WO2011080679A2

WO2011080679A2 - Dielectric barrier discharge lamp

Info

Publication number: WO2011080679A2
Application number: PCT/IB2010/056043
Authority: WO
Inventors: Gerardus Antonius Wilhelmus Van Mol; Maarten Walter Steinmann; Norbert Braun; Dirk Weyns; Harald Dielis
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2010-01-04
Filing date: 2010-12-23
Publication date: 2011-07-07
Also published as: BR112012016259A2; JP2013516730A; WO2011080679A3

Abstract

The invention relates to a dielectric barrier discharge lamp and a method for manufacturing such a lamp. An elongated electrode 20 with a dielectric cover 22 is arranged to project into an interior space 18 of an enclosure 16. The electrode 20 is fixed to the enclosure 16 by a pinch seal 28 formed at a first end 26 of the enclosure. A gas filling is provided in the interior space 18 so that a dielectric barrier discharge may be excited within the interior space.

Description

DIELECTRIC BARRIER DISCHARGE LAMP

FIELD OF THE INVENTION

The invention relates to the field of lamps, and more specifically to a dielectric barrier discharge lamp and a method of manufacturing such a lamp.

BACKGROUND OF THE INVENTION

In a dielectric barrier discharge lamp, a gas is arranged between two electrodes separated by an insulating dielectric barrier. By applying a high voltage alternating current to the electrodes, an electrical discharge is formed generating light. In the case of a xenon gas filling, the light generated comprises a large intensity of VUV radiation at approximately 172 nm. It is known to provide phosphors on a dielectric discharge lamp to convert the VUV light to other wavelengths. Dielectric barrier discharge lamps generating UV radiation are often use for treating and sterilizing fluids, such as water.

US 2004/0183467 discloses an elongate dielectric barrier discharge lamp with a discharge tube provided on its inner wall with two diametrically arranged inner electrodes formed as linear conductor tracks and covered with a dielectric barrier made from soldering glass. In manufacturing, a discharge tube of soda-lime glass initially open at a first end and closed at the other end is used. The electrodes are provided within a tube. An exhaust tube is arranged in the open end, which is subsequently closed off by a pinch, while the exhaust tube still initially remains open. The lamp is then evacuated and filled with xenon as discharge medium via the exhaust tube, which is finally fused shut. It is optionally possible for the wall of the discharge vessel to be at least partially provided with phosphor.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a dielectric barrier discharge lamp which is easy to manufacture.

This object is achieved by a lamp according to claim 1 and a method according to claim 10. Dependent claims refer to preferred embodiments of the invention.

In the dielectric barrier discharge lamp according to the invention, an enclosure is present comprising a first end and an interior space. The interior space is filled with a gas filling at a defined pressure, so the enclosure needs to be sealed gas tight. For the lamp to emit light, the enclosure will further be at least partly transparent to at least a spectral portion of the light generated. In a preferred embodiment, the enclosure may be quartz glass and the gas filling may be a rare gas, most preferably xenon.

Arranged within the interior space is an elongated electrode with a dielectric cover. The electrode projects into the interior space, i. e. it is not arranged at the wall of the enclosure but protrudes such that its largest portion is arranged at a distance from the enclosure. The conducting electrode, which is preferably made of metal, is preferably completely covered by dielectric material, which could be e. g. glass or a ceramic material, like AI₂O₃. The electrode is of elongated shape, i. e. the length of the electrode is larger than its width, preferably at least 5 times, more preferably at least 20 times.

The electrode is fixed to the enclosure at least by a pinch seal at the first end of the enclosure. A pinch seal is provided by pinching - in a softened state - opposing portions of the material of the enclosure together, thereby at least partly enclosing a portion of the elongated electrode and fixing it to the enclosure.

Such a pinch seal is especially advantageous in the preferred case of an elon- gated enclosure (i. e. with a length more than 2 times the width, preferably more than 5 times the width). The electrode may be securely held by the pinch seal. At the same time, the interior space may be effectively sealed to obtain an enclosed interior space to provide the gas filling at a predetermined pressure (as will be explained later, the seal may be incomplete in a first stage of the manufacturing process, leaving one or more channels). Especially in the case of quartz glass as the enclosure material, providing a pinch seal is a reliable yet simple manufacturing step.

