Patent Application
20090224674
The invention relates generally to lamps and, more particularly, techniques to reduce the potential for thermal stresses and cracking in ceramic high-intensity discharge (HID) lamps.
High-intensity discharge lamps are often susceptible to crack formation and failure due to various stresses within the lamp. In certain applications, such as automotive, it is desirable to provide a quick start of the lamp. Unfortunately, this quick start subjects the lamp to severe thermal shock. For example, the quick start causes a rapid increase in temperature and hot spots within the lamp. In turn, the rapid temperature changes and hot spots (i.e., temperature differentials) often lead to the formation of cracks in the lamp. These cracks can reduce the performance of the lamp, and eventually lead to lamp failure. In addition, the liquid dose often penetrates into these cracks and further deteriorates the lamp performance and limits its life. For example, the liquid dose may be corrosive to the material (e.g., metal) in the vicinity of the cracks. These temperature differentials can have more significant effects on lamps with poorly designed geometries, interfaces, and so forth. For example, compressive or tensile stresses can develop in certain geometries and interfaces. Unfortunately, existing lamps often have geometries and/or interfaces that abruptly change, e.g., step from one diameter to another, along a length of the lamp. As a result, the severe thermal shock associated with a quick start of the lamp can lead to significantly higher stresses, hot spots, and susceptibility to cracking in the vicinity of an abrupt change in geometry and/or interfaces.
A high intensity discharge lamp, in certain embodiments, includes a uniquely shaped shoulder in the vicinity of the electrode tip in the transition region between the arc chamber and the legs of the arctube, and dimensions selected to reduce stress and associated cracking. The uniquely shaped shoulder has a variable diameter, such as, e.g., a cup-shaped geometry, a curved funnel-shaped geometry, or a conical-shaped geometry. The selected or optimized dimensions may include a tip-to-neck distance, a tip-to-wall distance, and an internal diameter of the lamp. The selected or optimized dimensions also may include a uniform or non-uniform wall thickness, an arc gap distance, and an electrode thickness. These dimensions and shapes are selected to reduce undesirably high maximum stresses and temperatures in the lamp. As a result, the lamp is able to provide higher performance with a longer life due to a decreased risk of stress cracking during rapid start up and steady state operation.
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:
One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Moreover, the use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, but does not require any particular orientation of the components. Any examples of dimensions and shapes are not exclusive of other dimensions and shapes. Also, any examples of dimensions and shapes for various portions of an assembly (e.g., HID lamp) are intended to be used alone or in combination with one another.
As discussed in detail below, embodiments of the present technique relate to high intensity discharge (HID) lamps, such as those found, for example, in automotive applications. However, the lamps discussed below are not limited to any particular application. The disclosed embodiments provide the thermal and structural design space for a horizontally operated lamp, with a transparent or translucent ceramic envelope material, that is suitable as an automotive headlight (20-50 W) or other low watt (20-100 W) HID lamp application. The shape and the dimensions of the arc envelope or arc tube and the positions of the electrodes relative to the arc envelope are specified below, such that the stresses due to thermal shock during the fast start-up of the lamp and to thermal gradients during steady state operation are reduced far below the strength of the ceramic and below the strength of the end-seal structure. Therefore, the high performance of the lamp is not compromised by its reliability and life.
In many applications, such as automotive headlamps, it is desirable to design the lamps specially for quick start, e.g., in 4 seconds after the lamp is switched on it should generate about 80% of its steady state lumen output. As a result, the lamp should be able to withstand a severe thermal shock during the warm-up. In existing lamps, a fast increase of the temperature was observed in the electrode root regions that results in the formations of cracks during the time of warming up. The liquid dose then penetrates inside these cracks and reacts with the metal parts inside the legs, which significantly deteriorates the performance of the lamp and limits its life. The embodiments discussed below address these issues with optimized shapes and dimensions of the lamp.
