Preparation method of graphene-diamond composite structure to improve diamond field emission performance

Abstract

This application relates to the field of cold cathode field emission material preparation technology, and discloses a method for preparing a graphene-diamond composite structure to improve the field emission performance of diamond. The method involves first dispersing nanodiamonds and graphene oxide in a deep eutectic solvent; then adding boric acid to induce an in-situ sol-gel transformation under heating, forming a gel network that locks the dispersed phase; followed by microwave-assisted interface activation and gradient high-temperature carbonization, and finally cleaning and drying to obtain the product. This invention effectively suppresses phase separation and agglomeration during pyrolysis through a gel solidification mechanism, and utilizes the deep eutectic solvent and boric acid pyrolysis to achieve in-situ co-doping of nitrogen and boron, constructing a low-barrier continuous conductive channel on the diamond surface, and realizing tight electrical interconnection between components. This method is simple and controllable, and the resulting composite material possesses excellent conductivity, low turn-on electric field, and high emission stability, thus improving the electron emission efficiency of cold cathode devices.

Description

Technical Field

[0001] This invention relates to the field of cold cathode field emission material preparation technology, specifically to a method for preparing a graphene-diamond composite structure to improve the field emission performance of diamond. Background Technology

[0002] Nanodiamonds, with their unique electron affinity, excellent chemical stability, and high resistance to ion bombardment, are considered ideal cold cathode field emission materials, showing broad application prospects in vacuum microelectronic devices, flat panel displays, and X-ray sources. However, intrinsic diamond has a wide band gap, exhibiting insulating properties, which makes it difficult for electrons to transport from the back electrode to the emission sites on the diamond surface, severely limiting its field emission performance. To overcome this conductivity bottleneck, constructing composite structures of diamond and conductive carbon materials (such as graphene and carbon nanotubes) has become a current research hotspot, aiming to utilize the conductive carbon component to construct electron transport channels while retaining the excellent emission properties of diamond.

[0003] While existing composite strategies have improved the conductivity of materials to some extent, they still have technical limitations. In traditional liquid-phase mixing or physical doping processes, due to the significant differences in density, surface energy, and wettability between nanodiamond and graphene (or graphene oxide precursor), phase separation easily occurs during solvent evaporation or subsequent heat treatment, leading to disruption of the conductive network. Insulating diamond particles often exhibit severe spontaneous aggregation, making it difficult to be uniformly encapsulated by the conductive layer, resulting in uneven emission point distribution and insufficient electron supply capacity. Furthermore, the interfaces formed by traditional physical mixing rely primarily on van der Waals forces, resulting in weak interfacial bonding and high contact resistance. Electrons face extremely high interfacial barriers when transitioning from the highly conductive graphene layer to the diamond conduction band, causing low emission efficiency and Joule heat accumulation, thus affecting the long-term operational stability of the device. Simultaneously, current technologies for controlling the band structure of composite materials are relatively limited, often lacking effective heteroatomic doping mechanisms to optimize the Fermi level position. This means that composite cathode materials still require a relatively high turn-on electric field to achieve effective electron emission. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for preparing graphene-diamond composite structures that improves the field emission performance of diamond. This method solves the problems of discontinuous electron transport channels and high field emission activation electric fields caused by poor component phase separation and interfacial contact during the preparation of existing diamond composite cathode materials.

[0005] To achieve the above objectives, the present invention provides the following technical solution: Firstly, the present invention provides a method for preparing a graphene-diamond composite structure to improve the field emission performance of diamond, employing the following technical solution: A method for preparing a graphene-diamond composite structure with improved diamond field emission properties includes the following steps: Nanodiamond powder and graphene oxide powder are added to a deep eutectic solvent matrix and dispersed to form a uniform suspension. Boric acid was added to the suspension, and a cross-linking reaction was carried out under heating and stirring conditions until the viscosity of the system increased and the liquid state was transformed into a gel-like precursor. The gel-like precursor was placed in a microwave field and heated under an inert atmosphere to obtain a pre-carbonized precursor. The pre-carbonized precursor is placed in a high-temperature furnace and carbonized under an inert atmosphere, then cooled with the furnace. The carbonized product was acid-washed, water-washed, and dried to obtain a graphene-diamond composite structure.

