Preparation method of high-purity porous boron carbide ceramic based on high-frequency induction heating

Abstract

The application belongs to the technical field of boron carbide ceramic preparation, and particularly relates to a preparation method of high-purity porous boron carbide ceramic based on high-frequency induction heating. In the absence of sintering aids, the volume expansion caused by CO2 generated by the melt forces the plunger to move downward, thereby pressing the boron carbide powder. The effect of hot-pressing sintering can be achieved, the problems of low yield and high cost caused by the use of a hot-pressing sintering furnace can be avoided, the comprehensive performance of the prepared boron carbide ceramic can be improved, the process is easy to control, and the method is suitable for mass production of boron carbide ceramic round rods for high-temperature gas cooled reactor nuclear control rods. The prepared boron carbide ceramic has the advantages of pure phase, high porosity, uniform pore structure and stable mechanical properties, and especially has good pore connectivity. When used as a high-temperature gas cooled reactor nuclear control rod, the helium can be discharged in time, and the safety of the nuclear control rod is improved.

Description

Technical Field

[0001] This invention belongs to the field of boron carbide ceramic preparation technology, specifically relating to a method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating. Background Technology

[0002] Boron carbide ceramics, due to their high boron content, relatively low price, and abundant raw material sources, are often used to manufacture control rods for nuclear reactors. High-temperature gas-cooled reactors (HTGRs) generate high neutron radiation doses, and the nuclear control rods absorb neutrons, producing large amounts of helium gas. If this helium gas cannot be removed in time, the nuclear control rods are highly susceptible to swelling and damage. Therefore, boron carbide ceramics used for HTGR nuclear control rods should ideally have a connected porous structure.

[0003] Patent CN 201910042944.8 discloses a "method for preparing boron carbide porous ceramics for high-temperature gas-cooled reactor nuclear control rods". This method employs a pressureless sintering process suitable for mass production, and the prepared boron carbide ceramics exhibit excellent overall performance. Except for slightly poor pore connectivity, other properties meet the requirements for high-temperature gas-cooled reactor nuclear control rods. The drawbacks of this method are: the use of boron nitride as a sintering aid leads to reduced product purity; the use of small-particle-size boron carbide powder as raw material and expensive boron carbide whiskers as reinforcing agents results in high raw material costs; and the prolonged heating and cooling processes lead to high sintering costs. Summary of the Invention

[0004] To address the problems of high raw material and sintering costs, low purity and porosity, and poor pore connectivity in the production of boron carbide ceramics for nuclear control rods as described in the background art, this invention provides a method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating.

[0005] The technical solution of the present invention is as follows:

[0006] This invention provides a method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating, comprising the following steps:

[0007] (1) Add boron carbide powder, plunger and a mixture of quartz, borax and calcium carbonate to the sintering mold in sequence;

[0008] (2) The sintering mold is placed in the insulation tube, and high-purity argon gas is introduced into the insulation tube after the insulation tube is evacuated.

[0009] (3) The first high-frequency induction heating device heats the boron carbide powder, and the second high-frequency induction heating device heats the mixture. After a preset heating time, the second high-frequency induction heating device stops working, and the first high-frequency induction heating device cools down according to a preset rate. After the temperature drops to the preset temperature, the first high-frequency induction heating device stops working.

[0010] (4) Cool and demold to obtain high-purity porous boron carbide ceramic.

[0011] The weight ratio of boron carbide powder to the mixture of quartz, borax and calcium carbonate is 1:(0.46~0.60); the weight ratio of quartz, borax and calcium carbonate is 1:(0.33~0.42):(0.08~0.17); the first high-frequency induction heating device heats the boron carbide powder at a temperature of 2150~2200℃, and the second high-frequency induction heating device heats the mixture at a temperature of 670~690℃.

[0012] Furthermore, in the preparation method of high-purity porous boron carbide ceramics based on high-frequency induction heating as described above, the weight ratio of boron carbide powder to the mixture of quartz, borax and calcium carbonate is 1:(0.50~0.56); the weight ratio of quartz, borax and calcium carbonate is 1:(0.36~0.42):(0.11~0.17); the temperature at which the first high-frequency induction heating device heats the boron carbide powder is 2150~2185℃, and the temperature at which the second high-frequency induction heating device heats the mixture is 670~684℃.

