US2334718A - Lamp exhaust method - Google Patents

Description

943- E. F. LowRY ETAL 2,334,713

LAMP EXHAUST METHOD Filed Dec. 27, 1941 3 Sheets-Sheet l AEGON FLUSH zo'ueu VACUUM F'l NE VACUUM FINE VACUUM m EOUGH ROUGH VACUUM AQGON FILL.

EOUGH Q NITROGEN VACUUM I LEAK DETEcTo FLUSH Q [zoueu FACTORY VACUUM Fig.1

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ATTOIZNE Y Nov. 23, 1943. E. F. LOWRY ETAL Filed Dec. 27, 1941 5 Sheets-Sheet 2 Erwin F. Lowry AND Jam e s LCOX, INVENTORS Fig. 5

BY i? v ATToQA/EY No v. 23, 1943. E, F, LQWRY HA 2,334,718

LAMP EXHAUST METHOD Filed Dec. 2'7, 1941 3 Sheets-Sheet 3 Pre Furnac e sTeps T I fi'fm'i'? fim I v 6 l rough evacxmhom P r Fuhn QCQ sreps t I o 1 ron n evacuation 7 l mgi f 1 Furnace lrlfroduce merr 5 2| mm a evqcunflon 8 opporfum'fy for v H 9 Tes fo learwae introduce mgr w Introduce inerf gas 23 run n evacuah'cm H g mjrodqce meri' gas Introduce mart 18 26%) fme eyocuofion m Troduce inert 03 J9me evacumion 3| introduce, mart gas mrroduce l'nerr gas dnsmounf envehpe 2-3 from owner roun evacuation l c on '6 fne eva nah bremtmercury bomb E b ts lmfroduce inerr 520s I J mesb. COX INVENTORS BY;: '3 A ORNEY Patent'ed Nov. 23, 1943 I UNITED STATES PATENT orrlca zswm Danvers, Mala,

assignors to Srlvania Electric Products Inc., a corporation of Massachusetts Application December 27, 1941, Serial No. 424,808

6 Claims. (Cl. 176-3) This invention relates to electric gaseous discharge lamps and more particularly to fiuorescent lamps and the method of evacuation and filling thereof before they are finally sealed.

An object of. this invention is to provide a process for treating the lamps on an exhaust machine.

.Another object is to include in this process the heating of the lamp, a series of flushing and evacuating steps, the introduction of mercury into the lamp, the utilization of vaporized mercury in fiushing and evacuating, the introduction of a final filling of an inert gas, and the final sealing of the lamp.

A further object is to provide a process for treating the lamps which will eliminate some of the causes of yellow bands, dark ends, "snakers and other undesirable characteristics which may be found in some lamps. A snaker is a lamp cycle. These steps and processes are accomplished on a rotary turret type exhaust machine through a plurality of heads about the periphery thereof. A separate pump is used for each manifold, or group of interconnected positions.

Figure 2 is a plan view of the fixed valve plate and the plurality of manifolds associated therewith.

Figure 3 is a detail of the movable valve plate.

Figure 4 is a block diagram of the process steps of this invention.

Stations 2, 3, 4 and 5 may be used for loading and unloading the lamps. At these stations, a

lamp to be treated is connected to one of the v heads, or a lamp which has passed through the cycle and has been finally sealed at station I is withdrawn from the machine. At station 6 the lamp is connected to a vacuum pump. This connection has a restriction and a trap therein to prevent an evacuation with such speed that some of the coating of luminescent material on the walls of the lamp would be drawn off.

Stations 1 and 8 are plugged positions, the lamp at these two positions being disconnected from the rest of the exhausting apparatus in order to put it in condition to re tested at station 9 to determine whether or not it is leaking. By

providing one or more stations, after the first evacuation station, for air to seep into the lamp, the lamp may be connected at a subsequent station, such as station 9, to some suitable leakdetecting mechanism, thereby determining at the start of the exhausting cycle whether or not the lamp leaks. By providing suitable pinching-off mechanism to operate in conjunction with the leak-detecting mechanism, the head to which a leaky lamp is connected may be effectively disconnected from the system for the remainder of the cycle, thereby preventing the leaky lamp from deleteriously affecting any of the other lamps during the exhaust cycle.

