Silicon carbide coating progressively distributed on surface of fuel element and preparation method of silicon carbide coating

Abstract

The invention provides a gradually distributed silicon carbide coating on the surface of a fuel element and a preparation method thereof, the coating comprises a silicon carbide coating and a middle layer from outside to inside, and the middle layer is a bonding layer of silicon carbide and graphite. The preparation method comprises the following steps: impregnating and pyrolyzing a fuel element in an impregnant; wherein the impregnant comprises polycarbosilane or a solution thereof. According to the method provided by the invention, the dense gradually-distributed silicon carbide coating can be formed, the silicon carbide coating and the graphite matrix of the fuel element are tightly combined and are integrated, and the oxidation resistance, the wear resistance, the heat conductivity and the corrosion resistance of the fuel element can be improved. In addition, the preparation method is simple and easy to implement, the success rate and the preparation efficiency are high, and the coating thickness is controllable.

Description

Technical Field

[0001] The present invention belongs to the technical field of nuclear materials, and particularly relates to a silicon carbide coating with a progressive distribution on the surface of a fuel element and a preparation method thereof. Background Art

[0002] Nuclear power is a clean, efficient and economical energy source, and has developed into one of the important means to solve the world energy crisis. However, due to the long-term and huge harm that nuclear leakage or nuclear accidents can cause to the human body and the environment, therefore, countries around the world always adhere to the principle of "safety first" in the development of nuclear power.

[0003] As the core structure of a reactor, a fuel element is the first safety barrier for the operation of a nuclear power plant. The tristructural-isotropic (TRISO) fuel particle is a micro fuel with great application prospects. Its diameter is less than 1 mm, and it consists of a nuclear core (usually uranium oxide or thorium oxide), a loose pyrolytic carbon layer, an inner dense pyrolytic carbon layer, a SiC layer and an outer dense pyrolytic carbon layer from the inside to the outside.

[0004] TRISO particles can be used to manufacture various fuel elements, such as spherical, columnar, FCM fuel elements, etc. Among them, the spherical fuel element is to inject TRISO particles and graphite moderator materials into a mold, and after pre-pressing, it becomes a spherical core with a diameter of 50 mm, and a 5-mm-thick graphite spherical shell is wrapped outside it to become a fuel sphere with a diameter of 60 mm. This layer of graphite spherical shell is called the "fuel-free zone", which is another barrier to prevent the leakage of radioactive substances from the nuclear core. However, due to the large amount of graphite structures contained in the interior and outer layer of this fuel element, the injection of high-temperature air and water under accident conditions is likely to cause oxidation and corrosion of the graphite material. In addition, the graphite matrix materials on the surface of the fuel element collide and rub against each other and with other components in the reactor during the cycle of the high-temperature gas-cooled reactor, which will generate graphite dust, and these graphite dusts will accumulate and adsorb radioactive fission products.

[0005] So far, scientific research personnel are still committed to strengthening the surface protection of fuel elements to further improve their accident tolerance.

[0006] CN114937514A discloses a wear-resistant and oxidation-resistant coating on the surface of a spherical fuel element and its preparation method. First, the spherical fuel element is embedded in an embedding powder composed of graphite powder, high-purity silicon powder, and a cosolvent, and then heated to 1700 - 2300 °C for silicon infiltration reaction, so as to prepare a SiC coating with a gradient structure on the graphite matrix surface of the spherical fuel element, improve the wear resistance of the spherical fuel element, reduce the surface wear of the fuel element during cycling, reduce the formation of graphite dust, and prevent the oxidation of the graphite matrix of the spherical fuel element, further improving the safety of the pebble bed high-temperature gas-cooled reactor. However, the silicon infiltration process cannot obtain a SiC with uniform composition, and there is extremely easy to have local silicon enrichment or carbon enrichment. The existence of materials with non-uniform composition is extremely likely to cause cracks due to irradiation or mismatched thermal expansion coefficients under nuclear working conditions.

[0007] CN117373702A discloses a highly safe and complex-structured multi-coated dispersion fuel element, including TRISO particles, a SiC component for the fuel element, and a packaging cover plate. Its preparation method is as follows: load the TRISO particles and SiC powder into the complex SiC component; densify the matrix by chemical vapor infiltration; load the packaging cover plate and encapsulate it by vapor deposition; finally, polish the surface of the encapsulated fuel element. The SiC component is a hexagonal prism structure, with a height of 400 - 600 mm, an opposite side distance of 300 - 400 mm, a diameter of the porous channels of 10 - 16 mm, and the number of channels of 20 - 30. This process has a long manufacturing cycle, and the component size is large, which has a certain impact on the densification degree and heat transfer.

[0008] CN116705361A discloses a preparation method of an oxidation-resistant and wear-resistant spherical fuel element. Mix nuclear pure grade high-purity graphite powder with a silicon carbide precursor or an organic solution of the silicon carbide precursor, granulate, dry, and pulverize to obtain a matrix composite powder; mix the coated fuel particles TRISO into the matrix composite powder, and press it into the central sphere of the fuel element with a rubber mold; press the matrix composite powder around the central sphere to form an outer shell layer, forming a fuel-free zone to obtain a fuel element blank; heat-treat the blank at 1600 - 2500 °C to obtain a spherical fuel element. However, using this method cannot form a complete oxidation-resistant coating on the surface of the spherical fuel element.

