US3108621A - Evacuation of vacuum and gas-filled envelopes - Google Patents

Description

-- Oct. 29, 1963 J. H. o. HARRIES EVACUATION OF VACUUM AND GAS-FILLED ENVELOPES 2 Sheets-Sheet 1 Filed May 6. 1960 Oct. 29, 1963 J- H. o. HARRIES 3,103,621

ENACUA'I'ION 0F VACUUM AND GAS-FILLED ENVELQPES 2 Sheets-Sheet 2 Filed May 6. 1960 w a F v7 Inventor x/ w/W 7M Attorney Iillliill I'Illl I'll,

IIIIIIIIIII III! lllll United States Patent 3,108,621 EVACUATKGN 0F VACUUM AND GAS-FTLLED ENVELOPES John H. 9. Harries, Warwick, Bermuda Filed May 6, 196% Ser. No. 27,347 Claims priority, application Great Britain May 14, 1959 14 Claims. (Cl. 1418) This invention relates to the evacuation of vacuum and gas filled envelopes particularly, but not exclusively, for use as the envelopes of electric discharge tubes, to enclosed transistors, enclosed mechanical and electrical devices, and containers in general.

Hitherto it has been usual to evacuate such envelopes or containers firstly by means of a mechanical pump the mechanism of which runs in oil and which is capable of reducing the pressure to no lower than (typically) to 10 mm. Hg and, secondly, to reduce the pressure still further by means of an oil or mercury diffusion pump down to, at very best, about 10- mm. Hg. Still further lowering of the pressure, if needed, has to be obtained by the use of getters and/ or by means of an ion pump, or by gas clean-up during a protracted ageing schedule of the device if it is an electric discharge tube. This process is complicated and the equipment is elaborate and costly. In addition to these disadvantages, the parts or elements inside the envelope tend to be contaminated by oil and other waste products from the mechanical pump and by oil or mercury from the diffusion pump. Oil contamination tends to inhibit the action of getters and ion pumps. Oil or mercury contamination can only be reduced by relatively complicated ancillary devices such as refrigerated traps and battles. Even if traps and baffles are used, oil contamination in many cases is sufiicient to affect adversely the operation of the device as a whole, particularly when it is necessary to attain very low pressures. An object of my invention is to find means of evacuating envelopes to low pressures without these disad vagtages.

According to the present invention, an adsorbent substance in an appendix which is connected to and communicates with the interior of the envelope is heated in the presence of a flow of a carrier gas through an orifice in the envelope or the appendix, and the envelope and the appendix while still hot are filled with a gas, after which the orifice is sealed to close the space within the envelope and appendix, the appendix is cooled to cause the adsorbent substance to adsorb gas and thereby to reduce the pressure in the envelope, and the connection between the appendix and the envelope is sealed off so as to detach the appendix from the envelope. The gas used to fill the envelope before the orifice is sealed, which may be carbon dioxide, may also be used as the carrier gas which fiows through the envelope during heating stage. The adsorbent substance in the appendix may be activated charcoal. If desired, getters can be used to achieve further reduction of the pressure in the envelope.

The appendix containing the adsorbent material may be cooled portion by portion, so that the different portions of the adsorbent material are effective at different times to reduce the pressure in the envelope. This is particularly advantageous when electrodes within the envelope are being successively de-gassed.

In order that the invention may be understood, several embodiments thereof will now be described with reference to the accompanying drawings, in which:

FIGURES 1A, 1B, 1C and 1D illustrate successive stages in the exhaust of a glass vacuum envelope by a method embodying the present invention;

FIGURE 2 shows diagrammatically apparatus for difi fizl Patented @ct. 29, 1963 "ice 2 progressively cooling different portions of the appendix; and

FTGURES 3, 4 and 5 illustrate modifications of the appendix shown in FIGURES 1A and 1B.

