WO1998052210A1

WO1998052210A1 - Getter devices for halogen lamps and process for their production

Info

Publication number: WO1998052210A1
Application number: PCT/IT1998/000114
Authority: WO
Inventors: Michael Paul Hagen; Stefano Carlo Giorgi; Alessandro Gallitognotta
Original assignee: Saes Getters S.P.A.
Priority date: 1997-05-15
Filing date: 1998-05-08
Publication date: 1998-11-19
Also published as: AR012695A1; AU7547298A

Abstract

Getter devices for halogen lamps are formed by screen-printing a paste comprising tantalum, thorium, hafnium, platinum, niobium or zirconium, and preferably zirconium and tantalum, in strips onto a metallic foil and then cutting the strips perpendicularly. The resulting foil strips can be cut parallel to the getter material to form pieces that contain a small area of getter material and an area of foil. The foil area can be used to mount the getter device to a support structure in the inner quartz bulb of the halogen lamp. The getter device maintains the purity of the gas surrounding the tungsten filament. The getter material is suitable for use in any halogen lamp and especially for halogen lamps having small quartz bulbs.

Description

GETTER DEVICES FOR HALOGEN LAMPS AND PROCESS FOR THEIR PRODUCTION

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BACKGROUND AND SUMMARY OF THE INVENTION The invention relates to getter devices for use in halogen lamps and to a process for their production.

Halogen lamps, well known in the art, comprise a metal filament (generally tungsten) that acts as a light source by glowing incandescently when heated by a current passing through it. A sealed quartz bulb around the filament is filled with a predetermined amount of a halogen gas or a halogen gas mixture (generally bromine, iodine or, less commonly, chlorine, or mixtures thereof) that helps to ensure a long working life for the lamp.

The detailed mechanisms by which halogens prolong lamp life are quite complex, but may be summarized as follows: - tungsten atoms evaporate from filament surface due to its high working temperature; absent halogens, the filament is locally thinned and eventually breaks; - halogen atoms in the bulb react with evaporated tungsten atoms, forming tungsten halides, that are chemically stable enough not to dissociate in any internal part of the lamp but on the high temperature filament surface; - upon dissociation of the tungsten halide molecule, tungsten is returned to filament surface, and halogen atoms to the bulb atmosphere, ready for another halide formation-dissociation cycle.

The presence of some gases, particularly oxygenated species like O₂, H₂O, CO,

CO , may be harmful to the above mechanism, forming tungsten oxide upon contact with the filament; tungsten oxide has a higher vapor pressure than metallic tungsten. increasing the filament thinning rate.

Harmful gases may permeate the quartz bulb during lamp life, given the high temperatures reached by the bulb due to filament proximity; another source of harmful gases contamination of inner lamp atmosphere is the incomplete evacuation of the bulb during highly automated mass production. Whatever the source of the contamination, the harmful gases can be removed from the quartz bulb, both during its manufacture and during the operation of the lamp_, by placing a device comprising a getter material inside the quartz bulb The getter material acts to get πd of these low amounts of harmful gases by chemically binding or reacting with them

Most known getter materials do not work well within halogen lamps due to the highly aggressive chemical atmosphere inside the quartz bulb - namely halogen gases at a high temperature Metals such as tantalum (Ta), thorium (Th), hafnium (Hf), platinum (Pt), niobium (Nb) and zirconium (Zr), are used in this application since they are relatively inert with respect to hot halogen gases but are reactive with respect to oxygenated gases (e g , oxygen, water, and carbon oxides) Gettei systems made up of these metals or their mixtures for use in lamps are described for example, in German patent DE 2040122, in Japanese patent application JP- 52055856, and in former Soviet Union patent SU 1003199 Though the use of these metals or their alloys is known, it is difficult to produce halogen lamps that use these metals as getter materials in which the metals are both mechanically stable and retain their properties as gas sorbers Achieving good sorption generally requires a relatively high portion exposed surface of metal such as is generally obtained by using powders However, a powdered metal needs to be contained in some type of a container since it cannot simply be poured mside the quartz bulb in loose form It is known to prepare a paste of powders that is spread onto the surface of metal wires carrying the filament However, this pi ocess is not easy to implement in highly automated mass production since it requires making the metal powder into a paste and applying it to single, small metal parts by brushing on the paste

