Masahiro Egami

Investigation of YBCO Single Crystals Grown under 1 atm Oxygen Pressure and Air Atmosphere

The effects of oxygen partial pressure on Y123 single-crystal growth, morphology and properties were investigated. YBCO (Y123) single crystals were fabricated using the crystal pulling method under 1 atm oxygen pressure. Compared with crystals fabricated under air atmosphere, it was found that this high oxygen partial pressure played a significant role in increasing growth rate, which was 1.5–2.5 times higher than that of crystals fabricated under air atmosphere. A large crystal with size of 19.8×19.5 mm2 in the a–b plane and 16.5 mm in the c-axis direction was successfully obtained using a crucible 50 mm in diameter. In the YBCO system, critical temperature T c was not sensitive to the growth atmosphere.

Superconductivity and crystal growth of YBCO grown under 1 atm oxygen pressure

Abstract YBCO (Y123) single crystals were grown under 1 atm oxygen pressure by using the crystal pulling method. After annealing, critical temperature Tc was measured to be 92.7 K, which is nearly the same as those of the crystals grown in low oxygen partial pressure and air atmosphere. This indicates that the substitution of Y for Ba and Tc are not sensitive to the oxygen partial pressure of the growth atmosphere. Moreover, it was clarified that high oxygen partial pressure has a profound effect on growth rate, which is about 1.5–2.5 times larger under 1 atm oxygen pressure than under air atmosphere.

CRYSTALLINITY OF YBA2CU3O7-X SINGLE CRYSTALS GROWN BY THE PULLING METHOD

Large YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} (Y123) single crystals (larger than 13 mm cubed) have been grown along the {ital c}-axis reproducibly by the modified pulling method. The crystallinity of {ital Y}123 single crystal was investigated by a x-ray diffraction and a x-ray topography. Crystals grown from an MgO single crystal seed had some low angle subgrain boundaries which tilted 0.1{endash}0.8{degree} from each other. These grain boundaries originated from the seed crystal, and the subgrains were extended along the growth direction from the seed crystal. Y123 single crystals with no marked subgrains in the whole area were obtained by using Y123 single subgrain crystal seeds. FWHM of the x-ray rocking curve for the crystal so produced was about 0.14{degree}, which was much better than the spectrum consisted of several separated peaks obtained from the previous crystals.{ital T}{sub {ital c}} onset of the annealed sample was about 93.6 K, the transition width was about 0.9 K. The low angle subgrain boundaries did not seem not to be effective pinning centers for the magnetic flux. {copyright} {ital 1996 Materials Research Society.}

Single crystal growth and LPE thin film process for RE-123 superconducting materials

Abstract Fundamentals of solidification processing for high- T c superconductors (HTSC) and the progress of the crystal pulling and liquid phase epitaxy (LPE) processes were reviewed. RE-123 crystal is solidified by peritectic reaction with diffusion through the liquid. Based on this idea, a modified CZ (SRL-CP) method was developed for continuous single crystal growth. The large Nd123 single crystal, 25×25×25 mm 3 in size, was grown by optimizing the growth conditions. The high crystallinity in Y123 single crystal, which means monograin, was also realized by using Y123 seed crystals cut out of the best possible crystalline part of the single crystal grown separately. Additionally, the growth mechanism was discussed using the solidification model. In the LPE process, the highly aligned films in both c -axis and in-plane can be grown with the high superconducting characteristics, such as J c . The self-improvement mechanism for in-plane alignment due to preferential dissolution/growth was explained by the difference in the MgO/Y123 interface energies with different orientations.

Fabrication of meander shaped (Y,Yb)BCO LPE films for PCS materials

Abstract We have investigated the basic characteristics of the liquid phase epitaxy (LPE) grown (Y,Yb) Ba2Cu3Oy film and have proposed a meander shaped film to realize a high critical current as well as a high electric resistance in the normal-conducting state due to a long current path for the persistent current switch (PCS) materials. The LPE film exhibited an excellent critical current density in the superconducting state and a high electric resistance in the normal state which are both required for PCS applications. The meander shape of the seed films could be maintained during the further LPE growth. This could be explained by decreasing the lateral growth rate due to the combined phenomena of the step bunching and the difficulty of the overhanging growth above the MgO valley which was formed by the partial dissolution of the MgO surface to the solution. The meander shaped LPE film with a long current path on the substrate of 2″ in diameter could be successfully grown, which is a strong candidate as the PCS materials.

Investigation of the Crystal Shape Controllability of YBa2Cu3O7-x Single Crystals in the Pulling Method

The influence of the geometrical difference on the crystal growth rate of YBa2Cu3O7-x (Y123) single crystals in the modified pulling method was investigated. The growth rate of {001} faces appeared on the bottom of the crystals grown along direction (Rcvc) was about 1.6–2.0 times higher than that of {100} faces appeared on the bottom of the crystals grown along direction (Rava). Rava is more sensitive to the change of supersaturation than Raha (the crystal growth rate of {100} faces appeared on the side of the crystals grown along direction) for the low crystal pulling rate and for the low supersaturation regime, although these faces are crystallographically the same. These difference is attributed to the increase of the temperature at the crystal bottom face during the crystal growth. It is supposed from the experimental results that Y123 single crystals having long and nearly straight body parts with expanding about 25° can be obtained in the direction growth of the modified pulling method.

Enlargement of YBCO Single Crystals by Crystal Pulling

The melt convection phenomena in YBa2Cu3O7-x(Y123) single crystal growth by the modified pulling method (Solute Rich Liquid Crystal Pulling method; SRL-CP method)[1] was investigated by the experiments and the numerical calculations. Increase of the crystal diameter and the crystal rotation rate increased the strength of the forced convection in the melt, and as a result, the temperature at the crystal growth interface increased. This led to reduction of the crystal growth rate. Using a larger crucible could stabilize the melt flow pattern with the change of the crystal diameter and reduce the generation rate of the floating particles on the melt surface, and as a consequence the period of the single crystal growth could be extended. A Y123 single crystal which was 14.5mm*14.5mm and 13mm in length could be pulled along the c-axis with 100mm inner diameter crucible and a newly developed large furnace.

Phase-field analyses for Y123 crystal growth

Abstract A phase-field evolution equation (a kind of time dependent Ginzburg–Landau equation), in which order parameter ( φ ) transition in width restricted within a unit grid, was solved using an implicit finite difference method. The direct implicit scheme would have an ability to correspond to a long-range order parameter correlation, which appeared around a phase transition. When the transition width is restricted within a unit grid, a skew directional finite difference is necessary to obtain an isotropic growth rate. Using these schemes, an isotropic growth shape was obtained for a single composition system with a latent heat free condition. In the case of a large latent heat condition, which is the case for Y123 crystal growth, we could obtain a faceted growth shape without using any anisotropic parameters.

Fabrication of Large YBCO Single Crystal by SRL-CP Method under 1 atm Oxygen Pressure

The effects of oxygen partial pressure on the Y123 crystal growth were studied. Under 1 atm oxygen pressure YBCO(Y123) single crystals were fabricated by using the crystal pulling method. Compared with single crystals grown under air atmosphere, this high oxygen partial pressure played a significant role in increasing growth rate, which is 1.5–2.5 times higher than that in air atmosphere. Twin structures were observed on as-grown crystal surface. However, they did not appear when crystals were cooled in pure nitrogen. A large crystal -- size up to 19.8×19.5 mm2 in the a-b plane and 16.5 mm in the c-axis direction -- was successfully obtained by using a crucible of 50 mm in diameter.