Patent Application
20070120458
The present invention pertains to High Intensity Discharge (HID) lamps. More specifically, the invention pertains to quartz or ceramic metal halide discharge lamps.
A typical metal halide discharge lamp 10 is illustrated in
Before the chamber 14 is sealed, a composition including an inert gas, a metal halide dose and mercury is injected and sealed, under controlled atmosphere, in the chamber of the discharge lamp. The metal halide dose is typically a combination of metal halides such as sodium iodide and scandium iodide or sodium iodides, thallium iodide, dysprosium iodide, holmium iodide and thulium iodide. The metal halides serve as light emitting elements. While mercury contributes slightly to the emitted spectrum of a discharge lamp in the blue range, it mainly serves to increase the electrical resistance in the arc region in order to raise the voltage to a desired value. Raising the voltage to a desired value has two effects: 1) the lamp operating current can be maintained at a low value to minimize electrode erosion for better lumen maintenance and lamp life; and, 2) minimizing end-losses for better lamp efficiency. A desired operating voltage for a high intensity discharge lamp is typically from 70V to 150V so the current can be maintained from about 0.2 amps to about 3.5 amps depending on the type of lamp and a desired power.
When power is supplied to the electrodes, and an electric arc strikes between the electrode tips 15A and 16A, creating a plasma discharge within the chamber 14. Initially an arc discharge is created by the rare gas (typically argon or xenon) reaching a temperature of about 7000 K. The arc discharge heats the chamber 14 raising its temperature to about 1000° K or higher. Then the mercury and metal halide dose start evaporating. After this warm-up phase, the lamp reaches a steady state of operation, where the plasma discharge becomes a mixture of rare gas atoms (argon or xenon), Hg atoms and ions, metal atoms and molecules coming from the metal halide dose as well as their ions and the electrons. The temperature of the plasma discharge may range typically from about 1000° K to about 6000° K.
The lamp voltage depends strongly on the electrical conductivity of the gas mixture forming the arc. In typical HID lamps, mercury serves as a buffer gas by maintaining a certain desired lamp operating voltage. Mercury may achieve the desired voltage because of its relatively low electrical conductivity, which is the function of several parameters including atom density (or vapor pressure), electron density (or ionization energy) and electron-atom momentum transfer cross-section for the so-called buffer gas.
Mercury, as a buffer gas, has a high enough electron-atom momentum transfer cross-section and high enough vapor pressure to provide a sufficient electrical resistance at the arc region and therefore a desired lamp voltage. The collision between electrons and the metal halide compounds causes excitation of the metal atoms, which release photon energy in the form of light within the visible spectrum.
Despite the effectiveness of mercury, there are disadvantages to using this metal. Most notably, mercury is very toxic and raises health and environmental concerns. Laws and regulations have been adopted and/or proposed throughout the world limiting or, in some cases eliminating the use of mercury in all products. Accordingly, efforts are being made to replace mercury with other elements or compounds that have properties similar to mercury for purposes of generating light in a high intensity discharge lamp.
Zinc iodide has been disclosed as a substitute for mercury in the presence of metal halide additives sodium iodide (NaI) and scandium iodide (ScI3) in a quartz lamp. However, scandium is aggressive toward and reactive with alumina-based ceramics, which is the envelope material to be used in the next generation automotive headlamps.
Rare earth metal halides, such as dysprosium iodide and neodymium iodide have been disclosed as a substitute for scandium iodide (ScI3) in combination with a second metal halide that is substituted for mercury in a quartz lamp. The second metal halides include aluminum iodide, iron iodide, zinc iodide, antimony iodide, manganese iodide, chromium iodide, gallium iodide, beryllium iodide and titanium iodide.
With respect to the subject inventions various combinations of metal halides, including but not limited to zinc iodide, as a substitute for mercury, in combination with one or more rare earth metal halides, sodium iodide and thallium iodide as light emitting additives, were combined and tested in a ceramic metal halide lamp. The performance of these compounds were compared to metal halide ceramic lamps having a composition of mercury combined with the same combinations of the rare earth metal halides, sodium iodide and thallium iodide as the light emitting elements. Theoretical calculations supported by experimental tests have shown that mercury substitute metal halides disassociate into metal atoms and free iodine atoms within the arc region causing a high pressure of free iodine atoms. Iodine is known to be very electronegative. That is free electrons within the arc region attach relatively easily to the iodine atoms creating negative ions of iodine. This effect causes a significant reduction in the electrons density within the arc region. Furthermore, the iodine reacts with the rare earth metal forming stable compounds, i.e. dysprosium iodide, which causes the reduction in the density of rare earth metal atoms (light emitting species). The reduction of both electron density and light emitting species atoms (rare earth) caused by the high-pressure of free iodine affect directly in a negative way the lamp performance by reducing the amount of radiated power in the visible range (lamp lumens)
The pressure of the iodine and iodine negative ions in ZnI2 dosed lamp is almost one order of magnitude greater than in the mercury-dosed lamps. This means that the electron density in the arc region as well as the light emitting atom densities are significantly lower in a ZnI2 dosed lamp than in mercury lamp for instance. The net effect is reduced lumens because the electrons and the light emitting atoms are responsible for the creation of the excited states of light emitting metal atoms.
