MXPA00008413A - Field emission cathode fabricated from porous carbon foam material - Google Patents

Abstract

A field emission cathode (20) is provided comprising an emissive member (22) formed of a porous foam carbon material. The emissive member has an emissive surface (24) defining a multiplicity of emissive edges.

Description

CAYMENT OF EMISSION BY FIELD EFFECT MANUFACTURED OF POROUS CARBON FOAM MATERIAL FIELD OF THE INVENTION This invention relates generally to emission cathodes by field effect BACKGROUND OF THE INVENTION Electron emission devices are key components of many technologically modern products. For example, the concentrated "beams" of electrons produced by such devices are used in X-ray equipment, high-vacuum meters, televisions, large-area stadium screens, and electron beam analytical devices such as electron microscopes for exploration. Standard electron emission devices operate by extracting electrons from a cathode formed of a material that readily releases electrons when stimulated in a known manner. Typically, electrons are extracted from the cathode by applying either a thermal stimulus or an electric field to the cathode. The devices that operate through the application of an electric field are those mentioned to operate through the emission by field effect. The cathodes used in the emission devices by field effect are known, according to the above, as emission cathodes by field effect, and are considered "cold" cathodes since they do not require the use of a heat source to operate . Emissions by field effect offer several advantages over thermionic stimuli in many electron emission applications. An emission device by field effect (which creates an electric field) will typically require less energy than a thermionic device (which creates a heat source) to produce the same emission current respectively. The emission sources by field effect are typically in the order of 1000 times brighter than comparable thermionic sources. The added brightness can be highly advantageous in brightness applications, such as stadium screens, or in applications that require the use of electron beams that operate at intense concentration, such as microscopes. In addition, the heat sources used in the thermionic electron emission devices damage them, eventually leading to relatively rapid "melting". In applications that require the use of many electron emission devices, such as television screens for collective use in large areas, the use of thermionic emission devices is very expensive due to the need to frequently replace the devices that suffer of fast melting. Additionally, thermionic electron emission devices are not feasible for some applications. Thermionic devices depend on the temperature and therefore can not be used in applications that operate at extreme temperatures or where ambient temperature conditions vary substantially over time. For example, thermionic devices will not work properly on engines or machines where temperature conditions can range from 70 ° Fahrenheit to -60 ° Fahrenheit in a few minutes. In contrast, field emission devices that operate relatively independently of temperature conditions can be used in such applications. Thermionic devices are also unsuitable for use where the heat used to extract the electron beam can damage the environment within which the emission of electrons occurs. For example, in concentrated X-ray applications near the human body, the thermionic emission of electrons is undesirable, since the applied heat source can cause pain or damage to the person. The emission devices by field effect avoid these problems, since they apply and generate relatively little heat.

Among the various known materials that are suitable for the construction of emission cathodes by field effect, carbon-based materials have proven to be capable of producing significant emission currents through a long duration in relatively low vacuum environments (10 ~ 7 Torr or less). Carbon-based materials are particularly desirable for use in field emission because the chemical interaction products between coal and the most common waste gases (such as oxygen and hydrogen) are non-condensable gases (such as carbon monoxide, carbon dioxide and methane) which will not contaminate the surface of the emission cathode by field effect. Cathodes have been developed using diamond films, bulk coal and graphite, but have required the application of substantial voltages to the cathode before generating enough electron emission. Other cathodes having regular defined surface structures created from carbon materials include cathodes constructed of individual carbon fibers packaged together, cathodes made from carbon rods, and cathode mixtures with carbon surfaces formed by attack prores chemical of photolithography and thermochemicals. Although these cathodes can produce high current density in the application of low voltages, they are expensive to produce since they require sophisticated fabrication prores and / or manual assembly in their production. It is an object of the present invention to provide an emission cathode for Effective and durable field effect that can be manufactured simply and economically. Another object of the present invention is to provide an emission cathode by field effect comprising an emissive member formed of a porous carbon foam material having an emissive surface defining a multiplicity of emissive edges. "Other objects and advantages of the present invention will be apparent when the field emission cathode of the present invention is considered in conjunction with the accompanying drawings, the specification and the claims. SUMMARY OF THE INVENTION A transmitting