EP1204805A1

EP1204805A1 - Evacuated glass panel having a getter

Info

Publication number: EP1204805A1
Application number: EP00952793A
Authority: EP
Inventors: Richard Edward Collins
Original assignee: University of Sydney
Current assignee: University of Sydney
Priority date: 1999-08-18
Filing date: 2000-08-17
Publication date: 2002-05-15
Also published as: JP2003507845A; WO2001012942A1; CA2391831A1; AUPQ230499A0

Abstract

A method of fabricating an evacuated glass panel, in particular when in the form of evacuated glazing, which incorporates a chamber (18) that is defined by two spaced-apart glass sheets (10 and 12) and a fused edge seal (16). The method comprises positioning the glass sheets (10) and (12) in spaced-apart relationship using pillars (14), and locating a getter (20, 21 or 23) within a region (22) of the chamber that is to be formed between the glass sheets. A fusible sealant material (16) is deposited around the edges of the glass sheets (10 and 12) and air is displaced from between the glass sheets by admitting an inert gas to the space between the glass sheets. After admitting the inert gas the sealant material is fused to create the fused edge seal (16) around the glass sheets whilst maintaining the inert gas in the chamber that is formed with fusing of the sealant material. Thereafter, the chamber is evacuated and, depending upon the type of getter (20, 21 or 23) that is employed, the getter is activated either following evacuation of the chamber or during fusing of the edge seal around the glass sheets.

Description

EVACUATED GLASS PANEL HAVING A GETTER

Field of the Invention

This invention relates to an evacuated glass panel that incorporates a getter and to a method of constructing the panel. The evacuated glass panel is formed with a chamber located between two spaced-apart glass sheets and a fused seal surrounding the chamber.

The invention has particular application to vacuum glazing and is hereinafter described in that context. However, it will be appreciated that the invention does have broader application including, for example, in the fabrication of glass display panels.

Background of the Invention

In permanently sealed evacuated devices or structures such as electron tubes and vacuum glazing, the pressure level (i.e., vacuum pressure) required for effective performance is very low relative to atmospheric pressure.

In devices such as electron tubes the internal pressure is typically 10^~3 Pa or less, whereas in thermal insulating structures, such as vacuum glazing, the pressure is typically 10^"1 Pa or less.

The stability of a vacuum within a device or structure is dependent upon a low emission of gas from the internal surfaces of the device or structure after evacuation and sealing. Therefore, devices or structures in which a stable vacuum is to be maintained typically are subjected to a high temperature outgassing process, to release as much of the surface (adsorbed) and bulk (absorbed) gas as possible, whilst pumping is proceeding. In order to ensure that the outgassing occurs as quickly as possible, it typically is performed at the highest possible temperature.

In addition to the outgassing process, in some permanently evacuated devices such as electron tubes, a small quantity of a highly reactive material (i.e., a

"getter") is located inside the device to aid in the maintenance of the vacuum. Suitable getter materials include chemically active metals such as aluminium, barium, strontium, titanium and zirconium.

A getter provides for the formation of stable, inert, low vapour pressure compounds with most species of gas atoms that may be emitted from the devices, thereby effectively removing gas atoms from the internal volume of the devices. However, a getter may only be employed in circumstances where the getter itself will not be degraded during a manufacturing process.

Degradation of a getter will occur if it is heated to an elevated temperature of around 400°C in air and, as a result, the location of an active getter in an evacuated glass structure having a chamber defined by two spaced- apart glass sheets and a fused edge seal has not been feasible. This is because the formation of the fused edge seal would require heating of the entire structure to temperatures in excess of 400°C, with a getter in place, prior to evacuation of contained air. Localised heating at the edges of the glass structure (as an alternative to heating the entire structure) to form the fused seal would not be appropriate, as it would cause excessive stresses in the glass sheets, and localised heating to form a fused seal is generally only appropriate in cylindrical glass devices such as electron tubes. Summary of the Invention

The present invention seeks to avoid the above stated difficulties and it may be defined broadly as providing a method of fabricating an evacuated glass panel having a chamber defined by two spaced-apart glass sheets and a fused edge seal. The method comprises the steps of: (a) positioning the glass sheets in spaced-apart relationship and locating a getter within a region of the chamber to be formed between the glass sheets, (b) depositing a fusible sealant material around the edges of the glass sheets,

(c) displacing air from between the glass sheets,

(d) admitting an inert gas to the space between the glass sheets,

(e) fusing the sealant material to create a fused edge seal around the glass sheets whilst maintaining the inert gas in the chamber that is formed with fusing of the sealant material, (f) evacuating the chamber, and (g) activating the getter.

