EP0299230B1 - Cathode for a high-pressure discharge lamp - Google Patents

Cathode for a high-pressure discharge lamp Download PDF

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Publication number
EP0299230B1
EP0299230B1 EP88109786A EP88109786A EP0299230B1 EP 0299230 B1 EP0299230 B1 EP 0299230B1 EP 88109786 A EP88109786 A EP 88109786A EP 88109786 A EP88109786 A EP 88109786A EP 0299230 B1 EP0299230 B1 EP 0299230B1
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EP
European Patent Office
Prior art keywords
cathode
tip
cone
carbide layer
carbide
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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EP88109786A
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German (de)
French (fr)
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EP0299230A1 (en
Inventor
Wolfgang Dr. Pabst
Manfred Dr. Rehmet
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Osram GmbH
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Patent Treuhand Gesellschaft fuer Elektrische Gluehlampen mbH
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J61/00—Gas-discharge or vapour-discharge lamps
    • H01J61/02—Details
    • H01J61/04—Electrodes; Screens; Shields
    • H01J61/06—Main electrodes
    • H01J61/073—Main electrodes for high-pressure discharge lamps
    • H01J61/0735—Main electrodes for high-pressure discharge lamps characterised by the material of the electrode

Definitions

  • the invention relates to a cathode for a high-pressure discharge lamp according to the preamble of claim 1.
  • an electrode for high-pressure discharge lamps with gas or steam filling is known, which is made of a thorium dioxide-containing tungsten rod. Since these lamps are preferably used in devices with an optical beam path, the problems of arc disturbance and intensity fluctuation, associated with the premature destruction of the tip of the electrode serving as the cathode, occur more frequently than electrodes for electron tube construction. In recent years, increased demands have been placed on such lamps in this regard. Among other things this is due to the development of new areas of application.
  • the invention has for its object to reduce the intensity fluctuations of the arc, to contain the uneven arc and to prevent premature destruction of the cathode tip.
  • a region at the tip of the cathode can be particularly advantageously completely free of carbide.
  • Such cathodes are used in short-arc lamps (high-pressure xenon lamps and high-pressure mercury lamps), which may have temperatures in the area of the cathode tip which exceed the melting temperature of tungsten carbide (2710 ° C.). If tungsten carbide were present in this area, this would lead to a partial melting of the tip. The result would be a difficult post-diffusion of thorium and an increase in the work function, combined with an increase in the uneven arch.
  • GB-PS 929 668 and DE-OS 32 05 746 are generally concerned with electrodes for electron tube construction.
  • These electrodes consist of a high-melting material, usually tungsten, which is doped with an electron-emitting material, usually Th0 2 .
  • the proportion of Th0 2 can vary within wide limits (0.1-5% by weight) depending on the application.
  • elemental emitter material is formed due to the high temperature, which migrates to the surface preferably by diffusion along the grain boundaries. This process is crucial for the quality of the electrode and can be influenced by various measures.
  • the grain structure can also be changed by further doping (e.g. potassium, aluminum) so that the grain boundary diffusion is further facilitated.
  • further doping e.g. potassium, aluminum
  • FIG. 1 schematically shows a xenon short arc lamp 1 operated with direct current and having a low wattage (e.g. 150 W), which e.g. is used as a projection light source and in spectrophotometers and color reproduction devices.
  • the elliptical discharge vessel 2 made of quartz glass is filled with xenon (operating pressure approx. 50 bar).
  • the anode 3 and the cathode 4 are arranged axially in the discharge vessel at a distance of approximately 2 mm from one another.
  • Each electrode has a shaft 5.
  • the electrical supply takes place in a known manner via molybdenum foils 6, which are connected to the metal sleeve bases 7 via pins.
  • the molybdenum foils 6 are sealed in a vacuum-tight manner in the two ends of the discharge vessel 2.
  • another technique e.g. Rod melting or cup melting can be used.
  • the anode 3 is made as a solid cylinder block from hammered tungsten and has a wide, slightly bevelled end face.
  • the comparatively small cathode 4 is made of tungsten which is doped with 0.4% by weight Th0 2 . It is shown enlarged in FIG. 2 (but not to scale).
  • the cylindrical base body 8 of the cathode 4 tapers in the manner of a cone 9, the tip 10 of which is truncated.
  • the cone forms an opening angle a of 25 ° and has an overall length of approximately 4 mm.
  • the cone 9 is surrounded by a layer 11 of tungsten carbide over two thirds of its length. The layer thickness is approximately 10 ⁇ m.
  • the remaining third of the cone length is kept free of carbide.
  • the minimum width of the free zone at the cathode tip is 0.7 mm. This minimum width is essentially determined by the temperature distribution at the cathode tip. This free zone ensures that the tip cannot melt due to the lower melting temperature of the tungsten carbide layer compared to tungsten.
  • the carbide layer is produced by separating carbon from a carbon-containing gas, for example CH 4 (CVD process). To achieve a layer thickness of 10 ⁇ m, egg Maintain a gas flow of approx. 1 l / min over 10 minutes at 2100 ° C. The zone to be kept clear at the top of the cone is covered by depressions in the batch carrier. In this process, the cylindrical body is also partially covered with a carbide layer. However, this is meaningless for the essence of the invention.
  • a carbon-containing gas for example CH 4 (CVD process
  • lamps which were equipped with these cathodes were able to keep the luminance fluctuation caused by the uneven arc below 4% and the intensity drift occurring in continuous operation below 1% per hour. Premature failures due to melting of the tip were not observed.
  • the entire cone is covered by a carbide layer, the thickness of the layer continuously decreasing from the base to the tip of the cone. This can be done by dipping, brushing, spraying or the like. achieve, with adequate measures (drainage, etching) ensuring sufficient thinning towards the tip.
  • the shape of the cathode can be designed differently; e.g. can instead of a cone a hemisphere or similar be used.

