The characterisation and electrical properties of novel 8 mol% yttria-stabilised zirconias
The powder characteristics of the following 8 mol% yttria-stabilised zirconia samples were studied: Tioxide yttria-coated zirconia (coated-YSZ), Tioxide co-milled YSZ, Tosoh co-precipitated YSZ and A YSZ prepared in-house by a conventional solid state route. Co-milled YSZ powders containing 3-11 mol% yttria were also studied. The phase assemblage of as-received and reacted powders was investigated by XRD. As-received co-precipitated YSZ was cubic single phase. All other as-received powders consisted of a mixture of yttria and monoclinic zirconia. The phase evolution of the samples between 1000 and 1500° was studied. The coated and co-milled YSZ were cubic single phase only after reaction at 1300°. The rate at which the different powders reached equilibrium on heating decreased in the sequence: coated-YSZ > co-milled YSZ>YSZ prepared in-house. This difference in reactivity is discussed in terms of the yttria distribution in the as-received powder. A detailed study of the yttria-zirconia reaction in coated-YSZ was made. A model was devised to describe the counter-diffraction of yttria from the external coating and the zirconia. TEM analysis of a partially-reacted coated-YSZ sample showed that some grains contained to tetragonal inner core surrounded by a cubic outer shell. The kinetics of the yttria-zirconia reaction were studied: an activation energy of 470-555 kJmol-1 was determined, which was attributed to cation counter-diffusion. The formation of the metastable t'-phase in YSZ samples was studied. On rapid-cooling, cubic YSZ solid solutions containing between 3 and 7.7 mol% yttria transformed to the t'-phase: for compositions greater than 7.7 mol% the cubic solid solutions were retained to room temperature.