Thermal Stability of Sodium Aluminum Silicates with and without Alkali Carbonates

The thermal stability of solium aluminium silicates and their reactivity to alkali carbonates were investigated in view of their application as CO2 gas sensors. A nepheline phase was very stable, since the peak position of the X-ray diffraction pattern was hardly influenced by either heat treatment or humidification up to 800°C. A distorted carnegieite phase (like-carnegieite) transformed into the nepheline phase at 500°C under a humid condition, while under a dry condition, it was stable up to 800°C but transformed into the nepheline phase at 900°C. For mixtures with alkali carbonates, the nepheline phase was most stable even under the humid condition at temperatures below 500°C. When the mixtures were heated at 600°C, the nepheline phase reverted to the like-carnegieite phase and alkali silicates, such as Na2SiO3, Li2SiO3 and Li4SiO4, formed as by-products. For Na2O-Al2O3-4SiO2 sintering mixtures with Li2CO3, the like-carnegieite phase transformed into the nepheline phase at 500°C. The nepheline phase was stable up to 1000°C even under the humid condition and Li2SiO3 formed as a by-product.