Cathode material

Abstract

A cathode material for use in a high-pressure discharge lamp contains a matrix based on tungsten having a tungsten content of greater than or equal to 95% by weight, tungsten carbide, and oxides and/or predominantly oxidic phases of one or more emitter elements from the group of rare earth metals, Hf, and Zr. The cathode material additionally contains predominantly carbidic phases of the one or more emitter elements from the group of rare earth metals, Hf, and Zr. A high-pressure discharge lamp would contain such a cathode composed of the above cathode material.

Claims

1. A cathode material for use in a high-pressure discharge lamp, the cathode material comprising: 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 first emitter element selected from the group consisting of rare earth metals, Hf, and Zr, wherein said predominantly carbidic phases being present as shell or seam structure around an oxide of said at least one first emitter element; and predominantly carbidic phases of at least one second emitter element selected from the group consisting of said rare earth metals, said Hf, and said Zr.

2. The cathode material according to claim 1, wherein said first and second emitter elements are said rare earth metals.

3. The cathode material according to claim 1, wherein said tungsten carbide is W.sub.2C.

4. The cathode material according to claim 1, wherein a proportion of said tungsten carbide is in a range from 0.1 to 4% by volume.

5. The cathode material according to claim 1, wherein a carbon content being 50-3000 μg/g.

6. The cathode material according to claim 1, wherein a proportion of emitter material, based on a proportion by weight when added as an oxide, is in a range from 0.5 to 5% by weight.

7. The cathode material according to claim 1, wherein said at least one first emitter element which is present as an oxide and/or predominantly an oxidic phase is formed by lanthanum.

8. The cathode material according to claim 1, wherein said at least one second emitter element which is present as a predominantly carbidic phase is present in a predominantly carbidic form and/or as the predominantly carbidic phase and is formed by lanthanum.

9. The cathode material according to claim 8, wherein both said first emitter element which is present as an oxide and/or a predominantly oxidic phase and also said second emitter element which is present in the predominantly carbidic form and/or as the predominantly carbidic phase is formed by lanthanum.

10. The cathode material according to claim 1, wherein said predominantly carbidic phases of said at least one second emitter element adjoin said predominantly oxidic phases of said at least one first emitter element.

11. The cathode material according to claim 1, wherein said shell or seam structure has an average thickness of from 0.01 to 1 μm.

12. The cathode material according to claim 1, wherein the cathode material has a relative density of greater than or equal to 92%.

13. A high-pressure discharge lamp, comprising: a cathode composed of a cathode material according to claim 1.

14. A process for producing a cathode material, which comprises the steps of: producing a powder mixture containing tungsten powder, at least one emitter element from the group consisting of rare earth metals, Hf and Zr and at least one carbon source; pressing of the powder mixture; consolidation of the powder mixture in a consolidation step; performing a diffusion step in a form of a heat treatment to bring about homogeneous distribution of carbon in the cathode material; and performing a precipitation step in a form of cooling to provide a carbon depot adjacent to a phase containing the at least one emitter element.

15. The process according to claim 14, wherein the diffusion step takes place at temperatures of greater than or equal to 2200° C. but less than 3000° C.

16. The process according to claim 14, wherein predominantly carbidic phases of the at least one emitter element is formed by the precipitation step.

17. The process according to claim 14, wherein the precipitation step is carried out at a cooling rate of from 1 K/min to 500 K/min.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) The invention is illustrated in detail below by means of figures and production examples. The figures here show:

(2) FIGS. 1a and 1b a secondary electron SEM (1a) and EBSD phase chart (1b) of a cathode material comprising tungsten+2% by weight of La.sub.2O.sub.3+800 μg/g of C

(3) FIG. 2 result of the X-ray diffraction measurement (XRD) on a cathode material comprising W+2% by weight of La.sub.2O.sub.3+800 μg/g of C

(4) FIGS. 3a-3d secondary electron image of a fracture surface (3a) and element distributions determined by means of Auger Electron Spectroscopy (AES): C (3b), La (3c) and W (3d), diffusion step at 2200° C.

