Large-diameter thin-wall magnesium oxide insulating ring for high-pressure synthesis and preparation process thereof

Abstract

The invention belongs to the technical field of superhard material synthesis, and particularly relates to a large-diameter thin-wall magnesium oxide insulating ring for high-pressure synthesis and a preparation process thereof. The process comprises the following steps: adding magnesium oxide powder, an adhesive and a lubricant in a three-level particle size ratio into a solvent, uniformly mixing to obtain a mixture, and drying the mixture to obtain granulated powder; filling the granulation powder into an annular mold cavity of a mold in three layers, and performing four-stage two-way pressing to obtain a green body; and finally, carrying out stepped sintering on the green body to obtain the insulating ring. According to the technology, through the synergistic effect of raw material grading optimization, bidirectional balanced pressing and stepped controllable sintering, stress and shrinkage management of the whole process from powder to finished products is achieved, the problems of deformation and cracking of the large-diameter thin-wall magnesium oxide insulating ring in the forming and sintering process can be remarkably solved, and good controllability and expandability are achieved. The prepared insulating ring is low in defect rate, high in qualified rate, high in product size precision and consistency and excellent in mechanical and electrical performance.

Description

Technical Field

[0001] This invention belongs to the field of superhard material synthesis technology, specifically relating to a large-diameter thin-walled magnesium oxide insulating ring for high-pressure synthesis and its preparation process. Background Technology

[0002] In the high-temperature and high-pressure synthesis chamber of a six-sided or two-sided press, the magnesium oxide insulating ring is an indispensable core component. It integrates multiple functions such as high-voltage electrical insulation, high-temperature sealing, uniform pressure transmission, and thermal protection. Its performance directly determines the stability and safety of the synthesis process, as well as the quality and cost of the final product.

[0003] With the trend of large-scale synthesis cavities, the demanding requirements of "large diameter, thin wall thickness, and high precision" have been put forward for magnesium oxide insulating rings. However, traditional ceramic powder metallurgy processes face severe challenges in manufacturing such components: (1) Forming problems: During dry pressing, the powder flows unevenly in the mold cavity, resulting in a significant density gradient in the green body. Especially under unidirectional pressing, the density difference from top to bottom (usually more than 5%) is the root cause of warping and elliptical deformation (roundness error often exceeds 0.5%) in the sintered product. In addition, unreasonable demolding methods can easily introduce microcracks in the fragile green body. Furthermore, conventional liquid release agents are easily squeezed out or unevenly distributed under high pressure, which cannot effectively suppress the huge mold wall friction generated by thin-walled rings with large aspect ratios during the pressing process. This is one of the key factors that cause the density gradient of the green body and thus lead to sintering deformation. (2) Sintering deformation: Magnesium oxide will undergo a linear shrinkage of about 15-20% during the sintering process at 1000~1400℃. If the density of the green body is uneven, the shrinkage rate will vary in different areas, resulting in huge internal stress, which can lead to product deformation or even cracking. Conventional sintering processes have a fast heating rate and insufficient holding time, which cannot ensure that the green body is heated evenly and densified simultaneously, further amplifying the deformation.

