EP0757370A1 - Electric discharge tube or discharge lamp and scandate dispenser cathode - Google Patents

Abstract

An electrical discharge lamp, eg low pressure gas type, has a scandate dispenser cathode that is formed of a cathode body covered with an emitting surface. The cathode body is of a matrix formed from either a high melting point material or a high melting point alloy together with a barium binding material. Chemical reaction results in emission from the surface. A cover with one or more layers of tungsten and/or tungsten alloy, an intermediate layer of rhenium and a top coating of scandium oxide mixed with a rare earth metal oxide completes the structure

Description

The invention relates to an electric discharge tube, in particular a vacuum electron tube, or a discharge lamp, in particular a low-pressure gas discharge lamp, with at least one scan data storage cathode, which consists of a cathode body and a cover layer with an emitting surface, the cathode body being a matrix of at least one high-melting metal and / or a refractory alloy and a barium compound in contact with the matrix material to supply barium to the emitting surface by chemical reaction with the matrix material. It also relates to such a Scandat supply cathode.

Electron tubes, in particular 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 the construction of equipment and systems in every sector, in medical technology, in diagnostic and measuring equipment in workshops and also in gaming 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, high-frequency and microwave tubes.

abhängig von der Austrittsarbeit an der Kathodenoberfläche φ und der Betriebstemperatur T.The emission current density at a cathode is according to the Richardson equation $${\text{I.}}_{\text{0}}{\text{= A}}_{\text{R}}{\text{T}}^{\text{2nd}}\text{exp (-φ / kT)}$$

depending on the work function on the cathode surface φ and the operating temperature T.

A cathode with a lower work function φ can deliver a higher emission current density at the same operating temperature T. Alternatively, a cathode with a lower work function φ allows operation at lower temperatures with the same current density. A lower operating temperature has a positive effect on the service 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, a chemical reaction between tungsten, scandium oxide and the barium calcium aluminate forms a surface complex which causes and maintains the high electron emission. Since this surface complex is destroyed by ion bombardment in the tube, he is constantly being replicated. However, scandium oxide is not very mobile, so that the subsequent delivery (segregation) of scandium to form the surface complex is disturbed and the cathode emission is rapidly reduced during the operation of the discharge tube or discharge lamp. In order to overcome this disadvantage, EP 0 317 002 proposed to use scandium-containing metal compounds or alloys which are a compound of scandium with one or more of the metals rhenium, ruthenium, hafnium, nickel, cobalt, palladium, zirconium or tungsten Use scandium segregation in the surface of the cathode. The long-term behavior of the cathodes according to EP 0 317 002 is improved, but the reproducibility of the results leaves something to be desired.

Furthermore, EP 0 549 034 discloses a cathode with a matrix body impregnated with an alkaline earth metal compound, on the surface of which a cover layer is applied, which contains high-melting metal such as, in particular, tungsten and scandium. A high emission at a low operating temperature and at the same time a rapid recovery after ion bombardment and a long service life are achieved in that the cover layer contains at least two layers of different composition, with a purely metallic layer being applied to the impregnated matrix body, which contains scandium and a high-melting metal such as contains in particular tungsten and / or rhenium, and that a metallic layer made of a high-melting metal such as in particular tungsten is applied as the final layer. These cathodes are preferably produced by a process in which initially purely metallic layers of scandium and / or rhenium are produced by means of a plasma-activated CVD process in particular, preferably by means of a plasma generated by direct current glow discharge, and then by means of a metallic tungsten layer as the last layer CVD process is applied. However, the emission current density of this type of cathode is low.

The object of the invention is therefore to create an electrical discharge tube or discharge lamp which delivers reproducibly high emission current densities 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 scandate supply cathode, which consists of a cathode body and a cover layer with an emitting surface, the cathode body comprising 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 by chemical reaction with the matrix material and the top layer one or more times a layer composite of optionally a lower 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 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 scandate supply cathode which consists of a cathode body and a cover layer with an emitting surface, the cathode body comprising 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 from 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 a lower layer of tungsten and / or a tungsten alloy, an intermediate layer of rhenium and / or a rhenium alloy and an upper layer of scandium oxide, a mixture of scandium oxide Contains rare earth metal oxides, a scandate and / or a scandium alloy.

The scandate supply cathode according to the invention has little tungsten loss and the scandium supply into the emitting surface is not passivated during operation. 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 subsequent delivery of scandium to the emitting surface, has a particularly long service life.

It is preferred that the layer composite consist of ultrafine particles. Scandate supply cathodes with a cover layer made of ultrafine particles have a surface structure and surface modulation from particles in the diameter range from 1 to 100 nm, i.e. they have relatively small radii of curvature in dense particle and tip distribution on the macroscopic surface.

It is preferred that the layer composite in the top layer of the scandate supply cathode according to the invention is produced by a laser ablation deposition process. In contrast to known wet chemical processes, the laser ablation deposition process has short reaction times. In addition, the grain size distribution of the ultrafine particles is easy to control, in contrast to known evaporation processes.

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

The electron beam generation system of the discharge tubes or discharge lamps according to the invention contains an arrangement of one or more supply cathodes. For example, the electron gun can be one or more 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 surface. They can also contain the emitting supply cathode arrangement only in individual surface segments.

