Axial straight slot type electric heating element based on non-metal heating material and preparation method

Abstract

This invention belongs to the field of electric heating technology, specifically relating to an axial straight-slot electric heating element based on a non-metallic heating material and its preparation method. It includes an electric heating element body with a straight slot. The two side walls of the slot serve as the positive and negative electrodes of the electric heating element, respectively. Several high-temperature resistant ceramic gaskets are inserted into the slot in a pluggable manner, filling the gaps between the slots. A sealing ceramic ring is fitted on the top of the electric heating element body. A metal conductive sheet is connected to the top end face of the electric heating element body, and an insulating sleeve is fitted over the metal conductive sheet. The metal conductive sheet is connected to an external wire. The beneficial effect of this invention is that by embedding high-temperature resistant ceramic gaskets, the positive and negative electrodes of the electric heating element are effectively physically isolated, eliminating the risk of short circuits caused by structural deformation of the electric heating element after long-term operation at high temperatures.

Description

Technical Field

[0001] This invention belongs to the field of electric heating technology, specifically relating to an axial straight groove electric heating element based on non-metallic heating materials and its preparation method. Background Technology

[0002] Electric heating elements are core components that convert electrical energy into heat energy, and are widely used in nuclear reactor testing, industrial production, civilian applications, and special equipment. In nuclear reactor testing, such as fuel assembly performance testing, high-temperature aging testing of in-core materials, and coolant thermal-hydraulic simulation testing, electric heating elements are key components simulating the "heat release" function of a nuclear reactor. Their performance directly determines the safety of the test system and the accuracy and reliability of the test data. Therefore, the design optimization and performance improvement of electric heating elements is one of the important directions for technological breakthroughs in the field of nuclear reactor testing.

[0003] The requirements for electric heating elements in nuclear reactor testing far exceed those of conventional industrial scenarios. Based on the actual operating temperature of the reactor core, the nuclear reactor testing environment requires electric heating elements to generate stable heat within a wide temperature range of 300–1000°C and above, while also withstanding relatively rapid heating and cooling to simulate the thermal shock conditions during nuclear reactor startup. Furthermore, electric heating elements suitable for nuclear reactor testing must be capable of long-term operation in special cooling media such as boron-containing water, liquid metals (e.g., sodium-potassium alloys), or inert gases (e.g., helium-xenon mixtures), withstand media corrosion and radiation exposure, and release no radioactive materials. Considering that different test processes require simulating varying power levels in the nuclear reactor, corresponding to different heat flux densities in the electric heating elements, the elements must possess high precision in adjusting heating power and uniform surface temperature distribution to prevent test piece failure or data distortion due to localized overheating. Simultaneously, nuclear reactor testing also places extremely high demands on the insulation performance of electric heating elements to prevent system shutdowns or safety risks caused by short circuits in the heating elements.

[0004] In the field of nuclear reactor testing, the design level, performance stability, and environmental adaptability of electric heating elements have become crucial factors restricting the development of testing technology and the progress of nuclear reactor safety research, requiring continuous technical optimization and performance improvement. Traditional electric heating elements for testing mostly use metal-based heating elements such as nickel-chromium alloys and iron-chromium-aluminum alloys. These are prone to severe radiation swelling under high temperatures above 800℃ and radiation environments, leading to drastic fluctuations in element resistance, poor heating power control, and susceptibility to electrochemical corrosion or high-temperature oxidation in cooling media environments. This results in a short service life, and the corrosion products can contaminate the cooling medium, affecting the safety of the test and the accuracy of the data. In contrast, non-metallic heating elements, represented by nitrides, refractory metals, and carbides, have become the mainstream choice for electric heating element materials in high-temperature testing. Among them, silicon carbide, as a third-generation semiconductor material, has a melting point >2700℃, a much lower temperature coefficient of resistance than metal-based elements, and a dense crystal structure. It exhibits more stable heating performance in high-temperature environments while also possessing excellent radiation resistance. Furthermore, silicon carbide has a high thermal conductivity of 150-200 W / (m²). Silicon carbide (SiC) is an ideal material for electric heating elements in nuclear reactor experiments because it can rapidly conduct heat generated by high power density, meet the simulation requirements of higher heating rates, and quickly respond to the adjustment of heating power. However, existing silicon carbide-based electric heating elements are mostly monolithic or tubular structures, requiring external electrodes to draw current, which results in high electrode contact resistance and low heating uniformity. Furthermore, after prolonged high-temperature operation, existing electric heating element structures are prone to structural deformation due to differences in thermal expansion coefficients, leading to poor electrode contact or short circuits. This risk is particularly high for slender heating elements, severely impacting the safety and lifespan of the heating equipment. Therefore, it is necessary to design an axial straight-slot electric heating element suitable for nuclear reactor experiments based on silicon carbide, a non-metallic material. This element would utilize high-temperature resistant ceramic gaskets to physically isolate the positive and negative electrodes, preventing short circuits caused by structural deformation during high-temperature operation. This would allow for safe and accurate simulation of the reactor core heating state, providing support for technological breakthroughs in the field of nuclear reactor testing. Summary of the Invention