[0013] In step (1), the average particle size of boron carbide powder is 20-40 μm; the average particle size of the mixture of quartz, borax and calcium carbonate is 2-5 μm.

[0014] In step (3), the preset heating time is 60 to 90 minutes.

[0015] In step (3), the preset cooling rate is 20-30℃ / min, and the preset temperature is 800-900℃.

[0016] In step (2), the argon flow rate is set to 0.2-0.5 L / min.

[0017] The sintering mold is a graphite mold; in step (2), the insulation tube is a zirconium oxide tube.

[0018] This invention also provides a preparation system for high-purity porous boron carbide ceramics based on high-frequency induction heating, for implementing the aforementioned preparation method, comprising a sintering apparatus, a first high-frequency induction heating apparatus, and a second high-frequency induction heating apparatus.

[0019] The sintering apparatus includes a sintering mold, a heat insulation pipe, and a ventilation system. The heat insulation pipe is used to place the sintering mold, and the ventilation system is used to maintain an oxygen-free environment inside the heat insulation pipe.

[0020] The sintering mold includes a cylinder, a bottom cover, a plunger, and a top cover, for filling boron carbide powder and a mixture of quartz, borax, and calcium carbonate. The plunger is used to separate the boron carbide powder from the mixture. The bottom cover and the top cover are respectively connected to the bottom and top of the cylinder.

[0021] The first high-frequency induction heating device is used to heat the boron carbide powder in the sintering mold;

[0022] The second high-frequency induction heating device is used to heat the mixture of quartz, borax and calcium carbonate in the sintering mold.

[0023] The bottom and top of the cylinder are connected to the bottom cover and top cover respectively via threads.

[0024] Beneficial effects

[0025] The present invention discloses a method for preparing boron carbide ceramics for nuclear control rods. Without adding sintering aids, the method utilizes the volume expansion caused by CO2 generated in the melt to force the plunger to move downward, thereby pressurizing the boron carbide powder. This achieves the effect of hot pressing sintering while avoiding the problems of low yield and high cost associated with using a hot pressing sintering furnace. It also improves the overall performance of the prepared boron carbide ceramics, and the process is easy to control. It is suitable for mass production of boron carbide ceramic round bars for high-temperature gas-cooled reactor nuclear control rods.

[0026] This invention utilizes high-frequency induction for rapid heating, which enables boron carbide powder to heat up quickly, avoiding the problem of long heating times required when using high-temperature sintering furnaces, thereby significantly reducing the sintering cost of boron carbide ceramics.

[0027] This invention can change the volume expansion rate of the melt by controlling the raw material ratio and sintering temperature, thereby adjusting the pressure of the plunger on the boron carbide powder, and thus achieving the effect of adjusting the porosity of the prepared boron carbide ceramic rod.

[0028] The boron carbide ceramics prepared by the method of the present invention have the advantages of pure phase, high porosity, uniform pore structure and stable mechanical properties. In particular, they have excellent pore connectivity, which can ensure timely helium discharge when used as nuclear control rods in high-temperature gas-cooled reactors, thereby improving the safety of nuclear control rods. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the preparation process of the high-purity porous boron carbide ceramic of the present invention.

[0030] Figure 2 This is a scanning electron microscope (SEM) image of the high-purity porous boron carbide ceramic prepared in Example 1 of the present invention. Detailed Implementation

[0031] The following examples are intended to illustrate the present invention, and not to further limit the invention.

[0032] This invention provides a method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating, such as... Figure 1 As shown, it includes the following steps:

[0033] (1) Boron carbide powder, a plunger, and a mixture of quartz, borax, and calcium carbonate are added sequentially to a sintering mold, with the boron carbide powder and the mixture separated by the plunger. The sintering mold can be a graphite mold.

[0034] At high temperatures, quartz and borax form a continuous, viscous melt, and calcium carbonate slowly and continuously decomposes to produce CO2. The melt traps the CO2, causing the volume to expand continuously.

[0035] To control the volume expansion rate of the mixture, the proportions in the mixture need to be controlled; in addition, to ensure the performance of boron carbide ceramics, the ratio of boron carbide powder to the mixture also needs to be adjusted.

[0036] Preferably, the weight ratio of boron carbide powder to the mixture of quartz, borax and calcium carbonate is 1:(0.46-0.60); the weight ratio of quartz, borax and calcium carbonate is 1:(0.33-0.42):(0.08-0.17).

