METHOD OF MANUFACTURING A LOW-MELTING POINT ALLOY FOR SEALING IN A FLUORESCENT LAMP

Abstract: A method of manufacturing a low melting-point alloy (1) for sealing in a fluorescent lamp is disclosed. In this method, the low melting-point alloy is first melted and then dis­ charged through a nozzle (2) to be contacted with a coolant (6). A fluorescent lamp manufactured by the above method is also disclosed.

Description: The present invention relates to a method of manufacturing a low melting-point alloy which is sealed in a fluorescent lamp, particularly, a low-pressure mercury vapor discharge lamp, so as to control the mercury vapor pressure therein, and to a fluorescent lamp having a low melting-point alloy manufactured by this method sealed therein. A low-pressure mercury discharge lamp such as a fluorescent lamp is known to have a highest efficiency of converting supplied electric energy into ultraviolet radiation of 253.7 nm wavelength when a mercury vapor pressure in a sealed tube is 0,802 to 0,936 Pa (6 x 10⁻³ to 7 x 10⁻³ mmHg) and a discharge current is relatively low. Since the ultraviolet radiation of 253.7 nm wavelength has a high fluorescent excitation efficiency, the mercury vapor pressure is preferably kept to fall within the above range of 0,802 to 0,936 Pa (6 x 10⁻³ to 7 x 10⁻³ mmHg) and the temperature sealed tube wall at this time is preferably about 40°C. However, recently developed low-pressure mercury vapor discharge lamps such as fluorescent lamps have higher loads acting on the sealed tube wall due to a tendency toward a smaller tube diameter. For this reason, the temperature of the sealed tube wall is high and exceeds 100°C in some cases. When the sealed tube wall temperature becomes high, the mercury vapor pressure in the sealed tube exceeds 0,936 Pa (7 x 10⁻³ mmHg). Then, ultraviolet radiation components mainly having a wavelength of 253.7 nm are self-absorbed by mercury. This impairs the conversion ratio of the supplied energy into ultraviolet radiation and lowers the light output. As a countermeasure against this, amalgam is conventionally sealed in the sealed tube so as to suppress an increase in the mercury vapor pressure at high temperatures. Japanese Patent Publications No. 54-33215 and No. 54-38582 describe fluorescent lamps in which an amalgam consisting of Hg, In, and one metal selected from Li, Al, Zn, Sn, Pb and Bi; or an amalgam consisting of Hg, Bi, and Pb or of Hg, Bi, Pb, and Sn. Such an amalgam is sealed in the sealed tube by measuring a predetermined amount discharged from a vacuum suction thin tube having an inner diameter of about 2.0 to 2.5 mm. Therefore, conventionally, the amalgam is formed into particles by the atomizing method of spraying the amalgam in a molten state with a gas or by a method of pulverizing and granulating an amalgam ingot and measuring and sealing a predetermined amount of amalgam particles and sealing the measured amalgam particles in a sealed tube. However, amalgam particles obtained by the atomizing method have a nonuniform shape and size and must be sieved by a sieve of a predetermined size for size adjustment before measurement and sealing into a sealed tube. This results in a very low yield and an expensive method. With the method of pulverizing the ingot, the resultant particles similarly have a nonuniform particle size and shape and also have cracks. Furthermore, since the central portion of the ingot has a higher content of Hg than anywhere else, the composition of the particles varies, resulting in a variation in a suppression effect of mercury vapor pressure upon sealing in a sealed tube. The present invention has been made in consideration of this and has as its object to provide a method of manufacturing a low melting-point alloy for sealing in a fluorescent lamp, which has a uniform composition, a particle size or wire diameter falling within predetermined ranges, and allows easy measurement and introduction through a thin tube, and a fluorescent lamp in which a low melting-point alloy prepared by this method is sealed. According to the present invention, there is provided a fluorescent lamp into which particles of an amalgam are introduced through a thin tube and sealed, characterized in that the amalgam comprises mercury and at least one low melting-point metal selected from tin, lead, bismuth and indium, and is manufactured by the steps of: melting the amalgam; dripping the molten amalgam under an extrusion pressure of 490 to 1.96 x 10⁴ Pa, from a nozzle having an inner diameter of 0.15 to 1.0mm; and bringing the dripped molten amalgam into contact with a liquid coolant comprising water to rapidly cool the amalgam and to form the particles. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic sectional view of an apparatus which is used in a method of producing amalgam particles used in a lamp according to the present invention; and Fig. 2 is a graph showing the relationship between the sealed tube wall temperature and the mercury vapor pressure therein. By suitably selecting the ejecting conditions of a molten alloy from a nozzle and rapid cooling conditions, a sealing alloy in the form of particles of uniform diameter or a wire or uniform diameter may be formed. Fig. 1 illustrates an apparatus for forming a sealing alloy into particles. A low melting-point alloy raw material 1 is charged in a container 3 having a nozzle 2 at its lower end. The container 3 consists of a high-melting point material which will not react with the alloy raw material 1, for example, quartz or stainless steel. A high-frequency coil or an electric heater 4 is arranged to surround the container 3 and heat the raw material 1 held therein. A coolant container 5 is arranged below the nozzle 2 and holds therein a coolant 6 having a high cooling effect, such as water, a colloidal solution or an oil. The coolant 6 is preferably kept at a temperature ranging from room temperature to 80°C. Examples of the colloidal solution as the coolant 6 include an alumina colloidal solution and zirconia colloidal solution. Examples of the oil as the cooland 6 include silicone oil. The viscosity of such a coolant preferably should be as high as possible for example more than 10 Pa.s (1 poise) in order to obtain as spherical particles as possible. In the apparatus as described above, the alloy raw material 1 is first charged in the container 3 and is heated by the heater 4 to be melted. When the raw material 1 becomes molten and reaches a predetermined temperature, a gas is introduced into the container 3 from an inlet port at its upper portion so as to extrude the raw material 1 through the nozzle 2. The raw material 1 thus ejected from the nozzle 2 is sequentially dripped into the coolant 6 to be rapidly cooled thereby. Alloy particles 7 are thus prepared. The nozzle 2 preferably has an inner diameter of 0.15 to 1.0 mm and more preferably an inner diameter of 0.3 to 0.7 mm. When the inner diameter of the nozzle 2 is less than 0.15 mm, the extrusion resistance of the alloy becomes too great. However, when the inner diameter of the nozzle 2 exceeds 1.0 mm, the alloy droplets become too large, so that the resultant alloy particles 7 have diameters of 3 mm or more. Such large alloy particles cannot be inserted into a thin tube of the sealed tube. The distance between the nozzle 2 and a coolant surface level 8 is suitably about 2 to 100 mm and preferably falls within the range of 5 to 30 mm. When this distance is below 2 mm, the molten alloy is brought into contact with the coolant 6 and cooled thereby before it is dropped as droplets from the tip of the nozzle 2. However, when this distance exceeds 100 mm, the droplets are flattened upon colliding against the coolant surface level 8 and spherical particles cannot be obtained. The extrusion pressure of the molten alloy from the nozzle 2 preferably falls within the range of 490 to 1.96 x 10⁴ Pa (0.005 to 0.2 Kg/cm²) and more preferably falls within the range of 4900 to 9800 Pa (0.05 to 0.1 kg/cm²). When the extrusion pressure is below 490 Pa (0.005 kg/cm²), stable and continuous extrusion of alloy cannot be performed. However, when the extrusion pressure exceeds 1.96 x 10⁴ Pa (0.2 kg/cm²), the alloy is continuously extruded and a wire is obtained in place of particles. The alloy particles 7 thus obtained have a spherical shape of a diameter of about 1.5 to 2 mm. Furthermore, since these particles 7 are obtained by rapid cooling from the molten state, the particle composition is homogeneous. The thus obtained particles 7 are measured and inserted in a thin tube to be sealed in a sealed tube. Therefore, a conventional step of sieving the particles can be obviated, so that the method of the present invention is inexpensive and provides a better workability. When a plurality of nozzles 2 are formed at the lower end of the container 3, a number of droplets of the molten alloy can be formed simultaneously, resulting in a still better workability. A low melting-point alloy sealed in a fluorescent lamp, particularly, a low-pressure mercury vapor discharge lamp can be prepared by two processes. According to the first process, the alloy is formed and sealed in a sealed tube of the discharge lamp into which mercury is also separately sealed, and the alloy and mercury are alloyed (amalgamated). According to the second process, the alloy is alloyed with Hg and the resultant amalgam is formed into particles or wire to be sealed in the tube. The first process will first be described wherein particles of a low melting-point alloy is prepared by ejection cooling. The particles is then sealed in a sealed tube together with Hg, which are amalgamated when the tube is