GB2248526A

GB2248526A - Seal structure and method of sealing inner and outer members to each other

Info

Publication number: GB2248526A
Application number: GB9021782A
Authority: GB
Inventors: Ian Barry Blackburn
Original assignee: Arcotronics Ltd
Current assignee: Arcotronics Ltd
Priority date: 1990-10-06
Filing date: 1990-10-06
Publication date: 1992-04-08
Anticipated expiration: 2010-10-06
Also published as: GB9021782D0; GB2248526B; FR2689302B1; FR2689302A1

Abstract

An electrolytic capacitor comprises a housing (10) a tube (18) extending through an aperture in one end of the housing and containing an electric conductor (11) and a body of glass (19) for sealing the gap between the tube and the end wall (13). The tube carries an annular plate (22) which lies at the level of the surface of the body of glass and prevents the glass forming a small radius meniscus on the tube. <IMAGE>

Description

Title: Seal structure and method of sealing inner and outer members to each other Description of Invention The present invention relates to a structure comprising an outer member defining an opening, an inner member which extends through the opening and a body of glass or other sealant interposed between and fused to the inner and outer members to seal the members to each other, the body of glass or other sealant having a concave meniscus extending between the inner and outer members. Such a structure is referred to hereinafter as being a structure of the kind described.

One example of the use of a structure of the kind described is in electrical devices where the outer member is a housing of a device, for example a capacitor, and the inner member is or contains an electrical conductor which extends through the opening of the housing from a position outside the housing to an internal part of the device. The body of glass or other sealant is required to seal hermetically the housing and to insulate the conductor electrically from the housing. In devices of this kind, both the housing and the conductor are typically formed of metal.

Fusing a body of glass to the housing and the conductor is an effective way of hermetically sealing the housing. The molten glass wets the housing and the conductor and the glass adheres to these when the glass solidifies. The composition of the glass can be selected to provide a thermal co-efficient of expansion for the glass which is substantially the same as the thermal co-efficient of expansion of the metal of which the housing and conductor are formed.

Fusing of the glass to seal the conductor to the housing has the disadvantage that a concave meniscus is formed at the free surface of the glass between the conductor and the housing. In the region adjacent to the free surface of the glass where the conductor and body of glass meet, the body of glass is relatively thin and fragile. This constitutes a mechanical weakness in the device and the glass seal can fail if there is exerted on the device a force tending to move the conductor towards one side of the opening of the housing.

According to a first aspect of the present invention, in a structure of the kind described a peripheral surface of the inner member, which surface faces across the opening towards the outer member and to which peripheral surface the glass body extends, is so positioned that the meniscus of the glass body adjacent to the peripheral surface differs from the meniscus which would be formed adjacent to the peripheral surface if the peripheral surface was of infinite length.

It will be understood that, in known devices incorporating a structure of the kind described, the inner member protrudes beyond the body of glass so that the meniscus adjacent to the inner member is the same as the meniscus which would be formed adjacent to an inner member of infinite length, although the inner member is necessarily of finite length.

In a structure embodying the present invention, the peripheral surface of the inner member also is necessarily of finite length. An extremity of the peripheral surface which is remote from an interior of the outer member preferably lies sufficiently near to the adjacent free surface of the glass body to avoid the formation adjacent to the peripheral surface of a meniscus on the glass which would be formed if the peripheral surface was of infinite length.

The peripheral surface provided on the inner member may be a surface of a component formed separately from the inner member and applied to the inner member. Alternatively, the peripheral surface may be a surface of an integral part of the inner member.

According to a second aspect of the invention, there is provided a method of sealing to an outer member defining an opening an inner member which extends through the opening with a substantial clearance, wherein there is fitted onto the inner member a plate having an aperture in which the inner member is received, a body of glass, the inner member and the plate are inserted into the opening and the glass is fused to the outer member, the plate and the inner member.

An example of a structure embodying the present invention and which is formed by a method according to the second aspect of the invention will now be described, with reference to the accompanying drawings, wherein: FIGURE 1 is a partial cross section of an electrical device embodying the invention, FIGURE 2 shows on an enlarged scale certain parts of the device of Figure 1, FIGURE 3 is an illustration similar to Figure 2 of a known arrangement and FIGURE 4 illustrates a step in construction of the device.

The device illustrated in Figure 1 is an electrical device comprising a hollow housing 10 and internal components which are not shown in Figure 1.

The device may, for example, be a capacitor. A pair of electrically conductive leads 11, 12 is provided for conducting electric current to and from the device.

In the particular example illustrated, these leads are co-axial and extend from the device in opposite directions. Also in the device illustrated, the housing 10 constitutes one electrode of the device and the lead 12 is attached to the housing at one end thereof. The lead 12 may be secured to an external face of the housing. Alternatively, as illustrated in Figure 1, the lead may penetrate through the wall of the housing. The lead 11 extends from outside the housing to an internal component of the device and is electrically insulated from the housing 10.

