Abstract:
A discharge lamp, suitable for operating by means of a dielectrically impeded discharge, and having metal electrodes arranged on the wall of the discharge vessel has at least one dielectric impeding layer which covers at least a portion of the electrodes. According to the invention, the electrodes are additionally covered directly with a barrier layer, in particular made from sintered glass ceramic, that is to say the barrier layer is, if appropriate, respectively arranged between the electrode and the dielectric impeding layer. The result of this is to prevent metal ions from diffusing out of the electrodes into the dielectric impeding layer and undesirably influencing the properties thereof. Moreover, it is possible in this way to prevent the electrode tracks from evaporating or being sputtered away during operation of the lamp.

Description:
TECHNICAL FIELD 
     Here, the term “discharge lamp” covers sources of electromagnetic radiation based on gas discharges. The spectrum of the radiation can in this case comprise both the visible region and the UV (ultraviolet)/VUV(vacuum ultraviolet) region as well as the IR (infrared) region. Furthermore, it is also possible to provide a fluorescent layer for converting invisible into visible radiation. 
     What is involved here are discharge lamps with so-called dielectrically impeded electrodes. The electrodes as such are typically implemented in the form of thin metal strips or layer structures resembling conductor tracks, for example made from conductive silver, at least a portion of which layer structures is arranged on the inner wall of the discharge vessel, for example by means of a printing method such as screen printing or the like. At least a portion of these inner wall electrodes is entirely covered with respect to the interior of the discharge vessel with a dielectric layer which functions during operation of the lamp as a dielectric impediment with reference to the discharge. 
     If only the electrodes of a single polarity—preferably the anodes—are covered with such a dielectric impeding layer, in the preferred unipolar pulsed operation (WO94/23442) a so-called unilaterally dielectrically impeded discharge is formed which comprises a multiplicity of delta-shaped partial discharges. If, by contrast, all the electrodes, that is to say of both types of polarity, are covered with a dielectrically impeding layer, a so-called bilaterally dielectrically impeded discharge is formed both in the unipolar and in the bipolar operation. In the bipolar operation, for example with ac voltage, or else bipolar pulses (WO94/23442), each electrode alternately undertakes the role both of the anode and of the cathode. 
     However, it has proved in the case of such lamps that metal ions diffuse out of the electrodes into the dielectric impeding layer and can undesirably influence its properties with reference to its function as a dielectric impediment for the discharge. 
     In addition, unilateral dielectric impediment is further attended by the problem that metal particles of the unimpeded inner wall electrodes evaporate during the production of the lamp, for example during the burning-in process of the dielectric layer, thermal joining processes and the like, and can possibly be deposited in an uncontrolled fashion inside the lamp. Moreover, in some circumstances the conductivity of the electrode tracks decreases. Said problem is the more pronounced the higher the temperature (in particular, higher than 400° C.) and the longer the time during which this temperature prevails. During operation of the lamp, in addition, metal particles can be extracted from the unimpeded electrodes by sputtering processes, and can likewise be deposited on the discharge vessel wall. The metal deposition on the discharge vessel wall leads to a reduction in the luminous flux of the lamp. Moreover, the thickness and width of the typically strip-shaped electrodes influence the current-carrying capacity thereof, and this can become critical, in particular in the case of strong pulsed currents. In addition, the electrode width has the effect on the capacitance of the electrode arrangement which directly influences the dielectrically impeded discharge. Moreover, the striking distance can be partially reduced, something which has a negative influence on the uniformity of the discharge. This holds in particular for the case, explained in more detail in the exemplary embodiments, when the cathode tracks are provided with projections on which the delta-shaped partial discharges attach themselves. 
     A further functional layer, for example a layer made from a fluorescent material or a mixture of fluorescent materials and/or one or else a plurality of reflecting layers for visible radiation (light) and/or UV radiation can be applied to the dielectric impeding layer and, in general, also to further parts of the inner wall of the discharge vessel. If appropriate, the reflecting layer serves the aim of bringing visible light outside in a specific fashion, that is to say only in a specified preferred direction of the lamp. However, porous layers such as, for example, a layer of fluorescent material provide only a reduced protection against metal ions evaporating or sputtering off out of the electrode tracks. Moreover, the electrode tracks are, in any case, entirely unprotected up until these layers are applied during the burning-in processes. 
