Treatment method for blocking open pores of nuclear graphite by adopting silicon carbide nanostructure

Abstract

The invention discloses a treatment method for blocking open pores of nuclear graphite by adopting a silicon carbide nanostructure so as to prevent molten salt and molten metal from infiltrating graphite. Comprising the following steps: 1) mixing polycarbosilane and an organic solvent according to a mass concentration ratio of 50%-70% to form a solution, adding a catalyst to serve as a precursor solution, and completely immersing the precursor solution into graphite pores through vacuum infiltration or pressure infiltration; 2) placing the infiltrated graphite in a closed environment of 70-200 DEG C, hydrolyzing and curing the precursor solution to form a gel network, then drying in a vacuum environment of 0-300 DEG C, and forming an aerogel precursor in graphite holes; and 3) in an inert gas atmosphere or a vacuum environment, heating to 1300-1800 DEG C, and pyrolyzing the aerogel precursor to form the silicon carbide nanostructure. Compared with a traditional method, the method has the advantages of simple process, difficulty in failure, no change of the original performance of graphite and the like, and has relatively high economic value and application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of nuclear graphite materials, and more particularly to a treatment method for plugging open pores of nuclear graphite with silicon carbide nanostructures. BACKGROUND

[0002] Graphite is selected as the moderator and reflector material in molten salt reactors. Nuclear graphite is porous, and the invasion of liquid molten salt into the pores of nuclear graphite will damage the performance of graphite, thereby affecting the safety of the reactor. The current solutions mainly include the following three methods: 1. Surface coating. Surface coating can effectively block the invasion of molten salt into the pores. However, the coating is difficult to adapt to the size changes of graphite during irradiation, resulting in peeling off. Moreover, the surface coating method is difficult to process large-size and complex-shaped graphite components. 2. Resin or metal plugging. This method can also block the invasion of molten salt into the pores. However, this method greatly changes the original performance of nuclear graphite and has poor irradiation performance. 3. Currently, the development of ultra-fine particle graphite technology is difficult, and it is difficult to prepare large-size graphite components, with huge cost. The same problem also exists in molten salt energy storage applications.

[0003] In summary, there is an urgent need for a simple and effective method to process graphite to achieve the purpose of blocking molten salt. Silicon carbide nanostructures have low modulus and radiation resistance. The present application utilizes these characteristics to improve the blocking property of graphite to molten salt without changing the original performance of graphite. SUMMARY

[0004] The purpose of the present application is to provide a treatment method for plugging open pores of nuclear graphite with silicon carbide nanostructures, thereby solving the problem that the invasion of liquid molten salt into the pores of nuclear graphite will damage the performance of graphite, thereby affecting the safety of the reactor.

[0005] To solve the above technical problems, the present application adopts the following technical solutions:

[0006] The present application provides a treatment method for plugging open pores of nuclear graphite with silicon carbide nanostructures. The method blocks the infiltration of molten salt and molten metal into graphite, and is characterized in that the method comprises the following steps: 1) mixing polycarbosilane and an organic solvent at a mass concentration ratio of 50% to 70% to form a solution, adding a catalyst to serve as a precursor solution, and completely immersing the precursor solution into the pores of graphite through vacuum impregnation or pressure impregnation; 2) placing the impregnated graphite in a 70-200℃ sealed environment to make the precursor solution hydrolyze and solidify to form a gel network, aging for a certain period of time, and then drying in a vacuum environment at 0-300℃ to form an aerogel precursor in the pores of graphite; 3) heating to 1300-1800℃ at a heating rate of 1-10℃ / min in an inert gas atmosphere or a vacuum environment to make the aerogel precursor in the pores of graphite pyrolyze to form silicon carbide nanostructures.

[0007] Preferably, in step 1), the polycarbosilane is selected from at least one of dimethyldimethoxysilane, (3-aminopropyl)triethoxysilane, epoxy silane, vinyl silane, and the organic solvent is selected from at least one of ethanol, isopropanol, ethylene glycol, and methanol.

