JPH11195367A - Impregnated negative electrode structure, manufacture thereof, electron gun structure, and electron tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a impregnated negative electrode structure capable of conducting low temperature operation and high current density operation, of a long service life, and of a fast surface scandium concentration restoration time after an ion bombardment. SOLUTION: In a negative electrode for an electron tube provided with a negative electrode substrate in which electron emitting substrate 12 is impregnated, a negative electrode sleeve having the negative electrode substrate at an end part of it, and a heater to heat the negative electrode substrate, a thin film layer 13 including scandium and high fusion point metal, or a thin film layer 13 containing metal scandium or scandium compound formed by sputtering in a reducing atmosphere is formed on an electron emitting surface of the negative electrode substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impregnated cathode structure used for an electron tube such as a color picture tube or a traveling wave tube.
In particular, the present invention relates to a high-performance, long-life impregnated cathode assembly capable of operating at a high current density, a method of manufacturing the same, an electron gun assembly, and an electron tube.

[0002]

2. Description of the Related Art In recent years, there has been a demand for the development of a color picture tube in which the resolution is improved by increasing the number of scanning lines, and a picture tube compatible with ultrahigh frequency. Further, it is desired that the luminance of a projection tube and the like be improved. In order to meet these demands, it is necessary to increase the emission current density from the cathode much more than before.

Incidentally, the impregnated cathode has a higher emission current density than the oxide cathode, and has been used for electron tubes such as image pickup tubes, traveling wave tubes, and klystrons.
In the field of color picture tubes, it has been limited to only special applications such as HD-TV tubes and ED-TV tubes.
Demands for use and the like are increasing, and the uses are rapidly expanding.

[0004] The impregnated cathode structure used in the above color picture tube has conventionally been formed in a compact structure for the purpose of saving power. That is, for example, in the cathode structure shown in FIG. 1, the cup-shaped fixing member 2 is fixed inside one end of the cathode sleeve 1 so as to be substantially flush with the opening edge of the one end. In addition, a porous cathode substrate 3 impregnated with an electron emitting substance is fixed. Also,
A cylindrical holder 4 is coaxially arranged so as to surround the cathode sleeve 1.

A plurality of cathode sleeves 1 (3 in the drawing)
) Are coaxially supported inside the cylindrical holder 4. That is, one end of the strip-shaped strap 5 is attached to the outer surface of the other end of the cathode sleeve 1, and the other end is attached to the inside protrusion of one end of the cylindrical holder 4. In addition, a shielding tube 7 is arranged between the cathode sleeve 1 and the plurality of straps 5, and is attached to an inner protruding portion of one end of the cylindrical holder 4 by a support piece 6. Further, a heater 8 is inserted inside the cathode sleeve 1, and the porous cathode substrate 3
Is heated.

The porosity of the porous cathode substrate 3 is about 20%.
And the material is tungsten. The pores of the porous cathode substrate 3 are impregnated with an electron-emitting substance comprising BaO, CaO and Al ₂ O ₃ , and an Ir film is formed on the surface thereof by a sputtering method. In addition, this cathode structure
Strap 9 attached to the outer surface of cylindrical holder 4
Are fixed to the insulating support 10 together with a plurality of electrodes (only the first grid G1 is shown in the drawing) which are sequentially arranged at a predetermined interval via.

More recently, for the purpose of lowering the operating temperature and obtaining electron emission with a high current density, an impregnated cathode in which scandium oxide is dispersed on a cathode substrate, or a thin film layer of scandium oxide is applied to the surface of the cathode substrate. Scandium oxide-coated cathodes have been developed, but their reproducibility is poor, which is a barrier to practical application.

[0008]

As described above, the impregnated cathode structure conventionally used for a color picture tube has a compact structure for the purpose of power saving operation. Therefore, the thickness and diameter of the cathode substrate are necessarily limited, and the cathode substrate cannot be sufficiently impregnated with the electron emitting material.

In general, the life characteristics of an impregnated cathode are governed by the amount of evaporation of barium, which is a main component of an electron-emitting substance.
When barium is consumed by evaporation, the barium monoatomic layer coverage of the cathode substrate decreases, and the required long life characteristics cannot be obtained, which is a serious problem in practical use. For these reasons, there is a strong demand for the development of an impregnated cathode assembly capable of operating at a low temperature and at a high current density.

