Patent Publication Number: US-8110970-B2

Title: Light-emitting devices utilizing gaseous sulfur compounds

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No. 97144474, filed on Nov. 18, 2008, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to light-emitting devices, and in particular to light-emitting devices utilizing gaseous sulfur compounds, wherein a discharge chamber is provided with no plasma-media contacting electrodes built inside the chamber. 
     2. Description of the Related Art 
     There are various types of lighting sources, e.g., an incandescent lamp using radiation associated with a burning filament, a fluorescent lamp composed of an electric discharge tube and a fluorescent-powder coating for energy conversion, a high-intensity-discharge (HID) lamp that induces electrical discharge within a highly-pressurized gas or steam, and an electrodeless plasma lighting system (PLS) lamp that generates lighting plasma of gaseous media with no media-contacting electrodes. 
     The various types of lamps have their respective advantages. For example, incandescent lamps are excellent in color rendition and small in size. Switching circuits of the incandescent lamps are simple and low cost. However, compared to other lamps, incandescent lamps are less power efficient and have a shorter life span. In the other end, fluorescent lamps are more power efficient in emitting light and more durable than other lamps. However, while compared with incandescent lamps, fluorescent lamps are relatively large in size. Additionally, fluorescent lamps require also additional power-ballasting circuits to stabilize discharge current and light output thereof. Other gas-discharge lamps like HID lamps are also power efficient and durable. The HID lamps require, however, a relatively long time for restriking on upon switching off. In addition, HID lamps, similar to fluorescent lamps, requires additional power-ballasting circuits to assist switching. Electrodeless PLS lamps possess longest life among all the above-noted lamps. The electrodeless PLS lamps though are acceptably efficient in emitting light but relatively much expensive. The electrodeless PLS lamps require also additional power-ballasting (though similar but more complex) circuits for switching. 
     One type of electrodeless PLS lamps, called electrodeless sulfur lamp, is particularly efficient in emitting white light of broadband spectrum even closely resembling to natural sun light. 
     U.S. Pat. Nos. 5,404,076, 5,594,303, 5,847,517 and 5,757,130, issued to Fusion System Corporation, discloses an electrodeless sulfur lamp. 
     The electrodeless sulfur lamps disclosed in the above noted US patents consist of a of golf-ball sized quartz bulb containing ten to hundred milligrams of sulfur powers and argon gas at an end of a spindle for rotation. The bulb absorbs microwave energy of 2.45 GHz generated from a magnetron to excite buffering gas of low pressure argon therein and generates gaseous discharging plasma. As a consequent, the space within the quartz bulb is thus supplied with an appropriate amount of free electrons. The sulfur powers absorb the microwave energy to heat and vaporize itself, thereby raising the pressure inside the quartz bulb to 5˜10 times that of the surrounding atmosphere. The gaseous sulfur vapors elevate to a temperature in the quartz bulb under the continuous reaction with microwaves and plasmas of inert buffering gas and are thus stimulated to ionize and discharge. The sulfur ions vigorously oscillate within the space of a narrow mean free path and collapse within itself, thereby causing a molecular-type charge/discharge process, such a process is further aggravated by excitation and collision with highly energetic gas ions in the buffering gas plasma, thereby forming additional luminous thermal plasma of new media and emitting great amounts of photons, having a spectrum of about 73% of visible light, resembling to that of sunlight. 
     Nevertheless, the electrodeless sulfur lamps disclosed in the above noted US patents need a power source of more than 1.5 KW to reach a luminous efficiency of about 100 lumens per watt. As a result its application is confined to illuminate only large public spaces. In addition, the electrodeless sulfur lamps disclosed by the above noted US patents are normally large in size and appropriate means of electromagnetic shielding in most cases are mandatory, particularly for indoor applications. Therefore, the electrodeless sulfur lamps disclosed by the above noted US patents are not suitable for low power or planar luminance applications. 
     BRIEF SUMMARY OF THE INVENTION 
     Thus, a light-emitting device utilizing gaseous sulfur compounds is provided for low power or planar luminance applications. 
