A sintering temperature field regulation method for a silicon carbide mirror blank with a complex lightweight structure

Abstract

This invention relates to a method for controlling the sintering temperature field of a silicon carbide mirror blank with a complex lightweight structure, comprising: (1) analyzing the temperature field distribution during sintering of the silicon carbide mirror blank with a complex lightweight structure using a finite element thermal analysis simulation method; (2) covering the outside of the silicon carbide mirror blank with a complex lightweight structure with a material as a secondary heat source to homogenize the overall temperature field; (3) placing the material as a secondary heat source into each level of the lightweight structure in the silicon carbide mirror blank with a complex lightweight structure to compensate for the local temperature field; (4) first heating to 1300-1500℃ at 2-10℃ / min, and then heating to 1950-2200℃ at 0.5-5℃ / min for sintering to obtain the silicon carbide mirror blank with a complex lightweight structure.

Description

Technical Field

[0001] This invention relates to a method for preparing a silicon carbide mirror blank with a complex and lightweight structure, specifically a method for precisely controlling the sintering temperature of a silicon carbide mirror blank with a complex and lightweight structure, belonging to the field of optical component manufacturing. Background Technology

[0002] Lightweight mirrors are core components of space optical imaging systems such as aerospace / aviation remote sensing, astronomical optics, deep space exploration, and high-energy lasers. The performance of mirrors plays a decisive role in the resolution, sensitivity, and other indicators of optical systems. Mirror blanks are the basic materials for manufacturing mirrors.

[0003] Commonly used lightweight mirrors are made of materials such as low-expansion glass, microcrystalline glass, silicon carbide (SiC), and beryllium (Be). Among them, low-expansion glass and microcrystalline glass are both glass materials with very low coefficients of thermal expansion and have been widely used in Earth observation satellites. However, these materials have relatively weak structural strength, resulting in low weight reduction rates for the mirrors they produce, which is not conducive to reducing the weight of the payload of the optical system (space cameras, aerial cameras, etc.).

[0004] Silicon carbide ceramic materials possess advantages such as high thermal conductivity, high specific stiffness, and radiation resistance. Mirrors made from silicon carbide exhibit a high weight reduction ratio (typically greater than 60%). Therefore, in addition to significantly reducing the payload weight of optical systems, silicon carbide mirrors are better able to maintain surface stability in space environments, enabling stable and rapid adjustment of the optical system and providing better space operational stability, thereby improving the resolution and other technical indicators of the entire space optical system. Due to its unique comprehensive performance advantages, silicon carbide ceramics have become the preferred material for lightweight mirrors in space optical systems over the past two decades.

[0005] With the development of space remote sensing technology, reflectors are evolving towards larger sizes, lighter weight (greater weight reduction rates), and more complex lightweight structures. The size of a single reflector blank has increased from the original 500mm diameter to a maximum of 1550mm, and the weight reduction rate has increased from 60% to over 90%. The lightweight structure (i.e., weight-reducing structure design) has evolved from the original single-rib structure (see...). Figure 1 From the current high-drop, multi-level ribbed lightweight structure (see...) Figure 2During the sintering process, the interior of the mirror blank has a loose structure due to the presence of pores and other factors, resulting in extremely low internal heat transfer efficiency. Therefore, the sintering of the sample mainly relies on the thermal radiation of the furnace heating element. The large size of the blank reflects the differences in the relative positions of different regions of the sample with the heating element in the furnace during the sintering process. This results in a larger temperature gradient within the sample. The sample parts closer to the heating element (the outer region of the sample) heat up faster, while the central region heats up slower due to its greater distance from the heating element. The sample parts in contact with the furnace bottom plate heat up more slowly due to the obstruction of heat radiation by the furnace bottom plate and the sample itself. This temperature gradient leads to thermal stress, which can easily cause sample deformation and cracking, resulting in defective products. The large weight loss rate results in lower sample strength and a weaker ability to resist internal stress, making the sample more prone to deformation. Therefore, the control of internal stress in the sample is more stringent. The high-drop, complex lightweight structure causes different amounts of heat radiation energy received by different parts of the sample during sintering due to structural differences. The large internal temperature difference during sintering can easily lead to inconsistent sintering shrinkage in different parts, resulting in different deformations and making the sample prone to deformation, warping, and cracking. This makes sample preparation difficult and increases deformation. Large deformations can lead to dimensional defects such as out-of-tolerance dimensions. Summary of the Invention

