Silicon nitride ceramic and multi-step pressureless sintering process and application thereof

Abstract

The invention relates to silicon nitride ceramic as well as a multi-step pressureless sintering process and application thereof. The silicon nitride ceramic is prepared from the following raw material components: silicon nitride powder, a sintering aid and a binder, wherein the silicon nitride powder comprises alpha-silicon nitride powder and beta-silicon nitride powder; the sintering aid comprises oxides or nitrates of Y, Al and Bi. The silicon nitride ceramic adopts a Y-Al-Bi multi-element composite sintering aid, and the sintering aid with a low melting point not only reduces the liquid-phase sintering temperature, but also can introduce an interface energy level, increase sunken charges and improve the resistivity. According to the silicon nitride ceramic disclosed by the invention, beta-phase seed crystal introduction and multi-step pressureless sintering are matched, so that preparation of the low-temperature-sensitivity and high-toughness integrated silicon nitride ceramic is realized. The volume resistivity of the prepared silicon nitride insulating ceramic at 80 DEG C is greater than or equal to 1.0 * 10 < 14 > omega.m, the decrease rate is not higher than 55% compared with room temperature, and the fracture toughness is greater than or equal to 6.3 Mpa * m < 1 / 2 >.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of electrical insulating ceramics, and particularly relates to a silicon nitride ceramic, a multi-step pressureless sintering process thereof, and applications. Background Art

[0002] Due to its high strength, high toughness, high thermal conductivity, and excellent oxidation resistance, creep resistance, and high resistivity, silicon nitride ceramic has become one of the most widely used structural ceramics. With the development of powder metallurgy technology, silicon nitride ceramic, as a high-quality material with integrated structure and function, has attracted increasing attention. Its characteristics of high strength, high thermal conductivity, and high resistivity make it show significant application potential in the field of extra-high voltage power transmission.

[0003] DC wall bushings are key equipment in extra-high voltage DC power transmission systems. Among them, post insulators and basin insulators are the core components of gas internal insulation and undertake the important responsibility of supporting the conductor rod. These post insulators need to withstand the coupled action of high voltage, large temperature gradient, and multiple stresses for a long time and face severe insulation and mechanical property challenges. Compared with the existing epoxy materials, silicon nitride ceramic shows significant advantages in various indexes such as mechanical properties and volume resistivity, so it has the potential to be applied in the internal insulation scenario and solve the flashover fault problem. However, the current silicon nitride ceramic has defects such as poor resistivity temperature stability and low toughness. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems of poor resistivity temperature stability and low toughness existing in the current silicon nitride ceramic.

[0005] The purpose of the present invention is achieved by adopting the following technical solutions:

[0006] A silicon nitride ceramic is made from the following raw materials: silicon nitride powder, sintering aids, and binders;

[0007] Wherein, the silicon nitride powder includes α-silicon nitride powder and β-silicon nitride powder;

[0008] The sintering aids include oxides or nitrates of Y, Al, and Bi.

[0009] Preferably, the binder is one or more of polyvinyl butyral, polyvinyl alcohol, or polyvinylpyrrolidone.

[0010] Preferably, the mass ratio of the oxide or nitrate of Y, the oxide or nitrate of Al, the oxide or nitrate of Bi, the binder to the silicon nitride powder is (0.01 - 0.12):(0.01 - 0.08):(0.01 - 0.05):(0.01 - 0.10):1.

[0011] Preferably, the α-Si3N4 crystalline form of silicon nitride accounts for 80 to 99.5% of the weight of the silicon nitride powder.

