Method of Forming Shrink-Fit Compression Seal

This invention relates generally to methods for producing glass-to-metal seals and more specifically to such seals produced by shrink-fit compression which are adaptable to aerospace applications where space and weight are primary limitations as are cleanliness, simplicity of construction, and superiority of seal performance.

Shrink-fit compression seals have been developed and used successfully by others in operations involving glass-to-metal seals. These were primarily large diameter seals wherein a metal compressed on ring worked against a soft sealing material to form a seal with the glass. Obviously, if the necessity of the soft sealing material could be eliminated, a lighter, smaller seal, having inherent advantages with respect to aerospace application, would be produced. There is therefore a definite need in the art for a method which will produce a superior glass-to-metal seal and which eliminates the use of a third medium as a binder.

It is therefore an object of the present invention to provide a glass-to-metal seal method using no other material in the seal.

It is a further object of the present invention to provide a method for producing glass-to-metal seals which are clean yet does not compromise the cleanliness of surrounding parts and structures.

An additional object of the present invention is to provide a method which produces glass-to-metal seals of superior leak resistance.

These and other objects of the present invention are obtained by providing a method of producing glass-to-metal seals which minimizes the disadvantages of prior art methods and offers a novel parts preparation and attachment method producing glass-to-metal seals of superior leak resistance, and characterized by simplicity and cleanliness. The present invention describes the process for preparing a domed metal cylinder and a glass post; their subsequent positioning, and the manner by which they are bonded together.

Specifically, a domed niobium cylinder is prepared with a seal ring. That is, an annulus is cut into the base of the cylinder, thereby providing the cylinder with a seal edge of desired edge width and surface finish. A fused quartz cylinder is machined to a slight taper and desired surface finish. The domed cylinder is placed atop the quartz post; then placed in a vacuum chamber which is subsequently evacuated. The metal dome is heated by induction which causes the annulus to expand relative to the low expansion quartz block and result in the metal dome sliding down the quartz post. Heating the dome and quartz post under vacuum to high temperatures tends to keep the parts clean and free of contaminants while the seal edge tends to soften and flow slightly. Upon progression of the dome a desired distance down the post, the parts are permitted to cool causing the seal edge to crush against the glass post forming the desired seal whereupon the assembly is removed from the evacuated chamber.

FIG. 1 is a cross section of the niobium dome and quartz post; and

FIG. 2 is a plan view perspective of the niobium dome and quartz post placed inside a vacuum chamber, a portion shown cut away, and surrounded by an induction coil also having a portion shown cut away.

Referring now to the drawings and more particularly to FIG. 1, the present method consists in the first stage of the preparation of a domed niobium cylinder generally designated by reference numeral 10 and preparation of a fused quartz cylinder generally designated by reference numeral 20.

The domed niobium cylinder 10 is manufactured with a niobium seal ring 11 defining its base. The inner surface of seal ring 11 slopes downwardly and inwardly at an angle of 45 degrees to an annulus 12 machined at its base defined by a seal edge 13 of known width. The annulus is centered in the base of domed niobium cylinder 10 and posseses a known diameter, which, in the case of the preferred embodiment, is 0.375 inches.

In the preferred embodiment, seal edge 13 has a width of .004 inches. While this dimension is variable, a seal edge of lesser dimension, for example, less than 0.001 inches a "knife-edge", will result in small fractures in the quartz cylinder upon attachment of the niobium cylinder thereto. This condition reduces the leak resistance of the seal and may result in component failure. On the other hand, a seal edge width of too great a dimension, for example, greater than 0.008 inches, will reduce the leak resistance of the desired seal. Therefore, in the preparation of the seal ring the seal edge width is restricted to between 0.002 and 0.006 inches. This range has been found to practically eliminate the small fracture problem and to provide the best leak resistance.

The fused quartz cylinder 20 is machined such that it possesses a diameter slightly smaller than the annulus of the seal ring 21, in this instance less than 0.375 inches at its top and a 2.degree. downward and outward taper along its length. The outer surface 22 of quartz cylinder 20 is then polished transparent, that is, fine ground to a surface finish of better than 16 microinches.

As shown in FIG. 2, following preparation of the niobium dome and quartz cylinder, the parts are positioned upright with the seal ring 11 of niobium dome 10 resting on tapered outer surface 22 of quartz cylinder 20. The assembly is then placed in a vacuum chamber 30 and surrounded by an induction coil 40. Vacuum chamber 30 is then evacuated and power applied to induction coil 40 so that the niobium seal ring 11 is heated to a bright red at which point the niobium dome 10 slides down the tapered quartz cylinder 20. As soon as the niobium dome stops dropping, heating is stopped. The parts are allowed to cool to room temperature, approximately 68.degree. F., under vacuum, at which time the parts are removed from the vacuum chamber 30.

Heating to very high temperatures under vacuum tends to keep the parts clean and free of contaminants and, at high temperatures, the seal edge 13 tends to soften and flow slightly. In operation, the resulting seal is capable of sustaining a leak rate of less than 1 .times. 10.sup..sup.-9 standard cubic centimeters of helium per second.

It is to be understood that the preparatory steps taken, materials used and annulus configuration are merely the preferred embodiment of the present method. Various changes can be made in the shape, size and material as will be readily apparent to those skilled in the art without departing from the spirit and scope of the present invention.