WO2005109471A2 - High-pressure discharge lamp with a closing member comprising a cermet - Google Patents

Abstract

A high pressure discharge lamp, comprising a discharge vessel having a ceramic wall enclosing a discharge space and which is closed by a closing member comprising a cermet having a surface part pointing towards the discharge space and incorporating a feedthrough to an electrode positioned inside the discharge vessel, wherein the cermet is provided with a protective layer on its surface part pointing towards the discharge space.

Description

High-pressure discharge lamp

The invention relates to a high-pressure discharge lamp, comprising a discharge vessel having a ceramic wall enclosing a discharge space and which is closed by a closing member incorporating a feedthrough to an electrode positioned inside the discharge vessel. In the present description and claims ceramic wall means a wall of a crystalline metal oxide like monocrystalline sapphire and densely sintered polycrystalline alumina and YAG, or a metal nitride, for example A1N. These ceramics are well known in the art for their good translucent properties and their high degree of chemical resistivity. An important class of commercially available lamps in line with the lamp mentioned in the opening paragraph is formed by metal halide lamps with a ceramic discharge vessel. In these known lamps the closing member is made of the ceramic wall material and is shaped as an extended plug comprising a three part feedthrough of a tungsten (W) conductor to the electrode, a molybdenum (Mo) conductor and a niobium (Nb) conductor extending outwardly. The Nb conductor is hermetically sealed to the ceramic closing member by means of a sealing ceramic. Niobium (Nb) is used because it has a coefficient of expansion which closely matches the coefficient of expansion of the ceramic wall material. However, it needs protection against the halide salts of the discharge vessel filling. This is achieved by the sealing ceramic or sealing glass, which extends alongside the Nb conductor up to the Mo conductor and preferable also partly alongside the Mo conductor. By the length of the closing member it is achieved that the region of the sealing is kept relatively cool during lamp operation, which is of importance to reduce the rate of damage of the sealing ceramic by filling components of the discharge vessel as much as possible. A drawback of this type of lamp construction is its complexity.

The European patent application EP 981 151 provides a lamp similar to the lamp mentioned in the opening paragraph in which the closing member is formed by a cermet, wherein the cermet contains a material with a coefficient of linear expansion greater than or equal to the coefficient of expansion of the ceramic of the discharge vessel, in combination with a material having a smaller coefficient of linear expansion than the ceramic of the discharge vessel wall. Though the construction is substantionally simplified, it turns out that the cermet is sensitive to chemical corrosion by salt components of the filling of the discharge vessel. A further drawback of the known cermet closing member is its relatively large dimensions necessary to make the stresses between the cermet materials and the ceramic discharge vessel wall acceptably small. In both known lamp types the discharge vessel has relatively long protruding feedthrough constructions, which act as cooling fins on the discharge vessel, resulting in reduction of the luminous efficacy of the lamp. A further drawback of these constructions, which extend relatively far outward, is the presence of undesired capillarities alongside the current conductors to the electrodes inside the discharge vessel, which can be the cause of color instabilities due to local distillation of the discharge vessel filling salts in the capillarities. In addition, the manufacture of the protruding constructions is relatively labor- intensive and costly, which is also a drawback.

The present invention has for its object to obviate the above stated drawbacks. The invention provides for this purpose a high-pressure discharge lamp of the kind mentioned in the opening paragraph, in which the closing member comprises a cermet having a surface part pointing towards the discharge space, wherein the cermet is provided with a protective layer on its surface part pointing towards the discharge space. The lamp according to the invention comprises a cermet, with a coefficient of thermal expansion matching the coefficient of expansion of the ceramic of the discharge vessel. The lamp according to the invention can be constructed in alternative ways. In a first construction the closing member is formed by the cermet being provided with the protective layer at its surface. A separate feedthrough conductor is incorporated by a hermetic connection to the cermet closing member, preferably for instance by means of welding or by hard soldering. The length of the thus formed sealing of the discharge vessel can be shorter than is the case in the known lamp. This will improve the luminous efficacy of the lamp. In a preferred embodiment it is furthermore suitable for coupling the cermet to the ceramic wall of the discharge vessel by means of a sinter bond. Preferably this is realized in such a way that the ceramic wall of the discharge vessel is gastight sintered with shrinkage to the closing member. Alternatively the closing member is primarily formed by a ceramic body incorporating the cermet provided with the protective layer on its surface as the feedthrough. The said ceramic closing member is preferably sintered to the ceramic wall of the discharge vessel. Alternatively the closing member is hermetically sealed to the discharge vessel wall by means of a melting ceramic. An advantage is the use of proven techniques at a relatively cool spot, which promotes reliability in production. A sinter bond coupling between closing member and cermet feedthrough can have a shorter length than the hermetic sealing with sealing ceramic of the known lamp, which has an improving effect on the efficacy of the lamp. It is also an advantage in reaching a reduction in costs. In a preferred embodiment the cermet comprises MgO and Mo. It has turned out that with a mixture of these materials suitable cermets can be realized, having a coefficient of expansion which matches well those of the ceramics of which the discharge wall is made. In particular a mixture is preferred, which comprises between 65% and 75% Mo by volume. Not only has the thus formed cermet a desired coefficient of expansion but from experiments it was learned that other properties like thermal shock durability was highly advantageous when applied in a discharge vessel construction of a high-pressure discharge lamp. In a further preferred embodiment the protective layer is formed by Mo as a metal having a very good chemical resistance against halides. When the cermet comprises Mo, the protective layer is relatively easily formed during sintering of the cermet, wherein a passivation layer is built up on the surface of the mixture. Owing to the protective layer on the cermet it has been found possible to couple the assembly directly to a supply current conductor, whereby a lamp can be manufactured more simply and more rapidly. The protective layer prevents reactions with the filling of the discharge vessel during lamp operation and further prevents the metal oxide evaporating out of the cermet.

