WO1999009580A1 - Cathode obtenue a partir de materiau getter - Google Patents

Cathode obtenue a partir de materiau getter Download PDF

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Publication number
WO1999009580A1
WO1999009580A1 PCT/US1998/017064 US9817064W WO9909580A1 WO 1999009580 A1 WO1999009580 A1 WO 1999009580A1 US 9817064 W US9817064 W US 9817064W WO 9909580 A1 WO9909580 A1 WO 9909580A1
Authority
WO
WIPO (PCT)
Prior art keywords
cathode
getter
granular
conditioning
diamond
Prior art date
Application number
PCT/US1998/017064
Other languages
English (en)
Inventor
Victor I. Chornenky
Dale L. Schreiner
Original Assignee
Xrt Corp.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Xrt Corp. filed Critical Xrt Corp.
Priority to EP98940959A priority Critical patent/EP1005702A1/fr
Priority to JP2000510156A priority patent/JP2001516128A/ja
Priority to AU89118/98A priority patent/AU8911898A/en
Publication of WO1999009580A1 publication Critical patent/WO1999009580A1/fr

Links

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/30—Cold cathodes, e.g. field-emissive cathode
    • H01J1/304—Field-emissive cathodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J7/00—Details not provided for in the preceding groups and common to two or more basic types of discharge tubes or lamps
    • H01J7/14—Means for obtaining or maintaining the desired pressure within the vessel
    • H01J7/18—Means for absorbing or adsorbing gas, e.g. by gettering
    • H01J7/183—Composition or manufacture of getters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2201/00—Electrodes common to discharge tubes
    • H01J2201/30—Cold cathodes
    • H01J2201/304—Field emission cathodes
    • H01J2201/30446—Field emission cathodes characterised by the emitter material
    • H01J2201/30453—Carbon types
    • H01J2201/30457—Diamond

