EP0757370B1 - Electric discharge tube or discharge lamp and scandate dispenser cathode - Google Patents

Description

The invention relates to an electrical discharge tube, in particular a vacuum electron tube, or discharge lamp, in particular low-pressure gas discharge lamp, with at least one Scandat supply cathode, which consists of a cathode body and a cover layer with an emitting surface, the Cathode body a matrix of at least one high-melting metal and / or a high melting alloy and a barium compound in contact with the matrix material for supplying barium to the emitting surface chemical reaction with the matrix material. It continues to affect one such a Scandat supply cathode.

Electron tubes, especially vacuum electron tubes, are primarily used as Picture tubes in televisions, as monitor tubes, as X-ray tubes, as high-frequency and microwave tubes for various applications in device and plant construction every division, in medical technology, in diagnostic and measuring equipment in workshops and also used in play equipment.

TV and monitor tubes are subject to ever increasing demands in terms of greater brightness, increased resolution, constant image quality and better long-term operation. In order to achieve a higher image brightness and a better resolution of the electron beam, higher electron emission current densities in the tubes are necessary, which can only be achieved with improved electron sources, ie cathodes. In the mid-1980s, standard oxide cathodes with an emission current density of 2 A / cm ^{2 met} long-term operation, currently 10 A / cm ^{2 are} required and far higher emission current densities are required for the new high-performance tubes.

The same applies to the emission current density and long-term stability for X-ray, radio frequency and microwave tubes.

The emission current density at a cathode is according to the Richardson equation I. 0 = A R T 2nd exp (- / kT) depending on the work function on the cathode surface  and the operating temperature T.

A cathode with a lower work function  can have a higher emission current density deliver at the same operating temperature T. Alternatively, one allows Lower work function cathode Betrieb operation at lower temperatures at the same current density. A lower operating temperature affects thereby positive on the life of the cathode and the discharge tube.

Scandat supply cathodes are currently the cathodes with the highest electron emission. The two most important types of Scandat supply cathodes are the "Mixed Matrix Scandat Cathode" and the "Top Layer Scandat Cathode". The "Mixed Matrix Scandat Cathode" consists of a porous cathode body made of tungsten and scandium oxide, which is impregnated with 4 BaO.CaO.Al ₂ O ₃ . "Top Layer Scandat Cathodes" consist of a porous tungsten body, which is impregnated with 4 BaO.CaO.Al ₂ O ₃ and is covered with a thin cover layer made of tungsten and scandium oxide or Sc ₂ W ₃ O ₁₂ .

During the operation of the cathode, chemical reaction forms between Tungsten, scandium oxide and the barium calcium aluminate a surface complex, which causes and maintains the high electron emission. Because of this Surface complex must be destroyed by ion bombardment in the tube he is constantly being replicated. Scandium oxide is not very mobile, so the Subsequent delivery (segregation) of Scandium to form the surface complex is disturbed and the cathode emission increases during the operation of the discharge tube or discharge lamp quickly decreased. To fix this disadvantage, was proposed in EP 0 317 002, scandium-containing metal compounds or alloys that combine Scandium with one or more of the Metals rhenium, ruthenium, hafnium, nickel, cobalt, palladium, zirconium, or Tungsten are used to scandium segregation into the surface of the cathode. The long-term behavior of the cathodes according to EP 0 317 002 is improved, however, the reproducibility of the results leaves something to be desired.

Furthermore, EP 0 549 034 describes a cathode with an alkaline earth compound impregnated matrix body with a top layer on its surface is applied, which high-melting metal such as in particular tungsten and Scandium contains, known. A high emission at a low operating temperature and at the same time a quick recovery after ion bombardment as well as a long one Service life is achieved in that the cover layer has at least two layers contains different compositions, with a purely metallic layer is applied to the impregnated matrix body, which scandium as well contains high-melting metal such as in particular tungsten and / or rhenium, and that as a final layer, a metallic layer made of a high-melting Metal such as tungsten in particular is applied. These cathodes are preferred produced by a process in which initially purely metallic layers Scandium and / or rhenium by means of a plasma-activated CVD process in particular, preferably by means of a direct current glow discharge Plasmas are produced, and that then a metallic layer as the last layer Tungsten layer is applied by means of CVD processes. The emission current density however, this type of cathode is low.

The object of the invention is therefore an electric discharge tube or discharge lamp to create the reproducibly high emission current densities delivers over a long period of time.

