WO2022233093A1 - Source d'électrons à émission de champ à microfoyer basée sur un nanotube de carbone, et son procédé de préparation - Google Patents

Source d'électrons à émission de champ à microfoyer basée sur un nanotube de carbone, et son procédé de préparation Download PDF

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Publication number
WO2022233093A1
WO2022233093A1 PCT/CN2021/108337 CN2021108337W WO2022233093A1 WO 2022233093 A1 WO2022233093 A1 WO 2022233093A1 CN 2021108337 W CN2021108337 W CN 2021108337W WO 2022233093 A1 WO2022233093 A1 WO 2022233093A1
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Prior art keywords
carbon nanotube
microfocus
electron source
field emission
nickel
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PCT/CN2021/108337
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English (en)
Chinese (zh)
Inventor
董长昆
祝维
钱维金
黄卫军
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温州大学
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Publication of WO2022233093A1 publication Critical patent/WO2022233093A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
    • H01J37/06—Electron sources; Electron guns
    • H01J37/073—Electron guns using field emission, photo emission, or secondary emission electron sources
    • 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
    • H01J9/025—Manufacture of electrodes or electrode systems of cold cathodes of field emission cathodes

Definitions

  • the invention relates to a field emission electron source, in particular to a microfocus field emission electron source based on carbon nanotubes and a preparation method thereof.
  • Carbon nanotubes have excellent physical, chemical, structural and other properties, and are ideal field emission cathode materials. Compared with traditional metal tip (usually tungsten or molybdenum) emitters, they have tips that are almost close to the theoretical limit. Surface area, its tip size is only a few nanometers to tens of nanometers, it has a low field emission voltage (can be less than 100 volts), it can transmit extremely large current densities, and the current is stable and has a long service life, so it is very suitable as an excellent
  • the point electron source is used in the electron emission components of scanning electron microscope (Scanning Electron Microscope), transmission electron microscope (Transmission Electron Microscope), microfocus X-ray imager and other equipment.
  • the composite structure of graphene and CNT also showed certain advantages in improving the emission performance.
  • all kinds of CNT cold cathodes have good emission stability under the vacuum degree of less than 10 -6 Pa, stable emission under high pressure (P>10 -6 Pa) and high current is always a challenge.
  • micro-focus X-ray sources have appeared.
  • the micro-focus can prevent the blurring of X-ray images and provide sharp magnified images, which are used in X-ray non-destructive testing and other fields.
  • Microfocus ray sources have smaller focal sizes down to the micron level, minimizing image geometric unsharpness at high geometric magnifications to achieve resolutions up to micron level.
  • the focal point size of microfocus X-ray sources is still far from being small enough.
  • the purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art, and to provide a micro-focus field emission electron source based on carbon nanotubes and a preparation method thereof, the micro-focus field emission electron source has good strong current emission and high voltage Strong and stable characteristics.
  • the first object of the present invention is to provide a preparation method of a micro-focus field emission electron source based on carbon nanotubes.
  • the technical solution of the present invention is a preparation method of a carbon nanotube-based micro-focus field emission electron source, the cathode of which comprises the following steps:
  • a metal platinum layer is plated on the surface of the nickel substrate
  • a pulsed laser is used to ablate the surface of the nickel substrate coated with metal platinum with negative defocusing.
  • the laser focus is inside the nickel substrate, so that the internal nickel metal is melted. , which flows to the surface of the nickel substrate under the driving action of the nickel metal vapor and forms a spherical shell after cooling;
  • the carbon nanotube cathode film was directly grown on the spherical shell formed by laser ablation of the nickel substrate by chemical vapor deposition.
  • step (3) there is also a step (4): depositing graphene on the carbon nanotube cathode film by electrophoresis, and performing vacuum high temperature annealing.
  • the carbon nanotube cathode film is a multi-wall carbon nanotube cathode film.
  • pulsed laser ablation with negative defocus parameters wavelength 1064 nm, frequency 3 Hz, output current 50-150 A, and negative defocus distance at 0.00-0.50 In the mm range, this has a greater impact on the diameter and shape of the spherical shell.
  • the second object of the present invention is to provide a carbon nanotube-based microfocus field emission electron source prepared by the method.
  • the inner nickel metal flows to the surface to form a spherical shell with tiny dots protruding from the surface.
  • the material of the spherical shell is nickel, which has a catalytic effect, because chemical vapor deposition can form a carbon nanotube cathode film on the surface of the spherical shell of the nickel material, and the platinum layer on the edge of the spherical shell does not grow carbon nanotubes because it has no catalytic effect.
  • a thin film is formed, thereby forming carbon nanotubes with a controllable size, and by utilizing the field emission effect of the carbon nanotubes, the formed electron source has the effect of micro-focus technology with small size.
  • the micro-focus electron source of the technical solution of the present application has the advantages of low turn-on electric field ( ⁇ 1 V/ ⁇ m) and high current density ( ⁇ 1 A/cm 2 ).
  • ⁇ 1 V/ ⁇ m low turn-on electric field
  • ⁇ 1 A/cm 2 high current density
  • Figure 1 Decomposed schematic diagram of the preparation steps of the present invention, in which Figure 1 (a) magnetron sputtering platinum (b) laser ablation molten state (c) laser ablation solidified state (d) covered area of CNT thin film after growth by CVD ;
  • FIG. 3 Field emission test diagram, in which Fig. 3(a) E-J curve of field emission, (b) F-N curve, (d) imaging diagram of field emission emission site;
  • Fig. 4 is the test data graph of field emission stability under different pressures
  • Figure 6 is a side view of the field emission of the preferred embodiment of the present invention, wherein Figure 6 (a) field emission E-J curve, (b) F-N curve;
  • FIG. 7 is a data diagram of field emission stability test under different pressures according to the preferred embodiment of the present invention.
  • a layer of platinum metal was first plated on the surface of the nickel substrate by magnetron sputtering (vacuum degree: 9 Pa, sputtering current: 30 mA, sputtering time: 600 s), as shown in Figure 1 (a).
  • the nickel substrate was then point ablated with a pulsed laser at negative defocus (wavelength: 1064 nm, frequency: 3 Hz, output current: 50-150 A, negative defocus distance: 0.00-0.50 mm). Pulsed laser ablation can melt the internal particles of nickel metal and eject them to the surface to form spherical shells, as shown in Figures 1b and 1c.