Definitions

  • the invention relates to the field of cathodes, and more particularly, cathodes that are used for vacuum electronic applications. Background Of The Invention
  • a cathode is an electrode by which electrons enter a system, such as an electrolytic cell or electron tube.
  • Cathodes are also employed in x-ray devices, flat panel display systems, microwave sources, radar, communications, high power fast switches, electron beam processing of materials, high gradient accelerators, and many other applications.
  • Cathodes are generally divided into four types: thermionic cathodes, laser driven photo-cathodes, field emission cathodes, and exploding or plasma field emission cathodes.
  • the present invention relates to field emission cathodes, which are used in vacuum applications.
  • Vacuum field emission cathodes produce an electron beam by Fowler- Nordheim quantum tunneling of electrons from near the Fermi level into the vacuum.
  • a relatively large electric field is required compared to other cathode types.
  • the large electrical field that is required can be obtained from enhancements of the applied field due to surface irregularities.
  • One example of a device that may employ a field emission cathode is a miniature x-ray device.
  • a miniature x-ray device is designed for use inside a body, and the cathode operates inside a vacuum chamber.
  • the present invention is directed to a cathode for a vacuum electronic device, the cathode is composed of a material that also allows the cathode to act as a getter. Any form of the cathode surface will allow it to function as a cathode and getter.
  • the cathode preferably has a surface that may be smooth or granular.
  • One additional embodiment of the invention is a cathode for a vacuum electronic device, the cathode composed of a material that also allows it to act as a getter and containing diamond powder mixed with a getter material.
  • Another additional embodiment is an x-ray device constructed with the cathode which also acts as a getter.
  • the x-ray device includes a miniature vacuum housing with an anode and the cathode-getter disposed inside.
  • the x-ray device may also include an electrical connector for generation of an electric field between the anode and cathode.
  • FIG. 1 is a side view of one embodiment of a cathode of the present invention.
  • Fig. 2 is a side view of another embodiment of a cathode of the present invention.
  • Fig. 3 is a plot of voltage applied versus time for the current plots shown in Fig. 4 and Fig. 5, for the cathode of the present invention.
  • Fig. 4 is a plot of current versus time for the cathode of Fig. 1.
  • Fig. 5 is a plot of current versus time for the cathode of Fig. 2.
  • Fig. 6 is a side view of another embodiment of a cathode and gate electrode of the present invention.
  • Fig. 7 is a block diagram of a manufacturing process of one embodiment of the present invention. While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
  • the cathode is applicable to a variety of devices, and methods of fabrication thereof, which employ field emission cathodes.
  • the invention is particularly advantageous in devices using field emission cathodes in vacuum uses.
  • Examples of vacuum electronics that utilize electron emission include vacuum tubes, flat-panel displays, and microwave tube generators. While the present invention is not so limited, an appreciation of various aspects of the invention is best gained through a discussion of application examples operating in such environments.
  • One such application example is a miniature x-ray device which is suitable for insertion into a body to deliver localized x-ray radiation.
  • Such a device includes a miniature vacuum housing and an anode and cathode, which also acts as a getter, contained within the housing.
  • the device For use within a body such as within vessels of the body, the device also includes a connector to the anode and cathode.
  • the connector enables generation of an electric field between the anode and cathode.
  • the device with its connector may be disposed within a catheter for insertion into the body.
  • the miniaturized design of the device provides an outer cross-sectional dimension of the device (for example, the cross-sectional diameter of a cylinder, ellipse or spheroid shaped device) of less than or equal to about 5 millimeters, preferably no more than about 3 millimeters and especially preferably no more than about 1-2 millimeters. Such dimension enables access to the small vessels of the body.
  • the connector preferably is capable of carrying high voltage under insulated conditions yet maintaining a miniature construction suitable for the small vessels of the body.
  • a goal for any electronic emission component, and especially for the above described miniaturized x-ray device, is increased efficiency, for example, by reducing the required electric field.
  • Small irregularities on the surface of the cathode result in an increase in the magnitude of the electrical field for an applied voltage, thereby increasing the chance of electrical flashover.
  • the weaker the required electrical field at the cathode the more imperfections can be tolerated on the cathode surface without risking flashover.
  • Another goal in many electronic devices and especially for the above- described miniature x-ray device is to maintain a vacuum condition within the volume surrounding the cathode.
  • the components are assembled within a vacuum or the chamber is pumped out by conventional methods.
  • a getter may also be disposed within the volume.
  • the getter has an activation temperature and once activated it will react with stray gas molecules in the vacuum.
  • the getter eliminates stray gas molecules, improving the quality of the vacuum within the volume.