[0005] The purpose of this invention is to provide an axially grooved electric heating element based on non-metallic heating materials and its preparation method. This element can maintain relatively stable heating and insulation performance under high-temperature thermal shock conditions with rapid heating and cooling power. At the same time, the positive and negative electrodes of the electric heating element are physically isolated by high-temperature resistant ceramic gaskets. Even if the silicon carbide heating body undergoes slight deformation due to high temperature, short circuits between the positive and negative electrodes can be avoided. This enhances the ability and service life of the electric heating element to work for a long time in special cooling media such as boron-containing water, sodium-potassium alloys, and helium-xenon mixed gases. This enables the simulation testing of heating in key nuclear reactor experiments under various operating conditions, such as reactor fuel assembly performance, high-temperature aging of in-core materials, and thermal-hydraulic characteristics of coolants.

[0006] The technical solution of the present invention is as follows: An axial straight groove electric heating element based on a non-metallic heating material includes an electric heating element body, a straight groove on the electric heating element body, the two side walls of the straight groove serving as the positive electrode and negative electrode of the electric heating element, a plurality of high-temperature resistant ceramic pads being inserted into the straight groove in a pluggable manner to fill the gap of the straight groove, a sealing ceramic ring being fitted on the top of the electric heating element body, a metal conductive sheet being connected to the top end face of the electric heating element body, an insulating sleeve being fitted over the metal conductive sheet, and an external wire being connected to the metal conductive sheet.

[0007] The electric heating element has a slender cylindrical structure and is made of high-purity silicon carbide with a thermal conductivity ≥150W / m. K, melting point > 2700℃.

[0008] A straight groove is cut along the axial direction of the cylinder from top to bottom on the main body of the electric heating element, reaching the bottom cold end and 30mm away from the bottom of the electric heating element.

[0009] A method for preparing an axial straight groove electric heating element based on a non-metallic heating material includes the following steps: Step 1: Prepare high-purity SiC-based raw materials separately. For the heating part, mix coarse, medium and fine α-SiC powders according to the optimal gradation. For the cold part, add additional metallic silicon powder to the same SiC matrix. Step 2: Cold isostatic pressing. The uniformly mixed raw materials are loaded into an elastic mold, and pressure is applied evenly in three dimensions through the cold isostatic pressing process to achieve initial densification of the blank. Step 3: Cold end silicon diffusion treatment, local silicon diffusion is performed on the cold end area of ​​the formed blank; Step 4: High-temperature sintering; Step 5: Axial straight groove cutting, after sintering, the axial straight groove is processed by diamond toroidal wire cutting process; Step 6: Deburring and surface treatment of the tank walls; Step 7: Electrode preparation; Step 8: Ceramic gasket assembly.

[0010] In step 2, a local pressure mold is used at the cold end to increase the green density of the conductive section, thus ensuring the uniformity of the subsequent silicon diffusion process.

[0011] In step 3, a nitrogen atmosphere is used to maintain the temperature at 1750~2050℃ for 3~15 minutes, so that the silicon powder can fully penetrate into the gaps between SiC particles to form a low-resistance silicide layer.

[0012] In step 4, the silicon-infiltrated monolithic blank is placed in a vacuum or high-purity argon environment, heated to 2100~2220℃ according to a segmented heating curve and held for 40 minutes. Through solid-state sintering, the SiC particles recrystallize to form a continuous heat-conducting network.

[0013] In step 6, an abrasive flow extrusion grinding process is used to remove microscopic burrs from the tank wall and optimize the surface roughness. This avoids the risk of tip discharge during electrode spraying and increases the bonding area between the electrode and the tank wall, thereby reducing contact resistance.

[0014] In step 7, a metal electrode is prepared on the tank wall by plasma spraying. First, a NiCrAl transition layer is sprayed, and then a tungsten coating is sprayed to ensure high-temperature conductivity, ultimately achieving a reliable bond between the electrode and the substrate.

[0015] In step 8, high thermal conductivity ceramic powder is pre-formed into a high-temperature resistant ceramic gasket by dry pressing and high-temperature solid-state sintering. The sintering temperature is controlled at 1600~1800℃ and held for 2~4 hours. The prepared high thermal conductivity SiC-based ceramic gasket is then inserted into a straight groove.