[0004] Existing patent document CN107382283A discloses a method for dry pressing magnesium oxide ceramics, comprising the following steps: a) magnetically separating magnesium oxide powder using an electromagnetic separator, followed by ball milling to a certain fineness; b) granulating the magnesium oxide powder using an airflow granulation method; c) pressing the granulated magnesium oxide particles into a semi-finished product using a powder press; d) sintering the semi-finished product obtained in step c in a sintering furnace, obtaining magnesium oxide ceramics after sintering. This invention obtains a high-density semi-finished product by controlling the pressing conditions, and then controls the sintering conditions to prepare magnesium oxide ceramics with high density, high hardness, and good compactness. While this patent document discloses a conventional dry-pressing sintering process, it proposes an effective solution to suppress deformation of untreated magnesia ceramics, specifically addressing the unique structure of large-diameter, thin-walled rings. Patent document CN118637892A discloses an oxygen production method employing a granulation-forming-high-temperature sintering process. First, ceramic granules are obtained through binder-free granulation. Then, high-purity magnesia ceramic powder is loaded into a feeding hopper or mold, and magnesia ceramic green bodies are obtained through continuous dry pressing, isostatic pressing, or hot casting. After high-temperature sintering, dense, high-strength, and high-thermal-conductivity magnesia ceramics are obtained. In this method, rare earth oxides such as yttrium oxide, lanthanum oxide, cerium oxide, and gadolinium oxide, as well as silicon oxide, zirconium oxide, magnesium fluoride, titanium dioxide, alumina, and aluminum titanate, are used as sintering aids during ceramic preparation. The synergistic effect of these sintering aids lowers the sintering temperature and improves the mechanical strength of the magnesia ceramic. The use of binder-free granulation methods can greatly reduce the formation of porosity. Higher density magnesia ceramics have lower porosity, thus improving their thermal conductivity. This patent document focuses on improving thermal conductivity and mechanical strength through binder-free granulation and the use of doping sintering aids, but it doesn't delve deeply into dimensional accuracy control. Therefore, developing a dedicated process for the stable production of magnesia insulating rings with high dimensional accuracy and consistency has become an urgent industry need. Summary of the Invention

[0005] Based on this, the purpose of this invention is to provide a manufacturing process for large-diameter thin-walled magnesium oxide insulating rings for high-pressure synthesis. This process is a precision manufacturing process that can significantly suppress deformation and cracking of large-diameter thin-walled magnesium oxide insulating rings during molding and sintering. Through the synergistic effect of "raw material gradation optimization—bidirectional balanced pressing—step-controlled sintering," this process achieves stress and shrinkage management throughout the entire process from powder to finished product.

[0006] The present invention also provides a large-diameter thin-walled magnesium oxide insulating ring prepared by the above process. The insulating ring has a roundness error of <0.1% and excellent straightness, and can be used as a key component in equipment for high-temperature and high-pressure synthesis of superhard materials such as diamond and cubic boron nitride.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a process for preparing a large-diameter, thin-walled magnesium oxide insulating ring for high-voltage synthesis, comprising the following steps: S1, first add magnesium oxide powder with three particle size ratio, binder and lubricant together to solvent and mix well to obtain a mixture, then dry the mixture to obtain granulated powder; S2, the granulated powder described in S1 is filled into the annular cavity of the mold in three layers, and the green embryo is obtained by four-stage bidirectional pressing; S3, the green embryo described in S2 is subjected to step-by-step sintering to obtain the large-diameter thin-walled magnesium oxide insulating ring for high-pressure synthesis.

[0008] Furthermore, the proportion of the magnesium oxide powder with the three-stage particle size ratio mentioned in S1 is as follows: by mass percentage, 50-60 wt% of coarse particles with a particle size D50 of 45-65 μm, 25-35 wt% of medium particles with a particle size D50 of 10-20 μm, and 10-15 wt% of fine particles with a particle size D50 of 1-3 μm.

[0009] Furthermore, the proportions of the magnesium oxide powder with the three-stage particle size distribution are designed based on the Andreasen particle packing model. Fine particles fill the gaps between medium particles, and medium and fine particles together fill the gaps between coarse particles, thereby theoretically achieving the maximum packing density and reducing the overall shrinkage rate and shrinkage difference during sintering. Tests have shown that the tap density of the powder with this proportion can be increased by more than 18% compared to powder with a single particle size.

