Silicon nitride ceramic for high-power electric heating element and low-temperature normal-pressure sintering method thereof

Abstract

The application discloses a low-temperature normal-pressure sintering preparation method of silicon nitride ceramic for high-power electrothermal elements, and comprises the following steps: (1) mixing silicon nitride powder and nitride glass sintering additives, wherein the amount of the silicon nitride powder is 60%-93% based on the total weight of the mixed raw materials, and the amount of the nitride glass sintering additives is 7%-40%; (2) drying the mixture obtained in the step (1) to obtain dry materials; (3) forming the dry materials obtained in the step (2) to obtain formed materials; and (4) sintering the formed materials obtained in the step (3) under normal pressure. According to the method, the sintering temperature is reduced to a low-temperature interval of 1550-1680 DEG C by using the nitride glass sintering additives, the dependence on expensive and complex hot-pressing or gas-pressure sintering equipment is eliminated, the normal-pressure sintering of high-performance silicon nitride ceramic is realized while the energy consumption and cost are significantly reduced, and a process foundation is laid for large-scale and low-cost manufacturing.

Description

Technical Field

[0001] This invention relates to the field of electric heating element manufacturing technology, and in particular to a silicon nitride ceramic for high-power electric heating elements and its low-temperature atmospheric pressure sintering preparation method. Background Technology

[0002] Currently, strategic emerging industries such as new energy vehicles are developing rapidly, and traditional high-temperature industrial equipment is also continuously upgrading and iterating, placing higher demands on the performance of core electrothermal elements in thermal management systems. Heating elements not only need higher power density and faster start-stop response capabilities, but also need to maintain stability and reliability during long-term operation. However, the widely used alumina-based metal-ceramic heating elements are limited by their intrinsic material properties, exhibiting inherent defects such as low bending strength (≤350MPa) and poor thermal shock resistance (≤200℃), making it difficult to meet the aforementioned increasing comprehensive performance requirements. This has become a prominent bottleneck restricting the further development of related equipment and technologies.

[0003] Silicon nitride ceramics are considered an ideal alternative to alumina-based heating materials due to their excellent thermal and mechanical properties, especially their outstanding thermal shock resistance and high mechanical strength. However, the preparation of traditional silicon nitride ceramics mainly relies on hot pressing sintering or high-temperature (usually exceeding 1750°C) gas pressure sintering processes, which have problems such as complex processes, high energy consumption, and high costs, limiting their potential for large-scale application.

[0004] To address the aforementioned problems, this invention proposes a method for preparing silicon nitride ceramics by low-temperature (1550-1680℃) atmospheric-pressure sintering based on a nitride glass sintering aid. The densification of silicon nitride primarily relies on a liquid-phase sintering mechanism. This method utilizes the nitride glass sintering aid to form a homogeneous and uniformly distributed liquid phase within the 1350–1550℃ range, effectively promoting the densification process of silicon nitride particles. This method only requires controlling the composition and amount of the nitride glass sintering aid to achieve sufficient material densification without a high-pressure environment. Furthermore, the low-temperature atmospheric-pressure sintering process employed in this method significantly reduces sintering energy consumption and equipment requirements, effectively suppresses the decomposition reaction of silicon nitride at high temperatures, and helps refine the silicon nitride grain size, thereby further improving the material's thermal shock resistance and mechanical strength. The silicon nitride ceramics prepared by this method have a flexural strength greater than 550 MPa and a thermal shock resistance greater than 400℃. This method provides a new technical path for the large-scale, low-cost manufacturing of high-performance silicon nitride ceramics, especially for replacing alumina-based materials in heating elements. Summary of the Invention