According to the invention, the object is achieved by an electrical discharge tube or discharge lamp with at least one Scandat supply cathode consisting of a There is a cathode body and a cover layer with an emitting surface, wherein the cathode body is a matrix of at least one high-melting Metal and / or a high melting alloy and a barium compound in Contact with the matrix material for delivery of barium to the emitter Surface covered by chemical reaction with the matrix material and the top layer one or more times a layer composite of optionally one Bottom layer made of tungsten and / or a tungsten alloy, an intermediate layer made of rhenium and / or a rhenium alloy and a top layer made of scandium oxide, a mixture of scandium oxide with rare earth oxides, a scandate and / or a scandium alloy.

Such a discharge tube or discharge lamp has a long service life because it shows good resistance to ion bombardment with doses up to a few 10 ¹⁹ ions / cm ² . For example, it can be used as a high-resolution computer monitor (CMT), in high-definition television sets with a screen aspect ratio of 16: 9 and as a high-performance X-ray tube, because at 965 ° C, measured as the radiation temperature of the molybdenum cap of the cathode holder, it has a saturation emission current density i ₀ of ≥ 25 A / cm ² reached.

Another aspect of the invention relates to a Scandat supply cathode, which consists of a cathode body and a cover layer with an emitting surface consists, wherein the cathode body is a matrix of at least one high-melting Metal and / or a high-melting alloy and a barium compound in contact with the matrix material for the delivery of barium to the emitting surface by chemical reaction with the matrix material and the top layer one or more times a layer composite of optionally an underlayer made of tungsten and / or a tungsten alloy, an intermediate layer made of rhenium and / or a rhenium alloy and a top layer Scandium oxide, a mixture of scandium oxide with rare earth oxides, one Contains scandate and / or a scandium alloy.

The Scandat supply cathode according to the invention has little tungsten loss and that Scandium replenishment in the emitting surface occurs during operation not passivated. The layer structure prevents oxygen diffusion to the tungsten.

A scandate supply cathode according to the invention, in which the cathode body has a Scandium compound or a scandium alloy for scandium delivery to the emitting surface has a particularly long life.

It is preferred that the layer composite consist of ultrafine particles. Scandate supply cathodes with a cover layer made of ultrafine particles have one Surface structures and surface modulations from particles in the diameter range from 1 to 100 nm, so they have relatively small radii of curvature Particle and tip distribution on the macroscopic surface.

It is preferred that the layer composite in the top layer of the invention Scandat supply cathode produced by a laser ablation deposition process becomes. In contrast to known wet chemical methods, the laser ablation deposition method has short response times. In addition, the grain size distribution the ultrafine particles in contrast to known evaporation processes easily controllable.

It is further preferred that the lower layer, intermediate layer and upper layer each have a layer thickness of 5 to 150 nm. Scandat supply cathodes with such layers have excellent emitter properties.

It is particularly preferred that the cover layer of the scandate supply cathodes according to the invention a layer thickness of 50 to 1000 nm, preferably 400 to 600 nm. This achieves a cathode life of 10,000 hours.

The invention is explained further below and examples are given.

An electric discharge tube or discharge lamp consists of four functional groups: from electron beam generation, beam focusing, beam deflection and the fluorescent screen.

The electron gun of the discharge tubes according to the invention or discharge lamps contains an arrangement of one or more supply cathodes. For example, the electron gun can be one or multiple point cathodes or a system of one or more wire cathodes, Flat ribbon cathodes or surface cathodes. Wire cathodes, surface cathodes and Ribbon cathodes do not have to emit over their entire area. You can the emitting supply cathode arrangement also only in individual surface segments contain.

A supply cathode according to the invention consists of a cathode body and a top layer. The cathode body comprises a matrix of at least one refractory metal and / or a refractory alloy and a Barium compound in contact with the matrix material to supply barium the emitting surface by chemical reaction with the matrix material.

Storage cathodes are more suitable as cathode bodies for the invention Design like L-cathodes, M-cathodes and I-cathodes and mixed matrix cathodes.

I-cathodes and mixed-matrix cathodes are particularly suitable as cathode bodies.

The cover layer of the cathodes according to the invention contains one or more Layer composite consisting of a sub-layer made of tungsten and / or one Tungsten alloy, an intermediate layer made of rhenium and / or rhenium alloy and a top layer of scandium oxide, a mixture of scandium oxide with Rare earth metal oxides, a scandate and / or a scandium alloy. The The total thickness of the cover layer is dimensioned so that the cathode is adequate Has lifespan. The service life of supply cathodes is through erosion limited by sputtering reactions on the cathode surface. At the sputtering reaction ions are involved, which are caused by the electron beam from the residual gases in a vacuum of the discharge tube or discharge lamp. These ions will accelerated by the applied voltage against the cathode and erode it Surface. This erosion process by ion bombardment can be carried out using a Ion gun simulated and the erosion rate determined. From this erosion rate the total layer thickness of the top layer is estimated. Generally speaking the total thickness of the top layer is 600 to 1000 nm.