[0009] In addition, CN105185418A discloses a fully ceramic-coated fuel particle, its preparation method, and a fuel element made therefrom to solve the problems of oxidation and corrosion of existing coated fuel particles and fuel elements when air and water are injected in accident situations. The preparation method of the coated fuel particle includes: in a fluidized bed reactor, sequentially coat a layer with a density of 0.8 - 2.4 g / cm 3, a porous SiC layer with a thickness of 20 - 150 μm and a density of 2.68 - 3.1 g / cm 3 , a SiC transition layer with a thickness of 10 - 50 μm, and a density of 3.18 - 3.2 g / cm 3 , a dense SiC layer with a thickness of 20 - 150 μm. After sintering at 1600 - 1950 °C, the coated fuel particles are dispersed in the SiC matrix to obtain an all-ceramic fuel element. However, the thermal conductivity of SiC is lower than that of graphite, and the porous SiC layer and the SiC transition layer have a certain impact on the heat transfer generated by reactions such as nuclear fission of fuel particles.

[0010] In summary, the current solutions for improving the oxidation resistance, wear resistance, and corrosion resistance of the surface layer of fuel elements still have their respective deficiencies. Therefore, it is necessary to develop new and advanced fuel element coating structures and technologies. SUMMARY OF THE INVENTION

[0011] Problems to be Solved by the Invention

[0012] Aiming at the deficiencies of the prior art, the present invention aims to provide a silicon carbide coating with a progressive distribution on the surface of a fuel element and a preparation method thereof. The silicon carbide coating is tightly combined with the graphite matrix of the fuel element and can significantly improve the oxidation resistance, wear resistance, thermal conductivity, and corrosion resistance of the fuel element.

[0013] Solutions for Solving the Problems

[0014] [1]. A coating on the surface of a fuel element, wherein the coating on the surface of the fuel element includes a silicon carbide coating and an intermediate layer from the outside to the inside, and the intermediate layer is a bonding layer of silicon carbide and graphite.

[0015] [2]. The coating on the surface of the fuel element according to [1], wherein the silicon carbide coating contains silicon carbide converted from polycarbosilane and silicon carbide filler;

[0016] Preferably, the silicon carbide filler is one or more of silicon carbide powder, silicon carbide short fiber, and silicon carbide whisker.

[0017] [3]. The coating on the surface of the fuel element according to [1] or [2], wherein the silicon carbide coating further contains elemental silicon; and / or

[0018] The silicon carbide coating further comprises one or more of a sintering aid, a boride, a carbide, an oxide, a silicide, and a rare earth salt; the sintering aid is selected from one or more of Al2O3 - Y2O3, AlN - Re2O3, and Y3Si2C2; the boride is selected from one or more of ZrB2, HfB2, B4C, and TiB2; the carbide is selected from one or more of TaC, ZrC, and HfC; the silicide is selected from one or more of MoSi2 and CrSi2; the oxide is selected from one or more of SiO2, ZrO2, B2O3, Al2O3, TiO2, and HfO2; the rare earth salt is selected from one or more of Er2SiO5, LaMgAl 11 O 19 and Yb2SiO5.

[0019] [4]. The method for preparing the coating on the surface of the fuel element according to any one of [1] to [3], which comprises the following steps: impregnating the fuel element in an impregnating agent and pyrolyzing to obtain the coating on the surface of the fuel element; wherein, the impregnating agent contains polycarbosilane or its solution.

[0020] [5]. The method for preparing the coating on the surface of the fuel element according to [4], wherein the preparation method further comprises the following steps: preparing a ceramic coating on the surface of the fuel element, then impregnating the fuel element in an impregnating agent and pyrolyzing to obtain the coating on the surface of the fuel element.

[0021] [6]. The method for preparing the coating on the surface of the fuel element according to [4] or [5], wherein the impregnation time is 0.5 to 2 h; and / or

[0022] the pyrolysis temperature is above 900 °C, preferably 900 - 1800 °C, more preferably 1200 - 1600 °C.

[0023] [7]. The method for preparing the coating on the surface of the fuel element according to any one of [4] to [6], wherein the number of impregnation and pyrolysis times is 1 to 15 times, preferably 4 to 7 times.

[0024] [8]. The method for preparing the coating on the surface of the fuel element according to any one of [4] to [7], wherein the solvent in the polycarbosilane solution is one or more of tetrahydrofuran, methyltetrahydrofuran, n - heptane, n - hexane, and toluene.

[0025] [9]. The method for preparing the coating on the surface of the fuel element according to any one of [4] to [8], wherein the impregnating agent further contains silicon carbide filler;

[0026] Preferably, the silicon carbide filler is one or more of silicon carbide powder, silicon carbide short fiber, and silicon carbide whisker.

[0027]

[10] . The preparation method of the coating on the surface of the fuel element according to any one of [4] to [9], wherein the impregnating agent further contains elemental silicon; and / or

[0028] The impregnating agent further contains one or more of a sintering aid, a boride, a carbide, an oxide, a silicide, and a rare earth salt; the sintering aid is selected from one or more of Al2O3 - Y2O3, AlN - Re2O3, and Y3Si2C2; the boride is selected from one or more of ZrB2, HfB2, B4C, and TiB2; the carbide is selected from one or more of SiC, TaC, ZrC, and HfC; the silicide is selected from one or more of MoSi2 and CrSi2; the oxide is selected from one or more of SiO2, ZrO2, B2O3, Al2O3, TiO2, and HfO2; the rare earth salt is selected from one or more of Er2SiO5, LaMgAl 11 O 19 and Yb2SiO5.

[0029] Effects of the Invention

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The SiC coating formed by the present invention is in a progressive distribution, wherein the outer layer is a dense SiC coating, and the inner coating is a coating composed of SiC formed by pyrolysis of polycarbosilane infiltrated into the graphite and graphite composite. Therefore, the formed SiC coating is tightly combined with the graphite matrix of the fuel element and is integrated with each other.

[0032] (2) The SiC coating structure formed by the present invention is dense, which can play a good protective role for the graphite matrix of the fuel element, and improves the oxidation resistance, wear resistance, thermal conductivity, and corrosion resistance of the fuel element.

