Method and apparatus for forming discrete microcavities in a filament wire using microparticles

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.

FIELD OF THE INVENTION

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

DETAILED DESCRIPTION OF THE INVENTION

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.

Referring toFIG. 1, tungsten filament manufacturing system10includes heater14, swaging device18, microcavity forming device22, pulling device26, and coiling device32. In operation, tungsten material12is heated by heater14to form heated tungsten material16. The tungsten is heated by heater14to a malleable temperature (1,200° C. to 1,500° C.). The resulting tungsten material16is drawn, utilizing swaging device18, to reduce the diameter of the tungsten material. The heating and drawing steps are repeated until heated tungsten wire20of requisite diameter, typically between 40 microns and 100 microns, is formed. As explained below, microcavity forming device22is adapted to form microcavities on the outer surface of heated tungsten wire20. Microcavitied filament wire30is coiled by coiling device32to form filament coil34. The present invention includes several embodiments of microcavity forming device22, and is discussed in detail below.

Referring next toFIG. 2, an embodiment of microcavity forming device22, generally designated as22A, is illustrated. Microcavity forming device22A includes sprayer36for depositing particles38on heated filament wire20. Sprayer36includes a hollow, circumferential housing40, which is positioned at a distance from swaging device18and is adapted to receive heated tungsten wire20. Jet nozzles42, as shown, are mounted at various locations on the inner surface of housing40, and positioned to spray particles38in a radial direction toward tungsten wire20. Pressurized particle source44is adapted to supply particles38to distribution bars46, which in turn, deliver particles38to jet nozzles42. As heated tungsten wire20is drawn through circumferential housing40, jet nozzles42spray particles38onto tungsten wire20. Particles38are embedded in wire20to form tungsten wire having microcavities with particles38embedded therein. The tungsten wire with the particles embedded therein is generally designated as48.

FIGS. 3A and 3Bare cross-sectional views of sprayer36ofFIG. 2.FIG. 3Aillustrates a cross-sectional view of four rows of jet nozzles42arranged radially 90 degrees apart within housing40.FIG. 3Billustrates a cross-sectional view of eight rows of jet nozzles42arranged radially 45 degrees apart within housing40. The present invention, however, may have another number of rows of jet nozzles42different from that shown inFIGS. 3A and 3B.

Housing40may be made from silicon carbide or any other hardened material capable of withstanding the temperature of heated tungsten wire20and hardened to prevent damage from the jet sprays. The diameter of particles38is preferably 0.35–0.75 micron, and most preferably 0.5 micron. Particles38may be made from tantalum, rhenium, molybdenum, tungsten, silicon carbide, rare earth elements, glass beads, or any other hardened material.

In operation referring toFIGS. 1–3, heated tungsten wire20exits swaging device18and is drawn by pulling device26through sprayer36of microcavity forming device22A. High velocity jet nozzles42propel particles38toward the surface of heated tungsten wire20as it moves in direction A through housing40. Due to malleability from the heating process, as particles38contact the surface of heated tungsten wire20, they form and become embedded in microcavities therein.

As will be appreciated, the diameter of housing40and the spacing between jet nozzles42in a row may be adjusted based upon a desired density of the embedded particles38in the wire. Similarly, the pressure of jet nozzles42may be adjusted based upon a desired depth of the microcavity formed by each of the embedded particles38.

FIG. 5is a schematic diagram of particle remover54ofFIG. 4. The exemplary particle remover54includes reactor tube56, chemical flow control system58, and vacuum pumping system60. Reactor tube56is surrounded by heater62.

Particles38may be removed from wire48in several ways. In one approach, a chemical dissolution process may be used. Chemical solutions suitable for separating particles38from wire48, such as a mixture of nitric acid, sulphuric acid and water, may be placed in chemical flow control system58, and wire48may be placed in reactor tube56. 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.

Vacuum pumping system60may be utilized to provide a vacuum in reactor tube56and a flow of the chemical solutions through reactor tube56. Vacuum pumping system60may also provide suction to deliver the particles removed from the wire to a reservoir (not shown).

In operation, reactor tube56is sealed from the atmosphere, and a chemical solution is added through chemical flow control system58. Dissolution of particles38begins immediately and NOxgas is formed and mixes with air above the acidic surface. The NO gas combines with O2in the air and is dissolved. As a result, a low-pressure condition occurs in reactor tube56. This condition causes a caustic soda solution to be sucked into vacuum pumping system60. The process acid is removed through vacuum pumping system60to a waste reservoir (not shown). The removal of particles38results in voids in the outer surface of wire48, thereby producing microcavitied wire24.

In an alternate approach, particles38may be removed by melting the particles38. As shown inFIG. 4, microcavity former22B continues to pass wire48in direction A through particle remover54, heaters62may apply heat into reactor tube56, 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.

A further alternate approach for removing particles38may be via a blowing process. After cooling, wire48may be positioned in a chamber, such as reactor tube56. Particles38may be separated from wire48by blowing force of air-flow.

Referring next toFIG. 6, another embodiment of tungsten filament manufacturing system10is shown and is generally designated as70. Tungsten filament manufacturing system70includes system10with microcavity former22A (FIG. 2) and particle remover54positioned downstream after coiling device32, as shown inFIG. 6. System70, by way of microcavity former22A, may form wire48having microcavities with particles embedded therein. Wire48may then be coiled or wound on a mandrel, as disclosed in U.S. Pat. No. 4,291,444 to McCarty et al. AlthouahFIG. 6shows the particle remover54being positioned downstream of the coiling device32, it is contemplated that it may be positioned upstream of the coiling device32. In this alternative embodiment, the positions of blocks54and32would be switched inFIG. 6.

Coiled wire34may then be passed through particle remover54, as previously described, to form coiled microcavitied filament64.

It will be appreciated that if heated wire20is 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 remover54may use a heating approach to melt both the particles and the mandrel away from the tungsten wire.

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.

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.