JP2001006607A - Discharge tube and manufacture of its cathode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cathode of discharge tube having a stability in the electric discharging operation, a high rate of light flux maintenance, and a long lifetime. SOLUTION: A discharge tube includes a cathode and anode arranged in an electric discharge vapor and arc discharge is conducted, wherein the cathode 21 is formed by mixing a powder of an electron emissive material with a high melting point metal powder, followed by a compressive press shaping and sintering. The manufacturing process of this cathode comprises a first process as the above-mentioned emissive material mixing process, a second process to make compressive press shaping of the mixture powder, and a third process to sinter the obtained shape by the second process.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a discharge tube and a method for manufacturing a cathode for a discharge tube, and more particularly to a long-life discharge tube having high discharge stability and high luminous flux maintenance factor.

[0002]

2. Description of the Related Art As one example of a conventional discharge tube for a light source, as shown in FIG. 3, sealing tubes 17a and 17b are provided at both ends of a luminous tube 11 made of quartz.
The anode assembly 12 and the cathode assembly 13 are sealed inside through molybdenum foils 14a and 14b in the sealing tubes 17a and 17b, and leads 16a and 16b are connected to the outside.

An exhaust tube 15 is connected to the quartz light emitting tube 11. After exhausting through the exhaust tube 15, a rare gas such as xenon gas and a metal required for light emission are sealed and sealed. Can be

As a cathode attached to the tip of the cathode assembly 13, usually, about 1 to 4% by weight of thoriated tungsten containing thorium oxide as an electron emitting material is used. However, since thorium oxide is a radioactive substance, during the period from its production to disposal,
Including the problem of environmental pollution, a tungsten electrode containing a rare earth oxide, for example, lanthanum oxide, yttrium oxide, cerium oxide, etc., instead of thoriated tungsten containing thorium oxide has been conventionally proposed. I have.

[0005]

However, the temperature of the tip of the cathode becomes high during the discharge operation, the rare earth oxide contained therein evaporates, and the inner wall of the discharge tube is contaminated. There are many practical problems, such as a shortened life and an unstable discharge arc due to deformation of the cathode tip.

Similarly, a metal powder of a high melting point metal, for example, tungsten, is press-molded instead of a triplet tungsten cathode, fired in a vacuum or a hydrogen atmosphere, and then impregnated with an electron-emitting material in a porous metal substrate. A cathode that has been made has been conventionally proposed.

However, in such a cathode in which an electron-emitting material is impregnated in a porous metal substrate, it is difficult to make the amount of the electron-emitting material impregnated uniform, and the porous metal substrate contains the electron-emitting material. Due to non-uniform dispersion, lack of long-term discharge stability, and for large currents, the electron-emitting material impregnated in the porous metal substrate gradually evaporates and scatters from the porous part. As a result, there is a problem that the inner wall of the discharge tube is contaminated, and the life is shortened due to a decrease in the luminous flux maintenance factor.

As means for solving these problems, the inner surface area of the arc tube 11 is increased to reduce the adhesion density of the scattered substances, or the porous metal substrate is impregnated with an electron-emitting material, and then the cathode is removed. While various measures have been taken to prevent the scattering of the electron-emitting material from the porous metal substrate by winding a metal wire around the side surface in a coil shape, none of these methods is an essential solution.

The present invention has been conceived with the object of providing a long-life cathode of a discharge tube having high discharge stability and a high luminous flux maintenance factor.

[0010]

According to a discharge tube of the present invention, a cathode and an anode are arranged in a discharge vapor to perform arc discharge. Then, a cathode obtained by sintering a product obtained by compression press molding is provided.

The method of manufacturing a cathode for a discharge tube according to the present invention includes a first step of mixing a powder of an electron-emitting substance with a high melting point metal powder and a second step of compression-pressing the powder mixed in the first step. And a third step of sintering the product formed in the second step.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a discharge tube cathode according to the present invention will be described with reference to FIG. An example of a discharge tube to which the cathode of the present invention is applied is, similarly to the conventional light source discharge tube shown in FIG. 3, a quartz light emitting tube 11 having a maximum inner diameter of 20 mm, an anode structure 12 made of a tungsten rod having a diameter of 4 mm, and an anode structure 12. , FIG.
As shown in (a), a tungsten rod 20 for forming a conductive path having a diameter of 3 mm and a cathode assembly 13 comprising a cathode 21 having a diameter of 1.8 mm and a length of 3 mm attached to the tip thereof are enclosed. Note that the tungsten rod 20 serving as a conductive path
The method of attaching the cathode 21 and the cathode 21 is not limited to the structure shown in FIG. 1A as long as the heat from the cathode 21 is efficiently conducted to the conductive path.