[0008] Preferably, in step 1), the catalyst is selected from at least one of nitric acid, hydrochloric acid, and sulfuric acid, and the volume fraction ratio of the catalyst to the solvent is 1: (10-20).

[0009] Preferably, in step 1), the vacuum degree of the vacuum impregnation is >1000 Pa, and the pressure of the pressure impregnation is 0.5-2 MPa, so as to ensure that the precursor solution fills ≥95% of the connected pores inside the graphite.

[0010] Preferably, in step 2), the hydrolysis and curing process is performed in an environment with a solvent vapor partial pressure ≥80%, and the curing time is 48-120 hours; the vacuum degree of the vacuum drying process is ≤10 Pa, and the drying time is 24-72 hours. -1

[0011] Preferably, in step 3), the inert gas is selected from at least one of argon and nitrogen, and the gas flow rate is 0.5-2 L / min.

[0012] Preferably, in step 3), the pyrolysis holding time is 1-5 hours, and the cooling rate is 5-20℃ / min.

[0013] According to the present application, the silicon carbide nanostructure formed in step 3) is: (1) a three-dimensional network structure formed by entanglement of silicon carbide nanowires with a diameter of 50-200 nm and a length of 1-10 μm, or (2) a porous aggregate formed by aggregation of particulate silicon carbide with a particle size of 50-100 nm.

[0014] According to the present application, the method is suitable for preparing a graphite component of a molten salt reactor core, a structural material of a liquid metal reactor, or a graphite reflector of a high-temperature gas-cooled reactor.

[0015] According to the scheme of the processing method provided by the present application, in step 1), a solution of polycarbosilane and an organic solvent is used as a solution precursor after adding an appropriate amount of catalyst and is immersed into the interstices of the graphite; in step 2), the precursor solution is converted into an organic aerogel through curing and drying technology, the precursor sol in the pores of the graphite is converted into a gel under the action of the catalyst and the temperature, and after aging for a certain time, the gel is dried, and the aging is used to make the organic gel have a higher strength; in step 3), the gel after curing at 1300-1800℃ is further gasified to form a silicon carbide nanostructure.

[0016] ​The solidification process in step 2 refers to the process of changing the precursor from a liquid phase to a solid phase, in which the graphite needs to be placed in a closed container or in a solvent vapor atmosphere close to saturation; the temperature range of the solidification process is generally 70 to 200 ℃. The drying process is a process of removing the solvent after the solidification process. In this process, the temperature range of the graphite is generally 0 to 300 ℃, and the vacuum environment can reduce the drying temperature.

[0017] The pyrolysis process in step 3 refers to the process of converting the gel precursor in the pores into silicon carbide nanostructures. The heating rate in this process is between 1 and 10 ℃ per minute, and the final temperature should be between 1300 and 1800 ℃; the pyrolysis process sample should be in a vacuum environment or an inert gas atmosphere.

[0018] For the above experimental scheme, it is worth noting that first, the concentration of polycarbosilane in the solution is too low, which will cause the precursor organic aerogel to be unable to form in the pores of the graphite; and the concentration is too high, which will cause the graphite sample to crack during the subsequent high-temperature treatment process; second, the pyrolysis temperature must be within 1300 to 1800 ℃, which is too low to form silicon carbide nanostructures, but will produce silicon dioxide nanostructures, and the radiation resistance of silicon dioxide cannot be compared with that of silicon carbide.

