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The solid solubility limit of Al2O3 and its effect on densification and microstructural evolution in cubic-zirconia used as an electrolyte for solid oxide fuel cell

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Abstract

The solid solubility of Al2O3 in 8 mol% yttria-stabilized cubic zirconia (8YSCZ) was determined by XRD as a function of composition based on the lattice parameter change. It was seen that the average lattice parameter of 8YSCZ linearly decreased from 5.139 to 5.134 Å, as the Al2O3 content increased up to 0.3 wt% and the parameter did not vary with further increase in the Al2O3 content. This linearity shows that solid solubility limit of Al2O3 in 8YSCZ is about 0.3 wt%. The lower values of the lattice parameter found for Al2O3-doped 8YSCZ could be due to the dissolution in the cubic phase of some Al2O3 which has a smaller ionic radius than Zr+4 and Y+3 and thus replacing the smaller Al+3 ions with Zr+4 and Y+3 in the cubic lattice. It was also seen that the amount of solid solution affected densification and grain growth behaviour of 8YSCZ. Particularly, Al2O3 addition up to solubility limit of 0.3 wt% enhanced the densification and grain growth whereas further increase in Al2O3 content over solubility limit resulted in decreased densification and grain growth.

Introduction

Zirconia-based ceramics have attracted special attention because of their excellent mechanical properties and possibility of obtaining a nano-grained bulk ceramic with a controllable microstructure and improved properties. Pure zirconia exists in the three different crystal structure, i.e., monoclinic, tetragonal and cubic. These phases can be obtained depending on temperature and compositional ranges under equilibrium conditions [1], [2], [3], [4]. Monoclinic zirconia is present below 1240 °C and is the stable room temperature phase of pure zirconia. Tetragonal zirconia is an intermediate phase, which lies between 1240 and 2370 °C. Cubic zirconia is the highest temperature phase which is present in the temperature range of 2370–2680 °C. Among zirconia phases, yttria-stabilized cubic zirconia (YSCZ) with fluorite structure is well known as a solid electrolyte that possesses high oxygen ionic conductivity and chemical stability over wide ranges of temperature and oxygen partial pressure and thus it is widely used as an oxygen sensor, thermal barrier and solid oxide fuel cell (SOFC) [5]. Solid oxide fuel cells (SOFC) are being used for various applications in the automobile, power generation and aeronautics industries. A single SOFC unit consists of two electrodes (an anode and cathode) separated by a electrolyte. For SOFC applications, the thin solid electrolyte needs to be strong and tough enough to withstand room temperature assembly stresses and be mechanically stable for long periods at high temperatures in reducing and oxidizing atmospheres [6]. Therefore the enhancement of mechanical properties of the solid electrolyte is important problem to be solved. Many approaches have been made to enhance the mechanical strength and hinder the severe grain growth (it is known that cubic zirconia has large grain sizes and high grain growth rates [7]) in cubic zirconia [8]. One such approach is to use the composite way by dispersing a second phase particles. In the composites, it has been known that the remarkable mismatching of lattice parameters and thermal expansion coefficients between matrix and second phase particles creates grain growth inhibition and thus the enhancement of mechanical properties. In the present study, the solid solubility limit of Al2O3 and its effect on densification and microstructural evolution in cubic zirconia used as an electrolyte for SOFC was investigated.

Section snippets

Materials and procedures

The raw materials used in the present work were commercial 8 mol% yttria-stabilized cubic zirconia (8YSCZ) powder, Tosoh, Japan and high purity (>99.999%) α-Al2O3 powder, Sumitomo, Japan. The average particle sizes were 0.3 μm for 8YSCZ and 0.4 μm for α-Al2O3. The chemical composition of 8YSCZ was 13.6 wt% Y2O3 (equivalent to 8 mol%), 85.9 wt% ZrO2, and the following impurities (in weight %), Al2O3 0.25, SiO2 0.10, TiO2 0.12, Fe2O3 0.003, Na2O 0.002 and CaO 0.02. Colloidal processing was used for the

Results and discussion

Uniformly dispersed powder is generally desirable for producing dense ceramics, because of its close packing during green compaction and uniform densification during sintering [9]. Also in ceramic processing, fine powders usually tend to agglomerate and such agglomeration of the powder decreases the packing efficiency and reduces the green density. During sintering each agglomerated side sinters differently causing the formation of large pores. For these pores, long hours and high sintering

Conclusion

The solid solubility limit of Al2O3 and its effect on densification and microstructural evolution in cubic zirconia was investigated. It was seen that the solid solubility limit of Al2O3 in 8YSCZ is about 0.3 wt%. It was also seen that the amount of solid solution affected densification and grain growth behaviour of 8YSCZ. Particularly, Al2O3 addition up to solubility limit of 0.3 wt% enhanced the densification and grain growth whereas further increase in Al2O3 content resulted in decreased

Acknowledgements

This work has been supported by DPT (the State Planning Organization of Turkey) under project number 2003K120470-18. The author is grateful to the DPT for financial support and Gazi University for the provision of laboratory facilities.

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