An elongated curved oven is disposed to enclose the lamps from stations I to l9 As the machine indexes from station! to ,station 8, the

'lamp enters the oven, andas the machine indexes from station ill to station is, the lamp comes out of the oven. One of the purposes of heating the lamp is to assist the evacuating and flushing apparatus in obtaining a high purity atmosphere in the lamp.

At stations II), II, l2 and I3, the lamp is alternately connected to a supply of inert gas, such as argon, or a gas inert with respect to this particular process, such as nitrogen, and flushed, and then connected to a vacuum pump such as that marked "rough vacuum" and shown-as connected to stations H and I3 in the accompanying figure. In positions In and. I2 the lamp flushing takes place at one of the positions rather than between positions as is done subsequently to insure removal of as much of the residual gases originally in the lamp aspossible before the finer evacuation takes place.

During the rest of its stay in the oven, the lamp is subject to a series of flushing and evacuating steps. The use of more than one pump in this process enables the attainment of a higher degree of evacuation in the lamp. Thus between stations I4 and I5, I6 and I1, and i8 and IS, a

I quantity'of an inert gas such as argon is introduced into the lamp. It has been found that a suitable quantity of the inert gas may be introduced as the rotatable turret indexes to carry the lamps from one station to another. Thus there is made available a greater number of stations for evacuation. As may be notedfrom the accompanying schematic diagram the stations.

- immediately following the several flushing steps,

1. e., stations l5, l1 and I9 are connected to one pump whereas the stations further removed from the flushing points, 1. e., stations 14, i6, and iii are connected to a separate pump. This. ar-

rangement increases the efiiciency of the pumps and enables the attainment of a finer degree of vacuum, for the pump immediately following the fiush will draw off the major portion of the residual gases mixed with the flushing gas, thereby insuring a better degree of vacuum on the next pump and enabling a finer degree of purification and evacuation of the lamp at that position. If the same pump were used for both rough and fine vacuum, then the gases in the lamp on the rough vacuum might back up and get into the lamp connected to the fine vacuum, thereby preventing the attainment of a finer degree of vacuum at this position.

Not only is the lamp heated, flushed and evacu ated at the several stations in the oven, but the filamentary electrodes thereof are connected to a source of electrical energy while the lamp is in the 'oven.

The fundamental object in this heating or cathode break-down is to obtain a filamentary electrode or cathode with high emissive qualities. Since one or more of the alkaline earth oxides may be used for this purpose, it has been found that barium and strontium oxide is desirable.

Since it is advantageous to use a barium and strontium carbonate or acetate or other decomposable compounds of the alkaline earth roup and lacquer solution or other suitable dispersing or suspending media, when coating the coiled wire, usually of tungsten, which forms the surface on which the barium and strontium xide is desired, the barium and strontium carbonate or acetate and dispersing or suspending medium must be broken down so a to leave an active barium-strontium oxide with high emissive qualities. This break-down is accomplished by a series of filament heating steps in which a current flows through the filamentary electrodes. The heating of the filament will cause the reduction of the barium and strontium carbonates or acetates to their respective oxides. 'Carbon dioxide and de-- composition products of lacquer will be drawn off in th gaseous state.

The heating of the electrodes may be started at station l5 and continued on for several stations after the lamp leaves the oven. Care should be taken in heating the cathodes that they are not heated too quickly for if this is done, the tungsten wire may break, the walls of the lamp become discolored, or the electron emissive material may break up and chip off. Thus the voltage must be kept below the point where any one of these conditions might take place.