The individual layers of the layer composite, i.e. the lower layer with tungsten, the intermediate layer made of rhenium and the top layer with scandium oxide or one Scandium alloy should preferably be very thin. The mass-equivalent layer thickness the scandium layer should preferably be in the nanometer range between 5 and 20 nm, that of the tungsten- and rhenium-containing layer are between 20 and 200 nm. The mass-equivalent layer thicknesses become the theoretical densities and applied basis weights of the cover layer substances determined. This very thin individual layers result in a better bond between the individual phases and inhibit grain enlargement through sinter growth during operation. The Layers are then nanostructured, i.e. they consist of individual clusters of particles, which are separated by large, essentially open pores. As a result, the Top layer a slightly dissolved, radially and laterally structured surface. If successively the particles of the lower layer, the intermediate layer and the upper layer are deposited, their nanostructures interlock and it a combination of materials is created in the top layer, which has excellent emitter properties Has.

If the supply cathode according to the invention contains the layer composite only once, the lowest layer containing tungsten can also be determined by the matrix containing the tungsten Cathode body are formed.

Scandium oxide Sc ₂ O ₃ or scandium oxide, which is mixed with the oxides of other rare earth metals such as europium, samarium and cerium, and scandate, for example alkaline earth metal candate, can be used as the material for the scandium-containing upper layer. Alternatively, alloys containing scandium and / or intermetallic compounds such as Re ₂₄ Sc ₅ , Re ₂ Sc, Ru ₂ Sc, Co ₂ Sc, Pd ₂ Sc and Ni ₂ Sc can be used. However, these compounds, compound mixtures or alloys should not contain tungsten.

Metallic rhenium is used as the material for the rhenium-containing intermediate layer used.

The material for the underlayer is tungsten or a tungsten alloy Contains osmium, iridium, ruthenium, tantalum and / or molybdenum.

The manufacturing process for the supply cathode according to the invention is a two-stage process Method. It starts with the manufacture of the cathode body, then onto the in a second step, the emitting cover layer is applied.

Conventional I-cathodes or mixed matrix are preferred as cathode bodies Cathodes.

I cathodes are impregnated supply cathodes. They consist of a porous tungsten matrix produced by powder metallurgy from tungsten powder. This porous matrix is impregnated with a mixture of BaO, CaO and Al ₂ O ₃ . For this purpose, a mixture of BaCO ₃ , CaCO ₃ and Al ₂ O _{3 is} melted and the porous matrix is filled with the mixture by melt infiltration. The surface of the body is then cleaned by ultrasound and water from externally adhering oxide mixture.

Mixed matrix cathodes contain scandium in a common matrix of tungsten and scandium oxide. The matrix is produced by sintering a powder mixture of tungsten and scandium oxide, the sintering process being carried out in such a way that a porous body is formed. This porous sintered body is then impregnated with the same method as for the I cathodes with a mixture of BaO, CaO and Al ₂ O ₃ . The cleaning and activation procedures are also the same.

The cover layer can be produced using conventional coating processes become. These methods include CVD, PCVD, and sputtering. However, it is in the Within the scope of the present invention preferred that the individual layers of the cover layer from ultrafine particles in a laser ablation deposition process getting produced.

For this purpose, the cathode body is brought into the deposition chamber of a laser ablation deposition system. It is favorable to use an excimer laser as the laser, which unlike CO ₂ lasers also ablates tungsten without any problems. The tungsten-containing layer is optionally first deposited, the rhenium-containing layer second, and the scandium-containing layer third. It is favorable to use multitargets that contain all three components on a target arrangement. The emission properties of the finished scandate supply cathode are influenced favorably if the gas atmosphere in the ablation process consists of high-purity argon or argon / hydrogen. Furthermore, it can be expedient if the substrates (cathode bodies) for the cover layer are heated during the ablation deposition process. The conditions for the laser ablation deposition process are set so that the grain size of the ultrafine particles is in a medium to high range.

Generally, the emissive surface of the cathode is in another Process step activated.

