From waste glass to highly insulating glass foam: Foaming mechanisms and controlling the thermal conductivity (invited talk)

Waste glass can be a burden to the environment, however, for some products it is of great value. As an example, glass foam can be fabricated from various types of landfilled waste glass such as flat glass, container glass, and cathode ray tube (CRT) glass. Thus, glass foams are interesting materials from a sustainable point of view as they can be used in various applications depending on the final properties. One possible application is thermal insulation, which ensures energy saving from heating or cooling buildings.
The foaming process requires little energy compared to glass melting as the foaming typically occurs at < 900 °C. Highly porous glass foams can be obtained by matching the viscosity of the glass and the gas formation temperature by foaming agents, and various additives can be added to the powder mixtures prior to heating to alter the foaming temperature, closed/open porosity, and crystallization.
For thermal insulation applications, the thermal conductivity is the most important property. Generally, the thermal conductivity decreases with increasing porosity, which can be described through different models.[1] As the porosity increases, the composition of the gas phase becomes significantly more important, and it can be controlled through the foaming agents combined with a high degree of closed pores that trap gases in the macrostructure. To fully understand the thermal conductivity of porous materials, one should consider the contributions from the solid and gas phase, radiation, convection, and solid-gas coupling.
In this presentation, I will present two different foaming mechanisms and show how we can use the knowledge acquired from a physical foaming approach [2] to understand the typical chemical foaming. Furthermore, the effect of chemical composition of the solid phase and gas composition on the thermal conductivity will be discussed to optimize the thermal insulating properties of glass
foams.
1. Østergaard et al., High-speed synchrotron X-ray imaging of glass foaming and thermal conductivity
simulation. Acta Mater. 189 (2020) 85-92.
2. Østergaard et al., Foam glass obtained through high-pressure sintering. J. Am. Ceram. Soc. 101 (2018)
3917-3923.