cathode is provided by field effect comprising an emissive member formed of a porous carbon foam material The emissive member has an emissive surface defining a multiplicity of emissive edges SHORT DESCRIPTION OF THE DRAWINGS Figure 1 shows a photomicrograph of an electron microscope for exploration of an emissive member of the present invention formed of Reticulated Carbon Steel (Reticulated Vitreous Carbon ™) and having a vertical edge cut-off Figure 2 shows a modality of a field emission device using the cathode of the Fig. 3 is a perspective view of the cathode of the invention that A cut of the emissive surface to form parallel rectangular grooves. Figure 4 is a cross-sectional side view of the cathode of the invention within which a screw that serves as an electrical contact is incorporated. Figure 5 is a graph of the typical emission current / applied voltage characteristics of the RVC cathodes tested at low to medium DC voltages. Figure 6 is a cross-sectional side view of the cathode of the invention plainside a nickel capsule. DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 illustrates a porous carbon foam material 10 used to form the emitting member of the field emission emitting cathode of this invention. The microphotographed member is formed of Reticulated Vitreous Carbon (Reticulated Vitreous Carbon ™) ("RVC"). The RVC forms vitreous (crystalline) carbon in an open-cell reticulated structure having an irregular pore structure with good, uniformly pore distribution statistically. The typical characteristics of the currently available porous carbon foam materials are listed in Table I. Table I: Characteristics of Porous Carbon Foam Materials Currently Available Physical Characteristics Typical Range of Important Values 1 Porosity Degree 10 to 100 pores per inch 1 (ppi) with a potential additional 1 compression per J factor of 10 j Elevated Surface Area Up to 66 cm c "per 100 ppi High vacuum volume 90-97% for different grades 1 porosity | Compressive Force 40-170 psi (greater for J compressed materials) 1 Tensile Strength 25-150 psi (greater for I compressed materials 1 Hardness 6-7 Mohs Specific Resistivity 0.18- 0.27 Ohm-inches (0.47- 0.69 Ohm-cm) The emitting member of the cathode of the invention is prepared by forming the porous carbon foam material in an emissive surface defining a multiplicity of emissive edges. The emissive edges constitute the killed edges 12 of the individual pore structures 13 on the surface of the carbon material. These edges 12 can be produced in the emissive member according to various methods, including but not limited to sawing and conventional drilling of carbon foam material or precision milling techniques. Machined processing of the carbon foam material is preferred, since it forms well-defined hard and sharp edges within the structure of the three-dimensional emissive member. The carbon foam material can be machined into the desired cathode shape concurrent with the formation of the emissive surface. Figure 1 shows the cutting of RVC material to form a tridimiensional surface structure with a vertical edge 14. In operation, the electrons are extracted from the emissive edges 12 of the carbon foam material in the application of an electric field towards the cathode. Since the carbon foam material is porous, and does not have a continuous surface, each edge 12 is separated from each other edge 12, and the electric field applied to the carbon foam material will be improved around each edge 12, causing the emission of electrons from the carbon material at the edge 12. By taking advantage of the irregular pore distribution of the porous carbon foam material, the invention avoids the work and expense required to fabricate defined emission points on the cathode surface , while creating an emission cathode by a coal-based field effect that operates well at low voltages and in low vacuum environments (10 ~ 7 Torr or less). RVC cathodes have been successfully tested in vacuum environments as low as 10"Torr.The cathode of the invention provides long term stability in emission due to its use of large number of irregularly distributed pore edges on the emissive surface. Cathodes that use carefully defined emission tips formed in regular patterns typically do not use extremely large numbers of emission tips and can be devastated by the destruction of a few key emission sites, in contrast, since the cathode of the invention forms a large number of emissive edges, the loss of a few emissive edges will have a negligible impact on the emission current produced.Also at the cathode of the invention the destruction of an emissive edge will often create a new pore edge which will operate instead of the destroyed edge, the current density available from the cathode can be controlled by changing the number of emissive edges. This can be achieved by varying the porosity of the carbon foam material: materials with a higher degree of porosity will have more pores 13 per inch ("ppi") and correspondingly more edges 12 over the same surface area. According to the above, the porosity of the carbon foam material used should be chosen according to the level of emission current density desired for the application in which the field emission device employing the inventive cathode is used. The lower limits in the porosity of the material are dictated essentially by the decrease in the number of emission sites as the pore size of the material increases. Suitable porosities for the RVC materials of the invention are equal to or greater than 50 ppi. The upper limits in the porosity of the material are governed by an effect of current overload: if the emissive edges of the emissive surface