Further, even a scandium oxide impregnated cathode capable of operating at a low temperature has a drawback such as slow recovery of electron emission characteristics when subjected to ion bombardment. Surface analysis of the scandium oxide impregnated cathode by Auger electron spectroscopy revealed that scandium on the surface disappeared when subjected to ion bombardment, and it took time for the electron emission to recover to a favorable concentration.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems of the conventional scandium oxide impregnated cathode and the scandium oxide coated cathode, and is capable of operating at a low temperature and a high current density, and has a long life. It is another object of the present invention to provide an impregnated cathode assembly having a fast scandium surface concentration recovery time after ion bombardment.

Another object of the present invention is to provide a method for manufacturing such an impregnated cathode structure. Still another object of the present invention is to provide a highly reliable electron gun structure and electron tube.

[0013]

According to a first aspect of the present invention, there is provided a cathode substrate impregnated with an electron-emitting substance, and the cathode substrate is attached to an end thereof. In an impregnated cathode assembly comprising a cathode sleeve and a heater for heating the cathode substrate, scandium and / or a scandium compound and a high melting point metal and / or a high melting point metal compound are provided on the electron emitting surface side surface of the cathode base. And a thin film layer containing the following.

In the impregnated cathode structure according to the first aspect of the present invention, examples of the high melting point metal and / or high melting point metal compound contained in the thin film layer formed on the electron emission surface of the cathode base include molybdenum and the like. Can be A high melting point metal such as molybdenum has an effect of preventing oxidation of metal scandium.

The content of the high melting point metal and the high melting point metal compound in the thin film layer is preferably 0.1 to 5% by weight. When the content of the high melting point metal or the high melting point metal compound is less than 0.15% by weight, the effect of preventing the oxidation of scandium metal is insufficient, and when it exceeds 5% by weight, sufficient electron emission characteristics cannot be obtained.

The thickness of the thin film layer is preferably 100 to 10
00 angstroms. If the thickness of the thin film layer is less than 100 angstroms, the film itself may be lost by sputtering due to ion bombardment. On the other hand, when the thickness exceeds 1,000 angstroms, exudation of the electron-emitting substance from the base metal part is reduced, and it becomes difficult to obtain good electron-emitting characteristics.

The thin film layer can be formed by laminating a scandium layer, a scandium compound layer, a high melting point metal layer, and a high melting point metal compound layer in this order from the electron emission surface side. Even with such a laminate, oxidation of scandium and a scandium compound can be prevented. Also in this case, the ratio of the high melting point layer to the laminate is the same as the content ratio of the high melting point layer described above.

According to the first aspect of the present invention, the thin film layer coated on the electron emission surface of the cathode substrate is composed of scandium and / or
Alternatively, since it contains a scandium compound and a high melting point metal and / or a high melting point metal compound, that is, scandium is used, the recovery time of scandium after ion bombardment can be significantly reduced. Also, since the oxidation of scandium is prevented by a high melting point metal such as molybdenum,
The deterioration of the cathode due to oxidation during the manufacturing process of the cathode and the electron tube can be greatly improved.

According to a second aspect of the present invention, a cathode substrate impregnated with an electron emitting material, a cathode sleeve having the cathode substrate attached to an end thereof, and a heater for heating the cathode substrate are provided. The present invention provides an impregnated-type cathode structure comprising an electron tube cathode provided with a thin film layer containing metal scandium or a scandium compound formed by sputtering in a reducing atmosphere on an electron emission surface of the cathode substrate.

In the impregnated cathode structure according to the second aspect of the present invention, scandium oxide or scandium hydride can be used as the scandium compound. The thickness of the thin film layer is preferably between 100 and 1000 angstroms. If the thickness of the thin film layer is less than 100 angstroms, the film itself may be lost by sputtering due to ion bombardment. On the other hand, when the thickness exceeds 1,000 angstroms, exudation of the electron-emitting substance from the base metal part is reduced, and it becomes difficult to obtain good electron-emitting characteristics.

