Abstract
The effect of Sr-nonstoichiometry on phase composition, microstructure, defect chemistry and electrical conductivity of SrxZrO3−δ and SrxZr0.95Y0.05O3−δ ceramics (SZx and SZYx, respectively; x = 0.94–1.02) was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy followed by distribution of relaxation times analysis of impedance data. It was shown that at low Sr deficiency (x > 0.96 and 0.98 for SZx and SZYx, respectively) a solid solution of strontium vacancies in strontium zirconate crystal structure forms, whereas at higher Sr deficiency the secondary phase, zirconium oxide or yttrium zirconium oxide, is precipitated. Yttrium solubility limit in strontium zirconate was found to be close to 2 mol%. Y-doped strontium zirconates possess up to two orders of magnitude higher total conductivity than SZx samples. A-site nonstoichiometry was shown to have a significant effect on the electrical conductivity of SZx and SZYx. The highest total and bulk conductivity were observed at x = 0.98 for both systems. Increasing the conductivity with a rise in humidity indicates that proton conduction appears in the oxides in wet conditions. A defect model based on consideration of different types of point defects, such as strontium vacancies, substitutional defects and oxygen vacancies, and assumption of Y ions partitioning over Zr and Sr sites was elaborated. The proposed model consistently describes the obtained data on conductivity.
Highlights
Perovskite oxides exhibiting high proton conductivity are promising electrolyte materials for solid oxide fuel cells (SOFCs) which convert the chemical energy of fuel directly into electrical energy with high efficiency and low air pollutants emissions [1,2]
The purpose of this research was to clarify the effect of Sr-nonstoichiometry on phase composition, microstructure, defect chemistry and electrical conductivity of Y-doped SrZrO3
X-ray diffraction (XRD) patterns powders are shown in Figure theFor compositions with low with
Summary
Perovskite oxides exhibiting high proton conductivity are promising electrolyte materials for solid oxide fuel cells (SOFCs) which convert the chemical energy of fuel directly into electrical energy with high efficiency and low air pollutants emissions [1,2]. Acceptor-doped barium and strontium cerates exhibit the proton conductivity sufficiently high for practical applications at intermediate temperature range but their chemical stability in CO2 and water containing atmospheres is poor [4,5,6,7]. Zirconate-based perovskites (AZrO3 , A = Ba, Sr) combines high chemical stability with satisfactory proton conductivity [7,8]. Proton conductivity of strontium zirconate can be enhanced by substitution of Zr by acceptor dopants, such as Yb, Y and Dy, due to the formation of large concentrations of oxygen vacancies as Materials 2019, 12, 1258; doi:10.3390/ma12081258 www.mdpi.com/journal/materials. Y-doped SrZrO3 was reported to be a good protonic conductor exhibiting its maximum proton conductivity at the dopant concentration of 5 mol% [11,12,13]
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