Abstract:
The present invention relates to afield emission cathode, comprising an at least partly electrically conductive base structure, and a plurality of electrically conductive micrometer sized sections spatially distributed at the base structure, wherein at least a portion of the plurality of micrometer sized sections each are provided with a plurality of electrically conductive nanostructures. Advantages of the invention include lower power consumption as well as an increase in light output of e.g. a field emission lighting arrangement comprising the field emission cathode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 U.S. National Stage of International Application No. PCT/EP2011/055213, filed Apr. 4, 2011. This application claims the benefit of European Patent Application No. 10159139.4, filed Apr. 6, 2010. The disclosures of the above applications are herein incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to an arrangement for field emission cathode. More specifically, the invention relates to a cathode for a field emission lighting arrangement. 
     BACKGROUND OF THE INVENTION 
     There is currently a trend in replacing the traditional light bulb with more energy efficient alternatives. Florescent light sources also in forms resembling the traditional light bulb have been shown and are often referred to as compact fluorescent lamps (CFLs). As is well known, all florescent light sources contain a small amount of mercury, posing problems due to the health effects of mercury exposure. Additionally, due to heavy regulation of the disposal of mercury, the recycling of florescent light sources becomes complex and expensive. 
     Accordingly, there is a desire to provide an alternative to florescent light sources. An example of such an alternative is provided in WO 2005074006, disclosing a field emission light source containing no mercury or any other health hazardous materials. The field emission light source includes an anode and a cathode, the anode consists of a transparent electrically conductive layer and a layer of phosphors coated on the inner surface of a cylindrical glass tube. The phosphors are luminescent when excited by electrons. The electron emission is caused by a voltage between the anode and the cathode. For achieving high emission of light it is desirable to apply the voltage in a range of 4-12 kV. 
     The field emission light source disclosed in WO 2005074006 provides a promising approach to more environmentally friendly lighting, e.g. as no use of mercury is necessary. However it is always desirable to improve the design of the lamp to prolong the life time, and/or to increase the luminous efficiency of the lamp. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, the above is at least partly met by a field emission cathode, comprising an at least partly electrically conductive base structure, and a plurality of electrically conductive micrometer sized sections spatially distributed at the base structure, wherein at least a portion of the plurality of micrometer sized sections each are provided with a plurality of electrically conductive nanostructures. 
     In the context of this document, the term nanostructure is understood to mean a structure element with one or more dimensions of 100 nanometers (nm) or less, and a spatial arrangement of such elements. The term nanostructures include nanotubes, nanospheres, nanorods, nanofibers, and nanowires, where the nanostructures may be part of a nanonetwork. Furthermore, the term nanosphere means a nanostructure having an aspect ratio of at most 3:1, the term nanorod means a nanostructure having a longest dimension of at most 200 nm, and having an aspect ratio of from 3:1 to 20:1, the term nanofiber means a nanostructure having a longest dimension greater than 200 nm, and having an aspect ratio greater than 20:1, and the term nanowire means a nanofiber having a longest dimension greater than 1,000 nm. 
     Further definitions in relation to the nanostructures include the term aspect ratio, which means the ratio of the longest axis of an object to the shortest axis of the object, where the axes are not necessarily perpendicular. The term width of a cross-section is the longest dimension of the cross-section, and the height of a cross-section is the dimension perpendicular to the width. The term nanonetwork means a plurality of individual nanostructures that are interconnected. 
     As a comparison to prior art arrangements comprising nanostructures being grown on an essentially flat surface, the nanostructures according to the invention are instead provided on spatially distributed micrometer sized sections. Among other things, this has the advantage that it may be possible to provide an improved electron emission, for example when using the present field emission cathode in a field emission application. The improved electron emission is derived from the fact that the length of each of the individual nanostructures has less impact on the overall length provided by the micrometer sized sections in combination with the thereto arranged nanostructures. Accordingly, the micrometer sized sections will be the majority contributor for the total length, and the nanostructures will essentially only provide the micrometer sized sections with sharp tips of essence for achieving a high electron emission in a field emission application. An improvement in electron emission also lower power consumption in the case where the field emission cathode is comprised in a field emission arrangement, further allowing for an increased light output in case the field emission arrangement is a field emission light source and/or a field emission display. 
