A model for the texture development of high- T c superconductors under an elevated magnetic field
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A model for the texture development of high-Tc superconductors under an elevated magnetic field P.J. Ferreira, H.B. Liu, and J.B. Vander Sande Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 (Received 20 August 1998; accepted 19 April 1999)
A theoretical model is proposed to explain the degree of texture achieved in high-Tc superconductors during melt-processing under an elevated magnetic field. The degree of grain alignment is quantified through a factor F which is defined as ranging from 0 (random alignment) to 1 (completely oriented). Intermediate values of F clearly characterize intermediate states of alignment in which there is still some tendency for the grains to align their c axes with the magnetic field. The model suggests that the enhancement in texture is primarily obtained through grain rotation during the early stages of grain growth from the liquid. At the later stages of growth, grains interact with each other, which hinders the phenomena of magnetic-field–induced grain alignment.
I. INTRODUCTION
The principal limitation to technological applications of high-Tc polycrystalline oxide superconductors is the low critical current density (Jc) found in these materials. This limitation is strongly correlated with the misorientation among the grains. Hence, to minimize the number of intergranular weak links, a high degree of crystallographic texture must be obtained. One possible route by which a strong crystallographic texture can be produced is to melt-process the material under the effect of an elevated magnetic field. The magnetic alignment of nondoped superconductor grains results from anisotropy in the paramagnetic susceptibility associated with the Cu–O conducting planes. Since the susceptibility parallel to the superconductor grain crystallographic c axis (c) is higher than the susceptibility perpendicular to the c axis (ab),1–3 nondoped superconductor grains should align with the c axis parallel to the external field, H. For superconductors doped with magnetic rare-earth elements, the paramagnetic susceptibility is dominated by the R3+ ion, and the origin of anisotropy is single-ion anisotropy associated with crystal fields at the rare-earth site.4 Grain alignment induced by a magnetic field has been confirmed by various groups,5–11 although very little work has been done on BSCCO thick films or tapes processed under elevated magnetic fields. Recently, however, Ma and Wang12 obtained a high degree of texture in Bi-2223/Ag tapes melt-processed in a 4.5 T magnetic field. In addition, we have observed in two recent works13,14 an increase in the degree of texture and transport critical current (Jc) of Bi-2212 thick films meltJ. Mater. Res., Vol. 14, No. 7, Jul 1999
processed under the influence of an elevated magnetic field. To illustrate the effect of the magnetic field, Fig. 1 shows two Bi-2212 tapes that were melt-processed in the absence [Fig. 1(a)] and the presence [Fig. 1(b)] of a 10 T m
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