Abstract

1. Cathode for a high-pressure discharge lamp composed of a refractory metal which is doped with an electron-emitting material and, optionally, further additives, which cathode comprises a cylindrical body (8) which is conically tapered on the discharge side, the tapering region (9) being outwardly coated with a carbide layer (11), characterized in that the thickness of the carbide layer (11) decreases in the direction of the tip (10) of the cone (9).

Description

Die Erfindung betrifft eine Kathode für eine Hochdruckentladungslampe nach dem Oberbegriff des Anspruchs 1.The invention relates to a cathode for a high-pressure discharge lamp according to the preamble of claim 1.

Aus der DE-PS 1 088 155 ist eine Elektrode für Hochdruckentladungslampen mit Gas- oder Dampffüllung bekannt, die aus einem thoriumdioxidhaltigen Wolframstab gefertigt ist. Da diese Lampen vorzugsweise in Geräten mit optischem Strahlengang verwendet werden, treten im Vergleich zu Elektroden für den Elektronenröhrenbau verstärkt die Probleme der Bogenunruhe und der Intensitätsschwankung auf, verbunden mit der vorzeitigen Zerstörung der Spitze der als Kathode dienenden Elektrode. In den letzten Jahren werden in dieser Hinsicht erhöhte Anforderungen an derartige Lampen gestellt. U.a. ist dies auf die Erschließung neuer Anwendungsgebiete zurückzuführen.From DE-PS 1 088 155 an electrode for high-pressure discharge lamps with gas or steam filling is known, which is made of a thorium dioxide-containing tungsten rod. Since these lamps are preferably used in devices with an optical beam path, the problems of arc disturbance and intensity fluctuation, associated with the premature destruction of the tip of the electrode serving as the cathode, occur more frequently than electrodes for electron tube construction. In recent years, increased demands have been placed on such lamps in this regard. Among other things this is due to the development of new areas of application.

