JP2012099422A - Negative electrode for hort-arc discharge lamp and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron radioactive material to the tip of a negative electrode stably.SOLUTION: A body part 14A is constituted by tungsten including thorium oxide. A sinter junction layer 14B is constituted by tungsten having coarsened crystal grain in comparison to that of the body part 14A. The grain boundary diffusion in the sinter junction layer 14B is suppressed by covering the body part 14A including thorium with the sinter junction layer of the tungsten of the coarsened crystal grain that does not include thorium. In addition, a liner grain boundary 14D is formed between the body part 14A and the sinter junction layer 14B of a negative electrode 14. Therefore, the liner grain boundary diffusion of the grain boundary 14D is caused by the supply of thorium.

Description

  The present invention relates to a cathode for a short arc discharge lamp, and more particularly to a stable supply of an electron radioactive substance to the tip of the cathode.

As a cathode of a short arc discharge lamp, a cathode obtained by doping an electron radioactive substance (emitter) with a refractory metal by a mass ratio of several percent is adopted. In particular, tritium tungsten (usually 2 wt% ThO ₂ -W) containing thorium oxide (ThO ₂ ) in tungsten (W), or a refractory metal containing an electron-emitting substance in which the cathode body is formed of a refractory metal. A cathode is used that is embedded in the tip of the main body. This makes it possible to stabilize arc discharge formed between the cathode and the anode for a long time, and to obtain high-luminance light stably for a long time.

  In a cathode employing such an emitter, when the cathode becomes extremely hot, thorium oxide deposited on the cathode surface is likely to be evaporated and scattered. The evaporated and scattered thorium oxide releases oxygen in the discharge space, and the released oxygen combines with tungsten at the tip of the anode to form tungsten oxide. This tungsten oxide lowers the melting point of tungsten at the tip of the anode, thereby causing wear and deformation of the tip of the anode.

  For such a problem, Patent Document 1 discloses forming a thorium oxide-deficient layer from the outermost surface of the cathode to a depth of 50 μm to 100 μm. Thereby, evaporation of thorium oxide from the surface during lighting can be reduced.

JP 2003-257365 A

  However, in the cathode of Patent Document 1, thorium that has been internally diffused in the depleted layer of thorium oxide reaches the surface of the depleted layer, and thorium is supplied to the tip of the cathode by surface diffusion. Therefore, there is a problem that the emitters are exhausted in a short period of time.

  An object of the present invention is to solve such problems and to provide a cathode for a short arc discharge lamp capable of stably supplying an electron radioactive substance from the inside of the cathode to the tip of the cathode.

  (1) In the method for producing a cathode for a short arc discharge lamp according to the present invention, a step of preparing a cathode element having a tapered shape composed of a refractory metal containing an electron-emitting substance and having a tapered tip, A pressure sintering step of placing the profile in a sintering die and pressurizing and sintering the high melting point metal powder filled around the cathode profile to the surface of the cathode profile; In order to coarsen the crystal grains, there is provided a crystal grain coarsening step of recrystallizing the joint surface between the refractory metal and the cathode element shape member by heat treatment. Therefore, by covering the cathode element with a refractory metal layer having crystal grains larger than that of the cathode element, surface diffusion on the side surface of the cathode element can be prevented. Thereby, the electron radioactive substance from the inside of the cathode can be stably supplied to the tip of the cathode.

  (2) The method for manufacturing a cathode for a short arc discharge lamp according to the present invention removes the tip of the cathode base material on which the refractory gold is bonded and sintered before the grain coarsening step, Tapered shape. Therefore, the cathode shape can be easily configured.

  (3) A cathode for a short arc discharge lamp according to the present invention is a cathode for a short arc discharge lamp that is made of a refractory metal containing an electron radioactive substance and emits the electron radioactive substance from a tip portion thereof. The high melting point metal layer is made of a refractory metal that does not contain a radioactive substance, and the crystal grains are coarser than the metal constituting the cathode, and covers a region other than the arc discharge region at the tip of the cathode. Therefore, the movement to the front-end | tip part by the surface diffusion of the said electron radioactive substance can be prevented. In addition, since the refractory metal layer is sintered with the cathode surface, the electron-emitting material is supplied to the tip of the cathode by interfacial diffusion. Interfacial diffusion is slower than surface diffusion, and the electron-emitting material moves at a constant speed. Thereby, supply control of an electron radioactive substance becomes easy.