In designing the ceramic HID lamps discussed in detail below, the following circumstances are considered along with the operating conditions mentioned above. In any high-pressure ceramic HID lamp operating in a substantially horizontal orientation, circumferential tensile stresses develop on the outside part of the arc envelope during the operation because of the significant difference between top and bottom temperatures, which is a result of the discharge arc column bending due to the natural convection. As a result, it was found desirable to minimize the temperature difference between the top and bottom of the lamp to reduce temperature differentials, stresses, and potential crack formation. In general, an isothermal arc envelope is desirable for achieving long life. Moreover, it was found desirable to limit the compressive stresses and the temperatures on the inside of the arc envelope. Even though the ceramic material can withstand temperatures up to 1500K, the high compressive stresses at the location of the hottest spot on the inner surface of the arc envelope can result in creep deformation if the operating temperature is too high. As our models and experiments show, at a given power and for a given dose composition, the temperature distribution of an HID arc tube is controlled by the shape and the dimensions of the arc envelope, and these parameters can be carefully optimized to improve the lamp performance and increase the lamp life. For instance, it was found that an elliptical shape of the arc envelope operates more isothermally than a straight cylindrical tube of similar dimensions. Furthermore, the elliptical arc envelope enables a larger internal diameter of the lamp while maintaining the desired high temperature in the cooler regions of the arc chamber where the temperature of metal halide condensate determines the vapor pressure of the radiating metals that provide the photometric performance of the lamp. Electrode positioning relative to the arc envelope is also a factor controlling the stress formation in the ceramic. It was found desirable to position the electrode tip sufficiently far both from the arc envelope center and the neck of the arc envelope in order to prevent ceramic overheating and cracking. In addition, it was found that positioning the electrode tips sufficiently far from the bulb internal surface closest point is desirable (i.e., the shortest distance from the electrode tips to the internal surface of the bulb). This applies to lamps having electrodes centered along the axial centerline and, also, lamps having electrodes off-center or shifted from the axial centerline. It should be noted that conventional lamps are not optimized in the manner set forth below. Thus, the disclosed shapes and dimensions are not found in conventional lamps.
As discussed below, in certain embodiments, a ceramic HID automotive lamp has a rated power of 20-50 W, a shape of the arc envelope may or may not include a central cylindrical portion, and the arc envelope has curved shoulders of uniform or non-uniform wall thickness. The shoulders are curved in a way so that the temperature in the wall closest to the electrode tip is not too high and not too low, and the stresses generated both in the center of the arc envelope and in the neck region are essentially below the strength of the ceramic material. Furthermore, metal electrodes are sealed inside the legs of the lamp, and may have an arc gap equal to or greater than 2 mm and equal to or smaller than 8 mm.
In one exemplary embodiment discussed below, the lamp has the following design features. The arc gap is at least equal to or greater than the length of the cylindrical part of the arc envelope, and is at least 4 mm but smaller than 6 mm. The distance from the electrode tip to the arc envelope neck is not too large or too small, e.g., 0.25 mm≦TN≦1.55 mm. The arc envelope shoulder is curved in such a way so that the distance between the electrode tip and the closest wall point in a vertical direction is equal to or greater than 0.13 mm and smaller than half of the internal diameter (ID) of the lamp, e.g., 0.13 mm≦TW≦ID/2. The arc envelope internal diameter (ID) is not too small and not too large, e.g., 1.6 mm≦ID≦4 mm. If the internal diameter is greater than 2.5 mm, then an arc straightening technique (e.g., magnetic or acoustic straightening) can be used to straighten the bent/curved arc plasma between the electrode tips in a horizontally operated lamp. The wall thickness is inversely related to the internal diameter. For a design with the internal diameter 1.6 mm≦ID≦2.5 mm, a suitable arc envelope wall thickness is equal to or larger than 0.6 mm and smaller than 1.2 mm. For a design with 2.5 mm<ID≦4 mm, the wall thickness can be in the range of 0.3-0.8 mm. The diameter of the electrode shank is not too large to guarantee electron emission and not too small to avoid its melting, e.g., 0.25 mm<shank diameter<0.4 mm. Again, the various shapes and dimensions disclosed herein are intended to reduce stresses, reduce hot spots and other temperature differentials, and reduce the growth of cracks in the lamps. As a result, the disclosed embodiments of HID lamps provide relatively greater performance and longevity than existing lamps.
In certain embodiments, the ceramic arc envelope 12, which includes the shoulder portions 14 and 16 and the body portion 22, may be made from a variety of light-transmitting ceramics and other materials, such as polycrystalline alumina (PCA) or yttrium-aluminum-garnet (YAG). Other embodiments of the arc envelope 12 may be made from ytterbium-aluminum-garnet, microgram polycrystalline alumina (μPCA or MCA), AlN, sapphire or single crystal alumina, yttria, spinel, and ytterbia. The foregoing materials provide relatively low light absorption, high temperature capability, high strength, corrosion resistance and other desired characteristics.
