Hollow spherical nanometer silicon carbide powder, preparation method and application thereof

Abstract

The application belongs to the technical field of ceramic materials, and particularly relates to a hollow spherical nanometer silicon carbide powder and a preparation method and application thereof. The preparation method of the silicon carbide powder comprises the following steps: placing a resorcinol and formaldehyde solution in an alkaline alcohol-water mixed solution to perform a polycondensation reaction, then adding tetraalkoxysilane to perform a hydrolysis polycondensation reaction, obtaining phenolic resin coated silica microspheres through the co-polycondensation reaction, obtaining carbon coated silica microspheres through carbonization treatment, and finally obtaining the hollow spherical nanometer silicon carbide powder through a magnesium hot reduction reaction. The hollow spherical nanometer silicon carbide powder prepared by the application has a uniform structure and high sintering activity. In the process of sintering and preparing silicon carbide ceramics, the hollow structure is broken to generate a large number of new surfaces, the sintering driving force is enhanced, sintering aids do not need to be introduced, silicon carbide ceramics can be densified quickly at a low temperature, and the microstructure and mechanical properties of the ceramic material can be improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of ceramic materials, and particularly relates to a hollow spherical nano silicon carbide powder, a preparation method and application thereof, and is especially suitable for the preparation process of silicon carbide ceramic materials for improving ceramic density, reducing sintering temperature and improving performance. BACKGROUND

[0002] Silicon carbide ceramics are widely used in aerospace, armor protection, nuclear engineering, chemical equipment and semiconductor fields due to their low density, high melting point, excellent mechanical properties, thermal properties, oxidation resistance and chemical stability, and are a kind of structural ceramic materials with important strategic value.

[0003] The excellent performance of silicon carbide is mainly due to its strong covalent bond crystal structure. However, the content of covalent bond in silicon carbide is extremely high, and the surface self-diffusion coefficient is extremely low, which makes it difficult to achieve dense sintering under normal conditions. The preparation of fully dense silicon carbide ceramics by traditional sintering method usually requires high temperature sintering conditions of more than 2000℃, which not only brings huge energy consumption, but also may cause abnormal grain growth, thereby reducing the comprehensive mechanical properties of the material.

[0004] In order to solve the above problems, various technical schemes have been proposed to improve the sintering activity of silicon carbide, such as increasing the sintering pressure, prolonging the sintering time or adding sintering additives to promote densification. However, these methods generally have certain limitations, such as the need to use expensive high temperature and high pressure equipment, complex process conditions, and the addition of sintering additives may introduce impurities, affecting the purity and high temperature performance of silicon carbide ceramics.

[0005] In the process of ceramic sintering, the sintering activity of the powder is the key factor to determine the densification efficiency. Generally, it is believed that the use of ultra-fine nano powder with smaller particle size and larger specific surface area can significantly improve the sintering activity, thereby reducing the sintering temperature, shortening the holding time, improving the density and optimizing the microstructure. However, nano powder is prone to agglomeration in practical application, which leads to uneven ceramic microstructure and may induce abnormal grain growth, thereby seriously affecting the densification and mechanical properties of ceramic materials. SUMMARY

[0006] The purpose of the present application is to overcome the deficiencies in the prior art and provide a hollow spherical nano silicon carbide powder, a preparation method and application thereof. By controlling the pH value of the system, the hydrolysis rate of tetraalkoxysilane is precisely adjusted, the size of the silica core layer is precisely controlled, and the particle size of the final silicon carbide powder is effectively regulated. The obtained hollow spherical nano-structured silicon carbide powder has a large specific surface area and a high surface energy, and its sintering activity is significantly improved. It can be used to prepare dense, uniform microstructure and high-performance silicon carbide ceramics at low temperature without introducing impurities.

