WO1997018576A1

WO1997018576A1 - Diamond powder field emitters and field emitter cathodes made therefrom

Info

Publication number: WO1997018576A1
Application number: PCT/US1996/018140
Authority: WO
Inventors: Graciela Beatriz Blanchet-Fincher; Syed Ismat Ullah Shah
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1995-11-15
Filing date: 1996-11-13
Publication date: 1997-05-22
Also published as: AU7728696A

Abstract

Diamond powders prepared by shock synthesis are useful as electron field emitters. Field emitting cathodes made up of such diamond powders attached to the surface of a substrate are also provided. The field emitters and field emitter cathodes are useful in vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers, klystrons and lighting devices.

Description

DIAMOND POWDER HELD EMITTERS AND FIELD EMTΠΈR CATHODES MADE THEREFROM FIELD OF THE INVENTION The invention generally relates to the use of diamond powders prepared by shock synthesis as electron field emitters and more particularly to the use of said diamond powders in field emitter cathodes

BACKGROUND OF THE INVENTION Field emission electron sources, often referred to as field emission materials or field emitters, can be used in a variety of electromc applications, e g , vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers, klystrons and lighting devices

Display screens are used in a wide variety of applications such as home and commercial televisions, laptop and desktop computers and indoor and outdoor advertising and information presentations Flat panel displays are typically only a few inches thick in contrast to the deep cathode ray tube monitors found on most televisions and desktop computers Flat panel displays are a necessity for laptop computers, but also provide advantages rn weight and size for many of the other applications Currently laptop computer flat panel displays use liquid crystals which can be switched from a transparent state to an opaque state by the application of small electπcal signals It is difficult to reliably produce these displays in sizes larger than that suitable for laptop computers

Plasma displays have been proposed as an alternative to liquid crystal displays A plasma display uses tiny pixel cells of electrically charged gases to produce an image and requires relatively large electrical power to operate Flat panel displays having a cathode using a field emission electron source, 1 e , a field emission material or field emitter, and a phosphor capable of emitting light upon bombardment by electrons emitted by the field emitter have been proposed Such displays have the potenual for providing the visual display advantages of the conventional cathode ray tube and the depth, weight and power consumption advantages of the other flat panel displays U S Patents 4,857,799 and 5,015,912 disclose matrix-addressed flat panel displays using micro-tip cathodes constructed of tungsten, molvbdenum or silicon WO 94-15352. WO 94-15350 and WO 94-28571 disclose flat panel displays wherein the cathodes have relatively flat emission surfaces

What are needed are additional and/or improved field emitting materials suitable for use in field emitter cathodes which are. ln-tum, useful in display panels and other electronic devices Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the detailed description of the mvention which hereinafter follows

SUMMARY OF THE INVENTION The present mvention provides an electron field emitter comprised of diamond powder prepared by shock synthesis The mvention also provides a field emitter cathode comprised of diamond powder prepared by shock synthesis attached to the surface of a substrate

These field emitters and field emitter cathodes are useful in vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers, klystrons and lighting devices Such panel displays can be planar or curved DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The mvention provides a novel electron field emitter comprised of diamond powder prepared by shock synthesis and a novel electron field emitter cathode made therefrom

As used herein, "diamond powder means diamond in a fmely divided state, l e , particulate diamond, with a particle size less than about 20 μm The diamond powder used in this mvention preferably has a particle size less than about 10 μm In some cases, the diamond powder may be "nanoparticulate" meanmg that the particulate diamond powder has a particle size in the nanometer range (I e , less than 1 micron)

As used herein, "shock synthesis" means a synthesis in which a shock wave, l e , a compressional or detonation wave, is used to provide the pressure necessary for synthesis When an explosion is used to provide the pressure, the synthesis can occur withm mateπal which is subjected to the pressure or withm the explosive mateπal itself

Diamond powder can be prepared bv shock synthesis In one method carbon, e g , graphite, is sealed in a steel (product) tube which turn is placed concentrically in a larger, heavier (driver) tube This system is suπounded by several thousand pounds of high-velocity explosive confmed mside a cylmdrical culvert Detonation is instigated at one end of the charge of explosive and propagates along the cymdπcal charge of explosive to the other end As the explosive detonates, it progressively collapses the driver tube onto the product tube, subjecting the graphite to very high pressure This pressure can be as high as 7 x IO⁶ lb/in² (5 x 10¹⁰ Pa) This transforms the graphite mto microcrystallme diamond with crystal size of about 10 nm These crystallites bond together m random structures and. if the pressure pulse is sufficiently long, the resultmg particles can be up to 100 μm in size The heat generated must be controlled by incorporating a heat-sink material so that the temperature does not rise to the level sufficient to reconvert the diamond to carbon. Polycrystalline diamond powders made by this method are the commercially available Mypolex® diamond powders (E. I. du Pont de Nemours and Company. Wilmington, DE). V. L. Kuznetsov et al., Carbon 32 (5), 873 ( 1994), the contents of which are incorporated herein, describe another method for making polycrystalline diamond powder by shock synthesis. Soot resulting from the detonation of explosives in hermetic tanks filled with a gas that is inert toward elemental carbon contains diamond particles 2- 1 nm in size. The diamond powders prepared by shock synthesis are prepared under severe conditions and it is not surprising that the diamond product typically contains metal impurities. These impurities depend upon the apparatus used for the preparation and the particular explosive when soot from the explosive is the source of the diamond. The more prevalent impurities in diamond powders prepared by shock synthesis are copper, iron, silicon, chromium, titanium, aluminum, calcium and manganese.