[0016] The beneficial effects of this invention are as follows: The axial straight-slot electric heating element uses silicon carbide non-metallic material, which has a high melting point, a temperature coefficient of resistance far lower than that of metal-based elements, and a dense crystal structure. Simultaneously, by embedding high-temperature resistant ceramic gaskets, the positive and negative electrodes of the electric heating element are effectively physically isolated, eliminating the risk of short circuits caused by structural deformation of the electric heating element after prolonged high-temperature operation. This allows the electric heating element to maintain relatively stable heating and insulation performance even under high-temperature thermal shock conditions with rapid heating and cooling power. Compared to traditional electric heating elements, this silicon carbide straight-slot electric heating element can better balance high heating power and fast response rate, while also exhibiting superior stability and safety. It can effectively improve the accuracy of simulating the thermal state of nuclear reactor cores, providing support for technological breakthroughs in the field of nuclear reactor testing. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of an axial straight groove electric heating element based on a non-metallic heating material, provided by the present invention.

[0018] In the diagram, 1 is the main body of the electric heating element, 2 is the straight groove, 3 is the high-temperature resistant ceramic gasket, 4 is the sealing ceramic ring, 5 is the metal conductive sheet, 6 is the insulating sleeve, and 7 is the external wire. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] This invention provides an axial straight groove electric heating element based on non-metallic heating materials, which aims to overcome the problems of traditional experimental electric heating elements. It can maintain relatively stable heating and insulation performance under high-temperature thermal shock conditions with rapid heating and cooling power. At the same time, the positive and negative electrodes of the electric heating element are physically isolated by high-temperature resistant ceramic gaskets, which enhances the ability and service life of the electric heating element to work for a long time in special cooling media such as boron-containing water, sodium-potassium alloy, and helium-xenon mixed gas.

[0021] like Figure 1 As shown, an axial straight groove electric heating element based on a non-metallic heating material includes an electric heating element body 1. The electric heating element body 1 has a slender cylindrical structure, is made of high-purity silicon carbide, and is manufactured using a hot and cold end production process, with a thermal conductivity ≥150 W / m. K, with a melting point >2700℃, possesses both superior high-temperature resistance and heat transfer capabilities. To achieve built-in electrodes for the electric heating element and avoid problems such as high contact resistance and low heating uniformity caused by external electrodes, a straight groove 2 is formed along the cylindrical axis of the electric heating element body 1 from top to bottom, reaching the bottom cold end (30mm from the bottom of the electric heating element). The two sides of the groove 2 serve as the positive and negative electrodes of the electric heating element, respectively. No external electrodes are required; the current flows directly into the silicon carbide body through the groove walls, utilizing the resistive properties of silicon carbide to achieve heating.

[0022] Several high-temperature resistant ceramic gaskets 3 are inserted into the straight grooves 2 in a pluggable manner, filling the gaps in the straight grooves and preventing short circuits between the two groove walls (positive and negative electrodes) due to thermal deformation of the silicon carbide heating element at high temperatures. The thermal conductivity of the ceramic gaskets is relatively close to that of silicon carbide material, which can reduce the contact thermal resistance between the two and ensure uniform heat transfer from the heating element.

[0023] A sealing ceramic ring 4 is fitted on the top of the electric heating element body 1. A metal conductive sheet 5 is connected to the top end face of the electric heating element body 1. An insulating sleeve 6 is fitted over the metal conductive sheet 5. An external wire 7 is connected to the metal conductive sheet 5.