(5) FIGS. 4a-4d secondary electron image of a fracture surface (4a) and element distributions determined by means of Auger Electron Spectroscopy (AES): C (4b), La (4c) and W (4d), diffusion step at 2700° C.

(6) FIG. 5a-5c schematic depictions of the mechanism of formation of carbide of the emitter element for the example of lanthanum

(7) FIG. 6 phase analysis from EDX

(8) FIG. 7 a schematic depiction of the microstructure

(9) FIG. 8 a schematic depiction of a high-pressure discharge lamp

(10) FIG. 9 a graph of the course of the temperature in production of the cathode material

DESCRIPTION OF THE INVENTION

(11) FIG. 1a shows a secondary electron scanning electron micrograph (SEM) of a cathode material comprising W+2% by weight of La.sub.2O.sub.3+800 μg/g of C. FIG. 1b shows the corresponding phase chart from electron backscatter diffraction (EBSD).

(12) In this working example of the cathode material, lanthanum has thus been selected as emitter element. The dark-gray phase in FIG. 1b is the tungsten matrix, the light-gray phase is W.sub.2C and the white phase is La.sub.2O.sub.3.

(13) It can be seen that the major part of the lanthanum oxide particles is removed in the preparation for use of the EBSD method. The phase chart shows that the material is made up not only of the W (tungsten) matrix and the lanthanum oxide particles but also of at least a carbidic phase (W.sub.2C).

(14) The average proportion by area of the W.sub.2C phase is 0.5% according to the analysis of 5 images. Assuming that the grains have no preferential orientation, the proportion by volume is equal to the proportion by area. The proportion by area of W.sub.2C of 0.5% corresponds to a proportion by mass of carbon of about 150 μg/g. As will be shown later, smaller amounts of carbon are also bound in seam-like accumulations right around the lanthanum oxide particles. It is assumed that the additional content of carbon has been forced to dissolve in the tungsten matrix.

(15) On the basis of this information, the theoretical density of the material can be estimated. The composition based on W and containing 0.5% by volume of W.sub.2C and 2% by weight of La.sub.2O.sub.3 gives, taking into account the density of the individual phases (W 19.3 g/cm.sup.3, La.sub.2O.sub.3 6.51 g/cm.sup.3, W.sub.2C 17.2 g/cm.sup.3), a theoretical density of 18.56 g/cm.sup.3. It may be noted that the effect of these small amounts of C on the density is relatively small. For comparison, the theoretical density of the material without addition of C (W+2% by weight of La.sub.2O.sub.3) is 18.57 g/cm.sup.3.

(16) FIG. 2 shows the result of an X-ray diffraction measurement (XRD) on the cathode material comprising W+2% by weight of La.sub.2O.sub.3+800 μg/g of C. The W.sub.2C phase was able to be confirmed by agreement with the peak positions and heights recorded in the data bank. In the legend for the figure, the respective peak positions of the phases W (light gray), La.sub.2O.sub.3 (dark gray) and W.sub.2C (black) are indicated.

(17) FIGS. 3a to 3d show scanning electron micrographs of a fracture surface of a cathode material. In FIG. 3a, an analysis based on secondary electrons of a fracture surface was carried out. In FIGS. 3b to 3c, element distributions determined by means of Auger Electron Spectroscopy (AES) were analyzed on the same section of the image: C (3b), La (3c) and W (3d).

(18) To interpret the images, it should be noted that light-colored regions correspond to a relatively high concentration of the respective element.

(19) It can be seen from FIG. 3b that the carbon preferentially accumulates in the vicinity of the lanthanum-containing particles.

(20) The cathode material of this working example was subjected to a diffusion step at 2200° C.

(21) FIGS. 4a to 4d show images analogous to FIGS. 3a to 3d but in this case the carbon material was subjected to a diffusion step at 2700° C. It can be seen from a comparison with FIGS. 3a-3d that after a heat treatment at higher temperature, the carbon has formed marked seams around the lanthanum-containing particles.

