A New Carbon Nanotube-Based Field Enhanced Thermionic Cathode
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A New Carbon Nanotube-Based Field Enhanced Thermionic Cathode Feng Jin and Christopher Day Department of Physics and Astronomy Ball State University Muncie, IN 47306, U.S.A. ABSTRACT The electron emission properties and field enhancement effects of carbon nanotubes (CNTs) have been extensively studied. However, all of these studies focus only on the field emission aspect of the materials and its application in cold cathode electron emitters. So far, we have not seen any studies in the literature that link CNTs with thermionic cathodes, which are an equally important cathode type because of their many applications. We present a study of field enhanced electron emissions from a new type of cathode: the CNT-based field enhanced thermionic cathode. This new cathode consists of a metal substrate with CNTs grown on top of its surface. The CNTs are coated with thermionic emission materials (BaO, SrO, and CaO). This unique cathode structure takes advantage of both the field enhancement effect from CNTs and the high electron emission capability of thermionic materials. The electron emission properties of this new cathode, particularly the field enhancement factor and effective work function, are compared with the conventional thermionic cathodes that are made of same oxide coating. INTRODUCTION Carbon nanotubes (CNTs) have generated great interest and enthusiasm because of their many unique properties. One of their important applications is in field emitters, where field emission is governed by Fowler-Nordheim equation:
J = aF 2 e
−bϕ 3 / 2 βF
(1)
Where F is the electric field, φ is the work function, b is the field enhancement factor, and a = 1.51x10–6 A eV V-2, b = 6.83x109 eV–3/2 V m–1. The unique geometry and high aspect ratio of CNTs make them ideal candidates to be field electron emitters. It is estimated that the field enhancement factor due to their aspect ratio can be as great as 10,000 [1-6]. While the electron emission properties and field enhancement effect of CNTs have been extensively studied, these studies focus exclusively on the field emission aspects of the materials and their application in cold cathode electron emitters. There have been no studies linking CNTs with thermionic cathodes. Thermionic cathodes are widely used in vacuum and discharge devices that require high current density, such as x-ray tubes and fluorescent lamps [7,8]. Typically, thermionic cathodes are coated with low work function emission materials (usually barium-rich oxides), and operate at temperatures above 1000 0C to enhance thermionic electron emission. The thermionic emission is given by the Richardson-Dushman equation:
HH13.33.2
J s = 120T e 2
−11, 605ϕ T
e
4.4 E T
(2)
where T is the temperature, φ is the work function, E is the external electrical field in volts per centimeter, and the last exponential term is the field enhancement factor due to the Schottky effect. The field effect in thermionic emissions has been largely overlooked in the past due to the fact that it is usually insignificant in most app
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