Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
  • H01J9/02—Manufacture of electrodes or electrode systems
  • H01J9/022—Manufacture of electrodes or electrode systems of cold cathodes
• • 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
  • H01J1/13—Solid thermionic cathodes
  • H01J1/14—Solid thermionic cathodes characterised by the material
  • H01J1/144—Solid thermionic cathodes characterised by the material with other metal oxides as an emissive material
• • 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
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
  • H01J9/02—Manufacture of electrodes or electrode systems
  • H01J9/04—Manufacture of electrodes or electrode systems of thermionic cathodes

Definitions

• the present invention relates to the field of electron emission in a vacuum from a cathode in the general sense.
• the object of the invention thus relates to the field of electron sources in the general sense, such as the electron sources of televisions or electronic systems using electron sources (radiofrequency tubes) in a vacuum environment (10 "4 to 10 " 11 Torr) for consumer and professional applications.
• an electron extraction device comprises an anode and an emission cathode located at a distance from each other and between which there is a vacuum or an ultra-vacuum.
• the anode and the cathode are connected together using a polarization source making it possible to place them at a given relative potential.
• a polarization source making it possible to place them at a given relative potential.
• the extraction of electrons from the cathode can be obtained by a technique of heating the cathode, with a view to raising the energy of the electrons to a value exceeding the output work.
• This technique known as thermionic emission, has the disadvantage of placing the cathode at high temperature (2700 K in the case of a tungsten cathode for example) and, consequently, of presenting an energy consumption and relatively large heat dissipation.
• the object of the invention therefore aims to meet this need by proposing a method for producing an electron emission cathode, according to a relatively simple technique and at a reduced cost, and intended to operate at ambient temperature.
• the subject of the invention relates to a method for producing an electron-emitting cathode.
• the method consists:
• a substrate in the form of a metallic filament having a diameter between 50 and 400 ⁇ m and preferably, of the order of 100 ⁇ m or a flat metallic pellet whose emission surface is between 0 , 01 mm 2 and 100 mm 2 ,
• the object of the invention also aims to propose an electron emission cathode comprising a substrate produced in the form of a metallic filament having a diameter between 50 and 400 ⁇ m and, preferably, of the order of 100 ⁇ m, or a flat metal pellet whose emission surface is between 0.01 mm and 100 mm, the metal substrate being covered with a layer of metal oxide obtained from a soil containing a metallic alkoxide (M - (OR) felicitwhere M denotes a metal and R denotes an alkyl group), the metal oxide layer delimiting with the metallic substrate, an electronic junction having a potential barrier height of a few tenths of electron volts and having a thickness between 1 nm and 10 nm and, preferably, of the order of 5 nm.
• M - (OR) metallic alkoxide
• Figs. 1 and 2 are large-scale schematic views illustrating exemplary embodiments of emission cathodes having, respectively, a pin and a point shape.
• Fig. 3 is a diagram illustrating a device for drawing a layer, used in the production method according to the invention.
• Fig. 4 shows curves illustrating the emission currents i of a cathode according to the invention, as a function of the voltage V necessary to extract the electrons.
• Fig. 5 is a curve illustrating the stability over time of the emission current i of a cathode according to the invention.
• the emission cathode I comprises a metal substrate 1 in the form of a filament produced from a metal wire, for example made of platinum, having a diameter included between 50 and 400 ⁇ m and, preferably, of the order of 100 ⁇ m.
• the cathode I is shaped to have a hairpin geometry revealing a loop 2 whose radius of curvature can be between 0.5 mm and 5 mm.
• the metal filament 1 ends in a tip 3, the end of which has a radius of between 10 nm and 10 ⁇ m and, preferably, of the order of 100 nm.
• the substrate 1 can be produced in the form of a flat metallic pellet of millimeter dimensions.
• a flat metal pellet has an emission surface of between 0.01 mm 2 and 100 mm 2 .
• Such an emission cathode I is therefore formed from a filament or a metal pellet forming an electron reservoir.
