EP0882307B1 - Sintering electrode - Google Patents

Description

Technical field

The invention is based on a sintered electrode according to the preamble of claim 1. It is a sintered electrode for high-pressure discharge lamps such as metal halide lamps or High-pressure sodium discharge lamps.

State of the art

DE-OS 42 06 909 describes a thermionically emitting cathode element known for vacuum electron tubes, which consist of spherical particles 5 to 90% of the total volume is produced with an average grain size of less than 1 µm the sintered electrode consist of unfilled, to the environment open pores. The distances between neighboring particles (grains) are smaller than 1 µm.

From IEE Proceedings-1 / Solid-state and Electron Devices, Vol. 128, No. 1, February 1981, pages 19-32, XP002062073 is a thermionically emitting cathode known, which comprises a sintered body with porosity of about 20% The sintered body consists of tungsten powder with a grain size distribution from 2 to 14 µm. The use of a very narrow grain size distribution is considered proposed a way to achieve controlled porosity.

From US-A 2 721 372 a dispenser cathode is known which has a sintered body made of porous tungsten, which reaches 83% of theoretical density It is made from tungsten powder, the grains of which have a radius in the area May have 1.5 to 6.1 microns.

A sintered electrode is known from US Pat. No. 3,244,929 which, in addition to tungsten Proportions of emitter material such as oxides of aluminum, barium, calcium or Thorium contains The sintered body sits on a solid solid core pin Material.

From US-A 5 418 070 a cathode is known which consists of a porous Tungsten matrix exists, in whose pores emitter material is built Pores of the sintered matrix are filled with liquid Copper filled which will later be removed. The disadvantage This method is that the pores are irregular in shape and their properties are undefined. Manufacturing is complicated and time consuming.

DD-PS 292 764 is a cermet sintered body consisting of a mixture known from tungsten and thorium oxide or alkaline earth oxide, in which the porosity of the sintered body through the defined use of a binder is controlled during production. The particle size of the cermet powder is 80 to 550 µm.

A major problem with known sintered electrodes is that their porosity does not remain constant over the lifespan because of the sintering process the high temperature load during the operation of the electrode progresses. Therefore, such lamps have poor maintenance during the lifespan.

Because of this serious disadvantage, sintered electrodes have so far been used for high-pressure lamps cannot prevail across the board. Rather, it was up to now instructed to use spiral electrodes with a core pin made of thoriated tungsten or pin electrodes made of thoriated tungsten. The production So far, each was made of compact solid material.

Presentation of the invention

It is an object of the present invention to provide a sintered electrode according to the Provide preamble of claim 1 which dispenses with thorium and which achieves a longer lifespan and less bow unrest shows

This object is achieved by the characterizing features of claim 1 solved Particularly advantageous configurations can be found in the dependent Claims.

The sintered electrode according to the invention for high-pressure discharge lamps consists of a sintered body made of one of the high-melting metals Tungsten, tantalum, osmium, iridium, molybdenum or rhenium or an alloy of these metals. In addition, a metal or alloy known oxidic doping (up to 5% by weight) can be added, for example an oxide of lanthanum or yttrium.

The sintered body is made of a metallic powder, essentially spherical or the alloy, the average grain size between 2 and 100 microns, the grain size distribution by a maximum of 20% the mean fluctuates and is between 10 and 40 vol .-% of the total volume the sintered electrode consists of pores open to the environment

The pores can be unfilled or contain emitter additives. typical Emitter additives are oxides of alkaline earth, for example barium, calcium, Strontium and mixtures thereof. Aluminates are also suitable Oxides of hafnium or zirconium or rare earth metals (in particular Sc, Y, La, Ce, Nd, Gd, Dy and Yb).

The average grain size of the spherical powder is preferably between 5 and 70 µm.

In a particularly preferred embodiment, the grain size distribution fluctuates by a maximum of 10% around the mean.

In particular, the sintered body is on a in a manner known per se Solid metal core pin attached A particular advantage is that connection techniques such as No soldering or welding can be. The mechanical connection is only by shrinking or sintering

The material of the sintered body and of the core pin is preferably essentially the same, for example pure tungsten. The sintered body can be unfilled or contain emitter additives (e.g. lanthanum oxide). Pure tungsten doped with potassium is also suitable for the core pin or a rhenium-tungsten alloy.

