US3576671A - Method of making a cathode fluorescent lamp - Google Patents

Abstract

SO AS TO PRODUCE A BARRIER LAYER OF TUNGSTEN COMPOUND BETWEEN THE TUNGSTEN WIRE AND BARIUM OXIDE AND INHIBITING THE RELEASE OF FREE BARIUM.

DURING THE PROCESS OF HEATING A FLUORESCENT LAMP TUNGSTEN WIRE CATHODE AND REDUCING ITS ALKALINE EARTH CARBONATE TO THE CORRESPONDING OXIDE, A FLOW OF NITROGEN OR OTHER INERT GAS IS MAINTAINED PAST THE CATHODE, THE FLOW OF INERT GAS BEING CONTROLLED AT A SLOW RATE WHICH MAINTAINS AN ATMOSPHERE OF CARBON DIOXIDE ADJACENT THE CATHODE

Description

April 27, 1971 c L, TOQMEY ET AL I 3,576,671

METHOD OF MAKING A CATHODE FLUORESCENT LAMP Original Filed July 5 l966 INVENTORS I CHARLES L. TOOMEY BY GERALD D. BUTLER ROBERTS, CUSHMAN 6 GROVEH ATTORNEYS United States Patent 3,576,671 METHOD OF MAKING A CATHODE FLUORESCENT LAMP Charles L. Toomey, Danvers, and Gerald D. Butler,

Beverly, Mass., assigmors to Sylvania Electric Products Inc.

Original application July 5, 1966, Ser. No. 562,736. Divided and this application May 27, 1968, Ser. No. 739,942

Int. Cl. B32b 21/26; H0lj 9/12 U.S. Cl. 117-219 7 Claims ABSTRACT OF THE DISCLOSURE During the process of heating a fluorescent lamp tungsten wire cathode and reducing its alkaline earth carbonate to the corresponding oxide, a flow of nitrogen or other inert gas is maintained past the cathode, the flow of inert gas being controlled at a slow rate which maintains an atmosphere of carbon dioxide adjacent the cathode so as to produce a barrier layer of tungsten compound between the tungsten wire and barium oxide and inhibiting the release of free barium.

This is a division of application Ser. No. 562,736, filed July 5, 1966.

This invention relates to are discharge devices, such as fluorescent lamps, and particularly to the manufacture of such devices with improved emissively coated cathodes.

A fluorescent lamp, for example, comprises an elongate glass envelope with an internal phosphor coating and a fill of mercury, sodium or similar volatilizable, positively ionizable material and inert gas. At the ends of the envelope are cathodes to which an alternating current is applied to cause an alternating thermionic electron discharge between the electrodes. The discharge vaporizes and ionizes the mercury, exciting it to ultraviolet emission which, in turn, causes the phosphor to radiate visible light by fluorescence. To increase the electron emission of the cathodes they are coated with one or more of the oxides of the alkaline earth metals barium, strontium and calcium with a small amount, e.g. of zirconium dioxide, applied for example as described in U.S. Pat. No. 2,530,394.

In manufacture of the lamp the cathode coating is applied to a multiply coiled conductor of tungsten, nickel, tantalum or like refractory metal, as a suspension of the alkaline earth carbonates with a binder such as nitrocellulose and a solvent such as amyl acetate. 0n prior continuous production lines the cathode, mounted by lead in wires on a glass stern, has been secured in the lamp envelope by sealing the glass stem to the end of the envelope. After sealing the stem a heating current was passed through the cathode coil causing breakdown of the carbonates to electron emissive oxides accompanied by formation of carbon monoxide and dioxide. To avoid a residue of the carbon oxides and gases from the glass sealing flame, the envelope was flushed by evacuation. To maintain the rate of the production line this evacuation was necessarily rapid and thorough. Then the inert gas fill was admitted and the envelope finally sealed by fusing exhaust tubes opening through the glass stern. During heating of the cathode coil a further reaction took place between the newly formed alkaline earth oxide and the refractory metal of the cathode conductor with the formation of free, gaseous alkaline earth which was also pumped out thereby resulting in a reduction of alkaline earth available as oxide to render the cathode highly electron emissive. In operation of the lamp, subsequent to manufacture, the same reaction occurred with excessive release of free alkaline earth metal in the lamp during the first ice two to three thousand hours of normal operation. Not only was emissive oxide depleted, shortening the life of the lamp, but also the excess free alkaline earth deposited on the inner wall of the envelope, discoloring the Wall, contaminating the phosphor and reducing light emission.

It is one object of the present invention to provide a fluorescent lamp, and a way of making such a lamp, in which it is assured that free alkaline earth metals and other contaminants will be substantially absent at the completion of manufacture of the lamp, and that their formation will be minimized during operation of the lamp subsequent to manufacture. A further object is to form the cathode oxides at a rate which is independent of the other steps of lamp manufacture such as filling and sealing.

