Preparation method of high-purity boron carbide spheres for nuclear protection and high-purity boron carbide spheres

Abstract

The present invention belongs to the technical field of ceramic preparation, and particularly relates to a preparation method of high-purity boron carbide spheres for nuclear protection and the high-purity boron carbide spheres. The preparation method comprises the following steps: S1. Mix boron carbide powder and graphite powder; S2. Press the mixed powder into a rod-shaped green body; Place the green body-forming mold containing the green body in a heating container and wait for sintering; S3. Heat up to 1010-1130 °C, introduce a mixed gas of inert gas and boron trichloride into the heating container, control the air pressure at 1000-1500 Pa, and keep warm for 5-8 h to obtain a boron carbide rod; S4. Process the boron carbide rod into a semi-finished boron carbide sphere; S5. Heat up to 1920-2010 °C, and keep the boron carbide sphere warm for 2-3 h under the atmosphere of inert gas and hydrogen. The present invention solves the problems existing in the boron carbide spheres prepared in the prior art, such as great forming difficulty, high manufacturing cost, low production efficiency, and low compressive strength.

Description

Technical Field

[0001] The invention belongs to the technical field of ceramic preparation, and in particular relates to a preparation method of high-purity boron-carbon balls for nuclear protection and the high-purity boron-carbon balls. Background Art

[0002] A meltdown accident is one of the most serious potential risks in the nuclear energy field. When a meltdown occurs, the core temperature will rise sharply, causing the nuclear fuel to melt, releasing a large amount of high-intensity radiation such as neutrons and gamma rays, causing a devastating disaster to the surrounding environment and personnel life safety. In the protection system for meltdown accidents, efficient neutron absorbing materials are crucial. Boron-carbon materials have attracted much attention due to the high absorption characteristics of boron elements for neutrons. However, the existing boron-carbon materials for nuclear protection are mostly block or plate-shaped. When a meltdown accident occurs, it is difficult for block or plate-shaped boron-carbon materials to fill the complex space inside the core. In particular, when the boron-carbon material cannot fit the key parts around the fuel rods and the bottom of the core, it will cause protection loopholes, resulting in a large amount of neutron leakage, accelerating the deterioration of the meltdown accident. Recent studies have shown that making boron-carbon materials into spheres can effectively solve the above problems, but the existing technology cannot solve the many problems existing in the molding and firing of boron-carbon balls.

[0003] In terms of molding, mold closing is a conventional technology for preparing ceramic balls. Since each ceramic ball must go through a complex process of filling, mold closing, pressurizing, and demolding, the production efficiency is low. In addition, in order to ensure the sphericity of the ceramic ball, the mold needs to have a precise hemispherical cavity. On the one hand, this hemispherical cavity is very difficult to process and has high manufacturing costs. On the other hand, due to the extremely high hardness of boron carbide powder, the cavity will soon wear out after frequent mold closing, pressurizing, and demolding, which seriously shortens the service life of the mold. Moreover, frequent mold replacement will further increase production costs and lead to frequent interruptions in the production process.

[0004] In terms of firing, high-temperature sintering is a conventional method for preparing boron carbide ceramics. However, given that boron-carbon balls contain a large amount of graphite and a certain porosity, even hot pressing sintering is not enough to meet the compressive strength index of boron-carbon balls, causing boron-carbon balls to be easily damaged during transportation, storage and use. Although adding sintering aids can improve the compressive strength of boron-carbon balls to a certain extent, the actual effect of this approach is not ideal, and the introduction of sintering aids will inevitably reduce the purity of boron-carbon balls, thereby affecting their neutron absorption effect. In addition, in order to prevent the boron-carbon balls from collapsing and deforming during high-temperature sintering, the boron-carbon ball blank must be placed in the hemispherical groove of the graphite tray. Since each pit can only hold one boron-carbon ball blank, the output of boron-carbon balls per furnace is seriously restricted.

[0005] In view of the series of problems listed above, the application and promotion of boron carbon balls in the field of nuclear protection has never been realized. Summary of the Invention

[0006] In view of the deficiencies of the above-mentioned existing technologies, the present invention provides a preparation method of high-purity boron carbide balls for nuclear protection and the high-purity boron carbide balls, solving the problems of large forming difficulty, high manufacturing cost, low production efficiency, and low compressive strength existing in the boron carbide balls prepared by the existing technologies.

[0007] The specific technical solutions are as follows:

[0008] One of the objectives of the present invention is to provide a preparation method of high-purity boron carbide balls for nuclear protection, including the following steps:

[0009] S1. Mixing materials: Mix boron carbide powder and graphite powder evenly to obtain a mixed powder material.

[0010] S2. Forming a blank: Place the mixed powder material obtained in step S1 into a cylindrical blank-forming mold and press it into a rod-shaped blank; place the blank-forming mold containing the rod-shaped blank into a heating container and wait for sintering.

[0011] S3. Sintering: Raise the temperature to 1010 - 1130 °C, introduce a mixed gas of inert gas and boron trichloride into the heating container, control the internal air pressure of the heating container at 1000 - 1500 Pa, and keep it warm for 5 - 8 h to obtain a boron carbide rod with elemental boron deposited inside.

