Sioc modified boron carbide-alumina composite and method of making

Abstract

The application relates to a SiOC modified boron carbide-aluminum oxide composite material and a preparation method thereof. The SiOC modified boron carbide-aluminum oxide composite material comprises a porous structure boron carbide-aluminum oxide material and a SiOC layer coated and modified on the porous structure of the boron carbide-aluminum oxide material and the surface of boron carbide and aluminum oxide particles; the porosity of the porous structure boron carbide-aluminum oxide material is 20-40%.

Description

Technical Field

[0001] This invention belongs to the technical field of discrete combustible poison components for nuclear reactors and neutron shielding materials, specifically relating to a SiOC-modified boron carbide-alumina composite material and its preparation method. Background Technology

[0002] Boron carbide (B4C) is a widely used material for nuclear reactor control rods. It utilizes boron as a neutron absorber to regulate reactor power. After absorbing a neutron, boron carbide releases helium. Ceramic-based neutron absorbers containing boron carbide typically have some porosity to accommodate the helium release, preventing severe swelling or even disintegration. Boron carbide offers advantages as a neutron absorber, including low residual radioactivity after irradiation, low cost, and ease of processing. However, it suffers from poor oxidation resistance and is prone to deterioration during long-term storage in air, especially in porous structures. Under accident conditions, exposure to oxygen can also cause boron carbide to burn or pulverize.

[0003] Boron carbide-alumina composite pellets have also been used as wet annular combustible poisons (WABA). Boron carbide-alumina multiphase materials use alumina as the matrix material and boron carbide as a neutron absorber, which improves oxidation resistance. Alumina also has advantages such as a small neutron absorption cross-section, stable physicochemical properties, and low cost. However, current conventional techniques have not effectively solved the hygroscopic problem of boron carbide-alumina pellets. Due to the strong hygroscopicity of alumina itself and the porous structure of boron carbide-alumina, the material exhibits high hygroscopicity. Excessive moisture, acting as a hydrogen source, can cause hydrogen embrittlement of the zirconium cladding, leading to material leaching into the moderator medium and severely impacting the safe operation of the nuclear reactor. Earlier discrete combustible poison borosilicate glass designs, while cheaper to manufacture, could not be used with zirconium alloy cladding due to their high water absorption. Therefore, solving the hygroscopic problem of boron carbide-alumina pellets is crucial for the safety of zirconium alloy cladding tubes and even the safe operation of nuclear reactors.

[0004] Furthermore, boron carbide particles contain impurities such as free boron and soluble boron on their surface. During high-temperature sintering, boron also volatilizes and deposits on the alumina surface. In aerobic and humid environments, these surfaces undergo various chemical reactions, generating boric acid and other substances that further increase hygroscopicity. These issues complicate the material manufacturing process, increase processing steps and usage costs, and limit the application of boron carbide-alumina cores. Existing technologies do not offer corresponding solutions to limit or mitigate this increased hygroscopicity risk associated with boron carbide. Summary of the Invention

[0005] To address the shortcomings and defects in existing boron carbide-alumina material preparation technologies, this invention proposes a SiOC-modified boron carbide-alumina composite material and its preparation method. The method first involves molding and sintering boron carbide powder, alumina powder, and a sintering aid to prepare a porous boron carbide-alumina material. Then, a SiOC precursor is used to encapsulate the boron carbide-alumina, followed by drying and pyrolysis to obtain the SiOC-modified boron carbide-alumina composite structure. This preparation method can suppress surface side reactions of boron carbide-alumina, improve moisture resistance, and is simple, relatively low-cost, and easy to mass-produce. This material can be used as a discrete combustible poison for reactor burnup and power control, as well as neutron shielding.

[0006] In a first aspect, the present invention provides a SiOC-modified boron carbide-alumina composite material, the SiOC-modified boron carbide-alumina composite material comprising: a porous boron carbide-alumina material, and a SiOC layer coating and modifying the porous structure of the boron carbide-alumina material and the surface of the boron carbide and alumina particles; The porosity of the porous boron carbide-alumina material is 20-40%.