[0006] To address the challenges of temperature field inhomogeneity in the sintering process of larger, lighter, and more complex lightweight silicon carbide mirror blanks (which include at least primary, secondary, and tertiary lightweight structures), caused by size effects and differences in lightweight structure design, leading to internal stress, deformation, increased deformation, and cracking, this paper employs methods such as finite element thermal structure simulation, overall temperature field homogenization, local temperature field compensation, and sintering process control. These methods regulate the sintering temperature field of larger, more complex, and lighter silicon carbide mirror blanks, reducing the temperature field inhomogeneity in the sample during sintering and achieving precise control of the sintering temperature. This ultimately enables the sintering and preparation of complex, lightweight silicon carbide mirror blanks.

[0007] Specifically, the sintering method (i.e., sintering temperature field control method) for the silicon carbide mirror blank with a complex lightweight structure includes: (1) analyzing the temperature field distribution during sintering of the silicon carbide mirror blank with a complex lightweight structure using a finite element thermal analysis simulation method; (2) covering the outside of the silicon carbide mirror blank with a complex lightweight structure with a material as a secondary heat source (without contact) to homogenize the overall temperature field; (3) placing the material as a secondary heat source into each level of the lightweight structure in the silicon carbide mirror blank with a complex lightweight structure to compensate for the local temperature field; (4) first heating to 1300-1500℃ at 2-10℃ / min, and then heating to 1950-2200℃ at 0.5-5℃ / min for sintering to obtain the silicon carbide mirror blank with a complex lightweight structure.

[0008] Preferably, the height of the first-level lightweight structure is greater than the height of the second-level lightweight structure, which in turn is greater than the height of the third-level lightweight structure.

[0009] Preferably, the diameter of the silicon carbide mirror blank with the complex lightweight structure is 500-2100 mm and the thickness is 100-500 mm.

[0010] Preferably, the material covering the outside of the silicon carbide mirror blank that can serve as a secondary heat source during the sintering process can be monolithic or modular, such as at least one selected from graphite paper or graphite plate.

[0011] Furthermore, preferably, the thickness of the graphite paper is 0.5mm to 2mm, and the thickness of the graphite plate is 5cm to 10cm.

[0012] Preferably, the material placed in the lightweight structure can serve as a secondary heat source during the sintering process. The size and thickness of the material are matched with the size and thickness of the lightweight structure, and the material is selected from one or more of graphite blocks, graphite paper stacks, and carbon felt stacks.

[0013] Preferably, the sintering atmosphere is argon.

[0014] Preferably, the sintering time is 2 to 6 hours.

[0015] On the other hand, the present invention also provides a silicon carbide mirror blank with a complex and lightweight structure prepared according to the above-described preparation method. The silicon carbide mirror blank has a diameter of 500-2100 mm and a thickness of 100-2100 mm, and the dimensional deviation from the theoretically sintered sample is controlled within 0.3%.

[0016] Beneficial effects: One of the significant features of this invention is that, through precise control of the sintering temperature field, the stress caused by the size effect and the non-uniformity of the sintering temperature field due to the complex lightweight structure and multi-level drop in large-sized SiC mirror blank samples is reduced, thus avoiding sample cracking and deformation caused by stress. Another significant feature of this invention is that, after adopting this method, the size deviation of the sample is controlled within 0.3%, achieving precise control of the sample size, ensuring the dimensional accuracy of the sample, and reducing the amount of subsequent processing. Attached Figure Description

[0017] Figure 1 A schematic diagram of a traditional single-ribbed lightweight silicon carbide reflector; Figure 2 A schematic diagram of a lightweight silicon carbide reflector with a high drop and multi-level ribs (left image is a top view, right image is a right view); Figure 3 Model of the sintering and mounting structure for silicon carbide mirror blanks; Figure 4 The finite element analysis model of the silicon carbide mirror blank (symmetric structure is taken as 1 / 2); Figure 5 Finite element analysis of the firing temperature field distribution of silicon carbide mirror blank (firing temperature 1980℃, high temperature heating rate 5℃ / min); Figure 6 Finite element analysis of the firing temperature field distribution of silicon carbide mirror blank (firing temperature 2100℃, heating rate 1℃ / min); Figure 7 Temperature field distribution diagram during the sintering process of silicon carbide mirror blank (sintering temperature 1700℃, heating rate 1℃ / min) for finite element analysis; Figure 8 The measured temperature field distribution of the temperature measuring ring for the silicon carbide mirror blank (sintering temperature 1700℃, heating rate 1℃ / min); Figure 9 Samples that cracked during sintering due to the lack of temperature field homogenization measures; Figure 10 For sintered silicon carbide mirror blanks with large deformation that have undergone local temperature field compensation measures, the dimensional deviation of the sample is 0.5%. Figure 11 Photograph of a silicon carbide mirror blank sample (in good condition) sintered with overall temperature field homogenization and local temperature field compensation measures. The dimensional deviation of the sample is 0.25%. Detailed Implementation