[0006] By employing the above technical solution, and utilizing the multiple functions of deep eutectic solvents—dispersion medium, carbon source, and nitrogen source—combined with the cross-linking locking effect of boric acid and microwave-assisted activation, in-situ construction and interface optimization of the conductive layer on the surface of nanodiamonds were achieved. The specific mechanism of action is as follows: First, during the sol-gel locking stage, the hydroxyl components in the deep eutectic solvent undergo a condensation cross-linking reaction with boric acid molecules, constructing a three-dimensional network framework. This process increases the system viscosity, physically locking the nanodiamond particles and graphene oxide sheets within a homogeneous network. This effectively suppresses nanoparticle aggregation and phase separation caused by solvent flow during subsequent pyrolysis, ensuring a uniform distribution of components.

[0007] Secondly, during the microwave-assisted activation stage, the high dielectric loss characteristics of the deep eutectic solvent and graphene oxide are utilized to induce localized high-temperature hotspots at the diamond-solvent interface using a microwave field. This non-equilibrium heating mode promotes the etching effect of solvent decomposition products on the chemically inert diamond surface, increases surface active sites, strengthens the interfacial bonding force between the subsequently formed amorphous carbon layer and the diamond matrix, and reduces interfacial contact resistance.

[0008] Finally, during the gradient high-temperature carbonization stage, the locked deep eutectic solvent precursor is transformed in situ into a nitrogen / boron co-doped amorphous carbon layer. This amorphous carbon layer acts as a conductive binder, connecting the dispersed nanodiamond particles to the highly conductive graphene sheets, constructing a continuous electron transport channel. Furthermore, nitrogen atoms from urea and boron atoms from boric acid are simultaneously doped into the carbon lattice, introducing donor or acceptor energy levels in the band gap and modulating the electronic structure and Fermi level positions of the composite material. The nitrogen-boron synergistic doping effect effectively lowers the potential barrier for electrons to transition from the graphene conductive network to the diamond conduction band, thereby reducing the field emission turn-on electric field of the composite material and improving the stability of the emission current.

[0009] Preferably, the deep eutectic solvent matrix is ​​composed of a hydrogen bond acceptor and a hydrogen bond donor; the hydrogen bond acceptor is choline chloride, and the hydrogen bond donor is urea; the molar ratio of choline chloride to urea is 1:(1.8-2.2).

[0010] By employing the above technical solution and selecting a choline chloride urea system as the matrix, urea gradually decomposes and releases nitrogen-containing gaseous species (such as ammonia) during pyrolysis. These decomposition products not only provide an in-situ nitrogen doping source for carbon materials but also create a reducing and weakly etching atmosphere during microwave and high-temperature processing, which helps remove oxygen-containing impurities from the diamond surface and promotes sp. 2 The formation of hybrid carbon structures. A specific molar ratio range ensures the liquid flowability of the deep eutectic solvent at room temperature and its gelation ability after reaction with boric acid.

[0011] Preferably, the deep eutectic solvent matrix is ​​prepared in advance. The preparation steps are as follows: choline chloride and urea are mixed and stirred at 70℃-90℃ for 30-60 minutes until the mixture is completely converted into a homogeneous and transparent liquid, and then cooled to room temperature for later use.

[0012] By adopting the above technical solution, mild heating conditions promote the formation of hydrogen bond networks between choline chloride and urea molecules, establish a stable supramolecular solvent system, and avoid premature decomposition of components at high temperatures.

[0013] Preferably, the mass ratio of the nanodiamond powder to the graphene oxide powder is (5-15):1; the total mass of the nanodiamond powder and the graphene oxide powder is 0.5%-2.5% of the mass of the deep eutectic solvent matrix.

[0014] By employing the above technical solution, this ratio balances the relationship between emission site density (provided by diamond) and conductive network connectivity (provided by graphene). Graphene oxide, as a flexible two-dimensional substrate, can support and separate nanodiamond particles, preventing them from stacking and agglomerating, and maximizing the exposure of effective field emission edges.

[0015] Preferably, in the step of forming a uniform suspension, a catalyst, nickel acetylacetone, is added, wherein the amount of nickel acetylacetone added is 1.0%-3.0% of the total mass of the nanodiamond powder and the graphene oxide powder.

[0016] By adopting the above technical solution, the nickel catalyst plays a role in catalyzing graphitization during the high-temperature carbonization stage, promoting the transformation of amorphous carbon derived from deep eutectic solvent into a more ordered graphitic carbon structure, improving the overall electrical conductivity of the composite material, and facilitating the rapid transport of electrons.

[0017] Preferably, the dispersion treatment is implemented by first mechanically stirring and mixing nanodiamond powder, graphene oxide powder and deep eutectic solvent matrix for 10-20 minutes, and then ultrasonically dispersing under ice-water bath cooling conditions, with an ultrasonic power of 400W-800W and an ultrasonic time of 45-90 minutes.

