US2795905A

US2795905A - Method and apparatus for making evacuated envelopes

Info

Publication number: US2795905A
Application number: US524952A
Authority: US
Inventors: Robert E Berge
Original assignee: JAMES KNIGHTS Co
Current assignee: JAMES KNIGHTS Co
Priority date: 1955-07-28
Filing date: 1955-07-28
Publication date: 1957-06-18
Anticipated expiration: 1974-06-18

Description

June 18, 1957 BERGE 2,795,905

METHOD AND APPARATUS FOR MAKING EVACUATED ENVELOPES Filed July 28, 1955 4 Sheets-Sheet 1 June 18, 1957 R. E. BERGE 2,795,905

METHOD AND APPARATUS FOR MAKING EVACUATED ENVELOPES Filed July 28, 1955 4 sheets-sheet 2 IN V EN TOR.

June 18, 1957 R. E. BERGE 2,795,905

METHODAND APPARATUS FOR MAKING EVACUATED ENVELOPES Filed July 28, 1955 4 Sheets-Sheet 3 26 J5 iii,

F 'INVEINTOR. BY mm: m

June 18, 1957 BERGE 2,795,905

METHOD AND APPARATUS FOR MAKING EVACUATED ENVELOPES 4 Sheets-Sheet 4 Filed July 28, 1955 471 105. PRESSURE fiQ/Z f6 j 1 '26 14/ 4 zug 2* I I mmvrm BY cu p mfzg A @i United States Patent METHOD AND APPARATUS FOR MAKING EVACUATED ENVELOPES Robert E. Berge, Sandwich, 111., assignor to The James gnights Company, Sandwich, 111., a corporation of linois Application July 28, 1955, Serial No. 524,952

23 Claims. (Cl. 53-9) The present invention relates to evacuated containers, the art of making the same, and apparatus for practicing that art. In a more specific sense, the invention is concerned with evacuated, hermetically sealed glass envelopes for electrical components, particularly piezoelectric crystals, transistors, and similar temperature-sensitive components and includes an improved method and apparatus for making such envelopes.

It is the general aim of the invention to provide a novel method and apparatus for making structurally improved, sealed and evacuated containers without damage to the components placed therein before sealing.

More specifically, it is an object of the invention to provide an improved method and apparatus for making a sealed, evacuated envelope which requires no separate opening or tubulation for evacuating the same, and which presents no protuberance or ti'p-oif projecting from the assembly after it is evacuated and completely sealed. Such tip-offs or sealed tubulations, present in prior sealed glass envelopes, not only increase the outer dimension of the envelopes and render them less attractive in appearance, but also constitute weak points in the envelope walls which are susceptible to easy breakage.

Another object is to create a sealed, evacuated envelope, for piezoelectric crystals or similar electrical components, constructed in a manner such that necessarily high temperatures required for bonding the envelope parts are kept from the crystal or component so that the latter is not damaged in the sealing process.

Still another object is to create sealed, evacuated envelopes for heat sensitive components by evacuating the space in the envelope through the joint during the course of sealing two envelope parts together, thereby eliminating the separate steps of first bonding an envelope base to its bonnet, evacuating the envelope through a separate open ing or tribulation for that purpose, and subsequently heating and sealing the opening or tubulation.

It is a further object to provide a method of producing sealed, evacuated envelopes by which the parts to be sealed and the molten sealing material are properly sealed and disposed by the action of gases drawn from the interior of the envelope as it is evacuated, thereby providing a more reliable bond between the parts.

Additional objects are to produce a durable sealed envelope which may be very small in size and pleasing in appearance; to provide a quick, convenient, and economical method for producing such envelopes; and to provide apparatus for efficiently carrying out the method and which is readily embodied to permit production line techniques.

Other objects and advantages will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings, in which:

Figure 1 is an enlarged perspective view of an evacuated, sealed envelope produced by the method and apparatus embodying the features of the invention and housing a piezoelectric crystal;

Fig. 2 is an enlarged vertical cross section of the en- 2,795 ,905 Patented June 18, 1957 velope and crystal in Fig. l, the parts being shown in exploded relation prior to scaling;

Fig. 3 is a perspective view of a metal element employed in the sealing process and constituting a part of the completed envelope;

Fig. 4 is a general perspective view of the apparatus used in carrying out the inventive method;

Fig. 5 is a plan view of the apparatus in Fig. 4, shown partially in section and partially in schematic form;

Fig. 6 is a vertical section taken substantially along the line 6-6 in Fig. 5;

Figs. 7 and 8 are bottom and top views of two respective vacuum transfer plates, these views being taken substantially along the lines '77 and 8-8, respectively, in Fig. 6;

Fig. 9 is an enlarged vertical section taken substantially along the line 9-9 in Fig. 6;

Fig. 10 is a polar diagram of the operating cycle showing the variations in temperature of the frit and the degree of vacuum;

Fig. 11 is a fragmentary cross-sectional view of an envelope prior to sealing, being similar to Fig. 2 but illustrating a sealed envelope produced by a modified form of the present invention;

Fig. 12 is similar to Fig. 9 but illustrates the modified apparatus for producing the envelope of Fig. 11; and

Fig. 13 is a perspective view of a metal element employed in the apparatus shown in Fig. 12.