[0018] The present invention will be further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0019] In this disclosure, the sintering temperature field of silicon carbide mirror blanks with large size, complex and lightweight structure is regulated by finite element thermal structure simulation, overall temperature field homogenization, local temperature field compensation and sintering process control, thereby achieving precise control of its sintering temperature.

[0020] The following exemplarily illustrates a method for preparing a silicon carbide mirror blank with a complex lightweight structure. Unless otherwise specified, the method for preparing a silicon carbide mirror blank with a complex lightweight structure in this invention is detailed in ZL200810077877.5.

[0021] This invention employs existing finite element thermal analysis simulation methods to simulate the sintering temperature field, thereby obtaining the temperature field distribution of the silicon carbide reflector blank with a complex lightweight structure during sintering, which can then guide the actual sintering process. This invention defines material performance parameters, contact properties between different components, loads, and boundary conditions.

[0022] A material capable of serving as a secondary heat source during the sintering process is used to cover the exterior of a complex, lightweight silicon carbide reflector blank, thereby homogenizing the overall temperature field. This covering material, which acts as a secondary heat source during sintering, can be either monolithic or modular. The material serving as a secondary heat source during sintering can be one or more of graphite plates and graphite paper.

[0023] Simultaneously, a material capable of serving as a secondary heat source during the sintering process is placed within the lightweight structure of the silicon carbide mirror blank, which has a complex lightweight structure, to compensate for the local temperature field of the sample. The covering material, which acts as a secondary heat source during sintering, covering the lightweight structure of the silicon carbide mirror blank, can be integral or discrete. The material placed within the lightweight structure can be one or more of the following: graphite block, graphite paper stack, or carbon felt stack. The morphology of the material serving as the secondary heat source is consistent with the morphology of the lightweight structure within the mirror blank.

[0024] Finally, the sintering process is controlled, specifically by setting different temperature control rates within different temperature ranges to regulate the densification process of the silicon carbide mirror blank. During sintering, the temperature range is divided into a low-temperature sintering temperature region and a high-temperature sintering temperature region. In the low-temperature sintering temperature region, the temperature does not exceed 1500℃, and the heating rate can be 2-10℃ / min. In the high-temperature sintering temperature region, from the highest temperature in the low-temperature sintering temperature region to the firing temperature, the heating rate can be 0.5-5℃ / min. The firing temperature can be 1950-2200℃.

[0025] In this invention, temperature field compensation measures are used to reduce the sintering temperature gradient caused by size effects and complex structures during the sintering of ultra-large and complex samples, thereby reducing cracking and deformation of complex-shaped samples. Temperature regulation is used as a means of densification. The dimensional deviation of the resulting complex and lightweight silicon carbide mirror blank is controlled within 0.3%.

[0026] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0027] Example 1 uses a circular silicon carbide mirror blank with a lightweight structure and a diameter of 1000 mm. The weight reduction rate of the blank is approximately 70%. Ansys software was used to analyze the temperature field distribution of the sample at the high-temperature sintering temperature. A schematic diagram of the sintering process of the mirror blank sample is shown below. Figure 3 As shown, when performing finite element analysis of the high-temperature sintering temperature field, the analysis model (1 / 2 for symmetric structures) is as follows: Figure 4 As shown. The simulated sample was heated to 1500℃ at a rate of 10℃ / min, and then to 1980℃ at a rate of 5℃ / min. The finite element simulation results are shown below. Figure 5 As shown in the figure, the temperature distribution in the sample area is uneven, with a temperature difference of about 30℃. The bottom and middle areas of the sample have reduced heat radiation, resulting in lower temperatures, while the outer periphery of the sample has higher temperatures due to heat transfer through heat radiation. This indicates that there are temperature differences in different parts of the sample during the sintering process.

