GB2238653A

GB2238653A - Dispenser cathodes

Info

Publication number: GB2238653A
Application number: GB9024424A
Authority: GB
Inventors: Jong-In Jeong; Jong-Seo Choi; Hwan-Cheol No; Kyu-Nam Ju
Original assignee: Samsung Electron Devices Co Ltd
Current assignee: Samsung SDI Co Ltd
Priority date: 1989-11-09
Filing date: 1990-11-09
Publication date: 1991-06-05
Also published as: KR910010576A; KR920001333B1; GB9024424D0; JPH03173035A; DE4105295A1

Abstract

A dispenser cathode comprises an electron emissive material 2c including at least one material selected from the group consisting of Ba and Ba oxide and further including scandium tungstate, and a porous metal base body 1c having diffusing cavities therewithin and positioned on the upper portion of said electron emission material 2c. The activation aging time of the dispenser cathode is shortened greatly as compared with a conventional dispenser cathode, and the loss of Sc due to ion bombardment is effectively prevented, resulting in stable thermoelectron emission. <IMAGE>

Description

- s, --- C' fi DISPENSER CATHODES This invention relates to dispenser

cathodes.

Referring to Figures 1 and 2 of the accompanying diagrammatic drawing, a dispenser cathode generally may be classified as of cavity reservoir type (see Figure 1), impregnated type (see Figure 2), sintered type, etc., and these have common characteristics of a high-density beam current and a long lifetime.

These dispenser cathodes, however, have many problems to be employed in electron tubes such as Braun tubes because they are operated under a high temperature of 1100'C to 12000C. That is to say, these dispenser cathodes should have a heater having a large calorific value because a large amount of thermal energy is needed to enable thermoelectronic emission, and their parts should be made of materials which show no thermal deformation under the effect of the heater.

Further, neighbouring parts of the cathode, for example, a sup porting part for the body of the cathode, and electron beam generating source components such as a control grid and, a screen grid, should be made of heat resistant materials.

In order to overcome such problems, steady research and development have been made.

In U.S. Patent No. 4,823,044, an example as illustrated in Figure I is described, wherein a porous metal base body la containing the platinum group elements of Os, Ir, Re, Ru, etc., and tungsten is disposed on the surface of an electron emissive material 2a stored in a reservoir 3a.

- n - 2 The operating temperature of the dispenser cathode of this tv c. pe is approximately 1000'C, which is quite higher than that of an oxide cathode working at about 7SO'C to 8OTC, so that the above-mentioned problems still remain unsolved.

Meanwhile, Japanese Laid-Open Patent No. 86-13526A describes a Se impregnated type cathode with a low operating temperature of 80WC to 90WC. In this impregnated type cathode as illustrated in Figure 2. a thin film layer lb made mainly of tungsten and scandium is coated on the surface of an electron emissive 10 material 2b.

Impregnated type cathodes, of this type have the problem of an adverse effect caused by the reaction of a Ba oxide and a Sc oxide. When a Ba oxide and a Sc oxide of the thermoelectron emissive source react with each other, Ba3Sc4O, is produced as a by-product and is accumulated on the upper surface of the thermoelectron emissive material. This causes the thermoelectron emissive characteristic to become partially unstable. Further, the time required for forming a monatomic layer containing Ba-ScO over the electron emissive surface, that is, the activation aging time becomes very long, thereby resulting in low productivity -)o and local unequalization in the thermoelectron emission characteristics.

Preferred embodiments of the present invention aim to provide a dispenser cathode in which a high density beam current may be obtained even at low temperature and which has electron emissive characteristics which mav be 25 maintained stably for a longer period.

Another aim is to provide a dispenser cathode which greatly shortens the activation aging time and improves productivity 1 Y ilk According to one aspect of the present invention, there is provided a dispenser cathode comprising an electron emissive material including at least one material selected from the group consisting of Ba and Ba oxide, and a porous metal base body having diffusing cavities therewithin and positioned on said electron emissive material, wherein said electron emissive material further includes a scandium tungstate.

Preferably, said scandium tungstate includes at least one material selected from the group consisting of SCW3012 and SC.WO12.

Preferably, the content of said scandium tungstate is 2 to 30wt% based on the total weight of said electron emissive material.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to Figure 1 3 of the accompanying diagrammatic drawing, which is a cross-sectional view of one example of a cavity reservoir type dispenser cathode according to the present invention.

In Figure 3, there is shown a cavity reservoir type dispenser cathode which comprises a cup type reservoir 3C, an electron emissive material 2c stored in the reservoir, a porous metal base body Ic positioned at the upper portion of the electron emissive material 2c, and a sleeve 4c which supports and fixes the reservoir 3c, and stores a heater 5c.