[0012] The present invention also provides a multi-step pressureless sintering process for silicon nitride ceramics for preparing the silicon nitride ceramics, comprising the following steps:

[0013] Weigh silicon nitride powder, sintering aids and binders in proportion, and perform ball milling and mixing to obtain a slurry;

[0014] Dry and shape the slurry to obtain a green ceramic body;

[0015] Debind the green ceramic body in flowing air to obtain a debound green ceramic body;

[0016] Heat the debound green ceramic body from room temperature to a pre-sintering temperature in a flowing nitrogen atmosphere, hold for 1 to 5 h, continue to heat to a first holding temperature, hold for 1 to 8 h, then cool to a second holding temperature, hold for 1 to 8 h, and then naturally cool to obtain the silicon nitride ceramic.

[0017] Preferably, the pre-sintering temperature is 1400 to 1450 °C; and / or

[0018] The first holding temperature is 1780 to 1820 °C; and / or

[0019] The second holding temperature is 1350 to 1450 °C.

[0020] Preferably, the pre-sintering temperature is 1450 °C; and / or

[0021] The first holding temperature is 1800 °C; and / or

[0022] The second holding temperature is 1400 °C.

[0023] Preferably, heat from room temperature to the pre-sintering temperature at a rate of 0.3 to 15 °C / min; and / or

[0024] Heat to the first holding temperature at a rate of 3 to 5 °C / min; and / or

[0025] Cool to the second holding temperature at a rate of 3 to 5 °C / min.

[0026] Preferably, the shaping method is one or more of dry pressing and cold isostatic pressing.

[0027] Preferably, the debinding process specifically includes: heating from room temperature to 350 - 600 °C at a rate of 0.1 - 2 °C / min, holding for 1 - 6 h, then heating to 350 - 600 °C at a rate of 0.1 - 2 °C / min, and holding for 0.5 - 5 h. Among them, the required air flow rate is 0.5 - 5 L / min.

[0028] Based on the same inventive concept, the present invention also provides the application of the silicon nitride ceramic or the silicon nitride ceramic prepared according to the multi-step pressureless sintering process in the field of extra-high voltage power transmission.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] The present invention provides a silicon nitride ceramic, the raw materials of which include the following components: silicon nitride powder, sintering aids, and binders; among them, the silicon nitride powder includes α-silicon nitride powder and β-silicon nitride powder; the sintering aids include oxides or nitrates of Y, Al, and Bi. The silicon nitride ceramic adopts a Y-Al-Bi multi-component composite sintering aid. Among them, the low-melting-point sintering aid not only reduces the liquid-phase sintering temperature, but also can introduce interface energy levels, increase trapped charges, and improve the resistivity. The α-silicon nitride and β-silicon nitride act synergistically to improve the toughness while ensuring the strength.

[0031] The crystal structures of the α-silicon nitride and β-silicon nitride are different: the α-phase is a hexagonal crystal system, with higher activity, and is easy to react with additives during sintering, forming a liquid phase to promote particle rearrangement and densification, and can also refine grains to improve toughness. The β-phase is a hexagonal crystal system or a trigonal crystal system, with a more stable crystal structure, and can maintain the structural integrity of the material at high temperatures, providing high-temperature strength and wear resistance. The α-phase can promote densification during the sintering process, helping to improve the hardness and toughness of the material; the β-phase can improve the high-temperature stability of the material, enhance the strength and wear resistance of the material. The cooperation of the two can enable the silicon nitride ceramic to reach a high density faster, improve the sintering efficiency and quality, and the combined action can make the comprehensive mechanical properties of the silicon nitride ceramic more excellent.

[0032] The sintering aids can synergistically reduce the sintering temperature. The liquid phases formed by the oxides of Y, Al, and Bi have different chemical compositions and physical properties. They dissolve and interact with each other to form a composite liquid phase with better performance. This composite liquid phase has a lower melting point and better fluidity, enabling particle rearrangement and mass diffusion at a lower temperature, thereby promoting the sintering process. When acting together, they can significantly reduce the sintering temperature of silicon nitride ceramics. At the same time, Y2O3 stabilizes the β phase, Al2O3 promotes the transformation from the α phase to the β phase and improves the grain boundary properties, and Bi2O3 improves grain growth and microstructure. When used simultaneously, the mechanical properties such as the strength and toughness of silicon nitride ceramics are significantly improved, which is more obvious than when using only one or two additives. In addition, the grain boundary phase formed by Y2O3, Al2O3, and Bi2O3 has better stability at high temperatures, and they can inhibit grain boundary sliding and grain growth, thereby improving the creep resistance of the material. The nitrates of Y, Al, and Bi transform into oxides at high temperatures, and the principle of action is the same.