According to a second aspect of the present invention, there is provided an impregnated cathode assembly comprising: a step of impregnating the cathode substrate with an electron emitting substance; and a step of depositing a metal scandium or a scandium compound on the electron emitting surface of the cathode substrate by sputtering in a reducing atmosphere. Can be manufactured by a process of forming a thin film layer containing.

As the reducing atmosphere, a rare gas containing hydrogen gas can be used. According to the second aspect of the present invention, as a thin film layer coated on the electron emission surface of the cathode substrate,
Since a highly active metal scandium or scandium compound is used by sputtering in a reducing atmosphere, the recovery time of scandium after ion bombardment can be greatly reduced. In addition, since low-temperature operation is possible, the amount of Ba evaporated can be reduced, and sufficient life characteristics can be obtained even when the thickness of the cathode base is small.

The third (claim 8) and fourth (claim 9) aspects of the present invention relate to an electron gun assembly using the impregnated cathode assembly of the present invention and an electron tube using the electron gun assembly. It is. As a result, a highly reliable electron gun structure and electron tube having sufficient life characteristics can be obtained.

[0024]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows an impregnated cathode assembly according to one embodiment of the present invention. Its basic structure is
This is almost the same as the conventional impregnated cathode structure described above.
That is, in the cathode assembly shown in FIG. 1, the cup-shaped fixing member 2 is fixed inside one end of the cathode sleeve 1 so as to be substantially flush with the opening edge of the one end. A porous cathode substrate 3 impregnated with an electron emitting substance is fixed. A cylindrical holder 4 is coaxially arranged so as to surround the cathode sleeve 1.

A plurality of cathode sleeves 1 (3 in the drawing)
) Are coaxially supported inside the cylindrical holder 4. That is, one end of the strip-shaped strap 5 is attached to the outer surface of the other end of the cathode sleeve 1, and the other end is attached to the inside protrusion of one end of the cylindrical holder 4. In addition, a shielding tube 7 is disposed between the cathode sleeve 1 and the plurality of straps 5, and is attached to an inner protruding portion of one end of the cylindrical holder 4 by a support piece 6. Further, a heater 8 is inserted inside the cathode sleeve 1, and the porous cathode substrate 3
Is heated.

FIG. 2 is an enlarged view showing a part of the surface area of the porous cathode substrate described above. The porous cathode substrate is a porous tungsten substrate 11 made of a tungsten sintered body.
And a thin film layer 13 formed on the porous tungsten base 11. Note that an electron-emitting substance 12 is impregnated in the porous tungsten base 11.

The second embodiment of the present invention in which the thin film layer 13 is different will be described below.
7 shows an example according to one aspect. Example 1 This example relates to an embodiment in which the thin film layer contains scandium and a high melting point metal.

A barium oxide (BaO): calcium oxide (CaO): aluminum oxide (Al ₂ O ₃ ) = 4: 1 was applied to a porous tungsten substrate 11 made of a tungsten sintered body having a porosity of 20% as shown in FIG. The electron emitting material 12 consisting of a mixture in a molar ratio of 1: 1 is melt-impregnated into the holes of the cathode substrate in a hydrogen atmosphere.

The impregnated cathode substrate obtained by the above impregnation was attached to a binary sputtering apparatus having a scandium target and a molybdenum target, and was sputtered so that the amount of molybdenum in scandium was 0.5% by weight. An Angstrom thin film 13 is formed. The sputtering atmosphere is basically Ar gas, but a better thin film can be obtained by introducing a small amount of H ₂ gas.

An anode was attached to the impregnated cathode assembly thus prepared, and an electron tube having a diode structure was prepared. The electron emission characteristics were evaluated in the same manner as in Example 1. As a result, the conditions for evaluating the electron emission characteristics in the figure are as follows. The operating temperature of the cathode is about 1300 K, and 2
A pulse of 00V was applied. Here, the duty of the applied pulse was changed from 0.1% to 9.0%. When a voltage is applied to the anode, the cathode is subjected to ion bombardment by ions generated in the electron tube, and the consumption of scandium increases. When no voltage is applied to the anode, scandium is supplied for thermal diffusion on the surface of the cathode, and the recovery of the surface coverage increases.