[0007] To achieve the above technical purposes, the technical scheme adopted by the embodiments of the present application is:

[0008] In a first aspect, the embodiments of the present application provide a preparation method of hollow spherical nanometer silicon carbide powder, comprising the following steps:

[0009] (1) placing a resorcinol and formaldehyde solution in an alkaline alcohol-water mixed solution to perform a polycondensation reaction for 40-60 min, and then adding a tetraalkoxysilane to perform a hydrolytic polycondensation reaction, and separating, washing and drying the total polycondensation reaction product to obtain phenolic resin-coated silica microspheres;

[0010] (2) performing carbonization treatment on the phenolic resin-coated silica microspheres obtained in step (1) in an inert atmosphere to obtain carbon-coated silica microspheres;

[0011] (3) mixing the carbon-coated silica microspheres obtained in step (2) with magnesium powder, and performing a magnesium thermal reduction reaction in a protective atmosphere, and performing post-treatment to obtain hollow spherical nanometer silicon carbide powder;

[0012] In step (1), the pH value of the alkaline alcohol-water mixed solution is 8-12.

[0013] Further, in step (1), the volume ratio of anhydrous ethanol to deionized water in the alcohol-water mixed solution is 1:0.1-0.3.

[0014] Further, in step (1), the tetraalkoxysilane includes one or more of tetramethoxysilane, tetraethoxysilane and tetrapropoxysilane.

[0015] Further, in step (1), the mass ratio of the resorcinol to the formaldehyde solution is 1:1.2-1.5, the mass fraction of the formaldehyde solution is 30%-40%, and the mass ratio of the tetraalkoxysilane to the resorcinol is 6-9:1.

[0016] Further, in step (2), the carbonization treatment is performed at 700-800°C under a nitrogen atmosphere, and the holding time is 3-6 hours.

[0017] Further, in step (3), the magnesium thermal reduction reaction is performed by mixing the carbon-coated silica microspheres with magnesium powder at a mass ratio of 1:0.6-0.8, and then loading them into a sealed stainless steel reaction kettle, and performing the reaction at 650-850°C under an argon atmosphere, and the holding time is 4-8 hours.

[0018] Further, in step (3), the post-treatment comprises the following steps: using a 0.5-2 M hydrochloric acid solution for pickling to remove unreacted magnesium and generated magnesium oxide; after drying the pickling product, calcining in an air atmosphere at 600-800℃ to remove residual carbon; then using a 0.5-2 M hydrofluoric acid solution for etching to remove residual silicon dioxide, and after washing and drying, the hollow spherical nano-silicon carbide powder is obtained.

[0019] In a second aspect, the embodiment of the present application provides a hollow spherical nano-silicon carbide powder, which is prepared by the method of the first aspect, and the particle size of the hollow spherical nano-silicon carbide powder is 280-720 nm, and the specific surface area is 110-150 m 2 / g.

[0020] In a third aspect, the embodiment of the present application provides an application of the hollow spherical nano-silicon carbide powder of the second aspect, and the hollow spherical nano-silicon carbide powder is applied in the preparation of silicon carbide ceramic by low-temperature sintering, comprising the following steps:

[0021] The hollow spherical nano-silicon carbide powder is loaded into a graphite mold, the graphite mold is placed in a spark plasma sintering furnace, and sintering is performed under vacuum conditions, the sintering pressure is 40-70 MPa, the sintering temperature is controlled between 1700-1800℃, the heating rate is 50-200℃ / min, the holding time is 1-10 minutes, and after sintering, the silicon carbide ceramic material is obtained by grinding.

[0022] Further, the density of the silicon carbide ceramic material is 3.0-3.2 g / cm 3 , the relative density reaches 95%-99.9%, and the Vickers hardness is 25-33 GPa.

