EP0845154A1

EP0845154A1 - Fluorescent lamp

Info

Publication number: EP0845154A1
Application number: EP96931379A
Authority: EP
Inventors: Aris Kenneth Silzars; Robert William Springer
Original assignee: University of California; EI Du Pont de Nemours and Co
Current assignee: University of California; EIDP Inc
Priority date: 1995-08-14
Filing date: 1996-08-12
Publication date: 1998-06-03
Anticipated expiration: 2016-08-12
Also published as: AU7007596A; DE69605118T2; CN1199503A; EP0845154B1; JPH11510951A; DE69605118D1; WO1997007531A1; KR100397720B1; AU696412B2; KR19990028976A; CA2229067A1

Abstract

A lighting apparatus including a fibrous cold cathode field emitter wherein fibers of said cold cathode have a diameter of less than about 100 microns, an anode for attraction of electrons emitted by the fibrous cold cathode field emitter, a phosphor capable upon contact with emitted electrons from the cold cathode field emitter of generating a persistent light, and an evacuated enclosure of less than about 10-5 Torr containing within the enclosure the cold cathode field emitter, the anode and the phosphor is provided.

Description

TITLE FLUORESCENT LAMP FIELD OF INVENTION The present invention relates to lighting, and more particularly to lighting articles employing a suitable phosphor in combination with a cold cathode field emitter. This invention is the result of a contact with the Department of Energy (Contract No. W-7405-ENG-36).

BACKGROUND OF THE INVENTION Fluorescent lighting has been the standard illumination method in commercial buildings for many years. While it is used in a lessor degree in homes, it is generally applied where large areas need to be economically lighted. Although incandescent tungsten lighting is less efficient and more costly than fluorescent lighting, incandescent bulbs are the primary method of home lighting because of superior convenience and ascetics. Although fluorescent lighting is a highly efficient method of lighting, it does suffer from several deficiencies. Among these deficiencies are ecological concerns. Fluorescent light tubes are now classified as hazardous mateπals by the U.S. Environmental Protection Agency (EPA) as these tubes contain mercury, a highly toxic and regulated material. This problem has lead to developments such as in U.S. Patent Nos. 5,229,686 and 5,229,687 which describe the potential leaching problems ofthe mercury from such lights and the addition of a chemical agent to these lights for reaction with the mercury upon pulverization of the light. Further, the ballast resistor required in most, if not all, fluorescent lighting systems can contain polychlorinated biphenyl oils (PCB's) such materials being highly carcinogenic materials also regulated by the EPA.

Generally, the production of light in a fluorescent bulb takes several steps. First, liquid mercury within the tube is electronically heated to volatilize at least some of the mercury. Then, an electric current is passed through the mercury vapor to excite the mercury into a plasma state. The excited mercury plasma emits ultraviolet (UV) light. Finally, the UV light strikes a phosphor in the bulb with the phosphor converting the UV light energy into emitted visible light. This light production pathway has certain performance shortcomings. In comparison to conventional incandescent tungsten lighting systems, fluorescent lighting systems are slow to start as the mercury must first be heated to provide mercury vapor. Also, fluorescent lights are known to make acoustic noise due to the transformer and ballast register electronics needed to start and keep the current flowing through the mercury vapor. Oscillation in light output from fluorescent lighting can occur when the system is cold and first turned on thereby distracting some people. Further, fluorescent lighting systems are typically incompatible with conventional dimming technology used to adjust the light brightness output necessitating expensive dimmable fluorescent lighting using more exotic electronics. I. Brodie, U.S. 4,818,914, discloses a lamp comprising a cathode formed with an array of needle-like members projecting from one surface thereof, an accelerator electrode formed with an array of apertures there through, a layer of phosphor and an anode electrode. Voltages applied across the cathode and the accelerator electrode and across the cathode and the anode result in field emission from the cathode and collection of the electrons by the anode. Impingement of the electrons on the phosphor layer results in the emission of light. Dworsky et al., U.S. 5,180,951, discloses a uniform light source comprised of a substantially planar (flat) polycrystalline diamond film electron emitter. Y. Taniguichi et al., WO 94 28571, disclose a fluorescent tube light source comprising a layer of amorphic diamond film deposited over a conductive filament and an anode surrounding this filament and film which radiates light when struck by the emitted electrons. The amorphic diamond film is said to be comprised of a plurality of distributed localized electron emission sites, each sub-site having a plurality of sub¬ regions with differing electron affinities between sub-regions. It is an object of the present invention to provide a mercury-free fluorescent light employing a fibrous field emission element.