[0007] Preferably, the thickness of the SiOC layer modified on the surface of the boron carbide-alumina material is 1-10 nm, more preferably 2-5 nm.

[0008] Preferably, the SiOC-modified boron carbide-alumina composite material has a hydrogen content of 15-30 ppm after absorbing water for 12-72 hours under conditions of relative humidity of 80-90% and temperature of 20-30℃.

[0009] Secondly, the present invention provides a method for preparing the above-mentioned SiOC-modified boron carbide-alumina composite material, the method comprising the following steps: (1) Mix the raw material powder containing boron carbide powder, alumina powder and sintering aid and form it into a blank, and then sinter it to obtain a porous boron carbide-alumina material. (2) Introduce SiOC precursor into the pores and surface of the porous boron carbide-alumina material, and after drying and wrapping, obtain a boron carbide-alumina composite structure wrapped with SiOC precursor. (3) The boron carbide-alumina composite structure wrapped by the SiOC precursor is pyrolyzed to obtain the SiOC modified boron carbide-alumina composite material.

[0010] Preferably, in step (1), the sintering aid includes SiO2. x (x=0-2), at least one of MgO and CaO.

[0011] Preferably, in step (1), the mass percentage of boron carbide powder in the raw material powder is 5-50 wt%, more preferably 10-40 wt%, and even more preferably 12-20 wt%; the mass percentage of alumina powder is 49-94.9%; and the mass percentage of sintering aid is 0.1-1%.

[0012] Preferably, in step (1), the sintering atmosphere is an inert atmosphere or a vacuum environment, and the inert gas is preferably at least one of argon, nitrogen, helium and carbon dioxide; the sintering temperature is 1400-1700℃, and the sintering time is 1-6h.

[0013] Preferably, in step (2), the SiOC precursor includes at least one of polysiloxane, polysilsesquioxane, and silicone resin; the process of introducing the SiOC precursor includes impregnation, stirring, filtration, and spraying.

[0014] Preferably, in step (2), the drying and wrapping is carried out in an air atmosphere, the drying and wrapping temperature is 60-250℃, preferably 100-200℃, and the drying and wrapping time is 1-10h.

[0015] Preferably, in step (3), the pyrolysis is carried out under an argon, nitrogen, or vacuum atmosphere; the pyrolysis temperature is 900-1200℃, and the pyrolysis time is 1-5 hours.

[0016] Beneficial effects (1) This invention addresses the defects of boron carbide-alumina materials, such as strong hygroscopicity, high surface activity, and stringent requirements for storage and use environment. The SiOC-modified boron carbide-alumina composite structure prepared by this invention effectively improves the hygroscopic performance of porous boron carbide-alumina materials. (2) The SiOC-modified boron carbide-alumina boron carbide-alumina material of the present invention is made by coating the surface of boron carbide-alumina boron carbide-alumina with precursors containing Si, O and C such as polysiloxane, polysilsesquioxane, and silicone resin. The SiOC-modified boron carbide-alumina boron carbide-alumina material is obtained by drying and pyrolysis, and can obtain surface properties with excellent chemical and thermal stability. (3) The SiOC used in this invention can form a uniform distribution in the boron carbide-alumina porous structure, preventing the hydrophilic alumina from absorbing water vapor in the environment, and also inhibiting the generation, diffusion and adsorption of water vapor by boric acid, free boron and other substances on the surface of boron carbide particles. (4) The SiOC precursor used in this invention is low in cost, does not produce harmful substances, is environmentally friendly, and the preparation method of modified boron carbide-alumina materials is simple. It can process boron carbide-alumina materials of arbitrary shape and is easy to achieve large-scale production. Attached Figure Description

[0017] Figure 1 A schematic diagram of the microstructure of a SiOC-modified boron carbide-alumina composite material provided as an example of the present invention; Figure 2 This is a sample image before pyrolysis in Example 2; Figure 3 This is a sample image after pyrolysis in Example 3; Figure 4 The XRD pattern of the SiOC-modified boron carbide-alumina composite material prepared in Example 4; Figure label: 1. Boron carbide particles; 2. Alumina particles; 3. SiOC layer; 4. Pores. Detailed Implementation

[0018] The present invention will be further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0019] First, such as Figure 1 As shown, this invention provides a SiOC-modified boron carbide-alumina composite material. The SiOC-modified boron carbide-alumina composite material comprises: a porous boron carbide-alumina material, and a SiOC layer coating and modifying the porous structure of the boron carbide-alumina material and the surface of the boron carbide and alumina particles.

