Patent Publication Number: US-7915799-B2

Title: Field emission lamp having carbon nanotubes

Description:
RELATED APPLICATIONS 
     This application is related to commonly-assigned applications entitled, “FIELD EMISSION PLANE LIGHT SOURCE AND METHOD FOR MAKING THE SAME”, application Ser. No. 11/603,639 and filed on Nov. 21, 2006, “FIELD EMISSION LAMP AND METHOD FOR MAKING THE SAME”, application Ser. No. 11/603,640 and filed on Nov. 21, 2006, “FIELD EMISSION DOUBLE-PLANE LIGHT SOURCE AND METHOD FOR MAKING THE SAME”, application Ser. No. 11/603,627 and filed on Nov. 21, 2006, and “FIELD EMISSION ELECTRON SOURCE AND METHOD FOR MAKING THE SAME”, application Ser. No. 11/603,672 and filed on Nov. 21, 2006, the contents of each of which are hereby incorporated by reference thereto. 
     BACKGROUND 
     1. Technical Field 
     The invention relates generally to cold cathode luminescent field emission devices and, particularly, to a field emission lamp employing a getter to exhaust unwanted gas from therein, thereby ensuring a high degree of vacuum. The invention also relates to a method for making a field emission lamp. 
     2. Discussion of Related Art 
     Electrical lamps for daily living are usually incandescent lamps and/or fluorescent lamps. Ever since Thomas Edison invented the first viable incandescent lamps in 1879, the incandescent lamps have a long history for simple fabrication thereof. However, because an incandescent lamp emits light by incandescence of a tungsten filament, most of electric energy used therein is converted into heat and thereby is wasted. Therefore, a main drawback of the incandescent lamp is the low energy efficiency thereof. 
     A typical conventional fluorescent lamp generally includes a transparent glass bulb. The transparent glass bulb has a white or colored fluorescent material coated on an inner surface thereof and a certain amount of mercury vapor filled therein. In use, electrons are accelerated by an electric field, and the accelerated electrons collide with the mercury vapor. This collision causes excitation of the mercury vapor and causes radiation of ultraviolet rays. The ultraviolet rays irradiate the fluorescent material, whereby the ultraviolet rays are converted into visible light. Compared with the incandescent lamps, the fluorescent lamps have higher electrical energy utilization ratios. However, when the glass bulb is broken, the mercury vapor is prone to leak out and, thus, is poisonous and noxious to humans and is environmentally unsafe. 
     To settle the above problems, a kind of fluorescent lamps (i.e., field emission lamps) not adopting the mercury vapor has been developed. A conventional field emission lamp without the mercury vapor generally includes a cathode and an anode. The cathode has a number of nanotubes formed on a surface thereof, and the anode has a fluorescent layer facing the nanotube layer of the cathode. In use, a strong field is provided to excite the nanotubes. A certain amount of electrons is then accelerated and emitted from the nanotubes, and such electrodes collide with the fluorescent layer of the anode, thereby producing visible light. 
     For a field emission lamp, a high degree of vacuum in an inner portion (i.e., interior) thereof is a virtual necessity. In general, the better of the degree of vacuum of the field emission lamp is able to maintain during the sealing process and thereafter during use, the better of the field emission performance thereof is. To maintain the degree of vacuum of the field emission lamp within a desired range, a conventional way is to provide a getter in the inner portion thereof. Such a getter is able to exhaust a gas produced by the fluorescent layer and/or any other residual gas remaining within the field emission lamp upon sealing and evacuation thereof. The getter is generally selected from a group consisting of non-evaporable getters and evaporable getters. 