In particular, the electrode can do without thorium and is then radioactive.

The electrode according to the invention has a number of advantages:

The service life of the high-pressure discharge lamps equipped with it is lengthened, the increase in lamp lamp voltage is reduced and the Maintenance of the luminous flux significantly improved In addition, the blackening the wall of the discharge vessel is reduced when the lamps are in operation, there is a reduction in the restlessness of the arc and flickering In addition, the manufacture of the electrode is considerably simplified Compared to conventional electrodes, the electrode coil can be saved become.

a) Bereitstellen eines im wesentlichen sphärischen Metallpulvers aus einem der hochschmelzenden Metalle Wolfram, Tantal, Molybdän, Iridium, Osmium oder Rhenium oder einer Legierung dieser Metalle, wobei das Pulver folgende Eigenschaften besitzt

die mittlere Korngröße des Metallpulvers beträgt zwischen 2 und 100 µm;

die Korngrößenverteilung schwankt um maximal 20 % (typisch 10 %) um den Mittelwert; die dafür verwendeten sphärischen Partikel des Metallpulvers sind einkristallin;

b) Pressen des Pulvers; ein typischer Wert des dabei angewendeten Drucks ist 100 bis 400 MPa;

. c) Sintern des Preßlings bei einer Temperatur von etwa dem 0,6 bis 0,8-fachen der Schmelztemperatur des verwendeten Metalls (angegeben in Kelvin).

A particularly advantageous method for producing a sintered body according to claim 1 consists of the following method steps:

a) Providing an essentially spherical metal powder made of one of the refractory metals tungsten, tantalum, molybdenum, iridium, osmium or rhenium or an alloy of these metals, the powder having the following properties

the average grain size of the metal powder is between 2 and 100 µm;

the grain size distribution fluctuates by a maximum of 20% (typically 10%) around the mean value; the spherical particles of the metal powder used for this are single-crystalline;

b) pressing the powder; a typical value of the pressure used is 100 to 400 MPa;

, c) sintering the compact at a temperature of about 0.6 to 0.8 times the melting temperature of the metal used (specified in Kelvin).

The powder is single crystal. In process step b) Powder in particular be pressed around a core pin.

Method step c) can be preferred, for example, in the case of tungsten at temperatures from 2500 to 2800 K. In the event of of an alloy is the lowest with melting temperature melting component meant.

Due to the spherical shape of the metal powder, the result when filling Mold (die) favorable flow properties. This allows the pressing advantageously done without the addition of a binder. This saves an additional one Processing step and prevents possible contamination.

Another advantageous process is the metal injection molding process. This Technology is described in more detail in DE-OS 197 49 908. You can in modified form can also be used for the present invention. The course of the procedure can be summarized as follows: A suitable one Metal powder is mixed with so much plastic (the so-called binder) that this starting material, which is in the form of granules, has the flow properties of the plastic and analogous to plastic injection molding can be further machined by placing it in an injection mold with the contour of the desired future component. Then a metallic one To obtain the component, the green body is removed from the injection mold; the binder is then removed by heat or by solvent removed from the so-called green body. This process is called Dewaxing. Then the component is made accordingly classic powder metallurgy sintered to a very high density component.

The essentially spherical metal powder is produced in known way, being rounded or almost exactly spherical Particles can arise. One example is the carbonyl process (New Types of Metal Powders, Ed. H. Hausner, Gordon and Breach Science Publishers, New York 1963, published in the series Metallurgical Society Conferences as volume 23). Particularly good results are obtained with single crystal Metal powder achieved.

The spherical powder grains of homogeneous size develop during sintering Equilibrium surfaces in the form of polyhedra. For example, it is around [110] or [111] faces. Surprisingly, it turned out that these polyhedron surfaces do not sinter further, so that the porosity of this new sintered body is practically constant over the service life remains. It is a so-called sponge body with an open Porosity.

The mode of operation of the sintered body is explained in more detail below using an example in which the sintered body is produced from pure (that is to say ThO _2- free) tungsten.

