Directly heated cathode structure and manufacturing method thereof

Abstract: A directly heated cathode structure includes a porous pellet (500) in which cathode material is impregnated, a first metal member (510) being fixed to the lower surface of the porous pellet (500), a second metal member (520) being welded with the first metal member (510), and a filament (600) being interposed between the first and second metal members. A method for manufacturing a directly heated cathode structure includes the steps of manufacturing a porous pellet (500) having a multiplicity of cavities, welding a first metal member (510) to the lower surface of the porous pellet by a brazing layer (700), impregnating electron radiating material (600) into the cavities of the pellet, and welding a second metal member (520) to the first metal member (510) so that a filament (600) is disposed between the first and second metal members. Life of the cathode structure is prolonged by this because thermions are not emitted through the lower surface of the pellet.

Description: The present invention relates to a directly heated cathode structure for a cathode-ray tube (CRT), and, more particularly, to a directly heated dispenser cathode structure for use in a color CRT electron gun and to a manufacturing method for such a structure. Cathodes for absorbing heat energy and emitting thermions can be divided for the most part according to the manner of heating, into a directly heated type and an indirectly heated type. In the structure of the direct-heated cathode, the filament and the thermion emission source are in direct contact with each other, whereas in the indirect-heated cathode a separated structure is provided for the filament and thermion emission source. A directly heated cathode is disclosed in US-A-5 057 736. In contrast to the indirectly heated cathode, which is generally used for an electron gun requiring a great quantity of thermions, the directly heated cathode is used for an electron gun of a small CRT, such as that for a built-in viewfinder of a video camera. A directly heated cathode is generally fixed directly to a filament and provided with a base metal the surface of which is coated with electron-radiating material or a pellet into which cathode material is impregnated. In the structure shown in FIG. 1, developed by the applicant, a pair of filaments 102 and 102' are directly welded to the opposing sides of a porous pellet 101 in which electron-radiating material is impregnated. Alternatively, a single such filament may penetrate the porous pellet 101. In another structure developed by the applicant, the filaments are directly welded to (or penetrate at) at least three points on the outer sides of the porous pellet in which the electron-radiating material is impregnated. The above-mentioned directly heated cathode structures require only a very short interval after current is applied before starting thermion emission and exhibit a high-density thermion emission, since the filament is in contact with the pellet body itself and the porous pellet is heated directly by the filament current. However, there is a possibility of loss of thermions, since the thermion emission is made through the entire surface of the pellet, including the sides thereof. Moreover, thermion-radiating material evaporated from the pellet is attached to the filament, thereby embrittling the filament. Additionally, the process of securing the filament to the pellet (either by welding it to or passing it through the pellet) is difficult in practice, resulting in lower productivity in manufacture. The present applicant has also developed a directly heated cathode having an improved structure, as is shown in FIG. 2. Here, a filament 210 is fixed to a metal member 220 which is arranged under a pellet 200 in which electron radiating material is impregnated. Since metal member 220 covers the lower surface of pellet 200, thermion emission through the lower surface of pellet 200 is effectively blocked. Nonetheless, a small proportion of the thermions escape through minute gaps which exist between pellet 200 and metal member 220. Moreover, since the sides of the pellet also constitute thermion emission surface area, continuous and uniform thermion emission cannot be achieved. Furthermore, the life of pellet 200 is shortened due to the rapid consumption of the electron radiating material, and, as in the case of the aforementioned structure, the attached electron-radiating material evaporated from the sides of pellet 200 to the filament embrittles the filament. To solve the above problems, it is an object of the present invention to provide a directly heated cathode structure and a manufacturing method thereof wherein emission of electron radiating material through the lower surface of a pellet is prevented and the structure thereof is stabilized to thereby provide quality and productivity improvement. Accordingly, the invention provides a directly heated cathode structure comprising a porous pellet where cathode material is impregnated, a first metal member being fixed to the lower surface of the porous pellet, a second metal member being welded with the first metal member, and a filament being interposed between the first and second metal members. There is further provided according to the invention a method for manufacturing a directly heated cathode structure comprising the steps of manufacturing a porous pellet having a multiplicity of cavities, welding a first metal member to the lower surface of the porous pellet by a brazing layer, impregnating electron radiating material into the cavities of the pellet, and welding a second metal member to the first metal member so that a filament is fixed between the first and second metal members. Moreover, another method for manufacturing a directly heated cathode structure is provided according to the invention which comprises the steps of manufacturing a porous pellet having a multiplicity of cavities, impregnating electron radiating material into the cavities of the pellet, welding a first metal member to the lower surface of