The present invention is for a mercury-free metal halide discharge lamp, and/or a composition for the same. The discharge lamp comprises a discharge medium composition having a first metal halide that produces a luminous discharge and a second metal halide that generates a lamp voltage as a substitute for mercury. In one embodiment the composition also contains a metal in pure form that is not derived from either the first metal halide or the second metal halide.
During operation of a discharge lamp the first and second metal halides dissociate producing halogen atoms and metal atoms. The metal atoms of the first halide provide the desired light output of the lamp and the metal atoms of the second halide provide the desired lamp voltage. A portion of the halogen atoms of the second halide attaches to the electrons to form negative ions and another portion reacts with the metal of the first halide. The phenomenon results in a reduced amount of lumens because fewer electrons and the first metal halide atoms are available for collisions resulting in a lower lumens output. The excess metal in a pure form attracts, or reacts with the halogen, making available electrons and the first metal halide in a form that produces a luminous flux during operation of the lamp. In other words, the excess metal in a pure form acts as “getter” for the excess halogen free atoms.
A more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings.
The present invention for a mercury-free high intensity metal halide discharge lamp contains a discharge medium that comprises a rare gas (e.g., Ar or Xe), and a first metal halide as a light emitting element or additive that emits light within a desired range of the light spectrum and with a desired amounts of lumens. The medium also comprises a second metal halide that replaces mercury to maintain a desired operating voltage of the lamp. The discharge lamp structure comprises typical elements of a discharge lamp as illustrated in
In one embodiment, the invention also includes a metal that is reactive with a halogen and/or halogen ions that are generated during the operation of the discharge lamp. During the operation of the discharge lamp containing the above referenced discharge medium of rare gas, the first metal halide and second metal halide, the molecules of both metal halides dissociate within the arc region into metal atoms and halogen atoms. It has been determined that the largest portion of the free halogen atoms originates from the dissociation of the second metal halide: that is the voltage riser halide. The halogen atoms produced from the dissociation of the metal halides react with the metal of the first metal halide, forming stable molecular compounds that may not or will not release photons necessary for generating light thereby reducing the lumens output of the lamp.
Discharge lamps having a similar construction to the lamp illustrated in
The lamps tested included discharge lamps using the same amounts of a first metal halide that served as the light emitting material and various combinations and amounts of a second metal halide that served as a voltage “riser” or mercury substitute. The tests monitored the performance of the lamps in terms of lamp operating voltage and lumens considering various factors such as the dose type, amount, density and composition of the second metal halide, the lamp operating current and power. These test results were compared to similar tests conducted on standard ceramic metal halide lamps (Hg-CMH lamps) that included mercury as the voltage riser. The test lamps and the Hg-CMH lamps both included identical combinations and amounts of the light emitting elements or first metal halide as well as the amount or pressure of the rare gas. More specifically, all the lamps included NaI and rare earth metal halides TlI, DyI3, HoI3 and TmI3 as well as 200 torr of Ar. The first metal halide should refer to one or more light emitting elements or additives. In one embodiment, the total dose of the light-emitting element includes 10 mg, or about 36 mg/cc, including of 66.8 percent by weight of NaI, 9.2 percent by weight of TlI, 12 percent by weight of DyI3, 6 percent by weight of HoI3 and 6 percent by weight of TmI3. However, one skilled in the art will appreciate that the dose ratios, amounts or compounds may vary according to type of discharge lamp used. In addition, all lamps contained the inert gas argon sealed in the chamber at 200 torr. The pressure of argon in the lamp may range from about 100 torr to about 300 torr.
Prior to conducting the tests various metal iodides were selected having properties comparable to mercury, namely a high vapor pressure (or high atoms density), high ionization energy (or low electron density) and a large electron-atoms momentum transfer cross-section. The vapor pressures of various metal iodides were computed for a 1200° K cold spot temperature for an automotive ceramic metal halide lamp. The parameters chosen for computing the vapor pressure were determined by the specific discharge lamp used in the testing; however, these parameters may differ depending on the type of discharge lamp to be tested. In addition, other halogens may be used, such as bromine and chlorine, for providing an acceptable metal halide.
