Carbon nanotube field emission cathode and manufacturing method thereof

Abstract

The invention provides a carbon nanotube field emission cathode and a manufacturing method thereof. The manufacturing method comprises the following steps: preparing carbon nanotube slurry from the carbon nanotubes, the conductive particles and the bonding particles, printing the carbon nanotube slurry to the surface of the conductive substrate to form a carbon nanotube film, and carrying out surface treatment on the carbon nanotube film to enable the orientation of the carbon nanotubes to be along a preset direction. According to the method, an optimized carbon nano tube slurry formula is adopted, so that the adhesive force and the conductivity between the carbon nano tube and the substrate are improved. Furthermore, a microstructure is arranged on the surface of the conductive substrate, so that the total deposition area of the carbon nanotubes is remarkably increased, more field emission sites are provided, the emission current density is increased, and the starting electric field of the carbon nanotube cathode is reduced.

Description

Technical Field

[0001] The present application belongs to the field of field emission technology, and in particular, relates to a carbon nanotube field emission cathode and a manufacturing method thereof. Background Art

[0002] X-ray tubes have important applications in medical imaging, industrial and safety detection, and scientific research. Traditional X-ray tubes use thermionic cathodes. Thermionic cathodes need to be heated to above 2000°C to emit electrons when working. X-ray tubes using thermionic cathodes have the disadvantages of high power consumption, large size, slow response time, and divergence of X-ray beams, which greatly limit the improvement of spatial and temporal resolution of X-ray imaging. Field emission X-ray tubes using carbon nanotube nanomaterials as electron emission cathodes are expected to solve these problems. Carbon nanotube field emission cold cathode X-ray tubes have the advantages of low operating temperature, low power consumption, instantaneous response time, easy miniaturization, and programmable control. They are applied to X-ray imaging equipment to simplify the structure and obtain excellent image spatial and temporal resolution. Among the various methods for preparing carbon nanotube field emission cathodes, the screen printing method has the advantages of simple process, low cost, high controllability, and good repeatability, and has a high application prospect. At present, the main problem with the carbon nanotube cathode prepared by screen printing is the poor bonding strength and conductivity between the carbon nanotubes and the substrate. On the one hand, the carbon nanotubes are very easy to fall off from the substrate under the action of the electric field force. On the other hand, the carbon nanotubes generate a large amount of Joule heat during the field emission process, which causes the carbon nanotubes to be burned. As a result, the turn-on electric field of the carbon nanotube cathode increases and the emission current density is greatly reduced, which limits its application in X-ray imaging equipment. Summary of the invention

[0003] The technical problem solved by the present application is: how to provide a method for improving the bonding force and conductive performance between carbon nanotubes and a substrate.

[0004] The present application provides a method for manufacturing a carbon nanotube field emission cathode, the manufacturing method comprising:

[0005] Carbon nanotube slurry is prepared by using carbon nanotubes, conductive particles and bonding particles;

[0006] Printing the carbon nanotube slurry onto the surface of a conductive substrate to form a carbon nanotube film;

[0007] The carbon nanotube film is surface treated so that the carbon nanotubes are oriented in a predetermined direction.

[0008] Optionally, before printing the carbon nanotube slurry onto the conductive substrate, the manufacturing method further comprises:

[0009] The surface of the conductive substrate is roughened to form a microstructure on the surface of the conductive substrate, wherein the microstructure is a concave structure and / or a convex structure.

[0010] Optionally, the roughening process includes:

[0011] A partial area of ​​the flat surface of the conductive substrate is etched, and a microstructure is formed in the etched partial area, wherein the microstructure is a concave structure.

[0012] Optionally, the roughening process includes:

[0013] Part of the flat surface of the conductive substrate is etched, and other areas that are not etched form microstructures, which are convex structures.

[0014] Optionally, the carbon nanotube film covers the surface and microstructure of the conductive substrate.