[0012] S4. Processing: Process the boron carbide rod obtained in step S3 into a semi-finished boron carbide ball.

[0013] S5. Re-sintering: Raise the temperature to 1920 - 2010 °C, and under the atmosphere of inert gas and hydrogen, keep the boron carbide ball obtained in step S4 warm for 2 - 3 h to obtain the finished boron carbide ball.

[0014] The principle of the above preparation method is as follows: In step S3, the elemental boron generated by the high-temperature decomposition of boron trichloride is deposited inside the blank, so that the originally loosely packed boron carbide and graphite particles are firmly bonded together, ensuring the smooth progress of subsequent machining of the boron carbide rod; in step S5, the elemental boron in the semi-finished boron carbide ball reacts with graphite to generate boron carbide, so that the finished boron carbide ball only contains two components, boron carbide and graphite, and further improves the strength of the finished boron carbide ball.

[0015] Furthermore, in step S1: The average particle size of the boron carbide powder is preferably 2 - 5 μm, and the average particle size of the graphite powder is preferably 0.3 - 0.6 μm.

[0016] Furthermore, in step S1: The mass ratio of the boron carbide powder to the graphite powder is preferably 1:(1 - 9).

[0017] Furthermore, in step S2: Axial die pressing of the mixed powder material in the blank-forming mold is preferably carried out under a pressure of 2 - 5 MPa.

[0018] Further, in step S2: The blank-making mold needs to be made of a high thermal conductivity material and has openings at both ends, preferably a graphite tube, especially a graphite cylinder.

[0019] Further, in step S2: The heating container is preferably a quartz tube. When the cylindrical blank-making mold is placed in the heating container, the outer wall of the cylindrical blank-making mold should be in close contact with the inner wall of the heating container.

[0020] Further, in step S3: In the mixed gas of inert gas and boron trichloride, the molar ratio of inert gas to boron trichloride is preferably controlled at 1:(0.2 - 0.4).

[0021] Further, in step S3: The flow rate of the mixed gas of inert gas and boron trichloride is preferably controlled at 130 - 160 mL / min.

[0022] Further, in step S3: The gas is introduced from one end (the inlet end) of the heating container, and the other end (the outlet end) is evacuated to maintain the air pressure. The temperature of the blank-making mold is increased from 1010 - 1040 °C at the inlet end to 1100 - 1130 °C at the outlet end. Controlling the temperature of the blank-making mold (i.e., the graphite tube) from 1010 - 1040 °C at the inlet end to 1100 - 1130 °C at the outlet end is to make the boron deposition amounts at the front section (the inlet end) and the rear section (the outlet end) of the boron carbide rod produced the same, so as to ensure that the boron carbide balls processed from the front section and the rear section of the boron carbide rod have the same composition. Specifically, step S3 can be implemented using the device disclosed in the Chinese patent "A Preparation Method and Device for a High-Purity Porous Boron Carbide Control Rod" with the application number CN202510479801.9. The graphite sleeve in this patent application corresponds to the graphite cylinder in the present invention. When using this device, the temperature of the front section of the graphite cylinder is preferably 1010 - 1040 °C, the temperature of the middle section is 1050 - 1080 °C, and the temperature of the rear section is 1100 - 1130 °C.

[0023] Further, in step S3: Preferably during the heat preservation process, for every 1 h increase in time, the flow rate of the mixed gas is reduced by 8 - 10 mL / min. The above technical solution is to ensure that the flow rate of the mixed gas is adapted to the air pressure inside the quartz tube, so as to prevent the front part of the quartz tube from bursting due to excessive air pressure caused by the pores of the blank being blocked.

[0024] Further, in step S3: During the heating-up process, an inert gas is introduced into the heating container. The flow rate of the inert gas is preferably 130 - 160 mL / min, and the air pressure inside the heating container is preferably maintained at 2000 - 3000 Pa.

[0025] Further, in step S3: after heat preservation, cooling is carried out; during the cooling process, an inert gas with a flow rate of 60 - 90 mL / min is introduced into the heating container, the internal air pressure of the heating container is controlled at 5000 - 8000 Pa, the heating of the blank - making mold is stopped to allow it to cool naturally; when the temperature of the blank - making mold drops below 300 °C, the introduction of the inert gas into the heating container is stopped, the internal air pressure of the heating container is restored to normal pressure, and when the temperature of the blank - making mold drops below 100 °C, the blank - making mold is taken out to obtain a boron - carbon rod with elemental boron deposited inside.

[0026] Further, in step S4: a ball - making machine is used to process the boron - carbon rod into a sphere, and the cutter head of the ball - making machine is preferably made of boron carbide. The semi - finished boron - carbon rod has components of elemental boron, boron carbide, and graphite; when using a ball - making machine to process the boron - carbon rod into a sphere, the cutter head of the ball - making machine is made of boron carbide, and the powder ground from the boron - carbon rod also only contains elemental boron, boron carbide, and graphite. After recycling these powders, they can be used as raw materials repeatedly, thus avoiding the problem of raw material waste.

