High-pressure discharge lamp

Abstract

A high-pressure discharge lamp includes a cathode body composed of a cathode made of tungsten or tungsten alloy and a lead rod inserted in a lead rod insertion hole of the cathode. The cathode has a carbonized layer made of tungsten carbide (W.sub.2C) formed on a surface thereof exposed to a discharge space (except for a tip portion thereof) and on an inner surface of the lead rod insertion hole. The carbonized layer contains carbon in an amount of 0.44 g/cc to 0.53 g/cc.

Claims

1. A high-pressure discharge lamp comprising a cathode body composed of a cathode made of tungsten or tungsten alloy and a lead rod inserted in a lead rod insertion hole of the cathode, the cathode having a carbonized layer made of tungsten carbide (W.sub.2C) formed on a surface thereof exposed to a discharge space (except for a tip portion thereof) and on an inner surface of the lead rod insertion hole, the carbonized layer containing carbon in an amount of 0.44 g/cc to 0.53 g/cc.

2. The high-pressure discharge lamp according to claim 1, wherein the carbonized layer has a thickness of 20 to 40 sm.

3. The high-pressure discharge lamp according to claim 1, wherein the cathode is formed by joining tungsten and tungsten alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of a first embodiment of an electrode body used in the high-pressure discharge lamp of the present invention.

(2) FIG. 2 is a diagram for explaining the behavior of carbon at the cathode in the present invention.

(3) FIG. 3 is a cross-sectional view of a conventional high-pressure discharge lamp.

(4) FIG. 4 is a cross-sectional view of a conventional cathode.

(5) FIG. 5 is a diagram for explaining the behavior of carbon in a tip portion of the conventional cathode.

MODE FOR CARRYING OUT THE INVENTION

(6) FIG. 1 is a cross-sectional view in one embodiment of an electrode body used in the high-pressure discharge lamp of the present invention. A cathode body 1 includes a cathode 2 made of tungsten or tungsten alloy, and a lead rod 4 inserted into a lead rod insertion hole 3 bored at the rear end of the cathode. A carbonized layer 5 is formed on surfaces of the cathode 2 exposed to the discharge space except for a tip portion 2a.

(7) The tungsten may contain impurities as much as would be mixed in during the refining process of tungsten. The tungsten alloy is an alloy of tungsten W and thorium oxide ThO.sub.2, or the oxide of rare earths such as cerium Ce and lanthanum La. The alloy may contain intermetallic compounds between these oxides and tungsten.

(8) The cathode 12 may be formed by joining tungsten and tungsten alloy together.

(9) As described above, the carbonized layer 5 is formed on surfaces of the cathode 2 exposed to the discharge space. In so far as the effect of the present invention is achieved, there may be parts where no carbonized layer is provided, or where the carbonized layer is thin.

(10) In the case of using tungsten carbide for this carbonized layer 5, the carbonized layer should preferably contain carbon C in an amount of 0.44 to 0.53 g/cc.

(11) Too large an amount of carbon C would lead to excessive generation of CO on the cathode surface, which will cause the carbon C to be transported not only to the surface of the cathode tip but also to the surface of the anode tip, where tungsten carbides such as W.sub.2C and WC will be formed. Since the surface of anode tip is heated to a higher temperature than the surface of cathode tip during the time when the lamp is illuminated, the carbides on the surface of anode tip will vaporize, which leads to unwanted acceleration of blackening of inner surfaces of the discharge vessel.

(12) On the contrary, too small an amount of carbon C would lead to early irregularities of the tip of the cathode due to insufficient generation of CO on the cathode surface, which allows the arc discharge point to move more easily and causes flickering.

(13) The carbonized layer 5 should preferably have a thickness of 20 to 40 m.

(14) Too large a thickness of the carbonized layer would lead to excessive generation of CO on the cathode surface, which will cause the carbon C to be transported not only to the surface of the cathode tip but also to the surface of the anode tip, where tungsten carbides such as W.sub.2C and WC will be formed. These carbides vaporize and accelerate blackening of inner surfaces of the discharge vessel.

