Fabrication And Analysis On The Law Of Crystal Growth Of YBCO Bulk By Directional Process

In this paper, bulk directional pulling growth technology was proposed basing on the PMP method and traditional unidirectional solidification. This process contributed to the distribution of the single-domain crystal and improved the microstructure defects inside the crystal.The influence of the seed on crystal nucleation, growing and microstructure morphology was investigated. It was shown that the introduction of seed increased the texture forming ability of the sample and contributed to the control of the crystal orientation. Better single domain microstructure can be obtained at a lower pulling rate. The orientation of the Y123 single domains was controlled by the seed through a transitive growth mechanism at the contact area between the seed and sample. With increasing the distance between the layer microstructure inside the sample and the seed, the crystal changed from homogeneous growth to inhomogeneous growth.The inner growth drive of crystal was analyzed by the growth models based on diffusion of yttrium in the liquid phase; the drive force was induced mainly by the undercooling caused by solidification parameter and the differential concentration at the growth front. The growth velocity of the crystal depends on the undercooling, temperature gradient and original particles in the precursor. The liquid flow during the crystal growth mostly relied on the melting temperature; the liquid flow ,which did harm to the crystal nucleation and growing ,led to the liquid loss while it increased the diffusion velocity of yttrium,. The Y211-free region was related to the formation of spherical pores and the pores and liquid flow promoted the formation of Y211 -free region. The effects of the temperature gradient and pulling rate on the crystal growth, microstructure morphology and composition distribution were studied. The result shown that the positive lateral and axial temperature gradient were in favor of the heterogeneous nucleation of the crystal. The lesser (d77dx)/(dT /dz) ratio contributed to the formation of bulk texture. The inhomogeneous composition distribution along the axis of the sample was mainly caused by the pushing/trapping phenomena of the Y211 particles and the liquid flow. Pulling growth method contributed to the incremental growth of the bulk texture . The YBCO single domain with successive crystal grains and fine Y211 particles of dispersed distribution can be obtained under a lower pulling rate.