  • a getter may serve as an electron emitter when a voltage differential is applied. Therefore, the getter may be used as the cathode. This combination of elements results in a smaller electronic device with a very simple design.
  • a cathode made of metal materials may not require further processing in order to emit electrons at moderate electrical fields for x-ray applications.
  • an additional coating such as diamond, is used on the surface of a cathode to improve the cathode's field emission characteristics.
  • the physical characteristic of the cathode surface may be any that will allow the cathode to function in both capacities. This characteristic will range from smooth to granular.
  • a smooth surfaced cathode promote uniform emission of electrons and avoid high local field density.
  • a granulated surface of the cathode provides multiple microprotrusions capable of efficient field emission of electrons at moderate electrical fields.
  • a diamond coating is sometimes used on the surface of a cathode to improve field emission qualities.
  • This diamond coating may be produced by a laser sputtering technique, as described in U.S. Patent 4,987,007, issued January 22, 1991, invented by Wagal.
  • the laser sputtering technique produces diamond-like carbon, or amorphous diamond.
  • Amorphous diamond is less stable when exposed to heat above about 500°C to 600°C than other types of diamond, such as natural diamond, high pressure high temperature diamond or high quality CVD diamond. Therefore, it is preferable to use these types of diamond in a cathode, if that cathode is to be subjected to a high temperature process. For example, the resistivity of amorphous diamond is affected after being heated to about 500°C to 600°C. Natural diamond, high pressure high temperature diamond and high quality CVD diamond exhibit more stable thermal behavior.
  • Fig. 7 shows the general steps of forming the cathode of the present invention.
  • Granular diamond from natural diamond, high pressure high temperature diamond or high quality CVD diamond may be mixed in with getter material, according to one embodiment of the present invention in step 20 of Fig. 7.
  • the mixed material is annealed in a vacuum furnace.
  • An acceptable annealing procedure can include the steps of mixing the cathode material, filling cathode molds with the mixed material in step 22, thermally bonding the molds at approximately 1000°C to 1200°C for about one hour in step 24, and removing the cathodes from the molds in step 26.
  • Cathodes with diamond films may exhibit different emission characteristics for different geometrical configurations of the coating. Therefore, precise masking techniques are used to achieve the desired cathode configuration. This process is expensive, difficult and time-consuming.
  • the field emission characteristics of the present invention may be adjusted by changing the concentration of diamond material. The concentration of diamond in a cathode is simpler to adjust than the geometrical configuration of diamond coatings.
  • a typical cathode is configured to be spaced apart from an anode. This configuration is called a diode.
  • a triode configuration may also be used when there is enough space in the device to permit a third or gate electrode.
  • the third electrode may be positioned near the cathode and allows independent control of the anode- cathode current and the anode-cathode voltage. Therefore, more varied performance characteristics may be elicited from a cathode when a gate electrode is used.
  • One example of a gate electrode 10 is shown in Fig. 6.
  • the cathodes 14 are positioned in spaces in the gate electrode.
  • granular getter materials field emission current densities of approximately 0.5-5 milliamps per square millimeter are observed at the cathode at electrical fields of 10-50 volts per micron. This level of emission current is similar to that found in diamond-coated cathodes operating at moderate electrical fields for x-ray production.
  • a device using a granular getter material as the cathode is considerably less complicated to manufacture.
  • the cathode may be formed by thermal diffusion bonding of powdered getter material, or if a granulated surface is desired, use of getter powder having granular sizes of .5 to 50 micrometers in diameter.
  • a smoother cathode surface may be achieved by using conditioning processes and coating processes.
  • a granulated cathode surface may be achieved by use of granulated getter material and its mixture with diamond in proportions that allow the granules to protrude above the overall surface of the cathode.
  • low-electrical field x- ray radiation is produced from a cathode made by mixing diamond powder with the granulated getter materials before forming the cathode of the present invention.
  • Diamond materials display valuable properties as field emitters, losing electrons easily as a field is applied.
  • an electrical field of 10-50 volts per micron will produce current densities in the range of 1 - 10 milliamps per square millimeter.
  • the diamond material may be purchased commercially from numerous sources.
  • diamond powder material may be obtained from CVD processes, although it may be produced from natural beds of diamond or high temperature high pressure diamond.
  • the getter cathode 14 may be composed of many different types of getter materials.
  • the getter may include zirconium, aluminum, vanadium, iron, and/or titanium.
  • the getter materials may be composed of an alloy including vanadium, iron and zirconium.
  • One successful choice for the getter cathode is a material produced by SAES Getters, S.p.A., via Gallarate 215, 20151 Milano, Italy and referred to as ST707.
  • Getter alloy ST707 is produced by thermal diffusion bonding and is composed of 24.6% vanadium, 5.4% iron, and 70%) zirconium.