[0018] By adopting the above technical solution, the cavitation effect generated by high-power ultrasound can destroy the van der Waals forces between nanoparticles and achieve monodispersion; ice-water bath cooling prevents local overheating or volatilization of the solvent caused by ultrasonic heat generation, thus maintaining the stability of the system components.

[0019] Preferably, the amount of boric acid added is 3.0%-6.0% of the mass of the deep eutectic solvent matrix; the temperature of the heating and stirring conditions is 90℃-115℃, and the reaction time is 90-180 minutes.

[0020] By employing the above technical solution, the amount of boric acid added and the reaction conditions ensured the complete sol-gel transformation. Too little boric acid would fail to form an effective gel network to lock the particles, while too much would leave excessive boron oxide impurities in the final product, affecting conductivity. Temperature and time control aimed to regulate the cross-linking reaction rate and form a precursor gel with a suitable pore structure.

[0021] Preferably, the frequency of the microwave field is 2.0 GHz to 3.0 GHz; the process parameters for heating in the microwave field are: maintaining the temperature at 180℃ to 230℃ with pulse power, and holding the temperature for 15 to 30 minutes.

[0022] By employing the above technical solution, microwaves of a specific frequency can achieve efficient coupling with the polar groups in the deep eutectic solvent and graphene oxide. The pulsed heating method avoids thermal runaway, maintaining the temperature within the optimal range for rapid solvent decomposition and interface activation, thus achieving controlled pre-carbonization of the precursor.

[0023] Preferably, the carbonization process involves: heating at a rate of 2°C / min to 5°C / min until reaching 300°C; then heating at a rate of 2°C / min to 4°C / min until reaching the target carbonization temperature, which is 750°C to 950°C, and maintaining the temperature at the target carbonization temperature for 2-4 hours.

[0024] By adopting the above technical solution, the segmented heating process effectively releases the large amount of gas generated by the decomposition of the precursor, preventing structural collapse or microcracks from forming due to excessively rapid heating. The high-temperature treatment of 750℃-950℃ ensures that the amorphous carbon matrix has sufficient graphitization and conductivity, while retaining an appropriate amount of nitrogen / boron doping defect sites.

[0025] Preferably, the acid washing, water washing and drying are carried out as follows: the carbonized product is washed with deionized water to remove water-soluble salts, then soaked in a 1 mol / L dilute hydrochloric acid solution for 2-4 hours, finally washed with deionized water until neutral, and dried at 50℃-70℃ under vacuum for 12-24 hours.

[0026] By adopting the above technical solution, the acid washing step can effectively remove unreacted boron oxides and catalyst residues, open up the pore structure inside the composite material, and expose more effective emission area.

[0027] This invention provides a method for preparing graphene-diamond composite structures that improve the field emission properties of diamond. It has the following beneficial effects: 1. This invention employs boric acid to induce in-situ sol-gel transformation of a deep eutectic solvent, utilizing the resulting three-dimensional gel network to physically lock together nanodiamonds and graphene oxide, effectively suppressing phase separation and nanoparticle aggregation caused by solvent flow during subsequent high-temperature pyrolysis; ensuring that insulating diamond particles are uniformly encapsulated by conductive components, constructing a continuous and stable electron transport pathway at the microscale, and improving the uniformity of the composite material's microstructure and overall conductivity.

[0028] 2. This invention utilizes microwave-assisted heating to activate the interface of the precursor, combined with in-situ carbonization of the amorphous carbon layer by deep eutectic solvent, to achieve chemical bonding and electrical interconnection between graphene sheets and nanodiamond particles. This process effectively improves the contact state at the heterogeneous interface, removes the surface inert layer, and reduces the interfacial barrier for electron transmission from the highly conductive graphene network to the diamond emitter, thereby improving the electron transport efficiency and maximum emission current density of the composite material.

[0029] 3. This invention introduces nitrogen and boron heteroatoms in situ into the carbon matrix through the synergistic pyrolysis of the deep eutectic solvent component (urea) and the crosslinking agent (boric acid), constructing a nitrogen / boron co-doped defect carbon structure. The introduction of heteroatoms effectively modulates the band structure and Fermi level of the composite material, forming defect levels in the band gap that are conducive to electron transitions, reducing the energy required for electron tunneling and resulting in a composite structure exhibiting a lower turn-on electric field and excellent long-term emission stability. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Preparation Examples 1-3: Preparation Example 1: This preparation example demonstrates the preparation of a standard deep eutectic solvent matrix with a molar ratio of 1:2, denoted as DES-A. 139.63 g (1.0 mol) of choline chloride and 120.12 g (2.0 mol) of urea were weighed and placed in a 500 mL round-bottom flask equipped with a PTFE magnetic stirrer. The flask was placed in a constant-temperature oil bath, the heating temperature was set to 80°C, and the magnetic stirrer was turned on at a speed of 300 rpm. The reaction was carried out under continuous heating and stirring for 45 minutes, until the white solid powder in the flask was completely dissolved, and the system transformed into a homogeneous, transparent, and colorless viscous liquid. Heating was stopped, and the resulting liquid was transferred to a dry wide-mouth bottle, purged with nitrogen for protection, sealed, and allowed to cool naturally to room temperature for later use.

