Abstract:
A microcavity forming device is provided for making microcavities in a tungsten wire. The microcavity forming device includes a source of particles; a housing for receiving a heated tungsten wire; and a plurality of jet nozzles disposed in the housing for spraying the particles toward the heated tungsten wire. The particles are 0.35-0.75 micron in diameter. The heated tungsten wire is received in the housing and the jet nozzles spray the particles toward the tungsten wire to form the microcavities in the tungsten wire.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to forming microcavities in filament wires to improve their radiative efficiency. More particularly, this invention relates to a device and method for forming microcavities in a filament wire suitable for mass manufacturing environments.  
       BACKGROUND OF THE INVENTION  
       [0002]     The cost of producing and purchasing electricity has escalated to all-time highs worldwide. This is especially true in under-developed countries where electricity supply is limited, as well as in those countries with large populations where the demand for electricity is high. Driven by this demand is an ever-increasing desire to produce lighting sources that are energy efficient and minimize the cost of electric usage.  
         [0003]     Over the past two centuries, scientists and inventors have strived to develop a cost-effective, practical, long-life incandescent light bulb. Developing a long-life, high-temperature filament is a key element in designing a practical incandescent light bulb.  
         [0004]     Tungsten filaments have been found to offer many favorable properties for lighting applications, such as a high melting point (3,410° C./6,170° F.), a low evaporation rate at high temperatures (10-4 torr at 2,757° C./4,995° F.), and a tensile strength greater than steel. These properties allow the filament to be heated to higher temperatures to provide brighter light with favorable longevity, making tungsten a preferred material for filaments in commercially available incandescent light bulbs.  
         [0005]     The filament of an incandescent lamp emits visible and non-visible radiation when an electric current of sufficient magnitude is passed through it. The filament emits, however, a relatively small portion of its energy, typically 6 to 10 percent, in the form of visible light. Most of the remainder of the emitted energy is in the infrared region of the light spectrum and is lost in the form of heat. As a consequence, radiative efficiency of a typical tungsten filament, measured by the ratio of power emitted at visible wavelengths to the total radiated power over all wavelengths, is relatively low, on the order of 6 percent or less.  
         [0006]     Conventional techniques for increasing the amount of visible light emitted by an incandescent filament rely on increasing the amount of energy available from the filament by increasing the applied electrical current. Increasing the current, however, wastes even larger amounts of energy. What is needed is a tungsten filament that emits increased visible light, without increasing energy consumption.  
         [0007]     Another concern is the life span of a filament. A tungsten filament is very durable, but after a prolonged period of time large electrical currents cause excessive electron wind, which occurs when electrons bombard and move atoms within the filament. Over time, this effect causes the filament to wear thin and eventually break.  
         [0008]     It has been observed that the radiative efficiency of filament material such as tungsten may be increased by texturing the filament surface with submicron sized features. A method of forming submicron features on the surface of a tungsten sample using a non-selective reactive ion etching technique is disclosed by H. G. Craighead, R. E. Howard, and D. M. Tennant in “Selectively Emissive Refractory Metal Surfaces,” 38 Applied Physics Letters 74 (1981). Craighead et al. disclose that improved radiative efficiency results from an increase in the emissivity of visible light from the tungsten. Emissivity is the ratio of radiant flux, at a given wavelength, from the surface of a substance (such as tungsten) to radiant flux emitted under the same conditions by a black body. The black body assumes to absorb radiation incident upon it.  
         [0009]     Craighead et al. disclose that the emissivity of visible light from a textured tungsten surface is twice that of a non-textured surface, and suggest that the increase is a result of more effective coupling of electromagnetic radiation from the textured tungsten surface into free space. The textured surface of the tungsten sample disclosed by Craighead et al. has depressions in the surface separated by columnar structures projecting above the filament surface by approximately 0.3 microns.  
