John Drennan

Grain boundary resistivity in Y-TZP materials as a function of thermal history

Grain-boundary resistivity in yttria-containing tetragonal zirconia polycrystalline (Y-TZP) materials dominates the total resistivity. Impedance measurements combined with microstructural studies suggest that post-sintering heat treatments (in particular the cooling rate) influence the location of the grain-boundary phase which, in turn, has a significant effect on the grain-boundary resistivity. Higher cooling rates from the sintering temperature lead to reduction in the grain-boundary resistivity. In both alumina-containing and relatively pure tetragonal zirconia polycrystals, post-sintering heat treatments have a less conspicuous effect. The activation energy associated with the grain-boundary resistivity was independent of the post-sintering heat treatments but was 25 to 30 kJ mol−1 higher than that for the oxygen-ion conduction within the grains at low temperatures.

The system Y2O3-Sc2O3-ZrO2: Phase characterisation by XRD, TEM and optical microscopy

Abstract The phase assemblage in the ternary system Y2O3-Sc2O3-ZrO2 has been investigated as a function of the Sc 2 O 3 Y 2 O 3 ratio for several compositions with a constant total stabilizer (Y2O3 + Sc2O3) content of 8 mol%, using X-ray diffraction, transmission electron and optical microscopy. Considerable twinning was observed in as-sintered speciments with higher scandia content ( Sc 2 O 3 /Y 2 O 3 ≥ 5 3 . These compositions were indexed as having tetragonal symmetry. The twinning is associated with the formation of a dopant rich tetragonal phase (designated as the t′-ZrO2 phase) as a result of diffusionless transformation on cooling the material from the sintering temperature at which it had a cubic symmetry. On annealing Sc2O3-rich compositions at 1000°C for 2000 h, twinning disappeared and the specimens consisted of cubic matrix with fine precipitates of a low-dopant tetragonal phase (t-ZrO2) dispersed uniformly. The cubic matrix had a slightly higher dopant content than the corresponding t′-phase. The Y2O3-rich compositions in the as-sintered form had cubic symmetry and showed no twinning. On annealing, they decomposed to t-ZrO2 precipitates plus a cubic solid-solution matrix richer in the dopant. All the eight Y2O3/Sc2O3 compositions studied were in the two-phase field at the annealing temperature.

Microstructural features of the α to β-SiAlON phase transformation

Abstract The microstructural characteristics of Sm (α + β) -SiAlON ceramics after post-sintering heat treatment at 1450 °C are investigated by analytical electron microscopy (AEM). X-ray diffraction (XRD) analyses indicate that a significant amount of the α-SiAlON (α′) phase has transformed to β-SiAlON (β′) during the heat treatment. It is found that the transformation is nucleated on the existing β′ phase and the transformed β′ grains have unique microstructural features containing a high density of dislocations and ultra-fine spherical inclusions rich in Sm and O. Similar characteristics have also been observed in the Yb (α′ + β′) materials. It is suggested that the transformation from α′ to β′ proceeds via a nucleation-growth mechanism and may require only a small amount of liquid phase to promote atomic diffusion.

Evaluation of conducting properties of yttria-zirconia wafers

Abstract Grain boundary and volume resistivity of wafers supplied by ICI, UK (Daiichi powder) and discs and wafers of yttria-tetragonal zirconia prepared in our laboratory from TOSOH powder have been evaluated. The size and distribution of grains in the as-received ICI wafers and in TOSOH wafers and discs sintered at 1500°C were similar and uniform but the grain boundary resistivity of ICI wafers, in the 300–400°C temperature range, was higher by a factor of 20 to 50. The 1500°C heat treatment of ICI wafers resulted in inhomogeneous grain growth and considerable reduction in the grain boundary resistivity (by a factor of 3–7 depending on the cooling rate). Examination using analytical electron microscopy revealed that differences in the grain boundary resistivity between the samples could be directly related to the presence or absence of the glassy phase and its location in the respective materials.

The effect of thermal history on the grain boundary resistivity of YTZP materials

Abstract Impedance measurements have been combined with microstructural studies on YTZP materials to understand how the thermal history of the specimens has so much influence on the grain boundary resistivity.

Using Electron Microscopy and X-Ray Microanalysis to Study Wool Morphology and Composition:

Energy dispersive x-ray analysis has been used in combination with scanning electron microscopy and scanning transmission electron microscopy to investigate the distri bution of both cystine and tyrosine in the morphological components of wool fiber. The technique has also been used to show that the zirconium in Zirpro treated wool is deposited evenly throughout the fiber.

Some Interfacial Related Properties of Transformation Toughened Ceramics

The role of interfaces in the sintering, heat treatment and resulting mechanical properties of PSZ and some TZP materials are considered. This is first illustrated by the addition of SrO to Mg-PSZ to modify the grain boundary chemistry and resultant behaviour on heat treatment. The growth of t-ZrO2 precipitates in Ca-PSZ is controlled by the Wagner interfacial relationship. Finally, the role of interfaces, particularly grain boundaries on the strength and toughness of PSZ and TZP materials is discussed.