The mechanism of foaming and thermal conductivity of glasses foamed with MnO2

Abstract

We prepare foam glass from cathode ray tube (CRT) panels using MnO2 as foaming agent at different temperatures for various durations. The reduction of MnO2 to Mn2O3 leads to formation of O2 gas, and hence, causes initial foaming. The Mn2O3 particles dissolve into the glass melt and subsequently reduce, causing further formation of O2 gas and foaming of the glass melt. Increasing the treatment temperature and time enhances foam expansion, Mn2O3 dissolution, and lowers the closed porosity. Once the foam reaches a percolated stage, the foam continues to grow. This is caused by nucleation of new bubbles and subsequent growth. We discuss evolution of pore morphology in terms of pore number density, pore size and closed porosity. The thermal conductivity of the foam glasses is linearly dependent on density. The heat transfer mechanism is revealed by comparing the experimental data with structural data and analytical models. We show that the effect of pore size, presence of crystal inclusions and degree of closed porosity do not affect the overall thermal conductivity.