US2394919A - Metal-glass seal and sealing alloy - Google Patents

Description

Feb. 12, 1946.

ELONGATION CM. PER CM W. E. KINGSTON METAL-GLASS SEAL AND SEALING ALLOY Filed April 25, 1939 TEMPERATURE DEGREES CENTIGRADE INVENTQR Patented Feb. 12, 1946 2,394,919 METAL-GLASS SEAL AND SEALING ALLOY Walter E. Kingston, Emporium, Pa., assignor to sylvania Electric Products Inc., a corporation of Massachusetts Application April 25, 1939, Serial No. 269,855

2 Claims. (Cl. 75-128) This invention relates to'metal-glass seals and more especially to alloys which are capable of being sealed into glass bodies in a vacuum-tight manner.

A principal object is to provide a ternary ironnickel alloy of low chromium content.

Another object is to provide a new stainless alloy.

Another object is to provide an alloy which can be given a desired expansion coefficient over a wide temperature range rendering it useful in thermostats, thermoelectric couples and the like.

Another object is to provide an inexpensive alloy which is capable of being sealed into a soft glass, which seal maintains its vacuum-tight character over a wide range of temperature variations.

Another object is to provide an inexpensivealloy whose expansion characteristics closely match a wide variety of soft glasses.

Another object is to provide an alloy which can be treated to provide a tightly adherent oxide on its surface whereby a vacuum-tight seal can be effected with glass bodies.

A further object is to provide an alloy whos physical properties can be varied by cold working or annealing so as to adapt it to particular kinds of use.

A feature of the invention relates to inexpensive alloys that can be sealed in a vacuum-tight manner to a soft glass having the characteristics of Corning G-12 glass.

Another feature relates to a nickel -iron-chromium alloy which has a desirable coefficient of linear expansion rendering it suitable for scaling in a vacuum-tight manner to soft grades of glasses.

Another feature relates to a nickel-iron-chromium alloy wherein the greater part of the composition consists of nickel and iron with the remainer consisting of from 3 to 15 percent chromium.

A further feature relates to an improved soft glass header for radio tubes of the type having the contact prongs directly sealed therethrough.

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

In certain devices, for example electron discharge tubes, it is necessary to provide a glassto-metal seal which will remain vacuum-tight over long periods and over relatively wide temperature ranges.

While this istrue in the case true where the metal part to be sealed into the glass is of rigid construction. Thus there is disclosed in U. S. Patent No. 2,238,025, a radio tube header consisting of a glass button-like member through which are sealed the various contact prongs in the form of rigid metal rods. While devices of this category have been successfully manufactured in large quantities, the cost of manufacture has been increased over the ordinary radio tube because of the cost of the usual metal alloys that have been deemed necessary to effectuate the vacuum-tight seal. On the other hand, it is possible to seal a slightly harder glass, for example Corning 125 AJ" glass together with a chromium-iron alloy, e. g. Alleghany 55. One type of alloy that has been used in this kind of radio tube header consists of iron, nickel, cobalt and chromium and is quite costly to manufacture and work.

I have found that it is possible to make devices such as disclosed in said U. S. Patent 2,238,025, with soft glass headers and by using for the prongs an alloy which is relatively cheap to manufacture and work. While the invention is not limited to any particular kind of soft glass, it is particularly useful where the glass-to-metal seal employs a glass of the Corning (3-12 type. example of a soft glass that may be used may have the following composition,