FTGURE 1A shows the initial stage of the exhaust of a vacuum envelope it which is inside an oven 11. Attached to one end of the vacuum envelope is a vacuum gauge 12 which is of the general type described by Penning in U.S. Patent No. 2,197,079 dated April 16, 1940 and in British Patent No. 474,845. Inside the vacuum envelope are electrodes 13 and getters 14 and 1d. These getters are respectively a barium-aluminum getter l4 and a titanium getter 15. The reasons for the use of these gettering materials are discussed in my copending patent application No. 27,445 filed May 6, 1960. An appendix to is attached to the other end of the vacuum envelope by means of a glass tubulation 17 and inside the appendix 16 is a quantity 1% of activated charcoal, or other adsorbent material. Suitable activated charcoal is that supplied by Barnebey-Cheney Co. of Columbus 19, Ohio, under the trade description type lC-l (614 mesh). The amount of activated charcoal required is related to the total volume of the tube, gauge and appendix. Typically, for a total volume of 300 cc., the volume of activated charcoal used can be about cc., namely about half the total volume, but this ratio is not critical. Open- ended glass tubes 19 and 20 communicate with the interior of the gauge 12 and the appendix 16 respectively.

During the first stage of the exhaust (FIGURE 1A), the vacuum envelope, the appendix and the gauge are baked in the oven 11 at about 350 C. for approximately one hour. The glass tubes 19 and 20 are left open to the atmosphere. Air flows through the assembly by convection and carries off water vapour and other contaminants from the charcoal and the glass walls.

After baking at 350 C. for typically one hour, and while the tube, its gauge and the charcoal in the appendix are still at this temperature, carbon dioxide gas is injected for about two minutes into the system through the orifice it) until the appendix, tube and gauge are full of this gas. Orifices 1? and 2d are then sealed off in that order while the temperature is maintained. The carbon dioxide gas may be supplied from a pressure cylinder shown diagrammatically at 21.

Alternatively, instead of leaving the orifices 19 and 2% open to the atmosphere, gas may be fed through them and through the tube during the period of baking. This gas may be carbon dioxide and in that case it is unnecessary to introduce further carbon dioxide gas just prior to sealing up the orifices A and B.

FIGURE 13 shows the next stage of the exhaust. The appendix l6 and activated charcoal 18 are immersed in a refrigerant 22 which may be solid carbon dioxide contained in a flask 23. The pre-heated charcoal then adsorbs the gas and the pressure in the tube falls to a moder-ately low value. The appendix i6 is then sealed off at the tubulation 17 and detached from the vacuum tube. The tube and gauge are illustrated in FIGURE 1C. A reduced gas pressure then exists inside the tube, for example the pressure may lie between about l0 and 10- mm. Hg. This pressure will be sufficiently low for the flash getters 14- and 15 and the Penning vacuum gauge 12 to be operated. The pressure would not be low enough for the getters and gauge to work if air alone were present inside the envelope (i.e. had not been displaced by C0 gas at the end of the baking period) unless far colder refrigerants than solid CO were used, such as liquid nitrogen or liquid helium. These refrigerants are dangerous and costly.

FIGURE 11) illustrates the final stage of the exhaust.

The getters i and 15 are flashed in succession, as described and claimed in the above-mentioned co-pending a plication, to produce an evaporated layer 24 of barium and an evaporated layer 25 of titanium on the glass walls of the vacuum tube Ed, the barium aluminium getter being flashed first and the titanium getter second. The pressure in the vacuum envelope will then fall to a very low value, say 10* or 10- mm. Hg. This pressure may be measured by means of the Penning gauge 12 as shown in FZGURE 1D. The gauge consists of a plate cathode 26 and a ring anode 27 which are immersed in the transverse field of a magnet 28. Potentials are applied to electrodes 26 and 27 from a source 29 by means of the wires The vacuum gauge current is amplified and measured by a meter 31, and is eh ctively proportional to pressure. The Penning gauge may be operated continuously whilst the gettcrs 14 and 15 are flashed, and its pumping action will then assist substantially in reducing the pressure.

