Surface chemistry effects on the processing and superplastic properties of nanocrystalline oxide ceramics

The unusual bulk behavior of nanoparticle and nanograined systems often originates in surface chemistry effects. Three examples are used to illustrate this point. In the first, newly precipitated nanocrystalline titania is washed with ethanol, and the mixture of these two supposedly inert substances causes the titania to lose its anatase crystal structure and become amorphous. This phenomenon is attributed to a reverse hydrolysis reaction at the particle surface. In the second example, nanocrystalline ZrO2-3mol%Y2O3 is observed to partially dissolve on exposure to pH-adjusted water. due to the formation of soluble hydroxides at the particle surface. A major consequence of the dissolution is the formation of large, hard, multiparticle agglomerates on subsequent drying. In the final example. ZrO2-3mol%Y2O3 particles are intentionally surface-doped with submonolayer levels of Cu-containing ions from ammoniacal solutions. The ceramics fabricated from such powders exhibit superplastic strain rates 100 or so times faster than in comparable undoped systems, due to the dopant's role in lowering of the activation energy for diffusion along grain boundaries. (C) 1999 Acta Metallurgica Inc.