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

Abstract

The invention belongs to the technical field of preparation of boron carbide ceramics, and particularly relates to a preparation method of high-purity porous boron carbide ceramics based on high-frequency induction heating, under the condition that a sintering aid is not added, a plunger is forced to move downwards due to volume expansion caused by CO2 generated by a melt, so that boron carbide powder is pressurized, the effect of hot pressing sintering can be achieved, and the effect of high-purity porous boron carbide ceramics 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 the boron carbide ceramic round rod for the high-temperature gas cooled reactor nuclear control rod; the prepared boron carbide ceramic has the advantages of pure phase, high porosity, uniform pore structure and stable mechanical property, especially has very good pore connectivity, and can ensure timely discharge of helium and improve the safety of the nuclear control rod when being used as a high-temperature gas cooled reactor nuclear control rod.

Description

Technical Field

[0001] The invention belongs to the technical field of boron carbide ceramic preparation, and in particular relates to a method for preparing high-purity porous boron carbide ceramic based on high-frequency induction heating. Background Art

[0002] Boron carbide ceramics are often used to prepare control rods for nuclear reactors due to their high boron content, relatively low price, and abundant raw material sources. The neutron radiation dose of high-temperature gas-cooled reactors is large, and nuclear control rods will generate a large amount of helium after absorbing neutrons. If the helium cannot be removed in time, the nuclear control rods are prone to swelling and damage. Therefore, boron carbide ceramics used as nuclear control rods for high-temperature gas-cooled reactors should preferably have a connected porous structure.

[0003] Patent CN 201910042944.8 discloses "a method for preparing porous boron carbide ceramics for high-temperature gas-cooled reactor nuclear control rods". This method adopts a pressureless sintering process suitable for mass production. The prepared boron carbide ceramics have excellent comprehensive performance. Except for slightly poor pore connectivity, other properties meet the indicators of high-temperature gas-cooled reactor nuclear control rods. The shortcomings of this method are: using boron nitride as a sintering aid leads to reduced purity of the product; using small-particle boron carbide powder as raw material and expensive boron carbide whiskers as reinforcing agents, resulting in high raw material costs; long-term heating and cooling leads to high sintering costs. Summary of the invention

[0004] In order to solve the problems mentioned in the background technology that the existing technology for producing boron carbide ceramics for nuclear control rods has high raw material cost and sintering cost, and the prepared boron carbide ceramics have low purity and porosity and poor pore connectivity, the present invention provides a preparation method of high-purity porous boron carbide ceramics based on high-frequency induction heating.

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

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

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

[0008] (2) The sintering mold is placed in an insulation tube, and high-purity argon gas is introduced into the insulation tube after the 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 at a preset rate. After the temperature drops to a preset temperature, the first high-frequency induction heating device stops working;

[0010] (4) Cooling and demolding to obtain high-purity porous boron carbide ceramics.

[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 temperature at which the first high-frequency induction heating device heats the boron carbide powder is 2150-2200°C, and the temperature at which the second high-frequency induction heating device heats the mixture is 670-690°C.

[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°C, and the temperature at which the second high-frequency induction heating device heats the mixture is 670-684°C.

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

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

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

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

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

[0018] The present invention also provides a preparation system of high-purity porous boron carbide ceramics based on high-frequency induction heating, which is used to implement the preparation method, and comprises a sintering device, a first high-frequency induction heating device and a second high-frequency induction heating device.

[0019] The sintering device comprises a sintering mold, a heat preservation pipe and a ventilation system, wherein the heat preservation pipe is used to place the sintering mold, and the ventilation system is used to maintain an oxygen-free environment in the heat preservation pipe;

[0020] The sintering mold comprises a cylinder, a bottom cover, a plunger and a top cover, which are used to load 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, and the bottom cover and the top cover are connected to the bottom and the top of the cylinder respectively.

[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 the top of the cylinder are respectively connected with the bottom cover and the top cover via threads.

