BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The invention relates to a cathode material for use in a high-pressure discharge lamp having the features of the preamble of the independent claim.

The invention further relates to a high-pressure discharge lamp which comprises a cathode composed of the cathode material of the invention and a process for producing a cathode material.

The task of the cathode of a high-pressure discharge lamp is to provide electrons at a sufficient current density. At typical current densities of high-pressure discharge lamps, this can be realized only at sufficiently high cathode temperatures, so that tungsten-based materials, in particular, have become established here because of the high melting point.

Failure of the high-pressure discharge lamp or a reduction in its performance frequently occurs because of changes in the morphology, structure and shape of the cathode and also an increase and/or local change in the electron work function during use of the cathode material. One of more of the following effects can occur:an increase in the cathode and/or anode temperature which due to evaporation of cathode and/or anode material leads to blackening of the bulb;a brief local change in the electric arc and, associated therewith, fluctuations in the light output (arc instability, also known as flickering);burning back of the cathode;change in shape of the cathode tip.

For high-power high-pressure discharge lamps, thoriated tungsten (W—ThO2) is still preferably used as cathode material since this additive significantly reduces the electron work function (from, depending on the grain orientation, 4.6-5.4 eV for pure tungsten to 2.4-3.0 eV for W—ThO2). However, since thorium is a radioactive element which emits alpha radiation, efforts have been made for decades to replace this material. These materials which reduce the electron work function, which are generally rare earth oxides, will hereinafter be referred to as “emitter materials”. The emitter materials are usually added as oxide. During operation, a monolayer of the element concerned in elemental form is formed on the cathode tip as a result of the high temperatures. The effect of reducing the electron work function can be attributed to this layer. If, in the context of the present patent application, the actual chemical element (e.g. thorium or lanthanum) is meant, reference is made to “emitter element”. “Emitter material”, on the other hand, refers to the form in which the element is introduced. Thus, for example, ThO2is an emitter material and thorium is the corresponding emitter element.

Alternative emitter materials to ThO2have the disadvantage that they generally have a lower boiling point than ThO2and the cathode surface rapidly becomes depleted in the emitter material which reduces the electron work function. The oxide which reduces the electron work function cannot diffuse sufficiently quickly out from the volume of the electrode. Consequently, arc instability and increased burning away occur. The cathode does not attain a long life.

Thus, EP 1 481 418 A1 describes a tungsten cathode material which contains La2O3and ZrO2or HfO2as emitter materials. However, melting of these oxides occurs in the region of the start of the electric arc because of their lower thermal stability compared to ThO2. This leads to a local change in the coverage of the cathode with the emitter material, which is in turn reflected in increased arc instability. In addition, the originally fine particles become coarser during use of the cathode as a result of, for example, penetration of melt along grain boundaries. The inhomogeneous distribution of the emitter material resulting therefrom can also lead to instability of the electric arc.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a cathode material having the following properties:thorium-free;arc instability similar to or less than in the case of thoriated tungsten;low evaporation of the cathode material and thus low blackening of the bulb;shape stability of the cathode and low burning back of the cathode.

Furthermore, a high-pressure discharge lamp which displays a high and constant light yield over a long period of use and a similarly low or lower arc instability as a lamp having a thoriated cathode should be provided.

The invention preferably provides high-pressure discharge lamps for projection applications, in particular cinema projection, and also high-pressure discharge lamps for lithography applications. The high-pressure discharge lamps used for these purposes encompass, for example, short-arc xenon lamps and short-arc mercury vapor lamps.

This object is achieved by the features of the independent claims. Advantageous embodiments are subject matter of the dependent claims.

A cathode material according to the invention contains:a matrix based on tungsten having a tungsten content of greater than or equal to 95% by weight,tungsten carbide,oxides and/or predominantly oxidic phases of at least one or more emitter elements from the group of (rare earth metals, Hf, Zr),

wherein the cathode material additionally contains predominantly carbidic phases of at least one or more emitter elements from the group of (rare earth metals, Hf, Zr).