[0010] Further, the binder in S1 includes polyethylene glycol, the lubricant includes zinc stearate, and the solvent includes ethanol; the amount of binder added is 0.8~1.2 wt% of the weight of the magnesium oxide powder with the three particle size distribution, the amount of lubricant added is 0.3~0.5 wt% of the weight of the magnesium oxide powder with the three particle size distribution, and the amount of solvent added is 20~30 wt% of the total weight of the magnesium oxide powder, binder, and lubricant. This amount of addition ensures that all components are fully wetted and mixed evenly to form a suspension slurry suitable for subsequent spray drying.

[0011] Furthermore, in step S1, the mixing is performed by ball milling for 4-6 hours, and the drying is performed by spray drying. The inlet air temperature of the spray dryer is controlled at 180-220℃, and the outlet air temperature is controlled at 80-100℃. This temperature range ensures rapid evaporation of the ethanol solvent, prevents the slurry from sticking to the wall, and also prevents excessively high temperatures from causing the binder (such as polyethylene glycol) to coke or altering the surface properties of the magnesium oxide powder.

[0012] Furthermore, the granulated powder described in S1 is a spherical granulated powder with good flowability and uniform particle size distribution. This step aims to eliminate powder agglomeration and ensure uniform filling during subsequent pressing.

[0013] Furthermore, each of the three filling layers in S2 is supplemented with low-frequency vibration, the frequency of which is 30~50Hz; the mold is made of high-hardness alloy steel, and the surface of the annular mold cavity is mirror-polished and coated with a molybdenum disulfide (MoS2) lubricating layer. The molybdenum disulfide (MoS2) lubricating layer is a continuous, dense solid film with a thickness of 0.5~3.0μm. The combination of this solid lubricating layer and the mirror-polished surface can provide a continuous and stable ultra-low friction interface during high-pressure pressing, significantly reducing the mold wall friction effect, which is an important guarantee for achieving uniform stress transmission in bidirectional pressing and obtaining high-density uniform green blanks. Furthermore, in S2, the granulated powder is filled into the mold cavity using a "multi-point, multi-layer, equal-volume powder distribution method". This means that the powder is filled in three layers through multiple movable discharge ports, and low-frequency vibration (30~50Hz) is applied after each layer is filled to ensure that the powder is evenly distributed in the annular mold cavity, especially in the corner areas.

[0014] Furthermore, the four-stage bidirectional suppression described in S2 includes the following steps: S21, First stage pre-compression: The upper and lower punches simultaneously apply pressure to the granulated powder at a rate of 0.5~1.0 MPa / s to 20~30 MPa; the function of this stage is to complete the initial degassing and positioning. S22, Second Stage Main Pressure: The pressure increases at a rate of 1.5~2.5 MPa / s to a final pressing pressure of 100~150 MPa, and is held for 30~60 seconds; the role of this stage is the key to obtaining high-density green bodies; S23, Third stage of pressure relief and stress balance: The pressure of the upper and lower punches is reduced to 50 MPa at a rate of 0.1~0.2 MPa / s simultaneously; the purpose of this stage is to initially redistribute the elastic internal stress generated during the pressing process under the constraint of the mold. S24, Fourth Stage Progressive Demolding: Keep the lower punch in place and control the upper punch to rise at a speed of 0.05~0.1 mm / s; after the upper punch is completely detached from the blank, the lower ejection system is used to eject the blank at a speed of 0.05~0.1 mm / s; the flexible demolding method in this stage can effectively avoid lamination or transverse cracks caused by "demolding springback".

[0015] Furthermore, the stepped sintering in S3 sequentially includes a debinding stage, a pre-firing stage, a sintering stage, a homogenization stage, and a cooling stage; the temperature of the debinding stage is 25±5℃~450℃, the temperature of the pre-firing stage is 450℃~1100℃, the temperature of the sintering stage is 1100℃~1550℃, the temperature of the homogenization stage is 1530±2℃, and the temperature of the cooling stage is 1550℃~25±5℃.