In addition, the shoulder portions 14 and 16 of the ceramic arc envelope 12 may be shaped and dimensioned to reduce stresses, reduce temperature differentials (e.g., more isothermal temperature distribution), and reduce the potential for crack formation within the lamp 10. For example, the shoulder portions 14 and 16 have diameters or widths that vary relative to a longitudinal axis 50 of the lamp 10 between the respective necks 28 and 30 and body ends 18 and 20. In the illustrated embodiment, the shoulder portions 14 and 16 have a curved shaped, such as a cup-shaped geometry, a concave geometry, an elliptical geometry, or an egg-shaped or S-shaped geometry. As a result, the illustrated shoulder portions 14 and 16 gradually decrease in diameter from the body ends 18 and 20 along the longitudinal axis 50 toward the respective necks 28 and 30. The shoulder portions 14 and 16 are curved in a way so that the temperature in a wall 52 and 54 closest to the electrode tips 44 and 46 is not too high and not too low, and so that the stresses generated both in a center 56 of the arc envelope 12 and in the necks 28 and 30 are essentially below the strength of the ceramic material. The walls 52 and 54 in the shoulder portions 14 and 16 also have uniform or non-uniform thicknesses 58 and 60. Similarly, the body portion 22 of the arc envelope 12 has a cylindrical wall 62 disposed about the hollow interior 48, and the wall 62 has a uniform thickness 64.
Regarding optimization of the lamp 10, the wall thicknesses 58, 60, and 64 are inversely related to an internal diameter 66 of the central body portion 22. Based on various testing and optimization, a suitable dimension of the wall thicknesses 58, 60, and 64 may range between about 0.6 mm and 1.2 mm for a design with the internal diameter 66 ranging between 1.6 mm and 2.5 mm. For a design with the internal diameter 66 between about 2.5 mm and 4 mm, a suitable dimension for the wall thicknesses 58, 60, and 64 may range between about 0.3 mm and 0.8 mm.
Thus, based on various testing and design optimization, the illustrated arc envelope 12 may have the internal diameter 66 in a range between about 1.6 mm and 4 mm, which is not too small and not too large to cause undesirably high stresses and non-uniformity in the temperature distribution. If the internal diameter 66 of the illustrated horizontally operated arc envelope 12 is greater than about 2.5 mm, then the arc plasma between the electrode tips 44 and 46 can bend or curve beyond an acceptable limit within a horizontally oriented lamp 10. For example, undesirably high bending of the arc plasma can cause high temperature differentials (e.g., hot spots), high stresses, and a resulting formation of cracks in the lamp 10. Accordingly, one or more arc straightening techniques, such as magnetic or acoustic straightening, may be applied to the lamp 10 to straighten the bending arc plasma between the electrode tips 44 and 46 or just shift the arc center line downwards and thus reducing the “effective” bending value.
Furthermore, based on various testing and design optimization, an arc gap 68 between the electrode tips 44 and 46 is at least greater than or equal to a length 70 of the central body portion 22, e.g., between the ends 18 and 20 where the shoulder portions 14 and 16 extend toward the necks 28 and 30. The arc gap 68 is also less than an internal bulb length (IBL) 71, e.g., the distance between the interior portions of the necks 28 and 30 where the diameters begin changing from the legs 24 and 26 to the shoulder portions 14 and 16. For example, in certain embodiments, the illustrated arc gap 68 may range between about 2 mm and 8 mm. By further example, in certain embodiments, the illustrated arc gap 68 may range between about 4 mm and 6 mm.
The illustrated legs 24 and 26 may be an integral part of or coupled to the arc envelope 12. For example, in the illustrated embodiment, the arc envelope 12 and the legs 24 and 26 are a single piece structure, which may be formed of a single material (e.g., ceramic) in a single process without coupling together various separate components. In other words, the one-piece structure including the arc envelope 12 and the legs 24 and 26 may be free of seal interfaces between the various components. As a result, the arc envelope 12 and the integral legs 24 and 26 may be integrally made of a suitable ceramic, such as PCA, YAG, or another suitable ceramic as discussed in detail above with reference to the arc envelope 12. Alternately, the configuration of the one-piece structure can be achieved by joining two separately formed halves of the structure at some point between the ends 18 and 20, for example at or near the center 56. Again, these halves may be made of the same material, e.g., ceramic.