[0024] Beneficial Effects

[0025] The method for preparing boron carbide ceramics for nuclear control rods of the present invention utilizes the CO2 generated by the melt to cause volume expansion and force the plunger to move downward without adding a sintering aid, thereby pressurizing the boron carbide powder. The method can achieve the effect of hot pressing sintering, avoid the problems of low output and high cost caused by using a hot pressing sintering furnace, and improve the comprehensive performance of the prepared boron carbide ceramics. The process is easy to control and is suitable for mass production of boron carbide ceramic round rods for high-temperature gas-cooled reactor nuclear control rods.

[0026] The present invention utilizes high-frequency induction for rapid heating, which can rapidly heat the boron carbide powder, thereby avoiding the problem of a long heating time required in a high-temperature sintering furnace, thereby significantly reducing the sintering cost of the boron carbide ceramic.

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

[0028] The boron carbide ceramics prepared by the preparation method of the present invention have the advantages of pure phase, high porosity, uniform pore structure, and stable mechanical properties, especially good pore connectivity. When used as a nuclear control rod of a high-temperature gas-cooled reactor, it can ensure that helium is discharged in time, thereby improving the safety of the nuclear control rod. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It 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) photograph of the high-purity porous boron carbide ceramic prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0031] The following examples are intended to illustrate the present invention rather than to further limit the present invention.

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

[0033] (1) Add boron carbide powder, a plunger, and a mixture of quartz, borax, and calcium carbonate into a sintering mold in sequence, and separate the boron carbide powder and the mixture through the plunger. The sintering mold may be a graphite mold.

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

[0035] In order to control the volume expansion rate of the mixture, it is necessary to control the ratio of the mixture; in addition, in order to ensure the performance of boron carbide ceramics, it is also necessary to adjust the ratio of boron carbide powder to the mixture.

[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 the mixture of boron carbide powder and 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] In addition, the smaller the particle size of boron carbide powder, the more conducive it is to improving the density of boron carbide ceramics. However, if the porosity of boron carbide is to be increased, the particle size of boron carbide powder should be appropriately increased. Preferably, 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. This can not only increase the total porosity of boron carbide ceramic round rods, but also ensure that the pores have good connectivity.

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

[0040] In order to ensure that the reaction proceeds fully, preferably, the insulation tube is 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 exhausted and the reaction is in an oxygen-free environment to ensure the purity of the 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 at a preset rate. After the temperature drops to a preset temperature, the first high-frequency induction heating device stops working.

[0043] The present invention utilizes high-frequency induction for rapid heating, which can rapidly heat the boron carbide powder, thereby avoiding the problem of a long heating time required in a high-temperature sintering furnace, thereby significantly reducing the sintering cost of the boron carbide ceramic.

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

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

[0046] Pure boron carbide powder is difficult to sinter. Without adding sintering aids, boron carbide powder must be pressurized to promote its sintering. The present invention utilizes a mixture of quartz, borax and calcium carbonate to melt at 670-690°C to generate CO2, which causes volume expansion and forces the plunger to move downward, thereby pressurizing the boron carbide powder, thereby achieving the effect of hot pressing and sintering the boron carbide powder.

[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 then achieving the effect of adjusting the porosity of the prepared boron carbide ceramic round rod.

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

[0049] After sintering is completed, for 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°C / min and the preset temperature is 800-900°C. The purpose is to prevent the boron carbide ceramic round rod in the graphite mold from cracking due to too fast cooling.

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

[0051] The method for preparing boron carbide ceramics for nuclear control rods of the present invention utilizes the CO2 generated by the melt to cause volume expansion and force the plunger to move downward without adding a sintering aid, thereby pressurizing the boron carbide powder. The method can achieve the effect of hot pressing sintering, avoid the problems of low output and high cost caused by using a hot pressing sintering furnace, and improve the comprehensive performance of the prepared boron carbide ceramics. The process is easy to control and is suitable for mass production of boron carbide ceramic round rods 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 more than 99.9% and a density of 1.78 to 1.82 g / cm 3 The total porosity is 27.6% to 29.5%, the ratio of open pores to total pores is 98.8% to 99.2%, the compressive strength is 406 to 435 MPa, and the fracture toughness is 3.0 to 3.3 MPa·m 1 / 2 .

