Vacuum Insulation Glazing (VIG)

The potential and current limitations of Vacuum Insulation Glazing (VIG)

The insulation performance of windows has dramatically increased during the last fifty years. The U-value of a state-of-the-art triple glazing unit is around 0.7 W/(m²·K), which is still higher than that of a well-insulated wall, with U-values as low as 0.15 W/(m²·K).

Historical evolution of glazing performance. For comparison, a well-insulated wall has a U-value of 0.15 W/(m²·K).

Vacuum Insulation Glazing (VIG) have potential U-values lower than 0.3 W/(m²·K), but currently available products do not even approach these values due to technical limitations. One of the most critical aspects of VIG is the edge seal. For typical products, the edge between both panes is sealed by a PbO-SiO₂ based solder glass before evacuation and sealing of a pump-out hole. The result is a rigid edge seal and a visible pump-out hole in one of the corners of the glazing. The rigidity of the edge seal imposes limitations on the performance of the VIG unit: for large sizes of highly insulating glazing, large temperature differences between the inner and outer pane result in differential expansion of the panes, which may bend and break the VIG.

The Winsmart VIG

The Winsmart strategy overcomes these problems by using a flexible edge seal that consists of laser-welded metallic ribbons. In a first step, metallic ribbons are soldered onto both glass panes under atmospheric pressure. In the second step, the VIG is created by laser-welding the metallic ribbons inside a vacuum chamber. The result is a VIG unit with a flexible edge seal and without a pump-out hole.

Schematic cross-section of the Winsmart edge-sealing technology.

Glass-to-metal bonding by liquid solder anodic bonding

The bonding of metal to glass is notoriously difficult and often requires metallization of the glass surface before bonding. The ALTSAB process combines anodic bonding with the use of an activated liquid tin solder to produce strong hermetic seals. The liquid solder can accommodate surface roughness and negates the need for surface pre-treatment or the application of high mechanical force during the bonding process.

Schematic illustration of the ALTSAB process (Figure from Elrefaey et al. 2014). The application of a high electric field (500-1000 V) oxidizes the activating element in the solder (Al) near the glass-solder interface, dramatically improving bond strength and the wetting of the glass with liquid thin. At the same time, the metal-solder bond is forms by soft soldering.

The application of a high voltage (left) dramatically improves the wetting of the glass by the solder (Figure from Koebel et al. 2011).

Early glass-to-metal bonding prototype.

Laser welding of metallic ribbons under vacuum

After bonding the metallic ribbons, a pair of glass panes is transferred to a vacuum chamber (P < 1E-5 mbar). The spacers have to be placed before in a way that they don't influence the optical characteristic of a window. In the vacuum tunnel a plasma cleaning process is applied to avoid excessive degassing of the final VIG unit. Then, the top pane is lowered onto the spacers positioned on the bottom pane and the metallic ribbons are clamped together. Finally, the metallic ribbons are laser-welded to form a hermetic seal, while the laser itself is outside of the vacuum chamber.

Vacuum chamber with laser-welding equipment for 1m x 1m glass panes.

The advantage of this method is the hermetic sealing without pumping the VIG afterwards. The joint glass panes leave the vacuum chamber as a complete VIG.

Close-up (top-view) of the laser-welded edge seal. The high degree of overlap ensures a hermetic seal.

This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 314407.