JPH05205610A - Hot cathode and electron tube using it - Google Patents

Abstract

PURPOSE:To enable high current density operation and low temp. operation by supplying barium atoms from the lower part of a surface thin film layer, and specifying the area of the region to be supplied from the part directly under. CONSTITUTION:A surface thin film layer 2 is formed on the over-surface of a thick film layer 2, and besides tungsten the thin film layer 3 contains at least scandium or its oxide. Barium atoms are supplied from below the thin film layer 3, and a barium supplying body is provided in which the area of the region supplied with barium atoms occupies not less than 50% of the area of the region utilizing electrons emitted from the surface of a facial covering film layer. A base board 1 of high melting point metal is used a disc having a recess in the center, and the thin film layer 3 and base board 1 are connected electrically at the periphery of the disc. Accordingly barium is supplied to the thin film layer 3 uniformly in a large quantity to generate good electron emission, and temp. rise in the neighborhood of a cathode can be suppressed. It is also possible to make low temp. operation which enables large current density operation, to lead to enhancement of the reliability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot cathode suitable for electron tubes such as cathode ray tubes and image pickup tubes, and more particularly to the structure of a hot cathode which operates at a large current and at a low temperature.

[0002]

2. Description of the Related Art A cathode capable of operating at a large current density is required for increasing the output of an electron tube, particularly for increasing the brightness and definition of a cathode ray tube. Impregnated cathodes are generally used for this purpose. The impregnated cathode consists of a heat-resistant porous substrate made of tungsten, mainly composed of BaO, and other materials such as Al ₂ O ₃ and CaO.
Is melted by heating in a hydrogen atmosphere or in a vacuum and impregnated as a composite oxide. This melt impregnated material is
Generally called an impregnating agent.

The impregnated cathode always operates at 1000 ° C.
It has been heated to a certain degree, and the heat resistant porous substrate and the impregnating agent react with each other to release barium atoms. Barium atoms are supplied to the surface of the cathode by diffusion, and are adsorbed on the tungsten substrate, which is the surface of the cathode, together with oxygen atoms supplied from the inside of the cathode or the atmosphere inside the electron tube. Thus, when the adsorption layer composed of barium atoms and oxygen atoms is formed on the metal surface, the work function of the metal surface is substantially lowered, and the electron emission becomes easy. This is the principle of operation of the impregnated cathode, and detailed consideration therefor can be found in, for example, Journal of Physics D: Applied Physics, Volume 15 (1982), pages 1519 to 1529 (J. Phys.
D: Appl. Phys., 15 (1982) 151-1529).

The impregnated cathode has a small electric resistance because the electron emitting portion is basically a metal. Therefore, the impregnated cathode does not undergo cathode deterioration due to decomposition of the cathode material itself due to Joule heat generation, unlike a so-called high-resistance oxide cathode which is an oxide of an alkaline earth metal. Therefore, the impregnated cathode can operate at a higher current density than the oxide cathode. However, while the impregnated cathode is capable of high current density operation, it needs to be operated at a higher operating temperature than the operating temperature of about 750 ° C. for oxide cathodes. In the case of the impregnated cathode having the above-mentioned basic structure, it is necessary to heat it to about 1100 ° C. in order to obtain the current density of 10 A / cm ^{2 in} the space charge limited state.

Therefore, the impregnated cathode has been improved for the purpose of lowering its operating temperature. For example,
An impregnated cathode in which the surface of the impregnated cathode having the above-mentioned structure is coated with a platinum group such as osmium or iridium has been developed, and its operating temperature has dropped to about 1000 ° C. Regarding these, IEE Proceedings, Volume 128, Part 1, Number 1 (1981), pages 19 to 32 (IEE Pr
oc., 128, Pt1, No1 (1981) 19-32). Similarly, scandium is mainly used for tungsten and Sc
The impregnated-type cathode coated with a thin film containing a / W atomic ratio of 0.05 to 0.30 (hereinafter abbreviated as Sc-coated impregnated-type cathode) is 90
It can operate at 0 ° C. Regarding the cathode, Japanese Journal of Applied Physics, Volume 28,
Number 3 (1989), pages 490 to 494 (Jpn. J. App
l. Phys. 28, No. 3 (1989) 490-494).