[0023] Compared with the prior art, the present application has the following beneficial effects:

[0024] (1) The present application precisely adjusts the hydrolysis rate of tetraalkoxysilane by adjusting the ammonia water addition amount to control the pH value of the system during the reaction, thereby precisely controlling the size of the silicon dioxide core layer, and effectively controlling the particle size of the final silicon carbide powder. The obtained hollow spherical nano-structured silicon carbide powder has the characteristics of large specific surface area and high surface energy, and the sintering activity is significantly improved, which can prepare dense, microstructure uniform and high-performance silicon carbide ceramic at low temperature and fast speed without introducing impurities.

[0025] (2) In the preparation of silicon carbide ceramic by the spark plasma sintering process, the hollow spherical shell structure collapses and breaks under the action of temperature and pressure, forming more fine fragments. The existence of these fragments increases the surface energy of the system, thereby improving the sintering activity of the silicon carbide ceramic, enabling the sintering of high-density silicon carbide ceramic at a relatively low temperature, effectively inhibiting abnormal grain growth, and improving the uniformity of the ceramic material and the mechanical properties. The entire process does not require the introduction of sintering aids or impurities, and the purity of the ceramic material is high and the structure is controllable. The preparation process has strong controllability and reduces energy consumption, is suitable for large-scale preparation, and has broad application prospects. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The XRD spectrum of the hollow spherical nanometer silicon carbide powder obtained in Example 1.

[0027] Figure 2 The SEM image of the hollow spherical nanometer silicon carbide powder obtained in Example 1.

[0028] Figure 3 The TEM image of the hollow spherical nanometer silicon carbide powder obtained in Example 1.

[0029] Figure 4 The indentation image of the silicon carbide ceramic obtained in Example 1.

[0030] Figure 5 The SEM image of the fracture of the silicon carbide ceramic obtained in Example 1.

[0031] Figure 6 The SEM image of the hollow spherical nanometer silicon carbide powder obtained in Example 2.

[0032] Figure 7 The TEM image of the hollow spherical nanometer silicon carbide powder obtained in Example 2.

[0033] Figure 8 The indentation image of the silicon carbide ceramic obtained in Example 2.

[0034] Figure 9 The SEM image of the fracture of the silicon carbide ceramic obtained in Example 2.

[0035] Figure 10 The SEM image of the hollow spherical nanometer silicon carbide powder obtained in Example 3.

[0036] Figure 11 The TEM image of the hollow spherical nanometer silicon carbide powder obtained in Example 3.

[0037] Figure 12 The indentation image of the silicon carbide ceramic obtained in Example 3.

[0038] Figure 13 SEM image of the fracture of the silicon carbide ceramic obtained in Example 3.

[0039] Figure 14 Indentation map of the silicon carbide ceramic obtained in Example 4.

[0040] Figure 15 SEM image of the fracture of the silicon carbide ceramic obtained in Example 4.

[0041] Figure 16 Indentation map of the silicon carbide ceramic obtained in Example 5.

[0042] Figure 17 SEM image of the fracture of the silicon carbide ceramic obtained in Example 5. DETAILED DESCRIPTION

[0043] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and are not used to limit the present application.

[0044] Example 1

[0045] A method for preparing hollow spherical nanometer silicon carbide powder, comprising the following steps:

[0046] (1) At room temperature, 6.5 mL of 28% ammonia water is added to a mixed solution composed of 70 mL of anhydrous ethanol and 12 mL of deionized water, then 0.4 g of resorcinol and 0.5 mL of 37% formaldehyde solution are added to the system, and stirring is carried out for 40 min; then 3 mL of tetrapropoxysilane is added, and stirring is continued at room temperature for 24 h; after the reaction is completed, the product is centrifuged, washed with water and ethanol in sequence, and then dried to obtain phenolic resin-coated silica microspheres;

[0047] (2) The phenolic resin-coated silica microspheres obtained in step (1) are carbonized at 800°C for 3 h under a nitrogen atmosphere to obtain carbon-coated silica microspheres;

[0048] (3) Then the carbon-coated silica microspheres and magnesium powder are mixed at a mass ratio of 1:0.654, and then placed in a sealed stainless steel reaction kettle, heated to 660°C at a rate of 5°C / min under an argon atmosphere, and kept at 660°C for 360 min to complete the magnesium thermal reduction reaction;

[0049] The reaction product is washed with a 2 M hydrochloric acid solution to remove unreacted magnesium and generated magnesium oxide, then calcined at 750°C for 3 h under an air atmosphere to remove residual carbon, and finally etched with a 2 M hydrofluoric acid solution to remove residual silica, washed with deionized water, and dried to obtain hollow spherical nanometer silicon carbide powder.