         [0010]     Another method for enhancing incandescent lamp efficiency by modifying the surface of a tungsten lamp filament appears in a paper entitled “Where Will the Next Generation of Lamps Come From?”, by John F. Waymouth, pages 22-25 and FIG. 20, presented at the Fifth International Symposium on the Science and Technology of all Light Sources, York, England, on Sep. 10-14, 1989. Waymouth hypothesizes that filament surface perforations measuring 0.35 microns across, 7 microns deep, and separated by walls 0.15 microns thick, may act as waveguides to couple radiation in the visible wavelengths between the tungsten and free space, but inhibit emission of non-visible wavelengths. Waymouth discloses that the perforations on the filament may be formed by semiconductor lithographic techniques, but such perforation dimensions are beyond current state-of-the-art capabilities.  
         [0011]     Another method for reducing infrared emissions of an incandescent light source is described in U.S. Pat. No. 5,955,839 issued to Jaffe et al. As described, the presence of microcavities in a filament provides greater control of directivity of emissions and increases emission efficiency in a given bandwidth. Such a light source, for example, may have microcavities between 1 micron and 10 microns in diameter. While features having these dimensions may be formed in some materials using microelectronic processing techniques, it is difficult to form them in metals, such as tungsten, commonly used for incandescent filaments.  
         [0012]     Yet another method for reducing infrared emissions of an incandescent light source is disclosed in U.S. Pat. No. 6,433,303 issued to Liu et al. entitled Method and Apparatus Using Laser Pulses to Make an Array of Microcavity Holes. The method disclosed uses a laser beam to form individual microcavities in a metal film. An optical mask divides the laser beam into multiple beams and a lens system focuses the multiple beams onto the metal film and forms the microcavities.  
         [0013]     Still another method is disclosed in U.S. Pat. No. 5,389,853 issued, to Bigio et al., and describes a filament having improved emission of visible light. The emissivity of the tungsten filament is improved by depositing a layer of submicron-to-micron crystallites on its surface. The crystallites are formed from tungsten, or a tungsten alloy of up to 1 percent thorium and up to 10 percent of at least one of rhenium, tantalum, and niobium.  
         [0014]     While these conventional methods form microcavities and improve light emissivity, they are complex and costly. None of these methods is suitable for mass manufacturing environments where cost and efficiency are important factors. Consequently, a need still exists for a method of making microcavities in a filament that is suitable for mass manufacturing environments.  
       SUMMARY OF THE INVENTION  
       [0015]     A microcavity forming device is provided for making microcavities in a tungsten wire. The microcavity forming device includes a source of particles; a housing for receiving a heated tungsten wire; and a plurality of jet nozzles disposed in the housing for spraying the particles toward the heated tungsten wire with sufficient force to embed the particles into the tungsten wire. The heated tungsten wire is received in the housing and the jet nozzles spray the particles toward the tungsten wire to form the microcavities in the tungsten wire. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0016]     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:  
         [0017]      FIG. 1  is a block diagram of a system for making microcavities in a tungsten filament in accordance with the present invention;  
         [0018]      FIG. 2  is a partial perspective view of a microcavity forming device which forms a portion of the system of  FIG. 1 , including a sprayer in accordance with an embodiment of the present invention;  
         [0019]      FIG. 3A  is a cross-sectional view of the sprayer illustrated in  FIG. 2  in accordance with an embodiment of the present invention;  
         [0020]      FIG. 3B  is a cross-sectional view of the sprayer illustrated in  FIG. 2  in accordance with another embodiment of the present invention;  
         [0021]      FIG. 4  is a partial perspective view of a microcavity forming device, including a sprayer and a particle remover in accordance with another embodiment of the present invention;  
         [0022]      FIG. 5  is a schematic side view of the particle remover illustrated in  FIG. 4  in accordance with an embodiment of the present invention; and  
         [0023]      FIG. 6  is a block diagram of a system for making microcavities in a tungsten filament including a particle remover in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     Preferred features of embodiments of this invention are now described with reference to the figures. It will be appreciated that the invention is not limited to the embodiments selected for illustration. Also, it should be noted that the drawings are not rendered to any particular scale or proportion. It is contemplated that any of the configurations and materials described hereafter may be modified within the scope of this invention.  