The electrode is preferably provided such that it is in direct contact with the enclosure only at the pinch seal. It is further preferred that it extends parallel to the walls of the enclosure, such that in each case a substantially constant distance between the electrode and the enclosure is maintained. If necessary, e. g. for mechanical stability, it is possible to provide a - preferably electrically insulated - further mechanical fixture for the electrode.

In a preferred embodiment of the invention, the lamp has a phosphor coating arranged at least on a part of the inner surface of the enclosure. A phosphor coating is any material that, if stimulated by primary light out of a discharge generated inside the interior space, emits secondary light of a different wavelength. For example, the radiation of a dielectric barrier discharge in a xenon filling, which emits VUV radiation, may be converted to different wavelength UV radiation, such as UV-C radiation well suitable for a specific task, such as disinfecting water. Generally, a phosphor material comprises a host lattice doped with an optically active material. Phosphor material be provided in a suspension, such that it can be applied as a coating when dried.

The phosphor coating is preferably arranged at least on the largest part (i.e. at least 50%, preferably on more than 80%) of the inner surface of the enclosure. It is addition- ally possible to provide the phosphor coating on further parts of the lamp, especially preferred also on the dielectric cover of the electrode.

For manufacturing a dielectric barrier discharge lamp with a phosphor coating and a pinch seal, the inventors have recognized that many phosphor compositions do not withstand the high temperatures necessary to form a pinch seal in commonly used enclosure materials, such as quartz. Therefore, it is advantageous to provide the phosphor coating only after forming the pinch seal to avoid degradation of the phosphor coating at least in the region close to the pinch seal. In order to still be able to introduce phosphor into the interior space, it is proposed according to a preferred embodiment of the invention to provide the pinch seal such that at least one channel (preferably two channels) are formed when provid- ing the pinch seal. The channels connect the interior space to the outside, such that phosphor material in a fluid state may be introduced easily.

It should be noted that there may be at least one further opening in the enclosure, such as e. g. an exhaust tube provided preferably on the opposing end, which is later closed, e. g. fused shut. While it is preferred to introduce the phosphor through the one or more channels formed in the pinch seal, it may equally be possible to introduce the phosphor material through another opening and using the pinch seal channel as exhaust. The inventors have found, that a good distribution of fluid phosphor material within the interior space is achieved by introducing the material through two channels provided at the pinch sealed first end, while using a distant opening, such as an exhaust tube at the opposite end, to evacuate contained air displaced from the interior space.

According to a further embodiment, drying (by introducing a drying gas) and optionally cleaning (via a cleaning fluid) may also be effected using the channel or channels, where the drying gas and/or the cleaning fluid pass through at least one of the channels, either on the way into or on the way out of the interior space.

After the phosphor coating is provided (and, optionally, further steps such as cleaning and drying have been effected), the channel is (or all channels are) closed by a gas- tight barrier. Such a gas-tight barrier may be of any type, such as e. g. cement or molten material of the enclosure, but is preferably provided as a further (smaller) pinch seal provided at the channels. Since the channels are naturally smaller than the first end of the enclosure, clos- ing them off by a pinch seal requires a limited amount of heating, such that the phosphor layer is not seriously degraded.

Accordingly, a dielectric discharge lamp may be provided with a phosphor coating, where in a pinch seal one or more channel is provided, which in the final product is closed off by a gas-tight barrier.

Further preferred embodiments of the inventions relate to the electrode. Generally, the electrode comprises a conducting element and dielectric cover. In a preferred embodiment, the electrode is provided as a tube out of dielectric material (such as quartz or ceramics, e. g. AI₂O₃). Within the tube, a metal electrode is arranged.

