Cold cathodes for fluorescent lamps

Abstract: There is provided a metallic tape supporting a mercury vapour releasing material, preferably Ti₃Hg, admixed with a non-evaporable getter metal, preferably an alloy of 84% Zr - 16% Al, contained within a continuous series of depressions within the tape. The depressions form a successive series of pairs of depressions preferably each having an oval shape. Each pair of oval shaped depressions is separated by a distance greater than the distance separating the individual oval shapes. The tape can then be cut between each pair of depressions to form a small strip containing two depressions. Such a small strip can then be folded through an angle of approximately 180° about an axis, in the plane of the tape midway between the depressions, perpendicular to the tape length. This folded strip can then be welded to a support and used as a cold cathode electrode in a miniature fluorescent lamp. The cathode can be heated during a manufacturing process to release mercury and subsequently act as a cold cathode and getter device.

Description: Mercury dispensing getter devices are well known in the art. See for example Japanese Utility Model Application Publication No. 50-10679 and USA Patents No. 3,722,976; 3,657,589 and 3,733,194 among others. However such publications give no teachings regarding the miniaturization of cold cathode fluorescent lamps which are required, for instance, in the back-lighting of liquid crystal displays for automobile instruments panels, pocket television sets, and the like, or simply for lighting purposes. For example US-A-3 722 976 discloses a method for manufacturing a mercury dispensing device with T i3 Hg admixed with a Zr-Al alloy as mercury vapour releasing material, which is not adapted to be used as a cold cathode and can not be introduced in small tubes as required by miniaturized fluorescent lamps. One particular attempt to produce a miniaturized cold cathode fluorescent lamp has been to use two small pieces of a strip coated with both T i3 Hg and an alloy of 84% Zr - 16% Al spot welded to wire support leads. However these cathodes are still relatively large and they also have the defect that insufficient mercury is released to ensure the desirable lifetime of the fluorescent lamp. It is therefore an object of the present invention to provide an improved cold cathode for use in the manufacture of a miniaturized fluorescent lamp based on the use of a powdered mercury vapour releasing material, said cold cathode comprising a metallic tape of nickel plated iron supporting an intermetallic compound T i3 Hg admixed with a powdered non -evaporable getter material comprising an alloy of 84% Zr-16% Al. Such an object is achieved by means of the features of the characterizing portion of claim 1. According to a particular embodiment of the present invention the cold cathode comprises a wire support attached thereto. Fig. 1 shows a perspective view of a prior art cold cathode for use in a fluorescent lamp; Fig. 2 shows a plan view of a mercury releasing metal getter tape to be used for manufacturing a cold cathode of the present invention; Fig. 3 is an enlarged cross-sectional view along line 3-3′ of Fig. 2; Fig. 4 is a perspective view of a cold cathode produced using a portion of the tape shown in Fig. 2; Fig. 5 is a view of the cold cathode of Fig. 4 taken along arrow A of Fig. 4; and Fig. 6 is a cross sectional representation of a miniaturized fluorescent lamp utilizing a cold cathode of the present invention. Referring now to the drawings and in particular to Fig. 1 there is shown a prior art cold cathode electrode 100 comprising two strips 102, 102′ of nickel plated iron. Outer facing surface of strip 102 has pressure bonded thereon a layer 106 of powdered 84% Zr - 16% Al non-evaporable getter alloy. The outer surface (not shown) of strip 102′ has a similar layer. Inner facing surface 108 of strip 102′ has a layer 110 of powdered Ti₃Hg mercury releasing intermetallic compound. Strips 102, 102′ can be cut from continuous lengths of strip commercially available under the code name St 101-505/CTL/NI/6.3-3 from SAES GETTERS S.P.A. Milan, Italy. Strips 102, 102′ are welded to supports 112, 112′ at points 114, 114′. The welding points 114, 114′ must take place in the border of the strip as shown in the border 116 of strip 102. This is to avoid any possible release of mercury or destruction of the gettering properties of the Zr-Al alloy in the assembly process of the cold cathode prior to insertion in the fluorescent tube. As the dimensions of the strips 102, 102′ are approximately 0.65 cm width by 0.6 cm length, the accurate positioning of two such strips together with the support electrodes is extremely difficult. There is a severe risk of overheating, during welding, of both the mercury releasing alloy and the non-evaporable getter alloy. Furthermore attempts to miniaturize even further the cold cathode lead to a reduction of the quantities of Ti₃Hg present and