[0032] Preparation Example 2: This preparation example prepares a deep eutectic solvent matrix with a molar ratio of 1:1.8, denoted as DES-B. 139.63 g (1.0 mol) of choline chloride and 108.11 g (1.8 mol) of urea were weighed and placed in a reaction vessel. Except for the amount of urea added, the other preparation process parameters, such as heating temperature, stirring rate, and reaction time, were exactly the same as in Preparation Example 1. After the reaction was completed, a homogeneous and transparent liquid was obtained, cooled, and sealed for later use. This matrix has a higher viscosity than DES-A.

[0033] Preparation Example 3: This preparation example prepares a deep eutectic solvent matrix with a molar ratio of 1:2.2, denoted as DES-C. 139.63 g (1.0 mol) of choline chloride and 132.13 g (2.2 mol) of urea were weighed and placed in a reaction vessel. Except for the amount of urea added, the other preparation process parameters, such as heating temperature, stirring rate, and reaction time, were exactly the same as in Preparation Example 1. After the reaction was completed, a homogeneous and transparent liquid was obtained, cooled, and sealed for later use. This matrix contains a higher proportion of nitrogen source precursor compared to DES-A.

[0034] Examples 1-5: Example 1: This embodiment provides a method for preparing a graphene-diamond composite structure to improve the field emission performance of diamond, including the following steps: (1) Measure 100g of the deep eutectic solvent matrix (DES-A) obtained in Preparation Example 1 and place it in a beaker. Add 0.9g of nanodiamond powder and 0.1g of graphene oxide powder (i.e., the mass ratio of nanodiamond to graphene oxide is 9:1, and the total solid content is 1.0% of the mass of the DES matrix). First, mechanically stir and mix for 15 minutes, and then disperse the mixture using an ultrasonic cell disruptor under ice-water bath cooling. The ultrasonic power is 500W, the frequency is 20kHz, and the treatment time is 60 minutes to obtain a uniformly dispersed black suspension.

[0035] (2) Add 4.0 g of ground and sieved boric acid powder (the amount of boric acid added is 4.0% of the mass of the DES matrix) to the above suspension. Place the mixture in an oil bath, heat it to 105 degrees Celsius, and mechanically stir it at a speed of 100 rpm. React at a constant temperature for 120 minutes, at which point the system changes from a liquid state to a translucent gel-like precursor that has lost its fluidity.

[0036] (3) The gel-like precursor was transferred to an alumina crucible and placed in a microwave high-temperature reactor. Under nitrogen protection, it was rapidly heated to 150 degrees Celsius at 800 watts, and then the pulse power was automatically adjusted to maintain the temperature at 200 degrees Celsius for 20 minutes. During the reaction, the nitrogen flow rate was 100 ml / min. After the treatment, a porous solid pre-carbonized precursor was obtained.

[0037] (4) Transfer the pre-carbonized precursor to a tube furnace and introduce argon gas as a protective gas at a flow rate of 200 ml / min. Increase the temperature to 300°C at a rate of 5°C / min, and then increase it to 850°C at a rate of 3°C / min. Calcinate at this temperature for 3 hours. Then allow the furnace to cool naturally to room temperature.

[0038] (5) The obtained product was dispersed in deionized water at 80 degrees Celsius and washed 3 times. Then it was soaked and stirred in a 1 mol / L dilute hydrochloric acid solution for 4 hours. Finally, it was washed with deionized water until neutral and dried in a vacuum drying oven at 60 degrees Celsius for 12 hours to obtain graphene diamond composite structure powder material.

[0039] Example 2: This embodiment provides a method for preparing a graphene-diamond composite structure to improve the field emission performance of diamond, including the following steps: (1) Measure 100g of the deep eutectic solvent matrix (DES-B) obtained in Preparation Example 2 and place it in a beaker. Add 0.5g of nanodiamond powder and 0.1g of graphene oxide powder (i.e., the mass ratio of nanodiamond to graphene oxide is 5:1, and the total solid content is 0.6% of the mass of the DES matrix). At the same time, add 0.012g of nickel acetylacetone as a catalyst (accounting for 2.0% of the total mass of carbon material). First, mechanically stir and mix for 10 minutes, and then ultrasonically disperse at 400W power for 45 minutes under ice-water bath cooling to obtain a uniform suspension.

