JP2005533356A - Field emission cold cathode - Google Patents

Abstract

A field emission cold cathode (11) for use in a vacuum tube is provided. The carbon velvet material (25) is made of a high aspect ratio carbon fiber embedded perpendicularly to the base material. The carbon velvet material (25) is coated with a low work function cesium salt, and the base material of the carbon velvet material (25) is bonded to the cathode surface (27). The field emission cold cathode (11) emits electrons even at an operating temperature of less than 900 ° C. when an electric field is applied.

Description

In the context of the present invention, under the terms of paragraph 1 of Executive Order 10096, the United States government representing the Secretary of the Air Force is entitled to the rights, qualifications and benefits of the present invention including those in foreign countries. .
The present invention relates to the field of vacuum tubes, and more particularly to a field emission cold cathode that functions as an electron emitter (electron emission electrode) in a vacuum tube.

  The cathode is an electron emitter used in a wide range of vacuum tubes such as a cathode ray tube (CRT, Braun tube) used for television and various microwave tubes used for radar and communication. All of these cathodes must be kept under high vacuum and heated to very high temperatures, ie at least 900 ° C., for proper operation. Obtaining a high vacuum requires extensive baking (baking out) processes as well as special manufacturing techniques using sealed devices. In addition, this type of cathode is susceptible to contamination when removed from a vacuum environment. Thus, vacuum tubes impose considerable constraints on their handling, processing operations and storage because they have a high vacuum.

  There are also some design constraints because this type of cathode requires high temperature operation. First, a special material that can withstand high temperature operation must be used as the cathode material. In addition, heaters for heating the cathode are a factor that reduces energy efficiency and increases system volume, weight and structural complexity.

  Therefore, there is a demand for a cathode that can operate at a low temperature without requiring a strict vacuum and that exhibits the same field emission characteristics as a conventional new section cathode.

  The object of the present invention is to overcome the disadvantages of the prior art described above by providing a field emission cold cathode that can operate at low temperatures and at a lower vacuum than prior art field emission cathodes.

  More specifically, the present invention uses a carbon velvet material composed of high aspect ratio carbon fibers embedded perpendicularly to a base material (base material or base material). This carbon velvet material is adhered to the cathode. A cesiated salt is deposited on the carbon fiber. When a sufficient voltage is applied to the obtained cathode, electrons are emitted.

  According to a second aspect of the present invention, there is provided a method for manufacturing a field emission cathode comprising preparing a solution of a cesium salt, coating the carbon velvet material with the solution, and bonding the carbon velvet material to the cathode. The According to the third aspect of the present invention, the vaporized cesium salt solution is deposited on the fiber of the carbon velvet material to form a cesium salt crystal on the fiber, and the carbon velvet material is bonded to the cathode. A method of manufacturing a field emission cold cathode comprising the above is provided.

  According to a fourth aspect of the present invention, in a method for coating a carbon velvet material attached to a cathode to produce a field emission cold cathode, the carbon velvet material is sprayed with a solution of cesium salt and deionized water. The carbon velvet material coated with the cesium salt is baked (baked) at a temperature of at least 100 ° C. in a vacuum oven aerated to less than 1 Torr, and then dried nitrogen A method is provided that comprises using to evacuate the vacuum oven to atmospheric pressure.

  According to a fifth aspect of the present invention, a cesium salt film having a thickness of 1 angstrom to 10 microns is formed on each of the plurality of fibers of the carbon velvet material, and the carbon velvet material is bonded to the cathode. A method of manufacturing a field emission cold cathode is provided. According to the sixth aspect of the present invention, a carbon velvet material having fibers is attached to a cathode, the fibers are immersed in a molten cesium salt solution, and the solution is cooled while the fibers are immersed in the solution. This provides a method of manufacturing a field emission cold cathode. According to a seventh aspect of the present invention, an electric field is obtained by attaching a carbon velvet material having fibers to a cathode, immersing the fibers in a molten cesium salt solution, removing the fibers from the solution, and cooling the fibers. A method of manufacturing an emission cold cathode is provided.