are too close to each other, the electrons will not be released from each emissive edge, but rather they will be grouped in a few emission sites, decreasing the number of effective emissive edges and decreasing the density of the emission current. RVC samples that have a natural porosity of 100 ppi and undergo compression of 2x, 3x, 5x and lOx, have produced successful results in emission applications by field effect in the tests. The shape of the cathode emissive member can also be chosen to meet the requirements of the desired application in which it is used. Forms having a large flat emissive area from which a substantial emission current can be extracted will be suitable for many applications, such as screen illumination. Appropriate shapes for the cathode of the invention include, but are not limited to disks, cubes, cylinders, rods and parallelepipeds. The RVC is the preferred porous carbon foam material due to the characteristics it possesses, which are desirable in the emission by field effect. The RVC has a high vacuum volume (up to 97%) and a large surface area (up to 66 cm2 / cm3 per 100 ppi) which creates a large number of emissive edges on its emissive surface. In addition, the RVC presents a highly uniform micromorphology. As a crystalline material, the RVC has greater internal uniformity of its pore structures than natural graffiti. According to the above, the emission current drawn from an RVC emission surface has a more uniform distribution than that of a natural graphite material. RVC is also characterized by exceptional chemical inertness and resistance to oxidation. These properties reduce the danger of chemical reactions between the ions or molecules of the residual gases with the cathode surface, which can be a critical factor when using the emission cathode by field effect in modest vacuum environments. The hardness of the RVC, the rigid volume structure and the high compressive strength provide durability and allow the material to be easily machined to the desired shapes. Its high tensile strength resists the ponderomotrices forces created by strong electric fields that act to apply tensile action towards the structure of the cathode and create tension in the material. In addition, the RVC has a fairly high resistivity for a carbon material (0.18-0.27 Ohm-inch for the RVC as compared to 0.001-0.002 Ohm-inch for the solid vitreous carbon), which limits the localized currents and thus reduces the probability that these arc currents from the surface are formed. This increases the duration of the cathode. RVC is typically formed by high temperature pyrolysis under a controlled atmosphere from a natural polymer resin. RVC is currently commercially available from Energy Research and Generation, Inc. ("ERG") of Oaland, California. The Destech Corporation of Tucson, Arizona also sells an open cell crystalline carbon foam. However, it should be understood that the porous carbon foam material used to form the cathode of the invention does not need to be RVC or manufactured according to any specific method. The invention is directed to using the morphology of the surface of a porous carbon material to form a large number of edges that act as individual emission sites. The material must have a sufficiently low porosity in such a way that current overload does not occur, but sufficiently high porosity so that a significant emission current is reliably produced by the cathode. The inertness and resistance to oxidation of the material must be adequate to avoid chemical reaction hazards. The material must be durable and must have sufficient tensile strength to withstand the ponderomotor forces created within the cathode structure. Its resistivity must be high enough so that no significant surface arc currents are formed during the operation of the emission device by field effect in which the cathode is used. The cathode of the invention can utilize any porous carbon foam material produced according to any method having the characteristics described above. The cathode of the invention can be used in any application of the emission device by field effect. Figure 2 represents an example of a simple field emission device 20, in which the cathode of the invention can be used. The cathode 22 of the invention, which has the emissive surface 24 and the anode 26, is enclosed within a vacuum envelope 28 which operates at a high vacuum in a manner sufficient to prevent undesirable chemical reactions with the waste gases in the stimulation of the emission of electrons. A port 30 is placed between the cathode 22 and the anode 26, so that the emissive surface 24 of the cathode 22 is separated from the port 30 by a distance Ll, and the port 30 is separated from the anode 26 by a distance L2. The cathode 22 is preferably established within an insulation member 32 such that the insulation member 32 does not obstruct the paths between the emissive surface 24 and the port 30. The insulation member 32 acts to electrically isolate the cathode port 30. 