[0016] Furthermore, the adhesive removal stage involves heating at a rate of 0.5~1.0℃ / min and holding at 200℃ and 450℃ for 60~90 minutes respectively. The purpose of this stage is to thoroughly and gently remove the adhesive and lubricant, avoiding bubbling or cracking of the blank due to rapid volatilization of organic matter. The pre-firing stage involves heating at a rate of 1~2℃ / min and holding at 800℃ and 1100℃ for 120 minutes each. The long holding time ensures that the internal temperature of the blank is highly uniform, laying the foundation for synchronous shrinkage.

[0017] Furthermore, during the sintering stage, the temperature is increased at a rate of 0.8~1.5℃ / min and held at 1550℃ for 180-240 minutes. This high-temperature, long-duration holding ensures sufficient diffusion, resulting in a density close to the theoretical density (>3.50 g / cm³). 3 The microstructure of ) The homogenization stage is carried out at 1530±2℃ and held for 60±5 minutes. This unique "cooling homogenization" step helps to further relax the sintering stress caused by the small density difference at high temperature. The cooling stage first raises the temperature from 1530±2℃ to 1550℃ at a rate of 1~2℃ / min, and then lowers it from 1550℃ to 1000℃ at a rate of 1~2℃ / min. After the temperature reaches 1000℃, it is naturally cooled to 25±5℃.

[0018] The present invention further provides a large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis prepared using the above-described preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis.

[0019] Furthermore, the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis possesses excellent mechanical and electrical properties. The product is sintered and dense, with a bulk density ≥3.52 g / cm³ and a flexural strength ≥140 MPa. Even at 1000℃, its volume resistivity remains at 1×10⁻⁶. 8 Ω•cm or higher, meeting the insulation requirements for ultra-high voltage combined working conditions.

[0020] The beneficial effects of this invention are: 1. When preparing magnesium oxide insulating rings using the high-pressure synthesis process provided by this invention, extremely high product dimensional accuracy and consistency can be achieved. Through complementary particle size gradients and bidirectional flexible pressing, the uniformity deviation of green compact density can be controlled within ±0.5%. Combined with stepped speed-controlled sintering, the roundness error of the final product (e.g., an insulating ring with an outer diameter of 200 mm and a wall thickness of 10 mm) is less than 0.1%, with excellent straightness and a batch dimensional fluctuation CV value of <0.3%.

[0021] 2. The process for preparing large-diameter thin-walled magnesium oxide insulating rings for high-pressure synthesis provided by this invention exhibits extremely low defect rates and high yield rates. This process systematically controls the stress at each stage from powder to finished product, increasing the one-time molding yield (no cracks, no macroscopic deformation) from the industry average of below 75% to over 98%.

[0022] 3. The process for preparing large-diameter, thin-walled magnesium oxide insulating rings for high-voltage synthesis provided by this invention exhibits excellent process controllability and scalability. The process parameters are clearly defined and easily automated and digitally controlled, providing a reliable technical solution for the large-scale production of high-performance magnesium oxide insulating rings of different sizes and specifications.

[0023] 4. The magnesium oxide insulating ring prepared using the high-voltage synthesis process for large-diameter thin-walled magnesium oxide insulating rings provided by this invention possesses excellent mechanical and electrical properties. The product is sintered and dense, with a bulk density ≥3.52 g / cm³ and a bending strength ≥140 MPa. Even at 1000℃, the volume resistivity remains at 1×10⁻⁶. 8 With a strength of Ω•cm or higher, it fully meets the insulation requirements for ultra-high voltage combined working conditions. Attached Figure Description

[0024] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Appendix Figure 1 This is a product image of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis prepared in Example 1 of this invention; Appendix Figure 2 This is a product image of the magnesium oxide insulating ring prepared in Comparative Example 1 of this invention. Detailed Implementation

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below. All mentioned embodiments are implemented based on the technical solutions of the present invention, and detailed implementation processes are given. However, it should be stated that the scope of protection of the present invention is not limited to the following embodiments.