[0027] Further, in step S5: the molar ratio of the inert gas to hydrogen is controlled to be 1:(0.8 - 1.1).

[0028] Further, in step S5: during the heating - up process, an inert gas is introduced.

[0029] Specifically, in step S5: the semi - finished boron - carbon balls are laid flat on a graphite tray, and the graphite tray is placed in a high - temperature furnace for sintering.

[0030] Further, in each of the above steps, the inert gas is preferably argon.

[0031] The second object of the present invention is to provide a high - purity boron - carbon sphere obtained by using the above preparation method.

[0032] The beneficial effects of the present invention are as follows:

[0033] (1) In the chemical vapor deposition of boron carbide process, methane and hydrogen need to be introduced into the mixed gas. These two gases are flammable and explosive, greatly increasing the risk level of the equipment, and the boron carbide deposited by chemical vapor deposition has extremely high hardness, making the subsequent machining difficulty of the boron - carbon rod increase sharply. The elemental boron deposited in the present invention is in an amorphous state with relatively low hardness. Amorphous boron carbide can bond the particles of the blank together without overly increasing the hardness of the blank, thus ensuring the smooth progress of the subsequent machining of the boron - carbon rod. The finished product rate of the boron - carbon sphere processing in the present invention can basically reach 100%.

[0034] (2) By using the method of the present invention, the mixed gas can be forced to flow through the pores of the green body, and elemental boron can be uniformly deposited on the surfaces of the particles inside the green body. During the high-temperature sintering stage, boron carbide generated by the reaction of elemental boron and graphite can tightly wrap the graphite particles. On the one hand, it significantly improves the compressive strength of the boron-carbon balls, avoiding damage to the boron-carbon balls during transportation, storage, and use. On the other hand, it reduces the shrinkage rate of the green body during high-temperature sintering, thereby enabling the boron-carbon balls to maintain good sphericity.

[0035] (3) By using the method of the present invention, by adjusting the inner diameter of the graphite cylinder, controlling the deposition process parameters of elemental boron, and reasonably controlling the sintering temperature, boron-carbon balls with diameters and porosities meeting actual requirements can be accurately prepared.

[0036] (4) The method of the present invention avoids the problems of low production efficiency in the die closing process and high production costs caused by frequent die replacement. For semi-finished boron-carbon balls, although their strength is not outstanding, their supporting force is sufficient to ensure no collapse at high temperatures. Therefore, during high-temperature sintering, it is not necessary to place the semi-finished boron-carbon balls in spherical pits, effectively solving the problem of low loading per furnace and significantly increasing the output of boron-carbon balls per furnace.

[0037] (5) The semi-finished boron-carbon rods are composed of elemental boron, boron carbide, and graphite. When using a ball mill to process the boron-carbon rods into spheres, the cutter head of the ball mill is made of boron carbide, and the powder ground from the boron-carbon rods also contains only elemental boron, boron carbide, and graphite. These powders can be recycled as raw materials, thus avoiding waste of raw materials.

[0038] (6) If directly using the high-temperature sintering method to prepare boron-carbon balls, the temperature needs to be raised to 2100 - 2200 °C, and binders and sintering aids need to be added to the raw materials. The high-temperature conditions increase the energy consumption cost. Using binders and sintering aids not only increases the raw material cost but also reduces the purity of the boron-carbon balls. The sintering temperature of the method of the present invention is only 1920 - 2010 °C, and there is no need to add binders and sintering aids to the raw materials, which not only reduces the energy consumption cost but also improves the purity of the boron-carbon balls. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the preparation device of the high-purity porous boron carbide nuclear control rod used in the specific embodiment of the present invention;

[0040] Figure 2 It is a schematic diagram of the reaction chamber structure of the preparation device of the high-purity porous boron carbide nuclear control rod used in the specific embodiment of the present invention;

[0041] Figure 3 It is a microstructural photograph of the high-purity boron-carbon balls prepared in Example 1 of the present invention;

[0042] Figure 4 Macrophotograph of the high-purity boron carbide spheres prepared in Example 1 of the present invention;

[0043] In the figure: 1 - Flow rate adjustment module; 2 - Temperature control module; 3 - Pressure adjustment module; 4 - Reaction chamber; 5 - Graphite cylinder; 6 - Quartz tube; 7 - Valve. Detailed implementation manners

[0044] The principles and features of the present invention are described below in conjunction with examples. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0045] Using the preparation method of the present invention, boron carbide spheres with different diameters can be prepared by changing the inner diameter of the graphite cylinder. For the sake of convenience of description, in the detailed implementation manners, the inner diameter of the graphite cylinder will be fixed at 10 mm and the length at 100 mm in each example, and the diameter of the processed boron carbide spheres is 9 ± 0.2 mm.

[0046] The high-purity boron carbide spheres in each example are prepared using the device disclosed in Example 1 of the Chinese patent "Preparation method and device for a high-purity porous boron carbide nuclear control rod" with the application number CN202510479801.9. The graphite sleeve in this patent corresponds to the graphite cylinder in the detailed implementation manners of the present invention, and the atmosphere is adjusted according to the needs of the present invention.