  Conventional vacuum tubes require a high degree of vacuum and a cathode element that must be heated to at least 900 ° C. for proper operation. The “cold cathode” means not only a cathode that operates at room temperature or a temperature near room temperature, but also a cathode that operates at a temperature of 900 ° C. or less, but the cold cathode of the present invention operates at room temperature, It does not require the heating and high operating temperatures required in the prior art. Also, the cold cathode of the present invention operates at a lower vacuum level than prior art cathodes. The cold cathode of the present invention advantageously replaces the heated cathode of any type of vacuum tube, including klystrons (velocity modulation tubes), traveling wave tubes, magnetrons, magnicons, Christroad / IOT television transmitters, etc. be able to.

  Other aspects and advantages of the cold cathode of the present invention will become apparent from embodiments of the principles of the present invention described below with reference to the accompanying drawings.

  The attached figure is a schematic view of the experimental equipment used to test the field emission cold cathode 11 of the present invention and shows a cross section of the cold cathode 11. This equipment includes a vacuum chamber 13, a cathode attachment 17, a shaft 19 that protrudes into the vacuum chamber 13 in a telescopic manner with respect to the cold cathode 11, and an anode 21 attached to the inner end of the shaft 19. The gap 23 is a distance separating the cathode 11 and the anode 21 and can be adjusted by extending or contracting the shaft 19.

  The cold cathode 11 includes a high voltage bushing 24, a carbon velvet material 25, and a cathode surface 27. As will be described in detail later, the carbon velvet material 25 is treated with a low work function cesium salt and bonded to the cathode surface 27. The carbon velvet material 25 is made of a high aspect ratio carbon fiber embedded perpendicularly to the base material. A specific material of this type is Belblack® applique (a patented product of Energy Science Laboratories, Inc.). Belblack® applique consists of high aspect ratio carbon fibers implanted in an adhesive base (matrix) and has special optical properties, ie ultra-low reflectivity in optical systems It was developed as a black applique to secure stray light and suppress stray light.

  The carbon velvet material 25 is flexible and can be easily coupled to the cold cathode 11 of any shape. If the cathode surface 27 is made of metal, conductive epoxy can be used to bond the carbon velvet material 25 to the cathode surface 27. Alternatively, as a means for bonding the carbon velvet material 25 to the cathode surface 27, a high thermal bonding (pyro bonding) method can be used. Low temperature electron emission (field emission) is achieved by erasing the cesium salt crystal at the tip of the fiber of the carbon velvet material 25 or by depositing a cesium salt film having a thickness of 1 angstrom to 10 microns on the carbon velvet material. Can be obtained.

  In order to cover only the fiber axis of the carbon velvet material 25, a mask to which a cesium salt cannot adhere is placed on the tip of the fiber. The mask is removed by etching after the cesium salt is coated on the fibers of the carbon velvet material.

  Several different methods can be used for depositing the low work function cesium salt on the carbon velvet material 25. Two of these methods use highly pure cesium salt solution and deionized water as the medium for depositing the cesium salt. Specifically, the cesium salt is first mixed with deionized water, and then the carbon velvet material 25 is sprayed together with the cesium salt solution using an atomizer. Grade 5 dry nitrogen is used to set the back pressure for the nebulizer. Apply 2 to 4 coatings. The cold cathode 11 is then placed in a vacuum oven aerated to less than 1 Torr and fired at a temperature and time sufficient to evaporate deionized water (at a temperature of 100 ° C or higher for approximately 1 hour), then Evacuate to atmospheric pressure using grade 5 dry nitrogen.

As the low work function cesium salt, various cesium salts such as cesium iodide (CsI), cesium tellurate (CsTeO ₄ ), and cesium bromide (CsBr) can be used. Although the performance of the cathode can be improved and gas emission can be reduced even in one cycle, the performance of the cold cathode 11 can be further improved by repeating additional cycles. However, the performance improvement of the cold cathode is obtained up to the point where additional cycles increase the required turn-on electric field, i.e., the point where the electric field level at which electrons begin to be emitted from the cathode 11 to the anode 21 is increased.