[0040] (2) Add 3.0 g of boric acid powder to the suspension (the amount of boric acid added is 3.0% of the mass of the DES matrix). Heat the system to 95 degrees Celsius and stir at 60 rpm for 150 minutes until the viscosity of the system increases significantly to form a gel-like precursor.

[0041] (3) The precursor was placed in a microwave reactor and heated to 180 degrees Celsius under nitrogen protection, and kept at that temperature for 30 minutes. During the microwave treatment, a catalyst was used to assist in low-temperature activation to obtain a pre-carbonized precursor.

[0042] (4) Place the pre-carbonized precursor in a tube furnace and purge it with nitrogen at a flow rate of 150 ml / min. Heat the furnace from room temperature to 750°C at a rate of 2°C / min and calcine at this temperature for 4 hours, then allow it to cool naturally.

[0043] (5) The obtained product was washed with deionized water and acid-washed with dilute hydrochloric acid to remove boric acid residue and nickel catalyst. After being washed with water until neutral, it was vacuum dried to obtain graphene diamond composite structure powder material.

[0044] Example 3: This embodiment provides a method for preparing a graphene-diamond composite structure to improve the field emission performance of diamond, including the following steps: (1) Measure 100g of the deep eutectic solvent matrix (DES-C) obtained in Preparation Example 3 and place it in a beaker. Add 2.25g of nanodiamond powder and 0.15g of graphene oxide powder (i.e., the mass ratio of nanodiamond to graphene oxide is 15:1, and the total solid content is 2.4% of the mass of the DES matrix). After mechanical stirring and ultrasonic dispersion at 800W for 90 minutes, a high-concentration dispersion is obtained.

[0045] (2) Add 6.0 g of boric acid powder to the dispersion (the amount of boric acid added is 6.0% of the mass of the DES matrix). Heat the system to 115 degrees Celsius and stir at 150 rpm for 90 minutes to achieve rapid cross-linking and locking using high concentration of boric acid, forming a high viscosity gel precursor.

[0046] (3) Place the precursor in a microwave reactor, heat it to 230 degrees Celsius under nitrogen protection, and keep it at that temperature for 15 minutes to enhance the interface etching effect using a high-temperature microwave field.

[0047] (4) Place the pre-carbonized precursor in a tube furnace and purge it with argon gas at a flow rate of 300 ml / min. Heat the furnace to 950°C at a rate of 4°C / min and calcine at a constant temperature for 2 hours to promote the formation of a highly crystalline amorphous carbon layer.

[0048] (5) The obtained product was washed with water, acid washed and dried according to the post-processing process of Example 1 to obtain graphene diamond composite structure powder material.

[0049] Example 4: This embodiment provides a method for preparing a graphene-diamond composite structure to improve the field emission performance of diamond, including the following steps: (1) Measure 100g of the deep eutectic solvent matrix (DES-A) obtained in Example 1 and place it in a beaker. Add 1.4g of nanodiamond powder and 0.2g of graphene oxide powder (i.e., the mass ratio of nanodiamond to graphene oxide is 7:1, and the total solid content is 1.6% of the mass of the DES matrix). The ultrasonic dispersion conditions are the same as in Example 1.

[0050] (2) Add 5.0 g of boric acid powder to the suspension (the amount of boric acid added is 5.0% of the mass of the DES matrix). Heat the system to 110 degrees Celsius and stir for 100 minutes to form a stable gel network.

[0051] (3) The microwave processing temperature is set to 210 degrees Celsius and the holding time is 20 minutes.

[0052] (4) The high-temperature carbonization temperature was set to 900 degrees Celsius and calcined at a constant temperature for 2.5 hours. The rest of the heating procedure and atmosphere conditions were the same as in Example 1.

[0053] (5) The post-processing steps are the same as in Example 1, and finally the graphene diamond composite structure powder material is obtained.

[0054] Example 5: This embodiment provides a method for preparing a graphene-diamond composite structure to improve the field emission performance of diamond, including the following steps: (1) Measure 100g of the deep eutectic solvent matrix (DES-A) obtained in Example 1 and place it in a beaker. Add 1.2g of nanodiamond powder and 0.15g of graphene oxide powder (i.e., the mass ratio of nanodiamond to graphene oxide is 8:1, and the total solid content is 1.35% of the mass of the DES matrix). Mechanically stir and ultrasonically disperse according to the conditions of Example 1 to obtain a uniform suspension.