[0020] During the sintering process, boric acid, boron oxide, and Al are formed on the surface and interface of the boron carbide-alumina core. 18 B4O 33 The SiOC (Silicon-Oxide-Carbon) precursor is a type of ceramic with advantages such as high thermal stability, oxidation resistance, and high strength. It can be used as a high-temperature structural material, functional material, and structural microwave absorbing material. In this invention, the SiOC is uniformly distributed on the porous structure and surface of boron carbide-alumina. By introducing SiOC-modified particles, a hydrophobic protective layer is formed, inhibiting the adsorption of water molecules and preventing the highly hydrophilic alumina from absorbing moisture from the environment. Simultaneously, it inhibits the generation, diffusion, and moisture adsorption of boric acid and free boron on the surface of the boron carbide particles. Furthermore, the SiOC-modified boron carbide-alumina provided by this invention can utilize the inorganic components (such as SiO2, SiC, C, etc.) formed after the pyrolysis of SiOC precursors such as organosilicon to improve water absorption resistance.

[0021] In some embodiments, the porosity of the porous boron carbide-alumina material can be 20-40%, and the pore size can be 0.1-2 μm. If the porosity is too low, the SiOC precursor will have difficulty entering the pores and contacting the boron carbide or alumina particles during the preparation process; if the porosity is too high, the material will have poor moisture resistance.

[0022] In some embodiments, the thickness of the SiOC layer modified on the surface of the boron carbide-alumina material can be 1-10 nm, preferably 2-5 nm. If the SiOC layer thickness is too large, the content of impurity elements such as C and Si will be too high; if the SiOC layer thickness is too small, it will not be able to isolate moisture.

[0023] In some embodiments, the SiOC-modified boron carbide-alumina composite material has a hydrogen content of 15-30 ppm after absorbing water for 12-72 hours under conditions of 80-90% relative humidity and 20-30°C.

[0024] The following is an exemplary description of a method for preparing SiOC-modified boron carbide-alumina composite material provided by the present invention. The preparation method may include the following steps: (1) Mix the raw material powder containing boron carbide powder, alumina powder and sintering aid and form it into a blank, and then sinter it to obtain a porous boron carbide-alumina material. (2) Introduce SiOC precursor into the pores and surface of the porous boron carbide-alumina material, and after drying and wrapping, obtain a boron carbide-alumina composite structure wrapped with SiOC precursor. (3) The boron carbide-alumina composite structure wrapped by the SiOC precursor is pyrolyzed to obtain the SiOC modified boron carbide-alumina composite material.

[0025] In some embodiments, in step (1), the particle size of the boron carbide powder can be 2-50 μm, and the D50 of the alumina powder particle size can be 50-500 nm, preferably 100-200 nm. If the boron carbide particle size is too large, the neutron absorption performance will be weakened due to the self-shielding effect; if the size is too small, it will cause rapid swelling, stronger water absorption, and inhibit the sintering densification of alumina in the material. Moreover, the smaller the boron carbide particles and the higher their content, the more difficult it is to achieve densification of alumina-boron carbide, requiring a higher sintering temperature.

[0026] In some embodiments, in step (1), the sintering aid may include SiO2. x (x=0-2), at least one of MgO and CaO; preferably, the particle size of the sintering aid can be 20-100nm.

[0027] In some embodiments, in step (1), the mass percentage of boron carbide powder in the raw material powder can be 5-50 wt%, preferably 10-40 wt%, more preferably 12-20 wt%, the mass percentage of alumina powder can be 49-94.9%, and the mass percentage of sintering aid can be 0.1-1%.