According to one embodiment, the electrode is provided as metal grains filled within the tube. According to a further embodiment, the electrode comprises a metal layer, e. g. a silver layer applied in a chemical way or, alternatively, formed by sputtering a metal wire in a HF field to the wall. In a still further embodiment, the electrode comprises a metal wire or rod arranged within the tube, which has a number of protrusions on the surface (which may be made e. g. by laser engraving). The protrusions are in close contact with the tube material, while the rod or wire acts as conductive lead.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1 shows a partly symbolical drawing of a lamp according to a first embodiment of the invention arranged within a water reservoir;

Figs. 2a, 2b show side views of a quartz tube used for manufacturing the lamp of fig. 1, in different manufacturing steps;

Figs. 3a-3d show sectional views of the quartz tube of figs. 2a, 2b in different manufacturing steps with the section taken in the plane A.. A in figs. 2a, 2b;

Figs. 4a-4e show different embodiments of electrodes for the lamp of fig. 1. DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows an UV generating lamp 10 used in a device for treating water 12 within a container 14. The lamp 10 is a dielectric barrier discharge lamp with an outer enclosure formed by a quartz tube 16 enclosing an inner space 18. Centrally arranged within the quartz glass tube 16 is a central electrode 20 surrounded by a dielectric cover 22.

A filling gas is provided within the inner space 18 of the lamp 10. In the preferred embodiment, the filling gas is xenon at a pressure of e.g. 300 mbar. In operation of the lamp 10, an electrical power supply 24 supplies an alternating voltage to the electrode 20 and to the surrounding water 12 acting as a second electrode. Both the second electrode (wa- ter 12) and the first electrode 20 are separated from the xenon filling by a dielectric barrier

(dielectric cover 22 and the wall of the glass tube 16). A dielectric barrier discharge is excited in the gas filling of the interior space 18 which generates UV light, by which the water 12 is irradiated and disinfected.

The spectral composition of the emitted UV light depends on the filling gas and may be influenced also by a phosphor coating on the enclosure 16 of the lamp. In the case of a xenon filling, VUV radiation with an intensity maximum at 172 nm is generated. A phosphor material may be provided on the enclosure 16 which is excited by the VUV radiation to generate light of different wavelength, e. g. in the UV-C range.

Different phosphor materials are known or may be designed by the skilled per- son to obtain a desired spectral composition of the secondary light emitted from the lamp 10.

Examples for phosphor materials are given in US 6,398,970 and US 7,298,077. As explained there, a preferred phosphor comprises an activator in a host lattice. By a specific choice of materials, different wavelengths of secondary light may be obtained.

The phosphor material may be applied to the quartz tube 16 by a flow-coating process. The phosphor material may be provided as a fine-grained powder, and a coating suspension for the flow-coating process may further comprise a solvent, such as water or an organic compound. Further auxiliary agents, such as organic or inorganic binder, stabilizers or liquefiers may be added. Examples of solvents and other agents for the suspension are given in US 6,398,970 and US 7,298,077.

For forming a phosphor coating on the quartz tube 16, the thus prepared suspension is applied to the quartz tube 16 and dried.

While such a coating may be formed on the outside of the quartz tube 16, it is preferred to provide it on the inside, as will be explained below. The lamp 10 is a finger lamp, i. e. it has a cylindrical elongated shape. Preferred values for the diameter are 10-25 mm, in particular 15-20 mm. The diameter of the dielectric cover 22 of the electrode 20 is preferably 3-5 mm, where the wall thickness of the cover is 0.7-1 mm. The overall length of the lamp 10 is preferably 50 to 200 mm.

The central finger, comprised of the electrode 20 and the dielectric cover 22, is centrally arranged within the cylindrical enclosure 16. It is fixed to the enclosure 16 only at a first end 26, where a pinch seal 28 is formed. The electrode 20 and cover 22 extend parallel to the longitudinal axis of the lamp 10, so that the distance between the wall of the quartz tube 16 and the electrode 20 is essentially constant.

In the following, manufacture of the lamp 10 will be explained with regard to figs. 2a, 2b, 3a-3d for a first embodiment, where a phosphor layer is formed after a pinching operation.