The starting material is spherical W powder with a diameter that is as uniform as possible, i.e. with a narrow distribution width of the grain size. This homogeneity of the powder ultimately results in great stability of the sintered body at high temperatures and leads to correspondingly stable conditions during the life of the lamp. In particular, the powder can be pressed directly around a ThO ₂ -free core pin. Sintering is then carried out at the relatively low temperature of around 2350 (± 100) ° C. This low temperature, which corresponds approximately to 0.7 times the melting temperature of the tungsten, means considerable energy savings compared to the usual sintering temperatures of 2800-3000 ° C for compact tungsten material.

Further emitter additives are not necessary in many applications, but can be introduced into the cavities or pores if necessary.

The residual porosity of the finished sintered sponge electrode can be targeted can be set via the ball size of the starting material. Preferably are ball sizes from 5 to 70 µm for the sponge electrode used. A residual porosity of about 15 to 30% by volume can thus be achieved.

The special advantages of the sponge electrode in the lamp are in the listed the following:

With an electrode according to the invention, the discharge starts at a large one Area. The point-like approach known from conventional electrodes, who often there at very high temperatures and for hiking of the focal spot is avoided. The temperature distribution on the whole sponge body is largely even. On the other hand, one conventional electrode has a high temperature gradient. It has in particular at the top a typically 500 K higher temperature than in rear part of the electrode.

After the lamp has ignited, the transition from glow to glow occurs Arc discharge faster when using the sintered electrode than when conventional solid electrode because the heat dissipation from the top the electrode in the direction of crushing due to the small contact area between the sintered grains of the sintered body is greatly reduced.

With the sponge electrode, especially in the vertical operating position, better heating of the area of the discharge vessel close to the pinch reached. The cause is the larger surface of the electrode, that emits more light. Therefore, any reflective coating may be present the piston ends are dimensioned smaller or omitted entirely, whereby a higher luminous flux is achieved.

characters

Figur 1

eine Sinterelektrode, im Schnitt

Figur 2

eine Metallhalogenidlampe mit Sinterelektrode

In the following, the invention will be explained in more detail using an exemplary embodiment. Show it:

Figure 1

a sintered electrode, on average

Figure 2

a metal halide lamp with a sintered electrode

Description of the drawings

The sintered electrode 1 shown in FIG. 1 for a 150 W lamp consists of a cylindrical sintered body 2, in its half facing away from the discharge a solid core pin 5 made of tungsten is pressed axially. The sintered body 2 consists of tungsten, which is made of spherical metal powder with a medium Grain size of 10 microns is made. The grain size distribution fluctuates by 10% around the mean. The residual porosity is approximately 15% by volume.

The diameter of the core pin is approximately 0.5 mm, the outer diameter the sintered body is approximately 1.5 mm.

2 shows an example of a metal halide lamp 9 with a Power of 150 W. It consists of a quartz glass vessel 10, which is a metal halide filling contains. There are external power supplies at both ends 11 and molybdenum foils 12 embedded in bruises 13. On The core pins 5 of the electrodes 1 are fastened to the molybdenum foils 12. Latter protrude into the discharge vessel 10. The two ends of the outlet tion vessel are each with a heat reflective coating 14th made of zirconium oxide.

In another embodiment, the electrode consists of a sintered body, which is rounded on the discharge side or tapered to a point The sintered body consists of tungsten, while the pressed core pin is made of rhenium, rhenium-plated tungsten or molybdenum.

A particularly advantageous method for producing a sintered electrode according to the invention is based on the metal injection molding method known per se. The principle is explained in detail in DE-OS 197 49 908. An overview can be found in the article "Overview of Powder Injection Molding" by PJ Vervoort et al., In: Advanced Performance Materials 3 , pp. 121-151 (1996).

• Bereitstellen eines im wesentlichen sphärischen, einkristallinen, Metallpulvers aus hochschmelzendem Metall wie Wolfram, Tantal, Molybdän, Osmium, Iridium oder Rhenium oder einer Legierung dieser Metalle, wobei das Pulver folgende Eigenschaften besitzt:
• die mittlere Korngröße des Metallpulvers beträgt zwischen 2 und 100 µm;
• die Korngrößenverteilung schwankt um maximal 20 % um den Mittelwert;
• Herstellen einer Mischung (sog. "feedstock") aus Pulver und Binder (oft auch als "Wachs" bezeichnet) und evtl. Polymer;
• Spritzen der Mischung in eine Spritzgußform;
• chemisches und thermisches Entfernen des Binders ("Entwachsen", sog. "dewaxing")
• Sintern bei einer Temperatur von etwa.dem 0,6 bis 0,8-fachen der Schmelztemperatur des verwendeten Metalls.

