Patent Grant
10043649
The present invention relates to a field emission lighting arrangement. The present invention further relates to a method for selecting a shape of a field emission cathode for use in such a field emission lighting arrangement.
Traditional incandescent light bulbs are currently being replaced by other light sources having higher energy efficiency and less environmental impact. Alternative light sources include light emitting diode (LED) devices and fluorescent light sources. However, LED devices are relative expensive and complicated to fabricate and fluorescent light sources are known to contain mercury, thereby posing potential health problems due to the health risks involved in mercury exposure. Furthermore, as a result of the mercury content, recycling of fluorescent light sources is both complicated and costly.
An attractive alternative light source has emerged in the form of field emission lighting. A traditional field emission lighting arrangement comprises an anode structure and a field emission cathode, the anode structure consists of a transparent electrically conductive layer and a light conversion layer, such as a layer of phosphor coated on the inner surface of an evacuated envelope, provided in the form of e.g. a transparent glass tube. The phosphor layer emits light when excited by the electrons emitted from the cathode.
Previously known field emission lighting arrangements are often in the shape of tubes and seldom in the form of the traditional bulb. Hence there is a need to provide field emission lighting arrangements with a form factor suitable for retrofitting of e.g. traditional incandescent bulbs as well as corresponding compact fluorescent light sources.
With regards to the above-mentioned desired properties of field emission lighting arrangements, it is a general object of the present invention to enable improved performance of a field emission lighting arrangement for example by improved distribution of the light emitted.
The present invention is based upon the realization that an alternative shape and position of the cathode within the evacuated envelope may provide a more uniform electric field on the outer surface of the cathode, most importantly on the upper half of the cathode surface, which will in turn provide a more uniform distribution of electrons impinging upon an electron to light conversion layer used for converting electron energy into e.g. visible light. Accordingly, this selection and/or adaptation of shape and position may enable a uniform spatial distribution of the light emitted from the field emission lighting arrangement.
According to a first aspect of the invention these and other objects are achieved by 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 a shape that is selected based on the shape of the evacuated envelope and is arranged in a lower part of the evacuated envelope towards the base structure, such that a distance between the cathode and the anode structure is larger along the optical axis than along any other axis, whereby the distance between the cathode and the anode structure decreases with an increasing central angle from the optical axis, thereby improving the uniformity of the electric field.
In the context of the present invention the optical axis is defined as an axis around which there is rotational symmetry of the light output from an optical system, i.e. according to the invention being the inventive field emission lighting arrangement. An effect of selecting the shape and position of the field emission cathode based upon a (pre)determined shape of the evacuated envelope is the possibility of providing improvements in relation to the uniformity of light emitted by the field emission lighting arrangement. The form factor (i.e. shape) of the evacuated envelope is typically dictated by design considerations, possibly relating to the form factor used for retrofit lighting arrangements, e.g. retrofit light bulbs.
As such, the predetermined shape and the adaptation of the cathode structure results in a distance between the surface of the cathode and the anode structure being 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.
A commercially viable light source must preferably have a relatively long life time. In the technical area of field emission, the lifetime of the field emission lighting arrangement is at least partly determined by the degradation of the electron to light conversion layer, being for example a lighting powder (e.g. a so called phosphor layer), specifically due to the accumulated charge per unit area, i.e. impinging current density over time. It is therefore desirable to use an anode structure where the area covered by the electron to light converting layer being as large as possible. In addition, such a commercially viable light source typically comprises the necessary driving electronics provided in the same “unit”, possibly in the base of the lighting arrangement. Still further, as mentioned above, the lighting arrangement preferably has a form factor similar to light sources already commercially available today, typically light bulbs. Accordingly, as the form factor of the inventive field emission lighting arrangement preferably is similar to available light bulbs used today and at the same time the area of the anode structure should be maximized, the resulting evacuated envelope will typically be formed as a half sphere possibly with a cylindrical extension in a lower end towards the base structure in order to facilitate space for e.g. a so called pump stem (the bottom of the evacuated envelope) used for evacuating the envelope before its operation and usually supplying the electrical connection feed through to the anode and the cathode. Following the above discussion, the most natural position of the cathode is typically at the center of the sphere. As the design of the inventive lighting arrangement in this embodiment thus will differ from a full sphere, the result is that the electrical field on the cathode will be non-uniform.
