Preparation method of boron carbide ceramic for nuclear protection

Abstract

The invention belongs to the technical field of ceramic preparation, and relates to a preparation method of boron carbide ceramic for nuclear protection, which comprises the following steps: S1, using epoxy resin and boron carbide to obtain a mixed green body; s2, the mixed blank and boron oxide are placed in two different temperature zones of a double-temperature-zone high-temperature furnace correspondingly, and vacuumizing is conducted; s3, heating the mixed green body for degreasing and carbonization reaction; and S4, heating the mixed green body to a sintering temperature, heating boron oxide to a volatilization temperature, continuously vacuumizing from the end of the mixed green body for reaction, and obtaining the boron carbide ceramic for nuclear protection after the reaction is finished. Sintering densification is promoted in the mode that B2O3 steam reacts with carbon to generate B4C, no by-product is generated, the obtained product is high in purity and adjustable in density, and the process is simple, convenient and easy to implement.

Description

Technical Field

[0001] The invention belongs to the technical field of ceramic preparation, and in particular relates to a method for preparing boron carbide ceramics for nuclear protection. Background Art

[0002] Boron carbide (B 4 C) is a hexagonal structure (space group: ) low density (about 2.52g / cm 3 ) compounds, which have the advantages of ultra-high hardness (50GPa) second only to diamond and cubic boron nitride, high melting point (2450℃) and good physical and chemical stability, are widely used in protective armor, wear-resistant coatings, high-temperature industrial nozzles, and aero-engine blades. In addition, B 4 C ceramics have good thermal conductivity (30~50W / m·K), low thermal expansion (4.5×10 -6 ℃) and good neutron absorption performance (boron isotopes 10 B has a thermal neutron absorption cross section of 1000b, making it an ideal choice for neutron absorption materials for nuclear control rods and protective materials for key equipment such as nuclear power plants and nuclear reactors. 4 The excellent comprehensive properties of C ceramics make them important for application in many industrial fields. 4 The preparation technology of C ceramics has always received widespread attention.

[0003] B 4 Although C ceramics have the above excellent properties, 4 The low diffusion coefficient, high melting point, weak grain boundary sliding ability and the presence of an oxide layer on the surface of C make its sintering preparation relatively difficult. 4 The main preparation methods of C ceramics are pressureless sintering, hot pressing sintering and spark plasma sintering (SPS). Among them, pressureless sintering is carried out under normal pressure or vacuum conditions and has the advantage of producing ceramics with complex shapes without relying on post-processing, but pure B 4 Pressureless sintering of C is extremely difficult and needs to be carried out at a high temperature above 2200℃. The abnormal growth of grains caused by excessively high temperatures results in low density. Although adding sintering aids (carbon, boride, etc.) can reduce the sintering temperature and increase the density, impurities are usually introduced and affect the purity. Compared with pressureless sintering, hot pressing sintering can improve B 4 C ceramic density, reduce sintering temperature and partially inhibit abnormal grain growth, but the equipment requirements are extremely high, the production efficiency is low, and the production capacity of complex-shaped ceramics is weak due to the influence of molds. SPS has the advantages of fast sintering and high efficiency, but it cannot prepare large-sized and complex-shaped samples. 4 For C ceramics, large nuclear protection materials require a higher density (density> 2.2g / cm 3) to have good protection performance, and the neutron absorption core block needs to have a suitable density (2.0~2.1g / cm 3 ) and high purity, and for the pursuit of small and lightweight nuclear reactors, the nuclear protection materials of key equipment need to be lightweight (expected density <2.0g / cm 3 ), but the above main preparation methods are difficult to achieve high purity and density control at the same time, so the development of B 4 The new preparation technology of C ceramics is of great significance. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a method for preparing boron carbide ceramics for nuclear protection, which can effectively solve the problems of low density and purity of high-temperature sintered boron carbide ceramics.