[0024] A method for preparing an axial straight groove electric heating element based on a non-metallic heating material includes the following steps: Step 1: To meet the different functional requirements of the heating element and the cold end, high-purity SiC-based raw materials are prepared separately. The heating element is made by mixing coarse, medium and fine α-SiC powders in the optimal gradation to ensure high-temperature strength and thermal conductivity. The cold end is made by adding additional metallic silicon powder to the same SiC matrix to provide a reaction source for the subsequent silicon infiltration process, thereby reducing the resistivity of the cold end. Step 2: Cold isostatic pressing. The uniformly mixed raw materials are loaded into an elastic mold, and pressure is applied uniformly in three dimensions through the cold isostatic pressing process to achieve initial densification of the green body. The cold end uses a locally pressurized mold to precisely increase the green density of the conductive section, ensuring the uniformity of the subsequent silicon infiltration process. Step 3: Cold end silicon infiltration treatment. The cold end area of ​​the formed blank is locally silicon infiltrated under a nitrogen atmosphere and at a temperature of 1750~2050℃ for 3~15 minutes to allow silicon powder to fully penetrate into the gaps between SiC particles and form a low-resistance silicide layer, providing a reliable conductive basis for subsequent electrode connection. Step 4: High-temperature sintering. The silicon-infiltrated monolithic blank is placed in a vacuum or high-purity argon environment and heated to 2100~2220℃ according to a strictly segmented heating curve and held for 40 minutes. Through solid-state sintering, SiC particles recrystallize to form a continuous heat-conducting network, ensuring the high-temperature stability of the component. Step 5: Axial straight groove cutting. After sintering, the axial straight groove is machined using diamond wire cutting technology. This process must be carried out after sintering because the strength of the green blank before sintering is only about 1 / 10 of that of the sintered body. If the groove is cut in advance, defects such as edge chipping, cracking, or even overall fracture are very likely to occur. The hardness of SiC after sintering can reach 9.5 on the Mohs scale, and only diamond tools can achieve high-precision machining. Step 6: Deburring and surface treatment of the tank wall. Abrasive flow extrusion grinding process is used to remove micro-burrs from the tank wall and optimize the surface roughness. This avoids the risk of tip discharge during electrode spraying and increases the bonding area between the electrode and the tank wall, reducing contact resistance. Step 7: Electrode preparation. Metal electrodes are prepared on the tank wall by plasma spraying. First, a NiCrAl transition layer is sprayed to solve the problem of thermal expansion mismatch between SiC and metal. Then, a tungsten coating is sprayed to ensure high-temperature conductivity, and finally, a reliable bond between the electrode and the substrate is achieved. Step 8: Ceramic gasket assembly. High-temperature resistant ceramic gaskets are pre-made from high thermal conductivity ceramic powder through dry pressing and high-temperature solid-state sintering. The sintering temperature is controlled at 1600~1800℃ and held for 2~4 hours to ensure high density and good thermal stability of the gaskets. The prepared high thermal conductivity SiC-based ceramic gaskets are inserted into straight slots. The pluggable function is achieved through precise clearance fit, while ensuring a balance between thermal conductivity and electrical insulation.

[0025] Example: like Figure 1 As shown, an axial straight-slot electric heating element based on a non-metallic heating material is disclosed. Silicon carbide powder with a purity ≥99.5% is selected, and sintering aids (materials including but not limited to alumina) are added. After uniform mixing, the mixture is pressed into a slender cylindrical blank with an outer diameter of 20 mm and a total length of 1080 mm. The bottom cold end is 30 mm long, the heating body is 800 mm long, and the wiring cold end is 250 mm long. The blank is placed in a high-temperature sintering furnace and sintered at ≥2100℃ for 1 hour. After sintering, a straight slot is cut along the axial direction of the cylinder from the top down. The slot depth is 1050 mm (accounting for 97.2% of the total cylinder length; the remaining part serves as a bottom support section to prevent the slot from penetrating and causing structural breakage), the width is 2 mm, and the length is 12-14 mm, ensuring sufficient thickness of the slot wall to serve as an electrode. The two sides of the slot wall are polished to improve the surface smoothness.

[0026] Simultaneously, alumina ceramic powder with a purity of ≥99.5% is selected and pressed into long, thin sheet blanks with a width adapted to the straight groove. The length can be the same as the depth of the straight groove or the depth of the straight groove can be divided into multiple equal parts. The thickness is 1-3mm, and the width is 0.05-0.1mm smaller than the width of the straight groove. Sintering is carried out at a high temperature of ≥1600℃ for 2 hours. After sintering, the surface of the gasket is ground and polished to improve its fit with the inner wall of the straight groove, with a gap ≤0.1mm.

[0027] In addition, the electric heating element has a sealing ceramic ring structure on top for connecting the positive and negative electrode lead-out components and external terminals. The positive and negative electrode lead-out components include metal conductive plates (materials including but not limited to high-temperature resistant copper alloys) and insulating sleeves (materials including but not limited to quartz glass). The metal conductive plates are attached to the top of the groove walls on both sides of the straight groove, and the insulating sleeves are installed outside the metal conductive plates for insulation protection, exposing the connectors for connection to external circuits. Besides its connection function, the sealing ceramic ring structure also prevents high-temperature oxidation and dust ingress.

