Phosphate glasses as alternatives to Pb-based sealing frits

Glasses in the SnO–ZnO–P2O5 (SZP) ternary were examined as potential replacements to PbO–ZnO–B2O3 or PbO–ZnO–SiO2 frits for low temperature (<500°C) sealing applications. Glasses were melted in air at 900–1100°C across a wide region of the ternary using standard reagent grade materials and carbonaceous additives to maintain a reducing atmosphere. Following melting, glasses were evaluated both as solid samples and in fine-powdered frit form (mean particle size of 20–40 μm). Several commercial Pb-based sealing frits were used as comparisons. Data were obtained on glass transition temperature, softening point, expansion compatibility with potential commercial substrates, aqueous durability and crystallization tendency as a fine powder. An area of glass formation was found in the SZP ternary. Compositions in both the SZP- and Pb-based systems formed stable frits – i.e., glasses that in finely-powdered form have a range of viscous flow during sintering without evidence of crystallization. By design of composition, both the aqueous durability and crystallization of the SZP frits may be made comparable to that of Pb-based systems. By the addition of a suitable expansion-modifying filler, the expansions of the SZP frits may be adjusted to be compatible with a range of potential substrates, including fused silica.