Single glazed windows are a significant source of energy loss, resulting in $20 billion/year (in the US) of unnecessary energy expenditure1, and for a variety of reasons it is often not possible to simple replace these windows with multi-glazing alternatives. This work is a demonstration of the development of a flame pyrolysis tool, integrated with a low vacuum deposition chamber, for the purpose of synthesizing and depositing silica nanoparticles (NPs) in a one-step-process in order to fabricate thermally insulating and transparent thin-films. A primary focus for this technology would be as insulating layers on windows. Sufficient thermal insulation can be achieved by depositing a high porosity silica coating (analogous to an aerogel) on a standard window. To use these coatings for thermal insulation on windows it is a requirement that the coating be sufficiently thick so as to make a meaningful contribution to the thermal insulation of the window, relative to a standard single glazed window. Therefore, in order to maintain the mechanical strength of the coating it is necessary to include binding material for cross-linking. At this early stage of the project coatings of greater than 160 μm have been demonstrated, and initial thermal conductivity measurements are taking place.
In addition to thermal insulation, the optical properties of these coatings must be of high enough quality to maintain sufficient transmission in the visible region of the electromagnetic spectrum, so as to be considered practical for a window coating. This means that properties such as optical haze, reflection and transmission need to be carefully maintained. It is possible to control these optical properties by carefully controlling NP size and size distribution (allowing for even pore size distribution), varying the porosity of the NP coatings as the distance to material interfaces closes (thus preventing dramatic shifts in refractive index) and lastly by controlling the surface roughness to prevent significant scattering losses from the coatings surface. Our early samples currently show high haze and have highly diffuse transmittance in the visible spectrum, suggesting more needs to be done to control pore size uniformity. Pore size distribution and surface roughness are controllable by limiting the NP size, which in turn is controllable by varying the residence time of the particles within the synthesizing flame after the initial particle nucleation. Furthermore, by controlling the pressure differential between the upper and lower chambers of the deposition system, it is possible to vary the speed at which the silica NPs impact a substrate (mounted inside the deposition chamber), and thus, vary the porosity and refractive index of the NP coating.
1) Consumer prices for natural gas are compiled by the Bureau of Labor Statistics. For the current summary for major cities, see http://www.bls.gov/regions/midwest/data/AverageEnergyPrices_SelectedAreas_Table.htm .