• This emission cathode I also comprises an ultra-thin metal oxide layer 4 deposited on the substrate 1, in particular at its end, namely the loop 2 or the tip 3, in the case of the examples illustrated in FIGS. 1 and 2.
• the metal oxide layer 4 (of formula MO 2 , with M denoting a metal) forms a conduction medium for the electrons injected coming from the metal substrate 1.
• this layer of metal oxide 4 behaves like an n-type semiconductor defining with the metal substrate 1, a metal-semiconductor electronic junction (Schottky).
• This Schottky junction has a potential barrier height of a few tenths of electron volts, that is to say between 0.05 and 1 eN and, preferably, of the order of 0.1 eN.
• the characteristics of this Schottky junction impose the choice of the couple of adequate materials metal 1 and layer 4 of type n.
• the layer 4 can be either n-type SiC (silicon carbide), or n-type TiO 2 (titanium oxide).
• This metal oxide layer 4 thus has an emission surface for the electrons extracted in a vacuum using a polarization source.
• the metal oxide layer 4 has a thickness defined between the Schottky junction and the emission surface, substantially equal to the mean free path of the electrons in this metal oxide layer 4, for example, between 1 and 10 nm and, preferably, of the order of 5 nm for semiconductor layers of n-type SiC (silicon carbide) or n-type TiO- 2 (titanium oxide) on a platinum substrate 1.
• the deposition of an ultra-thin metal oxide layer 4 of nanometric thickness is carried out using the SOL-GEL method.
• the following description describes the production of chemical gels and their use for depositing ultra thin layers on a substrate 1.
• the SOL-GEL transition corresponds to the change in viscosity of a liquid or colloidal solution called SOL , until it massively occupies its entire container.
• the production of the metal oxide layer 4 consists of depositing the liquid SOL on the substrate 1 before gelling, and at a viscosity adapted to the thickness desired for this layer 4.
• the gelling (solidification by the successive chemical reactions which form the monomer chains) is produced during deposition on substrate 1.
• the solution is then stirred (magnetic stirrer) for 10 minutes.
• the water necessary for the hydrolysis reactions comes from the esterification reactions between the acid (CH 3 COOH) and the alcohol ((CH 3 ) 2 CHOH).
• the solution is stirred for 15 minutes. It should be noted that this solution is known, moreover.
• a dilution is carried out: if Vs is the volume of the solution at this time, a dilution is carried out in methanol CH 3 OH, and 11 Vs of CH 3 OH are added.
• the solution is stirred for two hours.
• the dilution carried out is a function of the thickness desired for the metal oxide layer 4.
• the latter Prior to the deposition of the metal layer 4 on the substrate 1, the latter is cleaned, for example in several ultrasonic baths composed of more and more solvents. volatile, namely for example alcohol, acetone and ether.
• the actual metal oxide layer 4 is deposited or drawn on the substrate 1.
• a drawing device a schematic example of which is illustrated in FIG. 3.
• a drawing device comprises an enclosure 6 whose atmosphere is controlled.
• the enclosure 6 is equipped with a hygrometer 7 making it possible to control the humidity inside the enclosure in order to control the speed of the reactions (hydrolysis).
• the enclosure comprises a source 8 for injecting dry neutral gas, such as argon or nitrogen. It should be understood that the drawing of the metal oxide layer 4 is carried out under a gas flow.
• the enclosure 6 is equipped with a metal wire 9 intended to support, at its free end, the substrate 1 on which a layer of metal oxide is to be deposited.
• a metal wire 9 is moved in an upward or downward movement by means of an electric motor 11 controlled to allow the speed of descent and the rate of ascent of the substrate to be controlled inside a container 12 containing the colloidal solution or SOL.
• the method therefore consists in immersing the substrate 1 in the SOL at a controlled speed and also removing it at a controlled speed.
• the immersion of the substrate 1 inside the SOL leads to the penetration, into the solution, of the filament 1 from its end in a loop 2 or at a point 3.
• the withdrawal speed is a governing parameter the thickness of the metal oxide layer 4.
• the faster the removal speed the thicker the metal oxide layer 4.
• a constant withdrawal speed of 8 cm / min has been chosen which is considered to be a speed which does not slow down the drawing procedure too much and which can be controlled relatively easily.