[0028] Example 2 uses a circular silicon carbide mirror blank with a lightweight structure and a diameter of 1000 mm. The weight reduction rate of the blank is approximately 70%. Ansys software was used to analyze the temperature field distribution of the sample at the high-temperature sintering temperature. The simulated sample was heated to 1500℃ at a rate of 2℃ / min, and then to 2100℃ at a rate of 1℃ / min. The finite element simulation results of the sintering temperature are shown below. Figure 6 As shown in the figure, with the decrease of sintering heating rate, the uneven temperature distribution in the sample area is significantly reduced, with a temperature difference of about 5℃.

[0029] Example 3 uses a circular silicon carbide mirror blank with a diameter of 2000 mm and a complex, lightweight, three-stage structure with a high drop. The lightweighting rate of the blank is approximately 90%. Ansys software was used to analyze the temperature field distribution of the sample during the high-temperature sintering process. The simulated sample was heated to 1500℃ at a rate of 5℃ / min, and then to 2180℃ at a rate of 1℃ / min. The intermediate sintering temperature of 1700℃ was used for finite element simulation of the sample's sintering temperature. The results are shown in […]. Figure 7 As shown in the figure, a temperature difference of approximately 6°C exists during the sintering process of the sample. A temperature measuring loop was used to detect the actual temperature at various points on the sample at this temperature; the test results are shown below. Figure 8The temperature difference measured by the temperature measuring ring during the sintering process of the sample was approximately 9°C, which is slightly higher than the simulated value.

[0030] Example 4 uses a circular silicon carbide mirror blank with a diameter of 2000 mm and a complex, lightweight, three-stage structure, exhibiting a high drop. The blank's weight reduction rate is approximately 90%. The sample is placed naked in a sintering furnace and heated to 1500°C at a rate of 5°C / min, then to 2100°C at a rate of 5°C / min. The sintered sample is shown in [reference needed]. Figure 9 As shown in the figure, the sample cracked due to the uneven sintering temperature field.

[0031] Example 5 uses a circular silicon carbide mirror blank with a diameter of 2000 mm and a complex three-stage lightweight structure and high drop, achieving a lightweight ratio of approximately 90%. Graphite paper is placed in the lightweight holes of the blank sample, and the temperature is increased to 1500°C at a rate of 3°C / min, then to 2100°C at a rate of 1.0°C / min. The sintered sample is shown in [reference needed]. Figure 10 As shown in the figure, due to the local temperature homogenization measures in the sintering temperature field and the control of the sintering temperature regime, the prepared sample is intact. However, the outer diameter of the sample after sintering is 1507.3 mm, while the outer diameter designed in the drawing is 1499.7 mm, with a dimensional deviation of 0.51%.

[0032] Example 6 uses a circular silicon carbide mirror blank with a diameter of 2000 mm and a complex, lightweight, three-stage structure, achieving a weight reduction rate of approximately 90%. The sample outer cover is made of separate graphite paper and placed in a sintering furnace. The temperature is increased to 1500 °C at a rate of 3 °C / min, and then to 2100 °C at a rate of 1.0 °C / min. Due to the overall temperature homogenization measures and controlled sintering temperature regime, the prepared sample is intact. However, the outer diameter of the sintered sample is 1506.0 mm, while the designed outer diameter is 1499.7 mm, resulting in a dimensional deviation of 0.42%.

[0033] Example 7 uses a 2000mm diameter circular silicon carbide mirror blank with a complex three-stage lightweight structure and high drop, achieving a lightweight ratio of approximately 90%. The sample is encased in a separate graphite paper sheet and placed in a sintering furnace. Graphite blocks are placed in the lightweight holes of the sample. The temperature is increased to 1500℃ at a rate of 2℃ / min, and then increased to 2100℃ at a rate of 0.5℃ / min. The sintered sample is shown in [the image / description]. Figure 11 As shown in the figure, the prepared sample was intact due to the overall temperature, local temperature homogenization measures, and control of the sintering temperature regime. The outer diameter of the sintered sample was 1503.4 mm, while the designed outer diameter was 1499.7 mm, with a dimensional deviation of only 0.25%. This achieved precise control of the sample size, ensuring dimensional accuracy and reducing subsequent processing.