22 although the junction port 30 and the cathode 22 are within a structure, ensuring the maintenance of the proper distance Ll. The cathode contact 34, the anode contact 36, and the port contact 38 are placed in contact with the cathode 22, the anode 26 and the port 30 respectively and extend through the vacuum shell 28 in such a manner that The voltage differentials can be applied between the cathode 22, the anode 26 and the port 30 by connecting a means to create a voltage differential (not shown) through the contacts. In operation, the first voltage differential is applied between the cathode 22 and the anode 26 creating an electric field between the cathode 22 and the anode 26, which tends to attract electrons from the surface of the cathode 22 and towards the anode 26 through the vacuum environment 40, but produces a negligible emission current when applied independently. When the emission is desired, a second voltage differential of the same polarity as the first voltage differential between cathode 22 and port 30 is applied, improving the electric field enough to produce the desired emission current. It is desirable to use port 30 in this manner, since the level of the emission current produced by the field emission device 20 can be controlled by modifying the second voltage differential in small increments. Both voltage differentials can be created by grounding the cathode 22 and applying the positive voltages to the port 30 and the anode 26, but it should be understood that other means of creating both voltage differentials can be employed. The distances Ll and L2 and the first and second voltage differential must be selected to meet the requirements of the specific application to which the emission device is oriented by field effect 20 while producing the emission effects described above. The simple field-emitting device 20 described above is suitably configured to act as a cathode-luminescent light source and can be constructed from the materials typically used in cathode ray tube type devices. For example, the vacuum envelope 28 may be a glass envelope, while the port 30 may be a mesh suspended from a structure supported by the ceramic insulators 32. Suitable materials for building the port 30 include, but are not limited to, refractory metals. Low vapor pressure such as platinum, gold, molybdenum, nickel or nichrome and non-conductive metals such as carbon mesh. It should be understood that the cathode of the invention can be used in a wide variety of emission applications by field effect and its use is not limited to the emission device by field effect 20. Potential applications in which the cathode of the invention include, but are not limited to, large-area stadium screens, X-ray sources (which can potentially be used in vitro), high vacuum meters, flat panel displays, digital or graphic indicators, background projectors for computer screens, LCD, UHV devices such as clistrodes or magnetrons, analytical tools such as scanning electron microscopes, and microfabrication tools such as evaporators or electron beam heaters. Experimental Results Several emitter configurations have been tested by reticulated vitreous carbon field effect. The RVC cathodes tested were prepared from bulk RVC material and formed by manually cutting the RVC material with a knife or razor blade or by machining. Several simple RVC cathode shapes were tested including cylinders having a diameter of approximately 3 millimeters, pyramids and cubes and rectangular blocks having sides ranging in length from 3 to 5 millimeters. An emission source was formed for each cathode of RVC during the cutting or machining of each RVC cathode. For some of the RVC cathodes tested, the emission surface was flat. Referring to Figure 3, the other RVC cathodes tested were cut or machined to produce a three dimensional emission surface 100, formed by cutting parallel rectangular grooves 102 in the emission surface. During the test, the RVC cathodes having a three-dimensional emission surface such as that shown in Fig. 3, generally large emission currents were produced when the same voltage was applied as to the cathodes having a flat emission surface. Each tested RVC cathode was provided with a contact in the form of a screw or a smooth flexible wire, formed of stainless steel or molybdenum. Referring to Figure 4, screw 104 or flexible wire was dipped at one end into a preparation of colloidal graphite and isopropanol, such as "Electrodag ™ or Aquadag ™, both manufactured by the Acheson Colloids Company of Port Huron, Michigan. The submerged screw or wire was then screwed or introduced into the side 106 of the RVC cathode 108 opposite the cathode emission surface 100, until the screw or wire was introduced into the cathode to a depth d, or two to Four millimeters The preparation of the colloidal graphite, which serves as an adhesive, was allowed to air dry without heating for three or four hours in some of the tested RVC cathodes Other tested RVC cathodes were heated to temperatures between 150 and 200 degrees Centigrade up to half an hour to dry the colloidal graphite preparation, either heated or air-dried, the preparation of colloidal graphite, which is both electrically and thermally This conductive, provided sufficient mechanical and electrical connection between the contact and the RVC cathode for the purposes of the test. After the test, it was determined that the use of a screw or a fluted wire or a filament, is preferred as a contact over the use of a smooth wire, since the threading of a screw or the fluted of the wire or the filament reinforces the connection between the contact and the RVC cathode. Low to Medium Voltage Testing Tests were conducted to verify the performance of RVC cathodes at low to medium voltages ranging from 500 to 6,000 volts. The RVC cathodes tested were formed from RVC materials in bulk and had porosities of 50, 60, 80 and 100 ppi, or were formed from RVC material from - - 100 ppi which was then compressed by a factor of two to ten times. Each RVC cathode was placed in a stainless steel vacuum chamber. No insulating material was used to encapsulate the RVC cathode, to avoid the risk of electrical surface leakage from the cathode to an insulating material. Each stainless steel vacuum chamber was equipped with an ion pump or turbo pump capable of reducing the pressure