In this context, “predominantly oxidic” means that the phase has a predominantly oxidic bonding character. The phase can deviate in respect of its composition from a nominal stoichiometry of an oxide. The same applies, mutatis mutandis, to the term “predominantly carbidic”.

The emitter elements are thus selected from among the elements scandium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, yttrium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and also hafnium and zirconium.

According to the invention, emitter elements are present both in an oxidic form and in a carbidic form in the microstructure.

The particular advantage of the presence of a carbidic phase of an emitter element is that reduction of an oxide of an emitter element by means of a carbide of an emitter element makes available increased molar amounts of the emitter element. When the reaction equation of the reduction of La2O3and W2C, namely
La2O3(s)+3W2C(s)→2La(s)+6W(s)+3CO(g),  (I)

is compared with the reaction equation of the reduction of La2O3by means of LaC2, namely
La2O3(s)+1.5LaC2(s)→3.5La(s)+3CO(g),  (II)

it is clear that an additional 1.5 mol of lanthanum compared to (I) are formed by the reduction of La2O3by means of LaC2, and this lanthanum is available as elemental lanthanum for diffusion into a surface region depleted in emitter element. It is also particularly advantageous that fewer moles of CO per mole of elemental lanthanum produced are formed in the reduction by means of a lanthanum carbide than in a reduction by means of W2C. Since CO as reactant inhibits the progress of the reaction and has to be removed, a reaction in which fewer moles of CO are formed per mole of lanthanum is more favorable for the reaction kinetics.

The principle is not restricted to lanthanum, but can instead also be applied to the other emitter elements mentioned. It is also not necessary for the emitter element present as oxide to be the same as the emitter element present as carbide.

Elemental emitter element is resupplied to the surface from the volume (bulk) of the cathode material by means of the above-described reduction mechanism, and a lamp equipped with a cathode composed of the cathode material according to the invention achieves a long life at a uniform burning-away behavior.

Emitter elements are preferably one or more rare earth metals.

The tungsten carbide is preferably present as W2C.

The proportion of tungsten carbide is preferably in the range from 0.1 to 4% by volume, particularly preferably from 0.5 to 2% by volume. The proportion of tungsten carbide in % by volume can be determined by means of quantitative analysis of the microstructure, as is described further below. A conversion into % by weight can be effected via the respective densities of the species. In a determination by chemical analysis, the content of tungsten carbide tends to be overestimated since part of the carbon is dissolved in the tungsten matrix.

The total carbon content of the cathode material is preferably 50-3000 μg/g, preferably 150-1500 μg/g and particularly preferably from 350 μg/g to 800 μg/g. At a carbon content below the limits indicated, a sufficient reducing action is not obtained; if the carbon content is higher, increased cathode deformation and/or increased blackening of the bulb is observed.

Furthermore, the proportion of the one or more emitter materials is preferably in the range from 0.5 to 5% by weight, preferably from 1.0 to 2.5% by weight and particularly preferably from 1.5 to 2% by weight.

The term emitter material is used to indicate the form in which the emitter element is introduced. The figures relate to the proportion by weight of the oxide of the emitter element when the latter is added as oxide. If different emitter materials are present, the figure relates to the proportion attained together. Indication of the proportion of emitter material in % by weight is particularly practical since the amount weighed in of the one or more emitter elements is effected via the respective emitter material.

It has been found that a very low solubility of the emitter element in the base material of the cathode material, generally tungsten, is desirable for sustained action of the emitter element during use. If the solubility is good, the emitter element diffuses into the volume of the cathode and is no longer available for formation of a monolayer on the cathode surface. The solubilities can be evaluated with the aid of the respective phase diagrams.

The emitter element which is present as oxide and/or predominantly oxidic phase is preferably, more preferably exclusively, formed by lanthanum.

Further preference is given to the emitter element which is present in predominantly carbidic form and/or as predominantly carbidic phase being formed by lanthanum.