  • the laser focus is inside the material, which melts the internal nickel metal, flows to the surface of the material under the driving action of nickel metal vapor, and forms a spherical shell after cooling, and protects the nickel from oxidation by high-purity nitrogen.
  • the morphology of the microfocus electron source was characterized by scanning electron microscopy.
  • the field emission test adopts a two-level structure, the anode is a metal molybdenum with a small thermal expansion coefficient, and the distance between the two poles is 300 ⁇ m.
  • a Keithley 248 power supply was used to supply voltage to the two electrodes.
  • high-purity nitrogen gas with a purity of 99.999 % was injected into the vacuum system through the inflation valve to change the pressure in the system, and the voltage was adjusted under different pressures to make the initial emission current 600 mA (443 mA/cm 2 ), Then keep the voltage unchanged and record the current data of continuous emission for 6 h.
  • the multi-layer graphene oxide of Suzhou Tanfeng Technology was selected and prepared into 0.01 g/l, 0.05 g/l, 0.10 g/l, 0.15 g/l, 0.20 g/l graphene suspension.
  • Graphene was plated on the top of CNTs by DC electrophoresis (electrophoresis time was 1 min), and then annealed at 750 °C under rough vacuum. Its morphology was characterized by scanning electron microscopy. Finally, I-V test and stability test were performed on the samples coated with different concentrations of graphene to measure the stability under different pressures.
  • the covered area of the multi-walled carbon nanotubes prepared by this method is a spherical shell surface with a diameter of about 350 ⁇ m, as shown in Fig. 2(a).
  • the surface area of the covered area of the multi-walled carbon nanotubes was calculated using the measurement data of the cross-sectional view in Figure 2(b), and the average surface area was 0.135 mm 2 by averaging multiple sets of data.
  • Nickel metal can form a dense oxide film at room temperature, which hinders the contact between nickel and carbon source gas, which is not conducive to the growth of carbon nanotubes. Platinum plating can further inhibit the growth of carbon nanotubes on the surface.
  • the unoxidized nickel particles on the surface of the spherical shell can catalyze the growth of carbon nanotubes, forming an electron point source cathode with a diameter of about 350 ⁇ m.
  • the diameter of CNT is directly related to the type of catalyst and particle diameter.
  • the nickel particles at the edge of the spherical shell become smaller and aggregated after melting, and the diameter of the catalytically grown carbon nanotubes is smaller ( ⁇ 10 nm) and form bundles, called bundled carbon nanotube regions.
  • the top of the spherical shell is formed by the solidification and agglomeration of completely melted nickel, so the carbon nanotubes catalyzed and grown in this area are evenly distributed, which is called the carbon nanotube thin film area.
  • the ITO imaging of the emission site under different emission currents is shown in Figure 3c.
  • the emission is enhanced from left to right, and all are relatively uniform circular images, indicating that the emission area of the cathode is concentrated in the micro-scale spherical shell part.
  • the field emission IV test data are shown in Fig. 3a, the threshold electric field of the first test was less than 1 V/ ⁇ m, and the threshold electric field of the second and third IV tests increased to 3 V/ ⁇ m compared with the first test.
  • the field emission current density reaches 400 mA/cm 2 .
  • the possible reason for the low field strength of the first emission is that the effective work function of CNTs is reduced due to gas adsorption, which increases the field emission current.
  • Figure 4 shows the stability of the continuous emission of the electron source for 6 h under the pressures of 3.7 ⁇ 10 -7 Pa, 2.9 ⁇ 10 -6 Pa and 4.3 ⁇ 10 -5 Pa and the initial current of 600 mA (443 mA/cm 2 ).
  • the current showed an upward trend after the second hour, and the current increased by 5.8% at the sixth hour.
  • the current increased by 7.5% in the first 2 h, and then decreased slowly to the initial current of 443 mA/cm 2 in 2 h-6 h.
  • the current continuously decayed by 68.9% for 6 h.
  • Figure 6 shows the I-V characteristics of CNTs after plating with 0.10 g/l graphene and annealing at 750 °C under high temperature vacuum.
  • the emission turn-on electric field is ⁇ 1 V/ ⁇ m; the threshold electric field is 2.3 V/ ⁇ m for the first time and 3.5 V/ ⁇ m for the second and third times.
  • the first and last two curves are closer compared to when no graphene is plated. It shows that the deposition of graphene reduces the influence of gas adsorption on the emission, which is beneficial to improve the emission stability.
  • Figure 7 shows the emission stability of the samples coated with 0.10 g/l graphene and annealed at 750 °C under different pressures and 600 ⁇ A emission (443 mA/cm 2 ).
  • the graphene-coated samples had a current decay of 14% after 6 h of testing, which was much lower than the 68.9% before untreated.
  • this CNT field emission cathode exhibits better emission stability at high current density at a pressure of 10 -5 Pa.
  • the emission stability of other CNT cathodes is generally carried out at a low pressure of ⁇ 10 -6 Pa, and the test at a pressure of 10 -5 Pa shows a large current decay.
  • Graphene has high electrical conductivity, thermal stability and excellent mechanical strength.
  • the larger bulk graphene covers the top of the CNT, which weakens the field-enhancing effect and leads to a higher turn-on electric field.
  • the graphene on the top of the CNT can prevent the direct bombardment of the CNT by charged particles during high electric field emission, resulting in the degradation of the cathode performance. This may be one of the main reasons why coating graphene can enhance the stability of CNTs under high pressure.
  • the addition of graphene increases the thermal conductivity in the radial direction of the CNT, which disperses the heat conduction at the emission site in time to protect the emission site.
  • vacuum high-temperature annealing at 750 °C can promote the defect repair of CNTs and improve the crystallinity.
  • the combination of graphene coating on CNT films and vacuum high temperature annealing at 750 °C can effectively improve the stability of the microfocus electron source under high pressure.
  • the present invention uses pulsed laser to ablate the nickel substrate to melt the internal nickel particles and spray them out to form micro-scale spherical shells, and uses the CVD method to directly grow MWNTs to obtain micro-focus CNT field emission cathodes.
  • the microfocus electron source has the advantages of low turn-on electric field ( ⁇ 1 V/ ⁇ m) and high current density ( ⁇ 1 A/cm 2 ).
  • ⁇ 1 V/ ⁇ m low turn-on electric field
  • ⁇ 1 A/cm 2 high current density
  • graphene was coated on the top of CNT by DC electrophoresis and annealed at 750 °C in high temperature vacuum, the high-voltage strong emission performance of the micro-focus cathode was improved, and it had good working stability at 10 -5 Pa.
  • the anti-ion bombardment performance of graphene and Excellent thermal conductivity plays an important role.
  • the invention provides an effective means for the development of microscale (focus) field emission cathodes.