inside the vacuum chamber to 10"9 Torr RVC cathodes were tested in both diode and triode configurations. In the diode configurations, an anode was placed inside the vacuum chamber at a distance of two to five millimeters from the cathode.Then three types of fluorescent screen were tested as anodes.The first type of fluorescent screen was formed by depositing emission phosphors by field effect P-22 on a metal disk The second type of fluorescent screen was formed by depositing emission phosphors by field effect P-22 on a glass disc covered with tin oxide of indium ("ITO"). The third type of fluorescent screen was formed by depositing emission phosphors by field effect P-22 on a glass disk and subsequently aluminizing the back side of the glass disk on phosphorus depo The P-22 Field Emission Matches are available from Osram Sylvania of Towanda, Pennsylvania. In the triode configurations, the anode was placed within the vacuum chamber at a distance of two to five centimeters from the cathode and a stainless steel modulating grid was placed between the RVC cathode and the anode. The modulating grid served the same purpose as the port 30 treated with respect to the field-emitting device 20 of FIG. 2. As the distance between the modulating grid and the RVC cathode decreases, the level of voltage that must be applied to the port is decreased to stimulate the emission from the RVC cathode, placing the grid too close to the cathode can short circuit the cathode due to the flexibility of the modulating grid. The indicated tests that separate the stainless steel modulating grid at a distance of one tenth of a millimeter to one millimeter of the RVC cathode work well. During the tests of both the diode and the triode configuration, the vacuum chamber was pumped up to a pressure of 10"s to 10 ~ 9 Torr.The two voltage schemes were used.In the first scheme, the RVC cathodes were maintained at a potential grounded, while a high positive voltage was applied to the anode, in the diode configuration, or to the modulating grid in the triode configuration.In the second scheme, a high negative voltage was applied to the cathodes of RVC while the anode remained at the grounded potential, in the diode configuration, or in the modulating grid that remained at the grounded potential in the triode configuration. applied voltages in the CD mode Figure 5 shows the emission current / applied voltage characteristics of the RVC cathodes tested in the diode configuration at low to medium DC voltages. The emission current by applied voltage produced in the initial application of voltage, while the line B indicates the emission current by applied voltage, produced after the voltage has been applied for 30 minutes. During testing, most cathodes of RVC produced an unstable emission current after the application of the initial voltage, which was characterized by a series of peaks in the emission current, as shown in line A of figure 5. The emission pattern by field effect visible on the fluorescent screen (the anode) corresponds to the variations in the emission current. The period of instability of the emission current varies between the RVC cathodes tested from a few minutes to approximately two hours. After the period of instability, the emission current stabilizes in such a way that it fluctuates from ten to twenty percent of an average value. These fluctuations remain present for the rest of the cathode test duration. Ballast resistors that have resistances in the range of between 10 and 500 megaohms were added in series to the cathode, anode or both the cathode and the anode during the test to reduce the magnitude of the fluctuations. The period of instability, which has been termed "preparation" of the emission stream, is considered to result from (i) the desorption of contaminants initially present at the RVC cathode emission surface and (ii) from the destruction of the sharpest emissive edges of the RVC material. After the contaminants have been desorbed and the sharpest emitting edges are destroyed, the current becomes more evenly distributed over the multitude of emission sites present in the cathode emissive surface. Ten to twenty percent of the fluctuations in the emission current present after the preparation period may result from the statistical equilibrium of the destruction of emission sites on the emission surface and the production of new emission sites as the destruction produces new emissive edges, resulting in a constant redistribution of the emission current network over the multitude of emission sites. The RVC cathodes can be cut from bulk RVC material using laser cut. RVC cathodes prepared by laser cutting can have a shorter test period before the emission current stabilizes, since laser cutting can introduce few pollutants to the emission surface and produce an emission surface that It has emissive edges that are more uniform than those formed by manual cutting or machining. Testing of High Voltage and Beam Concentration Six high voltage RVC cathodes were tested. These cathodes were formed from RVC materials in bulk, and had, porosities between 50 and 100 ppi, or were formed from RVP material of 100 ppi which was compressed by a factor of two to ten times. The RVC cathodes were each placed in a high vacuum stainless steel chamber equipped with an ion or turbo pump capable of reducing the pressure inside the vacuum chamber to 10 ~ 9 Torr. No insulating material was used to encapsulate the RVC cathode. An anode of 8 to 15 centimeters was placed away from the RVC cathode. The anode was formed from a round metal plate with a diameter of fifteen centimeters, which was covered with phosphorus emission by field effect P-22.

