GB1562092A

GB1562092A - Electrically insulated leadthrough assembly

Info

Publication number: GB1562092A
Application number: GB4180676A
Authority: GB
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 1976-01-29
Filing date: 1976-10-07
Publication date: 1980-03-05
Also published as: IT1064974B; JPS5293993A; FR2339941A1; JPS5610855B2; DE2603360C2; DE2603360B1; NL7609811A; SE441553B; SE7609244L; CH612545A5; FR2339941B1

Abstract

In the case of an electrically insulated bushing having a bushing part (1) which passes through an opening (3) in a wall part (2), the wall part (2) and the bushing part (1) consist of materials having essentially the same coefficients of thermal expansion, and the insulation which is fitted between the mutually facing bushing surfaces of the two parts (1, 2) is a thin-film layer (5) which adheres firmly to the bushing surface of at least one of the two parts (1, 2) and consists of electrically insulating material. The thin-film layer (5) may consist, for example, of glass, silicon oxide or aluminium oxide. If at least one of the two parts (1, 2) consists of aluminium, the thin-film layer is preferably an aluminium-oxide layer formed by surface oxidation of the aluminium. As a result of its small thickness and its firm adhesion, the thin-film layer can follow the temperature changes without any risk of splitting or peeling, as a result of which a material which is suitable for the required insulation can be selected without any need to take into account its coefficient of thermal expansion. <IMAGE>

Description

(54) IMPROVED ELECTRICALLY INSULATED LEAD-THROUGH ASSEMBLY (71) WE, ENDRESS & HAUSER GmbH & Co., a German Coporate body of Haupstrasse I, 7867 Maulburg, Federal German Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electrically insulated lead-through assembly by which a conductor member is taken in an insulated manner through an aperture in a wall member, with electrical insulation disposed between the mutually opposite surfaces of the two members.

Lead-through assemblies of this kind are required particularly for the purpose of leading electrical conductors for the supply of current or the transmission of signals through conductive walls of enclosed spaces. The conditions obtaining within the space may be very different from the normal conditions outside, and may even be extreme, such as very high, very low, or widely varying temperatures, high pressure or high vacuum, chemically aggressive environments, and so on.The lead-through assembly should therefore, while providing good insulation of the conductor from the wall, be resistant to widely differing temperatures, gas tight, pressure tight and resistant to chemical attack and should absorb and store as little moisture as possible; it should also resist temperature alternations, that is, the recited characteristics should be maintained even when the device is subjected to temperature alternations.

Known lead-through assemblies often fulfil only a part of the stated conditions at one time, since the selection of useful materials is very limited. The provision of adequate resistance to temperature alternations provides the greatest problem. In the most widely employed kind of lead-through assembly, the lead-through conductor is sealed by glass into the aperture in the wall. This method of manufacture assumes, however, that the lead-through conductor and the wall consist of a material that can withstand the softening temperature of glass and additionally has about the same thermal co-efficient of expansion as glass, since otherwise on cooling or chilling of the glass mechanical forces appear that lead to cracks and fissures in the insulation.For the available kinds of glass, these requirements can in general be met only by particular selections of metals and alloys, e.g. platinum and iron-nickel-cobalt alloys. It is true that the condition of resistance to temperature alternations is then also fulfilled; the use of these metals is, however, undesirable or even impossible for many applications, particularly where increased demands are laid on the electrical conductivity of the lead-through conductor or when these metals are excluded for reasons of cost.

Both requirements may be fulfilled by aluminium, for example, though aluminium cannot be sealed with glass because of its high thermal co-efficient of expansion and its low melting point.

The use of a plastics insulation is impossible in many applications; such lead-through assemblies are unsuitable for high-vacuum equipment, for example, because of their high moisture absorption and high leakage rate.

The object of the invention is the provision of a lead-through assembly that is simply to manufacture and which yields a sufficient resistance to temperature alternations even when used in combination with aluminium and similar metals.