The reservoir 3c and the sleeve 4c are made of a high-melting 'point metal such as Mo, Ta, etc., and the electron emissive material 2c is mainly made of barium-calcium aluminate and contains SC2W4012 and/or SCWO12 in a proper amount.

The porous metal base body Ic is made by sintering a heat resistant metal powder such as tungsten, and if necessary, said body may selectively contain platinum group elements of Ir, Os, Ru, Re, etc.

Q, The above-mentioned electron emissive material 2c mav be manufactured by the following procedure.

Bac03. Caca03 and A'203, after being mixed at a mole ratio of 4: 1: 1 or 5:3:2, are baked at a temperature of 1200'C to 14000C for 8 hours.

After baking, the baked body of barium-calcium aluminate is mixed with tungsten powder with a weight ratio of 20 to 50%.

W andScApowder are mixed at a proper ratio and then, are baked under an oxidizing atmosphere, thereby manufacturingSC'W30,2 or SC6WO12, or a mixture thereof.

Then, the mixture of barium-calcium aluminate and W powder is mixed with the SC2W3012 and/or SC6W012 powder, with a weight ratio of 2 to 30wt%.

)o The final metal powder mixture, manufactured by the above-mentioned steps, is put in the reservoir 3c, and then is press-molded with a press jig, thereby forming the electron emissive material 2c within the reservoir 3c.

The above porous metal base body lc is prepared by press-molding and sintering W powder having an ordinary particle diameter of 5Am, and then is attached to the reservoir 3c, by spot welding. It may contain platinumgroup elements of Ir, Os, etc.

1 As described above, the cavity reservoir type dispenser cathode of Figure 3 has an element generating Ba-Sc-0 monatomic layer of a lower work function than the Sc supply source in the bottom of the porous metal base body. According to this configuration, by-products obtained from the reaction of Sc oxides and Ba oxides are restrained from being formed on the electron emissive material surface, which is explained in detail in the following.

When the electron emissive material is heated by the heater, the reaction of scandium-tungstate and Ba positioned at the porous metal base body produces scandium (Sc) according to the following reaction equation:

SC,W3012+ 3Ba = 3BaWO4 + 2Sc Therefore, Sc together with diffuse Ba is diffused through the cavity of the porous metal base body and a monatomic layer containing Ba-Sc-0 is formed on the surface of the porous metal base body. The by-products of reaction of Ba oxide and Sc oxide during this process are generated at the lower portion of the porous metal base body, but fail to reach the surface of the porous metal base body. As a result, the monatomic layer having an uniform structure can be formed on the surface of the porous metal base body, thereby making possible stable thermoelectronic emission for a longer period.

The activation aging time of the dispenser cathode of Figure 3 is about 2 hours, greatly shortening the time required for aging compared with the conventional Sc impregnated type cathode of which the activation aging time is 10 hours.

In the conventional Se impregnated type cathode, a thin film layer containing W and SC203 is formed on the surface of the cathode material so that heat transfer is not facilitated, thereby retarding production of scandium tungstate. On the other hand. the dispenser cathode of Figure 3 may rapidly produce vaporized Sc because scandium-tungstate is included in or positioned on the upper portion of the electron emissive material.

Furthermore, the Sc supply source is not positioned on the surface of the porous metal base body suffering severe ion bombardment but positioned at the lower portion thereof and therefore, the loss of Sc due to ion bombardment does not occur, thereby resulting in stable emission of the thermal electrons.

Thus the embodiment of Figure 3 may provide a cavity reservoir type dispenser cathode with a high-density beam current and a longer lifetime.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any

0 - accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination. except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s).

The invention extends to any novel one, or any novel combination, of the features 16 I disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

CLAEVIS:

1. A dispenser cathode comprising an electron emissive material including at least one material selected from the group consisting of Ba and Ba oxide, and a porous metal base body having diffusing cavities therewithin and positioned on said electron emissive material, wherein said electron emissive material further includes a scandium tungstate.

1 2. A dispenser cathode as claimed in claim 1, wherein said scandium tungstate includes at least one material selected from the group consisting of Sc2W3012 and SC6WO12.

3. A dispenser cathode as claimed in claim 1 or 2, wherein the content of said scandium tungstate is 2 to 30wt% based on the total weight of said electron emissive material.

4. A dispenser cathode, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawing.

71 Published 1991 at 7be Patent Office. State House. 66/71 High Holborn. London WCIR47?. Further copies maybe obtained from Sales Branch. Unit 6. Nine Mile Point, Cwmfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray, Kent-