GB2253174A

GB2253174A - Seal

Info

Publication number: GB2253174A
Application number: GB9104223A
Authority: GB
Inventors: William Charles Maskell
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-02-28
Filing date: 1991-02-28
Publication date: 1992-09-02
Anticipated expiration: 2011-02-28
Also published as: GB9104223D0; GB2253174B

Abstract

A ceramic (or other solid) or metal seal is effected in a process in which the surface of a flat solid is prepared by rotating the solid about an axis perpendicular to its surface and to a flat grinding surface while pressing the solid and grinding surfaces together. This produces a pattern of concentric scratches 5 on the surface of the solid 1. A smooth metal, for example gold, is pressed against the prepared surface and held at an elevated temperature for an appropriate time. The pressure is then released and an hermetic seal has been formed between the solid and the metal which is stable over a range of temperatures. The process can also be applied with appropriate modifications to non flat surfaces where co-axial rather than concentric scratches are produced. A particular application of the process is the assembly of gas sensors incorporating one or more zirconia-gold seals. <IMAGE>

Description

SEAL This invention relates to the preparation of solid-metal seals It is well known that a metal-ceramic bond may be achieved by pressing an appropriate combination of metal and ceramic together at an elevated temperature for a suitable length of time. In order that the bond should form an hermetic seal the ceramic is normally ground flat and then polished to a smooth finish with suitable polishing compounds. This process is lengthy and consequently expensive. Furthermore, where surfaces are coated, e.g. by evaporation, sputtering, screen printing or a combination of these, either within the region of the seal or adjacent to it, the strength of the bond between the polished ceramic and the deposited material is low. This is because the polished surface provides little keying to the deposited material resulting in poor adhesion.

According to the present invention there is described a process for preparing a seal between a solid and a metal involving a non-polished solid surface, with consequent cost reduction while still resulting in an hermetic seal and providing improved adhesion of any deposited material. The solid may be a ceramic, a glass-ceramic, a metal or other material.

A normally ground solid surface does not enable an hermetic ceramic seal to be achieved with a metal because of the presence of scratches (grooves) on the solid surface. If one or more of these grooves traverses the seal then fluid may be physically transported across the seal by hydraulic pressure, diffusion, osmosis etc., and the seal is said to leak. In the grinding process described in this invention leakage is obviated by ensuring that grooves do not run from one side of the seal to the other.

The surface preparation required to achieve an hermetic seal according to this invention is as follows. The solid surface is first ground flat in the normal way. The purpose of this step is to remove gross undulations from the surface. This first step may be omitted provided gross undulations are not initially present or are removed at the second stage. The solid is then rotated, relative to a flat grinding surface, about an axis perpendicular to the surface being prepared and to the grinding surface. This is achieved by rotating the solid and holding the grinding surface fixed, by rotating the grinding surface and holding the solid fixed or by rotating both so that one rotates with respect to the other. The axis of rotation is chosen to be within the boundaries of the seal and ideally close to the centre of the seal when the seal is circular.

The resulting effect of this mode of grinding is to produce a rough surface on the solid with grooves following segments of concentric circles. Where the grinding medium is coarse this pattern is clearly visible on the surface to the naked eye.

Where the grinding medium is fine it may be necessary to use a microscope to observe the pattern. The presence of this pattern is evidence that the mode of grinding as described above has been employed. When grinding, points on the solid close to the axis of rotation move more slowly relative to the grinding medium than points further from the axis. Consequently the rate of grinding tends to increase with distance from the axis and, in some cases, particularly when employing a universal joint this may impart a slightly conical shape to the ground surface. However, this presents no problem as an hermetic seal is still readily achieved.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows the disposition of the solid rotating about an axis relative to a grinding surface.

Figure 2 shows the arrangement of Fig. 1 with the addition of a universal joint on the axis of rotation.

Figure 3 illustrates a typical pattern of grooves resulting from the grinding method in Figs. 1 and 2.

Figure 4 shows the assembly of a gas sensor with hermetic seals formed at the metal-ceramic interfaces.

Referring to Fig. 1 the solid 1 is mounted so as to rotate about an axis 2 perpendicular to the surface to be ground and to a flat grinding plate 3. Grinding may be accomplished in the normal ways, e.g. using carborundum or diamond materials. The solid is ground by rotating while applying pressure usually with the addition of a lubricant to prevent excessive heating at the grinding interface. In order to ensure that the solid surface is ground sufficiently it may be prepainted with a non-soluble ink prior to grinding. Removal of the ink during grinding indicates that the complete area to be sealed has been ground.

Ground surfaces other than flat, e.g. conical- or curved surfaces, are also encompassed within this invention. In such cases the surface of the metal used to form the seal with the solid must match the gross shape of the ground surface.

A seal is effected by placing the metal and solid together and holding under pressure at elevated temperature for a period sufficient to produce a strong bond. It is important to ensure that the pressure is uniformly applied in order to obtain an hermetic seal. Upon releasing the pressure the solid and the metal remain firmly bonded together and the bond provides an hermetic seal which is stable over a range of operating temperatures. In order to join the two pieces of solid together the two mating surfaces are both ground as described above. The join is effected by interposing a thin metal sealing washer between the two solid surfaces prior to applying pressure at elevated temperature as previously described. The smooth surfaces of the metal washer should have approximately the same gross shape as those of the two ground surfaces.This same procedure may also be used to join together dissimilar solids such as a metal to a ceramic or similar solids such as a metal to a metal.