It is another object ofthe present invention to provide a fluorescent light having a substantially instant tum-on.

Still another object ofthe present invention is to provide a low voltage, low power backlight.

SUMMARY OF THE INVENTION To achieve the foregoing and other objects, and in accordance with the purposes ofthe present invention, as embodied and broadly described herein, the present invention provides a lighting apparatus including a fibrous cold cathode field emitter wherein fibers of said cold cathode have a diameter of less than about 100 microns, an anode for reaction of electrons emitted by the fibrous cold cathode field emitter, a phosphor capable upon contact with emitted electrons from the cold cathode field emitter of generating a persistent light, an evacuated enclosure of less than about IO"⁵ Torr containing within the enclosure the cold cathode field emitter, the anode and the phosphor. The persistent light preferably has a luminous intensity of at least 20 lumens per watt. BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 shows an elongated tube device in accordance with the lighting apparatus of present invention.

FIGURE 2 shows an exploded view of a flat plate device in accordance with the lighting apparatus of present invention.

FIGURE 3 shows a bulb device in accordance with the lighting apparatus of present invention.

FIGURE 4 shows a light beam producing device in accordance with the lighting apparatus of present invention. FIGURE 5 shows a test device in accordance with the lighting apparatus of present invention.

FIGURE 6 shows driver circuits for the lighting device of the present invention.

FIGURE 7 shows a bulb device for use in standard light bulb sockets. DETAILED DESCRIPTION

The present invention relates to a field emission lighting apparatus and to a fiber field emission lighting (FFEL) apparatus.

The lighting apparatus ofthe present invention uses field emission to generate light output from a phosphor, e.g., a cathodoluminescent material. The field emission lighting apparatus involves a fibrous cold cathode field emitter. The field emission electron emitting material can be any material that can be provided in the form of a fiber. Preferably the field emission electron emitting material ofthe fibrous cathode is diamond, diamond-like carbon or glassy carbon. Diamond is especially preferred. Preferably the fibrous cathode is formed of one or more diamond, diamond-like carbon or glassy carbon composite fibers consisting essentially of diamond, diamond-like carbon or glassy carbon on non-diamond core fibers. The non-diamond core can be made of a conductive or semi-conductive material. Alternatively, the core can be made of a non-conductive material surrounded by a film coating of a conductive or semi-conductive material. The core material in the diamond fiber can be, e.g., a conductive carbon such as graphite or a metal such as tungsten, or can be, e.g., silicon, copper, molybdenum, titanium or silicon carbide. In another embodiment, the core may consist of a more complex structure, for example, a non-conductive material surrounded by a thin coating of conductive or semi-conductive material. A diamond, diamond-like or glassy carbon layer is then coated on the sheath. As examples, the non-conductive core can be a synthetic fiber such as nylon, KEVLAR® (KEVLAR® is a registered trademark of E. I. du Pont de Nemours and Company, Wilmington, DE), or polyester or inorganic materials such as ceramics or glass. In other embodiments, a diamond, diamond-like carbon or glassy carbon precursor can be coated onto the non-diamond core or the core can be a diamond, diamond-like carbon or glassy carbon precursor and the diamond, diamond-like carbon or glassy carbon is then formed by appropriate treatment ofthe precursor. In particular, the field emission lighting apparatus involves a fibrous cold cathode field emitter which can be of the type described by Valone, in U.S. patent application serial number 08/196,343, filed February 14, 1994, entitled "Diamond- Graphite Field Emitters" or by Blanchet-Fincher et al., in U.S. patent application serial number 08/387,539, filed February 13, 1995, entitled "Diamond Fiber Field Emitters" which is a continuation-in-part of Valone et al., U.S. patent application serial number 08/196,340, filed February 14, 1994, entitled "Diamond Fiber Field Emitters", such descriptions incorporated herein by reference. Further, the cold cathode field emitter can be any other suitable emitting fibrous material such as a suitable graphite fiber treated by exposure to intense ion beam treatment or a suitable graphite fiber treated by exposure to a laser as described by Friedmann, in U.S. provisional patent application number 60/002,277, entitled "Method for Creation of Controlled Field Emission Sites" filed August 14, 1995, or a diamond-coated or diamond-like-coated nickel-coated KEVLAR® fiber as described in U S. patent application number 08/387,539, filed February 13, 1995, or fibers containing glassy carbon, an amorphous material exhibiting Raman peaks at about 1380 cm^-1 and 1598 cm"¹. "Diamond-like carbon" is used herein to designate the material referred to in the literature as diamond-like carbon as well as glassy carbon and carbon containing microscopic inclusions of glassy carbon, all of which are diamond-like in their performance as field emission materials. Additionally, the fibrous cold cathode may generally be of an conductive material having an activated surface, i.e., capable of allowing electrons to be drawn off at a relatively low bias voltage, with suitable dimensions, i.e., diameters of generally less than about 100 microns, preferably less than about 15 microns, and more preferably less than about 10 microns. Among the suitable materials may be included thin fibers of magnesium oxide and the like, suitably with an activated surface based on treatment ofthe fibers by, e.g., flash heating.

