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Mayenite (Ca12Al14O33) was indicated as oxygen ion conductor because of its high ion conductivity by high temperature [1]. The ion conductivity of polycrystalline mayenite is about 1 order of magnitude smaller than that of yttrium stabilized ZrO2 [1]. Mayenite has a cubic crystal structure with the space group (I43d) [2]. The cubic unit cell of mayenite consists of two mayenite molecules with 12 cages altogether and can be expressed as [Ca24Al28O64]4+(2O2-). Two of these cages are occupied with two oxide ions respectively. The high ion conductivity of mayenite by elevated temperature can be explained by the high mobility of the free oxide ions in the mayenite crystal structure. The ion conductivity of the synthesized polycrystalline mayenite samples (in labor air, by 1351 and 1390°C) was determined in labor air using the impedance spectroscopy. The measured ion conductivities of the samples in this work correspond to the measured ion conductivity of the polycrystalline mayenite reported by Lacerda et al. [1]. The activation energies of the measured ion conductivities of the samples (synthesized by 1351 and 1390°C) are 0.61 and 0.59 eV respectively. Mayenite can form by the solid state reaction between CaAl2O4 and Ca3Al2O6 (diffusion couple between CaAl2O4 and Ca3Al2O6). In the labor air the activation energy of the parabolic growth rate constant (Kp) of the formed mayenite layer is 1.98 eV. The increase of the water partial pressure (in the dried synthesized air) has no effect on Kp. Kp remains constant in the observed water partial pressure interval (0.034, 0.056 and 0.094 bar). In the dried synthesized air the mayenite layer is instable. The kinetic and thermodynamic stability of the formed mayenite layer depends probably on the hydrogen ion concentration. The Hydrogen ions supposed has a stability effect on the mayenite crystal structure (thermodynamic stability). The diffusion coefficient of the layer growth rate determined mobile Ca2+-ions in mayenite was determined using a defined point defect model with the help of the kinetic and the irreversible thermodynamic to mayenite formation by the diffusion couple between CaAl2O4 and Ca3Al2O6. The calculated Ca2+-ion diffusion coefficients in mayenite on the right (in Ca3Al2O6 phase direction) and on the left (in CaAl2O4 phase direction) from the phases interface (CaAl2O4/Ca3Al2O6) are 2 to 3 order in magnitude smaller than the O2--ion diffusion coefficient calculated from the ion conductivity [1]. The Ca2+-ion diffusion coefficient in the right site of the phases Interface is 2 factors smaller than that of the left site because the Ca vacancy concentration depends only with the exponent (-14/18) on the Ca3Al2O6 activity. The activation energies of the Ca2+-ion diffusion coefficients right and left from the phases interface are 1.91 and 1.94 eV respectively.[1] M. Lacerda. J.T.S. Irvine and F.P. Glasser, Nature., 1988, 332, 525-526.[2] H.B. Bartl and T. Scheller, Neues Jahrb. Mineral. Monatsh., 1970, 35, 547. |