  (4) In the cathode for a short arc discharge lamp according to the present invention, the refractory metal layer has crystal grains coarser than the cathode by recrystallizing the sintered joining layer. Therefore, surface diffusion can be prevented.

  (5) The cathode for a short arc discharge lamp according to the present invention is composed of a refractory metal containing an electron radioactive substance, and the electron radioactive substance is diffused from the tip by surface diffusion from the side and internal grain boundary diffusion. A cathode for a short arc discharge lamp that emits the electron-emitting material, and in order to limit the surface diffusion, a refractory metal layer that does not contain the electron-emitting material is formed in a region other than the arc discharge region at the tip of the cathode. Sintered to the area. Therefore, it is possible to prevent surface diffusion on the side surface of the cathode element.

  In the present specification, tungsten, molybdenum or the like can be adopted as the “refractory metal”. Further, rare earth metals such as lanthanum, cerium, and yttrium, alkaline earth metals such as barium, calcium, and strontium, thorium, and oxides thereof can be used as the electron emitting material.

It is a figure which shows an example of the structure of the short arc type mercury discharge lamp provided with the cathode to which this invention is applied. 2 is a view showing a cross-sectional structure of a cathode electrode 14. FIG. It is a figure for demonstrating the spreading | diffusion effect | action in the said embodiment. It is the schematic which shows the apparatus for pressure sintering in the said embodiment. It is a figure which shows the manufacturing process of a cathode raw material.

  FIG. 1 shows a short arc type mercury discharge lamp 1 having a cathode according to the present invention. The quartz glass-made sealing body 10 bulges in a substantially spherical shape at the center, both ends are sealed via a sealing portion, and a base 11 is fitted. In addition to a predetermined gas such as argon or xenon, mercury is sealed in the envelope 10 at a predetermined pressure and amount. Tungsten rods 12 are inserted through both ends of the envelope 10. An anode 13 and a cathode 14 are attached to the tip of the tungsten rod 12, respectively. A predetermined gap is provided between the anode 13 and the cathode 14 so that arc discharge occurs.

  FIG. 2 shows the detailed structure of the cathode 14. The cathode 14 includes a main body portion 14A and a sintered bonding layer 14B that covers the main body portion 14A. The main body portion 14A is made of tungsten containing thorium oxide. The main body portion 14A has a truncated cone shape.

  In the sintered bonding layer 14B, the crystal grains are coarser than the crystal grains of the main body portion 14A. The sintered bonding layer 14B covers the main body portion 14A except for the arc discharge region 14C at the tip.

  Covering the main body portion 14A containing thorium with a tungsten layer of coarse crystal grains not containing thorium has the following effects. As shown in FIG. 3, the thorium contained in the main body portion 14A diffuses at the grain boundaries in the main body portion 14A. Since thorium that has reached the sintered bonding layer 14B has large crystal grains in the sintered bonding layer 14B, the grain boundary diffusion is suppressed accordingly. Therefore, thorium diffusing up to the surface of the sintered bonding layer 14B can be suppressed. As a result, surface diffusion from the surface of the sintered bonding layer 14B can be suppressed. Thereby, exhaustion of the emitter can be prevented. Naturally, evaporation from the surface of the sintered bonding layer 14B can also be prevented.

  Further, a linear grain boundary 14D is formed between the main body portion 14A of the cathode 14 and the sintered bonding layer 14B. Thereby, thorium that has moved from the main body portion 14A diffuses toward the arc discharge region 14C at the cathode tip along the linear grain boundary 14D between the main body portion 14A and the sintered bonding layer 14B. Thus, the thorium supply to the arc discharge region 14C is not the surface diffusion that moves on the surface of the main body portion 14A as in the prior art, but the grain boundary 14D is linearly diffused. Such grain boundary diffusion has a slower moving speed than surface diffusion. Therefore, thorium can be stably supplied even when the lamp is lit for a long time.

  The manufacturing method of the cathode 1 for short arc discharge lamps is demonstrated using FIG. 4, FIG. The manufacturing method in the present embodiment includes the following first to sixth strokes.

[First step]
In the first step, the tip of a tungsten rod containing thorium is processed into a conical shape to produce a cathode element 140 as shown in FIG.

[Second step]
In the second step, tungsten powder having a plurality of particle sizes is mixed. By mixing the tungsten powder having a plurality of particle sizes, the gap between the particles can be reduced when the raw material powder is filled in the sintering die. Thereby, a high-density (small pore) sintered joined body can be obtained in the subsequent sintering step. In order to mix the tungsten powder having a plurality of particle sizes, a known mixer such as a ball mill may be used.

