Masaki Sumida

UNIDIRECTIONAL PARTIAL MELTING AND SOLIDIFICATION OF SMBCO SUPERCONDUCTOR

Microstructure control of SmBCO superconductor was carried out using the floating zone partial melting and solidification method. It is generally recognized that finely and uniformly dispersed non-superconductive high temperature stable phase (Sm211) particles included in the superconductive Sm123 matrix act as effective pinning centers. Microstructure formation of the partial molten mixture (Sm211 particles and BaO{endash}CuO liquid) by decomposition of the precursor Sm123 on melting and solidification of Sm123 from the mixture have to be controlled concurrently to fabricate the 123/211 composite fiber with the optimum microstructure. During unidirectional solidification, planar crystal growth which provides the single crystal growth of Sm123 becomes unstable with increased growth rate. During unidirectional melting, the mean diameter of aligned Sm211 particles behind the melting interface decreases with increased growth rate and with decreased temperature gradient at the melting interface. Initial composition of the precursor significantly affects the formation behavior of Sm211 particles. The contribution of process parameters to the microstructure formation is also briefly discussed. {copyright} {ital 1997 Materials Research Society.}

Floating zone partial melting and solidification of SmBCO superconductor under low oxygen partial pressure

Microstructure control of SmBCO superconductor was carried out using the floating zone partial melting and solidification method under 0.01 atm oxygen partial pressure which is a preferable atmosphere to obtain a crystal with stoichiometric SmBCO. The growth rate, initial composition, and addition of small amount of platinum dependences on the microstructure formations of the (Sm211 + L) mixture during melting and the Sm123 or Sm123/211 during solidification were investigated. Furthermore, superconductive properties of the solidified Sm123/211 were measured by SQUID after appropriate oxygen annealing. Estimated critical current density of the single crystalline Sm123/211 was 3.6 × 10 4 A/cm at 77 K, 1 T.

Influences of oxygen partial pressure on YBCO grain growth by a zone melting method

This study reports investigation on influences of Y{endash}Ba{endash}Cu{endash}O compounds solidification under different oxygen partial pressures by a zone melting method. In Y{endash}Ba{endash}Cu{endash}O system, {ital P}(O{sub 2}) ranged from 0.02 to 1 atm, following reaction occurred during heating: YBa{sub 2}Cu{sub 3}O{sub 6+{delta}}(Y123){r_arrow}Y{sub 2}BaCuO{sub 5}(Y211)+L. With an increase of oxygen partial pressure or decrease of pulling rate, the morphology of solidified interface changed from mushy to equiaxed, cellular, and planar. The continuous Y123 grains were readily obtained under high oxygen partial pressure. Based on the constitutional supercooling theory and combining the result of the yttrium solubility limit for different oxygen partial pressures, the influences of oxygen partial pressure on Y123 morphological evolution were clarified. {copyright} {ital 1996 Materials Research Society.}

Coarsening of High Temperature Stable Phase Particles in RE(Nd,Sm,Y)BaCuo Melt

Development of a new material process to produce REBa2Cu3Ox (RE123, RE = Nd, Sm, Y)/RE422 (RE = Nd) or RE211 (RE = Sm, Y) composite bulk materials or fibers with high Jc has been required to apply to superconductive magnets and current leads. The Jc value of superconductors in the low magnetic fields increases with decreasing the size of high temperature stable phase particles (RE422 or RE211) in the superconductive matrix (RE123).

Effect of additives (Pt or CeO2) on Nd4Ba2Cu2O10 particles size in melt processed NdBa2Cu3Ox superconductor

Abstract Finely and uniformly dispersed high temperature stable phase particles in the single crystalline superconductive phase matrix can contribute to the improvement of the J c property. For the purpose of investigation of the Pt or CeO 2 addition effect on Nd 4 Ba 2 Cu 2 O 10 (Nd422) particles size, the experiments to study coarsening of Nd422 particles in the liquid was carried out. Microstructure observation and quantitative measurements showed that the size of Nd422 particles with Pt addition was approximately the same size as that without Pt addition, while CeO 2 addition has the significant effect on the size reduction of Nd422 particles in the liquid.

Interface Structure of Heteroepitaxially Grown Y123/Pr123 Crystals by the SRL-CP Method

Y123/Pr123 crystals were fabricated by the Solute Rich Liquid-Crystal Pulling (CRL-CP) method and the interface structure was investigated. Pr123 and Y123 single crystals were used as seed crystals. Y123 and Pr123 single crystals were grown on seed crystals of Pr123 and Y123 respectively. The decomposition temperatures of Pr123 and Y123 under air atmosphere are 972 °C and 1005 °C respectively. To prevent the decomposition of Pr123, the solution for the SRL-CP was changed from Ba3Cu5O8 (process temperature 1000 °C) to Ba2.85Cu7.15O10 (process temperature 930 ~ 980 °C). On the other hand it was not necessary to change the solution (BaCu3O4) when a Y123 seed was used for growing a Pr123 crystals. The interfaces of these bicrystals were investigated by polarized microscopy, electron probe microanalysis (EPMA).

The melting and solidification mechanism of R123 high Tc superconductor (R=Y,Sm,Nd•••)

The improvement of critical current density(Jc) of REBa2Cu3Ox (R=Y,Sm,Nd•••, R123 hereafter) has been performed by means of unidirectional melting and solidification. The high temperature stable phase crystals (Y211, Sm211, Nd422 under air atmosphere) in the bulky RE123 are expected to act as strong pinning centers and also to be important for the RE123 growth, accordingly it is necessary to control their micro/macro structures, based on the understandings of the growth mechanisms of both the melting and solidification interfaces.

Interface morphological stability of RE123 superconductor via peritectic reaction

Publisher Summary This chapter explores the interface stability of RE 123 superconductors which is analyzed in accordance with the constitutional supercooling criterion. As single crystal growth of the 123 phase is largely dependent on the interface stability, quantitative analysis is greatly required. Considering, at a certain supercooled situation from the peritectic temperature, a unidirectional peritectic solidification of the 123 phase from the 211 +liquid mixture by liquid diffusion limitation, solute concentration distribution around the 123 and an isolated 211 particles is derived precisely under the bispherical coordinate. Interface stability is determined with reference to competition of the liquidus temperature gradient and the imposed temperature gradient at the 123/L interface along the 123 growth direction. From the analysis above, it is clarified that the constitutional supercooling must exist when a 211 particle comes close to the 123 growth interface. It can also predict that larger 211 particles, larger growth rate, or smaller imposed temperature gradient causes easy occurrence of constitutional supercooling ahead of the 123 growth interface. This tendency agrees with the microstructure observation and itis concluded that the interface stability of the 123 phase is explainable quantitatively.

Fabrication of PrBa2Cu3O7-δ single crystal by SRL-CP method

Recently, we succeeded in producing single crystals of PrBa2Cu3O7-δ by the SRL-CP method. Y2O3 and MgO polycrystalline crucibles and MgO single crystal crucible were used to grow the single crystals. The temperature at the contact point was between 960°C and 1000°C. The grown crystals were identified to be PrBa2Cu3O7-δ by X-ray diffraction. Electron probe micro analysis (EPMA) shows that in the case of the Y2O3 crucible the composition of crystals were Pr1-xYxBa2Cu3O7-d (0.48 < × < 0.57) and in the case of the MgO crucible the contamination of Mg was relatively small with the composition of PrBa2Cu3O7-δMg0.06. According to the crystal growth model of the SRL-CP method, The maximum growth rate of 3 × 10−5 cm/s was calculated and is in reasonably good agreement with the experimental result.