As the duty becomes higher, the intensity of the ion bombardment increases. The conventional scandium-type impregnated cathode shows good emission current characteristics in the low duty region, but deteriorates in the high duty region,
This has been an obstacle to practical application to color picture tubes and the like that require operation in a higher duty range.

FIG. 3 shows the duty-emission current density characteristics of the cathode in this embodiment. In the figure, the curve X ₁ shows the measurement results in the case of using the conventional scandium oxide-coated cathode, curve Y ₁ scandium according to the embodiment - the measurement results obtained by using molybdenum thin film coating cathode, curve Z ₁ is The measurement results when a conventional non-scandium impregnated cathode is used are shown.

As is apparent from FIG. 3, in the conventional scandium oxide-coated cathode, the duty is 0.1% and 45%.
While the emission current density is A / cm ^2, the emission current density decreases to 12 A / cm ² at a duty of 9%.
On the other hand, in the scandium-molybdenum thin film-coated impregnated cathode according to the present embodiment, 28 A / c at a duty of 0.1%.
Although the emission current density is m ² , it is 16% at 9% duty.
It can be seen that the emission current density is reduced only to the emission current density of A / cm ² , and the electron emission characteristics in the high duty region are improved.

This is presumably because the use of scandium metal having a high diffusion rate in place of scandium oxide greatly improved the supply of scandium and the recovery of the coverage.

Next, an environmental resistance test was carried out as follows. As a result, the scandium-molybdenum thin film-coated cathode according to the present embodiment contained molybdenum, thereby preventing oxidation of metal scandium and using only metal scandium. It was found that the environment resistance was superior to that of the coated cathode.

The environmental resistance test was performed using a diode having a cathode and an anode mounted in a vacuum device. That is, the cathode mounted in the vacuum device is activated by a predetermined program, and the cathode temperature is set to 1
A current of 1.5 A / cm ² was obtained stably at 250K. Then, open the bell jar of the vacuum apparatus and set the humidity at 8
It was exposed to an atmosphere of 0% and a temperature of 25 ° C. for 10 minutes. Thereafter, the bell jar was closed, and the inside of the apparatus was evacuated to a high vacuum.

Next, the heater is turned on while the voltage at which the current density is obtained is set on the anode. The time when the heater is turned on is set to 0 second, and the change over time of the cathode current is measured. FIG. 4 shows the result. In the figure,
Curve A shows the case of a cathode sputtered with only scandium, and curve B shows the case of a cathode sputtered with molybdenum (0.5% by weight) and scandium according to the present example.

As can be seen from FIG. 4, in the case of a cathode sputtered with only scandium, it takes 5 minutes to recover to 1 A / cm ² , but in the case of this embodiment, it recovers to the same value in 2 minutes. doing. Further, in order to completely return to the original state, it is necessary to use 20% in the case of a cathode in which only scandium is sputtered.
It takes more than a minute, but in the case of the present embodiment, it can be seen that it recovers to almost the original value in 10 minutes.

From these results, it was found that the addition of molybdenum is very important for improving the moisture resistance of scandium. However, the addition amount of molybdenum is 0.1%
If less than, the effect of molybdenum addition is difficult to be exhibited,
The recovery time is not much different from scandium alone. Conversely, when the addition amount exceeded 5%, only a current of 20 A / cm ² or less could be obtained at a low duty.

In the above embodiments, scandium and molybdenum were simultaneously sputtered by a binary sputtering apparatus. However, the thin film layer was composed of a laminate of a scandium layer and a molybdenum layer, and the outermost surface was formed of a molybdenum layer. Even with such a configuration, the same effect of environmental resistance characteristics can be obtained.

Example 2 This example relates to an embodiment in which the thin film layer contains metal scandium or a scandium compound formed by sputtering in a reducing atmosphere.

A barium oxide (BaO): calcium oxide (CaO): aluminum oxide (Al ₂ O ₃ ) = 4: 1 was applied to a porous tungsten substrate 11 made of a tungsten sintered body having a porosity of 20% as shown in FIG. The electron emitting material 12 consisting of a mixture in a molar ratio of 1: 1 is melt-impregnated into the holes of the cathode substrate in a hydrogen atmosphere.

