CA2492853C

CA2492853C - Field emission cold cathode

Info

Publication number: CA2492853C
Application number: CA002492853A
Authority: CA
Inventors: Donald A. Shiffler, Jr.
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 2002-07-18
Filing date: 2002-07-18
Publication date: 2009-03-31
Anticipated expiration: 2022-07-18
Also published as: AU2002322392A2; AU2002322392B2; EA200500228A1; HK1078678A1; IL166173A0; CN100508100C; CA2492853A1; EA009410B1; HK1135794A1; AU2002322392A1; JP2005533356A; IL166173A; JP4295215B2; WO2004010450A1; BR0215809A; CN1639821A; CN101527238B; EP1523751A1; CN101527238A

Abstract

A field emission cold cathode (11) for use in vacuum tubes. A carbon velvet material (25) is comprised of high aspect ratio carbon fibers embedded perpendicular to a bas e material. The tips and/or the shafts of the carbon velvet material (25) are coated with a low work function cesiated salt. The base material of the carbon velvet material (25) is bonde d to a cathode surface (27). The cold cathode (11) emits electrons when an electric field i s applied, even at operating temperatures less than 900.degree.C.

Description

FIELD EMISSION COLD CATHODE
STATEMENT OF AMERICAN GOVERNMENT INTEREST
The conditions under which this invention was made are such as to entitle the Government of the United States of America under paragraph 1 (a) of Executive Order 10096, as represented by the Secretary of the Air Force, to the entire right, title and interest therein, including foreign rights.
TECHNICAL FIELD
The invention is in the field of vacuum tubes, and more particularly relates to a field emission cold cathode acting as an electron emitter in a vacuum tube.
BACKGROUND ART
Cathodes are electron emitters used in a wide variety of vacuum tubes, such as cathode ray tubes used in televisions and various microwave tubes used in radar and communications. All of these cathodes must be kept under a high vacuum and heated to a very high temperature, i.e., at least 900 °C, for proper operation. High vacuum necessitates the use of special manufacturing techniques, such as having a device that is sealed, as well as extensive baking out procedures. Further, these types of cathodes are susceptible to contamination if the cathode is removed from vacuum. The high vacuum thus gives rise to considerable restrictions associated with tube handling, operation, and storage.
The requirement for high temperature operation creates several significant design constraints. Firstly, the high temperature requires the use of special materials able to withstand the high temperature operation of the cathode. In addition, the heater reduces energy efficiency and increases system volume, weight, and complexity.
Accordingly, there is a need for a cathode that can operate at low temperatures with less stringent vacuum requirements, while delivering the same electron emission characteristics as conventional vacuum tube cathodes.
DISCLOSURE OF INVENTION
The present invention addresses the aforementioned shortcomings of the prior art by providing a field emission cold cathode that operates at cold temperatures and at a lower vacuum than the field emission cathodes of the prior art. More particularly, the invention includes a carbon velvet material comprised of high aspect ratio carbon fibers perpendicularly embedded in a base material. The carbon velvet material is attached to a cathode. A cesiated salt is deposited on the fibers. Electrons are emitted when a sufficient voltage is applied to the cathode.
A second aspect of the invention provides a method for making a field emission cathode comprising forming a solution of a cesiated salt, coating a carbon velvet material with the solution, and bonding the material to a cathode. The third aspect of the invention provides a method for making a field emission cold cathode comprising depositing a vaporized cesiated salt solution onto the fibers of a carbon velvet material, forming cesiated salt crystals on the fibers, and bonding the carbon velvet material to a cathode.
A fourth aspect of the invention provides a method for coating a carbon velvet material attached to a cathode to make a field emission cold cathode, comprising spraying the material with a solution of cesiated salt and de-ionized water;
baking the coated carbon velvet material at a temperature of at least 100 °C for approximately an hour in a vacuum oven evacuated to less that 1 tort., and then venting the vacuum oven to an atmospheric pressure using dry nitrogen.
A fifth aspect of the invention provides a method for making a field emission cold cathode by forming a film of cesiated salt having a thickness of 1 angstrom to microns on each of a plurality of fibers of a carbon velvet material, and bonding the carbon velvet material to a cathode. A sixth aspect of the invention provides making a field emission cold cathode by attaching a carbon velvet material having fibers to a cathode, immersing the fibers in a molten cesiated salt solution, and cooling the solution while the fibers are immersed in the solution. A seventh aspect of the invention provides for making a field emission cold cathode by attaching a carbon velvet material having fibers to a cathode, immersing the fibers in a molten cesiated salt solution, removing the fibers from the solution, and cooling the fibers.
Conventional vacuum tubes require a high vacuum and a cathode element that must be heated to at least 900 °C for proper operatjon. Although the term "cold cathode" refers to a cathode that operates at or near room temperature, as well as to cathodes that operate at temperatures below 900 °C, the cold cathode of the present invention operates at room temperature and thus eliminates the heating and high operating temperature requirements of the prior art. It also operates at a lower vacuum level than the cathodes of the prior art. The cold cathode of the present invention can replace, with attendant advantages, the heated cathode of any type of vacuum tube, including, klystrons, traveling wave tubes, magnetrons, magnicons, and klystrode/IOT
television transmitters.
Other aspects and advantages of the cold cathode of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing, illustrating by way of example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWING
The figure is a schematic of the laboratory setup used to test the field emission cold cathode of the present invention, and includes a cross-section of the field emission cold cathode of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The figure is a schematic drawing of the laboratory setup used to test field emission cold cathode 11 of the present invention, and shows a cross section of cold cathode 11. The setup includes vacuum chamber 13 and cathode mounting 17.
Shaft 19 protrudes into vacuum chamber 13, and can be retracted and exfiended relative to cold cathode 11. Anode 21 is mounted on the end of shaft 19. Gap 23 is the distance separating cathode 11 and anode 21, and is adjustable by means of retracting or extending shaft 19.
Cold cathode 11 is comprised of high voltage bushing 24, carbon velvet material 25, and cathode surface 27. As will subsequently be described in detail, carbon velvet material 25 is treated with a low work function cesiated salt and bonded to cathode surface 27. Carbon velvet material 25 consists of high aspect ratio carbon fibers embedded perpendicular to a base material. A particular material of this type is Vel-Black° applique, a proprietary product of Energy Science Laboratories, Inc. Vel-Black~
applique consists of high aspect ratio carbon fibers mounted in an adhesive base, and was developed for its optical characteristics, i.e., as a black applique for ultra-low reflectance and for stray-light suppression in optical systems.
Carbon velvet material 25 is flexible and can be readily bonded to any shape of cold cathode 11. A conductive epoxy can be used to bond carbon velvet material 25 to cathode surface 27 where cathode surface 27 is metallic. Alternatively, pyrobonding can be used to bond carbon velvet material 25 to a carbon substrate. Cold electron emission is obtained by forming cesiated salt crystals on the tips of the fibers of carbon velvet material 25, as well as by depositing a film having a thickness of 1 angstrom to microns on the fiber shafts.
To coat only the fiber shafts of carbon velvet material 25, a mask to which the cesiated salt cannot become attached is applied to the fiber tips. The mask is removed 5 be etching after the cesiated salt is applied to the fibers.
The low work function cesiated salt can be deposited on the carbon velvet material 25 by several different methods. Two of the methods employ a solution of highly purified cesiated salt and de-ionized water as the medium for cesiated salt deposition. More particularly, cesiated salt is first mixed with de-ionized water. Carbon 10 velvet material 25 is then sprayed with the cesiated salt solution using an atomizer.
Grade five dry nitrogen is used to provide the backpressure for the atomizer.
Two to four coats are applied. Cold cathode 11 is then placed in a vacuum oven, evacuated to less than 1 torn, baked at a sufficient temperature and duration to evaporate the de-ionized water (over 100 °C for approximately an hour), and then vented to atmospheric pressure using grade five dry nitrogen.
A number of low work function cesiated salts can be used, including cesium iodide (Csl), cesium tellurate (CsTe04), and cesium bromide (CsBr). While a single cycle will improve cathode performance and reduce out-gassing, additional cycles will further improve the operational performance of cold cathode 11. However, improvement is obtained only up to a point, whereupon additional cycles will increase the required turn-on field, i.e., the electric field level at which the electrons begin to flow from cathode 11 to anode 21.
Alternatively, the fibers of carbon velvet material 25 can be dipped into the cesiated salt solution. The assembly comprised of cold cathode 11 and the solution bath is then baked to approximately 100 °C at atmospheric pressure until the solution crystallizes on the tips and/or the shafts of carbon velvet material 25. At this stage, cold cathode 11 is withdrawn from the bath and baked in a vacuum oven to evaporate any remaining water from the tips and/or the shafts. The vacuum oven is then vented to the atmosphere using dry nitrogen.
In another alternative, the cesiated salt can be deposited on the fibers of carbon velvet material 25 by dipping cold cathode 11 into a crucible of molten cesiated salt so that the fibers are submerged. The molten cesiated salt is then allowed to cool with the fibers still submerged, until the cesiated salt crystallizes on the tips andlor the shafts.
Cesiated salt can also be deposited on the tips and/or the shafts of carbon velvet material 25 by chemical vapor deposition such that the cesiated salt crystals form on the tips and/or the shafts. Each of these processes is more expensive and time consuming than using the de-ionized water solution of cesiated salt. However, each results in a more uniform coating of the cesiated salt, and neither requires baking cold cathode 11 to remove excess water vapor.
When negative voltage is applied to cold cathode 11, electrons are emitted from the cathode surface 27, accelerated through anode-cathode gap 23, and then impinge anode 21. The turn-on field has been as low as 0.2 kV/cm. This is far less than the typical turn-on fields of conventional vacuum tubes. The voltage source may be pulsed or continuous. Cold cathode 11 can have any shape, e.g., spherical, cylindrical, or planar. .Anode-cathode gap 23 can be any interaction region or other region in which emitted electrons are used. Anode 21 can be any region or structure that collects emitted electrons.
The turn-on field of cold cathode 11 can be varied in several ways to suit the requirements of the device in which it is to be used. With respect to carbon velvet material 25, a longer, narrower fiber tip and a lower tuft density permit greater field enhancements at the fiber tips and hence reduce the turn-on field for cold cathode 11.
It has also been found that a distribution of fiber lengths tends to reduce the turn-on filed.
The turn-on field can also be varied by changing the density of cesiated salt in solution with de-ionized water and by varying the number of coats applied to the tips and shafts of the carbon fibers, i.e., increasing the density or the number of coats (up to a point) decreases the turn-on field. For example, in some microwave tubes it is desirable to not have electrons flow until the voltage reaches its full value.
This can be accomplished by increasing the tuft density as well as by decreasing the amount of cesiated salt applied, either by the decreasing the number of coats or by decreasing the density of the cesiated salt in solution with de-ionized water.