  • the getter will be sufficiently conductive and capable of electron emission to serve as an effective cathode at moderate electrical fields, for example, 10 volts per micron to 60 volts per micron.
  • a passivating layer such as a layer of oxidation, that shields the getter material from the atmosphere at normal conditions.
  • a passivating layer such as a layer of oxidation
  • the oxidation layer diffuses into the interior of the getter, revealing the active getter surface, which will react and bond with molecules.
  • the active getter surface reacts with most stray gas molecules and bonds them to the getter, thereby improving the quality of the vacuum.
  • the SAES ST707 alloy getter has an activation temperature of 400-500°C.
  • the shape of the cathode affects the electron emission qualities of the cathode, and may be chosen to best suit the particular cathode application.
  • the field emission properties of the cathode may be modified by a conditioning procedure in step 30 that alters the surface of the cathode to achieve predetermined field emission parameters.
  • a conditioning procedure in step 30 that alters the surface of the cathode to achieve predetermined field emission parameters.
  • spark conditioning multiple applications of high voltage are carried out at different distances and relative positions of the electrodes, causing electrical discharges between electrodes.
  • the discharges can remove unstable emitting sites at the cathode surface and drive the field emission parameters into the desired range.
  • Conditioning should not eliminate all electron-emitting sites.
  • emitting sites that are located far way from the tip such as more than 40- 45° off of the central axis of the cathode, may be eliminated to reduce the risk of flashover.
  • a conditioned cathode is capable of more consistent performance.
  • Conditioning of the cathode may be carried out according to the concepts set forth in High Voltage Vacuum Insulation: Basic Concepts and Technological Practice, R.V. Latham, Editor, Academic Press, 1995.
  • current conditioning includes applying a voltage over the cathode and a series resistor. The applied voltage is increased in small steps such that a "pre-breakdown" current in the cathode stabilizes before proceeding. The conditioning procedure is typically continued until the intended operating voltage is reached.
  • “glow-discharge” conditioning includes using the sputtering action of low-energy gas ions to remove contaminants from the cathode surfaces.
  • a low- voltage AC glow discharge is allowed to occur between the cathode and an electrode by raising the pressure in the vacuum chamber while alternating current is provided to the cathode and the electrode.
  • gas conditioning which includes progressively increasing the field at the gap near the cathode at currents of a few microampere and a pressure of about -10 "5 mbar.
  • the emission currents are allowed to quench for a 20 minute period to their asymptotic limit.
  • the procedure is repeated at increasing field levels until the intended operating field of the cathode is reached.
  • spark conditioning may be used, also known as "spot- knocking".
  • the method may be carried out essentially as the current conditioning method; although a reduced series resistor may be necessary to increase the rate of dissipation of energy in the spark.
  • the external capacitance associated with the gap should be minimized so that only a limited amount of energy ( ⁇ 10 J) is dissipated in the gap during sparking.
  • a more sophisticated form of spark conditioning includes using discharges in the nanosecond range under ultra high vacuum conditions.
  • Current conditioning may preferably be used to improve a cathode in one embodiment of the present invention.
  • the slow application of voltage involved in current conditioning may be the most preferable method for conditioning the cathode of the present invention.
  • current conditioning involves slow increases in the application of voltage across the cathode and an anode. Voltage application may begin at about 1 kilovolt per millimeter and gradually increase to about 60 kilovolts per millimeter, and perhaps as high as 100 kilovolts per millimeter. The voltage application may increase in increments of 1 kilovolt per millimeter in one embodiment of the present invention.
  • the procedure for conditioning one cathode may take about thirty minutes. The conditioning process may be carried out before the cathode is assembled with the housing.
  • Fig. 1 shows a cathode 14-1 surface as formed with granular materials.
  • FIG. 3 shows a linear application of voltage to the anode and cathode.
  • the voltage of Fig. 3 is applied to the anode and cathode configuration of Fig. 1, the current shown in Fig. 4 results. There are many spikes in the current plot of Fig. 4.
  • Fig. 2 shows a plot of current versus time for the cathode of Fig. 2, assuming the voltage application shown in Fig. 3. If the cathode will be operated to produce a current of 100 microamps, then it may be desirable to condition the cathode at currents of about 200 to 300 microamps.
  • test cathodes may be used to determine the exact time and method of conditioning, and these parameters will be applied to other conditioning processes.
  • the material mixed to form the cathode will include getter material in granular shapes of about .50 to 50 microns, or more particularly about 5 to 15 microns.
  • This material may include diamond granular material of approximately the same granular size.
  • the diamond powder may be present in the cathode material at about 0.5% to 20%) by weight. More particularly, the diamond powder may make up approximately 1%> to 10% of the cathode material by weight.