     For the evaporable getter, a high temperature evaporating process has to be provided during the fabrication of the field emission lamp, and a plane arranged in the inner portion of the field emission lamp has to be provided to receive the evaporated getter. Thus, the cost of the fabrication of the field emission lamp increases, and the cathode and anode are prone to shorting during the high temperature evaporating process, thereby causing the failure of the field emission lamp. For the non-evaporable getter, it is generally focused in a fixing head of the field emission lamp, which is typically located at a position away from the cathode, and, thus, the degree of vacuum of portions near to the cathode tends to be poorer, in the short-term, than that of portions near to the fixing head, at least until internal equilibrium can be reached, thereby decreasing the field emission performance of the cathode or at least potentially resulting in a fluctuating performance thereof. 
     What is needed, therefore, is a field emission lamp that overcomes the above-mentioned shortcomings to ensure a high degree of vacuum thereof, thus providing a better and more steady field emission performance during the use thereof. 
     What is also needed is a method for making such a field emission lamp. 
     SUMMARY 
     A field emission lamp includes a transparent bulb with an open end, a lamp head, an anode and a cathode. The lamp head is disposed on the open end of the bulb. An anode includes an anode conductive layer, a fluorescent layer, and an anode electrode. The anode conductive layer is formed on an inner surface of the bulb. The fluorescent layer is formed on a portion of a surface of the anode conductive layer, leaving an exposed portion on the anode conductive layer. The anode electrode is disposed on the open end of the bulb and electrically connecting the anode conductive layer with the lamp head. The cathode includes a cathode electrode and an electron emission element. An end of the cathode electrode is disposed on the open end of the bulb, insulated with the anode electrode, and electrically connected with the lamp head. The electron emission element is disposed on an opposite end of the cathode electrode and having an electron emission layer. The electron emission layer includes a glass matrix and a plurality of carbon nanotubes, getter powders, and metallic conductive particles dispersed therein. 
     A method for making a field emission lamp generally includes the steps of:
         (a) providing a transparent glass bulb with an open end and a bulb interior; an anode electrode; a cathode electrode; a metallic base body; a lamp head; and a certain number of carbon nanotubes, metallic conductive particles, glass particles, and getter powders (i.e., in particulate or granular form), the bulb having an anode conductive layer on an inner surface thereof and a fluorescent layer on an inner surface of the anode conductive layer, the fluorescent layer facing the bulb interior;   (b) mixing the nanotubes, the metallic conductive particles, the glass particles, and the getter powders in an organic medium to form an admixture;   (c) forming a layer of the admixture on a surface of the base body;   (d) drying and baking the admixture at a temperature of about 300° C. to about 600° C. to form an electron emission layer on the base body, thereby obtaining an electron emission element; and   (e) assembling the bulb, the anode electrode, the cathode electrode, and the electron emission element; and   (f) sealing the open end of the bulb at a temperature of about 400° C. to about 500° C. in order to secure the anode electrode and the cathode electrode and evacuating the bulb interior, assembling the lamp head and electrically connecting the lamp head with the anode electrode and the cathode electrode, respectively, thereby yielding the field emission lamp.       

     Other advantages and novel features of the present field emission lamp and the relating method thereof will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present field emission lamp and the relating method thereof can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present field emission lamp and the relating method thereof. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a cross-section view of a field emission lamp, in accordance with an exemplary embodiment of the present device; 
         FIG. 2  is an enlarged view of a circled portion II of  FIG. 1 ; 
         FIG. 3  is a cross-section view along a line III-III of  FIG. 1 ; and; 
         FIG. 4  is an enlarged view of a circled portion IV of  FIG. 3 . 
     
    
    
     The exemplifications set out herein illustrate at least one preferred embodiment of the present field emission lamp and the relating method thereof, in one form, and such exemplifications are not to be construed as limiting the scope of such a field emission lamp and a method for making such in any manner. 
     DETAILED DESCRIPTION 
     Reference will now be made to the drawings to describe, in detail, the field emission lamp and the method for making the same, according to the present embodiment. 
     Referring to  FIG. 1 , a field emission lamp  10 , in accordance with an exemplary embodiment of the present device, is provided. The field emission lamp  10  includes a transparent glass bulb  20 , an anode  30 , a lamp head  40 , and a cathode  50 . 