Abstract

Source d'électrons à émission de champ à microfoyer basée sur un nanotube de carbone, et son procédé de préparation. Le procédé comprend les étapes suivantes consistant à : (1) appliquer une couche de platine métallique sur la surface d'un substrat de nickel ; (2) sous la protection d'un gaz protecteur, réaliser, à l'aide d'un laser à impulsions et dans un mode de défocalisation négative, une ablation par point sur la surface du substrat de nickel qui est plaqué de platine métallique, le foyer du laser étant à l'intérieur du substrat de nickel lors d'une ablation par défocalisation négative, de telle sorte qu'un métal de nickel interne est fondu, s'écoule vers la surface du substrat de nickel sous une force de poussée d'une vapeur de métal de nickel, et forme une enveloppe sphérique après avoir été refroidi ; et (3) directement faire croître, à l'aide d'un procédé de dépôt chimique en phase vapeur, un film de cathode de nanotube de carbone sur l'enveloppe sphérique, qui est formée au moyen d'une ablation par laser, du substrat de nickel. La source d'électrons à microfoyer présente les avantages d'un faible champ électrique d'activation (< 1 V/μm), d'une densité de courant élevée (~ 1 A/cm2), d'une bonne stabilité de fonctionnement à émission haute pression, etc.
PCT/CN2021/108337 2021-05-05 2021-07-26 Source d'électrons à émission de champ à microfoyer basée sur un nanotube de carbone, et son procédé de préparation WO2022233093A1 (fr)

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