Vacuum glazing is a transparent thermal insulator with applications in energy efficient windows. It consists of two sheets of glass separated by a narrow evacuated space. The concept of vacuum glazing operates on the same principles as the conventional Dewar flask - the vacuum eliminates conductive and convective heat flow due to gas between the two glass plates, and internal low emittance coating reduces radiative heat transport to a low level.

The thermal insulating performance of windows is very important in the design of energy efficient buildings. Since windows are considerably less insulating than the other components of surrounding building structure, they play an important role in energy consumption.

The design and manufacture of vacuum glazing

Vacuum glazing consists of two flat sheets of glass hermetically sealed together around the edges, and separated by a narrow (~0.15 mm) evacuated space (see figure). Under the influence of atmospheric pressure, the internal surfaces of the glass sheets are kept apart by an array of small support pillars which are high strength material such as metal (stainless steel or Inconel 718) or ceramic (alumina or zirconia).

The actual glazing is made from two sheets of soda-lime glass. The glass sheets, typically 3 mm thick, are separated by a large number of small support pillars. The edge seal around the glazing periphery is made from solder glass, which is a low melting point glass with a coefficient of thermal expansion matching that of the soda-lime glass. The sample is evacuated through the small pumpout tube, which is sealed to the outer surface of the upper glass sheet with solder glass. The internal space is evacuated to a pressure of about 10-4 Torr. After completion of the evacuation and outgassing process, the end of the pumpout tube is melted and sealed.

Transparent low emittance coatings on the internal surfaces of one or both of the glass sheets are used to reduce radiative heat transport to a low level. Both pyrolytically deposited coatings (based on tin oxide), and sputtered coatings (multilayer stacks of silver and dielectric materials) have been successfully incorporated into vacuum glazing.

Research on vacuum glazing

The Department of Applied Physics in the University of Sydney was the first institution in the world to produce thermally insulating samples of vacuum glazing. This occurred in 1988. We have since made very substantial progress on the science and technology of these devices.

Over the years, research has been done on various aspects of vacuum glazing. Over 1000 samples of vacuum glazing, with dimensions up to 1 m x 1 m, have been made at the University of Sydney using constructions similar to that shown in the figure above. Research has focused on

• heat transport mechanisms
• stress due to atmospheric pressure and temperature differences
  
• stability of the internal vacuum
  
• measurement of heat flow
  
• production technology and costs
  

The University of Sydney vacuum glazing is being developed commercially in Japan under a licensing agreement. The technology has the potential to achieve substantial energy savings in buildings.

For further information on vacuum glazing project, please contact Dr. Nelson Ng, nelson@physics.usyd.edu.au