(22) In addition, it can be seen that a higher carbon concentration compared to FIG. 3 is present at the grain boundaries. Since the present figures show images of fracture surfaces of an intercrystalline fracture, the view is of the grain boundaries.

(23) FIGS. 5a to 5c schematically show the development of the microstructure in the cathode material due to the precipitation heat treatment (diffusion step and precipitation step). Beginning from the initial state comprising La.sub.2O.sub.3, W and W.sub.2C in FIG. 5a, the size of the W.sub.2C grains decreases at the maximum temperature (FIG. 5b) because of increased solubility of C in W. In FIG. 5b, the tungsten grains with dissolved carbon are denoted by W—C.sub.sol.

(24) FIG. 5c shows the microstructure after cooling:

(25) During cooling (precipitation step), the solubility of C in W decreases, which leads to precipitation at the grain boundaries. C is preferentially precipitated in the vicinity of the La.sub.2O.sub.3 grains, which is indicated by their white periphery in FIG. 5c. The carbon precipitated at grain boundaries (of La.sub.2O.sub.3 and of W) is denoted by C.sub.gb.

(26) It is not possible to draw any conclusions as to the grain size distribution or proportions by volume from this in-principle sketch.

(27) FIG. 6 shows, with the aid of a comparison of the profiles of the Auger electron emission over the energy spectrum which are characteristic of the different phases, that lanthanum is present in oxidic form, in carbidic form and in a mixed form (La with 0 and C).

(28) FIG. 7 schematically shows the microstructure after a heat treatment at 2700° C. A section comprising tungsten grains W with grain boundaries gb and a lanthanum oxide particle at a triple point is depicted. After the diffusion step at 2700° C., not only La.sub.2O.sub.3 but also a carbidic form of lanthanum (designated as La—carb.) is found. In addition, there are regions (denoted by La—ox.) in which the La.sub.2O.sub.3 has already been subjected to incipient reduction. The carbon accumulates, in particular, at the grain boundary triple points since the diffusion of carbon along grain boundaries is significantly faster than in the volume.

(29) FIG. 8 schematically shows a high-pressure discharge lamp 1 with a discharge vessel (bulb) 2. During operation, a discharge arc is formed between a cathode 3 and an anode 4. A high-pressure discharge lamp 1 having a cathode 3 composed of the cathode material of the invention is free of thorium and has an at least equally long life and a similarly low or lower arc instability as a lamp having a thoriated cathode.

(30) FIG. 9 schematically shows, by way of example, courses of the steps consolidation step K, diffusion step D, precipitation step A

(31) for producing the cathode material in a graph of temperature T over time t.

(32) In variant I, the consolidation step K and the diffusion step D are carried out in one step. In the variant II, the consolidation step K is carried out at a relatively low temperature and is followed by a separate diffusion step D at higher temperature.

(33) It is also conceivable for the consolidation step K and the diffusion step D to be separated in time and in space.

PRODUCTION EXAMPLES

(34) To produce the cathode material, the alloy constituents were used in the form of powders. Lanthanum was added in the form of lanthanum hydroxide, with a proportion by mass of 2.33% by weight being weighed in. The addition of C was effected in the form of flame black or as WC powder. The C content was varied in the range from 240 μg/g to 5800 μg/g in the working examples in order to examine the influence of this parameter on the burning behavior in the short electric arc lamp. The concentrations of carbon indicated are final contents in the finished cathode material. The powders were mixed in a conventional plowshare mixer.

(35) Different processes were employed for compaction of the powder mixture. One method is cold isostatic pressing (CIP). Here, the powder was introduced into the pressing tool consisting of a rubber hose and a metal cage, closed tightly and pressed at a pressure of 2000 bar. An alternative to compaction by means of the CIP process is hot pressing. Here, the powder was introduced into a cylindrical graphite mold and a pressure of 200 bar was applied at a temperature of 1000° C., which was attained by direct passage of electric current. This process took place in a protective gas atmosphere.