[0055] (2) Add 4.5 g of boric acid powder to the suspension (the amount of boric acid added is 4.5% of the mass of the DES matrix). Place the mixture in an oil bath, control the temperature at 90 degrees Celsius, and mechanically stir at a speed of 80 rpm. Extend the reaction time to 180 minutes to verify the stability of the sol-gel transition under low temperature and long-term conditions, and finally obtain a uniform gel precursor.

[0056] (3) Place the gel-like precursor in a microwave reactor and heat it to 190 degrees Celsius at 600 watts under nitrogen protection. Extend the holding time to 25 minutes to ensure that the boric acid is fully melted and decomposed under mild conditions.

[0057] (4) Place the pre-carbonized precursor in a tube furnace and purge it with argon gas at a flow rate of 200 ml / min. Heat the furnace to 800 degrees Celsius at a rate of 3 degrees Celsius / min and calcine at a constant temperature for 3 hours, then allow it to cool naturally in the furnace.

[0058] (5) The obtained product was washed with water, acid washed and dried according to the post-processing process of Example 1 to obtain graphene diamond composite structure powder material.

[0059] Comparative Examples 1-5: Comparative Example 1: Compared with Example 1, the difference is that boric acid powder is not added in step (2), and the dispersion obtained in step (1) is directly placed in a microwave reactor for subsequent processing. Due to the lack of boric acid-induced gelation process, the precursor remains liquid during heating until the solvent decomposes, and the final product does not contain boron doping elements. The rest of the preparation process is exactly the same as in Example 1.

[0060] Comparative Example 2: Compared to Example 1, the difference lies in the absence of a deep eutectic solvent as the reaction matrix and carbon source. Specifically, equal masses of nanodiamonds and graphene oxide are dispersed in anhydrous ethanol, and phenolic resin equivalent to the theoretical residual carbon content of the deep eutectic solvent in Example 1 is added as an external binder. After stirring, mixing, and rotary drying to remove the ethanol, the solid mixture is subjected to the high-temperature carbonization treatment in step (4) (steps (2) and (3) are omitted), with the remaining parameters being the same as in Example 1. This comparative example represents a traditional physical coating process for polymer binders.

[0061] Comparative Example 3: Compared to Example 1, the difference lies in that microwave heating is not used in step (3). Instead, the gel-like precursor is placed in a conventional forced-air drying oven and subjected to resistance radiation heating at 200 degrees Celsius for an equal duration. This comparative example aims to verify the special activation and etching effect of the microwave field on the diamond / solvent interface. The remaining preparation process is exactly the same as in Example 1.

[0062] Comparative Example 4: Compared to Example 1, the difference lies in the deep eutectic solvent matrix used in step (1), which is prepared by mixing choline chloride and ethylene glycol in a molar ratio of 1:2 (i.e., without urea). Since ethylene glycol does not decompose upon heating to produce ammonia species, this system lacks an in-situ nitrogen doping source and an ammonia-assisted etching environment. The remaining preparation process is the same as in Example 1. This comparative example is used to verify the synergistic effect of nitrogen / boron co-doping and urea decomposition products.

[0063] Comparative Example 5: Compared to Example 1, the difference lies in the use of a direct solid-state mixing method. Specifically, without using deep eutectic solvents and boric acid, nanodiamond powder and graphene oxide powder are directly mixed uniformly in a ball mill, pressed into shape, and then directly subjected to the high-temperature carbonization treatment in step (4). This comparative example represents a simple physical stacking technique without interface modification. Test Example 1-2: Test Example 1: Verification of Basic Physical Properties and Process Feasibility This test case aims to verify the conductive network construction and carbon structure characteristics of the composite material obtained by the above preparation method, in order to confirm the feasibility of this process route in transforming insulating nanodiamonds into conductive composites.

[0064] Experimental steps and testing methods: Powder conductivity test: Take 0.5 g of each of the composite structure powder materials prepared in Examples 1 to 5, and place them in the mold of a powder resistivity tester (model: ST2722). Press the powder into a circular disc sample with a diameter of 10 mm by holding it under a hydraulic pressure of 20 MPa for 60 seconds. Measure the volume resistivity of the sample at room temperature using the four-probe method, and calculate the corresponding conductivity (conductivity = 1 / volume resistivity). Measure each sample 5 times and take the arithmetic mean.