[0005] In view of this, the present invention addresses the deficiencies of existing technologies, and its main objective is to provide a silicon nitride ceramic for high-power electric heating elements and its low-temperature atmospheric pressure sintering preparation method. This method aims to achieve densification of silicon nitride ceramics under low-temperature atmospheric pressure sintering conditions by adding nitride glass sintering aids to form a uniformly distributed liquid phase. Compared to traditional hot pressing or high-temperature gas pressure sintering, the present invention not only significantly reduces sintering energy consumption and equipment requirements, but also effectively suppresses the decomposition reaction of silicon nitride at high temperatures, while simultaneously helping to refine the silicon nitride grain size, thereby further improving the material's thermal shock resistance and mechanical strength.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing silicon nitride ceramics for high-power heating elements by low-temperature atmospheric pressure sintering includes the following steps: (1) Silicon nitride powder is mixed with nitride glass sintering aid, wherein the amount of silicon nitride powder is 60%-93% and the amount of nitride glass sintering aid is 7%-40% based on the total weight of the mixed raw materials; (2) The mixture obtained in step (1) is dried to obtain a dry material; (3) The dried material obtained in step (2) is shaped to obtain a molded material; (4) The molding material obtained in step (3) is subjected to atmospheric pressure sintering. The sintering temperature is gradually increased from room temperature to the target sintering temperature of 1550-1680℃ at a rate of 1-10℃ / min. When the target sintering temperature is reached, the target sintering temperature is maintained for 1-6h.

[0007] As a preferred embodiment, the nitride glass sintering aid is glass obtained by melting and quenching silicon nitride with at least two compounds selected from magnesium oxide, magnesium fluoride, yttrium fluoride, and silicon oxide.

[0008] As a preferred embodiment, the mixing in step (1) is achieved by ball milling in a ball mill, wherein the ball milling conditions include: a ball milling speed of 200-600 r / min, a ball milling time of 1-12 h, and a ball milling medium of ethanol.

[0009] As a preferred embodiment, the drying temperature in step (2) is 60-120℃ and the drying time is 6-24h.

[0010] As a preferred embodiment, the molding pressure in step (3) is 10-80 MPa, and the molding pressure is maintained for 10-60 s.

[0011] As a preferred embodiment, the sintering in step (4) is carried out under an inert atmosphere, which is a nitrogen atmosphere or an argon atmosphere.

[0012] As a preferred embodiment, the sintering temperature increase process in step (4) is divided into two stages. In the first stage, the sintering temperature is increased to the target sintering temperature 1 of 800-1000℃ at a heating rate 1. In the second stage, the sintering temperature continues to increase to the target sintering temperature 2 of 1550-1680℃ at a heating rate 2, and the heating rate 2 is less than or equal to the heating rate 1.

[0013] As a preferred embodiment, the heating rate 1 is 5-10℃ / min.

[0014] As a preferred embodiment, the heating rate 2 is 1-5℃ / min.

[0015] A silicon nitride ceramic for high-power heating elements is prepared by the aforementioned low-temperature atmospheric pressure sintering method for silicon nitride ceramics for high-power heating elements.

[0016] Compared with the prior art, the present invention has obvious advantages and beneficial effects. Specifically, as can be seen from the above technical solution: I. The method of this invention uses nitride glass sintering aids to reduce the sintering temperature to a low temperature range of 1550-1680℃, eliminating the dependence on expensive and complex hot pressing or gas pressure sintering equipment. While significantly reducing energy consumption and costs, it realizes atmospheric pressure sintering of high-performance silicon nitride ceramics, laying the technological foundation for large-scale, low-cost manufacturing.

[0017] Second, the silicon nitride ceramic prepared by this invention has key properties of bending strength ≥550MPa and thermal shock resistance ≥400℃, which fundamentally overcomes the inherent shortcomings of existing alumina-based heating elements (bending strength ≤350MPa, thermal shock resistance ≤200℃), and can meet the ultimate requirements of material performance for demanding application scenarios such as high power density and rapid start-stop. Detailed Implementation

[0018] This invention discloses a method for preparing silicon nitride ceramics for high-power heating elements by low-temperature atmospheric pressure sintering, comprising the following steps: (1) The silicon nitride powder is mixed with the nitride glass sintering aid, and the amount of the silicon nitride powder is 60%-93% (e.g., 65% by weight, 70% by weight, 75% by weight, 80% by weight, 85% by weight or 90% by weight) based on the total weight of the mixed raw materials, and the amount of the nitride glass sintering aid is 7%-40% (e.g., 10% by weight, 15% by weight, 20% by weight, 25% by weight, 30% by weight or 35% by weight).

