Thermodynamic and structural aspects of equilibrium and mechanically milled YBa2Cu3O6+delta powder

Citation

Lee, David Soong-Hua (1993) Thermodynamic and structural aspects of equilibrium and mechanically milled YBa2Cu3O6+delta powder. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xzfk-gw94. https://resolver.caltech.edu/CaltechETD:etd-08302007-095221

Abstract

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The equilibrium relationships between partial pressure of oxygen, temperature, lattice parameters and oxygen content in the YBa2Cu3O6+[delta] superconductor were examined by differential scanning calorimetry, high-temperature x-ray diffraction, thermogravimetric analysis and measurement of pressure versus concentration isotherms. Oxygen pressures ranged from 1 mbar to 1013 mbar and temperatures ranged from 400°C to 750°C. Lattice parameters as a function of temperature and partial pressure of oxygen were measured. P(c) isotherms and TGA were used to measure oxygen content as a function of temperature. A phase line separating the low temperature orthorhombic phase and the high temperature tetragonal phase was determined. No evidence of a miscibility gap between the two phases was found in the range of temperatures and oxygen pressures explored. From the data, the excess enthalpy and entropy through the orthorhombic to tetragonal phase transition was calculated. It was found that the excess entropy is less than the entropy of mixing for an ideal solution. DSC was used to measure enthalpies for the transition and to calculate activation energies for the process. The enthalpy for the evolution of oxygen from the solid was found to be 5.2 kJ/mol in Argon, 2.0 kJ/mol in 16% oxygen and 1.7 kJ/mol in pure oxygen. Activation energies for the transition were 1.2 eV/atom in Argon, 1.8 eV/atom in 16% oxygen, and 2.9 eV/atom in 100% oxygen. DSC using a sealed sample pan (to maintain a constant concentration of oxygen in the solid during heating) was done and no signal arising from the order-disorder transition was seen.

High temperature x-ray diffraction and differential scanning calorimetry were used to study the effect of high energy mechanical deformation on the structural and thermal characteristics of YBa2Cu3O6+[delta] powder. A Warren-Auerbach analysis was done on the x-ray peaks to determine the grain size of the material as a function of milling time. A Phillips 430 transmission electron microscope equipped with an EDX detector was used to determine chemical composition and grain size of the ball-milled material.

Broadening of Bragg peaks due to the reduction of grain size makes the distinction between orthorhombic and tetragonal phases of YBa2Cu3O6+[delta] difficult after only one hour of ball milling. The equilibrium orthorhombic to tetragonal phase transition may occur within the first hour of ball-milling. Longer milling times (>5hrs) produce a cationic disorder on the yttrium and barium sites. A metastable cubic (Y1/3Ba2/3)CuO2+[delta] structure with a= 3.86[...] is formed. Further mechanical deformation does not induce the formation of an amorphous phase; rather, an eventual decomposition into the constituent oxides Y2O3 and BaCuO2 is observed. Annealing of the metastable cubic (Y1/3Ba2/3)CuO2+[delta] phase in relatively low pressures of oxygen ([...]100mbar) and at moderate temperatures (~400-600°C) produces an as yet unidentified cubic phase which is unstable in air and has a lattice constant of a= 5.47[...].

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