Claims

CLAIMS:

1. A field emission cold cathode comprising:
a cathode; and a carbon velvet material attached to the cathode;
wherein the carbon velvet material includes fibers having a cesiated salt deposited thereon;
wherein the carbon velvet material includes fibers having nonuniform lengths.

2. The cold cathode of claim 1, wherein:
each fiber is comprised of a shaft and a tip; and the cesiated salt is deposited either on the tips or on the shafts, or is deposited on the tips and also on the shafts.

3. The cold cathode of claim 2, wherein the deposition of the cesiated salt comprises a film having a thickness of 1 angstrom to 10 microns.

4. The cold cathode of claim 2, wherein the deposition of the cesiated salt comprises a film having a thickness of 10 microns or less.

5. The cold cathode of any one of claims 1 to 4, wherein the cesiated salt is a low work function cesiated salt.

6. The cold cathode of any one of claims 1 to 5, wherein the cesiated salt is selected from the group consisting of cesium iodide, cesium tellurate and cesium bromide.

7. The cold cathode of any one of claims 1 to 6, wherein the carbon velvet material is Vel-Black® applique.

8. The cold cathode of any one of claims 1 to 7, wherein the cathode is operated in a vacuum of at least 10 -3 torr.

9. The cold cathode of any one of claims 1 to 8, wherein:
the cathode includes a metallic surface; and the carbon velvet material is bonded to the surface with a conductive epoxy, or wherein the cathode includes a carbon substrate; and the carbon velvet material is attached to the cathode by means of pyrobonding with the substrate.

