High pressure discharge lamp having composite electrode

Details High pressure discharge lamp having composite electrode High pressure discharge lamp having composite electrode

1998-08-26

2002-06-18

Day, Michael H. (Department: 2879)

Electric lamp and discharge devices

With gas or vapor

Having electrode lead-in or electrode support sealed to...

C313S633000, C313S625000, C313S572000

Reexamination Certificate

active

06407504

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a high pressure discharge lamp, such as a high pressure sodium light-emitting lamp, a metal halide lamp, and to a method of manufacturing such a high pressure discharge lamp. The present invention also relates to a composite electrode for a high pressure discharge lamp, and a method of manufacturing such a composite electrode.
BACKGROUND ART
Conventionally, such a high pressure discharge lamp comprises a vessel made of a non-conductive material (e.g. alumina) which forms an inner space filled with an ionizable light-emitting material and a starting gas, and which has opening portions at the ends thereof. The high pressure discharge lamp also comprises a composite electrode having a substantially cylindrical current conductor made of a conductive material (e.g. molybdenum) with a diameter which is substantially the same as that of the opening portion at one end of the vessel, and an electrode electrically connected to the current conductor. In this instance, a gap between the current conductor and the vessel is tightly sealed.
In this type of high pressure discharge lamp, there is a significant difference between the coefficient of thermal expansion of the conductive material forming the current conductor and that of the non-conductive material forming the vessel (for example, the coefficient of thermal expansion of alumina is 8×10
−6
K
−1
, and that of molybdenum is 6×10
−6
K
−1
). Owing to such a difference, when the high pressure discharge lamp is heated such as when the high pressure discharge lamp is in operation, there may be formed a gap between the current conductor on one hand and the vessel and/or the plug on the other hand. In this instance, as the molecular movement of the ionizable light-emitting material and the starting gas in the vessel becomes more active, these ionizable light-emitting material and starting gas may leak through the gap to outside of the vessel.
To avoid such a drawback, JP-A-2-132750 discloses a high pressure discharge lamp wherein, instead of forming the current conductor with only conductive material, the current conductor comprises a substantially cylindrical non-conductive material (e.g. alumina), which is same as that forming the vessel and coated by tungsten with a substantially uniform thickness over the surface of the non-conductive material. In this case, the composite electrode is composed such that a concave portion is provided at the bottom of the current conductor and an electrode is buried in the concave portion, or the electrode is connected to the current conductor with another member such as a cap. Also, the vessel and the composite electrode have been subjected to a co-firing into an integrated body, with the current conductor inserted into the opening portion at one end of the vessel so that the electrode is exposed to the inner space with one end of the composite electrode exposed to outside of the vessel. In this way, by composing most of the current conductor of a non-conductive material which is the same as the that forming the vessel, the adverse influence of the difference in the coefficient of thermal expansion between the conductive material (in this case, tungsten) and the non-conductive material is made substantially insignificant.
Also, JP-A-7-211292 discloses a high pressure discharge lamp wherein the current conductor comprises a substantially cylindrical non-conductive material, which is the same as that forming the vessel, and covered by a layer of mixture of platinum and alumina, a layer of platinum, and a layer of a mixture of platinum and alumina, with a substantially uniform thickness and one above the other over the surface of the non-conductive material. In this case, also, the composite electrode is composed such that a concave portion is provided at the bottom of this current conductor and an electrode is buried in the concave portion, or the electrode is connected to the current conductor with another member such as a cap. Therefore, the adverse influence of the difference in the coefficient of thermal expansion between the conductive material and the non-conductive material is made substantially insignificant.
Also, JP-A-8-273616 discloses a high pressure discharge lamp wherein the current conductor is formed with the substantially cylindrical non-conductive material, which is the same as that forming the vessel material, and covered by a halide-resistant metal such as niobium, tungsten, etc., with a substantially uniform thickness over the surface of the non-conductive material. In this case, the composite electrode is also composed such that a concave portion is provided at the bottom of this current conductor and an electrode is buried in the concave portion, or the electrode is connected to the current conductor with another member such as a cap. Therefore, the adverse influence of the difference in the coefficient of thermal expansion between the conductive material and the non-conductive material is made substantially insignificant.
However, in the high pressure discharge lamp disclosed in JP-A-2-132750, the composite electrode having the current conductor metallized with tungsten, whose melting point (3400° C.) is higher than that of alumina (2015° C.), is co-fired with the vessel into an integrated body. In this case, the melting point of tungsten in metallization is much different from that of alumina in metallization, so that the firing speed of tungsten is different from that of alumina. Also, the mutual wetting property of tungsten and aluminum is poor, and it is thus difficult to form a tightly metallized layer. Therefore, such a high pressure discharge lamp does not have a fully gas-tight property.
Further, in the high pressure discharge lamps disclosed in JP-A-7-211292 and JP-A-8-273616, the vessel and the composite electrode are not co-fired into an integrated body at least at one end of the vessel. Thus, a stronger junction cannot be formed between the non-conductive material of the vessel and metallized layer on the composite electrode, as compared to an arrangement wherein the vessel and the composite electrode are co-fired into an integrated body. Therefore, such a high pressure discharge lamp does not have a fully gas-tight property, either.
Moreover, in the composite electrodes for the high pressure discharge lamps disclosed in JP-A-2-132750, JP-A-7-21 1292 and JP-A-8-273616, it is preferred that the composite electrode can be easily manufactured and has a uniform thickness of the metallized layer over the surface of the current conductor.
On the other hand, in the conventional high pressure discharge lamp, when the gap between the current conductor and the vessel is tightly sealed, there is used a frit seal. In this case, the proximity of the opening portion at one end of the vessel is heated to a predetermined temperature (e.g. 1500° C.). On such occasion, the other end of the vessel is cooled in order to prevent the molecular movement of the ionizable light-emitting material and starting gas from being active such that they are prevented from leakage through the frit seal of the vessel to outside of the vessel. However, in spite of such cooling, the inner part of the vessel is still heated to a substantial temperature (e.g. 300-400° C.) even for a limited period (e.g. 1-3 minutes). Thus, there still remains the possibility for the ionizable light-emitting material and starting gas to more or less leak through the frit seal of the vessel to outside of the vessel.
Moreover, in manufacturing the composite electrode by jointing the electrode to the current conductor, it is preferable to have a high bonding strength between the electrode and the current conductor, a high corrosion resistance, and a, high conductivity.
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide a high pressure discharge lamp having a satisfactory gas-tight property while fully maintaining the required conductivity, as well as a method of manufacturing the same.
It is a second object

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