     An exemplary light-emitting device utilizing gaseous sulfur compounds comprises a first substrate with an energy transmission coil disposed thereover. A dielectric barrier layer is formed over the first substrate to cover the energy transmission coil. A sealant wall circles around the dielectric barrier layer. A second substrate is disposed against the first substrate and supported by the sealant wall, thereby defining an inner chamber between the first and second substrates, wherein the second substrate is a transparent substrate. A gaseous reactant is filled in the inner chamber, wherein the gaseous reactant comprises an inert gas and a sulfur-containing gas. A high-frequency oscillating power supply is coupled to the energy transmission coil, thereby allowing the energy transmission coil to induce an electromagnetic field into the inner chamber for lighting up the light-emitting device. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram showing a top view of a light-emitting device according to an embodiment of the invention; 
         FIG. 2  shows a cross section taken along line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a schematic diagram showing a top view of an energy transmission coil according to an embodiment of the invention; 
         FIG. 4  is a schematic diagram showing a cross section of a light-emitting device according to another embodiment of the invention; and 
         FIGS. 5-10  are schematic diagrams showing top views of an energy transmission coil in various embodiments of the invention, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1  is a schematic diagram showing a top view of an exemplary light-emitting device  100 . As shown in  FIG. 1 , the light-emitting device  100  comprises a substrate  102 , an energy transmission coil (illustrated as two electrically isolated electrodes  106  and  108 ) disposed over the substrate  102 , a dielectric barrier layer  112  disposed over the substrate  102  and embedding the energy transmission coil, a sealant wall  110  which circles around the dielectric barrier layer  112  over the substrate  102 , and a high-frequency oscillating power supply  200 . An impedance matching circuit  300  may be optionally provided between the energy transmission coil and the high-frequency oscillating power supply  200  to improve energy transmission efficiency. The electrodes  106  and  108  of the energy transmission coil are connected to the impedance matching circuit  300  to receive power transmission from the high-frequency oscillating power supply  200 .  FIG. 1  illustrates the substrates  102  and  104  as substantially rectangular shape from a top view. However the shape of the substrates  102  and  104  are not limited thereto. The substrates  102  and  104  can be configured into, for examples, other orthogonal geometries or a substantially circular shape from a top view. 
       FIG. 2  shows a cross section taken along line  2 - 2  of the light-emitting device  100  in  FIG. 1 . As shown in  FIG. 2 , the substrate  104  and the substrate  102  are assembled together with a sealant wall  110  which thereby defines an inner chamber  114  there in between. The substrate  104  is a transparent substrate and the material thereof can be, for example, quartz, borosilicate, or translucent alumina which transmits visible light. The substrate  104  is formed with a thickness of about 1.5˜5.0 mm. On the other end, the substrate  102  is an electrically-insulative substrate and material thereof can be, for example, quartz, glass, or ceramic. In the inner chamber  114  defined between these two substrates ( 102  and  104 ), a gaseous reactant  150  is filled for purposed as media of lighting plasma. The gaseous reactant  150  is a mixture comprising of at least a buffer gas and a sulfur-containing gas. The buffer gas can be, for example, inert gases such as He, Ne, Ar, Kr, Xe, Rn, or combinations thereof. And the sulfur-containing gases can be, for example, SF 4  or SF 6 . The buffer gas may be formed with only one kind of inert gas or ideally a combination of at least two kinds of inert gases. A blending of more than one inert gases is for example combining Ar or Kr taken from a group of higher molecular weight with He or Ne taken from another group of low molecular weight. A plasma can be easily ignited at low power by an inert gas of lower molecule weight (M.W.) to rapidly reach enough free electron density at starting stage. Meanwhile upon excited within the plasma atmosphere, inert gas having high M.W. may supply energetic ions having high momentum to continuously strike the sulfur-containing gas which is relatively immobile, and by such an action to knock fluorine ions out of the sulfur-containing gas. The released active fluorine ions may then recombine with ions of the inert gas of high M.W. to temporarily produce metastable fluoride such as ArF or KrF. Such a decomposing/regeneration process gradually brings forth sulfur ions out of the plasma atmosphere. The buffer gas and the sulfur-containing gas in the gaseous reactant  150  are blended with a mix ratio of about 100:0.1˜2:1. The weighting of the inert gas having high M.W. in the buffer gas should be adjusted according to the proportioning of sulfur-containing gas in order to completely consume the fluorine ions released during the above noted decomposing/regenerating process. The inner chamber  114  is preferably sustained at a pressure of about 0.01˜1 atm. 
