Seleção de aditivos de sinterização e processamento cerâmico visando otimização da microestrutura do eletróllto sólido BaZr0,8Y0,2O3-δ

Abstract

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.