We have found that facility in evacuation and the attainment of a high degree of purity in the lamp may be furthered by the utilization of mercury vapor in driving off. the carbon dioxide and decomposition products of the lacquer. This may be accomplished by introducing mercury into the lamp preferably in the manner described in copending application Serial Number 400,216 filed June 28, 1941, issued May 19, 1942, as Patent No. 2,283,189, although the mercury may well be introduced in a diiferent manner, for example, by causing a quantity thereof to be dropped down through the exhausttubulation from a suitable dispensing device mounted in register with the exhaust tubulation. As shown in the co-pending application, the mercury is enclosed in a metal container or bomb and electrically connected to the filamentary electrode at that end of the lamp further removed from the tubulation through which the lamp is being evacuated.

The heat of the oven and the degree to which the filamentary electrode adjacent the mercury bomb is heated are adjusted to control the point of time at which the bomb breaks and releases the mercury. It has been found desirable to have the mercury released just before or just after the lamp leaves the oven because it i at this point in the exhaust cycle that the lamp is, at its highest temperature. flushing agent, of the released mercury which vaporizes is obtained, for it is when the lamp is hottest that the sweeping of the carbon dioxide and the decomposition products of the lacquer from the lamp is most desirable. If one waits until the lamp cools down before flushing with the mercury, some of the foreign gases are liable to be absorbed by the fluorescent powder on the walls of the lamp.

The mercury bomb may be broken to release the mercury contained therein by raising the current through the cathode to which it is connected, sufiiciently so that the voltage drop across the cathode is high enough to ionize the gas around it and cause a discharge to pass between the cathode and the mercury bomb connected to an end thereof. The point of time during the exhaust cycle at which the mercury is released is quite important. If the mercury is released and permitted to vaporize while a substantial portion of the carbon dioxide and decomposition products of the lacquer are still present in the lamp, there is a tendency for the mercury and oxygen to combine and form mercuric oxide which in turn has been one of the causes of discoloration, for example yellow bands around the wall of the lamp adjacent the cathode.

A highly emcient use of the mercury released by the bomb as a flushin means may be flushes with the same mercury as one of the flushing agents may be obtained.

When the lamp comes out of the oven, an arrangement, similar to the one employed when the lamp was in the oven, is used, with respect to flushing and evacuating. Stations l9, 2|, 23 and 25, are located immediately following the introduction of a quantity of argon and the sweeping back into the lamp of the condensed mercury for flushing purposes. The major portion of the foreign gases will be drawn oil by the vacuum pumps connected to these positions while separate pumps to which stations 20, 22 and 24 are connected will enable the attainment of a higher degree of vacuum before the next flush.

The cathode lighting steps mentioned above are continued after the lamp leaves the oven and are continued until the lamp reaches station 30. Although the number of stations at which the cathodes are heated may well vary, it has been found that a cathode with better emissive qualities may be obtained by gradually heating the cathode at several stations rather than subjecting it to intensive heating at a few stations. 1

Positions 26 to 30 inclusive are devoted to evacuating, the flushing being discontinued after station 25. The mercury which has been utilized Thus maximum eillciency, as a as a flushing agent will have condensed outside the lamp during the evacuation at station 25. Such other mercury as vaporizes while the lamp is being evacuated at stations 26 to 30 inclusive will also condense with the previously vaporized mercury. Two or more separate pumps are used during the evacuation at stations 26 to 30 inclusive in order to obtain the highest degree of vacuum before the introduction of a quantity of inert gas which is to be sealed in the lamp.

This filling of gas is introduced as station 3|.

The accurate measurement of the desired pressure of argon may be accomplished in any one of several ways. However, I have found that the introduction of argon in excess of the desired amount at station 3| and then drawing oif the excess at station 32 is preferable for a higher degree of accuracy is automatically assured. Apparatus such as that shown in U. S. Patents 2,267,274 or 2,267,594 is suitable for this purpose. This final filling of argon will sweep the condensed mercury back into the lamp in which it will be finally sealed at station i.