[0028] By controlling the appropriate raw material ratio, the function and structure of the material can be guaranteed. Excessive boron carbide content leads to increased water absorption, while insufficient content prevents it from fulfilling its intended neutron absorption function. Alumina acts as the matrix, forming a porous structure to contain helium release and swelling. Insufficient alumina content can easily cause structural collapse and damage in the later stages of the material's service life (18-24 months).

[0029] In some embodiments, in step (1), the sintering atmosphere can be an inert atmosphere or a vacuum environment, preferably the inert gas can be at least one of argon, nitrogen, helium and carbon dioxide; the sintering temperature can be 1400-1700℃, and the sintering time can be 1-6h.

[0030] In some embodiments, in step (2), the SiOC precursor may include at least one of polysiloxane, polysilsesquioxane, and silicone resin.

[0031] It should be noted that directly mixing SiOC with the raw materials easily forms an independent third phase, making it impossible to achieve in-situ modification of the porous boron carbide-alumina material surface. Introducing a liquid precursor allows for uniform coating of the porous boron carbide-alumina material, and the process is simple, requiring only a simple pyrolysis process to achieve the modification goal.

[0032] In some embodiments, step (2) of the process of introducing the SiOC precursor may include impregnation, stirring, filtration, and spraying. The number of impregnations may be 1-5.

[0033] In some embodiments, in step (2), the drying and wrapping can be carried out in an air atmosphere, the drying and wrapping temperature can be 60-250℃, preferably 100-200℃, and the drying and wrapping time can be 1-10 hours. The drying and wrapping can be performed 1-5 times. If the temperature is too low or the time is too short, the moisture and solvent in the precursor-wrapped core block cannot be completely removed, the initial curing cannot be completed, the uniformity of the wrapping layer and high-temperature pyrolysis are affected, and the effect of improving the water absorption resistance is not good.

[0034] In some embodiments, in step (3), the pyrolysis can be carried out under flowing argon, nitrogen atmosphere or vacuum conditions; the pyrolysis temperature can be 900-1200℃, and the pyrolysis time can be 1-5 hours. If the pyrolysis temperature is too low, the precursor cannot be completely decomposed; if the pyrolysis temperature is too high, it will result in an excessive amount of SiC generated and the product particles will be too large.

[0035] The SiOC-modified boron carbide-alumina composite material provided by this invention can significantly improve the moisture resistance of boron carbide-alumina when used as a discrete combustible poison, simplify the drying and storage conditions before tube loading, and reduce the cost of material use and storage.

[0036] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art.

[0037] Example 1

[0038] The preparation method of SiOC-modified boron carbide-alumina composite material provided in this embodiment includes the following steps: (1) Take 10g of boron carbide powder (5-35μm) and 90g of alumina powder (D50 is 300nm), add 0.2g of sintering aid magnesium oxide, then add 500g of water and binder, ball mill and mix evenly, spray granulation to obtain mixed powder; take 2g of mixed powder and press to obtain ring green blank, sinter the green blank in an argon atmosphere furnace at 1500℃ for 2 hours to obtain porous boron carbide-alumina material; (2) Prepare a solution containing SiOC precursor, using polysiloxane as solute and anhydrous ethanol as solvent, and prepare a 50% (wt) solution. Then, impregnate the porous boron carbide-alumina material obtained in step (1) with this solution for 2 min. After impregnation, crosslink the porous boron carbide-alumina material in air at 200°C for 1 hour and dry and wrap it to obtain a boron carbide-alumina composite structure wrapped with SiOC precursor. (3) The boron carbide-alumina composite structure wrapped with the SiOC precursor obtained in step (2) is pyrolyzed in a pyrolysis furnace under nitrogen atmosphere protection. The pyrolysis temperature is 1000℃ and the holding time is 1 hour. After cooling to room temperature, the SiOC-modified boron carbide-alumina composite material is obtained.

[0039] Tests showed that the porosity of the boron carbide-alumina composite material modified with SiOC prepared in this embodiment was 30%. After absorbing water in an oven at 80% relative humidity and 25°C for 48 hours, the hydrogen content (as an indicator of water absorption) of the SiOC modified boron carbide-alumina composite material prepared in this embodiment was 18 ppm, as measured by a nitrogen-oxygen-hydrogen meter.