Abstract

La présente invention concerne une cathode destinée à un dispositif électronique sous vide et réalisée à partir d'un matériau qui permet également à la cathode de se comporter comme getter. La cathode considérée peut comporter un mélange d'un matériau getter et d'une poudre de diamant.
PCT/US1998/017064 1997-08-18 1998-08-18 Cathode obtenue a partir de materiau getter WO1999009580A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
EP98940959A EP1005702A1 (fr) 1997-08-18 1998-08-18 Cathode obtenue a partir de materiau getter
JP2000510156A JP2001516128A (ja) 1997-08-18 1998-08-18 ゲッタ材料からなる陰極
AU89118/98A AU8911898A (en) 1997-08-18 1998-08-18 Cathode from getter material

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US5568297P 1997-08-18 1997-08-18
US60/055,682 1997-08-18

Publications (1)

Publication Number Publication Date
WO1999009580A1 true WO1999009580A1 (fr) 1999-02-25

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EP (1) EP1005702A1 (fr)
JP (1) JP2001516128A (fr)
AU (1) AU8911898A (fr)
WO (1) WO1999009580A1 (fr)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6320935B1 (en) 2000-02-28 2001-11-20 X-Technologies, Ltd. Dosimeter for a miniature energy transducer for emitting X-ray radiation
US6324257B1 (en) 1998-06-04 2001-11-27 X-Technologies Inc. Radiotherapeutical device and use thereof
WO2002007183A1 (fr) * 2000-07-19 2002-01-24 Saes Getters S.P.A. Procede de fabrication d'ensembles comprenant une base et un dispositif a fonction de cathode et de getter pour emetteurs de rayons x miniaturises et ensembles obtenus
US6463124B1 (en) 1998-06-04 2002-10-08 X-Technologies, Ltd. Miniature energy transducer for emitting x-ray radiation including schottky cathode
GB2387021A (en) * 2002-03-25 2003-10-01 Printable Field Emitters Ltd Creating field emission materials

Citations (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB230183A (en) * 1923-12-06 1925-03-06 Ruben Samuel Improvements in or relating to x-ray tubes
US2173258A (en) * 1937-11-27 1939-09-19 Rca Corp Active metal compound for vacuum tubes
GB997352A (en) * 1960-07-22 1965-07-07 Ind Distributors 1946 Ltd Abrasive bodies
US3582702A (en) * 1968-04-04 1971-06-01 Philips Corp Thermionic electron-emissive electrode with a gas-binding material
US4164680A (en) * 1975-08-27 1979-08-14 Villalobos Humberto F Polycrystalline diamond emitter
EP0359724A2 (fr) * 1988-09-12 1990-03-21 SAES GETTERS S.p.A. Cathodes froides pour tubes fluorescents
US5090043A (en) * 1990-11-21 1992-02-18 Parker Micro-Tubes, Inc. X-ray micro-tube and method of use in radiation oncology
FR2672734A1 (fr) * 1991-02-08 1992-08-14 Futaba Denshi Kogyo Kk Element d'emission de champ.
EP0572170A1 (fr) * 1992-05-28 1993-12-01 AT&T Corp. Dispositif de visualisation plat à émission de champ
JPH08153460A (ja) * 1994-11-29 1996-06-11 Canon Inc 電子源及びそれを用いた画像形成装置
EP0718864A1 (fr) * 1994-12-22 1996-06-26 AT&T Corp. Dispositif à émission de champ avec particules émitteurs de diamant ultra-fin
WO1997006549A1 (fr) * 1995-08-04 1997-02-20 Printable Field Emmitters Limited Materiaux et dispositifs d'emission electronique de champ
WO1997007740A1 (fr) * 1995-08-24 1997-03-06 Interventional Innovations Corporation Catheter a rayons x
EP0860180A2 (fr) * 1997-02-21 1998-08-26 Interventional Innovations Corporation Méthode d'irradiation X localisée vers l'intérieur d'un corps et sa méthode de fabrication
EP0860181A2 (fr) * 1997-02-21 1998-08-26 Interventional Innovations Corporation Dispositif à rayons X ayant une structure dilatable de délivrance de radiations localisées vers l'intérieur d'un corps

