CERAMIC ENVELOPE DEVICE FOR HIGH-PRESSURE DISCHARGE LAMP

Abstract: A ceramic envelope device for a high-pressure metal-­ vapor discharge lamp, has a translucent ceramic tube (6), a pair of electrically conducting end caps (14) closing opposite ends of the ceramic tube, and a pair of opposed discharge electrodes (16) each of which is supported at its one end by the corresponding end cap (14) such that the other end of the electrode protrudes from the inner surface of the corres­ ponding end cap in a longitudinally inward direction of the ceramic tube. Each of the end caps is covered at its inner surface with an electrical insulator (20). To avoid "arc-back" between the electrode and the end cap and also corrosion of the central portion of the end cap, the electrical insulator has a protruding portion (22) surrounding a part of the corres­ ponding electrode which protrudes from a radially central portion of the inner surface of the corresponding end cap, The surrounded part of the corresponding electrode is radially spaced from the protruding portion of the electrical insulator.

Description: The present invention relates in general to a discharge tube assembly for use in a high-pressure discharge lamp (hereinafter referred to as "HID lamp"; "HID" representing High Intensity Discharge), and more particularly to electrically conducting end caps or closure discs which close the opposite ends of a translucent ceramic tube which cooperates with the end caps to form a gas-tight envelope incorporated in a HID lamp. In the art of such HID lamps using a translucent ceramic tube, a pair of electrically conducting discs are known as end caps to close the opposite open ends of the translucent ceramic tube. Examples or such closure end caps are illustrated in US-A-4,155,757 and US-A-4,155,758. Such end caps are formed of an electrically conducting cermet obtained by mixing, for example, particles of tungsten with particles of aluminum oxide, and sintering the mixture. These electrically conducting cermet end caps support a pair of tungsten electrodes at their inner surfaces in the interior of the ceramic envelope so that the electrodes protrude from the inner surfaces of the end caps toward each other, i.e., longitudinally inwardly of the translucent ceramic tube. In the meantime, electrical contact rods or lead rods are connected or fixed to the outer surfaces of the cermet end caps by suitable methods, so that electric power is applied to the pair of opposed tungsten electrodes through the contact rods and through the cermet end caps. Such cermet end caps have been advantageously employed, for example, in high-pressure sodium lamps, because they eliminate the need of using expensive metallic niobium. It is further recognized that such cermet end caps have been used also advantageously for so-called metal halide lamps which employ translucent ceramic tubes charged with a suitable metal halide together with mercury and rare gas, because the cermet exhibits relatively high corrosion resistance to metal halides. However, such a HID lamp with a translucent ceramic tube closed by cermet end caps may suffer a problem generally known as "arc-back" phenomenon wherein an arc will take place between the electrodes and the corresponding cermet end caps, rather than between the opposed electrodes, when the HID lamp is initially turned on. This arc-back phenomenon causes the cermet end caps to crack, thereby causing the translucent ceramic tube to leak. In addition, the "arc-back" phenomenon gives rise to vaporization and scattering of refractory metal component of the cermet, and consequent deposition thereof on the inner surface of the ceramic tube, which results in blackening the wall of the translucent ceramic tube, thereby reducing the degree of its luminous flux. It is also recognized that supersaturated metal halide in the ceramic tube of a metal halide lamp may be condensed at the cold spot in the tube, e.g., at the lower end portion of the ceramic tube disposed vertically when the lamp is used in its upright position. The cermet end caps closing the end portions of the tube are subject to corrosion due to the liquid phase of condensed metal halide which is heated by thermal conduction from the heat electrodes, with a result that the end caps fail to stably support the electrodes in their upright posture, if the corrosion becomes severe. EP-A-74720 illustrates high pressure sodium-mercury amalgam and metal halide vapour lamps in which a conductive end cap and plug construction seals the tube and supports the electrodes. A one-piece end closure, rather than the two-part end plug and cap, is also envisaged. Insulators may be arranged covering the inner surfaces of the cermet plugs. EP-A-74188 describes sodium-mercury amalgam vapour lamps in which conductive end caps of the tubes also have insulators on their inner surfaces. The insulators have central protruding portions around inwardly projecting central portions of the end caps, in order to avoid rectification occurring during the starting period. US-A-3 892 993 discloses high pressure discharge lamps containing an amalgam of sodium and mercury having ceramic sealing members with supply conductors passing through them. The ceramic sealing members are shaped to provide compartments for the amalgam. A wall of the compartment, which may be a longitudinal projection of one of the sealing members, shields the electrode from the amalgam in the compartment, to prevent irregular glowing in the lamp caused by quickly evaporating drops of metal in the vicinity of the electrode. The present invention, which was made in view of the above-discussed inconveniences experienced in the prior art, has as its principal object the provision of a discharge tube assembly for a high-pressure metal halide metal-vapor discharge lamp, which avoids not only the "arc-back" phenomenon between its electrodes and the corresponding end caps but