[0047] The device is as shown in Figure 1 , Figure 2 shown. As shown in Figure 1 is a preparation device for a high-purity porous boron carbide nuclear control rod, including a temperature control module 2 and a flow rate adjustment module 1, a reaction chamber 4, and a pressure adjustment module 3 that are connected in sequence. A valve 7 is provided between the flow rate adjustment module 1 and the reaction chamber 4, and a valve 7 is provided between the reaction chamber 4 and the pressure adjustment module 3; the flow rate adjustment module 1 is used to control the flow rate of the gas; the pressure adjustment module 3 is used to adjust the gas pressure in the reaction chamber 4.

[0048] As shown in Figure 2 is a schematic structural diagram of the reaction chamber 4. The reaction chamber 4 is composed of a quartz tube 6 and a graphite cylinder 5. The graphite cylinder 5 is located in the quartz tube 6, and the outer wall of the graphite cylinder 5 is in close contact with the inner wall of the quartz tube 6; the graphite cylinder 5 is used to load the green body and is evenly divided into five regions, namely, a front section, a front middle section, a middle section, a middle rear section, and a rear section, starting from the gas inlet end. The length ratio of the front section, the front middle section, the middle section, the middle rear section, and the rear section is 1:1:1:1:1.

[0049] The temperature control module 2 is connected to the reaction chamber 4. In some embodiments, the temperature control module 2 includes a temperature controller, a thermocouple, and heating elements corresponding to the front section, middle section, and rear section of the graphite cylinder 5 respectively. The heating elements adjust the temperature of the graphite cylinder 5 in the reaction chamber 4 by changing the output power; the temperature controller is used to adjust the output power of the heating elements so as to control the temperature, and the thermocouple is used to detect the temperature of the graphite cylinder 5. The temperature controller can be used to collect the temperature data of the thermocouple.

[0050] In some embodiments, the temperature control module 2 includes a temperature controller, a thermocouple, and heating elements corresponding to the front section, front-middle section, middle section, middle-rear section, and rear section of the graphite cylinder 5 respectively. The heating elements adjust the temperature of the graphite cylinder 5 in the reaction chamber 4 by changing the output power; the temperature controller is used to adjust the output power of the heating elements so as to control the temperature, and the thermocouple is used to detect the temperature of the graphite cylinder 5. The temperature controller can be used to collect the temperature data of the thermocouple. Example 1

[0051] To prepare high-purity boron carbide spheres for nuclear protection, the method is as follows:

[0052] S1. Mixing: Mix boron carbide powder with an average particle size of 2 μm and graphite powder with an average particle size of 0.3 μm evenly according to a mass ratio of 1:1 to obtain a mixed powder.

[0053] S2. Billet making: Place the mixed powder in a graphite cylinder, axially press the mixed powder in the graphite cylinder with a pressure of 2 MPa to press the mixed powder into a rod-shaped blank, and insert the graphite cylinder containing the rod-shaped blank into a quartz tube so that the outer wall of the graphite cylinder is in close contact with the inner wall of the quartz tube.

[0054] S3. Sintering: Pass argon with a flow rate of 130 mL / min from one end of the quartz tube (hereinafter described as the gas inlet end), evacuate from the other end of the quartz tube (gas outlet end) to keep the internal pressure of the quartz tube at 2000 Pa, heat the graphite cylinder, and control the temperature of the graphite cylinder from the front end to the rear end to increase from 1010 °C to 1100 °C; among them, the temperature of the front section is 1010 °C, the temperature of the middle section is 1050 °C, and the temperature of the rear section is 1100 °C.

[0055] Stop passing argon, continuously pass a mixed gas of argon and boron trichloride from the gas inlet end of the quartz tube for heat preservation; control the flow rate of the mixed gas at 130 mL / min, control the molar ratio of argon to boron trichloride at 1:0.2, and control the internal pressure of the quartz tube at 1000 Pa; keep the molar ratio of argon to boron trichloride and the internal pressure of the quartz tube unchanged, and for every 1 h increase in time, lower the flow rate of the mixed gas by 8 mL / min.

[0056] When the heat preservation time reaches 5 h, argon with a flow rate of 90 mL / min is introduced from the gas inlet end of the quartz tube, the internal pressure of the quartz tube is controlled at 8000 Pa, and the heating of the graphite cylinder is stopped to allow it to cool naturally; when the temperature of the graphite cylinder drops below 300 °C, the introduction of argon into the quartz tube is stopped, the internal pressure of the quartz tube is restored to normal pressure, and when the temperature of the graphite cylinder drops below 100 °C, the graphite cylinder is taken out to obtain a boron-carbon rod with elemental boron deposited inside;

[0057] S4. Processing: Use a ball mill equipped with a boron carbide cutter head to process the boron-carbon rod into a semi-finished boron-carbon ball with a diameter of 9 ± 0.2 mm, use diamond sandpaper to grind the surface defects of the semi-finished boron-carbon ball smooth, and use an ultrasonic cleaner to clean and dry the semi-finished boron-carbon ball;