[0015] Optionally, the method for preparing carbon nanotube slurry using carbon nanotubes, conductive particles, and bonding particles comprises:

[0016] Adding conductive particles and bonding particles into an organic solvent and uniformly mixing them to form a conductive matrix slurry;

[0017] The carbon nanotubes are added into the conductive matrix slurry and mixed evenly to form the carbon nanotube slurry.

[0018] Optionally, the method for preparing carbon nanotube slurry using carbon nanotubes, conductive particles, and bonding particles comprises:

[0019] Adding conductive particles and bonding particles into an organic solvent and uniformly mixing them to form a conductive matrix slurry;

[0020] Adding carbon nanotubes to the conductive matrix slurry and uniformly mixing them to form a carbon nanotube slurry matrix;

[0021] The organic powder is added into the carbon nanotube slurry matrix and mixed evenly to form the carbon nanotube slurry.

[0022] Optionally, the uniform mixing is performed by ball milling.

[0023] Optionally, the method of printing the carbon nanotube slurry onto the surface of a conductive substrate to form a carbon nanotube film comprises:

[0024] The carbon nanotube slurry is printed on a conductive substrate using screen printing equipment to form a preset pattern, and then dried, vacuum annealed, and cooled to form a carbon nanotube film.

[0025] The present application also discloses a carbon nanotube field emission cathode, which is prepared by the above-mentioned manufacturing method.

[0026] The present application provides a carbon nanotube field emission cathode and a method for manufacturing the same, which have the following technical effects:

[0027] The method uses an optimized carbon nanotube slurry formula to improve the adhesion and conductivity between the carbon nanotubes and the substrate. Further, by setting a microstructure on the surface of the conductive substrate, the total deposition area of ​​the carbon nanotubes is significantly increased, thereby providing more field emission sites, increasing the emission current density, and reducing the start-up electric field of the carbon nanotube cathode. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flow chart of a method for manufacturing a carbon nanotube field emission cathode according to one or more embodiments;

[0029] Figure 2 is a side view of a conductive substrate of a carbon nanotube field emission cathode according to one or more embodiments;

[0030] Figure 3 The graphs show the current density-electric field intensity emission curve test results of the carbon nanotube field emission cathode before and after the conductive substrate is roughened according to one or more embodiments. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0032] Before describing the various embodiments of the present application in detail, the technical concept of the present application is briefly described first: the carbon nanotube cathode currently prepared by screen printing has problems such as poor bonding force and conductivity between the carbon nanotube and the substrate. To this end, the present application provides a method for manufacturing a carbon nanotube field emission cathode, wherein carbon nanotubes, conductive particles, and bonding particles are mixed to obtain a carbon nanotube slurry, followed by printing and surface treatment to form a carbon nanotube field emission cathode, wherein the conductive particles and bonding particles form a conductive matrix, most of the carbon nanotubes are embedded in the conductive matrix, and the carbon nanotubes are indirectly in contact with the conductive substrate through the conductive matrix. Since the conductive matrix has strong conductive properties and adhesion, the bonding force and conductive properties between the carbon nanotubes and the conductive substrate are enhanced, thereby preventing the carbon nanotubes from falling off from the conductive substrate under the action of the electric field force, reducing the large amount of Joule heat generated by the carbon nanotubes during the field emission process, and preventing the carbon nanotubes from being burned. The specific principles of the carbon nanotube field emission cathode and its manufacturing method of the present application are described below in conjunction with more embodiments.

[0033] Specifically, Figure 1 As shown, the method for manufacturing a carbon nanotube field emission cathode in this embodiment includes the following steps:

[0034] Step S10, preparing carbon nanotube slurry using carbon nanotubes, conductive particles, and bonding particles;

[0035] Step S20, printing the carbon nanotube slurry onto the surface of the conductive substrate to form a carbon nanotube film;

[0036] Step S30: performing surface treatment on the carbon nanotube film so that the carbon nanotubes are oriented in a predetermined direction.

