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Title:
OXIDE CATHODE
Document Type and Number:
WIPO Patent Application WO/2005/062334
Kind Code:
A1
Abstract:
This invention provides an oxide cathode comprising a body having a base layer, the body comprising a metallic component and a reducing component, and an electron-emitting layer on the base layer, wherein a diffusion inhibiting layer covers at least a proportion of at least the outer surface of the body not covered by the electron-emitting layer. The invention further provides a method of manufacturing an oxide cathode having an indirectly heated electron-emitting layer disposed on a base layer of a cathode body, which body comprises a reducing component, the method comprising forming a diffusion-inhibiting layer over at least substantially most of the surface of a cathode body, etching away the diffusion-inhibiting layer from the base layer, and forming an electron-emitting layer over the base layer.

Inventors:
BARRATT DAVID STEVEN (GB)
STOCKWELL MATTHEW (GB)
Application Number:
PCT/GB2004/005364
Publication Date:
July 07, 2005
Filing Date:
December 21, 2004
Export Citation:
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Assignee:
L G PHILIPS DISPLAYS NETHERLAN (NL)
BARRATT DAVID STEVEN (GB)
STOCKWELL MATTHEW (GB)
International Classes:
H01J1/142; H01J1/20; H01J9/04; (IPC1-7): H01J1/142; H01J9/04
Foreign References:
US3922179A1975-11-25
Other References:
PATENT ABSTRACTS OF JAPAN vol. 2000, no. 20 10 July 2001 (2001-07-10)
PATENT ABSTRACTS OF JAPAN vol. 2000, no. 09 13 October 2000 (2000-10-13)
Attorney, Agent or Firm:
Johnson, Graham Pierssene (15 Clare Road, Halifax HX1 2HY, GB)
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Claims:
CLAIMS
1. An oxide cathode comprising a body having a base layer, the body comprising a metallic component and a reducing component, and an electronemitting layer on the base layer, wherein a diffusion inhibiting layer covers at least a proportion of at least the outer surface of the body not covered by the electronemitting layer.
2. An oxide cathode as claimed in Claim 1 wherein the diffusion inhibiting layer covers a major part of the surface of the body not covered by the electronemitting layer.
3. An oxide cathode as claimed in Claim 1 or 2 wherein the diffusion inhibiting layer covers substantially the entire surface of the body not covered by the electron emitting layer.
4. An oxide cathode as claimed in any one of Claims 1 to 3 wherein the inner surface of the body is substantially covered by the diffusioninhibiting layer.
5. An oxide cathode as claimed in any one of Claims 1 to 4 wherein the diffusion inhibiting layer is formed from one of a carbide, nitride or oxide.
6. An oxide cathode as claimed in Claim 5 wherein the diffusion inhibiting layer is formed of boron nitride.
7. A method of manufacturing an oxide cathode having an indirectly heated electronemitting layer disposed on a base layer of a cathode body, which body comprises a reducing component, the method comprising forming a diffusioninhibiting layer over at least substantially most of the surface of a cathode body, etching away the diffusioninhibiting layer from the base layer, and forming an electronemitting layer over the base layer.
8. A method of manufacturing an oxide cathode as claimed in Claim 7 wherein the diffusioninhibiting layer covers the body's surface portions where escape of the diffused reducing component is not desired.
9. A method of manufacturing an oxide cathode as claimed in Claim 7 or 8 wherein the diffusioninhibiting layer is formed by oxidising the cathode body in the presence of nitrogen.
10. A method of manufacturing an oxide cathode as claimed in Claim 9 wherein impurities present in the body react with the nitrogen to produce a nitride at the surface thereof.
Description:
OXIDE CATHODE Field of the Invention This invention relates to an oxide cathode having an indirectly heated electron emitting layer disposed on a cathode base layer which comprises a reducing component. In particular, but not exclusively, the invention relates to oxide cathodes for use in electron guns.