A supply cathode according to the invention consists of a cathode body and a cover layer. The cathode body comprises 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 by chemical reaction with the matrix material.

Storage cathodes of a known type, such as L-cathodes, M-cathodes and I-cathodes and mixed-matrix cathodes, are suitable as cathode bodies for the invention.

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 layers of a composite of optionally a lower layer of tungsten and / or a tungsten alloy, an intermediate layer of rhenium and / or rhenium alloy and an upper layer of scandium oxide, a mixture of scandium oxide with rare earth metal oxides, a scandate and / or a scandium alloy. The total thickness of the cover layer is dimensioned so that the cathode has an adequate service life. The service life of supply cathodes is limited by erosion due to sputtering reactions on the cathode surface. Ions are involved in the sputtering reaction, which are formed by the electron beam from the residual gases in the vacuum of the discharge tube or discharge lamp. These ions are accelerated by the applied voltage against the cathode and erode its surface. This erosion process by ion bombardment can be simulated using an ion gun and the erosion rate can be determined. The total layer thickness of the cover layer is estimated from this erosion rate. In general, the total thickness of the top layer will be 600 to 1000 nm.

The individual layers of the layer composite, ie the lower layer with tungsten, the intermediate layer made of rhenium and the upper layer with scandium oxide or a scandium alloy should preferably be very thin. The mass-equivalent layer thickness of the scandium layer should preferably be in the nanometer range between 5 and 20 nm, that of the tungsten- and rhenium-containing layer between 20 and 200 nm. The mass-equivalent layer thicknesses are determined from the theoretical densities and applied basis weights of the cover layer substances. These 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, ie they consist of individual particle clusters that are separated by large, essentially open pores. As a result, the cover layer has a slightly dissolved, radially and laterally structured surface. If the particles of the lower layer, the intermediate layer and the upper layer are deposited one after the other, their nanostructures interlock and a material combination is created in the cover layer which has excellent emitter properties.

If the supply cathode according to the invention contains the layer composite only once, the lowermost tungsten-containing layer can also be formed by the tungsten-containing matrix of the cathode body.

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 scandates, for example alkaline earth metal scandates, can be used as 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.

Tungsten or a tungsten alloy containing osmium, iridium, ruthenium, tantalum and / or molybdenum is selected as the material for the underlayer.

The production process for the supply cathode according to the invention is a two-step process. It begins with the production of the cathode body, to which the emitting cover layer is then applied in a second step.

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

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

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. If necessary, the tungsten-containing layer is deposited first, the rhenium-containing layer second, and the scandium-containing layer third. It's cheap multi-targets to use, which contain all three components on a target arrangement. The emission properties of the finished scandate supply cathode are favorably influenced 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.

In general, the emitting surface of the cathode is activated in a further process step.

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 a cylindrical body 1.8 mm in diameter and 0.5 mm in height and with 7% by weight barium calcium aluminate powder having 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 is continued until a mass-equivalent total layer thickness of 600 nm is reached.

The pill with the cover layer according to the invention is welded onto a cathode shaft which contains a heating coil. This indirectly heated cathode is assembled with other components, such as radiation cylinders and ceramic insulation, to form a cathode unit. Three of these units are then installed in a color television tube.

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 a cylindrical body 1.8 mm in diameter and 0.5 mm in height and with 7% by weight barium calcium aluminate powder having 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 welded onto a cathode shaft which contains a heating coil. This indirectly heated cathode is assembled with other components, such as radiation cylinders and ceramic insulation, to form a cathode unit. Three of these units are then installed in a color television tube.

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

Claims (7)

Electrical discharge tube or discharge lamp with a Scandat supply cathode, which consists of a cathode body and a cover layer with an emitting surface, the cathode body comprising 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 by chemical reaction with the matrix material and
the top layer contains one or more layers of a composite of optionally a lower layer of tungsten and / or a tungsten alloy, an intermediate layer of rhenium and / or rhenium alloy and an upper layer of scandium oxide, a mixture of scandium oxide with rare earth metal oxides, a scandate and / or a scandium alloy. Scandat supply cathode, which consists of a cathode body and a cover layer with an emitting surface, the cathode body passing through 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 to supply barium to the emitting surface chemical reaction with the matrix material and
the top layer contains one or more layers of a composite of optionally a lower layer of tungsten and / or a tungsten alloy, an intermediate layer of rhenium and / or rhenium alloy and an upper layer of scandium oxide, a mixture of scandium oxide with rare earth metal oxides, a scandate and / or a scandium alloy. Scandate supply cathode according to claim 2,
characterized by
that the cathode body has a scandium compound or a scandium alloy for scandium delivery to the emitting surface. Scandat supply cathode according to claim 2 and 3,
characterized by
that the layer composite consists of ultrafine particles. Scandat supply cathode according to claim 3 and 4,
characterized by
that the layer composite is produced by a laser ablation deposition process. Scandat supply cathode according to claim 3 to 5,
characterized by
that the lower layer, intermediate layer and upper layer each have a layer thickness of 5 to 150 nm. Scandat supply cathode according to claim 3 to 6,
characterized by
that the cover layer has a layer thickness of 50 to 1000 nm, preferably 400 to 600 nm.