[0033] (3) The preparation method provided by the present invention is simple and easy to implement, with high success rate and preparation efficiency, controllable coating thickness, and the densification efficiency and the oxidation resistance and corrosion resistance of the coating can be improved by adjusting the composition of the impregnating agent. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a cross-sectional microscopic morphology diagram of the progressive distribution SiC coating prepared in Examples 1, 2, 3, and 4 of the present invention.

[0035] Figure 2 It is the scanning electron microscope EDS elemental scanning result of the outermost layer of the progressive distribution SiC coating prepared in Examples 1, 2, and 3 of the present invention.

[0036] Figure 3XRD pattern of the outermost layer of the gradually distributed SiC coating prepared in Examples 1, 2, and 3 of the present invention.

[0037] Figure 4 X-CT three-dimensional non-destructive detection map of the graphite sphere coated with the gradually distributed SiC coating prepared in Example 5 of the present invention.

[0038] Figure 5 Cross-sectional microscopic morphology map of the gradually distributed SiC coating prepared in Example 5 of the present invention.

[0039] Figure 6 Corrosion rate diagrams of the graphite spheres obtained in Comparative Example 1 and Examples 1, 2, 3, and 4 of the present invention in a 1000 °C, 20% water vapor and He mixed gas for 5 h. Detailed Description of the Invention

[0040] The following will detail various exemplary embodiments, features, and aspects of the present invention. The special term "exemplary" used here means "serving as an example, embodiment, or illustration". Any embodiment described as "exemplary" here does not necessarily have to be construed as superior or better than other embodiments.

[0041] In addition, to better illustrate the present invention, numerous specific details are given in the following detailed description. Those skilled in the art should understand that the present invention can still be implemented without some specific details. In other instances, methods, means, equipment, and steps well-known to those skilled in the art are not described in detail to highlight the gist of the present invention.

[0042] Unless otherwise stated, the units used in this specification are all international standard units, and the numerical values and numerical ranges appearing in the present invention should be understood to include the inevitable systematic errors in industrial production.

[0043] In this specification, the meaning expressed by using "can" includes both the meaning of performing a certain process and the meaning of not performing a certain process.

[0044] In this specification, the so-called "some specific / preferred embodiments", "other specific / preferred embodiments", "embodiments", etc. refer to the specific elements (e.g., features, structures, properties, and / or characteristics) related to the embodiment described, which are included in at least one of the embodiments described here, and may or may not exist in other embodiments. In addition, it should be understood that the elements can be combined in various embodiments in any suitable manner.

[0045] In this specification, the numerical range expressed by using "numerical value A to numerical value B" refers to the range including the endpoint numerical values A and B.

[0046] In this specification, when "normal temperature" or "room temperature" is used, the temperature can be 15 - 30°C, further 15 - 25°C, for example, 20°C.

[0047] Aiming at the deficiencies of the prior art, the present invention provides a gradually distributed SiC coating for fuel elements and a preparation method thereof. The gradually distributed SiC coating is prepared by converting polycarbosilane.

[0048] SiC ceramics have excellent properties such as high strength, high thermal conductivity, corrosion resistance, radiation resistance, and high temperature resistance. They are ideal high - safety structural materials in extreme environments such as high temperature, strong neutron irradiation, high - speed friction, and strong corrosion. Moreover, SiC has a good thermal expansion coefficient match with carbon - based materials.

[0049] Polycarbosilane is a precursor of SiC ceramics, which can be converted into SiC ceramics by high - temperature pyrolysis. Since polycarbosilane or its solution has a low viscosity and is easy to diffuse and infiltrate into the graphite matrix of fuel elements and fill the internal pores of graphite; moreover, the conversion of polycarbosilane into SiC ceramics can also act as a binder to improve the strength and wear - resistance of graphite. In addition, it can form a dense SiC coating on the surface of fuel elements, thereby improving the overall antioxidant, wear - resistant, and corrosion - resistant properties of fuel elements.

[0050] Coating on the Surface of the Fuel Element

[0051] The present invention provides a coating on the surface of a fuel element. The coating is a gradually distributed SiC coating. The coating on the surface of the fuel element includes an SiC coating and an intermediate layer from the outside to the inside. The intermediate layer is a bonding layer of SiC and graphite.

[0052] In some embodiments, there is no gap between the SiC coating and the intermediate layer.

[0053] In some specific embodiments, in the intermediate layer (the bonding layer of SiC and graphite), from the inside to the outside, the SiC content gradually increases.

[0054] In the present invention, the thickness of the coating has a certain impact on its performance. For example, if the thickness of the coating is too thin, the integrity of the coating cannot be guaranteed, and the antioxidant performance of the coating cannot be ensured; if the coating is applied to spherical fuel elements, if it is too thick, it will affect the heat transfer performance of graphite.

[0055] In some embodiments, the thickness of the SiC coating ranges from 0.01 to 10 mm (such as 0.02 mm, 0.05 mm, 0.1 mm, 0.2 mm, 0.5 mm, 1 mm, 2 mm, 5 mm, 8 mm, etc.). For facilitating the outward diffusion and transfer of heat from the fuel element, preferably, the thickness of the SiC coating ranges from 0.01 to 1 mm (such as 0.02 mm, 0.04 mm, 0.05 mm, 0.1 mm, 0.15 mm, 0.2 mm, 0.3 mm, 0.5 mm, 0.6 mm, 0.8 mm, 0.9 mm, etc.).

[0056] In some embodiments, the thickness of the intermediate layer is 50 - 1000 μm (such as 55 μm, 60 μm, 65 μm, 70 μm, 80 μm, 90 μm, 120 μm, 150 μm, 200 μm, 500 μm, 600 μm, 800 μm, etc.).

[0057] In some embodiments, the fuel element used in the present invention is composed of a fuel zone formed by TRISO particles and a matrix graphite and a non - fuel zone formed by the matrix graphite.