the porous pellet by a brazing layer, and welding a second metal member to the first metal member so that a filament is disposed between the first and second metal members. Specific embodiments of the invention are described below, by way of example, with reference to the attached drawings in which: FIG. 1 is a perspective view schematically illustrating a conventional directly heated cathode structure; FIG. 2 is a section schematically illustrating another conventional directly heated cathode structure; FIG. 3 is an exploded perspective view illustrating a directly heated cathode structure according to an embodiment of the present invention; FIG. 4 is a section illustrating the assembled directly heated cathode structure shown in FIG. 3; and FIGS. 5-9 are process drawings for explaining a method for manufacturing the directly heated cathode structure according to the present invention. FIGS. 3 and 4 show an exploded perspective view and a assembled sectional view, respectively, of a preferred embodiment of a directly heated cathode structure according to the present invention. The directly heated cathode structure comprises a porous pellet 500 of which cavity is impregnated with electron radiating material, a first metal member 510 being fixed to the lower surface of a pellet 500 by brazing, a filament 600 disposed under first metal member 510, and a second metal member 520 welded to first metal member 510 and for supporting filament 600 with filament 600 being in contact with the lower surface of first metal member 510. Here, the porous pellet 500 is made of tungsten (W), molybdenum (Mo), ruthenium (Ru), nickel (Ni) and/or tantalum (Ta), and the material used for first and second metal members 510 and 520 includes molybdenum (Mo), tantalum (Ta) and/or tungsten (W). On a surface of pellet 500 used in this embodiment of the present invention, a coating layer (not shown) including osmium (Os), ruthenium (Ru) and/or iridium (Ir) is formed. It is preferred that the diameter and thickness of pellet 500 are 0.4-2.0mm and 0.2-1.0mm, respectively, and that the diameter and thickness of first and second metal members 510 and 520 are 0.3-3.0mm and 20-200µm, respectively. It is also preferred that the diameter of filament 600 interposed between the first and second metal members is 30-200µm. For the welding of first metal member 510 and second metal member 520, laser welding, arc welding or plasma welding can be employed. Moreover, it is preferred that filaments are arranged either cross-wise or radially, to achieve more efficient pellet heating. A preferred embodiment of a manufacturing method of the directly heated cathode structure according to the present invention will be described now in detail. Primarily, as shown in FIG. 5, powder of tungsten (W), molybdenum (Mo), ruthenium (Ru), nickel (Ni) and/or tantalum (Ta) is shaped by compression into a column and is then sintered. When the sintering is completed, a columnar material 50 is severed at a predetermined length to obtain a unit porous pellet 500. Here, the cross section of the pellet may be circular or polygonal. Then, as shown in FIG. 6, porous pellet 500, contacted by cathode material 600, is heated at a high temperature so that the cathode material can be impregnated into cavities of the porous pellet. Next, as shown in FIG. 7, after setting the lower surface of pellet 500 upwards, a brazing weld layer 700 including ruthenium (Ru) and/or Molybdenum (Mo) is formed on the lower surface of the pellet to a thickness of 10-100µm. As shown in FIG. 8, first plate metal member 510 including molybdenum (Mo), tungsten (W) and/or tantalum (Ta) is contacted with brazing weld layer 700, and then first plate metal member 510 and brazing weld layer 700 are heated to a high temperature, so that first metal member 510 is attached to the lower surface of the pellet by the melted brazing weld layer 700. Then, as shown in FIG. 9, a single filament or crossed filament 600 is arranged on first metal member 510, and a second plate metal member 520 is put thereon. Then, the second metal member is welded to first metal member so that a cathode structure of the present invention is obtained. In an alternative embodiment of the present invention, the step in which the cathode material is impregnated into the pellet is performed after the first metal member is coupled to the pellet by the brazing weld, in contrast to the above-mentioned embodiment. Accordingly, the order of impregnation of the cathode material can be changed, if required, in a manufacturing method of the directly heated cathode according to the present invention. The cathode structure manufactured by the above method of the present invention has merits as discussed below. In this structure the filament is fixed to the lower surface of pellet 500 between the first and second plate members. Firstly, when the impregnation of cathode material is performed after the first-member brazing weld step, oxidation of the electron radiating material due to the brazing weld can be prevented. Secondly, since the lower surface of the pellet is completely closed by the first metal member which is brazing-welded, evaporation of the electron radiating material through the lower surface of the pellet can be blocked. Thus, continual thermion emission renders possible and life of the cathode structure is prolonged. Thirdly, the structure of the filament fixed to the pellet is stabilized so as to have a large strength against external impact. Fourthly, since thermion radiating material does not escape through the lower surface of the pellet, embrittlement of the filament can be prevented. As described above, cathode structures manufactured in accordance with manufacturing methods for directly heated cathode structure according to the present invention can contribute to the improvement of product quality and productivity due to the strong pellet structure and improved weld process. Cathode structures according to the present invention can also be used in color CRTs for large-screen televisions and computer monitors, as well as in small black-and-white CRTs.

