Electric lamp and discharge devices – With gas or vapor – Having electrode shield
Reexamination Certificate
1999-05-04
2001-08-14
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having electrode shield
C313S492000
Reexamination Certificate
active
06274981
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a low-pressure mercury vapor discharge lamp comprising a discharge vessel,
which discharge vessel encloses a discharge space provided with a filling of mercury and a noble gas in a gastight manner,
electrodes being arranged in the discharge space for generating and maintaining a discharge in the discharge space,
and an electrode shield at least substantially surrounding at least one of the electrodes.
In mercury vapor discharge lamps, mercury is the primary component for (efficiently) generating ultraviolet (UV) light. An inside wall of the discharge vessel may be provided with a luminescent layer comprising a luminescent material (for example a fluorescent powder) for converting UV to other wavelengths, for example UV-B and UV-A for tanning purposes (sun-couch lamps), or to visible radiation. For this reason, such lamps are also referred to as fluorescent lamps.
A low-pressure mercury vapor discharge lamp of the type mentioned in the opening paragraph is known from DE-A 1 060 991. In the known lamp, the electrode shield surrounding the electrode is made of titanium sheet. The use of an electrode shield, also referred to as anode shield or cathode shield, counteracts blackening at an inside wall of the discharge vessel. The titanium serves in this respect as the getter for chemically binding oxygen, nitrogen and/or carbon.
A disadvantage of the use of a metal or metal alloy is that it may cause a short-circuit of the pole wires of the electrode. In addition, the metals in the electrode shield may amalgamate with the mercury present in the lamp and, thus, absorb mercury. As a result, to obtain a sufficiently long service life of the known lamp, a relatively high mercury dose is necessary. If the lamp is unskilfully processed after its service life has ended, the environment is adversely affected.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a low-pressure mercury vapor discharge which consumes a relatively small quantity of mercury.
To achieve this, at least one electrode comprises an alkaline earth metal which, in operation, is partly liberated from the electrode, and the electrode shield includes a material which reacts with or forms an alloy with the alkaline earth metal originating from the at least one electrode.
To ensure proper operation of low-pressure mercury vapor discharge lamps, the electrodes of such discharge lamps comprise, in addition to a material with a high melting temperature (a much used metal is tungsten), an (emitter) material with a low so-called work function (reduction of the work function voltage) to supply (emit) electrons to the discharge (cathode function) and receive electrons from the discharge (anode function). Known emitter materials having a low work function are oxides of alkaline earth metals, such as oxides of barium (Ba), strontium (Sr) and calcium (Ca). It has been observed that during operation of low-pressure mercury vapor discharge lamps, (emitter) material is released, for example as a result of alkaline earth metals being liberated from the electrode(s) by evaporation or sputtering. In general, these materials are deposited on the inside wall of the discharge vessel. It has further been found that the alkaline earth metals deposited elsewhere in the discharge vessel no longer participate in the light-generating process. In addition, the deposited (emitter) material forms mercury-containing amalgams on the inside wall, so that the quantity of mercury available for discharge decreases (gradually), which adversely affects the service life of the lamp. In order to counteract such a loss of mercury during the life of the lamp, the lamp requires a relatively high dose of mercury, which is undesirable from the point of view of the environment. Experiments carried out by the inventors have shown that the alkaline earth metals in metallic form amalgamate with mercury and that oxides of alkaline earth metals do not react with mercury. For example, alkaline earth metals in the form of, for example, BaO, SrO, Ba
3
WO
6
, Sr
3
WO
4
, etc., do not form amalgams with mercury, while under comparable conditions, metallic alkaline earth metals bond with mercury, thereby forming, for example, Ba-Hg or Sr-Hg amalgam. The inventors have realized that by providing an electrode shield comprising a material which reacts with or forms an alloy with the alkaline earth metal originating from the electrode(s), the risk that mercury amalgamates is reduced considerably, so that the mercury remains available for discharge and, as a result, the mercury consumption of the discharge lamp is limited.
A preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention is characterized in that the material of the electrode shield comprises an oxide of a material which oxidizes the alkaline earth metal. By changing the chemical state of alkaline earth metals originating from the electrodes and deposited on the electrode shield from metallic to a suitable metal oxide, the mercury consumption of the discharge lamp is limited. Suitable materials include oxidic materials with more than one oxidation state, whereby the material is not in the lowest oxidation state. Further suitable materials are materials having an oxygen deficiency. Preferably, barium or strontium is used for the alkaline earth metal, and the oxide is selected from the group formed by MnO
2
, TiO
2
, Fe
2
O
3
, In
2
O
3
, SnO
2
, SnO
2
:Sb, ZrO
2
, Nb
2
O
5
, V
2
O
5
, Tb
4
O
7
and ZnO. In contact with metallic alkaline earth metal (originating from the electrode), the corresponding oxide of the alkaline earth metal, i.e. BaO and/or SrO, is formed.
A further preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention is characterized in that the material of the electrode shield comprises an oxide of a material which is nobler than the alkaline earth metal. Under normal operating conditions of the lamp, such a material oxidizes the alkaline earth metal. As a result, the alkaline earth metal is reduced and does not react with the mercury present in the discharge vessel. Preferably, the alkaline earth metal is barium or strontium, and the oxide is selected from the group formed by copper oxide and iron oxide.
A further preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention is characterized in that the material of the electrode shield includes a material which liberates water at a temperature which, in operation, is higher than a temperature of the electrode shield. Preferably, the alkaline earth metal is barium or strontium, and the material is an oxide selected from the group formed by SiO
2
, Al
2
O
3
(particularly suitable is the so-called Alon-C) and rare earth metal oxides (for example, La
2
O
3
).
A further preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention is characterized in that the material of the electrode shield includes a metal which forms an alloy with the alkaline earth metal and not with mercury. Preferably, the alkaline earth metal is barium or strontium, and the metal is selected from the group formed by aluminium, zinc, copper, iridium and rhodium.
The electrode shield itself may absorb only a negligible quantity of mercury. To achieve this, the material of the electrode shield comprises at least an oxide of at least one element of the series formed by magnesium, aluminium, titanium, zirconium, yttrium and the rare earths. A particularly preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention is characterized in that the electrode shield is made from a ceramic material. A particularly suitable electrode shield is manufactured from so-called densely sintered Al
2
O
3
, also referred to as DGA. As a result, the risk that materials in the electrode shield react with mercury present in the discharge vessel to form amalgams is reduced. In addition, the use of an electrically insulating material f
De Maagt Bennie J.
Denissen Cornelis J. M.
Groen Wilhelm A.
Ronda Cornelis R.
van Kemenade Wilhelmus M. P.
Day Michael H.
Halajian Dicran
U.S. Philips Corporation
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