Although the invention has been shown and is described in some detail with reference to particular embodiments thereof, there is no intention thus to limit it to such detail. On the contrary, it is intended here to cover all alternatives, modifications and equivalents falling within the spirit and scope of the invention as defined by the appended claims.

Referring now to Figs. 1 and 2, there is shown a sealed and evacuated glass envelope 15 formed in this instance of two parts adapted to fit together in space-enclosing relation and which may be termed a bonnet 16 and base 18, respectively.

The envelope 15 is shown as enclosing a piezoelectric crystal made, for example, as a circular wafer 19 of quartz having on its opposite sides centrally deposited electrodes 20 of a conductive material such as silver. Extending radially from each of the coatings to diametrically opposite edge portions of the wafer 19 are lead portions 20a which are engaged by respective spring clips 21. These clips both support the crystal and provide electrical connection to the respective electrodes 20. The wire forming the clips 21 extends downwardly for connection, as by soldering, to metal collars 22 slipped over and fixed to the inner ends of mounting pins 24 which extend in sealed relation through the base 18. The pins 24 thus support the crystal internally of the envelope 15 and are adapted to be received in an electrical socket (not shown) to provide electrical lead-in connections to the crystal.

Piezoelectric crystals of the type described are intended to operate with stability at a precise design frequency, determined by their size, the manner in which they are cut, and the thickness and disposition of the conductive electrodes 20. it is known, however, that the frequency of operation of these crystals tends to drift slightly with changes in the temperature and humidity of the space surrounding them. It is, therefore, most desirable that crystals employed for precision frequency control be mounted in inert evacuated envelopes or containers in order to eliminate variations in atmospheric humidity and to minimize fluctuations of temperature. Further, piezoelectric crystals of the type described cannot readily withstand temperatures in excess of about 300 C. without being damaged, i. e., their operating frequency changed from the intended and designed value. For

open at one end to present a flat lip surface 16a surround ing its opening. The base 18, in this instance, is also made of glass and shaped to have a peripheral edge surface 18a on its inner side for mating with the bonnet surface 16a. When thus assembled, the base 16 and bonnet 18 form an enclosed space 25 which surrounds the crystalwafer 19.

It is to be particularly noted that the bonnet 16 is perfectly smooth on its inner and outer surfaces, having no separate opening or tubulation for evacuation purposes, but providing relatively thick and symmetrical walls substantially immune to breakage. The base 18 is itself a flat part of relatively great thickness and high inherent strength. Prior to assembly of the two envelope parts, the mounting and connecting pins 24 may be bonded in extending relation through the base 18 in known manner, the spring clips 21 and collars 22 being attached to the inner ends of the pins 24, and with the crystal Wafer 19 inserted into the clips as illustrated in Fig. 2.

In accordance with the invention, the two parts of the envelope are structurally joined and sealed by a quantity of fusible bonding material 26 which is locally heated to its melting temperature; and while the bonding material 26 is in a molten state, the interior space 25 of the envelope is exhausted by creating a vacuum in the external region surrounding the junction of the two parts. This causes gases or air inside the envelope to pass or bubble out through the viscous bonding material. After substantially all of the gases have been removed from the interior of the encelope, the external vacuum is maintained while the bonding material 26 is allowed to cool and form a hermetic bond between the adjacent surfaces of the two parts. The envelope evacuation and assembly are thus complete, and separate exacuation, with or without a tribulation is unnecessary.

The fusible bonding material may be any that is capable of forming an hermetic seal between the two envelope parts, which has a lower melting point than those parts, and which has a similar temperature coefficient of expansion so that the completed bond is not fractured by ambient temperature variations. The material preferred and presently employed is a powdered glass frit available under the commercial designation Corning #7570, having a melting point in the order of 600 C. This frit forms a good bond between and matches well the coeificient of expansion of the base and bonnet, which preferably are made of glass available under the commercial designation Corning #0120.

To create the vacuum externally of the envelope and thus draw gases from within the latter through or around the molten frit 26, the assembled parts are preferably placed in an evacuatable enclosure connected to a vacuum pump, as explained more fully below.

In practicing the invention, also, heat is applied to the bonding material and the adjacent surfaces 16a, 18a in a manner such that it is substantially localized during the sealing process, so that the crystal wafer 19 remains relatively cool and undamaged. For this purpose, a heating arrangement is provided which has most of its components located externally of the enclosure and a heater element disposed in proximity to the bonding material 26. In short, a metallic induction element is placed in the enclosure in proximity to the frit 26 and heated by currents induced therein by the field of an induction coil or loop energized with relatively high frequency electrical energy.

In the specific form illustrated by Figs. 1-3, the metallic element is made as an oblong ring 28 which is nested in the frit 26 and interposed between the surfaces 16a and 18a of the bonnet 16 and base 18 as the latter are fitted together. The powdered frit may be placed in a volatile vehicle such as alcohol and painted on the ring 28 after the latter has been placed on the base 18. When the ring is brought to red heat by induced currents, it heats and melts the bonding frit 26 so that the evacuation of the envelope by bubbling of gasses through the molten frit may take place and a hermetic seal produced upon cooling of the molten frit. These steps result in the completed envelope 15 shown by Fig. 1 which is smooth and pleasing in appearance yet sealed by a hermetic and structurally strong bond between the two envelope parts. As an incident in bubbling of gases through the frit 26 when it is in liquid state, the two envelope parts are firmly seated and alined in proper relation so that a strong, air-tight seal is obtained.

