JP2024062715A - Discharge lamp - Google Patents

Abstract

[Problem] To provide a discharge lamp in which a coating is applied to the electrode surface, which can effectively exert its coating function while the lamp is lit. [Solution] In a discharge lamp 10, a coating layer 44 is formed on the surface of the body portion 34 of the electrode 30, and tungsten particles are attached to the surface of the coating layer 44 in a dotted or scattered manner. The amount of tungsten particles attached has a concentration distribution in which the amount increases the closer to the tip of the electrode along the electrode axis C. [Selected Figure] Figure 2

Description

The present invention relates to discharge lamps such as short arc type discharge lamps, and in particular to coatings for electrode surfaces.

When a discharge lamp is turned on, the electrode tip becomes very hot, causing the electrode material, such as tungsten, to melt and evaporate, blackening the discharge tube and reducing the lamp's illuminance. To prevent the electrode, including the electrode tip, from overheating, for example, a screw-shaped groove is formed on the side of the electrode body to increase the surface area, and tungsten powder is sintered on top of the groove to form a heat dissipation layer (see Patent Document 1).

There is also a known discharge lamp in which a heat dissipation member is sintered to the electrode body (see Patent Document 2). In this case, a hollow cylindrical heat dissipation member made of a ceramic material with higher thermal conductivity than the electrode body is sintered to the electrode body, and heat is dissipated from the side of the electrode, thereby suppressing the rise in electrode temperature.

JP 2000-306546 A JP 2008-186790 A

When the lamp is turned on, the electrode overheats, causing part of the electrode material (such as tungsten) to evaporate, and some of the evaporated material adheres to the electrode surface. This is true even if the electrode surface is formed of a heat dissipation member surface made of a ceramic material. Such adhesion of electrode material to the electrode surface leads to a decrease in the heat dissipation function of the ceramic material.

Therefore, there is a need to provide a discharge lamp that can effectively dissipate heat from the electrode surface while the lamp is lit.

A discharge lamp according to one aspect of the present invention comprises a discharge tube and a pair of electrodes arranged opposite each other within the discharge tube, at least one of the electrodes contains ceramics, and a coating layer having tungsten attached to the layer surface is formed on at least a portion of the surface of the electrode body, and the amount of tungsten attached to the coating layer near the electrode tip side is relatively greater than the amount of tungsten attached to the coating layer near the electrode support rod side.

The coating layer can be formed at various positions. For example, the coating layer can be formed on the surface of the electrode body so as to straddle the axial center of the electrode. In this case, it is possible to configure the coating layer so that the amount of tungsten adhering to the layer surface on the electrode tip side of the axial center is greater than the amount of tungsten adhering to the layer surface on the electrode support rod side of the axial center.

There are various possible methods for creating a difference in the amount of tungsten attached between the electrode tip side and the electrode support rod side, and it is possible to create a specified concentration distribution. For example, it is possible to determine the amount of tungsten attached so that there is a gradation in the coating layer in which the amount of tungsten attached gradually increases from the electrode support rod side to the electrode tip side.

The coating layer includes a single coating layer containing one or more ceramics as a material, and multiple coating layers. In the case of multiple coating layers, each layer contains one ceramic as a material, or each layer contains multiple ceramics.

There are various configurations for the multiple coating layers. For example, one coating layer (e.g., the bottom layer) can contain ceramics and electrode material (tungsten, molybdenum, etc.), while other coating layers (e.g., coating layers formed on the bottom layer) can contain only ceramic components, or components other than ceramics and electrode material. Alternatively, it is possible to configure multiple coating layers with different components at the tip and rear ends of the electrode along the lamp axial direction.

The ceramics contained in the coating layer may be at least one of nitrides, oxides, borides, carbides, and silicides. In other words, the coating layer may be made of one of the following ceramics: nitrides, oxides, borides, carbides, and silicides, or two or more of these ceramics.

For example, the ceramics can be made of at least one of silicon nitride, aluminum nitride, zirconium nitride, aluminum oxide, zirconium oxide, zirconium carbide, silicon carbide, tantalum silicide, and zirconium boride.

The coating layer can be made chromatic. For example, it can be made chromatic by adjusting the ratio of the nitrogen and zirconium atomic concentrations in zirconium nitride to the range 0.2<N/Zr<0.9.

The present invention provides a discharge lamp that can effectively perform its heat dissipation function while the lamp is lit.

1 is a schematic plan view of a discharge lamp according to an embodiment of the present invention. FIG. 2 is a schematic plan view partially showing an electrode of the present embodiment. 1 is a graph showing illuminance maintenance rates of an example and a comparative example.