Those metal halides selected as candidates for replacing mercury included metal halides having a vapor pressure of at least 1 atm and an ionization energy of at least 6 eV at a cold spot temperature of 1200° K. Those metals chosen included zinc, aluminum, indium, gallium, zirconium, hafnium, antimony, nickel, titanium, iron, magnesium, copper and beryllium. The selection parameters, such as a minimum vapor pressure or minimum ionization energy of the metal halide compound will differ according to the type of lamp tested or used.
The performance of the test lamps in terms of the operating voltage and lumens was compared to the performance of the Hg-CMH lamps to determine which of the metal halide mercury substitutes performed comparatively with mercury in terms of maintaining an acceptable voltage and lumens at an acceptable current.
Table I below provides a list of the metal iodides, including the dose amounts and test results of sample test lamps showing the performance of test lamps that operated within a range of power about 66 watts to about 71 watts, similar to that of the Hg-CMH lamps.
By way of example the Hg-CMH lamp included a dose of 4.4 mg of mercury, operated at a power of 66 watts, produced a voltage of 69 volts and maintained an efficacy of 84 lumens per watts. Test lamp 660 included a dose amount of 4.3 mg of indium iodide (InI3) as the second metal halide mercury substitute. At a power of 67.15 watts, the test lamp 660 maintained a voltage of 39 watts and an efficacy of 46 lumens per watts.
Test lamp 629 included a dose amount of 3.8 mg of ZnI2 and a dose amount of 3.5 mg of AlI3 as the second metal halide mercury substitute. This test lamp, operating at 69 watts, produced an operating voltage of 49 volts, and an efficacy of 48 lumens per watts.
The test lamps including MgI2, SnI4, CuI, SbI3, FeI2 or NiI2 did not operate at sufficiently high power to produce lumens output to serve as an acceptable substitute for mercury.
It was found that increasing the amount, or density, of the second metal halide did help in increasing the lamp operating voltage but did not necessarily result in increasing the lumens per watts of the test discharge lamps. Indeed, increasing voltage with the amount of the second metal halide the lumens degraded. With respect to Tables II test results are listed for eight test lamps each containing different amounts of GaI2.
As shown in Table II, test lamp 581 produced the highest lumens output of 37 lumens per watts, having a 4.0 mg dose of GaI2 or a density of 16.2 mg/cc as the second metal halide mercury substitute. The test lamp 582 contained a 4.5 mg dose of GaI2 and the lumens output dropped slightly to 35 lumens per watts. The lumens output dropped more significantly with test lamp 567 which contained a 6.2 mg dose or 22.3 mg/cc of GaI2 and produced 30 lumens per watts. Based on the tests conducted it was determined that dose amounts of the second metal halide mercury substitute may range from about 1 mg/cc up to about 100 mg/cc may produce sufficient voltage and lumens for operation of a metal halide discharge lamp. A preferred range of the dose amount is from about 5 mg/cc to about 20 mg/cc with a preferable dose amount being about 18 mg/cc.
Although the test lamps did not produce lumens output as high as the Hg-CMH lamps, increasing the cold spot temperature of the lamp chamber may increase the lumens. This may be accomplished by changing the geometry of the chamber namely reducing the length, diameter and/or volume of the chamber and or by changing the parameters related to the dose of light emitting metal halides (first halide). By increasing the cold spot temperature, the vapor pressure within the chamber of both the first metal halide and second metal halide can be increased leading to increased lumens output. Also, selecting an adequate dose type and composition of the light emitting metal halide elements can enhance the lumens.