In the grinding process illustrated in Fig. 1 alignment of the solid, the grinding plate and the axis of rotation requires considerable care. In a modification of the process a universal joint (UJ) 4 is incorporated as shown in the example in Fig. 2.

Alternatively the UJ may be incorporated into the support for the grinding surface. The UJ automatically aligns the solid and grinding surfaces.

In a specific example of the process using the UJ a zirconia ceramic disc was ground using 1000 grade carborundum paper supported on a flat metal surface. The ceramic was rotated at slow speed and the grinding interface lubricated with water. A satisfactorily ground surface was achieved when a waterproof ink applied to the ceramic prior to grinding was totally removed. A diagram of the pattern visible on the ceramic surface is shown in Fig. 3. Grooves 5 on the surface lie on concentric circles.

The above process may be used to fabricate the seal for a gas sensor. The fully-sealed device allows the seal to be leaktested as described below. Two zirconia ceramic discs are each ground on one side as shown in Fig. 2. Referring to Fig. 4 a circular hole 6 smaller than the diameter of the discs 1 is punched in a gold foil 7. Electrodes 8 of the same diameter, or a little larger than the hole in the gold are applied to the ground side of one or both discs and fired. The components are assembled with the ground faces against the gold : pressure is applied centrally via additional components 9 and 10. The assembly is then held at 10000C while maintaining the pressure for several hours. The sealed device is then returned to ambient temperature, the pressure is released and electrodes centrally applied to one or both outer ceramic faces complementing the inner electrode(s). The device is then fired to stabilise the outer electrode(s).

Tests, involving electrochemical pumping of oxygen into or out of the central volume and subsequent observation of. the emf generated between an inner and outer electrode, showed that there was no physical leakage between the enclosed volume and the outer atmosphere. This demonstrated that the seal produced by the process described in this invention was an hermetic one.

Modifications may be made to the above devices to produce alternative designs of sensors while using the same sealing technology. An example is the amperometric sensor in which a pore or pores connect the inner volume with the outer atmosphere and the device is operated in the limiting current mode.

Claims

1. A process for preparing an hermetic seal between a solid and a metal wherein the surface of the solid is ground by rotating the solid relative to a grinding surface resulting in a solid surface with concentric or co-axial grinding marks wherein the prepared surface is placed in contact with a metal having a smooth surface and approximately the same macro-geometry as the surface of the solid over the area to be sealed and the seal is effected by the application of pressure between the solid and the metal at elevated temp erature for an appropriate length of time.

2. A process as claimed in Claim 1 wherein the grinding surface is flat and the axis of rotation of the solid is perpend icular to the surfaces of the solid and of the grinding surface resulting in concentric grinding marks on the solid and the smooth metal surface is flat.

3. A process as claimed in Claim 2 wherein a universal joint is incorporated into the rotation axis either on the solid side or on the grinding surface side to aid alignment of the surfaces.

4. A process as claimed in Claim 1 wherein the grinding surface is conical and the axis of rotation is along the axis of the cone and the smooth metal surface has the same conical macro-geometry as the grinding surface.

5. A process as claimed in Claim 1 wherein the grinding surface is curved and symmetrical about an axis which is the same as the axis of rotation and the smooth surface of the metal has the same macro-geometry as the grinding surface.

6. A process for joining two solids together to form an hermetic seal wherein each solid is prepared as claimed in Claim 1 and a metal foil having smooth surfaces with the same macro-geometry as the two solid surfaces is interposed between the prepared surfaces and the seal is effected by the application of pressure between the two solids at an elevated temperature and subsequent removal of the pressure for operation of the seal over a range of temperatures.

7. A process as claimed in Claim 6 wherein each solid is prepared as claimed in Claim 2 and the metal foil is flat.

8. A process as claimed in Claim 6 wherein each solid is prepared as claimed in Claim 4 with two complementary grinding surfaces and the smooth metal foil has the same conical shape as the grinding surfaces.

9. A process as claimed in Claim 6 wherein each solid is prepared as claimed in Claim 5 with two complementary grinding surfaces and the smooth metal foil also has the same macro-geometry as the grinding surfaces.

10. A process for fabricating a gas sensor employing the sealing process as claimed in Claims 1, 2, 3, 4, 5, 6, 7, 8 or 9.

11. A process for fabricating a sensor composed of two or more ceramic discs wherein each pair of discs is joined together with a flat metal foil into which a hole is precut so as to enclose an hermetically sealed volume between two discs employing the sealing process as claimed in Claims 2 or 3.

12. A process as claimed in Claim 11 wherein electrodes are added to internal and external faces of one or both ceramic discs.

13. A process as claimed in Claims 11 or 12 in which one or more holes are laserdrilled or otherwise cut through one or more of the ceramic discs either prior to or after grinding.

14. A process for fabricating any article requiring the joining of one solid to another by the method as claimed in Claims 6, 7, 8 or 9.

15. A process for making a seal substantially as described herein with reference to Fig. 1-4 of the accompanying drawings.