Method for fabricating an impregnated type cathode

Abstract: A method for fabricating an impregnated type cathode comprises the steps of mixing metal powder having a high melting point and a heat proof property, and electron emission substance powder in a dry state, pressing the mixed powder to provide a pressed mixture, and applying an isostatic pressure to the pressed mixture contained in a sealed capsule. At the mixing stage, the metal powder is heated by a high temperature lower than the melting point, and at the mixing stage, a sintered mixture is obtained. In this method, the steps are simplified and decreased in number to decrease a fabricating cost. Furthermore, no infulence occurs in electron emission due to hydrooxides.

Description: FIELD OF THE INVENTION This invention relates to a method for fablicating an impregnated type cathode, and more particularly to, a method for fabricating an impregnated type cathode having a long life of electron emission and a stable current flowing property. BACKGROUND OF THE INVENTION An impregnated type cathode has been proposed to improve electric conduction of an oxide cathode. In this impregnated type cathode, the so-called impregnated dispenser cathode having a porous tungsten which is impregnated with electron emission substance has been dominant in this field. This impregnated dispenser cathode has been described, for instance, in the U. S. Patent Nos. 4,165,473 and 3,358,178. However, a method for fabricating an impregnated dispenser cathode has disadvantages in that steps are complicated, and a time of each step is long, so that a fabricating cost is increased. In addition, it has a disadvantage in that electron emission is badly affected by hydrooxides of metals in an emitter composed of barium oxide (BaO) calcium oxide (CaO) alumina (Al₂O₃), etc., because such oxides are easily changed into hydroxide in atmosphere during assembly process. The hydroxides melt and cover a surface of the cathode at evacuating stage at a low temperture of 100°C. The document IEEE Proceedings-I/Solid-State and Electron Devices, vol. 128, part I, No. 1, Feb. 1991, pages 19-32 discloses two methods of fabricating an impregnated type cathode. In the first method, tungsten powder, electron emission powder and a reducing agent are mixed and pressed, and the pressed powder is subsequently sintered at high temperture in hydrogen. In the second method tungsten powder is pressed and subsequently sintered in a hydrogen atmosphere. An inert plastic filler or copper is then infiltrated to act as a lubricant during machining, and is then removed. Thereafter an electron emissive powder is impregnated by melting. It is an object of the invention to provide a method for fabricating an impregnated type cathode which can be obtained with low fabricating costs and without producing hydroxide and which provides a long life of electron emission and a stable current flow. According to the invention this object is achieved by the method as defined in claim 1. The dependent claims relate to further advantageous features of the invention. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be explained in more detail in conjunction with appended drawings, wherein: Fig. 1 is a flow chart showing a conventional method for fabricating an impregnated dispenser electrode, Fig. 2 is a flow chart showing a method for fabricating an impregnated type cathode in a preferred embodiment according to the invention, Fig. 3 is a schematic cross sectional view showing a pressed mixture of particles contained in a capsule at a step of the method in the preferred embodiment, Fig. 4 is a schematic cross sectional view showing the capsule positioned in an HIP treating furnace, and Fig. 5 is a graph showing a condition of temperature and pressure in the HIP treating furnace. DESCRIPTION OF THE PREFERRED EMBODIMENT Before explaining a method for fabricating an impregnated type cathode in the preferred embodiment, a conventional method (,such as disclosed in the aforementioned IEEE document,) for fabricating an impregnated dispenser cathode will be explained in Fig. 1. At first, tungsten powder having an average particle diameter of several µm is pressed to provide rod shaped tungsten (STEP 1), and the rod shaped tungsten is sintered in an atmosphere of hydrogen at a temperature of 2500 °C (STEP 2). In the steps 1 and 2, the particle degree of the tungsten powder, the pressure, the sintering temperature, etc. are adjusted to provide a porous sintered product which is well controlled in quality. Next, the porous rod shaped tungsten is buried in copper powder and heated, so that the porous rod shaped tungsten is mechanically strengthened by the penetration of copper thereinto (STEP 3). Then the strengthened rod shaped tungsten is processed into a predetermined configuration of pellets (STEP 4), and the penetrated copper is melted and removed from the rod shaped tungsten by heating it in a vacuum state (STEP 5). Thereafter, electron emission substance which is defined to be an emitter obtained in the form of a mixture including barium carbonate (BaCO₃), calcium carbonate (CaCO₃), alumina (Al₂O₃), etc. having an appropriate mole ratio is heated to be impregnated into the pores of the pellets in an atmosphere of hydrogen at a temperature of 1600 to 1700°C (STEP 6). Finally, brushing, polishing, and cleaning are carried out to remove surplus emitter adhered on the surface of the pellets (STEP 7). Thus, the completed pellets are transferred to a following stage for assembling an impregnated dispenser cathode. As apparent from the process described above, each step is complicated and takes a long time, so that a fabricating costs are increased. In addition, the emitter composed of barium carbonate (BaCO₃), calcium carbonate (CaCO₃), alumina (Al₂O₃), etc. is molten to be impregnated into the porous tungsten pellet at a temperature of 1600 to 1700°C at the step 6, so that the above carbonates are decomposed to produce oxides such as BaO and CaO and compounds, which are liable to react with water component in the air atmosphere to produce barium hydroxide such as Ba(OH)₂. This hydroxide is molten to cover the surface of the cathode at a low temperature of several 100°C, so that electron emission is adversely affected, as described before. This is a reason why the above described disadvantages occur in the conventional method for fabricating an impregnated dispenser cathode. Next, a method for fabricating an impregnated type cathode according to a preferred embodiment of the invention will be explained in Figs. 2 to 5. At first, tungsten powder of 20 gr heated to a high temperature and having a high melting point, nickel particles of 0.12 gr and a mixture of 1.2 gr including BaCO₃ powder, CaCO₃ powder, Al₂O₃ powder which provide an emitter are dry-mixed (STEP 10), and the mixed powder is pressed in a dry and cold state under a pressure of approximately 1 ton/cm² to provide a cylindrical pressed mixture (STEP 11). This cylindrical pressed mixture 21 is inserted in a capsule 22 which is filled with boron nitride (BN) 23 as shown in Fig. 3, and the capsule 22 is sealed to provide a vacuum capsule 24 (STEP 12), and is put into a Hot Isostatic Press (HIP) treatment furnace 25 as shown in Fig. 4 (STEP 13). In this HIP treatment furnace 25, an isostatic pressure is applied in an atmosphere of argon gas to the pressed mixture 21 in accordance with temperature and pressure increasing schedule as shown in Fig. 5. As apparent from Fig. 5, the temperature is increased to 770°C, at which it is maintained for 15 minutes, and is again increased to 1,000°C, at which it is maintained for 90 minutes. During the time of 90 minutes, an increased pressure of 1,50 MPa (1,500 barometric pressure) is maintained along with maintaining of the temperature of 1,000°C to carry out a final HIP treatment, so that the pressed mixture 21 becomes a sintered product which is then processed into a predetermined configuration of pellets by a mechanical treatment (STEP 14). Then, the pellets are subjected to a cleaning process for cleaning the surface of the pellets (STEP 15), and are finally transferred to an assembling stage of an impregnated dispenser cathode (STEP 16). At this stage, barium (Ba) contained in the cathode is maintained in the form of barium carbonate (BaCO₃) which is then decomposed into barium oxide (BaO) and carbon dioxide gas (CO₂) at an evacuating stage, at which the temperature of the cathode is increased to evacuate a bulb including the cathode. The carbon dioxide gas thus formed is exhausted, and the barium (Ba) in the cathode of the bulb is changed to barium oxide (BaO) for the first time. Consequently, electron emission is not affected by hydroxide produced by a reaction of barium oxide (BaO) with water component in the method of the invention, although this is a serious problem in the conventional method. In this preferred embodiment, an impregnated type cathode is fabricated by the above described steps including the HIP treatment stage, at which it is remarkable that producing carbon dioxide gas is suppressed and explosion of capsules by CO₂ evolution is avoided. The parameters of the HIP treatment stage such as temperature and pressure, mixture ratio of Ni powder and emitter powder, etc. are one example. Therefore, these may be changed appropriately. As described above, steps which are complicated and take a long time as seen in a fabrication of a porous tungsten-sintered product, penetration and molten-out of copper, an impregnation of an emitter at a high temperature for a long time by heating, etc. are not necessary to be included in the invention. Furtheremore, an HIP treatment is carried such that a pressed mixture is contained in a vacuum-sealed capsule, so that a high pressure is unidirectionally applied to the pressed mixture from the outside of the capsule. Consequently, partial pressures of carbonates such as BaCO₃, and CaCO₃ become high, which is effective to suppress the production of oxides such as BaO, and CaO and of carbon dioxide gas CO₂ by thermal decomposition during the time of maintaining a high temperature. Even if the oxides are produced, the capsule is filled with carbon dioxide gas CO₂, so that the explosion of the capsule is definitely avoided. This avoids the decomposition of carbonates included in an emitter during the sintering process, so that the aforementioned influence of hydrooxides is avoided. In the preferred embodiment, carbonates are used as electron emitting substance. But oxides such as Ba₃Al₂O₆-CaO, BaAl₂O₄-BaO-CaO, BaO-CaO-Al₂O₃ can be used successfully. In this case, high density sintering by HIP prevents the invading of moisture, thus slowing down the adverse effect of hydroxide.