[0019] The key point of the present application is that it is the first time to use low-modulus silicon carbide nanostructures to fill the pores of graphite to solve the problem of molten salt infiltration in molten salt reactors and other applications without changing the original thermal, mechanical properties and radiation resistance of graphite. So far, the existing methods include resin, molten salt and metal impregnation, surface coating and carbon nanostructure filling pores. The above substances will cause the graphite to crack after impregnation, which is difficult to prepare, and the neutron radiation resistance is poor. The surface coating is difficult to adapt to the size change of the graphite during use, so it is easy to fall off and difficult to be applied in practice. According to the research of the present application, it is found that the carbon nanostructure can indeed improve the molten salt barrier property of the graphite, and the original thermal, mechanical properties of the graphite are not changed, but the radiation resistance of the carbon nanostructure is poor, and the molten salt barrier property of the graphite will be reduced with the increase of the neutron radiation dose. The treatment method provided by the present application successfully realizes the impregnation of the graphite to block the molten salt and the molten metal, while the properties of the graphite, especially the radiation behavior, are not changed. The present application first uses silicon carbide nanostructures to block the nuclear graphite, which can successfully block the molten salt. Most importantly, this method is suitable for all types of graphite on the market, and provides a new scheme for the development of high-density graphite for reactors.

[0020] The present application realizes a technical breakthrough by replacing "external coating" with "in-situ transformation". Unlike traditional surface coating methods which are prone to peeling off and difficult to handle complex shapes, the present application grows silicon carbide nanostructures from the inside of graphite pores, realizes internal plugging, and solves the problem of handling large size / complex components. When filling pores with nanostructures, the original heat conduction channels of the graphite are not damaged, avoiding the problem of "changing the properties of graphite" caused by resin / metal plugging. In addition, the flexible entanglement of silicon carbide nanowires and the discrete accumulation of nanoparticles can alleviate the volume change of graphite under irradiation, solving the poor irradiation performance of traditional methods.

[0021] In summary, the processing method for plugging open pores of core graphite with silicon carbide nanostructures provided by the present application has the following significant beneficial effects compared to the prior art:

[0022] 1) The present application not only enhances the molten salt barrier properties of graphite while maintaining its original properties, but also the silicon carbide nanostructures inside the pores have excellent radiation resistance;

[0023] 2) The chemical vapor deposition process mainly involves the sublimation of precursor organic aerogel solids into gas and then the deposition of silicon carbide nanostructures in the pores of core graphite, while ensuring the uniformity of the silicon carbide nanostructures filling the pores;

[0024] 3) This method is low in cost and simple in process;

[0025] 4) This method can also be used to treat graphite materials in molten salt energy storage systems, and has high economic value and application prospects. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 A process flow diagram of the processing method for plugging open pores of core graphite with silicon carbide nanostructures provided by the present application is shown;

[0027] Figure 2 A scanning electron microscope (SEM) photo of the silicon carbide nanowires treated in Example 1 is shown;

[0028] Figure 3 An X-ray diffraction (XRD) spectrum of the silicon carbide nanowires treated in Example 1 is shown;

[0029] Figure 4 A scanning electron microscope (SEM) image of the silicon carbide nanostructure material treated in Example 2 is shown;

[0030] Figure 5 A scanning electron microscope (SEM) image of the silicon carbide nanostructure material treated in Example 3 is shown. DETAILED DESCRIPTION

[0031] The application will be further described in conjunction with specific examples. It should be understood that the following examples are only used to illustrate but not limit the scope of the application.

[0032] Example 1

[0033] In this experimental example, dimethyl dimethoxysilane and ethanol are mixed in a mass ratio of 3:1, and an appropriate amount of nitric acid solution with a concentration of 2 mol / L is added. The volume fraction ratio of the nitric acid solution to the solvent is 1:10. The processed graphite component (true diameter 50 mm, height 100 mm) is placed in the immersion kettle, vacuum is drawn, and when the vacuum degree reaches 1000 Pa, the dispersed precursor solution is introduced, the precursor solution needs to immerse the graphite completely, then gas is pressurized to 1 MPa, and the pressure in the kettle is maintained for 1 hour before being taken out. The graphite is kept in a sealed container at 100 ℃ for 96 hours, and then dried at 150 ℃. Finally, under an argon atmosphere, the temperature is raised to 1300 ℃ at a rate of 5 ℃ per minute and maintained for 2 hours before cooling.

