Seleção de aditivos de sinterização e processamento cerâmico visando otimização da microestrutura do eletróllto sólido BaZr0,8Y0,2O3-δ
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2018-11-12Autor
Wendler, Leonardo Pacheco
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In this thesis, we investigated the production of Yttrium-doped Barium Zirconate
proton-conducting electrolytes with 20 at.% of Yttrium (BaZr0.8Y0.2O3-δ) via solid state
reaction. The great challenge presented by this material is its refractory nature, which
limits densification as well as grain growth at temperatures below 1700⁰C. The
influence of sintering additives on the microstructural development and on the
formation of desired phase were also investigated. The literature is lacking on this
subject, since there is no consensus concerning the mechanisms of action of sintering
additives, and most times, ceramic processing fundaments are not taken in to
consideration. In this thesis Aluminum oxide, Zinc oxide and Yttrium-Barium cuprate
were selected as additives, being that the latter has not yet been reported in the
literature. Compositions without additives were also prepared for comparison
purposes. Samples with densification greater than 97% after sintering at 1600⁰C were
obtained even without sintering additives and with a mean grain size of 1.40 μm. The
results obtained in this thesis show that the real purpose of using certain sintering
additives is to affect the formation of yttrium-rich secondary crystalline phase, which
remains in grain boundary, hinders grain growth, and damages electrical conductivity.
The addition of Al2O3 favored the formation of the secondary phase, limiting grain
growth severely. The addition of ZnO minimized the formation of the secondary phase
and promoted liquid phase formation, which in turn favored densification from as early
as 1300⁰C, but effective grain growth occurred only at 1600⁰C, reaching an average
grain size of 2.70 μm. However, the presence of liquid phase lead to grain boundaries
with a blocking effect on protonic movement. Yttrium-Barium cuprate (YBC) worked
effectively as sintering additive, leading to samples with densification higher than 95%
after sintering at 1300⁰C and free of the yttrium-rich secondary phase. The best
electrical performance was obtained with samples sintered at 1600⁰C, with a mean
grain size of 2.40 μm, non-blocking grain boundary, and total proton conductivity of
9.0x10-3S.cm-1 at 500⁰C, which is about 20% higher than samples without additives.