JPS648892B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oxide cathode assembly, particularly to a long-life, high-performance oxide cathode assembly suitable for use in color television picture tubes and the like.

Usually, the base metal that makes up the oxide cathode structure is mainly nickel (Ni), with trace amounts of reducing agents such as magnesium (Mg), silicon (Si), aluminum (Al), zirconium (Zr), and tungsten (W). Ni−Mg, Ni−Si, Ni−
Mg-Si, Ni-Al, Ni-W, Ni-Al-W, Ni-
Alloys such as W-Mg and Ni-Zr are used. A base metal made of this alloy is fixed to the tip of the cathode sleeve, and on this base metal are carbonates of alkaline earth metals such as barium (Ba), strontium (Sr), calcium (Ca), etc. Ba, Sr.
After this _carbonate is incorporated into a color television picture tube, etc., it is heated to a predetermined temperature in a vacuum to form alkaline earth metal oxides (Ba, Sr, Ca)O. It is decomposed into Among these alkaline earth metal oxides, BaO contributes most to electron emission.

This BaO oxide diffuses into the base metal during the operation of the oxide cathode, including the above-mentioned Mg, Si,
It is reduced at the boundary between the base metal and the oxide by a reducing agent such as Al, Zr, or W. For example, when Mg is used as a reducing agent, free Ba is formed which causes electron emission by the reaction of the following formula. It has been said.

BaO (s) + Mg (in Ni) → Ba (in BaO) + MgO
(s)...(1) Therefore, in such an oxide cathode, as mentioned above, the reaction between the reducing agent and the oxide, which is an electron emitting substance, proceeds at the interface between the base metal and the oxide. ,
Whether you like it or not, a compound layer called an intermediate layer is formed between the two, such as MgO.

Now, recently, especially regarding television picture tubes,
There are demands for higher performance such as ultra-fast operation, higher definition, and higher brightness, and higher performance of oxide cathodes is desired. One way to meet these demands is to
Although attempts have been made to reduce the thickness of the base metal and increase the amount of reducing agent added, there is a limit to how much the oxide cathode having an intermediate layer as described above can do.

However, as a result of several detailed studies on oxide cathodes, the present inventors have found that (i) the dissociation reaction of oxygen from the electron-emitting substance and the site of the reaction between this oxygen and the reducing agent have been separated; We have found that (ii) it is possible to form a control layer for the diffusion of oxygen and reducing agent between both reaction sites.

That is, the inventors set Mg and Si at 0.03 each.
Using a thin plate of Ni-based alloy with a thickness of 150 μm containing weight percent as the base metal, and without applying an electron emitting substance, a mixed gas of carbon monoxide (CO) + carbon dioxide (CO ₂ ) (CO:CO ₂ = 1) was used as the base metal. :20, partial pressure ratio)
A sample was prepared and held at 1000℃ for 1 hour in an air stream,
The metal structure of the cross section of the sample was observed using an EPMA (electron beam excited X-ray microanalyzer). An example of a typical cross-sectional photograph of a metallographic structure is shown in FIG. 1A, and a schematic diagram thereof is shown in FIG. 1B. Furthermore, Figure 1B
In the figure, the expression of grain boundaries is omitted.

The observation results show that [] Mg and Si in the base metal 1 are internally oxidized, and all Mg and Si from the base metal surface to a depth of about 20 μm are dispersed in Ni as MgO and SiO ₂ particles 2. . (The part where the oxide particles 2 are dispersed in the cross section of the base metal is shown below.
It is called internal oxide layer 3. ). []In the crystal grains existing in the internal oxide layer 3, oxides of MgO and SiO ₂ are generated along the grain boundaries, and even if the base metal 1 is continued to be heated, crystal growth is not possible, and the grain boundaries are remains fixed. [] If there are many grain boundaries fixed in the internal oxide layer 3 in this way, the cross section of the base metal will be a cross section in which grain boundaries with oxides exist near the surface and almost parallel to the surface. It becomes a structure. It can be summarized as above. In addition, by examining various data regarding diffusion coefficients, we found that oxygen diffuses faster in Ni than in the reducing agents Mg and Si, and that oxygen diffuses faster in oxides such as MgO and _SiO2 . The diffusion rates of Mg, Si, and O were significantly slower than those in Ni. Furthermore, the matter [ ] is based on the fact that the oxidation phenomenon of the reducing agent that occurs during the operation of a base metal having a composition similar to that of the observation sample and an oxide cathode having an alkaline earth metal oxide or the like as an electron emitting substance on the base metal. The reason is that rather than surface oxidation that occurs when Mg or Si diffuses to the interface between the base metal and the electron emitting material, internal oxidation can occur when oxygen thermally dissociated from the electron emitting material becomes solid solution and diffuses into the base metal. , and the above-mentioned observation results [ ] support this possibility. Furthermore, the item [ ] indicates that if an oxide layer substantially parallel to the surface of the base metal is continuous in the vicinity of the surface, this layer becomes a diffusion control layer for Mg, Si, and O.

