JPH0778548A - Direct heat type oxide negative electrode - Google Patents

Abstract

PURPOSE:To provide a direct heat type oxide negative electrode which achieves excellent emission ability at small power consumption. CONSTITUTION:In a direct heat type oxide negative electrode such as a fluorescent display tube in which a layer of electron emission material is applied on the surface of metal core wire such as tungsten wire and rhenium tungsten wire, thickness of the electron emission material layer comprising ternary carbonate of barium, strontium, calcium, etc., is set to be within a range of 6.5-7.5mum.

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 for a large-screen display device to which the principle of the fluorescent display tube is applied.
The present invention relates to a direct heating oxide cathode used for a printer power source, a light emitting device such as a backlight, and more particularly to a direct heating oxide cathode having excellent emission characteristics.

[0002]

2. Description of the Related Art FIG. 1 is an overall perspective view of a fluorescent display tube, and FIG. 2 is a sectional view of the fluorescent display tube shown in FIG. 1. The fluorescent display tube comprises a plate glass 21, a windshield 31, and a spacer glass 32. This is a three-electrode vacuum tube in which an anode composed of a conductive layer 22 and an anode electrode 23, a grid 27, and a filament cathode 29 as a direct heating oxide cathode are arranged in the vacuum container. Further, in the figure, 24 is an insulating layer, 2
5 is a phosphor layer formed on the anode electrode 3, 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 filament cathode 29 has a diameter of 5 to 3
Barium (Ba), strontium (Sr), calcium (C
Powder of mixed carbonate of alkaline earth metal such as a)
After being deposited to a thickness of m and arranged in the container of the fluorescent display tube, the filament is electrically heated while the container is evacuated to a vacuum, and the carbonate is thermally decomposed into an oxide to form a cathode. It is a direct heating type oxide cathode.

In such a fluorescent display tube, the electrons emitted from the filament cathode 29 are transferred to the grid 27.
Layer 25 formed on the anode electrode 3 accelerated by
And the accelerated electrons excite the phosphor, and the phosphor emits light.

In order to make the phosphor emit light efficiently, the filament cathode 29 has an improved emission capability,
Also, characteristics such as reduction of power consumption are required.

Generally, when an oxide cathode is heated, the thermoelectron flow (saturation current) emitted from the surface of the oxide cathode per unit time is expressed by the following Richardson-Dashman equation.

[0007]

[Equation 1] Here, I _s : Saturation current (maximum current that can be taken out from the object at that temperature) (A) S: Thermionic emission area (cm ² ) of the cathode A: Thermionic emission constant (A / cm ² K ² ) T: temperature of cathode (K) e: charge of electron φ: work function (eV) k: Boltzmann constant, respectively.

As is clear from this equation, the saturation current I _s
In order to increase, the cathode temperature T is high, the thermionic emission surface S is large, and the work function φ is small.

Here, the work function φ is a characteristic peculiar to the material of the electron emitting material, and is constant at 0.9 eV in the case of the ternary oxide of (Ba, Sr, Ca) O. Therefore, in the case of an oxide cathode using a ternary oxide, by increasing the thermoelectron emission area S of the cathode and raising the temperature T of the cathode,
It can be seen that the emission ability can be improved.

The thermionic emission area S can be increased by increasing the amount of oxide deposited, that is, the deposited thickness. However, on the other hand, by increasing the amount of the deposited oxide, the radiant heat from the surface of the oxide cathode is increased at the same time, and as a result, the cathode temperature that affects the emission capability is lowered. Therefore, when the power consumption is kept constant, the emission capability is not necessarily improved by simply increasing the thickness of the oxide deposited.

Conventionally, as such a direct heating type oxide cathode,
JP-A-52-127151 discloses that a rhenium tungsten wire having a diameter of 7 to 9 μm and an electron emitting substance having a thickness of 1.5 to
What is deposited to 6 μm is disclosed in JP-A-63-32836, in which a tungsten wire is coated with an electron emitting substance in an amount of 10 to 15 μm.
What was applied to a thickness of m
No. 1 discloses a carbonate thickness of 6 μm, 12 μm, and 1
Those having a thickness of 8 μm have been proposed. However, according to the experiments by the present inventor, none of the direct heating oxide cathodes described above has the maximum emission capability with low power consumption.