[0040] Example 2

[0041] The preparation method of the SiOC-modified boron carbide-alumina composite material provided in this embodiment is the same as that in Example 1, with the main difference being: In step (1), 0.5g of magnesium oxide is used and the sintering temperature of the green blank is 1450℃.

[0042] Figure 2 This is a sample image from Example 2 before pyrolysis. As can be seen from the image, the annular boron carbide-alumina core encapsulated by the precursor is light gray in color.

[0043] Tests showed that the porosity of the boron carbide-alumina material in the SiOC-modified boron carbide-alumina composite material prepared in this embodiment was 35%. After absorbing water in an oven at 80% relative humidity and 25°C for 48 hours, the hydrogen content (as an indicator of water absorption) of the SiOC-modified boron carbide-alumina composite material prepared in this embodiment was 30 ppm, as measured by a nitrogen-oxygen-hydrogen meter.

[0044] Example 3

[0045] The preparation method of the SiOC-modified boron carbide-alumina composite material provided in this embodiment is the same as that in Example 1, with the main difference being: In step (2), the concentration of the solution containing the SiOC precursor is 20% (wt).

[0046] Figure 3 This is a sample image after pyrolysis in Example 3. As can be seen from the image, the annular boron carbide-alumina core is dark gray in color.

[0047] Tests showed that the porosity of the boron carbide-alumina material in the SiOC-modified boron carbide-alumina composite material prepared in this embodiment was 29%. After absorbing water in an oven at 80% relative humidity and 25°C for 48 hours, the hydrogen content (as an indicator of water absorption) of the SiOC-modified boron carbide-alumina composite material prepared in this embodiment was 21 ppm, as measured by a nitrogen-oxygen-hydrogen meter.

[0048] Example 4

[0049] The preparation method of the SiOC-modified boron carbide-alumina composite material provided in this embodiment is the same as that in Example 1, with the main difference being: In step (1), the sintering aid is silicon dioxide, and the amount added is 0.5g. In step (2), a 30% (wt) solution is prepared using polysilsesquioxane as the solute; the drying temperature of the package is 150°C.

[0050] Figure 4 The image shows the XRD pattern of the SiOC-modified boron carbide-alumina composite material prepared in Example 4. As can be seen from the image, boron carbide and alumina are the main diffraction peaks, with boron carbide having a relatively weaker peak intensity.

[0051] Tests showed that the porosity of the boron carbide-alumina composite material modified with SiOC prepared in this embodiment was 28%. After absorbing water in an oven at 80% relative humidity and 25°C for 48 hours, the hydrogen content (as an indicator of water absorption) of the SiOC modified boron carbide-alumina composite material prepared in this embodiment was 19 ppm, as measured by a nitrogen-oxygen-hydrogen analyzer.

[0052] Example 5

[0053] The preparation method of the SiOC-modified boron carbide-alumina composite material provided in this embodiment is the same as that in Example 1, with the main difference being: In step (1), the sintering aid is silicon dioxide; In step (2), a solution is prepared using silicone resin as the solute; In step (3), the pyrolysis atmosphere is argon.

[0054] Tests showed that the porosity of the boron carbide-alumina material in the SiOC-modified boron carbide-alumina composite material prepared in this embodiment was 26%. After absorbing water in an oven at 80% relative humidity and 25°C for 48 hours, the hydrogen content (as an indicator of water absorption) of the SiOC-modified boron carbide-alumina composite material prepared in this embodiment was 15 ppm, as measured by a nitrogen-oxygen-hydrogen analyzer.

[0055] Example 6

[0056] The preparation method of the SiOC-modified boron carbide-alumina composite material provided in this embodiment is the same as that in Example 1, with the main difference being: In step (1), 20g of boron carbide powder, 79.8g of alumina powder, and 0.2g of silicon oxide are used as sintering aids.

[0057] Tests showed that the porosity of the boron carbide-alumina composite material modified with SiOC prepared in this embodiment was 29%. After absorbing water in an oven at 80% relative humidity and 25°C for 48 hours, the hydrogen content (as an indicator of water absorption) of the SiOC modified boron carbide-alumina composite material prepared in this embodiment was 19 ppm, as measured by a nitrogen-oxygen-hydrogen meter.

