Effect of Post‐Precipitation Treatment on the Pore‐Structure Stability of Sol‐Gel‐Derived Lanthanum Zirconate

Abstract

The importance of post-precipitation treatments (porefluid exchange and its removal) on the evolution of the texture of coprecipitated lanthanum zirconate has been investigated. The nature of the pore fluid and the type of fluid-removal (drying) process have shown a profound effect on the aggregate structure (packing of primary particles within the aggregates of the gel). Exchanging the pore fluid with organic fluids such as ethanol or tetrahydrofuran resulted in higher specific surface area, larger pore volume, and, most frequently, larger pore radii. The effect has been attributed to the surface tension of the pore fluid rather than to the polycondensation effect. Freeze drying and microwave drying of ethanol-washed lanthanum zirconate gel also resulted in higher surface area, compared to the ovendried gel. Freeze drying and microwave drying yielded a surface area of ∼30 m 2 /g, whereas oven drying yielded a surface area of 20 m 2 /g after heating at 800°C.

Aluminum hydroxide microstructural units in gelled media aged, or nonaged, with alcohol and water

“…2. In agreement with previous studies [8][9][10][11][12][13], the alcoholic gelled media give xerogels with higher specific surface area than the ones obtained from aqueous gelled media.…”

“…Thus, the beneficial action of alcohol was demonstrated experimentally, and nowadays alcohol washing is commonly used to obtain ultrafine dispersed powders [9]. To explain the effect of ethanol washing on precipitates, a number of authors argued that, since it has a surface tension less than water, the maximum capillary tension inside the network of the evaporating gel is kept at a minimum level; accordingly, the resulting dried xerogels are crack-free and have larger specific surface area [5,10,11]. According to other authors [7,9], assuming that the formation of agglomerates is the result of bridging among individual particles due to surface hydroxyl groups [12,13], alcohol washing would prevent or limit such a bridging action by substitution of hydroxyls by ethoxy groups.…”

Azeotropic distillation, ethanol washing, and freeze drying on coprecipitated gels for production of high surface area 3Y–TZP and 8YSZ powders: A comparative study

“…For example, the S BET values of the 8FD and 8OD samples are very similar (66.82 m 2 g À 1 versus 61.20 m 2 g À 1 ). Likewise, Nair et al [54] reported that coprecipitated lanthanum zirconate precipitates dehydrated by FD and OD present essentially identical surface areas. The higher agglomeration in powders dehydrated by FD than by other methods is visible in the SEM images (Fig.…”

Sintering of Lanthanum Zirconate

“…The initial endothermic peak at ∼90°C in the DTA trace is due to the loss of volatile water; two exothermic peaks at ∼240°a nd 450°C are probably due to the decomposition of ethanol. 8 This corresponds to a continuous weight loss in the TGA trace up to a temperature of ∼800°C. The final weight loss is ∼24% of the initial sample weight.…”