From the PCT published patent application WO 98/03987, in the name of the same Assignee of the present application, it is known to produce supported layers of getter materials by screen-printing

It is an objective of the present invention to provide getter devices for use inside the quartz bulb of halogen lamps, as well as to provide a process for the production of said getter devices The foregoing objectives can be achieved by preparing the getter device by screen-printing a paste comprising tantalum, thorium, hafnium, platinum, niobium or zirconium, and preferably a mixture of zirconium and tantalum, onto a metallic foil, generally consisting of titanium or molybdenum, that is then cut into pieces or strips. The foil may be completely covered with the getter material deposit, but the foil is preferably partially covered. In particular, a preferred embodiment consists of a metal foil covered with parallel tracks of getter material deposit, the foil is then cut perpendicular to the tracks to form strips. The strips can then be cut into smaller pieces (using cuts parallel to the tracks, both between and along the axes of the tracks) such that each piece can have only one getter-covered area These pieces present a foil area that is free of getter material which can be used for mounting the getter device inside the quartz bulb

The getter devices of the present invention are suitable for use in any halogen lamp However, the small dimensions of the getter devices according to the present invention, especially their thinness, have particular significance, and make the present invention particularly well suited for use in small size quartz bulb lamps

All of the foregoing objectives, features and advantages of the present invention, and more, are explained below with the aid of the following illustrative figures and exemplary embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a halogen lamp comprising a metal filament in an inner quartz bulb;

Figure 2 shows a detail of a metal foil that is covered with getter material, Figure 3 shows how to cut the getter-covered metal foil shown in figure 2, Figure 4 shows the resulting foil strip obtained by cutting the foil shown in

Figure 3; and

Figure 5 shows a getter device that is suitable for mounting in a halogen lamp made from the strip shown in Figure 4.

DETAILED DESCRIPTION Figure 1 shows a halogen lamp, generally referred to as 1, comprising a quartz bulb 2. The quartz bulb 2 contains a filament 3 that is mounted to supports 4, 4' The space 5 within the quartz bulb is generally filled with a halogen gas or halogen gas mixture, typically a mixture of bromine, and iodine, and less commonly chlorine, to a predetermined pressure Quartz bulb 2 is depicted as being sealed at crimping region 6 which surrounds the supports 4, 4' which mount filament 3 The quartz bulb 2 is formed from an open end fused-silica tube whose properties have been adjusted to conform to those needed for uses as a halogen lamp in any of several ways known in the art The metal leads 4, 4', with the tungsten filament 3 already affixed, are inserted into an open end of the tube The tube is then pinched, by hot pressing, around the leads 4, 4' The leads may be either normal wires (circular cross section) for big- and medium-size tubes For small size tubes, where the thickness of the fused-silica is reduced, it is preferred to use leads that ai e flattened in the zone around which silica will be fused The leads are made of metal, often molybdenum, that have a thermal expansion behavior with temperature that is similar to that of the quartz The end of the tube opposite supports 4, 4' can be reduced in known ways The quartz tube is then evacuated and back-filled with the desired halogen gas, most often a mixture of bromine and iodine The quartz tube can then be hot-crimped to produce the final quartz bulb 2

