CÁTODO OF EMISSION OF FIELD, METHOD TO PRODUCE IT AND SOURCE OF LIGHT
FIELD OF THE INVENTION This invention relates to a method for producing a field emission cathode for a light source, according to the preamble of claim 1. It also relates to a field emission cathode thus produced and to a source of light that includes this field emission cathode. BACKGROUND OF THE INVENTION
WO 96/25753 relates to field emission lighting devices, which employ a cold cathode and in particular to a method according to the foregoing. Lighting devices with field emission have great advantages over other types of lighting devices such as fluorescent tubes, since the latter require complicated external electrical devices for the function and since they typically contain materials that have negative environmental effects. In fluorescent tubes, gas discharges are used to emit radiation on a fluorescent material that in turn emits visible light. The lighting device with field emission, on the other hand, allows the inclusion of
REF: 134917 Environmentally friendly materials and can work using simpler and more economical equipment. Furthermore, in WO 96/25753, it is provided that a cold field emission cathode for use in a field emission light source is provided with a surface geometry that facilitates obtaining high local electric field strengths for emission of electron field. It is also planned to form a field emission cathode with a emitting surface having a particular topography, which facilitates the emission of electron field. In addition, it is expected that the irregularities are trained in such a way that high mechanical and electrical durability as well as long operating life and high energy emission per unit surface area are obtained. According to WO 96/25753, this is achieved by irradiating the emitting ends of a fiber with ions of a low working material, in order to reduce the electron working function of the emitting ends. This irradiation step causes marked irregularities at the emitting ends. A modification stage is then carried out in order to train very high irregularities and marked in a rounded shape that will result in emitting ends that are
Effective and durable to result in a cathode that has long operational life. The method according to the cited document results in a cathode of good operation, its manufacture however is complicated, time consuming and thus relatively expensive. SUMMARY OF THE INVENTION A primary objective of the present invention is to provide a method for the production of a cold field emission cathode for a light source, which does not have the aforementioned disadvantages of the previously known method. Also a general objective is to provide a method for producing a cathode of cold field emission of long duration and effective for a light source. This objective is achieved in a method according to the above through the characterizing portion of claim 1. In this way the manufacture of a cold field emission cathode for a field emission light source is radically simplified and in this way it is more economical. Contrary to what might be expected, it is possible to avoid time-consuming and complicated steps according to the prior art. The treatment carried out by bringing a beam of laser light in contact with the emitting surface of
field of the emitting bodies, results in a thermochemical treatment that ensures better control than the desired result obtained with respect to the resulting dimensions and distribution of the field emitting elements, when the method of the invention is carried out. This is in contrast to the previous method where weaknesses inherent in the irradiation and modification stages, can not guarantee even results in separate parts of bundles of fibers, which in practice are subject to treatment. The surprising reason for the good result for this treatment is explained by the synergistic effect of the energy of the laser beam and on the length of the beam, in combination with a slightly heterogeneous structure of the material in the emitting bodies, where in practice different regions They have different properties. These different regions respond differently to laser treatment, which subsequently results in the creation of irregularities that have the desired topography high and rounded. With respect to carbon fibers, this material has oriented phase microstructures, more or less ordered, such as graphite crystallites, providing high strength, elastic modulus, conductivity and stability to chemical products in a
AJtÜ * * ^ ** ***. . rí? -jÁ ». W? A¿-ai áta-tijáB ^ matrix and surface layer of amorphous carbon. The invention provides for the modification of the material by laser treatment whereby the crystallites and the amorphous material respond in such a manner to the laser treatment, that the desired topography of the irregularities is obtained. In addition, the electrical properties of the surface layer of carbon fibers, which is less than the ordered phase, can be improved by laser treatment. The fact that the body is configured at the same time as its surface is modified provides several additional advantages. Cutting the emitter body with laser radiation in order to cut its dimensions, its length, etc., and simultaneously modify the emitting surface with laser radiation, provides a rational and economical method. In this way, a mechanical cut or the like can be avoided, whereby the disadvantages of mechanical cutting are such that deformation of the portions of the body surface is avoided. Laser cutting gives the advantage over shaping a mechanic, such as mechanical cutting, that dust formation can be reduced to a minimum. In light sources that include emitters produced according to the prior art, it has been found that the efficiency and operational lifetime are reduced due to the presence of dust emanating from cuts and mechanical cuts of the emitters.