         [0025]     Referring to  FIG. 1 , tungsten filament manufacturing system  10  includes heater  14 , swaging device  18 , microcavity forming device  22 , pulling device  26 , and coiling device  32 . In operation, tungsten material  12  is heated by heater  14  to form heated tungsten material  16 . The tungsten is heated by heater  14  to a malleable temperature (1,200° C. to 1,500° C.). The resulting tungsten material  16  is drawn, utilizing swaging device  18 , to reduce the diameter of the tungsten material. The heating and drawing steps are repeated until heated tungsten wire  20  of requisite diameter, typically between 40 microns and 100 microns, is formed. As explained below, microcavity forming device  22  is adapted to form microcavities on the outer surface of heated tungsten wire  20 . Microcavitied filament wire  30  is coiled by coiling device  32  to form filament coil  34 . The present invention includes several embodiments of microcavity forming device  22 , and is discussed in detail below.  
         [0026]     Referring next to  FIG. 2 , an embodiment of microcavity forming device  22 , generally designated as  22 A, is illustrated. Microcavity forming device  22 A includes sprayer  36  for depositing particles  38  on heated filament wire  20 . Sprayer  36  includes a hollow, circumferential housing  40 , which is positioned at a distance from swaging device  18  and is adapted to receive heated tungsten wire  20 . Jet nozzles  42 , as shown, are mounted at various locations on the inner surface of housing  40 , and positioned to spray particles  38  in a radial direction toward tungsten wire  20 . Pressurized particle source  44  is adapted to supply particles  38  to distribution bars  46 , which in turn, deliver particles  38  to jet nozzles  42 . As heated tungsten wire  20  is drawn through circumferential housing  40 , jet nozzles  42  spray particles  38  onto tungsten wire  20 . Particles  38  are embedded in wire  20  to form tungsten wire having microcavities with particles  38  embedded therein. The tungsten wire with the particles embedded therein is generally designated as  48 .  
         [0027]      FIGS. 3A and 3B  are cross-sectional views of sprayer  36  of  FIG. 2 .  FIG. 3A  illustrates a cross-sectional view of four rows of jet nozzles  42  arranged radially 90 degrees apart within housing  40 .  FIG. 3B  illustrates a cross-sectional view of eight rows of jet nozzles  42  arranged radially 45 degrees apart within housing  40 . The present invention, however, may have another number of rows of jet nozzles  42  different from that shown in  FIGS. 3A and 3B .  
         [0028]     Housing  40  may be made from silicon carbide or any other hardened material capable of withstanding the temperature of heated tungsten wire  20  and hardened to prevent damage from the jet sprays. The diameter of particles  38  is preferably 0.35-0.75 micron, and most preferably 0.5 micron. Particles  38  may be made from tantalum, rhenium, molybdenum, tungsten, silicon carbide, rare earth elements, glass beads, or any other hardened material.  
         [0029]     In operation referring to  FIGS. 1-3 , heated tungsten wire  20  exits swaging device  18  and is drawn by pulling device  26  through sprayer  36  of microcavity forming device  22 A. High velocity jet nozzles  42  propel particles  38  toward the surface of heated tungsten wire  20  as it moves in direction A through housing  40 . Due to malleability from the heating process, as particles  38  contact the surface of heated tungsten wire  20 , they form and become embedded in microcavities therein.  
         [0030]     As will be appreciated, the diameter of housing  40  and the spacing between jet nozzles  42  in a row may be adjusted based upon a desired density of the embedded particles  38  in the wire. Similarly, the pressure of jet nozzles  42  may be adjusted based upon a desired depth of the microcavity formed by each of the embedded particles  38 .  
         [0031]     Referring next to  FIG. 4 , another embodiment, generally designated as  22 B, of microcavity forming device  22  is illustrated. Microcavity forming device  22 B includes microcavity forming device  22 A (illustrated in  FIG. 2 ) and particle remover  54 . Cross-sectional views of an exemplary sprayer  36  are illustrated in  FIGS. 3A and 3B . Particle remover  54  is disposed downstream of sprayer  36  and removes particles  38  from particled-wire  48  as the wire is drawn from sprayer  36  through particle remover  54 . The removal of particles  38  forms microcavitied wire  24 .  