     The geometric shape of the discharge vessel is not subject to any particular restrictions. Tubular or else flat discharge vessels are customary, for example. The latter are suitable, inter alia as so-called flat lamps for backlighting liquid-crystal display screens (LCD). Reference may be made, for example, to DE 197 18 395 C1 or WO 98/43277 with regard to the technical details of such lamps. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to avoid the disadvantages mentioned and to provide a discharge lamp which has an improved design with regard to the long-term performance, in particular also with regard to thinning of the electrodes by diffusion and reducing the influence of the metal electrodes on the dielectric layer. 
     According to the invention, in the case of the unilaterally dielectric impediment of at least that portion of the inner wall electrodes which is covered with a dielectric impeding layer, is additionally directly covered with a barrier layer, that is to say that the additional barrier layer is respectively arranged between the inner wall electrodes and the dielectric impeding layer. In other words, in this case the arrangement of the layers is as follows, viewed starting from the inner wall of the discharge vessel: electrode layer, barrier layer, dielectric impeding layer. In order to prevent metal particles from evaporating and sputtering off from the electrodes, which was mentioned at the beginning, it is advantageous also to cover the dielectrically unimpeded inner wall electrodes with such a barrier layer. 
     In the case of bilateral dielectric impediment, according to the invention all the inner wall electrodes, that is to say electrodes of both types of polarity, are directly covered with the barrier layer. The barrier layer is finally followed by the customary dielectric impeding layer. 
     The barrier layer should cover at least the entire electrode in each case, but can, if appropriate, also be applied “over the entire surface”, that is to say that in the latter case all the electrodes including the discharge vessel wall on which the electrodes are arranged are covered with a single coherent barrier layer. The application of the typically initially pasty barrier layer is performed by standard methods such as spraying, dispensing, rolling, screen printing, or silk screen printing etc. 
     The barrier layer comprises a dielectric, for example a glass solder, which in addition to preventing evaporation and sputtering away, also prevents metal ions of electrodes from diffusing through the barrier layer into the dielectric impeding layer which is important for the dielectrically impeded discharge. At least partially crystallized or crystallized glass solders, so-called sintered glass ceramics, in particular bismuth borosilicate glass (Bi—B—Si—O), have proved to be suitable in this regard. Further suitable crystallized glass solders are, for example, zinc bismuth borosilicate glass (Zn—Bi—B—Si—O), tin zinc phosphate glass (Sn—Zn—P—O) and zinc borosilicate glass (Zn—B—Si—O). For the sake of brevity, and in the interest of better terminological delimitation by comparison with the dielectric impeding layer provided for dielectrically impeding the electrodes, the dielectric layer acting as a diffusion, evaporation and sputtering-off barrier layer is also denoted below simply as a (dielectric) barrier layer. 
     It has proved to be sufficient for the effect according to the invention when the thickness of this barrier layer is of the order of magnitude of at least approximately 1 μm. The thickness of the barrier layer is typically in the range of between 1 μm and 40 μm, preferably in the range of between 1 μm and 30 μm, particularly preferably in the range of between 5 μm and 20 μm. In practice, thicknesses of typically a few μm, for example 6 μm, have proved to be effective. In any case, the thickness of the barrier layer is smaller than the thickness of the impeding layer. Moreover, it is essential that the barrier layer be present in a truly partially crystallized state. 
     The dielectric impeding layer can be applied to the individual electrodes both in a strip-shaped fashion (for unilateral and bilateral dielectric impediment) and—in the case of the bilaterally dielectrically impeded discharge—“over the entire area” by means of a single coherent barrier layer which covers the entire inner wall electrodes including adjoining parts of the discharge vessel wall. 
     The selection of the suitable thickness of the dielectric impeding layer is essentially determined by physical discharge requirements and is of the order of magnitude of 50 μm and several hundred μm, in particular in the range of between 50 and 200 μm. The material of the dielectric layer is likewise determined essentially by physical discharge requirements, in particular by the desired dielectric properties, for example dielectric coefficient, electric strength etc. Lead borosilicate glass (Pb—B—Si—O), for example, is suitable. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The aim below is to explain the invention in more detail with the aid of a plurality of exemplary embodiments. In the drawing: 
     FIG. 1 a  shows a schematic representation of a partially cutaway plan view of a flat discharge lamp according to the invention and having electrodes arranged on the base plate, 
     FIG. 1 b  shows a schematic representation of a side view of the flat lamp from FIG. 1 a,    
     FIG. 1 c  shows a partial sectional schematic representation of the flat lamp from FIG. 1 a  along the line AA, and 
     FIG. 2 shows a partial sectional schematic representation of a variant of the flat lamp from FIG. 1 a  along the line AA, 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 a ,  1   b  and  1   c  show schematic representations of a plan view, a side view or a partial section along the line AA of a flat fluorescent lamp which emits white light when in operation. It is intended as background illumination for an LCD (Liquid Crystal Display). 
