Today, the best cellular glass is for example produced by ZES FOAMGLASS. ZES 500 has an average thermal conductivity of 0.038 W/mK at a density of 110 kg/m³ and an average compressive strength of 500 kPa. Other thermal insulations can have thermal conductivities halve the above value. How can we improve cellular glass?
Lowering further the density is probably not an option. Even if we keep a fine cell structure, the compressive strength lowers quadratically with the density like shown in this graph from the paper from Lorna Gibson, famous MIT professor and cellular solids expert. Indeed, we can assume the compressive strength is linear with the Young modulus because glass breaks at a certain strain above the static fatigue limit, independent of the density of the cellular glass.
Besides the solid heat conduction in the glass, we have the gas thermal conduction and radiation. Radiation can be improved by more walls absorbing and radiating or an improved cell structure.
Gas thermal conduction is sensitive to the weight of the molecules. If we assume that we have only CO2 in the cells with a carbonaceous foaming agent like carbon black or glycerin, we can improve with a receipe generating SO2. Indeed, CO2 has a thermal conductivity 0.015 and SO2 0.009 W/mK like shown here. But I doubt that an SO2 containing foam will be popular.
Inducing a vacuum in the cells could be the solution. For example, open cell XPS, evacuated to 10 Pa has a large improvement like shown in the graph of this interesting paper of the EMPA institute in Switzerland. It looks simple but a vacuum of 10 Pa is not easy to generate in a fine cells foam. This low pressure is needed to get a situation where the mean free path of the gas molecules is a lot larger than the cell size.
But the graph shows that the thermal conductivity of XPS with air (0.031 W/mK) decreases to 0.007 W/mK with the vacuum. The difference = 0.024 W/mK is the thermal conductivity of air. If we apply this on a cellular glass (0.038), filled for 100% with CO2 (0.015), we arrive at the lowest thermal conductivity = 0.023 W/mK possible for cellular glass based on soda lime glass.
In principle, we can dream about the following material:
- stable thermal conductivity = 0.023 W/mK
- compressive strength = 500 kPa or permanent load = 250 kPa
- absolute vapor tight
- free from water absorption
- extreme good ecology balance due to the use of waste glass without remelting
I guess this nice dream is a nightmare for the other thermal insulation materials and after 70 years cellular glass, I doubt it is realistic. But it is maybe not impossible ….