In the particular example illustrated, the housing is cylindrical and the lead 12 is attached to an end wall of the housing which is integral with a side wall thereof. An opposite end wall 13 of the housing is formed separately from the side wall and is attached to the side wall after internal components have been inserted into the housing.

The end wall 13 of the housing includes a substantially cylindrical portion 14 at the periphery of the end wall which bears on an internal surface of the side wall of the housing. An intermediate portion 15 of the end wall 13 extends from the portion 14 a part of the way towards an axis of the housing. In the example illustrated, this intermediate portion 15 is substantially flat and has a surface facing away from the interior of the housing which lies in a plane V V indicated in Figure 2. A central portion 16 of the end wall 13 defines a well 29 having an open mouth which faces away from the interior of the housing. At the bottom of the well, there is an aperture 17 through which the lead 11 extends.

That portion of the lead 11 which extends through the end wall 13 is enclosed in a rectilinear tube 18. One end of the tube 18 lies at the inside of the housing and may be flush with an inwardly facing surface of the end wall 13. An opposite end of the tube 18 lies outside the housing and is spaced substantially from the end wall 13. The tube 17 is spaced by an annular gap from the end wall 13. A body 19 of glass which occupies the well 29 defined by the end wall 13 is fused both to that end wall and to the tube 18 to seal the housing hermetically.

The housing 10, the lead 11 and the tube 18 are formed of metal, preferably the same metal. In the example illustrated, these components of the device are all formed of tantalum. The lead 12 may be formed of a different metal, preferably one which can be soldered, and is welded to the adjacent end wall of the housing. The peripheral portion 14 of the end wall 14 is welded to the side wall of the housing and the tube 18 is welded to the lead 11 at that end of the tube which is remote from the interior of the housing. An extension of metal which can be soldered may be welded to the lead 11 and the tube 12.

Capacitors arranged generally as described thus far with reference to Figure 1 are known. In manufacture of the known devices, the body of glass 19 is fused to the tube 18 and the end wall 13 prior to assembly of that end wall with the side wall of the housing and to insertion of the lead 11 into the tube 18.

When the glass is fused, there is formed at the free surface of the glass between the tube 18 and the end wall 13 a meniscus as illustrated in Figure 3. It will be seen that the meniscus is concave. Adjacent to the tube 18, the meniscus has a height represented by the dimension Y in Figure 3. By the height of the meniscus adjacent to the tube, we mean the distance measured along the tube between the extremity of the body of glass at the surface of the tube and a plane indicated by the roman numeral XX in Figure 3 which is the plane nearest to the central portion 16 of the end wall which touches the free surface of the glass.It will be noted that the height of the meniscus adjacent to the intermediate portion 15 of the end wall of the housing is somewhat less than the dimension Y, because the surface 21 of the housing which lies at the periphery of the well and faces towards the tube 18 terminates in the vicinity of the free surface of the glass, where the central portion 16 merges with the substantially flat intermediate portion 15 of the end wall. In contrast with this, the tube 18 continues without interruption from the interior of the housing to a position well beyond the free surface of the body of glass. Adjacent to the tube 18, the meniscus is the same as it would be if the tube 18 was of infinite length.

As shown in Figures 1 and 2, the device embodying the invention differs from the known device represented in Figure 3 by the provision on the tube 18 of a further component which, in the example illustrated, has the form of a substantially flat plate 22. The outside diameter of the plate 22 is substantially less than the diameter of the well 29 defined by the end wall 13.

The plate is annular and is a close fit on the outside of the tube 18. The plate is positioned on the tube to lie at the mouth of the well 29. The outside diameter of the plate 22 is substantially less than the diameter of the well so that there is between the plate and the end wall 13 a radial gap which is sufficiently wide to prevent significant leakage of electric current across the gap when the device is in use. The peripheral surface 23 of the plate 22 is preferably cylindrical and co-axial with the housing 10. This surface faces across the well towards the intermediate portion 15 of the end wall 13 and towards the side wall of the housing 10.

That dimension of the peripheral surface 23 which extends in a direction along the axis of the housing 10 and corresponds to the thickness of the plate 22 is small, as compared with the corresponding dimension of the body of glass 19. In the example illustrated, the dimension of the plate is within the range 0.1 to imam. As shown in Figure 2, the plate is so positioned that the body of glass extends to the peripheral surface 23. However, the surface 23 terminates at the flat surface 24 of the plate 22 which faces away from the interior of the housing 10 and the position of this flat surface relative to the free surface of the glass is such that the depth of the meniscus adjacent to the surface 23, represented by the dimension Z in Figure 2, is considerably less than the depth of the meniscus adjacent to the tube 18 in the arrangement of Figure 3.

Consequently, the shape of the meniscus on the glass immediately adjacent to the peripheral surface 23 differs from the shape of the meniscus shown in Figure 3 adjacent to the tube 18. That part of the body of glass which embraces the surface 23 is thicker (as measured in a direction perpendicular to the axis of the housing 10) and more robust.