also the corrosion of the central portions of the end caps around the fixed ends of the electrodes, and wherein the electrodes are stably supported in position by the end caps for a long period of time. According to the invention, there is provided a discharge tube assembly as set out in claim 1. In the discharge tube assembly of the invention, the electrical insulator covering the inner surface of the corresponding end cap will effectively protect the ceramic envelope device against the "arc-back" phenomenon at the moment when the lamp is turned on. That is, the electrical insulator will serve to protect the end cap against damage due to such "arc-back" phenomenon, thus contributing to improvement in the operating reliability of the lamp. Furthermore, the prevention of the "arc-back" by the electrical insulator results in solving the conventionally experienced problem of blackening of the inner surface of the translucent ceramic tube, thereby maintaining a high degree of luminous flux of the translucent ceramic tube. More specifically, the protruding portion of the electrical insulator keeps the liquid phase of metal halide of the metal halide lamp which may be condensed near the end cap, away from the exposed end portion of the heated discharging electrode. At the same time, the predetermined distance between the electrode and the corresponding electrical insulator protects the liquid metal halide against thermal conduction of the high-temperature electrode. As a result, the reactivity of the liquid metal halide is inhibited. The central portion of the end cap around the fixed end of the electrode is protected against corrosion by the liquid metal halide. Hence, the instant ceramic tube overcomes the conventional failure of the end cap to stably support the electrode. In accordance with one embodiment of the invention, the electrical insulator is made of a refractory ceramic material selected from the group consisting of alumina, beryllia, spinel, boron nitride and glass frit. Above all, it is recommended that the electrical insulator is made of white and opaque alumina. Preferably the electrical insulator has an annular peripheral portion of a constant thickness, as measured from the inner surface of the corresponding end cap. In this case, the thickness of the annular peripheral portion is preferably held within a range of 0.05-0.8 mm. Preferably the protruding portion of the electrical insulator is positioned at a radially central part of the corresponding end cap, and is of tubular shape having a central bore through which the corresponding electrode extends. The thickness of the protruding portion is preferably within a range of 1.0-3 mm, as measured from the inner surface of the corresponding end cap. The protruding portion may have a variable-diameter part which diameter decreases as it protrudes from the inner surface of the end cap. The electrical insulator may have a second protruding portion which contacts a central part of the corresponding end cap and protrudes radially into the central bore, while the second protruding portion is radially spaced apart from the corresponding electrode. In another embodiment, the electrical insulator may be of substantially frusto-conical shape having a central bore through which the corresponding elecrode extends with a radial gap therebetween. The predetermined distance between the electrode and the corresponding electrical insulator is preferable not more than a half of a radius of the end cap, more preferably within a range of 0.1-2 mm. The foregoing and other optional objects, features and advantages of the invention will be apparent from reading the following description of illustrative embodiments taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic elevational view partly in cross section of an example of a HID lamp incorporating one embodiment of a discharge tube assembly (ceramic envelope device) of the invention which includes a translucent ceramic tube and end caps closing the opposite ends of the tube; Fig. 2 is a fragmentary view partly in cross section, showing in enlargement one end portion of the envelope device of the HID lamp of Fig. 1; and Figs. 3-5 are views corresponding to Fig. 2, illustrating modified embodiments of the invention. Referring first to Fig. 1, there is schematically illustrated a complete assembly of a HID lamp which incorporates one preferred embodiment of a discharge tube assembly 6 (ceramic envelope device) of the invention which will be described. In the figure, reference numeral 2 designates a bulbiform translucent jacket which is generally made of glass or similar material. This translucent jacket 2 is closed at its open end by a base 4. The jacket 2 and the base 4 cooperate to form a gas-tight enclosure which is charged with a suitable inert gas such as nitrogen, or maintained under vacuum. As is well known in the art, electric power applied to the base 4 is supplied, via electrical conductor members 10, 10, to electrically conducting lead members in the form of electrical contact rods 8, 8 which are disposed at the opposite ends of the ceramic envelope device 6 acccommodated in the translucent jacket 2. The ceramic envelope device 6 includes a translucent ceramic arc tube 12 and a pair of closure discs in the form of end caps 14, 14 which are secured to the opposite ends of the ceramic arc tube 12 such that the end caps 14, 14 close the opposite ends of the arc tube 12 so as to maintain gas-tightness of the ceramic envelope 6. The translucent ceramic arc tube 12 is a tubular member made of alumina or other ceramic materials as disclosed in US-A-3,026,210 and US-A-3,792,142. The end caps 14, 14 are formed of an electrically conducting cermet. The ceramic arc tube 12 of the gas-tight ceramic envelope device 6 is charged with a suitable gas, and suitable metal or its compound which is selected depending upon the specific type of the HID lamp, from the