SiOg PbO A120 C80 N820 K20 M11203 In accordance with the invention, the alloy percent chromium, and the balance substantially entirely of iron except, if desired, for the addition of a small percentage of manganese to facilitate melting and casting. When the alloy is to be sealed to a glass such as Corning G-12 glass, the preferred proportions are 42 percent nickel; 4 to 8 percent chromium, and the balance substantially entirely of iron and a small percentage of manganese of the order of one-fourth of one percent. I have found that such an alloy in addition to having desirable expansion coefficients over a wide range of temperatures rendering it suitable to form a vacuum-tight weld with Corning G-12" glass, also has a relatively high tensile strength of from 125,000 to 150,000 pounds per square inch. This is particularly desirable where the alloy forms the rigid contact prongs of ordinary flexible lead-in wires, it is especially of a radio tube. .If desired, the alloy can be annealed at a temperature of 1000 C. so as to render it ductile. Furthermore, by heating the alloy at a temperature of 900 C. in a suitable atmosphere, for example an atmosphere of moist Hz, a tightly adherent chromium oxide layer is formed on the exterior surface, which oxide facilitates wetting by the molten glass and the formation of a vacuum-tight bond or weld upon subsequent cooling. I have found that by using the range of proportions of alloy constituents as mentioned above, it is possible to vary them to provide a corresponding series of alloys whose mean linear expansion between zero and 300" centigrade can be given any desired value between 60 10- centimeters per degree centigrade, and 150 10-" centimeters per degree centigrade.

Thus there is shown in Fig. 1, a family of curves illustrating the characteristics of three separate alloys having compositions within the range of constituents above mentioned. The dotted curve of Fig. 1 represents the expansion characteristic of a soft glass, such for example a Corning (3-12 glass.

.Curve No. I represents the expansion characteristics upon heating and coolin of an alloy of the above mentioned composition containing approximately 42 percent nickel; 4 to 8 percent chromium, the balance being iron with a small percent of manganese. It will be seen that this alloy matches quite closely the characteristics of Corning G-12" glass. I have found that the match is sufficiently close to enable the manufacture of satisfactory radio'tube headers of the type disclosed in said U. S. Patent No. 2,238,025, which are free from undesirable strain around the prongs, up to the softening point of the glass, withth'e result that the vacuum-tight character of the seal remains permanent.

Curve No. 2 shows the expansion characteristic of an alloy consisting of 42 percent nickel; 8 to 12 percent chromium, and the balance iron.

Likewise curve No. 3 illustrates the expansion characteristics of an alloy consisting of 42 percent nickel and the balance iron.

Referring to Fig. 2, there is shown in cross section, a typical radio tube header embodying the invention. The header consists of a glass cup-shaped member having a circular button-.

shaped bottom I and a smoothly curved rim 2, and which terminates in a substantially cylindrical lip 4, the bottom portion at least of the cup-shaped member being preferably formed of a soft grade of glass such as "Corning G-12. Sealed through the member I are a plurality of rigid metal rods 3 which extend outwardly to form rigid contact prongs for the radio tube to which the header is sealed in any well-known manner. The prongs are preferably formed of the alloy above mentioned consisting of approximately 42 percent nickel; 4 to 8 percent chromium, and the balance substantially of iron.

Fig, 3 is a bottom view of Fig. 2, from which it will be seen that the portions of the header through which the prongs are sealed, are provided on both sides with integral beads 5. Preferably, one of the beads is oval-shaped and provided with a. rib 6 which cooperates with a. correspondingly shaped perforation in a metal cup base such as disclosed in U. S. Patent No. 2,250,- 184, so as to properly locate the prongs with respect to the base. While particular materials and parts have been described however, it will be understood that various changes and modifications may be made without departing from the spirit and scope of the invention. Furthermore, while the alloy according to the invention has been described as applied to a radio tube header, it is capable of other useswhere a stainless and easily workable alloy is required. Furthermore, because of its widely adjustable expansion coefficient, it is capable of use as a thermostat element such as in a bi-metallic thermostat. and also as one element of a thermocouple the other of which may be iron or other base metal. In the foregoing description the percentages of the various constituents are in terms of weights.

What I claim is:

1. An alloy for effecting a vacuum-tight seal to glass, said alloy comprising approximately 42 percent nickel, .4 to 8 percent chromium, and the balance substantially entirely iron.

2. A ternary iron-nickel-chromium alloy having the property of effecting a seal of vacuum tightness such as used in radio tubes to soft glass such as Corning G-12 which glass has an elongation coefiicient of from zero to 0.005 cm./cm.

: over a temperature range of zero to approximately 500 C., the elongation characteristic curve of said alloy being substantially coincident with its contraction characteristic curve over said temperature range, said alloy being approximately 42% nickel, from 4 to not more than 8% chromium and the balance iron. I

WALTER E. KINGSTON.

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