(15) Too small a thickness of the carbonized layer would lead to early irregularities of the tip of the cathode due to insufficient generation of CO on the cathode surface, which causes flickering.

(16) This carbonized layer 5 is not formed to the tip portion 2a of the cathode 2. Thoriated tungsten which is the material of the cathode 2 has a melting point of 3420 C., while the substances that form the carbonized layer 5 have a lower melting point than that (for example, the melting point of tungsten carbide is about 2800 C.). If the carbonized layer 5 is formed to the tip portion 2a of the cathode 2, the carbonized layer 5 on the tip portion 2a will melt excessively during the illumination, leading to earlier occurrence of the flickering.

(17) This is why the carbonized layer 5 is not formed at the point where it will reach a temperature at which it melts, i.e., the tip portion 2a. Specifically, the carbonized layer 5 is not formed in an area of 3 to 5 mm from the tip in the case of a xenon lamp of about 2 to 6 kW.

(18) In the present invention, the carbonized layer 5 is formed also on inner surfaces of the lead rod insertion hole 3 of the cathode 2.

(19) As illustrated in FIG. 2, while the carbon C in the carbonized layer 5 formed on the surface of the cathode 2 is consumed and reduced with the illumination time, the carbon C in the carbonized layer 5 on the inner surface of the lead rod insertion hole 3 is gradually diffused into the cathode 2.

(20) This carbon C diffused from the lead rod insertion hole 3 into the electrode gradually reaches the cathode surface and contributes to the generation of CO on the cathode surface.

(21) The inner surface temperature of this lead rod insertion hole 3 is lower than the temperature on the surface of the cathode, because of which the carbon diffusion from the carbonized layer 5 of the lead rod insertion hole 3 and from the carbonized layer 5 of the cathode surface occur with a time difference. This allows the carbon to be replenished at about the time when carbon in the carbonized layer 5 on the cathode surface is consumed and reduced, so that depletion of carbon in the carbonized layer 5 on the cathode surface is inhibited.

(22) In so far as the effect of the present invention is achieved, there may be parts where no carbonized layer is provided, or where the carbonized layer is thin, on the inner surface of the lead rod insertion hole 3.

(23) For the formation of such a carbonized layer 5, a gas-phase carbonization method may be utilized. The gas-phase carbonization is a method wherein a mixture gas of benzene and hydrogen is made to react with the cathode heated in a high-frequency heating device.

(24) A process gas which is a mixture of benzene and hydrogen is supplied to a reaction chamber to flow at a rate of about 2 L/min. The cathode inside the reaction chamber is heated to about 1900 C. by high-frequency heating and kept at the high temperature for 5 minutes. The mixture gas is replaced with hydrogen gas, the temperature is reduced to about 1700 C. and this temperature is maintained for 5 minutes. This process is repeated several times until a cathode body with a carbonized layer of about 30 m formed thereon is obtained.

(25) The carbonized layer 5 is thus formed entirely on outer surfaces of the cathode 2 as well as on inner surfaces of the lead rod insertion hole 3. After that, the carbonized layer is removed by a machining process from the tip portion 2a of the cathode 2.

(26) In the high-pressure discharge lamp according to this invention, a carbonized layer is formed on the surface of the cathode exposed to the discharge space (except for the tip portion) to maintain generation of CO on the cathode surface. Moreover, the carbonized layer formed on the inner surface of the lead rod insertion hole allows for carbon diffusion from inside of the cathode toward the surface during illumination, which makes it possible to sustain generation of CO on the cathode surface over a long time.

DESCRIPTION OF REFERENCE SIGNS

(27) 1 Cathode body 2 Cathode 2a (Cathode) tip portion 3 Lead rod insertion hole 4 Lead rod 5 Carbonized layer