[0019] As is well known in the art, silicon nitride is generally classified into two crystal forms: α-phase silicon nitride (needle-like crystals, white or grayish-white) and β-phase silicon nitride (darker in color, dense granular polyhedrons or short prisms). Both are hexagonal crystals, both based on [SiN4]. 4- A three-dimensional spatial network formed by tetrahedrons sharing vertices. At high temperatures, the β phase is thermodynamically more stable, so the α phase silicon nitride will undergo a phase transition and can transform into the β phase. Therefore, in this step (1), silicon nitride powder refers to α phase silicon nitride powder with room temperature crystal structure.

[0020] The nitride glass sintering aid is glass obtained by melting and quenching silicon nitride with at least two compounds selected from magnesium oxide, magnesium fluoride, yttrium fluoride, and silicon oxide.

[0021] In this step (1), mixing can be carried out by any conventional method in the art, as long as it enables the raw materials to be mixed uniformly. In a preferred embodiment, the mixing can be achieved by ball milling in a ball mill. For the ball milling process, in a preferred embodiment, the ball mill can be a planetary ball mill, and more specifically, an XGB2 type planetary ball mill. In another preferred embodiment, the ball milling conditions may include: a ball milling speed of 200-600 r / min (e.g., 250 r / min, 300 r / min, 350 r / min, 400 r / min, 450 r / min, 500 r / min or 550 r / min), a ball milling time of 1-12 h (e.g., 5 h, 7 h or 10 h), and a ball milling medium of ethanol.

[0022] (2) The mixture obtained in step (1) is dried to obtain a dry material. The drying temperature in this step is 60-120℃ (e.g., 70℃, 80℃, 90℃, 100℃, 110℃), and the drying time is 6-24h (e.g., 8h, 12h, 16h, 20h).

[0023] (3) The dried material obtained in step (2) is molded to obtain a molded material. The molding pressure in this step is 10-80MPa (e.g., 20MPa, 30MPa, 40MPa, 50MPa, 60MPa, 70MPa), and the molding pressure is maintained for 10-60s (e.g., 20s, 30s, 40s, 50s).

[0024] (4) The molding material obtained in step (3) is subjected to atmospheric pressure sintering. The sintering temperature is gradually increased from room temperature to a target sintering temperature of 1550-1680℃ (e.g., 1570℃, 1600℃, 1630℃, 1660℃), with a heating rate of 1-10℃ / min (e.g., 2℃ / min, 4℃ / min, 6℃ / min, 8℃ / min). When the target sintering temperature is reached, the target sintering temperature is maintained for 1-6 hours (e.g., 2 hours, 3 hours, 4 hours, 5 hours). The sintering in this step is carried out under an inert atmosphere, which is a nitrogen atmosphere or an argon atmosphere. The sintering temperature increase process in this step is divided into two stages. In the first stage, the sintering temperature is increased at a heating rate 1 to a target sintering temperature 1 of 800-1000℃ (e.g., 850℃, 900℃, 950℃). In the second stage, the sintering temperature continues to increase at a heating rate 2 to a target sintering temperature 2 of 1550-1680℃, and the heating rate 2 is less than or equal to the heating rate 1. The heating rate 1 is 5-10℃ / min (e.g., 5℃ / min, 7℃ / min, 8℃ / min, 9℃ / min). The heating rate 2 is 1-5℃ / min (e.g., 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min).

[0025] The present invention also discloses a silicon nitride ceramic for high-power heating elements, which is prepared by the aforementioned low-temperature atmospheric pressure sintering method for silicon nitride ceramics for high-power heating elements.

[0026] The present invention will be described in detail below through multiple embodiments and comparative examples.

[0027] Example 1: 1) Raw material ratio: Weigh 25.5g of silicon nitride powder and 4.5g of nitride glass sintering aid, wherein the molar ratio of nitride glass components is silicon nitride: magnesium oxide: silicon oxide: yttrium fluoride = 20:51:23:6.

[0028] 2) Ball milling: The prepared raw materials were mixed and homogenized using an XGB2 planetary ball mill at a speed of 500 r / min for 3 hours; and ethanol was used as the ball milling medium.

[0029] 3) Drying: Drying temperature 100℃, drying time 12h.

[0030] 4) Molding: The dried powder is pressed into shape (forming pressure is 30MPa, and the pressure is held for 30s) to obtain ceramic green body.