Generally, the fibers of the fibrous cathode each have diameter of less than about 100 microns, preferably less than about 15 microns, and more preferably less than about 10 microns. Smaller diameter fibers reduce the voltage necessary to generate the field emission. Preferably, the diameter exceeds 1 micron. Generally, diameters ofthe fibers ofthe cold cathodes are substantially smaller in dimensions than the metallic filaments commonly used in presendy available lighting apparatus. While a single fiber can be used as the fibrous cathode, it is generally preferred to use more than a single fiber as the fibrous cathode to provide redundancy in electron emission.

The phosphor used in the lighting apparatus of the present invention can generally be of any type suitable to generate visible light upon being struck by electron emission. For example, the phosphor can be zinc oxide:zinc, zinc sulfide, cadmium sulfide, zinc cadmium sulfide, zinc selenide, zinc cadmium selenide yittrium silicate: cesium, zinc phosphatermaganese, or other well known materials which emit light following suitable excitation. Blends or combinations of phosphors may also be employed. The phosphor used in the present invention is further capable of producing a persistent light, i.e., the light from a particular point of the phosphor does not readily fade wid time as it is excited. Additionally, the output from the excited phosphor may be capable of generating this persistent visible light with a luminous intensity of at least 20 lumens per watt. The lighting apparatus of the present invention includes a fibrous cold cathode field emitter, a phosphor, an anode for attraction of emitted electrons from the cold cathode field emitter, all generally contained within an evacuated enclosure. Unlike a standard mercury vapor fluorescent light, the lighting apparatus ofthe present invention can turn on instantly without the need for heating the mercury to form a plasma. Without the need for rnaintaining a plasma, the light generated by the lighting apparatus of the present invention can be readily and easily dimmed or brightened by adjustment ofthe voltage applied to the fiber. When more voltage is applied, more electrons are emitted and impinge upon the phosphor material resulting in additional light output, i.e., a brighter light. When less voltage is applied, but above the minimum turn-on voltage for the fibrous field emitter, less electrons are emitted and impinge upon the phosphor material resulting in a reduction in light output, i.e., a dimmer light. No starting circuits are necessary for the lighting apparatus ofthe present invention, only a rectifying voltage step-up circuit and a simple current limiting circuit. Such a current limiting circuit can consist, e.g., of a small resistor/inductor in series with the fibrous cold cathode.

The evacuated enclosure typically maintains a low pressure of least as low as about IO"⁵ Torr. Such an evacuated enclosure can be, e.g., a glass bulb or multiple glass sheets with appropriate spacer material therebetween. The lighting apparatus of the present invention uses electron emission induced by a directed, shapable applied field. This is in contrast with lighting using a plasma which results in a non-directed light source. This allows practical, but varied, shape configurations ofthe present lighting apparatus without limitations to the traditional elongated tube. No matter what shape the lighting apparatus has, electron emission from the fibers ofthe fibrous cathode occurs along the length of the fibers utilized and not from the fiber tip or end.