[0006]

As described above, the impregnated cathode is capable of high-current-density operation, but the Sc-coated impregnated cathode operating at the lowest temperature has a temperature of 900 ° C. In comparison, the operating temperature is as high as 150 ° C. Such a high cathode temperature causes an increase in the temperature of the electrode near the cathode, which causes a change in the electrode spacing due to thermal expansion and unnecessary thermionic emission. Therefore, the electron gun of the cathode ray tube using the impregnated cathode is apt to cause a decrease in reliability as compared with the electron gun using the oxide cathode.

Further, the electron emission from the Sc-coated impregnated cathode is non-uniform, and the high electron emission region corresponds to the region where barium is mostly supplied. This is 1991 S.
ID International Symposium Digest of Technical Papers pp. 707-710 (1991 SID International Symposium Digest of Te
chnical Papers pp.707-710). If the barium supply to the cathode surface is made uniform and the density is high and the characteristics of the high electron emission region can be obtained over the entire area of the cathode surface, a better electron emission characteristic can be obtained. In other words, it is clear that the operating temperature can be reduced. However, in the Sc-coated impregnated cathode, the barium supply to the coating film is limited to the pores of the porous substrate. Moreover, since the porous substrate is a sintered body of powder of a high melting point metal, it was practically impossible to make the hole portion 50% or more. Therefore, it was difficult to make uniform the supply of barium to the surface of the cathode and measure the decrease in operating temperature with the basic structure of the conventional impregnated cathode using a porous substrate.

Therefore, it is an object of the present invention to provide a hot cathode capable of high current density operation and low temperature operation, and a highly reliable electron tube using the hot cathode. Another object of the present invention is to provide a hot cathode capable of high current density operation of about 10 A / cm ² required for a cathode ray tube and operating at a lower temperature than an impregnated cathode, and a highly reliable electron tube using the hot cathode. To do.

[0009]

In order to achieve the above object, the hot cathode and the hot cathode of the electron tube of the present invention include a surface thin film layer mainly containing a refractory metal and containing scandium or scandium oxide, and the above surface. The thin film layer is composed of a layer that supplies barium atoms, and the area of the layer that supplies barium atoms occupies 50% or more of the area of the region that utilizes the electrons emitted from the surface of the surface coating film. It was composed.
Therefore, in the first configuration of the hot cathode of the present invention, a layer for supplying the barium atom is a refractory metal substrate and a thick film layer having at least an oxide containing barium as a constituent element is laminated on the refractory metal substrate. Then, the surface thin film layer was formed on the upper surface of the thick film layer. In the second configuration of the hot cathode of the present invention, an intermediate thin film layer mainly containing a refractory metal and containing barium oxide is formed between the thick film layer and the surface thin film layer of the first configuration. Furthermore, the third configuration of the hot cathode of the present invention is a configuration in which the thick film layer of the hot cathode of the first or second configuration contains refractory metal powder.

Here, the refractory metal of the refractory metal substrate, the intermediate thin film layer, and the refractory metal powder is typically tungsten, but is not limited to this. It is composed of molybdenum, tungsten, or an alloy containing at least one of molybdenum as a main component. The oxide containing barium in the thick film layer on the refractory metal substrate is the same as the impregnating agent for the impregnated cathode, and the molar ratio of BaO, Al ₂ O ₃ , and CaO is 4: 1: 1 or 5 : 2: 3 is preferable.

The surface thin film layer contains scandium in tungsten at a Sc / W atomic ratio of 0.05 to 0.30. The hot cathode of the present invention exhibits good electron emission characteristics in a wide range of the thickness of the surface thin film layer of 0.05 μm to 1 μm as compared with the impregnated cathode having the above-mentioned basic structure.
In order to obtain characteristics exceeding that of the conventional Sc-coated impregnated cathode, 0.1 μm to 0.5 μm is desirable. In the case of the hot cathode of the present invention, the retention of the surface thin film layer may become unstable when barium or other components in the thick film layer are consumed by evaporation at the end of the life of the cathode. In general, when used in a cathode ray tube, it is unlikely that a large impact will occur so that the surface thin film layer is damaged. In applications where this is a problem, the surface thin film layer may be thickened to increase the film strength. However, the thickness of the surface thin film layer is set in order to exhibit high electron emission characteristics,
It is preferably 0.5 μm or less.

