Optimised NASICON ceramics for Na+ sensing

NASICON dense ceramics were obtained from solid state reaction between SiO₂, Na₃PO₄·12H₂O and two different types of zirconia: monoclinic ZrO₂ and the yttria-doped tetragonal phase (ZrO₂)_0.97(Y₂O₃)_0.03. Higher temperatures were needed to obtain dense samples of the yttrium free composition (1265 °C). The electrical conductivity, at room temperature, of the yttria-doped samples sintered at 1230 °C (0.20 S/m) is significantly higher than the value obtained with the material prepared from pure ZrO₂. The impedance spectra show that the differences in conductivity are predominantly due to the higher grain boundary resistance of the undoped ceramics, probably due to formation of monoclinic zirconia and glassy phases along the grain boundary. Further improvement of the electrical conductivity could be achieved after optimization of the grain size and density of grain boundaries. A maximum conductivity value of about 0.27 S/m at room temperature was obtained with the yttria-doped samples sintered at 1220 °C for 40 h. Yttria-doped and undoped ceramics were tested as Na⁺ potentiometric sensors. The detection limit of the yttria-doped sample (10⁻⁴ mol/l) was one order of magnitude lower than the obtained with the undoped material. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.