standpoints of radiant efficacy, color-rendering properties, etc. In a metal halide lamp, the arc tube 12 is charged with metal halide (such as dysprosium iodide, thallium iodide, sodium iodide, indium iodide, etc.), together with mercury and rare gas. The subject matter of the invention is particularly related to the electrically conducting end caps 14, 14 which serve as closure members for the translucent arc tube 12. As illustrated in Fig. 2 on an enlarged scale, an electrically conducting end cap 14 is fixedly fitted in one end of a translucent ceramic arc tube 12, by shrinkage differential between the end cap 14 and the arc tube 12 during a sintering process. The contact rod 8 is embedded at its one end in the outer portion of the end cap 14, such that the other end of the rod 8 protrudes outwardly from the outer surface of the end cap 14. In the meantime, a known electrode 16 of tungsten or some other metal is similarly embedded at its one end in the inner portion of the end cap 14, such that the other end of the electrode 16 protrudes from an inner surface 18 of the end cap 14 in the longitudinally inward direction of the translucent arc tube 12. The electrode 16 is positioned at a radially central portion of the end cap 14. The inner surface 18 from which the electrode 16 protrudes is covered with an electrical insulator 20, except the central portion thereof around the fixed end of the electrode 16. In this embodiment, at least the electrical insulator 20 for the lower end cap 14 (the lower one when the lamp is oriented upright as shown in Fig. 1) has a central protruding portion 22 of tubular shape which protrudes, longitudinally inwardly of the ceramic arc tube 12, so as to surround a longitudinally intermediate part of the centrally located discharge electrode 16 which protrudes from the inner surface 18 of the corresponding (lower) end cap 14. Stated more specifically, the central protruding portion 22 protrudes from an annular peripheral portion 23 of the electrical insulator 20, and has a thickness larger than that of the peripheral portion 23, as measured from the inner surface 18 of the end cap 14. The discharge electrode 16, which is embedded over a suitable length in the central portion of the corresponding end cap 14, extends through a central bore 24 defined by the protruding portion 22 while being spaced a predetermined distance from the protruding portion 22 (the electrical insulator 20), in the radial direction of the electrode. In the ceramic envelope device 6 described above, the electrical insulator 20 which has the central protruding portion 22 provided therein with the central bore 24 and covers the inner surface 18 of the end cap 14 is effective to prevent an "arc-back" phenomenon which is an electrical discharge between the electrode 16 and the inner surface 18 upon application of a voltage between the opposed electrodes 16, 16 through the contact rods 8, 8 at the moment when the HID lamp is turned on. Therefore, the electrical insulators 20, 20 permit normal arcing between the opposed ends of the discharge electrodes 16, 16, while making it possible to prevent the conventionally experienced troubles of cracking and consequent leaking at the end caps 14, 14 due to the "arc-back" phenomenon, and to avoid vaporization and scattering of refractory metal of the cermet end caps 14, 14. Accordingly, the electrical insulators 20, 20 are capable of solving the conventionally encountered problem of blackening of the inner surface of the translucent arc tube 12 due to deposition of the refractory metal, and thereby overcoming the resulting problem of reduced luminous flux of the arc tube 12. Moreover, when the lamp is on, the central bore 24 (more strictly, the predetermined radial distance between the electrode 16 and the inner or bore-defining surface of the protruding portion 22 of the electrical insulator 20) effectively cuts off thermal conduction from the heated electrode 16 so as to keep at a comparatively low temperature the liquid phase of supersaturated metal halide condensed around the inner surface of the peripheral portion 23 of the electrical insulator 20 and thereby inhibit the reactivity of the liquid metal halide. The liquid metal halide around the central bore 24 is gasified by the high-temperature electrode 16, and the gasified metal halide is condensed in the cold spot spaced from the electrode 16, i.e. at the peripheral portion of the ceramic arc tube 12. As a result, the central portions of both the end cap 14 and the electrical insulator 20 around the electrode 16 are advantageously protected against corrosion by the liquid metal halide. Hence, the durability of the lamps is increased. Furthermore, the central protruding portion 22 of the electrical insulator 20 keeps the liquid phase of metal halide condensed in the vicinity of the end cap 14, away from the exposed end portion of the discharge electrode 16, so that the central portion of the cermet end cap 14 around the fixed end of the electrode 16 is protected against exposure to the liquid metal halide and consequent corrosion thereof. Hence, the conventional failure of the end cap 14 to stably support the electrode 16 is effectively avoided. On the other hand, even though the thermal-expansion properties of the electrical insulator 20 (e.g., alumina) may not match those of the electrode 16 (e.g., tungsten, molybdenum), this fact may not cause the electrical insulator 20 to crack or suffer similar problems when the lamp, more specifically the envelope device, is manufactured. This is because the predetermined distance between the electrode 16 and the inner surface of the protruding portion 22 prevents the electrical insulator 20 from suffering such problems due to the thermal-expansion differential therebetween. The electrically conducting end caps 14, 14 closing the translucent ceramic arc tube 12 of the ceramic envelope device 6 are formed of suitable