The short arc type discharge lamp 10 is a large discharge lamp capable of outputting high-intensity light, and includes a transparent quartz glass, approximately spherical discharge tube (light emitting tube) 12, in which a pair of tungsten electrodes 20, 30 are arranged facing each other (coaxially). On both sides of the discharge tube 12, quartz glass sealing tubes 13A, 13B are connected to the discharge tube 12 and formed integrally with it. Mercury and rare gases such as halogen and argon gas are sealed in the discharge space DS within the discharge tube 12.

The cathode electrode 20 is supported by an electrode support rod 17A. Sealed in the sealed tube 13A are a glass tube (not shown) through which the electrode support rod 17A is inserted, a lead rod 15A that connects to an external power source, and metal foil 16A that connects the electrode support rod 17A and the lead rod 15A. Similarly, the anode electrode 30 is sealed with mounting parts such as a glass tube (not shown) through which the electrode support rod 17B is inserted, metal foil 16B, and lead rod 15B. In addition, caps 19A and 19B are attached to the ends of the sealed tubes 13A and 13B, respectively.

When a voltage is applied to the pair of electrodes 20, 30, an arc discharge occurs between the electrodes 20, 30, and light is emitted toward the outside of the discharge tube 12. Here, a power of 1 kW or more is input. The light emitted from the discharge tube 12 is guided in a predetermined direction by a reflector (not shown).

Figure 2 is a schematic plan view partially showing the electrode (anode) 30. Note that the electrode (cathode) 20 can also have a similar structure.

The electrode 30 has an electrode tip surface 32T and is composed of a tapered portion (hereinafter referred to as the tip tapered portion) 32 and a columnar portion (hereinafter referred to as the body portion) 34 that is connected to the electrode support rod 17B. The electrode 30 can be composed of tungsten, molybdenum, or an alloy of these. Here, the electrode 30 is composed as a single unit.

A coating layer 44 is formed on the side surface 34S of the body portion 34. Here, the coating layer 44 is formed over the entire body portion 34, that is, from the electrode tip end portion 34E1 of the body portion 34 to the electrode support rod end portion (not shown in FIG. 2). Note that in FIG. 2, the coated portion is indicated by diagonal lines.

The coating layer 44 is configured as a coating layer containing ceramics with thermal performance (including heat resistance and heat dissipation). It is particularly preferable that the coating layer has a high melting point of 2000°C or more. The ceramics can be composed of nitrides such as zirconium nitride (ZrN), silicon nitride (Si3N4), and aluminum nitride (AlN), oxides such as aluminum oxide (Al2O3) and zirconium oxide (ZrO2), carbides such as zirconium carbide (ZrC) and silicon carbide (SiC), silicides such as tantalum silicide (TaSi2), or borides such as zirconium boride (ZrB2), or can be composed of a combination of at least two or more of these.

Here, ceramics made of zirconium nitride or zirconium carbide, or ceramics made of both, are contained in the coating layer 44. Note that the coating layer 44 may contain the same metal as the electrode 30, such as tungsten or molybdenum.

Due to the properties of zirconium nitride, the coating layer 44 is visible as a chromatic colored side area. In other words, it is identified as a color different from the electrode base. This makes it easy to check for color unevenness and film condition from the outside, and makes it possible to inspect whether the coating layer 44 has been properly formed without measuring emissivity or conducting lighting experiments.

For example, if the ratio of the nitrogen and zirconium atomic concentrations in zirconium nitride is in the range of 0.2<N/Zr<0.9, the coating layer 44 is formed as a brownish chromatic color. The ratio of the atomic concentrations can be determined by measuring and analyzing the surface of the body portion 34 using, for example, energy dispersive X-ray analysis (EDS).

Tungsten particles (see symbol W) are attached to the surface of the coating layer 44 (hereinafter referred to as the layer surface). Here, they are scattered (dispersed) across the entire layer surface of the coating layer 44. Furthermore, the amount of tungsten particles attached to the layer surface of the coating layer 44 is configured to gradually increase along the electrode axis C from the electrode support rod side end of the body portion 34 toward the electrode tip side end 34E1.

In other words, the concentration of the scattered tungsten along the electrode axis C increases from the electrode support rod end toward the electrode tip end 34E1, forming a concentration distribution that is close to a gradation. In Figure 2, the difference in the amount (concentration) of tungsten particles attached is shown diagrammatically by the enlarged areas (symbols A1 and A2).

By providing a coating layer 44 in which the amount of tungsten particles attached gradually increases toward the electrode tip, it is possible to suppress the deterioration of the heat dissipation function of the coating layer 44 while the lamp is lit. This will be explained below.

