Shaped cathode for a field emission arrangement

Abstract

The present invention relates to a field emission lighting arrangement, comprising an anode and a cathode, where the shape of the cathode is selected based on the shape of a evacuated envelope in which the anode and cathode is provided. The inventive shape of cathode allows for an improved uniformity of an electric field provided between the anode and cathode during operation of the field emission lighting arrangement. The invention also relates to a corresponding method for selecting a shape of such a cathode.

Claims

1. A field emission lighting arrangement, comprising: a bulb shaped evacuated envelope, comprising: a field emission cathode arranged along the optical axis of the field emission lighting arrangement, and an anode structure arranged along an inside of the evacuated envelope, the anode structure comprising a transparent electrically conducting layer and an electron to light conversion layer, and a base structure provided at a bottom end of the evacuated envelope, the base structure comprising a power supply electrically integrated within the base structure and connected to the anode structure and the cathode, wherein the power supply is configured to apply a voltage such that electrons are emitted from the cathode to the anode structure, wherein the field emission cathode has an essentially ellipsoidal form factorthat is selected based on a predetermined shape of the evacuated envelope and is arranged in a lower part of the evacuated envelope towards the base structure, wherein a distance between the surface of the cathode and the anode structure is largest along the optical axis and the shortest distance between a surface of the cathode and the anode structure decreases with an increasing central angle from the optical axis; and the cathode has an essentially circular cross-section on the plane which has a normal aligned with the optical axis, and the ratio between the semi axis aligned with the normal and the other two semi axes is between 1.05 and 2.

2. The field emission lighting arrangement according to claim 1, wherein the distance between the surface of the cathode and the anode structure varies between 0.1 and 100 mm.

3. The field emission lighting arrangement according to claim 1, wherein the selection of cathode shape provides an electrical field strength that differ less than 50% at all points of the cathode surface.

4. The field emission lighting arrangement according to claim 1, wherein the selection of cathode shape provides electron paths resulting in a uniform electric current density in the anode structure.

5. The field emission lighting arrangement according to claim 1, wherein the field emission lighting arrangement further comprises: an electrically conductive structure arranged between the evacuated envelope and the base structure.

6. The field emission lighting arrangement according to claim 5, wherein the electrically conductive structure is arranged at an electrical potential V.sub.p with respect to an electrical potential of the cathode V.sub.c such that V.sub.pV.sub.c is positive, and based on an electrical potential of the anode structure V.sub.a such that (V.sub.pV.sub.c)/(V.sub.aV.sub.c) is in the range of 0 to 2.

7. The field emission lighting arrangement according to claim 1, wherein the cathode further comprises: an array of protruding base structures arranged on a substrate of the cathode, wherein the protruding base structures are arranged to have a center-to-center distance of 10 m to 100 m, and at least one nanostructure arranged on at least a portion of the protruding base structures.

8. The field emission lighting arrangement according to claim 7, wherein the nanostructure comprises at least one ZnO nanorod.

9. The field emission lighting arrangement according to claim 7, wherein the nanostructure comprises at least one carbon nanotube.

10. The field emission lighting arrangement according to claim 7, wherein the protruding base structure are shaped as square pyramids.

11. The field emission lighting arrangement according to claim 10, wherein the protruding base structure shaped as square pyramids having a base size of 10 m to 100 m.

12. The field emission arrangement according to claim 1, wherein the bulb shaped evacuated envelope is half-spherical, half-parabolic or half-ellipsoidal and has a cylindrical, conical or straight connection to the base structure.

13. The field emission lighting arrangement according to claim 7, wherein the base structures are provided with a plurality of nanostructures at least partly randomly arranged thereon.

14. A method for selecting a shape of a field emission cathode for use in a field emission lighting arrangement, the field emission lighting arrangement comprising: a bulb shaped evacuated envelope having an anode structure arranged along an inside of the evacuated envelope, the anode structure comprising a transparent electrically conducting layer and an electron to light conversion layer, and a base structure provided at a bottom end of the evacuated envelope, wherein the field emission cathode is arranged along the optical axis of the field emission lighting arrangement and in a lower part of the evacuated envelope towards the base structure, wherein the method comprises: determining a shape of the inside of the evacuated envelope covered by the anode structure; determining a spatial relation between the position at which the field emission cathode is arranged in the lower part of the evacuated envelope in correlation with the anode structure; selecting an essentially ellipsoidal form factor of the field emission cathode such that a distance between the field emission cathode and the anode structure at the inside of the evacuated envelope is largest along the optical axis and the shortest distance between a surface of the cathode and the anode decreases with an increasing central angle from the optical axis; arranging an array of protruding base structures on a substrate of the cathode, wherein the protruding base structures are arranged to have a center-to-center distance of 10 m to 100 m; and arranging at least one nanostructure on at least a portion of the protruding base structures.

15. The method according to claim 14, wherein the field emission lighting arrangement further comprises an electrically conductive structure arranged between the evacuated envelope and the base structure.