     The glass bulb  20  includes a main portion  22  and a neck portion  24  extending from the main portion  22 , the neck portion  24  having an open end  26 . The main portion  22  is generally shaped as a ball/spherical shape, an ellipsoid shape, or another chosen shape that helps produce a desired distribution of light from the glass bulb  20 . A ball shaped main portion  22  is shown in  FIG. 1 . The open end  26  of the neck portion  24  is sealed by an end piece  28 , thereby forming a closed-off/sealed inner portion (i.e., interior) of the bulb  20 . The sealed interior can be evacuated and such a vacuum maintained, facilitating the operation of the field emission lamp  10 . 
     The lamp head  40  is secured on an outer portion of the neck portion  24  of the bulb  20 . The lamp head  40  is advantageously made of a conductive and oxidation-resistant material (e.g., aluminum, copper, stainless steel, etc.). The lamp head  40  includes a securing portion (not labeled) and a bottom portion (not labeled). In order to fixing thereof with a predetermined device (not shown in the drawings), the securing portion is beneficially provided with a latch configuration, a screw-thread configuration, or another attachment means. A screw-thread securing portion is shown in  FIG. 1 . A thermally insulative medium  42  is formed on a middle portion of the bottom portion of the lamp head  40 , thereby insulating the middle portion from other portions of the lamp head  40 . 
     Referring to  FIG. 2 , the anode  20  includes an anode conductive layer  32  formed directly on the inner surface of the bulb  20 , a fluorescent layer  34  deposited in contact with a surface of the anode conductive layer  32  facing the bulb interior, and an anode electrode  36  electrically connected with the anode conductive layer  32 . 
     The anode conductive layer  32  entirely covers an inner surface of the main portion  22  of the bulb  20 , extends towards the open end  26  of the neck portion  24 , and covers an inner surface of the neck portion  24 , partly or entirely. The anode conductive layer  32  is a transparent conductive film, such as an indium tin oxide (ITO) film. The fluorescent layer  34  partly covers the anode conductive layer  32  (e.g., advantageously the entirety thereof on the main portion  22 ), leaving the anode conductive layer  32  exposed at the neck portion  24  of the bulb  20 . The fluorescent layer  34  is advantageously made of one of a white and color fluorescent material with such a fluorescent material usefully having many satisfactory characteristics (e.g., a high optical-electrical transferring efficiency, a low voltage, a long afterglow luminescence, etc.). Alternatively, an aluminum layer (not shown in the drawings) is formed on a surface of the fluorescent layer  34 , in order to improve the brightness of the field emission lamp (due, e.g., to its high conductivity and its reflective nature) and to help prevent the fluorescent layer  34  from premature failure, reinforcing the layer and reducing the chances of spalling thereof. 
     The anode electrode  36  includes an anode down-lead ring  360 , an anode down-lead pole  362 , and a pair of anode down-lead wires  364 . The anode down-lead ring  360  is disposed on an exposed portion of the anode conductive layer  32  and thus electrically connected therewith. The anode down-lead pole  362  is disposed and secured on the end piece  28  of the neck portion  22 , with one end thereof in the inner portion of the bulb  20  and an opposite end thereof in the lamp head  40 . One of the anode down-lead wires  364  electrically connects the end of the anode down-lead pole  362  with the anode down-lead ring  360 , and the other anode down-lead wire  364  electrically connects the opposite end of the anode down-lead pole  362  with a portion of the lamp head  40  away from the thermally insulative medium  42 . The anode down-lead ring  360 , anode down-lead pole  362 , and anode down-lead wires  364  are respectively made of a conductive material (e.g., copper, etc.), and the arrangements thereof are done in a manner so as to electrically connect the anode conductive layer  32  with the lamp head  40 . Alternatively, the anode electrode  36  can have other configurations, such as a pole or a wire provided to electrically connect the anode conductive layer  32  with the lamp head  40  or such as a ring provided on a portion of the anode conductive layer  32  and a wire or a pole provided to electrically connect the ring with the lamp head  40 . 