[0058] In some embodiments, the fuel element is a spherical fuel element, a columnar fuel element, an FCM fuel element, etc.

[0059] In some embodiments, the intermediate layer is formed by SiC embedded in the graphite matrix of the fuel element and tightly combined with the graphite matrix, and the SiC embedded in the graphite matrix of the fuel element and tightly combined with the graphite matrix is obtained by the conversion of polycarbosilane.

[0060] In some embodiments, the SiC coating is prepared by the conversion of polycarbosilane.

[0061] In the present invention, "polycarbosilane" is a linear or branched high - molecular compound whose main chain is composed of silicon and carbon atoms, and hydrogen and / or other organic groups are connected to the silicon and carbon atoms.

[0062] Due to the pyrolysis shrinkage of polycarbosilane, preferably, the SiC coating includes SiC obtained by the conversion of polycarbosilane and SiC filler.

[0063] In some embodiments, the SiC filler is selected from one or more of SiC powder, SiC short fiber, and SiC whisker.

[0064] Due to the blockage of the graphite matrix, the SiC filler is mainly distributed outside the graphite matrix of the fuel element, which has a significant promoting effect on improving the density and densification efficiency of the SiC coating.

[0065] In some embodiments, the SiC coating may further contain elemental silicon.

[0066] In some specific embodiments, the content of elemental silicon in the SiC coating is 5 wt% to 30 wt%, such as 8 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, etc.

[0067] In some embodiments, the SiC coating may further contain a sintering aid, and the sintering aid includes but is not limited to known systems such as Al2O3 - Y2O3, AlN - Re2O3 (where Re2O3 is usually an oxide of rare earth elements such as Y2O3, Er2O3, Yb2O3, Sc2O3, Lu2O3, etc.), Y3Si2C2, etc. The sintering aid can improve the densification efficiency and reduce the occurrence probability of cracks and pores.

[0068] In some specific embodiments, the content of the sintering aid in the SiC coating is 1 wt% to 30 wt%, such as 2 wt%, 5 wt%, 8 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, etc.

[0069] In some embodiments, the SiC coating may further contain compound fillers that have excellent antioxidant properties after oxidation or intrinsically, including but not limited to (1) borides such as ZrB2, HfB2, B4C, TiB2, etc.; (2) carbides such as SiC, TaC, ZrC, HfC, etc.; (3) silicides such as MoSi2, CrSi2, etc.; (4) oxides such as SiO2, ZrO2, B2O3, Al2O3, TiO2, HfO2, etc.; (5) rare earth salts such as Er2SiO5, LaMgAl 11 O 19 , Yb2SiO5, etc.

[0070] In some specific embodiments, the content of the above - mentioned compound fillers that have excellent antioxidant properties after oxidation or intrinsically in the SiC coating is 5 wt% to 20 wt%, such as 8 wt%, 10 wt%, 12 wt%, 15 wt%, 18 wt%, etc.

[0071] Preparation Method of the Coating on the Surface of the Fuel Element

[0072] The present invention also provides a method for preparing the coating on the surface of the above - mentioned fuel element, which comprises the following steps: impregnating the fuel element in an impregnating agent and then pyrolyzing to obtain the coating on the surface of the fuel element; wherein, the impregnating agent contains polycarbosilane or its solution.

[0073] The polycarbosilane or its solution can penetrate into the graphite matrix of the fuel element and form a SiC coating on the surface of the intermediate layer after pyrolysis.

[0074] In some embodiments, the following preparation method can be directly adopted: impregnating a fuel element in an impregnating agent containing polycarbosilane or its solution, and then pyrolyzing.

[0075] In other embodiments, the preparation method of the present invention includes: first preparing a ceramic coating on the surface of the fuel element, and then impregnating the fuel element in an impregnating agent and pyrolyzing; the impregnating agent contains polycarbosilane or its solution. The polycarbosilane or its solution can penetrate into the graphite matrix of the fuel element through the gaps of the ceramic coating. After pyrolysis, not only a bonding layer (i.e., an intermediate layer) of silicon carbide and graphite can be formed in the graphite matrix, but also a SiC coating can be formed on the surface of the intermediate layer.

[0076] The solvent in the polycarbosilane solution can be one or more of tetrahydrofuran, methyltetrahydrofuran, n-heptane, n-hexane, and toluene.

[0077] In some embodiments, a ceramic coating is prepared on the surface of the fuel element by a gas-phase method, an embedding method, or a molten salt method (for example, the loose SiC layer obtained on the surface of the graphite sphere by the embedding method in step 1 of Example 1).

[0078] The ceramic coatings formed by these gas-phase methods, embedding methods, or molten salt methods are not tightly bonded to the graphite matrix on the outer layer of the fuel element. Especially in spherical fuel elements, the formed coatings inherently have cracks or pores, or cracks or pores are generated due to collisions or thermal expansion and contraction during use. If used alone for the surface protection of fuel elements, the effect is not good. However, through polycarbosilane treatment, not only can the cracks be repaired or the pores be filled, but also the bonding force between the intermediate layer and the graphite matrix on the outer layer of the fuel element can be improved.

[0079] In some embodiments, the method for preparing the ceramic coating is a gas-phase method. Further, the gas-phase method includes methods such as chemical vapor infiltration, chemical vapor deposition, magnetron sputtering, and ion beam sputtering. Therefore, the ceramic coating can be a ceramic coating prepared by methods such as chemical vapor infiltration, chemical vapor deposition, magnetron sputtering, and ion beam sputtering (for example, a SiC coating).

[0080] In some embodiments, the method for preparing the ceramic coating is an embedding method. Therefore, the ceramic coating can be a ceramic coating formed by the embedding method, such as a SiC coating, a ZrC coating, a ZrB2 coating, a MoSi2 coating, a mullite coating, etc. formed by the embedding method.