According to the invention a method of making arc emissive conductor for an arc discharge device comprises the method of making an emissive conductor for an arc discharge device which comprises coating a refractory metal conductor with a suspension of alkaline earth metal carbonate, resistively heating the conductor in a confined chamber while flushing the chamber with an inert gas, thereby to convert the carbonate to metallic oxide and a gaseous carbon oxide and to form an interface on said conductor between the conductor and metallic oxide in the presence of gaseous carbon oxide, said interface comprising a reaction compound of the refractive metal and alkaline earth metal carbonate, characterized in that said heating and flushing is continued until the gaseous carbon oxide is removed and the interface substantially entirely covers the coated conductor and forms a reaction barrier between said metallic oxide and refractory metal conductor, thereby to minimize reduction and release of free alkaline earth metal during heated operation of said conductor in an arc discharge device.

For the purpose of illustration typical embodiments of the invention are shown in the accompanying drawing in which:

FIG. 1 is an elevation of a fluorescent lamp having a coated cathode;

FIG. 2 is an elevation of apparatus, partly broken away, showing apparatus for preparing the cathode coat- 51G. 3 is an enlarged section on line 3--'3 of FIG. 2; an

FIG. 4 is a schematic diagram of the electrical circuit of the apparatus.

As shown in FIG. 1 a conventional fluorescent lamp comprises an elongate glass envelope 1 with an interior phosphor coating 2. Each end of the envelope is closed by glass stem 3 which includes an exhaust tube 4 openingv into the interior of the envelope, and a stern press 5 through which lead wires 6 and 7 enter the envelope. The lead wires are connected to contact pins 8 and 9 insulatively mounted on a base 10 which covers the end of the envelope 1. Inside the envelope the lead wires 6 and 7 support a filamentary electrode 11 electrically connected between the lead wires, and lead wire 6 supports an auxiliary electrode 14. The cathode coil is formed of a refractory metal such as tantalum, nickel or preferably tungsten.

Prior to sealing the stem 3 in the envelope end, a stem subassembly is prepared with the exhaust tube open at its outer end. All alkaline earth carbonate cathode coating is applied over the surface of the coils, and hitherto the stems were sealed in the envelope ends and the envelope was evacuated through an open exhaust tube while heating the cathodes by passing current through the lead wires '6 and 7 and coil 11. The open exhaust tube was sealed by a flame after filling the envelope with an inert gas at low pressure.

Shown in FIGS. 2 to 4 is apparatus for breaking down the alkaline earth metal carbonates in a way which prevents substantial loss of alkaline earth metal and results in an improved coated cathode coil. The apparatus comprises a base 21 with a lip 22 provided with a gasket 23. A cover 24 has an edge 26 which rests on the gasket 23 sealing the Compartment within the base and cover. The compartment is divided by an insulating wall 27 into an inlet portion or gas manifold 28 connected by an inlet valve 29 to a source of inert gas such as nitrogen, and an outlet portion 29 having an outlet port 31.

Extending through the wall 27 are a plurality of openings 32. As shown in FIG. 3 each opening has a narrow circular bore 33 with a shoulder 34 for receiving and holding the exhaust tube 3. An O-ring 35 seals the gap between the glass exhaust tube and the walls of the openings, and also holds the tube and the stem 3 so that the ends of the lead wires 6 and 7 extending from the stem may be held jammed between the tube 4 and adjacent conductive bus bars 36.

As shown in FIG. 4, each bus 'bar 36 is fixed on the insulating wall on either side of an opening 33. The lead wires 6 and 7 are jammed in electrical contact with respective bars by inserting the tube 4 in a wall opening 33 with the wires loosed in the space between bars, and then rotating the stem and tube until the wires individually jam between the tube and a bar. The bus bars are arranged in three series connected by wires 37, 3'8, 39 and 40 to an autotransformer T having alternating current line input terminals C. The autotransformer T has an adjustable tap tfor varying the current supplied to the three series of bus bars. In each series is an ammeter A measuring the current through each series. When each opening adjacent the bus bars of a series holds a stem with the lead wires jammed against opposed bus bars, the electrodes on the stems are connected in the series.

With stems mounted in all the wall openings and the cover 24 sealed on the base 21, the cathodes on the stems are processed as follows. Nitrogen or other inert gas is admited through the valve 29 to the inlet, manifold chamber portion 28. Thence the gas flows through each exhaust tube 4 past the cathode on each stem into the outlet chamber, whence the gas is exhausted through the cover opening 31. After flushing atmosphere from the chamber portions and during continued flow of the inert gas, heating current is supplied through the cathodes, the amount of current and the rate of flow of gas being adjusted to maintain gaseous carbon oxides present adjacent the cathodes in quantities dependent on the amount of refractory metal used in the cathode.