Therefore, in order to improve the film strength, an intermediate thin film layer mainly containing a refractory metal and containing barium oxide may be provided as a base of the surface thin film layer. Here, the reason why barium oxide is included in the intermediate thin film layer is to ensure the supply of barium to the surface thin film layer in the initial heating. Further, even after the initially contained barium is supplied to the surface thin film layer, the diffusion path in the intermediate thin film layer is already secured, so it is easy to supply barium from the thick film layer to the surface thin film layer. .. The reduction of the barium supply amount by the intermediate thin film layer does not pose a practical problem if the intermediate thin film layer has a thickness of 3 μm or less.

The surface thin film layer and the intermediate thin film layer are good conductors because they are mainly composed of metal such as tungsten. However, in order to enable high current density operation, the thin film layer needs to be electrically connected to a wiring for supplying electrons from outside the cathode. It is not impossible to wire directly to the surface thin film layer, but the wires are generally connected to the part that holds the cathode material. Therefore, it is practically preferable that the thin film layer is electrically connected to the cathode holding portion or the refractory metal substrate. To this end, a portion of the refractory metal substrate which is not covered with the thick film layer is provided on the outer peripheral portion of the electron emission region of the cathode, and the surface thin film layer is continuously formed from the portion. As another means for establishing conduction between the thin film layer and the refractory metal substrate, unevenness is formed on the refractory metal substrate, and the projections are exposed on the surface of the thick film to establish conduction with the surface thin film layer.

[0014]

In the conventional impregnated cathode, barium atoms are generated by the reaction between the impregnating agent and tungsten inside the porous substrate pellet of refractory metal such as tungsten. On the other hand, in the hot cathode of the present invention, barium atoms are generated by the reaction between the surface of the refractory metal substrate and the oxide containing barium. Further, in the case where the thick film layer contains the refractory metal powder, barium atoms are also generated by the reaction between the refractory metal powder and the oxide containing barium. A hot cathode of the present invention in which a thin film layer of tungsten containing scandium or scandium oxide is provided on the upper surface of a thick film layer which is a barium supply layer, or an intermediate thin film layer of refractory metal containing barium oxide is further provided therebetween. The barium is uniformly and much supplied to the surface thin film layer corresponding to the coating film of the Sc-coated impregnated cathode. As a result, good electron emission can be obtained and low temperature operation becomes possible. Therefore, the electron tube using this hot cathode can suppress the temperature rise in the vicinity of the cathode, and can achieve higher reliability than the electron tube using the impregnated cathode.

[0015]

Embodiments of the present invention will be specifically described below with reference to the drawings. Example 1 FIG. 1 is a sectional view of a first example of a hot cathode according to the present invention. In the figure, the sleeve 4 and the heater 5 for holding and heating the cathode material portion are also shown. In this example, the surface thin film layer 3 was formed on the upper surface of the thick film layer 2.
As the refractory metal substrate 1, a tungsten disk having a concave portion at the center was used. The refractory metal substrate 1 and the surface thin film layer 3 are electrically connected at the outer peripheral portion of the disk 1. Tungsten metal substrate 1 used in this example
Has a disk diameter of 1.5 mm, a recess diameter of 1.2 mm,
The recess depth is 0.2 mm. BaO, Ca
_Filled with powder prepared by mixing O and _{Al 2} O ₃ in a molar ratio of 4: 1: 1, heated to 1700 ° C. in hydrogen and melted, and then cooled to form a complex oxide composed of barium, aluminum and calcium. Thick film layer 2 was formed. After cooling, the surface was ground flat, and the surface thin film layer 3 was formed on the upper surface. The surface thin film layer 3 was formed by a sputtering film forming method using argon as a discharge gas. As a sputter target,
A composite target of tungsten and Sc ₂ W ₃ O ₁₂ was used.