[0037] Furthermore, the weight ratio of boron carbide powder to the mixture of quartz, borax and calcium carbonate is 1:(0.50-0.56); the weight ratio of quartz, borax and calcium carbonate is 1:(0.36-0.42):(0.11-0.17).

[0038] Furthermore, smaller boron carbide powder particle size is more beneficial for improving the density of boron carbide ceramics. However, to increase the porosity of boron carbide, the particle size of the boron carbide powder should be appropriately increased. Preferably, the average particle size of the boron carbide powder is 20–40 μm; the average particle size of the mixture of quartz, borax, and calcium carbonate is 2–5 μm. This improves both the total porosity of the boron carbide ceramic rods and ensures good pore connectivity.

[0039] (2) The sintering mold is placed in the insulation tube, and high-purity argon gas is introduced into the insulation tube after the insulation tube is evacuated.

[0040] To ensure the reaction proceeds fully, the insulation tube is preferably a zirconia tube.

[0041] Furthermore, the argon flow rate is set to 0.2–0.5 L / min to ensure that the oxygen in the zirconia tube is completely purged and the reaction is carried out in an oxygen-free environment, thus ensuring the purity of boron carbide.

[0042] (3) The first high-frequency induction heating device heats the boron carbide powder, and the second high-frequency induction heating device heats the mixture. After a preset heating time, the second high-frequency induction heating device stops working, and the first high-frequency induction heating device cools down according to a preset rate. After the temperature drops to the preset temperature, the first high-frequency induction heating device stops working.

[0043] This invention utilizes high-frequency induction for rapid heating, which enables boron carbide powder to heat up quickly, avoiding the problem of long heating times required when using high-temperature sintering furnaces, thereby significantly reducing the sintering cost of boron carbide ceramics.

[0044] Preferably, the first high-frequency induction heating device heats the boron carbide powder at a temperature of 2150–2200°C, and the second high-frequency induction heating device heats the mixture at a temperature of 670–690°C.

[0045] Furthermore, the first high-frequency induction heating device heats the boron carbide powder at a temperature of 2150–2185°C, and the second high-frequency induction heating device heats the mixture at a temperature of 670–684°C.

[0046] Boron carbide powder alone is difficult to sinter. Without the addition of sintering aids, pressure must be applied to the boron carbide powder to promote sintering. This invention utilizes a mixture of quartz, borax, and calcium carbonate that melts at 670–690°C to generate CO2, causing volume expansion and forcing a plunger to move downwards. This pressurizes the boron carbide powder, thereby achieving the effect of hot-pressing sintering.

[0047] In addition, by controlling the sintering temperature, the volume expansion rate of the melt can be changed, thereby adjusting the pressure of the plunger on the boron carbide powder, and thus achieving the effect of adjusting the porosity of the prepared boron carbide ceramic rod.

[0048] In the high-frequency induction heating process, the preset heating time for the first high-frequency induction heating device and the second high-frequency induction heating device is 60 to 90 minutes.

[0049] After sintering, during the cooling process of the first high-frequency induction heating device, the heating current is gradually reduced. Preferably, the preset cooling rate is 20-30℃ / min, and the preset temperature is 800-900℃. This is to prevent the boron carbide ceramic rod in the graphite mold from cracking due to excessively rapid cooling.

[0050] (4) Cooling, for example, when the temperature drops to 50°C, demold to obtain high-purity porous boron carbide ceramic.

[0051] The present invention discloses a method for preparing boron carbide ceramics for nuclear control rods. Without adding sintering aids, the method utilizes the volume expansion caused by CO2 generated in the melt to force the plunger to move downward, thereby pressurizing the boron carbide powder. This achieves the effect of hot pressing sintering while avoiding the problems of low yield and high cost associated with using a hot pressing sintering furnace. It also improves the overall performance of the prepared boron carbide ceramics, and the process is easy to control. It is suitable for mass production of boron carbide ceramic round bars for high-temperature gas-cooled reactor nuclear control rods.

[0052] The high-purity porous boron carbide ceramic has a total carbon and boron content of over 99.9% and a density of 1.78–1.82 g / cm³. 3 The total porosity is 27.6%–29.5%, the ratio of open pores to total porosity is 98.8%–99.2%, the compressive strength is 406–435 MPa, and the fracture toughness is 3.0–3.3 MPa·m. 1 / 2 .