From the results of the above experiments and studies, it is possible to construct an oxide cathode having the structure shown in FIG. In the figure, reference numeral 11 indicates a base metal 12 which is an electron emitting material, and reference numeral 13 indicates a diffusion control layer formed by a series of grain boundaries in which an oxide of a reducing agent exists.

In the present oxide cathode, the electron emitting substance 12
The oxygen dissociation reaction for generating free Ba from BaO occurs outside the base metal 11, and the reaction between this dissociated oxygen and the reducing agent inside the base metal occurs at the diffusion control layer 13, so that both reactions occur at the sites of both reactions. are separated. Therefore, the solid solution of oxygen generated by the dissociation of BaO into the base metal is regulated by the reaction between oxygen and the reducing agent in the base metal.

Furthermore, the diffusion control layer 13 inside the base metal 11 has the role of controlling the diffusion of oxygen and reducing agent.

Based on the above considerations, an object of the present invention is to separate the position of the reaction between the reducing agent and oxygen from the electron emitting material layer by arbitrarily setting the position of the diffusion control layer, and to separate the reducing agent from the electron emitting material layer. and to provide an oxide cathode structure with high performance and long life by controlling oxygen diffusion.

Hereinafter, the oxide cathode structure according to the present invention will be explained in detail using FIGS. 3 to 5. FIG. 3 is a sectional view of essential parts of the oxide cathode structure of the present invention. In FIG. 3, 22 is an electron emitting material, 21 is a base metal,
23 is a diffusion control layer formed inside the base metal. A method for forming the diffusion control layer 23 will be described later using Examples. 24 is a cathode sleeve, and 25 is a heater. Among the electron emitting substances 22, BaO, which contributes the most to electron emission, dissociates near the interface between the electron emitting substances 22 and the base metal 21 as BaO(s)Ba(in BaO)+O(in Ni)...(2) The reaction generates free Ba, and at the same time, oxygen becomes a solid solution in Ni, which is the base metal 21, and diffuses toward the inside of the base metal. On the other hand, the reducing agent added to the base metal diffuses toward the base metal surface, but the diffusion control layer 23 acts as a barrier to the diffusion of oxygen and the reducing agent. When the agent is Mg, it is possible to proceed with the reaction of the following formula.

Mg (in Ni) + O (in Ni) → MgO ... (3) According to Le Chatelier's law, which provides a guideline for the equilibrium conditions of chemical reactions, as the reaction of equation (3) progresses, In equation (2), the equilibrium shifts to the right, and the reaction to generate free Ba progresses.

One of the greatest advantages of the present invention is that by setting the position of the diffusion control layer 23 to an arbitrary depth,
It is possible to arbitrarily control the electron emission characteristics and lifetime of the oxide cathode. i.e. distance
When there is a concentration difference dc in dx, the amount of solute dm that diffuses through cross-sectional area A during time dt is expressed as dm/dt=-DAdc/dx (4). In equation (4), D is the solute diffusion coefficient. Hereinafter, the operating state of the oxide cathode structure according to the present invention will be considered using equation (4). The concentration of oxygen dissolved in the base metal Ni at the interface between the base metal and the electron emitting material is C ₁ , the depth of the diffusion control layer is l,
If the solid solution oxygen concentration at this position is C ₂ and the diffusion coefficient of oxygen in Ni is Doin _Ni , equation (4) can be rewritten as the following equation.

dm/dt=-D _pioNi AC ₁ -C ₂ /l ...(5) D _pioNi is a function only of temperature, and under conditions where C ₂ can be assumed to be substantially zero,
Since the solubility of oxygen in Ni is determined by temperature,
At a constant temperature, the amount of oxygen that diffuses toward the diffusion control layer will depend on the depth l of the diffusion control layer.