[0037] In one or more embodiments, before printing the carbon nanotube paste onto the conductive substrate, the manufacturing method further comprises:

[0038] Step S00: roughening the surface of the conductive substrate to form a microstructure on the surface of the conductive substrate, wherein the microstructure is a concave structure and / or a convex structure.

[0039] The carbon nanotube film prepared in step S10 covers the surface and microstructure of the conductive substrate. By setting the microstructure, the surface area of ​​the conductive substrate is significantly increased, and the total deposition area of ​​the carbon nanotubes is increased, thereby providing more field emission sites, increasing the emission current density, and reducing the turn-on electric field of the carbon nanotube cathode.

[0040] In one embodiment, the roughening process includes: etching a part of the flat surface of the conductive substrate, and forming a microstructure in the part after etching, wherein the microstructure is a concave structure, such as Figure 2 As shown. Exemplarily, the conductive substrate can be a metal substrate such as stainless steel, nickel, cobalt, titanium, copper, molybdenum, tungsten, tantalum and alloys thereof, and the conductive substrate is ultrasonically cleaned with acetone, ethanol and deionized water in sequence, and blown dry with nitrogen. The conductive substrate is etched to form a uniform concave structure on the surface. The etching process can be chemical etching, laser etching or plasma etching, and the cross-sectional shape of the concave structure can be circular, triangular or elliptical, etc., and its size can be 100-400 microns.

[0041] In another embodiment, the roughening treatment includes: etching a part of the flat surface of the conductive substrate, and forming a microstructure in other areas that are not etched, and the microstructure is a convex structure. Exemplarily, a mask with a predetermined pattern is used to block other areas, exposing some areas to be etched, and after etching, the remaining other areas are used as microstructures, and the microstructure here is a convex structure. Among them, the conductive substrate can be a metal substrate such as stainless steel, nickel, cobalt, titanium, copper, molybdenum, tungsten, tantalum and alloys thereof, and the conductive substrate is ultrasonically cleaned with acetone, ethanol and deionized water in turn, and blown dry with nitrogen. The conductive substrate is etched to form a uniform concave structure on the surface. The etching process can be chemical etching, laser etching or plasma etching, and the cross-sectional shape of the convex structure can be circular, triangular or elliptical, etc., and its size can be 100-400 microns.

[0042] In order to prove the effect of the roughening treatment, the carbon nanotube field emission cathode was tested before and after the roughening treatment to obtain the current density-electric field intensity emission curve. Figure 3 As shown in the figure, according to the test results, the current density-electric field intensity emission curve of the roughened carbon nanotube field emission cathode is steeper, that is, under the same electric field intensity, the emission current density of the roughened carbon nanotube field emission cathode is greater. In other words, compared with the carbon nanotube field emission cathode that has not been roughened, the roughened carbon nanotube field emission cathode only needs a smaller start-up electric field to obtain a higher emission current density.

[0043] In one embodiment, the method for preparing carbon nanotube slurry using carbon nanotubes, conductive particles, and bonding particles in step S10 comprises the following steps:

[0044] Step S101, adding conductive particles and bonding particles into an organic solvent and uniformly mixing them to form a conductive matrix slurry;

[0045] Step S102: adding carbon nanotubes to the conductive matrix slurry and uniformly mixing them to form carbon nanotube slurry.

[0046] Exemplarily, the carbon nanotubes may be single-walled carbon nanotubes or multi-walled carbon nanotubes with a length of 5-30 microns. For single-walled carbon nanotubes, the diameter is preferably 1-2 nanometers, and for multi-walled carbon nanotubes, the diameter is preferably 3-10 nanometers. The bonding particles are preferably low-melting point glass powder, and the conductive particles are preferably a mixture of metal nanoparticles such as nickel, cobalt, titanium, silver, copper, aluminum, platinum, and indium tin oxide nanoparticles, and the particle size of the conductive particles is preferably 20-50 nanometers. The organic solvent is preferably pine oil. Among them, the uniform mixing adopts ball milling mixing to uniformly mix the carbon nanotubes, bonding particles, conductive particles, organic powder and organic solvent, wherein the mass ratio of metal particles to indium tin oxide nanoparticles in the conductive particles is 2:1, and the mass ratio of carbon nanotubes, conductive particles, bonding particles, and organic powder is 1:3-5:0.5-1.