[0053] The boron carbide ceramics prepared by the method of the present invention have the advantages of pure phase, high porosity, uniform pore structure and stable mechanical properties. In particular, they have excellent pore connectivity, which can ensure timely helium discharge when used as nuclear control rods in high-temperature gas-cooled reactors, thereby improving the safety of nuclear control rods.

[0054] This invention also provides a preparation system for high-purity porous boron carbide ceramics based on high-frequency induction heating, for implementing the aforementioned preparation method, comprising a sintering apparatus, a first high-frequency induction heating apparatus, and a second high-frequency induction heating apparatus.

[0055] The sintering apparatus includes a sintering mold, a heat insulation pipe, and a ventilation system. The heat insulation pipe is used to place the sintering mold, and the ventilation system is used to maintain an oxygen-free environment inside the heat insulation pipe.

[0056] The sintering mold includes a cylinder, a bottom cover, a plunger, and a top cover, for filling boron carbide powder and a mixture of quartz, borax, and calcium carbonate. The plunger is used to separate the boron carbide powder from the mixture. The bottom cover and the top cover are respectively connected to the bottom and top of the cylinder, preferably by threads.

[0057] The first high-frequency induction heating device is used to heat the boron carbide powder in the sintering mold;

[0058] The second high-frequency induction heating device is used to heat the mixture of quartz, borax and calcium carbonate in the sintering mold.

[0059] Both the first and second high-frequency induction heating devices are equipped with thermocouples. The current of the induction heating device is adjusted according to the temperature change measured by the thermocouples, thereby maintaining a stable temperature.

[0060] During high-temperature sintering, under the pressure of the graphite plunger and the constraint of the graphite cylinder wall, the boron carbide ceramic only shrinks in height, and its diameter always matches the inner diameter of the cylinder. Therefore, by machining the inner diameter of the sintering mold cylinder to the diameter required for the nuclear control rod, the prepared boron carbide ceramic rod can directly meet the diameter requirements of the nuclear control rod, thereby reducing the amount of subsequent grinding of the boron carbide ceramic rod and lowering processing costs.

[0061] To facilitate comparison of the comprehensive performance of the high-purity porous boron carbide ceramics prepared in the following four embodiments, the dimensions of the graphite molds and the filling heights used in the four embodiments were fixed. The outer diameter, inner diameter, and total height of the cylindrical graphite molds used were 70 mm, 50 mm, and 400 mm, respectively. The thread length of the top and bottom caps screwed into the cylinder was 18 mm, and the diameter and height of the plunger were 49.9 mm and 180 mm, respectively. After determining the dimensions of the graphite molds, the total height of the mixture of boron carbide powder, quartz, borax, and calcium carbonate filled in the cylinder was 184 mm (400-18-18-180=184). In the following four embodiments, based on the general requirement that the height of the boron carbide ceramic rods used for high-temperature gas-cooled reactor nuclear control rods is ≥100 mm, and the optimal weight ratio of boron carbide powder to the mixture composed of quartz, borax, and calcium carbonate is 1:0.53, the filling heights of the boron carbide powder and the mixture composed of quartz, borax, and calcium carbonate were set to 120 mm and 64 mm, respectively. In addition, depending on actual needs, when using thermocouples to adjust the current of the induction heating device, the height of the graphite cylinder and plunger can be increased to increase the heat conduction path of the boron carbide powder filling section and the mixture filling section, so as to achieve a large temperature difference between the graphite mold in the two reaction processes of the boron carbide powder filling section and the mixture filling section.

[0062] Example 1

[0063] (1) Screw a graphite bottom cap into the bottom of the graphite cylinder, pour boron carbide powder with an average particle size of 30μm into the graphite cylinder, and then put in a graphite plunger.

[0064] (2) Mix quartz, borax and calcium carbonate in a weight ratio of 1:0.36:0.11 and ball mill them into ceramic powder with an average particle size of 4μm. Pour the ceramic powder into the cavity above the plunger of the graphite cylinder and tighten the top cover of the graphite cylinder.