First, a glass tube 16 of appropriate length is provided (fig. 2a). The glass tube which becomes the enclosure 16 of the lamp 10 has a wall thickness of preferably 0.8-1.5 mm, most preferred 1.0-1.25 mm. While a second end 30 (see fig. 1, not shown in figs. 2a- 3d) is closed except for an open exhaust tube 36, an electrode finger comprised of a metal conducting electrode 20 and dielectric cover 22 is introduced into the interior space 18 centrally within the tube 16 (fig. 3a).

The electrode finger 20, 22 is than fixed to the tube 16 by forming a pinch seal 28 at the first end 26. The quartz material is heated to a softening temperature. Then, pinching blocks deform the first end 26 of the tube 16 so that opposing wall portions are pressed together, enclosing the electrode finger 20, 22 and forming a pinch seal 28

(figs. 2b, 3b).

However, the pinch seal 28 is formed such that a first channel 40a and a second channel 40b remain open at the first end 26 of the tube 16 (fig. 3b).

After forming the pinch 28, a phosphor layer 34 is applied. An exhaust tube 36 (see fig. 1) is opened at the second end 30 and a phosphor material in liquid form is introduced through the channels 40a, 40b into the interior space 18. The phosphor material adheres to the inside of the wall of the quartz tube 16 and to the dielectric cover 22 of the electrode 20. After drying, which is achieved by passing a drying gas through the channels 40a, 40b and the exhaust tube 36, the phosphor material forms a phosphor layer 34 (fig. 3c).

After forming the phosphor layer, the channels 40a, 40b are sealed by providing further, smaller pinches 32. The quartz tube 16 is locally heated at the first end 26 to form the smaller pinches 32. Because of the limited size of the pinches 32, the phosphor layer 34 is not seriously degraded by forming the pinches 32. The lamp is now closed at its first end 26 (fig. 3d).

In a subsequent step, the interior space 18 is evacuated through exhaust tube 36 and the desired gas filling is introduced. After this, the exhaust tube 36 is sealed.

In the following, different methods for forming the central finger consisting of a metal electrode 20 and a dielectric coating 22 will be described.

In a first embodiment (fig. 4a, 4b), a hollow quartz glass tube 22a, which later forms the dielectric cover, is provided and filled with metal grains 42 and a conductive wire 44. The tube 22a is closed with cement 46. In this way, a metal electrode in close contact with the dielectric (tube 22a) is provided.

In a second embodiment (fig. 4b), the dielectric is again provided as a hollow quartz tube 22a. A conductive metal layer of silver is applied in a chemical way, e. g. from AgNC"3 solution in water on the inner wall of the hollow quartz finger 22a, thus forming a metal layer 48 on the inside. In an alternative embodiment, the tube 22a may be made of ceramic material, such as AI2O3 with an inner diameter of 1.5 to 5 mm and a wall thickness of 0.3 to 0.5 mm, which is covered by a thin tungsten layer. The ceramic material and the tungsten layer are co-sintered to obtain a strong adhesion.

As shown in figs. 4b, 4c, the metal layer 48 thus provided on the dielectric tube 22a may be contacted with a conductive wire 44 in different ways, e. g. by a pressure contact 50 within the tube (fig. 4b) or by soldering, conductive gluing or conductive cement 52 (fig. 4c).

In a third embodiment, a tungsten wire rod 56 is introduced into a tube 22a of quartz or other dielectric material. The wire rod has protrusions 58 on the surface, which are made by laser engraving, and which are in close contact with the dielectric material of the tube 22a.

In a fourth embodiment (fig. 4e), a metal wire 58, preferably of molybdenum or niobium of a diameter between 1.0 and 1.5 mm is covered with an AI₂O₃ layer 60 of a thickness in the range of 40-500 μιη. The AI₂O₃ layer may be applied e. g. by EPD or CVD. The AI₂O₃ tube thus formed may be co-sintered with the molybdenum or niobium wire.