In addition to the above-described tests, the partial pressures for iodine, iodine negative ions, electrons, and dysprosium species were calculated for a metal halide (ZnI2) test lamp and a standard Hg-CMH lamp for temperatures ranging from about 1000° K to about 6000° K. This is the range of operating temperatures of the arc region depending on the location within the arc region from which the temperature is measured. With respect to
Similarly, the partial pressure of the iodine negative ions in the chamber of the ZnI2 test lamp was higher than in the Hg-CMH lamp. With respect to
The increased iodine partial pressure in the test lamp indicates that dissociation of the ZnI2 takes place producing iodine and thereafter iodine negative ions. Given the high electronegative nature of iodine, the electron partial pressure was calculated at temperature ranges from about 3000° K to about 6000° K. The
In addition, the partial pressures of the dysprosium species were calculated within the temperature. At such high temperatures the dysprosium iodide dissociates like the zinc iodide. The iodine will react with dysprosium atoms forming more stable DyI, DyI2 and DyI3 molecules, which do not emit light or do not emit light as well as the dysprosium atoms. With respect to
The effect of high-pressure of free iodine on the reduction of the partial pressure of the light emitting elements in the ZnI2 lamps has been illustrated here for the dysprosium but the same effect was found for the other light emitting elements, namely sodium, thallium, Holmium and thulium
In order to overcome the effect of iodine and iodine negative ions in reducing the pressures and/or amounts of electrons and light emitting elements, a metal in its pure form (not metal halide) was added to the discharge medium composition of the metal halide test lamps. For example, zinc was included with a zinc iodide dose. Other metals added included aluminum, gallium and indium, or a combination two, three or four of these metals. Table III below lists sample test lamps that included a dose of zinc iodide as a mercury substitute and a dose of zinc. The same light emitting elements (first metal halide) at the same dose amounts were used in these test lamps as in all other test lamps. In addition, argon was also injected into the chamber at the same pressure.
When combined with zinc iodide, the dose amount of zinc ranged from about 4 mg up to about 14.5 mg; however different amounts of zinc, other metals and combinations can be used in combination with one or metal halide mercury substitutes.
The test results of those test lamps that operated at voltages similar to that of Hg-CMH lamps, or in a range of about 65 watts to about 71 watts, were compared to the test results of the other test lamps having a metal halide mercury substitute and the Hg-CMH lamps. Table IV below lists sample test lamps having a metal dose in combination with doses of one or more metal halide mercury substitutes. The zinc was added as an “iodine collector.” That is zinc reacted with available iodine or iodine ions forming zinc mono-iodide and other zinc iodide species; thereby, preventing a significant portion of iodine atoms from collecting or reacting with free electrons and metal atoms of the first metal halide available to produce a light discharge.
The test lamps having the excess metal consistently produced higher voltage and lumens values at acceptable currents. The highest lumens output for those test lamps having a metal halide mercury substitute dose without a dose of a metal was from test lamp 629. This test lamp included a combination of ZnI2 and AlI3 in dose amounts of 3.8 mg and 3.5 mg respectively. The lumens output was 48 lumens per watts; however, the voltage was relatively low at 49 volts. The highest voltage output for such test lamps was from test lamp 565. This lamp included an 11.2 mg dose of GaI2 as the mercury substitute and produced a voltage of 79 volts; however the lumens was relatively low at 19 lumens per watts.
In comparison, test lamp 677 included a 13.5 mg dose of Zn and a 6.1 mg dose of ZnI2. This lamp produced a voltage of 75 volts and lumens of 55 lumens per watts. Indeed, each of the test lamps 695 and 705 that included a dose amount of zinc in combination with a dose amount of one or more of the second metal halides produced higher voltages and lumens than test lamps not having the excess metal combined with the second metal halide. The dose amount of excess metal in the chamber may range from about 1 mg to about 15 mg, or may have a density ranging from about 3.6 mg/cc to about 72 mg/cc. Preferably, the dose amount of the excess metal may range from about 2 mg to about 5 mg, or the density may range from about 7.2 mg/cc to about 18 mg/cc.
The partial pressure for dysprosium was calculated within temperature ranges of 1000° K to about 6000° K. With respect to
Most mercury ceramic metal halide used in general lighting typically operate with a ballast that produces a current sine waveform. In as much as the test lamps were replicas of ceramic metal halide lamps, the ballast used produced a current sine waveform. It was found that the test lamps could not operate in a stable manner or not operate at all employing a current sine waveform. Most of the lamps extinguished after operating about thirty seconds to about a minute.
The re-ignition voltage was too high with a current sine waveform. This was due to the high pressure of halogen and to its electronegative effect. With any AC current waveform, the applied current goes through zero during the polarity change and thereby the plasma temperature and electron density is significantly reduced. Just after the polarity change, the plasma “re-ignites” again and the electron density is increased again. This phenomenon usually manifests itself on the waveform of the lamp operating voltage with a spike called “re-striking voltage”. In the presence of high-pressure of iodine, as it is the case of Hg-free lamps where Hg is substituted by a metal halide dose, the electrons density is further reduced during the polarity change due to the electronegative effect of iodine. This makes it difficult for the plasma to “re-ignite”, which leads to an extremely high “re-striking voltage” spike. The net effect is that the Hg-free lamps operated with a sine waveform are either unstable or they extinguishes about thirty seconds to sixty seconds after they start.
It has been found in the work related to this invention that this problem can be solved by changing the current waveform from a sine shape to a square shape. With respect to
While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