The current follows the Fowler-Nordheim equation:
where
Ar is the effective emitter area, a is the first Fowler-Nordheim constant;
b is the second Fowler-Nordheim constant
Ø is the work function in eV and β is a dimensionless enhancement factor Accordingly, changes in electrical field will results in changes in current.
To achieve field emission at reasonable applied voltages (typically below 10 kV) specially designed structures may in one embodiment be used in order to locally enhance the field strength. A common rule of thumb is that 1 GV/m is needed for field emission to be achieved. In the present invention this may for example in one embodiment be achieved in one or preferably two steps. The macroscopic field, as defined here, is provided by the basic macroscopic geometry and the applied voltage. In this invention it is generally defined as the electrical field of a spherical symmetry, (albeit for a full spherical symmetry) given by:
where V is the applied voltage, R is the radius of the outer sphere (the anode) and r is the radius of the inner sphere (the cathode). V is generally in the range of 1-20 kV and preferably in the range of 1-10 kV and r and R for example are determined by the desired form factor of the evacuated envelope (as discussed above). For the sake of briefness, the above macroscopic electrical field is further now simply referred to as the “field” or the “electrical field”.
In order to reach sufficient field strength to achieve field emission the macroscopic field is preferably, in one embodiment of the present invention, amplified by adding geometries down to the nanometer level. The first (and in some cases optional) step is to enhance the macroscopic electrical field locally on the cathode surface by adding microscopic protrusions to the cathode spherical surface. The second step is to use nanostructures. Both are described briefly further below.
As light output in general can be regarded as proportional to the current (within certain limits), it is vital to keep the macroscopic electrical field uniform on the cathode surface, if a uniform light output is desired. A non-uniform electrical field strength at the cathode surface may typically result in a non-uniform emission of electrons, which will result in a non-uniform irradiation of the electron to light conversion layer and in turn a non uniform light output. In order to achieve this in a practical manner the macroscopic field should preferably be as uniform as possible over the relevant area of the cathodes surface, generally approximately the upper half of the cathode. Alternatively, it may according to the invention be possible to carefully control the distribution of the size of the microscopic protrusions over the relevant part of the cathode surface.
As discussed above, the electron to light conversion layer may for example comprise a phosphor layer configured to convert energy from impinging electrons to light. Alternatively, it may also be possible to introduce or instead use quantum dots for converting energy from impinging electrons into light.
According to one embodiment of the invention, the distance between the cathode and the anode structure varies between 0.1 and 100 mm, preferably between 0.2 and 70 mm and most preferably between 0.5 mm and 40 mm. Furthermore, a field emission lighting arrangement in this size may for example be comparable to a standard A19 light bulb, which may make it suitable for many lighting fixtures in use today. Other types of predetermined shapes are of course possible and within the scope of the invention.
According to another embodiment of the invention, the cathode is shaped essentially ellipsoidal, with 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 (a) and the other two semi axes (b) is such that the ratio b/a lies in the range between 1.05 and 2. Making the cathode into a flattened spherical shape may provide a uniform electrical field strength within the evacuated envelope due to the essentially ellipsoidal shape, i.e. well in line with the above discussion.
In one embodiment of the invention, the selection of cathode shape provides an electrical field strength that differ less than 50%, more preferably less than 20% and most preferably less than 10% at all relevant points of the cathode surface. The selection of cathode shape typically provides electron trajectories resulting in a uniform electric current density in the anode structure.
According to yet another embodiment of the invention the field emission lighting arrangement may further comprise an electrically conductive structure arranged between the evacuated envelope and the base structure (i.e. typically outside of the evacuated envelope). The electrically conductive structure is according to the invention preferably arranged at an electrical potential Vp with respect to an electrical potential of the cathode Vc such that Vp−Vc is positive, and based on an electrical potential of the anode structure Va such that (Vp−Vc)/(Va−Vc) is in the range of 0 to 2, thereby further adjusting the electron trajectories to be received by a lower area of the anode structure, i.e. being closer to the base structure, in order to further improve the area of the anode structure receiving electrons from the cathode. Such an electrically conductive structure may in addition protect the power supply from electrons impinging towards the base structure, and also protect the cathode and the evacuated envelope from disturbing and varying electromagnetic fields originating from the power supply. Furthermore such an electrically conductive structure may be made so that its upper surface reflect light which has been emitted inwards instead of outwards from the anode structure and further enhance the total light emitted from the field emission lighting arrangement. As an alternative, the electrically conductive structure may be arranged on the inner surface of the bottom part of the evacuated envelope.