Figure 1 also shows two possible mounting positions for a getter device according to the present invention. It has been difficult to insert a getter device into a small halogen lamp Larger or medium size lamps are large enough to mount a getter device in any of several positions However, small-size halogen bulbs only have enough room to mount the getter device in one of two basic mounting positions A getter device 7 can be fixed to support 4 by, for example, welding so that it does not come free, move around inside the quartz bulb and possibly damage filament 3. Alternatively, a getter device 8 can be inserted independent of support 4 (or 4') through the crimped region 6 of the quartz bulb 2 The getter device can function as an independent rod through the crimped region Either, or both, mounting structures could be used for a quartz bulb However, it is contemplated that only one getter device structure will be attached to any particular bulb for purposes of minimizing the amount of space occupied by the getter device in the quartz bulb 2. Figures 2-5 show details of the getter devices 7 or 8 in Figure 1 The strips of getter material 20 can be formed by screen-printing the getter material powders onto metallic substrates This technique involves preparing a suspension of a non- evaporable getter (NEG) metal in a dispersing medium NEG metals include, foi example, metals such as Zr, Ti, Ta, Nb, and V

When used in a suspension, the NEG metal can be a powder having a particle size not greater than about 150 mm, a preferred range of particle sizes is between about 5 μm and about 70 μm The NEG metal particles can be dispersed in a solution having an aqueous, alcoholic, or hydroalcoholic base and which contains not more than about 1 wt% of high boiling point organic compounds which have a boiling temperature of at least about 250°C An example of a suitable aqueous base is distilled water Suitable alcoholic bases include, but are not limited to, low molecular weight alcohols such as ethanol, propanol, and butanol(s) Suitable hydroalcoholic bases have a solvent which is a mixture of water and the previously described alcohols The amount of high-boiling point organic compounds is preferably not more than about 0.8 wt% Dispersing media used for serigraphy usually have high contents of organic components, which are used as binders

The organic components left in the deposit after drying can decompose to form gases such as CO, CO₂, or nitrogen oxides at a temperature of from about 200°C to 400°C during the subsequent sintering phase At such temperatures, the particles of NEG metal are already at least partially activated and can therefore sorb these gases, which results in a reduction of the sorption capacity of the resultant getter device

Thin layers of NEG metal serigraphically deposited using a dispersing medium containing more than about 1 wt% of high-boiling point organic compounds have poor gas sorption properties. On the other hand, the dispersing medium preferably contains at least about 0 2 wt% of high-boiling point organic compounds At lowei concentrations of such compounds, the viscosity of the suspension is too low Under these conditions, the final form of the deposit is defined by the surface tension of the solvent and by the solvent wettability of the metallic substrate and of the web of the serigraphic screen The solvent's surface tension tends to form suspension drops on the substrate, in larger proportion when the solvent wettability of the substrate is low Moreover when the seπgraphic screen is formed of a material having high solvent wettability, during peeling of the screen from the deposit the suspension tends to stick to the threads of the screen to a greater extent which results in an accumulation of excessive amounts of NEG metal in the l egion of the meniscus formed between the suspension and the screen The result of these effects cannot be forecast and changes as a function of the material used for the substrate and for the seπgraphic screen but nonetheless coincides with the formation of an uneven deposit The thus prepared suspension is deposited onto a substrate by a seπgiaphic technique This technique is known for other applications, such as, for example the reproduction of drawings on adapted surfaces or the deposition of conductive tracks for a printed circuit Suitable materials for the formation of the substrate include, but are not limited to, metals such as titanium and molybdenum which can withstand the extreme chemical reactivity of the atmosphere of hot halogen gas that is created by the operation of the tungsten filament in the quartz bulb The substrate can have a thickness between about 20 μm and about 500 μm The deposit may be in the form of a continuous layer covering an entire surface of the substrate or if desired, the substrate's edges may be left uncovered to facilitate handling of the final sheet Those skilled in the art will recognize, that the seπgraphic technique also enables the formation of partial deposits on the surface of the substrate so that many different geometries for the NEG metal deposits can be obtained

