Stability and Thermal Expansion of Na+-Conducting Ceramics


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Abstract

An impedance spectroscopy study of sodium cation-conducting ceramics, including layered compounds Na0.8Ni0.4Ti0.6O2, Na0.8Fe0.8Ti0.2O2, Na0.8Ni0.6Sb0.4O2 (structural type O3), Na0.68Ni0.34Ti0.66O2 (P2 type), and NASICON-type Na3Si2Zr1.88Y0.12PO11.94 and Na3.2Si2.2Zr1.88Y0.12P0.8O11.94, showed that their transport properties are essentially independent of partial water vapor pressure at temperatures above 420 K. In the low-temperature range, increasing vapor partial pressure from approximately 0 (dry air) up to 0.46 atm. leads to a reversible increase in the conductivity. The sensitivity of studied materials to air humidity is strongly affected by the ceramic microstructure, being favored by larger boundary area and porosity. Maximum stability in wet atmospheres was found for NASICON ceramics, which also exhibit the highest cationic conduction. The average thermal expansion coefficients at 300–1173 K are in the range (13.7–16.0) × 10−6 K−1 for the layered materials and (5.9–6.5) × 10−6 K−1 for NASICON-type ceramics.

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