[0005] The specific technical solutions are as follows: A method for preparing boron carbide ceramics for nuclear protection comprises the following steps: S1 uses epoxy resin and boron carbide to obtain a mixed green body; S2 placing the mixed green body and boron oxide in two different temperature zones of a double-temperature zone high temperature furnace and evacuating the furnace; S3 heating the mixed body to perform degreasing and carbonization reactions; S4: heating the mixed green body to a sintering temperature, heating the boron oxide to a volatilization temperature, and continuously evacuating the mixed green body end to react, and obtaining boron carbide ceramics for nuclear protection after the reaction is completed.

[0006] Specifically, in step S1, epoxy resin and boron carbide are mixed, and a mixed body of epoxy resin and boron carbide is obtained by granulation and pressing. Among them, epoxy resin is an excellent granulation binder with a high carbon content (about 70%). Using epoxy resin as a carbon source can achieve granulation and carbon introduction in one step, and no new impurities need to be introduced during the blank making process; boron carbide is preferably a powder with an average particle size of 0.5~3μm, which can not only regulate the density of boron carbide ceramics, but also ensure uniform granulation. In order to achieve B 4 The density of C ceramic and the content of boron carbide can be controlled, and the amount of epoxy resin added needs to be reasonably controlled. Preferably, in step S1, the mass ratio of epoxy resin to boron carbide is (0.01-0.3):1, and the epoxy equivalent of the epoxy resin is 100-700 g / eq. Preferably, the epoxy resin is a bisphenol A epoxy resin with an epoxy equivalent of 300-600 g / eq.

[0007] The present invention uses boron oxide as a boron source, introduces boron oxide vapor into the mixed body to fully react with carbon, and in order to ensure that the carbon in the body fully reacts, the boron oxide should be excessive. Preferably, in step S2, the molar ratio of the total amount of carbon in the epoxy resin in the mixed body to the total amount of boron in the boron oxide is 1:(0.05-0.4); in step S2, the vacuum degree can be 10Pa, which is conducive to the degreasing and carbonization of the mixed body in step S3.

[0008] To ensure the degreasing and carbonization effects, the temperature and time need to be reasonably controlled. Preferably, in step S3, the degreasing and carbonization reaction temperature is 700-1500°C, the time is 1-12 hours, and the heating rate is 2-10°C / min; more preferably, the degreasing and carbonization reaction temperature is 800-1500°C, and the time is 2-10 hours.

[0009] To ensure B 2 O 3 Fully volatilized, B 2 O 3 Steam reacts fully with carbon to achieve controllable adjustment of ceramic density. Both sintering and volatilization temperatures should be reasonably controlled. Preferably, in step S4, the sintering temperature is 1500-2400°C, the sintering heating rate is 2-10°C / min, and the volatilization temperature is 800-1800°C; more preferably, the sintering temperature is 1600-2400°C, and the volatilization temperature is 1000-1800°C. To achieve B 4 The density of C ceramic can be controlled and the reaction time needs to be reasonably controlled. Preferably, in step S4, the reaction time is 1 to 12 hours, and more preferably, the reaction time is 2 to 10 hours. During the reaction of the mixed green body and boron oxide, vacuum is continuously drawn from the end of the mixed green body, and the boron oxide vapor can pass through the green body with the air flow and fully react with carbon, and the vacuum environment can promote B 2 O 3 of volatilization.

[0010] The beneficial effects of the present invention are as follows: (1) The present invention is based on B 2 O 3 The boron source and epoxy resin were used as carbon source. The degreasing and carbonization were first carried out by heating and calcining in a vacuum environment. Then, the B 2 O 3 Volatilize, use B 2 O 3 Steam reacts with carbon to produce B 4 C, which can promote the sintering of ceramics while ensuring that no other phases are generated, thus achieving high abundance B 4 C ceramics can be prepared, and the porosity can be controlled by adjusting the carbon content and sintering process, which is a high abundance and density controllable B 4 The preparation of C ceramics provides a new idea.