In alternative embodiments, the legs 24 and 26 may be made from different materials than the arc envelope 12. For example, the legs 24 and 26 may be made from a different ceramic, a cermet, a metal, or a combination thereof. Furthermore, the legs 24 and 26 may be coupled to the arc envelope 12 at the respective necks 28 and 30 via diffusion bonding without a seal material, with a seal material such as a sealing glass, with a plurality of sealing materials having progressively changing coefficients of thermal expansion, or another suitable sealing technique. In one specific embodiment, the legs 24 and 26 may be made from a ductile metal or alloy, such as molybdenum, rhenium, molybdenum-rhenium alloy, or a combination thereof. For example, an exemplary molybdenum-rhenium alloy has about 35-55% by weight of rhenium. In certain embodiments, the molybdenum-rhenium alloy has about 44-48% by weight of rhenium. In such embodiments with different materials and separate components, the legs 24 and 26 may be coupled to the arc envelope 12 by a crimping and/or a focused heating technique. For example, a laser, an induction heating coil, or another suitable technique, may be used to focus heat in the desired seal region without requiring the entire lamp 10 to be placed inside a furnace.
The illustrated electrode assemblies 32 and 34 are configured to reduce stresses and improve the seal with the legs 24 and 26, such that the lamp 10 can operate over a broader range of power input, internal pressures, and temperatures without forming cracks in the legs 24 and 26. For example, the coils 36 and 38 may be made from a ductile metal to provide resiliency or flexibility in the seal between the shanks 40 and 42 and the legs 24 and 26. For example, the coils 36 and 38 may be made from molybdenum, rhenium, molybdenum-rhenium alloy, or a combination thereof. Thus, the ductile material and the partial freedom to move provided by the coils 36 and 38 is able to absorb at least some of the stresses between the electrode assemblies 32 and 34 and the legs 24 and 26. As a result, the possibility of stress cracks developing within the legs 24 and 26 is substantially reduced by these electrode assemblies 32 and 34. The shanks 40 and 42 also may be made from a variety of materials, such as tungsten, or doped tungsten, or a tungsten alloy. In addition, the material of the coils 36 and 38 and/or the shanks 40 and 42 may be made entirely of or coated with a corrosion resistant material, such as molybdenum, to reduce the possibility of corrosion by a dosing material disposed within the hollow interior 48 of the lamp 10. The electrode tips 44 and 46 also may be made from a variety of materials, such as tungsten, molybdenum, rhenium, or a combination thereof, or with additional dopants. Furthermore, the electrode tips 44 and 46 may include coils or other configurations suitable for high intensity discharge electrode tips.
As appreciated, the electrode assemblies 32 and 34 may be inserted lengthwise into the legs 24 and 26 along the longitudinal axis 50, such that precise control of the arc gap 68 can be achieved during the assembly of the lamp 10. For example, if the legs 24 and 26 are made of a ductile material, then the legs 24 and 26 may be crimped and laser welded about the electrode assemblies 32 and 34. However, if the legs 24 and 26 are made of a non-ductile metal or ceramic, then the electrode assemblies 32 and 34 may be sealed or co-sintered within the legs 24 and 26 via focused heating or placement of the entire lamp 10 within a furnace. In either case, the ductile material and/or the partial freedom to move provided by the coils 36 and 38 absorbs various stresses within the legs 24 and 26 during operation of the lamp 10. As illustrated in the embodiment of
Various features of the electrode assemblies 32 and 34 also may be optimized for the illustrated lamp 10. For example, the shanks 40 and 42 have diameters or thicknesses 76 and 78, which are selected to be sufficiently small to guarantee electron emission and sufficiently large to avoid melting or excessive evaporation or sputtering loss of the shanks 40 and 42. In certain embodiments, based on various testing and design optimization, the diameters 76 and 78 of the shanks 40 and 42 may be in a range of about 0.25 mm to about 0.4 mm. Again, as discussed above, the arc gap 68 also may be selected to optimize performance, reduce stresses, improve temperature uniformity, and reduce the potential for cracking within the lamp 10. For example, the arc gap 68 of the illustrated lamp 10 is selected to be greater than or equal to the length 70 of the central body portion 22. In the specific embodiment discussed herein, the arc gap 68 may be in a range of about 4 mm to about 6 mm. Furthermore, as discussed in further detail below with reference to
In the illustrated embodiment, the electrode assemblies 32 and 34 (including the electrode tips 44 and 46) are generally aligned along the longitudinal axis 50 (e.g., centerline). However, in alternative embodiments, the electrode assemblies 32 and 34 may be mounted at positions off-axis or generally offset from the longitudinal axis 50 of the lamp 10. For example, the legs 24 and 26 may be positioned off-axis or generally offset from the longitudinal axis 50, such that the electrode assemblies 32 and 34 are also off-axis when mounted within the respective legs 24 and 26. By further example, the shanks 40 and 42 may bend at an angle or curve away from the longitudinal axis 50 toward the respective electrode tips 44 and 46, such that the tips 44 and 46 are off-axis or generally offset from the longitudinal axis 50. In this manner, the off-axis positions of the tips 44 and 46 may improve the performance of the lamp 10 by centering the arc within the body portion 22 of the arc envelope 12. In other words, depending on the radius of the arc between the tips 44 and 46, the tips 44 and 46 may be offset from the axis 50 to generally center the arc about the axis 50.