• the viscosity of the SOL was chosen to adjust the viscosity of the SOL in order to obtain, in the end, a metal oxide layer 4 of between 1 and 10 nm and, preferably, of the order of 5 nm. Furthermore, it is planned to choose a reduced immersion speed, so as to prevent any risk of damaging the substrate 1. Thus, the immersion speed is chosen to be lower than the withdrawal speed. For example, a constant immersion speed of 4 cm / min can be chosen.
• the substrate 1 is advantageously inverted to avoid the phenomenon of capillary rise of the liquid which can lead to the formation of drops.
• the substrate 1 is thus placed vertically with the looped end 2 or at the point 3 directed upwards.
• the emission surface is held horizontally and directed upwards.
• the substrate 1 then undergoes, in this position, a drying operation insofar as, following its removal from the ground, the metal oxide layer 4 contains liquid residues of the reactions (water). This metal oxide layer 4 must therefore be dried uniformly.
• the substrate 1 is placed inside a drying enclosure controlled at a temperature which can be between 80 and 120 ° C and, preferably, equal to 100 ° C for a period which can be between 10 and 30 min and, preferably equal to 15 min.
• an annealing operation is carried out on the deposited metal oxide layer 4.
• Such an operation aims to control the crystallographic structure and the densification of the ultra thin layer 4.
• the SOL-GEL method leads to porous amorphous materials containing organic residues.
• annealing the metal layer 4 is to densify it (closing the porosity so that the layer is no longer permeable and obtaining nanometric pores) and to ensure its stoichiometric purity (MO 2 ).
• this annealing operation is carried out by means of an infrared lamp (15 V and 150 Watts) making it possible to subject the entire metallic layer 4 to a temperature which may be between 200 and 750 ° C. and, preferably of the order of 350 ° C. for a period which may be between 10 and 30 min and, preferably, of the order of 15 min.
• the annealing operation is carried out under a flow of oxygen which can simply be produced by forced ventilation.
• Annealing the entire layer under an infrared lamp makes it possible to take advantage of the good reflectivity of the metal in this wavelength, leading to direct heating of the metal oxide layer 4.
• an electron emission cathode I can be obtained comprising a dense, homogeneous metal oxide layer, of the order of 5 nm thick on the end of a filament. in the form of a point or loop or on a flat pellet. A metal oxide layer is thus obtained without shrinkage or cracking with a uniform thickness.
• the method described above makes it possible to develop an electron-emitting cathode I according to a simple method which can be implemented industrially, insofar as it is carried out in the open air.
• Such a cathode I can advantageously be used as a source of electrons, the emission of which is regulated or controlled by means of a polarization source creating a magnetic field in vacuum and making it possible to control the height of the barrier of potential of surface of layer 4 behaving like an n-type semiconductor, so as to reversibly modify the electronic surface affinity of the metal oxide layer 4.
• a cathode I has the same emission performance current than those obtained with ultra thin layers deposited by vacuum techniques.
• Fig. 4 illustrates the emission currents i of a pin cathode produced according to the method according to the invention and for different distances (curves gfn012 to gfh017) from the cathode relative to a measurement anode, as a function of the voltages extraction V.
• Fig. 5 makes it possible to show the stability over time t of the emission current î of a cathode produced in the form of a tip and according to the manufacturing method according to the invention.
• Another advantage of the invention relates to the fact that the emission cathode I can take the geometric form of current cathodes operating either in thermionic mode or in field emission mode. So, such an emission cathode I can come to replace advantageously the current cathodes in their configuration, without geometrical modification.
• the cathode according to the invention can thus replace the current cathodes used in electron tubes or electron guns.

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• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Cold Cathode And The Manufacture (AREA)

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• 1999
  • 1999-11-12 FR FR9914474A patent/FR2801135B1/fr not_active Expired - Fee Related
• 2000
  • 2000-11-10 WO PCT/FR2000/003138 patent/WO2001035434A1/fr not_active Application Discontinuation
  • 2000-11-10 JP JP2001537082A patent/JP2003514346A/ja active Pending
  • 2000-11-10 AU AU17105/01A patent/AU1710501A/en not_active Abandoned
  • 2000-11-10 EP EP00979705A patent/EP1228521A1/de not_active Withdrawn

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