[0033] The multi-step pressureless sintering process of the present invention first pre-sinters the ceramic green body after debinding at about 1400 - 1450 °C to promote the formation of a bimodal morphology and increase the toughness of the ceramic. Then, it is heated to about 1780 - 1820 °C for a long time of heat preservation to achieve densification through grain boundary diffusion. Then, it is cooled to 1350 - 1450 °C and kept warm for a period of time to promote the recrystallization of the second phase, reducing the temperature sensitivity of the volume resistivity by reducing the glass phase content. The silicon nitride insulating ceramic obtained by this method has a volume resistivity ≥ 1.0×1014 Ω·m at 80 °C, and the decrease rate compared with room temperature is not higher than 55%, and the fracture toughness ≥ 6.3 Mpa·m 1 / 2 。

[0034] The silicon nitride ceramic of the present invention uses a Y-Al-Bi multi-component composite sintering aid, and through the cooperation of β-phase seed introduction and multi-step pressureless sintering, the preparation of a low-temperature-sensitivity and high-toughness integrated silicon nitride ceramic is realized. The process is simple, the cost is low, it is more suitable for practical use, and it has industrial utilization value. Description of the Drawings

[0035] Figure 1 It is a SEM electron micrograph of the fracture surface of the silicon nitride ceramic prepared in Example 1 of the present invention. Detailed Embodiments

[0036] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. The following embodiments are provided to better understand the present invention, and are not limited to the described best embodiment. They do not limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with those of other prior arts falls within the protection scope of the present invention.

[0037] Embodiment 1

[0038] The silicon nitride ceramic of this embodiment is made from the following raw materials: silicon nitride powder, sintering aids, and binder; wherein, the silicon nitride powder includes α-silicon nitride powder and β-silicon nitride powder; the α-silicon nitride powder accounts for 98% by weight of the silicon nitride powder. The sintering aids include Y2O3, Al2O3, and Bi2O3; the binder is polyvinyl alcohol; the mass ratio of Y2O3, Al2O3, Bi2O3, the binder to the silicon nitride powder is 0.05:0.03:0.01:0.05:1.

[0039] The preparation method of the silicon nitride ceramic of this embodiment is as follows: respectively take 9.8 kg of α-Si3N4 powder and 0.2 kg of β-Si3N4 powder, 0.05 kg of sintering aid Y2O3, 0.03 kg of Al2O3, 0.01 kg of Bi2O3, and 0.05 kg of binder polyvinyl alcohol, and ball mill for 10 hours with alcohol as the medium. The slurry is dried by spray drying process, then dry-pressed into shape under a pressure of 100 MPa, and further cold isostatically pressed under a pressure of 200 MPa. The formed green body is degummed in flowing air, the air flow rate is 3 L / min, heated from room temperature to 400 °C in 3 hours, held for 6 hours, then heated to 550 °C in 5 hours and held for 3 hours. The degummed green body is heated to 1450 °C at a rate of 10 °C / min in a vacuum carbon tube furnace under a nitrogen atmosphere and held for 3 hours. Subsequently, it is heated to 1800 °C at a rate of 3 °C / min and held for 6 hours. Then it is cooled to 1400 °C at a rate of 3 °C / min and held for 5 hours, and then naturally cooled to obtain the silicon nitride ceramic. After testing, the silicon nitride ceramic prepared in this embodiment has a volume resistivity of 8.0×1014 Ω·m at 80 °C, a 52% decrease compared with room temperature, and a fracture toughness of 6.3 Mpa·m 1 / 2 。