[0081] In some embodiments, the steps of the embedding method are as follows:

[0082] (1) Mixing powders (such as Si powder, SiC powder, Al2O3 powder, and graphite powder) to obtain an embedded powder.

[0083] (2) Embed the fuel element in the embedding powder obtained in step (1), compact it, and then sinter it at high temperature.

[0084] In some specific embodiments, the compaction time is 1 to 6 h, such as 2 h, 3 h, 5 h, etc.

[0085] In some specific embodiments, the high-temperature sintering temperature is 1200 to 2500 °C, such as 1500 °C, 1700 °C, 2000 °C, 2200 °C, etc.

[0086] In some specific embodiments, the high-temperature sintering time is 1 to 6 h, such as 2 h, 3 h, 5 h, etc.

[0087] In some specific embodiments, the high-temperature sintering is carried out in an inert gas atmosphere.

[0088] In some specific embodiments, the mass ratio of Si powder, SiC powder, Al2O3 powder and graphite powder in the powder is (40 - 80):(10 - 30):(5 - 20):(5 - 20), such as 50:25:10:15, 60:20:10:10, 70:15:5:10, etc.

[0089] In some embodiments, the method for preparing the ceramic coating is the molten salt method. Therefore, the ceramic coating can be a ceramic coating formed by the molten salt method, such as a SiC coating, a TiC coating, a TiN coating, a ZrB2 coating, a TiB2 coating, a HfC coating, etc. formed by the molten salt method.

[0090] The impregnation described in the present invention is to immerse the fuel element in the impregnating agent. To improve the impregnation efficiency, vacuum or pressure can be assisted. In some embodiments, the pressure during impregnation can be from -0.001 MPa to -0.1 MPa, such as -0.005 MPa, -0.008 MPa, -0.01 MPa, -0.02 MPa, -0.05 MPa, etc.

[0091] In some embodiments, the polycarbosilane is a liquid polycarbosilane (the liquid in the present invention means that the polycarbosilane is liquid at room temperature). In some specific embodiments, the polycarbosilane is a liquid polycarbosilane containing unsaturated groups, and the unsaturated group is an alkenyl group. For example, the polycarbosilane can be [SiH 1.9 (CH2CH=CH2) 0.1 CH2] n .

[0092] In some other embodiments, the polycarbosilane can also be a solid polycarbosilane. If it is in solid form, it can be dissolved in a solvent to form a solution for use. The solvents in the polycarbosilane solution include but are not limited to tetrahydrofuran, methyltetrahydrofuran, n-heptane, n-hexane, toluene, etc.

[0093] In some embodiments, the number-average molecular weight of the polycarbosilane is 500 - 800 g / mol, such as 550 g / mol, 600 g / mol, 650 g / mol, 665 g / mol, 700 g / mol, 750 g / mol, etc.

[0094] In some embodiments, the weight-average molecular weight of the polycarbosilane is 2000 - 2500 g / mol, such as 2100 g / mol, 2200 g / mol, 2300 g / mol, 2376 g / mol, 2400 g / mol, etc.

[0095] When it is necessary to appropriately reduce the viscosity, an organic solvent can be added to the polycarbosilane to form a polycarbosilane solution. The organic solvent can be an organic solvent that is miscible with the polycarbosilane.

[0096] In some embodiments, the solvents in the polycarbosilane solution include but are not limited to tetrahydrofuran, methyltetrahydrofuran, n-heptane, n-hexane, toluene, etc.

[0097] In some embodiments, the impregnation time is 0.5 - 2 h, such as 0.5 h, 1 h, 1.5 h, etc.

[0098] In some embodiments, the impregnation temperature is room temperature.

[0099] In some embodiments, after the impregnation, the fuel element is dried and then pyrolyzed. Specifically, the drying can be carried out at 50 - 150 °C (such as 60 °C, 70 °C, 80 °C, 100 °C, 120 °C, etc.) for 0.5 - 10 h (such as 1 h, 3 h, 5 h, 8 h, etc.).

[0100] In the present invention, the pyrolysis refers to heating and treating the fuel element impregnated with the impregnating agent in an inert atmosphere.

[0101] In some embodiments, the pyrolysis temperature ≥ 900 °C; to improve the crystallinity of the SiC coating and thus improve the thermal conductivity and corrosion resistance, preferably, the pyrolysis temperature ≥ 1100 °C, such as 1150 °C, 1200 °C, 1400 °C, 1500 °C, 1600 °C, 1700 °C, etc.

[0102] In some embodiments, the pyrolysis temperature ≤ 1800 °C, and preferably, the pyrolysis temperature ≤ 1600 °C.

[0103] In some embodiments, the pyrolysis temperature is 900 to 1800 °C. Preferably, the pyrolysis temperature is 1200 to 1600 °C.

[0104] In some embodiments, the pyrolysis time is 0.5 to 4 h, such as 0.8 h, 1 h, 1.5 h, 2 h, etc.

[0105] In some embodiments, the number of impregnation and pyrolysis can be 1 to 15 times, preferably 4 to 7 times, such as 5 times, 6 times, 7 times, 8 times, 9 times, etc. In the present invention, the more the number of impregnation and pyrolysis, the denser the coating. In the examples of the present invention, after 7 times of impregnation, the impregnating solution (impregnating agent) cannot enter the surface of the fuel element. In addition, by controlling the number of impregnation and pyrolysis, the thickness of the coating of the present invention can also be controlled. The more the number of impregnation and pyrolysis, the greater the thickness of the coating.

[0106] In some preferred embodiments, the impregnating agent contains SiC filler, that is, the impregnating agent contains polycarbosilane or its solution and SiC filler.

[0107] In some embodiments, the SiC filler is selected from one or more of SiC powder, SiC short fiber, and SiC whisker.

