ANISOTROPIC THERMAL-EXPANSION OF THE 1-2-4 YTTRIUM BARIUM COPPER-OXIDE SUPERCONDUCTOR

The 1:2:4 yttrium barium copper oxide superconductor has been prepared and sintered to high density by high pressure techniques. This material is phase pure and shows a sharp transition temperature of 80 K. High temperature X-ray powder diffraction of the 1:2:4 material shows a small anisotropy in thermal expansion. This is compared to the anisotropy in thermal expansion of the 1:2:3 superconductor. A critical grain size, above which thermal expansion anisotropy causes cracking, is calculated for 1:2:4 and compared to that for 1:2:3. The results predict that the critical grain size is over twice as large for the 1:2:4 material. This should result in less microcracking in polycrystalline materials due to thermal expansion anisotropy in devices based on 1:2:4.