Simulations of the kinetic growth of YBa2Cu3O7 − δ grains (original) (raw)
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Simulations of the kinetic growth of YBa2Cu3O7 − grains
Journal of Crystal Growth, 1996
Using simple kinetic and geometric arguments, we have developed a computer model which simulates the growth of melt-textured YBa2Cu307_ ~ grains along the a-b planes. A chemical bonding anisotropy along the main lattice directions is taken into account by the model. We have also considered the presence of Y2BaCuO5 particles becoming trapped or not in the solidifying YBaaCU3OT_ ~ front. The model simulates the formation of 211 segregation patterns in the 123 matrix. An interpretation of these is attempted in terms of trapping anisotropy due to intrinsic crystallographic anisotropy.
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.
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 .
Subgrain formation, 211 particle segregation and high-angle 90° boundaries in melt-grown YBaCuO
Physica C-superconductivity and Its Applications, 1997
Microstructural features such as subgrains, Y2BaCuO5 particles, and high-angle boundaries were studied in single- and multiple-domain melt-textured YBaCuO levitators. Subgrains elongated in the c- or a-direction and rectangular in cross section were formed during solidification by cellular growth. Formation of subgrains by combination of a- and c-growth was also observed. Their boundaries were predominantly low-angle, and were not cracked or wetted by second phases. Macrosegregation of Y2BaCuO5 particles developed mainly in the regions formed by c-axis growth. The segregation is related to Y2BaCuO5 particle pushing by the YBa2Cu3O7−x growth front. The structure of 90° high-angle grain boundaries appeared to be similar to that of the low-angle grain boundaries.
Entrapment of Y2BaCuO5 particles in melt-textured YBa2Cu3O7−δ crystals and its effect on J
Physica C: Superconductivity, 1996
In order to investigate the entrapment phenomenon of Y2BaCuO5 (Y211) particles and its effect on the properties of the critical current density (Jc) in YBa2Cu307_~ (Y123) crystals, Pt-added Y123 crystals were grown by two different methods; the constant undercooling method and the continuous cooling method, with top-seeded growth. It was revealed that the amounts and size distribution of trapped Y211 particles in solidified Y123 crystals depended on the growth direction ((001), (100)) and the growth rate (R) as a function of undercooling (AT). These phenomena could be at least qualitatively explained by the prevalent trapping/pushing theory. The Jc values of the Y123 crystals were found to vary with their Y211 distributions, which in turn depended on growth conditions such as growth direction and/or growth rate even when the nominal composition was kept constant. Considering the effect of R or AT on the Y211 particle entrapment/pushing and Jc properties, it was shown that the conventional continuous cooling methods (i.e. MPMG/QMG methods) were favorable for Y211 particle entrapment, leading to high Jc values.
Simulated growth and microstructure of DyBa2Cu3O7−x with and without Dy2BaCuO5 addition
Journal of Materials Research, 1995
We present optical observations of magnetically melt-textured DyBa2Cu3O7−x with and without 20 wt. % excess of Dy2BaCuO5. From these observations, we propose some kinetic mechanism of the growth of 123 compounds. Kinetic processes can be simulated on computers. Two (very) simple models derived from the well-known Eden model are presented. They simulate the growth of the grain front. The simulated patterns agree with the observations. The microstructure of such materials cannot be explained by thermodynamic and chemical considerations alone, but explanations must include the kinetics of the growth front as well. From our observations, we conclude that the growth probability ratios g110/g100 and g100/g001 are of the order of 10 and 50, respectively.
Spiral growth mechanisms in partially melted bulk YBa2Cu3O7??
Journal of Materials Science, 1994
Large domains with platelets almost parallel to each other were obtained in bulk YBa2Cua07_~ by a single-step partial melting procedure. The mechanisms of nucleation and growth of platelets are discussed. The nucleation of peritectic material from the liquid phase is favoured by heterogeneities in the melt. Experimental evidence of spiral growth of the nuclei in the [0 01 ] direction is given. Furthermore, structures of growth, which could also be an indication of spiral growth in the [01 0]/[1 00] directions, are shown. The final morphology of the domains can be explained on the basis of the periodic bond chain (PBC) theory if the growth rates of flat (F) faces of the platelets are dominated by kinetic coefficients which differ between them. The morphology of the as-grown (001) surface is explained in the framework of the PBC theory with the shape of the steps of macrospirals governed by the transition from roughness to smoothness of the liquid-solid interface. An account of large step heights is given by the model of giant screw dislocations caused by an impurity-induced lattice-constant gradient. Even higher step heights are correlated to the presence of obstacles and lack of liquid phase.
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.
Microstructural development of partial-melt processed YBa2Cu3O7−δ
Proceedings of the Samahang Pisika ng Pilipinas, 2002
YBa 2 Cu 3 O 6.5 (YBCO123) was synthesized, subjected to a pre-melt process, and its microstructures were characterized via SEM. The microstructural evolution was observed by pre-melting YBCO123 at the melting temperature onset of 1020°C and cooling it down with crystallization cooling rates (CR) 100, 50, 33.3, 20, 10, 5, and 2°C/hr. Microstructural development from a brittle fissured faceted phase with no defined grain boundary and densified melted features to column-like grains and biphasic grains with hexagonal grain boundaries were observed as the CR was varied from fastest to slowest CR.
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.