US3458748A

US3458748A - Field-enhanced thermionic emitter

Info

Publication number: US3458748A
Application number: US632159A
Authority: US
Inventors: Hans Hieslmair
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1967-04-17
Filing date: 1967-04-17
Publication date: 1969-07-29
Anticipated expiration: 1986-07-29

Description

July 29, 1969 H. Hl

FIELD-ENHANCED ESLMAIR 3,458,748

THERMIONIC EMITTER Filed April 1'7, 1967 ACCELERATING mmme ELECTRODE SURFACE 4 4 H lllhiul \I I I I 4mm Li, I z.- Low RESISTIVITY r g BODY "EATER A E ELECTRODE '2 IO 8 s INVENTOR, HANS HIESLMAIR.

United States Patent 3 458,748 FIELlJ-ENHANCED THERMIONIC EMITTER Hans Hreslmair, Elberon, N.J., assignor to the United States of America as represented by the Secretary of the Army Filed Apr. 17, 1967, Ser. No. 632,159 Int. Cl. H01j 1/14, 19/06 US. Cl. 313-346 2 Claims ABSTRACT OF THE DISCLOSURE Background of the invention This invention relates to a field-enhanced thermionic emitter utilizing thermionic emission from anisotropic materials characterized by a low work function and a long mean free path of electrons.

Many modern electron tubes are limited by the emission density of present cathode sources. Approaches such as multiple beaming have been tried but have not proved to be satisfactory in that they involve rather complex structures.

The general object of this invention is to provide a simply constructed thermionic emitter that is characterized by a high emission density. A more particular object of this invention is to provide a simply constructed thermionic emitter in which there is no metal layer through which electrons must tunnel in order to provide emission.

Summary of the invention It has now been found that the aforementioned objects can be attained by providing a field-enhanced thermionic emitter utilizing a disc of anisotropic material whose rectangular cross-section is characterized by a relatively low electrical resistivity, or a high electrical conductivity, in the longitudinal direction and a relatively high electrical resistivity, or low electrical conductivity, in the transverse direction.

More particularly, the field-enhanced thermionic emitter according to the invention comprises a heater, a disc of anisotropic material whose rectangular cross-section is characterized by a relatively low electrical resistivity in the longitudinal direction and a relatively high electrical resistivity in the transverse direction, and a heat conductive base electrode intermediate the heater and the disc of anisotropic material. The free and electron emitting surface of the anisotropic disc is provided with an accelerating electrode and means are provided for applying a positive potential to the accelerating electrode. When the disc of anisotropic material has been heated to temperature of thermionic emission (600 to 1000 C.) and a small positive potential then applied to the accelerating electrode, as for example 4 volts, electrons are accelerated transversely toward the emitting surface with enough energy to overcome the surface barrier thereof.

Brief description of the drawing The invention can perhaps best be understood by referring to the accompanying drawing wherein there is shown a diagrammatic sketch of a thermionic emitter.

Description of the preferred embodiment Referring to the drawing, a base electrode separates a heater 12 from a disc 14 composed of anisotropic material and having a rectangular cross-section characterized by a relatively low electrical resistivity in the longitudinal direction and a relatively high electrical resistivity in the transverse direction. The free electron emitting surface of the anisotropic disc is provided with the accelerating electrode 16. A battery 18 is provided having its negative terminal connected to the base electrode 10, and its positive terminal connected to the accelerating electrode 16 so that the potential field is along the transverse or thickness direction of disc 14. With such an arrangement, there is provided a high electrical conductivity perpendicular to the emission direction and lower conductivity to the emission direction.

In operation, the anisotropic disc 14 is heated to temperatures such that electrons are accelerated towards the free surface of disc 14. The entire emitting surface assumes the positive potential applied by battery 18 due to the excellent longitudinal conductivity in the disc 14. A field in the emission direction, indicated by the arrows, is established in this manner and the electrons are rapidly accelerated toward the free emitting surface. If the accelerating potential is sufficiently high, the electrons are supplied with enough energy to overcome the surface barrier and hence become emissive. Thus an accelerating potential gradient is established in the emission direction.

Under ordinary conditions, it is not possible to accelerate the electrons to escape velocities, since collisions occur before the electrons reach the necessary energies. However, if a disc or any other emitter configuration is heated to temperatures of thermionic emission, a very small accelerating field can change the emission appreciably because the effective work function for thermionic emission is reduced by the amount of energy the electrons absorb from the field. When the number of collisions of the electrons in the transverse direction is reduced and when the longitudinal conductivity is improved, the efficiency of the emission process is greatly enhanced.

As the heater of the thermionic emitter, any conventional heating device can be used such as a helical tungsten filament.

The base electrode of the thermionic emitter is composed of a metal having a melting point higher than the temperature of thermionic emission. Metals that may be used as the base electrode include nickel, platinum and tungsten. Similarly, the accelerating electrode is composed of a metal having a higher melting point than the temperature of thermionic emission. A thin film of iridium about 0.075 millimeter in thickness has been found to be very suitable as the accelerating electrode.

As the anisotropic material or emissive source of the thermionic emitter, any form can be used provided it is characterized by a low Work function and a long mean free path of electrons. This can be achieved by an emission source having a cross-section having a relatively low electrical resistivity in the longitudinal direction and a relatively high resistivity in the transverse directon. The ratio of the two resistivities may vary from 100 to 1 to 1000 to l. A disc composed of a mixture of part by weight boron nitride and 20 parts by weight pyrolytic graphite in a thickness of about 1000 to 10,000 angstroms has been found very suitable as the emissive source. This mixture has a resistivity of 10- megohm centimeters in the longitudinal direction and a resistivity of 10- megohm centimeters in the transverse or thickness direction. Other emissive sources that can be used include boron nitride and pyrolytic graphite.

The foregoing description is to be considered merely as illustrative of the invention and not in limitation thereof.

What is claimed is:

1. A field-enhanced thermionic emitter comprising a heater, a disc of anisotropic material having a rectangular cross-section and characterized by a relatively low electrical resistivity in the longitudinal direction and a relatively high electrical resistivity in the transverse direction and wherein said disc of anisotropic material consists of a mixture of 80 parts by weight of boron nitride and 20 parts by Weight of pyrolytic graphite, a heat conductive base electrode intermediate said heater and said body of anisotropic material, an accelerating electrode on the free end and electron emitting surface of the anisotropic disc, and means for applying a positive potential to the accelerating electrode whereby when the disc of anisotropic material is heated to temperatures of thermionic emission, electrons are accelerated toward the emitting surface with enough energy to overcome the surface barrier can be emitted.

2. A field-enhanced thermionic emitter according to claim 1 wherein the ratio of the resistivity in the transverse direction to the resistivity in the longtiudinal direction of the disc of anisotropic material is from 100 to 1 to 1000 to 1.

References Cited UNITED STATES PATENTS 3,373,307 3/1968 7 Zalm 31 3346 3,134,924 4/ 1964 Henderson 313-346 3,278,782 10/1966 Kanter 313--94 10 JOHN W. HUCKERT, Primary Examiner M. EDLOW, Assistant Examiner US. Cl. X.R.