The cathode 21 is manufactured by uniformly mixing and stirring the powder of an electron-emitting substance with tungsten powder of about 4 μm to 10 μm as a refractory metal, placing the mixture in a required molding die, and compressing and pressing. I do. The compression-pressed cathode is heated at about 2000 ° C. in an argon gas atmosphere.
Sinter for a minute.

If the desired shape as the cathode 21 cannot be obtained by compression press molding, after the compression press molding,
After provisional sintering at about 1400 ° C. for 15 minutes in an argon gas atmosphere, grinding into a desired shape is performed, and then main sintering at about 2000 ° C. in an argon gas atmosphere.

Although the mixing ratio of the electron emitting material to be mixed with the tungsten powder as the high melting point metal was set to 6% by weight, it became clear that the range of 4 to 12% by weight can be put to practical use. If the mixing ratio of the electron-emitting material is less than 4%, the supply of the electron-emitting material during discharge is insufficient,
Long-term stable discharge cannot be maintained. On the other hand, when the mixing ratio of the electron-emitting material exceeds 12%, the amount of the electron-emitting material evaporated and scattered from the cathode increases, thereby contaminating the inner wall of the discharge tube and lowering the luminous flux retention rate. It turned out to be.

In this embodiment, BaO: Ca is used as an easily dischargeable radiant for tungsten powder.
A mixture of O: Al ₂ O ₃ at a molar ratio of 2: 1: 1 was used. Since these electron emitting materials form a solid body during the sintering process, they act as a binder between tungsten powders, which are high melting point metals, and increase the physical strength of the cathode and increase the discharge potential of the cathode tip. The heat conductivity is improved against heat generation, suppressing the rise in temperature at the cathode tip discharge part.Furthermore, in the sintering process, the electron emitting material on the cathode surface layer evaporates and dissipates and disappears. When used as a cathode 21 for a tube, an effect of reducing the amount of evaporation of the electron-emitting material from the cathode surface layer is obtained.

As described above, by mixing the powder of the electron emitting material with the tungsten powder, which is a high melting point metal, before molding and sintering, the dispersion of the electron emitting material inside the cathode is uniform. And enhances physical strength and thermal conductivity, minimizes the amount of electron-emissive material evaporated and scattered from the cathode surface layer, and emits electron easily into the porous metal substrate. By eliminating the need for impregnation treatment with a substance, a simple and inexpensive cathode can be provided.

The temperature at the cathode tip can be reduced to a work function of about 1.7 eV by the action of the electron-emitting material to prevent recrystallization of the cathode tip discharge. And stable discharge can be maintained for a long period of time without deformation of the tip.

(Embodiment) The shape of the cathode 21 after sintering is shown in FIG.
As shown in (a), the diameter is 1.8 mm and the length is 4 mm, and the shape of the tip is tapered toward the electron emission point as shown in FIG. It has an R shape in contact with this taper shape, and the taper angle θ is 4
0 ° and the tip R was 0.5 mm.

(First Comparative Example) The size and shape of the cathode were the same as those of the embodiment, and the cathode material used was a conventional cathode made of triplet tungsten containing ₂ % by weight of ThO ₂ .

(Second Comparative Example) The same dimensions and shapes as those of the example were used. A porous tungsten substrate was mixed with BaO: CaO: Al ₂ O ₃ at a molar ratio of 2: 1: 1 as an electron emitting material. The impregnated one was used as a cathode.

Using the cathodes of the embodiment, the first comparative example, and the second comparative example, argon gas was used as a starting gas in the arc tube 11 at a pressure of 0.4 atm, and mercury was added in an amount of 40 mg per 1 cc of the inner volume of the arc tube 11. And the discharge interval between the cathode 21 and the anode is 2 mm.
, A mercury short arc lamp having a discharge voltage of 42 V was operated at an input power of 250 W. The measurement results of the lighting time and the luminous flux maintenance ratio are as shown in FIG.

When the luminous flux at the beginning of lighting is set to 100, the luminous flux maintenance rate decreases in the first comparative example and the second comparative example every time the lighting time increases. In particular, the second comparative example
Although the luminous flux maintenance factor sharply decreases at the beginning of lighting, it is clear that the embodiment of the present invention is superior to the comparative examples.

Next, in order to examine the relationship between the shape (taper angle θ and tip R) of the tip of the sintered cathode 21 and the discharge stability, (first experimental example) The shape of the tip is 20 ° for the taper angle θ and 0.1 mm for the tip R.
(Second Experimental Example) The shape of the tip of the cathode 21 after sintering was such that the taper angle θ was 20 ° and the tip R was 0.9.
(Third Experimental Example) The shape of the tip of the cathode 21 after sintering was such that the taper angle θ was 100 ° and the tip R was 0.2 mm.
Less than 1 mm, (Fourth experimental example) Cathode 21 after sintering
The shape of the tip part of the taper angle θ is 100 ° and the tip R
Was set to 0.9 mm, and the discharge stability of each experimental example was compared and examined.