In an alternative embodiment (not shown in the drawings), the pinch seal 28 may be formed after application of a phosphor layer 34. This may be the case if a phosphor material is used that is less sensitive to the temperature of the pinching operation, or if a certain degradation of a phosphor layer in the area of the pinch seal 28 may be tolerated. In this case, the above described manufacturing process may start with a tube 16 which already comprises a phosphor layer 34. Here, the pinch seal 28 fixing the electrode 20 and dielectric cover 22 may be applied in a way completely sealing the first end 26 of the tube 16, i. e. no channels are formed in this alternative embodiment.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

As a variation to the above described embodiments, the filling gas may have a different composition, e. g. be a mixture of xenon with additional rare gases of which the ionization energy is smaller, in order to reduce the ignition voltage of the lamp.

Further, the pressure of the gas filling may be chosen at different values depending on the lamp geometry. Preferred values of the gas pressure are on the right hand side of the Paschen curve for a given distance between the dielectric cover 22 and the enclosure 16. In particular, pressures between 100 and 500 mbar are preferred.

As a further variation, the lamp 10 may comprise a second electrode provided on the outside of the quartz tube 16. The electrode may be provided in a way such that the enclosure of the lamp 10 remains transparent to UV radiation. For example, the electrode may be a wire or wire mesh or a transparent, conductive, inorganic compound such as ITO.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Dielectric barrier discharge lamp with

- an enclosure (16) comprising a first end (26) and an interior space (18),

- a gas filling provided within said interior space (18),

- an elongated electrode (20) with a dielectric cover (22) arranged to project into said interior space (18),

- said electrode (20) being fixed to said enclosure (16) at least by a pinch seal (28) at said first end (26) of said enclosure (16).

2. Lamp according to claim 1, where

- a phosphor coating (34) is arranged at least on a part of an inner surface of said enclo- sure (16),

- and where in said pinch seal (28) at least one channel (40a, 40b) is provided between said first end (26) and said interior space (18), said channel (40a, 40b) being closed by a gas- tight barrier (32).

Lamp according to claim 2, where

two channels (40a, 40b) are provided at said first end (26),

where both channels (40a, 40b) are closed by gas-tight barrier (32).

4. Dielectric barrier discharge lamp according to one of the above claims 2, 3, where

- said gas-tight barrier is formed by a pinch seal (32).

5. Lamp according to one of the above claims 2-4, where

- said phosphor coating (34) is arranged also at least on a part of said dielectric cover (22) of said electrode (20).

6. Lamp according to one of the above claims, where

- said electrode (20) is provided as a hollow tube (22a, 60) out of the material of said dielectric cover,

- where a metal electrode (20) is arranged within said tube (22a, 60).

7. Lamp according to claim 6, where

- said electrode is provided as metal grains (42) filled within said tube (22a).

8. Lamp according to claim 6, where

- said electrode comprises a metal layer (48) formed on the inner surface of said tube (22a).

9. Lamp according to claim 6, where

- said electrode comprises a metal wire (56) arranged within said tube (22a),

- where protrusions (58) are provided on said metal wire (56), said protrusions (58) being in contact with said tube (22a).

10. Method of manufacturing a dielectric barrier discharge lamp, comprising

- providing an enclosure (16) with a first end (26) and an interior space (18),

- arranging an elongated electrode (20) with a dielectric cover (22) to project into said interior space (18),

- and fixing said electrode (20) to said enclosure (16) by providing a pinch seal (28) at said first end (26) of said enclosure (16).

11. Method according to claim 10, where

- when providing said pinch seal (28), at least one channel (40a, 40b) is formed between said first end (26) and said interior space (18),

- and where a phosphor material is introduced into said interior space (18) for forming a phosphor coating (34) on at least a part of an inner surface of said enclosure (16),

- where after said phosphor coating (34) is provided said channel (40a, 40b) is closed by gas-tight barrier (32).

12. Method according to claim 11, where

- said pinch seal (28) is provided forming two channels (40a, 40b) at said first end (26),

- where after said phosphor coating (34) is provided, both channels (40a, 40b) are closed by a gas-tight barrier (32).

13. Method according to claim 11 or 12, where

- said gas-tight barrier (32) is formed by pinch seal.

14. Method according to one of claims 11-13, where

- after said phosphor material has been introduced into said interior space (18), a drying gas is introduced into said interior space (18), where said drying gas passes through at least one of said channels (40a, 40b).