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB230183A (en) * 1923-12-06 1925-03-06 Ruben Samuel Improvements in or relating to x-ray tubes
US2173258A (en) * 1937-11-27 1939-09-19 Rca Corp Active metal compound for vacuum tubes
GB997352A (en) * 1960-07-22 1965-07-07 Ind Distributors 1946 Ltd Abrasive bodies
US3582702A (en) * 1968-04-04 1971-06-01 Philips Corp Thermionic electron-emissive electrode with a gas-binding material
US4164680A (en) * 1975-08-27 1979-08-14 Villalobos Humberto F Polycrystalline diamond emitter
EP0359724A2 (fr) * 1988-09-12 1990-03-21 SAES GETTERS S.p.A. Cathodes froides pour tubes fluorescents
US5090043A (en) * 1990-11-21 1992-02-18 Parker Micro-Tubes, Inc. X-ray micro-tube and method of use in radiation oncology
FR2672734A1 (fr) * 1991-02-08 1992-08-14 Futaba Denshi Kogyo Kk Element d'emission de champ.
EP0572170A1 (fr) * 1992-05-28 1993-12-01 AT&T Corp. Dispositif de visualisation plat à émission de champ
JPH08153460A (ja) * 1994-11-29 1996-06-11 Canon Inc 電子源及びそれを用いた画像形成装置
EP0718864A1 (fr) * 1994-12-22 1996-06-26 AT&T Corp. Dispositif à émission de champ avec particules émitteurs de diamant ultra-fin
WO1997006549A1 (fr) * 1995-08-04 1997-02-20 Printable Field Emmitters Limited Materiaux et dispositifs d'emission electronique de champ
WO1997007740A1 (fr) * 1995-08-24 1997-03-06 Interventional Innovations Corporation Catheter a rayons x
EP0860180A2 (fr) * 1997-02-21 1998-08-26 Interventional Innovations Corporation Méthode d'irradiation X localisée vers l'intérieur d'un corps et sa méthode de fabrication
EP0860181A2 (fr) * 1997-02-21 1998-08-26 Interventional Innovations Corporation Dispositif à rayons X ayant une structure dilatable de délivrance de radiations localisées vers l'intérieur d'un corps

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* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 096, no. 010 31 October 1996 (1996-10-31) *

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6324257B1 (en) 1998-06-04 2001-11-27 X-Technologies Inc. Radiotherapeutical device and use thereof
US6463124B1 (en) 1998-06-04 2002-10-08 X-Technologies, Ltd. Miniature energy transducer for emitting x-ray radiation including schottky cathode
US6320935B1 (en) 2000-02-28 2001-11-20 X-Technologies, Ltd. Dosimeter for a miniature energy transducer for emitting X-ray radiation
WO2002007183A1 (fr) * 2000-07-19 2002-01-24 Saes Getters S.P.A. Procede de fabrication d'ensembles comprenant une base et un dispositif a fonction de cathode et de getter pour emetteurs de rayons x miniaturises et ensembles obtenus
GB2387021A (en) * 2002-03-25 2003-10-01 Printable Field Emitters Ltd Creating field emission materials
GB2387021B (en) * 2002-03-25 2004-10-27 Printable Field Emitters Ltd Field electron emission materials and devices

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JP2001516128A (ja) 2001-09-25
AU8911898A (en) 1999-03-08
EP1005702A1 (fr) 2000-06-07

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