Method of producing a low-pressure mercury vapour discharge lamp

Abstract

 Method of producing a low-pressure mercury vapour discharge lamp having a discharge vessel (1), a metal or an alloy which forms an amalgam with mercury being provided in the discharge vessel (1) before the desired gas atmosphere is established in the discharge vessel (1) and the discharge vessel (1) is sealed in a vacuum-tight manner, at leastthe metal or the alloy (11) being in a closed container (8) which is provided near a portion (9) of the interior wall of the discharge vessel (1), the container (8) being opened after the discharge vessel (1) has been sealed in a vacuum-tight manner, at least a portion of the metal or the alloy (11) then leaving the container (8) and moving to the interior wall portion (9) located near the container (8).

Description

[0001] The invention relates to a method of producing a low-pressure mercury vapour discharge lamp having a discharge vessel, at least a metal or an alloy which forms an amalgam with mercury being disposed in the discharge vessel before the desired gas atmosphere is established in the discharge vessel and the discharge vessel is closed in a vacuum-tight manner. Such a method is disclosed in USP 4,157,485 (PHN 8057). The invention further relates to lamps produced by means of such a method.

[0002] In the prior art method an alloy of indium and bismuth (the amalgam-forming component) are pressed to form a ductile wire the end of which is thereafter heated to a temperature just above the melting point of said alloy and is sprayed onto a portion of the lamp wall. According to the said Patent Specification this portion is preferably the foot of the stem. Such a stem carries, for example, an electrode, an auxiliary electrode, leading-in wires and similar elements. The use of the said stems (which are often provided with an exhaust tube) is customary in the production of low-pressure mercury vapour discharge lamps. The stems and accescries (such as leading-in wires, electrodes etc.) are first produced separately from the other lamp components. The indium and bismuth alloy is applied onto the foot of the stem before the stem is sealed in a vacuum-tight manner to the discharge vessel. Thereafter the lamp is pumped (that is to say the walls and the other lamp components are outgassed, the electrodes are annealed and outgassed, the discharge vessel is filled with the required gas atmosphere etc.). Thereafter the discharge vessel is closed in a vacuum-tight manner by sealing the exhaust tube. Thereafter the discharge vessel is filled with mercury, for example by heating a glass container which comprises the mercury and is present in the discharge vessel (see GB-PS 1,267,175). Then at least a portion of the mercury in combination with the amalgam-forming component forms an amalgam for the control of the mercury vapour pressure in the discharge vessel during operation of the lamp.

[0003] One oqthe problems encountered in said method is that the consecutive operations during the production of the lamp, during which the temperature rises to a com- peratively high value (for example during sealing of the stem to the wall of the discharge vessel or during the indispensable heating of components, for example out- gassing of the electrodes, activation of the electrodes etc.) oxidizing of the amalgam-forming alloy or the amalgam-forming metal easily occurs, a thin oxide layer then being formed on the metal or the alloy, which during operation of the lamp greatly impedes the absorption and release of mercury (which is necessary for a proper operation of an amalgam). In addition, with the known method it is difficult to dose the quantity of the amalgam-forming component accurately.

[0004] The invention has for its object to-provide a method of producing a low-_Iressure mercury vapour discharge lamp which mitigates these drawbacks, wherein attack of the amalgam-forming alloy and the amalgam-forming metal, respectively, by oxidation is prevented from occurring.

[0005] According to the invention, the method of producing a low pressure mercury vapour discharge lamp is characterized in that at least the amalgam-forming alloy or the amalgam-forming metal is present in a closed container which is provided near a portion of the interior wall of the discharge vessel, which container is opened after the discharge vessel has been closed in a vacuum-tight manner, at least a portion of the alloy or the metal then leaving the container and moving to the interior wall portion located near the container.

[0006] In the method in accordance with the invention the amalgam-forming metal and the amalgam-forming alloy, respectively are not exposed to a hot, oxygen-containing atmosphere, so that no oxide layer is formed on the alloy and metal, respectively. Namely, during the entire procedure the metal or the alloy are located in a closed container which is not opened until the discharge vessel has been closed in a vacuum-tight manner.

[0007] An additional advantage of the method in acordance with the invention is that the amalgam-forming metal or the alloy can be applied in very accurate doses in the discharge vessel. The ratio of the quanity of metal and alloy, respectively, to the quantity of mercury in the lamp is accurately determined. The temperature range in which the mercury vapour pressure is substantially stable during operation of the lamp is then as wide as possible.

[0008] In this method it is not necessary that the entire content of the container moves to the wall portion located near the container. An adequate control of the mercury vapour pressure is obtained even if a portion of the content, for example half the content, remains in the container. During operation of the lamp, the absorption and release of mercury from the portion of the amalgam located in the container proceeds via the opening in the container wall.