     For achieving an improved electron emission, the nanostructures preferably comprise nanostructures of at least one of conductive oxides, borides, nitrides, carbides, metallic alloys, silicides. Most preferably, the nanostructures comprise ZnO nanostructures. Also, at least a portion the pluralities of nanostructures have a shape of polygonal or circular cylinders, having a diameter between 5-500 nanometer and a length up to 500 nanometers. 
     The least partly electrically conductive base structure may comprise a matrix (or network) of interconnected of addressable conductors connecting to the micrometer sized sections. In between the conductors there may for example be provided insulating segments allowing groups or “clusters” of micrometer sized sections to be individually addressed. The base structure may be provided in different forms, for example arranged to comprise a wire, or being provided to comprise at least one of a grid, and a perforated or solid thin sheet, depending on the specific application in mind. 
     As discussed above, the micrometer sized sections are spatially distributed at the base structure. Preferably, for improving the electron emission, the micrometer sized sections are distributed with a distance exceeding an average diameter of the plurality of micrometer sized sections. The shape of the micrometer sized sections are preferably selected such that at least a portion of the plurality of micrometer sized sections have a shape of polygonal or circular cylinders, having a diameter between 5-500 micrometer and a length up to 10 millimeters, or at least having an aspect ratio of at least 3:1. 
     Depending on the application in mind, at least a portion of the plurality of micrometer sized sections preferably comprises an electrically conducting or semiconducting, optically transparent or reflecting material. 
     The field emission cathode according to the invention preferably forms part of a field emission arrangement, further to the above discussed field emission cathode comprising an anode structure and an evacuated envelope, for example in the form of a container comprising a transparent glass section. The field emission arrangement may for example be comprised in at least one of a field emission light source, a field emission display, an X-ray source. 
     The field emission cathode and the anode structure are both arranged inside of the evacuated envelop. Additionally, the anode structure may preferably be at least partly covered by a phosphor layer and comprising a thermally conductive material having a reflective coating. Such an arrangement is for example disclosed in EP09180339 by the same applicant and incorporated by reference in its entirety. 
     Furthermore, the anode structure is preferably configured to receive electrons emitted by the field emission cathode when a voltage is applied between the anode structure and field emission cathode and to reflect light generated by the phosphor layer out from the evacuated envelope, e.g. through the glass section of the above mentioned container. The voltage is preferably in the range of 2-12 kV 
     The voltage may for example be provided by a power supply comprised with the field emission arrangement, e.g. arranged together with (such as for example within a socket in the case the field emission arrangement is a field emission light source) with or in the vicinity of the field emission arrangement. The power supply may be connected to the field emission cathode and the anode structure and configure to provide a drive signal for powering the field emission lighting arrangement. The drive signal may be provided with a first frequency, where the first frequency is selected to be within a range corresponding to the half power width at resonance of the field emission lighting arrangement. In accordance with the invention, the selection of the first frequency to be such that the half power width at resonance of the field emission lighting arrangement is achieved is understood to mean that the first frequency is selected to be centered around the resonance frequency of the field emission lighting arrangement and having a range such that half of the total power is contained. Put differently, the first frequency is selected to be somewhere within the range of frequencies where drive signal has a power above a certain half the maximum value for its amplitude. This is further discussed in EP09180155, by the applicant, which is incorporated by reference in its entirety. 
     Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled addressee realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which: 
         FIG. 1  illustrates the general concept of a field emission cathode according to the invention; 
         FIGS. 2   a  and  2   b  illustrates different conceptual embodiments of the inventive field emission cathode, and  FIG. 2   c  illustrates a detailed view of the field emission cathodes; 
         FIGS. 3   a  and  3   b  illustrates different embodiments of emission lighting arrangements comprising a field emission cathode according to a currently preferred embodiment of the invention; and 
         FIG. 4  illustrates another embodiment of a field emission lighting arrangement. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like elements throughout. 
