FR2801135A1 - PROCESS FOR PRODUCING A TRANSMISSION CATHODE USING THE SOL-GEL TECHNIQUE AND CATHODE OBTAINED BY SUCH A METHOD - Google Patents

Abstract

The electron emission cathode according to the invention comprises a substrate (1) made in the form of a metal filament having a diameter of between 50 and 400 m and, preferably, of the order of 100 m, or of a flat metal pellet whose emission surface is between 0.01 mm2 and 100 mm2, the metal substrate (1) being covered with a layer of a metal oxide (4) obtained from a sol containing a metal alkoxide (M - (OR) n where M denotes a metal and R an alkyl group), the layer of metal oxide (4) delimiting with the metal substrate (1), an electronic junction having a barrier height of potential of a few tenths of electron volts and having a thickness comprised between 1 and 10 nm and, preferably, of the order of 5 nm.

Description

The present invention relates to the field of electron emission in

the

  empty 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 101 Torr) for consumer and professional applications. Conventionally, 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 interconnected using

  a polarization source making it possible to place them at a given relative potential.

  In order to obtain the emission in vacuum of a constant flow of electrons from the cathode, it is necessary to extract the electrons from the potential in which they are trapped in the material of the cathode. 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 dissipation

  relatively large heat.

  It is known, moreover, a second technique for extracting electrons by deformation of the surface potential barrier of the cathode by an intense electric field. This technique called field emission makes it possible to obtain the emission of electrons at a so-called cold temperature (300 K or less). A disadvantage of this technique lies in the need to implement a vacuum

  important (10'- Torr) to stabilize the emission current of the electrons.

  Furthermore, to obtain an intense electric field, the cathode must necessarily have a point-shaped geometry whose practical realization of

  point networks pose relatively significant problems.

  Analysis of the state of the art shows that there appears to be a need for an emission cathode capable of being manufactured industrially, at low cost, while being suitable for working at temperature. ambient,

  to limit energy consumption and 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.

  Thus, the object of the invention relates to a method for producing an electron-emitting cathode. According to the invention, the method consists in: producing a substrate in the form of a metallic filament having a diameter of between 50 and 400 μm and preferably, of the order of 100 μm or of a flat metallic pellet whose emission surface is between 0.01 mm2 and 100 mm2, - to produce a soil from a metal alkoxide (M - (OR), where M denotes a metal and R an alkyl group), in order to form a layer of a metal oxide, - to immerse at least part of the substrate in the soil, - to remove the substrate from the soil at a controlled speed to obtain, ultimately, a layer of metal oxide deposited on the substrate between I and nm and, preferably, of the order of 5 nm, - to dry the substrate,

  - And to anneal the substrate.

  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 gm and, preferably, of the order of 100 lim, or of a flat metal pellet whose emission surface is between 0.01 mm2 and 100 mm2, the metal substrate being covered with a layer of

  metal oxide obtained from a soil containing a metal alkoxide (M -

  (OR) no 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 of between 1 nm and 10 nm and preferably of the order of nm.

  Various other characteristics emerge from the description given below in

  reference to the appended drawings which show, by way of nonlimiting examples, embodiments and implementation of the subject of the invention. Figs. I and 2 are large-scale schematic views illustrating exemplary embodiments of emission cathodes having, respectively, a shape

pin and point.

  Fig. 3 is a diagram illustrating a layer drawing apparatus, 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.

  1 5 As shown more precisely in FIGS. 1 and 2, the emission cathode I according to the invention comprises a metal substrate 1 in the form of a filament produced from a metal wire, for example made of platinum, having a diameter between 50 and 400 gtm and preferably around 100 gtm. In the exemplary embodiment illustrated in FIG. 1, 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. In the exemplary embodiment illustrated in FIG. 2, the metal filament 1 is terminated by a tip 3, the end of which has a radius of between 10 nm and lim and, preferably, of the order of 100 nm. It should be noted that the substrate 1 can be produced in the form of a flat metallic pellet of millimeter dimensions. Thus, such a flat metal pellet has an emission surface of between 0.01 mm2 and 100 mnm2. 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 MO2, with M denoting a metal) forms a conduction medium for the electrons injected coming from the metal substrate 1. It should be considered that this metal oxide layer 4 is comprises as 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

  that is to say between 0.05 and 1 eV and preferably of the order of 0.1 eV. The characteristics of this Schottky junction impose the choice of the couple of adequate materials metal 1 and layer 4 of type n. For example, for a metal I which is platinum, the layer 4 can be either SiC (silicon carbide) of type n, or TiO2 (oxide of

titanium) of type n.