The porous substrate 11 obtained by the above impregnation
A scandium hydride (ScH ₂ ) layer 13 is formed on the surface of the substrate by a sputtering method (after removing an unnecessary electron-emitting substance from the surface of the base if necessary). The thickness of the scandium hydride thin film layer is 20 nm, and as a sputtering gas,
1 vol. % Ar gas containing H ₂ gas was used.

The impregnated cathode assembly thus prepared was provided with an anode, an electron tube having a diode configuration was prepared, and the electron emission characteristics were evaluated in the same manner as in Example 1. as a result,
A duty-emission current density characteristic as shown in FIG. 5 was obtained. In the figure, a curve X ₂ shows the measurement result of the conventional scandium-type impregnated cathode, a curve Y ₂ shows a measurement result of the scandium-type impregnated cathode according to the present invention, and a curve Z ₂ shows the measurement result of the conventional non-scandium-type impregnated cathode.

As is apparent from FIG. 5, the conventional scandium-type impregnated cathode has a duty ratio of 0.5% and a current of 45 A / A.
cm ² emission current density and duty 9
% To 13 A / cm ² emission current density. on the other hand,
In the scandium-type impregnated cathode according to the present embodiment, the emission current density is 32 A / cm ² at a duty of 0.5%.
At a duty of 9%, the emission current density is reduced to only 18 A / cm ² , indicating that the electron emission characteristics in a high duty region are improved.

This is probably because the activity of scandium atoms present on the surface of the cathode substrate was high, and the supply of scandium and the recovery of the coverage after ion bombardment were greatly improved. Further, the scandium-type impregnated cathode according to the present embodiment has better emission current characteristics in both low and high duty regions than the conventional non-scandium-type impregnated cathode.

In the above embodiment, scandium hydride was used as a material for sputtering. According to the experiments of the present inventors, sputtered scandium hydride showed the best characteristics as a scandium-type impregnated cathode, but instead of using metal scandium or scandium oxide, scandium hydride was used. It showed good characteristics, very close to the case.

In the above embodiment, the gas composition at the time of sputtering is 1 vol. However, according to experiments by the present inventors, as the proportion of hydrogen in the argon gas is increased, the reducibility of the atmosphere is increased, and the activity of scandium atoms in the thin film is increased. It was also found that the film formation rate of the thin film during sputtering was decreased by increasing the hydrogen component.

Further, according to an experiment by the present inventors, as the mixing ratio of hydrogen gas to argon gas as a sputtering gas is increased, 10 vol. %, The reducibility of scandium increases steadily, but if the proportion of hydrogen gas is further increased, the activity of scandium atoms is almost saturated. When the mixing ratio of hydrogen gas is 10 vol. %, The deposition rate of scandium is significantly reduced, and the production efficiency as an industrial product becomes impractical.

Considering the increase in the reducing property and the decrease in the film formation rate, that is, the product cost and the product quality, the ratio of the hydrogen gas in the argon gas is 1 vol. % Is considered the most appropriate.

The method of manufacturing the impregnated cathode structure according to the present embodiment includes forming a cathode base (sintering if necessary).
, Followed by impregnation with an electron-emitting substance, and finally, forming a scandium metal or scandium compound thin film layer.

That is, according to the experiments of the present inventors, when a metal scandium or a scandium compound layer is formed and then impregnated with an electron-emitting substance, the highly active scandium atoms present on the electron-emitting surface become thin films. Are exposed to a high temperature after the formation, and at the same time come into contact with a refractory metal such as tungsten of the cathode substrate or an impregnated electron emitting material having a very high degree of oxidation, these materials react with each other, and the activity of the scandium atom is increased. , And further oxidation of scandium atoms. Therefore, effective scandium diffusion was not exhibited during operation of the cathode assembly, and it was confirmed that the operation reduction effect was not obtained.

An electron gun assembly was obtained using the impregnated cathode assemblies according to Examples 1 and 2, and an electron tube was obtained using the electron gun assembly. These electron gun assemblies and electron tubes had sufficient life characteristics and high reliability.

[0054]

As described above, according to the first aspect of the present invention, since the thin film layer containing scandium and the high melting point metal is formed on the surface of the cathode substrate, the thickness after ion bombardment is reduced. It is possible to greatly reduce the recovery time of scandium, and also to significantly improve environmental resistance characteristics, which are a problem with a thin film layer composed of only scandium metal.

