Methods of construction of evacuated glazing

A thermally insulating glass panel comprises two spaced-apart sheets of glass enclosing a low pressure space and interconnected by a glass solder edge seal and an array of support pillars, wherein the glass sheets are sized or configured, or both, such that their edges are not in registration, and, at any point around the periphery of the panel, the edge of one of the sheets extends beyond the edge of the other in order to provide a peripheral stepped cavity in which solder glass material for forming the edge seal is deposited prior to being melted and fused. A method of forming the edge seal for a thermally insulating glass panel comprises the steps of: (a) preparing first and second sheets of glass such that when they are brought together their edges are not in registration, at any point around the periphery of the panel the edge of one of the sheets of glass extending beyond the edge of the other; (b) depositing a strip of solder glass around the periphery of the panel on the extending edges of the glass sheets; (c) heating the panel to melt the solder glass such that it runs between the glass sheets; and (d) cooling the panel to solidify the solder glass.

The invention described in this specification concerns an improvement to 
the design of thermally insulating glass panels. These panels typically 
comprise two spaced apart sheets of glass enclosing a low pressure space 
and interconnected by an edge seal and an array of support pillars. The 
panels are used for glazing, and such thermally insulating windows are 
generally referred to as evacuated glazing. 
BACKGROUND 
Thermally insulating panels are typically constructed by depositing a strip 
of solder glass around the periphery of the glass sheets, depositing an 
array of support pillars onto one or other of the sheets of glass, 
bringing the sheets together or permitting them to move together, heating 
the panels to melt the solder glass around the periphery so that the two 
sheets settle onto the pillars, and then cooling the panel to solidify the 
edge seal. The panel is the evacuated by pumping out through a tube 
positioned either through one of the glass sheets or through the edge 
seal, and finally the pump-out tube is melted and sealed off. 
The improvement concerns the edge seal, and in particular a method of 
constructing a thermally insulating glass panel with a superior edge seal, 
and a panel incorporating the superior seal. 
Several conditions must be satisfied in the construction of evacuated 
glazing. First, it is necessary to produce an hermetic (leak-free) edge 
seal around the periphery of the panel. Second, in order to prevent the 
glass sheets from being forced into contact with each other, by the large 
atmospheric pressure forces acting, it is essential to include an array of 
support pillars within the glazing. Third, the space between the glass 
sheets must be evacuated to a high level, and this level of vacuum 
maintained over the life of the glazing. Fourth, in order to reduce 
radiative heat flow through the glass, a low emittance coating is applied 
to the inner surfaces of one or both sheets of glass. 
One of the methods of forming the edge seal around the periphery of 
evacuated glazing is to use a low melting point glass (solder glass). In 
order to manufacture such an edge seal, solder glass is deposited as a 
liquid slurry onto the surface of one or both of the glass sheets, around 
the edges. The glass sheets are brought together and the entire assembly 
is heated to a temperature at which the solder glass melts, wets the 
surface of the glass sheets, and flows to produce an hermetic seal between 
the sheets. 
One of the difficulties is that even at the high temperatures used the 
solder glass possesses a relatively high viscosity, and it does not flow 
readily between the glass sheets. If there is too much solder glass in the 
gap between the sheets, it may not flow out of the gaps sufficiently to 
allow the upper sheet to settle down onto the pillars during the edge 
sealing process. This leads to large tensile stresses in the glass sheets 
near their edges when the vacuum is eventually created. 
SUMMARY OF THE INVENTION 
The invention provides a method of forming the edge seal for a thermally 
insulating glass panel comprising two spaced apart sheets of glass 
enclosing a low pressure space interconnected by an edge seal and an array 
of pillars. The method includes the steps of: 
(a) preparing first and second sheets of glass such that when they are 
brought together their edges are not in registration, and at any point 
around the periphery of the panel the edge of one of the sheets of glass 
extends beyond the edge of the other; 
(b) depositing a strip of solder glass around the periphery of the panel on 
the extending edges of the glass sheets; 
(c) heating the panel to melt the solder glass such that it runs between 
the glass sheets; and 
(d) cooling the panel to solidify the solder glass. 
In this way solder glass does not intrude between the sheets before it is 
melted, thereby overcoming the current problem in conventional 
construction methods. 
The first and second glass sheets are conveniently prepared by making one 
slightly smaller than the other. For instance the upper glass sheet may be 
5 mm less than the lower on all sides; in other words 10 mm less in each 
orthogonal direction. 
The solder glass strip may be deposited on the extending edges such that it 
bridges the gap between the sheets and contacts the end edge of the other 
sheet. 
Another advantage of the method is that the solder glass flows into the gap 
between the two glass sheets a uniform distance all around the periphery 
of the panel. The edge seals are therefore formed with a relatively 
constant width which gives the finished panels an excellent appearance and 
a high mechanical strength. Typically the width of the bonded region 
between the glass sheets is between 3 and 10 mm. 
The invention may further be defined as providing a thermally insulting 
glass panel comprising two spaced apart sheets of glass enclosing a low 
pressure space interconnected by an edge seal and an array of pillars 
wherein: the glass sheets are sized or configured, or both, such that 
their edges are not in registration, and at any point around the periphery 
of the panel the edge of one of the sheets extends beyond the edge of the 
other. Preferably one of the sheets is slightly smaller than the other, 
for example, by 5 mm on all sides.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The lower glass sheet 10 is slightly larger than the upper glass sheet 11 
which rests on an array of support pillars 12 at a predetermined spacing 
above lower sheet 10. A strip of solder glass 13 is laid on the extending 
edge 14 by which the edge of lower sheet 10 extends beyond the edge of 
upper sheet 11. The solder glass is deposited either as a liquid slurry, a 
powder, or a rod. The strip of solder glass 13 bridges the gap 15 between 
the glass sheets and contacts and the end edge 16 of upper glass sheet 10. 
When the construction is heated and the solder glass melts, it flows 
between the two glass sheets 10 and 11 by the action of capillary forces. 
The distance the solder glass flows can be controlled by varying the time 
and temperature of the process. In this way it is not necessary for solder 
glass between the two glass sheets to be expelled by the weight of the 
upper glass sheet. 
It should be appreciated that although this invention has been described 
with reference to a specific example it need not be limited to that 
embodiment. For example, one glass sheet need not be a slightly different 
size to the other and both sheets could be the same size provided they 
were translated out of registration with each other in both orthogonal 
direction.