[0028] As can be seen, this electric heating element uses silicon carbide material, which combines superior heat transfer capacity with high-temperature stability and durability. The combination of the axial straight groove type inside the element and the high-temperature resistant ceramic gasket effectively prevents short circuits between the positive and negative electrodes under high-temperature thermal shock conditions with rapid heating and cooling power. At the same time, the use of insulating sleeves to protect the electrode lead-out components improves the electric heating element's ability to balance heating power and response rate, making it more suitable for the requirements of nuclear reactor core heating state simulation tests. Test results show that the axial straight groove electric heating element of this invention can operate stably for ≥24 hours at temperatures exceeding 1100℃, exhibiting excellent high-temperature stability and maintaining good heating and insulation performance during rapid heating and cooling processes.

Claims

1. An axial straight groove electric heating element based on a non-metallic heating material, characterized in that: The device includes an electric heating element body with a straight groove. The two sides of the groove serve as the positive and negative electrodes of the electric heating element, respectively. Several high-temperature resistant ceramic gaskets are inserted into the straight groove in a pluggable manner to fill the gaps in the groove. A sealing ceramic ring is fitted on the top of the electric heating element body. A metal conductive sheet is connected to the top end face of the electric heating element body. An insulating sleeve is fitted over the metal conductive sheet. The metal conductive sheet is connected to an external wire.

2. The axial straight groove electric heating element based on non-metallic heating material as described in claim 1, characterized in that: The electric heating element has a slender cylindrical structure and is made of high-purity silicon carbide with a thermal conductivity ≥150 W / m. K, melting point > 2700℃.

3. The axial straight groove electric heating element based on non-metallic heating material as described in claim 1, characterized in that: A straight groove is cut along the axial direction of the cylinder from top to bottom on the main body of the electric heating element, reaching the bottom cold end and 30mm away from the bottom of the electric heating element.

4. A method for preparing an axially grooved electric heating element based on a non-metallic heating material, characterized in that, Includes the following steps: Step 1: Prepare high-purity SiC-based raw materials separately. For the heating part, mix coarse, medium and fine α-SiC powders according to the optimal gradation. For the cold part, add additional metallic silicon powder to the same SiC matrix. Step 2: Cold isostatic pressing. The uniformly mixed raw materials are loaded into an elastic mold, and pressure is applied evenly in three dimensions through the cold isostatic pressing process to achieve initial densification of the blank. Step 3: Cold end silicon diffusion treatment, local silicon diffusion is performed on the cold end area of ​​the formed blank; Step 4: High-temperature sintering; Step 5: Axial straight groove cutting, after sintering, the axial straight groove is processed by diamond toroidal wire cutting process; Step 6: Deburring and surface treatment of the tank walls; Step 7: Electrode preparation; Step 8: Ceramic gasket assembly.

5. The method for preparing an axial straight groove electric heating element based on a non-metallic heating material as described in claim 4, characterized in that: In step 2, a local pressure mold is used at the cold end to increase the green density of the conductive section, thus ensuring the uniformity of the subsequent silicon diffusion process.

6. The method for preparing an axial straight groove electric heating element based on a non-metallic heating material as described in claim 4, characterized in that: In step 3, a nitrogen atmosphere is used to maintain the temperature at 1750~2050℃ for 3~15 minutes, so that the silicon powder can fully penetrate into the gaps between SiC particles to form a low-resistance silicide layer.

7. The method for preparing an axial straight groove electric heating element based on a non-metallic heating material as described in claim 4, characterized in that: In step 4, the silicon-infiltrated monolithic blank is placed in a vacuum or high-purity argon environment, heated to 2100~2220℃ according to a segmented heating curve and held for 40 minutes. Through solid-state sintering, the SiC particles recrystallize to form a continuous heat-conducting network.

8. The method for preparing an axial straight groove electric heating element based on a non-metallic heating material as described in claim 4, characterized in that: In step 6, an abrasive flow extrusion grinding process is used to remove microscopic burrs from the tank wall and optimize the surface roughness. This avoids the risk of tip discharge during electrode spraying and increases the bonding area between the electrode and the tank wall, thereby reducing contact resistance.

9. The method for preparing an axial straight groove electric heating element based on a non-metallic heating material as described in claim 4, characterized in that: In step 7, a metal electrode is prepared on the tank wall by plasma spraying. First, a NiCrAl transition layer is sprayed, and then a tungsten coating is sprayed to ensure high-temperature conductivity, ultimately achieving a reliable bond between the electrode and the substrate.

10. The method for preparing an axial straight groove electric heating element based on a non-metallic heating material as described in claim 4, characterized in that: In step 8, high thermal conductivity ceramic powder is pre-formed into a high-temperature resistant ceramic gasket by dry pressing and high-temperature solid-state sintering. The sintering temperature is controlled at 1600~1800℃ and held for 2~4 hours. The prepared high thermal conductivity SiC-based ceramic gasket is then inserted into a straight groove.

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