An experimental determination of the stability diagrams of the superconducting oxide yttrium barium(2) copper(3) oxygen(7-x) and neodymium(2-x) cesium(x) copper oxygen systems
Abstract
In view of the performance dependence of superconducting oxides on processing conditions and that superconducting properties are related to the formal oxidation state of Cu in the oxides, quantitative knowledge of thermodynamic stability diagrams is advantageous. To elucidate the phase stability limits in YBa$\sb2$Cu$\sb3$O$\sb{\rm 7-x}$ (123), the dependence of the nonstoichiometry, i.e., the variation of x, has been determined by thermogravimetric measurement over the range of temperature, 400-950$\sp\circ$C and the range of oxygen partial pressure, 10$\sp{-6}$ $-$ 1 atm. The solid-state decomposition of the 123 at 750-950$\sp\circ$C and oxygen partial pressures, P$\sb{\rm O\sb2}$ = 10$\sp{-6}$ $-$ 10$\sp{-2}$ atm has been characterized by TGA and x-ray diffraction (XRD) analysis. The 123 decomposes to various phase fields as the oxygen partial pressure is decreased and four different three-phase fields have been identified over a range of P$\sb{\rm O\sb2}$ = 10$\sp{-6}$ $-$ 1 atm in the present study. The solidus line of the incongruent melting over the range of oxygen partial pressure of P$\sb{\rm O\sb2}$ = 10$\sp{-6}$ $-$ 1 atm has been determined by differential thermal analysis (DTA) and the phases transformed by melting have been identified by XRD. The complete phase stability diagram has been constructed from the experimental results. With the resultant stability diagram of 123, application to the processing is discussed with respect to the existence of a thermodynamic stability limit for the 123 growth. In parallel work, similar experiments conducted with YBa$\sb2$Cu$\sb3$O$\sb{\rm 7-x}$ have been extended to a T$\sb{\rm c}$ = 24 K superconductivity system, Nd$\sb{\rm 2-x}$Ce$\sb{\rm x}$CuO$\sb4.$ Oxygen nonstoichiometries in Nd$\sb2$CuO$\sb{\rm 4-x}$ and Nd$\sb{1.85}$Ce$\sb{0.15}$CuO$\sb{\rm 4-x}$ have been measured in the range of temperature, 350-950$\sp\circ$C and oxygen partial pressure, 10$\sp{-6}$ $-$ 1 atm. For this new n-type superconductor, difficulties were experienced in the determination of the maximum oxygen content, mainly due to effects of impurities and inhomogeneity. The CuO impurity in the samples of Nd$\sb2$CuO$\sb{\rm 4-x}$ and Nd$\sb{1.85}$Ce$\sb{0.15}$CuO$\sb{\rm 4-x},$ observed by SEM and EPMA micrographs have been determined quantitatively from the results of oxygen nonstoichiometry measurement. The complete stability diagrams of Nd$\sb2$CuO$\sb4$ and Nd$\sb{1.85}$Ce$\sb{0.15}$CuO$\sb4$ have been determined by TGA-DTA and XRD in the same manner as for the 123. The Nd$\sb2$CuO$\sb4$ decomposes to NdCuO$\sb2$ + Nd$\sb2$O$\sb3,$ then to Nd$\sb2$O$\sb3$ + Cu$\sb2$O. The Nd$\sb{1.85}$Ce$\sb{0.15}$CuO$\sb4$ decomposes to Nd$\sb2$O$\sb3$ + NdCeO$\sb{3.5}$ + Cu$\sb2$O. The dependence of the onset of superconductivity as oxygen partial pressure and annealing temperature under which the sample was treated, have been examined by the standard four-probe technique of resistivity measurement.
Degree
Ph.D.
Advisors
Gaskell, Purdue University.
Subject Area
Engineering|Materials science|Condensation
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