The advantages and features described above and further aspects of the invention will be elucidated with reference to a drawing, in which: Fig. 1 shows a perspective view of a lamp according to the present invention in a first preferred embodiment Fig. 2 shows an image of a cermet in accordance with the present invention; Fig. 3 shows the temperature curve of the coefficient of expansion of alumina for values corresponding with the coefficients of expansion of a mixture according to the present invention; and Fig. 4 shows in detail a cross section part of a second preferred embodiment.

In Fig. 1 a high-pressure discharge lamp according to the invention is shown, comprising a discharge vessel 24 having a ceramic wall 26 enclosing a discharge space 18 and which is closed by a closing member 28, 30 comprising a cermet 34, 35 having a surface part 37 pointing towards the discharge space. In the embodiment shown the cermets provide a feedthrough to an electrode 21, 23 positioned inside the discharge vessel. According to the invention the cermet 34, 35 is provided with a protective layer on its surface part 37 pointing towards the discharge space. Each feedthrough is connected with a current conductor 20, 22 for supplying current to the lamp. The discharge vessel is filled with a buffer gas, such as an inert gas in combination with metal halogenides. In Fig. 2 part of a cermet 10 and closing member 14 is shown of a practical embodiment of a lamp as described in Fig. 1. The cermet 10 has a protective layer 12 of Mo, which is directly sintered to the closing member 14. In Fig. 3 a graph is shown of mixing ratios of cermets versus temperature having coefficients of expansion matching the coefficient of expansion of alumina. These lamps operate at a temperature above 1000°C. Because of difference in temperature level when the lamp is in operative condition and when the lamp is switched off as well as temperature gradients in the lamp during operation, it is important that the closing member and the feedthrough construction as well as the ceramic wall of the discharge vessel have thermal expansion coefficients which mutually match in order to limit (mechanical) stress in the discharge vessel. Besides, the lamp will be exposed to relatively large dynamical thermal gradients, also described as thermal shocks, due to ignition and switching off of the lamp. Aside from a few precious metals, there are only a few metals, such as W and Mo, which do have an acceptable chemical resistance against halides as part of the filling of the discharge vessel. However, these metals have a coefficient of expansion which is lower than the coefficient of expansion of the ceramic of the wall of the discharge vessel. The coefficient of expansion of W and Mo is thus in the order of 5 ppm/K, against a coefficient of expansion of polycrystalline alumina (PC A) of about 8 ppm/K. By sintering a mixture of such resistant metals with a ceramic material with a higher coefficient of expansion, greater than 8 ppm/K, a cermet can be obtained with a coefficient of expansion which fairly matches that of the ceramic wall of the discharge vessel. Magnesium oxide (MgO) is found to be a suitable ceramic material with a coefficient of expansion in the order of 13 ppm/K. A suitable mixing ratio can be determined from a plotting of values at which the coefficients of expansion of the mixtures and of alumina correspond with the temperature, as shown in Fig. 3. The discharge vessel wall of high-pressure discharge lamps according to the invention reaches a temperature between 1000°C and 1500°C during operation. Consequently, in a preferred embodiment the cermet comprises a mixture having between 65% and 75% Mo by volume. A percentage by volume in this range is furthermore advantageous with respect to thermal shock durability. A suitable method manufacturing the cermet comprises manufacturing a mixture of a metal oxide as ceramic material and a metal in a desired ratio, wherein the mixture has a coefficient of thermal expansion with a desired temperature curve; applying a protective layer to the outside of the mixture. The protective layer can be applied by sputtering or vapor deposition. Preferably the protective layer is applied by subjecting the mixture to a heat treatment. The mixture is heated to a temperature at which the ceramic material evaporates, which creates a metal-rich layer (Fig. 2; 12) on the surface of the mixture. Due to the high temperature the metal layer sinters and compacts, whereby the layer prevents further evaporation of ceramic material out of mixture and thus provides passivation of the cermet. In a preferred method of manufacturing, the