US2446277A - Glass to metal seal in electrical devices - Google Patents

Description

Aug. 3, 1948. R. GORDON 2,446,277

GLASS TO METAL SEAL IN ELECTRICAL DEVICES Filed Sept. 24, 1945 ATTORNEY Patented Aug. 3, 1948 GLASS TO METAL SEAL IN ELECTRICAL DEVICES Ronald Gordon, San Carlos, CaliL, assignor to Eitel-McCnllough, Inc., San Bruno, CallL, a corporation of California Application September 24, 1945, Serial N 0. 618,349

My invention relates to electrical devices embodying glass-to-metal seals, such as vacuum tubes.

In electrical devices, such as electron tubes and the like, wherein a metallic conductor member is sealed through a glass wall of an enclosure or envelope, there are several conditions which must be satisfied for optimum operation, particularly at the higher frequencies. One is that a gas impervious or vacuum-tight seal must be established between the metallic member and the glass. This requires a metallic member having an expansion characteristic matching that of the glass, and a surface oxide which is soluble in the glass. Another condition is good electrical conductivity through the member, since such metallic member usually function as lead-in conductors to an internal part such as an electrode within the envelope.

The problem of satisfying these conditions is complicated by the fact that metals or alloys having the desired expansion characteristic for matching the glass usually have a relatively high electrical resistance. This resistance is a serious factor in limiting the operation of the tube, particularly at high frequencies. A partial solution to the problem, proposed in the past, was to coat the member with copper to provide a conducting layer on the surface for high frequency currents. In order to be fully effective, however, the conducting layer :must be continuous, including the area underlying the glass at the seal. This introduces another problem, namely, that of getting a good seal to the glass.

It is difficult to get a permanent seal between glass and a copper coated member for several reasons. Firstly, the only sealing oxide available is. the copper oxide formed when the seal is made. At best, copper oxide is not a very good glass sealing agent. Also. in attempting to provide a suflicient amount of the oxide there is grave danger of oxidizing the full depth of the coating, thus destroying its conducting properties. Controlling the depth of oxide penetration in a copper coating is almost impossible because of the readimess with which copper oxidizes.

The broad object of my invention is to over- 5 Claims. (Cl. 250-275) come the difliculties above mentioned, and prov 2 The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of my invention. It is to be understood that I do not limit myself to this disclosure of species of my invention as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure l is a vertical sectional view of a vacuum tube embodying the improvements of my invention; and

Figure 2 is an enlarged detail view of a lead-in conductor adjacent a seal.

In terms of broad inclusion, my electrical device comprises a section of glass sealed to a metallic member composed mainly of a material having an expansion characteristic matching that of the glass. A multi-layer coating is provided on the member, extending under the glass at the seal, which coating comprises an outer sealing layer of a metallic oxide having good glass scaling properties, such as oxidized chromium, and an inner conductor layer of a low resistance metal,

such as copper. This multi-layer coating makes it possible to select for the inner layer a metal chosen particularly for its electrical conducting properties, and for the outer layer a metal chosen for the sealing properties of its oxide.

In greater detail, and referring to the drawing, an external anode type of vacuum tube embodying my invention comprises a cup-shaped metallic anode 2 carrying a flange 3 sealed to a glass wall section 4 of the envelope. A lower wall 6, also of glass, has a reentrant stem 1 to which is sealed a pair of leads 8 for cathode or filament 9. Another electrode in the envelope, such as grid H, is connected to a ring-shaped metallic member i2 interposed between and sealed to glass sections t and 6 at the glass-to-metal seal structure generally indicated by reference numeral 1'31 Member l2 thus extends through the envelope wall and serves as a lead-in conductor for making electrical connection with electrode ll.

Evacuation of the envelope is accomplished through a suitable tribulation, not shown. A base 16 cemented to the lower portions of the envelope carries prongs H for filament connections. A suitable cooler I8 is provided on anode 2 for heat dissipation.

This tube structure is merely for purposes of illustration, and may be varied within wide limits.

actuary Thus, the tube may have an internal rather than an external anode, and may have an unipotential cathode rather than the filamentary type shown; it being understood that the improvements of my invention are concerned chiefly with a metallic conductor member sealed to glass, such as shown by the metallic terl ring ill sealed to glass envelope sections 6 and 6 m the tube illustrated.

Figure 2 shows the glass-to-mctal seal struc= turc it in greater detail, the parts being enlarged and accentuated to illustrate the invention more clearly. The glass in sectioned and 6 is a hard glass such as a hero-silicate glass. A hard glass has a higher melting point than ordinary lead glass, and is preferred for electrical devices such as vacuum tubes. The use of hard glass, however, seriously limits the sort of materials usable for number it because the latter must have expansion characteristics matching that of the glass. Special alloys have been developed to obtain the desired expansion properties. For example, an iron-nichel-cobalt alloy known as Kovar matches Corning 705% glass, and this combination is widely used in the electron tube industry. Other alloys compoundm to match particular kinds of hard glass may be employed. Also, some pure metals such as molybdenum may be used, but these are less desirable from the expansion standpoint.