16. A field emission lighting arrangement, comprising: a bulb shaped evacuated envelope, comprising: a field emission cathode arranged along the optical axis of the field emission lighting arrangement, and an anode structure arranged along an inside of the evacuated envelope, the anode structure comprising a transparent electrically conducting layer and an electron to light conversion layer, a base structure provided at a bottom end of the evacuated envelope, the base structure comprising a power supply electrically integrated within the base structure and connected to the anode structure and the cathode, wherein the power supply is configured to apply a voltage such that electrons are emitted from the cathode to the anode structure, wherein the field emission cathode has a shape that is selected based on a predetermined shape of the evacuated envelope and is arranged in a lower part of the evacuated envelope towards the base structure, wherein a distance between the surface of the cathode and the anode structure is largest along the optical axis and the shortest distance between a surface of the cathode and the anode structure decreases with an increasing central angle from the optical axis; and the distance between the surface of the cathode and the anode structure varies between 0.5 and 40 mm.

17. The field emission lighting arrangement according to 16, wherein the selection of cathode shape provides an electrical field strength that differ less than 10% at all points of the cathode surface.

18. The field emission lighting arrangement according to claim 17, wherein the field emission lighting arrangement further comprises: an electrically conductive structure arranged between the evacuated envelope and the base structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 schematically illustrates a cross-section of the field emission lighting arrangement according to an embodiment of the invention;

(3) FIGS. 2a-2e illustrates examples of not applying as well as applying the inventive concept of an adequately shaped cathode, possibly in combination with an electrically conductive structure as discussed above, and

(4) FIG. 3 is a view of the field emission lighting arrangement according to a currently preferred embodiment of the invention.

DETAILED DESCRIPTION

(5) 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.

(6) In the present detailed description, an embodiment of a field emission lighting arrangement according to the present invention is mainly discussed with reference to a field emission lighting arrangement comprising a cathode with an essentially elliptical shape. It should be noted that this by no means limit the scope of the invention, which is also applicable in other circumstances, for example for use with otherwise shaped evacuated envelopes or cathodes.

(7) The invention will now be described with references to the enclosed drawings where first attention will be drawn to the structure, and secondly, functions of the field emission lighting arrangement will be described.

(8) In FIG. 1, the field emission lighting arrangement 118 is represented through a cross-section (i.e. side-view), where the evacuated envelope 100 and an anode structure 104 along an inside of the evacuated envelope 100 are shown. The anode structure 104 comprises a transparent electrically conducting layer and an electron to light conversion layer, such as a phosphor layer, e.g. using standard phosphors such as P22 (and/or e.g. quantum dots as mentioned above). Furthermore a field emission cathode 102 having a slightly elliptical form (as is discussed above as well as elaborated below) is arranged along the optical axis 116 of the field emission lighting arrangement 118, and is arranged in the lower end of the evacuated envelope 100 adjacently to a base structure 106 of the field emission lighting arrangement 118. It should be noted that the field emission cathode 102 in the illustrated embodiment, and preferably according to the present invention, has a circular form when seen from above (i.e. top-view, also visible from FIG. 3). The optical axis extends through a center point within the cathode.

(9) The base structure 106 comprises a power supply 108 which is electrically connected (not shown) to the transparent electrical conductive layer of the anode structure 104 and to the cathode 102. The power supply may preferably deliver a DC (direct current) voltage to the anode structure 104 and the cathode 102. Other alternatives are possible and within the scope of the invention. In the embodiment shown in FIG. 1, the field emission lighting arrangement 118 further comprises an electrically conductive structure 110 in the form of e.g. a conductive shield, foil or plate being electrically connected (not shown) to the power supply 108.

(10) A first arrow 112 shows the distance from the cathode 102 to the anode structure 104 along the optical axis 116, and a second arrow 114 shows the distance from a surface of the cathode 102 to the anode structure 104 along another axis also extending through the center point of the cathode. That is, the second arrow 114 is angled as compared to the optical axis 116. The distance along the first arrow 112 is larger than along the second arrow 114, this is due to the shape and position of the cathode 102. Furthermore the distance between the cathode 102 and the anode structure 104 decreases smooth and continuously as a function of the central angle from the optical axis 116 indicated by the second arrow 114. In FIG. 1, a typical pump stem 120 for the evacuated envelope 100 is additionally shown. As such, the shortest distance between the surface of the cathode and the anode structure increases with the angle between the first 112 and the second 114 arrow.

(11) In FIG. 2a, a graph of the electric field strength along a circumference of a cathode is shown; the electric field strength values (please note, absolute values are not of interest as they depend on the voltage applied) in FIG. 2a are calculated from spherical cathode geometry (i.e. a typical prior art field emission cathode). The arc length described starts at a 90 degree angle from the optical axis and ends at a +90 degree angle from the optical axis (as is indicated by the point-bolded line at the upper end surface of the cathode). It is apparent from FIG. 2a that the largest values of the electric field strength in the case of a spherical cathode are in the direction of the optical axis and that the perpendicular direction from the optical axis has lower electrical field strength, and more importantly that the variation is high. In use, a spherical cathode will then produce an increased emission of electrons towards the optical axis and less at the directions perpendicular to the same axis and will not provide a uniform distribution of the light emitted. The corresponding electron trajectories provided in relation to a prior art field emission lighting arrangement are seen in FIG. 2b.