[0027] The following embodiments provide detailed implementation procedures for the technical solutions of the present invention. Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; unless otherwise specified, the materials and reagents used are commercially available.

[0028] Example 1 The production of magnesium oxide insulating rings with an outer diameter of Φ100mm, an inner diameter of Φ96mm, and a height of 50mm includes the following steps: S1, Ingredients: Take 55 wt% coarse magnesium oxide powder with D50=55μm, 30 wt% medium granules with D50=15μm, and 15 wt% fine granules with D50=2μm. Add 1 wt% polyethylene glycol and 0.4 wt% zinc stearate to the total powder weight, ball mill for 5 hours with anhydrous ethanol as the medium, and spray dry to obtain granulated powder. The inlet air temperature of the spray dryer is controlled at 200℃ and the outlet air temperature is controlled at 90℃.

[0029] S2, Suppression: 1. High-hardness alloy steel molds are used. The mold cavity surface is mirror-polished and coated with a molybdenum disulfide lubricating layer with a thickness of 1.5μm. The granulated powder is filled into the mold cavity using a "multi-point, multi-layer, equal-volume powder distribution method," that is, it is filled in three layers through multiple movable feeding ports. After each layer is filled, a 40Hz low-frequency vibration is applied to ensure that the powder is evenly distributed in the annular mold cavity, especially in the corner areas.

[0030] 2. Implement four-stage bidirectional pressing on a 160-ton hydraulic press: (1) First stage pre-compression: The upper and lower punches apply pressure to the powder at a rate of 0.8MPa / s to 25MPa, completing the initial venting and positioning.

[0031] (2) Second stage main pressure: The pressure increases at a rate of 2.0 MPa / s to the final pressing pressure of 120 MPa, and is held for 45 seconds. This stage is the key to obtaining high-density green billets.

[0032] (3) Third stage of pressure relief and stress balance: The pressure of the upper and lower punches is reduced to 50MPa at an extremely slow rate of 0.15MPa / s, so that the elastic internal stress generated during the pressing process is initially redistributed under the constraint of the mold.

[0033] (4) Fourth stage progressive demolding: Keep the lower punch in place and control the upper punch to slowly rise at a speed of 0.08 mm / s. After the upper punch is completely detached from the blank, use the lower ejection system to smoothly eject the green blank at the same slow speed to obtain the green blank. This flexible demolding method can effectively avoid lamination or transverse cracks caused by "demolding springback".

[0034] S3, Sintering: The green billet is placed on a high-precision alumina pad and sent into an atmosphere sintering furnace. The sintering process is strictly carried out according to the five-stage curve.

[0035] (1) Low temperature adhesive removal stage (25~450℃): The temperature is increased at a very slow rate of 1.0℃ / min, and the temperature is maintained at 200℃ and 450℃ for 80 minutes respectively. The purpose of this stage is to thoroughly and gently remove the adhesive and lubricant, so as to avoid the bubbling or cracking of the green body caused by the rapid volatilization of organic matter.

[0036] (2) Medium-temperature pre-firing stage (450℃~1100℃): The heating rate is increased to 1.5℃ / min. The temperature is held at 800℃ (the activation temperature of MgO lattice) and 1100℃ (the sintering start temperature) for 120 minutes each. The long holding time makes the internal temperature of the green body highly uniform, laying the foundation for synchronous shrinkage.

[0037] (3) High-temperature sintering stage (1100℃~1550℃): This is the densification core stage. The heating rate is controlled at 1.0℃ / min, and the temperature is held at the target sintering temperature of 1550℃ for 210 minutes. The long-term high-temperature holding ensures sufficient diffusion and obtains a density close to the theoretical density (>3.50 g / cm³). 3 The microstructure of ).

[0038] (4) High-temperature homogenization stage: After holding at 1550℃, the temperature is lowered by 20℃ to 1530℃ and held again for 60 minutes. This unique "cooling homogenization" step helps to further relax the sintering stress caused by the small density difference at high temperature.