[0065] Raman spectroscopy analysis: The powder samples from Examples 1 to 5 were characterized using a micro-laser Raman spectroscopy system. The excitation wavelength was 532 nm, and the scanning range was 800 to 2000 wavenumbers. The intensities of the D peak (approximately 1350 wavenumbers, representing defects and disordered structures) and the G peak (approximately 1580 wavenumbers, representing sp2 hybrid graphitic carbon structures) in the spectra were recorded, and the intensity ratio (ID / IG) was calculated to assess the defect density and graphitization degree of the carbon materials.

[0066] Test results: The test data of the samples obtained in Examples 1 to 5 are recorded in Table 1.

[0067] Table 1. Electrical conductivity and Raman spectral parameters of the composite powder materials obtained in Examples 1-5

[0068] Results Analysis and Conclusions: The data in Table 1 show that, after processing by the process of this invention, the originally insulating nanodiamond and graphene oxide precursors were successfully transformed into a composite material with good conductivity.

[0069] Data analysis shows that the conductivity of all samples in the examples reached above 8 S / cm, with Example 3 (under high-temperature carbonization conditions) reaching 16.05 S / cm. This confirms that the gel network formed by the deep eutectic solvent and boric acid effectively constructs continuous conductive channels between diamond particles and graphene sheets after pyrolysis. The boric acid-induced sol-gelation process restricts the phase separation of the precursor, allowing the carbonized amorphous carbon matrix to uniformly encapsulate diamond particles and connect them to graphene.

[0070] The ID / IG ratios in the Raman spectra generally ranged from 0.98 to 1.26. A high ID / IG ratio indicates a large number of structural defects in the material. Combined with raw material composition analysis, these defects mainly originate from in-situ introduced nitrogen atoms (derived from urea decomposition) and boron atoms (derived from boric acid decomposition) at doping sites in the carbon lattice. This hybrid carbon structure with high defect density is beneficial for increasing the electronic state density near the Fermi level, consistent with the material design expectation of improving field emission performance. The fluctuations in data between examples were within a reasonable range, indicating that the preparation process has stable reproducibility under different parameters. Test Example 2: Field Emission Performance Comparison Test This test case mainly focuses on the systematic testing of the electron field emission performance of the composite materials prepared in Example 1 and various comparative examples, in order to evaluate their application potential as cold cathode emission materials.

[0071] Experimental steps and testing methods: Cathode sample preparation: Accurately weigh 50 mg of each of the powder materials obtained in Example 1 and Comparative Examples 1 to 5. Mix the powder with the ethyl cellulose terpineol organic carrier at a mass ratio of 1:1, and grind in an agate mortar for 30 minutes to prepare a uniform slurry. Use screen printing technology to coat the slurry onto a clean conductive indium tin oxide (ITO) glass substrate, with a coating area of ​​1 square centimeter. Place the printed sample in an oven at 120 degrees Celsius for 30 minutes to dry, and then anneal at 400 degrees Celsius for 30 minutes under an argon atmosphere to remove the organic carrier, thus obtaining the cathode sample to be tested.

[0072] Field launch performance test: The test was conducted in a vacuum chamber with a diode structure. The anode was a stainless steel probe, and the cathode was the sample prepared above. The distance between the anode and cathode was fixed at 150 micrometers using a mica gasket. The vacuum level was maintained at 1.0 × 10⁻⁶ during the test. -5 Below Pascal. A voltage was applied using a high-voltage DC power supply (Keithley 248), and the emission current was recorded using a picoammeter.

[0073] Before testing, the samples were subjected to high-voltage aging until the emission current stabilized. A voltage scan was then performed, recording the JE curve of current density versus electric field strength. The turn-on electric field (Eto) was defined as the macroscopic electric field strength when the emission current density reached 10 μA / cm²; the threshold electric field (Eth) was defined as the macroscopic electric field strength when the emission current density reached 1 mA / cm².

[0074] The stability test was conducted with an initial current density of 2 mA / cm², and the system was continuously operated under constant voltage for 10 hours. The fluctuation range of the current density was recorded, and the current decay rate was calculated.

[0075] Test results: The field emission characteristics test data of Example 1 and each comparative sample are listed in Table 2.

[0076] Table 2. Summary of field emission performance test data of samples from Example 1 and Comparative Examples 1-5

[0077] Results Analysis and Conclusions: Table 2 provides a clear picture of the significant impact of different preparation processes on the field emission performance of the material.

[0078] Example 1 exhibits the best overall performance, with an on-state electric field as low as 2.34 V / μm and maintaining extremely low volatility (3.4%) even at high current densities. The difference in data compared to Comparative Example 1 confirms the crucial role of the boric acid-induced gelation step. Comparative Example 1, lacking a gel-locking mechanism, exhibits greater fluidity of the deep eutectic solvent before pyrolysis, leading to microscopic phase separation between the diamond and graphene components, reducing interfacial contact sites, and consequently resulting in an increased on-state electric field and a significant decrease in emission stability.