As described above, the anode 30 of the short arc type discharge lamp 10 is configured to contain tungsten as the electrode material. Therefore, when the lamp is turned on, the electrode 30 becomes hot, causing some of the tungsten, which is a component of the electrode 30, to evaporate. The evaporated tungsten moves along the convection of the gas (see symbol F) in the discharge tube 12, and some of it adheres to the surface of the coating layer 44.

On the other hand, the tungsten particles attached to the coating layer 44 are a different substance from the tungsten that is the electrode material, and therefore do not become one with the material (base material), but merely adhere to it, resulting in a weak bond. Therefore, even with regard to the tungsten particles that have been attached in advance to the surface of the coating layer 44 so as to have a concentration distribution, some of them evaporate when the electrode 30 is heated by lighting the lamp. Some of the evaporated tungsten then adheres to the surface of the coating layer 44.

Here, whether or not the tungsten particles attached to the surface of the coating layer 44 undergo crystal growth depends on whether or not the entire system is moving in a thermodynamically stable direction, and it is necessary for the critical nucleus, which is the boundary between stability and non-stability, to become a stable nucleus and undergo crystal growth. The frequency with which tungsten particles become stable nuclei from among the critical nuclei is closely related to crystal growth.

As described above, the surface of the coating layer 44 is configured so that the amount of tungsten particles attached increases closer to the electrode tip. Therefore, the amount of tungsten evaporating from the electrode 30 also increases closer to the electrode tip. In other words, the concentration of tungsten is high near the surface of the coating layer 44. The higher the tungsten concentration, the higher the probability of stable tungsten nuclei reaching the surface of the coating layer 44 being formed. This means that there are more stable nuclei on the surface of the coating layer 44 closer to the electrode tip.

The more stable nuclei are formed on the surface of the coating layer 44, the lower the probability that tungsten particles attracted to and adsorbed on the surface of the layer will evaporate again and fall away, settling on the surface of the layer. As a result, crystal growth becomes faster and more vigorous, leading to a higher tungsten concentration. The concentration distribution of the adhering tungsten along the electrode axis C is roughly the same as that before the lamp was turned on, i.e., a gradient concentration distribution.

The closer to the electrode support rod side of the electrode 30, the less tungsten particles adhere to it. Therefore, the coating layer 44 exerts its heat dissipation function, and the decrease in emissivity is suppressed. On the other hand, the closer to the electrode tip side of the electrode 30, the more tungsten particles adhere to it, which has the effect of suppressing the evaporation of tungsten from adhering to the inner wall of the discharge tube 12 and blackening it as the gas flows inside the discharge tube 12.

As described above, in the discharge lamp 10 of this embodiment, a coating layer 44 is formed on the surface of the body portion 34 of the electrode 30, and tungsten particles are attached to the surface of the coating layer 44 in a dotted or scattered manner. The amount of tungsten particles attached has a concentration distribution that increases the closer to the tip of the electrode along the electrode axis C.

In this embodiment, the coating layer 44 is formed on the entire surface of the body 34 of the electrode 30, but the coating layer 44 may be formed on a portion of the surface. Considering that gas convection occurs when the short arc type discharge lamp 10 is arranged straight (along the vertical direction), the coating layer 44 may be formed across the center CL along the electrode axis C of the body 34 of the electrode 30. On the other hand, the coating layer 44 may be formed closer to the electrode tip than the center CL.

In addition to the configuration in which tungsten is attached to the surface of the coating layer 44 so as to have a gradation-like concentration distribution, other variations are also possible. In the surface region in which the layer surface of the coating layer 44 is formed, the amount of tungsten attached at a location relatively closer to the electrode tip side may be greater than the amount of tungsten attached at a location relatively closer to the electrode support rod side. For example, a configuration in which the amount of tungsten attached near the center portion CL is greater than that near the electrode support rod side is also possible. Note that the particle size does not need to be constant, and can be configured to be between about 0.1 and 20 μm, for example. For example, a configuration in which tungsten with a larger particle size is attached closer to the electrode tip side along the electrode axis C, and tungsten with a smaller particle size is attached closer to the electrode support rod side may be configured.

The electrodes 30 of such a discharge lamp can be manufactured as follows.

First, an electrode with a cylindrical body and a tapered tip is formed. Next, a coating layer is formed by coating. Any method can be used to attach tungsten particles to the layer surface. For example, tungsten powder is dispersed in a solvent and then sprayed to attach the tungsten. The concentration distribution can be adjusted by spraying in stages with different amounts of tungsten. Other possible methods include sprinkling tungsten powder or dipping the electrode in a solvent with a different tungsten concentration.