[0034] Example 8 used a circular silicon carbide mirror blank with a diameter of 1500 mm and a two-stage complex lightweight structure. The lightweighting rate of the blank was approximately 80%. The sample, consisting of a split graphite plate, was placed in a sintering furnace. Laminated carbon felt (carbon fiber interwoven structure) was placed in the lightweighting holes of the sample. The temperature was increased to 1500 °C at a rate of 2 °C / min, and then increased to 2200 °C at a rate of 0.5 °C / min. The sintered sample was intact. The outer diameter of the sintered sample was 1127.1 mm, while the designed outer diameter was 1124.8 mm, with a dimensional deviation of only 0.20%.

[0035] Example 9 used a circular lightweight silicon carbide mirror blank with a diameter of 1200 mm. The lightweighting rate of the blank was approximately 70%. An integral graphite plate was placed over the sample in a sintering furnace, and graphite blocks were placed in the lightweighting holes of the sample. The temperature was increased to 1500 °C at a rate of 5 °C / min, and then increased to 2200 °C at a rate of 0.5 °C / min. The sintered sample was intact. The outer diameter of the sintered sample was 914.2 mm, while the designed outer diameter was 912.3 mm, with a dimensional deviation of only 0.21%.

[0036] Example 10 used a circular lightweight silicon carbide mirror blank with a diameter of 1500 mm. The lightweighting rate of the blank was approximately 70%. The sample was covered with a graphite plate around its circumference and graphite paper on top, and placed in a sintering furnace. Stacked graphite paper was placed in the lightweighting holes of the sample. The temperature was increased to 1500 °C at a rate of 5 °C / min, and then increased to 2200 °C at a rate of 0.5 °C / min. The sintered sample was intact. The outer diameter of the sintered sample was 1130.3 mm, while the designed outer diameter was 1127.1 mm, with a dimensional deviation of only 0.28%.

Claims

1. A method for controlling the sintering temperature field of a silicon carbide mirror blank with a complex and lightweight structure, characterized in that, The silicon carbide mirror blank includes at least a primary lightweight structure, a secondary lightweight structure, and a tertiary lightweight structure, wherein the height of the primary lightweight structure is greater than the height of the secondary lightweight structure, which is greater than the height of the tertiary lightweight structure. The method for controlling the sintering temperature field of the silicon carbide mirror blank with a complex and lightweight structure includes: (1) The temperature field distribution during the sintering of the silicon carbide reflector blank with the complex lightweight structure was analyzed by the finite element thermal analysis simulation method. (2) A material serving as a secondary heat source is used to cover the outside of the silicon carbide reflector blank with a complex and lightweight structure to homogenize the overall temperature field. (3) A material used as a secondary heat source is placed in each stage of the lightweight structure of the silicon carbide reflector blank with a complex lightweight structure to perform local temperature field compensation. (4) First, heat the temperature to 1300-1500℃ at 2-10℃ / min, and then heat it to 1950-2200℃ at 0.5-5℃ / min for sintering to obtain the silicon carbide mirror blank with the complex lightweight structure.

2. The method according to claim 1, characterized in that, The silicon carbide reflector blank with a complex and lightweight structure has a diameter of 500–2100 mm and a thickness of 100–500 mm.

3. The method according to claim 1, characterized in that, The material covering the silicon carbide reflector blank that can serve as a secondary heat source during the sintering process is selected from at least one of graphite paper or graphite plate.

4. The method according to claim 3, characterized in that, The thickness of the graphite paper is 500 micrometers to 2 mm; the thickness of the graphite plate is 5 cm to 10 cm.

5. The method according to claim 1, characterized in that, The material placed inside the lightweight structure of the green blank, which can serve as a secondary heat source during the sintering process, is selected from at least one of graphite blocks, graphite paper stacks, and carbon felt stacks.

6. The method according to claim 1, characterized in that, The sintering atmosphere is argon.

7. The method according to claim 1, characterized in that, The sintering time is 2 to 6 hours.

8. A silicon carbide mirror blank with a complex and lightweight structure prepared by the sintering temperature field control method according to any one of claims 1-7, characterized in that, The diameter of the silicon carbide mirror blank is 300-1550 mm, the thickness is 80-400 mm, and the dimensional deviation from the theoretical sintered sample is controlled within 0.3%.

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