[0058] S5. Sintering again: Lay the semi-finished boron-carbon balls flat on a graphite tray, place the graphite tray in a high-temperature furnace, introduce argon into the high-temperature furnace and heat it up to 1920 °C under normal pressure, introduce a mixed gas of argon and hydrogen into the high-temperature furnace, control the molar ratio of argon to hydrogen to be 1:0.8, and keep it warm for 2 h under normal pressure to complete the sintering of the semi-finished boron-carbon balls, and finally obtain finished high-purity boron-carbon balls. Example 2

[0059] To prepare high-purity boron-carbon balls for nuclear protection, the method is as follows:

[0060] S1. Mixing: Mix boron carbide powder with an average particle size of 3 μm and graphite powder with an average particle size of 0.4 μm evenly according to a mass ratio of 1:4 to obtain a mixed powder;

[0061] S2. Blanking: Place the mixed powder in a graphite cylinder, axially mold the mixed powder in the graphite cylinder with a pressure of 3 MPa, press the mixed powder into a rod-shaped blank, and insert the graphite cylinder containing the rod-shaped blank into the quartz tube so that the outer wall of the graphite cylinder is in close contact with the inner wall of the quartz tube;

[0062] S3. Sintering: Introduce argon with a flow rate of 140 mL / min from one end of the quartz tube (hereinafter described as the gas inlet end), evacuate from the other end (the gas outlet end) of the quartz tube to keep the internal pressure of the quartz tube at 2300 Pa, heat the graphite cylinder, and control the temperature of the graphite cylinder from the front end to the rear end to increase from 1020 °C to 1110 °C; among them, the front section temperature is 1020 °C, the middle section temperature is 1060 °C, and the rear section temperature is 1110 °C;

[0063] Stop introducing argon gas, and introduce a mixed gas of argon and boron trichloride from the inlet end of the quartz tube for heat preservation; control the flow rate of the mixed gas at 140 mL / min, control the molar ratio of argon to boron trichloride at 1:0.3, and control the internal pressure of the quartz tube at 1150 Pa; keep the molar ratio of argon to boron trichloride and the internal pressure of the quartz tube unchanged, and for every 1 h increase in time, lower the flow rate of the mixed gas by 9 mL / min.

[0064] When the heat preservation time reaches 6 h, introduce argon gas with a flow rate of 80 mL / min from the inlet end of the quartz tube, control the internal pressure of the quartz tube at 7000 Pa, stop heating the graphite cylinder and let it cool naturally; when the temperature of the graphite cylinder drops below 300 °C, stop introducing argon gas into the quartz tube, restore the internal pressure of the quartz tube to normal pressure, and when the temperature of the graphite cylinder drops below 100 °C, take out the graphite cylinder to obtain a boron-carbon rod with elemental boron deposited inside.

[0065] S4. Processing: Use a ball mill equipped with a boron carbide cutting head to process the boron-carbon rod into a semi-finished boron-carbon ball with a diameter of 9 ± 0.2 mm, use diamond sandpaper to grind the surface defects of the semi-finished boron-carbon ball flat, and use an ultrasonic cleaner to clean and dry the semi-finished boron-carbon ball.

[0066] S5. Re-sintering: Lay the semi-finished boron-carbon balls flat on a graphite tray, place the graphite tray in a high-temperature furnace, introduce argon gas into the high-temperature furnace and heat it up to 1950 °C under normal pressure, introduce a mixed gas of argon and hydrogen into the high-temperature furnace, control the molar ratio of argon to hydrogen at 1:0.9, and keep it warm for 2 h under normal pressure to complete the sintering of the semi-finished boron-carbon balls, and finally obtain finished high-purity boron-carbon balls. Example 3

[0067] To prepare high-purity boron-carbon balls for nuclear protection, the method is as follows:

[0068] S1. Mixing: Mix boron carbide powder with an average particle size of 4 μm and graphite powder with an average particle size of 0.5 μm evenly according to a mass ratio of 1:7 to obtain a mixed powder.

[0069] S2. Blanking: Place the mixed powder in a graphite cylinder, axially mold the mixed powder in the graphite cylinder with a pressure of 4 MPa, press the mixed powder into a rod-shaped blank, and insert the graphite cylinder containing the rod-shaped blank into the quartz tube so that the outer wall of the graphite cylinder is in close contact with the inner wall of the quartz tube.

[0070] S3. Sintering: Argon with a flow rate of 150 mL / min is introduced from one end of the quartz tube (hereinafter described as the intake end), and the other end of the quartz tube (the outlet end) is evacuated to keep the internal pressure of the quartz tube at 2700 Pa. The graphite cylinder is heated, and the temperature of the graphite cylinder is controlled to increase from 1030 °C to 1120 °C from the front end to the rear end; among them, the temperature of the front section is 1030 °C, the temperature of the middle section is 1070 °C, and the temperature of the rear section is 1120 °C;

[0071] Stop introducing argon, introduce a mixed gas of argon and boron trichloride from the intake end of the quartz tube, and keep warm; control the flow rate of the mixed gas at 150 mL / min, control the molar ratio of argon to boron trichloride at 1:0.3, and control the internal pressure of the quartz tube at 1300 Pa; keep the molar ratio of argon to boron trichloride and the internal pressure of the quartz tube unchanged, and for every 1 h increase in time, lower the flow rate of the mixed gas by 9 mL / min;