[0031] 5) Atmospheric pressure sintering: The sintering process is carried out in a tube furnace with nitrogen atmosphere. The target sintering temperature is 1650℃. In the first stage, the temperature rises from room temperature to 800℃ at a rate of 5℃ / min. In the second stage, the temperature rises from 800℃ to 1650℃ at a rate of 3℃ / min. The target sintering temperature is maintained for 3 hours.

[0032] Example 2: 1) Raw material ratio: Weigh 24.0g of silicon nitride powder and 6.0g of nitride glass sintering aid, wherein the molar ratio of nitride glass components is silicon nitride: magnesium oxide: silicon oxide: yttrium fluoride = 20:51:23:6.

[0033] 2) Ball milling: The prepared raw materials were mixed and homogenized using an XGB2 planetary ball mill at a speed of 500 r / min for 3 hours; and ethanol was used as the ball milling medium.

[0034] 3) Drying: Drying temperature 100℃, drying time 12h.

[0035] 4) Molding: The dried powder is pressed into shape (forming pressure is 30MPa, and the pressure is held for 30s) to obtain ceramic green body.

[0036] 5) Atmospheric pressure sintering: The sintering process is carried out in a tube furnace with nitrogen atmosphere. The target sintering temperature is 1650℃. In the first stage, the temperature rises from room temperature to 800℃ at a rate of 5℃ / min. In the second stage, the temperature rises from 800℃ to 1650℃ at a rate of 3℃ / min. The target sintering temperature is maintained for 2 hours.

[0037] Example 3: 1) Raw material ratio: Weigh 24.6g of silicon nitride powder and 5.4g of nitride glass sintering aid, wherein the molar ratio of nitride glass components is silicon nitride: magnesium oxide: silicon oxide: magnesium fluoride = 18:60:20:2.

[0038] 2) Ball milling: The prepared raw materials were mixed and homogenized using an XGB2 planetary ball mill at a speed of 400 r / min for 4 hours; and ethanol was used as the ball milling medium.

[0039] 3) Drying: Drying temperature 100℃, drying time 12h.

[0040] 4) Molding: The dried powder is pressed into shape (forming pressure is 40MPa, and the pressure is held for 50s) to obtain ceramic green body.

[0041] 5) Atmospheric pressure sintering: The sintering process is carried out in a tube furnace with nitrogen atmosphere. The target sintering temperature is 1620℃. In the first stage, the temperature rises from room temperature to 900℃ at a rate of 5℃ / min. In the second stage, the temperature rises from 900℃ to 1620℃ at a rate of 2℃ / min. The target sintering temperature is maintained for 3 hours.

[0042] Comparative Example 1: The difference between the comparative example and Example 1 above is that the nitride glass sintering aid is replaced with a mixture of yttrium oxide and alumina sintering aid, wherein the molar ratio of yttrium oxide and alumina is 1:1. The rest of the contents of this comparative example are exactly the same as those of Example 1, and will not be repeated here.

[0043] Comparative Example 2: The difference between the comparative example and Example 2 above is that 3 mol% of yttrium oxide is added to the composition of Example 2. The rest of the contents of this comparative example are exactly the same as those of Example 2, and will not be repeated here.

[0044] Comparative Example 3: The difference between the comparative example and Example 3 above is that the sintering temperature is increased to 1750°C. The rest of the contents of this comparative example are exactly the same as those of Example 3, and will not be repeated here.

[0045] The density, flexural strength, and thermal shock resistance of the silicon nitride ceramics prepared in the above embodiments and comparative examples were tested using conventional methods. The test results are shown in the table below:

[0046] The test results show that the silicon nitride ceramic prepared using this invention is superior to the comparative example in terms of density, flexural strength, and thermal shock resistance, exhibiting excellent performance and quality. Comparing Example 1 with Comparative Example 1, which uses different sintering aids, it can be seen that adding nitride glass sintering aids results in better sinterability and performance than conventional oxide sintering aids. Comparing Example 2 with Comparative Example 2, it can be seen that adding additional oxide sintering aids is detrimental to sinterability and performance. Comparing Example 3 with Comparative Example 3, it can be seen that increasing the sintering temperature leads to excessive decomposition of silicon nitride, which is detrimental to sinterability and performance.