In one embodiment of the present invention, shown in Fig. 1, a light IQ included a glass tube 12 as an evacuable enclosure. The inner or interior surface of glass tube 12 can be coated with a transparent conductor as an anode and a phosphor or cathodoluminescent material 12. A fibrous cathode element 14, the fibrous field emitter, is situated widiin glass tube 12. An end cap 1£ includes electrodes connected to fibrous cathode element 14 and to the transparent conductor coating or anode. The fibrous cathode element can consist of a single fiber, can include a multiple of fibers or can include a thicker single fiber. Generally, the single fiber or individual fibers making up the multiple fibers can have a diameter of from about 1 micron to about 20 microns, preferably about 5 microns to about 10 microns. The glass tube can be a circular cylinder as shown or it can have a configured surface.

In another embodiment of the present invention, shown in Fig. 2, light 20 has a flat plate design with a flat plate 22 having a fibrous cathode element array 24 thereon. A transparent second flat plate 26. includes a coating of transparent conductor 2& upon the surface of flat plate 26 facing cathode element array 24 and a coating of a phosphor or cathodoluminescent material 20. upon transparent conductor coating 2&. A spacer plate 2£ separates flat plate 22 and flat plate 2ή and provides an evacuable enclosure for fibrous cathode element array 24. transparent conductor coating 2& and phosphor or cathodoluminescent material 30. Conductor electrodes are connected to transparent conductor coating 28. and fibrous cathode element array 24.

In another embodiment of die present invention, shown in Fig. 3, a bulb- shaped light 4Q includes a glass globe 42 having an interior coating of a transparent conductor 44 and a coating of a phosphor or camodoluminescent material 4£. A fibrous cathode element 4&, the fibrous field emitter, is situated within glass globe 42. Conductors are connected to fibrous cathode element 4S and to the transparent conductor coating or anode 44- The fibrous cathode element 4£ can consist of a single fiber or can include a multiple of fibers. Generally, the single fiber or individual fibers making up the multiple fibers can have a diameter offrom about 1 micron to about 20 microns, preferably about 5 microns to about 10 microns.

In another embodiment of the present invention, shown in Fig. 4(a) and Fig. 4(b), a light 50 capable of producing a light beam output 52 includes glass hemispherical support £4 having coatings upon the concave inner surface of a reflector material 5ή, a transparent conductive material 58_., and a phosphor or cathodoluminescent material 6Ω- A fibrous cathode element £2- the fibrous field emitter, is situated within glass hemisphere 54. Conductors are connected to fibrous cad ode element 62 and to the transparent conductor coating or anode 53.- Varying the shape of the glass support 54 can result in a more concentrated light beam. For example, a parabolic support would accomplish this result.

Figure 5 shows a simple test device used to test field emission variables of the fibrous cad odes including a clear plastic, e.g., Lucite® plastic, tube 2Ω capped with end caps 22 and 24 to form an evacuable enclosure. End cap 22 includes an opening 26 connected to a vacuum pump. Suspended within the evacuable enclosure is a grounded copper screen mesh 28 coated with a phosphor or cathodoluminescent material £0_. A fibrous cathode element £2 is situated within grounded copper screen mesh 2S- Conductors are connected to fibrous cathode element £2 and to copper screen mesh or anode 2S- This test device can prove useful for determining emission uniformity of emissive fibers.

Figure 6 shows a current limiting circuit for use with the lighting apparatus of the present invention. The current limiting circuit 9_Ω includes resistor 22 and an inductor 24 in series with the emissive fiber or fibrous cathode element 26. Power source 9_£ is connected through a rectifying voltage step-up circuit IQO to the anode 102 and the current limiting circuit 9_Ω in series with cathode element 26_*

In another embodiment of me present invention, shown in Fig. 7, light 120 is in the form of a standard light bulb with a screw type base. The inner surface of the glass bulb 121 is coated with a transparent conducting oxide 122 and a phosphor or cathodoluminescent material 123. A fibrous cathode field emitter comprised of a field emission electron emitting fiber 124 is in the central region of the bulb. The fiber emitter is shown in a triangular configuration but could be in other configurations, e.g., a circle or a figure having four or more sides. The fiber emitter is supported by a non-emitting current carrier 125. Emitted electrons are shown by the arrows 126. The screw type base 127 is essentially the same as used for standard incandescent bulbs.