     Referring now to the drawings and to  FIG. 1  in particular, there is depicted the general concept of a field emission cathode  100  according to the invention. The field emission cathode  100  comprises an at least partly electrically conductive base structure  102  and a plurality of electrically conductive and spaced apart micrometer sized sections  104  extending from the base structure  102  and being formed as preferably (but not necessary) polygonal or circular cylinders. At the tips of preferably each of the micrometer sized sections  104  there are provided a plurality of electrically conductive nanostructures  106 . Not only the tip of the micrometer sized sections  104  may be provided with the nanostructures  106 , e.g. also the sides of the cylinder shaped micrometer sized sections  104  and also possible at the spaces in between the micrometer sized sections  104  (e.g. on the base structure  104 ). 
     As discussed above, the micrometer sized sections  104  preferably have a diameter between 5-500 micrometer and a length up to 10 millimeters or at least having an aspect ratio of at least 3:1. The micrometer sized sections  104  may extend at a right angle from the base structure  102 , or may as is shown in  FIG. 1  extend from the base structure  102  with slightly different angles. The micrometer sized section  104  in itself may also be straight (from a length perspective) and/or slightly bent. Different combinations are of course possible and within the scope of the invention. 
     The nanostructures  106  are preferably, or at least comprise, ZnO nanostructures and grown on the micrometer sized sections  104  using any one of prior known growth methods suitable in combination with the material selection for the base structure  102  and the micrometer sized sections  104 . The nanostructures  106  may also or instead comprise nanostructures of at least one of conductive oxides, borides, nitrides, carbides, metallic alloys, silicides. Additionally, at least a portion the pluralities of nanostructures have a shape of polygonal or circular cylinders, having a diameter between 5-500 nanometer and a length up to 500 nanometers. 
       FIG. 2  illustrates two conceptual embodiments of a field emission cathode according to the invention. The first embodiment illustrated in  FIG. 2   a  shows a field emission cathode  202  also comprising a base structure  102 , in this embodiment in the form of twisted wire  204 . In between the twisted wires  204  there are provided a plurality of micrometer sized sections  104 , in this embodiment in the form of a plurality of micrometer sized conductive treads  206  (preferably thinner than the twisted wires  204 ) of for example metal. On the outer tips of the conductive treads  206 , there are provided nanostructures, such as ZnO nanostructures  208  (not seen in  FIG. 2   a ). 
     The second embodiment of a field emission cathode  210  according to the invention as is illustrated in  FIG. 2   b  shows a flat base structure  210  being provided with “clusters”  214  of micrometer sized sections, for example of conductive treads  206 , and grouped in 10-100 conductive treads  206 . Larger or smaller clusters  214  may of course be provided. Similar to  FIG. 2   a , the conductive treads  206  of  FIG. 2   b  are provided with ZnO nanostructures  208  (not seen in  FIG. 2   b ). The ZnO nanostructures  208  may however of course extend in different directions. 
     However, in  FIG. 2   c  there can be seen a detailed view of the ZnO nanostructures  208  being arranged on the tips of the conductive treads  206 . As may be seen, the ZnO nanostructures  208  extend in a direction being essentially the same as the extending direction of the micrometer sized sections  206 . 
     It should be noted that the micrometer sized sections  104 / 206  may at least partly be of the same material as the base structure  102 / 204 / 212 , and may also be a direct material extension from the base structure  102 / 204 / 212 . In other embodiments the micrometer sized sections  104 / 206  may be of other materials than the base structure  102 / 204 / 212 , where the base structure  102 / 204 / 212  for example only partly may be conductive whereas the micrometer sized sections  104 / 206  are fully conductive. An opposite combination may of course be possible. That is, the base structure  102 / 204 / 212  may be comprised of a mixture of conductors and isolators, thereby allowing for e.g. separate clusters of micrometer sized sections  104 / 206  to be individually addressable and thereby individually controllable in a field emission application. 
