Catalytic properties, densification and mechanical properties of nanocrystalline yttria-zirconia-based materials

Abstract

Alumina, titania, ceria and manganese oxide were either coated onto or doped in cubic 7 mol% Y203-ZrO2 (7YZ) nanocrystals to form nanocomposites for methane combustion. These novel catalysts were very active and thermally stable. In particular, 25 wt% Mn203-coated 7YZ and 25 wt% Mn203-doped 7YZ showed remarkably low light-off temperatures of 3750C and 3580C, respectively. These catalysts were highly attractive as they were competitive with the much more expensive supported noble metal catalysts. Their catalytic activity could be attributed to the availability of active surface oxygen species, which facilitated the methane activation at low temperatures. Nanocrystalline 3 mol% and 8 mol% Y203-ZrO2 (3YZ and 8YZ) were successfully densified with an ultrafine grain size of < 90 nm by pressureless sintering at 11000C and 11500C, respectively. The low-temperature sinterability could be attributed to the well-defined nanocrystalline particles obtained via hydrothermal synthesis, and the effective elimination of secondary porosity through the dry compact processing. Submicron-sized 3 mol% Y203-ZrO2 ceramics with a grain size of - 150 nm was also obtained with commercial TOSOHC powders. Grain growth during densification of TOSOH© powders was successfully suppressed by presintering to 93% density under an argon atmosphere, followed by hot isostatic pressing at a temperature lower than the presintering temperature. The grain sizes of dense 3YZ and 8YZ ceramics were controlled between 100 nm and 5 glm. This allowed for the systematic study of 3YZ and 8YZ in indentation hardness, Young's modulus and fracture toughness as a function of grain size through micro-indentation and instrumented nano-indentation.

(cont.) The Hall-Petch effect was found to be extended to the nanocrystalline regime for 3YZ. 8YZ showed the Hall-Petch effect only in the micrometer and submicrometer regime. Maximum Hv values of 19 and 20 GPa were achieved for 3YZ and 8YZ, respectively. A continuous decrease in Young's modulus with decreasing grain size was observed in both 3YZ and 8YZ. This could be partially explained by the percolation theory. Transgranular fracture was observed in 3YZ as the grain size approached - 100 nm. This was in contrast with the dominant intergranular fracture mode observed in ceramics with fine grain sizes. Transgranular fracture was found in 8YZ over an even broader range of grain sizes (150 nm to 5.0 glm). A significant reduction in fracture toughness from 7.9 MPam-1/2 to 3.1 MPa-m1/2 was observed as the grain size was reduced from 1.1 im to 100 nm in 3YZ. Fracture toughness was much lower for 8YZ than for 3YZ, and showed little dependence on grain size. The stability of tetragonal phase at small grain sizes could account for the considerable reduction in the fracture toughness in 3YZ, and the transgranular fracture mode as grain size approached 100 nm.