[0034] As shown in Figure 1 , it is a process flow chart of a treatment method for plugging open pores of nuclear graphite with silicon carbide nanostructure according to the application. As can be seen from the figure, the treatment method mainly includes three steps: 1) mixing polycarbosilane and organic solvent to form a solution, adding a catalyst as a precursor solution, and completely immersing the precursor solution into the graphite pores by vacuum impregnation or pressure impregnation; 2) placing the impregnated graphite in a sealed environment, allowing the precursor solution to hydrolyze and solidify to form a gel network, aging for a certain period of time, and then drying in a vacuum environment to form an aerogel precursor in the graphite pores; 3) heating to 1300-1800 ℃ at a heating rate of 1-10 ℃ / min in an inert gas atmosphere or a vacuum environment to pyrolyze the aerogel precursor in the graphite pores to form silicon carbide nanostructure.

[0035] As shown in Figure 2 , it is a scanning electron microscope (SEM) photo of silicon carbide nanowires treated according to the method of the present embodiment. As can be seen from the figure, there are a large number of filamentous and intertwined structures, which are silicon carbide nanowires, with a diameter of nanoscale, about 50-200 nm, and a length of several microns, about 1-10 μm. They intertwine to form a three-dimensional network structure, which can be used to plug the open pores of nuclear graphite.

[0036] As shown in Figure 3 , it is an X-ray diffraction (XRD) spectrum of silicon carbide nanowires treated according to the method of the present embodiment. As can be seen from the figure, silicon carbide forms a relatively regular crystal structure.

[0037] Example 2

[0038] In this example, dimethyldimethoxysilane, (3-aminopropyl)triethoxysilane, and ethanol were mixed in a mass ratio of 2:1:1. A nitric acid solution with a concentration of 2 mol / L was added, and the volume fraction ratio of the nitric acid solution to the solvent was 1:10. A processed graphite component (50 mm true diameter, 100 mm height) was placed in an impregnation kettle and evacuated. Once the vacuum reached 1000 Pa, the dispersed precursor solution was introduced, completely submerging the graphite. The solution was then pressurized with gas to 1 MPa, maintained within the kettle for 1 hour, and then removed. The graphite was placed in the solution in a sealed container at 90°C for 4-6 hours, until the solution reached a viscosity of approximately 12 mPa·s. The sample was then placed in a natural environment for 12 hours to solidify into a gel. The sample was then dried in a vacuum environment at 70°C for 24 hours. Finally, under an argon atmosphere, the temperature was increased at a rate of 5°C / min to 1500°C, held for 2 hours, and then cooled.

[0039] like Figure 4 As shown, a scanning electron microscope (SEM) image of the silicon carbide nanostructure obtained by the method of this embodiment can be seen from the figure. The pore network formed by the interweaving of silicon carbide nanowires can be seen; these pores are adapted to the micron-scale pores of the core graphite, thereby filling and sealing the graphite voids.

[0040] Example 3

[0041] In this example, liquid polycarbosilane and nickel acetate were mixed at a mass ratio of 20:1. A processed graphite component (50 mm true diameter, 100 mm height) was placed in an impregnation kettle and evacuated. After the vacuum reached 1000 Pa, the dispersed solution was introduced, completely submerging the graphite. The solution was then pressurized to 1 MPa with gas, maintained within the kettle for 1 hour, and then removed. The graphite was tightly wrapped with carbon fiber and placed in a tube furnace. Finally, the temperature was raised to 1800°C under an argon atmosphere at a rate of 5°C per minute, maintained for 2 hours, and then cooled.

[0042] like Figure 5 As shown in FIG, a scanning electron microscope (SEM) image of the silicon carbide nanostructure obtained by the method of this embodiment is shown. From the image, it can be seen that the silicon carbide nanowires are interwoven to form a pore network. This embodiment proves that the method of the present invention can also be used to treat graphite with larger pore sizes.