[0039] (5) Controlled cooling stage (1550℃~25℃): The initial cooling stage (1550℃~1000℃) is crucial. The heating rate is controlled at 1.5℃ / min to raise the temperature from 1530℃ to 1550℃, and then the cooling rate is 1.5℃ / min to lower the temperature from 1550℃ to 1000℃ to help the material overcome its brittle temperature range. Below 1000℃, the material is allowed to cool naturally to room temperature in the furnace.

[0040] In this embodiment, 100 magnesium oxide insulating rings were prepared in one batch. Figure 1 As shown in the image. Testing revealed that the molding qualification rate of this batch of insulating rings was 98%. The average roundness error of the products was 0.08%, with excellent straightness and a batch dimensional fluctuation CV value of <0.3%. The products are sintered densely, with a bulk density of 3.54 g / cm³ and a flexural strength of 148 MPa. No cracking or insulation failure occurred under a 15 GPa high-pressure environment, and the performance fully meets the design requirements. The volume resistivity at 1000℃ is 2.8 × 10⁻⁶. 8 Ω·cm, fully meeting the insulation requirements of ultra-high voltage combined working conditions.

[0041] Comparative Example 1 Magnesium oxide insulating rings of the same specifications were prepared using traditional unidirectional dry pressing and conventional sintering processes. The specific steps are as follows: S1, Ingredients and Granulation: 100 wt% of single-grade magnesium oxide powder with an average particle size D50≈45μm was used, along with 1.0 wt% of polyethylene glycol and 0.4 wt% of zinc stearate. After dry mixing for 2 hours, mechanical granulation was performed to obtain granulated powder.

[0042] S2, Molding: On a conventional hydraulic press, the granulated powder is added to the mold in one go using a unidirectional pressing method. The pressure is increased from 1.5 MPa / s to 120 MPa, and after holding the pressure for 10 seconds, the mold is quickly demolded to obtain the green body.

[0043] S3, Sintering: The green billet is placed directly into an air atmosphere sintering furnace and heated to 1550°C at a rate of 3°C / min. After holding at this temperature for 180 minutes, the power is turned off and the furnace is allowed to cool naturally.

[0044] One batch of 100 magnesium oxide insulating rings was prepared using the above process. The finished product morphology is as follows: Figure 2 As shown, obvious elliptical deformation, warping, and edge defects are visible. Testing revealed that the first-pass yield of this batch of products was only 43% (i.e., products without macroscopic cracks or severe deformation). The average roundness error of the qualified products was 0.85%, indicating poor dimensional consistency (CV value > 1.0%). The average bulk density of the qualified products was 3.48 g / cm³, and the average flexural strength was 125 MPa. In a 15 GPa high-voltage environment simulation test, approximately 15% of the qualified products showed cracks or insulation failure. At a high temperature of 1000℃, its average volume resistivity was 6.5 × 10⁻⁶. 7 Ω·cm. This result fully demonstrates that traditional processes cannot effectively control the forming and sintering stress of large-diameter thin-walled magnesium oxide insulating rings, resulting in low product qualification rate, poor dimensional accuracy, unstable performance, and insufficient reliability. This, in turn, confirms the systematic advantages of the process of this invention.

[0045] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A process for preparing a large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis, characterized in that, Includes the following steps: S1, first add magnesium oxide powder with three particle size ratio, binder and lubricant together to solvent and mix well to obtain a mixture, then dry the mixture to obtain granulated powder; S2, the granulated powder described in S1 is filled into the annular cavity of the mold in three layers, and the green embryo is obtained by four-stage bidirectional pressing; S3, the green embryo described in S2 is subjected to step-by-step sintering to obtain the large-diameter thin-walled magnesium oxide insulating ring for high-pressure synthesis.