Therefore, according to Le Chatelier's law mentioned above, the amount of free Ba produced depends on the depth of the diffusion control layer. According to equation (5), the amount of free Ba produced increases if l is reduced, but the optimal value of l is determined by taking into account the amount of free Ba in BaO required for electron emission and the evaporation rate of free Ba. Must be decided. For base metals with a plate thickness of 150μm or less,
It is desirable that l is 3 to 20 μm.

If l is less than 3μ, it is not preferable because it is shallow and a reaction product between the electron emitting substance and the oxide is formed near the surface of the base metal, and the same adverse effect on emission as in the conventional one remains.

If it exceeds 20μ, the diffusion time of oxygen in the base metal becomes longer, and as a result, it becomes difficult for new oxygen to diffuse, resulting in a decrease in the amount of free Ba produced and a decrease in emission.

Another advantage of the present invention is that since the oxidation reaction of the reducing agent occurs inside the base metal, there is a strong possibility that the generated oxide will react with the electron emitting substance and form a harmful substance. Since the amount of the reducing agent is small, the range of choices for the reducing agent is greatly expanded in terms of both type and amount.

Examples will be described below.

Example 1 Plate thickness 150 μm containing 0.06 weight percent Si
The surface of the cold-rolled Ni-Si alloy was heat-treated at 1000℃ for 1 hour in an atmosphere with a specified oxygen partial pressure using a mixed gas of CO and CO ₂ (mixture partial pressure ratio 1:20). Internal oxidation treatment was applied to a depth of approximately 20 μm. Through this treatment, internal oxidation and recrystallization phenomenon based on the processing strain accumulated in the Ni-Si alloy during rolling proceed simultaneously, and the grain growth of the crystal grains existing in the internal oxidation layer is suppressed and the grain boundaries oxidized and the grain boundaries were fixed. As a result, as seen in the micrograph in Figure 4, the
A base metal was obtained which had a diffusion control layer having a cross-sectional structure of a series of crystal grain boundaries where oxide existed almost parallel to the surface at a depth of 12 μm, on average about 10 μm.

After this, it was punched out into a disk with a diameter of 1.3 mm, welded to the tip of a cathode sleeve, and after coating the surface with an electron emitting material, it was assembled into the electron gun of a 13-inch color television picture tube. Thereafter, a color television picture tube was fabricated through the usual steps such as evacuation, sealing, carbonate decomposition, getter flash, etc., and a life test was conducted.

As a result, it has been found that the electron emission characteristics of this color television picture tube are superior to those of conventional tubes, especially during long-term operation.
A typical example of this result is shown in FIG. 5A. The broken line in the figure shows a conventional oxide cathode structure without a diffusion control layer, and the solid line shows a structure in which a diffusion control layer is formed by the process described above. The grain boundaries fixed by the above-mentioned treatment act as a barrier to the diffusion of oxygen and the reducing agent, and the reaction between the reducing agent and oxygen proceeds at this location, as shown in the cross-section of the base metal after the life test.
Confirmed by EPMA measurements.

Example 2 Plate thickness 120μ containing 0.08 weight percent Mg
In order to coarsen the crystal grains of the Ni-Mg alloy of
It was ground by 20 μm and processing strain was applied near the surface. After this, a mixed gas of CO and CO ₂ (mixture partial pressure ratio, 1:25)
1000 in an atmosphere with a specified oxygen partial pressure using
By applying heat treatment at ℃ for 1 hour, internal oxidation treatment was performed from the surface to a depth of approximately 20 μm. As a result, 15 to 20 μm from the surface, on average about
A base metal was obtained which had a cross-sectional structure in which grain boundaries with oxides were continuous at a depth of 18 μm and approximately parallel to the surface. The reason why the grain boundaries are fixed at deeper positions despite the same internal oxide layer thickness compared to Example 1 is because the vapor pressure of Mg in the alloy is high, so the sample reaches equilibrium temperature and oxygen Mg is selectively evaporated from the surface until it is supplied, and the Mg concentration near the surface decreases, reducing the density of internal oxidation grain precipitation.
This is because the effect of suppressing grain growth was reduced. After that, a circular plate with a diameter of 1.3 mm was punched and fixed to a sleeve so that the ground surface, that is, the surface on which the diffusion control layer was formed, was the surface coated with the electron emitting material, and the same process as in Example 1 was carried out. After that, a television picture tube was manufactured and a lifespan test was conducted. As a result, as in Example 1, it was found that the tube exhibited stable electron emission characteristics even after a long period of operation compared to the conventional tube.

