Patent Application
20100026184
The invention relates to a ceramic burner for a metal halide lamp, which ceramic burner comprises a discharge vessel enclosing a discharge space in a substantially gastight manner and being provided with an ionisable filling comprising one or more halides, the discharge vessel comprising a ceramic wall including two end portions, a conductor being embedded in each of the end portions for supplying electric current to respective electrodes arranged in the discharge space for maintaining a discharge.
Ceramic metal halide lamps contain fillings which comprise, besides a buffer gas, also metal halide salt mixtures such as NaCe iodide, NaTl iodide, NaSc iodide, NaTlDy iodide or combinations of these salts. These metal halide salt mixtures are applied to obtain, inter alia, a high lamp efficacy, a specific color corrected temperature and/or a specific color rendering index. Such a ceramic metal halide lamp comprises a discharge vessel enclosing a discharge space comprising the filling of the metal halide salt mixtures. The ceramic discharge vessel may have a substantially cylindrical tube-like portion both ends of which are closed by means of a ceramic end-plug, which ceramic plugs are co-sintered with the ceramic material of the tube-like portion. Thus, the ceramic wall of the discharge vessel is formed by the tube-like portion and the two end-plugs. The ceramic discharge vessel may also have another shape, for example, a shape such that the diameter of the central portion of the discharge vessel is larger than the diameter of the end portions of the discharge vessel.
The discharge space comprises two electrodes between which a discharge is maintained during operation of the lamp. Typically, the electrodes pierce through the end portions of the discharge vessel. In order to fill the ceramic metal halide lamp with the metal halide salt mixture, a filling-opening can be provided in the ceramic wall of the discharge vessel, which opening is closed, after the filling of the discharge space has taken place, by means of a closing-plug.
An embodiment of such a ceramic metal halide lamp is known from Japanese patent application JP10284002. In this known metal halide discharge lamp, the lamp consists of a gastight ceramic discharge vessel having two end-plugs made of a material having almost the same coefficient of thermal expansion; each end-plug guides an electrode. The ceramic discharge vessel further comprises a filling-opening. The filling is introduced into the discharge vessel through the filling-opening, which opening is subsequently closed by means of a T-shaped plug fitting into the filling-opening. The T-shaped plug is fused with the wall of the discharge vessel by exposing it to radiation from a laser. A disadvantage of the known ceramic metal halide lamp is that the T-shaped plug cannot be closed without substantially increasing the temperature of further portions of the discharge vessel, heating up the filling of the discharge vessel, in particular when the burner has relatively small dimensions.
An object of the invention is to provide a ceramic burner for a ceramic metal halide lamp having a sealed filling-opening which has been closed without heating up the ionisable filling of the discharge vessel.
Another object of the invention is to provide a burner for a ceramic metal halide lamp having a sealed filling-opening which has been closed without causing cracks in the ceramic material of the discharge vessel.
Another object of the invention is to provide a burner for a ceramic metal halide lamp having a sealed filling-opening, the filling-opening of the burner being sealed relatively fast, i.e. in a short operation.
Another object of the invention is a metal halide lamp having means for facilitating the starting of the discharge process in the discharge vessel.
To accomplish one or more of these objects, the ceramic wall of the discharge vessel comprises a tube for introducing the ionisable filling into the discharge vessel during the manufacturing of the ceramic burner, the tube protruding outside the ceramic wall of the discharge vessel, and said tube being sealed gastight.
The use of a tube-like member, in this specification referred to as tube, enables the gastight seal to be arranged away from the ceramic wall of the discharge vessel at the protruding end of the tube. Due to the distance between the gastight seal and the ceramic wall, the tube can be sealed without damaging the ceramic wall of the discharge vessel and without heating up the ionisable filling of the discharge vessel too much.
In the ceramic burner according to JP10284002, the filling-opening is a bore in the wall of the discharge vessel. Sealing of the filling-opening is done by inserting a T-shaped plug into the bore and subsequently fusing the T-shaped plug to the ceramic wall of the discharge vessel by means of laser irradiation. Said laser irradiation causes an increase of the temperature of the T-shaped plug and of the temperature of a portion of the wall of the discharge vessel to the melting temperature of the ceramic material, which is around 2100° Celsius. This increase of the temperature creates a relatively large local temperature gradient which may result in cracks in the ceramic material of the discharge vessel. To reduce the occurrence of cracks, the known discharge vessel is sealed while a portion of the discharge vessel is heated up to approximately 800° Celsius for reducing the temperature gradient near the sinter location of the T-shaped plug. However, a further portion of the discharge vessel must be kept at a temperature below 350° Celsius to ensure that the ionisable filling does not evaporate and is blown out of the discharge vessel through the filling-opening before the discharge vessel is sealed gastight. To overcome this problem, said further portion of the discharge vessel has to be cooled.
