US 3826634 A
Description (OCR text may contain errors)
July 30, 1974 H. L. BLUST ETAL 3,826,634
PLUG SEALING 0F 'HERMETIC ENCLOSURES Filed Feb 5, 1973 United States Patent Ofiice 3,826,634 Patented July 30, 1974 3,826,634 PLUG SEALING OF HERMETIC ENCLOSURES Henry Leo Blust, Lyndhurst, and Norman Lee Lindburg, Berkeley Heights, N.J., assignors to RCA Corporation Filed Feb.5, 1973, Ser. No. 329,616 I Int. Cl. C03h 29/00 U.S. Cl. 65-34 2 Claims ABSTRACT OF THE DISCLOSURE A plug comprising a viscous mass of glass powder in a liquid carrier is disposed within an exhaust port of the envelope of a device to be sealed. The plug mass is heated at a low temperature to drive-oif the carrier leaving a plug adherent to and completely closing the exhaust port, the plug, however, being porous to gasses. The device is exhausted through the porous plug, and the plug then heated to fuse together the glass particles to hermetically seal the port.
BACKGROUND OF THE INVENTION This invention relates to the exhausting and back-filling of hermetically or vacuum sealed devices, and particularly to the sealing of such devices by means of a fused plug.
Various hermetically sealed devices, such as electron discharge devices, are completely evacuated or contain a pre-selected atmosphere other than the ambient air. In the fabrication of such devices, it is generally the practice to hermetically seal the device within its envelope while leaving open an exhaust port, evacuate the device through theexhaust port, back-fill the device with the desired atmosphere, if used, and seal the port to completely seal the device.
In some devices, the exhaust port comprises an elonggated tubulation fo glass, copper, or the like which communicates with the interior of the device through it envelope.v After the exhausting operation (which is meant to mean eitherexhausting alone or exhausting followed by a back-filling operation), the tubulation is pinched or tipped off close to the device envelope to both sever the tubulation and seal it. A disadvantage in the use of such tubulations, however, it that at least a small portion of the tubulation remains extending from the device envelope. With small devices in particular, the extending tubulation is unsightly and frequently interferes with the mounting and use of the device.
In other devices, the exhaust port comprises simply an Opening through the envelope which is plugged after the exhaust procedure. While various schemes have been devised, the use of plugs according to the prior art processes generally involves some complexity and attendant cost with respect to manipulating the plug into place after the exhausting operation but prior to removal of the device from the exhausting apparatus.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a view, in cross-section, of a portion of the envelope of a device prior to the exhausting thereof, the envelope portion having an exhaust opening containing a plug in accordance with the instant invention.
FIG. 2 is a view similar to that of FIG. 1 but showing the envelope portion after the exhausting procedure and with the exhaust port hermetically sealed in accordance with the instant invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The instant invention has utility in the exhausting (and, if desired, the back-filling) of any number of 'known devices, such as receiving tubes, display devices, or the like, having an hermetically or vacuum sealed envelope. Ow-
ing to the known structural details of such devices, description of specific devices is not provided.
With reference to FIG. 1, a portion of an envelope 10 of a device is shown, the envelope portion having an opening 12 therethrough communicating with the interior of the device can be, for example, an otherwise conventional receiving tube, such as the RCA 6SN7, having the opening 12 through its glass envelope in lieu of the usual exhaust tubulation. In the usual case, at that step in the fabrication of the device at which the process of the instant invention is used, the device has been completely assembled and otherwise completely hermetically sealed Within its envelope.
Then, in accordance with the instant invention, a plug 14 of a sealing material is disposed over and within the opening 12. The plug 14 comprises finely ground particles of glass in a liquid carrier or vehicle, e.g., water. The plug 14, at this point, is actually in liquid form, the viscosity thereof, however, being adequately high to prevent the plug from either falling into the device through the opening 12 or migrating therefrom.
To apply the plug 14 to the exhaust hole 12, an elongated wire can be used having a loop formed at one end thereof. The looped end of the wire is dipped into a bath or slurry of the plug material, and a quantity of the plug material adheres to the loop upon its removal from the bath. In one embodiment of the invention, for example, a 15 mil diameter wire is used having a 40 mil diameter loop at its end. The quantity of plug material picked up by the loop is fairly uniform from dip to dip.