Background to the Invention Conventional oxide cathodes generally comprise an oxide- containing electron-emitting layer (or coating) disposed on a metal base layer provided by a cathode body. A characteristic feature of the electron-emitting coating materials of oxide cathodes is that they comprise an alkaline earth metal in the form of an alkaline earth metal oxide. This is typically barium-oxide BaO but may comprise others such as SrO, CaO, SC203, Th02, La203, Y203. The cathode body typically comprises nickel as a main component with a small quantity of reducing component such as magnesium Mg and/or silicon Si. Other suitable materials for the main component of the body include Mg, Al, Si, Re, Mo and Pt for example. Impurities such as boron, carbon, sulphur, copper and/or zinc are also present in the body material. The body comprises a sleeve portion which is capped at one end by the base layer. A heater, generally contained in the sleeve, serves to heat the base layer and the electron-emitting oxide layer.

In order to emit electrons from the cathode, reducing reactions occur at the interface between the base and the oxide- containing electron-emitting layer. The alkaline earth metal oxide components in the electron-emitting layer are reduced by reacting with the reducing components, or"activators", present in the body. For example, BaO may be reduced in the following reactions:

(i) BaO + Mg ~ MgO + Ba T (ii) 4BaO + Si Ba2SiO4 + 2 Ba t thus liberating free barium which serves to emit electrons at the emission surface. Such reactions occur when the cathode is heated to a working temperature of around 700-850°C. The rate of reaction determines the maximum current which the cathode can supply.

The reducing component, e. g. Mg, contained in the body, diffuses through the base layer and reacts with the alkaline metal oxide components. However, the reducing component can also diffuse through other surfaces of the body which are not in contact with the electron-emitting layer. For example, the reducing component may diffuse into a surrounding vacuum through the outer walls of the sleeve portion. This is undesirable as the supply of the reducing component to the electron-emitting layer is essential for the continued operation of the cathode. Therefore, the loss of reducing component through the surfaces of the body reduces the lifetime of the cathode.

It is an object of the present invention to provide an improved oxide cathode.

It is a further object of the present invention to provide an oxide cathode having an extended lifetime.

The present invention provides an oxide cathode that offers or permits improvements over known oxide cathodes. Various novel concepts, inventive concepts and specific embodiments are disclosed herein, particularly but not exclusively with reference to the accompanying drawings.

Summary of the Invention According to a first aspect of the invention there is provided an oxide cathode comprising a body having a base layer, the body comprising a metallic component and a reducing component, and an electron-emitting layer on the base layer, wherein a diffusion inhibiting layer covers at least a proportion of at least the outer surface of the body not covered by the electron-emitting layer. By providing a diffusion inhibiting layer over the surface, comprising the base layer and sleeve walls of the body, undesired diffusion of the reducing component from the body is reduced thus extending the lifetime of the cathode. The diffusion inhibiting layer desirably covers a major part, preferably substantially the entire, surface of the body not covered by the electron-emitting layer.

Preferably, the inner surface of the body is substantially covered by the diffusion inhibiting layer.

In a preferred embodiment, the diffusion inhibiting layer is formed of boron nitride. This serves to prevent diffusion of the reducing component (e. g. magnesium) therethrough. Other materials which substantially reduce the escape of the reducing component and which are suitable for use in this manner on the cathode body surface may be used for the diffusion inhibiting layer, such as a carbide, nitride or oxide layer.

According to a second aspect of the invention there is provided a method of manufacturing an oxide cathode having an indirectly heated electron emitting layer disposed on a base layer of a cathode body, which body comprises a reducing component, the method comprising forming a diffusion inhibiting layer over at least substantially most of the surface of a cathode body, etching away the diffusion inhibiting layer from the base layer, and forming an electron-emitting layer over the base layer. Advantageously, the diffusion-inhibiting layer covers the body's surface portions where escape of the diffused reducing component is not desired. The reducing component can

then only escape the cathode body via the interface with the electron-emitting layer where the reducing reactions occur.

This increases the lifetime of the cathode.