     The flat lamp  1  comprises a flat discharge vessel  2  with a rectangular base surface, four strip-like metal cathodes  3 ,  4  (−) and anodes (+), of which three are formed as elongated double anodes  5  and two are formed as individual strip-like anodes  6 . The discharge vessel  2  comprises, for its part, a base plate  7 , a front plate  8  and a frame  9 . The base plate  7  and front plate  8  are respectively connected in a gastight fashion to the frame  9  by means of glass solder  10  in such a way that the interior  11  of the discharge vessel  2  is of cuboid construction. The base plate  7  is larger than the front plate  8  in such a way that the discharge vessel  2  has a circumferential free-standing edge. The perforation in the front plate  8  merely serves representational purposes and exposes the view onto a portion of the cathodes  3 ,  4  and anodes  5 ,  6 . 
     The cathodes  3 ,  4  and anodes  5 ,  6  are arranged alternately and parallel on the inner wall of the base plate  7 . The anodes  6 ,  5  and cathodes  3 ,  4  are lengthened in each case at one end and are guided outward on both sides on the base plate  7  from the interior  11  of the discharge vessel  2 . At the edge of the base plate  7 , the electrode strips  3 ,  4 ,  5 ,  6  merge in each case into a bus-like external supply lead on the cathode side  13  or anode side  14 , respectively. The two outer supply leads  13 ,  14  serve as contacts for connecting to an electric supply source (not illustrated). 
     In the interior  11  of the discharge vessel  2 , the electrodes  3 - 6  and the adjacent discharge vessel wall are completely covered (compare FIG. 1 c ) with a sintered glass ceramic layer  61  made from Bi—B—Si—O which acts as barrier layer and whose thickness is approximately 6 μm. For its part, the barrier layer  61  is completely covered with a dielectric impeding layer  62  made from Pb—B—Si—O whose thickness over the electrodes is approximately 250 μm. What is involved here, therefore, is a bilateral dielectric impediment. The barrier layer  61  prevents metal ions diffusing out of the electrodes  3 - 6  into the dielectric impeding layer  62 . A reflector layer  63  made from TiO 2  and whose thickness is approximately 4 μm is applied to the dielectric impeding layer  62 . Applied to the reflector layer  63  itself and to the inner wall of the front plate  8  is a fluorescent mixed layer  64  (for the sake of clarity, the layers are not illustrated in FIG. 1 a ; compare, however, FIG. 1 c ) which converts the UV/VUV radiation generated by the discharge into visible white light. What is involved here is a three-band phosphor with the blue component BAM (BaMgAl 10 O 17 :Eu 2+ ), the green component LAP (LaPO 4 : [Tb 3+ ,Ce 3+ ]) and the red component YOB ([Y,Gd]BO 3 :Eu 3+ ). The thickness of the fluorescent mixed layer  63  is approximately 30 μm. 
     In one variant (not illustrated), a further reflecting layer made from Al 2 O 3  is arranged between the TiO 2  layer and the fluorescent layer. The reflecting effect is improved in this way. The thickness of the Al 2 O 3  layer is approximately 5 μm. 
     The electrodes  3 - 6  including lead-throughs and external supply leads  13 ,  14  are constructed in each case as a coherent cathode-side or anode-side layer-like structure resembling a conductor track. These two layer-like structures and the further functional layers following thereupon—barrier layer  61 , dielectric impeding layer  62 , reflecting layer  63  and fluorescent layer  64 —are applied by screen printing in the specified sequence directly onto the base plate  7  or, if appropriate, to the front plate  8 . 
     After application of the layers  61 - 64 , the base plate  7  together with the frame  9 , and the latter, in turn, together with the front plate  8  are respectively fused by means of glass solder  10  to form the complete flat lamp  1 . The thermal joining operation is performed, for example, in a vacuum oven. Before the components of the discharge vessel are fused, the interior  11  of the flat lamp  1  is filled with xenon at a filling pressure of 10 kPa. 