[0058] Example 7

[0059] The preparation method of the SiOC-modified boron carbide-alumina composite material provided in this embodiment is the same as that in Example 1, with the main difference being: In step (1), the sintering aid is silicon dioxide, the amount added is 0.8g, and the sintering temperature of the green blank is 1600℃; In step (2), the porous boron carbide-alumina material is impregnated with the solution for 5 minutes, dried and wrapped at 100°C, and the impregnation, drying and wrapping process is repeated twice.

[0060] Tests showed that the porosity of the boron carbide-alumina composite material modified with SiOC prepared in this embodiment was 35%. After absorbing water in an oven at 80% relative humidity and 25°C for 48 hours, the hydrogen content (as an indicator of water absorption) of the SiOC modified boron carbide-alumina composite material prepared in this embodiment was 15 ppm, as measured by a nitrogen-oxygen-hydrogen meter.

[0061] Example 8

[0062] The preparation method of the SiOC-modified boron carbide-alumina composite material provided in this embodiment is the same as that in Example 1, with the main difference being: In step (1), 15g of boron carbide powder, 84.5g of alumina powder, 0.5g of silicon dioxide as sintering aid, and the sintering temperature of the green blank is 1550℃; In step (2), the temperature for drying the package is 250°C; In step (3), the drying atmosphere is argon atmosphere and the pyrolysis temperature is 1100℃.

[0063] Tests showed that the porosity of the boron carbide-alumina composite material modified with SiOC prepared in this embodiment was 32%. After absorbing water in an oven at 80% relative humidity and 25°C for 48 hours, the hydrogen content (as an indicator of water absorption) of the SiOC modified boron carbide-alumina composite material prepared in this embodiment was 21 ppm, as measured by a nitrogen, oxygen and hydrogen analyzer.

[0064] Comparative Example 1

[0065] The method for preparing the composite material provided in this comparative example is the same as in Example 3, with the main difference being: SiOC was not modified in steps (2) and (3).

[0066] Tests showed that the boron carbide-alumina composite material prepared in this comparative example absorbed water for 48 hours in an oven at 80% relative humidity and 25°C, and the hydrogen content (used as an indicator of water absorption) was 45 ppm as measured by a nitrogen-oxygen-hydrogen meter.

[0067] Comparative Example 2

[0068] The method for preparing the composite material provided in this comparative example is the same as in Example 1, with the main difference being: In step (1), the sintering temperature of the green blank is 1350℃.

[0069] Tests showed that the boron carbide-alumina composite material prepared in this comparative example absorbed water for 48 hours in an oven at 80% relative humidity and 25°C, and the hydrogen content (used as an indicator of water absorption) was 48 ppm as measured by a nitrogen-oxygen-hydrogen meter.

[0070] Comparative Example 3

[0071] The method for preparing the composite material provided in this comparative example is the same as in Example 1, with the main difference being: In step (1), the sintering holding time of the green blank is 0.5 hours, and the porosity of the obtained boron carbide-alumina material is 42%.

[0072] Tests showed that the boron carbide-alumina composite material prepared in this comparative example absorbed water for 48 hours in an oven at 80% relative humidity and 25°C, and the hydrogen content (used as an indicator of water absorption) was 72 ppm as measured by a nitrogen-oxygen-hydrogen meter.

[0073] The comparative examples show that the boron carbide-alumina material has excessive porosity, which increases its water absorption capacity.

[0074] Comparative Example 4

[0075] The method for preparing the composite material provided in this comparative example is the same as in Example 1, with the main difference being: In step (2), the concentration of the solution containing the SiOC precursor is 80% (wt).

[0076] Tests showed that the boron carbide-alumina composite material prepared in this comparative example absorbed water for 48 hours in an oven at 80% relative humidity and 25°C, and the hydrogen content (used as an indicator of water absorption) was 32 ppm as measured by a nitrogen-oxygen-hydrogen meter.

[0077] Excessive precursor solution concentration leads to excessively thick and uneven modified membranes, pore blockage, and membrane rupture, which increases water absorption capacity.