[0043] In summary, the graphite treatment method provided by the present application can be used for blocking the infiltration of molten salt or molten metal in graphite, can be used for blocking molten salt treatment of graphite in a reactor core of a molten salt reactor, blocking molten salt treatment of graphite components in a molten salt energy storage system, and the like, and in particular, when used for treating graphite in a reactor core of a molten salt reactor, has little influence on the force, thermal performance and radiation behavior of the graphite, can avoid or reduce the cost caused by re-evaluation of the graphite material, and has obvious advantages over other treatment methods.

[0044] The above merely describes preferred embodiments of the present application, and is not intended to limit the scope of the present application. The above-described embodiments of the present application can be variously changed. Any simple, equivalent changes and modifications made in accordance with the content of the present application, as well as the claims, fall within the scope of the present application. The present application is not described in detail, and is conventional technical content.

Claims

1. A method for blocking open pores in nuclear graphite using silicon carbide nanostructures, wherein the method blocks molten salt and molten metal from infiltrating graphite, characterized in that: The method comprises the following steps: 1) mixing polycarbosilane and an organic solvent at a mass concentration ratio of 50% to 70% to form a solution, adding a catalyst to form a precursor solution, and completely impregnating the precursor solution into the pores of graphite by vacuum infiltration or pressure infiltration; 2) The impregnated graphite is placed in a sealed environment at 70-200°C to allow the precursor solution to hydrolyze and solidify to form a gel network. After a certain period of aging, it is then dried in a vacuum environment at 0-300°C to form an aerogel precursor in the graphite pores. 3) In an inert gas atmosphere or vacuum environment, heat to 1300~1800℃ at a heating rate of 1~10℃ / min to pyrolyze the aerogel precursor in the graphite pores to form silicon carbide nanostructures.

2. The processing method according to claim 1, characterized in that In step 1), the polycarbosilane is selected from at least one of dimethyldimethoxysilane, (3-aminopropyl)triethoxysilane, epoxysilane, and vinylsilane, and the organic solvent is selected from at least one of ethanol, isopropanol, ethylene glycol, and methanol.

3. The processing method according to claim 1, characterized in that In step 1), the catalyst is selected from at least one of nitric acid, hydrochloric acid, and sulfuric acid, and the volume fraction ratio of the catalyst to the solvent is 1: (10-20).

4. The processing method according to claim 1, characterized in that: In step 1), the vacuum degree of the vacuum impregnation is greater than 800 Pa, and the pressure of the pressure impregnation is 0.5-2 MPa, ensuring that the precursor solution fills ≥95% of the interconnected pores inside the graphite.

5. The processing method according to claim 1, characterized in that In step 2), the hydrolysis curing process needs to be carried out in an environment where the solvent vapor partial pressure is ≥80%, and the curing time is 48 to 120 hours; the vacuum degree of the vacuum drying process is ≤10 -1 Pa, drying time is 24~72 hours.

6. The processing method according to claim 1, characterized in that In step 3), the inert gas is selected from at least one of argon and nitrogen, and the gas flow rate is 0.5-2 L / min.

7. The processing method according to claim 1, characterized in that In step 3), the pyrolysis holding time is 1 to 5 hours, and the cooling rate is 5 to 20°C / min.

8. The processing method according to claim 1, characterized in that The silicon carbide nanostructure formed in step 3) is: (1) A three-dimensional network structure formed by entanglement of silicon carbide nanowires with a diameter of 50-200 nm and a length of 1-10 μm, or (2) Porous aggregates formed by the aggregation of granular silicon carbide with a particle size of 50~100nm.

9. The processing method according to claim 1, characterized in that: The method is applicable to preparing molten salt reactor core graphite components, liquid metal reactor structural materials or high-temperature gas-cooled reactor reflective layer graphite.

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