[Third step]
In the third step, a tungsten film is formed around the cathode base material 140.

  The cathode element 140 manufactured in the first step is set on the punch 132 shown in FIG. 4 and inserted from below the die 130. Tungsten powder 133 prepared in the second step is filled in the gap between the die 130 and the cathode element 140. The upper punch 131 is inserted into the opening on the die 130. These are set in the discharge plasma sintering apparatus 100, and the inside of the apparatus chamber 134 is evacuated.

  Between the upper and lower punch electrodes 135, a pulsed large current of several hundreds of A to several tens of thousands of A is applied at a low voltage of several V to several 10V. Thereby, the tungsten powder 133 and the cathode raw material 140 are heated at a rate of increase of, for example, 100 K (Kelvin) / min, and finally heated to, for example, 1500 K.

  Further, the tungsten powder 133 is sintered by applying a pressure of several tens of MPa for several tens of minutes between the upper and lower punch electrodes 135. As a result, the sintered tungsten layer is bonded to the cathode element 140. The density is 60 to 99.5% of the theoretical density.

  Tungsten powder is sintered and bonded by a large pulsed current. Here, the principle of the connection between the cathode element 140 and the tungsten 133 is not clear. The inventor has developed a high energy density field of discharge generated by a spark discharge phenomenon at the interface between the two, thermal diffusion due to Joule heat, and electric field diffusion that promotes high-speed mass transfer due to generation of an electromagnetic field along the direction of ON-OFF pulse current flow. We infer that it is an effect.

  In carrying out the present invention, the discharge plasma sintering method is optimal, but it is not particularly limited as long as the tungsten powder can be sintered and the cathode shape member 140 can be diffusion bonded. (Hot isostatic pressing) may be used.

  The temperature during sintering is preferably 1300K or more and 2300K or less. This is because if it is less than 1300 K, a desired sintered density cannot be obtained, or sufficient bonding cannot be performed. On the other hand, if the temperature exceeds 2300 K, the recrystallization of the cathode element shape proceeds rapidly, the crystal grain size increases, and thorium diffusion is inhibited during lighting.

  If it exceeds 2000K, depending on the pressure during sintering, the strength of the punch made of graphite may be insufficient, or the sublimation of graphite may start. Therefore, in order to avoid such a problem, the temperature during sintering may be set to 1300K or more and 2000K or less.

  Further, since the tungsten powder having a plurality of particle sizes is mixed in the second step, the pores of the sintered bonding layer 14B of tungsten after sintering can be reduced. Therefore, it is possible to prevent the gas and impurities accumulated in the pore portion from being released during the arc discharge. Thereby, the deformation | transformation and consumption of the cathode 14 are prevented, the evaporation accompanying the temperature rise of the cathode 14 can be prevented, and the inner surface of the envelope can be prevented from being blackened. The sintered bonding layer 14B desirably has a density of 90% or more of the theoretical density.

  Thereby, the cylindrical composite material 150 shown in FIG. 5A is obtained.

[Fourth step]
In the fourth step, shape processing as a cathode is performed. The composite material 150 obtained in the third step is processed into a cathode shape in which a circular truncated cone is combined on a cylinder, and an arc discharge region 14C having a predetermined diameter is formed at the tip. For this processing, a normal metal processing method such as cutting using a lathe or wire-cut electric discharge processing can be employed. As a result, a cathode as shown in FIG. 5B is obtained.

  The thickness of the sintered bonding layer 14B is preferably in the range of 0.01 mm to 3.0 mm, but is not limited thereto. If it is less than 0.01 mm, the diffusion of the electron emissive material to the surface cannot be effectively suppressed. If the cathode has an input power of about 25 KW, if it exceeds 3.0 mm, the temperature of the entire cathode decreases, This is because the temperature at which electrons are emitted is not reached and arc discharge becomes unstable. Note that the thickness of the sintered bonding layer 14B may be varied depending on the size (number of W) of the cathode.

  Moreover, it is preferable to process so that the thickness of the sintered joining layer 14B may become so thin that it becomes a front-end | tip. This is because by reducing the diameter of the cathode tip as much as possible, the locations where arc discharge occurs are concentrated and the brightness is improved.

