Sodium-Ion Conductivity and Humidity-Sensing Properties of Na₂O-MoO₃-P₂O₅ Glass-Ceramics

A series of glass-ceramics were prepared by heat-treatments of 40Na₂O-30MoO₃-30P₂O₅ (in mol%) glass in a temperature range from 380 (T_g) to 490 °C (T_c) and for 1-24 h. The prepared glass-ceramics contain from 2 to 25 wt.% of crystalline NaMoO₂PO₄. The sodium-ion conductivity in these materials decreases up to one order of magnitude with an increase in the degree of crystallization due to the immobilization of sodium ions in crystalline NaMoO₂PO₄. The transport of sodium ions in these materials occurs primarily through the dominant continuous glassy phase, and it is weakly affected by the sporadically distributed crystalline grains. However, the prepared glass-ceramics exhibit high proton conductivity in a humid atmosphere and remarkable humidity-sensing properties; this could be related to crystalline NaMoO₂PO₄, which provides sites for water adsorption. The glass-ceramic prepared at 450 °C for 24 h shows the best humidity-sensing performance among all samples, showing an increase in proton conductivity for more than seven orders of magnitude with the increase in relative humidity from 0% to 95%. Under a highly humid atmosphere (95% relative humidity and 25 °C), the proton conductivity of this glass-ceramic reaches 5.2 × 10^-3 (Ω cm)^-1. Moreover, the electrical response of these materials on the change in the relative humidity is linear and reversible in the entire range of the relative humidity, which indicates that they are novel promising candidates for application as humidity sensors.