JPH07122177A - Oxide cathode - Google Patents

Abstract

PURPOSE:To prevent the sintering of an oxide and improve emission performance by setting the composition ratio of Ba, Sr and Ca, in a (Ba, Sr, Ca)CO3 ternary carbonate, to the range represented by a prescribed composition. CONSTITUTION:Ba(NO3)2, Sr(NO3)2, and Ca(NO3)2.4H2O are blended to have a prescribed composition ratio of carbonate, and the mixture is dissolved in pure water in such a manner that the concentration of alkali earth metal is A mole/l. While stirring this aqueous solution at high speed, 1.0 mole/l of (NH4)2CO3 aqueous solution is added thereto to precipitate and synthesize ternary carbonate (Ba, Sr, Ca)CO3 by coprecipitation, and the precipitate is sufficiently washed with water and then dried. The composition ratio of alkali earth metal carbonate is set within the segment connecting 6 points of Ba:Sr:Ca=A (50:40:10), C(50:30:20), G(45:30:25), N(35:40:25), L(35:50:15), and H(40:50:10) (mole %) in ternary composition constitutional diagram.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluorescent display tube and a light source unit of a large-screen display device to which the principle of the fluorescent display tube is applied, a light source for a printer, an oxide cathode used for a light emitting device such as a backlight, and particularly to emission characteristics. Excellent oxide cathode.

[0002]

2. Description of the Related Art As shown in a perspective view of FIG. 1 and a cross-sectional view of FIG. 2, a fluorescent display tube has a conductive structure in which a vacuum container composed of a plate glass 21, a windshield 31, and a spacer glass 32 is electrically conductive. It is a triode vacuum tube provided with an anode composed of a layer 22 and an anode electrode 23, a grid 27, and a filament cathode 29 as an oxide cathode. The phosphor layer 25 is formed on the anode electrode 23, and the electrons emitted from the filament cathode are transferred to the grid 27.
Are accelerated and collide with the phosphor layer 25. At this time, the accelerated electrons excite the phosphor and the phosphor emits light. Also, in the figure, 24
Is an insulating layer, 26 is an exhaust pipe, 28 is a conductive paste, 30 is a getter, 33 is a lead pin, and 34 is an anchor for disposing and supporting the filament cathode 29.

The oxide cathode is composed of a mixed carbonate of an alkaline earth metal having a low work function such as barium (Ba), strontium (Sr) and calcium (Ca) on the surface of an extremely thin tungsten wire having a diameter of 5 to 30 μm. Powder to a few μm
After coating the thickness of the inside of the container of the fluorescent display tube, the container is heated by energizing while evacuating to a vacuum,
The carbonate is thermally decomposed into an oxide and used as a cathode.

The main source of electron emission from the oxide cathode is Ba.
It is known that when O is caused by stoichiometrically excessive Ba, the emission is further increased when SrO and CaO are mixed as compared with BaO alone. Sr, C of these family members
Since a is a divalent cation like Ba, it occupies a position in place of Ba, but since it has a different ionic radius, it slightly disturbs its surroundings and gives an unstable high potential to oxygen ions, which causes reduction and temperature. It is placed in a mobile excited state, and activation is easier.

Regarding the composition of (Ba, Sr, Ca) O,
Nature Vol. 167 P522 March
1951 by L.H. E. For Gray, the molecular weight ratio with the best emission is BaO: SrO: Ca.
O = 47: 43: 10 is reported. Most of the oxide cathodes of fluorescent display tubes are close to this composition, and the practical carbonate ratio is BaCO ₃ : SrCO.
₃ : CaCO ₃ = 48.7: 44.55: 6.75 (mol%).

[0006]

As described above, as the oxide cathode, one obtained by thermally decomposing a mixed carbonate of an alkaline earth metal such as Ba, Sr, or Ca into an oxide is used. In this thermal decomposition step, the partial mutual melting of the oxide particles also proceeds at the same time, so that the oxide cathode sinters to reduce the surface area of the oxide that emits electrons, and the emission amount also decreases.

Particularly, in a fluorescent display tube, components of an insulating layer formed by a thick film printing method, a carbon layer of an anode, components of a conductor layer and components of a sealing glass for assembling and sealing a glass container are
Since it evaporates in the sealing heat treatment step at about 450 to 500 ° C. and is adsorbed by the carbonate of the cathode, it is an environment where sintering is easy.

Further, even if the ZnS-based or (Zn, Cd) S-based phosphor used in the multi-color fluorescent display is decomposed and sulfide gas is adsorbed, the oxide cathode is similarly sintered. Emissions are reduced.

Carbonate decomposes when heated and has the following formula

[Chemical 1] In the reaction formula, the temperature at which the partial pressure of CO ₂ becomes 1 atm is 1350 ° C. for BaCO _{3 and} 11 for SrCO _3.
55 ° C, CaCO ₃ is 898 ° C. Therefore, B
The decomposition temperature of aCO ₃ is the highest, and the thermal decomposition temperature can be lowered by replacing Ba with Sr or Ca.

