US2909662A - Dielectric field emission methods and apparatus - Google Patents

Description

Get. 20, 1959 R. vo IP L ETAL 2,909,662

DIELECTRIC FIELD EMISSION METHODS AND APPARATUS Filed July 18, 1955 amps cm.

No RA 0 3O 6O 90 I20 I50 I60 kv/crn.

amps/cm.

" AC INVENTORS SIGNAL ARTHUR R. von HIPPEL IN 'Z- DEAN A. POWERS SIGNAL WILLIAM B. GREEN OUT W, m wm Fig. 4 BY I ATTORN EYS 'and contact electrode.

United States Patent DIELECTRIC FIELD EMISSION METHODS AND APPARATUS Arthur R. Von Hippel, Weston, Dean A. Powers, Concord, and William B. Green, Arlington, Mass., assignors to Research Corporation, New York, N.Y., a corporation of New York Application July 18, 1955, Serial No. 522,812

13 Claims. (Cl. 25083.3)

emission from an electrode into a suitable dielectric medium.

In a co-pendin'g application filed March 15, 1954 by one of the present inventors, Serial No. 416,351, there'is a discussion of the term field emission as commonly understood in'the art. In general, it may briefly be said to relate to obtaining the release of electrons from an electrode by reducing the potential barrier that binds the electrons to the electrode.

In the aforesaid prior application, the electron emission is brought about through use of a polarizable dielectric material in which the charge distribution-is modified to produce a concentration of charges adjacent the electrode. This requires a dielectric medium of somewhat special properties. a

In the present invention the reduction of the potential barrier is accomplished by establishing a positive ion layer in the boundary region between dielectric medium The ion layer is established by the-migration of ions through the dielectric under the influence of an applied electric field. Contributing to the formation-of the requisite ion layer is a thin=insulating layer or barrier on the surface of the electrode, which, for example, may be an oxide or a bromide-film, :depending on the base metal used. Since this filmor surface layer is electrically insulatinga poor ionic conductorit retards further migration of ions to the electrode, resulting in the formation of a positive ion film adjacent said surface layer. The concentration of ions gives rise to a high potential gradient close to-the electrode, such as to bring about the emission of electrons from the electrode by reducing the potential barrier confining them.

Heretofore, apparatus utilizing field emission as an electron release mechanism, as noted in the aforementioned copending application, has had limited application inasmuch as the potential gradient-requiredto extract-electronsis extremely high. In addition, such emission-has notbeen subject to control inasmuch as breakdown .of

the medium occurs, once emission is initiated.

It is, therefore, an object of the invention to provide novel apparatus and devices operative by non-destructive field emission phenomena, utilizing relatively simple and convenient apparatus and techniques.

A-further object-is to provide a wide variety of devices and apparatus operative by field emission by the generation-of a copious supply of electrons in a new and useful manner at relatively low potentials.

It is likewise anobject of the invention to provide electronemission apparatus operative by field emission and responsive to radiant energy over a Wide range. g

In furtherance of the foregoing, and such other objects-and features as may hereinafter appear, :the present invention involves the provision of apparatus wherein a suitable dielectrics.

dielectric medium is employed in conjunction with an electrode so that under the influence of an applied pothereby to effect the release of electrons.

More specifically, the invention contemplates the provision of apparatus and devices operative by field emission, characterized by the utilization of a dielectric medium having migratory ions, such as a gas or liquid medium, including also solid dielectrics such as silver halides, in contact with an electrode having an insulating barrier or film, wherein a positive ion layer may be created adjacent the electrode to establish a strong local field and thereby bring about the controlled release of electrons from the electrode into the dielectric medium.

As a further feature of the invention, the control of the electron emission or release may be effected by an .applied control potential, by radiant energy, by thermal plication broadly to a wide variety of embodiments employing controllable, non-destructive high field emission 'phenomena through the use of an ionizable dielectric.

medium of special'properties wherein the number of ions: in the medium is a small fraction of the total number of molecules. The ion concentration, although sufficient to provide the requisite field distribution, should not be: suchas to neutralize the electrons passing through the medium.

In general, dielectrics of the desired properties are found notably among the hydrocarbons, and may be either liquid or gaseous. By way of example, hexane has been found particularly suitable, although benzene, carbon disulfide, and chlorocyclohexane are likewise In addition, polar solvents, such as water and alcohol, also possess the desirable properties.

In suitable dielectric materials for use in the present invention, and in hexane by way of example, there exists relatively-free moving ions. These may be the result of thermal dissociation or dissociation of impurity molecules.