[0050] The BET analysis shows that the specific surface area of the powder is 133.746 m 2 / g. The X-ray diffraction analysis shows that the powder is mainly 2H, 3C phase silicon carbide with high purity, as shown in Figure 1 The SEM observation shows that the obtained silicon carbide powder has a spherical structure with a particle size of 492 nm, as shown in Figure 2 The TEM observation of a single hollow silicon carbide sphere shows that it has a significant hollow structure, as shown in Figure 3 Therefore, the above obtained silicon carbide powder is a hollow spherical nano-porous structure.

[0051] The above hollow spherical nano-silicon carbide powder is used for low-temperature sintering to prepare a silicon carbide ceramic, including the following steps:

[0052] The obtained hollow spherical nano-silicon carbide powder is loaded into a graphite mold coated with graphite paper, and is initially compacted at 40 MPa, and then is placed in a discharge plasma sintering furnace, and is heated to 1750°C at a heating rate of 100°C / min, and the uniaxial pressure is gradually increased to 60 MPa, and after holding for 5 minutes, it is naturally cooled to room temperature. After cooling, the ceramic block is taken out, the surface carbon paper is removed, and grinding and polishing are performed, and finally a dense and smooth silicon carbide ceramic is obtained.

[0053] The Archimedes drainage method is used to measure the density of the silicon carbide ceramic, which is 3.1908 g / cm 3 , and the relative density reaches 99.4%. The indentation diagram of the silicon carbide ceramic is shown in Figure 4 , and the Vickers hardness is measured to be 31.13 GPa. As shown in Figure 5 , it is the SEM diagram of the fracture surface of the silicon carbide ceramic. It can be seen that the ceramic fracture structure is dense, the grain distribution is uniform, the particle size is in the nanometer level, and no obvious pores are seen.

[0054] Example 2

[0055] This example is basically the same as Example 1, except that the amount of ammonia water added in step (1) is adjusted to 1.85 mL. The specific surface area of the finally obtained silicon carbide powder is 148.887 m 2 / g. The SEM observation shows that the obtained silicon carbide powder has a spherical structure with a particle size of 294 nm, as shown in Figure 6 The TEM observation of a single hollow silicon carbide sphere shows that it has a hollow structure, as shown in Figure 7 Therefore, the above obtained silicon carbide powder is a hollow spherical nano-porous structure.

[0056] The hollow spherical nano-sized silicon carbide powder is used to prepare silicon carbide ceramic by low-temperature sintering, and the steps and process conditions are the same as those in Example 1. After sintering treatment of the obtained hollow spherical porous silicon carbide powder, the density of the obtained silicon carbide ceramic is 3.1549 g / cm3, and the relative density is 98.59% by Archimedes drainage method. The Vickers indentation of the silicon carbide ceramic is shown in Figure 8 , and the Vickers hardness thereof is measured to be 28.85 GPa. The SEM image of the cross section of the silicon carbide ceramic is shown in Figure 9 , and it can be seen from the image that the ceramic cross section is dense, and the grains are uniform and in nanometer level.

[0057] Example 3

[0058] This example is basically the same as Example 1, and only the amount of ammonia water added in step (1) is adjusted to 7.4 mL. The specific surface area of the obtained silicon carbide powder is 114.443 m 2 / g. The obtained silicon carbide powder is observed by SEM to have a spherical structure, and the particle size is 686 nm as shown in Figure 10 . The single hollow silicon carbide sphere is observed by TEM to have a significant hollow structure as shown in Figure 11 . It can be seen that the obtained silicon carbide powder is a hollow spherical porous structure.