It can be ensured that the adsorbent material, the interior of the vacuum envelope, the electrodes, and the gases introduced during exhaust, are all clean and uncontaminated, and contamination by oil or mercury from mechanical or diffusion pumps is absent. Largely because of this freedom from contamination, I have found that extremely low pressures can be readily obtained and maintained.

In some cases, however, consicerable contamination may be present due, for example, to unavoidably inadequate cleaning and to the evolution of dirty materials and gas from structures inside the vacuum envelope. 1 have found that the sequential flashing of the getters M and 15, and the pumping action of the Penning gauge, will enable quite low pressures (typically 10 to 10- mm. H to be obtained despite considerable contamination. In contaminated tubes, the pressure may rise somewhat when the tube is not in use, but will fall rapidly when the Penning gauge is switched on, in the manner illustrated in FIGURE lD.

Electrodes or other devices within the vacuum envelope may be processed while the appendix containing the adsorbent material is immersed in the refrigerant and before it is detached from the envelope. However, although gas will be initially adsorbed by the charcoal in the appendix when it is refrigerated, after refri eration the charcoal or other adsorbent will no longer adsorb gas very effectively, and gas subsequently produced from the electrodes and internal parts of the envelope or from other sources will not be adequately adsorbed. Therefore, when gas is to be adsorbed from successively degassed parts, I provide means successively to refrigerate different portions of the adsorbent during the process of exhaust so that they become operative one after the other. FIGURE 2 shows a method of gradually immersing an appendix 16 containing activated charcoal 13 in a refrigerant 22 by means of a servo-mechanism 32. This servo-mechanism is in turn operated in accordance with the gas pressure observed by means or" the vacuum gauge 12 and amplified in the amplifier 33. Thus, for example, the envelope It) may contain an oxide coated cathode 34 which may be heated by current from a source 35. After the envelope has passed through the baking stage indicated in F1 URE 1A, and has been sealed off from the atmosphere at the orifices 9 and 2b, the appendix and activated charcoal may be partially immersed as shown in FIGURE 2. This will reduce the pressure in the envelope to a substantial extent. A heater current from the source may then be used to heat up the cathode so as to break down the carbonates and produce an emissive oxide coating in the usual manner. The gas produced during this operation is then adsorbed by immersing a further portion of the adsorbent in the refrigerant. This second immersion may be arranged to be effected automatically by means of the servo-mechanism 32 in response to the increased gas pressure produced by the vacuum gauge 12 amplified by the amplifier 33. This process may also be 4 performed with respect to gas driven out from gctters when they are heated to de-gas them before they are flashed. In this way the envelope and internal electrodes or other devices may be successively rte-gassed and the gas adsorbed successively by the adsorbent as various parts are successively refrigerated. Alternatively, of course, the successive refrigeration of different portions of the adsorbent material may be performed manually.

The refrigerant may be of any convenient kind, such as, for example, liquid nitrogen, solid carbon dioxide in trichlorethylene or in acetone, liquid helium. Instead of immersing the appendix in a refrigerant, cooling coils with circulating cooling iluid may be arranged to be in contact with the adsorbent. Any other suitable gas which can be frozen by the chosen refrigerant may be used instead of carbon dioxide during the baking period (FEGURE 1A) but carbon dioxide has a relatively high freezing temperature and therefore does not require a very low temperature refrigerant which is costly and may be dangerous.

To avoid cooling the adsorbent material as a whole in those cases where it is desired to cool it piecemeal, for example, in the manner of FIGURE 2, baflies 36 of low heat conductivity may be inserted between appropriate portions or" the adsorbent (FIGURE 3), or the different portions may be contained in separate compartments (FIGURE 4). In a further modification, to enable the gases to be adsorbed to reach remote parts of the adsorbent material, this material may be traversed by a pierced tube 37 (FIGURE 5).

Automatic and repetitive exaust cycles can be arranged in accordance with the method described by successive automatic connection of hot and cold appendices to suecessive envelopes by means of valves.