[0108] In some specific embodiments, the SiC filler is SiC powder, and its average particle size is less than 100 μm. Further preferably, its average particle size is not more than 20 μm. Still further preferably, it is composed of powders with an average particle size of 1 to 20 μm (such as 15 μm, 10 μm, 5 μm, 1 μm, etc.) and an average particle size of 20 to 1000 nm (such as 30 nm, 50 nm, 100 nm, 200 nm, 500 nm, 800 nm, etc.). In some preferred embodiments, the average particle size of the SiC powder is 20 to 1000 nm (such as 30 nm, 50 nm, 100 nm, 200 nm, 500 nm, 800 nm, etc.).

[0109] In some specific embodiments, the SiC filler is SiC short fiber, and its average length is not more than 300 μm (preferably 10 to 200 μm, such as 15 μm, 20 μm, 50 μm, 80 μm, 100 μm, 150 μm, etc.), and the average diameter is less than 20 μm (preferably 0.5 to 10 μm, such as 1 μm, 1.5 μm, 2 μm, 5 μm, 8 μm, etc.).

[0110] In some specific embodiments, the SiC filler is SiC whiskers with a diameter less than 1 μm (preferably 10 - 800 nm, such as 30 nm, 50 nm, 100 nm, 200 nm, 500 nm, 600 nm, etc.), and the aspect ratio is not less than 10 (preferably 10 - 500, such as 30, 50, 100, 200, 400, etc.).

[0111] In some specific embodiments, the mass ratio of the polycarbosilane to the SiC filler is 10:90 - 70:30, such as 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, etc.

[0112] In some embodiments, the impregnating agent further contains silicon, that is, the impregnating agent contains polycarbosilane or its solution and silicon.

[0113] In some embodiments, the impregnating agent not only contains SiC filler but also contains silicon, that is, the impregnating agent contains polycarbosilane or its solution, SiC filler and silicon.

[0114] In the present invention, silicon can be introduced into the surface coating of the fuel element, which can react with the residual carbon in the SiC ceramic to form SiC, thereby reducing or removing the residual carbon in the SiC ceramic.

[0115] In some preferred embodiments, the molar content of the silicon is less than or equal to the molar content of the residual carbon in the SiC ceramic obtained after pyrolysis of the polycarbosilane.

[0116] In some specific embodiments, the mass ratio of the polycarbosilane to the silicon is 95:5 - 80:20, such as 92:8, 90:10, 85:15, 82:18, etc.

[0117] In some preferred embodiments, the average particle size of the silicon is in the micrometer range (preferably 1 - 5 μm, such as 2 μm, 3 μm, 4 μm, etc.). As a further preferred experimental scheme, the average particle size of the silicon is in the nanometer range (preferably 20 - 500 nm, such as 30 nm, 50 nm, 80 nm, 100 nm, 150 nm, 200 nm, 300 nm, 400 nm, etc.), which is beneficial to improving the reaction degree with the residual carbon.

[0118] In some embodiments, the impregnating agent may further contain sintering aids, including but not limited to known systems such as Al2O3 - Y2O3, AlN - Re2O3 (where Re2O3 is usually an oxide of rare earth elements such as Y2O3, Er2O3, Yb2O3, Sc2O3, Lu2O3, etc.), Y3Si2C2, etc.

[0119] In some specific embodiments, the mass ratio of the polycarbosilane to the sintering aid is 95:5 to 80:20, such as 92:8, 90:10, 85:15, 82:18, etc.

[0120] In some embodiments, the impregnating agent may further contain the following compounds that have excellent oxidation resistance after oxidation or intrinsically: (1) Borides: ZrB2, HfB2, B4C, TiB2, etc.; (2) Carbides: SiC, TaC, ZrC, HfC, etc.; (3) Silicides: MoSi2, CrSi2, etc.; (4) Oxides: SiO2, ZrO2, B2O3, Al2O3, TiO2, HfO2, etc.; (5) Rare earth salts: Er2SiO5, LaMgAl 11 O 19 、Yb2SiO5, etc.

[0121] In some specific embodiments, the mass ratio of the polycarbosilane to the above-mentioned compounds that have excellent oxidation resistance after oxidation or intrinsically is 90:10 to 10:90, such as 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, etc.

[0122] Corrosion Rate

[0123] In the present invention, the calculation formula for the surface corrosion rate of the fuel element is as follows:

[0124] Corrosion rate = (W0 - W1) / (A × t)

[0125] Wherein, W0 is the original weight of the fuel element, W1 is the weight of the fuel element after corrosion, A is the surface area of the fuel element, and t is the corrosion time.

[0126] Examples

[0127] The embodiments of the present invention will be described in detail below in conjunction with examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. For those not specified in the examples, they are carried out under conventional conditions or conditions recommended by the manufacturer. For the reagents or instruments not specified for the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0128] The fuel element used in the examples, namely the graphite ball sample, is composed of a fuel zone composed of TRISO particles and matrix graphite and a non-fuel zone composed of matrix graphite.

[0129] The polycarbosilane used in the examples has an approximate compositional formula of [SiH 1.9 (CH2CH=CH2) 0.1 CH2] n, where n is the degree of polymerization. According to elemental analysis, the composition of the polycarbosilane is m(Si):m(C):m(O):m(H) = 60.18:30.88:0.1:8.84 (mass ratio), and the chemical formula is SiC 1.2 H 4.1 O 0.003 , its number-average molecular weight and weight-average molecular weight are 665 g / mol and 2376 g / mol respectively, and it is in a liquid state at room temperature.

[0130] The average particle size of the SiC powder used in the examples is 40 nm.

[0131] The average particle size of the Si powder used in the examples is 40 nm.

[0132] Example 1

[0133] Step 1: Embedding and Sintering

[0134] Mix 60 g of Si powder, 20 g of SiC powder, 10 g of Al2O3 powder and 10 g of graphite powder evenly to obtain the embedding powder.

