Development and Investigation of Evacuated Windows Based on Monolithic Silica Xerogel Spacers

The objective of the project is to develop and investigate insulating glazings based on evacuated monolithic silica xerogel spacers. Low density monolithic silica xerogel is dried at atmospheric pressure. The process results in an increased density of xerogels compared to aerogels which leads to a slightly higher thermal conductivity with typical values of 0.030 W/(m K) measured in air at atmospheric pressure. If evacuated below 50-100 hPa the thermal conductivity will be approximately 0.013 W/(m K). The silica xerogel has to be protected against liquid water, that will demolish the pore structure of the material due to the surface tensions.
The project has been carried out as a co-operation between institutes and companies in Denmark, France, Germany, Norway and Sweden. Scientific developments have made it possible to prepare low density monolithic silica xerogels, only from about 1990, and developments in both the production process as well as size of the samples are necessary for a commercial use of the material. The improvement of the production process has as the main goals to improve the optical quality and the thermal conductivity of the monolithic silica xerogel by decreasing the density. Secondary an increase of the sample size should be achieved primarily by means of an optimisation of the drying process.
The thermal properties make the monolithic silica xerogel a well suited material for insulating glazings. Using the material as spacer between two layers of glass with a vacuum tight sealing of the rim combined with an internal gas pressure below 50-100 hPa result in an insulating glazing having a heat loss coefficient comparable with that of the surrounding walls, but at the same time offers a large solar heat gain possibility. Development of an airtight and vapour tight rim seal with negligible thermal bridge effect is one of the main goals of the project.