Effect of the process atmosphere on glass foam synthesis: A high-temperature environmental scanning electron microscopy (HT-ESEM) study

The effect of the process atmosphere composition on the foam formation of cathode-ray tube (CRT) glass containing graphite and MnO2 was studied using in situ environmental scanning electron microscopy at high temperature (HT-ESEM). When compared to He+4% H2, O2 or air, water steam facilitates glass grain sintering. This is probably due to the formation of hydroxyl groups at the glass grain surface that locally decrease the glass viscosity. We have shown that increasing the steam pressure from 50 Pa to 750 Pa decreases both sintering and foaming onset temperatures by approximately 100 degrees C, favouring the formation of closed pores in viscous glass. At high temperature, the presence of water steam or oxygen promotes foam formation, while the presence of a reducing atmosphere (He+4%H2) limits glass melt foaming. A synergetic effect of O2 and H2O on the onset temperature of glass sintering and foam formation is evidenced.