Patent Application
20080185963
The invention relates generally to lamps and, more particularly, techniques to reduce the potential for thermal stresses and cracking in high intensity discharge (HID) lamps.
High-intensity discharge lamps are often formed from a ceramic tubular body or arc tube that is sealed to one or more end structures. The end structures are often sealed to this ceramic tubular body using a seal glass, which has physical and mechanical properties matching those of the ceramic components and the end structures. Sealing usually involves heating the assembly of the ceramic tubular body, the end structures, and the seal glass to induce melting of the seal glass and reaction with the ceramic bodies to form a strong chemical and physical bond. The ceramic tubular body and the end structures are often made of the same material, such as polycrystalline alumina (PCA). However, certain applications may require the use of different materials for the ceramic tubular body and the end structures. In either case, various stresses may arise from the sealing process, the interface between the joined components, and the materials used for the different components. For example, the component materials may have different mechanical and physical properties, such as different coefficients of thermal expansion (CTE), which can lead to residual stresses and sealing cracks. These potential stresses and sealing cracks are particularly problematic for high-pressure lamps.
The geometry of the interface between the ceramic tubular body and the end structures also may attribute to the foregoing stresses. For example, the end structures are often shaped as a plug or a pocket, which interfaces both the flat and cylindrical surfaces of the ceramic tubular body. If the components have different coefficients of thermal expansion and elastic properties, then residual stresses arise because of the different strains that prevent relaxation of the materials to stress free states. For example in the case of the plug type end structure, if the plug has a lower coefficient of thermal expansion than the ceramic tubular body and seal glass, then tensile stresses arise in the ceramic-seal glass region while compressive stresses arise in the plug region upon cool-down after sealing.
In addition to the ceramic tubular body and end structures, high-intensity discharge lamps also include a variety of internal materials (e.g., gases) and electrode materials to create the desired high-intensity discharge for lighting. The particular internal materials disposed in the high-intensity discharge lamps can affect the sealing characteristics, the light characteristics, and the type of materials that may be workable for the lamp components and the seal glass. For example, certain internal materials, such as halides and metal halides, may be desirable for lighting characteristics, but they are corrosive to some of the ceramic and metallic components that comprise the tubular body and end structure.
A lamp is provided with an axially and radially graded structure to reduce the possibility of thermal stresses, cracks, and other defects in the lamp. In one embodiment, a system includes a ceramic lamp having a ceramic arc envelope and an end structure coupled to the ceramic arc envelope, wherein the end structure is graded both axially and radially into a plurality of regions. In another embodiment, a system includes a lamp having a layered end structure with a plurality of layers disposed one over another and that extend in both axial and radial directions relative to an axis of the lamp, wherein the plurality of layers include different materials having different coefficients of thermal expansion, Poisson's ratios, or elastic moduli, or a combination thereof.
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.
As illustrated, the lamp 10 includes a hermetically sealed assembly of a hollow body or arc envelope 12, a dosing structure 14 having a dosing tube 16 extending through an end structure 18, and an end structure 20. As illustrated, the arc envelope 12 has a cylindrical or tube-shaped geometry, while the end structure 20 has an axially and radially graded plug-shaped geometry. The lamp 10 also has lead wires 22 and 24 extending into the end structures 18 and 20 toward an interior of the arc envelope 12. In the arc envelope, arc electrodes 21 and 23 are coupled to the end structures 18 and 20 and terminate at arc tips 26 and 28. The arc gap between these arc tips 26 and 28 is set according to the distance of insertion of the arc electrodes 21 and 23 into the end structures 18 and 20. An internal dosing material 30 also may be disposed inside the hermetically sealed assembly. For example, certain embodiments of the dosing material 30 include a rare gas and mercury. Other embodiments of the dosing material 30 further include a halide, such as bromine, or a rare-earth metal halide.
As discussed in detail below, various embodiments of the end structures 18 and 20 include a plurality of layers of different materials, such as a ceramic, a metal, and various cermets having different ceramic-metal compositions. Specifically, these different materials are arranged to provide both a radial gradient and an axial gradient in the materials and their characteristics (e.g., coefficient of thermal expansion, Poisson's ratio, or elastic modulus, or a combination thereof) throughout the end structures 18 and 20. For example, the illustrated end structure 18 includes axially and radially graded regions 32, 34, and 36, which may include a ceramic, a first cermet (e.g., a first percent by volume metal cermet), and a different second cermet (e.g., a second percent by volume metal cermet), respectively. Similarly, the illustrated end structure 20 includes axially and radially graded regions 38, 40, and 42, which may include a ceramic, a first cermet (e.g., a first percent by volume metal cermet), and a different second cermet (e.g., a second percent by volume metal cermet), respectively. In certain embodiments, the ceramic may be the same or similar to the arc envelope 12, such as polycrystalline alumina, as discussed in further detail below. In addition, the first and second cermets may include a suitable metal, such as tungsten and/or molybdenum, that is also used at least to some extent for the arc electrodes 21 and 23, the lead wires 22 and 24, and/or the dosing tube 16.