FIG. 2 is a measurement diagram showing the electron emission characteristics of the hot cathode of the first embodiment, particularly the dependence of the cathode current density on the thickness of the surface thin film 3. Here, the composition of the surface thin film layer 3 was adjusted to an atomic ratio of Sc / W of about 0.13. Further, in the measurement, the anode was placed 7 mm away from the cathode electron emission surface, and a pulse voltage of 4 kV was applied to the anode. The cathode temperature at the time of measurement is 820 ° C. As can be seen from the figure, the emission current density is 0.15 μm to 0.
The maximum is 20 μm. Further, in order to obtain better electron emission characteristics than the characteristics of the conventional Sc-coated impregnated cathode, the thickness of the surface thin film layer 3 should be approximately 0.1 μm or more and 0.5 μm.
It turns out that the following are desirable:

Embodiment 2 FIG. 3 is a sectional view of a second embodiment of the hot cathode according to the present invention. In this embodiment, the intermediate thin film layer 6 is formed as the base of the surface thin film layer 3. The intermediate thin film layer 6 was formed by a sputtering film forming method using argon as a discharge gas. A composite target of tungsten and barium carbonate was used as the sputter target. Since the manufacturing method, structure and function of the part excluding the intermediate thin film layer 6 are the same as those of the embodiment shown in FIG. 1, the same parts are designated by the same reference numerals and the description thereof is omitted (the same applies to the following embodiments. ).

FIG. 4 shows the electron emission characteristics of the hot cathode of the second embodiment, particularly the dependence of the cathode current density on the thickness of the intermediate thin film 6. The composition of the surface thin film layer 3 was adjusted to a Sc / W atomic ratio of about 0.13 and the film thickness was adjusted to about 0.20 μm, and the composition of the intermediate thin film layer 6 was adjusted to a Ba / W atomic ratio of about 0.2. In the measurement, the anode is placed 7 m from the cathode electron emission surface.
The electrodes were arranged at a distance of m, and a pulse voltage of 4 kV was applied to the anode. The cathode temperature at the time of measurement is 820 ° C. As can be seen from the figure, the emission current density is such that the thickness of the intermediate thin film layer 6 is 1 μm.
Although there is a decreasing tendency above, no significant decrease is observed up to about 3 μm.

Embodiment 3 FIG. 5 is a sectional view of a third embodiment of the hot cathode according to the present invention. In the embodiment shown in Fig. 5, the thick film layer 2'includes the refractory metal powder 2 ". This allows not only the interface 7 between the refractory metal substrate 1 and the thick film layer 2 ', but also the thick film 2'. Barium atoms can also be generated in the layer 2 '.

Embodiment 4 FIG. 6 is a sectional view of a hot cathode according to a fourth embodiment of the present invention. In this embodiment, a plurality of irregularities are formed on the refractory metal substrate 1 ′, and the thin film layer 3 is formed at the convex portion 8 of the substrate 1 ′.
And has electrical continuity. Thereby, electrical conduction with the thin film layer 3 can be made more reliable. There is also an advantage that the area of the interface between the refractory metal substrate 1 ′ and the thick film layer 2 is increased, and the amount of barium atoms produced is increased.

In the embodiment described above, Ba0, CaO and Al ₂ O ₃ are used as the material of the thick film layer ₂ in a molar ratio of 4: 1 :.
Although a mixture of 1 and 2 was used, the same effect can be obtained with a mixture of 5: 2: 3 and other compositions generally used as an impregnating agent for an impregnated cathode. Further, the operating principle of the hot cathode of the present invention is the same as that of the Sc-coated impregnated cathode,
The composition and manufacturing method of the surface thin film layer 3 may be the same as the coating film of the Sc coating type impregnated cathode. Therefore, the preferred composition is
The Sc / W atomic ratio is in the range of 0.05 to 0.30. In the embodiment, the composite target of tungsten and Sc ₂ W ₃ O ₁₂ was used as the sputtering target in the production of the surface thin film layer 3, but this may be tungsten and Sc ₂ O ₃ or Sc ₆ WO ₁₂ , or the film formation. The method is not limited to the sputter film forming method. Similarly, the method of forming the intermediate thin film layer is not limited to the sputtering target of tungsten and barium carbonate, and is not limited to the sputtering film forming method.

Further, the refractory metal substrate 1, the thick film layer 2 ', and the refractory metal powder 2 "in the intermediate thin film layer 6 used tungsten, but these are refractory metals and react with barium oxide to produce barium atoms. For example, when molybdenum is used as the refractory metal in the cathode having the same structure as in Fig. 1, the electron emission characteristics can be almost the same as when tungsten is used. Therefore, an alloy mainly composed of tungsten or molybdenum can be used as well.The refractory metal substrate 1 may be a porous body of a conventional impregnated cathode.
In this case, the area of the interface with the thick film layer 2 increases and the amount of barium atoms generated increases, as in the refractory metal substrate 1 ′ having a plurality of irregularities. However, since barium atoms are generated inside the porous body as well, the atom supply becomes non-uniform in the vicinity of the upper part of the porous body, but the thick film layer 2 or the thick film layer 2 ′ and the intermediate thin film layer 6 contain barium atoms. The distribution is homogenized during passage. In addition, although the above-mentioned embodiment describes the hot cathode,
The hot cathode of the present invention is used for an electron tube, and naturally includes an electron tube using the hot cathode of the present invention.

