Electron emission devices are key components of many modern technological products. For example, focused "beams" of electrons produced by such devices are used in X-ray equipment, high-vacuum gauges, televisions, large-area stadium-type displays, and electron beam analytical devices such as scanning electron microscopes.
Standard electron emission devices operate by drawing electrons from a cathode formed from a material that readily releases electrons when stimulated in a known manner. Typically, electrons are drawn from the cathode by the application of either a thermionic stimulus or an electric field to the cathode. Devices operating through application of an electric field are said to operate by field emission. Cathodes used in field emission devices are accordingly known as field emission cathodes, and are considered "cold" cathodes, as they do not require the use of a heat source to operate.
Field emission offers several advantages over thermionic stimulus in many electron emission applications. A field emission device (which creates an electric field) will typically require less power than a thermionic device (which creates a heat source) to produce the same emission current, respectively. Field emission sources are typically on the order of 1000 times brighter than comparable thermionic sources. The added brightness can be highly advantageous in lighting applications, such as stadium displays, or in applications which require the use of electron beams operating at intense focus, such as microscopes.
Further, the heat sources used in thermionic electron emission devices eventually damage them, leading to relatively quick "burnout." In applications requiring the use of many electron emission devices, such as in large area collective usage television screens, use of thermionic emission devices is very expensive because of the need to replace frequently devices suffering from rapid burnout.
Additionally, thermionic electron emission devices are not feasible for some applications. Thermionic devices are temperature dependent, and thus cannot be used in applications operating in extreme temperatures or where the ambient temperature conditions vary substantially over time. For example, thermionic devices will not work properly in motors or engines where temperature conditions may swing from 70.degree. Fahrenheit to -60.degree. Fahrenheit within a few minutes. In contrast, field emission devices, which operate relatively independently from temperature conditions, can be used in such applications. Thermionic devices are also inappropriate for use where the heat used to draw the electron beam may damage the environment within which electron emission is to occur. For example, in X-ray applications focused near a human body, thermionic emission of electrons is undesirable as the heat source applied could cause pain or damage to the subject. Field emission devices avoid these concerns as they apply and generate relatively little heat.
Among various materials known to be suitable for the construction of field emission cathodes, carbon-based materials have proven to be capable of producing significant emission currents over a long lifetime in relatively low-vacuum environments (10.sup.-7 Torr or less). Cathodes utilizing diamond films, bulk carbon, and graphite have been developed, but have required the application of substantial voltages to the cathode before generating significant electron emission. Other cathodes having regular, defined surface structures created from carbon materials include cathodes constructed from individual carbon fibers bundled together, cathodes machined from carbon rods, and matrix cathodes with carbon surfaces formed by photolithography and thermochemical etching procedures. While these cathodes can produce high current density upon application of low voltages, they are expensive to produce, as they require sophisticated fabrication procedures and/or manual assembly in their production.
It is an object of the current invention to provide an efficient and durable field emission cathode which may be manufactured simply and inexpensively.
Another object of the current invention is to provide a field emission cathode comprising an emissive member formed of a porous carbon foam material having an emissive surface which defines a multiplicity of emissive edges.
Other objects and advantages of the current invention will become apparent when the field emission cathode of the present invention is considered in conjunction with the accompanying drawings, specification, and claims.