JPH11339713A - Electrode for discharge tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode capable of realizing stability of discharge, long life, ease of fabrication, etc., without causing contradictions among these properties. SOLUTION: In an electrode for a discharge tube wherein a negative electrode and a positive electrode facing to each other are sealed in a discharge gas atmosphere and arc discharge is performed between both electrodes, the negative electrode is formed of either an impregnated metal wherein a porous high melting metal is impregnated with a readily emissive material or a sintered metal wherein a high melting metal is impregnated with a readily emissive material and then sintered. Further, a tip part 22 facing the positive electrode is composed of a metal base body 221 having a pointed head and a cap 222 covering a major portion of this metal base body 221 except the pointed head protruding through the cap 222, being disposed in close contact with the tip part 22, which is made up of an insulating ceramic free of readily emissive materials.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrode of a discharge tube for a light source mainly used for a light source, and more particularly to an electrode of a discharge tube for a light source such as a xenon lamp or a xenon mercury lamp.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, Japanese Patent Application Laid-Open No. 1-213952 (conventional example 1), Japanese Patent Application Laid-Open No. 1-213953 (conventional example 2),
JP-A-8-273622 (conventional example 4), JP-A-9-92201 (conventional example 5),
The technology disclosed in each publication of -283341 (Conventional Example 6) is known.

[0003] In prior art example 1, a discharge tube for a light source having a cathode in which a metal substrate made of porous tungsten or the like contains an electron emitting material such as an alkaline earth metal is disclosed. Then, a coating made of a high melting point metal is formed on the tip of the cathode facing the anode. It is described that this suppresses excessive evaporation of the electron-emitting material and improves the stability of the arc.

[0004] In prior art example 2, a technique is disclosed in which an electron-emitting material layer and a high-melting-point metal layer are laminated on the front end surface of a metal base made of a high-melting-point metal. It is described that this stabilizes the supply of the electron-emitting material, suppresses excessive evaporation thereof, and improves the stability of the arc.

The prior art 3 discloses a technique of forming a metal film for facilitating electron emission on the surface of a so-called impregnated electrode substrate. It is described that the arc discharge is thereby stabilized.

[0006] In prior art example 4, a technique is disclosed in which an impregnated electrode made of porous tungsten or the like is arranged around a sharp electrode made of a high melting point metal, and a tungsten film is formed on the surface. According to this document, the thermoelectron emission capability is improved and the cathode drop voltage is reduced.

In Conventional Example 5, an impregnated portion impregnated with an electron-emitting material is formed around an electrode made of porous tungsten, and the periphery of the impregnated portion is made of a high-melting metal so as to expose the tip of the porous tungsten. A coating technique is disclosed. According to this, unnecessary evaporation of the electron-emitting material is suppressed, and a stable arc discharge can be performed.

In prior art example 6, a technique is disclosed in which porous tungsten is buried in the center of a hollow rod-shaped molybdenum electrode, and the buried porous tungsten is exposed by processing to manufacture. It is also described that also in this case, unnecessary evaporation of the electron emitting material is suppressed, and stable arc discharge can be performed.

[0009]

As described above, the cathode of the discharge tube for the light source has been variously improved and devised as described above. However, the light source such as easiness of manufacture, discharge stability, and long life has been considered. There has not yet been provided a product that satisfies various requirements required for a discharge tube for use without inconsistency with each other.

In each of the above-mentioned conventional examples, there is a problem in terms of stability of the discharge position. This is because the cathode surface is coated with a high-melting metal or the like, so that a discharge may occur in a wide area of the electrode surface. Further, when the discharge current per unit surface area is increased, the electrode temperature may be increased, and the evaporation of the electron-emitting substance may be increased. In addition, some of them are not always easy to manufacture.

Therefore, the present invention has been made in view of the above problems,
An object of the present invention is to provide an electrode for a discharge tube capable of realizing discharge stability, long life, ease of manufacture, and the like without contradiction.