According to the second aspect of the present invention, a thin film layer containing metal scandium or a scandium compound formed by sputtering in a reducing atmosphere is formed on the surface of the cathode substrate. The electron emission surface of this is a surface in which scandium atoms having a very active diffusion rate and a high diffusion rate are uniformly distributed, and thus the scandium recovery time after ion bombardment is greatly reduced. In addition, since low-temperature operation is possible, the amount of thermal evaporation of barium can be reduced even if the size of the cathode substrate is compact, and the life characteristics resulting from insufficient absolute impregnation amount, which has been a problem with conventional power-saving impregnated cathodes, have been improved. The obtained impregnated cathode is obtained. According to the third and fourth aspects of the present invention, it is possible to obtain a highly reliable electron gun structure and electron tube having sufficient life characteristics.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an impregnated cathode according to one embodiment of the present invention.

FIG. 2 is an enlarged sectional view showing a part of the impregnated cathode of FIG. 1;

FIG. 3 is a characteristic diagram showing duty emission current density characteristics of one embodiment of the present invention and a conventional scandium-type impregnated cathode assembly.

FIG. 4 is a characteristic diagram showing the results of an environmental resistance test of a scandium-coated cathode according to one embodiment of the present invention containing molybdenum and a cathode coated with scandium only.

FIG. 5 is a characteristic diagram showing duty emission current density characteristics of another embodiment of the present invention and a conventional scandium-impregnated cathode assembly.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Cathode sleeve 2 ... Cup-shaped fixing member 3 ... Porous cathode base 4 ... Cylindrical holder 5 ... Strip-shaped strap 6 ... Support piece 7 ... Shielding cylinder 8 ... Heater 11 ... Porous tungsten base 12 ... Electron emitting material 13 ... Thin film layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Ikuyo Oyama 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Inside the Toshiba Yokohama office (72) Inventor Toshiharu Higuchi 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Inside the Toshiba Yokohama Office (72) Inventor Sadao Matsumoto 8 Shinsugita-cho, Isogo-ku, Yokohama, Kanagawa Prefecture Inside the Toshiba Yokohama Office (72) Yoshiaki Ouchi 8 Shin-Sugita-cho, Isogo-ku, Yokohama, Kanagawa Toshiba Yokohama Office

Claims (9)

[Claims] 1. A cathode substrate impregnated with an electron emitting material,
A cathode for an electron tube, comprising: a cathode sleeve having a cathode substrate attached to an end thereof; and a heater for heating the cathode substrate, wherein scandium and / or a scandium compound and a high melting point An impregnated cathode assembly comprising a thin film layer containing a metal and / or a high melting point metal compound. 2. The impregnated cathode assembly according to claim 1, wherein the content of the high melting point metal in the thin film layer is 0.1 to 5% by weight. 3. The thin film layer has a thickness of 100 to 10,000.
3. The impregnated cathode assembly according to claim 1, wherein the cathode is an Angstrom. 4. The method according to claim 1, wherein the thin film layers are arranged in order from an electron emission surface side.
4. The impregnated cathode assembly according to claim 1, wherein the cathode assembly comprises a laminate of a scandium layer and a high melting point layer. 5. A cathode substrate impregnated with an electron emitting material,
In a cathode for an electron tube comprising a cathode sleeve having a cathode substrate attached to an end thereof and a heater for heating the cathode substrate, a metal formed by sputtering in a reducing atmosphere on an electron emission surface of the cathode substrate. An impregnated cathode assembly having a thin film layer containing scandium or a scandium compound. 6. The impregnated cathode assembly according to claim 5, wherein the scandium compound is scandium oxide or scandium hydride. 7. A method comprising: impregnating a cathode substrate with an electron-emitting substance; and forming a thin film layer containing metal scandium or a scandium compound on the electron-emitting surface of the cathode substrate by sputtering in a reducing atmosphere. A method for producing an impregnated cathode structure. 8. An electron gun assembly comprising the impregnated cathode assembly according to claim 1. 9. An electron tube using the electron gun assembly according to claim 5.