The plug material on the loop is transferred to the exhaust opening 12 simply by contacting the material with an edge of the opening. As shown in FIG. 1, the plug material penetrates into the opening 12 and is disposed around the edge thereof on the outside surface of the envelope 10. It is necessary that the plug 14 completely close the opening 12 in order that the opening can be eventually hermetically sealed.
After the plug 14 is placed within the opening 12, the liquid carrier of the plug 14 is removed, as by gentle heating. With water as the liquid carrier, the carrier removal is accomplished by heating the device to a temperature of approximately C. (utilizing an air oven or hot plate) for 10 minutes or more.
After removal of the liquid carrier, the plug 14 comprises a porous mass of relatively loosely adhered particles. The plug 14 adheres quite well to the envelope 10, however, and the device can be moved quite freely without danger of the plug 14 being displaced.
The plugged device, is then placed within an exhaust chamber, e.g., a bell jar, and the chamber, evacuated. Owing to the. porosity of the plug 14," vapors within the device are exhausted through the opening 12. If desired, various members within the device can be heated to out-gas them in known fashion. Preferably, however, heating of the plug to the melting point should be avoided until the device is completely exhausted. For example, if the device contains electrical resistance elements offlthe type used in filamentary display devices, out-gasing of the filaments can be accomplished by passing current through the filaments via the device leads. The heating of the filaments is quite' localized, whereby significant heating of the plug 14 is avoided.
The porosity of the plug 14 is not critical, but fshiguld be as high as possible to minimize the time required to exhaust the device. The porosity of the plug is related to such factors as the size and shape of the plug material particles, the composition of the plug material and carrier, and the addition of any binder to the carrier.
After the device is exhausted, it can be back-filled, if desired, by admitting the selected device atmosphere into the exhausting chamber in known fashion. The atmosphere enters the device by diffusion through the still porous plug 14 in the opening 12.
Thereafter, the plug 14 is heated to melt and fuse the particles therein to provide an hermetic seal of the opening 12, as shown in FIG. 2. Although some compacting of the plug takes place during the melting and fusing process, no separation of the plug 14 from the opening 12 walls has been observed to occur. As previously noted, such separation is avoided as long as the unmelted plug 14, as initially disposed within the opening 12, completely closes it.
In one embodiment of the invention, for example, the plug comprises 1 part of water to 3 parts of powdered glass, the glass being of a conventional solder-type lead glass, such as Owens-Illinois No. 00560 glass, and being of such size that 80% of the particles pass through a 400 mesh screen and 100% pass through a 100 mesh screen. The material of the envelope is also a conventional lead glass, such as Corning 012 glass (conventionally used in vacuum tube stems), the envelope 10 having a thickness of 65 mils and containing a cylindrical opening 12 therethrough of 12 mils diameter.
Other glasses having different physical characteristics can be used for the plugs. The principle characteristics involved in the selection of the glass are the glass melting temperature and thermal expansion. That is, the melting temperature should be high enough to allow adequate out-gassing of the device, and the thermal expansion of the plug should adequately match that of the device envelope to prevent cracking of either owing to thermal stresses. Also, in general, the plug should comprise a material that adheres to itself and to the envelope during andvafter the melting processes, i.e., the plug should not break contact with the opening wall due to surface 4 tension of the plug material. Also, the plug should not be of such excessive length or depth as to trap and prevent the escape of any gases evolved from the plug during the melting process.
Liquid carriers other than water can also be used; a principle criteria in the selection of such carrier being that it leave no residue which might be harmful to the device. For example, heat decomposible carriers such as alcohol and nitrocellulose can be used.
1. A method of exhausting and sealing a device having an exhaust port comprising:
closing the exhaust port with a porous plug of glass,
exhausting the device by diffusion through said plug,
fusing said plug for hermetically sealing said port.
2. The method of claim 1, wherein said closing step comprises disposing said plug within said port in the form of a viscous slurry of glass particle in a liquid vehicle and thereafter removing said vehicle while maintaining adherence between said plug and said port.
References Cited UNITED STATES PATENTS 2,248,644 7/1941 Reger et al -18 X 3,222,150 12/1965 Shepard 6518 3,374,076 3/1968 Smith 65--18 3,778,126 12/1973 Wilson 31620 S. LEON BASHORE, Primary Examiner R. PACE, Assistant Examiner US. Cl. X.R.