[0012]

In order to solve the above-mentioned problems, an electrode for a discharge tube according to the present invention is characterized in that its cathode is an impregnated type or a high-melting-point metal in which a porous high-melting-point metal is impregnated with an electron emitting material. A metal substrate made of a sintered metal containing an electron-emitting substance and sintered, and having a pointed tip at the tip end facing the anode, and exposing only the tip of the pointed tip of the metal base to protrude. And a cap made of a high-melting-point metal that does not contain an electron-emitting material and that is placed in close contact with the surface of the metal substrate.

According to this configuration, since only the tip of the spire portion of the metal base is exposed so as to protrude from the high-melting-point metal cap, the electron-emitting material is supplied intensively to this exposed portion. Therefore, unnecessary evaporation of the electron-emitting material is suppressed, and the discharge position is concentrated on the protruding tip portion. Further, since the heat capacity of the cap portion is large, a rise in temperature due to heat generation of the metal base is suppressed.

The electron emitting material is preferably a mixture of oxides of alkaline earth metals such as barium, calcium and strontium, or a mixture of these with addition of aluminum oxide.

According to this, the work function of the metal substrate can be reduced, and accordingly, the temperature of the protruding tip of the electrode during operation decreases. Therefore, it is possible to make the protruding portion smaller as compared with the case where another electron emitting material is used, and it is possible to concentrate the discharge position in a narrower region.

It is preferable that a metal or metal carbide coating covering the entire surface of the metal substrate except for the protruding tip is further provided. In any case, the effect of providing the cap described above is further enhanced because the coating itself suppresses evaporation of the electron-emitting substance. In particular, when a carbide coating is used, conduction between the cap and the metal substrate is cut off, so that unnecessary electron emission is removed.

Further, it is preferable that a metal or metal carbide coating covering the outer surface of the cap is further provided. According to this, the heat radiation from the cathode is further improved. In particular, in the case of a carbide coating, discharge on the cap is prevented.

[0018]

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. To facilitate understanding of the description, the same constituent elements are denoted by the same reference numerals as much as possible in each drawing, and redundant description will be omitted.

FIG. 1 is a longitudinal sectional view showing a configuration of a xenon short arc lamp using an electrode according to the present invention, and FIG. 2 is a side view showing a configuration of a cathode tip portion in a partially crushed section. is there. A hollow gas sealing portion 11 is formed in the middle of the rod-shaped quartz glass bulb 1, and xenon is sealed therein as a discharge gas at a pressure of about 10 atm. Cathode 2 and anode 3 inside gas filling section 11
External terminals 4, 5 electrically connected to the cathode 2 and the anode 3, respectively, are attached to both ends of the glass bulb 1 in the bar-shaped longitudinal direction.

The cathode 2 has a 2.4 mm diameter tungsten lead rod 21 having a base fixed to the quartz glass bulb 1 and a cathode tip 22 having a base fixed to the tip of the lead rod 21. ing. The cathode tip 22 includes a metal base 221 and a cap 222. Among them, the metal substrate 221 is formed by impregnating a porous tungsten (high melting point metal) with an alkaline earth metal or the like (electron emitting material), and is basically formed in a conical shape pointed toward the anode 3. It has a cannonball shape with a point of 2.0 mm in diameter and 2.0 mm in length. On the other hand, the cap 222 is made of, for example, tungsten, has a thickness of 0.3 mm, and is disposed in close contact with the surface of the metal base 221. An opening having a diameter of 0.4 mm is provided at the tip of the anode 3. The tip of the metal base 221 is 0.2 mm from this opening.
It is protruding.

This metal substrate 221 has an average particle size of 2 μm.
It is manufactured by press forming a tungsten powder of ８8 μm and firing in a vacuum or a hydrogen atmosphere. The porosity is about 10 to 35%. Average particle size is 2μm ~ 8μ
By using tungsten powder of m, the porous body can be easily manufactured, and by setting the porosity to about 10 to 35%, a necessary and sufficient amount of the electron-emitting material can be filled. . As the electron emitting material, for example, a material obtained by mixing BaO, CaO, and Al ₂ O ₃ at a ratio of 4: 1 to 1 is used.