One application of my present invention is in the provision of containers for the storage of materials and devices which are sensitive to contamination. Such containers may be exhausted, and may afterwards he filled with gas at a predetermined pressure, and the orifices sealed off as described above. The interior of the envelope will provide contamination-free storage space. The envelope may conveniently be made of metal or glass. Automatic processes may be used for the filling and sealing of these appendices.

I claim:

1. A method of evacuating an envelope to reduce the pressure therein to at least about 10 mm. Hg, comprising the steps of heating an adsorbent substance in an appendix which is connected to said envelope for gas flow between the appendix and the interior of said envelope, flowing a carrier gas over the adsorbent substance while said substance is undergoing said heating, replacing the gas in said envelope and appendix with a second gas having a condensation point greater than the condensation point of said carrier gas, cooling said adsorbent substance to cause it to adsorb gas from within said envelope and thereafter sealing the envelope against gas flow.

2. A method of evacuating an envelope without use of mechanical pumping devices to produce a vacuum therein of the order of 10*" mm. Hg which comprises providing an appendix in communication with said envelope for gas flow between the appendix and the interior of said envelope, providing an adsorbent substance in said appendix, heating said envelope, heating said adsorbent substance, allowing gas to pass through said heated envelope by convection, allowing gas to pass over said heated adsorbent substance by convection, replacing the gas in said envelope and said appendix after said gas passage which a second gas which has a higher condensation point than said convection passed gas, cooling said adsorbent substance, cooling said envelope, sealing the envelope against gas flow, providing separate first and second flash getters of different materials in communication with the interior of said envelope, flashing the first getter after reduction of pressure within the envelope by cooling of said adsorbent substance and flashing the second getter after an interval of time sufficient to allow a substantial reduction in pressure within the envelope created by the flashing of said first getter.

3. A method of evacuating an envelope wtihout use of mechanical pumping devices to produce a vacuum therein of the order of mm. Hg, which comprises providing an appendix in communication with said envelope for gas flow between the appendix and the interior of said envelope, providing gas adsorbing carbon in said appendix, heating said envelope to a temperature of the order of 350 F. for about one hour, heating said appendix and contained carbon to a temperature of the order of 350 F. for about one hour, allowing air to pass through said heated appendix and said heated envelope by convection to remove water vapour formed therein by said heating, replacing the air in said heated appendix and said heated envelope with carbon dioxide, cooling said adsorbent carbon to cause it to adsorb gas from within said envelope, sealing the envelope against gas flow to or from its interior, providing separate barium and titanium flash getters within said envelope, flashing the barium getter after said sealing of the envelope and then flashing the titanium getter after an interval of time suflicient to allow a substantial reduction in pressure within the envelope created by the flashing of said barium getter.

4. A method of evacuating an envelope comprising:

A. heating an adsorbent substance in an appendix which is connected to and communicated with the interior of the envelope in the presence of a flow of a carrier gas through an orifice which is in fluid communication with the interior of the envelope,

B. filling the envelope and the appendix while still hot vw'th a gas, then C. sealing said orifice to close the space within the envelope and appendix,

D. cooling the appendix to cause said adsorbent substance to absorb gas and thereby to reduce the pressure in the envelope,

E. sealing 011' the connection between the appendix land the envelope so as to detach the appendix from the envelope,

F. disposing first and second flash getters of different materials in communication with the interior of the envelope to be evacuated,

G. flashing the first getter after the reduction of pressure within the envelope by the coolingof the appenldix to a value at which flash getters can be used, an

H. flashing the second getter after an interval of time sufiicient to allow a substantial reduction in the rate of fall of pressure within the envelope following the flashing of the first getter, the first getter being of a material which is less inhibited than the material of the second getter by the presence of gases remaining after the sealing of said connection between the appendix and the envelope, and the second getter being of a material capable of further reducing the pressure after the flashing of the first getter.