To form a shaped deposit, the ports of the seπgraphic screen are selectively blocked in a desired pattern by means of a gel which cannot be etched by the suspension to be deposited The obtained deposit will have the geometry of the gel negative, I e , the geometry corresponding to the ports of the screen which ai e not blocked with gel In this manner, continuous deposits having complicated shapes such as, for example, a spiral can be obtained, as well as discontinuous deposits l e , deposits forming a plurality of discrete deposit zones on the same substrate with, for example, circular, square, or linear shapes The thus obtained deposit is then dried to eliminate as much of the dispersing medium as possible. Drying may be performed in an oven at a temperature between about 50°C and about 200°C, in a gaseous flow or in a static atmosphere. During drying, the volatile components of the dispersing medium are evaporated. The dried deposit is then sintered under a vacuum at a temperature between about 800°C and 1000°C, depending on the type of NEG metal. Preferably, sintering occurs in a vacuum oven at a residual pressure lower than 0. 1 mbar. Depending on the ultimate temperature reached, the sintering time may be from about 5 minutes to about 2 hours. At the end of the sintering treatment, the deposit may be cooled under vacuum or, to accelerate the rate of cooling, in a stream of inert gas. Cooling also may be accomplished using a combination of these two conditions.

If desired, the drying and sintering treatments may occur as subsequent steps of a single thermal treatment. For example, the sample may be placed in a vacuum oven and, after the oven is exhausted to a pressure lower than 0.1 mbar, heated to a temperature between about 50°C and about 200°C. The sample may be held at such temperature for a predetermined time between about 10 minutes and about one hour. Alternatively, the variation of pressure values in the oven may be monitored. In this case, the drying step is considered complete when pressure increases, which occur as the result of the evaporation of volatile components of the dispersing medium, come to occur. Upon completion of drying, the sample may be heated under vacuum to the sintering temperature. Depending on the chemical nature of the components of the dispersing medium and of the NEG metal, more complicated thermal cycles also may be used. By way of example, treatment periods at a constant temperature at temperatures between the drying temperature and the sintering temperature may be used. These treatments may be particularly useful in the elimination of the last traces of organic components, by allowing them to decompose at a temperature at which the NEG metal is not yet activated.

During sintering, the surface of the dried deposit is covered with a refractory material to inhibit scaling of the surface. As used in connection with the description of the invention, the term "refractory material" means any material which is physically and chemically inert, i.e., is not subjected to any physical or chemical alteration, under vacuum over the temperature range of the sintering cycle If the surface of the dried deposit is exposed during sintering, then scaling of the surface occurs. Although the reason for such scaling is not yet fully understood, it has been found that covering the dried deposit's surface with a plane surface of a refractory material, i.e., a physically and chemically inert material as defined above, prevents the phenomenon from occurring. Any suitable material can be used to cover and thereby protect the deposit, provided the material does not melt or in any way suffer from physical or chemical conversions or alterations under vacuum throughout the temperature range of the sintering cycle. By way of example, molybdenum and graphite can be used to cover the deposit's surface to inhibit scaling thereof Those skilled in the art will recognize that the sintering of several supported deposits in the same thermal cycle may be accomplished by overlapping several sheets of supported deposit, interposing refractory material amongst such sheets or plane surfaces, and covering the surface of the uppermost sheet with a refractory material. For further details about the production of getter devices by screen-printing, reference is made to the PCT published patent application WO 98/03987.

The screen printing can be used to form a paste comprising zirconium and tantalum. Depositing these metals onto a metallic foil of molybdenum produces a structure depicted in Figure 2. The foil 21 can be completely covered with the getter material 20. However, it is advantageous to cover the foil 21 only partially with the getter material 20 as shown in Figure 2.

Depositing the getter material 20 in parallel tracks on metallic foil 21 in the manner shown in Figure 2 offers an advantage. The metallic foil 21 can be cut perpendicular to the tracks along dashed lines 22, 22', .., as shown in Figure 3 (showing only a portion of the foil of Fig. 2). The perpendicular cuts along lines 22, 22', .., can be made using any suitable cutting tool. However, mechanical cutting tools may contaminate the getter material 20. Such contamination can be entirely avoided using a laser cutting tool. Also, mechanical cutting usually cut by shearing Such shearing may act to remove the getter material 20 from the metallic foil 21 Laser cutting tools do not produce such shearing force and are thus less likely to remove the getter material 20 during cutting Suitable mechanical cutting tools and laser cutting tools are known in the art