The method for the ring 28 may be any metal which is susceptible of induction heating. However, it is one which matches closely the temperature coefficient of expansion of the frit, base and bonnet glass, in order that ambient temperature variations will not affect the completed envelope. Preferably, the ring material has a fairly high resistivity and the ring itself is made thin, c. g, about .005 inch thick, so that inductive heating occurs rapidly. It has been found that, with the glass materials designated, the ring may be satisfactorily made of a stainless steel available commercially under the designation Carpenter #446.

The method and apparatus for producing the completed envelope 15 in a continuous production-line technique may be described in more detail with reference to Figs. 410.

For holding the individual evacuation enclosures, a pinrality of supports 30 are provided and mounted on a turntable 32 by screws 31 or the like. Centered on the supports 30 are respective pedestals 34 having an upper surface corresponding in shape and size with the envelope base 18 and held in place by screws 36. Each pedestal has a pair of holes 35 for registering with the downwardly projecting pins 24 on the base 18. The turntable 32, the supports 30 and the pedestals 34 may be made of suitable electrically inert material, such as plastics, pressed asbestos fiber, or wood.

In order to create a vacuum in the space externally surrounding a base and bonnet supported on each of the pedestals 34, an evacuatable enclosure or receptacle is provided for that space. As shown particularly in Fig. 9, a bell jar 38, preferably made of a non-conductive material such as glass, is seated on the support 30, sealing being enhanced by a gasket 39. An evacuation passage is provided by a brass tube 40 or the like inserted through alined holes drilled through the support 30 and the turntable 32. A conduit such as a rubber hose 41 is connected with the lower end of the tube 40, leading to a vacuum pump, as more fully described below. In order to prevent any undue vibration or wobbling of the honnet 16 on the base 18 as air is bubbled through the molten frit, a quantity of wadding material such as glass wool or fiber 42 may be located in the upper end of the bell jar 38 to bear lightly against the top of the bonnet 16.

For the purpose of heating the fusible bonding frit 26 to its melting point, the metallic ring 28 is initially disposed, as explained above, in the frit between the adjacent surfaces of the base and bonnet. As shown in Figs. 4-6, an inductive loop 44, of a suitable conductive material such as copper tubing, is arranged such that the several bell jars 38 pass successively through its field. Preferably, the inductive loop 44 is of hairpin shape with generally parallel legs bent into arcuate form to straddle the path of movement of the bell jars 38. Moreover, the ends of the loop are bent upwardly so that the bell jars may have clearance as the turntable 32 rotates. For the purpose of supporting the loop, the insulating drop links 45 are provided which are held at their upper ends on arms 46 extending horizontally from support posts 48 projecting from the surface of a casing 49 which peripherally surrounds the turntable 32. The ends of the inductive loop 44 extend into a suitable high frequency generator located at one side of the casing 49 and which, as illustrated schematically in Fig. 5, includes a suitable oscillator or other high frequency source 51 connected in series with a voltage-adjusting rheostat 52. It has been found that the frequency of the electrical source 51 is not critical and the frequencies between 500 kc. and 7 me. work well to provide the desired induction heating, a frequency of 500 kc. being preferred. For the equipment illustrated, the oscillator source may produce 750 watts at 3,000 volts, causing each ring 28 to dissipate about 25 watts.

It will be seen that as the turntable 32 rotates in a counterclockwise direction it causes each of the bell jars 38 and the assembled envelope parts on the respective pedestals 34 to pass successively into the right end of the loop where, due to the high frequency electric field between the two sides of the loop, the metal rings 28 are brought to high temperatures suflicient to melt the surrounding bonding frit 26. In order that the heating of the ring 28, the frit 26, and the adjacent portions of the bonnet 16 and base 18 may not be unduly abrupt so as to risk fracture of these latter parts, the sides forming the entrance portion of the loop 44 are widely spaced and taper progressively toward one another to a necked portion 44a (Fig. where they are relatively close together and spaced only slightly from the moving bell jars. It is at this point that the temperature of the metal ring 28 reaches its maximum. The sides of the loop 44 then diverge so that the heating action is reduced until the bell jars pass out the left end of the loop.

The physical organization of the turntable and the means for sequentially applying a vacuum to several bell jars as they progress may best be considered with reference to Figs. 6-8. As there shown, the turntable 32 is rotatably supported by a stand 55 having an upstanding shank 56 separated by a radial shoulder 58 from a stud 59. The table 32 is held by a plurality of suitable bolts 60 to a spider 61 having a plurality of arms extending radially from a central hub 62 which is slipped over and journaled by the stud 59 with its lower end bearing against the shoulder 58. The spider 61 may be locked in place by a nut 64 and washer 65 disposed in a central recess 66 of the hub and engaged with a threaded portion 59a extending upwardly from the stud 59.