[0009] Preferably the container is provided in a comparatively cool spot in the discharge vessel, for example in a position behind the electrode near an end of the discharge vessel. In one embodiment the opening is providedin such a place in the container that a portion of the content moves to a position near the electrode (for example the pinch or a ring or band around the electrode). During operation of the lamp an amalgam which has almost exclusively a vapour pressure-controlling function forms in the container with the mercury present in the discharge vessel, the material present in a place near the electrode being free from mercury-during operation. Only during extinguishing of the lamp said material absorbs some mercury which, however, when the lamp is switched on, is released from the amalgam in response to the rapid increase of the temperature there, as a result of which the lamp readily ignites.

[0010] The container is of such a shape and is so positioned in the discharge vessel that the material which is released on the opening of the container moves as rapidly as possible to a discharge vessel wall portion near the container. The container consists, for example, of glass or metal. In a practical embodiment the container is formed by two metal parts which are welded together, one part being provided with a recess in which the amalgam-forming material is present. The container is connected to the stem or to a supply wire of an electrode by means of, for example, a wire.

[0011] A container of this type metal is opened by, for example, inductive heating. The container then bursts open in a weak spot in the wall specially provided for that purpose. The weak spot faces, for example, and is located at a small distance from the wall portion of the discharge vessel on which at least the amalgam-forming component is provided. In a practical embodiment of the method the container is opened by means of a laser beam which is focused onto the wall of the container.

[0012] The mercury required for the operation of the lamp may be dosed in the discharge vessel separatelv from the amalgam-forming alloy and the amalgam-forming metal, respectively, for example by means of a glass mercury container described in the United Kingdom Patent Specification 1,267,175 mentioned already in the foregoing. Preferably, in the method in accordance with the invention, the mercury required for the operation of the lamp is placed together with the said amalgam-forming component in the container and provided on the wall of the discharge vessel. No additional step for the separate dosing of the mercury is then required. A further advantage in this embodiment is that the container holding the amagam opens rapidly when being heated as a result of the mercury vapour pressure which rapidly increases as a function of temperature. The mutual ratio of the elements which are part of the amalgam may moreover be determined very accurately, so that a proper operation of the amalgam during operation of the lamp is obtained.

[0013] The amalgam may be composed of mercury and a metal, such as indium, tin, or lead or alternatively of mercury and an alloy, such as an alloy of indium and bismuth. In a practical embodiment of the method in accordance with the invention a mixture of an alloy of indium and mercury is first placed together with bismuth in a closed metal container, a spherical bismuth member being positioned on a ductile tablet of the indium and mercury alloy. The spherical members and the tablets can be mass- produced tn a simple way. When the spherical bismuth member is positioned in the holder it is enveloped substantially wholly by the ductile mass of the indium and mercury alloy. The spherical member and the ductile mass are thereafter heated for some time, which results in a proper mixing of the said elements. The holder is of such a shape that the amalgam is tightly enclosed and unwanted trapping of gas is avoided. It has been found that the said elements can be easily positioned in the container in a mechanized process.

[0014] In low-pressure mercury vapour discharge lamps produced by means of the method the mercury vapour pressure remains reasonably stable over a wide temperature range around 6 x 10^-3 Torr (0.8 Pascal). Lamps of this type, having an amalgam, are, for example, suitable for use in places where the ambient temperature is high.

[0015] In addition, the method can be used with great advantage in small discharge lamps, for example in lamps having a shape as described in DE-OS 2,940,563 (PHN 9242). In this type of lamp the temperature in the discharge vessel is rather high during operation. The presence of an outer envelope reduces the heat discharge from the discharge vessel.

[0016] An opening of the container is performed, as mentioned in the foregoing, wholly separately from the further operations on the lamp, it is optionally possible to market discharge vessels which are closed in a vacuum-tight manner, but which still include a closed container.

[0017] This container may then be opened later at a suitable moment, for example by the buyer of these "lamps". This operation is namely so simple (particularly if high-frequency inductive heating or a converging laser beam is used) that a buyer can perform these operations without elaborate equipment. All this has the advantage that during transportation of the "lamps" the amalgam-forming metal or the amalgam-forming alloy cannot become detached from the lamp wall; should these "lamps" containing amalgams break during transportation no mercury vapour can be released into the environment. The invention therefore also relates to discharge vessels of this type.

[0018] The invention will now be further described by way of example with reference to a drawing in which

Fig. 1 shows an embodiment of a low-pressure mercury vapour discharge lamp produced by means of a method in accordance with the invention and

Fig. 2 shows one end, partly in cross-sectional view, of a discharge vessel containing a container still to be opened.

[0019] In Fig. 1 reference numeral 1 denotes the wall of a tubular discharge vessel of a low-pressure mercury vapour discharge lamp, stems 2 and 3 with electrodes 4 and 5, respectively being provided one at each end. The interior wall of the discharge vessel is coated with a luminescent layer 6 consisting, for example, of a mixture of trivalent europium-activated yttrium-oxide and trivalent terbium-activated cerium magnesium aluminate. A metal wire 7, which is connected to a metal container 8 located behind the electrodes is welded to one of the current supply wires of electrode 4. The wall portion near said container (the foot 9 of the stem 2) is provided with an amalgam. Said amalgam is applied by means of a method in accordance with the invention. The amalgam is provided in a cool spot, which is advantageous for the control of the mercury vapour pressure in the discharge vessel, behind the electrode 4.