     Turning now to  FIGS. 3   a  and  3   b , which illustrates two different conceptual field emission lighting applications according to currently preferred embodiments of the invention, the field emission lighting application  302  illustrated in  FIG. 3   a  is based on the concept of using a transparent field emission anode, such as an ITO layer  304  being provided on a transparent envelope, such as an evacuated cylindrical glass tube  306 . For emission of light, a layer of phosphor  308  is provided inside of the ITO layer  304 , in the direction towards the above and in relation to  FIG. 2   a  discussed field emission cathode  202 . The field emission lighting arrangement  302  further comprises a base  310  and a socket  312 , allowing for the field emission lighting arrangement  302  to be used for e.g. retrofitting conventional light bulbs. The base  310  preferably comprises a control unit for providing controlling a drive signals (i.e. high voltage) to the cathode  202 . 
     During operation of the field emission lighting application  302 , an electrical field is applied between the cathode  202  and the anode layer, e.g. the ITO layer  304 . By application of the electrical field, the cathode  202  emits electrons, which are accelerated toward the phosphor layer  308 . The phosphor layer  308  may provide luminescence when the emitted electrons collide with phosphor particles of the phosphor layer  308 . Light provided from the phosphor layer  308  will transmit through the transparent ITO/anode layer  304  and the glass cylinder  306 . The light is preferably white, but colored light is of course possible and within the scope of the invention. The light may also be UV light. 
     In the second type of field emission lighting application  314  shown in  FIG. 3   b , the field emission lighting application  314  similar to the field emission lighting application  302  of  FIG. 3   a  comprises one or a plurality of field emission cathodes  202 . The cathode(s)  202  are arranged in a similar evacuated cylindrical glass tube  306  and also comprises a base  310  and a socket  312 . However, the concept of the field emission lighting application  314  is based on reflecting light rather than arranging the anode layer to be transparent. Instead, the anode  316  is arranged essentially centrally within the cylindrical glass tube  306  and provided with a reflecting coating (or being comprised of a reflective material, such as comprising a metal or being made of a metal structure). On top of the anode  316 , there is then provided a phosphor layer  318  having similar characteristics as the phosphor layer  308  discussed in relations to  FIG. 3   a . The concept of the reflective anode of the field emission lighting application  314  shown in  FIG. 3   b  is further discussed in detail in the above referenced EP09180339. 
     Accordingly, during operation of the field emission lighting application  314 , the anode  316  is made to reflect light rather than to transmit light as is the case with the field emission lighting application  302  shown in  FIG. 3   a . This may for example allow for high heat dissipation during operation. The heat will be conducted away from the anode  316  to an anode contact acting as a thermal bath. 
     Finally, in  FIG. 4  there is depicted a conceptual flat field emission lighting arrangement  400  comprising a field emission cathode being based on the concept shown in relation to  FIG. 2   b . In essence, the flat field emission lighting arrangement  400  comprises the layered structure as is shown in  FIG. 3   a , e.g. comprising a transparent layer, such as a glass layer  402  (materials having similar characteristics are of course possible and within the scope of the invention), a transparent anode layer, such as an ITO layer  404  and a phosphor layer  406 . 
     The field emission cathode may comprise a conductive and possibly addressable matrix base structure where there are arranged a plurality of clusters  214  of micrometer sized sections  206  provided in a similar manner as in relation to  FIG. 2   b . The micrometer sized sections  206  are in turn provided with a plurality of nanostructures, for example comprising ZnO nanostructures. 
     The operation of the flat field emission lighting arrangement  400  is similar to the field emission lighting arrangement  302  of  FIG. 3   a , however, the phosphor layer may be sectioned and of comprising different types of phosphor emitting light of different color. Also, as mentioned, base structure may be of an addressable matrix type and may together with the sectioned phosphor layer  406  may be used for emitting lights of different color, for example simultaneously. Accordingly, the flat field emission lighting arrangement  400  may be used as a multi color display. 
     Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Variations to the disclosed embodiments can be understood and effected by the skilled addressee in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Furthermore, 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.