[0053] The boron carbide ceramics prepared by the preparation method of the present invention have the advantages of pure phase, high porosity, uniform pore structure, and stable mechanical properties, especially good pore connectivity. When used as a nuclear control rod of a high-temperature gas-cooled reactor, it can ensure that helium is discharged in time, thereby improving the safety of the nuclear control rod.

[0054] The present invention also provides a preparation system of high-purity porous boron carbide ceramics based on high-frequency induction heating, which is used to implement the preparation method, and comprises a sintering device, a first high-frequency induction heating device and a second high-frequency induction heating device.

[0055] The sintering device comprises a sintering mold, a heat preservation pipe and a ventilation system, wherein the heat preservation pipe is used to place the sintering mold, and the ventilation system is used to maintain an oxygen-free environment in the heat preservation pipe;

[0056] The sintering mold comprises a cylinder, a bottom cover, a plunger and a top cover, which are used to load 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 the 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] The first high-frequency induction heating device and the second high-frequency induction heating device are both provided with thermocouples. According to the temperature change measured by the thermocouples, the current of the induction heating device is adjusted to maintain the temperature stable.

[0060] During high-temperature sintering, due to 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 is always consistent with the inner diameter of the cylinder. Therefore, by processing the inner diameter of the sintering mold cylinder into the diameter required for the nuclear control rod, the prepared boron carbide ceramic round rod can directly meet the diameter requirements of the nuclear control rod, thereby reducing the subsequent grinding of the boron carbide ceramic round rod and reducing the processing cost.

[0061] In order to facilitate the comparison of the comprehensive performance of the high-purity porous boron carbide ceramics prepared in the following four embodiments, the size of the graphite mold and the filler height used in the following four embodiments are fixed. The outer diameter, inner diameter and total height of the cylinder of the graphite mold used are 70mm, 50mm and 400mm respectively, the length of the thread of the top cover and the bottom cover screwed into the cylinder is 18mm, and the diameter and height of the plunger are 49.9mm and 180mm respectively. After determining the size of the graphite mold, the total height of the mixture of boron carbide powder and quartz, borax and calcium carbonate filled in the cylinder is 184mm (400-18-18-180=184). In the following four embodiments, according to the height of the boron carbide ceramic round rod for high temperature gas-cooled reactor nuclear control rods, which is generally ≥100mm, and the optimal weight ratio of boron carbide powder and the mixture composed of quartz, borax and calcium carbonate is 1:0.53, the filling height of the boron carbide powder and the mixture composed of quartz, borax and calcium carbonate is set to 120mm and 64mm respectively. In addition, according to actual needs, when a thermocouple is used to adjust the current of the induction heating device, the height of the graphite cylinder and the plunger can be increased to increase the heat conduction path of the boron carbide powder filling part and the mixture filling part of the device, so as to achieve a large temperature difference in the reaction process of the graphite mold in the boron carbide powder filling part and the mixture filling part.

[0062] Example 1

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

[0064] (2) quartz, borax and calcium carbonate are mixed in a weight ratio of 1:0.36:0.11 and ball-milled into a ceramic powder with an average particle size of 4 μm, the ceramic powder is poured into the cavity above the plunger of the graphite cylinder, and the top cover of the graphite cylinder is tightened;

[0065] (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 three times, and 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 graphite mold at a location containing boron carbide powder to 2185° C., and then the second high-frequency induction heating device rapidly heats the graphite cylinder at a location containing ceramic powder to 684° C.;

[0067] (5) After maintaining the temperature at 2185°C and 684°C for 70 minutes respectively, the second high-frequency induction heating device is powered off, 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 is cooled at a rate of 27°C / min. When the temperature of the position of the graphite mold containing boron carbide powder drops to 870°C, the first high-frequency induction heating device is powered off;