As previously mentioned, metals having suitable expansion properties are seriously lacking in other respects for use in electrical devices, par ticularly when high frequency currents are involved. This is especially so with alloys such as Kovar, which alloys exhibit a high electrical resistance. Attempts made to overcome the dimculty by coating the member with copper have been unsuccessful because of the glass sealing problems, as hereinbefore discussed.

In my improved structure, member i2 is composed mainly of a material having the desired expansion properties, such as Kovar. and is provided with a multi-layer coating which extends under the glass at the seal. inner conductor layer i9 is of a metal having a low electrical re sistance such as copper or silver. Outer sealing layer 2!] is of a metallic oxide which is readily soluble in the glass. Chromium oxide makes a particularl good surface for sealing to a hard glass. other metallic oxides such as iron oxide may be employed, but chromium oxide ,is pre- 'ferred.

The structure is preferably made by first electro-depositing a thin layer of say i or 2 mils of copper on member '82. in order to reove occluded gas in the copper, the copper plated memher is preferably heated to about loco 6. in hydrogen. A thin layer of say mil of chromium is next preferably electrodeposited over the copper. The chromium is then oxidized. as by heating the member in air, after which the glass is sealed to the oxide layer.

The resulting structure provides an excellent glass-to-metal seal which is mechanically strong and vacuum tight. It also provides a continuous conductor layer 8 9 on the member, including that area which underlies the seal, insuring an adequate path for electric current from an external circuit to the electrode within the envelope. The fact that the conductor layer is covered by a metallic oxide of high resistance does not detract from the electrical conducting properties of the rfln'al structure. because high frequency currents willtake a path of least resistance if that path is closely adjacent to the surface. While examining into the influences oi surface films on the so-called skin eflect of high frequency conduction, it has been demonstrated that my multilayer structure conducts high frequency currents as well as one having a surface layer of pure copper.

Ac hereinbefore described, I prefer to fabrica e the structure by applying two layers of dlfiferent metals, such as chromium over copper, because this enables selection at metals for certain desired characteristics, namely. in the inner layer a metal of low resistance, and in the outer layer a metal whose oxide has the requisite glass seal lng properties.

Because oi" the excellent scaling properties of chromium oxide, such a sealing layer may be used to advantage directly over leaol-in conductors, such as tungsten or the like. Also, since the penetration of oxide in a chromiiun layer may be controlled, an inner portion oi non oxidized chromium may be left for current condeletion. The desired final results are easier to accomplish by applying the multi-=layer coating or diderent metals, however, and the latter procedurc is preferred While my invention is particularly useful in vacu devices such as electron tubes, vacuum condensers and the like, it is understood that the improvements may be incorporated in any electrical device wherein a metallic conductor member is sealed through the wall of an envelope or other glass enclosure.

1 claim: 1. electrical device comprising an envelope including a section of glass sealed to a metallic member, an electrode in the envelope electrically connected to the membensaid member being composed mainly of a material having an expansion characteristic matching that of said glass, and a multi-layer coating on the member underlying the glass at the seal, said coating comprising an outer sealing layer of oxidized metal and an inner conductor layer of a different metal.

2. An electrical device comprising an envelope including a section of glass sealed to a metallic member, an electrode in the envelope electrically connected to the member, said member being composed mainly of a material having an expansion characteristic matching that of said glass, and a multi-layer coating on the member underlying the glass at the seal, said coating comprising an outer sealing layer of oxidized chromium and an inner conductor layer of a difierent metal.

' 3. An electrical device comprising an envelope including a section of glass sealed to a metallic member, an electrode in the envelope electrically connected to the member, said member being composed mainly of a material having an expansion characteristic matching that of said glass, and a multi-layer coating on the member underlying the glass at the seal, said coating comprising an outer sealing layer of oxidized chromium and an inner conductor layer of copper.

4. The method of making an electrical device in which a body of glass is sealed to a metallic memher, which comprises forming the member mainly of a material having an expansion characteristic matching that of said glass, applying a layer of low resistance metal on the member, applying an outer layer of a diiferent metal whose oxide is soluble in said glass, oxidizing the metal of the outer layer, and then sealing the glass to said outer layer.

5 5. The method of making an electrical device UNITED STATES PATENTS in which a body of glass is sealed to a metallic me member, which comprises forming the member mg? s 1916 mainly of a material having an expansion char- 1'724465 hula 3 1929 acteristic matching that of said glass, applyini; a I vmm 1937 layer of copper on the member. applyinl an outer 1, Thomas Aug 1937 layer of chromium, oxidizing the chromium, and 2'35373 m July 19 then sealing the glass to the outer layer.

RONALD GORDON. FOREIGN PATENTS Number Country Date REFERENCES CITED 249,084 Great Britain July 1, 1926 The following references are of record in the 250L927 Great Britain Apr. 2, 1928 file of this patent:

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