Particular preference is given to both the emitter element which is present as oxide and/or predominantly oxidic phase and also the emitter element which is present in predominantly carbidic form and/or as predominantly carbidic phase being formed by lanthanum.

In other words, the emitter element is here formed by lanthanum which is present in oxidic and carbidic form in the cathode material.

Preference is given to the predominantly carbidic phases of one emitter element adjoining the oxidic phases of an emitter element. This results in particularly short diffusion paths in the reduction of the emitter oxide by the carbidic phase of the emitter element which takes place during operation.

Furthermore, it is preferred that the predominantly carbidic phases are configured as a shell or seam structure around particles of an emitter element present in oxidic form. In other words, any proportion of an emitter element which is present as predominantly carbidic phase is preferably present as shell or seam around the emitter elements present as oxide. This is particularly advantageous in respect of the diffusion paths for a reaction between the emitter elements present in carbidic form and oxidic form.

Particular preference is given to the shell or seam structure having an average thickness in the range from 0.01 to 1 μm, preferably from 0.05 to 0.8 μm, particularly preferably from 0.1 to 0.5 μm.

The cathode material preferably has a relative density of greater than or equal to 92%, preferably greater than or equal to 97%, particularly preferably greater than or equal to 99%. The relative density is the complement of the porosity. A relative density of, for example, 92% thus corresponds to a porosity of 8%. A high residual porosity results in a reduced thermal conductivity of the cathode material, which can lead to a rise in the peak temperature and premature failure of a lamp equipped therewith. In addition, the residual porosity decreases the strength and creep resistance of the cathode material at high temperature. The cathode material therefore particularly preferably has a relative density of greater than or equal to 99%, corresponding to a residual porosity of 1% or less.

Protection is also sought for a high-pressure discharge lamp containing a cathode composed of a cathode material according to any of the preceding claims.

Protection is also sought for a process for producing a cathode material, comprising the steps:production of a powder mixture containing tungsten powder, at least one emitter element from the group of (rare earth metals, Hf, Zr) and also at least one carbon source,pressing of the powder mixture,consolidation of the powder mixture in a consolidation step,

wherein the process contains at least the following steps:a diffusion step in the form of a heat treatment to bring about homogeneous distribution of carbon in the cathode material,a precipitation step in the form of cooling to provide a carbon depot adjacent to a phase containing at least one emitter element.

The at least one emitter element can be added as hydride, oxide, hydroxide or nitride. The form in which it is introduced can be pulverulent. Doping with a liquid is also possible.

As carbon source, it is possible to add, for example, tungsten carbide or carbon black. Doping in liquid form as a carbon-containing suspension is also conceivable.

Consolidation of the powder mixture refers to a step which leads to a stable bond in the powder mixture. One customary method is sintering. Alternative methods are hot isostatic pressing (HIP) or powder forging.

The diffusion step is carried out as a heat treatment (also “ignition treatment”) in which carbon goes into solution in the matrix formed by tungsten. The diffusion step can be carried out during the course of the consolidation step or be performed as a separate heat treatment step.

The diffusion step preferably takes place at temperatures of greater than or equal to 2200° C. but less than 3000° C. Below 2200° C., insufficient carbon goes into solution; above 3000° C., increased evaporation of carbon occurs.

In the following precipitation step in the form of cooling, the consolidated powder mixture which has been subjected to the diffusion step is cooled, as a result of which the solubility of carbon decreases and the carbon precipitates next to a phase containing at least one emitter element.

The precipitation step results in predominantly carbidic phases of the one or more emitter element being precipitated. The predominantly carbidic phases are formed as seam or shell around the emitter elements present in oxidic form. The precipitation step is carried out at a cooling rate of from 1 K/min to 500 K/min, preferably from 10 K/min to 100 K/min, particularly preferably from 20 K/min to 50 K/min.

Cooling rates in the range from 20 K/min to 50 K/m in have been found to be particularly advantageous for precipitation of carbidic emitter element around the emitter elements present in oxidic form.