  Alternatively, the fibers of the carbon velvet material 25 may be immersed in a cesium salt solution. Next, the combination of the cold cathode 11 and the cesium salt solution bath is fired to a temperature of approximately 100 ° C. under atmospheric pressure until the cesium salt solution crystallizes on the tip and / or axis of the carbon velvet material 25. To do. At this stage, the cold cathode 11 is pulled out of the cesium salt solution bath and fired in a vacuum oven to evaporate residual water from the tip and / or shaft of the carbon velvet material 25. The vacuum oven is then aerated with dry nitrogen to atmospheric pressure.

  As yet another method, the cesium salt is incorporated into the fibers of the carbon velvet material 25 by immersing the cold cathode 11 in the molten cesium salt crucible so that the fibers of the carbon velvet material 25 are completely immersed in the cesium salt solution. It may be deposited. Next, the crucible of the molten cesium salt is naturally cooled until the cesium salt solution crystallizes on the tip and / or the shaft of the carbon velvet material 25 while the fibers of the carbon velvet material 25 are immersed. The cesium salt may be deposited on the tip and / or shaft of the carbon velvet material 25 by chemical vapor deposition so that the crystal is formed on the tip and / or shaft of the carbon velvet material. Each of these methods is more costly and time consuming than the method using a deionized aqueous solution of cesium salt, but it allows a more uniform coating of the cesium salt and is also a cold cathode for removing excess water vapor. There is no need to fire 11.

  When a negative voltage is applied to the cold cathode 11, electrons are emitted from the cathode surface 27, accelerated through the cathode / anode gap 23, and collide with the anode 21. The required turn-on electric field required for emitting electrons was as low as 0.2 kV / cm. This is much lower than the typical turn-on field of conventional vacuum tubes. The voltage source may generate a pulse voltage or may continuously generate a voltage. The cold cathode 11 may have an arbitrary shape such as a spherical shape, a cylindrical shape, or a planar shape. The cathode / anode gap 23 may be an interaction space or other area where electrons are used. The anode 21 may be any region or structure that collects the emitted electrons.

  The turn-on field of the cold cathode 11 can be changed in various ways to suit the requirements of the device using the cold cathode. In connection with the carbon velvet material 25, the longer and thinner the fiber tip, the smaller the tuft density, the higher the electric field enhancement at the fiber tip, and thus the cold cathode. 11 turn-on electric field can be lowered. It has also been found that the fiber length distribution (making the length of each fiber non-uniform) acts to lower the turn-on field.

  The turn-on field can also be changed by changing the concentration of the cesium salt in the deionized water solution, or by changing the number of times the cesium salt is applied to the tip and shaft of the carbon fiber. That is, if the concentration of cesium salt or the number of coating times of cesium salt is increased (up to a certain point), the turn-on electric field can be lowered. For example, in certain types of microwave tubes it may be desirable not to emit electrons until the voltage reaches its set maximum value. The amount of cesium salt applied by increasing the tuft density of the carbon fiber, or by reducing the number of cesium salt coatings, or by reducing the concentration of cesium salt in the deionized water solution. Can be achieved by reducing.

FIG. 1 is a schematic cross-sectional view of an experimental facility used to test the field emission cold cathode of the present invention.

Explanation of symbols

11 Cold cathode 13 Vacuum chamber 17 Cathode fixture 19 Shaft 21 Anode 23 Gap, gap between anodes 24 High voltage bushing 25 Carbon velvet material 27 Cathode surface

Claims (24)