         [0032]      FIG. 5  is a schematic diagram of particle remover  54  of  FIG. 4 . The exemplary particle remover  54  includes reactor tube  56 , chemical flow control system  58 , and vacuum pumping system  60 . Reactor tube  56  is surrounded by heater  62 .  
         [0033]     Particles  38  may be removed from wire  48  in several ways. In one approach, a chemical dissolution process may be used. Chemical solutions suitable for separating particles  38  from wire  48 , such as a mixture of nitric acid, sulphuric acid and water, may be placed in chemical flow control system  58 , and wire  48  may be placed in reactor tube  56 . The wire, which may be wound on a mandrel to form a cassette, may be chemically treated with the chemical solutions to dissolve, or remove the embedded particles. One, or several cassettes may be used.  
         [0034]     Vacuum pumping system  60  may be utilized to provide a vacuum in reactor tube  56  and a flow of the chemical solutions through reactor tube  56 . Vacuum pumping system  60  may also provide suction to deliver the particles removed from the wire to a reservoir (not shown).  
         [0035]     In operation, reactor tube  56  is sealed from the atmosphere, and a chemical solution is added through chemical flow control system  58 . Dissolution of particles  38  begins immediately and NO x  gas is formed and mixes with air above the acidic surface. The NO gas combines with O 2  in the air and is dissolved. As a result, a low-pressure condition occurs in reactor tube  56 . This condition causes a caustic soda solution to be sucked into vacuum pumping system  60 . The process acid is removed through vacuum pumping system  60  to a waste reservoir (not shown). The removal of particles  38  results in voids in the outer surface of wire  48 , thereby producing microcavitied wire  24 .  
         [0036]     In an alternate approach, particles  38  may be removed by melting the particles  38 . As shown in  FIG. 4 , microcavity former  22 B continues to pass wire  48  in direction A through particle remover  54 , heaters  62  may apply heat into reactor tube  56 , and melt the particles. This approach is effective if the particles have a lower melting point than the tungsten wire. For example, if the particles are of molybdenum, the particles may be removed by heating since tungsten has a higher melting point than molybdenum.  
         [0037]     A further alternate approach for removing particles  38  may be via a blowing process. After cooling, wire  48  may be positioned in a chamber, such as reactor tube  56 . Particles  38  may be separated from wire  48  by blowing force of air-flow.  
         [0038]     Referring next to  FIG. 6 , another embodiment of tungsten filament manufacturing system  10  is shown and is generally designated as  70 . Tungsten filament manufacturing system  70  includes system  10  with microcavity former  22 A ( FIG. 2 ) and particle remover  54  positioned downstream after coiling device  32 , as shown in  FIG. 6 . System  70 , by way of microcavity former  22 A, may form wire  48  having microcavities with particles embedded therein. Wire  48  may then be coiled or wound on a mandrel, as disclosed in U.S. Pat. No. 4,291,444 to McCarty et al.  
         [0039]     Coiled wire  34  may then be passed through particle remover  54 , as previously described, to form coiled microcavitied filament  64 .  
         [0040]     It will be appreciated that if heated wire  20  is sprayed with molybdenum particles and then coiled or wound on a molybdenum mandrel, as disclosed in U.S. Pat. No. 4,291,444 to McCarty et al., particle remover  54  may use a heating approach to melt both the particles and the mandrel away from the tungsten wire.  
         [0041]     The present invention provides an improvement over conventional methods of forming microcavities in a filament, as it is suitable for mass manufacturing environments where cost and efficiency are important factors. The present invention does not require complicated and costly devices, and instead utilizes simple mechanical structures to form microcavities. The present invention may also be implemented with minimum changes to a conventional filament manufacturing production line.  
         [0042]     It will be appreciated that other modifications may be made to the illustrated embodiments without departing from the scope of the invention, which is separately defined in the appended claims.