Field emission tests on samples of diamond powder were carried out using a flat-plate emission measurement unit comprised of two electrodes, one serving as the anode or collector and the other serving as the cathode. This will be referred to in the Examples as Measurement Unit I. The two electrodes, copper plates 1.5 in x 1 in x 1/8 in (3.8 cm x 2.5 cm x .32 cm), were separated by ceramic insulator spacers. The thickness of the insulators determines the distance or gap between the electrodes and spacers of thicknesses from about 0.055 cm to about 1.0 cm were available. Electrical contacts with the electrodes were made with screws at the backs of the electrodes. In order to measure the emission properties of a sample of the diamond powder, the diamond powder was attached to an electrically conducting substrate and the substrate was placed on the copper plate serving as the cathode. A negative voltage was applied to the cathode and the emission current was measured as a function of the applied voltage using an ammeter connected to the anode. Since the separation distance between the plates d and the voltage V were measured, the electric field E could be calculated (E=V/d) and the current could be plotted as a function of the electric field. Another emission measurement unit (referred to in the Examples as Measurement Unit II) was used when wires or fibers were employed as the substrate. Electron emission from wires having attached diamond powder particles was measured in a cylindrical test fixture. In this fixture, the conducting wire to be tested (cathode) was mounted in the center of a cylinder (anode). This anode cylinder typically consisted of a fine mesh cylindrical metal screen coated with a phosphor. Both the cathode and anode were held in place by an aluminum block with a semi-cylindrical hole cut therein.

The conducting wire was held in place by two 1/16 inch-diameter stainless steel tubes, one at each end These tubes were cut open at each end, forming an open trough in the shape of a half cylinder of length 1/2 inch and diameter 1/16 mch, and the wire was placed in the open trough that results and held in place with silver paste. The connecting tubes were held in place within the aluminum block by tight fitting polytetrafluoroethylene (PTFE) spacers, which served to electrically separate the anode and cathode. The total length of exposed wire was generally set at 1 0 cm, although shorter or longer lengths could be studied by controlling the placement of the holder tubes T e cylmdπcal screen mesh cathode was placed m the semi-cylindrical trough in the aluminum block and held m place with copper tape The cathode was m electπcal contact with the aluminum block. Electπcal leads were connected to both the anode and cathode The anode was maintained at ground potential (0 V) and the voltage of the cathode was controlled with a 0-10 kV power supply Electrical cuπent emitted by the cathode was collected at the anode and measured with an electrometer The electrometer was protected from damagmg current spikes by an rn-senes 1 MΩ resistor and in-parallel diodes which allowed high current spikes to bypass the electrometer to ground.

Samples for measurement of length about 2 cm were cut from longer lengths of processed wires With the flexible stainless steel screen with phosphor removed, they were inserted into the cylindrical troughs of the two holder arms Silver paste was applied to hold them in paste The silver paste was allowed to dry and the phosphor screen was reattached and held in place with copper tape at the two ends. The test apparatus was inserted mto a vacuum system, and the system was evacuated to a base pressure below 3 x IO^"6 ton.

Emission cuπent was measured as a function of applied voltage Electrons emitted from the cathode create light when they stroke the phosphor on the anode. The distribution and intensity of electron emission sites on the coated wire were observed by the pattem of light created on the phosphor/wire mesh screen. The average electric field E at the wire surface was calculated through the relationship E = V/[a In (b/a)], where V was the voltage difference between the anode and cathode, a was the wire radius, and b was the radius of the cylmdπcal wire mesh screen

Typically, the diamond powder is attached to the surface of a substrate to form a field emitter cathode The substrate may be of any shape, e g , a plane, a fiber, a metal wire, etc. Suitable metal wires include nickel, copper and tungsten. The means of attachment must withstand and maintain its integrity under the conditions of manufacturing, in the apparatus into which the field emitter cathode is placed, and under the conditions surrounding its use. e.g., typically vacuum conditions and temperaures up to about 450°C. As a result, organic materials are not generally applicable for attaching the particles to the substrate and the poor adhesion of many inorganic materials to carbon further limits the choice of materials that can be used.