Der Erfindung liegt die Aufgabe zugrunde, die Intensitätsschwankungen des Lichtbogens zu mindem, die Bogenunruhe einzudämmen und eine vorzeitige Zerstörung der Kathodenspitze zu verhindern.The invention has for its object to reduce the intensity fluctuations of the arc, to contain the uneven arc and to prevent premature destruction of the cathode tip.

Diese Aufgabe wird bei einer Kathode für Hochdruckentladungslampen durch das kennzeichnende Merkmal des Anspruchs 1 gelöst.This object is achieved in a cathode for high-pressure discharge lamps by the characterizing feature of claim 1.

Die mit der Erfindung erzielten Vorteile bestehen insbesondere in einer verbesserten Stabilität des Lichtbogens und in einer erhöhten Lebensdauer.The advantages achieved with the invention consist in particular in an improved stability of the arc and in an increased service life.

Weitere Ausgestaltungen der Erfindung sind in den kennzeichnenden Merkmalen der abhängigen Ansprüche angegeben.Further refinements of the invention are specified in the characterizing features of the dependent claims.

Besonders vorteilhaft kann ein Bereich an der Spitze der Kathode völlig frei von Carbid sein. Solche Kathoden werden bei Kurzbogenlampen (Xenonhochdrucklampen und Quecksilberhochdrucklampen) verwendet, bei denen u.U. im Bereich der Kathodenspitze Temperaturen auftreten, die die Schmelztemperatur von Wolframcarbid (2710 °C) übersteigen. Wäre in diesem Bereich Wolframcarbid vorhanden, würde dies zu einem teilweisen Aufschmelzen der Spitze führen. Die Folge wäre eine erschwerte Nachdiffusion von Thorium und ein Ansteigen der Austrittsarbeit, verbunden mit einer Erhöhung der Bogenunruhe.A region at the tip of the cathode can be particularly advantageously completely free of carbide. Such cathodes are used in short-arc lamps (high-pressure xenon lamps and high-pressure mercury lamps), which may have temperatures in the area of the cathode tip which exceed the melting temperature of tungsten carbide (2710 ° C.). If tungsten carbide were present in this area, this would lead to a partial melting of the tip. The result would be a difficult post-diffusion of thorium and an increase in the work function, combined with an increase in the uneven arch.

Als Grundlage zum besseren Verständnis der Wirkungsweise der Erfindung wird auf die GB-PS 929 668 und die DE-OS 32 05 746 verwiesen, die sich allgemein mit Elektroden für den Elektronenröhrenbau beschäftigen. Diese Elektroden bestehen aus einem hochschmelzenden Material, in der Regel Wolfram, das mit einem elektronenemittierenden Material, meist Th02, dotiert ist. Der Anteil des Th02 kann je nach Anwendungszweck in weiten Grenzen (0,1 - 5 Gew.-%) variieren. Beim Betrieb der Lampe wird aufgrund der hohen Temperatur elementares Emittermaterial gebildet, das bevorzugt durch Diffusion entlang der Komgrenzen an die Oberfläche wandert. Dieser Prozeß ist entscheidend für die Qualität der Elektrode und kann durch verschiedene Maßnahmen beeinflußt werden.As a basis for a better understanding of the operation of the invention, reference is made to GB-PS 929 668 and DE-OS 32 05 746, which are generally concerned with electrodes for electron tube construction. These electrodes consist of a high-melting material, usually tungsten, which is doped with an electron-emitting material, usually Th0 2 . The proportion of Th0 2 can vary within wide limits (0.1-5% by weight) depending on the application. During operation of the lamp, elemental emitter material is formed due to the high temperature, which migrates to the surface preferably by diffusion along the grain boundaries. This process is crucial for the quality of the electrode and can be influenced by various measures.

Durch weitere Dotierungen (z.B. Kalium, Aluminium) kann das Komgefüge zusätzlich so verändert werden, daß die Korngrenzendiffusion weiter erleichtert wird.The grain structure can also be changed by further doping (e.g. potassium, aluminum) so that the grain boundary diffusion is further facilitated.

