EVACUATED GLAZING

Flat evacuated glazing with very good insulating properties requires: (i) a leak-free edge seal around the periphery of two glass sheets; (ii) a low, and stable pressure within the space between the sheets; (iii) a low thermal conductance array of support pillars to maintain the separation of the glass sheets under the influence of atmospheric pressure forces; and (iv) transparent, low emittance coatings on the inner surface of one, or both glass sheets. Experimental samples of evacuated glazing up to 200 mm x 250 mm have been produced which incorporate the first three features. It has been shown that an array of pillar supports having thermal conductance less than 0.3 W m-2 K-1 can be produced which is quite unobtrusive and which results in tolerably low levels of mechanical stress within the glass sheets. The optical properties of windows incorporating such supports are very similar to conventional windows. Levels of vacuum have been achieved without getters in several devices for which contributions to thermal conductance due to heat flow through the gas are less than the limit of resolution of the measuring device (0.2 W m-2 K-1). Durable, vacuum compatible low emittance coatings exist which should result in a thermal conductance due to radiative heat flow of less than 0.3 W m-2 K-1. The overall thermal conductance of a window incorporating such coatings should be less than 0.6 W m-2 K-1. The ability of these windows to withstand mechanical load and temperature differentials appears to be adequate for most architectural applications.