In order to produce slow rotation of the spider 61 and table 32 around the stud 59, the sleeve portion 70a of t a plate 70 is slipped over and splined or keyed as at 7% to the lower end of the hub 62. At its outer periphery, the plate 70 is formed with a worm gear 700 which is drivingly engaged by a worm 71 on the output shaft of a speed-reducing gear box 72 powered by a suitable elect tric motor '74. As the motor runs, therefore, the plate 70, the spider 62, and the table 32 all rotate at a slow speed in' a counterclockwise direction as viewed in Fig. 5.

In order to apply a vacuum to each bell jar 38 during a selected portion of one revolution of the turntable, a pair of ported vacuum transfer plates are use-d. Thus a lower vacuum transfer plate 76 is mounted stationarily on the shank 56, and an upper vacuum transfer plate 78 is connected, as by bolts, to the underside of the motordriven plate 70. The upper plate '78 is provided with a plurality of circularly spaced vertical holes 79 (Fig. 7) communicating with the respective bell jars. Such communication is provided by fittings 80 each of which is connected with one of the rubber hoses 41 leading to one of the bell jar tubes 40 previously described. The stationary vacuum transfer plate 76 is slidable axially with respect to the shank 56 but keyed to a stationary collar 85 by a plurality of dowels 84, the collar being adjustably fixed to the shank by a setscrew 86. The lower vacuum transfer plate 76 is urged upwardly into snug engagement with the upper plate 78 by means of compression springs 88. surrounding the dowels 84. A suitable vacuum grease is placed on the mating surfaces of the two vacuum plates.

For establishing the vacuum cycle the lower vacuum plate 76 is formed with a series of three annular grooves 91-93 (Fig. 8) in its upper surface, the first extending through a relatively short arc, the second through a larger arc of approximately 70, and the third extending over a very short arc. Bored upwardly into the bottom of the first groove 91 is a passageway 94 which is connected by means of a suitable conduit 95 to an initial or roug vacuum pump 96 diagrammatically illustrated in Fig. 6. Similarly, a passageway 98 is bored upwardly from the bottom of the plate 76 into the groove 92 and connected by means of a conduit 99 with a final or fine vacuum pump 100. A passageway 101 bored upwardly through the plate 76 to communicate with the third groove 93 is left open to the atmosphere. Thus, it will be apparent that whenever a given one of the holes 79 in the top vacuum plate 78 is in registry with the groove 91, the rough vacuum pump 96 will be connected to reduce the air pressure in the particular bell jar 38 connected by a conduit 41 to that hole in the top plate. As the turntable advances and that particular hole in the top plate 78 registers with and passe over the longer second groove 92, the fine vacuum pump 100 will be connected to exhaust the space within the corresponding bell jar 38. Finally, as the turntable advances and the particular hole 79 in the top plate registers with the third groove 93, thecorresponding bell jar will be vented to the atmosphere through the passageway 101 so that the vacuum existing in that bell jar is quickly released.

To understand the preferred timing relationship between the temperature and pressure, reference is made to Fig. 10. It will be noted that the stationary vacuum transfer plate 76 is so phased on the shank 56 with relation to the arc subtended by the inductive loop 44 that the vacuum is applied to a particular bell jar 38 as it reaches the necked region 44:: of the loop 44. At this time the metal ring 28 is at its maximum temperature and the fusible frit 26 is melted. The subsequent rotation of the table transfers the bell jar connection from the rough pump 96 to the fine vacuum pump 100 which then creates a high vacuum in the bell jar and the envelope, the vacuum within the latter causing bubbling of gases through the bonding material when it i in its molten state. Finally, as the envelope assembly passes from beneath the exit end of the inductive loop 44 and the molten bonding material has solidified through cooling, the vacuum in the corresponding bell jar is destroyed by communication with the atmosphere through the groove 93 and the passage 101.

The overall operation may best be described by following the complete sequence, i. e., the steps taken and the results produced at each of several successive positions of a given pedestal 34 as the table rotates. It will be understood, however, that each of the pedestals 34 may be loaded with envelope components as they progress with the table so that a continuous sealing and evacuation process of a plurality of envelopes results. The rotative cycle is preferably about four and a half minutes.

As an unoccupied pedestal 34 progresses from the rotational station marked A to that marked B (Figs. 5 and 10), a base 18 is added thereto, its pins 24 being inserted into the openings 35 of the pedestal. At the station C a metal ring 28 is fitted on the base 18, and then surrounded with the powdered glass frit 26 as it progresses to the station D. Preferably, the powdered frit, carried in a suitable volatile vehicle such as alcohol, is painted directly on the ring and the adjacent surface portion of the base.

A bonnet 16 is placed over the crystal so that its surface 16a rests on the powdered frit 26 and the ring 28 when the pedestal reaches station E. Then, one of the bell jars 38 is added to the corresponding support 30,

being seated on the gasket 39.

After leaving the station F in Fig. 5, the base 18 and bonnet 16 and the surrounding bell jar 38 are carried beneath and into the energized inductive loop 44. As the assembly enters the loop, inductive heating gradually increases the temperature of the ring from room temperature (about 26 C.) to above the melting point of the frit. As shown by the polar curve T for ring temperature in Fig. 10, the maximum is approximately 650 C. in the necked region 44a of the inductive loop. Heating of the ring 28 takes a period of approximately one minute. At this point, the powdered frit 26 surrounding the ring 28 has been heated beyond its melting point, the adjacent portions of the glass bonnet 16 and base 18 being only softened. The molten bonding frit 26, while fluid, has a relatively high viscosity.