[0135] The embedding sintering process is carried out in a graphite sintering furnace. The treated (washed once with deionized water and once with alcohol respectively) graphite ball samples are buried in a graphite crucible filled with the embedding powder, so that the graphite ball samples are completely buried and the graphite ball samples cannot be in contact with the crucible, and are compacted on a shaker for 2 h. Subsequently, the crucible is placed in a high-temperature furnace, the furnace is sealed and evacuated, then argon is filled, and then the temperature is raised to 1700 °C at a rate of 20 °C / min, held for 3 h, and then cooled to room temperature at a rate of 20 °C / min. After the furnace is cooled, the graphite balls are taken out to obtain graphite balls coated with a porous SiC layer.

[0136] Step 2: Impregnation and Pyrolysis

[0137] Put the graphite balls coated with a porous SiC layer obtained in step 1 into a suspension of liquid polycarbosilane + SiC powder (mass ratio 25:75), so that the suspension completely covers the samples, and impregnate at room temperature under vacuum (-0.1 MPa) for 0.5 h. Take out the impregnated samples, dry them at 50 °C for 10 h. After drying, pyrolyze them in a tube furnace. During the pyrolysis process, argon is used as the protective gas, the temperature is raised to 1200 °C at a rate of 5 °C / min, and held for 1 h, and then cooled with the furnace. Take out the graphite balls, repeat the above impregnation and pyrolysis steps, and impregnate and pyrolyze 7 times in total to obtain graphite balls coated with a gradually distributed SiC coating.

[0138] Example 2

[0139] The pyrolysis process in Example 1 was adjusted as follows: During the pyrolysis process, argon was used as the protective gas, and the temperature was raised to 1200 °C at a rate of 5 °C / min, then raised to the target temperature of 1600 °C at a rate of 2 °C / min and held for 1 h, and then cooled at a rate of 2 °C / min. After cooling to 1200 °C, it was cooled with the furnace. Other conditions were the same as in Example 1, and graphite spheres coated with a gradually distributed SiC coating were obtained.

[0140] Example 3

[0141] The target temperature in Example 2 was adjusted to 1800 °C, and other conditions remained unchanged, and graphite spheres coated with a gradually distributed SiC coating were obtained.

[0142] Example 4

[0143] The SiC powder in Step 2 of Example 2 was replaced with Si powder, and the mass ratio of liquid polycarbosilane to Si powder was 88:12. Other conditions remained unchanged, and graphite spheres coated with a gradually distributed SiC coating were obtained.

[0144] The cross-sectional micro-morphology of the surface coatings of the graphite spheres obtained in Examples 1-4 was detected, and the results are shown in Figure 1 In the figure, “(a)” is the result of Example 1, “(b)” is the result of Example 2, “(c)” is the result of Example 3, and “(d)” is the result of Example 4. The light gray area in the figure is SiC. From Figure 1 It can be seen that the outermost layer of the coating is entirely a light gray phase and is a whole. Only sporadic light gray is distributed in the area close to the graphite, proving that in the intermediate layer (the bonding layer of SiC and graphite), from the inside to the outside, the SiC content gradually increases. Therefore, the method of the present invention can form a gradually distributed SiC coating on the surface of the graphite sphere, with the outermost layer being the SiC coating and the intermediate layer being the bonding layer of graphite and SiC; in the intermediate layer, SiC penetrates into the graphite gaps, and from the inside to the outside, the SiC content gradually increases.

[0145] The composition analysis of the outermost layer of the surface coatings of the graphite spheres obtained in Examples 1-3 was carried out, and the EDS element scanning results in the scanning electron microscope are shown in Figure 2 (In the figure, “(a)” is the result of Example 1, “(b)” is the result of Example 2, and “(c)” is the result of Example 3). From Figure 2 It can be known that the main component of the outermost layer coating obtained by the present invention is silicon carbide and also contains a small amount of oxygen. Its XRD pattern is shown in Figure 3 (In the figure, “1200 °C” is the result of Example 1, “1600 °C” is the result of Example 2, “1800 °C” is the result of Example 3, and “α-SiC” and “β-SiC” are two common crystal forms of silicon carbide), Figure 3 It also proves that the main component of the outermost layer coating obtained by the present invention is silicon carbide.

[0146] Example 5

[0147] Take a graphite ball sample, without embedding and sintering, and directly carry out impregnation and pyrolysis according to the method in Example 2 to obtain graphite balls coated with a gradually distributed SiC coating.

[0148] Figure 4 Figure 9 is a three-dimensional non-destructive X-CT detection image of the graphite balls coated with a gradually distributed SiC coating obtained in Example 5. From Figure 4 it can be seen that by using the method of the present invention, a very uniform coating can be formed on the surface of the graphite balls.

[0149] Perform cross-sectional microscopic morphology detection on the surface coating of the graphite balls obtained in Example 5, and the results are shown in Figure 5 . From Figure 5 it can be seen that a gradually distributed SiC coating is formed on the surface of the graphite balls. The outermost layer is the SiC coating, and the middle layer is the bonding layer of graphite and SiC; in the middle layer, SiC penetrates into the graphite gaps, and from the inside to the outside, the content of SiC gradually increases.

[0150] Therefore, it can be seen from Examples 1-5 that whether or not through the embedding and sintering step, the gradually distributed SiC coating of the present invention can be formed on the surface of the graphite balls.

[0151] Oxidize the graphite balls obtained in Example 5 in air at 1500 °C for 200 h, and its mass only increases by 0.752%, indicating that the gradually distributed SiC coating prepared by the present invention has excellent antioxidant properties.