     Still referring to  FIG. 2 , the electrodes  106  and  108  disposed over the substrate  102  constitutes an energy transmission coil which is connected to the high frequency oscillating power supply  200  (See  FIG. 1 ). The high frequency oscillating power supply  200  may be, for example, an acoustic frequency oscillator, a radio frequency (RF) oscillator, or a microwave frequency oscillator. An impedance matching circuit  300  may be optionally provided between the electrodes ( 106  and  108 ) and the high-frequency oscillating power supply  200  to improve energy transmission efficiency. The impendence matching circuit  300  matches impendence between the high-frequency oscillating power supply  200  and loadings inferred from the electrodes  106  and  108 . If so arranged, the energy transmission coil could then be supplied with electro-magnetic power of pulses having a frequency of about 1 KHz˜20 MHz, or more preferably of about 5 KHz˜2 MHz. The electromagnetic pulses so provided can be either in DC or AC pulses. Such a powering up generates capacitively coupling effects, as forming a local electrical field within the inner chamber  114  to excite the gaseous reactant therein and cause charging/discharging process cycles which result in the emitting of light  180 . Herein, the dielectric barrier layer  112  embedding the electrodes  106  and  108  must be transmissive to the electromagnetic field having a frequency between 1 KHz˜20 MHz supplied from the high-frequency oscillating power supply  200 . The dielectric barrier layer  112  is made out of hybrid compounds by mixing inorganic dielectric powders such as silicon dioxide, barium titanate, aluminum oxide, titanium dioxide, magnesium oxide, or glass, with an organic binder such as silicon resin, epoxy resin, acrylic resin, PU resin or furan resin, etc. The dielectric barrier layer  112  is deposited on the substrate  102  by first screen printing of a raw-mixture paste and later applying high temperature baking to burn off the binder as causing particle sintering to form a continuous thick film which buries the energy transmission coil (displayed as electrodes  106  and  108 ) underneath. Similar to the dielectric barrier layer  112 , the sealant wall  110  may be made out of hybrid compounds by mixing inorganic powders such as silicon dioxide, magnesium oxide, aluminum oxide, silica gel or glass with an organic binder such as silicon resin, epoxy resin, acrylic resin, PU resin or furan resin, etc. The sealant wall  110  is formed by first screen-printing or casting, or dispensing a raw mixture pastes on the substrate  102  and later applying high temperature baking to burn off the binder as causing particle sintering to form a continuous and airtight sealing support between the substrates  102  and  104 . Herein, the dielectric barrier layer  112 , the sealant wall  110  and the substrate  102  must have close thermal expansion characteristics to avoid undesired deformation which might otherwise result in bending or leakage, while operating the light-emitting device  100 . 
     In addition, an optional light reflection layer  115  and/or a secondary-electron emitting layer  116  can be sequentially deposited over the top of the dielectric barrier layer  124  as directly meeting with the gaseous reactant  150  to direct illumination and to improve power utilization efficiency. The light reflection layer  115  may be made from simple metal oxides such as titanium dioxide (TiO 2 ) or from a multi-layered dichroic coating, which utilizes interference of light via media of contrast refraction, such as TiO 2 —SiO 2  to redirect out-scattered light for illumination. The secondary-electron emitting layer  116  may be made from aluminum oxide or magnesium oxide to purposely increase electron density and lighting plasma intensity of the light-emitting device  100 . Individual thicknesses of the light reflection layer  115  or the secondary electron emitting layer  116  is preferably no more than 1 μm. Similar to the dielectric barrier layer  112 , the above supplementary layers ( 115  and  116 ) must also be transmissive to the input electromagnetic wave from the high-frequency oscillating power supply  200  for excitation of the gaseous reactants  150  to form lighting plasma. 
     A possible reaction mechanism of the light-emitting device  100  as shown in  FIGS. 1 and 2  is described as follows. Inside the inner chamber  114 , the energy transmission coil is biased to first excite the inert gas with relatively low molecular weight (M.W) to form a starting plasma thereof which rapidly raises the free electron density in the plasma to a sufficient level. Meanwhile upon excited within the plasma atmosphere, another inert-gas ingredient of relatively high M.W. may subsequently supply energetic ions having high momentum to continuously strike the sulfur-containing gas which is relatively immobile. And by such an action it causes decomposition to occur which allows free fluorine atoms or ions to escape from the sulfur-containing gas molecules. Consequently such frequent and vigorous collisions produces various charged molecules such as SF 5   + , SF 4   + , SF 2   +  or SF +  as products in sequence of progressive stages. The released active fluorine ions (negatively charged) may then recombine with positively charged ions of the inert gas of high M.W. to temporarily produce metastable fluoride such as ArF or KrF. Such a decomposing/regenerating process gradually brings forth sulfur ions out of the plasma atmosphere. Since the molecular weight of the sulfur containing reactant is reduced due to the decomposition and liberation of the fluorine ions, vigorous oscillation of the resulting free sulfur ions thus become possible in response to the electromagnetic field just like ions of other inert gases. Excited by a electromagnetic wave of suitable frequency (e.g., 1 KHz˜1 MHz) and vigorously self-vibrated within space of narrow mean free path, the free sulfur ions with high momentum would frequently collide with other free sulfur ions and recombine into another multi-atomic molecular species. Such a three-body collision of atoms, ions, and electrons eventually forms charged diatomic sulfur radicals in an metastable and/or excited state. These ionization and recombination process cycles continuously increases in intensity and releases great amounts of photons which emit light  180 . High luminous efficacy is achieved as greater than 73% of light  180  is located within the visible range. Unlike traditional sulfur lamp which use solid-state sulfur powder as discharge media requiring preheating to vaporize, the light-emitting device  100  of this invention does not waste energy for phase exchange or intentionally raise temperature to trigger the three-body collision process. Therefore, operating temperature in the light emitting device  100  is greatly reduced as achieving a non-thermally equivalent plasma lighting at a low pressure. 
