Formation Mechanism of Zinc Metaphosphate Hydrogels by a Chemicovectorial Method and Their Proton Conductivities

Abstract

When zinc metaphosphate glass powders were mixed with water, their hydration immediately started with an exothermic reaction; the mixture of the glass powders and water was completely converted into a translucent hydrogel (ZP gel) after 2 d. No hydrogelation occurred when water was mixed with beryllium metaphosphate, strontium metaphosphate, or barium metaphosphate glass powders. The long-chain phosphate structures in beryllium metaphosphate glass powders were almost not broken after mixing water, while those in strontium metaphosphate and barium metaphosphate glass powders were easily broken as shown in our earlier work. This result suggested that hydrogelation occurs after mixing water and the metaphosphate glasses with adequate bonding strength between bivalent metal ion and non-bridging oxygen in the phosphate chain structure. The phosphate chains in the ZP gel were gradually broken, resulting in an increase of orthophosphate amount. Crystalline phases were observed in the ZP gel after 12 d of mixing with water the glass powders. The proton conductivity of the ZP gel, which showed 5.7 mS/cm at 30°C, was not influenced by the precipitation of the crystalline phases.