Patent Application
20070205723
The invention relates generally to electrodes for electric plasma discharge devices.
Low-pressure metal halide electric discharge plasmas have the potential to replace the mercury electric discharge plasma in conventional fluorescent lamps. However, many conventionally used electron emission materials, such as barium oxide, are not chemically stable in the presence of a metal halide plasma. Although the applicants do not wish to be bound by any theory, it is believed, for example, that a barium oxide (BaO) electron emissive material may react with a metal halide (MeX, wherein Me is the metal and X is the halogen) vapor, such as indium iodide vapor present in a discharge medium, leading to the formation of barium halide (BaX) vapor and a condensed metal oxide (MeO). Other conventionally used electron emission materials, such as calcium oxide and strontium oxide, may be less reactive with metal halide vapors. However, most electron emissive materials are expected to react with the metal halide vapor to some degree.
Even in conventional mercury-based fluorescent lamps, reactions which occur between the electrode material and the discharge material (mercury) are disadvantageous. In particular, mercury can react or amalgamate with electron emissive materials such as barium oxide, or with reaction products of the emissive materials. It is believed that electrode material deposits are formed on the inner wall of the lamp, as the lamp ages, and the mercury in the discharge amalgamates with the electrode material that has deposited on the wall. After this reaction or amalgamation, the mercury is more strongly bound and cannot evaporate as easily from the wall during normal operation, and hence is effectively removed from participation in the light-generation mechanism of the lamp. Undesirably, additional mercury must be placed into the lamp during its manufacture, to compensate for the mercury that is effectively lost to reaction or amalgamation, and ensure that the lamp meets its rated operational life. The reaction and amalgamation of mercury can be managed through the use of shields, which can provide both a physical as well as a chemical barrier to the loss of mercury, but the addition of shields also adds undesirably to the cost and complexity of the lamp.
Metal electrodes, such as tungsten electrodes, without electron emissive material coatings, are known in the art for high-pressure high-intensity-arc-discharge (HID) lamps. Some non-thermionic metal electrodes are also known in the art for low-pressure discharge plasmas, but only when the electrodes are relatively cold, below their thermionic electron emission temperature (for example, less than 1500 degree K). In the case of non-thermionic metal electrodes, the electrons are emitted from the electrode by “secondary electron emission” (in response to an incident high-energy ion, where typically the ion energy is 100-150 electron volts), or photoelectron emission (in response to a photon of sufficiently high energy). Such “cold cathodes” are used in neon signs and in “cold cathode fluorescent lamps” for display backlights, but because of the high cathode-fall voltage, the lamp discharge voltage is typically very high (>1 kV) to achieve good device efficiency. For general lighting, hot-cathode fluorescent lamps are commonly used instead of cold-cathode lamps, because of their higher efficiency and lower operating voltage.
Therefore there is a need for an electrode design which addresses one or more of the foregoing problems with electrodes used in low-pressure plasma discharge devices.
In one aspect of the present invention is an all-metal electron emissive structure consisting of one or more metals, wherein the electron emissive structure is operable to emit electrons in a discharge medium in response to a thermal excitation, wherein an active region of the electron emissive structure under steady state operating conditions has a temperature greater than about 1500 degree K, wherein the discharge medium under steady state operating conditions produces a total pressure less than about 1×105 Pascals, and wherein the cathode fall voltage in the discharge medium under steady state operating conditions is less than about 100 volts.
In another aspect of the present invention is an electrode including an all-metal electron emissive structure consisting of one or more metals, wherein the electron emissive structure is operable to emit electrons in a discharge medium in response to a thermal excitation, wherein an active region of the electron emissive structure under steady state operating conditions has a temperature greater than 1500 degree K, wherein the discharge medium under steady state operating conditions produces a total pressure less than about 1×105 Pascals, and wherein the cathode fall voltage in the discharge medium under steady state operating conditions is less than about 100 volts, and a supporting structure for the all-metal electron emissive structure.
In still another aspect of the present invention is a lamp including an envelope, a discharge medium disposed within the envelope, and an electrode, wherein the electrode comprises an all-metal electron emitting structure, wherein the electron emissive structure is operable to emit electrons in a discharge medium in response to a thermal excitation, wherein an active region of the electron emissive structure under steady state operating conditions has a temperature greater than 1500 degree K, wherein the discharge medium under steady state operating conditions produces a total pressure less than about 1×105 Pascals, and wherein the cathode fall voltage in the discharge medium under steady state operating conditions is less than about 100 volts.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Embodiments of the present invention include all-metal electron emitting structures and plasma discharge devices including such electron emitting structures.