[0152] The thicknesses of the SiC coating and the middle layer in each example are:

[0153] Examples Thickness of the SiC Coating Thickness of the Intermediate Layer Example 1 100μm 100μm Example 2 100μm 80μm Example 3 120μm 80μm Example 4 120μm 100μm Example 5 50μm 500μm

[0154] Comparative Example 1

[0155] Take a graphite ball sample, carry out embedding and sintering according to the method in Example 1 to obtain graphite balls coated with a porous SiC layer.

[0156] Place the graphite balls obtained in Comparative Example 1 and Examples 1-4 in a mixed gas of 1000 °C, 20% water vapor and He for 5 h, and then measure the corrosion rate. The results are shown in Figure 6 . From Figure 6 it can be known that in a high-temperature environment, the corrosion rate of the graphite balls obtained in Comparative Example 1 is as high as 0.81 mg / cm 2 ·h, while the corrosion rate of the graphite balls coated with the gradually distributed SiC coating prepared by the present invention is relatively low, which can be reduced to below 0.7 mg / cm 2 ·h, and even 0.05 mg / cm 2·h or less, it is described below that the coating prepared by the present invention has excellent corrosion resistance.

[0157] The present invention can improve the oxidation resistance and corrosion resistance of the coating by regulating the composition of the impregnating agent. Both Example 4 and Example 2 were pyrolyzed at 1600 °C. Si powder was added to the impregnating agent in Example 4, and SiC powder was added to the impregnating agent in Example 2. The corrosion rate of the graphite balls obtained in Example 4 was 0.61 mg / cm 2 ·h, and the corrosion rate of Example 2 was 0.04 mg / cm 2 ·h, indicating that the coatings obtained by adding Si powder or SiC powder to the impregnating agent both have good corrosion resistance, and the effect of adding SiC powder is better than that of adding Si powder.

[0158] It should be noted that although the technical solutions of the present invention are introduced by specific examples, those skilled in the art can understand that the present invention should not be limited thereto.

[0159] The embodiments of the present invention have been described above. The above description is exemplary and not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations are obvious to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of the terms used herein is intended to best explain the principles of the embodiments, practical applications, or improvements to technologies in the market, or to enable other ordinary skilled persons in the art to understand the disclosed embodiments.

Claims

1. A coating on the surface of a fuel element, characterized in that, The coating on the surface of the fuel element includes a silicon carbide coating and an intermediate layer from the outside to the inside, and the intermediate layer is a combined layer of silicon carbide and graphite.

2. The coating on the surface of the fuel element according to claim 1, characterized in that, The silicon carbide coating contains silicon carbide converted from polycarbosilane and silicon carbide filler; Preferably, the silicon carbide filler is one or more of silicon carbide powder, silicon carbide short fiber and silicon carbide whisker.

3. The coating on the surface of the fuel element according to claim 1 or 2, characterized in that, The silicon carbide coating also contains elemental silicon; and / or The silicon carbide coating further comprises one or more of a sintering aid, a boride, a carbide, an oxide, a silicide, and a rare earth salt; the sintering aid is selected from one or more of Al2O3-Y2O3, AlN-Re2O3, and Y3Si2C2; the boride is selected from one or more of ZrB2, HfB2, B4C, and TiB2; the carbide is selected from one or more of TaC, ZrC, and HfC; the silicide is selected from one or more of MoSi2 and CrSi2; the oxide is selected from one or more of SiO2, ZrO2, B2O3, Al2O3, TiO2, and HfO2; the rare earth salt is selected from one or more of Er2SiO5, LaMgAl 11 O 19 and Yb2SiO5.

4. The method for preparing the coating on the surface of the fuel element according to any one of claims 1-3, characterized in that, It includes the following steps: Immerse the fuel element in an impregnating agent and pyrolyze it to obtain the coating on the surface of the fuel element; wherein, the impregnating agent contains polycarbosilane or its solution.

5. The method for preparing the coating on the surface of the fuel element according to claim 4, characterized in that, The preparation method further includes the following steps: prepare a ceramic coating on the surface of the fuel element, then immerse the fuel element in an impregnating agent and pyrolyze it to obtain the coating on the surface of the fuel element.

6. The method for preparing a coating on the surface of a fuel element according to claim 4 or 5, characterized in that, The impregnation time is 0.5 - 2h; and / or The pyrolysis temperature is above 900°C, preferably 900 - 1800°C, more preferably 1200 - 1600°C.

7. The method for preparing the coating on the surface of the fuel element according to any one of claims 4-6, characterized in that, The number of times of impregnation and pyrolysis is 1 - 15 times, preferably 4 - 7 times.

8. The method for preparing the coating on the surface of the fuel element according to any one of claims 4-7, characterized in that, The solvent in the polycarbosilane solution is one or more of tetrahydrofuran, methyltetrahydrofuran, n-heptane, n-hexane and toluene.

9. The method for preparing a coating on the surface of a fuel element according to any one of claims 4-8, characterized in that, The impregnating agent also contains silicon carbide filler; Preferably, the silicon carbide filler is one or more of silicon carbide powder, silicon carbide short fiber and silicon carbide whisker.

10. The method for preparing a coating on the surface of a fuel element according to any one of claims 4-9, characterized in that, The impregnating agent also contains elemental silicon; and / or The impregnating agent further comprises one or more of a sintering aid, a boride, a carbide, an oxide, a silicide, and a rare earth salt; the sintering aid is selected from one or more of Al2O3 - Y2O3, AlN - Re2O3, and Y3Si2C2; the boride is selected from one or more of ZrB2, HfB2, B4C, and TiB2; the carbide is selected from one or more of SiC, TaC, ZrC, and HfC; the silicide is selected from one or more of MoSi2 and CrSi2; the oxide is selected from one or more of SiO2, ZrO2, B2O3, Al2O3, TiO2, and HfO2; the rare earth salt is selected from one or more of Er2SiO5, LaMgAl 11 O 19 and Yb2SiO5.

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