A coating layer of a different component may be layered on top of the coating layer 44 to form multiple coating layers. For example, a lower layer made of zirconium nitride may contain tungsten particles or molybdenum particles, while a surface layer made of zirconium nitride does not contain tungsten particles or molybdenum particles, forming multiple layers with different compositions. Alternatively, a coating layer made of zirconium nitride may be formed on a coating layer made of zirconium carbide, forming multiple layers made of different materials. It is also possible to configure one of the multiple coating layers to contain ceramics.

The electrode 30 can be constructed by joining a member having a tip taper portion 32 and a member having a body portion 34 by solid-phase joining such as diffusion bonding. It is also possible to join them via an intermediate member.

A heat dissipation structure may be provided on the side surface 34S of the body portion 34 or the side surface 32S of the tapered portion. The heat dissipation structure has a higher emissivity than the base surface, i.e., a surface that does not purposely employ a special heat dissipation structure, and functions to enhance heat dissipation. The heat dissipation structure may be configured, for example, with grooves formed at a predetermined pitch along the circumferential direction (around the electrode axis), and the grooves may be formed, for example, by laser or cutting. Alternatively, it may be configured with grooves along the electrode axis direction. Heat dissipation structures other than grooves may also be employed.

For example, the end of the coating layer 44 can be located at a position a predetermined distance away from the electrode tip end 34E1 of the body 34 along the lamp axis C, and a heat dissipation structure can be formed between that end and the electrode tip end 34E1, while not forming the coating layer 44. This can prevent the coating layer 44 from peeling off due to the heat of the arc discharge. The value of the predetermined distance is determined according to the size of the electrode, the rated power, etc.

Below, we will explain the heat dissipation performance of electrodes with a coating layer using examples.

The discharge lamp of the example is a short arc type discharge lamp equipped with an electrode (anode) having a configuration corresponding to the above embodiment, and the electrode is configured with a body length of 57 mm along the lamp axis direction and a diameter of 35 mm. A coating layer is formed on part of the side of the body of the electrode.

The coating layer is formed by dissolving zirconium nitride powder in a solvent containing ethyl cellulose, applying it to the side of the body, drying it, and then heating it. Tungsten particles are attached on top of the coating layer using the method described above.

A comparative experiment was conducted between the above-mentioned embodiment and a discharge lamp of a comparative example. The electrode shape of the discharge lamp of the comparative example is substantially the same as that of the electrode of the embodiment. On the other hand, in the comparative example, an electrode without a coating layer was used in order to confirm the heat dissipation function of the coating layer of the embodiment.

Figure 3 is a graph showing the illuminance maintenance rate of the Example and Comparative Example. The discharge lamp was turned on for 600 hours, and the blackening state of the discharge tube was visually confirmed. 600 hours after the start of lighting, the illuminance maintenance rate was 69% for the Comparative Example and 76% for the Example. This confirmed that the Example was able to suppress blackening better than the Comparative Example.

The fact that effective blackening suppression was achieved in the electrode of the embodiment suggests that the tungsten attached to the surface of the coating layer does not actively contribute to the blackening phenomenon, and indicates that tungsten evaporation from the electrode material other than the coating layer is suppressed, that is, the heat dissipation function of the coating layer is effectively exerted. The fact that such results were obtained in the electrode of the comparative example in which no coating layer was formed means that the electrode of this embodiment is more effective in terms of radiation function, even when compared to an electrode in which only a coating layer is formed and no tungsten is attached as in this embodiment.

10 Discharge lamp 30 Electrode (anode)
44 Coating layer

Claims (5)

A discharge tube;
A pair of electrodes disposed opposite each other within the discharge tube,
In at least one of the electrodes, a coating layer containing ceramics and having tungsten attached to the layer surface is formed on at least a portion of the surface of the electrode body,
A discharge lamp characterized in that the amount of tungsten adhering to a portion of the coating layer near the tip of the electrode is relatively greater than the amount of tungsten adhering to a portion of the coating layer near the electrode support rod. The coating layer is formed on a surface of an electrode body portion so as to straddle an axial center portion of the electrode,
2. A discharge lamp according to claim 1, wherein the amount of tungsten adhering to the layer surface closer to the tip of the electrode than the axial center portion is greater than the amount of tungsten adhering to the layer surface closer to the electrode support rod than the axial center portion. The discharge lamp of claim 1, characterized in that the amount of tungsten deposited in the coating layer gradually increases from the electrode support rod side toward the electrode tip side. A discharge lamp according to any one of claims 1 to 3, characterized in that the coating layer is chromatic. 4. The discharge lamp according to claim 1, wherein the ceramic is made of at least one of a nitride, an oxide, a boride, a carbide, and a silicide.

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