The invention may also be defined as providing an evacuated glass panel per se when formed by the above defined method. Preferred Feature of the Invention

The method steps (c) and (d) preferably are combined, so that the inert gas is admitted to the space between the glass sheets in a manner to displace the air from between the glass sheets . The air preferably is displaced through the sealant material that is deposited around the edges of the glass sheets prior to fusing of the sealant material.

Depending upon the nature of the getter, it may be activated at any time following displacement of the air and admission of the inert gas. When the getter is of a non- evaporable type, the getter preferably is activated at the same time as method step (e) is performed; that is by heating the glass sheets in a substantially uniform manner to a temperature sufficient to effect fusing of the sealant material and to activate the getter. When the getter is of an evaporable type, the getter preferably will be activated following evacuation of the chamber.

When the getter is in the form of an evaporable type, the getter may be heated by passing an electrical current through the getter by way of electrical connections. Alternatively, heating current may be induced to flow through the getter by an induced heating process. In this latter case, an induction coil may be located outside of the glass panel and be energised in a manner to couple radio frequency energy into the getter. The invention will be more fully understood from the following description of preferred, alternative embodiments of performing the invention. The description is provided with reference to the accompanying drawings .

Brief Description of the Drawings In the drawings:

Figure 1 shows a diagrammatic representation of a panel in the form of evacuated glazing incorporating a getter,

Figure 2 is a flow chart showing sequential steps in fabrication of the evacuated glazing when employing a non- evaporable getter,

Figure 3 is a flow chart showing sequential steps in fabrication of the evacuated glazing when employing an evaporable getter, Figures 4A to 4D show sectional elevation views of a portion of the evacuated glazing during fabrication by a first process,

Figure 5 shows a variation of the arrangement shown in

Figure 4, Figures 6A to 6D show sectional elevation views of a portion of the evacuated glazing during fabrication by a second process,

Figures 7A to 7D show sectional elevation views of a portion of the evacuated glazing during fabrication by a third process, and

Figures 8A to 8D show sectional elevation views of a portion of the evacuated glazing during fabrication by a fourth process.

Detailed Description of Embodiments of the Invention As illustrated in Figure 1 of the drawings, the evacuated glazing comprises two planar sheets 10 and 12 of glass that are maintained in spaced-apart relationship by an array of pillars 14. The glass sheets 10 and 12 typically have the following dimensions -

Surface area 0.02 - 4.00m² Sheet thickness 2.0 - 5.0mm

Sheet spacing 0.10 - 0.20mm.

The upper glass sheet 10 has a peripheral dimension that is marginally smaller than the lower glass sheet 12, and fused solder glass is employed to form an edge seal 16 around the periphery of the glass sheets. A chamber 18 is defined by the spaced-apart glass sheets 10 and 12 and the fused edge seal 16, and the chamber is evacuated to a pressure in the order of 10^"1 Pa or below. This provides for heat conduction between the glass sheets 10 and 12 that is negligible relative to other heat flow mechanisms.

A getter 20, 21 or 23 is located within a region 22 of the chamber 18 between the glass sheets and is activated for the purpose of sustaining the vacuum within the chamber. The getter may be of a non-evaporable type or an evaporable type, and Figures 2 and 3 present flow charts containing the sequential steps of fabricating the evacuated glazing using the respective types of getters.

The procedural steps indicated in Figures 2 and 3 may be performed with any of the arrangements that are shown in Figures 4 to 8.

In the arrangement shown in Figures 4A to 4D the two glass sheets 10 and 12 are initially placed in spaced-apart confronting relationship and the solder glass 16, in paste form, is deposited around the marginal edges of the sheets in a manner to contact the vertical edge of the upper glass sheet 10 and to rest on a horizontal projecting portion of the lower glass sheet 12. As indicated previously, the glass sheets 10 and 12 are maintained in spaced-apart relationship by the pillars 14 as shown in Figure 1, with the chamber 18 being located between the confronting glass sheets . A pellet-form non-evaporable getter 20 is located in the region 22 of the chamber 18. The region 22 is formed as a recess in the lower glass sheet 12.

A pump-out channel 24 is provided in the upper glass sheet 10, and a glass pump-out tube 26 is connected to the upper end of the channel 24 to extend beyond the upper surface of the upper glass sheet 10. A further deposit 28 of solder glass in paste form is located around the pump- out tube 26 on the upper surface of the glass sheet 10. An evacuating tool 30 is removably mounted to the upper surface of the upper glass sheet 10. The evacuating tool incorporates an annular recess 32 from which air is evacuated by a pump (not shown) which is connected to an evacuating line 34. With evacuation of the recess 32, seals are established between the upper surface of the glass sheet 10 and lands 36 of the evacuating tool.

