Thermion emitting oxide cathode and method for making the same

Abstract: A thermion emitting oxide cathode comprises a metal cap (5) and a cathode sleeve (3), the metal cap being coated with a thermion emitting material layer (9) comprising a titanate of a barium-based alkaline earth metal. The preferred titanate is made by mixing titanium tetrachloride (TiCl 4 ) with an aqueous solution of barium-strontium-calcium dichloride [(Ba-Sr-Ca)C1 2 ], dropping the mixture into an oxalate solution of 80°C, thereby precipitating a barium-strontium-calcium titanate hydrate [(Ba-Sr-Ca)TiO(C 2 O 4 ) · 4H 2 O], and treating the precipitated barium-strontium-calcium titanate hydrate at a temperature of 500 to 700°C to remove water molecules, thereby producing a suspension of barium-strontium-calcium titanate [(Ba-Sr-Ca)TiO 3 ].

Description: The present invention relates to a method for making a thermion emitting oxide cathode, and more particularly to a substance for making a thermion emitting oxide cathode widely used as a thermion emitting source such as an electronic or pick-up tube. Figure 1 shows a conventional thermion emitting cathode which is used as a thermion emitting source for Braun tubes in televisions. As shown in Figure 1, the thermion emitting cathode includes a heater 1, a cathode sleeve 3 having a tube construction surrounding the heater 1, and a metal cap 5 capping the cathode sleeve 3. The metal cap 5 is made of a nickel-based compound containing traces of a reducer such as silicon or magnesium. Over the metal cap 5 is coated a thermion emitting material layer 7, made of a three-element carbonate. This three-element carbonate used as the cathode substance is produced using a starting material which primarily comprises a three-element-stabilized carbonate containing barium, strontium and calcium in the form of a solid solution [(Ba-Sr-Ca)CO 3 ]. In order to allow the three-element-stabilized carbonate to act as a thermion emitting source, the cathode is heated to a temperature of about 1,000°C by the heater 1 at the step of producing a vacuum in an electronic tube to be manufactured. By this heating, carbon dioxide is decomposed from the carbonate which is, in turn, changed into an oxide. That is, barium carbonate (BaCO 3 ) is thermally decomposed as expressed by the following formula so that it is changed into barium oxide (BaO) and carbon dioxide. BaCO 3 → BaO + CO 2 ↑ The reason why oxides such as (Ba-Sr-Ca)O are not used as a starting material of the cathode substance is because they absorb carbon dioxide from the atmosphere and are thereby easily changed into the form of barium carbonate. The reason why the above-mentioned carbonate is used as the starting material is because it is kept stable at room temperature and pressure so that it can be easily handled. In spite of many advantages of such carbonates, the cathode is problematic in that cracks are generated at the cathode substance when the -temperature of the cathode substance is abruptly raised to change the cathode substance into an oxide with a low work function capable of emitting thermions. Such cracks result in a degradation in the electron emitting characteristic. To this end, the step of decomposing carbon dioxide from the cathode substance should be slowly and sequentially carried out. This means that the manufacture of electronic tubes essentially involves a lengthened and slow carbon dioxide decomposition and exhaustion. As a result, a long manufacturing time is required. DE-C-721 937 describes some of the advantages of a titanate of a barium based alkaline earth metal. Another object of the invention is to provide a method for making a thermion emitting oxide cathode. In accordance with the present invention a method for making a thermion emitting oxide material, comprises the steps of: (a) mixing a titanium tetrachloride with an aqueous solution of barium-strontium-calcium (or other barium-based alkaline earth metal) dichloride, and dropping the mixture into an oxalate solution at 80°C, thereby precipitating a barium-strontium-calcium (or said other metal) titanate hydrate; and (b) treating the precipitated barium-strontium-calcium (or said other metal) titanate hydrate obtained at step (a) at a temperature of 500 to 700°C to remove water molecules, thereby producing a suspension of barium-strontium-calcium (or said other metal) titanate. Other objects and aspects of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which: Figure 1 is a sectional view illustrating a conventional oxide cathode; Figure 2 is a sectional view illustrating an oxide cathode; and Figure 3 is a flow chart illustrating a method for making the oxide cathode in accordance with the present invention. Figure 2 shows a thermion emitting oxide cathode made in accordance with the method of the present invention. In Figure 3, elements respectively corresponding to those in Figure 1 are denoted by the same reference numerals. As shown in Figure 2, the thermion emitting cathode includes a heater 1, a cathode sleeve 3 surrounding the heater 1, and a metal cap 5 capping the cathode sleeve 3. The metal cap 5 is coated with a thermion emitting material layer 9 made of a barium-strontium-calcium-based titanate containing barium as a chief element therefor and a solid solution of alkaline earth metals such as strontium and calcium. Figure 3 is a flow chart illustrating a method for producing a barium-strontium-calcium-based titanate in accordance with the present invention. The method shown in Figure 3 is carried out using a compound precipitating process. This method includes a first procedure of mixing titanium tetrachloride (TiCl 4 ) with an aqueous solution of barium-strontium-calcium dichloride [(Ba-Sr-Ca)Cl 2 ], and dropping the mixture into an oxalate solution of 80°C, thereby producing a white precipitate which is barium-strontium-calcium titanate hydrate [(Ba-Sr-Ca)TiO(C 2 O 4 ) 2 · 4H 2 O]. The aqueous solution of barium-strontium-calcium dichloride has the same composition as the existing three-element-stabilized carbonate. Where the aqueous solution of barium-strontium-calcium dichloride is used to manufacture Braun tubes, it preferably has a composition having a barium:strontium:calcium ratio of 57.2:38.0:4.8 (wt%). The method also includes a second procedure of treating the white precipitate, namely, the barium-strontium-calcium titanate hydrate obtained at the first procedure at a temperature of 500 to 700°C to remove water molecules, thereby producing a suspension of barium-strontium-calcium titanate [(Ba-Sr-Ca)TiO 3 ]. The barium-strontium-calcium titanate hydrate may be produced using the well-known hydrolysis or citrate-utilizing method in place of the compound precipitating method. Now, the method for manufacturing the thermion emitting oxide cathode using the compound precipitation process will be described in detail. In the first step, an aqueous solution of barium-strontium-calcium dichloride [(Ba-Sr-Ca)Cl 2 ] with a composition having a barium:strontium:calcium ratio of 57.2:38.0:4.8 (wt%) is mixed with titanium tetrachloride (TiCl 4 ). The resulting mixture is then dropped into an oxalate solution at 80°C, thereby producing a white precipitate which is barium-strontium-calcium titanate hydrate [(Ba-Sr-Ca)TiO(C 2 O 4 ) 2 · 4H 2 O]. In the next step, the barium-strontium-calcium titanate hydrate is treated at a temperature of 500 to 700°C, thereby producing a suspension of barium-strontium-calcium titanate [(Ba-Sr-Ca)TiO 3 ]. This suspension is then sprayed onto the metal cap 5 mounted to the cathode sleeve 3 so as to form a thermion emitting material layer 9. Thus, an oxide cathode as shown in Figure 2 is manufactured. When the oxide cathode- manufactured in accordance with the above-mentioned method is subjected to a carbon dioxide decomposing and exhausting process in the manufacture of an electrode tube, ingredients of the suspension used in the manufacture of the cathode structure are removed as they are heated to a high temperature. Also, the exhausting of gaseous carbon dioxide from the cathode is completed in a short time. Further heating the cathode will not exhaust any more gas. Thus, the present invention basically solves the factor of lengthening the manufacturing time by changing the starting material. In accordance with the present invention, the time taken to decompose and exhaust the carbon dioxide can be remarkably reduced. The cathode material used in accordance with the present invention is stable not only at room temperature, but also at a higher temperature. In particular, the titanate of the cathode material contributes to the thermion emission of the cathode. Where the present oxide cathode is employed in an electronic tube, it exhibits a superior thermion emission characteristic. That is, the oxide cathode of the present invention exhibits a superior evaporation of barium, which is a primary thermion emitting source, over existing cathodes while emitting thermions in an approximately equal amount to the existing cathodes. As is apparent from the above description, the present invention provides a method for making the cathode, being capable of greatly reducing the carbon dioxide decomposing and exhausting time required in the manufacture of electronic tubes, thereby greatly reducing the manufacturing cost of the electronic tubes. In accordance with the present invention, it is possible to improve the characteristic of the cathode such as the characteristic distribution depending on the carbon dioxide decomposition degree. Accordingly, a uniform thermion emission characteristic is obtained. In addition, an improved barium evaporation characteristic is obtained. As a result, the life of the cathode is greatly lengthened. Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope of the invention as disclosed in the accompanying claims.

