Structure-processing-property relationships in air plasma -sprayed yttrium barium(2)copper(3)oxygen(7-delta)

摘要

Plasma spray processing has proven to be a versatile technique for applying thick ceramic coatings. These ceramic overlayers are wear and corrosion resistant, electrically insulating, and are excellent thermal barriers. Coatings of the superconducting perovskite cuprates are very different from other ceramic coatings in that they must be electrically continuous (i.e. minimal porosity and cracking). Moreover, the quantity and distribution of secondary phases must be carefully monitored. The attainment of high critical current densities will almost certainly require a strongly textured deposit. These constraints require an unprecedented level of control over the processing parameters used to fabricate the coatings.;This study examines the effect of processing conditions on the microstructure, chemistry and phase content of air plasma sprayed Y-Ba-Cu-oxide. As-sprayed deposits contain metastable phases and fine grain sizes, typical of the rapid solidification associated with plasma spraying. At least five parameters significantly affect the extent of porosity and cracking in deposited coatings: feedstock powder size, plasma power, substrate to gun distance, substrate temperature, and substrate composition. The chemical homogeneity of as-deposited coatings is poor, showing 10-20% variations over distances of tens of microns. These fluctuations are largely due to copper vaporization during a particle's flight through the plasma flame.;The superconducting YBa$sb2$Cu$sb3$O$sb{7-delta}$ crystal structure does not form immediately during post-deposition annealing. Instead, a complicated sequence of phase transformations leads first to the formation of the solid substitutional Y(Ba$sb{rm 2-x}$Y$sb{rm x}$)Cu$sb3$O$sb{7+delta}$. This phase then dissociates to form YBa$sb2$Cu$sb3$O$sb{7-delta}$. The temperature and oxygen partial pressure during heat treatment are found to be critical to the success of this step. An annealing treatment which yields $Tsb{c} >$ 90K and intragranular $Jsb{c} approx$ 10$sp6$ A/cm$sp2$ at 10K and H = 1 T is given. Intergranular $Jsb{c}$'s (i.e. transport $Jsb{c}$'s) are less than 100 A/cm$sp2$ at 77K and H = 0 T. Suggestions for improving $Jsb{c}$ are also given.