[0059] The hollow spherical nano-sized silicon carbide powder is used to prepare silicon carbide ceramic by low-temperature sintering, and the steps and process conditions are the same as those in Example 1. After sintering treatment of the obtained hollow spherical porous silicon carbide powder, the density of the obtained silicon carbide ceramic is 3.1549 g / cm3, and the relative density is 98.59% by Archimedes drainage method. The Vickers indentation of the silicon carbide ceramic is shown in Figure 12 , and the Vickers hardness thereof is measured to be 28.85 GPa. The SEM image of the cross section of the silicon carbide ceramic is shown in Figure 13 , and it can be seen from the image that the ceramic cross section is dense, and the grains are uniform and in nanometer level.

[0060] Example 4

[0061] The preparation method of the hollow spherical nano-sized silicon carbide powder in this example is the same as that in Example 1, and the difference is only that the sintering conditions for preparing the silicon carbide ceramic by low-temperature sintering are different:

[0062] The hollow spherical nanometer silicon carbide powder obtained in Example 1 was loaded into a graphite mold coated with graphite paper, and was initially compacted at 40 MPa, and then was placed in a discharge plasma sintering furnace, and was raised to 1700°C at a temperature raising rate of 100°C / min, and the uniaxial pressure was maintained to 40 MPa, and after 1 minute of heat preservation, was naturally cooled to room temperature, and after cooling, the ceramic block was taken out, the surface carbon paper was removed, and was ground and polished, and finally a smooth silicon carbide ceramic was obtained.

[0063] The density of the silicon carbide ceramic was determined by the Archimedes drainage method to be 3.0486 g / cm 3 , and the relative density reached 95.26%. The Vickers indentation diagram of the silicon carbide ceramic is shown in Figure 14 , and the Vickers hardness thereof was measured to be 25.01 GPa. The SEM diagram of the fracture surface of the silicon carbide ceramic is shown in Figure 15 , and it can be seen that the grains are small and uniformly distributed, but there are a small amount of pores.

[0064] Example 5

[0065] The preparation method of the hollow spherical nanometer silicon carbide powder in this example is the same as that in Example 1, and the only difference is that the sintering conditions for sintering the silicon carbide ceramic at low temperature are different:

[0066] The hollow spherical nanometer silicon carbide powder obtained in Example 1 was loaded into a graphite mold coated with graphite paper, and was initially compacted at 40 MPa, and then was placed in a discharge plasma sintering furnace, and was raised to 1800°C at a temperature raising rate of 100°C / min, and the uniaxial pressure was gradually raised to 70 MPa, and after 10 minutes of heat preservation, was naturally cooled to room temperature, and after cooling, the ceramic block was taken out, the surface carbon paper was removed, and was ground and polished, and finally a dense and smooth silicon carbide ceramic was obtained.

[0067] The density of the silicon carbide ceramic was determined by the Archimedes drainage method to be 3.1943 g / cm 3 , and the relative density reached 99.82%. The indentation diagram of the silicon carbide ceramic is shown in Figure 16 , and the Vickers hardness thereof was measured to be 30.59 GPa. The SEM diagram of the fracture surface of the silicon carbide ceramic is shown in Figure 17 , and it can be seen that the structure of the ceramic fracture surface is dense, and the grains are uniformly distributed, and the grain size increases with the increase of the sintering temperature and the extension of the heat preservation time.

[0068] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present application and not to limit, and although the present application is described in detail with reference to the examples, those skilled in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present application, and they should be covered in the scope of the claims of the present application.