The interface between the plate 22 and the body of glass 19 at the surface 23 is spaced much further from the axis of the housing 10 than is the interface between the tube 18 and the body of glass shown in Figure 3. This also contributes to robustness of the device because any laterally directed force exerted on the tube 18 and tending to displace the tube from the axis of the housing 10 is distributed over an interface with a larger radius of curvature.

In Figure 2, the body of glass 19 is represented as covering the peripheral surface 23 substantially entirely. Alternatively, the volume of glass present may be such that the peripheral surface 23 is only partly covered by the glass.

The depth of the meniscus adjacent to the peripheral surface 23 of the plate is preferably no greater than 0.25 mm. The flat surface 24 of the plate 22 which terminates the peripheral surface at the margin thereof furthest from the bottom of the well may, as shown in Figure 2, lie in a plane which is slightly nearer to the bottom of the well than is the plane V-V defined by the intermediate portion 15 of the end wall 13 at the outside of the housing.

Alternatively, the surface 24 may lie slightly further from the bottom of the well than does the plane V-V defined by the intermediate portion 15. However, the surface 24 preferably lies no more than 0.2 mm beyond the plane V-V.

Furthermore, the distance by which the spacing of the surface 24 from the bottom of the well exceeds the spacing from the bottom of the well of the plane V-V is preferably no greater than the radial clearance between the peripheral surface 23 and the side wall of the well 29.

It will be understood that the tube 18 extends beyond the plane V V a considerable distance, certainly more than the radial spacing of the surface 23 from the side wall of the well.

Figure 4 illustrates assembly of the tube 18 and plate 22 with the end wall 13. The end wall is placed in a recess in a block 25 with the open mouth of the well 29 facing upwards. The recess in the block is dimensioned to receive the end wall with a close sliding fit but does not grip the end wall. The end wall can be removed from the block without difficulty. The block also supports a guide rod 27 which extends along the axis of the end wall 13, when located by the block. A pre-formed, annular body of glass 28 is placed in the well 29 and the tube 18, already bearing the plate 22, is lowered along the guide rod 27 so that a lower end portion of the tube enters the hole in the glass annulus and the plate 22 comes to rest on the glass annulus. The assembly is then placed in a furnace and is heated sufficiently to melt the glass.The glass fuses to the end wall 13, the tube 18 and the plate 22. In order to ensure that the components occupy the required positions and voids do not remain in the well, a weight (not shown) may be placed on the upwardly protruding part of the tube 18 to bear on the plate 22 and exert downward pressure on the glass.

After the glass has been fused, the assembly is allowed to cool and is subsequently assembled with the side wall of the housing 10 by placing the tube 18 over the lead 11 protruding from the housing and fitting the end wall 13 inside the side wall of the housing. The end wall is then welded to the housing and the tube is welded to the lead 11. During welding of the tube to the lead 11 at the end of the tube remote from the interior of the housing, heat is inevitably conducted along the tube towards the body 19 of glass. The plate 22 serves as a heat sink and reduces the thermal shock to which the body of glass would be subjected in the absence of the plate. This further reduces the risk of failure of the glass seal.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS:

1. A structure comprising an outer member defining an opening, an inner member which extends through the opening and a body of glass or other sealant interposed between and fused to the inner and outer members to seal the members to each other, the body of glass having a concave meniscus extending between the inner and outer members and a peripheral surface of the inner member which faces across the opening towards the outer member, and to which peripheral surface the glass body extends, being so positioned that the meniscus of the glass body adjacent to the peripheral surface differs from the meniscus which would be formed adjacent to the peripheral surface if the peripheral surface was of infinite length.

2. A structure according to Claim 1 wherein said peripheral surface has an extremity which is remote from an interior of the outer member and which lies sufficiently near to the adjacent free surface of the glass body to avoid the formation adjacent to the peripheral surface of a meniscus on the glass which would be formed if the peripheral surface was of infinite length.

3. A structure according to Claim 2 wherein said extremity of the peripheral surface lies substantially at a mouth of the opening.

4. A structure according to Claim 1 wherein said peripheral surface of the inner member has an extent in a direction through the opening which is within the range 0.1 to Imm.

5. A structure according to Claim 1 wherein the depth of the meniscus adjacent to said peripheral surface of the inner member does not exceed 0.2mm.

6. A structure according to any preceding claim wherein the inner member comprises a central, elongated portion which extends through the opening and a lateral projection from the elongated portion, the projection being approximately at the level of a free surface of the body of glass.

7 An electrolytic capacitor comprising a structure as defined in any preceding claim.

8. A method of sealing to an outer member defining an opening an inner member which extends through the opening with a substantial clearance, wherein there is fitted onto the inner member a plate having an aperture in which the inner member is received, a body of glass, the inner member and the plate are inserted into the opening and the glass is fused to the outer member, the plate and the inner member.

9. A structure substantially as hereindescribed with reference to and as shown in Figures 1 and 2 of the accompanying drawings.

10. Any novel feature or novel combination of features disclosed herein or in the accompanying drawings.