THE MICROSTRUCTURE AND OPTICAL TRANSMITTANCE THERMAL ANALYSIS OF SODIUM BOROSILICATE BIO-GLASSES

摘要

The conditions to fabricate the bulk porous specimens have been studied on account ofrnsodium borosilicate (NBS) glasses. Glass composition heat treatment at phase separation andrnTiO2 additive have been considered in this study. Original glass samples of nominal compositionrnin mol%: sample A: 9.19 Na_2O – 23.58 B_2O_3 – 67.23 SiO_2, sample B: 9.29 Na_2O – 3.17 TiO_2 –rn23.82 B_2O_3-63.72 SiO_2 were prepared by melting reagent grade chemicals (Na_2CO_3, HBO_3,rnSiO_2 and AgNO_3) in platinum crucibles at 1480℃ for 1h in air. The melts were poured ontornstainless steel plates and were annealed at 500℃ for 0.5h after cooling. Thus, obtained samplesrnwere phase separated at 700℃ for 2, 15, 25 and 50h to study their microstructure by scanningrnelectron microscope (SEM). Besides the direct study of the microstructure by SEM, valuablerninformation on glass structural changes of samples are provided by measuring in situ changes byrnthe optical transmittance thermal analysis, heat treated isothermally at 700, 720 and 740℃. Therntemperature of phase separation, the leaching and nucleator additive (TiO_2), significantlyrninfluence the microstructure of the resulting leached product. TiO_2 additive seems to suppressrncrystallization of cristobalite: especially at the extended above heat treatment phase separationrnruns. The phase-separated domains of glasses containing above 80 mole of SiO_2 are so small thatrncan be hardly observed by SEM. The glass composition in our case was selected in a way to havernrelatively large phase separated areas easily observed by SEM at magnification approximatelyrn20,000x. The influence of TiO_2 is not too pronounced. It seems to suppress the cristobaliterncrystallization, especially of longer heating runs. The image analysis of leached glasses showsrnthe prevailing content of the skeletal phase in a comparison to pores. The TiO_2 contentrndiminishes slightly the content of the skeletal phase.