EXAMPLE 1

An I-cathode body is produced in the form of a porous pill by sintering tungsten powder at 1500 ° C. in a hydrogen atmosphere to form a cylindrical body 1.8 mm in diameter and 0.5 mm in height and containing 7% by weight of barium calcium aluminate powder with the composition 4 BaO- CaO-Al ₂ O _{3 is} impregnated. The pill is inserted into a molybdenum bowl and placed in the ablation chamber of a laser ablation deposition apparatus. A cylindrical multitarget is used as the target, which contains Sc ₂ O ₃ , rhenium and tungsten side by side. The laser is a UV excimer laser with a wavelength of 248 nm and an average power of 100 W, which produces a cold ablation on the rotating target. A mixture of high-purity argon and hydrogen is used as the carrier gas. The total pressure in the ablation chamber was 1 mbar. During the ablation, the multitarget is translated and the three partial areas of the target are scanned continuously in the order of tungsten, rhenium, and scandium oxide. To fix the coating, the tungsten pills are heated to 800 ° C. during the coating process.

The ablation deposition process continues until a mass equivalent Total layer thickness of 600 nm is reached.

The pill with the cover layer according to the invention is placed on a cathode shaft welded on, which contains a heating coil. This indirectly heated cathode will with other components, such as radiation cylinders and ceramic insulation, into one Cathode unit assembled. Three of these units are then divided into one Color television tube installed.

The measured emission current density of the cathode was 120 A / cm ² at a cathode temperature of 950 ° C.

EXAMPLE 2

An I-cathode body is produced in the form of a porous pill by sintering tungsten powder at 1500 ° C. in a hydrogen atmosphere to form a cylindrical body 1.8 mm in diameter and 0.5 mm in height and containing 7% by weight of barium calcium aluminate powder with the composition 4 BaO- CaO-Al ₂ O _{3 is} impregnated. The pill is inserted into a molybdenum bowl and placed in the ablation chamber of a laser ablation deposition apparatus. A cylindrical multitarget containing Sc ₂ O ₃ and rhenium side by side is used as the target. The laser is a UV excimer laser with a wavelength of 248 nm and an average power of 100 W, which produces a cold ablation on the rotating target. A mixture of high-purity argon and hydrogen is used as the carrier gas. The total pressure in the ablation chamber was 1 mbar. A Re layer with a mass-equivalent layer thickness of 120 nm and a scandium oxide layer with a mass-equivalent layer thickness of 20 nm are deposited in each case. This sequence of layers is repeated five times. To fix the coating, the tungsten pills are heated to 800 ° C. during the coating process.

The pill with the cover layer according to the invention is placed on a cathode shaft welded on, which contains a heating coil. This indirectly heated cathode will with other components, such as radiation cylinders and ceramic insulation, into one Cathode unit assembled. Three of these units are then divided into one Color television tube installed.

The measured emission current density of the cathode was 25 A / cm ² at a cathode temperature of 980 ° C.

Claims (7)

1. An electric discharge tube or discharge lamp having a scandate dispenser cathode, which is composed of a cathode body and a coating having an emissive surface, said cathode body comprising a matrix of at least one refractory metal and/or a refractory alloy and a barium compound which is in contact with the matrix material to supply barium to the emissive surface by means of a chemical reaction with said matrix material, and the coating containing one or more multilayers which may include a bottom layer of tungsten and/or a tungsten alloy, an intermediate layer of rhenium and/or a rhenium alloy and a top layer of scandium oxide, a mixture of scandium oxide and rare-earth metal oxides, a scandate and/or a scandium alloy.
2. A scandate dispenser cathode which is composed of a cathode body and a coating having an emissive surface, said cathode body comprising a matrix of at least one refractory metal and/or a refractory alloy and a barium compound which is in contact with the matrix material to supply barium to the emissive surface by means of a chemical reaction with the matrix material, and the coating containing one or more multilayers which may include a bottom layer of tungsten and/or a tungsten alloy, an intermediate layer of rhenium and/or a rhenium alloy and a top layer of scandium oxide, a mixture of scandium oxide and rare-earth metal oxides, a scandate and/or a scandium alloy.
3. A scandate dispenser cathode as claimed in Claim 2, characterized in that the cathode body comprises a scandium compound or a scandium alloy to dispense scandium to the emissive surface.
4. A scandate dispenser cathode as claimed in Claims 2 and 3, characterized in that the multilayer is composed of ultrafine particles.
5. A scandate dispenser cathode as claimed in Claims 3 and 4, characterized in that the multilayer is manufactured by means of laser-ablation deposition.
6. A scandate dispenser cathode as claimed in Claims 3 to 5, characterized in that the bottom layer, intermediate layer and top layer each have a thickness in the range from 5 to 150 nm.
7. A scandate dispenser cathode as claimed in Claims 3 to 6, characterized in that the top coating has a thickness in the range from 50 to 1,000 nm, preferably 400 to 600 nm.