Doctoral dissertation
Solution synthesis of nanoparticles in multicomponent oxide systems
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Abstract
The study of PbZrO3 (PZ), PbTiO3 (PT) and La2Zr2O7 (LZ) prepared by alkoxide sol-gel synthesis is described.
PZ and PT sols were prepared from lead acetate, zirconium and titanium n-butoxide, in n-butanol and hydrolysed with Rw (molar ratio of water per lead acetate) between 2 and 15 in neutral medium and Rw = 15 in alkaline medium. Different Rw were used to find the correlations between the hydrolysis conditions, morphology and homogeneity of PZ and PT.
The partial charge of Zr in the partially hydrolysed PZ product is higher than that of Ti in corresponding PT sample, which means that the former is more reactive. The partial charges of the OH groups are similar in all cases, with negative values; therefore the OH groups are nucleophilic enough to initiate further condensation. We assume that condensation stops predominantly due to the steric hindrance.
At low Rw dense and at high Rw porous gel fragments are obtained, and at Rw 15 and alkaline pH about 10 nm sized particles. The organic groups in the former decompose upon heating with intermediate carbonaceous residues, forming a locally reducing atmosphere and segregation of lead from the amorphous matrix. In contrast, the porous structure of Rw 15 powders allows a one-step oxidation of organic groups. The dried PT powders are composed of a porous network of nano-particles regardless of the hydrolysis conditions.
The morphology of the dried powders is retained upon heating. The PZ and PT powders crystallize in perovskite phase upon heating at 700 oC and 500 oC, respectively.
The LZ sol was prepared from dehydrated lanthanum nitrate and zirconium n-butoxide in 2-methoxyethanol. A strongly exothermic reaction between the nitrates and organic groups yielded a friable, porous powder which crystallized in pyrochlore phase upon heating to 800 oC. The powder, heated at
900 oC contained 70 nm sized particles and could be sintered to 97.9 % relative density at 1400 oC, about 200 oC below the usually reported temperatures for air-sintering of this material.
The structural evolution from the sol to the amorphous dried and heat treated PZ and LZ powders was investigated. In the PZ sol, the lead environment is dominated by two Pb-O-Zr links, while the Zr environment is populated by six Zr-O-Zr links. In the dried powders, one of the two Pb-O-Zr links is retained while Zr - Zr correlations are retained regardless the hydrolysis conditions. In the powder heated at 400 oC / 1h there is no Pb-Zr correlation, while the Zr - Zr correlations remain almost unchanged.
La environment in both La nitrate solution and LZ sol is similar to that of lanthanum nitrate hydrate. Zr environment is populated by six Zr-O-Zr links and it remains almost unchanged in the transition from the sol to the dried and heated amorphous powder. La-O-Zr links have not been established. Zr local environments are similar in both PZ and LZ sols and amorphous powders. Upon synthesis of the respective sols the Zr-O-Zr links are established and they are stable in the investigated temperature range. In both cases the reaction between individual metal compounds proceeds as a solid-state reaction.