A New Thermionic Cathode Based on Carbon Nanotubes with a Thin Layer of Low Work Function Barium Strontium Oxide Surface
- PDF / 298,707 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 18 Downloads / 185 Views
1142-JJ15-41
A New Thermionic Cathode Based on Carbon Nanotubes with a Thin Layer of Low Work Function Barium Strontium Oxide Surface Coating Feng Jin, Yan Liu, Scott A Little and Christopher M Day Department of Physics and Astronomy, Ball State University, Muncie, IN 47306 ABSTRACT We have created a thermionic cathode structure that consists of a thin tungsten ribbon; carbon nanotubes (CNTs) on the ribbon surface; and a thin layer of low work function barium strontium oxide coating on the CNTs. This oxide coated CNT cathode was designed to combine the benefits from the high field enhancement factor from CNTs and the low work function from the emissive oxide coating. The field emission and thermionic emission properties of the cathode have been characterized. A field enhancement factor of 266 and a work function of 1.9 eV were obtained. At 1221 K, a thermionic emission current density of 1.22A/cm2 in an electric field of 1.1 V/µm was obtained, which is four orders of magnitude greater than the emission current density from the uncoated CNT cathode at the same temperature. The high emission current density at such a modest temperature is among the best ever reported for an oxide cathode. INTRODUCTION Carbon nanotubes (CNTs) are natural field emitters; their unique geometry and high aspect ratio give rise to a high field enhancement factor . The field emission properties of CNTs have been extensively studied in recent years. [1-4] However, there are few reports on the thermionic emission properties of CNTs in the literature. The benefit of the large field enhancement factor introduced by CNTs has not been exploited for thermionic emission and thermionic cathode applications. [5-7] The governing equation for thermionic emission is the Richardson-Dushman equation:
J s = 120T 2 e −11605 ϕ T e 4.4
E T
(1)
The last exponential term in the equation represents the field effect in thermionic emission and is referred to as the Schottky Effect. The Schottky Effect is usually not very large and has been largely overlooked. However, with the aid of modern noanotechnology, it is possible to create a much larger Schottky effect by introducing a large field enhancement factor, which could potentially lead to a dramatic increase in thermionic emission. In this paper, we report a thermionic cathode structure based on CNTs, and its electron emission properties. The cathode consisted of three major components: the metal substrate, which was a thin tungsten ribbon that could be heated as a filament by flowing through an electric current; CNTs on the surface of the tungsten ribbon that provided a large field enhancement factor; and a thin layer of low work function oxide (BaO/SrO) materials. The basic idea of this oxide coated CNT cathode structure was to combine the benefits of the large field enhancement factor introduced by CNTs and the low work function from the oxide coating to improve the overall thermionic electron emission.
EXPERIMENTAL DETAILS CNTs were grown on the tungsten ribbon in a region approximately 0.012 cm2 in ar
Data Loading...