       FIG. 3  is a schematic drawing showing a top view of the electrodes  106  and  108  which constitutes the energy transmission coil. The electrodes  106  and  108  are electrically isolated two ends of opposite polarity. The electrodes  106  and  108  are configured as an interconnecting comb-like pattern when viewed from a top view. The electrodes  106  and  108  can be make from conductive metals such as copper foil or sintered thick films of pastes containing conductive particles such as silver, palladium or transparent conductive oxides such as indium tin oxide (ITO). Each segment of electrodes  106  and  108  may be formed with a line width W of about 0.1˜5 mm and a pitch P of about 0.05˜25 mm there in between. A terminal  130  of the electrode  106  and a terminal  140  of the electrode  108  are connected onto output terminals (not shown) of the high-frequency oscillating power supply  200  (not shown). Herein, the energy transmission coil is illustrated as a power conducting device disposed over the substrate  102  and protruding thereof. But it is not limited to only such a configuration. For example, the energy transmission coil can also be embedded within the substrate  102 . Such a buried configuration may improve flatness of composing elements and ease to integrate the light-emitting device  100  for particular applications such as in flat panel displays or projectors. 
     Moreover, to overcome the high dielectric strength before breakdown of the sulfur-containing reactant, a shorter pitch P may be applied as to effectively increase local electrical-field strength between the two electrodes  106  and  108 . Such an arrangement is beneficial in promoting the excitation and stability of the plasma. In addition, the short-pitched electrodes are also accommodative to be buried under a thin dielectric barrier layer  112 , as illustrated in  FIG. 2 , by means of sequential screen printing to form a structure of dielectric barrier discharge (DBD). By such, the gaseous reactant  150  in the inner chamber would be most effectively excited by the energetic electrons without causing arcing or streaming within a local region of relatively high electrical field which is located just above the embedded electrodes under the thin dielectric barrier  112 . And a critical voltage for igniting the plasma and power consumption can be greatly reduced. In addition, since the high electrical field suitable for excitation is just above a top surface of the electrodes of the energy transmission coil, the plasma so formed is non-diffusive but constricted within a local neighborhood between the electrodes. Thus the space height L for filling the gaseous reactants  150  can be shortened to even below 1 mm. Consequently, the overall thickness of the light-emitting device  100  can be reduced by applying a tabular sealant wall  110  of a small aspect ratio. Such a configuration greatly simplify supporting and sealing process for a planar vacuum device of a large area. 
     Arrangement of the electrodes  106  and  108  of the energy transmission coil in the light emitting device  100  are not limited by the coplanar interconnecting comb-like configuration as illustrated in  FIG. 3 . 
       FIG. 4  is a schematic drawing showing the electrodes  106  and  108  disposed on different substrates, respectively. As shown in  FIG. 4 , the electrodes  106  and  108  are disposed on the substrates  104  and  102  and are embedded under the dielectric barrier layers  112   a  and  112   b , respectively. A gaseous reactant  150  is filled within an inner chamber defined between the dielectric barrier layers  112   a  and  112   b . In this embodiment, a lower structure comprising the dielectric barrier layer  112   a  and the electrode  180  is similar to that disclosed in the previous embodiment shown in  FIG. 3 . An upper structure comprising electrode  106  and the dielectric barrier layer  112   b  must, however, be made with light transparent materials. The electrode  106  may be made from transparent conductive materials such as tin oxide, indium oxide, zinc oxide, or tin fluoride, etc. The dielectric barrier layer  112   b , in the other end, may be made from transparent insulative materials such as silicon resin, glass, acrylic resin, or epoxy resin, etc. 