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

[0069] Example 2

[0070] (1) Screw the graphite bottom cover 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 the graphite plunger;

[0071] (2) quartz, borax and calcium carbonate are mixed in a weight ratio of 1:0.33:0.08 and ball-milled into a ceramic powder with an average particle size of 5 μm, the ceramic powder is poured into the cavity above the plunger of the graphite cylinder, and the top cover of the graphite cylinder is tightened;

[0072] (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 three times, and 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 graphite mold at a position containing boron carbide powder to 2200° C., and then the second high-frequency induction heating device rapidly heats the graphite cylinder at a position containing foamed ceramic powder to 690° C.;

[0074] (5) After maintaining the temperature at 2200°C and 690°C for 60 minutes respectively, the second high-frequency induction heating device is powered off, 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 is cooled at a rate of 30°C / min. When the temperature of the position of the graphite mold containing boron carbide powder drops to 900°C, the first high-frequency induction heating device is powered off;

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

[0076] Example 3

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

[0078] (2) quartz, borax and calcium carbonate are mixed in a weight ratio of 1:0.39:0.14 and ball-milled into a ceramic powder with an average particle size of 3 μm, the ceramic powder is poured into the cavity above the plunger of the graphite cylinder, and the top cover of the graphite cylinder is tightened;

[0079] (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 three times, and 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 graphite mold at a position containing boron carbide powder to 2160° C., and then the second high-frequency induction heating device rapidly heats the graphite cylinder at a position containing foamed ceramic powder to 677° C.;

[0081] (5) After maintaining the temperature at 2160°C and 677°C for 80 min respectively, the second high-frequency induction heating device is powered off, 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 is cooled at a rate of 23°C / min. When the temperature of the position of the graphite mold containing boron carbide powder drops to 835°C, the first high-frequency induction heating device is powered off;

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

[0083] Example 4

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

[0085] (2) quartz, borax and calcium carbonate are mixed in a weight ratio of 1:0.42:0.17 and ball-milled into a ceramic powder with an average particle size of 2 μm, the ceramic powder is poured into the cavity above the plunger of the graphite cylinder, and the top cover of the graphite cylinder is tightened;

[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 three times, and 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 graphite mold at a position containing boron carbide powder to 2150° C., and then the second high-frequency induction heating device rapidly heats the graphite cylinder at a position containing foamed ceramic powder to 670° C.;

[0088] (5) After maintaining the temperature at 2150°C and 670°C for 90 minutes respectively, the second high-frequency induction heating device is powered off, and the current of the first high-frequency induction heating device is gradually reduced, so that the position of the graphite mold equipped with boron carbide powder is cooled at a rate of 20°C / min. When the temperature of the position of the graphite mold equipped with boron carbide powder drops to 800°C, the first high-frequency induction heating device is powered off;

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

[0090] Experimental Results

[0091] 1. Morphology Analysis

[0092] The morphology of the boron carbide porous ceramics prepared in Examples 1-4 was analyzed, among which the boron carbide porous ceramics prepared in Example 1 ( Figure 2 ) presents a porous structure, the pores are 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 porous boron carbide ceramics of Examples 1-4 were tested, and three examples in the patent with publication number CN109704771A (a method for preparing porous boron carbide ceramics for nuclear control rods of high temperature gas-cooled reactors) 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 to 4 are 27.6% to 29.5% and 1.78 to 1.82 g / cm 3 The total porosity and density of the boron carbide ceramics prepared in Comparative Examples 1 to 3 are 10.7% to 17.5% and 2.08 to 2.25 g / cm 3 , it is obvious that Examples 1 to 4 are lighter. As for the ratio of openings to total pores, Examples 1 to 4 are 98.8% to 99.2%, while Comparative Examples 1 to 3 are ≥50%. Since the higher the opening ratio of the nuclear control rod for a high temperature gas-cooled reactor is, the more conducive it is to the discharge of helium and the better the safety of the control rod is, it is obvious that Examples 1 to 4 are better.