With respect to the discharge stability, the mercury short arc lamp of each experimental example was fixed so that the cathode tip was located at the first focal point of the elliptical rotating mirror having a diameter of about 150 mm, and the elliptical rotating mirror was fixed. 3mm diameter on the second focal point of the
A light receiver having a light receiving surface of No. 3 was arranged to receive ultraviolet light of 365 nm, and the stability of the received ultraviolet light illuminance was defined as arc stability S. The arc stability S was obtained by the following equation. S (%) = {(Imax−Imin) / I)} where Imax is the maximum condensing illuminance, Imin is the lowest condensing illuminance, I is the average condensing illuminance, and Imax and Imi
An example of a temporal change of n and I is as shown in FIG.

The arc stability S obtained in the examples and the first to fourth experimental examples was as shown in Table 1 below.

[Table 1]

According to these data, the cathode tip angle θ is 2
If the temperature is less than 5 °, the amount of evaporation of the electron-emitting material due to the high temperature at the tip of the discharge is large, and recrystallization is caused by a rise in the temperature of the cathode with the decrease in the amount of the electron-emitting material at the cathode. As it proceeds, the supply of the electron-emissive material in the cathode decreases, and the arc stability deteriorates with the lighting time. Also, when the cathode tip angle θ exceeds 90 ° and when the shape of the cathode tip R exceeds 0.8 mm, the electron emission point of the cathode is not determined, and it is apparent that the arc stability S is poor from the beginning of lighting. Became.

From the above, regarding the tip shape of the cathode, it is desirable that the angle θ of the tapered portion is 25 ° to 90 °, and the R shape in contact with the tapered portion is in the range of 0.1 to 0.8 mm. It became clear.

[0029]

As described above, according to the present invention, as a method for producing a cathode of a discharge tube, a high-melting-point metal powder is mixed with an electron-emitting material, then compression-pressed, and then sintered. Since the dispersion of the electron-emitting material after sintering is uniform in the high melting point metal and there is no cavity for impregnating the electron-emitting material from the outside into the porous metal substrate, It is possible to provide a cathode of a long life discharge tube in which the amount of evaporative scattering of the substance can be suppressed to a minimum, and a decrease in the luminous flux maintenance rate due to contamination of the inner wall of the arc tube by the electron-emitting substance can be prevented. In addition, the method is simple and can be manufactured at a low cost. Since no radioactive substance such as thorium oxide is used, environmental pollution can be prevented.

Also, by appropriately selecting and combining the taper angle θ of the cathode tip and the shape of the cathode tip R in contact with the tapered portion, a long-life discharge tube with good arc stability can be obtained.

[Brief description of the drawings]

FIG. 1A is a cross-sectional view showing an embodiment of a discharge tube cathode according to the present invention, and FIG.

FIG. 2 is a characteristic curve diagram showing measurement results of lighting time and luminous flux maintenance factor of a discharge tube using the cathode material of the present invention and a conventional discharge tube,

FIG. 3 is a front view showing an example of a conventional discharge tube for a light source in a cross section of a central portion;

FIG. 4 is a diagram for explaining the definition of arc stability.

[Explanation of symbols]

 11 Arc tube 12 Anode 13 Cathode assembly 21 Cathode 17a, 17b Seal tube 14a, 14b Molybdenum foil 20 Tungsten rod

Claims (6)

[Claims] 1. A discharge tube in which a cathode and an anode are arranged in a discharge vapor to cause arc discharge, wherein a powder of an electron emitting material is mixed with a high melting point metal powder, and a compact formed by press molding is sintered. A discharge tube comprising a reduced cathode. 2. The discharge tube according to claim 1, wherein the tip of the cathode has a tapered shape toward the electron emission point, and has an R shape in contact with the taper. 3. The tip shape of the cathode is such that the angle of the tapered portion is 25 ° to 90 ° and the tip R is 0.1 to 0.8 m.
The discharge tube according to claim 2, wherein m is m. 4. A method for producing a discharge tube cathode in which a cathode and an anode are arranged in discharge steam and arc discharge is performed, wherein a first powder of an electron emitting material is mixed with a high melting point metal powder.
And a second step of compression-pressing the powder mixed in the first step.
And a third step of sintering the product formed in the second step. 5. The method for producing a cathode for a discharge tube according to claim 4, wherein the mixing ratio of the electron-emitting substance to the refractory metal powder is 4 to 12% by weight. 6. The method for producing a cathode for a discharge tube according to claim 4, wherein the electron-emitting substance is composed of barium, alkaline earth and alumina.