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Issue 93, 2015
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Vapour pressure and mixing thermodynamic properties of the KNbO3–NaNbO3 system

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Abstract

Equilibrium vapour pressures of sodium and potassium over a KxNa1−xNbO3 solid solution within its whole compositional range at temperatures between 1173 K and 1303 K were determined by Knudsen Effusion Mass Spectrometry. It should be noted that the thermodynamic equilibrium between the condensed and the vapour phase could be established only after prolonged annealing (more than 10 h at 1263 K). The equilibrium vapour pressure of potassium over K0.5Na0.5NbO3 (KNN) is a few times larger than that of sodium, i.e., 8 × 10−3 Pa as compared to 3 × 10−3 Pa at 1263 K. From the obtained results, the excess thermodynamic functions for the pseudo-binary KNbO3–NaNbO3 system were evaluated. The excess Gibbs energy was found to be positive, the excess enthalpy is close to zero, while the negative excess entropy indicates a partial ordering of alkaline ions in the solid solution. The comparison of the obtained results to the well-established lead-based piezoelectric systems revealed, that the vapour pressure of alkalis over the respective niobates at 1200 K is almost three orders of magnitude lower as compared to the values reported for lead oxide over Pb(Zr,Ti)O3.

Graphical abstract: Vapour pressure and mixing thermodynamic properties of the KNbO3–NaNbO3 system

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Publication details

The article was received on 19 Jun 2015, accepted on 01 Sep 2015 and first published on 01 Sep 2015


Article type: Paper
DOI: 10.1039/C5RA11874C
Author version available: Download Author version (PDF)
Citation: RSC Adv., 2015,5, 76249-76256
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    Vapour pressure and mixing thermodynamic properties of the KNbO3–NaNbO3 system

    A. Popovič, L. Bencze, J. Koruza and B. Malič, RSC Adv., 2015, 5, 76249
    DOI: 10.1039/C5RA11874C

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