10. The cold calthode of any one of claims 1 to 9, further comprising:
means for spraying a solution of the cesiated salt and de-ionized water onto the carbon velvet material; and means for baking the cathode at a temperature of at least 100°C in a vacuum of less than 1 torr. for a period sufficient to remove the de-ionized water from the carbon velvet material, whereby the cesiated salt is crystallized and deposited on the fibers.

11. A method for coating a carbon velvet material attached to a cathode to make a field emission cold cathode, comprising:
forming a solution of a low work function cesiated salt and de-ionized water;
spraying the carbon velvet material with the cesiated salt solution to form a coated carbon velvet material;
baking the coated carbon velvet material at a temperature of at least 100°C
for approximately an hour in a vacuum oven evacuated to less than 1 torr.; and venting the vacuum oven to an atmospheric pressure using dry nitrogen.

12. The method of claim 11, wherein the spraying step includes pressurizing a spraying means with dry nitrogen.

13. The method of claim 11 or 12, wherein the cesiated salt is selected from the group consisting of cesium iodide, cesium tellurate and cesium bromide.

14. The method of any one of claims 11 to 13, wherein the steps of forming, spraying, baking, and venting are repeated until a film of cesiated salt having a thickness of 1 angstrom to 10 microns is formed on a plurality of fibers of the carbon velvet material.

15. The method of any one of claims 11 to 13, wherein the steps of forming, spraying, baking, and venting are repeated until a film of cesiated salt having a thickness of 10 microns or less is formed on a plurality of fibers of the carbon velvet material.

16. A method of making a field emission cold cathode, comprising:
forming a solution of a cesiated salt;
coating a carbon velvet material with the cesiated salt solution; and pyrobonding the carbon velvet material to a cathode.

17. A method of making a field emission cold cathode, comprising:
forming a solution of a cesiated salt;
coating a carbon velvet material with the cesiated salt solution by spraying or vapour deposition; and bonding the carbon velvet material to a cathode.

18. A method of making a field emission cold cathode, comprising:
depositing a vaporized cesiated salt solution onto fibers of a carbon velvet material;
forming cesiated salt crystals on the fibers; and bonding the carbon velvet material to a cathode;
wherein the solution includes de-ionized water and the forming step is comprised of evaporating the de-ionized water.

19. A method of making a field emission cold cathode, comprising:
depositing a vaporized cesiated salt solution onto fibers of a carbon velvet material by vapour deposition or spraying;
forming cesiated salt crystals on the fibers; and bonding the carbon velvet material to a cathode.

20. A method of making a field emission cold cathode, comprising:
forming a film of cesiated salt having a thickness of 1 angstrom to 10 microns on a plurality of fibers of a carbon velvet material by vapour deposition or spraying of a vaporized solution of cesiated salt; and bonding the carbon velvet material to a cathode.

21. A method of making a field emission cold cathode, comprising:
forming a film of cesiated salt having a thickness of 10 microns or less on a plurality of fibers of a carbon velvet material by vapour deposition or spraying of a vaporized solution of cesiated salt; and bonding the carbon velvet material to a cathode.

22. A method of making a field emission cold cathode, comprising:
pyrobonding a carbon velvet material having fibers to a cathode;
immersing the fibers in a molten cesiated salt solution; and cooling the solution while the fibers are immersed in the solution, or removing the fibers from the solution; and cooling the fibers.

23. A field emission cold cathode comprising:
a cathode; and a carbon velvet material attached to the cathode;
wherein the carbon velvet material includes fibers having a cesiated salt deposited thereon;
wherein the deposition of the cesiated salt comprises a film having a thickness of 10 microns or less.

24. A field emission cold cathode comprising:
a cathode; and a carbon velvet material attached to the cathode;

wherein the carbon velvet material includes fibers having a cesiated salt deposited thereon;
wherein the deposition of the cesiated salt comprises a film having a thickness of 1 angstrom to 10 microns.