known electrically conducting materials having a coefficient of thermal expansion which is intermediate between that of the material of the translucent ceramic arc tube 12, and those of the refractory metal of the electrodes 16, 16 and contact rods 8, 8. For example, a composite material of metallic tungsten or molybdenum and aluminum oxide, or tungsten carbide, or tungsten boride may be suitably used for the end caps 14, 14. In particular, it is recommended to use a cermet which is a composite material of a non-metallic material and a metal and is variable in refractoriness (heat resistance), corrosion resistance, thermal expansion coefficient and electric resistance by changing its composition. Preferably, the cermet consists of 8-50 % by weight of refractory metal such as tungsten or molybdenum, and the balance being aluminum oxide. The cermet containing not more than 8 % by weight of a metallic material is excessively high in electrical resistance, while the cermet containing the same in an amount exceeding 50 % by weight can not be a sufficiently densified body, and renders the end caps 14, 14 poor in gastightness. The electrical insulators 20, 20 provided to cover the inner surfaces 18, 18 of the end caps 14, 14 on the side of the electrodes 16, 16, are made of known suitable electrically insulating materials, preferably refractory and electrically insulating ceramics having a thermal expansion coefficient close to that of the material of the end caps 14, 14. For example, the electrical insulators 20, 20 are made of alumina, beryllia, spinel, boron nitride, or glass frit. In particular, it is recommended to use white and opaque alumina, because the material reflects advantageously radiant heat of the electrodes 16, 16 and thereby keeps the liquid phase of supersaturated metal halide at a lower temperature than other materials. These insulators 20, 20 are formed in a suitable known process. For instance they are molded and sintered, simultaneously as an integral part of the end caps 14, 14 or separately from the end caps 14, 14. They may be formed by applying a coating of a selected insulating material to the pre-sintered material of the end caps 14, 14, by using a glass-frit sealing layer, by a spraying method or other suitable methods. In accordance with the present invention, it is essential that the electrical insulator 20 be formed with a protruding portion 22 protruding along a longitudinal axis of the electrode 16 and surrounding a part of the electrode 16. Therefore, although the central protruding portion 22 of the illustrated embodiment of Fig. 2 is provided as a stepped portion which protrudes from the annular peripheral portion 23 of the electrical insulator 20, it is possible that the electrical insulator 20 be formed as shown in Fig. 3, such that the protruding portion 22 is a variable-diameter part which has a thickness increasing in a radially inward direction toward the central bore 24, as measured from the inner surface 18 of the end cap 14. In other words, the diameter of the variable-diameter part of this type protruding portion 22 decreases as it protrudes from the inner surface 18. While at least the inner surface 18 of each end cap 14 must be covered with the electrical insulator 20 according to the invention, it is possible to cover all surfaces of the end cap 14 with the electrical insulator 20. The thickness of the peripheral portion 23 of the electrical insulator 20 of Fig. 2 is selected within an appropriate range so as to effectively restrain the "arc-back" phenomenon, generally within an approximate range of 0.05-0.8 mm. On the other hand, the thickness of the central protruding portion 22 surrounding the longitudinally intermediate part of the electrode 16 is chosen within a range of 1.0-3 mm measured from the inner surface of the end cap, in order to protect the exposed portion of the electrode 16 against exposure to the condensed metal halide, and to thereby protect the central portion of the end cap 14 around the fixed end of the electrode 16. However, the thickness of the central protruding portion 22 should be determined so that the top of the protruding portion 22 will not contact a coil 17 wound on the exposed portion of the electrode 16. The diameter of the central bore 24 is selected so that the electrode 16 and the protruding portion 22 of the electrical insulator 20 do not contact each other. To this end, the distance ℓ between the two members is determined to be not more than 1/2 a radius of the end cap 14, more preferably, approximately within the range 0.1-2 mm. Another embodiment of the invention is illustrated in Fig. 4. The central bore 24 defined by the protruding portion 22 of the electrical insulator 20 avoids more effectively the arc-back phenomenon if the insulator 20 is provided with a second protruding portion 26 which contacts an annular central part of the corresponding end cap 14 and protrudes radially inwardly from a part of the protruding portion 22 into the central bore 24. In this case, the second protruding portion 26 defines a second central bore 28 and is radially spaced a shorter distance from the corresponding electrode 16 than the distance ℓ. The closure end caps 14, 14 covered with the electrical insulators 20, 20 which have been described hitherto, are suitably applicable to the translucent ceramic tube 12 used in high pressure metal halide HID lamps. While the end caps 14 of Figs. 2, 3 and 4 are secured to the ceramic arc tube 12 by utilizing a shrinkage differential between the two members during a sintering process, it will be obvious that the end cap 14 may be fixed to the ceramic tube 12 with the help of a sealing layer 30 of glass frit, for example, as illustrated in Fig. 5. While the present invention has been illustrated in its preferred embodiments, it is to be understood that the invention is not limited by the details of description of construction and arrangement.