hence a reduction of the quantity of mercury released within the fluorescent lamp. Also a reduction of the quantity of non-evaporable getter material present does not ensure sufficient removal of the dangerous residual gases released during the life of the fluorescent lamp. Referring now to Fig. 2 there is shown a metallic tape 200 to be used for preparing cold cathodes of the present invention. Metallic tape 200 can be of any metal suitable for supporting a mercury vapour releasing material and a non-evaporable getter metal. Preferably tape 200 is of nickel plated iron. Tape 200 has a continuous series of depressions 202, 202′, 204, 204′, 206, 206′. Depressions 202, 202′ form within said tape a pair of individual depressions. Successive depressions 204, 204′ provide another pair of individual depressions. Again successive individual depressions 206, 206′ form a further pair of depressions, and so on. Each pair of depressions is separated by a distance greater than the distance separating the individual depressions. Preferably the depressions are of approximately oval shape as shown in Fig. 2 such that their length is greater than their width. The oval shape shown in the drawings can also be described as the shape of a race track or a cartouche. Referring now to Fig. 3 there is shown a cross-section 300 along lines 3-3′ of Fig. 2 of depression 204. Cross section 300 shows a metallic tape containing depression 204. Into depression 204 there is compressed a mercury vapour releasing material 302. Mercury vapour releasing material 302 preferably comprises a powdered mixture of intermetallic compound Ti₃Hg with a powdered non-evaporable getter metal preferably comprising an alloy of 84% Zr - 16% Al. Referring now to Fig. 4 there is shown a cold cathode 400 of the present invention. Cold cathode 400 is formed from a small length of tape 402 formed for instance by cutting a small length of tape 200 as shown in Fig. 2 along the lines indicated by alpha and beta in Fig. 2. Small strip of tape 402 is then folded along line gamma as indicated in both Fig. 4 and Fig. 2. Line gamma lies in the plane of the tape 402 midway between the two depressions within the tape 402 and perpendicular to the tape length. The tape 402 is bent through an angle of approximately 180°. As shown in Fig. 4 the bending has taken place so that the depressions face outwardly. This allows the cathode to occupy the smallest possible space. The cathode 400 may be welded to a wire support 404 as shown in both Figs. 4 and 5. In this case the wire support 404 is shown as being internal to the cathode 400 but it will be realized that a small length of tape 402 may be more closely folded and wire support 404 may be external to the cold cathode 400. Referring now to Fig. 6 there is shown a miniature fluorescent lamp 600 comprising a cylindrical glass envelope 602 and end seals 604, 604′. Miniature fluorescent lamp 600 incorporates two cold cathodes 400, 400′ being held within the miniature fluorescent lamp by means of seals 604, 604′ respectively. EXAMPLE A continuous length of nickel plated iron strip having a width of 2.5 mm is taken and a continuous series of depressions are formed therein such that the depressions form successive pair of individual depressions. Each depression has a width of approximately 1.5 mm and a length of 3.5 mm. The separation between the individual depression of each pair is 2.5 mm whereas the separation between successive pairs of depressions is 5 mm. Each depression is filled with approximately 5 mg of a 50% by weight mixture of an intermetallic compound Ti₃Hg and 50% by weight of 84% Zr - 16% Al non-evaporable getter material. A cold cathode is produced by cutting a small length of the tape and folding it as described above and it is welded to a wire support. Two such electrodes are used to produce a miniature fluorescent lamp as shown in Fig. 6. The cathode can be heated during a manufacturing process to release mercury and subsequently act as a cold cathode and getter device.

Claims: A cold cathode electrode comprising a metallic tape (200) of nickel plated iron supporting a powdered mercury vapour releasing material (302) comprising intermetallic compound Ti₃Hg admixed with a powdered non-evaporable getter material comprising an alloy of 84% Zr - 16% Al, characterized in that said vapour releasing material is deposited within two depressions (204, 204′) formed in said tape (200), wherein the depressions have an essentially oval shape, and the tape (200) is folded through an angle of approximately 180° about an axis (γ), in the plane of the tape, midway between the depressions (204, 204′), perpendicular to the tape length, the depressions thereby facing outwardly of the cathode. A cold cathode according to claim 1 characterized in that it comprises a wire support (404) attached thereto.

Inventors: RABUSIN ELIO, RABUSIN, ELIO
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1989
Main classification: H01J7
IPC codes: H01J61, H01J7