[0072] When the heat preservation time reaches 7 h, introduce argon with a flow rate of 70 mL / min from the intake end of the quartz tube, control the internal pressure of the quartz tube at 6000 Pa, stop heating the graphite cylinder and let it cool naturally; when the temperature of the graphite cylinder drops below 300 °C, stop introducing argon into the quartz tube, restore the internal pressure of the quartz tube to normal pressure, and when the temperature of the graphite cylinder drops below 100 °C, take out the graphite cylinder to obtain a boron carbide rod with elemental boron deposited inside;

[0073] S4. Processing: Use a ball mill equipped with a boron carbide cutter head to process the boron carbide rod into a semi-finished boron carbide ball with a diameter of 9 ± 0.2 mm, use diamond sandpaper to grind the surface defects of the semi-finished boron carbide ball smooth, and use an ultrasonic cleaner to clean and dry the semi-finished boron carbide ball;

[0074] S5. Re-sintering: Lay the semi-finished boron carbide balls flat on a graphite tray, place the graphite tray in a high-temperature furnace, introduce argon into the high-temperature furnace and heat it up to 1980 °C under normal pressure, introduce a mixed gas of argon and hydrogen into the high-temperature furnace, control the molar ratio of argon to hydrogen at 1:1, and keep warm for 3 h under normal pressure to complete the sintering of the semi-finished boron carbide balls, and finally obtain finished high-purity boron carbide balls. Example 4

[0075] To prepare high-purity boron carbide balls for nuclear protection, the method is as follows:

[0076] S1. Mixing: Mix boron carbide powder with an average particle size of 5 μm and graphite powder with an average particle size of 0.6 μm evenly according to a mass ratio of 1:9 to obtain a mixed powder;

[0077] S2. Blanking: Place the mixed powder in a graphite cylinder, and perform axial die pressing on the mixed powder in the graphite cylinder at a pressure of 5 MPa to press the mixed powder into a rod-shaped blank. Then, insert the graphite cylinder containing the rod-shaped blank into a quartz tube so that the outer wall of the graphite cylinder is in close contact with the inner wall of the quartz tube.

[0078] S3. Sintering: Pass argon with a flow rate of 160 mL / min from one end of the quartz tube (hereinafter described as the inlet end), and evacuate from the other end of the quartz tube (the outlet end) to keep the internal pressure of the quartz tube at 3000 Pa. Heat the graphite cylinder, and control the temperature of the graphite cylinder to increase from 1040 °C to 1130 °C from the front end to the rear end; among them, the temperature of the front section is 1040 °C, the temperature of the middle section is 1080 °C, and the temperature of the rear section is 1130 °C.

[0079] Stop passing argon, and pass a mixed gas of argon and boron trichloride from the inlet end of the quartz tube for heat preservation; control the flow rate of the mixed gas at 160 mL / min, control the molar ratio of argon to boron trichloride at 1:0.4, and control the internal pressure of the quartz tube at 1500 Pa; keep the molar ratio of argon to boron trichloride and the internal pressure of the quartz tube unchanged, and for every 1 h increase in time, lower the flow rate of the mixed gas by 10 mL / min.

[0080] When the heat preservation time reaches 8 h, pass argon with a flow rate of 60 mL / min from the inlet end of the quartz tube, control the internal pressure of the quartz tube at 5000 Pa, stop heating the graphite cylinder and let it cool naturally; when the temperature of the graphite cylinder drops below 300 °C, stop passing argon into the quartz tube, restore the internal pressure of the quartz tube to normal pressure, and when the temperature of the graphite cylinder drops below 100 °C, take out the graphite cylinder to obtain a boron carbide rod with elemental boron deposited inside.

[0081] S4. Processing: Use a ball mill equipped with a boron carbide cutting head to process the boron carbide rod into a semi-finished boron carbide ball with a diameter of 9 ± 0.2 mm, use diamond sandpaper to grind the surface defects of the semi-finished boron carbide ball flat, and use an ultrasonic cleaner to clean and dry the semi-finished boron carbide ball.

[0082] S5. Re-sintering: Lay the semi-finished boron carbide balls flat on a graphite tray, place the graphite tray in a high-temperature furnace, and under normal pressure, pass argon into the high-temperature furnace and heat it up to 2010 °C. Pass a mixed gas of argon and hydrogen into the high-temperature furnace, control the molar ratio of argon to hydrogen at 1:1.1, and keep it at a constant temperature for 3 h under normal pressure to complete the sintering of the semi-finished boron carbide balls, and finally obtain finished high-purity boron carbide balls.