The cap 222 is connected to the metal base 22.
1 is formed in advance into a shape conforming to the outer shape of the lead rod 21 and is put on the tip of a metal base 221 which is fixed to the tip of the lead rod 21 by high melting point brazing or press fitting, and is subjected to electric spot welding, laser welding, arc welding. The cathode 2 can be formed by fixing to the equal lead rod 21.

On the other hand, the anode 3 is made of a tungsten rod having a diameter of 3.0 mm, and the distance between the cathode 2 and the tip of the anode 3 is adjusted to 2.0 mm. By applying a predetermined voltage between the anode 3 and the cathode 2, arc discharge occurs.

According to the present invention, the metal substrate 221 of the cathode 2
The discharge can be concentrated on the exposed portion of the tip. The inventor of the present application has confirmed by experiments that the concentration of the discharge positions improves the luminance of the light emission by the discharge more than twice as compared with the case where the cap 222 is not provided.

If the opening of the cap 222 is too large, the effect of specifying the discharge position is weakened. Rated 150 W (8.5 A, 18
In the case of the xenon lamp electrode of V), the diameter is 0.8 m.
When it is more than m, the effect of suppressing evaporation is weakened. The effect of improving the brightness was remarkable when the diameter was set to 0.4 mm or less. On the other hand, when the thickness was 0.1 mm or less, the electron emitting material was not sufficiently supplied, and the discharge could not be maintained. Since the optimal size of the aperture diameter depends on the input voltage and current to the lamp, it needs to be set appropriately according to the application.

If the amount of protrusion of the metal base 221 is small or does not protrude, the arc due to discharge comes into contact with the periphery of the opening of the cap 222, and the periphery of the opening melts and evaporates. There is a possibility that it will become large. Therefore, it is necessary to adjust the amount of protrusion so that the arc does not contact the cap 222.

The cap 222 prevents evaporation of the electron-emitting material from the surface of the metal base 221 other than at the discharge position, thereby extending the life. Further, blackening of the bulb 1 caused by the vaporized electron emitting substance adhering to the inner surface of the bulb 1 can be prevented. Also, the metal base 221
The heat generated by the cap 222 effectively has a large heat capacity.
And the heat is dissipated, so that overheating of the metal base 221 can be prevented, and the temperature of the distal end portion is stabilized. This also contributes to stabilization of the discharge and an increase in luminance.

FIGS. 3 and 4 are views showing another embodiment of the cathode 2 of the present invention. As shown in FIG. 3, even when the cap 222 is placed only on the tip of the metal base 221 or when the outer surface of the cap is cylindrical as shown in FIG. 4, the same as in the embodiment shown in FIG. The effect of is obtained.

The metal substrate 221 or the cap 2
The surface of 22 may be coated with metal or metal carbide. FIG. 5 shows an example in which both are coated. Reference numeral 221a indicates a coating on the metal substrate 221 and reference numeral 222a indicates a coating on the surface of the cap 222.

As the metal coating, it is preferable to use a metal having a high melting point such as tungsten, molybdenum, tantalum, iridium, rhenium or an alloy thereof.
Further, as the metal carbide, it is preferable to use a carbide of these refractory metals or alloys thereof.

Table 1 shows combinations of the presence and absence of the coating.

[0032]

[Table 1] Among them, the coating 221a on the metal substrate 221
Is preferably performed with the protruding tip exposed, and this may be performed by masking the tip or depositing or removing the coating formed on the tip by polishing or the like after deposition. The method of exposing the tip portion of the substrate can be used. The thickness of the coating is a thin film of several μm to several tens μm. For this deposition, a CVD method or the like can be used. The coating 22 on the cap 222 side
2a can be performed similarly.

When the coating 221a is applied to the metal substrate 221, the coating 221a has the effect of suppressing the evaporation of the electron-emitting material and the emission of electrons from portions other than the exposed portion, and the effect of concentrating the discharge position by the cap 222. Can be even higher. In addition, there is an effect that heat transfer between the metal base 221 and the cap 222 is improved.