The positive ions can be mobilized so as to move or drift toward the cathode upon the application of an external field. In the case of an electrode in direct conductive sufficiently intense to produce electron emission. It is to be noted at this point that the ion layer concentration may be controlled solely by the applied potential, by an 55auxiliary potential, by illumination or irradiation, or by the injection of position ions or holes, or combination thereof.

It is appropriate for a better .understanding of the At the outset, it is important to note the environmental distinctions between the inventions.

In the invention of the co-pending application, electron emission from an emitter is effected by polarization of a dielectric; that is, the field gradient established by an applied voltage is modified upon forming a'positive space charge region within a relatively small portion of the dielectric volume. Creation of the gradient is accomplished by using a dielectric medium wherein electrons are located in the matrix of the crystal in a manner fully set forth in the prior application. Upon exposureto an energy source, such as light, the electrons migrate to the anode, the ions remaining fixed as part of'the crystal Patented Oct. 20, 1 959 3. field, leaving a positive space charge region in the vicinity of the electrode thereby to effect electron release.

In the invention of the present application, the nature of the dielectric employed is such that under the influence of an energy source, as will hereinafter appear, ionization occurs. There is suflicient ion mobility such that positive ions pass through the interstices of the molecules to migrate toward the insulated electrode. The thin insulating film on the electrode prevents the neutralization of the positive ions which then establish a strong local field adjacent the electrode. If sufficiently intense, the field can bring about electron emission from the electrode or emitter into the dielectric.

Other features of the invention, and the mode of operation thereof, will be set forth in conjunction with the following description, taken in conjunction with the accompanying drawings, in which Fig. 1 is a representative circuit and schematic illustration of the ionic matrix of a liquid dielectric subjected to an applied potential to form a positive ion layer in the region of the cathode electrode.

Fig. 2 is a graphic representation of the field distribution within the dielectric medium when a potential is applied to the circuit of Fig. 1.

Fig. 3 is a graph of the output current as a function of applied field in a dielectric medium.

Fig. 4 is a circuit diagram of a field emission dual diode adapted to generate an amplified A.C. signal output.

Fig. 5 is a schematic diagram of an embodiment of the invention suitable for use as an electric switch or pulser wherein auxiliary grids are employed as control elements.

Fig. 6 is a schematic diagram of another specific embodiment of the invention.

Fig. 7 is a graph of output current as a function of an applied field in a dielectric medium influenced by gamma radiation.-

Fig. 8 is a schematic showing of a further embodiment, wherein the field emission phenomenon is utilized to excite electro-luminescence within the medium.

The circuit of Fig. 1 includes the ionizable dielectric medium 12, disposed between the coated electrode, indicated as a unit by 14, and the conventional electrode 16, to which electrodes a voltage V can be applied by closing the switch 18. The electrode 14 is provided with a thin insulating film 20, which may be an oxide, bromide, or a sulfide film, or other insulatory film, depending on the dielectric and the base material of the electrode, an oxide film suitable on copper or iron, for example, and a bromide film on a silver electrode. With the switch 18 open, positive and negative ions are distributed throughout the dielectric, due to thermal dissociation of the dielectric molecules, or of impurity moleculm that may be present to increase the number of ions. Upon the application of the voltage V positive ions are at tracted to the electrode 14. Since the surface layer or film 20 is electrically insulating, further migration of cations to the electrode 14 is prevented, resulting in the formation of the positive ion field 24 adjacent said surface layer. The electrode surface layer 20 impedes neutraliza- .tion of the ion layer 24 so that the ions maintain a strong local field across the surface layer. The concentration of this field adjacent the emitter is shown in Fig. 2, its width indicated by X. The narrower X is, or alternately expressed, the steeper the slope Y is, the greater the reduction of the surface potential barriers so as to effect electron release from the cathode.

From the foregoing it is seen that the creation of the requisite field concentration is a function of the number of positive ions available in the medium 12. Assuming a fixed set of conditions, as shown in Fig. 1, the quantity of cations migrating to the cathode 14 is a function of the applied voltage V This .means, therefore, that the output current is an implicit function of the applied voltage, a feature similar to electron tubes to perform like useful functions. This fact has been established experimentally, as shown by the graph of Fig. 3.

It was pointed out above that the creation of the field concentration is a function of the number of ions available in the medium. Accordingly, the characteristics of the dielectric, like an electron tube, are predetermined by manufacture. Since the dielectric is ionizable, that is, molecules dissociate into parts of opposite charge, the output is variably dependent on the intensity and spectral distribution of an ionizing source as well as the magnitude of the applied voltage. In this connection, it is also to be noted that since this invention utilizes energy absorption for ion control, other means such as controlled heat or auxiliary electrodes may be used, as well as incident radiation.