By the way, the sintering of oxides after thermal decomposition is affected by their melting points, and the melting point of each oxide is 1 for BaO.
923 ℃, SrO 2430 ℃, CaO 2572 ℃
Thus, when the melting point of BaO is the lowest and the ratio of BaO in the composition of the oxide cathode is high, the sintering becomes easier.

On the other hand, the main component of electron emission of the oxide cathode is due to the excess Ba of BaO as described above, so that the ratio of BaO must be above a certain level in order to secure the emission amount. From such a viewpoint, for example, in Japanese Patent Publication No. 28218/1990, the ratio when synthesizing a carbonate of an oxide cathode of a fluorescent display tube is CaCO ₃ : SrC.
_{O 3: BaCO 3 = 50~60:} 25~35: 5~20
Is said to be desirable. However, according to the follow-up test by the present inventors, sufficient emission could not be obtained in this composition range.

The values of the thermal decomposition temperature and the melting point described above are general values measured in the air atmosphere, and the state diagram and the like in a vacuum such as a fluorescent display tube have not been clarified yet. Further, the carbonate to be used is a ternary carbonate synthesized by a coprecipitation method or the like, and is a solid solution. Therefore, BaCO ₃ , SrC is used.
The thermal decomposition temperature and the state of sintering are different from the mixed carbonate obtained by powder-mixing O ₃ and CaCO ₃ in a predetermined ratio.

Therefore, the present inventor has searched for a ternary carbonate capable of obtaining the highest emission in the manufacturing process of a fluorescent display tube, and has found an oxide cathode having excellent emission ability.

In view of the above-mentioned problems of the prior art, an object of the present invention is to improve the sintering of the oxide cathode in the thermal decomposition process and the like, and to provide an oxide cathode having excellent emission ability. is there.

[0015]

In order to achieve the above object, the present invention provides a fluorescent display tube obtained by coating a metallic core wire of tungsten or the like with a ternary carbonate of an alkaline earth metal and thermally decomposing it. In the oxide cathode such as the above, the composition ratios of the alkaline earth metal carbonates are shown as Ba: Sr: Ca = A (50:40:10) and C in the ternary composition diagram.
(50:30:20) and G (45:30:25) and N
(35:40:25) and L (35:50:15) and H
(40:50:10) (mol%) is characterized by being within a line segment connecting 6 points.

[0016]

In the present invention, the composition ratio of Ba, Sr, and Ca in the (Ba, Sr, Ca) CO ₃ ternary carbonate is set within the range shown by the above composition, so that the temperature at a temperature lower than that in the prior art is achieved. Pyrolysis is possible, and the oxide cathode can be prevented from sintering and an oxide cathode having excellent emission performance can be obtained.

Further, since the thermal decomposition temperature can be lowered, the reaction of the intermediate product formed at the interface between the tungsten wire and the oxide can be suppressed, and an effective effect for improving the emission capability can be expected.

[0018]

Example: Ba (NO ₃ ) ₂ , Sr (NO ₃ ) ₂ and Ca
(NO ₃₎ the ₂ · 4H ₂ O were blended so as to have the composition ratio of carbonate shown in Table 1, the concentration of the alkaline earth metal is dissolved in purified water to a 0.5 mol / l. While stirring this aqueous solution at high speed, 1.0 mol / l of (NH ₄ ) ₂ CO ₃
Add an aqueous solution to add ternary carbonate (Ba, Sr, Ca) CO
₃ was precipitated by the coprecipitation method to synthesize, and the precipitate was washed thoroughly with water and then dried.

The ternary carbonate thus prepared and an acrylic resin (manufactured by DuPont: Elvasite 2) as a binder
045), 45% by weight of acetone (special grade reagent),
24% by weight of MIBK (special grade), 24% by weight of IPA (special grade), 5% by weight of Elvacite 2045,
2% by weight of ternary carbonate is mixed by a well-known mixing method,
An electrodeposition solution was formed.

Using this electrodeposition solution, a tungsten wire having a diameter of 15 μm was coated with a carbonate to a thickness of 7 μm by electrophoresis to prepare a filament cathode.

The electrodeposition conditions were a constant current power supply, a current value of 60 ± 10 μA and a voltage value of 60 to 100 V.
The tungsten wire was passed at 16 m / min.

Using this, the fluorescent display tube shown in FIGS. 1 and 2 was manufactured, and the characteristics of the filament cathode were evaluated. To evaluate the filament cathode, pyrolysis of carbonate was performed at 1000 ° C to 1300 ° C for about 60 seconds while evacuation.

Since the thermal decomposition temperature differs depending on the composition of the carbonate, the thermal decomposition conditions were determined as follows.