As one example of a cathode for use in a 48 inch VHO fluorescent lamp, a triple coiled filament is formed as follows. A fine, 1.5 mg. tungsten wire is wound at 338 t.p.i. on a 45 8 mg. tungsten mandrel in parallel with a 7 mil. molybdenum mandrel (later removed by acid). The resulting primary coil is wound at 50 t.p.i. on a 14.25 molybdenum mandrel (also later removed). About 4.5 turns of the resulting secondary coil are wound at 17.5 t.p.i. on a 50 mil steel mandrel, which is then removed. The total, clean coil weight of the triple coil and 45.8 mg. tungsten mandrel is 48 to 56 mg. of tungsten. The triple coil is then coated with 23 mg. of a mixture of the combination of alkaline earth metals such as barium, calcium and strontium and a small amount of zirconium dioxide, the alkaline earth metal carbonates being in excess with respect to the refractory metal of the cathode.

For such a cathode a typical schedule of processing steps is as follows, wherein the current in amperes through each meter A is timed in seconds during four steps while nitrogen is flowing continuously at one cubic foot per hour.

Step

During processing the following primary reactions take place when barium carbonate is the principal cathode coating ingredient. First, the barium carbonate is broken down, by heating current through the cathode, into barium oxide and carbon dioxide. The barium carbonate reacts with the refractory metal of the cathode, preferably tungsten, to yield barium tungstate and carbon monoxide. And, most importantly, in the presence of carbon dioxide maintained by low rate of nitrogen flow the reduced barium oxide reacts with tungsten to produce barium tungstate, and incidentally carbon monoxide. As compared with prior processes, this latter reaction is continued until it produces at the interface between the tungsten cathode and barium carbonate coating a layer of barium tungstate which forms a barrier to further reaction between the reduced barium oxide coating and the tungsten cathode conductor. Further, the. presence of carbon dioxide substantially inhibits a reaction present in prior process, namely the reaction between barium oxide and tungsten releasing free barium which is removed by flushing or evacuation and thereby removed from the emissive cathode coating.

In contrast with prior processes in which the barium tungstate was produced in incidental amounts and with barium loss, the present process produces a complete reaction barrier between coating and cathode. The processed cathode may be stored in a normal atmospheric environment, and later sealed within a lamp envelope by conventional exhausting and sealing procedures. Thereafter the interface barrier prevents the presence and further production of free barium in the carbon-dioxide-free fill of the finished lamp, whereas, in the operation of prior lamps, continued reaction between barium oxide in the coating and the tungsten cathode released barium to reduce the available emissive oxide and discolor and contaminate the phosphor coating of the lamp envelope.

In a lamp so processed the barium is conserved for its principal function of providing electron emissive oxide, increasing the life of the lamp and decreasing its discoloration and loss of phosphor emission. In life tests, very high output (VHO) lamps have been increased 40 percent in life, and their discoloration has been decreased by one-third.

Although barium oxide and tungsten reactions have been described as examples, other alkaline earth metal oxides and refractory metals will produce lamps with longer lives in comparison with prior lamps whose cathodes are processed at the same time as final filling, stem sealing and evacuation under conditions controlled by requirements of finishing rather than cathode processing. Although control by batch processing of cathode mounts has been described, continuous cathode processing on the lamp production line can be carried out under control conditions separated from the final sealing.

Thus, while one desirable embodiment of the invention has herein been disclosed by way of example, it is to be understood that the invention is broadly inclusive of any and all modifications falling within the terms of the appended claims.

We claim:

1. The method of making an emissive conductor for an arc discharge device which comprises:

coating a refractory metal conductor with a suspension of alkaline earth metal carbonate,

resistively heating the conductor in a confined chamber, thereby to convert the carbonate to metallic oxide and a gaseous carbon oxide, and to form an interface on said conductor between the conductor and metallic oxide, said interface comprising a reaction compound of the refractory metal and alkaline earth metal oxide,

characterized in that said heating is continued while producing a flow of inert gas past the conductor, and said inert gas flow is controlled at a slow rate to maintain gaseous carbon oxide adjacent the conductor until said gaseous carbon oxide is removed and said interface substantially entirely covers the coated conductor and forms a reaction barrier between said metallic oxide and refractory metal conductor,

thereby to prevent reduction and release of free alkaline earth metal by reaction with the refractory metal during heated operation of said conductor in an arc discharge device.

2. The method according to claim 1 wherein said conductor is selected from the group consisting of tungsten, tantalum and nickel.

3. The method according to claim 1 wherein said alkaline earth metal carbonate is selected from the group consisting of barium carbonate, calcium carbonate and strontium carbonate and mixtures thereof.

4. The method according to claim 1 wherein said conductor is selected from the group consisting of tungsten, tantalum and nickel, and said alkaline earth metal carbonate is selected from the group consisting of barium carbonate, calcium carbonate, strontinum carbonate and mixtures thereof.

References Cited UNITED STATES PATENTS 11/1927 Myers 1l7223X 4/1969 Weiss 117223 WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R.

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