[0011] (2) The present invention is based on B 2 O 3 Steam reacts with carbon to produce B 4 C method promotes sintering densification, no by-products are generated, the obtained product has high purity, the density is adjustable, and the process is simple and easy; the present invention ensures the high abundance of boron carbide by controlling the epoxy resin content, reaction temperature and time, and realizes the regulation of pore structure and density. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is the XRD diagram of the boron carbide ceramic for nuclear protection in Example 2; Figure 2 This is the XRD diagram of the boron carbide ceramic for nuclear protection in comparative example 2; Figure 3 This is a SEM image of the boron carbide ceramic for nuclear protection in Example 2; Figure 4 This is the SEM image of the boron carbide ceramic for nuclear protection in comparative example 2. DETAILED DESCRIPTION

[0013] The principles and features of the present invention are described below in conjunction with examples, which are only used to explain the present invention and are not used to limit the scope of the present invention. The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, etc. used in the following examples, unless otherwise specified, can all be obtained from commercial sources. Example 1

[0014] A method for preparing boron carbide ceramics for nuclear protection comprises the following steps: S1. Mix bisphenol A epoxy resin with an epoxy equivalent of 700 g / eq and boron carbide powder with an average particle size of 0.5 μm at a mass ratio of 0.01:1, granulate, and then press the granulated powder into a mixed green body with a size of φ20×10 mm through a steel die; S2. Place the mixed body and excess boron oxide in the high and low temperature zones of a dual-temperature zone tubular high-temperature furnace, respectively, and start a mechanical pump to draw the vacuum degree in the furnace to 10 Pa; wherein the molar ratio of the total amount of carbon in the epoxy resin to the total amount of boron in the boron oxide is 1:0.1; S3: raising the temperature of the high temperature zone to 1000°C at a heating rate of 8°C / min and keeping the temperature for 6 hours to heat the mixed green body separately for degreasing and carbonization reaction; S4 After degreasing and carbonization are completed, the temperature of the high temperature zone is raised to 1600°C at a heating rate of 2°C / min to heat and sinter the mixed green body, and the temperature of the low temperature zone is raised to 1000°C to heat the boron oxide to promote B 2 O 3 Volatilize, and continue to vacuum from the high temperature area port to make B2 O 3 Steam passes through the body and fully reacts with carbon to produce B 4 C, keep warm for 6 hours, and after the reaction is completed, boron carbide ceramics for nuclear protection are obtained. Example 2

[0015] A method for preparing boron carbide ceramics for nuclear protection comprises the following steps: S1. Mix bisphenol A epoxy resin with an epoxy equivalent of 600 g / eq and boron carbide powder with an average particle size of 2 μm at a mass ratio of 0.05:1, granulate, and then press the granulated powder into a mixed green body with a size of φ20×10 mm through a steel die; S2. Place the mixed body and excess boron oxide in the high and low temperature zones of a dual-temperature zone tubular high-temperature furnace, respectively, and start a mechanical pump to draw the vacuum degree in the furnace to 10 Pa; wherein the molar ratio of the total amount of carbon in the epoxy resin to the total amount of boron in the boron oxide is 1:0.1; S3: raising the temperature of the high temperature zone to 1500°C at a heating rate of 8°C / min and keeping the temperature for 12 hours to heat the mixed body separately for degreasing and carbonization reaction; S4 After degreasing and carbonization are completed, the temperature of the high temperature zone is raised to 2400℃ at a heating rate of 2℃ / min to heat and sinter the mixed green body, and the temperature of the low temperature zone is raised to 1800℃ to heat the boron oxide to promote B 2 O 3 Volatilize, and continue to vacuum from the high temperature area port to make B 2 O 3 Steam passes through the body and fully reacts with carbon to produce B 4 C, keep warm for 8h, and after the reaction is completed, boron carbide ceramics for nuclear protection are obtained. Example 3

