Graduation date: 1996

The effect of oxygen partial pressure and temperature on the solid state sintering kinetics of the superconducting ceramic YBa₂Cu₃O₇₋₈ has been investigated. The isothermal contraction behavior of YBa₂Cu₃O₇₋₈ in the temperature range 930 - 960°C with oxygen partial pressures varying from 0.001 to 1.0 atmospheres was studied. The change in the unit cell lattice parameters of YBa₂Cu₃O₇₋₈ over this oxygen partial pressure range was also determined in order to evaluate its effect on the sintering behavior. The sintering kinetics of YBa₂Cu₃O₇₋₈ were investigated through the variation of the isothermal steady state contraction rate with respect to oxygen partial pressure at different temperatures. Isothermal contraction rates between 930 and 955°C reached a maximum at some critical oxygen partial pressure, PO₂CRIT, with decreasing contraction rates both above and below this oxygen partial pressure. This behavior was not observed for YBa₂Cu₃O₇₋₈ sintered at 960°C. SEM micrographs of the fracture surface of these samples suggest that a liquid phase was formed at this temperature. The unit cell volume and a-lattice parameter of YBa₂Cu₃O₇₋₈ decreased with decreasing oxygen partial pressure, while the c-lattice parameter increased. The activation energy for sintering at oxygen partial pressures above PO₂CRIT has been estimated to be approximately 191 kJ/mole, which is similar to oxygen ion diffusion in this compound, indicating that for oxygen partial pressures above PO₂CRIT, oxygen diffusion is the rate controlling mechanism for mass transport. The activation energy for sintering below PO₂CRIT was found to be ∽130 kJ/mole, which could also be due to oxygen ion diffusion. Several possible factors have been considered for the sintering behavior of YBa₂Cu₃O₇₋₈ for oxygen partial pressures below PO₂CRIT. These are formation of copper vacancies, positive hole creation, and lattice strain effect. However, although the oxygen partial pressure dependence on the sintering rate can be correlated with either the formation of Cu²⁺ vacancies in the lattice or hole formation, neither of these mechanisms are considered to be probable. Mathematically, the contraction rate was found to depend exponentially on the lattice strain (Δɛlat), ie. έ∞еΔɛlat. The significance of this relationship is not clearly understood. Intuitively, it appears that as the vacancies are created due to the removal of oxygen ions from the lattice, the lattice is not contracting to the extent needed to accommodate the vacancies. This elastic strain hinders the migration of the oxygen vacancies through the lattice, thus affecting the sintering rate of the YBa₂Cu₃Ox compound below PO₂CRIT. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate