DISSOLUTION OF HYDROGEN INTO TUNGSTEN PHOSPHATE GLASSES THROUGH PALLADIUM COATING

Dissolution of hydrogen into tungsten phosphate glasses through coated Pd layers was observed upon heating below the glass transition temperatures. The color of the glasses was varied from pale yellow to dark blue during the heat-treatments under hydrogen atmosphere, and the center responsible for the coloration was found to be W5+ formed by electron trapping from the optical and ESR absorption measurements. Here, a proton works as the charge compensator of W5+. Hydrogen was reversibly released from the glasses through coated Pd under low partial pressures of hydrogen such as N-2 atmosphere. Proposed was mechanisms of hydrogen dissolution into the glasses; 1) decomposition of H-2 to 2H by coated Pd, 2) dissolution of the resulting hydrogen atom through the reaction with W6+ ions to form W5+/H+ redox pair in the glasses, and 3) diffusion of hydrogen in the form of proton and electron trapped on W6+ hopping between W6+ and W5+ into the glass. The diffusion coefficient of hydrogen was enhanced with decreasing ion radius of modifiers such as alkali and alkali-earth ions. The electrical conductivity increased with increasing the concentration of hydrogen. The W-containing glass dissolving hydrogen was found to be a mixed conductor of proton and electron.