I Just after the assembly passes the narrow portion of he loop 44, the corresponding hole 79 in the upper rotating vacuum transfer plate 78 comes into registry with the groove 91 in the stationary vacuum plate 76. The pressure within the bell jar thus drops from atmospheric to a rough vacuum of about 200 microns shown by the polar graph P in Fig. 10. Shortly thereafter, the same hole passes from the groove 91 to the groove 92 so that the fine vacuum pump 100 reduces the pressure within the bell jar 38 to about 50 microns as shown by the graph P. While this vacuum is present in the bell jar, and the bonding frit 26 still in liquified state, air from the interior of the bonnet 16 is drawn out by bubbling through the molten, viscous bonding material. Thus, the pressure within the envelope is also reduced to a value in the order of 50 microns. In the process, the bonnet 16 is jiggled so that it is accurately seated in place on the bonnet, the fiber wadding 42 preventing the bonnet from being tossed out of place during the bubbling action.

The vacuum in the envelope is reduced to 50 microns in a relatively short time, e. g., as the connecting hole 79 in the rotating vacuum plate 78 passes over the first onethird of the groove 92, as indicated in Fig. 10. Then, the assembly passes out from beneath the inductive loop 44 and the molten frit immediately begins to cool and solidify while the vacuum is maintained in the bell jar 38 and the envelope. Finally, after the frit has frozen" to create a hermetic bond between the bonnet 16 and base 18, the bell jar is connected with the atmosphere through the groove 93 and passageway 101, thereby releasing the vacuum in the bell jar. The latter may then be removed and the completed envelope lifted ofi of the pedestal 34. This pedestal then reaches the original rotational station A and the above-described sequence of operations is repeated.

While in the procedure described, the bell jars 38 are added to each support 30 as the envelope assemblies on the pedestals 34 enter the induction loop 44, it has also been found that the bell jars may be added to each support 30 after the latter have entered the loop 44 and just as the corresponding tube 40 is about to be connected to the rough vacuum pump. The inductive heating thus takes place with the bell jars absent and the sides of the loop 44 may be more closely spaced in the region 44a.

In order that a specific application may be understood, the following tabulation of values is given by way of example for producing a sealed, evacuated envelope of a particular size.

Bonnet size:

long, wide, high, Wall thickness.

Base size:

,1 long, A wide, thick. Base and bonnet material:

Glass available commercially as of March 1, 1955, under designation Corning #0120. Expansion coefi'icient 89 10 Melting point approximately 800 C.

8. Frit material:

Powdered glass available commercially as of March 1, 1955, under designation Corning #7570 (disclosed in US. Patent 2,642,663). Expansion coefficient 84 10' Melting point 550 C.

Metal ring 28:

Shaped to overlie edge portion of base surface; V wide and .005 thick; made of stainless steel alloy available commercially under designation Carpenter #446.

Induction heater:

Loop 44 approximately 18 long tapered between 3" at widest points and 1.5" at narrowest point. Input power approximately 750 watt-s at 3000 volts and 500 kc. frequency. Approximately 25 Watts liberated in each ring 28.

Heating time:

1.5 minutes. Cooling time:

1 minute.

Final vacuum applied and held:

Approximately 1.5 minutes.

Final vacuum pressure: 50 microns Hg.

Turning now to Figs. ll-l3, a modified form of the invention is there illustrated. Insofar as like parts are shown, the same reference numbers are employed which appear in the preceding figures.

In this modification, the metallic element is not disposed directly in the frit 26 which is interposed between the base 16 and bonnet 18, and thus there is no metal element embedded in the completed envelope after it is exhausted and sealed. This eliminates the necessity of employing a metallic element which has a temperature coefficient of expansion which closely matches the temperature coefficient of expansion for the glass bonnet 16 and base 18. In lieu of the embedded metal ring, a metal plate is placed on top of each of the pedestals 34, and provided with a pair of openings 111 through which the pins 24 project with clearance space. The base 18 is thus placed directly on top of the metal plate 110 as shown in Fig. 8.

As the assembly passes through the inductive loop 44 shown in Fig. 5, the metal plate 110 is brought to a red heat and, by conduction, heats the frit 26 on the upper surface of the base 18 above its melting point. The bubbling action through the molten frit as a vacuum is created within the bell jar 38 occurs substantially as described above. It will be noted that since the two holes 111 in the metal plate leave considerable clearance around the mounting pins 24, the latter are not heated to an appreciable degree. Their sealed connection through the base 18 and the crystal wafer 19 are thus left relatively cool and undamaged.

While the above procedure has been described with particular reference to so-called soft glass, in order to minimize the risk of damage to the crystal, the invention is by no means limited thereto and would include the claimed procedure and apparatus even though hard" glass is employed. The term hard glass is well understood in the art, an example being a glass commercially known as Corning 7052 and having an expansion coefiicient ranging from 4 to 5 parts per million degrees centigrade. A frit having a similar expansion contact is used with this glass, for example a powdered frit known commercially as Corning 1826. The ring 28 may also be made of a variety of metals, for example, a nickle-copper-iron alloy known in the trade as Kovar.