     The two anode strips  5   a ,  5   b  of each anode pair  5  are widened in the direction of the two edges  15 ,  16  of the flat lamp  1  which are oriented perpendicularly to the electrode strips  3 - 6  and, to be precise, asymmetrically exclusively in the direction of the respective partner strip  5   b  or  5   a . The maximum distance between the two strips of each anode pair  5  is approximately 4 mm, and the smallest distance is approximately 3 mm. The two individual anode strips  6  are each arranged in the immediate vicinity of the two edges  17 ,  18  of the flat lamp  1  which are parallel to the electrode strips  3 - 6 . 
     The cathode strips  3 ;  4  have nose-like semicircular projections  19  facing the respective adjacent anode  5 ;  6 . They cause locally limited amplifications of the electric field, and consequently cause the delta-shaped individual discharges (not illustrated in FIG. 1 a ) produced in the unipolar pulsed operation in accordance with WO 94/23442 to be struck exclusively at these points. The distance between the projections  19  and the respective immediately adjacent anode strip is approximately 6 mm. The radius of the semicircular projections  19  is approximately 2 mm. Since the concrete configuration of the electrodes is only of subordinate importance here, reference is made in this connection to DE 196 36 965 A1 and/or DE 197 11 892 A1. 
     FIG. 2 shows a partial representation of a variant of the flat lamp from FIG. 1 a  along the line AA. Identical features are provided with identical reference numerals. By contrast with the representation in FIG. 1 c , what is involved here is a unilateral dielectric impediment, that is to say that only respectively the anodes ( 5   a ,  5   b ) are covered with a 250 μm thick dielectric impeding layer  62 ′ made from lead borosilicate glass. At the same time, in this exemplary embodiment, as well, all the electrodes, that is to say also the cathodes  3 ,  4  are covered directly with a 6 μm thick barrier layer  61  made from Bi—B—Si—O. The dielectric impeding layer  62 ′ is thus arranged above the barrier layer  61  and, moreover, only in the region of the anodes ( 5   a ,  5   b ). During the “burning in” of the dielectric impeding layer  62 ′, and also later during the operation of the lamp, the barrier layer  61  protects the cathodes, which are not provided with the dielectric impeding layer  62 ′, against evaporation. An approximately 150 μm thick fluorescent mixed layer  64  is arranged directly on the dielectric impeding layer  62 ′ (anodes) or the barrier layer  61  (cathodes and adjacent inner wall between the electrodes). This relatively thick fluorescent mixed layer  64  acts at the same time as the UV reflecting layer. Consequently, a separate reflecting layer is dispensed with in this simple variant. By contrast, the fluorescent mixed layer  64  is thinner on the inner wall of the front plate  8  so that it is transparent to visible light. 
     The fluorescent layer on the base plate and the front plate is omitted for pure UV radiators. For reasons of efficiency, however, in this case one or more UV-reflecting layers will not be omitted on the base plate. 
     Within the scope of the invention, it is possible to conceive of even further additional layers and layer arrangements without losing the advantageous effect of the invention. The only essential thing here is that the barrier layer, which prevents the metal ions of the electrodes from diffusing into the upper layers and, in particular, into the dielectric impeding layer which is critical for the dielectrically impeded discharge is arranged directly on the electrodes. 
     It may be pointed out again at this juncture that the layers illustrated in a very schematic fashion in FIGS. 1 c  and  2  need not necessarily be extended over the entire surface of the base plate. All that is essential is that at least the respectively relevant or, if appropriate, each electrode is completely covered with the corresponding layers. Furthermore, the individual layers need not necessarily be entirely flat, as they are illustrated in a simplified way in FIGS. 1 c  and  2 . However, the individual layers, in particular the very thin layers, can in practice also be inherently non-flat. This is to be seen, in particular, whenever one or more layers are thinner than the electrodes, and the layer(s) consequently still recognizably form the surface shape of the base plate with the electrodes. 
     A further exemplary embodiment (not represented), is a tubular aperture lamp. Apart from the different shape of the discharge vessel, the main difference by comparison with the flat lamp from FIG. 1 is the method of production, which is coordinated with the altered shape of the vessel. In particular, here the phosphor can no longer be applied to the inner wall or the further functional layers previously arranged thereon using a printing technique but, for example, by means of a slurry. The principle of the sequence and the function of the individual functional layers, in particular the effect according to the invention of the barrier layer, which limits the diffusion of the metal ions of the electrodes into the dielectric impeding layer, correspond to those from FIG.  1 .