According to the present invention there is provided an electrically insulated leadthrough assembly comprising an electrically conductive through-lead member extending through an aperture in an electrically conductive wall member, with insulation between the mutually opposite surfaces of the two members, wherein said members consist of the same material or of materials with substantially equal co-efficients of thermal expansion and wherein the insulation is a thin layer of electrically insulating material having a thickness not exceeding 100 ,um applied to at least one of the two members before their assembly.

In electrical lead-through assemblies in accordance with the invention the tolderance to temperature alternations is dependent only upon the fact that the lead-through member and the wall member consist of materials having substantially the same coefficient of thermal expansion; this condition is naturally intrinsically fulfilled if these portions consist of the same material.On the other hand, the coefficient of thermal expansion of the material forming the insulating layer is substantially without effect upon the tolerance to temperature alternations, except when this co-efficient of thermal expansion is very different from those of the lead-through portion and of the wall portion; as a result of its small thickness (which preferably amounts to between 10 and 100 ,um) and its secure fastening, the insulating layer can follow the expansions, and contractions, resulting from temperature changes without danger of splitting or breaking.

A material can therefore be chosen for the insulating layer, without reference to its coefficient of thermal expansion, that desirably fulfills the conditions for the insulation of the lead-through conductor. This material should preferably be a good insulator, have a low moisture absorption, i.e. not be hygroscopic, and good mechanical characteristics, especially good adhesion to the substrate and good elasticity.

These requirements are met by a few materials that are employed in thin-film technology, such as particular kinds of glass, silicon dioxide and aluminium oxide. The insulating layer may be made of these materials by, for example, sputtering in vacuum, though other processes for applying securely adherent layers are known and may be used.

It is possible in this manner to manufacture electrical lead-through assemblies with good temperature resistance, that are pressuretight up to 200 Bar or more and have a helium leakage rate of 10-8 millibar.litre per second or less and retain these characteristics even with temperatures alternating over a wide range.

Particular advantages as regards the construction and characteristics of the leadthrough assembly are produced if the oxide of the metal of one of the two metal portions of the lead-through assembly is used as the material for the insulating layer; in this case a particularly adherent insulating layer may be formed in a simple manner by surface oxidation of the metal.

This embodiment is particularly suitable for those cases in which the lead-through conductor or the wall member, or both, consists of aluminium, since aluminium oxide is an eminently suitable material for this purpose.

In a preferred embodiment of the invention at least one of the two metal portions of the lead-through assembly consists of aluminium and the insulating layer is a layer formed by surface oxidation of the aluminium.

The aluminium oxide is preferably formed by anodic oxidation.

Anodic oxidation yields a thin aluminium oxide layer that is sufficiently elastic and adheres so well to the aluminium that it can follow the expansion and contraction resulting from temperature changes without danger of splitting or laminating despite the different co-efficients of thermal expansion of aluminium and aluminium oxide.

As a result of its known characteristics of high temperature stability and resistance to chemical attack, aluminium oxide fulfils in a remarkable manner the requirements demanded of a lead-through assembly. Care must, however, be taken that the aluminium oxide layer formed by anodic oxidation is dense and non-porous, so that it does not absorb and retain moisture. The coatings of aluminium oxide normally obtained by anodic oxidation are in fact generally porous, which is undesirable for many applications; in the present state of the art it is, however, known and possible to produce by anodic oxidation layers of aluminium oxide that are wholly dense and non-porous.

Accordingly, as the lead-through conductor, the wall portion, or both, consist of aluminium, the aluminium oxide layer may be formed on the outer surface of the leadthrough portion, on the internal surface of the wall portion or also on both surfaces.

This embodiment of the invention can be used not only when at least one of the two portions of the lead-through consists of aluminium, but is suitable also for cases in which neither the lead-through member nor the wall member consists of aluminium. In accordance with a preferred embodiment of the invention this is attained by at least one of the two members having a layer of aluminium applied to it in the lead-through region, and the insulating layer being formed by oxidation of the surface of the aluminium layer. The aluminium layer may be applied for example by metal sputtering, evaporation or electrolytically.