Claims: A method for making a thermion emitting oxide material, comprising the steps of: (a) mixing titanium tetrachloride with an aqueous solution of barium-strontium-calcium (or other barium-based alkaline earth metal) dichloride, and dropping the mixture into an oxalate solution at 80°C, thereby precipitating a barium-strontium-calcium (or said other metal) titanate hydrate; and (b) treating the precipitated barium-strontium-calcium (or said other metal) titanate hydrate obtained from step (a) at a temperature of 500 to 700°C to remove water molecules, thereby producing a suspension of barium-strontium-calcium (or said other metal) titanate. The method in accordance with claim 1, wherein the aqueous solution is of barium-strontium-calcium dichloride having a barium:strontium:calcium ratio of 57.2:38.0:4.8 (wt%). A method for making a thermion emitting oxide cathode comprising the steps of (a) making a thermion emitting oxide material in accordance with any preceding claim, (b) applying the thermion emitting oxide material to a metal cap (5), (c) mounting the cap (5) on a cathode sleeve (3) (d) providing heating means (1) for heating the cap (3).

Inventors: CHOI JONG-SEO, CHOI KWI-SEUK, JOO KYU-NAM, KIM GEUN-BAE, LEE KWANG-MIN, LEE SANG-WON, CHOI, JONG-SEO, CHOI, KWI-SEUK, JOO, KYU-NAM, KIM, GEUN-BAE, LEE, KWANG-MIN, LEE, SANG-WON
Applicants: SAMSUNG DISPLAY DEVICES CO LTD, SAMSUNG DISPLAY DEVICES CO., LTD.
Application year: 1995
Main classification: H01J1
IPC codes: H01J1, H01J9, C01G23