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The microstructure in Y 2 O 3 -stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°-1500°C was examined to clarify the role of Y 3؉ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y 2 O 3 concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grainboundary faces in Y-TZP and Y 3؉ ions segregated at their grain boundaries over a width of ϳ10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y 3؉ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y 3؉ ions segregating along the grain boundary, respectively.

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Y 2 O 3 -stabilised ZrO 2 (YSZ) ceramics have been used for various engineering applications since Garvie et al. discovered phase-transformation toughening in 1975. The performance of YSZ ceramics depends on the YSZ microstructure. In the present review, the tetragonal-to-cubic phase transformation and grain growth are first discussed, organised according to existing hypotheses of microstructure-development mechanisms in YSZ during sintering. We demonstrate that the phase transformation and grain growth can be most reasonably understood by a grain boundary segregation-induced phase transformation (GBSIPT) mechanism and that the solute-drag effect of Y 3+ ions segregating along the grain boundaries, respectively. Next, the Al 2 O 3 -doping effect is discussed with emphasis on the microstructure-development behaviour in a small amount of Al 2 O 3 -doped YSZ, which is widely used in engineering applications. We further discuss the effect of GBSIPT on lowtemperature degradation resistance and the effect of grain size on superplasticity, focusing on the nanocrystalline YSZ, which was created by applying the Al 2 O 3 -doping effect.

The Raman spectra of hydrous‐zirconia fine particles produced by the hydrolysis of various ZrOCl2 solutions were investigated. The Raman spectra of hydrous zirconia synthesized at HCl concentrations below 1 mol/L were similar to those of monoclinic, crystalline ZrO2; those of hydrous zirconia synthesized at HCl concentrations greater than 1 mol/L showed a crystal structure change. The line width of the Raman bands increased with increasing H+ ion concentration. Analyzing the relationship between Raman band width and particle size revealed that the primary particle size of hydrous zirconia was controlled by the H+ and Cl− ions, because these ions interfered with the polymerization in a hydrolysis reaction. Based on the experimental results, the formation mechanism for primary particles of hydrous zirconia was determined.