In the ceramic burner according to the invention, the discharge vessel comprises the tube protruding away from the outer surface of the ceramic wall of the discharge vessel. After filling the discharge vessel with the ionisable filling through the tube, the protruding end of the tube is sealed. The protruding end of the tube extends away from the ceramic wall of the discharge vessel, so that it can be sealed without the ceramic wall being heated up to a temperature at which the ionisable filling of the discharge vessel evaporates, or at which the ionisable filling will expand in such a way that the plug is blown off the end of the tube. Furthermore, the relatively small temperature increase of the ceramic wall prevents cracks in the ceramic wall due to material stress and tension, which would result from a large temperature gradient.
Furthermore, by applying a tube as described above, the production time of the ceramic burner can be reduced, because only the relatively small protruding end of the tube has to be heated up in order to seal the tube.
As used herein, “ceramic” means a refractory material such as a monocrystalline metal oxide (e.g. sapphire), polycrystalline metal oxide (e.g. polycrystalline densely sintered aluminum oxide and yttrium oxide), and polycrystalline non-oxide material (e.g. aluminum nitride). Such materials allow wall temperatures of 1500° to 1700° Kelvin and resist chemical attacks by halides and other filling components. For the purpose of the present invention, polycrystalline aluminum oxide (PCA) has been found to be a very suitable material.
In a preferred embodiment, the tube is made of ceramic material, preferably substantially the same ceramic material as that used for the ceramic wall of the discharge vessel. By virtue thereof, stress and/or tension between the ceramic wall and the tube during operation of the ceramic metal halide lamp, and during the increase in temperature when applying the gastight seal is extremely low or even avoided.
Preferably, the ceramic tube protrudes more than 0.5 mm, preferably more than 1 mm, away from the outside surface of the ceramic wall of the discharge vessel. It has been found that such a length of the ceramic tube enables the required high temperature at the protruding end of the tube when sealing the tube, while the ceramic wall of the discharge vessel remains at a relatively low temperature.
In a preferred embodiment, the inner diameter of the ceramic tube is between 0.25 mm and 0.4 mm, and the wall thickness of the tube is between 0.15 mm and 0.25 mm. On the one hand, the inner diameter of the tube is large enough to introduce the ionisable filling into the ceramic vessel. On the other hand, too large an inner diameter requires too much tube-material to be molten for creating a gastight seal, resulting in a relatively high thermal strain during forming the gastight seal. Furthermore, on the one hand, the wall thickness of the ceramic tube must be sufficient to make the tube strong enough to withstand the thermal gradient occurring during the formation of the gastight seal and/or to allow enough ceramic wall material to be molten close to the protruding end of the tube. On the other hand, the wall thickness of the tube should not be too large, because melting the tube for creating the gastight seal would take a relatively long time, which also results in a relatively high thermal strain which might damage the tube during the formation of the gastight seal and/or which may result in too large an expansion of the ionisable filling. Preferably, the wall thickness should be substantially half the diameter of the tube.
In a preferred embodiment, the discharge vessel including the ceramic tube is made by injection molding. The injection molding process enables to produce the discharge vessel such that the ceramic tube is an integral part of the ceramic wall of the discharge vessel. In addition, the production process of the discharge vessel can be simplified, and the connection between the wall and the tube is very strong.
In another preferred embodiment, the material of the tube is metal. By making use of a metal tube, the time required for sealing the tube is shorter than when using a ceramic tube. The material used for the tube may be for example Mo (molybdenum) or a Mo-alloy, but preferably, the material used for the tube is an alloy comprising Ir (iridium), preferably comprising more than 95% iridium. Good results are obtained by making use of a metal tube comprising substantially iridium.
In a preferred embodiment, the metal tube protrudes at least 0.5 mm away from the outside surface of the ceramic wall of the discharge vessel. The length of the metal tube outside the surface of the ceramic vessel can be very small, because the sealing operation is relatively short, so that the temperature increase of the ceramic wall during the sealing operation is limited.
Preferably, the inner diameter of the metal tube is between 0.25 mm and 0.4 mm, and the wall thickness of the metal tube is between 0.075 mm and 0.2 mm. In experiments it has been found that such dimensions provide good results.
In a preferred embodiment, the tube protrudes from the inner surface of the ceramic wall into the discharge vessel, so that an end of the tube extends a little inside the discharge vessel. It has been found that thus a strong and gastight connection between the ceramic wall of the discharge vessel and the tube can be easily achieved.
In a preferred embodiment, the gastight seal of the tube is formed of molten material of the tube. In this process, the protruding end of the tube can be heated up by means of laser irradiation during a short time, which is a relatively simple process, which does not require any additional materials such as frit. The irradiation time depends on the material of the tube and on the dimensions of the tube and the power of the laser beam.