Claims: A discharge tube assembly (6) for a high-pressure metal halide discharge lamp, comprising a translucent ceramic tube (12), a pair of electrically conducting cermet end caps (14) closing opposite ends of the ceramic tube and at least partly located within the end of the ceramic tube, and a pair of opposed discharge electrodes (16) each of which is supported at one end by one said end cap such that its other end protrudes from a radially central portion of an inner surface of the end cap in a longitudinal direction of the ceramic tube, said end caps each being covered at their inner surfaces with an electrically insulating member (20), at least one of said electrically insulating members being formed with a portion (22) protruding longitudinally of the ceramic tube (12) and surrounding a part of the corresponding electrode (16), said part of the corresponding electrode not being embedded in the end cap (14) and being radially spaced a predetermined distance from the electrically insulating member (20). A discharge tube assembly as claimed in claim 1, wherein said electrically insulating members (20) are made of a refractory ceramic material selected from the group consisting of alumina, beryllia, spinel, boron nitride, and glass frit. A discharge tube assembly as claimed in claim 2, wherein said electrically insulating members (20) are made of white and opaque alumina. A discharge tube assembly as claimed in any one of claims 1 to 3 wherein said at least one electrically insulating member (20) has an annular peripheral portion (23) of a constant thickness from which said protruding portion (22) protrudes. A discharge tube assembly as claimed in claim 4, wherein said annular peripheral portion (23) has a thickness of 0.05-0.8 mm, as measured from the inner surface of the corresponding end cap (14). A discharge tube assembly as claimed in any one of claims 1 to 5 wherein said protruding portion (22) is positioned at a radially central part of the corresponding end cap, and is of tubular shape having a central bore (24) through which the corresponding discharge electrode (16) extends. A discharge tube assembly as claimed in claim 6, wherein said protruding portion (22) has a thickness of 1.0-3 mm, as measured from the inner surface of the corresponding end cap (14). A discharge tube assembly as claimed in claim 6 or claim 7 wherein said protruding portion (22) has a variable-diameter part which has a thickness increasing in a radially inward direction toward said central bore (24), as measured from the inner surface of the corresponding end cap. A discharge tube assembly as claimed in any one of claims 6 to 8, wherein said at least one electrically insulatin member (20) has a second protruding portion (26) which contacts a central part of the corresponding end cap and protrudes radially into said central bore (24), said second protruding portion (26) being radially spaced from the corresponding electrode. A discharge tube assembly as claimed in claim 1, wherein said at least one electrically insulating member (20) is of substantially frusto-conical shape having a central bore (24) through which the corresponding discharge electrode extends with a radial gap therebetween. A discharge tube assembly as claimed in any one of claims 1 to 10 wherein said predetermined distance is not more than half of the radius of said end cap. A discharge tube assembly as claimed in claim 11, wherein said predetermined distance is within the range of 0.1-2mm. A high-pressure metal halide discharge lamp having a discharge tube assembly according to any one of the preceding claims.

Inventors: ATSUMI, SENJI, IZUMIYA, HIROTSUGU, KAJIHARA, TAKEHIRO
Applicants: NGK INSULATORS, LTD.
Application year: 1985
Main classification: H01J61
IPC codes: H01J61