[0083] Comparative Examples 1 - 4

[0084] Comparative Examples 1 to 4 are respectively Examples 1 to 4 in the Chinese invention patent “A high-purity porous boron carbide ceramic for nuclear control rods and a preparation method thereof” with application number CN202411124474.7. Comparative Example 5

[0085] The carbon-boron ball was prepared by the mold-jointing method, and the raw material components were referred to Example 1, and the method was as follows:

[0086] S1. Mixing: Boron carbide powder with an average particle size of 2 μm and graphite powder with an average particle size of 0.3 μm are uniformly mixed in a mass ratio of 1:1 to prepare a mixed powder;

[0087] S2. Blanking: The mixed powder was pressed into a 9 mm spherical blank by a mold closing method, and the molding pressure was 50 MPa;

[0088] S3. Sintering: The spherical body is placed in a hemispherical groove with a diameter of 9 mm in a graphite tray, and the graphite tray is placed in a high-temperature furnace. Argon is introduced into the high-temperature furnace under normal pressure and the temperature is raised to 2240°C and maintained for 3 h to complete the sintering, thereby obtaining the finished boron carbon ball.

[0089] Test 1 Performance test of semi-finished boron carbon balls

[0090] The porosity and compressive strength of the semi-finished boron carbon spheres obtained in step S4 of Examples 1 to 4 were tested. The test results are shown in Table 1. The porosity was measured using the bulk density method in national standard GB / T25995-2010; the compressive strength was measured using the compression test method in national standard GB / T4740-1999.

[0091] Table 1 Performance test results of semi-finished boron carbon balls in various embodiments

[0092] Case Porosity (%) Compressive strength (MPa) Example 1 40.4 15 Example 2 35.5 14 Example 3 31.1 15 Example 4 26.8 14

[0093] As shown in Table 1, the porosity of the semi-finished boron carbon spheres can be effectively regulated by adjusting the process parameters. For Examples 1 to 4, although the porosity of the semi-finished boron carbon spheres decreased from 40.4% to 26.8%, the compressive strength of the semi-finished boron carbon spheres was maintained at 14 to 15 MPa due to the gradual increase in the graphite content in the raw materials. This compressive strength range is very suitable for machining using the ball mill described in the present invention.

[0094] Test 2: Morphology Analysis

[0095] The high-purity boron-carbon spheres obtained in Example 1 were subjected to morphological analysis, and microstructure photos and macroscopic optical photos were taken.

[0096] Figure 3 This is a microscopic photograph of the high-purity boron carbon sphere prepared in Example 1. Figure 1As shown, the high-purity boron-carbon spheres have a uniform porous structure. There is good connectivity between the pores, which can ensure that the helium gas generated inside can be discharged smoothly. The particles are firmly bonded together, which helps to improve their mechanical properties.

[0097] Figure 4 Figure is a macroscopic optical photograph of the high-purity boron-carbon spheres obtained in Example 1. It can be seen that the high-purity boron-carbon spheres prepared by the method of the present invention have a very high sphericity, and their surfaces are very smooth, without defects such as cracks, pits and burrs.

[0098] Test 3 Performance Test and Analysis

[0099] The porosity, density, compressive strength, total boron-carbon content, and carbon content of the products obtained in Test Examples 1 to 4 and Comparative Examples 1 to 5 were tested, and the shrinkage rate after sintering of the boron-carbon spheres was calculated. The test results are shown in Table 2. The porosity and density were measured by the volume density method and the Archimedes drainage method in GB / T 25995-2010 respectively; the compressive strength was measured by the compression test method in GB / T 4740-1999; the total boron-carbon content was jointly measured by inductively coupled plasma emission spectrometry and high-frequency combustion infrared absorption method; the carbon content was measured by high-frequency combustion infrared absorption method.

[0100] The shrinkage rate of each example was calculated by the following formula:

[0101] (Diameter of semi-finished boron-carbon sphere - Diameter of finished boron-carbon sphere) / Diameter of semi-finished boron-carbon sphere × 100%;

[0102] The shrinkage rate of Comparative Example 5 was calculated by the following formula:

[0103] (Diameter of spherical blank - Diameter of finished boron-carbon sphere) / Diameter of spherical blank × 100%.

[0104] Table 2 Performance test results of each example and comparative example

[0105] Case Sintering temperature (°C) Porosity (%) <![CDATA[Density (g / cm 3 )]]> Compressive strength (MPa) Total boron and carbon content (wt%) Carbon content (wt%) Shrinkage rate (%) Example 1 1920 31.4 1.64 72 ≥99.9 48.5 8.7 Example 2 1950 27.8 1.66 66 ≥99.9 78.1 7.4 Example 3 1980 24.1 1.73 62 ≥99.9 85.2 6.5 Example 4 2010 20.6 1.81 57 ≥99.9 87.1 5.6 Comparative example 1 2160 30.9 1.74 420 99.8 20.2 - Comparative example 2 2180 35.3 1.63 379 99.8 20.3 - Comparative example 3 2140 27.8 1.82 474 99.8 20.2 - Comparative example 4 2120 23.4 1.93 583 99.8 20.1 - Comparative example 5 2240 31.3 1.64 17 ≥99.9 49.8 12.8

[0106] The porosity and density of Examples 1 to 4 were 20.6% to 31.4% and 1.64 to 1.81 g / cm 3 respectively, and the porosity and density of Comparative Examples 1 to 4 were 23.4% to 35.3% and 1.63 to 1.93 g / cm 3 respectively. Generally, the porosity and density ranges of Examples 1 to 4 and Comparative Examples 1 to 4 are basically equivalent.