[0047] The key design features of this invention are as follows: First, by employing nitride glass sintering aids, the sintering temperature is lowered to a low-temperature range of 1550-1680℃, eliminating the reliance on expensive and complex hot-pressing or gas-pressure sintering equipment. This significantly reduces energy consumption and costs while achieving atmospheric pressure sintering of high-performance silicon nitride ceramics, laying the technological foundation for large-scale, low-cost manufacturing. Second, the silicon nitride ceramics prepared by this invention exhibit key properties such as flexural strength ≥550MPa and thermal shock resistance ≥400℃, fundamentally overcoming the inherent shortcomings of existing alumina-based heating elements (flexural strength ≤350MPa, thermal shock resistance ≤200℃). This meets the ultimate material performance requirements of demanding applications such as high power density and rapid start-stop.

[0048] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing silicon nitride ceramic for high-power electric heating elements by low-temperature atmospheric pressure sintering, characterized in that: Includes the following steps: (1) Mix silicon nitride powder with nitride glass sintering aid. Based on the total weight of the mixed raw materials, the amount of silicon nitride powder is 60%-93%, and the amount of nitride glass sintering aid is 7%-40%; wherein the molar ratio of nitride glass components is silicon nitride: magnesium oxide: silicon oxide: yttrium fluoride = 20:51:23:6, or the molar ratio of nitride glass components is silicon nitride: magnesium oxide: silicon oxide: magnesium fluoride = 18:60:20:2; (2) The mixture obtained in step (1) is dried to obtain a dry material; (3) The dried material obtained in step (2) is shaped to obtain a molded material; (4) The molding material obtained in step (3) is subjected to atmospheric pressure sintering. The sintering temperature is gradually increased from room temperature to the target sintering temperature of 1550-1680℃ at a rate of 1-10℃ / min. When the target sintering temperature is reached, the target sintering temperature is maintained for 1-6h.

2. The method for preparing silicon nitride ceramic for high-power heating elements by low-temperature atmospheric pressure sintering according to claim 1, characterized in that: The nitride glass sintering aid is glass quenched by melting silicon nitride, magnesium oxide, silicon oxide and yttrium fluoride, or glass quenched by melting silicon nitride, magnesium oxide, silicon oxide and magnesium fluoride.

3. The method for preparing silicon nitride ceramic for high-power heating elements by low-temperature atmospheric pressure sintering according to claim 1, characterized in that: In step (1), the mixing is achieved by ball milling in a ball mill. The ball milling conditions include: a ball milling speed of 200-600 r / min, a ball milling time of 1-12 h, and the ball milling medium is ethanol.

4. The method for preparing silicon nitride ceramic for high-power heating elements by low-temperature atmospheric pressure sintering according to claim 1, characterized in that: In step (2), the drying temperature is 60-120℃ and the drying time is 6-24h.

5. The method for preparing silicon nitride ceramic for high-power electric heating elements by low-temperature atmospheric pressure sintering according to claim 1, characterized in that: In step (3), the molding pressure is 10-80 MPa and is maintained for 10-60 s.

6. The method for preparing silicon nitride ceramic for high-power electric heating elements by low-temperature atmospheric pressure sintering according to claim 1, characterized in that: In step (4), sintering is carried out under an inert atmosphere, which is a nitrogen atmosphere or an argon atmosphere.

7. The method for preparing silicon nitride ceramic for high-power electric heating elements by low-temperature atmospheric pressure sintering according to claim 1, characterized in that: The sintering temperature increase process in step (4) is divided into two stages. In the first stage, the sintering temperature is increased to the target sintering temperature 1 of 800-1000℃ at a heating rate 1. In the second stage, the sintering temperature continues to increase to the target sintering temperature 2 of 1550-1680℃ at a heating rate 2, and the heating rate 2 is less than or equal to the heating rate 1.

8. The method for preparing silicon nitride ceramic for high-power electric heating elements by low-temperature atmospheric pressure sintering according to claim 7, characterized in that: The heating rate 1 is 5-10℃ / min.

9. The method for preparing silicon nitride ceramic for high-power electric heating elements by low-temperature atmospheric pressure sintering according to claim 7 or 8, characterized in that: The heating rate 2 is 1-5℃ / min.

10. A silicon nitride ceramic for high-power electric heating elements, characterized in that: It is prepared by the low-temperature atmospheric pressure sintering method for high-power electric heating element silicon nitride ceramic as described in any one of claims 1-9.

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