     The cathode  50  includes an electron emission element  52  and a cathode electrode  54 . The electron emission element  52  is arranged in an inner portion of the main portion  22  of the bulb  20 . The cathode electrode  54  includes a cathode electrode head  540 , a cathode down-lead wire  542 , and a hollow insulative glass column  544 . The cathode electrode head  540  is disposed on a middle of the thermally insulative medium  42  of the lamp head  40  and is insulated from the lamp head  40 . The cathode down-lead wire is received in the column  544  and electrically connects the electron emission element  52  with the cathode electrode head  540 . An end of the column  544  directly, attachedly supports the electron emission element  52 , and the other end of the column  544  is secured in place, via the end piece  28  of the neck portion  24  of the bulb  20 . 
     In an alternative configuration, a metallic base column (not shown in the drawings) is provided to replace the glass column  544  and the cathode down-lead wire  542 . One end of the metallic base column would support the electron emission element  52 , a lower portion thereof would be secured via the end piece  28  of the bulb  20 , and the other end (proximate the lower portion) thereof would electrically connect with the cathode electrode head  540 . 
     Referring to  FIGS. 3 and 4 , the electron emission element  52  includes a metallic base body  520  and an electron emission layer  522  formed on a surface of the base body  520 . The base body  520  is beneficially shaped corresponding to the shape of the main portion  22  of the bulb  20  (e.g., the base body  520  is preferably ball shaped if bulb  20  is ball shaped). 
     The electron emission layer  522  includes a plurality of carbon nanotubes  530 , metallic conductive particles  534  and getter powders  536 ; and a glass matrix  532 . Preferably, a length of each of the nanotubes  530  is in the approximate range from 5 micrometers to 15 micrometers, a diameter thereof is about in the range from 1 nanometer to 100 nanometers. An end of each nanotube  530  is advantageously exposed out from a top surface of the electron emission layer  522  and extends toward the bulb  20 . Meanwhile, the remainder of each is anchored/embedded within the electron emission layer  522 . The metallic conductive particles  534  are usefully made of a conductive material such as silver (Ag) or indium tin oxide (ITO) and are used to electrically connect the base body  520  with the nanotubes  530 . The getter powders  536  are most suitably made of a non-evaporating getter material (e.g., a material selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), thorium (Th), aluminum (Al), thulium (Tm), and alloys substantially composed of at least two such metals.). The average diameter of the getter powders  536  is in the range from about 1 micrometer to about 10 micrometers. 
     In use, the lamp head  40  is grounded, and an appropriate negative voltage is applied to the cathode electrode head  540 , resulting in a strong field between the anode conductive layer  32  of the anode  30  and the electron emission layer  522  of the cathode  50 . The strong electrical field excites the carbon nanotubes  530  in the electron emission layer  522  to emit electrons. The electrons bombard the fluorescent layer  34 , thereby producing visible light. Furthermore, the getter powders  536  exhaust gases produced by the fluorescent layer  34  and/or any residual gas in the field emission lamp  10  remaining upon evacuation, thus ensuring the field emission lamp  10  with a high degree of vacuum throughout its usage lifetime. 
     A method for making the above-mentioned field emission lamp  10  generally includes:
         (a) providing a transparent glass bulb  20  with an open end  26 ; an anode electrode  36 ; a cathode electrode  54 ; a metallic base body  520 ; a lamp head  40 ; and a certain number of carbon nanotubes  530 , metallic conductive particles  534 , glass particles (later melted to form a glass matrix  532 ), and getter powders  536 , the bulb  20  having an anode conductive layer  32  on an inner surface thereof and a fluorescent layer  34  on a surface of the anode conductive layer  32 ;   (b) mixing the nanotubes  530 , the metallic conductive particles  534 , the glass particles, and the getter powders  536  in organic medium to form an admixture;   (c) forming a layer of the admixture on a surface of the base body  520 ;   (d) drying and then baking the admixture at a temperature of about 300° C. to about 600° C. to soften and/or melt the glass particles to result in the glass matrix  532  with the nanotubes  530 , the metallic conductive particles  534 , and the getter powders  536  dispersed therein, in order to yield the electron emission layer  522  on the base body  520  thereby obtaining an electron emission element  52 ; and   (e) assembling the bulb  20 , the anode electrode  36 , the cathode electrode  54  and the electron emission element  52 ; and   (f) thereafter, sealing the open end  26  of the bulb  20  at a temperature of about 400° C. to about 500° C. in order to secure the anode electrode  36  and the cathode electrode  54  and evacuating the bulb  20  interior, assembling the lamp head  40  and electrically connecting the lamp head  40  with the anode electrode  36  and the cathode electrode  54 , respectively, thereby yielding the field emission lamp  10 .       