I claim:

1. The method of producing an hermetically sealed, evacuated glass envelope for a piezoelectric crystal, the envelope having no pinched tubulation thereon and including an impervious glass bonnet having ariopening therein and an impervious glass base having metal pins extending therethrough for mounting the crystal on one side and providing electrical connection to the crystal on the other side, the base being adapted to close the opening in the bonnet with mounted crystal disposed within the latter, said method comprising the steps of placing the base in closing relation to the bonnet with a quantity of fusible bonding frit interposed between the adjacent surfaces thereof, the frit having a lower melting point than the base and bonnet, locating a metal element in proximity to the frit but spaced from the pins, placing the assembly in a bell jar, locating the bell par within a conductive loop energized with high frequency current to thereby induc tively heat the metal element and effect melting of the frit, evacuating said bell jar to cause evacuation of the honnet by passage of gases through the molten frit, and maintaining the vacuum in the bell jar while letting the molten frit cool to form an hermetic bond between the base and bonnet. I

2. The method set forth in claim 1 further characterized in that the metal element is a ring, and the ring is located with the frit between adjacent surfaces of the base and bonnet.

3. The method set forth in claim 1 further character ized in that the metal element is a plate having apertures therein, and the plate is located in juxtaposition to the outer surface of the base with the pins projecting through the apertures.

4. The method of producing an hermetically sealed, evacuated envelope from a glass bonnet having one end open, and a glass base adapted to fit in closing relation over the open end of the bonnet, said method comprising the steps of placing the base in closing relation to the end of the bonnet with a-quantity of fusible "bonding glass having a lower melting point than the base and bonnet interposed between the adjacent surfaces thereof, slowly heating said bonding glass until its temperature exceeds the melting point thereof, removing the heating source, creating a vacuum in the external space surrounding the bonnet and base and holding it until the bonding glass cools to thereby exhaust the interior of the envelope by bubbling of gases from within the envelope through the molten material during the period that it temperature drops to its melting point, after which it forms an hermetic seal between the bonnet and base.

5. The method of producing an hermetically sealed glass envelope from a glass bonnet having one open end and a base adapted to overlie and close that open end, said method comprising the steps of placing the base in closing relation to the open end of the bonnet with a quantity of fusible powdered glass fn't having a melting point below that of the envelope glass interposed between theiradjacent surfaces, heating the frit to such temperature that it becomes fluid, creating a vacuum in the space extcrnally surrounding said bonnet and base to cause evacuation of the interior .of the envelope by bubbling of gases from its interior to its exterior through the molten frit, and maintaining the vacuum while letting the frit cool untilv it forms an hermetic bond between the bonnet and the base.

6. The method of producing an hermetically sealed envelope for an electrical component from a glass bonnet open at one end and a glass base having mounting and connecting pins sealed in extending relation therethrough, said base being adapted to close the open end of the bonnet with the mounted electrical component therein, said method comprising, in combination, the steps of placing the base over the open end of the bonnet with a quantity of fusible bonding glass having a lower melting point than the bonnet and base'interposed between the adjacent surfaces of the latter, locating a metal plate having oversize apertures through which the mountingpins project in continuous relation to the external surface of the base, inductively heating said plate to heat the bonding glass until it becomes fluid, creating a vacuum in the external space surrounding the bonnet and base to cause bubbling of gases from within the space enclosed by the latter through the molten bondl 10 ing glass, and maintaining the vacuum while permitting the bonding glass to cool and form an hermetic seal between the bonnet and the base.

7. The method of producing an hermetically sealed envelope for an electrical component from a glass bonnet open at one end and a glass base having mounting and connecting pins sealed in extending relation therethrough, said base being adapted to close the open end of the bonnet with the mounted electrical component therein, said method comprising, in combination, the steps of placing the base in closing relation to the open end of the bonnet with a quantity of fusible bonding glass having a lower melting point than the bonnet and base interposed between the adjacent surfaces of the latter, locating a metal ring in said bonding glass between the adjacent surfaces of the bonnet and base, inductively heating said ring to heat the bonding glass until it becomes fluid, creating a vacuum in the external space surrounding the bonnet and base to cause bubbling of gases from Within the space enclosed by the latter through the molten bonding glass, and maintaining the vacuum while permitting the bonding glass to cool and form an hermetic seal between the bonnet and the base.

8. The method of producing an hermetically sealed envelope for a piezoelectric crystal from a glass bonnet open at one end and a glass base having mounting and connecting pins for the crystal sealed in extending relation therethrough, said base being adapted to close the open end of the bonnet with the mounted crystal therein, said method comprising, in combination, the steps of placing the base in closing relation to the open end of the bonnet with a quantity of fusible powdered frit having a lower melting point than the bonnet and base interposed between the adjacent surfaces of the latter, locating a metal element in proximity to said frit, inductively heating said element to heat the frit until it becomes fluid, creating a vacuum in the external space surrounding the bonnet and base to cause bubbling of gases from within the space enclosed by the latter through the molten frit, and maintaining the vacuum while permitting the frit to cool and form an hermetic seal between the bonnet and the base.