According to another embodiment of the invention the cathode may further comprise, an array of protruding base structures arranged on a substrate, wherein the protruding base structures are arranged to have a center-to-center distance of 10 μm to 100 μm, more preferably 10 μm to 60 μm, and most preferably 10 μm to 40 μm and a height of 5 to 60 μm and at least one nanostructure arranged on each of the protruding base structures.
A protruding base structure may be advantageous regarding the voltage that needs to be applied over the cathode in order to achieve field emission from the nanostructure arranged on the base structure as described above. For a surface without protruding base structures, a higher voltage is required to achieve field emission in contrast to the presented structure where the voltage is concentrated to the protruding base structures thereby resulting in a higher electric field at the position of the nanostructures acting as field emitters.
In the present context, the term nanostructure refers to a structure where at least one dimension is on the order of up to a few hundreds of nanometers. Such nanostructures may for example include nanotubes, nanorods, nanowires, nanopencils, nanospikes, nanoflowers, nanobelts, nanoneedles, nanodisks, nanowalls, nanofibres and nanospheres. Furthermore, the nanostructures may also be formed by bundles of any of the aforementioned structures. The preferred direction of the nanostructures is in a direction essentially perpendicular to the cathode surface. According to one embodiment of the invention the nanostructures may comprise ZnO nanorods.
According to an alternative embodiment of the invention the nanostructure may include carbon nanotubes. Carbon nanotubes may be suitable as field emitter nanostructures in part due to their elongated shape which may concentrate and produce a higher electric field at their tips and also due to their electrical properties.
In one embodiment of the invention the protruding base structures are shaped as square pyramids. Preferably the protruding base structures are shaped as square pyramids which may provide a sharp well defined tip which may further concentrate the electrical field, and may provide a higher electrical field for the nanostructures as field emitters. Other types of protruding base structures, such as cylinders, square protrusions, any irregular protruding geometry or the like, are of course possible and within the scope of the invention. According to one embodiment of the invention the protruding base structure shaped as square pyramids having a base size of 20 μm to 40 μm.
According to another embodiment of the invention the bulb shaped evacuated envelope has a form as half-spherical, half-parabolic or half-ellipsoidal and comprising a cylindrical, conical or straight connection to the base structure. The connection to the base structure may provide the ability to position the cathode along the optical axis at different points within the evacuated envelope, advantageously this may allow for a uniform electric field when the cathode shape is limited.
Furthermore, this feature may also provide the ability to use the field emission lighting arrangement as a retrofit into standard incandescent light bulb sockets e.g. an Edison screw base. In addition, a field emission lighting arrangement in this size may be comparable to a standard A19 light bulb, which may make it suitable for many lighting fixtures in use today.
According to another aspect of the invention there is provided 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, and selecting the shape 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 larger along the optical axis than along any other axis, whereby the distance between the field emission cathode and the anode structure decreases with an increasing central angle from the optical axis. This aspect provides similar advantages as in relation to the previous aspect of the invention.
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.
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:
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.
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.
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.
In
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
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
In
In
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
Functional aspects from the features of the field emission lighting arrangement 118 will now be explained together with
In
Furthermore the cathode 102 in
Moreover the electrically conductive structure 110 is shown in the currently preferred embodiment in
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13160768 | Mar 2013 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/055124 | 3/14/2014 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2014/154505 | 10/2/2014 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4818914 | Brodie | Apr 1989 | A |
| 6873095 | Kjellman | Mar 2005 | B1 |
| 20020070648 | Forsberg | Jun 2002 | A1 |
| 20060091782 | Liu | May 2006 | A1 |
| 20120176024 | Yang | Jul 2012 | A1 |
| Number | Date | Country |
|---|---|---|
| 0247104 | Jun 2002 | WO |
| 2005074006 | Aug 2005 | WO |
| Entry |
|---|
| Ausra Kaveckaite, International Search Report of parent PCT Application No. PCT/IB2014/055124, dated Apr. 14, 2014, European Patent Office, Rijswijk Netherlands. |
| Number | Date | Country | |
|---|---|---|---|
| 20160020084 A1 | Jan 2016 | US |