[0079] The high turn-on electric fields (5.14 V / μm and 6.78 V / μm, respectively) of Comparative Examples 2 and 5 demonstrate that neither physical mixing nor the use of conventional polymer binders can establish an effective electron transport channel on the diamond insulating surface. Example 1, through in-situ conversion of an amorphous carbon layer using a deep eutectic solvent, achieved chemical bonding at the interface, effectively reducing the interfacial contact barrier.

[0080] The performance of Comparative Example 3 (without microwave treatment) and Comparative Example 4 (without urea / nitrogen source) falls between that of Example 1 and Comparative Example 2. This demonstrates that microwave-assisted surface etching and the energy level modulation effect introduced by nitrogen / boron co-doping substantially contribute to optimizing the electron tunneling process from graphene to the diamond conduction band. The lone pair electrons of nitrogen atoms and the electron-deficient structure of boron atoms form donor-acceptor pairs in the carbon matrix, improving the overall electronic structure of the composite material and thus enhancing the field emission efficiency.

Claims

1. A method for preparing a graphene-diamond composite structure to improve the field emission properties of diamond, characterized in that, Includes the following steps: Nanodiamond powder and graphene oxide powder are added to a deep eutectic solvent matrix and dispersed to form a uniform suspension. Boric acid was added to the suspension, and a cross-linking reaction was carried out under heating and stirring conditions until the viscosity of the system increased and the liquid state was transformed into a gel-like precursor. The gel-like precursor was placed in a microwave field and heated under an inert atmosphere to obtain a pre-carbonized precursor. The pre-carbonized precursor is placed in a high-temperature furnace and carbonized under an inert atmosphere, then cooled with the furnace. The carbonized product was acid-washed, water-washed, and dried to obtain a graphene-diamond composite structure.

2. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 1, characterized in that, The deep eutectic solvent matrix is ​​composed of hydrogen bond acceptors and hydrogen bond donors; the hydrogen bond acceptor is choline chloride, and the hydrogen bond donor is urea. The molar ratio of choline chloride to urea is 1:(1.8-2.2).

3. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 2, characterized in that, The deep eutectic solvent matrix is ​​prepared in advance, and the preparation steps are as follows: Mix choline chloride with urea and stir at 70℃-90℃ for 30-60 minutes until the mixture is completely converted into a homogeneous and transparent liquid. Cool to room temperature for later use.

4. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 1, characterized in that, The mass ratio of the nanodiamond powder to the graphene oxide powder is (5-15):1; The total mass of the nanodiamond powder and the graphene oxide powder is 0.5%-2.5% of the mass of the deep eutectic solvent matrix.

5. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 1, characterized in that, In the step of forming a uniform suspension, a catalyst, nickel acetylacetone, is also added, wherein the amount of nickel acetylacetone added is 1.0%-3.0% of the total mass of the nanodiamond powder and the graphene oxide powder.

6. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 1, characterized in that, The distributed processing is implemented as follows: First, the nanodiamond powder, graphene oxide powder and deep eutectic solvent matrix are mechanically stirred and mixed for 10-20 minutes. Then, ultrasonic dispersion is carried out under ice-water bath cooling conditions, with an ultrasonic power of 400W-800W and an ultrasonic time of 45-90 minutes.

7. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 1, characterized in that, The amount of boric acid added is 3.0%-6.0% of the mass of the deep eutectic solvent matrix; The heating and stirring conditions are characterized by a temperature of 90℃-115℃ and a reaction time of 90-180 minutes.

8. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 1, characterized in that, The frequency of the microwave field is 2.0 GHz to 3.0 GHz; The process parameters for heating in a microwave field are as follows: maintain the temperature at 180℃-230℃ using pulsed power, and hold for 15-30 minutes.

9. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 1, characterized in that, The carbonization process is as follows: The heating rate is 2℃ / min-5℃ / min, reaching 300℃; The temperature is then increased at a rate of 2℃ / min-4℃ / min until the target carbonization temperature of 750℃-950℃ is reached, and the temperature is maintained at the target carbonization temperature for 2-4 hours.

10. The method for preparing a graphene-diamond composite structure with improved diamond field emission performance according to claim 1, characterized in that, The pickling, washing, and drying processes are implemented as follows: The carbonized product was washed with deionized water to remove water-soluble salts, then soaked in a 1 mol / L dilute hydrochloric acid solution for 2-4 hours, and finally washed with deionized water until neutral. It was then dried at 50℃-70℃ under vacuum for 12-24 hours.

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