JPH04286827A - Impregnated cathode - Google Patents

Abstract

PURPOSE:To provide a low-temperature-operation impregnated cathode which is coated with a mixed thin film formed of W and oxide containing W, Sc, O to reduce nonuniform distribution of work function on the surface of the cathode, or to reduce batch field effect. CONSTITUTION:A porous base 1 for a substrate-impregnated cathode pellet 3 is manufactured by heat resistant metal sintered material sized 1-5mum, smaller than that for conventional cathodes. The cavity percentage of the porous base 1 is within 25-32%, to provide a low work function (Ba, Sc, O) composite layer 8 uniformly on the surface of the cathode during operation.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an impregnated cathode used in electron tubes such as display tubes, cathode ray tubes, image pickup tubes, and traveling wave tubes, and in particular, it is a low-temperature operation type and is capable of obtaining high current density. Regarding an impregnated cathode.

0002

2. Description of the Related Art An impregnated cathode basically has a structure in which a heat-resistant porous substrate made of tungsten (W) or the like is impregnated with an electron-emitting substance made of a barium (Ba) compound. As a method of lowering the operating temperature of the cathode, Japanese Patent Publication No. 47-2134
As described in Japanese Patent No. 3, a common method is to coat the surface of the cathode with an osmium-ruthenium (Os-Ru) alloy. This porous substrate is made of a sintered body consisting of particles with a particle size of 6 to 10 μm, and the porosity is selected in the range of 18 to 20%. However, in such a method, the operating temperature of the cathode is as high as about 1000° C., which has been a major obstacle to promoting practical use.

Furthermore, as a method of lowering the operating temperature, Os-Ru
Tungsten (W), scandium (S) instead of alloy
c) and a method of coating a thin film containing oxygen (O). According to this method, the saturation current density is 1 at an operating temperature of 800°C.
0 A/cm 2 or more can be obtained, and as a result, the operating temperature can be lowered by 150 to 200° C. compared to the former method. However, an impregnated cathode coated with a thin film containing W, Sc, and O requires a high electric field to be applied to the cathode surface in order to obtain high electron emission performance. This is due to the existence of a patch field effect caused by the distribution of parts with high electron emission ability and parts with low electron emission ability, that is, parts with small and large work functions. This non-uniform distribution of the work function, ie the presence of the patch field effect, makes the boundary between the space charge limited region and the temperature limited region unclear, making it difficult to use the cathode at low temperatures.

0004

[Problems to be Solved by the Invention] As mentioned above, although the operating temperature of conventional impregnated cathodes is low, it is difficult to obtain high electron emission performance due to the non-uniform distribution of work functions on the cathode surface. The problem was that a high electric field had to be applied, making it difficult to put it into practical use.

[0005] An object of the present invention is to provide an impregnated cathode that can reduce the non-uniform distribution of the work function on the surface of an impregnated cathode coated with a thin film containing W, Sc, and O, that is, reduce the patch electric field effect, and that can withstand mounting. It's about doing.

[0006]

[Means for solving the problem] The above object is to make the heat-resistant porous substrate of the impregnated cathode a heat-resistant metal sintered body consisting of particles with a particle size of 1 to 5 μm, and to have a porosity of 25 to 32%. This can be achieved by

[0007]

[Function] FIG. 1 is a sectional view showing the schematic structure of the impregnated cathode of the present invention, which includes a base impregnated cathode pellet 3 consisting of a heat-resistant porous substrate 1 and an electron-emitting substance 2, a barrier layer 4, a sleeve 5, and a heater 6. , and a thin film 7 containing W and Sc2W3O12. First, by heating the cathode by energizing the heater 6, the heat-resistant porous substrate 1 and the electron-emitting substance 2 react with each other in the base-impregnated cathode pellet 3.
Generate a. For example, Ba3Al is used as the electron emitting material 2.
When 2O6 is used and W is used for the porous substrate 1, Ba is generated according to the following formula.