9. The method of producing an hermetically sealed envelope for an electrical component from a glass bonnet open at one end and a glass base having mounting and connecting pins sealed in extending relation therethrough, said base being adapted to close the open end of the bonnet with the mouthed electrical component therein, said method comprising, in combination, the steps of placing the base in closing relation to the open end of the bonnet with a quantity of fusible bonding glass having a lower melting point than the bonnet and base interposed between the adjacent surfaces of the latter, locating a metal element in proximity to said bonding glass, inductively heating said element to heat the bonding glass until it becomes fluid, creating a vacuum in the external space surrounding the bonnet and base to cause bubbling of gases from within the space enclosed by the latter through the molten bonding glass, and maintaining the vacuum while permitting the bonding glass to cool and form an hermetic seal between the bonnet and the base.

10. The method of producing an evacuated glass envelope for an electrical component from an impervious glass bonnet having one open end and a base adapted to close that open end, said method comprising, in combination, the steps of placing the base in closing relation to the open end of the bonnet with a fusible bonding material having a lower melting point than that of the honnet and base between the adjacent surfaces thereof, placing the assembled parts in an evacuatable enclosure, heating the material until it becomes fluid, evacuating the enclosure to cause bubbling of gases from within the bonnet outwardly through the molten material, and cooling the material while holding the vacuum until the material solidifies and forms an hermetic seal between the bonnet and base,

11. The method of producing an hermetically sealed glass envelope from a glass bonnet having one open end and a base adapted to overlie and close that open end, said method comprising the steps of placing the base in closing relation to the open end of the bonnet with a quantity of fusible bonding glass having a melting point below that of the envelope glass interposed between their adjacent surfaces, heating the bonding glass to such temperature that it becomes fluid, creating a vacuum in the space externally surrounding said bonnet and base to cause evacuation of the interior of the envelope by bubbling of gases from its interior to its exterior through the molten bonding glass, and maintaining the vacuum while letting the bonding glass cool until it forms an hermetic bond between the bonnet and the base.

12. The method of producing an hermetically sealed evacuated envelope from two envelope parts adapted to fit together to form an enclosed space, said method comprising the steps of fitting the two parts together with a fusible bonding material interposed between their adjacent surfaces, heating such material until it becomes fluid, creating a vacuum in the space externally surrounding said parts to cause evacuation of the enclosed space by bubbling of gases through the molten material, and main taining the vacuum while letting the material cool until it forms an hermetic bond between the envelope parts.

13. The method of producing an hermetically sealed, evacuated container from mating impervious container parts which comprises the steps of placing the two container parts in mating relation with fusible bonding material interposed therebetween, heating such material until it becomes fluid, applying a vacuum externally to the junction between said parts to cause evacuation of their interior by passage of gases through the molten material, and letting said material cool while holding said vacuum until the material forms an hermetic bond between the container parts.

14. Apparatus for producing an hermetically sealed, evacuated glass envelope for a piezoelectric crystal, the envelope having no pinched tubulation thereon and including an impervious glass bonnet having an opening therein and an impervious glass base having metal pins extending therethrough for mounting the crystal on one side and providing electrical connection to the crystal on the other side, the base being adapted to close the opening in the bonnet with the mounted crystal disposed within the latter and being sealed to the bonnet by fused powdered frit of bonding glass having a melting point lower than that of the base and bonnet, said apparatus comprising, in combination, a support for holding the base in closing relation with the bonnet with the powdered frit interposed between the adjacent surfaces thereof, a metal element located in proximity to the frit but spaced from the crystal and its mounting pins, an evacuatable bell jar adapted to surround said support, metal element, base and bonnet, a conductive loop energizable with high frequency current located externally of said bell jar to inductively heat said metal element and thereby melt the frit, a vacuum pump for evacuating said bell jar to withdraw gases from within the bonnet by bubbling through the molten frit, and means for maintaining the vacuum in said bell jar while letting the molten frit cool to form an hermetic bond between the base and bonnet.

15. The apparatus set forth in claim 14 further characterized in that the metal element is a plate having apertures therein, and the plate is held by the support in juxtaposition to the outer surface of the base with the mounting pins projecting through its apertures.

16. The apparatus set forth in claim 14 further characterized in that the metal element is a ring which is disposed in the frit between the adjacent surfaces of the base 12 V and bonnet and which becomes an integral part of the envelope when the frit is cooled.

l7.vApparatus for producing an hermetically sealed, evacuated envelope for a piezoelectric crystal from a glass bonnet open at one end and a glass base having mounting and connecting pins for the crystal sealed in extending relation therethrough, said base being adapted to fit in space-enclosing relation against the open end of the bonnet with the mounted crystal disposed in the latter and the pins extending externally for electrical connection, and a quantity of powdered glass frit having a lower melting point that the base and bonnet and susceptible of bonding the latter together, said apparatus comprising, incombination, a support for holding the base in spaceenclosing relation with the open end of the bonnet and with the powdered frit interposed between the adjacent surfaces thereof, a metal plate held by said support in juxtaposed relation to the external surface of said base and having apertures through which the mounting pins project, an evacuatable enclosure for surrounding the support, the plate, the base, and the bonnet, electric means located externally of said receptacle for inductively heating said plate so that the frit is melted, a vacuum pump for evacuating said receptacle to cause bubbling of gases from within the space enclosed by the bonnet and base through the molten frit, and means for maintaining the vacuum while permitting the frit to cool and form an hermetic bond between the bonnet and the base.