[0012]

Therefore, the problem to be solved in the present invention is to provide a direct heating type oxide cathode having a low power consumption and an excellent emission capability.

[0013]

In order to solve the above problems, the present inventor has conducted various experiments on the relationship between the oxide deposition thickness and the emission characteristics, and as a result, the electron emitting material layer has a thickness of 7 μm. In the case of, it was found that a peak value is exhibited and the present invention has been completed. That is, in the direct heating oxide cathode of the present invention, the thickness of the electron emission material layer is 6.
It is characterized by being 5 to 7.5 μm.

Here, a tungsten wire or a rhenium tungsten wire can be used as the metal core wire.
In particular, in the case of adding about 3 to 5% by weight of rhenium to tungsten, the resistance of the core wire increases, and the temperature of the direct heating oxide cathode can be raised without increasing the power consumption. If the amount of rhenium added is less than 3% by weight, the effect cannot be exhibited, and if it exceeds 5% by weight, the workability of the metal core wire deteriorates.

The electron emission material layer has a constant power consumption and an emission capability when the thickness of the ternary oxide composed of barium, strontium and calcium, which has been widely used in the past, is 6.5 to 7.5 μm. Excellent balance.

[0016]

As described above, in order to improve the emission capability, it is necessary to increase the deposition thickness of the oxide to increase the thermionic emission area of the direct heating type oxide cathode. At the same time, increasing the thickness increases the heat dissipation area and increases the heat radiation loss, resulting in a decrease in the temperature of the directly heated oxide cathode. The thickness of the electron emission material layer of the present invention is 6.5 to
The 7.5 μm direct-heating oxide cathode has the greatest emission capability with a constant power consumption, suppressing thermal radiation loss while increasing the thermionic emission area.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be specifically described below based on an example applied to the fluorescent display tube shown in FIGS.

A ternary carbonate of (Ba, Sr, Ca) CO ₃ and an acrylic resin (Eulvsite 2045 made by DuPont) as a binder are used, and acetone (special grade reagent) is added to 45
% By weight, MIBK (special grade reagent) 24% by weight, IPA
24% by weight of (special grade reagent) and 5 of Elbasite 2045
% And ternary carbonate were set to 2% by weight and mixed by a known mixing method to prepare an electrodeposition liquid.

Using this electrodeposition solution, a ternary carbonate of 3 to 12 was applied to a tungsten wire having a diameter of 15 μm by electrophoresis.
A filamentary cathode was prepared by depositing it to a thickness of μm. Further, this was placed in a fluorescent display tube, and while being evacuated, it was thermally decomposed at a temperature of 1000 ° C. to 1300 ° C. for 50 seconds to form a direct heating type oxide cathode, and after the chip off, the barium getter was heated to Ba. Residual gas was adsorbed on the film to produce a high vacuum fluorescent display tube. Here, the length of the filament cathode was 120 mm, and three filament cathodes having the same thickness were arranged in parallel in one display tube. The characteristics of the filament cathode of the fluorescent display tube were evaluated as follows.

First, the filament cathode voltages were set so that the surface temperature of each filament cathode was 620 ° C., which is a normal operating condition, and the thermoelectrons were emitted from the filament cathode. The condition for extracting the emission is that the grid and anode voltages are D
C100V, pulse width 100 μsec, Duty = 1 /
It was set to 10. This condition is a condition in which the emission characteristic is in the temperature limit region and the saturation currents of the filament cathodes can be compared.

The results are shown in FIG. Here, the saturation current was shown as a relative value, with the value when the carbonate thickness was 7 μm being 100.

As shown in FIG. 3, when the surface temperature of the filament cathode is the same, it can be seen that the saturation current increases as the deposition thickness increases, as indicated by the above-mentioned Richardson-Dashman equation.

Next, the temperature and the saturation current on the surface of the filament cathode were compared when the power consumption was made constant for each carbonate thickness.