Darüber hinaus ist es bekannt, den Metallkörper mit Kohlenstoff zu dotieren, um die Reduktion des Emittermaterials zu erleichtern. Weiterhin kann auch eine äußere Carbidschicht auf den Metallkörper aufgetragen werden, wobei die hohe Diffusionsrate des Kohlenstoffs ein Eindringen in den Metallkörper sicherstellt (G.H. Gessinger, Ch. Buxbaum, Mater. Sci. Res. 1Q (1975), S. 295 ff.).In addition, it is known to dope the metal body with carbon in order to facilitate the reduction of the emitter material. Furthermore, an outer carbide layer can also be applied to the metal body, the high rate of diffusion of carbon ensuring penetration into the metal body (G.H. Gessinger, Ch. Buxbaum, Mater. Sci. Res. 1Q (1975), p. 295 ff.).

Die Erfindung wird anhand eines Ausführungsbeispiels näher beschrieben. Es zeigt

  • Figur 1 eine Xenonkurzbogenlampe
  • Figur 2 eine besonders bevorzugte Ausführungsform einer Kathode
The invention is described in more detail using an exemplary embodiment. It shows
  • Figure 1 shows a xenon short arc lamp
  • Figure 2 shows a particularly preferred embodiment of a cathode

In Figur 1 ist schematisch eine mit Gleichstrom betriebene Xenonkurzbogenlampe 1 niedriger Wattstufe (z.B. 150 W) gezeigt, die z.B. als Projektionslichtquelle und in Spektralphotometern und Farbreproduktionsgeräten Verwendung findet. Das elliptische Entladungsgefäß 2 aus Quarzglas ist mit Xenon (Betriebsdruck ca. 50 bar) gefüllt. Im Entladungsgefäß sind die Anode 3 und die Kathode 4 in einem Abstand von ca. 2 mm zueinander axial angeordnet. Jede Elektrode weist einen Schaft 5 auf. Die elektrische Zuleitung erfolgt in bekannter Weise über Molybdänfolien 6, die über Stifte mit den metallischen Hülsensockeln 7 verbunden sind. Die Molybdänfolien 6 sind vakuumdicht in die beiden Enden des Entladungsgefäßes 2 eingeschmolzen. Statt einer Einschmelzung mit Molybdänfolien kann auch eine andere Tecknik, z.B. Stabeinschmelzung oder Bechereinschmelzung, verwendet werden.FIG. 1 schematically shows a xenon short arc lamp 1 operated with direct current and having a low wattage (e.g. 150 W), which e.g. is used as a projection light source and in spectrophotometers and color reproduction devices. The elliptical discharge vessel 2 made of quartz glass is filled with xenon (operating pressure approx. 50 bar). The anode 3 and the cathode 4 are arranged axially in the discharge vessel at a distance of approximately 2 mm from one another. Each electrode has a shaft 5. The electrical supply takes place in a known manner via molybdenum foils 6, which are connected to the metal sleeve bases 7 via pins. The molybdenum foils 6 are sealed in a vacuum-tight manner in the two ends of the discharge vessel 2. Instead of melting with molybdenum foils, another technique, e.g. Rod melting or cup melting can be used.

Die Anode 3 ist als massiver Zylinderblock aus gehämmertem Wolfram gefertigt und weist eine breite, außen leicht angeschrägte Stirnfläche auf.The anode 3 is made as a solid cylinder block from hammered tungsten and has a wide, slightly bevelled end face.