In another preferred embodiment, the gastight seal comprises a plug sealed to the tube; preferably the material of the plug is the same as the material of the tube. A benefit of this embodiment is that the use of a plug considerably reduces the area that must be sealed to generate the gastight seal. When a plug is applied in the protruding end of the tube, only the contact area between the plug and the tube has to be sealed. In general, this requires less time and less sealing material to be molten.
The plug has preferably a T-shape, or, in another preferred embodiment, a conical shape, or, in another preferred embodiment, a spherical shape. A benefit when using a T-shaped plug is that when applying the plug, the plug cannot be pushed into the discharge vessel. A benefit when using a conical shape is that tolerances of the dimensions of the protruding end of the tube may be less accurate. A benefit when using a substantially spherical shape is that when using placement tools for placing the plug on to the protruding end of the tube, the spherically shaped plug can be easily picked and placed by a placement tool.
Preferably, the plug is directly fused to the tube, without using additional material. By fusing the plug to the tube the use of a sealing frit is avoided. The protruding tube enables the plug to be directly fused to the protruding end of the tube, for example, by means of a short irradiation operation with a laser beam, while an increase of the temperature of the remainder of the discharge vessel is limited.
To start the discharge process in the burner, a relatively high voltage is required between the two electrodes, being a much higher voltage than the voltage that is required for maintaining the discharge process in the burner. The discharge process can be started by using a lower voltage, when the distance between the electrodes is smaller. For this purpose, a so-called starting electrode can be used, being a third electrode located nearer to one of the two main electrodes than the distance between the two main electrodes. As a result, the discharge process can be started by a relatively small voltage between one of the main electrodes and the starting electrode, and the discharge process can subsequently be maintained between the two main electrodes; at this stage the starting electrode is switched off.
In a preferred embodiment, such a starting electrode is inserted through the tube, so that the end of the starting electrode is located near one of the two main electrodes. Also, the electric current supply conductor passes through the tube, and the tube can be sealed by melting the material of the tube and the material of said conductor.
The invention furthermore relates to a method of manufacturing a ceramic burner for a metal halide lamp, which ceramic burner comprises a discharge vessel enclosing a discharge space in a substantially gastight manner, the discharge vessel comprising a ceramic wall including two end portions, a conductor being embedded in each of the end portions for supplying electric current to respective electrodes arranged in the discharge space for maintaining a discharge, and an ionisable filling comprising one or more halides being introduced into the discharge vessel through an opening in the wall of the ceramic burner, the ceramic wall of the discharge vessel being provided with a tube for supplying the ionisable filling into the discharge vessel, said tube protruding outside the ceramic wall of the discharge vessel and being sealed gastight after the discharge vessel has been filled.
The invention will now be further elucidated by means of a description of some embodiments of a ceramic burner for a metal halide lamp, which ceramic burner comprises a discharge vessel surrounded by a ceramic wall, with reference to the drawing comprising seven diagrammatic Figures, wherein:
The diagrammatic Figures are not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar parts in the different Figures are denoted by the same reference numerals.
In the embodiments shown in
The ceramic burners shown in
Furthermore, the temperature of the ionisable filling in the discharge vessel 20 should not exceed a certain value before the discharge vessel 20 is completely sealed, in order to prevent part of the ionisable filling from flowing out of the discharge vessel 20. A further benefit of applying the tube 60,62,64 is that local heating of the protruding end of the tube 60,62,64 can be achieved in a relatively short time, reducing the processing time for producing the ceramic burner.
As an alternative embodiment, each of the two identical parts 22C may include one half of the tube 68, so that part 22C and half of the tube 68 form an integral part of the discharge vessel 22, and the two parts 22C, including the half tubes, can be fixed together by a sinter operation. In this embodiment, a single mould is required for producing both parts 22C of the discharge vessel 22.
The material of the tube 60,62,65,68 can be ceramic material or metal. The corresponding plug 72,74,78 is made of the same or similar material. Preferably, the ceramic material is the same as or similar to the material of the wall 30 of the discharge vessel 20. The metal tube can be made of iridium or molybdenum. The tube 60,62,65,68 is sintered in a bore in the ceramic wall 30 of the discharge vessel 20, in order to obtain a sealed connection with the ceramic wall 30. In the case of a metal tube 60,62,65,68, the tube and the wall 30 can be united by shrinking.
The described embodiments are only examples of the ceramic burner according to the invention; many other embodiments are possible.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06126720.9 | Dec 2006 | EP | regional |
| 07111178.5 | Jun 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB07/55075 | 12/13/2007 | WO | 00 | 6/16/2009 |
