Composite reaction texturing

Scanning Electron Microscope image of Bi2Sr2CaCu2O8+x material prepared by the Composite Reaction Texturing method. The dark areas are the MgO whiskers. The material is etched to reveal the grain boundaries. The Bi2Sr2CaCu2O8+x grains are flake-shaped with the c-axis aligned perpendicular to the whiskers

Composite reaction texturing is a process invented at the University of Cambridge by Prof. J.E. Evetts and Dr. B.A. Glowacki by which ceramic Bi2Sr2CaCu2O8+x superconductor with induced microstructural texture can be used for high current applications such as current leads or resistive fault current limiter.

Ceramic superconductors such as Bi2Sr2CaCu2O8+x with critical current temperature above 77K, the temperature of liquid nitrogen, became extremely popular in eighties after the discovery of YBCO materials. One drawback of the ceramic superconductors in polycrystalline materials is the low critical current density. The crystal structure of these materials consists of unit cells in which the current flows predominantly in the ab planes compared with the c-axis. It is thus important to have the grains in a polycrystalline material aligned with their c-axis perpendicular to the direction of the current flow.

Superconductors are known for their ability to carry a current with no resistance. Critical current density is the maximum current, averaged over the cross section of the material, which can flow in a superconductor before a significant voltage is produced. For this reason they are attractive candidates for high current applications, where the development of resistance and thus of power consumption must be minimized.

The Bi2Sr2CaCu2O8+x compound is such a candidate. With critical current temperature, temperature below which the material loses its ohmic resistance, of about 95K, this anisotropic material can be successfully used for high current applications provided that the grains are textured to a high degree.

In 1989 Prof. J.E. Evetts and Dr. B.A. Glowacki at the Device Materials Group of the Department of Materials Science and Metallurgy of the University of Cambridge, developed a novel technique for the alignment of the grains in BSCCO-2212 and potentially for any polycrystalline material. The method consists of the dispersal in the ceramic powder of inert fibers, which after being mechanically aligned, force the developed microstructure to be aligned correspondingly.

The fibers used are MgO whiskers. The whiskers are mixed with precursor powder of BSCCO-2212. The alignment is performed by applying a pressure to the mixture of precursor powder and the fibres. As result the MgO whiskers form a mesh with their long axis aligned randomly in planes perpendicular to the direction of pressure. The mixture of powder and MgO whiskers is subsequently heated to a proper temperature profile allowing the precursor powder to be melt partially. On allowing the material to cool, the solidification of grains progresses. The development of the superconducting grains is, however, prohibited by the MgO whiskers which inhibit the growth of the fast-growing (ab) planes perpendicular to their long axes and the growth of the superconducting grains is allowed mainly along the long axes of the aligned fibres.

The external current is applied in the plane where the MgO whiskers are aligned, and therefore it flows mainly in the ab planes with low transfer in the c-axis of the grains, and thus high values of critical current density can be achieved.

The method was initially applied to pellets of dimensions of about 2 cm but very quickly was extended to robust rods, sheets, and cylinders. The conductors were tested by various methods in order to verify their ability for high critical current densities.

The project was initially funded by British Technology Group and then by the European Community. The technology is now successfully transferred to the industry.

References

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