Na₂SO₄-based solid electrolytes for SO_x sensors

Na2SO4-based solid electrolytes with compositions Na2SO4 + x mol% Y2 (SO4)3 (x = 0, 3,5,6,9,12,15) and Na2SO4 + y mol% La2(SO4)3 (y = 0, 2, 4, 6, 8) were synthesized by solid state reaction. The crystal struCture, phaseS, and, electrical conductivity of these solid electrolytes were studied by XRD, SEM, EDX, DTA, TGA and impedance spectroscopy.

Upon doping with La2 (SO4)3 or Y2(SO4)3 the high-temperature fast-ion conducting phase NaSO4-I can be stabilized down to room temperature. Doping increases the ionic conductivity as well. Of these solid electrolytes the Na2SO4 + 5 mol% Y2(SO4)3 and Na2SO$ + 4 mol% La2(SO4)3 solid solutions exhibit the highest ionic conductivity values (at 400-degrees-C, 1.76 X 10(-2) S cm-1 and 2.95 X 10(-2) S cm-1, respectively) and lowest activation enthalpies (0.38 eV and 0.37 eV, respectively).

When x greater-than-or-equal-to 12 or y greater-than-or-equal-to 8, the Na2SO4-I structure can be completely stabilized down to room temperature. For the solid electrolytes with x = 5 and y = 4, Na2WO4 was used to further stabilize phase I, and to improve the mechanical properties. In the Na2SO4 + 5 mol% Y2(SO4)3 + m mol% Na2WO4 (M = 5,10,15,20) and Na2SO4 + 4 mol% La2(SO4)3 + n mol% Na2WO4 (n = 5, 10,15, 20) samples, XRD analysis reveals the Na2SO4-I structure to be stable at room temperature as is illustrated by DTA.

Impedance measurements show that doping these solid solutions with Na2WO4 decreases the ionic conductivity. However, their conductivity is still higher than that of undoped Na2SO4. In the ionic conductivity measurements of Na2SO4 + 4 mol% La2 (SO4)3 + Na2WO4 system, an unusual Arrhenius behaviour has been observed, i.e. the activation enthalpy at higher temperatures being larger than that at lower temperatures.

This behaviour is discussed based on the phases present and the structure of the specimens. From a point of view of practical application as SOx sensor material, the Na2SO4 + 5 mol% Y2(SO4)3 + Na2WO4 and Na2SO4+ 4 mol% La2(SO4)3 + Na2WO4 materials are better than undoped Na2SO4 because of their higher conductivity, and absence of a phase transition.