[0065] (3) Place the assembled graphite mold into a zirconia tube wrapped with zirconia insulation cotton, evacuate the zirconia tube and then introduce high-purity argon gas. Repeat the operation 3 times. Finally, set the argon gas flow rate in the zirconia tube to 0.3 L / min.

[0066] (4) The first high-frequency induction heating device rapidly heats the position of the graphite mold containing boron carbide powder to 2185°C, and then the second high-frequency induction heating device rapidly heats the position of the graphite cylinder containing ceramic powder to 684°C.

[0067] (5) After maintaining the temperature at 2185℃ and 684℃ for 70 minutes respectively, the second high-frequency induction heating device is de-energized, and the current of the first high-frequency induction heating device is gradually reduced so that the position of the graphite mold containing boron carbide powder cools down at a rate of 27℃ / min. When the temperature of the position of the graphite mold containing boron carbide powder drops to 870℃, the first high-frequency induction heating device is de-energized.

[0068] (6) When the temperature of the part of the graphite mold containing boron carbide powder is cooled down to 50°C, the graphite mold is demolded to obtain high-purity porous boron carbide ceramic.

[0069] Example 2

[0070] (1) Screw a graphite bottom cap into the bottom of the graphite cylinder, pour boron carbide powder with an average particle size of 40 μm into the graphite cylinder, and then put in a graphite plunger.

[0071] (2) Mix quartz, borax and calcium carbonate in a weight ratio of 1:0.33:0.08 and ball mill them into ceramic powder with an average particle size of 5μm. Pour the ceramic powder into the cavity above the plunger of the graphite cylinder and tighten the top cover of the graphite cylinder.

[0072] (3) Place the assembled graphite mold into a zirconia tube wrapped with zirconia insulation cotton, evacuate the zirconia tube and then introduce high-purity argon gas. Repeat the operation 3 times. Finally, set the argon gas flow rate in the zirconia tube to 0.2 L / min.

[0073] (4) The first high-frequency induction heating device rapidly heats the position of the graphite mold containing boron carbide powder to 2200°C, and then the second high-frequency induction heating device rapidly heats the position of the graphite cylinder containing foamed ceramic powder to 690°C.

[0074] (5) After holding at 2200℃ and 690℃ for 60 minutes respectively, the second high-frequency induction heating device is de-energized and the current of the first high-frequency induction heating device is gradually reduced so that the position of the graphite mold containing boron carbide powder cools down at a rate of 30℃ / min. When the temperature of the position of the graphite mold containing boron carbide powder drops to 900℃, the first high-frequency induction heating device is de-energized.

[0075] (6) When the temperature of the part of the graphite mold containing boron carbide powder is cooled down to 50°C, the graphite mold is demolded to obtain high-purity porous boron carbide ceramic.

[0076] Example 3

[0077] (1) Screw a graphite bottom cap into the bottom of the graphite cylinder, pour boron carbide powder with an average particle size of 25μm into the graphite cylinder, and then put in a graphite plunger.

[0078] (2) Mix quartz, borax and calcium carbonate in a weight ratio of 1:0.39:0.14 and ball mill them into ceramic powder with an average particle size of 3μm. Pour the ceramic powder into the cavity above the plunger of the graphite cylinder and tighten the top cover of the graphite cylinder.

[0079] (3) Place the assembled graphite mold into a zirconia tube wrapped with zirconia insulation cotton, evacuate the zirconia tube and then introduce high-purity argon gas. Repeat the operation 3 times. Finally, set the argon gas flow rate in the zirconia tube to 0.4 L / min.

[0080] (4) The first high-frequency induction heating device rapidly heats the position of the graphite mold containing boron carbide powder to 2160°C, and then the second high-frequency induction heating device rapidly heats the position of the graphite cylinder containing foamed ceramic powder to 677°C.

[0081] (5) After holding at 2160℃ and 677℃ for 80 minutes respectively, the second high-frequency induction heating device is de-energized and the current of the first high-frequency induction heating device is gradually reduced so that the position of the graphite mold containing boron carbide powder cools down at a rate of 23℃ / min. When the temperature of the position of the graphite mold containing boron carbide powder drops to 835℃, the first high-frequency induction heating device is de-energized.

[0082] (6) When the temperature of the part of the graphite mold containing boron carbide powder is cooled down to 50°C, the graphite mold is demolded to obtain high-purity porous boron carbide ceramic.