The following process steps are used in detail for the sintered electrode according to the invention:

• Provision of an essentially spherical, single-crystalline, metal powder made of refractory metal such as tungsten, tantalum, molybdenum, osmium, iridium or rhenium or an alloy of these metals, the powder having the following properties:
• the average grain size of the metal powder is between 2 and 100 µm;
• the grain size distribution fluctuates by a maximum of 20% around the mean;
• Producing a mixture (so-called "feedstock") from powder and binder (often also referred to as "wax") and possibly polymer;
• Injecting the mixture into an injection mold;
• chemical and thermal removal of the binder ("dewaxing")
• Sintering at a temperature of about 0.6 to 0.8 times the melting temperature of the metal used.

In a particularly preferred embodiment, the mixture in the Injection mold injected around a core pin and with this during sintering connected.

Such electrodes show a much better life behavior. Studies on metal halide lamps with 150 W output show that maintenance of the luminous flux after 1000 hours of use of metal powders with a grain size of 5 or 20 µm to 95% each of the initial luminous flux. In comparison, the state of the Technology (conventional stick electrode made of doped tungsten material) Luminous flux drops to values between 83 and 90% after 1000 hours to observe.

Claims (12)

1. Sintered electrode (1) for high-pressure discharge lamps, which comprises a sintered body (2), where the sintered body (2) has a cylindrical external contour and has been produced from an essentially spherical powder of one of the high-melting metals tungsten, tantalum, osmium, iridium, molybdenum or rhenium or an alloy of these metals, where the mean particle size of the powder is from 2 to 100 µm, and from 10 to 40% by volume of the total volume of the sintered electrode consists of pores which are open to the surroundings, characterized in that the sintered body (2) is made up homogeneously of single-crystalline powder and the particle size distribution of the powder covers a range from at most 20% below to at most 20% above the mean.
2. Sintered electrode according to Claim 1, characterized in that the pores are unfilled or contain emitter additives.
3. Sintered electrode according to Claim 1, characterized in that the mean particle size is from 5 to 70 µm.
4. Sintered electrode according to any of the preceding claims, characterized in that the particle size distribution covers a range from at most 10% below to at most 10% above the mean.
5. Sintered electrode according to Claim 1, characterized in that the sintered body (2) is fixed on a core pin (5) of solid metal.
6. Sintered electrode according to Claim 5, characterized in that the material of the sintered body (2) and the core pin (5) is the same.
7. Sintered electrode according to Claim 1, characterized in that the metal contains up to 5% by weight of dopants.
8. Process for producing a sintered electrode according to Claim 1, comprising the following process steps:

   provision of an essentially spherical, monocrystalline, metal powder of one of the high-melting metals tungsten, tantalum, molybdenum, osmium, iridium or rhenium or an alloy of these metals, where the powder has the following properties:

   the mean particle size of the metal powder is from 2 to 100 µm;

   the particle size distribution covers a range from at most 20% below to at most 20% above the mean;

   pressing the powder;

   sintering at a temperature of 0.6 to 0.8 times the melting point of the metal used.
9. Process according to Claim 8, characterized in that the powder is pressed around a core pin (5) and is joined to the latter during sintering.
10. Process according to Claim 8, characterized in that pressing is carried out without addition of a binder.
11. Process for producing a sintered electrode according to Claim 1, comprising the following process steps:

    provision of an essentially spherical, monocrystalline, metal powder of one of the high-melting metals tungsten, tantalum, molybdenum, osmium, iridium or rhenium or an alloy of these metals, where the powder has the following properties:

    the mean particle size of the metal powder is from 2 to 100 µm;

    the particle size distribution covers a range from at most 20% below to at most 20% above the mean;

    preparation of a mixture, often referred to as feedstock, of powder and binder, often also referred to as wax;

    injection of the feedstock into an injection moulding tool;

    chemical and thermal removal of the binder;

    sintering at a temperature of from 0.6 to 0.8 times the melting point of the metal used.
12. Process according to Claim 11, characterized in that the mixture is injected around a core pin in the injection moulding tool and is joined to this core pin during sintering.

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