Even with these embodiments of the electrically insulated lead-through assembly the previously stated advantageous characteristics are obtained. To produce good stability under alternating temperature changes it is only necessary that the lead-through member and the wall member shall consist of the same material, or of materials with substantially the same co-efficient of thermal expansion; the tolerance of the lead-through assembly to temperature alternations is not deleteriously affected if this co-efficient of thermal expansion is different from the coefficients of thermal expansion of aluminium and aluminium oxide, since because of their small thicknesses the layers of aluminium and of aluminium oxide can follow the expansions and contractions due to temperature changes without danger of damage.

Particularly good pressure-tightness and gas-tightness of the lead-through assembly in accordance with the invention may be achieved by the lead-through member being made a force-fit in the aperture in the wall member. Preferably the force fit is produced by a shrinking process of heating and subsequent cooling.

In this kind of joint, any microscopic unevennesses that may initially be present on the contiguous lead-through surfaces are pressed smooth, so that no gap remains. It is therefore advantageous if the metal of one at least of the two members of the lead-through assembly is relatively soft and easily deformable, as is the case for aluminium.

If it is wished to be completely certain that, even without perfect preparation of the surfaces, a completely gap-free lead-through assembly will be obtained, an adhesive, for example an epoxy adhesive, can be introduced between the surfaces of the two members of the lead-through assembly, that come into contact. In general, however, this is not necessary.

Exemplary embodiments of the invention are shown in the drawing, in which: Figure 1 is a longitudinal section through an electrically insulated lead-through assembly in accordance with the invention; Figure 2 shows the portions of the embodiment of Figure 1 before assembly; Figure 3 shows a modification of the embodiment of Figures 1 and 2; Figure 4 shows another modification of the embodiment of Figures 1 and 2; Figure 5 shows a longitudinal section through another embodiment of electrically insulated lead-through assembly; Figure 6 shows a longitudinal section through a further embodiment of leadthrough assembly according to the invention.

The embodiment of insulated leadthrough assembly shown in Figure 1 includes a lead-through conductor of rod form that is lead in an insulated manner through a conductive plate or wall member 2. In the wall member 2 there is provided a conically taper ing aperture 3 and the conductor I is pro vided with a slightly tapering conical portion 4 exactly fitting in this aperture 3. Between the opposed peripheral surfaces of the aperture 3 and the tapering portion 4 there is formed an insulating layer 5 that insulates the conductor 1 from the wall 2.

Figure 2 shows the componants of the insulated lead-through assembly of Figure 1 before the insertion of the conductor 1 into the aperture 3. The conductor 1 and the wall 2 consist either of the same material or of materials with substantially equal coefficients of thermal expansion. The insulating layer 5 is formed on the conductor 1 so that the whole circumference of the conical portion 4 and the neighbouring peripheral regions of the adjacent cylindrical portions of the conductor 1 are covered. As seen in Figure 1, there thus result after the assembly of the members, annular outwardly extending portions of the insulating layer, that prevent short-circuits or leakage currents over the margin of the lead-through assembly.

Any material having good insulating properties that can be applied in the form of a thin layer adherent to the material of the lead 1 and has good elasticity in this form, may be employed as the insulating layer 5. The material should preferably also be resistant to chemical attack and have low moisture absorption, that is, it should not be hygroscopic and not be porous. On the other hand the choice of the material may be made without reference to the co-efficient of thermal expansion.

These requirements are fulfilled, for example, by different materials that are employed in thin-film technology, such as certain kinds of glass, silicon dioxide and aluminium oxide.

The thickness of the insulating layer 5 depends upon the nature of the insulating material used and the required voltage resistance of the insulation. To produce resistance to alternating temperatures it is also better, the thinner is the insulating layer. Its thickness will be less than 100,um and may advantageously lie in the range of 10 to 100 meal, though even thinner insulating layers may sometimes be employed. In the drawing the thickness of the insulating layer 5 is greatly exaggerated for the sake of clarity.

The insulating layer 5 may be formed by applying the material to the conductor 1 by any known process whatever that ensures good adhesion of the layer to the substrate.

This is particularly the case for sputtering in vacuum.