The current flow, its direction and magnitude, are controlled by the establishment of a strong local field. By varying the nature and arrangement of the cathodes, suitable interacting field patterns may be created to perform varying functions. For example, cathodes of different materials, as a semi-conductor and metal, or variations in the surface treatment of the cathode, permit special types of field patterns. In addition, the cathodes may be selectively energized to further vary the field to produce other field distributions. Modulation of the conductivity of the electrode surface film, thermally or optically, is likewise effective to control the field.

In addition, inasmuch as thisinvention contemplates as one of its principal teachings the ionization of dielectrics to produce internal high fields, a hole conductor may be used in place of the dielectric. In this event, the holes may be used for the positive charging of the treated electrode instead of positive ions brought in by conduction or created by photo effect.

From what has been said it is clear that a wide variety of embodiments for sensing and control purposes may be constructed in accordance with the fundamentals of the invention. Figs. 4 through 6, and Fig. 8, are examples of such embodiments which, with the accompanying descriptions, will further set forth the nature and mode of operation of the invention.

Throughout the following discussion like elements will be designated in common with the elements of Fig. 1'.

In Fig. 4 the ionizable dielectric 12 is disposed between the anode 16 and the twin cathodes 14a and 14b. In this embodiment the cathodes 14a and 14b are of different materials, or have different insulating surface layers, so that their respective potential barriers are at different levels. As a consequence, assuming the cathode 14a is the one of lower potential barrier, it will emit more electrons than cathode 14b if a fixed potential is applied to both. The generated anode current, however, appears as the combined output of the respective cathodes, establishing a datum or reference current level.

By utilizing the property of the cathodes discussed immediately above, electric control may be exercised of the output current. Thus, if the cathode 14a is made positive with respect to the cathode 14b, electric flux lines are transferred from the cathode 14b to cathode 14a. As a result of the transferal of field strength to the cathode 14a, it will emit current more heavily with a small decrease of current from the cathode 14b, there by providing an overall increase in anode current. A reversal of polarity, that is, the cathode of 14b, is made positive with respect to the cathode 14a, results in an overall decrease of anode current.

In the network of Fig. 4, the field emission dual diode utilizes. the principles of the invention disclosed in the foregoing. An A.C. signal is fed into the network across the transformer 26, the secondary of which has an electrical tap leading to the anode 16. In accordance with well known circuit theory, upon the excitation of an A.C. signal across the tapped secondary, the cathodes have different instantaneous voltage levels impressed across them. This being so, when the cathode 14a is positive with respect to 14b, the generated current output is above the datum level noted before. When the cathode 14b is positive with respect tothe cathode 14a, the generated output current is below the aforesaid datum level. In this manner the circuit of Fig. 4 produces an amplified A.C. signal output.

Another embodiment utilizing a further feature of the invention is shown in Fig. 5 wherein the field emission device functions as an electric switch or pulser. In this application, in addition to the dielectric 12, the cathode 14, and the anode 16, the grid 28 is disposed in the region of electrode 14. The grid 28 is normally maintained negative by the battery 13 with the switch closed. In this event the positive ions migrating through the dielectric 12.are attracted to the grid, away from the cathode. If the grid is a poor emitter, negligible the energy spectrum; by variations in potential applied to grids or auxiliary electrodes of similar or differing nature; or by suitable combinations thereof.

current will flow. When the switch 15 is open, indicated by the broken line, no negative bias is applied to the grid, permitting the cations to travel to the cathode.

A large field emission current Will then flow since the.

electrode 14 is a good emitter. This manifestation, of course, corresponds to sensitive, controlled grid action in a traditional electron tube.

It can also be pointed out that if the switch 15 were to be replaced by a conventional control pulse circuit, the field emission device will, in accordance with the above explanation, be converted to relatively high current wave pulses whose rate and duration is determined respectively by the control pulse rate and control pulse amplitude and duration.

Still another embodiment constructed in accordance with this invention is shown in Fig. 6, the elements being the electrodes 14 and 16, the ionizable dielectric 12, the battery V and in addition, the energizing source 17. In this environment, the device may function as a radiation detector since the dielectric medium 12 used is one sensitive to ionization by radiation, as, for example, chlorocyclohexane, or hexane, as hereinbefore referred to. Thus, when the dielectric 12 is exposed to the radiation of the source 17, which may be a source of gamma, beta, or alpha rays, including as well other regions of the energy spectrum depending on the sensitivity of the medium, a number of free positive ions will be created. In accordance with principles already discussed, these ions are attracted to the surface layer and cause field emission. The derived field emission will be much larger than the current that would be due generated by V alone, producing in effect a sensitive radiation detector. The influence of gamma radiation on emission characteristics of the field emission device as the one of Fig. 6 is shown in Fig. 7, the curves of Fig. 7 being derived by experimental observations.