One method of attaching the diamond powder to a substrate is by pressing it against a conductor, e.g., a silver foil, with sufficient pressure to embed the diamond powder in the conductor. Alternatively, the diamond powder can be attached to a substrate by creating a thin metal, layer of a conductive metal, such as gold or silver, on the substrate with the diamond powder embedded in the thin metal layer. The thin metal layer anchors the diamond powder to the substrate. Tiie Applicants have found that in order for a diamond particle to be effective as an electron emitter it is necessary to have at least one surface of the particle exposed, i.e., be free of metal and protrude from the surface of the thin metal layer. The cathode surface should be comprised of the surfaces of an array of diamond powder particles with the metal filling the interstices between the diamond powder particles, The quantity of diamond particles and the thickness of the metal layer must be chosen to promote the formation of such a surface. In addition to providing means to attach the diamond particles to the substrate, the conducting metal layer also provides means to apply a voltage to the diamond powder particles. One process for creating a thin metal layer of a conductive metal, such as gold or silver, on a substrate with the diamond powder embedded in the thin metal layer comprises depositing a mixture of diamond powder and a conductive paste or composition of the type used in the electronics industry in producing printed circuit boards. An example of such a paste is 5007 Silver Conductor composition commercially available from E. I. du Pont de Nemours and Company, Wilmington. DE.

Another process for creating a thin metal layer of a conductive metal, such as gold or silver, on a substrate with the diamond powder embedded in the thin metal layer comprises depositing a solution of a metal compound in a solvent and the diamond powder onto the surface of the substrate. The solution can be applied to the surface first and the diamond particles then deposited, or the diamond particles can be dispersed in the solution which is then applied to the surface. The metal compound is one which is readily reduced to the metal, e.g.. silver mtrate, silver chloride, sdver bromide, silver iodide and gold chloride Additional description of this process is provided m provisional Application No 60/ C&a Hf entitled "Process for Making A Field Emitter Cathode Usmg a Particulate Field Emitter Material" filed simultaneously herewith, the contents of which are incorporated herein by reference

In many mstances it will be desireable to mcrease the viscosity of the solution by addmg an organic binder mateπal so that the solution readily remains on the substrate Examples of such viscosity modifiers include polyethylene oxide, polyvmyl alcohol and nitrocellulose The substrate with the solution and the diamond particles deposited on it is then heated to reduce the metal compound to the metal When an organic binder material is used it is boiled away (decomposed) duπng such heatmg The temperature and the time of heatmg are chosen to result m the complete reduction of the metal compound Typically, reduction is carried out at temperatures from about 120°C to about 220°C A reducing atmosphere or air can be used Typically, the reduc g atmosphere used is a 98% argon and 2% hydrogen mixture and the gas pressure is about 5- 10 psi (3 5-7 x 10⁴ Pa)

The product is a substrate coated with a thin layer of the metal with the diamond powder (e g , Mypolex® polycrystallme diamond powder) embedded therem and anchored to the substrate Such a product is suitable for use as a field emitter cathode

The following non-lumtmg examples are provided to further illustrate, enable and describe the mvention In the following examples, the flat -plate emission measurement unit or the coated wire emission measurement unit descnbed above were used to obtain emission characterisics for these materials

EXAMPLE 1 A 1 g portion of Mypolex® polycrystallme diamond powder (commercially available from E I du Pont de Nemours and Company, Wilmington, DE) with 6 μm particle size was evenly dispersed on a stπp of silver foil and a second silver foil was placed on top of the diamond powder The two silver foils were 0 2 cm thick and 1 1 cm wide The two silver foils with the Mypolex® polycrystallme diamond powder between them were manually pressed together usmg a pestle The pestle was rubbed on the foils untd essentially all the diamond powder was embedded in the fods as determined by pulling apart the foils and tapping them to determme if there was any unembedded diamond powder The foils were then blown with compressed air to remove any unembedded diamond particles 1 cm x 1 cm pieces were cut from each foil for emission testing Each 1 cm x 1 cm piece of silver foil with diamond powder embedded in it was placed on the cathode of the flat-plate emission measurement unit described above as Measurement Unit I Colloidal graphite (DAG conductive graphite pamt commercially available from SPI Supplies, West Chester, PA) was used to hold the piece in place by painting the non-emittmg side of the foil to ensure electπcal contact Care was taken to ensure that all the colloidal graphite was between the piece of foil and the cathode and that none was exposed. The plate separation distance was 0.063 cm The voltage was applied and the emission current was measured The results obtained are shown in Table I below where the same sample was used but 20 current readings for each voltage were taken to provide an average emission cuπent The results show that diamond powder prepared by shock synthesis can serve as an electron field emitter and that a field emitter cathode can be made therefrom