Die vergleichsweise kleine Kathode 4 ist aus Wolfram gefertigt, das mit 0,4 Gew.-% Th02 dotiert ist. Sie ist in Figur 2 vergrößert (jedoch nicht maßstäblich) wiedergegeben. Um eine hohe Bogenstabilität - zu sichern, verjüngt sich der zylindrische Grundkörper 8 der Kathode 4 (Durchmesser ca. 2 mm) nach Art eines Kegels 9, dessen Spitze 10 abgestumpft ist. Der Kegel bildet einen Offnungswinkel a von 25° und weist eine Gesamtlänge von etwa 4 mm auf. Der Kegel 9 ist, ausgehend vom Grundkörper 8, auf zwei Drittel seiner Länge von einer Schicht 11 aus Wolframcarbid umgeben. Die Schichtdicke beträgt etwa 10 µm. Zur Spitze 10 hin ist das restliche Drittel der Kegellänge (eine ca. 1,3 mm breite Zone) frei von Carbid gehalten.The comparatively small cathode 4 is made of tungsten which is doped with 0.4% by weight Th0 2 . It is shown enlarged in FIG. 2 (but not to scale). In order to ensure high arc stability, the cylindrical base body 8 of the cathode 4 (diameter approx. 2 mm) tapers in the manner of a cone 9, the tip 10 of which is truncated. The cone forms an opening angle a of 25 ° and has an overall length of approximately 4 mm. Starting from the base body 8, the cone 9 is surrounded by a layer 11 of tungsten carbide over two thirds of its length. The layer thickness is approximately 10 µm. Towards the tip 10, the remaining third of the cone length (an approximately 1.3 mm wide zone) is kept free of carbide.

In der gezeigten Konfiguration beträgt die Mindestbreite der freien Zone an der Kathodenspitze 0,7 mm. Diese Mindestbreite ist im wesentlichen durch die Temperaturverteilung an der Kathodenspitze bestimmt. Durch diese freie Zone wird sichergestellt, daß keine Aufschmelzung der Spitze durch die im Vergleich zu Wolfram niedrigere Schmelztemperatur der Wolframcarbidschicht erfolgen kann.In the configuration shown, the minimum width of the free zone at the cathode tip is 0.7 mm. This minimum width is essentially determined by the temperature distribution at the cathode tip. This free zone ensures that the tip cannot melt due to the lower melting temperature of the tungsten carbide layer compared to tungsten.

Die Herstellung der Carbidschicht erfolgt durch Abschiedung von Kohlenstoff aus einem kohlenstoffhaitigen Gas, z.B. CH4 (CVD-Verfahren). Zum Erreichen einer Schichtdicke von 10 µm wird eine Gasdurchfluß von ca. 1 I/min über 10 Minuten bei 2100 °C aufrechterhalten. Die freizuhaltende Zone an der Spitze des Kegels wird dabei durch Vertiefungen im Chargenträger abgedeckt. Bei diesem Verfahren wird auch der zylindrische Körper teilweise mit einer Carbidschicht überzogen. Dies ist für das Wesen der Erfindung jedoch bedeutungslos.The carbide layer is produced by separating carbon from a carbon-containing gas, for example CH 4 (CVD process). To achieve a layer thickness of 10 µm, egg Maintain a gas flow of approx. 1 l / min over 10 minutes at 2100 ° C. The zone to be kept clear at the top of the cone is covered by depressions in the batch carrier. In this process, the cylindrical body is also partially covered with a carbide layer. However, this is meaningless for the essence of the invention.

Während einer Betriebsdauer von 1000 Stunden konnte bei Lampen, die mit diesen Kathoden bestückt waren, die durch die Bogenunruhe bedingte Leuchtdichteschwankung unter 4 % und die im kontinuierlichen Betrieb auftretende Intensitätsdrift unter 1 % pro Std. gehalten werden. Vorzeitige Ausfälle durch Aufschmelzen der Spitze wurden nicht beobachtet.During an operating period of 1000 hours, lamps which were equipped with these cathodes were able to keep the luminance fluctuation caused by the uneven arc below 4% and the intensity drift occurring in continuous operation below 1% per hour. Premature failures due to melting of the tip were not observed.

Bei einer anderen Ausführungsform der Erfindung ist der gesamte Kegel von einer Carbidschicht überzogen, wobei die Dicke der Schicht von der Basis zur Spitze des Kegels hin kontinuierlich abnimmt. Dies läßt sich durch Tauchverfahren, Pinseln, Sprühen o.ä. erreichen, wobei durch geeignete Maßnahmen (Ablaufen, Ätzen) eine ausreichende Verdünnung zur Spitze hin sichergestellt wird.In another embodiment of the invention, the entire cone is covered by a carbide layer, the thickness of the layer continuously decreasing from the base to the tip of the cone. This can be done by dipping, brushing, spraying or the like. achieve, with adequate measures (drainage, etching) ensuring sufficient thinning towards the tip.