  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 1 5 of 5 nm for semiconductor layers of SiC (silicon carbide) of n type or

  TiO2 (titanium oxide) type n on a platinum substrate 1.

  In accordance with the invention, the deposition of an ultra-thin metal oxide layer 4 of nanometric thickness is carried out using the SOL-GEL method. The

  description which follows describes the production of chemical gels and their use for deposition

  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

  that it massively occupies its entire container.

  In the case of a chemical gel, this evolution is the product of a succession of chemical reactions of hydrolysis and condensation starting from a metal alkoxide (M- (OR), where R denotes an alkyl group and M denotes the metal of which we want to form the oxide MO2) according to the process: M (OR), + H20 (HO-M- (OR) n-1 + R-OH HO-M- (OR) n- 1 + HO- M- (OR) n- 1 ((OR) n- 1 -MOM- (OR) n- 1 + H20 The production of ultra thin layers requires control of the viscosity of the

  SOL and the possibility of diluting it in an appropriate solvent without precipitation of the SOL.

  It is typically an alcohol (R-OH).

  The production of the metal oxide layer 4 consists of depositing the liquid SOL on the substrate 1 before gelation, and at a viscosity adapted to the thickness desired for this layer 4. Gelification (solidification by the successive chemical reactions which form the monomer chains) is produced during deposition on the substrate 1. According to an exemplary embodiment, provision is made for depositing a layer of TiO 2 on a platinum substrate 1 (tip or pin). According to this example, the colloidal solution or SOL is obtained by the mixture of titanium isopropoxide Ti [OCH (CH3) 2] 4 and propanol-2 (CH3) 2CHOH in a volume ratio of Ti [OCH (CH3) 2] 4 / (CH3) 2CHOH = 2.9. The solution is then stirred (magnetic stirrer) for 10 minutes. Cold CH3COOH acetic acid is added, in a CH3COOH / Ti = 6 molar ratio, so as to avoid precipitation of TiO2 particles; it acts as a complexing agent for titanium isopropoxide. The water necessary for the hydrolysis reactions comes from the esterification reactions between the acid (CH3COOH) and the alcohol ((CH3) 2CHOH). The solution is stirred for 15 minutes. he

  it should be noted that this solution is known, moreover.

  Then, a dilution is carried out: if Vs is the volume of the solution at this time, a dilution is carried out in methanol CH3OH, and 11 Vs of CH3OH are added. The solution is stirred for two hours. The dilution performed depends on

  the desired thickness for the metal oxide layer 4.

  Before the metal layer 4 is deposited on the substrate 1, the latter is cleaned, for example in several ultrasonic baths composed of solvents

  increasingly volatile, namely for example alcohol, acetone and ether.

  After the operation for preparing the substrate 1, the actual metal oxide layer 4 is deposited or drawn on the substrate 1. Such a deposition or drawing of the metal oxide layer 4 requires placing of a

  drawing device, a schematic example of which is illustrated in FIG. 3.

  Such a drawing device comprises an enclosure 6 whose atmosphere is controlled. For this purpose, 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). Preferably, 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 layer

  of metal oxide 4 is produced 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. Such a metal wire 9 is moved in an upward or downward movement by means of an electric motor 11 controlled to enable the speed of descent and the rate of ascent of the substrate to be controlled inside a

  tray 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. It should be noted that the speed of

  shrinkage is a parameter governing the thickness of the metal oxide layer 4.

  Thus, the faster the withdrawal speed, the thicker the metal oxide layer 4. For example, 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. From this determined speed, it was chosen to adjust the viscosity of the SOL in order to obtain, in the end, a layer of metal oxide 4 between 1 and 10 nm and, preferably, of the order of 5 nm. Furthermore, provision is made 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, it can be chosen a speed

immersion constant of 4 cm / min.