[0047] Exemplarily, conductive particles and bonding particles are added to an organic solvent and mixed by ball milling at a speed of 350-450 rpm for 60-120 min to form a conductive matrix slurry; after cooling, carbon nanotubes are added to the conductive matrix slurry and the ball milling is continued at a speed of 200-300 rpm for 90-150 min to obtain a carbon nanotube slurry.

[0048] In another embodiment, the method for preparing carbon nanotube slurry by using carbon nanotubes, conductive particles, and bonding particles in step S10 comprises the following steps:

[0049] Step S103: adding conductive particles and bonding particles into an organic solvent and uniformly mixing them to form a conductive matrix slurry;

[0050] Step S104: adding carbon nanotubes to the conductive matrix slurry and uniformly mixing them to form a carbon nanotube slurry matrix;

[0051] Step S105: adding organic powder to the carbon nanotube slurry matrix and uniformly mixing them to form carbon nanotube slurry.

[0052] Among them, the material selection and size of carbon nanotubes, conductive particles, bonding particles, and organic solvents can be the same as those in the previous embodiment. Exemplarily, the organic powder is preferably ethyl cellulose, which is used to adjust the viscosity and fluidity of the slurry. The carbon nanotubes, bonding particles, conductive particles, organic powder and organic solvent are uniformly mixed by ball milling, wherein the mass ratio of metal particles to indium tin oxide nanoparticles in the conductive particles is 2:1, and the mass ratio of carbon nanotubes, conductive particles, bonding particles, and organic powder is 1:3-5:0.5-1:2-3.

[0053] Exemplarily, conductive particles and bonding particles are added to an organic solvent and mixed by ball milling at a speed of 350-450 rpm for 60-120 min to form a conductive matrix slurry; after cooling, carbon nanotubes are added to the conductive matrix slurry and the ball milling is continued at a speed of 200-300 rpm for 90-150 min to obtain a carbon nanotube slurry matrix; then, organic powder is added to the carbon nanotube slurry matrix and the ball milling is continued at a speed of 200-300 W for 200-400 min to obtain the desired carbon nanotube slurry.

[0054] In one or more embodiments, the method of printing the carbon nanotube slurry onto the surface of the conductive substrate to form a carbon nanotube film in step S20 includes: using a screen printing device to print the carbon nanotube slurry on the conductive substrate to form a preset pattern, and then drying, vacuum annealing, and cooling to form a carbon nanotube film. Exemplarily, the drying temperature is 200°C and the drying time is 60-120 minutes; the annealing temperature is 400-600°C, the annealing time is 90-180 minutes, and the vacuum degree is 10 -4 –10 -6 Pa; and naturally cool to obtain a carbon nanotube film.

[0055] In one or more embodiments, the method of surface treating the carbon nanotube film in step S20 so that the carbon nanotubes are oriented in a predetermined direction is: mechanically treating the surface of the carbon nanotube film with 3M tape to remove particles that are not firmly attached to the surface, and aligning the carbon nanotubes perpendicular to the surface of the conductive substrate to obtain a carbon nanotube field emission cathode. It should be noted that only a portion of many carbon nanotubes are buried in the conductive matrix, and the other portion is exposed on the surface and is randomly oriented. By tearing the tape, the exposed portion can be oriented vertically.