     In step (a), the carbon nanotubes  530  are formed by an appropriate technology (e.g., a chemical vapor deposition (CVD) method, an arc-discharge method, a laser ablation method, gas phase combustion synthesis method, etc.). Preferably, the average length of the nanotubes is in the range from about 5 micrometers to about 15 micrometers. The glass particles are selected from glass powders with a low melting temperature (e.g., glass powders with a low melting temperature in the range of about 350° C. to about 600° C., and preferably composed, in part, of silicon oxide (SiO 2 ), boric trioxide (B 2 O 3 ), zinc oxide (ZnO), and vanadium pentoxide (V 2 O 5 )). The average diameter of the glass particles is preferably in the range of about 10 nanometers to about 100 nanometers. The metallic conductive particles  534  are ball-milled, yielding particle diameters in the range from about 0.1 micrometer to about 10 micrometers. The getter powders  536  are also ball-milled, forming powder diameters in the range from about 1 micrometer to about 10 micrometers. Preferably, the getter powders are made of a getter material with an activity temperature of about 300° C. to about 500° C. (e.g., an alloy containing Zr and Al). 
     The bulb  20  includes a main portion  22  and a neck portion  24  with an open end  26 . The anode conductive layer  32  is formed directly on an inner surface of the bulb  20  (i.e., a surface facing the bulb interior and the cathode  50 ) by, e.g., a sputtering method or a thermal evaporating method. The fluorescent layer  34  is formed on and in contact with the anode conductive layer  32  by a depositing method. 
     In step (b), the organic medium is composed of a certain number of solvent (e.g., terpineol, etc.), and a smaller amount of a plasticizer (e.g., dimethyl phthalate, etc.) and stabilizer (e.g., ethyl cellulose, etc.). The percent by mass of the getter powders  536  is in the range of about 40% to about 80% of the admixture. The process of the mixing is preferably performed at a temperature of about 60° C. to about 80° C. for a sufficient period of time (e.g., about 3 hours to about 5 hours). Furthermore, low power ultrasound is preferably applied in step (b), to improve the dispersion of the carbon nanotubes  530 , as well as the metallic conductive particles  534  and the getter powders  536 . 
     Step (c) is performed in a condition of low dust content (e.g., being preferably lower than 1000 mg/m 3 ). 
     In step (d), the process of drying volatilizes the organic medium from the base body  520 , and the process of baking is melts or at least softens the glass particles to permit the flow thereof in order to form the glass matrix  532  of the electron emission layer  522 . The processes of drying and baking are performed in a vacuum condition and/or in a flow of a protective/inert gas (e.g., noble gas, nitrogen). An outer surface of the electron emission layer  522  is advantageously abraded and/or selectively etched, in order to expose ends of at least a portion of the nanotubes  530 . The exposure of such ends increases the field emission performance of the electron emission layer  522 . 
     In step (f), a sealing material (e.g., a glass with a melting temperature of about 350° C. to about 600° C.) is applied for the open end  26  of the bulb  20  and softened/formed at a temperature of about 400° C. to about 500° C. The sealing material forms the end piece  28  after cooling, to establish a chamber within the field emission lamp  10  that can then be evacuated. 
     Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope thereof.