Ceramics - Research

David L. Milius

The discovery of high temperature superconductivity (HTSC) in YBa₂Cu₃O₇ (123) in the mid-1980s began an intensive worldwide research and development effort. Much of this work attempts to understand the fundamental issues concerning the processing and properties of such materials. Better and more consistent methods of synthesizing these materials enable the practical use of these materials in real applications. One barrier to 123 use has been the inability to grow high-quality superconducting single crystals, a process that presents serious, both in fabricating required precursor materials and in developing and optimizing the crystal growth process. We have developed a modified Czochralski growth method for the production of single crystals that involves seeding a precursor compact with samarium-123 seeds combined with slow cooling below the peritectic temperature.

Our precursor YBCO (yttrium-barium-carbon-oxygen) powders were synthesized using a combustion spray pyrolysis (CSP) process developed for producing high-purity, ultra-fine, stoichiometric materials with highly reproducible characteristics. The effects of a number of process variations (solution concentration, fuel content, oxidizer concentration, and atomization pressure) on the characteristics of individual 123 and 211 powders were studied, and numerous composite compacts of 123/211 were produced and examined.

Schematic illustration of the combustion spray pyrolysis (CSP) process employed for producing high-purity, stoichiometric powders of HTSC.

HTSC picture of single crystal.

Two critical, inter-related factors influencing the growth of YBCO single crystals are examined in detail: (1) the loss of barium cuprate liquid from the system during the crystal growth process following the peritectic melting of 123 at temperatures above 1010°C, and (2) the distribution and role of platinum in the overall process. The loss of liquid is found to decrease significantly with increasing additions of platinum. In the absence of platinum, the extent of liquid loss is so great (more than 50% of the total sample weight) that insufficient material remained for reaction with 211 particles to form the desired 123 phase; the growth of single crystals under these conditions is thus not possible. Significant liquid losses can occur even when Pt is present in the compacts if it is not optimally distributed (e.g., as characteristic of material introduced by mechanical mixing). However, the addition of small amounts of platinum (0.1 to 1% by weight) directly into the CSP feed solutions to achieve a much more uniform distribution is found to have a pronounced stabilizing effect by more effectively refining the 211 particles and promoting the formation of dense 211 particle networks. These networks are thought to inhibit liquid loss through capillary forces exerted on the liquid in the small pores between 211 particles.

Identified potential applications include magnetic bearings for superconducting motors and generators, flywheel energy-storage systems, and vibration isolation systems; magnetic levitation systems; electromagnetic pumping; magnetic separation, magnetic shielding; material forming and joining; high-field-strength permanent magnets; and many other special-purpose devices.

References

1. J.S. Beck et al., J. Am. Chem. Soc. 359 (1992).

2. D.M. Dabbs and I.A. Aksay, Ann. Rev. Phys. Chem. 51 601-22 (2000).

3. K.M. McGrath, D.M. Dabbs, N. Yao, I.A. Aksay, S.M. Gruner, Science 277 (1997).

4. G. Porte, J. Marignan, R. Bassereau, J. May, J. Phys France 49 (1988).

5. K.M. McGrath, D.M. Dabbs, N. Yao, K.J. Edler, I.A. Aksay, S.M. Gruner, Langmuir 16 [2] 398-406 (2000).

6. A.-S. Malik, D.M. Dabbs, I.A. Aksay unpublished results.

7. J.M. Jarvis, I.A. Aksay, J.D. Carbeck, S. Bhansali, D.M. Dabbs, unpublished results.

8. K.M. McGrath Langmuir 13 [7] 1987-95 (1997).

9. M.D. McGehee, S.M. Gruner, N. Yao, C.M. Chun, A. Navrotsky, I.A. Aksay, Proc. 52nd Ann. Mtg. MSA, eds.: G. W. Bailey and A. J. Garratt-Reed, (Microscopy Society of America, 1994) pp. 448-49.

For more information, please contact David L. Milius.