5. A method of evacuating an envelope which comprises:

A. providing an appendix connected to and communicating with the interior of the envelope,

B. providing two orifices in the envelope and appendix combination forming a pathway for fluid to flow from without the combination into one of the orifices, through the interior of the envelope and appendix and out the second orifice,

C. heating an adsorbent substance in said appendix,

D. flowing air by connection through said combination pathway,

E. filling said combination with carbon dioxide while said adsorbent is still in heated condition, then F. sealing said orifices to close the space within the entvelope and appendix,

G. cooling the appendix to cause the adsorbent substance to absorb gas and thereby reduce the pressure in the envelope, and

H. sealing oif the connection between the appendix and the envelope so as to detach the appendix from the envelope.

6. A method as claimed in claim 5 in which, after said sealing off step H, the pressure within the envelope is still further reduced by the flashing of at least one flash getter.

7. A method as claimed in claim 5 in which at least portions of the interior of the envelope are heated during the cooling step G and gases evolved thereby are re moved by the cooled adsorbent material.

8. A method as claimed in claim 5 in which said adsorbent is subjected to the action of a refrigerant during said cooling step G.

9. A method as claimed in claim 8 in which solid carbon dioxide is used as the refrigerant.

10. A method as claimed in claim 5 in which progressive cooling of said adsorbent in step G is automatically controlled in response to the level of gas pressure within said envelope.

11. A method of evacuating an envelope comprising the ste s:

2. heating an absorbent substance in an appendix which is connected to and communicative with the interior of said envelope for gas flow between said appendix and the interior of said envelope;

B. flowing a carrier gas through an orifice which is in fluid communication with the interior of said envelope over the absorbent substance while said sub stance is undergoing heating;

C. filling the envelope and appendix while still hot with a gas having a condensation point greater than the condensation point of air;

D. sealing said orifice to close the space within the envelope and appendix;

E. cooling the appendix to cause said absorbent substance to absorb gas and thereby to reduce the pressure in said envelope and thereafter;

F. sealing off the connection between the appendix and the envelope so as to detach the appendix from the envelope.

12. A method as claimed in claim 11 in which said absorbent substance is activated charcoal.

13. A method according to claim 11, in which the gas used for filling the envelope and appendix before cooling is carbon dioxide.

14. A method according to claim 11, in which the gas which flows through the envelope during heating and with which the envelope and appendix are filled before cooling is carbon dioxide.

References Cited in the file of this patent UNITED STATES PATENTS 815,942 Dewar Mar. 27, 1906 1,189,664 Claude July 4, 1916 1,952,717 Lederer Mar. 27, 1934 2,730,280 Spencer Jan. 10, 1956 2,744,808 Ruedy May 8, 1956 2,757,840 Weissen'berg et al Aug. 7, 1956 2,791,104 Duz May 7, 1957 2,791,888 Vani May 14, 1957

Claims (1)

1. A METHOD OF EVACUATING AN ENVELOPE TO REDUCE THE PRESSURE THEREIN TO AT LEAST ABOUT 10-7 MM. HG, COMPRISING THE STEPS OF HEATING AN ADSORBENT SUBSTANCE IN AN APPENDIX WHICH IS CONNECTED TO SAID ENVELOPE FOR GAS FLOW BETWEEN THE APPENDIX AND THE INTERIOR OF SAID ENVELOPE, FLOWING A CARRIER GAS OVER THE ADSORBENT SUBSTANCE WHILE SAID SUBSTANCE IS UNDERGOING SAID HEATING, REPLACING THE GAS IN SAID ENVELOPE AND APPENDIX WITH A SECOND GAS HAVING A CONDENSATION POINT GREATER THAN THE CONDENSATION POINT OF SAID CARRIER GAS, COOLING SAID ADSORBENT SUBSTANCE TO CAUSE IT TO ADSORB GAS FROM WITHIN SAID ENVELOPE AND THEREAFTER SEALING THE ENVELOPE AGAINST GAS FLOW.

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