Metals, such as refractory metals including tungsten, have a higher work function (greater than 4 eV) relative to conventional oxide-electron emissive materials, and consequently have to be operated at higher temperatures to emit the desired level of electrons in low-pressure discharge environments. To heat such metals to their thermionic temperature, the cathode fall voltage may have to be increased to increase the bombardment of higher energy ions on the cathode. If the cathode fall voltage is too high the incident ions will bombard the electrode surface and physically destroy the electrode by sputtering or otherwise removing material from the electrode surface. Another mechanism, which may also lead to damage of the electrode, is ion-impact-assisted etching. If all-metal electron emissive structures were to be designed in the shape of conventional electrode structures known in the low-pressure plasma discharge device art, the structures would dissipate heat at levels not useable in low-pressure discharge environments. Embodiments of the present invention include smaller, heat-conserving, all-metal electron emissive structures for low-pressure plasma discharge devices that may be brought to thermionic temperature with less total heat input.
In accordance with one embodiment of the present invention an all-metal electron emissive structure consisting of one or more metals is described, wherein the electron emissive structure is operable to emit electrons in response to a thermal excitation. As used herein, the term “all-metal” refers to a structure consisting of only metals, mixtures of metals, alloys of metals, without the presence of any metal compounds such as metal oxides, in which all reasonable measures are taken during manufacture to avoid the presence of metal compounds in the electron emissive structure. The electron emissive structure under steady state operating conditions may be configured to have an active region with balanced heating and cooling fluxes. As used herein, the term “active region” refers to the surface with area A at the interface between a gaseous plasma region (hereinafter referred to as the “gas”) and the hot, electron-emitting portion of the electron emissive structure (hereinafter referred to as the solid), when the electron emissive structure is used in a plasma discharge device. Electrons are emitted from the solid into the gas.
Although the applicants do not wish to be bound by any particular theory, the following analysis is presented to provide a method for configuring an all-metal electron emissive structure to have desirable thermionic properties. That is, heat and current transfer are continuous at the surface that separates the gas from the solid, and at the same time the cathode fall voltage is low, so as to decrease damage caused by incident ions and increase operating life.
As will be described in further detail below, the electron emissive material properties and gas material properties may be used to configure the electron emissive structure to have an active region with desirable thermionic properties. For example, thermal-radiative emittance of the active region of the electron emissive structure surface e, work function of the electron emissive structure surface φ, ionization threshold of the gas Vion, and electron temperature at the boundary between the cathode fall and the bulk plasma, expressed in energy units Te, may be used to configure the electron emissive structure to have an active region with desirable thermionic properties
The active region may be cooled by at least three thermal transport channels: conduction (to both the gas and the remainder of the electron emissive structure structure), convection (to the surrounding gas), and thermal radiation (to the surrounding structures, which may include other parts of the electron emissive structure itself).
To estimate the thermal radiative cooling of the active region, the thermal radiative emission Prad may be calculated.
Prad=ε σ T4, (1)
where ε is the thermal emittance of the active region material, σ is the Stefan-Boltzmann constant (5.67×10−2 W cm−2 K4), and T is the active region material temperature. For example, the emittance of metals like tungsten is typically 0.2-0.4, so the thermal-radiative cooling of the active region ranges from 6 to 425 W/cm2 for temperatures in the range 1500 K to 3700 K. For example, the active region may also include the immediately underlying solid bulk material. The temperature in the active region may be assumed to be uniform or the temperature distribution in the active region may also be taken into account.
To consider additional heating and cooling mechanisms that are active when the structure is operating as a cathode in a plasma environment, the gas volume may be separated into two analysis regions: (i) the “cathode fall” region, a thin layer immediately adjacent to the electrode surface, and (ii) the “bulk plasma” region beyond the cathode fall. The “bulk plasma” may in fact be any of the regions of a discharge plasma, such as the presheath or negative glow or positive column. The “bulk plasma” may be treated as a quasineutral region with known plasma parameters, where the electric field strength is low. The bulk plasma contrasts with the cathode fall, where the net charge density is high and positive, and the electric field strength is comparatively large. The bulk plasma as a whole is the region that determines the properties of the device, such as the total current, and the efficiency of conversion of electricity into light (for a lamp). The calculations follow methods commonly used in the art to analyze the interaction of plasmas with electrodes and other boundaries.