Claims: A directly heated cathode structure comprising: a porous pellet (500) in which cathode material is impregnated; a first metal member (510) fixed to the lower surface of said porous pellet (500); a second metal member (520) welded to said first metal member (510); and a filament (600) interposed between said first and second metal members. A directly heated cathode structure according to claim 1, wherein said pellet (500) and said first metal member (510) are fixed by a brazing weld layer. A directly heated cathode structure according to claim 2, wherein said brazing weld layer is formed of at least one metal selected from the group consisting of ruthenium (Ru) and Molybdenum (Mo). A directly heated cathode structure according to any of claims 1 to 3, wherein said filament fixed between said first and second metal members is arranged cross-wise or radially. A directly heated cathode structure according to any preceding claim, wherein said pellet (500) includes as a main constituent at least one metal selected from the group consisting of tungsten (W), ruthenium (Ru), molybdenum (Mo), nickel (Ni) and tantalum (Ta). A directly heated cathode structure according to any preceding claim, wherein said filament (600) includes as a main constituent one metal selected from the group consisting of tungsten (W) and molybdenum (Mo). A directly heated cathode structure according to any preceding claim, wherein at least one of said first and second metal members includes at least one metal selected from the group consisting of molybdenum (Mo), tungsten (W) and tantalum (Ta). A directly heated cathode structure according to any preceding claim, wherein the diameter and thickness of said porous pellet (500) are 0.4-2.0mm and 0.2-1.0mm, respectively. A directly heated cathode structure according to any preceding claim, wherein the diameter and thickness of said second metal member (520) are 0.3-3.0mm and 20-200µm, respectively. A method for manufacturing a directly heated cathode structure comprising the steps of: manufacturing a porous pellet (500) having a multiplicity of cavities; welding a first metal member (510) to the lower surface of said porous pellet by a brazing layer (700); impregnating electron radiating material (600) into said cavities of said pellet (500); and welding a second metal member (520) to the first metal member (510) so that a filament (600) is fixed between the first and second metal members. A method for manufacturing a directly heated cathode structure comprising the steps of: manufacturing a porous pellet (500) having a multiplicity of cavities; impregnating electron radiating material (600) into said cavities of said pellet (500); welding a first metal member (510) to the lower surface of said porous pellet by a brazing layer (700); and welding a second metal member (520) to the first metal member (510) so that a filament (600) is fixed between the first and second metal members. A method for manufacturing a directly heated cathode structure according to claim 10 or claim 11, wherein said brazing weld layer (700) is formed of metal powder including one metal selected from the group consisting of ruthenium (Ru) and molybdenum (Mo). A method for manufacturing a directly heated cathode structure according to any of claims 10 to 12, wherein said brazing weld layer is formed to the thickness of 10-100µm.

Inventors: BONG-UK JEONG, CHANG-SEOB KIM, SANG-KYUN KIM, SEOK-BONG SON, BONG-UK,JEONG, CHANG-SEOB, KIM, SANG-KYUN, KIM, SEOK-BONG, SON
Applicants: SAMSUNG DISPLAY DEVICES CO LTD, SAMSUNG DISPLAY DEVICES CO., LTD.
Application year: 1995
Main classification: H01J1
IPC codes: H01J29, H01J1, H01J9