Hermetically Sealed Coaxial Connecting Means

Abstract

A hermetically sealed coaxial four-conductor feedthrough connecting device having glass insulating rings heat-bonded between the conductors. The outer conductor is chosen to have a thermal coefficient of expansion sufficiently greater than that of the glass rings and inner conductors so as to cause a resultant compressive force to be exerted thereon which is sufficient to achieve reliable hermetic sealing of the device despite any mismatching between the thermal coefficients of expansion of the glass and the conductors.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to coaxial connecting means, and more particularly to hermetically sealed coaxial connecting means of the feed-through type.

Various applications exit where it is required to provide hermetically sealed coaxial connections between the inside and outside of an enclosure. One such application, for example, requires that a triaxial cable penetrate a bulkhead or enclosure wall in a manner which provides hermetic sealing and low electrical loss, while maintaining electrical isolation of the outer shield. Such an application thus requires a four-conductor coaxial arrangement.

Since it is not easy to obtain a high quality hermetic seal even in a simple two-conductor coaxial connection, it will be recognized that a very much greater problem is presented in providing a hermetically sealed four-conductor coaxial connection as would be required in the above exemplary application

SUMMARY OF THE PRESENT INVENTION

The present invention is directed to a method and construction which permits obtaining a high quality, low loss, matched, hermetically sealed coaxial connection of relatively small diameter and having as many as four coaxial conductors, or even greater. In exemplary embodiment of the invention, glass insulators are heat-bonded between the coaxial metal conductors, the coefficients of thermal expansion of the metal conductors and the glass being chosen to have predetermined relationships so that the cooling following the glass heat-bonding operations causes coaction among the conductors and glass in a manner which results in the obtaining of a high quality hermetically sealed, low loss, coaxial structure.

The present invention has the further advantage of permitting the use of materials for the glass and the conductors having significantly different coefficients of thermal expansion. Such a freedom of choice can be of particular significance where it is important to be able to choose the conductors and glass based on other properties thereof, such as, for example, a low dielectric constant and/or a low power factor in the case of the glass, and mechanical characteristics and/or conductivity in the case of the conductors.

The specific nature of the invention as well as other objects, uses, and advantages thereof will become apparent from the following description of a preferred embodiment of the invention taken in conjunction with the accompanying drawing, which is a longitudinal cross-sectional view of a cylindrically shaped, hermetically sealed, feed-through receptacle, with a connector plug being additionally shown mating with one of the two substantially identical plug-receiving ends of the receptacle.

Like characters refer to like elements throughout the drawing. Also, to facilitate understanding of the invention, elements of the feed-through receptacle 10 are designated by numbers less than 100, elements of the cable connector 100 are designated by numbers greater than 100, and elements of a triaxial cable 200 to which the connector plug 100 is attached are designated by numbers greater than 200. In addition, since the present invention is primarily directed to the hermetic sealing problem, well-known mechanical and assembly details of the structure will be considered only generally herein, it being understood that such details may readily be provided by those skilled in the art from the drawing and the general description provided.

Initially, it is to be noted that the overall feed-through connecting structure comprises a generally cylindrical receptacle 10 with a cable connector plug 100 being provided for mating with each plug-receiving end of the receptacle 10, each connector plug 100 being in turn attached to a triaxial cable 200. The drawing shows only one triaxial cable 200 with its connector plug 100 mating with one plug-receiving end of the receptacle 10, but it is to be understood that a like connector plug and triaxial cable are provided to mate with the other plug-receiving end of the receptacle 10. In the exemplary embodiment illustrated in the drawing, both plug-receiving ends of the receptacle 10 are the same and mate with identical connector plugs 100.

Considering the construction of the receptacle 10 in more detail, it will be seen to comprise a cylindrical metal shell 12 serving as an outer electrical conductor; the shell 12 has a flange 13 depending therefrom suitable to permit securing to an enclosure wall (not shown) through which hermetically sealed, coaxial feed-through electrical connections are to be provided in accordance with the invention. Within the shell 12 of the receptacle 10 are three coaxial conductors 16, 18 and 20 secured by heat-bonded glass rings 21, 22, and 23. The conductors 16 and 18 are cylindrical metal shells, and the innermost center conductor 20 is a metal pin having a reduced portion 24 at each end. The glass rings 21, 22 and 23, along with dielectric insulator rings 26, silicone rubber O-rings 28, and silicone rubber interfacial seal rings 30, serve to provide the required electrical insulation for the shell 12 and the conductors 16, 18 and 20, as well as to provide hermetic sealing between the plug-receiving ends of the receptacle 10.