2. The preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis as described in claim 1, characterized in that, By mass percentage, the proportion of magnesium oxide powder with the three-stage particle size distribution in S1 is as follows: 50-60 wt% of coarse particles with a particle size D50 of 45-65 μm, 25-35 wt% of medium particles with a particle size D50 of 10-20 μm, and 10-15 wt% of fine particles with a particle size D50 of 1-3 μm.

3. The preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis as described in claim 1, characterized in that, The adhesive in S1 includes polyethylene glycol, the lubricant includes zinc stearate, and the solvent includes ethanol; the amount of adhesive added is 0.8~1.2 wt% of the weight of the magnesium oxide powder with the three particle size distribution, the amount of lubricant added is 0.3~0.5 wt% of the weight of the magnesium oxide powder with the three particle size distribution, and the amount of solvent added is 20~30 wt% of the total weight of the magnesium oxide powder, adhesive, and lubricant.

4. The preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis as described in claim 1, characterized in that, The mixing in S1 is carried out by ball milling for 4 to 6 hours, and the drying is carried out by spray drying. The inlet air temperature of the spray drying is controlled at 180 to 220°C, and the outlet air temperature is controlled at 80 to 100°C.

5. The preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis as described in claim 1, characterized in that, Each of the three filling layers in S2 is supplemented with low-frequency vibration after filling, and the frequency of the low-frequency vibration is 30~50Hz; the mold is made of high-hardness alloy steel, and the surface of the annular mold cavity is mirror polished and coated with a molybdenum disulfide lubricating layer, which is a solid film with a thickness of 0.5~3.0μm.

6. The preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis as described in claim 1, characterized in that, The four-stage bidirectional suppression described in S2 includes the following steps: S21, First stage pre-compression: The upper and lower punches simultaneously apply pressure to the granulated powder at a rate of 0.5~1.0 MPa / s to 20~30 MPa; S22, Second Stage Main Pressure: The pressure increases at a rate of 1.5~2.5 MPa / s to the final pressing pressure of 100~150 MPa, and is held for 30~60 seconds; S23, Third stage of pressure relief and stress balance: Simultaneously reduce the pressure of the upper and lower punches to 50MPa at a rate of 0.1~0.2 MPa / s; S24, Fourth Stage Progressive Demolding: Keep the lower punch in place and control the upper punch to rise at a speed of 0.05~0.1 mm / s; after the upper punch is completely detached from the blank, use the lower ejection system to eject the green blank at a speed of 0.05~0.1 mm / s.

7. The preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis as described in claim 1, characterized in that, The stepped sintering described in S3 includes, in sequence, a debinding stage, a pre-firing stage, a sintering stage, a homogenization stage, and a cooling stage; the temperature of the debinding stage is 25±5℃~450℃, the temperature of the pre-firing stage is 450℃~1100℃, the temperature of the sintering stage is 1100℃~1550℃, the temperature of the homogenization stage is 1530±2℃, and the temperature of the cooling stage is 1550℃~25±5℃.

8. The preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis as described in claim 7, characterized in that, The debinding stage involves heating at a rate of 0.5~1.0℃ / min and holding at 200℃ and 450℃ for 60~90 minutes respectively; the pre-firing stage involves heating at a rate of 1~2℃ / min and holding at 800℃ and 1100℃ for 120 minutes each.

9. The preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis as described in claim 7, characterized in that, The sintering stage involves heating at a rate of 0.8~1.5℃ / min and holding at 1550℃ for 180-240 minutes; the homogenization stage involves heating at 1530±2℃ and holding for 60±5 minutes. The cooling stage first raises the temperature from 1530±2℃ to 1550℃ at a rate of 1~2℃ / min, and then lowers it from 1550℃ to 1000℃ at a rate of 1~2℃ / min. After the temperature reaches 1000℃, it is naturally cooled to 25±5℃.

10. A large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis prepared using the preparation process of the large-diameter thin-walled magnesium oxide insulating ring for high-voltage synthesis according to any one of claims 1 to 9.

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