Having established a seal between the evacuating tool 30 and the upper glass sheet 10, an inert gas such as Argon is directed into the chamber 18 by way of a gas supply line 38. The gas is caused to flow into a central recess 40 of the evacuating tool 30 and through the channel 24 to enter the chamber 18.

The solder glass 16, as indicated in Figure 4A, remains permeable prior to fusing, and admission of the inert gas to the chamber 18 causes displacement of air from the chamber 18 through the unfused solder glass. When all air has been displaced from the chamber 18, the getter 20 is surrounded by the inert gas.

Having displaced the air from the chamber 18, the complete assembly as shown in Figure 4A, including the contained inert gas, is exposed to heat within an oven chamber (not shown) and the complete structure is heated to a level sufficient to create the fused solder glass seals 16 and 28, as indicated in Figure 4B. The inflow of inert gas is maintained until such time as the solder glass deposits 16 and 28 are fused to form impermeable seals around the edges of the glass sheets and, also, around the pump-out tube 26. Thus, the getter 20 is maintained in an inert gas environment at all times during the heating process . The complete assembly is heated to a temperature in the order of 450°C to fuse the solder glass seals 16 and 28, and this temperature is sufficient to activate the non- evaporable getter 20.

Activation of the getter 20 occurs as a consequence of rupturing of a native oxide layer on the surface of the getter. This results in exposure of un-reacted reactive material to gas molecules surrounding the getter 20.

The native oxide layer on the surface of the getter is formed originally when the getter is exposed to air, even at room temperature. Therefore should the getter 20 be surrounded by air at the time of activation, the reactive material of the getter 20 would react rapidly with gas molecules of the air. However, as the gas molecules of the inert gas do not form compounds with the material of the getter 20, no reaction occurs for such time as the getter is enveloped by the inert gas, this ensuring that the getter will remain active after the native oxide layer has been ruptured and the inert gas is evacuated from the chamber 18. The complete assembly is then cooled progressively to cause solidification of the fused seals 16 and 28 whilst maintaining the inert gas within the chamber 18. When the fused seals have been solidified, which typically occurs at a temperature around 400°C, the supply of inert gas is terminated and the chamber 18 is evacuated by way of the glass pump-out tube 26 and what previously was the gas supply line 38, as indicated in Figure 4C. For this purpose a vacuum pump (not shown) is connected to the line 38. During the evacuating process the complete arrangement is maintained at an elevated temperature, typically in the order of 250°C or greater, to effect outgassing of the internal surfaces of the chamber 18. Upon completion of the evacuating process, the glass pump- out tube 26 is sealed by melting its open end, so as to form the structure as shown in Figure 4D. For this purpose a resistive heating coil 42 is located within the evacuating tool 30 and is connected electrically with an external power supply.

After the glass pump-out tube 26 has been closed, the central recess 40 and the annular recess 32 of the evacuating tool are vented, and the evacuating tool is removed from the evacuated glazing.

Figure 5 shows an alternative arrangement to that of Figure 4 (in particular Figure 4A) but in which a slender getter 21 is provided and is sandwiched between the two glass sheets 10 and 12. This arrangement avoids the need to provide the recess 22 which is shown in the embodiment illustrated in Figure 4.

Figures 6A to 6D show an arrangement that is similar to that in Figures 4A to 4D and like reference numerals are used to identify like parts. However, the arrangement of Figures 6A to 6D comprises an embodiment in which an evaporable (as distinct from a non-evaporable) getter 23 is employed ultimately for sustaining a vacuum within the chamber 18. The evaporable getter 23 is contained within a cylindrical ring-like housing 44 that is formed from a metal such as stainless steel. The housing 44 has an open lower end that faces the bottom of the recess 22 in the lower glass sheet 12. Also, the housing 44 is held in place within the recess 22 by slender supporting fingers 46 which provide for low thermal conduction between the housing and the glass sheet 12.

The arrangement as shown in Figures 6A to 6D is fabricated and processed in much the same manner as described previously in the context of Figures 4A to 4D. That is, air is displaced from within the chamber 18 by admission of an inert gas to the chamber 18, and the complete arrangement is heated to establish the fused seals 16 and 28. However, whilst it is necessary to use the inert gas to protect the getter 23 from degradation during formation of the fused seals, the level of heat required to effect the glass solder fusing is not sufficient to activate the evaporable getter. Activation of an evaporable getter occurs only with evaporation of unreacted bulk material in the getter. The need to protect the getter 23 prior to its activation is explained as follows.

