Materials Engineering Problems in Crystal Growth and Epitaxy of Cuprate Superconductors
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empirically, not by physical understanding of the HTSC phenomenon. Applications of HTSC have been limited to very specific areas, but a concerted effort should reveal opportunities for the materials research and superconductivity communities. Significant applications could be developed
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Temperature [K] Figure 1. Temperature dependences of the upper critical fields of the classical superconductors NbTi and Nb3Sn, for the Chevrel phase PbMo6Ss, and for YBCO with the layer structure (after Reference 2).
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which in the long run would also benefit fundamental physical investigations of the HTSC phenomenon. This article discusses the causes of the materials engineering problems in HTSC development: complexity and limited stability ranges of HTSC compounds in combination with stringent requirements based on the very short coherence lengths of high Tc superconductors. The article then discusses specific materials engineering problems related to crystal growth and epitaxy, such as phase diagrams, crucible corrosion, growth stability, and strain problems in connection with epitaxy, oxidation, and phase transitions. Complexity and Limited Stability of HTSC Cuprates Superconductivity was first discovered in metals (Hg, Pb), then in binary compounds (NbC, NbN, A15 type compounds like Nb3Sn) with critical temperatures up to 23 K in Nb3Ge. Superconducting NbTi wires and coils are applied in medicine and in various technologies; the materials engineering problems could be solved for these relatively simple intermetallic compounds and alloys. The upper critical fields, which limit the current-carrying capability of NbTi and Nb3Sn, are 12 T and 22 T at 4.2 K, respectively. With the discovery of the ternary Chevrel phases like PbMo6S8 with critical temperatures of about 15 K and critical fields above 50 T (see Figure 1), the development of very high magnetic field applications was envisaged. However, unsolved chemical stoichiometry and materials engineering problems have so far prevented large-scale applications despite serious efforts. The discovery of the cuprate superconductors brought a Tc above the boiling temperature of liquid nitrogen, and also the vision of ultrahigh magnetic fields (see Figure 1), even at liquid-nitrogen temperatures. However, wires with the corresponding crystallite orientation (Figure 1) and electron percolation could not yet be developed because of the difficulty of stoichiometry control and complexity of the composition with four, five, or even more constituents. Figure 2 shows the historical development of critical temperatures of superconductors which become increasingly complex with increasing Tc. The relatively strong metal-oxygen bonds require processing the oxide compounds at high temperatures to mobilize the species for recrystallization, epitaxial growth, and bulk crystal growth. Figure 3 shows that with increasing Tc the thermodynamic stabilities of the superconductors, as indicated by the melting or decomposition temperatures, decrease. This can be attributed to the
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