iaia ^^,., ^ .- ^ -.,. ^.,. ^ j ^^^^ A. ^ BJjfe aM g ^^ aa ^ A, ^ - ^^ * ^^^ tfetffjÉfiM i é L In addition, a lighting device that includes a field emission cathode, manufactured in accordance with the method of the invention, wherein the emitter body is shaped with laser and simultaneously the modified emitting surface will be more effective. The reason for this is that more current can be conducted through the cathode and in this way more light can be emitted from the light source than in a prior art light source under otherwise similar conditions because the surface is more uniform. It will result in a more durable light source with extended operating life. For the emitting surface that is more uniform, a better electron-emitting action will result. This in turn results in a greater current and subsequently more light. As a comparison, it can be mentioned that in the case of fibers, fibers treated in accordance with this invention have been shown to be capable of allowing a current that is five times as large as the current through a device according to the prior art. This can be used to build light sources that have longer operational lifetimes than before and more intense light sources than before. Providing a uniform emitting surface is also an advantage due to a more uniform and pleasing light distribution, compared to a prior art light source. The invention can be applied to different types of field emitting bodies, such as fibers in strands, of the type that is used according to WO 96/25753, so-called carbon nanotubes (CNT = Carbon nanotubes), so-called diamond-like carbon (DLC = Diamond like carbon), porous carbon foam material such as reticulated vitreous carbon (RVC = Reticulated vitreous carbon). Materials to be subjected to the method in general are substantially carbon materials, but other similarly operating materials can be used as the emitter body material. Other similar materials and other body forms are thus not excluded from this invention. Basically, the invention is a one-stage process, wherein cleaning, cutting, shaping and surface modification are carried out simultaneously. In some cases, however, laser cutting and the somewhat modified surface may require further laser treatment in order to obtain a more effective emitting surface. According to one aspect of the invention, this can be achieved by contacting the pre-treated surface with more laser radiation, which may be of a different type and / or intensity to the first laser radiation for cutting, in order to optimize the treatment. The method is also applicable to integral porous bodies such as bodies of carbon foam material. This body is constituted by thin interconnected structures, which are referred to as an integral structure. For example, it can conform to flat or cylindrical emitting surfaces by laser shaping or cutting. The resulting surface is modified in order to achieve preferred electron emitting surfaces, similar to those described above for fibers. A field emission cathode, manufactured in this way is comparatively easy and inexpensive to manufacture. The invention is also applicable to woven structures made from filaments or threads containing carbon or a similar action material. DE-A1-196 53 820 describes the production of field emission surfaces, in particular for flat screen applications. According to this document, laser radiation is applied locally, for example through a masking technique to a diamond or diamond-like carbon layer. In this way, areas that are projected on the neighboring areas are
^^^^ HH ^^ ta. ^^ TUüa M * í will obtain, something that is highly undesirable with respect to the present invention. Additional advantages are obtained by this invention and it will be clear from the following detailed description of modalities given by way of examples and with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a field connection light source according to the invention, Figure 2 shows on an amplified scale, a emitting surface of an emitter body, Figure 3 shows a source of emission light Field according to the second embodiment, Figure 4 schematically illustrates a first embodiment of the method according to the invention, and Figure 5 illustrates schematically a second embodiment of the method according to the invention. DESCRIPTION OF MODALITIES In a preferred method according to the invention, a field emission cathode is made from bundles of fibers that are commercially available as polyacrylonitrile carbon fibers. Another suitable material containing carbon or similar material having a diameter in the range of a few microns (μm) can be used in an equivalent manner. As illustrated in Figure 1, a light source having a field emission cathode in the form of fiber bundles 1 is illustrated, the bundles are arranged in a matrix form and arranged on a conductive substrate 17. In the same plane as the matrix, and in close proximity to it, in the order of tenths of millimeters, and on the emitting ends of the bundles 1, a modular electrode 12 is provided, having a central opening around each bundle. The substrate 17 and the modulator 12 are supported on dielectric supports 18 within an evacuated glass vessel, with an upper border glass plate 15 and a lower border glass plate 16. Opposite to beams 1 and the modulator, are provided in the inside of the upper border 15, an anode layer 13 and a luminescent layer 14. The anode layer 13, the modulator 12 and the substrate 17 have electrical terminals A, B, C, respectively, for voltage application leading to electrons from the bundles 1 through the modulator openings to the luminescent layer 14 in connection with the anode layer 13. When the electrons enter the luminescent layer 14, the light is emitted by escaping the transparent anode 13 and the glass container.
The light source can also be constructed as a diode, that is without a modulator. It is important that the fiber bundle matrix disposed on the substrate 17 has a uniform surface and that the surface of each emitter body is provided with a number of irregularities configured to easily reject electrons when the substrate is subjected to a potential. In addition, the surfaces should be provided with irregularities configured, rounded, such that no exaggerated deformation of the irregularities during use is carried out. This deformation can otherwise be essentially detrimental to the operational lifetime of the light source. According to the invention, the unit consists of the substrate 17 and the fiber bundles are shaped and treated with laser light, whereby the fibers are cut precisely to form a total uniform surface, the surface by the laser treatment, is provided with the desired rounded irregularities. If necessary, to perfect the surface, an additional laser treatment through and for example scanning the cutting surface with laser radiation, may be desired.
