No abstract

The crystallization, transformation, and partitioning of amorphous PbO-ZrO 2 -TiO 2 powders produced by pyrolytic decomposition of partially hydrolyzed, mixed alkoxide precursors were investigated. Materials have the general formulation Pb 1؉ Ti [1/(1؉)] Zr [/(1؉)] O 3؉ , where ؊0.2 < < 0.2 is the fraction of PbO nominal excess/deficiency and 0 < < 1 is the Zr/Ti molar ratio. Most compositions first crystallized as a metastable fluorite structure with varying degrees of pyrochlore-like cation ordering, which transformed to a single perovskite phase upon additional heat treatment. Higher Zr/Ti ratios enhanced the retention of fluorite and reduced the incidence of cation ordering. Compositions with offstoichiometric amounts of PbO often yielded extended solid solutions prior to partitioning. For example, metastable perovskites with as much as 20% PbO deficiency ( ‫؍‬ ؊0.2) could be prepared for 0 < < 1, but only ϳ10% PbO excess could be incorporated in solid solution for 0.33 < < 1. Increasing PbO content was found to promote crystallization, suggesting that this oxide acts as a network modifier enhancing mobility within the initial amorphous precursor powder. Higher PbO was also noted to favor cation ordering in the metastable phase and to accelerate the transformation to perovskite, as well as to promote partitioning for hyperstoichiometric compositions. The findings are discussed in light of structural relationships between the fluorite, pyrochlore, and perovskite phases, as well as current understanding of the thermodynamics of the system.

Aqueous solutions of Al3+ and Fe3+ nitrates were pyrolyzed to produce (Al,Fe)2O3 amorphous solid solutions with up to 50 mol% Fe2O3. Following subsequent heat treatments, all compositions first crystallized to γ‐(Al,Fe)2O3 and those with ≥20% Fe2O3 ultimately partitioned into a mixture of A12O3 and Fe2O3‐rich α phases as predicted by the equilibrium diagram. The transformation path between the γ phase and the final microstructure was dependent on composition. Materials with 10%–20% Fe2O3 formed extended α solid solutions which partitioned only sluggishly at 1100°C. In contrast, materials with compositions above ∼30% Fe2O3 formed an intermediate metastable orthorhombic phase (O) based on the equiatomic AlFeO3 compound. The O‐phase also decomposed into a mixture of two α phases upon further heating. It appears that the α1+α2 mixture may evolve directly from γ for compositions near 30% Fe2O3, but clearly forms from O as the Fe content increases. Resistance to partitioning also reaches a minimum at ∼30% Fe2O3. At 900°C, the size of the α grains was ≥1 μm for compositions ≤5% Fe2O3, and decreased to ≤200 nm for compositions ≥10% Fe2O3, reflecting the strong effect of Fe2O3 on grain refinement relative to pure A12O3. Contrary to literature, the formation of single‐phase α in compositions ≤20% Fe2O3 appears to occur without the prior nucleation of Fe2O3‐rich “seeds.” Partitioned α mixtures exhibit orientation relationships suggestive of epitaxial nucleation of one phase on the other. The present results bear on important thermodynamic and kinetic issues related to the synthesis of fine‐grained polycrystalline alumina fibers.

Three different chemical precursor routes were investigated to synthesize Pb(Zr05Ti05)O3: mixing hexanoates, acetate complexing of alkoxides, and the synthesis of a mixed alkoxide by the reaction of titanium alkoxide and zirconium alkoxide with lead acetate. For each, elemental Pb and PbO were the first crystalline phase observed during pyrolysis conditions that involved rapid heating (e.g., to 400°C). The formation of Pb (and PbO) could be avoided by first heattreating hydrolyzed, mixed alkoxide precursor powders at 300°C for 1 h. This treatment was not effective for the two other precursors. It is concluded that both the carbonaceous content of the precursor (lowest for the hydrolyzed, mixed alkoxides) and the rate of hydrocarbon release during pyrolysis are critical to avoid the formation of elemental Pb during pyrolysis.