Influence of foaming agents on both the structure and the thermal conductivity of silicate glasses

Foam glass is one of the most promising insulation materials for constructions since it has low thermal conductivity, high compressive strength, non-water permeability, and high fire resistance. They can be produced using cullet sources, e.g., cathode ray tubes (CRT) panel glass, and foaming agents such as metal carbonates, or oxidizing transition metal oxides combined with carbonaceous sources. In this work, we mix CRT panel glass powder with different foaming agents: CaCO3 (0-4 wt%), Fe2O3 (0-6 wt%), and MnxOy (0-10 wt%). The powder mixtures are sintered in the range between the glass transition temperature (Tg) and the foaming temperature (corresponding to the viscosity range of 1012-106 Pa s) at 10 K/min and cool down to 773 K (below Tg) at 30 K/min and naturally down to room temperature. Upon sintering, the foaming agents are partially incorporated into the glass structure. Afterwards we measure the thermal conductivity of the sintered samples with Laser Flash Technique to see its dependence on the degree of incorporation of the foaming agents into the glass structure. In parallel we prepare glass samples by adding the above-mentioned foaming agents to the CRT panel glass via high temperature (about 1500 C) melting and subsequent quenching. We compare the thermal conductivity of re-melted samples with that of the sintered samples to study the influence of the structural incorporation of the foaming agents on the thermal conductivity. The samples could crystallize during the heating process, and thereby their thermal conductivity can be influenced. The crystallinity of the samples is determined by means of X-ray diffraction. The change of the glass structure can be indirectly reflected by Tg change which is measured using a differential scanning calorimeter.