* - "'" --riw? Iti? Ltir ^, ^ > A "^ -" ^ * t- Figure 2 shows a profile 10 of a fiber 9 after cutting and treatment with laser beam. The emitting end has high irregularities, but not slightly rounded 11. FIG. 3 shows a field emission light source 20, which employs a field emission cathode 21, made from carbon foam material. indicates as 22 and the anode layer as 23, while the phosphor layer is indicated as 24. The emitting cathode 21, the modulating grid 22 and the anode layer as 23, are provided with terminals A, B and C, respectively, to subject these elements to appropriate potential. The surface 25 is cut uniformly so as to provide a emitting surface located substantially in a plane and at the desired dimensions as the aid of laser radiation. If necessary, the surface 25 is further treated with subsequent laser radiation, in order to perfect the emitting surface. As indicated above, the method can be used to differently and otherwise differentiate field emission cathodes. In the case of a circular cylindrical cathode for radial emission, either made from fibers as illustrated in WO 98/57344 or WO 98/57345 or from a circular cylindrical porous body of carbon foam material, cutting can be applied and / or laser treatment of the emitting surfaces for example by rotating the cathode body and simultaneously sweeping with the laser beam. This is illustrated in Figure 4. A cylindrical body 30 having its peripheral surface provided with material to be configured and modified, is rotated about its axis. A laser 31 sweeps the surface of the body, being pivotable around an axis, with laser radiation, uninterrupted, so that the laser beam essentially follows the tangent of the cylindrical body 30. In this way the body is configured, thereby the material that is projected on a certain level, is removed by the effect of laser radiation, and modified according to the previous explanation. An alternate method is illustrated in Figure 5, wherein a flat body 32 is swept with laser radiation from the laser 33. The sweep is obtained by shifting to the sides of the laser beam with respect to the body on the upper surface, which leaves to configure and modify. The same effect is obtained as in the method illustrated in Figure 4. The scanning of a laser beam can also be obtained by moving optical devices such as mirrors.
and lenses, according to methods known per se. Regarding this invention, in this way it is important that the laser radiation is directed on the surface to be treated, so that it is tangent to that surface in case of a curved surface or that coincides with directions of the surface in case of being flat . An advantage of the invention is that the resulting material can be controlled for purity and / or addition of some desired characteristics such as different properties can be supplied to the emitting surface. This is referred to as etching. This depends on the composition of the atmosphere surrounding the body treated during treatment and the beam intensity and point size as well as the characteristics of the atmosphere flow. In an inert atmosphere, the evaporation or sublimation of amorphous carbon can be achieved. In the presence of oxygen or hydrogen, there may be catalytic graphitization of the amorphous carbon. In an atmosphere that includes nitrogen or air, surface groups with oxygen and / or bound nitrogen can be formed in a moderate flow atmosphere. Erosion can be carried out at high temperature under the condition of high flow atmosphere.
It is also possible, using processes known per se, to add substances to the emitting surfaces in order to alter their properties. The invention can be included in methods involving other steps known per se such as those referred to in the aforementioned PCT documents. In the case where the carbon fibers are surface treated for example with a polymeric material, and if that surface treatment material needs to be removed, this may be supported in some cases already during the laser treatment or otherwise in a heat treatment process. or of any other convenient processes. In principle, any type of laser can be used, that has sufficient intensity and works in the area of visible light or near that area. The light beam can be circular or elliptical or any other convenient section. The configuration of the laser beam can be adapted to the material to be modified. For the total process of cutting, etching and surface treatment, as an example, power in the range of approximately 0.01 to 0.1 m / s, can be used with beam intensities of 0.1 to 10 kW / mm2. Diode laser and JAG commercially available between approximately 100 to 1000 w can be used. The beam can be pressed or not pressed. The shape of the emitter body is achieved by the evaporation of undesired material and local fusion on the emitting surface of the material to be treated by the laser radiation. This will result in a cleaning effect. Small amounts of excess material as well as other possible non-stable material present, will be fixed by a mordanting or evaporating effect. Such a clean surface, without loose particles, increases the stability and operational life of a resulting light source. The form can be any desired. As examples, flat and cylindrical bodies can be formed. Flat emitting surfaces (which have the emitting portions on a plane) can be produced by a linear sweep displacement on the sides of a laser beam relative to a body (Figure 5) or alternatively of a body to be configured and modified with respect to a stationary laser beam. Methods to produce cylindrical emitting surfaces (which have the emitting portions on a cylinder surface) different to the method mentioned above (Figure 4) are as examples: either rotate the body to shape and modify or with respect to a stationary laser beam or make A laser beam is bar sideways in a circle with respect to the body.
It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.
'"' '^^ IlÉlíil