That is, when a voltage is applied to both ends of the filament cathode while evacuating, the temperature rises due to the resistance of the filament cathode, and the thermal decomposition of carbonate starts. Since the thermal decomposition reaction is an endothermic reaction, when the reaction is in progress, the filament temperature is low, but when the reaction is completed, it becomes a red hot state. The surface temperature in the red hot state is defined as the thermal decomposition temperature, and the applied voltage is set so that the time from the application of the voltage to both ends of the filament cathode until the entire filament cathode becomes red hot is 30 seconds. That is, the time until the whole became a red hot state was set to 30 seconds, the red hot state was kept for 30 seconds, and a total of 60 seconds was set as the pyrolysis condition.

After completion of the thermal decomposition, the exhaust pipe is closed to complete the exhaust. After that, the barium getter was heated to adsorb the residual gas on the Ba film, and a high vacuum fluorescent display tube was produced.

The characteristics of the filament cathode were evaluated using this fluorescent display tube. The filament cathode voltage was set so that the surface temperature of the filament cathode was 600 ° C., which is a normal operating condition. At this time, the power consumption per 1 cm of the filament cathode is 7.5 mW
Met. In order to keep the conditions constant, the voltage was determined with the filament cathode shown in Comparative Example 1 in Table 1, and all other measurements were performed with the same filament cathode voltage as in Comparative Example 1.

The grid and the anode voltage for extracting the emission are DC 100V and pulse width 100.
μsec and Duty = 1/10. The emission characteristics are in the temperature limited region, which is a condition under which the emission performance of each filament cathode can be compared.

The results are shown in Table 1. As Comparative Example 1, BaCO ₃ : SrCO ₃ : Ca, which has been most often used conventionally,
The emission amount of each filament cathode was shown as a relative value, with the emission amount of the composition of CO ₃ = 48.7: 44.55: 6.75 being 100. Further, the surface of the filament cathode was observed with an SEM image by an electron microscope, and the sintering states were compared. In the present invention, no sintering was observed and the amount of emission was larger than that of Comparative Example 1 as claimed.

[0029]

[Table 1] As is clear from Table 1, in Comparative Example 1 and Comparative Examples 2 and 3, sintering was observed and the emission was insufficient. Although Example 1 had a tendency to be slightly sintered, the emission ability was superior to that of the conventional example.

Further, as in Comparative Examples 5, 6 and 7, which are compositions not included in the claims of the present invention, when the mol% of Ca was 25% or more, the emission ability was drastically reduced. On the other hand, when the mol% of Ba was 35% or less, excess Ba of BaO, which is a main component of electron emission, was insufficient, and sufficient emission was not obtained. Furthermore, the mol% of Ba is 50
%, The tendency was to sinter, and sufficient emission could not be obtained.

On the other hand, the composition ratio of Ba: Sr: Ca is A (50:40:10), C (50:30:20), G.
(45:30:25), N (35:40:25), L
(35:50:15) and H (40:50:10) 6
In Examples 1 to 14 in which the line segment connecting the points was not sintered, the emission amount was all Comparative Example 1 or more. Particularly, in the composition of Example 6 in which the composition ratio of Ba: Sr: Ca was 45:35:20, the emission amount showed a relative value of 302, which was the maximum value of the present invention.

Next, SiO ₂ , Al ₂ O ₃ , ZrO ₂ and ScO ₃ as a fourth component are added to these ternary carbonates in an amount of 0.
When the amount of emission and the state of sintering were observed by adding 01 to 5%, the same result as above was obtained.

[0033]

In the oxide cathode of the present invention, the composition ratio of the alkaline earth metal carbonate is represented by Ba:
Sr: Ca = A (50:40:10) and C (50: 3)
0:20) and G (45:30:25) and N (35: 4)
0:25) and L (35:50:15) and H (40: 5)
By setting it within a line segment connecting 6 points of (0:10) (mol%), sintering in the thermal decomposition step of the oxide cathode was improved, and the oxide cathode was excellent in emission ability.

[Brief description of drawings]

FIG. 1 is an overall perspective view of a fluorescent display tube.

FIG. 2 is a cross-sectional view of the fluorescent display tube shown in FIG.

FIG. 3 is a diagram showing a composition of ternary carbonate.

[Explanation of symbols]

 22 conductive layer 23 anode electrode 25 phosphor layer 27 grid 29 filament cathode (oxide cathode)

Front Page Continuation (72) Inventor Tatsuo Matsuyama 3-36 Noritake Shinmachi, Nishi-ku, Nagoya-shi, Aichi Noritake Company Limited Limited

Claims (1)

[Claims] 1. A composition of the alkaline earth metal carbonate in an oxide cathode such as a fluorescent display tube obtained by coating a metallic core wire of tungsten or the like with a ternary carbonate of an alkaline earth metal and thermally decomposing the ternary carbonate. The ratio is expressed as Ba: Sr: Ca = A (50:40:10) and C in the ternary composition phase diagram.
(50:30:20) and G (45:30:25) and N
(35:40:25) and L (35:50:15) and H
(40:50:10) (mol%) within a line segment connecting 6 points, an oxide cathode.