[0020] During the production of the lamp the interior wall of a tube is first coated by means of a known method with a luminescent layer, whereafter the stems are connected in a vacuum-tight manner to the wall of the tube. Said stems have a closed metal container 8 which contains an amalgam. Thereafter the discharge vessel is exhausted (by means of an exhaust tube 10 connected to one of the stems), the rare gas atmosphere is established and the discharge vessel is closed in a vacuum-tight manner by sealing the exhaust tube. Not until thereafter is the metal container 8 opened by directing a focussed laser beam onto the wall of the container so as to make an opening therein. At least a portion of the amalgam leaves the container through the opening as a syrupy liquid and moves to the foot 9 of the stem, where it adheres.

[0021] Fig. 2 shows one end of a discharge vessel at the moment at which all operations on the lamps except the release of the amalgam have been finished. Said amalgam is in the container 8 which is still in the closed condition. The container 8 is formed by two sheet metal portions 8a and 8b (for example consisting of iron or nickel) which are welded together, 8b having been provided with a recess in which an amalgam 11 consisting of indium, bismuth and mercury is provided. The amalgam is obtained by heating a small sphere of bismuth which is enclosed by a quantity of a ductile alloy of indium and mercury. It has been found that the bismuth is then very rapidly absorbed by the alloy and forms a homogeneou/somewhat liquid compound. The metal portions 8a and 8b fit around amalgam 11 in such a way that no unwanted gasses are trapped in the container. The container 8 is opened by means of a laser arranged outside the discharge vessel. An opening is drilled in a portion 12 of 8a by focussing the laser beam. The temperature then increases to such a value that a portion of the amalgam flows from the container 8.

[0022] In a practical embodiment, the wall portion 9 where a portion of the amalgam arrives is located at only a few mm (for example 2mm) from portion 12. The use of a converging laser beam to form the opening has the advantage that the further portions (for example 1 and 2) of the glass wall of the discharge vessel are not attacked by the laser light. Moreover, the energy of the laser light can be concentrated such that even amalgam-forming components having a low vapour pressure (such as indium or bismuth) become syrupy when the opening is formed.

[0023] In an alternative embodiment the container is inductively heated, a weak spot having been provided in the container wall where the container is opened.

[0024] A practical embodiment of a lamp shown in Fig. 1 produced by means of a method in accordance with the invention contained 26.8 mg Bi, 13.2 mg In and 3.0 mg Hg (in wt.% 62.3% Bi, 30.7% In, 7% Hg). The container was formed of two metal portions which are welded together and one of which is provided with a recess. An opening (diameter 0.47 mm) was formed in the container by means of a pulsed Nd-YAG laser (pulse energy approximately 2 J). At an applied power of 13 W the lamp (having argon, 267 Pa, as rare gas filling) had a luminous flux of approximately 900 Lumen.

Claims

1. A method of producing a low-pressure mercury vapour discharge lamp having a discharge vessel, at least a metal or an alloy which forms an amalgam with mercury being disposed in the discharge vessel before the desired gas atmosphere is established in the discharge vessel and the discharge vessel is closed in a vacuum-tight manner, characterized in that at least the metal or the alloy is present in a closed container which is provided near a portion of the interior wall of the discharge vessel, which container is opened after the discharge vessel has been closed in a vacuum-tight manner, at least a portion of the metal or the alloy then leaving the container and moving to-the interior wall portion near the container.

2. A method as claimed in Claim 1, characterized in that the container is provided behind an electrode near an end of the discharge vessel.

3. A method as claimed in Claim 1 or 2, characterized in that the container also contains mercury.

4. A method as claimed in Claim 1, 2 or 3, characterized in that the container is filled with an amalgam consisting of indium, bismuth and mercury.

5. A method as claimed in Claim 4, characterized in that the amalgam is formed by heating a spherical member of bismuth and a quantity of a ductile alloy of indium and mercury.

6. A method as claimed in Claim 1, 2, 3, 4 or 5, characterized in that the container is opened by means of a laser beam focussed onto the wall of the container.

7. A method as claimed in Claim 1, 2, 3, 4 or 5, characterized in that the holder is made of metal and is inductively heated, a weak spot having been provided in the container wall where the container is opened.

8. A low-pressure mercury vapour discharge lamp produced by means of a method as claimed in Claim 1, 2, 3, 4, 5, 6 or 7.

9. A discharge vessel comprising a closed container comprising at least an amalgam-forming metal or an amalgam forming alloy for performing the method as claimed in Claim 1, 2, 3, 4, 5, 6 or 7.

Drawing