Die Erfindung ist nicht auf die gezeigten Ausführungsbeispieie beschränkt. Insbesondere kann die Form der Kathode anders gestaltet sein; z.B. kann statt eines Kegels eine Halbkugel o.ä. verwendet werden.The invention is not restricted to the exemplary embodiments shown. In particular, the shape of the cathode can be designed differently; e.g. can instead of a cone a hemisphere or similar be used.

Claims (5)

1. Cathode for a high-pressure discharge lamp composed of a refractory metal which is doped with an electron-emitting material and, optionally, further additives, which cathode comprises a cylindrical body (8) which is conically tapered on the discharge side, the tapering region (9) being outwardly coated with a carbide layer (11), characterized in that the thickness of the carbide layer (11) decreases in the direction of the tip (10) of the cone (9).
2. Cathode according to Claim 1, characterized in that a region at the tip of the cone (9) is free of carbide.
3. Cathode according to Claim 2, characterized in that the carbide-free region at the tip of the cone comprises about one third of the total cone length.
4. Cathode according to Claim 1, characterized in that the thickness of the carbide layer decreases continuously in the direction of the tip.
5. Cathode according to Claim 1, characterized in that tungsten is used as refractory metal and thorium dioxide as electron-emitting material.
EP88109786A 1987-07-14 1988-06-20 Cathode for a high-pressure discharge lamp Expired - Lifetime EP0299230B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE19873723271 DE3723271A1 (en) 1987-07-14 1987-07-14 CATHODE FOR A HIGH PRESSURE DISCHARGE LAMP
DE3723271 1987-07-14

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EP0299230A1 EP0299230A1 (en) 1989-01-18
EP0299230B1 true EP0299230B1 (en) 1990-09-12

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EP (1) EP0299230B1 (en)
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DE (2) DE3723271A1 (en)

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DE29823366U1 (en) * 1998-08-06 1999-07-08 Patent Treuhand Ges Fuer Elektrische Gluehlampen Mbh Electrode for a high-pressure discharge lamp with a long service life
NL1010374C2 (en) * 1998-10-22 2000-04-26 Orc Manufacturing Co High pressure discharge lamp cathode has a locally carbonized conical front end portion
JP2000306546A (en) * 1999-04-21 2000-11-02 Ushio Inc Short arc discharge lamp
JP2000323091A (en) 1999-05-12 2000-11-24 Hamamatsu Photonics Kk Discharge lamp for light source
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JP3596453B2 (en) * 2000-09-28 2004-12-02 ウシオ電機株式会社 Short arc discharge lamp
JP3926211B2 (en) * 2002-05-29 2007-06-06 日本碍子株式会社 High pressure mercury lamp and sealing material for high pressure mercury lamp
JP4295527B2 (en) * 2003-02-27 2009-07-15 株式会社アライドマテリアル Discharge lamp and its electrode structure
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JP5293172B2 (en) * 2008-12-26 2013-09-18 ウシオ電機株式会社 Discharge lamp
JP5299132B2 (en) * 2009-07-07 2013-09-25 ウシオ電機株式会社 Xenon short arc lamp for digital projector
CN102366837A (en) * 2011-08-10 2012-03-07 厦门虹鹭钨钼工业有限公司 Method for manufacturing thorium tungsten-tungsten composite electrode used for high pressure gas discharge lamp
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DE3860599D1 (en) 1990-10-18
EP0299230A1 (en) 1989-01-18
JPH0586026B2 (en) 1993-12-09
DE3723271A1 (en) 1989-01-26
US4906895A (en) 1990-03-06
JPS6424355A (en) 1989-01-26

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