A cathode,
A carbon velvet material attached to the cathode;
The field emission cold cathode, wherein the carbon velvet material includes fibers coated with a cesium salt. Each of the fibers consists of a shaft and a tip,
The field emission cold cathode according to claim 1, wherein the cesium salt is applied to a tip of the fiber. Each of the fibers consists of a shaft and a tip,
The field emission cold cathode according to claim 1, wherein the cesium salt is deposited on a shaft of the fiber.   The field emission cold cathode according to claim 2, wherein the cesium salt is also deposited on the axis of the fiber.   5. The field emission cold cathode according to claim 3, wherein the cesium salt deposition layer deposited on the fiber axis is a film having a thickness of 1 angstrom to 10 microns.   The field emission cold cathode according to claim 5, wherein the carbon velvet material includes fibers having non-uniform lengths.   The field emission cold cathode according to claim 1, 2, 3, or 4, wherein the carbon velvet material includes fibers having non-uniform lengths.   The field emission cold cathode according to claim 1, wherein the cesium salt is a cesium salt having a low work function.   The field emission cold cathode according to claim 1, wherein the cesium salt is selected from the group of cesium iodide, cesium tellurate, and cesium bromide. 5. A field emission cold cathode according to claim 1, 2, 3 or 4, wherein the cold cathode is operated under a vacuum of at least ^10-3 torr.   The field emission cold cathode according to claim 1, 2, 3, or 4, wherein the carbon velvet material is Velblack (registered trademark) applique.   The field emission cold cathode according to claim 1, 2, 3, or 4, wherein the cold cathode has a metal cathode surface, and the carbon velvet material is bonded to the cathode surface by a conductive epoxy.   The field emission according to claim 1, 2, 3, or 4, wherein the cold cathode has a carbon base material, and the carbon velvet material is attached to the cathode by high-temperature bonding to the base material. Cold cathode. Means for spraying a solution of the cesium salt and deionized water onto the carbon velvet material;
Sufficient to allow the cold cathode to remove the deionized water from the carbon velvet material at a temperature of at least 100 ° C. under a vacuum of less than 1 Torr, thereby crystallizing and depositing the cesium salt on the fibers. The field emission cold cathode according to claim 1, comprising means for firing for a long time. In a method for coating a carbon velvet material attached to a cathode to produce a field emission cold cathode,
Prepare a low work function cesium salt and deionized water solution,
Spraying a solution of the cesium salt and deionized water on the carbon velvet material to form the carbon velvet material coated with the cesium salt;
Firing the carbon velvet material coated with the cesium salt in a vacuum oven aerated to less than 1 Torr at a temperature of at least 100 ° C. for approximately 1 hour;
Evacuating the vacuum oven to atmospheric pressure using dry nitrogen.   The method of claim 15, wherein the spraying is performed by pressurizing the sprayer with dry nitrogen.   16. The method of claim 15, wherein the cesium salt is selected from the group of cesium iodide, cesium tellurate, and cesium bromide.   The solution preparation step, the spraying step, the firing step, and the exhausting step are repeated until a cesium salt film having a thickness of 1 angstrom to 10 microns is formed on each of the plurality of fiber axes of the carbon velvet material. The method described in 1. Prepare a solution of cesium salt,
Coating the carbon velvet material with the cesium salt solution;
A method of manufacturing a field emission cathode comprising bonding the carbon velvet material to a cathode. Vaporized cesium salt solution is deposited on carbon velvet fiber,
Forming cesium salt crystals on the fibers;
A method of manufacturing a field emission cold cathode comprising bonding the carbon velvet material to a cathode.   21. The method of claim 20, wherein the cesium salt solution comprises deionized water and the cesium salt crystal forming step comprises evaporating the deionized water. Forming a cesium salt film having a thickness of 1 angstrom to 10 microns on each of the plurality of fiber axes of the carbon velvet material;
A method of manufacturing a field emission cold cathode comprising bonding the carbon velvet material to a cathode. A carbon velvet material with fibers is attached to the cathode,
Soaking the fibers in a molten cesium salt solution;
A method for producing a field emission cold cathode, comprising cooling the solution while the fiber is immersed in the solution. A carbon velvet material with fibers is attached to the cathode,
Soaking the fibers in a molten cesium salt solution;
Removing the fibers from the solution;
A method for producing a field emission cold cathode comprising cooling the fiber.

Images