Preferably, the diffusion inhibiting layer is formed by oxidising the cathode body in the presence of nitrogen.

Impurities present in the body can then react with the nitrogen to produce a nitride at the surface, such as boron nitride.

This may use conventional cathode manufacturing apparatus, thus not requiring new and expensive equipment.

Brief Description of the Drawings Further features and advantages of the present invention will become apparent from reading of the following description of preferred embodiments, given by way of an example only, and with reference to the accompanying drawings, in which: Figure 1 shows schematically an oxide cathode in accordance with the invention; and, Figure 2 shows schematically a process step in the manufacture of an oxide cathode in accordance with the invention.

It should be noted that the Figures are not drawn to scale. The same reference numerals are used throughout the Figures to denote the same or similar parts.

Description of the Preferred Embodiments With reference to Figure 1, an oxide cathode 10 comprises a metallic body having a tubular sleeve portion 1 which houses a helical-shaped heater element 2. The body also comprises a metal base 3 which caps the upper end of the sleeve 1. The body is preferably formed from a metal alloy selected from the group of Ni, Co, Ir, Re, Pd, Rh and Pt. Traditionally, a nickel alloy is used for the body material. The body also comprises a

reducing component such as Mg or Si. Other suitable reducing elements or"activators"include Mn, Fe, W, Mo, Cr, Ti and Zr.

An electron-emitting layer 5 is formed on the base 3 by spraying a paste of oxide-containing material onto the base 3 using conventional deposition processes. The layer contains a main component of a rare earth metal oxide such as BaO, CaO or SrO. During operation of the cathode, reduction reactions occur between the reducing component in the base 3 and the alkaline earth metal oxide elements in the electron-emitting layer 4 to produce electrons. Therefore, the continued supply of the reducing component to the electron-emitting layer is essential to the operation of the cathode.

A diffusion inhibiting layer 5 covers at least substantially most of the surface of the body not covered by the electron- emitting layer. This serves to prevent diffusion of the reducing component from the body surface into the surrounding vacuum. The layer 5, in this particular example, covers both the entire outside and inside surfaces of the sleeve portion 1 and the underside of the capping base 3. Therefore, the only surface of the body from which the reducing component can diffuse is that which contacts with the electron-emitting layer 4. By preventing undesired diffusion and loss of the reducing component from the body surfaces, the reducing reactions can occur for a longer period of time thus increasing the lifetime of the cathode.

The diffusion inhibiting layer 5 can be formed of a carbide, nitride or oxide such as, for example, BN (boron nitride) and should be to a substantially constant thickness of approximately 5-50 nm over the body surface.

With reference to Figure 2, an example method of forming the diffusion inhibiting layer will be described. A body comprised of nickel having a tubular sleeve 1 and a capping base layer 3 is subjected to a coating process wherein the body is immersed

in a coating bath. This covers the entire surface of the body, including both the inside and outside surfaces of the sleeve 1 and base 3. The body may then be thermally treated to"fix"the coating thereon. This layer 5 is then etched away from the upper surface of the base layer 3, preferably using an ion- etching technique 100. An electron-emitting layer 5 having a main component of an alkaline earth metal-oxide is then formed by spraying a paste onto the exposed base 3. The remaining surfaces of the cathode body are left covered by the diffusion inhibiting layer 5.

From reading the description, modifications and variations will be apparent to persons skilled in the art. Such modifications and variations may involve other features which are already known in the art and which may be used instead of or in addition to features already disclosed herein. No specific patent claims have yet been formulated in this application to particular combinations of features, and it should be understood that the scope of the disclosure of the present application includes any and every novel feature or combination of features disclosed herein either explicitly or implicitly and together with all such modifications and variations, whether or not relating to the main inventive concepts disclosed herein and whether or not it mitigates any or all of the same technical problems as the main inventive concepts.

The applicants hereby give notice that patent claims may be formulated to such features and/or combinations of such features during prosecution of the present application or of any further application derived or claiming priority therefrom.