(2/3)Ba3Al2O6+(1/3)W→(1/3
)BaWO4+(2/3)BaAl2O4+Ba A part of the generated Ba diffuses to the cathode surface, and at the same time, other Ba reacts with the oxide containing W and Sc in the thin film 7 to generate Sc. For example, if the thin film 7 is W and Sc2W3
In the case of O12, Sc2W3O12 and Ba react to generate Sc according to the following formula.

Sc2W3O12+3Ba→3BaWO4+2Sc Ba and Sc diffused on the cathode surface combine with oxygen generated by thermal decomposition of the electron-emitting substance 2 and oxygen in the atmosphere,
On the thin film 7, an extremely thin (Ba, Sc, O) composite layer 8 of about a monomolecular layer to several molecular layers is formed. This composite layer 8 has a small electron emission work function of 1.2 eV, which is a factor in obtaining high electron emission ability. However, in the conventional impregnated cathode, this composite layer 8 is not uniformly formed over the entire surface area of the cathode, but is formed in an island shape. (Ba, Sc, O)
In the part where the composite layer 8 was not present, the work function was as large as 2.0 eV, and a non-uniform distribution of the work function was observed. Therefore, a patch electric field effect exists between the composite layer 8 and the thin film 7, and a high electric field must be applied to obtain high electron emission performance.

When the formation of the (Ba, Sc, O) composite layer 8 was carefully observed, it was found that the composite layer 8 was formed in an island shape centered on the pores of the porous substrate 1, that is, the portion of the electron-emitting material 2. . This is because the generated Ba, Sc has a slow diffusion rate and short residence time on the cathode surface.
O) Composite layer 8 could not cover the entire cathode surface area.

In order to cover the entire cathode surface area with the (Ba, Sc, O) composite layer 8 by compensating for the diffusion rate and residence time of Ba and Sc, it is necessary to By reducing the gap in the region, the coverage of the (Ba, Sc, O) composite layer 8 is improved, the non-uniform distribution of the work function is eliminated, and the patch electric field effect can be reduced. Therefore, the porous substrate 1 may be a metal sintered body made of finer grains than conventional ones. That is, regarding practical particle diameters, significant effects were observed in the range of 1 to 5 μm. Also,
The porosity governs the amount of Ba produced, and also influences the amount of Sc produced. In order to smoothly form the (Ba, Sc, O) composite layer 8, it is advantageous for the porosity to be larger than that of a conventional cathode. Regarding the porosity experimentally determined by measuring electron emission characteristics, a remarkable effect was observed in 25 cases.
In the range of ~32%, the electron emission properties were best at 28%.

0012

[Examples] The impregnated cathode of the present invention will be explained in detail below with reference to Examples.

First, the production of the impregnated cathode of the present invention will be explained based on FIG. We prepared W powder with particle sizes of 1, 3, and 5 μm and W powder with particle size of 7 μm, which is conventionally used for cathodes, and performed press molding, preliminary sintering in hydrogen, and main sintering in vacuum to form pores. Heat-resistant porous substrate 1 with a diameter of 1.2 mm and a thickness of 0.45 mm with a ratio of 20, 23, 25, 28, and 32%
was produced. In addition, the porosity can be adjusted by press molding pressure,
This was done by varying the sintering temperature. 4BaO・CaO・Al2O was applied to the prepared porous substrate 1 in a hydrogen atmosphere.
The electron-emitting material 2 having the composition of 3 is heated and melted and impregnated,
Substrate-impregnated cathode pellet 3 was manufactured. Next, the pellets 3 were inserted into a cup-shaped barrier layer 4 made of molybdenum (Mo), and further into a Mo sleeve 5 for enclosing the heater 6, and then fixed together to produce a cathode body.

Next, using an apparatus (not shown) capable of sputtering multiple substances simultaneously, W, Sc2
Using two types of W3O12 as sputter targets, the above W and Sc2W3O12 are applied to the surface of the pellet 3 of the cathode body.
A mixed thin film 7 was formed.