Another possibility for forming the insulating layer 5 consists in its being produced by chemical conversion, particularly by oxidation of the material of the conductor 1 at its surface. This embodiment is particularly advantageous when the conductor 1 consists of aluminium. In this case the insulating layer 5 is preferably formed by anodic oxidation of the aluminium surface.

As a result of the anodic oxidation there is produced on the aluminium surface a layer of aluminium oxide (A1203) that is a very hard material of high temperature stability, a good electrical insulator and very resistant to chemical attack. The aluminium oxide produced by anodic oxidation adheres very well to the aluminium and is sufficiently elastic to prevent lamination.

It is, however, important that care be taken in the anodic oxidation that the aluminium oxide layer produced is thick and non-porous. The aluminium oxide layers normally produced by anodic oxidation consist of a dense blocking layer and a porous covering layer, that is required for colouring, for example. For use as through-lead insulation, on the contrary, the porous covering layer would be unsuitable, since it would lead to the absorption and retention of moisture.

It is known and possible in the existing state of the technology to produce by anodic oxidation aluminium oxide layers that consist only of a dense and non-porous blocking layer and are well suited to the purpose contemplated.

After the application to it of the insulating layer 5, the conductor 1 is fastened in the aperture 3 in the wall 2 so that no gap exists between the insulating layer and the circumferential surfaces of the aperture. Preferably the conductor 1 is force fitted in the aperture 3. With the conical form shown in Figures 1 and 2 this may be effected by pressing the conductor 1 in its longitudinal direction into the aperture 3, or driving it in with light hammer blows. Preferably the force fit is effected by a heat-shrinking process. For this purpose the wall 2 is heated so that it expands and the aperture 3 becomes wider; the conductor 1 is then inserted in the aperture 3. and on cooling of the wall 2 this shrinks on to the conductor. Excess material then flows along the surfaces that are in contact, and microscopic unevennesses of the surfaces are pressed flat, so that no gap remains.In order to ensure complete sealing with still greater certainty an adhesive - for example an epoxy resin - may further be applied to the surfaces coming into contact; however. this is in general unnecessary. In any case, however the surface of the aperture 3 should be worked as smooth as possible.

If in the embodiment of Figures 1 and 2 the conductor 1 consists of aluminium, the wall 2 may consist of any material that has about the same coefficient of thermal expansion as aluminium and naturally may be likewise of aluminium.

An insulated through-lead assembly constructed in this manner in which the insulating layer 5 consists of aluminium oxide, that is formed by anodic oxidation, gives particularly good electrical insulation of the conductor 1 with respect to the wall 2. The throughlead assembly is gas tight with a helium leak age rate of less than 10 - 8 millibar.litre/sec- ond. It is pressure tight for pressures of at least 200 Bar and therefore is suitable for use in pressurized apparatus. The insulation is stable at high temperatures and extremely resistant to chemical attack. Since also the insulation absorbs very little moisture, the through-lead is suitable even for highvacuum apparatus.Of particular advantage is the fact that, despite the high co-efficient of thermal expansion of aluminium, the leadthrough assembly retains all these characteristics under alternating temperatures, if conductor and wall have about the same co-efficient of thermal expansion, since owing to the small thickness and good adhesion of the aluminium oxide layer the different co-efficients of thermal expansion have no effect.

In Figure 3 there is represented another embodiment in which an insulating layer 6 is applied instead of to the conductor 1 to the internal surface of the conical aperture 3, in the manner before described. If the wall 2 consists of aluminium, the insulating layer 6 is preferably formed by anodic oxidation. In this case the conductor 1 may consist of any material that has about the same co-efficient of thermal expansion as aluminium, especially naturally of aluminium also. In this case the insulating layer 6 is advantageously extended over the marginal portions of the wall 2 bounding the aperture 3.

Finally it is also possible to form insulating layers 6 both on the outer surface of the conductor 1 and also on the peripheral surface of the aperture 3, as is represented in Figure 4. When both the conductor 1 and also the wall 2 consist of aluminium, these insulating layers are again preferably formed by anodic oxidation.

In all these cases the manufacture and connection of the components can be effected as described for the embodiment of Figures 1 and 2 and the stated advantageous characteristics of the insulated lead-through assembly are also always obtained.