Claims

1. A method for preparing hollow sphere-shaped nanosized silicon carbide powder, characterized by, The method comprises the following steps: (1) placing resorcinol and formaldehyde solution in an alkaline alcohol-water mixed solution to perform a condensation reaction for 40-60 min, then adding tetraalkoxysilane to perform a hydrolytic condensation reaction, and separating, washing and drying the total condensation reaction product to obtain phenolic resin-coated silica microspheres; (2) performing carbonization treatment on the phenolic resin-coated silica microspheres obtained in step (1) in an inert atmosphere to obtain carbon-coated silica microspheres; (3) mixing the carbon-coated silica microspheres obtained in step (2) with magnesium powder, and performing a magnesium thermal reduction reaction in a protective atmosphere, and obtaining a hollow spherical nano silicon carbide powder through post-processing, wherein the particle size of the hollow spherical nano silicon carbide powder is 280-720 nm, and the specific surface area is 110-150 m 2 / g. In step (1), the pH value of the alkaline alcohol-water mixed solution is 8-12. In step (1), the mass ratio of resorcinol to formaldehyde solution is 1:1.2-1.5, the mass fraction of the formaldehyde solution is 30%-40%, and the mass ratio of tetraalkoxysilane to resorcinol is 6-9:

1.

2. The method of claim 1, wherein the hollow spherical nanocarbons are carbonized silicon powder. In step (1), in the alcohol-water mixed solution, the volume ratio of anhydrous ethanol to deionized water is 1:0.1-0.

3.

3. The method for preparing hollow spherical nano-silicon carbide powder according to claim 1, characterized in that, In step (1), the tetraalkoxysilane comprises one or more of tetramethoxysilane, tetraethoxysilane and tetrapropoxysilane.

4. The method of claim 1, wherein the hollow spherical nanocarbons are carbonized silicon powder. In step (2), the carbonization treatment is performed at 700-800°C under a nitrogen atmosphere, and the holding time is 3-6 hours.

5. The method for preparing hollow spherical nano-silicon carbide powder according to claim 1, characterized in that, In step (3), the magnesium thermal reduction reaction is performed by mixing the carbon-coated silica microspheres with magnesium powder at a mass ratio of 1:0.6-0.8, then loading the mixture into a sealed stainless steel reaction kettle, and performing the reaction at 650-850°C under an argon atmosphere, and the holding time is 4-8 hours.

6. The method of claim 1, wherein the hollow sphere-shaped nanosized silicon carbide powder is prepared by the steps of: preparing a mixture of a silicon source and a carbon source; and heating the mixture to a temperature of 1,000°C to 1,500°C in an inert gas atmosphere. In step (3), the post-treatment comprises the following steps: acid washing with a 0.5-2 M hydrochloric acid solution, calcining the acid washing product at 600-800°C in an air atmosphere after drying, then etching with a 0.5-2 M hydrofluoric acid solution, and obtaining the hollow spherical nano silicon carbide powder after washing and drying.

7. A hollow sphere-shaped nanosized silicon carbide powder, characterized by The method is prepared by any one of claims 1-6.

8. Use of the hollow sphere-shaped nanocrystalline silicon carbide powder according to claim 7, characterized in that The hollow spherical nano silicon carbide powder is applied in low-temperature sintering to prepare silicon carbide ceramics, comprising the following steps: loading the hollow spherical nano silicon carbide powder into a graphite mold, placing the graphite mold in a spark plasma sintering furnace, and performing sintering under vacuum conditions, the sintering pressure is 40-70 MPa, the sintering temperature is controlled between 1700-1800°C, the heating rate is 50-200°C / min, the holding time is 1-10 min, and the silicon carbide ceramic material is obtained after grinding after sintering is completed.

9. Use of hollow sphere-shaped nanocrystalline silicon carbide powder according to claim 8, characterized in that The density of the silicon carbide ceramic material is 3.0-3.2 g / cm 3 , the relative density reaches 95%-99.9%, and the Vickers hardness is 25-33 GPa.

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