Figure 4 shows a portion of one of the pieces cut from the foil sheet in Figure 3 This piece can be further cut along lines 23, 23', , as shown in Figure 4 to produce the individual getter device 50 shown in Figure 5 Cuts along lines 23, 23 ^' are made, alternatively, between (and preferably half way) next NEG metal powder tracks and along the axes of same tracks The piece of uncoated foil 21 of device 50 can be used to mount the device to either the supports 4, 4' or crimped region 6 as discussed above in connection with Figure 1 It is to be appreciated that the dimensions of two-dimensional device 50 should preferably not exceed 3 mm at its maximum dimension Thus, the getter device of the present invention repiesents a particularly compact getter structure for any application

The principles, preferred embodiments and modes of operation of the present invention have been set forth in the foregoing specification The embodiment disclosed herein should be interpreted as illustrating the present invention and not as restricting it The foregoing disclosure is not intended to limit the range of equivalent structures available to a person of ordinary skill in the art in any way, but rather to expand the range of equivalent structures in ways not previously thought of Numerous variations and changes can be made to the foregoing illustrative embodiments without departing from the scope and spirit of the present invention as set forth in the appended claims

Claims

1 A getter device (50) for use in halogen lamps, comprising

- a metallic foil capable of being sheared along orthogonal axes, - a getter material covering a portion of the metallic foil, wherein the metallic foil has a maximum dimension of 3 mm

2 A getter device according to claim 1, wherein the getter material is chosen among tantalum, thorium, hafnium, platinum, niobium and zirconium and mixtures thereof 3 A getter device according to claim 2, wherein the getter material is a chosen among tantalum, zirconium and mixtures thereof

4 A getter device according to claim 1. wherein the metallic foil is made of titanium or molybdenum

5 A process for the production of getter devices for use in halogen lamps, comprising the steps of

- preparing at least one suspension of particles of a NEG metal chosen among tantalum, thorium, hafnium, platinum, niobium and zirconium and mixtures thereof, with a particle size lower than about 150 μm, in a dispersing medium having an aqueous, alcoholic or hydroalcoholic base, containing a weight percentage of organic compounds, having a boiling temperature higher than 250°C. which is lower than 1%, wherein the ratio of the metal particles weight to the weight of dispersing medium is comprised between 4 1 and 1 1 ,

- depositing at least one layer of metal particles suspension onto a metal substrate by serigraphic technique; - drying the thus obtained deposit by allowing the volatile components to evaporate, and

- sintering in a vacuum oven the dried deposit at a temperature comprised between 800 and 1000°C and operating under vacuum, covering the deposit by means of a material not suffering from physical or chemical alterations under vacuum at any process temperature 6 A process according to claim 5, wherein the metal particles deposit is in form of parallel tracks

7 A process according to claim 6, further comprising the steps of

- cutting the metal foil carrying the NEG metal powder tracks along parallel lines (22, 22', ) perpendicular to said tracks,

- cutting the so-obtained metal foil pieces along lines parallel to the tracks (23, 23', ), alternatively between the tracks and along the axes of same tracks

8 A process according to claim 7 wherein the NEG metal is chosen among tantalum, zirconium and mixtures thereof 9 A halogen lamp (1), comprising

- a quartz bulb (2) being sealed at a region (6) so as to maintain a predetermined atmosphere comprising bromine, iodine, chlorine or mixtures thereof,

- a support structure (4, 4') extending through the seal of the quartz bulb into the predetermined atmosphere, supporting a metallic filament (3),

- a getter device inside the quartz bulb and in contact with the predetermined atmosphere for removing impurities therefrom

10 A halogen lamp according to claim 9, wherein the getter device (7) is mounted on the support structure (4, 4') 11 A halogen lamp according to claim 9, wherein the getter device (8) is fixed to the sealing region (6)