     The arrangement of the electrodes  106  and  108  on different substrates can be configured as an interconnecting comb on two different planes as illustrated in  FIG. 4 . Herein the electrodes  106  and  108  are embedded by different dielectric barrier layers  112   a  and  112   b , respectively. The electrodes  106  and  108  may also be arranged in other configuration (not shown) such as positioned the two electrodes in a same orientation with no horizontal displacement, or in a perpendicular manner like a chessboard when viewed from a top view. The electrodes can be so configured as interconnecting fingers or grids for a proper capacitively coupling of input energy for excitation of gas plasma where the intensity of an induced electrical field varies with vertical distance L in space filled with the gaseous reactants  150 . 
     Moreover, the energy transmission coil in the light emitting device  100  is not limited only to configurations for achieving capacitively coupling effects of input energy as described above. The energy transmission coil may also be configured for obtaining an inductively coupling effect of input energy using a single continuous electrode  109  such as those shown in  FIG. 5-10 . As shown in  FIG. 5-10 , the electrode  109  of the energy transmission coil in the light emitting device  100  is configured as other shapes such as a substantially rectangular helix loop (See  FIG. 5 ), a substantially circular helix loop (See  FIG. 6 ), a U-shaped line (See  FIG. 7 ), a meander (serpentine) line (See  FIG. 8 ), a S-shaped line (See  FIG. 9 ) or even multiplexed parallel lines (See  FIG. 10 . All such are capable of inducing inductively coupling effects for a lighting plasma. 
     The light-emitting device  100  has a high luminous efficacy and a color rendition that resembles sunlight. The light-emitting device  100  shows a wavelength distribution better match with the luminous sensitivity equivalence of human eyes than most of conventional fluorescent lamps does. Since the light-emitting device of current invention may directly emit visible white light, there is no need to coat fluorescent conversion materials on the chamber wall of the inner chamber  114  or to use environmentally hazardous mercury material. The light-emitting device  100  also shows a minimal aging characteristics over the life span thereof in both color and brightness of the emitted light. 
     Thus, planar lighting sources with high energy efficiency may be fabricated using the light-emitting device  100  of the invention adopting high efficient luminous discharge of sulfur molecules. The light-emitting device  100  of the invention incorporates a planar energy transmission coil to provide capacitively-coupled electrical fields for a powerful excitation. Besides, because there is no electrode contacting with the gaseous reactant  150  inside the inner chamber  114  in the light emitting device  100 , degradation of electrodes with plasma atmosphere is completely avoided. In addition, since the inner chamber  114  is fully sealed, no chemical contaminants could be formed therein during the plasma discharging process, thereby ensuring a durable life span and reliability thereof. 
     The light emitting device  100  may also take advantages of metastable products formed by the recombination of liberated fluorine ions with the ions of the inert buffer gases (e.g. Ar or Kr) to modulate the colors of emitted light. For example upon excited, a metastable product like KrF radiates a UV light peaking at a wavelength of about 249 nm which is so close to the 254 nm from mercury used in common fluorescent lamps. Therefore, traditional tri-chromatic (RGB) rare-earth-doped phosphors may be applied extensively to modify the spectrum of light output thereof without need of using mercury. Such a UV-to-visible converting fluorescent layer  118  (as illustrated in  FIG. 2 ) capable of enhancing brightness or modify color spectrum of the light output may be optionally deposited over an inner surface of substrate  104  at location in close contact with the gaseous reactant  150  and its associated plasma. The UV-to-visible converting fluorescent layer  118  which adopts traditional rare-earth doped phosphors may converts UV radiation from intermediate products (ArF or KrF) in the plasma into visible light by taking advantage of its similar UV emission peak as from mercury in most fluorescent lamps. 
     The light-emitting device  100  of the invention is thus applicable in applications such as concentrated type or planar type lighting sources. For applied the light emitting device  100  of the invention as a planar lighting source in a backlight module, no diffusion plates or brightness enhancing films would be required as normally necessary while using conventional tubular CCFL as light-emitting source. Therefore, fabrication costs could be decreased, while increasing luminous efficacy and power utilization efficiency of the backlight module. In addition, the light-emitting device  100  of the invention can served as an alternative which directly emits visible light using no wavelength converting fluorescent materials as commonly adopted in conventional cold cathode fluorescent lighting (CCFL) or in flat FED displays. Therefore unfavorable effects such as poor uniformity, aging of phosphors, instability and distortion of color, and erosion of electrodes commonly observed in conventional fluorescent lighting may then be prevented. The energy input to the light-emitting device  100  of the invention is directly converted into visible white light with no other middle stages for adjusting wavelength. The light-emitting device  100  of the invention can be further improved by adding peripheral electromagnetic shields (not shown) or other complementary components outside of the substrates  102  and  104  to enrich functionality of the light emitting device  100 . 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.