With the thickness of the carbonate being 7 μm as a reference, the filament cathode applied voltage was set to 4.6 V so that the filament cathode would have a surface temperature of 620 ° C. which is a normal operating condition. The same voltage was applied to all filament cathodes. The grid and anode voltages are DC 100 V, pulse width 100 μsec, Duty = 1
The driving condition was / 10. FIG. 4 shows the relationship between the thickness of the ternary carbonate and the surface temperature of the filament cathode when the filament cathode voltage is constant, and FIG. 5 shows the relationship between the thickness of the ternary carbonate and the saturation current under the same conditions. . Here, the characteristics of the respective filament cathodes were compared and evaluated, with the value of the saturation current being 100 when the thickness of the carbonate was 7 μm.

As shown in FIG. 4, when the applied voltage is the same, the surface area of the filament cathode decreases due to the increase of the radiation area. However, when the thickness of the ternary carbonate is up to 7 μm, the increase in the saturation current due to the increase in the thermoelectron emission area S of the cathode, which is represented by the above-mentioned Richardson-Dashman equation, is due to the decrease in the surface temperature. Overcome the decrease in saturation current. As a result, as clearly shown in FIG. 5, it was confirmed that the saturation current had a peak value when the thickness of the ternary carbonate was 7 μm, and the saturation current had a high value in the range of ± 0.5 μm before and after this. Here, the plot is an actual measurement value, and the curve is a quadratic regression curve based on the least square method.
The vicinity of the peak is smooth and stable, but the value of this saturation current begins to decrease at 6 μm or less and 8 μm or more.
On the other hand, if the thickness is 4 μm or less, the temperature of the filament cathode rises excessively, the electron emission layer deteriorates remarkably and the life is shortened, and if it is 10 μm or more, the surface temperature decreases and sufficient emission cannot be obtained.

Then, the same test as above was carried out using a normal tungsten wire and a rhenium tungsten wire obtained by adding 1 to 5% by weight of rhenium as an impurity in tungsten.

The results show that the thickness of the ternary carbonate is 7 ±.
High emission characteristics were obtained in the range of 0.5 μm, and the saturation current value of the rhenium tungsten wire added with 3 wt% of rhenium increased by about 5% compared to the tungsten wire.

Furthermore, in the ternary carbonate, as the fourth component,
It was confirmed that the emission peak was exhibited when the thickness of the carbonate was 7 μm, even when the fine powder of the oxide of Mg, Al, Zr, Sc or the metal oxide capable of becoming an oxide by thermal decomposition or the like was added. .

[0029]

According to the present invention, the following effects can be obtained.

(1) The thickness of the electron emission material layer is 6.5 to
By setting the thickness to 7.5 μm, a direct heating type oxide cathode having excellent emission capability with low power consumption was obtained.

(2) In particular, by using a rhenium tungsten wire, a directly heated oxide cathode having excellent emission ability with less power consumption was obtained in a controlled deposition thickness.

[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 graph showing the relationship between the thickness of the ternary carbonate and the saturation current when the filament cathode temperature is 620 ° C.

FIG. 4 is a graph showing the relationship between the thickness of the ternary carbonate and the filament cathode surface temperature when the filament cathode voltage is constant.

FIG. 5 is a graph showing the relationship between the thickness of ternary carbonate and the saturation current when the filament cathode voltage is constant.

[Explanation of symbols]

 22 Conductive Layer 23 Anode Electrode 25 Phosphor Layer 27 Grid 29 Filament Cathode (Direct Heat Oxide Cathode)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tatsuo Matsuyama 3-36 No. 1 Noritake Shinmachi, Nishi-ku, Nagoya-shi, Aichi Noritake Company Limited

Claims (3)

[Claims] 1. In a direct heating oxide cathode such as a fluorescent display tube in which an electron emission material layer is coated on the surface of a metal core wire, the thickness of the electron emission material layer is 6.5 to 7.5 μm. A directly heated oxide cathode. 2. The directly heated oxide cathode according to claim 1, wherein the metal core wire is a tungsten wire or a rhenium tungsten wire. 3. The directly heated oxide cathode according to claim 1, wherein the electron emission material layer is a ternary oxide composed of barium, strontium and calcium.