  Following the drawing of the metal oxide layer 4, the substrate 1 is advantageously inverted to avoid the phenomenon of capillary rise of the liquid which can lead to the formation of drops. In the exemplary embodiments of FIGS. 1 and 2, the substrate 1 is thus placed vertically with the looped end 2 or at the point 3 directed upwards. When the substrate I is produced in the form of a flat pellet, the emission surface is held horizontally and

directed upwards.

  The substrate 1 then undergoes, in this position, a drying operation to the extent that 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. For this purpose, 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. Following the drying operation, 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. In fact, the SOL-GEL method leads to porous amorphous materials containing organic residues. The purpose of 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 (MO2). Preferably, 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. Preferably, the annealing operation is carried out under a flow of oxygen which can be simply

  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 a

  direct heating of the metal oxide layer 4.

  In accordance with the method described above, 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. Such a cathode I has the same current emission performance 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 gfnO 12 to gfnO17) of the cathode relative to a measurement anode, as a function of the voltages d 'extraction V. Fig. 5 makes it possible to show the stability over time t of the emission current i of a cathode produced in the form of a tip and according to the method of

  manufacturing 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. Thus, such an emission cathode I can come to replace advantageously the current cathodes in their configuration, without geometric modification. The cathode according to the invention can thus replace the

  Current cathodes used in electron tubes or electron guns.

  The invention is not limited to the examples described and shown, since various

  modifications can be made without departing from its scope.

Claims (6)

CLAIMS:   1- A method for producing an electron emission cathode, characterized in that it consists in: - producing a substrate (1) in the form of a metallic filament having a diameter between 50 and 400 gm and , preferably, of the order of! m, or of a flat metallic pellet whose emission surface is between 0.01 mm2 and 100 mm2, to produce a sol from a metallic alkoxide (M - (OR). O M denotes a metal and R denotes an alkyl group), in order to form a layer of a metal oxide, - to immerse at least part of the substrate in the soil, - to remove the substrate from the soil according to a speed controlled to obtain, in the end, a layer of metal oxide (4) deposited on the substrate (1) of between 1 and 10 nm, and preferably of the order of 5 nm, - drying the substrate,   - And to anneal the substrate.   2- A method according to claim 1, characterized in that it consists, after removing the substrate (1) from the ground, to turn the substrate so that it undergoes the drying operation. 3- A method according to claim 1 or 2, characterized in that it consists in drying the substrate (1) in an enclosure placed at a temperature which may be between 80 and 120 C and, preferably, equal to 100 C, for a duration which may be between 10 and 30 min and, preferably, equal to 15 min.  4- A method according to claim 1, characterized in that it consists in annealing the substrate (1) under a flow of oxygen and by means of an infrared lamp making it possible to obtain an annealing temperature which can be between 200 and 750 C and preferably of the order of 350 C, for a period which can be between 10 and 30 min and, preferably, of the order of min. - Method according to claim 1, characterized in that it consists in immersing the substrate (1) in the SOL at a speed lower than the withdrawal speed of the SOIL substrate.   6- A method according to claim 1, characterized in that it consists in removing the substrate (1) inside an enclosure (6) with a controlled atmosphere. 7- Electron emission cathode, characterized in that it comprises a substrate (1) produced in the form of a metallic filament having a diameter between 50 and 400 gm and, preferably, of the order of 100 gm, or a flat metal pellet whose emission surface is between 0.01 mm2 and 100 mm2, the metal substrate (1) being covered with a layer of metal oxide (4) obtained from of a soil containing a metal alkoxide (M - (OR), where M denotes a metal and R denotes an alkyl group), the metal oxide layer (4) delimiting with the metallic substrate (1), an electronic junction having a potential barrier height of a few tenths of electron volts and having a thickness   between 1 and 10 nm and, preferably, of the order of 5 nm.   8- Emission cathode according to claim 7, characterized in that it comprises a substrate (1) produced from a metallic filament terminated in a point (3)   or shaped like a pin.   9- Cathode according to claim 7, characterized in that the electrical junction has a potential barrier height of between 0.05 and I eV   and preferably of the order of 0.1 eV.   - Application of a cathode according to one of claims 7 to 9 to the   production of electron beams for an electron tube or a barrel electrons.