[0040] Embodiment 2

[0041] The silicon nitride ceramic of this embodiment comprises the following raw materials: silicon nitride powder, sintering aids, and binder; wherein, the silicon nitride powder includes α-silicon nitride powder and β-silicon nitride powder; the α-silicon nitride powder accounts for 99% of the weight of the silicon nitride powder. The sintering aids include Y(NO3)3, Al(NO3)3, and Bi2O3; the binder is polyvinyl alcohol; the mass ratio of Y(NO3)3, Al(NO3)3, Bi2O3, the binder to the silicon nitride powder is 0.08:0.05:0.02:0.05:1.

[0042] The preparation method of the silicon nitride ceramic of this embodiment is as follows: respectively take 9.9 kg of α-Si3N4 powder and 0.1 kg of β-Si3N4 powder, 0.08 kg of sintering aid Y(NO3)3, 0.05 kg of Al(NO3)3, 0.02 kg of Bi2O3, and 0.05 kg of binder polyvinyl alcohol, and ball mill for 10 hours with alcohol as the medium. The slurry is dried by spray drying process, and then further cold isostatically pressed under a pressure of 220 MPa. The formed green body is degummed in flowing air, the air flow rate is 3 L / min, heated from room temperature to 400 °C in 3 hours, held for 6 hours, then heated to 550 °C in 5 hours and held for 3 hours. The degummed green body is heated to 1500 °C at a rate of 10 °C / min in a vacuum carbon tube furnace under a nitrogen atmosphere and held for 3 hours. Then it is heated to 1800 °C at a rate of 3 °C / min and held for 6 hours. Then it is cooled to 1400 °C at a rate of 3 °C / min and held for 5 hours, and then naturally cooled to obtain the silicon nitride ceramic. After testing, the silicon nitride ceramic prepared in this embodiment has a volume resistivity of 6.5×1014 Ω·m at 80 °C, a decrease rate of 50% compared with room temperature, and a fracture toughness of 6.7 Mpa·m 1 / 2 。

[0043] Example 3

[0044] The silicon nitride ceramic of this embodiment comprises the following raw materials: silicon nitride powder, sintering aids, and binder; wherein, the silicon nitride powder includes α-silicon nitride powder and β-silicon nitride powder; the α-silicon nitride powder accounts for 95% of the weight of the silicon nitride powder. The sintering aids include Y2O3, Al2O3, and Bi2O3; the binder is polyvinyl butyral; the mass ratio of Y2O3, Al2O3, Bi2O3, the binder to the silicon nitride powder is 0.05:0.03:0.05:0.062:1.

[0045] The preparation method of the silicon nitride ceramic in this embodiment is as follows: 9.5 kg of α-Si3N4 powder, 0.5 kg of β-Si3N4 powder, 0.05 kg of sintering aid Y2O3, 0.03 kg of Al2O3, 0.05 kg of Bi2O3, and 0.062 kg of binder polyvinyl butyral are taken respectively, and ball-milled for 10 hours with alcohol as the medium. The slurry is dried by spray drying process, then dry-pressed into shape under a pressure of 100 MPa, and further cold isostatically pressed under a pressure of 180 MPa. The green body after forming is degummed in flowing air, the air flow rate is 3 L / min, heated from room temperature to 400 °C in 3 hours, held for 6 hours, then heated to 550 °C in 5 hours and held for 3 hours. The degummed green body is heated to 1450 °C at a rate of 10 °C / min in a vacuum carbon tube furnace under a nitrogen atmosphere and held for 3 hours. Then it is heated to 1820 °C at a rate of 3 °C / min and held for 6 hours. Then it is cooled to 1400 °C at a rate of 3 °C / min and held for 8 hours, and then naturally cooled to obtain the silicon nitride ceramic. After testing, the silicon nitride ceramic prepared in this embodiment has a volume resistivity of 7.3×1014 Ω·m at 80 °C, a decrease rate of 46% compared with room temperature, and a fracture toughness of 7.2 Mpa·m 1 / 2 。

[0046] Examples 4 to 8

[0047] The differences between the silicon nitride ceramic compositions of Examples 4 to 8 and Example 1 are shown in Table 1, and the specific preparation method is the same as that of Example 1.