[0056] The manufacturing method of the carbon nanotube field emission cathode of this embodiment introduces a uniform microstructure on the surface of the substrate by roughening the conductive substrate, so that the surface area of ​​the substrate is significantly increased, and the total deposition area of ​​the carbon nanotubes is increased, thereby providing more field emission sites and increasing the emission current density. In addition, the carbon nanotube slurry formula is optimized, low-melting point adhesive particles and multi-component conductive particles are introduced, and after high-temperature annealing treatment, the carbon nanotube roots are embedded in the conductive matrix formed by the adhesive particles and the conductive particles, further improving the adhesion and conductivity between the carbon nanotubes and the substrate, reducing the turn-on electric field of the carbon nanotube cathode, and increasing the emission current density.

[0057] The specific implementation methods of the present application are described in detail above. Although some embodiments have been shown and described, those skilled in the art should understand that these embodiments can be modified and improved without departing from the principles and spirit of the present application whose scope is defined by the claims and their equivalents. These modifications and improvements should also be within the scope of protection of the present application.

Claims

1. A method for manufacturing a carbon nanotube field emission cathode, characterized in that: The manufacturing method comprises: Carbon nanotube slurry is prepared by using carbon nanotubes, conductive particles and bonding particles; Printing the carbon nanotube slurry onto the surface of a conductive substrate to form a carbon nanotube film; The carbon nanotube film is surface treated so that the carbon nanotubes are oriented in a predetermined direction.

2. The method for manufacturing a carbon nanotube field emission cathode according to claim 1, characterized in that: Before printing the carbon nanotube slurry onto a conductive substrate, the manufacturing method further comprises: The surface of the conductive substrate is roughened to form a microstructure on the surface of the conductive substrate, wherein the microstructure is a concave structure and / or a convex structure.

3. The method for manufacturing a carbon nanotube field emission cathode according to claim 2, characterized in that: The roughening treatment comprises: A partial area of ​​the flat surface of the conductive substrate is etched, and a microstructure is formed in the etched partial area, wherein the microstructure is a concave structure.

4. The method for manufacturing a carbon nanotube field emission cathode according to claim 2, characterized in that: The roughening treatment comprises: Part of the flat surface of the conductive substrate is etched, and other areas that are not etched form microstructures, which are convex structures.

5. The method for manufacturing a carbon nanotube field emission cathode according to any one of claims 2 to 4, characterized in that: The carbon nanotube film covers the surface and microstructure of the conductive substrate.

6. The method for manufacturing a carbon nanotube field emission cathode according to claim 1, characterized in that: The method for preparing carbon nanotube slurry by using carbon nanotubes, conductive particles and bonding particles comprises: Adding conductive particles and bonding particles into an organic solvent and uniformly mixing them to form a conductive matrix slurry; The carbon nanotubes are added into the conductive matrix slurry and mixed evenly to form the carbon nanotube slurry.

7. The method for manufacturing a carbon nanotube field emission cathode according to claim 1, characterized in that: The method for preparing carbon nanotube slurry by using carbon nanotubes, conductive particles and bonding particles comprises: Adding conductive particles and bonding particles into an organic solvent and uniformly mixing them to form a conductive matrix slurry; Adding carbon nanotubes to the conductive matrix slurry and uniformly mixing them to form a carbon nanotube slurry matrix; The organic powder is added into the carbon nanotube slurry matrix and mixed evenly to form the carbon nanotube slurry.

8. The method for manufacturing a carbon nanotube field emission cathode according to claim 6 or 7, characterized in that: The uniform mixing is performed by ball milling.

9. The method for manufacturing a carbon nanotube field emission cathode according to claim 1, characterized in that: The method of printing the carbon nanotube slurry onto the surface of a conductive substrate to form a carbon nanotube film comprises: The carbon nanotube slurry is printed on a conductive substrate using screen printing equipment to form a preset pattern, and then dried, vacuum annealed, and cooled to form a carbon nanotube film.

10. A carbon nanotube field emission cathode, characterized in that: The carbon nanotube field emission cathode is prepared by the manufacturing method according to any one of claims 1 to 9.

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