(36) Sintering of the compact is typically carried out in an H.sub.2 atmosphere. As an alternative, the use of hot pressing was demonstrated in further working examples. Here, the cylindrical compact was heated via the end faces with direct passage of electric current and sintered with application of pressure. This process took place in a protective gas atmosphere. In general, a temperature of greater than or equal to 2200° C. was employed for sintering. Alternative methods for further densification of the material were also demonstrated in the working examples. One possibility is hot isostatic pressing (HIP). A density close to the theoretical density is achieved thereby. As an alternative thereto, densification can be achieved by forming. In some working examples, flat forging of a cylindrical geometry was demonstrated. A density close to the theoretical density was achieved in this way, similarly to the HIP process.

(37) In the following working examples, compositions and relative densities of cathode materials were varied and the cathode materials were subsequently evaluated in lamp tests.

(38) The studies were carried out on mercury discharge lamps with lithography applications having a nominal power of 3.5 kW. To compare the performance of the various experimental lamps, the dimensional stability of the cathodes (measured by means of the plateau enlargement), the burning-back of the cathode and also the stability of the arc behavior (flickering) are examined. To assess flickering, the arc voltage U of the lamp was measured. The background here is that flickering of the electric arc is associated with a fluctuation in the arc voltage, with the latter being significantly easier to measure than the fluctuation in the light yield. The lamps were operated for a time of 1500 h (nominal life).

(39) Variation of the Carbon Content of the Cathode Material

(40) The results on 7 lamps A, B, C, D, E, F and G are described below. These are lamps having different cathode materials but are otherwise of identical construction. Lamp A contains a thoriated cathode (prior art) having a ThO.sub.2 content of 1.8% by weight. The cathode of the lamp B was made of the material WLZ (W+2.5% by weight of La.sub.2O.sub.3+0.07% by weight of ZrO.sub.2). Lamp C has the same construction as B but in the case of lamp C the WLZ cathode was carburized on the surface and the tip region (to 3 mm behind the plateau) was etched free.

(41) In the case of the lamps D, E, F and G, carbon was added to the bulk of the cathode material. The concentrations of carbon were 240 μg/g (D), 350 μg/g (E), 750 μg/g (F) and 5800 μg/g (G). Table 1 shows the results for these lamps in respect of flickering, dimensional stability and burning-back of the cathode.

(42) TABLE-US-00001 TABLE 1 C content and test results of the lamps A to G Diameter of the Electrode C content cathode plateau spacing in the cathode Flicker-free after 100 h after 1000 h material operating relative to relative to Lamp [μg/g] time [h] 0 h [%] 0 h [%] A — >1500 206 108 B — 40 382 124 C Surface 580 293 113 carburization D 240 50 500 h: 312 500 h: 117 E 350 >1500 160 107 F 750 >1500 170 109 G 5800  >1500 309 115

(43) The lamp having a WLZ cathode (lamp B) begins to flicker after only 40 hours, see 3.sup.rd column in table 1 above. Exterior carburization of this cathode (lamp C) increases the flicker-free time to 580 hours, but the times of the thoriated lamp (lamp A) cannot be attained. At higher carbon contents (lamps E, F and G), the lamps can be operated flicker-free over the entire nominal life and beyond. It is apparent here that a minimum amount of carbon has to be present. Thus, the lamp containing 240 μg/g of carbon (lamp D) does not behave any better than the lamp having the WLZ cathode (lamp B) and begins to flicker after only 50 hours. The carbon content of lamp D is obviously too low.