The solid line in FIG. 5B shows the life test results of Example 5, and the broken line shows the life test results of Comparative Example.

Example 3 A Ni-Zr alloy with a thickness of 55 μm containing 0.13% Zr was heated at 1000°C for 10 minutes in a stream of dry hydrogen, and then one surface was heated.
5μm grinding, machining strain applied near the surface. After this,
Internal oxidation treatment is applied to a depth of approximately 8 μm from the surface by heat treatment at 1000℃ for 10 minutes in an atmosphere with a specified oxygen partial pressure using a mixed gas of CO and CO ₂ (mixture partial pressure ratio, 1:20). was applied. As a result, a base metal was obtained which had a cross-sectional structure in which grain boundaries with oxides existed approximately parallel to the surface at a depth of about 5 μm on average from the surface. Thereafter, a television picture tube was manufactured through the same steps as in Example 2, and a life test was conducted.

In contrast to the case where the same treatment was performed except for the internal oxidation treatment, the emission suddenly decreased after a relatively short period of time (dashed line in Figure 5C).
The solid line in Figure C) showed stable emission even after long-term operation.

In the above embodiments, cold rolling and surface grinding were used as a method of applying processing strain near the surface, but the method of applying processing strain is not limited to these, and any method that can provide uniform processing strain can be used. , shot peening, sandblasting, glass honing, etc. may also be used. Further, the gas that can be used for the internal oxidation treatment is not limited to CO--CO ₂ , but may be any gas that can specify the oxygen partial pressure, such as inert gas + oxygen gas, H ₂ + H ₂ O gas, etc.

In addition, in this example, a Ni-based alloy containing Si, Mg, and Zr as reducing agents was used, but in addition, W,
Including Al, Ti, Y, Ce, La, Hf, Ba, Ca,
A similar diffusion control layer can also be formed with a Ni-based alloy containing Sr and Sc.

As explained above, the present invention focuses on the metal structure of the cross-section of the base metal for an oxide cathode structure, and is based on the dissociation reaction of oxygen from an electron emitting substance obtained as a result of detailed investigation of the oxide cathode. This invention is based on the discovery that it is possible to separate the reaction site between oxygen and the reducing agent, and furthermore, it is possible to form a diffusion control layer for oxygen and the reducing agent between the two reaction sites. That is, the present invention provides a diffusion control layer inside the base metal that serves as a barrier to the diffusion of oxygen and a reducing agent in the base metal, and prevents the dissociation of oxygen from the electron emitting substance.
This method separates the reaction site between oxygen and reducing agent and controls the diffusion of oxygen and reducing agent. By changing the formation depth of the diffusion control layer, the electron emission characteristics of the oxide cathode and This has the advantage that the life span can be controlled arbitrarily and the reducing agent can be selected from a wide range.

[Brief explanation of the drawing]

Figure 1A is a cross-sectional photograph illustrating the conventional metallographic structure of the base, B is its schematic diagram, Figure 2 is a diagram illustrating the operating mechanism of the oxide cathode of the present invention, and Figure 3 is the oxide cathode according to the present invention. FIG. 4 is an enlarged micrograph of the diffusion control layer of the base metal of the present invention, and FIG. 5 is a diagram illustrating the electron emission characteristics of the oxide cathode structure of the present invention. 12,22...Electron emitting substance, 11,21...
Base metal, 13, 23... Diffusion control layer, 24...
Cathode sleeve, 25...heater.

Claims (1)

[Scope of Claims] 1. An oxide cathode structure having an electron-emitting substance and a base metal on which the electron-emitting substance is coated on one surface, containing a trace amount of a reducing element and mainly consisting of nickel, wherein the base metal is at least A diffusion control layer for oxygen and/or a reducing agent is provided inside the base metal near the side to which the electron emitting material is applied, and the reducing element is
Mg, Zr, W, Al, Si, Ti, Ba, Sr, Ca, Sc,
oxidation comprising at least one selected from Y, Ce, La, and Hf, and characterized in that the diffusion control layer is made of an oxide of the reducing element and is formed substantially parallel to the surface of the base metal. Cathode structure. 2. The oxide cathode structure according to claim 1, wherein the base metal has a thickness of 150 μm or less. 3. The oxide cathode structure according to claim 1, wherein the diffusion control layer is formed within a range of 3 to 20 μm from the base metal surface on the side to which the electron emitting material is applied.