18. Apparatus for producing an hermetically sealed, evacuated envelope for a piezoelectric crystal from a glass bonnet open at one end and a glass base having mounting and connecting pins sealed in extending relation therethrough to afford support of the crystal on one side and external electrical connection to the crystal on the other side, said base being adapted to fit in spaceenclosing relation against the open end of the bonnet with the mounted crystal disposed within the latter and a quantity of powdered glass bonding frit interposed between the adjacent surfaces of the base and bonnet, said frit having a lower melting point than the bonnet and base, said apparatus comprising, in combination, a support for holding base and bonnet in space-enclosing relation with the powdered frit interposed between the adjacent surfaces thereof, a metal ring disposed within the frit and also interposed between the adjacent surfaces of the base and bonnet, an evacuatable receptacle for surrounding said support, the base, bonnet, and ring, electrical means disposed externally of said receptacle for inductively heating said ring to melt the powdered frit, a vacuum pump for evacuating said receptacle so that gases within the bonnet are withdrawn by bubbling through the molten frit, and means for maintaining the vacuum in the receptacle while permitting the frit to cool @d form an hermetic bond between the bonnet and the base.

19, Apparatus for producing an hermetically sealed, evacuated envelope for a piezoelectric crystal from a glass bonnet open at one end and a glass base having mounting and connecting pins for the crystal sealed in extending relation therethrough, the base being adapted to close the open end of the bonnet and being joined thereto by an initially powdered glass frit fusible at temperatures below the melting point of the base and bonnet, said apparatus comprising, in combination, a support for holding thebase in space-enclosing relation to the bonnet with the powdered frit interposed between adjacent surfaces thereof, a metallic element disposed in proximity to the frit, an evacuatable receptacle surrounding said support, said metallic element, and :the base and bonnet, electrical means external of the receptacle for inductively heating said metallic element so that said frit is melted, a vacuum pump for creating a vacuum within said receptacle thereby causing bubbling of gases from Within this base enclosed 'by the base and bonnet through the molten frit, and means for maintaining the vacuum in 13 said receptacle while permitting the frit to cool and form an hermetic seal between the bonnet and the base.

20. Apparatus for producing an hermetically sealed, evacuated glass envelope for an electrical component and having an impervious glass bonnet open at one end and a glass base adapted to fit over that open end with a fusible bonding material joining the adjacent surfaces of the base and bonnet, said apparatus comprising, in combination, means supporting the base with the bonnet resting thereon in space-enclosing relation therewith and with the bonding material interposed between adjacent surfaces thereof, enclosure means for surrounding said support and the assembled base and bonnet, means for heating the bonding material until it becomes fluid, means for exhausting the interior of said enclosure to cause bubbling of gases from within the bonnet outwardly through the molten bonding material, and means for maintaining the vacuum in said enclosure while permitting the bonding material to solidify and form an hermetic bond between the bonnet and the base.

21. Apparatus for producing an hermetically sealed, evacuated container having two pants adapted to fit together in space-enclosing relation with a fusible bonding material joining their adjacent surfaces, the bonding material having a melting point lower than that of the two parts, said apparatus comprising, in combination, a support for holding the two parts with one resting on the other in space-enclosing relation and the bonding material interposed between adjacent surfaces thereof, a receptacle for surrounding said support and the pants held thereby, a heater having physically disconnected components one of which is in the receptacle in proximity with the bonding material and another of which is outside the receptacle, said heater being operative to melt said bonding material, and a vacuum pump operatively connected to evacuate said receptacle while the material is melted so that the space enclosed by the two parts is evacuated by bubbling of gases through the molten material.

22. Apparatus for producing an hermetically sealed, evacuated container having two parts adapted to fit together in space-enclosing relation with a fusible bonding material joining their adjacent surfaces, the bonding material having a melting point lowertthan that of the two parts, said apparatus comprising, in combination, means for supporting the two parts in space-enclosing relation with the bonding material interposed between their adjacent surfaces, means for locally heating the bonding material to a liquid state, enclosure means surrounding the two pants, mean-s for evacuating said enclosure means while the material is liquid so that the space enclosed by the parts is evacuated by bubbling of gases through the molten material, and means for holding the vacuum in said enclosure means while letting the material cool and solidify to form an hermetic seal between the two parts.

23. Apparatus for producing hermetically sealed evacuated container having two parts adapted to fit together in space-enclosing relation with a fusible bonding material joining their adjacent surfaces, said apparatus comprising, in combination, means for supporting the two parts in space-enclosing relation with the bonding material between their adjacent surfaces, means for locally heating the bonding material until it becomes fluid, and means for creating a vacuum externally of the parts in the region of their junction to cause withdrawal of gases from their interior by bubbling through the molten material.

References Cited in the file of this patent UNITED STATES PATENTS 2,223,031 Edwards Nov. 26, 1940