[0016] A method for preparing boron carbide ceramics for nuclear protection comprises the following steps: S1. Mix bisphenol A epoxy resin with an epoxy equivalent of 300 g / eq and boron carbide powder with an average particle size of 3 μm at a mass ratio of 0.1:1, granulate, and then press the granulated powder into a mixed green body with a size of φ20×10 mm through a steel die; S2. Place the mixed body and excess boron oxide in the high and low temperature zones of a dual-temperature zone tubular high-temperature furnace, respectively, and start a mechanical pump to draw the vacuum degree in the furnace to 10 Pa; wherein the molar ratio of the total amount of carbon in the epoxy resin to the total amount of boron in the boron oxide is 1:0.4; S3: raising the temperature of the high temperature zone to 700°C at a heating rate of 8°C / min and keeping the temperature for 1h to heat the mixed body separately for degreasing and carbonization reaction; S4 After degreasing and carbonization are completed, the temperature of the high temperature zone is raised to 2100°C at a heating rate of 2°C / min to heat and sinter the mixed green body, and the temperature of the low temperature zone is raised to 1200°C to heat the boron oxide to promote B 2 O 3 Volatilize, and continue to vacuum from the high temperature area port to make B 2 O 3 Steam passes through the body and fully reacts with carbon to produce B 4 C, keep warm for 1h, and after the reaction is completed, boron carbide ceramics for nuclear protection are obtained. Example 4

[0017] A method for preparing boron carbide ceramics for nuclear protection comprises the following steps: S1. Mix bisphenol A epoxy resin with an epoxy equivalent of 100 g / eq and boron carbide powder with an average particle size of 1 μm at a mass ratio of 0.3:1, granulate, and then press the granulated powder into a mixed green body with a size of φ20×10 mm through a steel die; S2. Place the mixed body and excess boron oxide in the high and low temperature zones of a dual-temperature zone tubular high-temperature furnace, respectively, and start a mechanical pump to draw the vacuum degree in the furnace to 10 Pa; wherein the molar ratio of the total amount of carbon in the epoxy resin to the total amount of boron in the boron oxide is 1:0.05; S3: raising the temperature of the high temperature zone to 1000°C at a heating rate of 8°C / min and keeping the temperature for 10 hours to heat the mixed green body separately for degreasing and carbonization reaction; S4 After degreasing and carbonization are completed, the temperature of the high temperature zone is raised to 1500°C at a heating rate of 2°C / min to heat and sinter the mixed green body, and the temperature of the low temperature zone is raised to 800°C to heat the boron oxide to promote B 2 O 3 Volatilize, and continue to vacuum from the high temperature area port to make B 2 O 3 Steam passes through the body and fully reacts with carbon to produce B 4 C, keep warm for 12 hours, and after the reaction is completed, boron carbide ceramics for nuclear protection are obtained. Comparative Example 1

[0018] The specific steps are as shown in Example 1, except that in step S1, the mass ratio of epoxy resin to boron carbide powder is 0.35:1. Comparative Example 2

[0019] The specific steps refer to Example 2, except that: in step S4, the sintering temperature of the mixed green body is 1400° C., and the other operations remain unchanged. Comparative Example 3

[0020] The specific steps are as in Example 1, except that: in step S3, the degreasing and carbonization temperature is 1600°C and the time is 4 hours; in step S4, the sintering temperature of the mixed green body is 1600°C, the boron oxide volatilization temperature is 1600°C, and the holding time is 8 hours. Characterization and testing

[0021] The boron carbide ceramics for nuclear protection prepared in Example 2 and Comparative Example 2 were subjected to phase characterization analysis. Figure 1~2 The XRD diagrams of boron carbide ceramics for nuclear protection in Example 2 and Comparative Example 2 are shown respectively. Figure 1~2 It can be seen that the boron carbide ceramic prepared in Example 2 has a high boron carbide content, and the boron carbide ceramic prepared in Comparative Example 2 contains a large amount of free carbon and a low amount of boron carbide. Therefore, the boron carbide ceramic prepared by the method of the present application has a high boron carbide content and high purity.