One of the major advantages of the above-described exhaust cycle is that it enables the evacuation of large lamps to a high degree of purity of atmosphere in an exceedingly short time. For example, a lamp of approximately 75 cubic inches in volume may be passed through this exhaust cycle in about 5 /2 minutes. This is made possible to a great extent by the mercury vapor released from the bottom of the lamp to act as a difiusion pump. From the time the mercury is released at or about station [9, there is a continuous vaporization thereof thus serving as a difiusion pump. Not only does the vaporized mercury act in conjunction with the argon at stations to where argon is periodically introduced and exacuated but it also is very efl'ective in sweeping residual gases from the lampat stations 26 to 30 where the sole operation on the lamp is one of evacuation. It is important that this mercury be located and vaporized at the bottom of the lamp, i. e., at the end further removed from the end through which evacuation takes place, because otherwise it will hinder rather than aid in the obtaining of a high degree of purity of atmosphere in the lamp due it actin as a ceiling over the residual gases.

In Figure 2, the multiplicity of manifolds are shown connected to the fixed portion of the valve through which the evacuation and gas-filling of the lamp during the exhaust cycle takes place. The sweeps 33 connect the flushing manifolds 34 and the rough and fine vacuum manifolds 35 to the fixed valve plate 38. The plurality of ports 38 in the movable valve plate 39 shown in Figure 3 are successively placed in register with the plurality of ports 31 in the fixed valve plate 35 as the rotatable turret to which the movable valve plate 39 is connected carries the lamps connected thereto through the exhaust cycle. It is in this manner that a lamp which has been placed in register with one of the ports 38 in the valve plate 38 is successively brought into register with the 5 What we claim is:

1. A process for exhausting an electric discharge device having electrodes containing a quantity of elect'ron-emissive material, said process comprising: heating said device in an oven; alternately flushing with an inert gas and evacuating said device while in said oven; electrically heating said electrodes while said device is in said oven; removing said device from said oven; alternately in short steps flushing, with a combinatlon of an inert gas and mercury vapor, and evacuating said device after it is removed from said oven; introducing a final quantity of an inert gas into said device; and sealing said device.

2. A process for exhausting an electric discharge device having electrodes containing a quantity of electron-emissive material, said process comprising: heating said device; alternately flushing with an inert gas and evacuating said device; heating the electrodes of said device; aiternately in short steps flushing, with a combination of mercury vapor and an inert gas, and evacuating said device; introducing a final quantity of an inert gas into said device; and sealing said device.

3. A process for exhausting an electric discharge lamp having filamentary electrodes, said process comprising the following steps: exhausting the lamp to a rough vacuum; baking the lamp in an oven; alternately flushing the lamp with an inert gas and evacuating the same, said alternation comprising introducing a quantity of inert gas into said lamp, evacuating said lamp to a rough vacuum on one pump, and evacuating said lamp to a fine vacuum on a second pump; heating the filamentary electrodes of said lamp; alternately in short steps flushing said lamp, with a combination of an inert gas and vaporized mercury, and evacuating the same, said alternation being the same as said first alternation; and sealing said lamp.

4. A process for exhausting electric discharge devices having electrodes containing a quantity of electron-emissive material, said process comprising: heating said device in an oven; alternately flushing with an inert gas and evacuating said 7 device while it is in said oven; heating the electrodes of said device while said device is in said oven; withdrawing said device from said oven; alternately in short steps flushing with mercury vapor and evacuating said device after it is withdrawn from said oven; introducing a final quantity of an inert gas into said device; and sealing said device.

- 5. In a process for exhausting an electric discharge device, a series of short steps of alternately flushing said device with a combination of an inert gas and mercury vapor and evacuating said device.

6. In a process for exhausting an electric discharge device, the steps of vaporizing a quantity of mercury at the end thereof further removed from the end of evacuation, and alternately in short steps flushing said device with a combination of an inert gas and mercury vapor and evacuating said device.

ERWIN F. LOWRY. JAMES L. COX.

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