[0048] Table 1 Differences in raw material ratios

[0049]

[0050]

[0051] Examples 9 - 13

[0052] The silicon nitride ceramic compositions of Examples 9 to 13 are the same as those of Example 1, and the differences in the preparation methods are shown in Table 2.

[0053] Table 2 Differences in process parameters

[0054] Process parameters Example 9 Example 10 Example 11 Example 12 Example 13 Pre-sintering temperature (°C) 1400 1420 1450 1430 1450 Pre-sintering holding time (h) 1 3 4 5 2 First holding temperature (°C) 1780 1800 1790 1810 1820 First holding time (h) 1 3 5 7 8 Second holding temperature (°C) 1350 1360 1400 1420 1450 Second holding time (h) 5 1 4 6 8

[0055] After testing, the silicon nitride ceramics prepared in Examples 4 to 13 have a volume resistivity of ≥1.0×1014 Ω·m at 80 °C, a decrease rate compared with room temperature of not more than 55%, and a fracture toughness of ≥6.3 Mpa·m 1 / 2 。

[0056] Comparative Example 1

[0057] This example is basically the same as Example 1, except that β-phase silicon nitride seeds are not used. The fracture toughness of the silicon nitride ceramic obtained under the same preparation process decreases, and is only 5.8 Mpa·m 1 / 2 .

[0058] Comparative Example 2

[0059] This example is basically the same as Example 1, except that BiO2 sintering aid is not used, and the selected sintering aids are 5.0 wt.% Y2O3 and 3.0 wt.% Al2O3. The volume resistivity and resistivity temperature stability of the silicon nitride ceramic obtained under the same preparation process both decrease. The volume resistivity at 80 °C is 4.5×1014 Ω·m, and the decrease rate compared with room temperature is 66%.

[0060] Comparative Example 3

[0061] This example is basically the same as Example 1, except that only a one-step sintering process is used. Under a nitrogen atmosphere, it is heated to 1500 °C at a rate of 10 °C / min without heat preservation treatment. Subsequently, it is heated to 1800 °C at a rate of 3 °C / min and held for 6 hours. The volume resistivity temperature stability and fracture toughness of the silicon nitride ceramic obtained under the same preparation process both decrease. The volume resistivity at 80 °C is 5.1×1014 Ω·m, and the decrease rate compared with room temperature is 63%. The fracture toughness is only 5.6 Mpa·m 1 / 2 .

[0062] Comparative analysis

[0063] The test results of the silicon nitride ceramics prepared in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 3.

[0064] Table 3 Test results

[0065]

[0066] As can be seen from Table 3, for the silicon nitride ceramic obtained by using the raw material composition and preparation method of the present invention, the volume resistivity at 80 °C ≥ 1.0×1014 Ω·m, the decrease rate compared with room temperature is not higher than 55%, and the fracture toughness ≥ 6.3 Mpa·m 1 / 2。It can be seen from the comparison between Comparative Example 1 and Example 1 that the introduction of β-phase silicon nitride seeds significantly improves the fracture toughness of silicon nitride ceramics; it can be seen from the comparison between Comparative Example 2 and Example 1 that the Y-Al-Bi multi-component composite sintering aid can significantly improve the volume resistivity and resistivity temperature stability of silicon nitride ceramics; it can be seen from the comparison between Comparative Example 3 and Example 1 that the volume resistivity temperature stability and fracture toughness of the silicon nitride ceramics obtained by the multi-step pressureless sintering process described in the present invention are both significantly improved. At the microscopic level, the fracture surface of the silicon nitride ceramics obtained in Example 1 was observed with a scanning electron microscope, and the results are as Figure 1 shown. It can be seen that the ceramic microstructure is uniform, without obvious defects, and the cross-interlocked β-phase columnar crystals ensure excellent fracture toughness. In summary, the present invention uses a Y-Al-Bi multi-component composite sintering aid, and through the cooperation of β-phase seed introduction and multi-step pressureless sintering, the preparation of low-temperature-sensitive and high-toughness integrated silicon nitride ceramics is realized.

[0067] The above are only examples of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the scope of the claims of the present invention pending approval.

Claims

1. A silicon nitride ceramic, characterized in that, Its raw materials include the following components: silicon nitride powder, sintering aids, and binders; Among them, the silicon nitride powder includes α-silicon nitride powder and β-silicon nitride powder; The sintering aids include oxides or nitrates of Y, Al, and Bi.

2. The silicon nitride ceramic according to claim 1, wherein The binder is one or more of polyvinyl butyral, polyvinyl alcohol, or polyvinylpyrrolidone.

3. The silicon nitride ceramic according to claim 1, wherein The mass ratio of the oxides or nitrates of Y, the oxides or nitrates of Al, the oxides or nitrates of Bi, the binder to the silicon nitride powder is (0.01 - 0.12):(0.01 - 0.08):(0.01 - 0.05):(0.01 - 0.10):

1.

4. The silicon nitride ceramic according to claim 1, characterized in that, The α-silicon nitride powder accounts for 80 - 99.5% of the weight of the silicon nitride powder.

5. A multi-step pressureless sintering process for silicon nitride ceramics, which is used to prepare the silicon nitride ceramics according to any one of claims 1 to 4, characterized in that, It includes the following steps: Weigh the silicon nitride powder, sintering aids, and binders in proportion, and carry out ball milling and mixing to obtain a slurry; Dry and shape the slurry to obtain a green ceramic body; Debind the green ceramic body in flowing air to obtain a debound green ceramic body; Heat the debound green ceramic body from room temperature to the pre-sintering temperature in a flowing nitrogen atmosphere, hold for 1 - 5 h, continue to heat to the first holding temperature, hold for 1 - 8 h, then cool to the second holding temperature, hold for 1 - 8 h, and then cool naturally to obtain the silicon nitride ceramic.

6. According to the multi-step pressureless sintering process described in claim 5, characterized in that The pre-sintering temperature is 1400 - 1450 °C; and / or The first holding temperature is 1780 - 1820 °C; and / or The second holding temperature is 1350 - 1450 °C.

7. According to the multi-step pressureless sintering process described in claim 6, characterized in that The pre-sintering temperature is 1450 °C; and / or The first holding temperature is 1800 °C; and / or The second holding temperature is 1400 °C.

8. According to the multi-step pressureless sintering process described in claim 5, characterized in that Heat from room temperature to the pre-sintering temperature at a rate of 0.3 - 15 °C / min; and / or Heat to the first holding temperature at a rate of 3 - 5 °C / min; and / or Cool to the second holding temperature at a rate of 3 - 5 °C / min.

9. The multi-step pressureless sintering process according to claim 5, wherein The drying method is spray drying; the shaping method is one or more of dry pressing and cold isostatic pressing.

10. The multi-step pressureless sintering process according to claim 5, wherein The debinding process specifically includes: heating from room temperature to 350 - 600 °C at a rate of 0.1 - 2 °C / min, holding for 1 - 6 h, then heating to 350 - 600 °C at a rate of 0.1 - 2 °C / min, holding for 0.5 - 5 h, where the required air flow rate is 0.5 - 5 L / min.

11. The application of the silicon nitride ceramic described in any one of claims 1 - 4 or the silicon nitride ceramic prepared by the multi-step pressureless sintering process described in any one of claims 5 - 10 in the field of ultra-high voltage power transmission.

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