[0022] The boron carbide ceramics for nuclear protection prepared in Example 2 and Comparative Example 2 were subjected to morphological characterization and analysis. Figure 3~4 The following are SEM images of boron carbide ceramics for nuclear protection of Example 2 and Comparative Example 2. Figure 3~4 It can be seen that the boron carbide ceramic prepared in Example 2 is almost completely dense, while the boron carbide ceramic prepared in Comparative Example 2 contains a large number of pores.

[0023] The porosity, density and boron carbide content of the boron carbide ceramics prepared in Examples 1 to 4 and Comparative Examples 1 to 3 are compared, as shown in Table 1.

[0024] The density and porosity of boron carbide ceramics were measured by the Archimedes drainage method and bulk density method in the national standard GB / T25995-2010, respectively; the boron carbide content was determined by Rietveld refinement of XRD data using TOPAS software.

[0025] Table 1 Porosity, density and boron carbide content of boron carbide ceramics <![CDATA[Density (g / cm 3 )]]> Boron carbide content (wt%) Porosity Example 1 2.02 95 19.8% Example 2 2.41 98 4.4% Example 3 2.36 98 6.3% Example 4 1.88 87 25.4% Comparative Example 1 1.67 41 33.7% Comparative Example 2 1.82 48 27.8% Comparative Example 3 2.01 72 20.2% As shown in Table 1, compared with Comparative Examples 1 to 3, the boron carbide ceramics obtained by the method of the present invention have high purity, a boron carbide content greater than 87wt%, and a density of 1.88 to 2.41 g / cm 3 It can be adjusted within a wide range, suitable for B 4 C density requires different application fields; and compared with comparative examples 1 to 3, the present invention can effectively reduce the porosity of boron carbide ceramics by adjusting the carbon content, sintering process, etc.

[0026] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing boron carbide ceramics for nuclear protection, characterized in that: The steps include: S1 uses epoxy resin and boron carbide to obtain a mixed green body; S2 placing the mixed green body and boron oxide in two different temperature zones of a double-temperature zone high temperature furnace and evacuating the furnace; S3 heating the mixed body to perform degreasing and carbonization reactions; S4: heating the mixed green body to a sintering temperature, heating the boron oxide to a volatilization temperature, and continuously evacuating the mixed green body end to react, and obtaining boron carbide ceramics for nuclear protection after the reaction is completed; Among them, the sintering temperature is 1500~2400℃, and the volatilization temperature is 800~1800℃.

2. The preparation method according to claim 1, characterized in that: In step S1, the mass ratio of epoxy resin to boron carbide is (0.01-0.3):1, and the epoxy equivalent of the epoxy resin is 100-700 g / eq.

3. The preparation method according to claim 2, characterized in that: In step S1, the epoxy resin is a bisphenol A epoxy resin with an epoxy equivalent of 300-600 g / eq.

4. The preparation method according to claim 1, characterized in that: In step S1, epoxy resin and boron carbide are mixed, and a mixed body of epoxy resin and boron carbide is obtained through granulation and pressing.

5. The preparation method according to claim 4, characterized in that: In step S1, the boron carbide is a powder with an average particle size of 0.5-3 μm.

6. The preparation method according to claim 1, characterized in that: In step S2, the molar ratio of the total amount of carbon in the epoxy resin in the mixed body to the total amount of boron in the boron oxide is 1:(0.05-0.4).

7. The preparation method according to claim 1, characterized in that: In step S3, the reaction temperature is 700-1500° C. and the reaction time is 1-12 hours.

8. The preparation method according to claim 1, characterized in that: In step S4, the sintering temperature is 1600-2400°C, and the volatilization temperature is 1000-1800°C.

9. The preparation method according to claim 1, characterized in that: In step S4, the reaction time is 1 to 12 hours.

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