TABLE I

Applied Voltatre (Volts ) Averaee Emission Cuπent (μA )

1500 0 3

1600 1 1

1700 2 7

1800 4 0

1900 5.2

2000 6 2

2100 10.6

2200 19.8

EXAMPLE 2

A homogeneous mixture of 0 1 g of Mypolex® polycrystalline diamond powder and 1 9 g of a screen-printable thick film silver conductor composition, 5007 Silver Conductor (E I du Pont de Nemours and Company, Wilmington, DE), was deposited with a spatula onto a 1 in x 1 m (2.5 cm x 2.5 cm) alumina substrate. The mixture was deposited m the form of two lmes 1/8 in (0.32 cm) wide, 1/16 in (0.16 cm) thick and 3/4 ( 1.9 cm) long The substrate, with these lmes, was heated to 1 0°C and mamtamed at that temperature for 20 mmutes and then cooled in the furnace to ambient temperature, about 20°C, before it was removed from the furnace The substrate was placed on the cathode of the flat- plate emission measurement unit described first above and held there with conducting Cu tape Two additional pieces of conducting Cu tape were used to hold the cathode Cu plate Electrical connection to each of the prmted silver lines containing the embedded diamond powder was made by a screen-prmtable thick film silver conductor composition. 5007 Silver Conductor (E. I. du Pont de Nemours and Company, Wilmington, DE). The sample was air dried. The plate separation distance was 0.063 cm. The voltage was applied and the emission cuπent was measured. The results obtained are shown in Table II below where the same sample was used but 20 cuπent readings for each voltage were taken to provide an average emission cuπent. It is believed that lower emission cuπent was obtained here, compared to those of Example 1. because too much of the diamond powder was covered with the printed silver conductor; the small portion of the diamond powder which had portions of the particles exposed provided considerably less emission cuπents.

TABLE II

Applied Voltage (Volts) Average Emission Cuπent (μA)

2000 0.02

2200 0.03

2400 0.04

2500 0.05

2600 0.06

2700 0.07

2800 0.08

2900 0.09

3000 0.11

3100 0.17

EXAMPLE 3

This example illustrates a method for attaching 6 μm diamond powder particles (Mypolex® polycrystalline diamond powder from E. I. du Pont de Nemours and Company) onto a metal wire (2 mil nickel wire commercially available from Goodfellow Coφoration, Berwyn. PA) by using gold compound (commercially available from Aesar 12943, Ward Hill, MA) that was brushed onto the support wire following the manufacturer's suggestions. Immediately after depositing the gold compound, the wire was immersed into the diamond powder. The nickel wire covered with gold compound and diamond particles was then placed in a furnace for firing in an air atmosphere. The wire was heated to 540°C at a 25°C/minute heating rate in an air atmosphere for 30 minutes to bum off all organic materials. The wire was then cooled to room temperature. The fired samples comprised a thin gold metal layer that anchored the diamond particles onto the nickel wire. The emission was measured using Measurement Unit II previously described. Emission data is shown in Table HI below where the same sample was used but 20 cuπent readings for each voltage were taken to provide an average emission cuπent The results show that diamond powder prepared by shock synthesis can serve as an electron fielder emitter and that a field emitter wire cathode can be made therefrom

TABLE UI

Applied Voltage Average Emission Cuπent

(V) (μA)

1700 0 0015

1800 0 002

1900 0 004

2000 0 008

2100 0 016

2200 0 032

2300 0 068

2350 0 09

2400 0 13

Although particular embodiments of the present mvention have been descnbed m the foregomg descπption, it will be understood by those skilled in the art that the mvention is capable of numerous modifications, substitutions and reaπangements without departing from the spirit or essential attπbutes of the mvention Reference should be made to the appended claims, rather than to the foregomg specification, as indicating the scope of the mvenuon

Claims

CLAIMS:

1. A field emission electron emitter comprising diamond powder prepared by shock synthesis.

2. The field emission electron emitter of Claim 1 wherein the diamond powder has a particle size less than about 20 μm.

3. The field emission electron emitter of Claim 1 wherein the diamond powder has a particle size less than about 10 μm.

4. The field emission electron emitter of Claim 1 wherein the diamond powder has a particle size less than about 1 μm.

5. A field emission cathode comprised of diamond powder prepared by shock synthesis attached to the surface of a substrate.

6. The field emission cathode of Claim 5 wherein the substrate is planar.

7. The field emission cathode of Claim 5 wherein the substrate is a fiber.

8. The field emission cathode of Claim 5 wherein the substrate is a metal wire.

9. The field emission cathode of Claim 8 wherein the metal wire is nickel.

10. The field emission cathode of Claim 8 wherein the metal wire is tungsten.

11. The field emission cathode of Claim 8 wherein the metal wire is copper.