The heating flux q to the active region when the electrode is operating as a cathode (i.e. negative with respect to the bulk plasma) is given by
q=ji(Vion−φ)+ji(VCF+5Te/2)−jeφ, (2)
where ji is the ion current density (A/m2), je electron current density, VCF is the cathode fall voltage. The cathode fall voltage is the difference in electric potential between the surface and the bulk plasma. Te is the electron temperature at the boundary between the cathode fall and the bulk plasma, expressed in energy units. Equation 2 can also be written in terms of the total current j and the parameter fi, the fraction of current in the gas at the electrode surface that is carried by the ions:
q=j[fi(Vion+VCF+5Te/2)−φ]. (3)
The heating flux q should be sufficiently high to raise the active region to the proper temperature for thermionic emission, and offset the thermal-radiative cooling at that temperature.
The Richardson equation may be used to estimate the electron emission current density at the electrode surface. A simple form of the thermionic electron current emitted from the active region is given by
je=ART2e−φT (4)
where T is the temperature of the active region. In one example, material (tungsten) and plasma parameters in Equation (3) are used to generate distributions for both heat loss and heating flux.
Table 1 is a comparative listing of electron emissive structure parameters for low-pressure discharge electrodes of the present invention with known types of electrodes. Compared with the prior art, embodiments of the present invention operate at a much higher heat flux to the surface, so that the surface can be heated to high temperature without the need for destructive heating mechanisms, and can supply sufficient current density and total current from the emitting surface. The operating temperature, heat flux, and emission current density are all higher than in conventional low-pressure discharge devices.
In one embodiment of the present invention as illustrated in
In another embodiment of the present invention as illustrated in
Alternatively, as shown in
As shown in
In some embodiments the electron emissive structure may include two or more sub-structures with one or more metals independently or in combination being present in each sub-structure. In still other embodiments the electron emissive structure may have a multilayered structure. Some metals may be chemically attacked by certain discharge compositions such as halogen vapor. In one embodiment, the structure may include a metal substrate with a metal coating. For example, a tungsten structure may be coated with rhenium.
In one embodiment of the present invention, the one or more metals included in the electron emissive structure are selected from the group of transition and rare-earth metals. In one embodiment of the present invention, the metal selection is dependent on the discharge medium the electron emissive structure is expected to operate in. In a chemically less reactive atmosphere, such as argon-mercury, the work function, melting point, vapor pressure, evaporation rate of the electrode material are some of factors determining the material selection for the electron emissive structure rather than chemical reactions with the gas and removal of the reaction products.
In a more reactive atmosphere, such as a metal halide discharge medium, the reactivity of the one or more metals used in the electron emissive structure along with other factors such as but not limited to the work function, melting point, vapor pressure, evaporation rate of the electron emissive structure material are used to select the electron emissive structure material.
In a non-limiting example, as a first step to determining a metal for use in an electron emissive structure operable in a halide environment, metals such as Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Hf, Ta, W, Re, Gd, Dy, Er, Tm, Th, with known work functions are selected so heat flux calculations can be performed.
In a following step, for example, the reactivity of the metal in an iodine atmosphere may be assessed by determining the partial pressure of the metal halide at 1500 K gas with a gas discharge composition to what may be present in a low-pressure gallium iodide lamp operating near its highest radiant efficiency. For example, a cutoff threshold for the partial pressure of the metal compounds may be chosen to be 0.1 millitorr, which may lead to selection of metals including Fe, Co, Ni, Nb, Mo, Ta, W, and Re.
In another step in the process of metal selection, the operating temperatures required to provide a nominal current emission, for example 10 A/cm2 may be determined using the equation 4 along with a determination of whether the metal is a solid at that temperature. This may lead to the selection metals such as Mo, Ta, W, and Re.
The flux calculations may be rerun at the required operating temperatures and a further selection of metals based the partial pressure at the operating temperatures may be performed to select the metal or metals for use in all-metal electron emissive structures in low-pressure discharge environments. In a non-limiting example the metals selected for use may be W and Ta.
In one embodiment, the one or more metals in the all-metal electron emissive structure have a vapor pressure under standard operating temperature of less than 0.1 Pascals. In a further embodiment, the one or more metals have a vapor pressure under standard operating temperature of less than 0.01 Pascals. In a non-limiting example, the vapor pressure of tungsten vapor over a condensed phase of tungsten, is about 0.01 Pascals at a active region temperature of about 3100 K, which is the temperature at which the heating and cooling flux balance. In another non-limiting example, the vapor pressure of tantalum vapor over a condensed phase of tantalum is about 0.01 Pascals at 2900 K active region temperature. In a further embodiment of the present invention, two or more metals may be alloyed such that the total vapor pressure above a condensed phase of the alloy is lower than the vapor pressure of any single component of the alloy over a condensed phase of itself.