Regarding the impregnated cathode formed as described above,
Two parallel plate electrodes consisting of an anode and a cathode are arranged in a high vacuum container with a vacuum degree of 10-7 Pa class, and the cathode temperature is set to 115
After activating the cathode by heating it to 0℃, the cathode temperature was increased to 8℃.
The emission current of the cathode was measured when the temperature was lowered to 50° C. and a positive pulse voltage was applied to the anode. FIG. 2 shows the relationship (Schottky curve) between the cathode emission current density and the square root of the applied voltage. In the figure, curve 21 is the emission current density characteristic of the impregnated cathode according to the present invention, curve 22 is the characteristic of the conventional impregnated cathode, and curve 23 is the characteristic of the conventional impregnated cathode.
This is a characteristic of a coated impregnated cathode.

Table 1 shows the results of a qualitative evaluation of the electron emission characteristics of the cathode according to the manufacturing conditions of the heat-resistant porous substrate explained in the above examples, and the symbols are A>B> in order of superior characteristics.
C>D. A and B are of a practical level as cathodes.

[0017]

[Table 1]

As can be seen from FIG. 2, the emission current density characteristic 21 of the impregnated cathode according to the present invention provides a higher emission current density than the characteristic 22 of the conventional impregnated cathode, especially in a low electric field. It's becoming noticeable. Furthermore, similar to characteristic 23 of the conventional Os-Ru coated impregnated cathode at a practical level, the transition from the space charge limited region to the temperature limited region became clear. This means that the coverage of the low work function (Ba, Sc, O) composite layer 8 on the cathode surface was improved and the non-uniform distribution of the work function was reduced. Furthermore, even when Auger analysis of the cathode surface was performed at the operating temperature (cathode temperature of 850° C.), it was clearly confirmed that the coverage of the (Ba, Sc, O) composite layer 8 was improved.

[0019]

As described above, in the impregnated cathode having a mixed film of oxides containing W and Sc according to the present invention, the heat-resistant porous substrate is a sintered body made from powder with a smaller particle size than conventional ones, and By increasing the porosity to 25% to 32%, which is higher than before, the cathode surface has low work function (Ba, Sc, O
) A composite layer can be easily formed with a high coverage rate, and the non-uniform distribution of work function can be reduced, that is, the patch electric field effect can be reduced. Therefore, the impregnated cathode according to the present invention can obtain a high emission current density even in a small electric field, and can be used as a practical level impregnated cathode operating at low temperatures.

Furthermore, when the impregnated cathode according to the present invention is mounted in an electron tube, the operating temperature can be further lowered by 150 to 200°C than the conventional Os-Ru coated impregnated cathode, so that Ba and The evaporation rate of BaO can be reduced by about one order of magnitude, and deterioration of tube characteristics caused by grid emissions etc. can be prevented. Furthermore, since the impregnated cathode according to the present invention operates at low temperatures, the reliability of the heater that heats the cathode is greatly improved.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a schematic structure of an impregnated cathode of the present invention.

FIG. 2 is a diagram comparing the emission current density characteristics of an impregnated cathode constructed according to the present invention and a conventional impregnated cathode.

[Explanation of symbols]

1... Heat-resistant porous substrate, 2... Electron-emitting material, 3...
... Pellet, 7... Mixed thin film of oxide containing W and Sc, 8... (Ba, Sc, O) composite layer, 21...
Characteristics of the impregnated cathode of the present invention, 22, 23...Characteristics of the conventional impregnated cathode.

Claims (1)

[Claims] Claim 1: A heat-resistant porous substrate with an electron-emitting substance in its pores and tungsten (W) and scandium (
In the impregnated cathode provided with a thin film containing Sc) and oxygen (O), the heat-resistant porous substrate is made of a heat-resistant metal sintered body of particles with a particle size of 1 to 5 μm, and has a porosity of 25 to 32%. An impregnated cathode comprising: