High-thermal-conductivity silicon nitride ceramic and preparation method thereof

Abstract

The invention relates to the technical field of ceramic materials, in particular to high-thermal-conductivity silicon nitride ceramic and a preparation method thereof. The invention provides a preparation method of high-thermal-conductivity silicon nitride ceramic. The preparation method comprises the following steps: (1) carrying out high-energy ball milling on silicon nitride powder; (2) carrying out oxygen reduction treatment on the silicon nitride powder obtained by high-energy ball milling treatment in the step (1); and (3) carrying out hot pressed sintering on the silicon nitride powder obtained by the oxygen reduction treatment in the step (2). The invention provides silicon nitride ceramic with high thermal conductivity and a preparation method thereof. The method comprises the following steps: firstly, carrying out surface activation treatment on silicon nitride powder through high-energy ball milling to remarkably improve the reaction activity; in the hot pressing sintering process, the carbon atmosphere generated by the graphite mold and the heating body is used in cooperation with the flowing nitrogen environment, and efficient oxygen reduction treatment is achieved at the temperature of 1300-1450 DEG C. And then, carrying out short-time sintering densification at 1700-1850 DEG C to finally obtain the silicon nitride ceramic which has excellent thermal conductivity and does not contain the sintering aid.

Description

Technical Field

[0001] The present invention relates to the technical field of ceramic materials, and in particular to a high thermal conductivity silicon nitride ceramic and a preparation method thereof. Background Art

[0002] Silicon nitride (Si3N4) ceramics are widely used in high-end industrial applications due to their high strength, hardness, excellent high-temperature resistance, corrosion resistance, thermal shock stability, and wear resistance. Silicon nitride's thermal conductivity is outstanding among non-oxide ceramics, making it a key heat dissipation material in high-end electronics, semiconductors, and new energy. With advances in manufacturing technology, its application in thermal management of high-power devices is expected to expand further.

[0003] The theoretical value of silicon nitride (Si3N4) ceramics is higher than 300 W / (m·K), but due to the influence of oxygen impurities, its thermal conductivity is lower than 100 W / (m·K). Oxygen impurities are the core factor affecting the thermal conductivity of silicon nitride. Oxygen atoms will dissolve in the silicon nitride lattice during the sintering process, forming silicon vacancies and causing lattice distortion, resulting in increased phonon scattering and a significant reduction in thermal conductivity. At present, sintering aids such as MgO and Y2O3 are not only conducive to promoting sintering densification, but also have the effect of purifying the lattice, which can effectively reduce the influence of oxygen impurities and significantly improve thermal conductivity. Therefore, by introducing sintering aids and combining them with conventional sintering techniques such as gas pressure sintering, the thermal conductivity of silicon nitride ceramics can reach more than 90W / (m·K). The thermal conductivity of sintering aid-containing ceramics prepared by hot pressing is usually 50-60 W / (m·K). However, pure silicon nitride ceramics without sintering aids require hot pressing and sintering technology. However, due to the lack of sintering aids to purify the lattice, the thermal conductivity is only 20-30 W / (m·K). How to improve the thermal conductivity of pure silicon nitride ceramics without adding sintering aids has become a difficult problem in the industry.

[0004] Therefore, how to prepare silicon nitride ceramics with high thermal conductivity and high mechanical properties without adding sintering aids has become the main technical difficulty faced by current silicon nitride preparation technology. Summary of the Invention

[0005] In order to solve the above-mentioned problem in the prior art of preparing silicon nitride ceramics with high thermal conductivity and high mechanical properties without adding sintering aids, the present invention provides a high thermal conductivity silicon nitride ceramic and a preparation method thereof.

[0006] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a method for preparing a high thermal conductivity silicon nitride ceramic, comprising the following steps: (1) High-energy ball milling of silicon nitride powder; (2) subjecting the silicon nitride powder obtained by high-energy ball milling in step (1) to oxygen reduction treatment; (3) hot pressing and sintering the silicon nitride powder obtained by the deoxidation treatment in step (2).

[0007] In one embodiment of the present invention, the mass ratio of the silicon nitride powder to the grinding balls in step (1) is 1 / 5-1 / 8.

[0008] In one embodiment of the present invention, the rotation speed of the high-energy ball milling in step (1) is 150-250 rpm; Preferably, the high-energy ball milling time in step (1) is 6-12 hours.

[0009] In one embodiment of the present invention, the oxygen reduction treatment in step (2) comprises: introducing a nitrogen-containing atmosphere at a pressure of ≤1 MPa until the pressure of the nitrogen-containing atmosphere reaches 3 KPa-5 KPa, and heating to a first temperature of 1300° C.-1450° C.; When the target temperature is reached, the pressure of the nitrogen atmosphere is increased to 10-20 KPa and maintained at this temperature for 3-5 hours.

[0010] In one embodiment of the present invention, the heating rate in step (2) is 2°C / min.

[0011] In one embodiment of the present invention, the nitrogen-containing atmosphere is nitrogen, ammonia, or a mixture of nitrogen, ammonia, and an inert gas.

[0012] In one embodiment of the present invention, the hot pressing sintering in step (3) includes: adjusting the pressure of the nitrogen-containing atmosphere to 1.5 KPa-1.8 KPa, heating to a second temperature, wherein the second temperature is 1700° C.-1850° C.; When the temperature reaches the second temperature, the sintering pressure is increased to 25-30 MPa and kept at this temperature for 5-10 hours.

[0013] In one embodiment of the present invention, the heating rate in step (3) is 5°C / min.

[0014] In a second aspect, the present invention provides a high thermal conductivity silicon nitride ceramic, which is prepared by the method described above.

[0015] In one embodiment of the present invention, the thermal conductivity of the silicon nitride ceramic is ≥100 W / (m·K), and the flexural strength is ≥900 MPa.

[0016] Based on the above, compared with the prior art, the high thermal conductivity silicon nitride ceramics prepared by the preparation method provided by the present invention have high thermal conductivity and high bending strength.

[0017] Other features and beneficial effects of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood through practice of the present invention. The objectives and other beneficial effects of the present invention can be achieved and obtained through the structures particularly pointed out in the description and claims. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments; the technical features designed in the different implementation modes of the present invention described below can be combined with each other as long as they do not conflict with each other; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0019] In the description of the present invention, it should be noted that all terms used in the present invention (including technical terms and scientific terms) have the same meanings as those generally understood by ordinary technicians in the field to which the present invention belongs, and should not be understood as limiting the present invention; it should be further understood that the terms used in the present invention should be understood to have the same meanings as these terms in the context of this specification and the relevant field, and should not be understood in an idealized or overly formal sense, unless explicitly defined as such in the present invention.

[0020] Currently, high thermal conductivity silicon nitride ceramics with a thermal conductivity of over 100 W / (m·K) require the addition of sintering aids such as MgO and Y2O3, but the introduction of sintering aids changes the microstructure of the silicon nitride ceramic. Furthermore, currently commonly used methods such as hot pressing and spark plasma sintering can produce silicon nitride ceramics without sintering aids. However, due to the lack of sintering aids, the thermal conductivity of the resulting silicon nitride ceramics is only 60-80 W / (m·K).

[0021] To this end, the present invention provides a method for preparing a high thermal conductivity silicon nitride ceramic, comprising the following steps: Step (1) high-energy ball milling of silicon nitride powder; In the present invention, silicon nitride powder is dry-milled using a planetary ball mill. Specifically, the silicon nitride powder is placed in a ball mill jar, along with silicon nitride grinding balls, with the mass ratio of silicon nitride powder to grinding balls being 1 / 5-1 / 8. Nitrogen is then introduced into the jar. The jar containing the silicon nitride powder and grinding balls is then milled in the planetary ball mill for 6-12 hours at a speed of 150-250 rpm. After the milling is complete and the jar temperature has cooled to room temperature, the powder is removed and quickly moved to the next step.

[0022] It should be noted that the raw materials used in the method provided by the present invention have wide adaptability, do not rely on ultrafine / high-purity high-quality powders, and have low requirements on raw material performance; the process is highly safe and completely avoids the use of dangerous or harmful gases such as hydrogen and ammonia.

[0023] It should also be noted that the method provided by the present invention significantly increases the surface activity of silicon nitride powder by subjecting it to high-energy ball milling. Due to this characteristic, the present invention only requires a relatively low deoxidation temperature of 1300-1450°C during the hot pressing sintering process, without the need for the addition of reducing agents such as hydrogen, ammonia, or carbon black. By utilizing the naturally occurring carbon atmosphere of the sintering system and combining it with a flowing nitrogen environment, the oxygen impurity content in the material can be effectively reduced.

[0024] Step (2) deoxidizing the silicon nitride powder obtained by high-energy ball milling in step (1); In a specific implementation, silicon nitride powder that has undergone high-energy ball milling is loaded into a hot-pressed graphite mold, and a pressure of ≤1 MPa is applied. The powder in the mold is not compacted, and sufficient gaps are left between the powders to facilitate the circulation and full contact of the reducing gas. A flowing nitrogen-containing atmosphere is introduced, and when the pressure of the nitrogen-containing atmosphere reaches 3 kpa-5 kpa, the temperature is increased at a rate of 2°C / min. At this time, the deoxidation reaction has not yet occurred, and a lower gas pressure can be used to achieve an atmosphere protection effect on the powder. After reaching a temperature of 1300°C-1450°C, the pressure of the nitrogen-containing atmosphere is increased to 10 kpa-20 kpa, which is conducive to sufficient circulation and contact of the gas between the powders to improve the deoxidation efficiency, and the temperature is maintained for 3-5 hours.

[0025] The nitrogen-containing atmosphere is nitrogen, ammonia, or a mixture of nitrogen, ammonia, and an inert gas; for example, N2, NH3, NH3 / Ar, etc., preferably nitrogen or ammonia.

[0026] In another embodiment of the present invention, the nitrogen-containing atmosphere is nitrogen, ammonia, or a mixture of nitrogen, ammonia, and a reducing gas; for example, N2, NH3, NH3 / H2, etc.

[0027] It should be noted that the deoxidation temperature of the method provided by the present invention is much lower than the sintering and densification temperature of silicon nitride, which effectively avoids the problem of abnormal grain growth caused by high-temperature deoxidation treatment, ensures that the material maintains excellent bending strength, and overcomes the problem of strength loss caused by grain coarsening in traditional processes.

[0028] (3) hot pressing and sintering the silicon nitride powder obtained by the deoxidation treatment in step (2).

[0029] In specific implementation, after the deoxidation treatment of the silicon nitride powder is completed, the flowing nitrogen pressure is adjusted to 1.5KPa-1.8Kpa, and the temperature is raised to 1700℃-1850℃ at a heating rate of 2-8℃ / min. After the temperature reaches 1700℃-1850℃, the sintering pressure is loaded to 25MPa-30MPa and kept warm for 5-10 hours. After the insulation is completed, the pressure is released and the temperature is lowered with the furnace to prepare high thermal conductivity silicon nitride ceramics.

[0030] In a second aspect, the present invention provides a high thermal conductivity silicon nitride ceramic, which is prepared by the method described above.

[0031] In one embodiment of the present invention, the thermal conductivity of the silicon nitride ceramic is ≥100 W / (m·K), and the flexural strength is ≥900 MPa.

[0032] Experimental results showing the technical advantages of the present invention will be described below using Examples and Comparative Examples.

[0033] It should be noted that the specifications of the aluminum nitride powder used in the following examples are all oxygen content <2.0 wt.%, and D50 = 1.5 to 3.0 μm.

[0034] Example 1 Silicon nitride powder raw material was dry-milled in a planetary ball mill with silicon nitride balls as the grinding balls, a material-to-ball ratio of 1:10, and nitrogen was placed in the milling jar for protection. The milling time was 8 hours at a speed of 200 rpm. After the milling was completed and the temperature of the milling jar cooled to room temperature, the powder was removed and quickly loaded into a hot-pressed graphite mold. A pressure of ≤1 MPa was applied to the mold while flowing nitrogen. When the nitrogen pressure reached 3 kPa, the temperature was increased at a rate of 2°C / min. After reaching 1400°C, the nitrogen pressure was increased to 15 kPa and the temperature was maintained for 5 hours. After the temperature was maintained, the flowing nitrogen pressure was adjusted to 2 kPa, and the temperature was increased at a rate of 5°C / min. At the temperature reached 1800°C, the sintering pressure was applied to 30 MPa and maintained for 8 hours. After the temperature was maintained, the pressure was released and the temperature was cooled with the furnace, producing high thermal conductivity silicon nitride ceramics.

[0035] The silicon nitride ceramic prepared in this embodiment was tested for thermal conductivity according to ASTM E1461-13 (2022), and its thermal conductivity was 110 W / (m·K). The flexural strength was tested according to GB / T 4741-1999, and its flexural strength was 923 MPa.

[0036] Example 2 Silicon nitride powder raw material was dry-milled in a planetary ball mill, using silicon nitride balls as the grinding balls at a material-to-ball ratio of 1:10. The mill was protected by nitrogen. The milling time was 6 hours at a speed of 230 rpm. After the milling was completed and the temperature of the mill cooled to room temperature, the powder was removed and quickly loaded into a hot-pressed graphite mold. A pressure of ≤1 MPa was applied to the mold while flowing nitrogen. When the nitrogen pressure reached 5 kPa, the temperature was increased at a rate of 2°C / min. After reaching 1450°C, the nitrogen pressure was increased to 10 kPa and the temperature was maintained for 3 hours. After the temperature was maintained, the flowing nitrogen pressure was adjusted to 2 kPa, and the temperature was increased at a rate of 5°C / min. At the temperature reached 1750°C, the sintering pressure was applied to 30 MPa and maintained for 8 hours. After the temperature was maintained, the pressure was released and the furnace was cooled to cool, producing high thermal conductivity silicon nitride ceramics.

[0037] The silicon nitride ceramic prepared in this embodiment was tested for thermal conductivity according to ASTM E1461-13 (2022), and its thermal conductivity was 106 W / (m·K). The flexural strength was tested according to GB / T 4741-1999, and its flexural strength was 953 MPa.

[0038] Example 3 Silicon nitride powder raw material was dry-milled in a planetary ball mill with silicon nitride balls as the grinding balls, a material-to-ball ratio of 1:10, and nitrogen was placed in the milling jar for protection. The milling time was 12 hours at a speed of 180 rpm. After the milling was completed and the temperature of the milling jar cooled to room temperature, the powder was removed and quickly loaded into a hot-pressed graphite mold. A pressure of ≤1 MPa was applied to the mold while flowing nitrogen. When the nitrogen pressure reached 4 kPa, the temperature was increased at a rate of 2°C / min. After reaching 1350°C, the nitrogen pressure was increased to 20 kPa and the temperature was maintained for 5 hours. After the temperature reached 1700°C, the sintering pressure was applied to 30 MPa and maintained for 8 hours. After the temperature was reduced to 1700°C, the pressure was released and the temperature was cooled with the furnace to produce high thermal conductivity silicon nitride ceramics.

[0039] The silicon nitride ceramic prepared in this embodiment was tested for thermal conductivity according to ASTM E1461-13 (2022), and its thermal conductivity was 102 W / (m·K). The flexural strength was tested according to GB / T 4741-1999, and its flexural strength was 968 MPa.

[0040] Comparative Example 1 Silicon nitride powder that has not undergone high-energy ball milling is placed in a hot-pressed graphite mold, and a pressure of ≤1MPa is applied to the mold while flowing nitrogen. When the nitrogen pressure reaches 3KPa, the temperature is increased at a rate of 2°C / min. When it reaches 1400°C, the nitrogen pressure is increased to 15KPa and the temperature is maintained for 5 hours. After the temperature is maintained, the flowing nitrogen pressure is adjusted to 2KPa, and the temperature is increased at a rate of 5°C / min. When the temperature reaches 1800°C, the sintering pressure is applied to 30MPa and maintained for 8 hours. After the temperature is maintained, the pressure is released and the temperature is lowered with the furnace to produce high thermal conductivity silicon nitride ceramics.

[0041] The silicon nitride powder in this comparative example was not subjected to high-energy ball milling. Other processes were consistent with those in Example 1. The silicon nitride ceramic prepared in this comparative example was tested for thermal conductivity according to ASTM E1461-13 (2022), yielding a thermal conductivity of 83 W / (m·K). The flexural strength was tested according to GB / T 4741-1999, yielding a flexural strength of 921 MPa. This indicates that the thermal conductivity of the silicon nitride ceramic not subjected to high-energy ball milling is relatively low.

[0042] Comparative Example 2 Silicon nitride powder raw material was dry-milled in a planetary ball mill with silicon nitride balls as the grinding balls, a material-to-ball ratio of 1:10, and nitrogen was placed in the milling jar for protection. The milling time was 8 hours at a speed of 200 rpm. After the milling was completed and the temperature of the milling jar cooled to room temperature, the powder was removed and quickly loaded into a hot-pressed graphite mold. A pressure of ≤1 MPa was applied to the mold while flowing nitrogen. When the nitrogen pressure reached 3 kPa, the temperature was increased at a rate of 2°C / min. After reaching 1400°C, the nitrogen pressure was increased to 5 kPa and the temperature was maintained for 5 hours. After the temperature was maintained, the flowing nitrogen pressure was adjusted to 2 kPa, and the temperature was increased at a rate of 5°C / min. When the temperature reached 1800°C, the sintering pressure was applied to 30 MPa and maintained for 8 hours. After the temperature was maintained, the pressure was released and the temperature was cooled with the furnace, producing high thermal conductivity silicon nitride ceramics.

[0043] In this comparative example, after the temperature reached 1400°C, the nitrogen pressure was relatively low (3-5 kPa), failing to meet the 10-20 kPa requirement. The other processes were consistent with those in Example 1. The silicon nitride ceramic prepared in this comparative example was tested for thermal conductivity according to ASTM E1461-13 (2022), yielding a thermal conductivity of 92 W / (m·K). The flexural strength was tested according to GB / T 4741-1999, yielding a flexural strength of 925 MPa. This indicates that insufficient nitrogen pressure does not significantly improve the thermal conductivity of the silicon nitride ceramic.

[0044] In summary, the present invention provides a silicon nitride ceramic with high thermal conductivity and a preparation method thereof. The method first performs surface activation treatment on silicon nitride powder by high-energy ball milling, significantly improving its reactivity. During the hot pressing sintering process, the carbon atmosphere generated by the graphite mold and the heating element itself is combined with a flowing nitrogen environment to achieve efficient oxygen reduction treatment at 1300-1450°C. Subsequently, short-term sintering and densification are carried out at 1700-1850°C, and finally a silicon nitride ceramic without sintering aids and having excellent thermal conductivity is obtained.

[0045] In addition, those skilled in the art should understand that, although there are many problems in the prior art, each embodiment or technical solution of the present invention may be improved in only one or several aspects, without having to simultaneously solve all the technical problems listed in the prior art or background art. Those skilled in the art should understand that any content not mentioned in a claim should not be construed as limiting the claim.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing high thermal conductivity silicon nitride ceramics, characterized in that: The following steps are involved: (1) High-energy ball milling of silicon nitride powder; (2) subjecting the silicon nitride powder obtained by high-energy ball milling in step (1) to oxygen reduction treatment; (3) hot pressing and sintering the silicon nitride powder obtained by the deoxidation treatment in step (2).

2. The method for preparing high thermal conductivity silicon nitride ceramics according to claim 1, wherein: The mass ratio of the silicon nitride powder to the grinding balls in step (1) is 1 / 5-1 / 8.

3. The method for preparing high thermal conductivity silicon nitride ceramics according to claim 1, wherein: The rotation speed of the high-energy ball mill in step (1) is 150-250 rpm; Preferably, the high-energy ball milling time in step (1) is 6-12 hours.

4. The method for preparing high thermal conductivity silicon nitride ceramics according to claim 1, wherein: The deoxygenation treatment in step (2) comprises: introducing a nitrogen-containing atmosphere at a pressure of ≤1 MPa until the pressure of the nitrogen-containing atmosphere reaches 3 KPa-5 KPa, and heating to a first temperature of 1300° C.-1450° C.; When the target temperature is reached, increase the pressure of the inert / reducing gas to 10-20 KPa and keep it at this temperature for 3-5 hours.

5. The method for preparing high thermal conductivity silicon nitride ceramics according to claim 4, characterized in that: The heating rate in step (2) is 2°C / min.

6. The method for preparing high thermal conductivity silicon nitride ceramics according to claim 4, characterized in that: The nitrogen-containing atmosphere is nitrogen, ammonia, or a mixture of nitrogen, ammonia, and an inert gas.

7. The method for preparing high thermal conductivity silicon nitride ceramics according to claim 1, characterized in that: The hot pressing sintering in step (3) includes: adjusting the pressure of the nitrogen-containing atmosphere to 1.5 KPa-1.8 KPa, heating to a second temperature, wherein the second temperature is 1700° C.-1850° C.; When the temperature reaches the second temperature, the sintering pressure is increased to 25-30 MPa and kept at this temperature for 5-10 hours.

8. The method for preparing high thermal conductivity silicon nitride ceramics according to claim 7, characterized in that: The heating rate in step (3) is 5°C / min.

9. A high thermal conductivity silicon nitride ceramic, characterized in that: The high thermal conductivity silicon nitride ceramic is prepared by the method according to any one of claims 1 to 8.

10. The high thermal conductivity silicon nitride ceramic according to claim 9, characterized in that: The thermal conductivity of the silicon nitride ceramic is ≥100 W / (m·K), and the flexural strength is ≥900 MPa.