As in the case of non-evaporable getters, the outer layers of an evaporable getter react immediately with gas molecules when the getter is first exposed to air, even at room temperature. Reaction at a level sufficient to degrade the getter would then occur if the getter were to be exposed to air during the seal-forming heating process. As in the case of a non-evaporable getter, the reacted outer layers of an evaporable getter rupture at temperatures around 450°C, thereby exposing the un-reacted bulk material to air. This would result in immediate reactions between the bulk material and the gas molecules in the air and consequential degradation of the evaporable getter . Following admission of the inert gas, formation of the fused seals 16 and 28 and complete evacuation of the chamber 18, the evaporable getter 23 is activated by heating the getter to a temperature in the order of 800°C. This may be achieved by passing electrical current through the housing 44 or by coupling radio frequency energy into the housing material from a coil (not shown) that is located outside of the lower glass sheet 12.

The supporting fingers 46 for the housing 44 serve during heating of the getter to minimise thermal conduction between the housing 44 and the glass sheet 12. Any significant thermal conduction between the housing 44 and the glass sheet 12 would make it almost impossible to heat the evaporable getter 23 to the required temperature for evaporation. Furthermore, minimisation of heat transfer between the housing 44 and the glass sheets 10 and 12 will ensure that the glass sheets will not be subjected to excessive stresses that might otherwise be induced by the localised heating.

As a consequence of the evaporation process, a film 48 of active getter material is deposited at the bottom of the recess 22, this providing for the maintenance of the required vacuum within the chamber 18 over long periods of time .

Figures 7A to 7D and 8A to 8D show arrangements that are similar to those in Figures 4A to 4D and 6A to 6D respectively. However, in the Figures 7 and 8 embodiments, the use of the evacuating tool 30 is obviated, and the admission and evacuation of gases are effected by way of a single glass tube 50. The glass tube 50 has a length sufficient to locate its upper end outside of a heating chamber (not shown) in which the evacuated glazing is located during the fabrication process . This is required to enable coupling of gas delivery/extracting fittings (not shown) to the glass tube 50 when the fittings incorporate O-rings that would be degraded by the heating processes . Other variations and modifications may be made in the invention as above described without departing from the scope of the invention as defined in the appendant claims.

Claims

THE CLAIMS

1. A method of fabricating an evacuated glass panel having a chamber defined by two spaced-apart glass sheets and a fused edge seal, the method comprising the steps of:

(a) positioning the glass sheets in spaced-apart relationship and locating a getter within a region of the chamber to be formed between the glass sheets,

(b) depositing a fusible sealant material around the edges of the glass sheets,

(c) displacing air from between the glass sheets,

(d) admitting an inert gas to the space between the glass sheets ,

(e) fusing the sealant material to create a fused edge seal around the glass sheets whilst maintaining the inert gas in the chamber that is formed with fusing of the sealant material,

(f) evacuating the chamber, and

(g) activating the getter.

2. The method as claimed in claim 1 wherein the inert gas is admitted to the space between the glass sheets in a manner to displace the air from between the glass sheets .

3. The method as claimed in claim 2 wherein the sealant material comprises glass solder and the air is displaced through the sealant material prior to fusing the sealant material .

4. The method as claimed in any one of claims 1 to 3 wherein the fusing of the edge seal is effected by heating the complete structure comprising the glass sheets, the getter and the contained inert gas within an oven chamber.

5. The method as claimed in claim 4 wherein the getter comprises a non-evaporable getter and wherein the getter is heat activated during heating of the complete structure.

6. The method as claimed in claim 4 wherein the getter comprises a non-evaporable getter and wherein the getter is heat activated following evacuation of the chamber.

7. The method as claimed in any one of the preceding claims wherein the region within which the getter is located comprises a recess which is formed in one of the glass sheets .

8. The method as claimed in any one of claims 1 to 6 wherein the getter is sandwiched between the glass sheets.

9. The method as claimed in claim 6 wherein the getter is contained within a metal housing that is positioned within a recess that is formed in one of the glass sheets and which opens into the chamber.

10. The method as claimed in claim 9 wherein the housing is positioned within the recess by fingers which function to minimise heat conduction between the housing and the glass sheet in which the recess is located.

11. The method as claimed in claim 9 or claim 10 wherein the getter is heat activated by passing heating current through the housing from an external current source.

12. The method as claimed in claim 9 or claim 10 wherein the getter is heat activated by an external induction coil that is removably positioned adjacent the getter.

13. An evacuated glass panel in the form of evacuated glazing when fabricated by the method as claimed in any one of the preceding claims.

14. A method of fabricating evacuated glazing substantially as hereinbefore described with reference to any one of Figures 4 to 8 of the accompanying drawings.

15. A glass panel when in the form of evacuated glazing and when fabricated by the process as hereinbefore described with reference to any one of Figures 4 to 8 of the accompanying drawings .