[0083] Example 4

[0084] (1) Screw a graphite bottom cap into the bottom of the graphite cylinder, pour boron carbide powder with an average particle size of 20μm into the graphite cylinder, and then put in a graphite plunger.

[0085] (2) Mix quartz, borax and calcium carbonate in a weight ratio of 1:0.42:0.17 and ball mill them into ceramic powder with an average particle size of 2μm. Pour the ceramic powder into the cavity above the plunger of the graphite cylinder and tighten the top cover of the graphite cylinder.

[0086] (3) Place the assembled graphite mold into a zirconia tube wrapped with zirconia insulation cotton, evacuate the zirconia tube and introduce high-purity argon gas. Repeat the operation 3 times. Finally, set the argon gas flow rate in the zirconia tube to 0.5 L / min.

[0087] (4) The first high-frequency induction heating device rapidly heats the position of the graphite mold containing boron carbide powder to 2150°C, and then the second high-frequency induction heating device rapidly heats the position of the graphite cylinder containing foamed ceramic powder to 670°C.

[0088] (5) After holding at 2150℃ and 670℃ for 90 minutes respectively, the second high-frequency induction heating device is de-energized, and the current of the first high-frequency induction heating device is gradually reduced so that the position of the graphite mold containing boron carbide powder cools down at a rate of 20℃ / min. When the temperature of the position of the graphite mold containing boron carbide powder drops to 800℃, the first high-frequency induction heating device is de-energized.

[0089] (6) When the temperature of the part of the graphite mold containing boron carbide powder is cooled down to 50°C, the graphite mold is demolded to obtain high-purity porous boron carbide ceramic.

[0090] Experimental results

[0091] 1. Morphological analysis

[0092] Morphology analysis was performed on the boron carbide porous ceramics prepared in Examples 1-4. Among them, the boron carbide porous ceramic prepared in Example 1 ( Figure 2 It exhibits a porous structure with pores evenly distributed between the boron carbide particles, and the pores have good connectivity.

[0093] 2. Performance Analysis

[0094] The compressive strength, porosity, density, purity, and fracture toughness of the boron carbide porous ceramics in Examples 1-4 were tested. Three examples from patent CN109704771A (A method for preparing boron carbide porous ceramics for nuclear control rods in a high-temperature gas-cooled reactor) were used as Comparative Examples 1-3 for comparison. See Table 1 for details.

[0095] Table 1. Performance Comparison of Different High-Purity Porous Boron Carbide Ceramics

[0096]

[0097] As shown in Table 1, in terms of physical properties, the total porosity and density of the boron carbide ceramics prepared in Examples 1-4 are 27.6%-29.5% and 1.78-1.82 g / cm³, respectively. 3 The total porosity and density of the boron carbide ceramics prepared in Comparative Examples 1–3 were 10.7%–17.5% and 2.08–2.25 g / cm³, respectively. 3 Clearly, Examples 1-4 are lighter. Regarding the ratio of open holes to total pores, Examples 1-4 have 98.8%-99.2%, while Comparative Examples 1-3 have ≥50%. Since a higher percentage of open holes in the nuclear control rods of high-temperature gas-cooled reactors is more conducive to the venting of helium and better for the safety of the control rods, Examples 1-4 are obviously superior in this respect.

[0098] In terms of mechanical properties, the compressive strength and fracture toughness of the boron carbide ceramics prepared in Comparative Examples 1–3 were 634–705 MPa and 3.9–4.3 MPa·m, respectively.1 / 2 The boron carbide ceramics prepared in Examples 1-4 exhibited slightly lower compressive strength and fracture toughness, ranging from 406 to 435 MPa and 3.0 to 3.3 MPa·m, respectively. 1 / 2 Since nuclear control rods used in high-temperature gas-cooled reactors do not need to bear external loads, the boron carbide ceramics prepared in Examples 1-4, although not outstanding in mechanical properties, fully meet the structural strength requirements for use as nuclear control rods.

[0099] In terms of purity, the total carbon and boron content of Examples 1-4 is above 99.9%, which is higher than the ≥98% of Comparative Examples 1-3. The higher the purity of the nuclear control rods, the higher the shielding efficiency against neutrons; therefore, Examples 1-4 are slightly superior.