One of the factors that may adversely affect the life of a lamp is the total rate of material removal from the electrode. The removal rate of one or more metals from the electron emissive structure during operation is proportional to the product of the area of the active region and the thermodynamic vapor pressure of the material. It is therefore desirable to reduce the surface area of the active region, so as to reduce the total rate of material removal, and improve the operational life of a lamp. A lower rate of material removal may also desirably reduce the accumulation of material on the inner surface of the envelope, where it can form an absorbing or reflecting film and reduce light output. During operation, if material is removed from the electrode, the location of the active region continuously adjusts itself so as to provide about the same current density and surface area. Satisfactory operation will continue until enough material is removed from the electrode to cause a significant change in the thermal balance of the active region. Accordingly it is further desirable to lower the rate of material removal to prevent undesirable changes in the thermal properties of the electrode structure. In one embodiment of the present invention, the area of the active region may be less than about 10 mm2. In a further embodiment of the present invention, the area of the active region may be less than about 1 mm2. In a still further embodiment of the present invention, the area of the active region may be less than about 0.1 mm2.
In one embodiment of the present invention, the cathode fall voltage in the plasma discharge device is less than about 100 volts. In a further embodiment, the cathode fall voltage is less than about 50 volts. In a still further embodiment, the cathode fall voltage is less than about 20 volts. In some embodiments, the cathode fall voltage is in a range from about 20 volts to about 10 volts. In some other embodiments, the cathode fall voltage is less than about 10 volts.
In one embodiment of the present invention, an electrode including an all-metal electron emissive structure may be used in an electric plasma discharge device. Non-limiting examples of electric plasma discharge devices include discharge lamps. In a further embodiment of the present invention, an electrode including the all-metal electron emissive structure is disposed within a lamp having an envelope and a discharge medium disposed within the envelope. Non-limiting examples of lamps suitable for use in accordance with teachings of the present invention include linear fluorescent lamps, compact fluorescent lamps, circular fluorescent lamps, mercury free lamps, and xenon lamps.
Plasma discharge devices typically include an envelope containing a gas discharge medium through which a gas discharge takes place, as well as two metallic electrodes that are sealed in the envelope. While a first electrode supplies the electrons into the discharge space, a second electrode provides the electrons with a path out of the discharge space, to complete the electric circuit with the power source. Discharge lamps are typically energized by an external current-limiting power supply or “ballast”. Discharge devices may be energized either with direct current or with alternating current. In direct-current operation one electrode (the cathode) always supplies electron current, and the other always absorbs electron current (the anode). In alternating current operation, each electrode alternately functions as a cathode and then an anode as the external device alternates the polarity of the current through the device. Non-limiting examples of discharge devices include a discharge medium such as but not limited to rare gases such argon and neon. Other devices include materials such as mercury and metal halides, where the discharge medium may be present as both gas and condensed material, and the partial pressure of the mercury or metal halide during steady-state operation is several times higher than when the device is at room temperature.
Electron emission generally takes place via thermionic emission, although many physical processes contribute to electron emission, including the electric field at the surface (field emission, or field-enhanced thermionic emission), ion bombardment (ion-induced secondary electron emission), and photon bombardment (photoelectron emission). Here we use the term ‘thermionic emission’ to denote materials and conditions where the relatively high temperature (>1500 K) of the electron-emission material contributes a majority of the total electron current emitted by the cathode.
Discharge medium may include discharge materials such as buffer gases and ionizable discharge compositions. Buffer gases may include material such as but not limited to rare gases such as argon, neon, helium, krypton and xenon, whereas ionizable discharge compositions may include materials such as but not limited to, metals and metal compounds. In some embodiments, ionizable discharge compositions may include rare gases. Non-limiting examples of discharge materials suitable for use in a lamp equipped with an all-metal electron emissive structure may include metals, such as but not limited to Hg, Na, Zn, Mn, Ni, Cu, Al, Ga, In, Tl, Sn, Pb, Bi, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Re, or Os or any combinations thereof. Other discharge materials suitable for use include rare gases such as but not limited to neon and argon. Still other discharge materials include but are not limited to compounds such as halides or oxides or chalcogenides or hydroxides or hydrides or organometallic compounds or any combinations thereof of metals such as but not limited to Hg, Na, Zn, Mn, Ni, Cu, Al, Ga, In, Tl, Ge, Sn, Pb, Bi, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Re, or Os or any combinations thereof. Non-limiting examples of metal compounds include zinc halides, gallium iodide, gallium bromide, indiumbromide and indium iodide. In some embodiments, in metal halide discharge lamps, the metal and halogen may be present in a non-stoichiometric ratio. For example, in a gallium iodide lamp, iodine and gallium may be present in a molar ratio (I/Ga) equal to about ⅓. In another example, iodine and gallium may be present in a molar ratio (I/Ga) in a range greater than about 2 to less than about 3. In another example, the discharge is composed of one or more rare gases with mercury as the ionizable composition. In one embodiment, the lamp is a mercury lamp. In another embodiment, the lamp is a mercury-free lamp.