in use. The alloy composition in this case consists of Bi, In and one or both of Sn and Pb. The composition ratio is preferably 15 to 57% by weight of Sn, 5 to 40% by weight of Pb, 30 to 70% by weight of Bi, and 4 to 50% by weight of In. The alloy composition in the second process consists of Bi, In, and Hg and one or both of Sn and Pb. The composition ratio is preferably 15 to 57% by weight of Sn, 5 to 40% by weight of Pb, 30 to 70% by weight of Bi, 4 to 50% by weight of In, and 4 to 25% by weight of Hg. The elements Sn, Pb, Bi and In are respectively low melting-point metals and form an amalgam with Hg so as to lower its melting point. When the addition amounts of these metals fall within the prescribed ranges described above, a solid-liquid amalgam can be obtained at a temperature range of 50 to 130°C. Fig. 2 shows a graph showing the relationship between the sealed tube wall temperature and the mercury vapor pressure when the amalgam alloy in the form of particles is sealed in the sealed tube. As can be seen from curve A in Fig. 5, the amalgam alloy of the present invention is converted into a solid-liquid amalgam within a temperature range of 50 to 130°C. In this state, the mercury vapor pressure can be stably held at 0,802 to 0,936 Pa (6 x 10⁻³ to 7 x 10⁻³ mmHg) at which a maximum light output can be obtained. In contrast to this, in a lamp wherein only Hg is sealed, a mercury vapor pressure abruptly increases with an increase in temperature and the efficacy is lowered, as can be seen from curve B. The low melting-point alloy to be used herein is not limited to that having the composition described above but can be other composition. For example, an alloy consisting of 30 to 70% Bi, 4 to 50% In and 2 to 25% Hg can be employed. When this alloy is employed, the solid phase-liquid phase thereof will be shifted to a higher temperature side. Examples 1 - 6 The alloy raw material used consisted of 56% by weight of Bi, 16% by weight of In, 16% by weight of Sn, and 10% by weight of Hg. The raw material was charged in a quartz container 3 of an apparatus shown in Fig. 1 to prepare alloy particles 7. In this case, the nozzle inner diameter, the extrusion pressure, and a distance between a nozzle 2 and a coolant surface level 8 were varied as shown in Table 1 below to prepare alloy particles 7 using water as a coolant 6. The shape and size of the obtained alloy particles 7 were measured. For the purpose of comparison, alloy particles 7 were also prepared following the same procedures as in these examples but under conditions following outside the prescribed ranges according to the present invention. The results obtained are also shown in Table 1 below. According to a method of manufacturing a low melting-point alloy for sealing in a fluorescent lamp of the present invention, particles having a diameter of 1.5 to 2.0 mm or a wire or strip having a diameter or thickness of 0.1 to 2.0 mm can be obtained. Thus, measurement and sealing of the alloy is easy, and the obtained particles or wire or strip have a homogeneous composition due to rapid cooling from the molten state.

Claims: A fluorescent lamp into which particles of an amalgam are introduced through a thin tube and sealed, characterized in that the amalgam comprises mercury and at least one low melting-point metal selected from tin, lead, bismuth and indium, and is manufactured by the steps of: melting the amalgam; dripping the molten amalgam under an extrusion pressure of 490 to 1.96 x 10⁴ Pa from a nozzle having an inner diameter of 0.15 to 1.0 mm; and bringing the dripped molten amalgam into contact with a liquid coolant comprising water to rapidly cool the amalgam and to form the particles. A fluorescent lamp according to claim 1, characterized in that a distance between said nozzle and a coolant surface level is 2 to 100 mm. A fluorescent lamp according to claim 1, characterized in that the coolant is a member selected from the group consisting of water, an aqueous glycerin solution, alumina colloid and zirconia colloid. A fluorescent lamp according to claim 1, characterized in that the coolant has a viscosity of more than 10 Pa.s (1 poise.) A fluorescent lamp according to claim 1, wherein the amalgam comprises mercury, bismuth and indium. A fluorescent lamp according to claim 5, wherein the amalgam further comprises at least one of tin and lead. A fluorescent lamp according to claim 6, wherein the amalgam comprises 15 to 57% by weight of tin, 5 to 40% by weight of lead, 30 to 70% by weight of bismuth, 4 to 50% by weight of indium, and 4 to 25% by weight of mercury.

Inventors: HAGA, MASAKATSU PATENT DIVISION, OSHIMA, TERUO PATENT DIVISION, YORIFUJI, TAKASHI PATENT DIVISION, YOSHINO, HISASHI PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1984
Main classification: B22F9
IPC codes: B22D11, H01J61, C22C1, B22F9