichiometry in Y123 Sintered Bulks and Thin Films

To clarify cation nonstoichiometry in Y123, Y123 sintered bulks and Y123 thin films starting from various yttrium compositions were synthesized. These samples were annealed under moderately high <i>P</i>_O2 to promote yttrium substitution for barium-site. Y123 sintered bulks showed decreases in <i>T</i>_c accompanying increases in <i>a</i>-axis length and decreases in <i>c</i>-axis length when they were synthesized from Y-excess composition or annealed at ~ 900°C under high <i>P</i>_O2 . Y123 thin films with various yttrium compositions showed different <i>T</i>_c behavior against oxygen annealing conditions, which might originate from the cation nonstoichiometry. These results suggest the existence of Y-excess composition even in Y123. Therefore, more attention must be paid for the cation nonstoichiometry in Y123 materials.

New Approaches for Enhancement of

New two methods to fabricate Y123 melt-solidified bulks with excellent superconducting properties have been developed. One is the new crystal growth method from the seed crystal which was put its c-axis parallel to the top surface and the other is addition of high-temperature annealing process in moderately reducing atmosphere after the crystal growth. The former method increased high-Jc regions and the latter enhanced Jc particularly at positions far from the seed crystals and at low temperatures through suppression of yttrium substitution for barium in the present study. The field trapping property was also improved by introduction of the reductive annealing process prior to the oxygen annealing at low temperatures.

J_{\rm c}

Most of studies on pinning properties and relating vortex nature of the RE123 system were performed at high temperatures around 77 K. However, the pinning characteristics of RE123 at low temperatures have not been well understood yet, while extensive applications using the cryocooling system are also strongly expected. In the present study, the Jc-H-T properties have been studied for Gd123 single crystal with a low level Gd substitution for Ba and Y123 single crystals substituted by small amount of impurity ions, such as Sr and Co, which improve Jc properties accompanying huge peak effect in their Jc-H curves. It was found that the increase in Fp with decreasing temperature of the impurity doped Y123 samples were much larger than that of the Gd123 sample, while the Gd123 sample exhibited higher Jc than these Y123 samples above 77 K. The Jc performance below 50 K of the Gd123 sample was even lower than that of the undoped Y123 sample. This result strongly indicates that the RE123 with RE/Ba substitution does not appropriate for low temperature applications, such as high field magnets despite their good Jc performance at high temperatures e.g. 77 K.