[0100] In terms of cost, the sintering temperatures of Comparative Examples 1-3 and Examples 1-4 were 2090-2140℃ and 2150-2200℃, respectively, with the sintering cost of Examples 1-4 being slightly higher. However, Comparative Examples 1-3 added boron nitride and expensive boron carbide whiskers, while Examples 1-4 did not add any sintering aids, resulting in lower raw material costs for Examples 1-4.

[0101] In summary, the preparation method of this invention features easy process control, low energy consumption, low cost, and high yield, making it highly suitable for mass production of boron carbide ceramic rods for high-temperature gas-cooled reactor nuclear control rods. The boron carbide ceramics prepared using the method described in this invention possess advantages such as phase purity, high porosity, uniform pore structure, and stable mechanical properties. In particular, they exhibit excellent pore connectivity, ensuring timely helium venting when used as nuclear control rods in high-temperature gas-cooled reactors, thus improving the safety of the nuclear control rods.

Claims

1. A method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating, characterized in that, Includes the following steps: (1) Add boron carbide powder, plunger and a mixture of quartz, borax and calcium carbonate to the sintering mold in sequence; (2) The sintering mold is placed in the insulation tube, and high-purity argon gas is introduced into the insulation tube after it is evacuated; (3) The first high-frequency induction heating device heats the boron carbide powder, and the second high-frequency induction heating device heats the mixture. The melting produces CO2, which causes the volume to expand and forces the plunger to move downward, thereby pressurizing the boron carbide powder. After a preset heating time, the second high-frequency induction heating device stops working, and the first high-frequency induction heating device cools down according to a preset rate. After the temperature drops to the preset temperature, the first high-frequency induction heating device stops working. (4) Cool and demold to obtain high-purity porous boron carbide ceramic.

2. The method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 1, characterized in that, The weight ratio of boron carbide powder to the mixture of quartz, borax and calcium carbonate is 1:(0.46~0.60); the weight ratio of quartz, borax and calcium carbonate is 1:(0.33~0.42):(0.08~0.17); the first high-frequency induction heating device heats the boron carbide powder at a temperature of 2150~2200℃, and the second high-frequency induction heating device heats the mixture at a temperature of 670~690℃.

3. The method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 2, characterized in that, The weight ratio of boron carbide powder to the mixture of quartz, borax and calcium carbonate is 1:(0.50~0.56); the weight ratio of quartz, borax and calcium carbonate is 1:(0.36~0.42):(0.11~0.17); the first high-frequency induction heating device heats the boron carbide powder at a temperature of 2150~2185℃, and the second high-frequency induction heating device heats the mixture at a temperature of 670~684℃.

4. The method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 1, characterized in that, In step (1), the average particle size of boron carbide powder is 20~40μm; the average particle size of the mixture of quartz, borax and calcium carbonate is 2~5μm.

5. The method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 1, characterized in that, In step (3), the preset heating time is 60~90min.

6. The method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 1, characterized in that, In step (3), the preset cooling rate is 20~30℃ / min and the preset temperature is 800~900℃.

7. The method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 1, characterized in that, In step (2), the argon flow rate is set to 0.2~0.5L / min.

8. The method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 1, characterized in that, The sintering mold is a graphite mold; in step (2), the insulation pipe is a zirconium oxide pipe.

9. A preparation system for high-purity porous boron carbide ceramics based on high-frequency induction heating, used to implement the preparation method according to any one of claims 1-8, characterized in that, It includes a sintering apparatus, a first high-frequency induction heating device, and a second high-frequency induction heating device. The sintering apparatus includes a sintering mold, a heat insulation pipe, and a ventilation system. The heat insulation pipe is used to place the sintering mold, and the ventilation system is used to maintain an oxygen-free environment inside the heat insulation pipe. The sintering mold includes a cylinder, a bottom cover, a plunger, and a top cover, for filling boron carbide powder and a mixture of quartz, borax, and calcium carbonate. The plunger is used to separate the boron carbide powder from the mixture. The bottom cover and the top cover are respectively connected to the bottom and top of the cylinder. The first high-frequency induction heating device is used to heat the boron carbide powder in the sintering mold; The second high-frequency induction heating device is used to heat the mixture of quartz, borax and calcium carbonate in the sintering mold.

10. The preparation system for high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 9, characterized in that, The bottom and top of the cylinder are respectively connected to the bottom cover and the top cover by threads.

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