Claims: A method for fabricating an impregnated type cathode formed of a pressed and sintered mixture of tungsten powder and electron emission substance powder, the method comprising the steps of: mixing electron emission substance powder to tungsten powder or to a powder mixture of tungsten powder and nickel powder in a dry state to provide a mixed powder, said electron emission substance powder being selected from BaCO₃ - CaCO₃ - Al₂O₃ powder Ba₃Al₂O₆ - CaO powder, BaAl₂O₄ - BaO - CaO powder, and BaO - CaO - Al₂O₃ powder; pressing said mixed powder in a dry state to provide pressed powder (21); sealing said pressed powder (21) in a capsule (22); and sintering said pressed powder sealed in said capsule to provide a sintered mixture by a hot isostatic press treatment. A method as claimed in claim 1, wherein boron nitride powder (23) is additionally sealed in said capsule. A method as claimed in claim 1, wherein the sintering is carried out under a pressure of 152 x 10⁶ Pa (1500 barometric pressure) at a temperature of 1.000°C for 90 minutes in atmosphere of argon gas. A method as claimed in claim 1, further comprising the steps of: processing said sintered mixture mechanically to form a predetermined configuration of pellets; and cleaning the surfaces of said pellets.

Inventors: HIDEFUMI YAMAMOTO C O NEC KANS, MASAAKI YABUTA C O NEC KANSAI, MASAMI HORIUCHI C O NEC KANSAI, TOSHIKAZU SUGIMURA C O NEC KAN, YOSHIO TAKESHIMA C O NEC KANSA, HIDEFUMI, YAMAMOTO, C/O NEC KANSAI, LTD., MASAAKI, YABUTA, C/O NEC KANSAI, LTD., MASAMI, HORIUCHI, C/O NEC KANSAI, LTD., TOSHIKAZU, SUGIMURA, C/O NEC KANSAI, LTD., YOSHIO TAKESHIMA, C/O NEC KANSAI, LTD.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: H01J9
IPC codes: H01J9