In one embodiment of the present invention, an all-metal electron emissive structure is operable in a discharge medium, wherein the discharge medium under steady state operating conditions produces a total vapor pressure less than about 1×105 Pascals. As used herein, the term “steady state operating conditions” refers to operating conditions of a lamp which is in thermal equilibrium with its ambient surroundings, and wherein a majority of radiation from the discharge comes from the ionizable discharge compositions. In some embodiments of the present invention, the discharge medium in a lamp under steady-state operating conditions produces a total vapor pressure of less than about 1×105 Pascals. Typically, the buffer gas pressure during steady-state operation is higher than when at ambient temperature. The pressure rise is proportional to the temperature rise in the device. For example, for a mercury based discharge medium, an increase of about 5% in the pressure of buffer gas is seen when the operating temperature is increased to 40° C. operating from a temperature of 25° C. (non-operational). In a non-limiting example, in a mercury-free discharge medium such as gallium iodide, about 25% increase in the pressure of buffer gas is seen when the operating temperature is increased to about 100° C. from a temperature of about 25° C. (non-operational) and about 100% increase in buffer gas pressure is seen at an operating temperature of about 275° C. for indium and zinc halides. In some embodiments, the discharge medium under steady-state operating conditions produces a total vapor pressure in a range from about 20 Pascals to about 2×104 Pascals. In some other embodiments, the discharge material under steady-state operating conditions produces a total pressure in a range from about 20 Pascals to about 2×103 Pascals. In some embodiments the discharge material under steady-state operating conditions produces a total pressure in a range from about of about 1×103 Pascals. In some embodiments, the partial pressure under steady state operating conditions of the ionizable discharge composition in the discharge medium is less than about 1×103 Pascals. Typically, ionizable discharge composition pressure during steady state operation is several times higher than it was when the lamp was at ambient temperature, and often orders of magnitude higher, as the vapor pressure depends exponentially on the temperature. In further embodiments, the partial pressure under steady state operating conditions of the ionizable discharge composition in the discharge material is in a range from about 0.1 Pascals to about 10 Pascals. In one embodiment, the lamp is a mercury lamp. In another embodiment, the lamp is a mercury free lamp. In a non-limiting example, the discharge material includes argon buffer gas and gallium iodide ionizable discharge composition. At an ambient temperature of 20° C., the total pressure is about 670 Pascals, primarily due to the buffer gas, and the partial pressure of the ionizable discharge composition is about 1×10−4 Pascals. At steady state operating condition temperature of 100° C., wherein the conversion efficiency of electric power into radiation is high, at least 25 percent. The total pressure is about 1000 Pascal and the partial pressure of the ionizable discharge composition is about 1 Pascal.
In some embodiments, an all-metal electron emissive structure may be provided in a lamp including a cathode, a ballast, a discharge medium and an envelope or cover containing the discharge material. The lamp may optionally include one or more phosphors or phosphor blends. The lamp may comprise a linear lamp 100 as illustrated in
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following examples are included to provide additional guidance to those skilled in the art in practicing the claimed invention. The examples provided are merely representative of the work that contributes to the teaching of the present application. Accordingly, these examples are not intended to limit the invention, as defined in the appended claims, in any manner.
In one example, an all-metal electrode is made for use in a discharge lamp. The electrode includes a tungsten rod-like or wire-like electron emissive structure as shown in
The electrode includes a tungsten, loop-like electron emissive structure as shown in
A plasma discharge device using a vitreous silica or glass is made. The electron emissive structure-glass joint is designed such that residual conducted thermal power can pass from the wire, through the wire-glass joining area, and into the bulk of the silica or glass, consistent with a temperature in the bulk region that is equal to the envelope temperature. A design parameter which may be used for matching the heat transfer at the location where the metal rod enters the envelope is the diameter of the rod-like electron emissive structure.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