The coating 222a is provided on the cap 222 side.
Is applied, there is an effect that heat radiation is promoted by the coating 222a. Thereby, the temperature stability of the metal base 221 can be improved.

In particular, when a carbide is coated,
It is possible to prevent occurrence of abnormal discharge from the cap 222 due to conduction.

In the above description, an example in which tungsten is used as the metal base and the cap has been described in detail. However, similar effects can be obtained by using molybdenum, rhenium, tantalum, or the like as a material. The material of the metal base and the cap may be the same or different.

Although the impregnation type electrode in which the porous metal substrate is impregnated with the electron emitting material has been described as an example, the electrode is not limited to this. You may use the sintered type electrode sintered simultaneously.

Alternatively, a so-called tritan electrode in which thorium oxide is added to a tungsten base material may be used.

The electron emitting material is preferably a simple substance or a mixture of oxides of alkaline earth metals such as calcium, barium and strontium. Alternatively, oxides of Group IIIa of the periodic table such as scandium, yttrium, lanthanum, cerium, and thorium may be used.

Although an example in which the cathode is used for a xenon short arc lamp has been described, similar effects can be obtained when the cathode is used as a cathode for various other discharge tubes.

[0041]

As described above, according to the present invention,
On the base end side surface of the tip of the cathode tip, evaporation of the electron-emitting material is prevented during operation, while diffusion of the electron-emitting material to the point is promoted to facilitate electron emission. .
For this reason, electrons can be efficiently emitted at a relatively low temperature, so that the discharge is stable, and furthermore, the evaporation of the electron-emitting material is suppressed, and the life can be greatly extended. On the other hand, since the discharge region can be narrowed, the energy density per unit area increases, and the luminance improves. Furthermore, since the cathode tip can be realized with a simple configuration, a highly practical electrode for a light source discharge tube can be provided.

Therefore, according to the present invention, various requirements for the cathode of a discharge tube for a light source, such as ease of production, stability of discharge, and long life, are satisfied at once without inconsistency. Becomes possible.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a xenon short arc lamp using a cathode according to the present invention.

FIG. 2 is a cross-sectional view showing a configuration of a cathode according to the present invention.

FIG. 3 is a cross-sectional view showing another configuration of the cathode according to the present invention.

FIG. 4 is a sectional view showing still another configuration of the cathode according to the present invention.

FIG. 5 is a cross-sectional view showing still another configuration of the cathode according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Glass bulb, 11 ... Gas sealing part, 2 ... Cathode, 21
... lead rod, 22 ... cathode tip, 221 ... metal substrate, 2
22: cap, 3: anode, 4, 5: external terminal.

Claims (6)

[Claims] 1. An electrode for a discharge tube in which a cathode and an anode are sealed in a discharge gas atmosphere with facing each other, and arc discharge is performed between the electrodes. A metal base made of an impregnated type impregnated with a radioactive substance or a sintered type metal obtained by sintering a high-melting-point metal containing an electron-emitting substance, and having a point at a tip end facing the anode; A cap made of a refractory metal that does not contain the electron-emissive substance and that is arranged so as to protrude and expose only the tip of the pointed tip of the base and cover the surface of the metal base in close contact. An electrode for a discharge tube, characterized in that: 2. The electrode for a discharge tube according to claim 1, wherein the electron emitting material is a mixture of alkaline earth oxides or the mixture to which aluminum oxide is added. 3. The electrode for a discharge tube according to claim 1, further comprising a metal coating covering an entire surface of the metal base except for a protruding tip. 4. The electrode for a discharge tube according to claim 1, further comprising a coating made of a metal carbide covering an entire surface of the metal substrate except for a protruding tip end of the metal substrate. 5. The apparatus according to claim 1, further comprising a metal coating covering an outer surface of said cap.
An electrode for a discharge tube according to any one of the above. 6. The electrode for a discharge tube according to claim 1, further comprising a metal carbide coating covering an outer surface of said cap.