[0098] In terms of mechanical properties, the compressive strength and fracture toughness of the boron carbide ceramics prepared in Examples 1 to 3 are 634 to 705 MPa and 3.9 to 4.3 MPa·m1 / 2 The compressive strength and fracture toughness of the boron carbide ceramics prepared in Examples 1 to 4 are slightly lower, 406 to 435 MPa and 3.0 to 3.3 MPa·m 1 / 2 Since the nuclear control rods for high temperature gas-cooled reactors do not need to bear external loads, the boron carbide ceramics prepared in Examples 1 to 4, although their mechanical properties are not outstanding, fully meet the structural strength requirements for use as nuclear control rods.

[0099] In terms of purity, the total content of carbon and boron in Examples 1 to 4 is above 99.9%, which is higher than ≥98% in Comparative Examples 1 to 3. The higher the purity of the nuclear control rod, the higher the shielding efficiency against neutrons, so Examples 1 to 4 are slightly better.

[0100] In terms of cost, the sintering temperatures of Comparative Examples 1 to 3 and Examples 1 to 4 are 2090 to 2140° C. and 2150 to 2200° C., respectively, and the sintering costs of Examples 1 to 4 are slightly higher. However, Comparative Examples 1 to 3 add boron nitride and expensive boron carbide whiskers, while Examples 1 to 4 do not add any sintering aids, and the raw material costs of Examples 1 to 4 are lower.

[0101] In summary, the preparation method of the present invention has the characteristics of easy process control, low energy consumption, low cost and high yield, and is very suitable for mass production of boron carbide ceramic round rods for high temperature gas-cooled reactor nuclear control rods. The boron carbide ceramic prepared by the method of the present invention has the advantages of pure phase, high porosity, uniform pore structure and stable mechanical properties, especially good pore connectivity. When used as a nuclear control rod for a high temperature gas-cooled reactor, it can ensure the timely discharge of helium and improve the safety of the nuclear control rod.

Claims

1. A method for preparing high-purity porous boron carbide ceramics based on high-frequency induction heating, characterized in that: The following steps are involved: (1) Adding boron carbide powder, a plunger, and a mixture of quartz, borax, and calcium carbonate into a sintering mold in sequence; (2) The sintering mold is placed in an insulation tube, and high-purity argon gas is introduced into the insulation tube after the tube 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. After a preset heating time, the second high-frequency induction heating device stops working, and the first high-frequency induction heating device cools down at a preset rate. After the temperature drops to a preset temperature, the first high-frequency induction heating device stops working; (4) Cooling and demolding to obtain high-purity porous boron carbide ceramics.

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 temperature at which the first high-frequency induction heating device heats the boron carbide powder is 2150-2200°C, and the temperature at which the second high-frequency induction heating device heats the mixture is 670-690°C.

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 temperature at which the first high-frequency induction heating device heats the boron carbide powder is 2150-2185°C, and the temperature at which the second high-frequency induction heating device heats the mixture is 670-684°C.

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

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 the step (3), the heating time is preset to be 60 to 90 minutes.

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 the step (3), the preset cooling rate is 20-30°C / min, and the preset temperature is 800-900°C.

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 the step (2), the argon gas flow rate is set to 0.2-0.5 L / 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 the step (2), the insulation tube is a zirconia tube.

9. A preparation system of 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 to 8, characterized in that: It includes a sintering device, a first high-frequency induction heating device and a second high-frequency induction heating device, The sintering device comprises a sintering mold, a heat preservation pipe and a ventilation system, wherein the heat preservation pipe is used to place the sintering mold, and the ventilation system is used to maintain an oxygen-free environment in the heat preservation pipe; The sintering mold comprises a cylinder, a bottom cover, a plunger and a top cover, which are used to load 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, and the bottom cover and the top cover are connected to the bottom and the top of the cylinder respectively. 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 of high-purity porous boron carbide ceramics based on high-frequency induction heating according to claim 9, characterized in that: The bottom and the top of the cylinder are respectively connected with the bottom cover and the top cover via threads.

Images