In operation of the lighting apparatus of the present invention, power density of about 1.5 watts per inch from the cathode can generally be necessary to generate sufficient electron emission. Generally, if the bias voltage on the fibrous cathode is at least 1500 Volts, then the emission current per inch must be at least about 1 milliamperes.

The present invention is more particularly described in the following example which is intended as illustrative only. EXAMPLE 1 A lighting apparatus was assembled essentially as shown in Figure 5 using a carbon fiber that was exposed to a single intense ion beam treatment. The carbon fiber was prepared as follows. Untreated graphite fibers (commercially available IM7 graphite fibers from

Hercules, Inc., Wilmington, DE) from a graphite yam were spread out across a frame and the frame placed in me padi of an intense ion beam operated in accordance with the teachings of Rej et al., Rev. Sci. Instrum. 64(10), pp. 2753-2760, Oct. 1993. The voltage was about 300 kilovolts. The distance of the frame from me focus of the ion beam was varied between about six inches to 18 inches. The energy density ofthe ion beam was estimated at from about 2 joules per square centimeter to about 10 joules per square centimeter. The time of a pulse ofthe ion beam was about one microsecond. After a single pulse, the frame was tumed over (180°) and the reverse side of the fibers was exposed to a single pulse of the intense ion beam. The resultant fibers were tested and shown to be excellent field emission electron emitters.

A fiber was then attached to conductor "A" shown in Figure 5. A zinc oxide :zinc phosphor was coated onto the copper mesh screen. A potential difference of about 3.5 keV was applied to the cathode and anode, i.e., to conductors "A" and "B". A current of 2-3 mA was obtained over a one inch

(2.5 cm) length of fiber together with a persistent light emission. About 10 watts per inch (2.5 cm) was obtained for lighting purposes.

Aldiough the present invention has been described with reference to specific details, it is not intended diat such details should be regarded as limitations upon the scope of the invention, except as and to die extent mat ey are included in die accompanying claims.

Claims

WHAT IS CLAIMED IS:

1. A lighting apparatus comprising: a fibrous cold cathode field emitter including one or more fibers, said one or more fibers of said fibrous cold cathode having a diameter of less than about 100 microns; an anode for attraction of electrons emitted by the fibrous cold cathode field emitter; a phosphor capable upon contact wim emitted electrons from the fibrous cold camode field emitter of generating persistent light; an evacuated enclosure of less than about 10"⁵ Torr containing within the enclosure, the fibrous cold camode field emitter, d e anode and die phosphor.

2. The lighting apparatus of Claim 1 wherein said one or more fibers have diameters offrom about 1 micron to about 15 microns.

3. The lighting apparatus of Claim 1 wherein said fibrous cold cad ode field emitter includes more than one fiber.

4. The lighting apparatus of Qaim 3 wherein said fibers have diameters of from about 1 micron to about 15 microns.

5. The lighting apparatus of Qaim 1 wherein said evacuated enclosure is of a globe shape.

6. The lighting apparatus of Qaim 1 wherein said evacuated enclosure is of a hemispherical shape.

7. The lighting apparatus of Qaim 3 wherein said evacuated enclosure is of a globe shape.

8. The lighting apparatus of Qaim 3 wherein said evacuated enclosure is of a hemispherical shape.

9. The lighting apparatus of Qaim 1 wherein said evacuated enclosure is of a flat plate shape.

10. The lighting apparatus of Qaim 3 wherein said evacuated enclosure is of a flat plate shape.

11. The lighting apparatus of Qaim 1 wherein said evacuated enclosure is of a cylindrical shape.

12. The lighting apparatus of Claim 3 wherein said evacuated enclosure is of a cylindrical shape.

13. The lighting apparatus of Claim 1 wherein said apparatus is further characterized as mercury-free.