[0107] The compressive strengths of Examples 1-4 were only 57-72 MPa, while those of Comparative Examples 1-4 were as high as 379-583 MPa. Considering that the nuclear control rods prepared in Comparative Examples 1-4 are used to regulate the chain reaction rate, and their components are all boron carbide, resulting in a carbon content of only 20.1%-20.3%, they should have high compressive strength. The boron carbide balls prepared in Examples 1-4 are used for the protection of the core meltdown accident. Due to the large amount of graphite contained, the carbon content is as high as 55.7wt%-87.2wt%, resulting in their unremarkable compressive strength. Nevertheless, the compressive strength of 57-72 MPa is sufficient to ensure that the boron carbide balls will not be damaged during transportation, storage, and use.

[0108] The total boron-carbon contents of Examples 1-4 and Comparative Examples 1-4 were ≥99.9wt% and 99.8wt% respectively, indicating that Examples 1-4 have the advantage of less impurity content. The sintering temperature ranges of Examples 1-4 and Comparative Examples 1-4 were 1920-2010°C and 2120-2160°C respectively. The lower sintering temperature of Examples 1-4 means lower energy consumption costs.

[0109] By setting the sintering temperature of Comparative Example 5 to 2240°C, Comparative Example 5 and Example 1 had the same porosity, density, and total boron-carbon content. The deposition of elemental boron in Example 1 would increase the boron content of the boron carbide balls, making the carbon content of Example 1 slightly lower than that of Comparative Example 5. Although the sintering temperature of Comparative Example 5 was as high as 2240°C, its compressive strength was only 17 MPa, far lower than 72 MPa of Example 1. Moreover, the shrinkage rate of Comparative Example 5 was 12.8%, much higher than 8.7% of Example 1. In addition, the sintering temperature of Comparative Example 5 as high as 2240°C would greatly increase the sintering cost and severely reduce the service life of the sintering furnace.

[0110] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of high-purity boron carbide spheres for nuclear protection, characterized in that, It includes the following steps: S1. Mixing: Mix boron carbide powder and graphite powder evenly to obtain a mixed powder material; S2. Billet making: Place the mixed powder material obtained in step S1 into a cylindrical billet-making mold and press it into a rod-shaped billet; Place the billet-making mold containing the rod-shaped billet into a heating container and wait for sintering; S3. Sintering: Heat up to 1010 - 1130 °C, introduce a mixed gas of inert gas and boron trichloride into the heating container, control the internal air pressure of the heating container at 1000 - 1500 Pa, and keep it warm for 5 - 8 h to obtain a boron carbide rod; S4. Processing: Process the boron carbide rod obtained in step S3 into a semi-finished boron carbide ball; S5. Re-sintering: Heat up to 1920 - 2010 °C, and keep the boron carbide ball obtained in step S4 warm for 2 - 3 h in an atmosphere of inert gas and hydrogen.

2. The preparation method according to claim 1, wherein In step S1: The average particle size of the boron carbide powder is 2 - 5 μm, and the average particle size of the graphite powder is 0.3 - 0.6 μm; The mass ratio of the boron carbide powder to the graphite powder is 1:(1 - 9).

3. The preparation method according to claim 1, wherein, In step S2: Axially mold press the mixed powder material in the billet-making mold with a pressure of 2 - 5 MPa; The said cylindrical billet-making mold is a graphite cylinder; The said heating container is a quartz tube.

4. The preparation method according to claim 1, characterized in that, In step S3: In the mixed gas of inert gas and boron trichloride, control the molar ratio of the inert gas to boron trichloride at 1:(0.2 - 0.4); Control the flow rate of the mixed gas of inert gas and boron trichloride at 130 - 160 mL / min.

5. The preparation method according to claim 1, characterized in that, In step S3: The gas is introduced from one end of the heating container, and the other end is evacuated to maintain the air pressure, and control the temperature of the billet-making mold from the gas inlet end to the gas outlet end to increase from 1010 - 1040 °C to 1100 - 1130 °C.

6. The preparation method according to claim 1, characterized in that, In step S3: During the heat preservation process, for every 1 h increase in time, lower the flow rate of the mixed gas by 8 - 10 mL / min.

7. The preparation method according to claim 1, wherein In step S3: During the heating-up process, introduce inert gas into the heating container, the flow rate of the inert gas is 130 - 160 mL / min, and the internal air pressure of the heating container is maintained at 2000 - 3000 Pa.

8. The preparation method according to claim 1, characterized in that, In step S4: Use a ball-making machine to process the boron carbide rod into a ball, and the cutter head of the ball-making machine is made of boron carbide material.

9. The preparation method according to claim 1, characterized in that, In step S5: Control the molar ratio of the inert gas to hydrogen at 1:(0.8 - 1.1).

10. A high-purity boron carbide sphere for nuclear protection, characterized in that, Obtained by using the preparation method described in any one of claims 1 - 9.

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