In some embodiments, the end structures 18 and 20 may include a cermet-ceramic structure having a number of layers extending about one another in both axial and radial directions, wherein the number is greater than two, and wherein a volume fraction of metal is changed from a low to a high either continuously or in steps either from an outside to an inside of the end structure or, alternatively, from the inside to the outside of the end structure. For example, the low may be 0 percent and the high may be 100 percent. In one specific embodiment, a first layer includes a ceramic, a second layer includes the ceramic and about 5 to 15 volume percent refractory metal, and a third layer includes the ceramic and about 35 to 65 volume percent refractory metal. For example, the metal may include a refractory metal, such as tungsten, molybdenum, rhenium, or tantalum, or a combination thereof. By further example, the ceramic may include polycrystalline alumina (PCA), yttrium-aluminum-garnet, ytterbium-aluminum-garnet, microgram polycrystalline alumina (μPCA), alumina or single crystal sapphire, yttria, spinel, ytterbia, or a combination thereof. Furthermore, the number of layers may be selected to minimize the potential for thermal stress in the end structures 18 and 20, or such that the thermal stress generated within the end structures 18 and 20 is not significant to cause cracking.
As a result, the regions 32, 34, 36, 38, 40, and 42 provide a stepwise change in the material characteristics (e.g., coefficient of thermal expansion, Poisson's ratio, or the elastic modulus, or a combination thereof) in both the radial and axial directions, thereby decreasing the possibility of thermal stresses and cracking within the lamp 10. For example, the coefficient of thermal expansion, Poisson's ratio, or elastic modulus, or a combination thereof, may increase from one portion/layer to another across the plurality of portions at each end structure 18 and 20. In addition, the different materials may have a ceramic percentage that decreases and a metallic percentage that increases from one portion/layer to another across the plurality of portions at each end structure 18 and 20. In one embodiment, the first portion/layer is made of alumina, the second layer is made of alumina and about 5-15% tungsten by volume, and the third layer is made of alumina and about 35-65% by volume of molybdenum. In addition, the geometry of these portions or at the interface between these portions 32, 34, 36, 38, 40, and 42 may include a generally convex geometry, a generally concave geometry, a cup-shaped geometry, a plug-shaped geometry, a stepped geometry, a conical geometry, a disc-shaped geometry, or a combination thereof. Various embodiments and details of these radial and axial gradients are discussed in further detail below with reference to the subsequent figures.
In addition, certain embodiments of the lamp 10 are bonded or sealed together by one or more seal materials, a material diffusion or co-sintering process, localized heating, and/or other suitable techniques. For example, one embodiment of the lamp 10 has a seal material applied between the end structures 18 and 20 and opposite end portions of the arc envelope 12. In another embodiment, the end structures 18 and 20 are bonded to opposite end portions of the arc envelope 12 via material diffusion without using any seal material (e.g., a seal material free bond). Similarly, some embodiments of the lamp 10 have the dosing tube 16 and the lead wires 22 and 24 bonded to the respective end structures 18 and 20 by the application of one or more seal materials, material diffusion, and/or localized heating. After injecting the dosing material 30 into the arc envelope 12, the dosing tube 16 is sealed by localized heating, cold welding, crimping, and/or other suitable sealing techniques.
Various embodiments of the lamp 10 also 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 light, a vehicle, or a street light, among other things. As mentioned above, the lamp 10 is uniquely sealed 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 200, 300, or 400 bars and temperatures exceeding 1000, 1300, or 1400 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. These higher internal pressures are also particularly advantageous to short arc lamps, which are capable of producing a smaller (e.g., it gets smaller in all directions) arc as the internal lamp pressure increases. Different embodiments of the lamp 10 also hermetically retain a variety of dosing materials 30, such as a rare gas and mercury. In some embodiments, the dosing material 30 comprises a halide (e.g., bromine, iodine, etc.) or a rare earth metal halide.
The components of the lamp 10 are formed from a variety of materials, which are either identical or different from one another. For example, embodiments of the arc envelope 12 are formed from a variety of transparent ceramics and other materials, such as yttrium-aluminum-garnet, ytterbium-aluminum-garnet, micrograin polycrystalline alumina (μLPCA), alumina or single crystal sapphire, yttria, spinel, and ytterbia. Other embodiments of the arc envelope 12 are formed from conventional lamp materials, such as polycrystalline alumina (PCA). However, the foregoing materials advantageously provide lower light scattering and other desired characteristics. Various embodiments of the arc envelope 12 also have different forms, such as a bulb, a cylinder, a semi-sphere, or any other suitable hollow body.