[0078] Comparative Example 5

[0079] The method for preparing the composite material provided in this comparative example is the same as in Example 1, with the main difference being: In step (3), the pyrolysis temperature is 800℃.

[0080] Tests showed that the boron carbide-alumina composite material prepared in this comparative example absorbed water for 48 hours in an oven at 80% relative humidity and 25°C, and the hydrogen content (used as an indicator of water absorption) was 50 ppm as measured by a nitrogen-oxygen-hydrogen meter.

[0081] If the pyrolysis temperature is too low, the precursor reaction will be incomplete, leading to an increase in the water absorption of the composite material.

[0082] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A SiOC-modified boron carbide-alumina composite material, characterized in that, The SiOC-modified boron carbide-alumina composite material includes: a porous boron carbide-alumina material, and a SiOC layer coating and modifying the porous structure of the boron carbide-alumina material and the surface of the boron carbide and alumina particles; The porosity of the porous boron carbide-alumina material is 20-40%.

2. The SiOC-modified boron carbide-alumina composite material according to claim 1, characterized in that, The thickness of the SiOC layer modified on the surface of the boron carbide-alumina material is 1-10 nm.

3. The SiOC-modified boron carbide-alumina composite material according to claim 2, characterized in that, The thickness of the SiOC layer modified on the surface of the boron carbide-alumina material is 2-5 nm.

4. The SiOC-modified boron carbide-alumina composite material according to claim 1, characterized in that, The SiOC-modified boron carbide-alumina composite material has a hydrogen content of 15-30 ppm after absorbing water for 12-72 hours under conditions of relative humidity of 80-90% and temperature of 20-30℃.

5. A method for preparing the SiOC-modified boron carbide-alumina composite material according to claim 1, characterized in that, The preparation method includes the following steps: (1) Mix the raw material powder containing boron carbide powder, alumina powder and sintering aid and form it into a green blank, and then sinter it to obtain a porous boron carbide-alumina material. (2) Introduce SiOC precursor into the pores and surface of the porous boron carbide-alumina material, and after drying and wrapping, obtain a boron carbide-alumina composite structure wrapped with SiOC precursor. (3) The boron carbide-alumina composite structure wrapped by the SiOC precursor is pyrolyzed to obtain the SiOC modified boron carbide-alumina composite material.

6. The preparation method according to claim 5, characterized in that, In step (1), the sintering aid includes SiO₂. x And x = 0 - 2, MgO, CaO or at least one of them.

7. The preparation method according to claim 5, characterized in that, In step (1), the mass percentage of boron carbide powder in the raw material powder is 5-50 wt%; the mass percentage of alumina powder is 49-94.9%; and the mass percentage of sintering aid is 0.1-1%.

8. The preparation method according to claim 7, characterized in that, In step (1), the mass percentage of boron carbide powder in the raw material powder is 10-40 wt%.

9. The preparation method according to claim 8, characterized in that, In step (1), the mass percentage of boron carbide powder in the raw material powder is 12-20 wt%.

10. The preparation method according to claim 5, characterized in that, In step (1), the sintering atmosphere is an inert atmosphere or a vacuum environment. The inert atmosphere uses at least one of argon, nitrogen, and helium. The sintering temperature is 1400-1700℃ and the sintering time is 1-6h.

11. The preparation method according to claim 5, characterized in that, In step (2), the SiOC precursor includes at least one of polysiloxane, polysilsesquioxane, and silicone resin; the process of introducing the SiOC precursor includes impregnation, stirring, filtration, and spraying.

12. The preparation method according to claim 5, characterized in that, In step (2), the drying and wrapping is carried out in an air atmosphere, the temperature of the drying and wrapping is 60-250℃, and the drying and wrapping time is 1-10h.

13. The preparation method according to claim 12, characterized in that, In step (2), the temperature for drying the package is 100-200℃.

14. The preparation method according to claim 5, characterized in that, In step (3), the pyrolysis is carried out under an argon, nitrogen atmosphere or a vacuum; the pyrolysis temperature is 900-1200℃ and the pyrolysis time is 1-5 hours.

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