During testing, the vacuum chamber is pumped to a base pressure of approximately 5 x 10 ~ 8 Torr. Negative voltages of up to 55 kV were applied to the RVC cathode, while maintaining the anode at a potential grounded potential. Negative voltages were applied in both pulsed mode and CD mode. The production of the emission stream resulted in significant degassing of the anode, resulting in a total vacuum chamber pressure of approximately 10"Torr.The stainless steel vacuum chamber was surrounded by lead plates during the operation due to the risk of X-ray generation, and a portable X-ray detector was used to continuously monitor the X-ray level outside the lead plates. All six samples produced emission currents of up to 10 mA which were stable (fluctuating within 10-20% of an average value) through the observation periods of between two and four hours., the diffused beam of the emitted electron beam, was large enough so that the impact of the beam exceeded the diameter of the anode, so that part of the electron beam was captured by the walls of the vacuum chamber. Referring to Figure 6, tests were conducted in which the diffused beam was significantly reduced by placing the RVC cathodes inside a capsule 110. Capsules 110 formed of nickel and capsules of stainless steel were tested. The emission surface 100 of the RVC cathode was slotted into the capsule 110, at a distance from the front edge 112 of the capsule 110 of approximately 4 millimeters. However, the use of the capsule 110 makes it necessary to increase the negative voltage applied to the cathode to produce the same average emission current. For example, a voltage of -55 kV applied to the RVC cathodes when placed in a capsule 110 to 4 millimeters from the front edge of the capsule, produces the same average emission current as a voltage of -37 kV applied to the RVC cathodes without the capsule 110. Although the above invention has been described in some details by way of illustration for purposes of clarity and understanding, it will be readily apparent to those of ordinary experience in the art in light of the teachings of this invention, that certain changes and modifications may be made to it, without departing from the spirit or scope of the appended claims.

Claims (15)

1. CLAIMS 1. An emission cathode by field effect, comprising: an emissive member formed of a porous carbon foam material, the emissive member having an emissive surface defining a multiplicity of emissive edges.
2. 2. The field emission cathode of claim 1, wherein the emissive member contains a multiplicity of pores, the emissive edges projecting from the pores in the emissive surface.
3. 3. The field-effect emitting cathode of claim 2, wherein the porous foam carbon material has a porosity, the porosity being greater than or equal to 50 pores per inch.
4. 4. The field emission cathode of claim 3, wherein the porosity of the porous carbon foam material is less than or equal to 1000 pores per inch.
5. 5. The field emission cathode of claim 4, wherein the porous carbon foam material has a vacuum volume in the range of 90 to 97 percent.
6. 6. The field emission cathode of claim 5, wherein the porous carbon foam material has a compressive force of at least forty pounds per square inch.
7. The field emission cathode of claim 6, wherein the porous carbon foam material has a tensile strength of at least 25 pounds per square inch.
8. 8. The field-effect emitting cathode of claim 7, wherein the porous carbon foam material has a hardness of at least six Mohs.
9. 9. The field-effect emitting cathode of claim 8, wherein the porous carbon foam material has a specific resistivity in the range of 0.18 to 0.27 Ohms per square inch.
10. 10. The field emission cathode of claim 9, wherein the porous carbon foam material is Reticulated Vitreous Carbon ™.
11. 11. An emission device by field effect, comprising: a cathode formed of a porous carbon foam material, the cathode having an emissive surface defining a multiplicity of emissive edges; an anode; a vacuum environment that encloses the cathode and the anode; and means for maintaining the cathode and the anode at a voltage differential so that a plurality of electrons are emitted from the emitting-cathode edges toward the anode.
12. The field emission device of claim 11, further comprising a capsule within the vacuum environment, the capsule having a front edge, the cathode being placed within the capsule such that the emissive surface of the cathode It slots from the front edge of the capsule.
13. 13. The field emission device of claim 12, wherein the capsule is formed of nickel.
14. 14. The field emission device of claim 12, wherein the capsule is formed of stainless steel.
15. 15. The field-emitting device of claim 12, wherein the cathode emissive surface defines a series of parallel rectangular grooves in the porous carbon foam material.