In addition, the illustrated arc electrodes 21 and 23 and the lead wires 22 and 24 penetrate the end structures 18 and 20 without extending entirely through the end structures 18 and 20 and without directly contacting one another. Thus, the electricity passes through the body of the end structures 18 and 20 due to the percent by volume of metal in the cermets. In other embodiments, the arc electrodes 21 and 23 may directly connect with the respective lead wires 22 and 24. The arc electrodes 21 and 23 and the lead wires 22 and 24 are made of any suitable materials, such as tungsten or molybdenum. Some embodiments of the lamp 10 have these arc electrodes 21 and 23 and lead wires 22 and 24 diffusion bonded to the end structures 18 and 20. For example, exemplary diffusion bonds include a tungsten diffusion bond or a molybdenum diffusion bond between the respective components.
The dosing tube 16 also can be made of a variety of materials, such as an alloy of molybdenum and rhenium, both of which are stable against hot halides disposed within the lamp 10. An exemplary molybdenum-rhenium alloy has about 35-55% by weight of rhenium. In certain embodiments, the molybdenum-rhenium alloy comprises about 44-48% by weight of rhenium. Alloys of molybdenum and rhenium are also sufficiently ductile to allow the dosing tube 16 to be hermetically sealed via a crimping process, a cold welding process, or any other suitable mechanical deformation technique. However, in an exemplary embodiment of the lamp 10, the initial hermetic seal of the dosing tube 16 is made without external heat (e.g., via cold welding). In this manner, the volatile components of the dosing materials 30 are not unduly heated within the arc envelope 12. Moreover, the cold welding substantially eliminates thermal shock to the arc envelope 12 and the other components of the lamp 10. If desired, localized heating is applied to the cold weld to facilitate a stronger seal of the dosing tube 16. For example, if a crimping tool is used to provide the cold weld, then one technique of applying localized heat is to heat the crimp jaws of the tool. Another localized heating technique involves applying localized heat to the cold weld after its initial creation by the tool. In this manner, the localized heat ensures that the cold welded or crimped dosing tube 16 withstands higher pressures, such as internal pressures exceeding 1 to 500 atmospheres (e.g., 200, 300 or 400 atm.) Laser welding is one exemplary localized heating technique.
During assembly of the lamp 10, the dosing tube 16 enables the volume of the arc envelope 12 to be evacuated and back filled with the desired dosing material 30, such as a rare gas, mercury, halides, and metal halides. An exemplary embodiment of the evacuation and back fill process is performed by attaching the dosing tube 16 to a suitable processing station, as opposed to handling the assembly in a dry box and/or furnace. This is particularly advantageous when the room temperature rare gas pressure in the arc envelope 12 is substantially above one bar.
As illustrated in
Again,
Overall, the end structure 18 of
Regarding the other portions 34 and 36, the second portion 34 of the end structure 18 may include a conductive cermet (e.g., alumina 8% by volume of tungsten) that is fabricated to provide similar thermal expansion behavior to alumina and to be electrically conductive. The third portion 36 of the end structure 18 may include a conductive cermet with a higher volume fraction of a refractory metal, for example, alumina 50% by volume of molybdenum or alumina 50% by volume of tungsten. Thus, the composition of the third portion 36 provides a less than optimal thermal expansion match with alumina, but the higher volume fraction of metal provides for improved weldability to metallic components, such as the lead wire 22 for connection to external devices. In other embodiments, the portions 32, 34, and 36 may have other compositions of ceramics, metals, cermets, and combinations thereof, such that the thermal expansion characteristics increase or decrease in a stepwise manner for bonding with different components of the lamp 10.
In the illustrated embodiment, the first portion 32 is entirely ceramic, such as alumina, to provide a close thermal expansion match and effective diffusion bonding with the ceramic of the arc envelope 12. The second portion 34 is a low percent by volume of metal cermet (e.g. less than 20% by volume of metal), while the third portion 36 is a relatively higher percent by volume of metal cermet (e.g., 20-70% by volume of metal). The portions 38, 40, and 42 of the end structure 20 also have similar compositions to those of the portions 32, 34, and 36. In the illustrated embodiment, the arc electrodes 21 and 23 extend entirely through the first portions 32 and 38 and partially into the second portions 34 and 40 without connecting with the corresponding lead wires 22 and 24, respectively. Similarly, the lead wires 22 and 24 extend through the third portions 36 and 42 and partially into the second portions 34 and 40 without connecting with the arc electrodes 21 and 23, respectively. Again, the second and third portions 34, 36, 40, and 42 are electrically conductive cermets, which enable electricity to pass through the end structures 18 and 20 to the arc electrodes 21 and 23 to form an arc between the tips 26 and 28.
In step 2 of
In step 5 of
In steps 6 of
In alternative embodiments, in step 5 of the process 100 shown in
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.