(44) The deformation and the burning-back are listed in the two right-hand columns of the above table. Lamp G containing 5800 μg/g of carbon displays great plateau enlargement and high burning-back: both values are even above those of lamp C having the carburized WLZ cathode. The reason for this is that the high-temperature strength and creep resistance of the cathode material decrease with increasing proportion by volume of W.sub.2C. The cathodes of the lamps without and with a carbon content which is too low (lamps B, C and D) likewise deform severely. In these lamps, the flickering indicates temporary depletion of emitter element (lanthanum or lanthanum oxide) at the cathode tip. As a result, the temperature increases at the cathode tip, which leads to greater deformation. The cathodes of the lamps E and F display very low deformation and also low burning-back. Both parameters are comparable to those for the thoriated reference (lamp A) and are sometimes even lower.

(45) A carbon content of 350 μg/g or 750 μg/g obviously ensures constant resupply of the emitter element to the cathode tip, without adverse effects on the dimensional stability and the burning-back behavior.

(46) Variation of the Relative Density of the Cathode Material

(47) The effect of redensification of the cathode material after the sintering process will now be demonstrated with the aid of the test results for lamps H, I and J, which apart from the cathode material have an identical construction to the lamps A-G. The cathode of lamp H was used in the sintered state. The cathode materials used in the lamps I and J were densified by flat forging or by an HIP process. Accordingly, these two materials have a higher density than the material which has merely been sintered. The production route for these three cathode materials is comparable, and the cathode content of 630 μg/g is in the identified target range. The lamp test data are summarized in table 2.

(48) TABLE-US-00002 TABLE 2 Test data for the lamps H to J having different densities of the cathode material Diameter of the Electrode cathode plateau spacing Density after 1000 h after 1100 h Lamp Densification [g/cm.sup.3] relative to 0 h relative to 0 h H None 17.86 213% 106.4% (96.2%) after 500 h after 500 h I Flat 18.25 150% 105.9% forging (98.3%) J HIP 18.52 180% 107.0% (99.8%)

(49) The lamps H, I and J burned in a stable manner during the entire test time and no flickering occurred. The test on lamp H was stopped after 500 hours because of excessive deformation of the cathode plateau. This negative test result is attributed to the residual porosity of the cathode material which leads to a rise in the tip temperature because of the reduced thermal conductivity. In addition, the residual porosity decreases the strength and creep resistance at high temperature. The densified cathode materials display characteristics which are comparable to those of the thoriated cathode or are superior to these in respect of some properties.

(50) Variation of the Maximum Temperature during the Diffusion Step in the Production of the Cathode Material

(51) As demonstrated above with the aid of the figures, a higher temperature in the diffusion step in the production of the cathode material leads to increased formation of carbidic phases of the emitter element. In the following, two cathode materials (cathodes of lamps K and L) for which the temperature during the diffusion step was varied will be compared. The cathodes of lamps K and L have the same construction as the specimens A to J. Density and carbon content of both materials are in the optimal range of values.

(52) TABLE-US-00003 TABLE 3 Results of lamp test on lamps H and I having a different maximum temperature in the diffusion step Diameter of the cathode Electrode spacing Maximum temperature Flicker-free plateau after 1000 h after 1000 h Lamp in diffusion step [° C.] operating time [h] relative to 0 h in % relative to 0 h in % K 2100 540 207 112.8% L 2200 >1500 170% .sup. 110%

(53) The lamp K with a maximum temperature of 2100° C. in the diffusion step attains a flicker-free operating time of 540 hours. The lamp L with a maximum temperature of 2200° C. in the diffusion step, on the other hand, attains a flicker-free operating time of more than 1500 hours.

(54) This can be interpreted as meaning that there is insufficient reduction of emitter oxide and formation of carbidic phases of the emitter element in the cathode material at a maximum temperature of less than 2200° C. in the diffusion step and the lamp is not supplied sufficiently with emitter element during operation.

(55) If the process is carried out at a maximum temperature of 2200° C. or above, the reduction of lanthanum oxide in the material is accelerated. A carbidic bonding state of lanthanum is observed to an increased extent. This bonding state is preferentially attained at triple points of the microstructure because the diffusion of C is accelerated along the grain boundaries compared to the bulk diffusion in the interior of the grain.