[0023]

According to the constitution of the hot cathode of the present invention, sufficient barium is supplied to the surface thin film layer mainly containing tungsten and containing scandium or scandium oxide. Therefore, there is an effect that the large current density operation required for the electron tube can be performed at a lower temperature than the conventional Sc-coated impregnated cathode.
Further, this can improve the reliability of the electron tube which requires a cathode capable of high current density operation.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of a first embodiment of a hot cathode according to the present invention.

FIG. 2 is a measurement diagram showing electron emission characteristics of the hot cathode of the first embodiment.

FIG. 3 is a sectional view showing the configuration of a second embodiment of the hot cathode according to the present invention.

FIG. 4 is a measurement diagram showing electron emission characteristics of the hot cathode of the second embodiment.

FIG. 5 is a sectional view showing the structure of a third embodiment of the hot cathode according to the present invention.

FIG. 6 is a sectional view showing the structure of a fourth embodiment of the hot cathode according to the present invention.

[Explanation of symbols]

 1, 1 '... Refractory metal substrate 2, 2' ... Thick film layer 2 "... Refractory metal powder 3 ... Surface thin film layer 4 ... Sleeve 5 ... Heater 6 ... -Intermediate thin film layer.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Tomio Yaguchi 1-280 Higashi Koigokubo, Kokubunji, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Emiko Yamada 1-280 Higashi Koikeku, Kokubunji, Tokyo Hitachi Ltd. Central research institute

Claims (13)

[Claims] 1. A surface thin film layer containing tungsten and at least scandium or scandium oxide, a region in which barium atoms are supplied from a lower portion of the surface thin film layer, and a barium atom is supplied from a portion directly below the surface thin film layer. And a barium supplier that occupies 50% or more of the area of the region that utilizes the electrons emitted from the surface of the surface coating film layer. 2. The barium supplier is formed of a thick film layer containing an oxide containing barium on a refractory metal substrate,
The hot cathode according to claim 1, wherein the surface thin film layer is laminated on an upper surface of the thick film layer. 3. The barium supplier comprises a refractory metal substrate, a thick film layer containing an oxide containing barium, and an intermediate thin film containing barium oxide mainly containing a refractory metal on an upper surface of the thick film layer. And a surface thin film layer formed on the upper surface of the intermediate thin film layer.
The hot cathode described. 4. The hot cathode according to claim 2, wherein the thick film layer further contains powder of refractory metal. 5. The hot cathode according to claim 2, wherein the refractory metal substrate is an alloy containing at least one of tungsten and molybdenum as a main component. 6. The hot cathode according to claim 3, wherein the refractory metal serving as a main component of the intermediate thin film layer is an alloy containing at least one of tungsten and molybdenum as a main component. 7. The hot cathode according to claim 4, wherein the powder of the refractory metal is an alloy containing at least one of tungsten and molybdenum as a main component. 8. The hot cathode according to claim 2, wherein the oxide containing barium forming the thick film layer is a composite oxide of barium with one or both of aluminum and calcium. .. 9. A surface of the refractory metal substrate on which the thick film layer is formed has a single or a plurality of irregular shapes, and at least a part of the convex portion is electrically connected to the intermediate thin film layer or the surface thin film layer. The hot cathode according to claim 2, wherein the hot cathode is connected to the hot cathode. 10. The hot cathode according to claim 2, wherein the atomic ratio of scandium contained in the surface thin film layer to tungsten is in the range of 0.05 to 0.30. 11. The hot cathode according to claim 2, wherein the surface thin film layer has a thickness of 0.1 μm or more and 0.5 μm or less. 12. The hot cathode according to claim 3, wherein the thickness of the intermediate thin film layer is 3 μm or less. 13. An electron tube using the hot cathode according to claim 1.