The described construction of the insulated lead-through assembly is suitable for any desired kind and shape of the leadthrough components. As an example, Figure 5 shows a through-lead portion in the form of a cylindrical conductor la, so that the aperture 3a is naturally also cylindrical. Here the insulating layer 5a is formed on the conductor la, by anodic oxidation for example if the conductor la consists of aluminium. The wall 2a can consist of the same material as the conductor la or of any material that has about the same co-efficient of thermal expansion as the conductor la. The different modifications shown in Figures 2, 3 and 4 may naturally also be similarly constructed.

The embodiment of Figure 5 is particularly suitable for connection of the components by the above described shrinkage process; the aperture 3a is for this purpose constructed with an appropriately tight fit.

The formation of the insulating layer by anodic oxidation of aluminium is not limited to cases in which at least one of the two components (lead-through member, wall member) consists of aluminium. This principle may often be employed if neither of the two members consists of aluminium, as is shown in Figure 6. As an example there is again shown in Figure 6, a lead-through assembly of the same form as that of Figure 1, though now neither the conductor 1 nor the wall 2 consist of aluminium, the two components may consist of any materials that have about the same co-efficient of thermal expansion.

Over the whole conical portion 4 and the adjacent marginal regions of the cylindrical portions of the conductor 1 is applied an aluminium layer 7 and a layer 8 of aluminium oxide is formed by anodic oxidation of the surface of this aluminium layer 7. The aluminium layer 7 may be applied for example by metal sputtering, evaporation in vacuum or electrolytic application of aluminium from an aprotic organic aluminium electrolyte.

After the formation of the aluminium oxide layer the two portions may then be assembled in one of the above described manners. Because of the small thickness of the aluminium layer 7 and of the aluminium oxide layer 5 the advantageous characteristics of the insulated through-lead including the stability under temperature alternations are obtained even when the co-efficient of thermal expansion of the material of the conductor 1 and of the wall 2 is different from the co-efficients of thermal expansion of aluminium and of aluminium oxide.

In analagous transformations of the modifications of Figures 3 and 4, an aluminium layer can be applied to the circumference of the aperture 3 instead of to the conductor 1, or even to each of the two portions, and subsequently provided with an aluminium oxide layer by anodic oxidation.

In all the above description the term "aluminium" is to be understood as meaning not only pure aluminium but also any aluminium alloy on which a layer of aluminium oxide may be formed by surface oxidation, especially anodic oxidation.

WHAT WE CLAIM IS: 1. An electrically insulated lead-through assembly comprising an electrically conductive through-lead member extending through an aperture in an electrically conductive wall member, with insulation between the mutually opposite surfaces of the two members, wherein said members consist of the same material or of materials with substantially equal co-efficients of thermal expansion and wherein the insulation is a layer of electrically insulating material hav ing a thickness not exceeding 100 ,um applied to at least one of the two members before their assembly.

2. An assembly in accordance with claim 1, wherein the insulating layer has a thick ness in the range from 10 to 100 clam.

3. An assembly in accordance with claim 1 or claim 2, wherein the insulating layer extends over a portion of those surfaces of the member to which it is applied that adjoin the surface opposite a surface of the other said member.

4. An assembly in accordance with any one of claims 1-3, wherein the insulating layer consists of glass, silicon dioxide or an oxide of the metal of the member to which it is applied.

5. An assembly in accordance with any one of claims 1-4, wherein the insulating layer has been formed by sputtering in vacuum.

6. An assembly in accordance with any one of claims 1-3, wherein the insulating layer has been formed by surface oxidation of the respective member.

7. An assembly in accordance with claim 6, wherein at least one of the two members consists of aluminium and the insulating layer is provided by a layer of aluminium oxide that has been formed by oxidation of the aluminium.

8. An assembly in accordance with claim 7, wherein the aluminium oxide has been formed by anodic oxidation.

9. An assembly in accordance with claim 7 or 8, wherein the lead-through member consists of aluminium, the wall member consists of a material having the same coefficient of thermal expansion as aluminium and the insulating layer is a surface oxide layer formed on the outer surface of the lead-through member.