The Fig. 8 shows the principles of the invention adapted as an electroluminescent light source. The elements of this application are similar to arrangements discussed before, as is the mode of operation. However, the dielectric 12 has the further property of being an electroluminescent material, as zinc sulfide, so that the electrons provided by field emission are accelerated by the field within the medium 12. Collision of the accelerated electrons with atoms of the electroluminescent material yield visible radiation that passes through the transparent anode 16, thus serving as a light source.

There has thus been disclosed a novel and useful method and apparatus, operative by field emission, wherein electron emission may be effected by the use of a dielectric medium in accordance with discovered phenomena. Through the controlled establishment of a positively charged layer within the dielectric medium adjacent a treated electrode, strong local fields may be readily created which are effective to procure electron release from the said electrode.

It is also apparent that new and useful control and sensing properties have been discovered, wherein electron emission may be effected by potential variation; by control of or response to radiation over a wide range of Accordingly, it will be understood that the invention has been described in terms of illustrative embodiments only, with the further purpose of more fully teaching the principles of the invention, and that the invention contemplates a wide variety of embodiments and applications of field emission phenomena wherein electron emission is obtained by the controlled migration of positive charges to an electrode provided with an electrically insulating or poorly conducting film to establish a strong local field within a dielectric medium adjacent said electrode.

We-claim as our invention:

1. Apparatus for electron emission comprising an ionizable dielectric medium, electrodes in contact therewith, one of said electrodes being electrically insulated from the medium by a thin film, means for applying a potential to said electrodes 'to form a positive charge concentration adjacent the insulated electrode and provide a strong local field to effect the emission of electrons into the dielectric medium.

2. Apparatus for electron emission comprising an ionizable dielectric medium, spaced electrodes therefor, one of said electrodes being electrically insulated from the medium by a thin film, means for applying a potential to said electrodes to mobilize the ions in the dielectric medium, thereby to cause an ion concentration adjacent the insulated electrode to form a strong local field within said film.

3. Apparatus for electron emission comprising an ionizable dielectric medium, said medium being a hydrocarbon having mobile ions therein, spaced electrodes therefor, one of said electrodes being electrically insulated from the medium by a thin film, means for applying a potential to said electrodes to mobilize the ions in the dielectric medium, thereby to cause an ion concentration adjacent the insulated electrode to form a strong local field within said film.

4. Apparatus for electron emission comprissing an ionizable dielectric medium, spaced electrodes therefor, at least one of said electrodes being electrically insulated from the medium by a thin film, said medium having a concentration of positive charges disposed adjacent said insulated electrodes to form a high local field within said films, and supplementary means for modifying the said positive charge concentration.

5. Apparatus for electron emission comprising an ionizable dielectric medium, spaced electrodes therefor, at least one of said electrodes being electrically insulated from the medium by a thin film, said medium having a concentration of positive charges disposed adjacent said insulated electrodes to form a high local field within said films, and supplementary electrodes to modify the positive charge concentration.

6. Apparatus for electron emission comprising an ionizing dielectric medium, spaced electrodes therefor, at least one of said electrodes being electrically insulated from the medium by a thin film, said medium having a concentration of positive charges disposed adjacent said insulated electrodes to form a high local field within said films, and radiant energy means for modifying the positive charge concentrations.

7. Apparatus for electron emission comprising an ionizing dielectric medium, spaced electrodes therefor, at least one of said electrodes being electrically insulated from the medium by a thin film, said medium having a concentration of positive charges disposed adjacent said insulated electrodes to form a high local field within said films, and grid means for modifying the positive charge regions.

8. Radiation detection apparatus operative by field emission comprising an ionizable dielectric medium, spaced electrodes therefor, at least one of said electrodes being electrically insulated from the medium by a thin -film, said medium having a concentration of positive charges disposed adjacent said insulated electrodes to form a high local field within said films, current responsive apparatus connected to the electrode to respond to currents flowing between the electrodes upon exposure of the dielectric medium to radiant energy.

9. Apparatus for controlled electron emission comprising an ionizable dielectric medium, a plurality of electrodes therefor, one of said electrodes having an insulating film barrier adjacent said medium, and means for controlling the concentration of ions in said medium thereby establishing by ion migration 21 high potential gradient adjacent said insulated electrode to induce electron emission across said barrier.

10. Apparatus as defined in claim 9 wherein said con- 12. Apparatus as defined in claim 9 wherein said control means includes a wire grid positioned within said dielectric medium adjacent said insulated electrode and References Cited in the'file of this patent UNITED STATES PATENTS Joffe Oct. 13, 1931 Ahearn July 22, 1952 Marshall et al Sept. 30, 1952

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