[Fifth step]
In the fifth step, the crystal grains of the sintered bonding layer 14B are coarsened. The sintered joint layer 14B obtained in the fourth step is heat-treated in a vacuum atmosphere, so that the crystal grains of the sintered joint layer 14B are coarsened. The heat treatment temperature is preferably from 1300K to 2300K. If it is less than 1300K, recrystallization hardly occurs and the crystal does not become coarse. If the temperature exceeds 2300 K, the recrystallization of the cathode element shape proceeds rapidly, the crystal grain size becomes too large, and thorium grain boundary diffusion is inhibited during lighting.

  In general, the crystal grain size of the main body portion of the cathode 14 is approximately 15 to 50 μm, and the crystal grain size of the sintered bonding layer 14B is preferably 100 μm or more.

  In addition, what is necessary is just to set the heat processing time from the range for about 5 minutes-360 minutes according to process temperature.

[Sixth step]
In the sixth step, degassing treatment is performed.

  In this embodiment, the crystal grains are coarsened after shape processing as a cathode. This is because tungsten is a difficult-to-process material, and if it is cut after the crystal grains are coarsened, it may be peeled off during processing. In such a situation, a protrusion is formed on a part of the cathode to avoid problems such as abnormal discharge from the protrusion. Further, the reason why the degassing process is performed thereafter is to avoid the problem that cutting oil and impurities are attached when cutting or the like after the degassing process is performed.

  Therefore, the order of the fourth to sixth steps is not limited as long as the above problem can be avoided.

  In this embodiment, although the case where it comprised with the sintered joining layer as a coating layer was demonstrated, it is not necessarily limited to this.

  In the present embodiment, the tungsten powder 133 is formed into a cylindrical shape and then processed into a tapered shape. Therefore, it takes time and effort to cut into a tapered shape. Therefore, the shape of the punch 131 may be changed so as to have a tapered shape when the tungsten powder 133 is formed. Thereby, the amount of processing can be reduced.

  In this embodiment, thorium oxide is used as the electron-emitting material and tungsten is used as the refractory metal. However, the present invention is not limited to this.

  Although the case where the inside of the apparatus chamber 134 is evacuated has been described in the present embodiment, a hydrogen atmosphere or an inert gas atmosphere may be used.

  In the above embodiment, the case where the present invention is applied to an ultra-high pressure mercury discharge lamp has been described, but the present invention can also be applied to other short arc discharge lamps.

14... Cathode 14 A... Main part 14 B... Sintered bonding layer 14 C.

Claims (6)

A step of preparing a cathode base material composed of a refractory metal containing an electron-emitting substance and having a tapered tip;
A pressure sintering step of placing the cathode stock material in a sintering die and pressurizing and sintering a high melting point metal powder filled around the cathode stock material on the surface of the cathode stock material;
In order to coarsen the crystal grains on the joint surface, a crystal grain coarsening step of recrystallizing the joint surface between the refractory metal and the cathode body shape material by heat treatment,
A method for producing a cathode for a short arc discharge lamp, comprising: In the manufacturing method of the cathode for short arc discharge lamps of Claim 1,
Removing the tip of the cathode base material in which the refractory gold is bonded and sintered before the grain coarsening step, and having a step where the tip is tapered.
A method for producing a cathode for a short arc discharge lamp. It is composed of a refractory metal containing an electron radioactive substance, and is a cathode for a short arc discharge lamp that discharges the electron radioactive substance from a tip part,
The high melting point metal layer which is composed of a high melting point metal not containing the electron-emitting substance and whose crystal grains are coarser than the metal constituting the cathode covers an area other than the arc discharge area at the tip of the cathode. ,
A cathode for a short arc discharge lamp. The cathode for a short arc discharge lamp according to claim 3,
The refractory metal layer has a crystal grain coarser than the cathode by recrystallizing the sintered joining layer of the refractory metal,
A cathode for a short arc discharge lamp. A cathode for a short arc discharge lamp, which is composed of a refractory metal containing an electron-emitting substance, and the electron-emitting substance emits the electron-emitting substance from its tip by surface diffusion from the side surface and internal grain boundary diffusion. There,
In order to limit the surface diffusion, a refractory metal layer not containing the electron-emitting material was sintered and bonded to a region other than the arc discharge region of the cathode tip,
A cathode for a short arc discharge lamp.   The short arc discharge lamp which has a cathode for short arc discharge lamps in any one of Claims 3-5.

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