In certain embodiments, the tip-to-neck distance 80 is in a range of about 0.25 mm to about 1.55 mm. Similarly, the tip-to-wall distance 84 is in a range of about 0.13 mm to about one half of the internal diameter 66 (i.e., the internal radius) of the arc envelope 12. Thus, in the present embodiment, given that the internal diameter 66 is in a range of about 1.6 mm to 4 mm, the tip-to-wall distance 84 is in a range of about 0.13 mm to about 0.8 mm-2 mm depending on the selected internal diameter 66. In this particular embodiment, one or both of these distances 80 and 84 may be used to characterize and optimize the location of the electrode tip 46 within the lamp 10. In the same manner, these distances 80 and 84 may be used to optimize and characterize the location of the electrode tip 44 on the opposite end of the lamp 10. In the illustrated embodiment, the distance 80 is generally identical for both of the electrode tips 44 and 46, and the distance 84 is generally identical for both of the electrode tips 44 and 46. However, certain embodiments may employ different dimensions at the different ends and electrode tips 44 and 46 in the lamp 10.
As illustrated in
In the illustrated embodiments of
In other embodiments, one or both of the shoulder portions 14 and 16 may include a complex geometry including variations of a particular geometry, e.g., varying angles of the conical geometry (
Furthermore, in some embodiments, one or both of the shoulder portions 14 and 16 may include a complex or multi-type geometry, such as a combination of two or more of the geometries shown in
For example, as illustrated in
As a result, the maximum stress and the maximum temperature are generally inversely proportional relative to one another as functions of internal diameter. Thus, an optimal design of the lamp 10 generally has an internal diameter 66 that limits both the maximum stress and the maximum temperature within the lamp 10. In the illustrated embodiment of
In certain embodiments of the lamps 10 discussed above, the lamp 10 may have a variety of different lamp configurations and types, such as a high intensity discharge (HID) or an ultra high intensity discharge (UHID) lamp. For example, certain embodiments of the lamp 10 comprise a high-pressure sodium (HPS) lamp, a ceramic metal halide (CMH) lamp, a short arc lamp, an ultra high pressure (UHP) lamp, or a projector lamp. Thus, the lamp 10 may be part of a video projector, a vehicle headlight, or a street light, among other things. As mentioned above, the lamp 10 is uniquely shaped and dimensioned to accommodate relatively extreme operating conditions. Externally, some embodiments of the lamp 10 are capable of operating in a vacuum, nitrogen, air, or various other gases and environments. Internally, some embodiments of the lamp 10 retain pressures exceeding 20, 100, 200, 300, or 400 bars and temperatures exceeding 1000, 1300, 1400 or 1500 degrees Kelvin. For example, certain configurations of the lamp 10 operate at internal pressure of 400 bars and an internal temperature at or above the dew point of mercury at 400 bars, i.e., approximately 1400 degrees Kelvin. Different embodiments of the lamp 10 also hermetically retain the variety of dosing materials, such as a rare gas and mercury. In some embodiments, the dosing material comprises a halide (e.g., bromine, iodine, etc.) or a rare earth metal halide. Certain embodiments of the dosing material also include a buffer gas, such as xenon, krypton, or argon gas. In other embodiments, the lamp 10 is mercury free. For example, the lamp 10 may be dosed with a rare gas (e.g., Xe), metal halides, and zinc or zinc iodide.
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.