heat treatment of the mixture is combined with a process step of sintering the cermet to the ceramic closing member and/or the ceramic discharge vessel wall. The final thickness of the thus formed metal layer depends on the mixing ratio of mixture and on the temperature curve during the sintering and the subsequent passivation. Consequently, in the embodiment shown of Fig. 2 the thickness of layer 12 is chosen to be smaller than 50 μm. A relatively thin protective layer suffices in practice, since such layer is relatively elastic. The assembly of cermet 10 with layer 12 is suitable for use as a feedthrough in a high-pressure discharge lamp. The metal protective layer allows the current conductor for supply of a lamp current, which is preferably of molybdenum wire, to be connected by for instance soldering to the feedthrough, which results in a further simplification. The lamp constructed thus is created without the use of a sealing ceramic or sealing glass. The discharge vessel closing and feedthrough constructions thus obtained can be shorter in length than with the known constructions, resulting in less cooling and an increased efficacy. The preferred process can proceed more quickly and comprises fewer steps than known methods, and therefore will be cheaper. In a second preferred embodiment shown partly in cross-section in Fig. 4, a discharge vessel 50 comprises a cylindrical ceramic wall 52 of alumina. A closing member formed by a cermet disc 54 of MgO and Mo with a protective layer 56 substantially comprising Mo over its entire surface including the surface part 560 pointing towards the discharge space 58, is sintered to the cylindrical ceramic wall 52 in a gastight manner. Preferably, the ceramic wall has been sintered with shrinkage on to the cermet closing member. After the filling has been introduced into the discharge space 58, a conductor 60, being the feedthrough conductor to which an electrode 62 with filament 64 is welded, has been fixed to the cermet closing member 54 by means of hard soldering 66, which hermetically seals the discharge space 58. The thus formed discharge vessel 50 is a HID lamp, which might be provided with an outer bulb (not shown). The invention finds an application in, among other articles, ceramic metal halogenide lamps. The invention is not limited to the above-described preferred embodiments, in which many changes and modifications can be envisaged within the scope of the appended claims. In these descriptions and claims it is understood that "comprising" does not exclude other elements or steps and, that "a" or "an" does not exclude a plurality. It is also understood that any reference signs in the claims shall not be construed as limiting the scope.

Claims

CLAIMS:

1. A high pressure discharge lamp, comprising a discharge vessel having a ceramic wall enclosing a discharge space and which is closed by a closing member comprising a cermet having a surface part pointing towards the discharge space and incorporating a feedthrough to an electrode positioned inside the discharge vessel, wherein the cermet is provided with a protective layer on its surface part pointing towards the discharge space.

2. The lamp as claimed in claim 1, wherein the protective surface layer is formed by Mo.

3. The lamp as claimed in claim 1 or 2, wherein the cermet comprises MgO and Mo.

4. The lamp as claimed in any one of the claims 1-4, wherein the cermet comprises between 65% and 75% Mo by volume.

5. The lamp as claimed in any one of the claims 1-4, wherein the ceramic wall of the discharge vessel has a sintered bond with the closing member.

6. The lamp as claimed in any one of the preceding claims, wherein the feedthrough is connected to an external conductor by means of welding or hard soldering.

7. The lamp as claimed in any one of the preceding claims, wherein the closing member is hermetically connected by means of a melting ceramic to the ceramic wall of the discharge vessel.

8. The lamp as claimed in any one of the preceding claims, wherein the closing member is formed by the cermet being provided with the protective layer on its surface.

9. The lamp as claimed in claim 8, wherein the feedthrough is a separate conductor incorporated by a hermetic sealing to the cermet closing member, preferably by means of welding or by hard soldering.

10. The lamp as claimed in any one of the preceding claims, wherein the closing member is primarily formed by a ceramic body incorporating the cermet provided with the protective layer on its surface as the feedthrough.

11. The lamp as claimed in claim 10, wherein there is a sinter bond coupling between closing member and cermet feedthrough.