An exemplary method for providing the conductors 16, 18 and 20 and glass rings 21, 22 and 23 in the receptacle 10 is to initially provide preformed glass rings corresponding to rings 21, 22 and 23 in the drawing; these are inserted into the shell 12 of the receptacle 10 along with the conductors 16, 18 and 20 and held in proper location therein using an appropriate jig provided for this purpose. The resulting assembly is then heated to a temperature sufficient to melt the glass and effect bonding thereof to the respective adjacent conductor surfaces, following which the structure is cooled.

It will be understood that the achievement of a high quality hermetic seal between the opposite ends of the receptacle 10 is crucially dependent upon the success of the heat-bonding of the glass rings 21, 22 and 23 to the respective adjacent surfaces of the shell 12 and conductors 16, 18 and 20. However, because of the expansion and contraction occurring during the heat-bonding operation among the relatively large number of conductors and glass rings, it will be understood that it is difficult, if not impossible, to predict in advance whether or not a high quality hermetic seal will be obtained (for example, a seal providing a leakage of the order of 1 .times.10.sup..sup.-9 cc. helium/second). One approach which can be used to simplify the problem is to employ the well-known expedient of choosing the materials used for the conductors and the glass rings so that they have the same coefficients of linear expansion. Such a choice does not necessarily insure obtaining a high quality hermetic seal since, even if perfect matching of the coefficients could be obtained (which is unlikely), differences in the volume and geometry of the conductors and glass rings will still cause differences in actual amounts of expansion and contraction obtained. Furthermore, as mentioned previously, the requirement that the conductors and glass rings have the same coefficient of thermal expansion can be a severe disadvantage in that it may prevent use of materials having significantly better characteristics from the viewpoint of the electrical connection means being provided.

Since the present invention make possible the use of conductors and glass rings having significantly different coefficients of thermal expansion, it will be understood that the choice of materials therefor is greatly extended so as to permit greater optimization of other characteristics, such as mechanical and electrical properties.

In the exemplary embodiment of the present invention being considered herein, successful hermetic sealing has been obtained by the use of stainless steel for the shell 12 having a coefficient of thermal expansion of approximately 110 .times.10.sup.-.sup.7 in./.degree. C., "Kovar" for the conductors 16, 18 and 20 having a coefficient of thermal expansion of approximately 49 .times.10.sup..sup.-7 /in. .degree.C. (Kovar being the trademark of an alloy containing 20 percent nickel, 17 percent cobalt, 0.2 percent manganese, and the balance iron), and a low electrical loss borosilicate glass for the glass rings 21, 22 and 23 having a Corning number 7070 and a coefficient of thermal expansion of approximately 32. The use of Corning number 7070 glass is advantageous in that it not only has low electrical loss, but also has a relatively low dielectric constant so as to permit electrical impedance matching to be obtainable while providing a relatively small outer diameter for the receptacle 10. The conductors 16, 18 and 20 are preferably preplated with rhodium prior to the heat glass bonding operation to inhibit oxidation and to provide corrosion resistance.

It will be understood that, because the Kovar internal conductors 16, 18 and 20 have a significantly greater coefficient of thermal expansion than the Corning 7070 glass rings 21, 22 and 23, the conductors will radially shrink by a significantly greater amount than the glass rings (once they solidify during the cooling following the previously described exemplary glass heat-bonding method. As a result, during cooling each of the conductors 18 and 20 will shrink away from its respective immediately adjacent larger diameter glass ring 21 or 22, while contracting toward its respective immediately adjacent smaller diameter glass ring 22 or 23. Also, during cooling, the center conductor 20 will shrink away from its immediately adjacent larger diameter glass ring 22, and the outer shell conductor 12 will contract toward its immediately adjacent diameter ring 21.

These various expansions and contractions would be expected to greatly reduce the possibility of obtaining a successful hermetic seal for the receptacles, as well they might under normal circumstances. However, because the coefficient of expansion of the stainless steel shell 12 is so much larger than that of the conditions 16, 18 and 20, it radially contracts during cooling by a sufficient amount to cause a resultant compressive force to be exerted on the conductors 16, 18 and 20 and the glass rings 21, 22 and 23 despite the mismatching of their thermal coefficients of expansion. This resultant compressive force not only aids in achieving good glass bonding to the conductors during the glass bonding operation so as to permit achieving a high quality hermetic seal, but also aids in reliably maintaining this high quality hermetic seal during subsequent use of the structure.

After the conductors 16, 18 and 20 and the glass rings 20, 21, and 22 have been provided within the shell 12 of the receptacle 10 as described above, the dielectric rings 26, the O-rings 28, and the interfacial seal rings 30 are inserted, as illustrated in the drawing. Circular contact springs 32 of, for example, beryllium copper are also inserted in appropriately shaped grooves 33 provided in the inner surface of the conductor 18 at opposite ends thereof.