10. An assembly in accordance with claim 7 or 8, wherein the wall member consists of aluminium, the lead-through member consists of a material having the same coefficient of thermal expansion as aluminium and the insulating layer is a surface oxide layer formed on the peripheral surface of the aperture.

11. An assembly in accordance with claim 7 or 8 wherein both the lead-through member and the wall member consist of aluminium and the mutually opposite surfaces of both members are provided with an insulating aluminium oxide layer formed by oxidation of the metal surface.

12. An assembly in accordance with claim 7 or 8, wherein each of said leadthrough member and said wall member consists of a material other than aluminium, said surface of at least one of said members has a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. suitable for connection of the components by the above described shrinkage process; the aperture 3a is for this purpose constructed with an appropriately tight fit. The formation of the insulating layer by anodic oxidation of aluminium is not limited to cases in which at least one of the two components (lead-through member, wall member) consists of aluminium. This principle may often be employed if neither of the two members consists of aluminium, as is shown in Figure 6. As an example there is again shown in Figure 6, a lead-through assembly of the same form as that of Figure 1, though now neither the conductor 1 nor the wall 2 consist of aluminium, the two components may consist of any materials that have about the same co-efficient of thermal expansion. Over the whole conical portion 4 and the adjacent marginal regions of the cylindrical portions of the conductor 1 is applied an aluminium layer 7 and a layer 8 of aluminium oxide is formed by anodic oxidation of the surface of this aluminium layer 7. The aluminium layer 7 may be applied for example by metal sputtering, evaporation in vacuum or electrolytic application of aluminium from an aprotic organic aluminium electrolyte. After the formation of the aluminium oxide layer the two portions may then be assembled in one of the above described manners. Because of the small thickness of the aluminium layer 7 and of the aluminium oxide layer 5 the advantageous characteristics of the insulated through-lead including the stability under temperature alternations are obtained even when the co-efficient of thermal expansion of the material of the conductor 1 and of the wall 2 is different from the co-efficients of thermal expansion of aluminium and of aluminium oxide. In analagous transformations of the modifications of Figures 3 and 4, an aluminium layer can be applied to the circumference of the aperture 3 instead of to the conductor 1, or even to each of the two portions, and subsequently provided with an aluminium oxide layer by anodic oxidation. In all the above description the term "aluminium" is to be understood as meaning not only pure aluminium but also any aluminium alloy on which a layer of aluminium oxide may be formed by surface oxidation, especially anodic oxidation. WHAT WE CLAIM IS:

1. An electrically insulated lead-through assembly comprising an electrically conductive through-lead member extending through an aperture in an electrically conductive wall member, with insulation between the mutually opposite surfaces of the two members, wherein said members consist of the same material or of materials with substantially equal co-efficients of thermal expansion and wherein the insulation is a layer of electrically insulating material hav ing a thickness not exceeding 100 ,um applied to at least one of the two members before their assembly.

layer of aluminium deposited thereon and an insulating layer has been formed by surface oxidation of said aluminium layer.

13. An assembly in accordance with claim 12, wherein said aluminium layer consists of sputtered aluminium.

14. An assembly in accordance with claim 12, wherein said aluminium layer consists of evaporated aluminium.

15. An assembly in accordance with claim 12, wherein said aluminium layer consists of electrolytically deposited aluminium.

16. An assembly in accordance with any one of the preceding claims, wherein the lead-through member is a force fit in the aperture in the wall member.

17. An assembly in accordance with claim 16, wherein the lead-through member has been secured in the aperture of the wall member by heating the wall member prior to assembly of the two members and subsequent cooling of that member to grip the lead-through member.

18. An assembly in accordance with any one of the preceding claims, wherein an adhesive is present between the opposed surfaces of the two members.

19. An assembly in accordance with claim 18, wherein the adhesive is an epoxy resin.

20. A lead-through assembly substantially as described with reference to Figures 1 and 2, Figure 3, Figure 4, Figure 5 or Figure 6 of the drawing. - .. -