Effect of the calcium dopant on oxide ion diffusion in yttria ceramics
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Tracer oxygen diffusion coefficients, D*, in polycrystalline yttria doped with Ca have been determined by a gas–solid exchange technique and secondary ion mass spectrometry. Samples containing few pores were used to avoid their influences on diffusion profiles. The resulting profiles were assigned only to volume diffusion; no grain boundary diffusion was observed. According to the effects of Ca doping on D*, the Ca contents are divided into three regions. In a Ca content region of 0–0.17 mol%, D* changed a little with Ca doping and took a minimum experimentally at 0.02 mol%. D* increased significantly within a range of 0.17–0.54 mol% and saturated at 0.54 mol% or above because of a solubility limit. The activation energies of oxygen diffusion were estimated at 249–282 kJ/mol.
I. INTRODUCTION
II. EXPERIMENTAL
Yttria, Y2O3, has a C-type rare-earth sesquioxide structure where one-fourth of the oxygen sites in the fluorite-type structure are vacant.1 Ando et al. have proposed that oxygen diffusion in yttria occurs by interstitialcy rather than vacancy migration.2 On substitution of Zr4+ ions for Y3+ ions, the concentration of oxygen interstitials increases, and oxide ions diffuse through interstitial oxygen sites as well.3 On the other hand, oxide ions are regarded as diffusing via oxygen vacancies when doping with Ca2+ ions.4 Polycrystalline yttria is a potential candidate for an industrial material used at high temperature and/or in corrosive atmospheres. Dense ceramic bodies are indispensable for improving its durability, and several sintering aids have been investigated so far. Because Ca is one of the most effective additives,5 its influence on oxide ion diffusion is worthy of a detailed study. Moreover, oxygen diffusion coefficients are generally very sensitive to the content of a dopant. They are often a good measure of the solubility limit of the dopant because they change considerably with increasing amount of substitution, saturating around the solubility limit. In the present study, we have determined tracer oxygen diffusion coefficients of the Y2O3–CaO system by a gas–solid exchange technique6 and secondary ion mass spectrometry (SIMS). To make measurements as accurate as possible, transparent samples of yttria with few pores and with homogeneous distribution of Ca have been prepared by a carbonate precursor method.7 We shall discuss defect structures associated closely with oxide ion diffusion and a solubility limit of Ca on the basis of the resulting data.
To obtain highly dense polycrystalline yttria, sinterable powders were synthesized as follows.7 Y(NO3)3 ⭈ 6H2O (99.99%) and Ca(NO3)2 ⭈ 4H2O were dissolved in deionized water to get a 0.7 mol/dm3 mother solution. Yttrium carbonate was precipitated by dripping 300 cm3 of a 1.4 mol/dm3 solution of NH4HCO3 into 300 cm3 of the mother solution at a dripping speed of 10 cm3/min. To crystallize the carbonate precipitate, it was aged for 2 d at room temperature while being agitated with a magnetic stirrer. The product was filtered off and washed repeatedly w
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