(12) In FIG. 2c a graph of the electric field strength along a circumference comprising the optical axis of a cathode is shown, the electric field strength values in FIG. 2c are from an essentially ellipsoidal cathode, positioned in a more ideal manner below the centre of the half sphere part of the evacuated envelope (preferably between 0-5 mm below) according to the present invention, e.g. as shown in relation to the field emission lighting arrangement 118 of FIG. 1. The information provided through the illustration of FIG. 2c teaches that the electrical field strength along a circumference comprising the optical axis of a cathode according to the present invention will provide an (improved and) essentially uniform electrical field strength on the surface of the cathode as compared to the prior art illustration of FIGS. 2a and 2b. The resulting field strength will, in use, provide essentially uniform distribution of the electrons emitted towards the anode structure. The electrons impinging upon the anode structure (typically comprising the electron to light conversion layer, such as the phosphor layer), will produce light upon impact of the electron to light conversion layer through excitations of e.g. the phosphor material used for the conversion process, and thereby produce an essentially uniform spatial distribution of the light emitted from the field emission lighting arrangement. In a corresponding manner, the adjusted electron trajectories provided in line with the inventive concept are illustrated in relation to FIG. 2d.

(13) Introducing the novel cathode shape having an optimized shape and arranged at an optimized position the field uniformity may be greatly improved, as illustrated in FIG. 2c to be around +/5%. As can be seen from FIG. 2d, the corresponding electron trajectories are adjusted in a corresponding manner such that they now cover almost the half sphere of the evacuated envelope. When additionally introducing the electrically conductive structure 110 (e.g. using a potential of V=V(anode)) towards the lower end of the evacuated envelope, the electron trajectories are still further improved such that more than the half sphere will be covered by emitted electrons. This concept is further illustrated in FIG. 2e.

(14) Functional aspects from the features of the field emission lighting arrangement 118 will now be explained together with FIG. 3 which represents a currently preferred embodiment of the field emission lighting arrangement 118 illustrated in FIG. 1.

(15) In FIG. 3, the power supply 108 electrically connected to the cathode 102 and the anode structure 104, will supply a potential difference between the cathode 102 and the anode structure 104. Typical values of the potential difference are within the range of 4-12 kV, (the anode potential being more positive than the cathode potential) which will be adapted to the specific application and embodiment of the invention, smaller or larger potential differences might be preferred or other ranges are also within the scope of the invention. The potential difference will during operation of the field emission lighting arrangement 118 effect the emission of electrons from the cathode 102 towards the anode structure 104, the electrons impinging upon the anode structure 104, which comprises the above discussed transparent electrically conducting layer as well as the electron to light conversion layer, will first encounter the electron to light conversion layer and cause photons to be emitted from/by the electron to light conversion layer. The photons will travel through the transparent electrically conducting layer and will reach an observer, light a room or another area where light is desired.

(16) Furthermore the cathode 102 in FIG. 3 is shaped and positioned according to the present invention, it has an elliptical shape and position within the evacuated envelope selected based on the bulb shaped evacuated envelope 100 in such a way that the uniformity of the electric field strength is improved which will provide an uniform spatial distribution of the light emitted from the field emission lighting arrangement 118. That is, the process for determining the shape of the field emission cathode 102 typically include determining the shape of the inside of the evacuated envelope 100 covered by the anode structure 104, determining a spatial relation as shown with the arrows of FIG. 1 between the position at which the field emission cathode 102 is arranged in the lower part of the evacuated envelope 100 in correlation with the anode structure 104, and then selecting the shape of the field emission cathode 102 such that a distance between the field emission cathode 102 and the anode structure 104 both arranged at the inside of the evacuated envelope 100 is larger along the optical axis than along any other axis, whereby the distance between the field emission cathode 102 and the anode structure 104 decreases with an increasing central angle from the optical axis, thus resulting in the essentially elliptically shaped cathode as seen in all of FIGS. 1, 2b, 2d, 2e and 3.

(17) Moreover the electrically conductive structure 110 is shown in the currently preferred embodiment in FIG. 3, being connected to the power supply 108 and biased by a potential adapted to the specific application. The electrically conductive structure 110 is configured to protect the power supply from electrons emitted by the cathode 102; by biasing the electrically conductive structure 110 with a potential further protection of the power supply 108 will be achieved. Another purpose of biasing the electrically conductive structure 110 with a potential might be further increase of the electric field strength. In the currently preferred embodiment shown in FIG. 3, a connecting portion 120 of the base structure 106 is also included; the connecting portion is adapted to fit into a standard light bulb socket.

(18) Although the figures may show a specific order of method steps, the order of the steps may differ from what is depicted. Also two or more steps may be performed concurrently or with partial concurrence. Such variation may for example depend on system and design considerations. All such variations are within the scope of the disclosure. Additionally, 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.