Isothermal growth of large YBa2Cu3O7−x single domains up to 93 mm (original) (raw)
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Direct observation of textured YBa2Cu3O7−δ crystal growth from the melt
Physica C: Superconductivity, 1992
The behavior and kinetics of textured growth of the YBaZCu@_d (123) compound from Y,BaCuO, (21 I) +liquid have been investigated by hot-stage optical microscopy and high-temperature x-ray diffraction methods. The formation of the 123 is found to begin in the liquid of the peritectic melt, not nucleated from 2 11 crystals as is widely assumed. The quantity of the liquid phase strongly influences the crystal growth and final morphology of the 123. Dynamic studies show that the optimum temperature for 001 orientation of 123 is 940°C on an MgO substrate. The growth rate of 123 from the melt is rapid; the rate constant was determined to be 0.526x 10m2 s-l.
Physica C: Superconductivity, 1998
Parameters affecting the nucleation and growth of a Y123 single-domained crystal were studied in top-seeded melt processing. Based on the parametric study, the two-step undercooling technique was developed. The key points techniques are the controlled nucleation of Y123 at the primary undercooling step and the accelerated growth at the secondary undercooling temperature. Optimization of the process parameters in the two-step undercooling technique considerably reduced the processing time required for the growth of the Y123 single crystal without degradation of the levitation property.
Growth of YBaCuO single-domains up to 7 cm
Materials Science and Engineering: B, 1998
The growth of large top seeded YBaCuO single-domain has been studied by means of in situ video recording during the process. The growth for samples (a <4 cm), textured under a thermal gradient, follows a model based on yttrium diffusion through the melt. The growth appears to be limited (3 x 3 cm2) for an isothermal texturation whereas single-domains up to 7 cm are grown using whether a radial thermal gradient or a overheating process. 0 1998 Elsevier Science S.A. All rights reserved.
Physica C: Superconductivity, 1996
A modified melt-textured-growth (MTG) method which has a relatively low working temperature and short reaction time compared with the conventional MTG method has been developed. The number of YBa2CU3OT_y (123) nuclei during the initial slow-cooling process has been minimized which leads to the enhancement of the 123 grain growth rate and a better homogenized and smaller Y2BaCuOs (211) particles over the samples. Magnetization measurement shows that the 123 crystals have a high Jc and a good performance in high magnetic fields.
Nucleation and growth mechanisms of textured YBaCuO and the influence of Y2BaCuO5
Physica C-superconductivity and Its Applications, 1994
123 texturation has been achieved with success on Y203 without application of an external thermal gradient for the purpose of a better understanding of nucleation and growth mechanisms. We have studied the effects of thermal parameters such as the maximal applied temperature and crystallization speed. Their respective contributions to the peritectic recrystallization (211 consumption), and to the textured domain size have been made precise by using X-ray data and semi-quantitative analysis on the micrographs. The interface reaction between the Y203 substrate and 123 has been analyzed. The substrate promotes a seeded type growth of 123 owing to an interracial 211 layer. The mechanism of crystallization has been analyzed in the light of existing theoretical models. Our data confirm a crystallization of 123 directly from a liquid as in a peritectic reaction where a major part is played by the yttrium diffusion in the liquid. Y atoms are supplied by the dissolution of 211 particles. According to Uhlmann-Chalmers-Jackson (UCJ) theory, coarsened 211 grains are trapped by the solidification front in contrast to particles smaller than a critical radius that are consumed in the peritectic reaction. (This process explains why such large 211 particles are obtained in MTG contrary to QMG or MPMG methods. ) Moreover, EPR results show a preferential orientation of 211 inclusions according to the 123 matrix which could be favorable to flux pinning in the superconducting state. 211 oriented inclusions make possible, to some extent, heterogeneous nucleation at platelet-211 junctions. On the other hand, microstructural studies show that the liquid/solid interface is rather unstable being frequently cellular or dendritic, consistent with a model proposed by .
Isothermal growth of large YBaCuO single domains through an artificial array of holes
Journal of Crystal Growth, 2005
For various applications, single domains (centimetric crystals with defects) of the YBa 2 Cu 3 O x superconductor oxide are grown using the top seeding melt growth (TSMG) method. During the oxygen annealing inducing superconductivity, cracks open at the surface layer and propagate into the bulk. Since the cracking can be reduced on thin samples, we proposed to grow single domains with thin wall geometry prior to oxygenation.
Growth rate limiting mechanisms of YBa2Cu3O7 films manufactured by ex situ processing
Physica C: Superconductivity, 2001
YBa 2 Cu 3 O 7 ®lms were fabricated on SrTiO 3 substrates using the BaF 2 ex situ process. Precursor ®lms 1, 3 and 5 lm thick were processed in an atmospheric pressure reactor using a gas mixture of oxygen, nitrogen and water vapor. The ®lms were processed at dierent water vapor pressures and it was observed that the ®lm growth rate was independent of ®lm thickness and proportional to the square root of the water vapor pressure. The dependence of the ®lm growth rate on ®lm area was also investigated for ®lm areas varying from 10 to 160 mm 2. Surprisingly, it was observed that the growth rate was inversely proportional to square root of the area of the ®lm. A theoretical model is developed and applied to the experimental results presented in this paper. The model correctly predicts the inverse square root dependence of the ®lm growth rate on sample area. In addition, it predicts that it will be dicult to process long samples, such as tapes, in simple reactor geometries.
Physica C: Superconductivity, 2003
The influence of the ratio of small and large size YBa 2 Cu 3 O x (Y123) starting powders on the microstructure development in the sintered and melt-textured YBa 2 Cu 3 O 7 /Y 2 BaCuO 5 (Y211) bulk superconductors with the CeO 2 addition was studied. It was shown that Y211 low concentration regions in the microstructure of the melt-textured samples are formed due to the presence of large multigrain particles and/or agglomerates of small 123 particles in the starting Y123 powder. Moreover, it was observed that large Y211 particles in the melt-textured samples are nucleated on the surface of large Y123 grains during the sintering stage. It is suggested that the insufficient mixing of the powders with higher portion of small Y123 particles causes that the CeO 2 concentration in the sample is more inhomogeneous. Consequently the Y211 particles can locally grow during peritectic melting and temperature dwell.
Growth shape of melt-textured YBaCuO crystal
Journal of Crystal Growth, 1995
A growth shape of the melt-textured YBa2Cu3Oy (1-2-3) domain was constructed with two-and three-dimensional shapes of the local parts of the 1-2-3 domain. The 1-2-3 domain has a polygonal shape with a square pyramid consisting of stepped terraces at the top and a tetragonal columnar body. X-like linear traces are observed on the surface of the 1-2-3 domains, which often meet at the corners and the center of the plane. The melt-textured 1-2-3 domain is believed to be a single-crystalline body including some microstructural defects such as microcracks, entrapped liquid phase and YzBaCuO5 inclusions.