The connector plug 100 shown mating with the left end of the receptacle 10 in the drawing will next be considered along with the triaxial cable 200 to which it is attached. It is to be understood that the triaxial cable 200 may typically be of a conventional type such as, for example. Amphenol P/N 21-529 triaxial cable. As illustrated in the drawing, this triaxial cable 200 has two braided coaxial cylindrical conductors 216 and 218, a center conductor 220, suitable insulative material 222 for conductor insulation purposes, and an outer layer 205 of silicone rubber shrink tubing. For greater clarity, the center conductor 220 is illustrated in the drawing only by its external pin portion.

As also illustrated in the drawing, the connector plug 100 is constructed and arranged for cooperation with the triaxial cable 200 and the receptacle 10 so that, when the connector plug 100 mates with the receptacle 10, electrical connections are made between the conductors 16, 18 and 20 of the receptacle 10 and respective conductors 216, 218 and 220 of the triaxial cable 200 via respective conductors 116, 118 and 120 of the connector plug 100. More specifically, the center conductor 120 of the connector plug 100 has a central bore 124 at the right end (as viewed in the drawing) for making a releasable electrical connection with the left reduced portion 24 of the receptacle center conductor 20, and has a central bore 126 at the left end for providing a crimped electrical connection with the external pin portion 220 of the triaxial cable 200. Also, the connector plug 100 has a circular contact spring 132 in a respective groove 133 at the right end of the conductor 116 for making a releasable electrical connection with the left end of the receptacle conductor 16, while the right end of the conductor 118 makes a releasable electrical connection with the left end of the receptacle conductor 18 via the receptacle contact spring 32 provided in the groove 33 thereof. The left ends of the plug conductors 116 and 118 are electrically connected to the respective braided connectors 216 and 218 of the triaxial cable 20 by means of conventional crimped connections generally indicated at 141 and 143. The connector plug 100 also includes suitable dielectric material 139 for conductor insulation purposes, and silicone rubber ring 142 for use as an interfacial seal.

As illustrated in the drawing, the connector plug 100 further includes a coupling ring 115 having inner threads 114 for cooperation with outer threads 14 provided on the shell 12 of the receptacle 10 in order to permit a releasable locked mating to be provided between the receptacle 10 and the connector plug 100. Since the coupling ring 115 and the receptacle shell 10 are insulated from the other conductors, the desired electrical isolation of the outer shield is achieved.

Although the invention has been described herein with respect to a particular exemplary embodiment, it is to be understood that various modifications in construction, arrangement and/or use may be made within the scope of the invention as defined in the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Hermetically sealed coaxial connecting means comprising:

a cylindrical outer metal shell,

a plurality of metal conductors disposed within said shell coaxial with each other and with said shell, said conductors including at least two spaced coaxial shells and cylindrical member at the center thereof,

a plurality of glass members interposed between respective ones of said conductors and said shell for providing electrical insulation therebetween, said glass members including at least three glass rings having their inner and outer surfaces heat-bonded to respective adjacent surfaces of said conductors and said outer shell,

said outer metal shell and said conductors and said glass members being constructed and arranged to provide a cylindrically shaped feed-through receptacle having opposite connector plug-receiving ends, of like construction and a connector plug for mating with each plug-receivingg end of said receptacle,

said shell having a sufficiently large thermal coefficient of expansion with respect to those of said conductors and said glass members so as to cause a resultant compressive force to be exerted on said conductors and said glass members after the cooling following said heat-bonding operations to thereby provide a high quality hermetic seal therebetween despite differences in the expansions and contractions of said conductors and said glass members occurring during said heat-bonding operations.

2. The invention in accordance with claim 1, wherein the coefficient of thermal expansion of said conductors is at least 20 percent larger than that of said glass members.

3. The invention in accordance with claim 2,

wherein the coefficient of thermal expansion of said outer metal shell is at least twice as large as that of said conductors.

4. The invention in accordance with claim 3,

wherein said outer metal shell is stainless steel, said conductors are an alloy containing 20 percent nickel, 17 percent cobalt, 0.2 percent manganese, and the balance iron, and said glass members are a low electrical loss borosilicate glass having a coefficient of thermal expansion of approximately 32.

5. The invention in accordance with claim 1,

wherein said connector plug includes electrically insulated coaxial conductors

wherein to make electrical connection with corresponding coaxial conductors in said receptacle when said connector plug is mated with a plug-receiving end of said receptacle.

6. The invention in accordance with claim 5,

wherein a triaxial cable is mechanically and electrically coupled to said connector plug.