Silicon carbide mirror reaction sintering

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Silicon carbide ceramics, as one of the candidate materials for large-sized light-weight mirrors for space, has excellent comprehensive advantages compared with traditional glass and metal materials, such as high specific stiffness, small thermal expansion coefficient, good thermal conductivity, and can be obtained. Good optical processing surface and non-toxic, shock-resistant, corrosion-resistant, the preparation process does not require special equipment. Therefore, foreign space optical detection instruments all use SiC ceramics as the preferred material for the mirror body. With the development of space technology, in order to improve the resolution of space detectors and achieve higher imaging quality, an optical system with a larger aperture must be used, and the quality is also increasing. Therefore, large-diameter, light-weight silicon carbide mirrors have become the focus of research. my country’s space exploration instruments are also entering the world’s advanced ranks.

At present, the SiC ceramics used for space light mirror base materials mainly include hot pressing sintering SiC (HP-SiC), reaction sintering SiC (RB-SiC), pressureless sintering SiC (Sintering SiC, SSiC) and chemical vapor deposition SiC ( CVD-SiC) four. Reaction sintering (RB-SiC) of silicon carbide blank is carried out in a high-temperature vacuum sintering furnace, and the manufactured silicon carbide (SiC) mirror body has a small shrinkage and is not easy to chip, crack and deform. The obtained sintered body has a complete geometric shape. Homogeneous internal organization and good mechanical properties and optical processing properties.

Reaction Sintering Mechanism

Many foreign researchers have done a lot of research on the mechanism of reaction sintered silicon carbide (RB-SiC). In the 1950s, Popper invented reactive sintered silicon carbide. The basic principle is that reactive liquid silicon and silicon alloy infiltrate into the carbon-containing porous ceramic green body under the action of capillary force, and react with the carbon in it to form silicon carbide. , The newly generated silicon carbide is in-situ combined with the original silicon carbide particles in the green body, and the infiltration agent fills the remaining pores in the green body to complete the ceramic densification process.

When the temperature rises to 1420℃, the melting point of Si, molten Si will appear, contact with the carbon on the surface of the green body, and a chemical reaction will occur to form SiC. gap. After cooling down, silicon carbide is precipitated, and a nearly completely dense silicon carbide mirror body is obtained.

Vacuum sintering furnace

Average temperature zone size: 1000mm×700mm×400mm

Ultimate vacuum degree: <6.5×10-Pa

Maximum heating temperature: 1850℃

Installed power: 150kW

Graphite box size: 1200mm×760mm×300mm

Gas medium: Argon (Ar)

Sintering process

The main operating steps of the high temperature vacuum sintering furnace are as follows:

(1) First, protect the inner wall of the graphite box with 40% boron nitride (BN) to prevent silicon vapor during the sintering process spill reacts with the graphite box to form silicon carbide.

(2) Place the green body in the center of the graphite box.

(3) Add silicon powder (purity>99.9%) according to 30%~40% of the weight of the green body, and evenly stack it on the upper surface of the green body.

(4) Cover the top cover of the graphite box, push the graphite box into the center of the vacuum furnace, and close the furnace door.

(5) Control the heating rate, cooling rate and holding time according to the high temperature vacuum furnace reaction sintering process and the vacuum degree control curve.

(6) Eliminate the stress of the mirror body due to high temperature sintering.

(7) Complete the sintering process and take out the sintered body—the silicon carbide mirror body.

The sintering process is determined according to the amount of sintered material in the high-temperature vacuum furnace. The main parameters are temperature, vacuum value, heating rate, cooling rate and time. The maximum temperature of reaction sintering is determined to be 1560 °C. The heating steps are shown in the figure.

The first stage of green body reaction sintering (RB-SiC) starts from 200 °C, the organic compounds in the green body begin to decompose, enter the rapid development zone above 450 °C, reach a maximum value around 600 °C, and then decrease significantly. Therefore, the heating rate of this section should not be too fast, and it is best to carry out under the protection of inert gas (Ar). In this way, the loss of organic matter in the green body during carbonization is small, and a uniform carbon supplement can be obtained, which is beneficial to the progress of reaction sintering, otherwise the green body will be cracked and even scrapped. Therefore, a heat preservation section still needs to be set at about 600°C to make the organic compounds evenly and completely decomposed. The second stage is the carbonization process after the organic compounds are decomposed. It only needs to heat up evenly and appropriately increase the vacuum degree in the vacuum furnace to completely carbonize the organic compounds in the green body. The third stage enters the substantial stage of reaction sintering (RB-SiC). After entering the melting point of silicon powder (1420°C), the vacuum value and heating rate in the furnace must be strictly controlled. Silicon infiltration, the occurrence of the reaction is limited to a certain depth of the surface of the green body. If the vacuum value is too high, it will cause uneven silicon infiltration of the green body, that is, the green body is partially cracked due to the large reaction concentration and the fast speed. Therefore, the third stage is the key to the reaction sintering of the green body. When the furnace temperature rises to 1560°C, the degree of vacuum is increased, and the temperature is kept for 1 hour, so that the liquid silicon can effectively fill the voids in the green body, so as to achieve the effect of complete densification of the green body. The fourth stage is that when the green body is cooled to 1420 ℃, it must be kept warm or carried out at a slow speed, which mainly plays the role of eliminating the internal stress of the green body.

The finished product is as follows:

Equipment selection: The RVS vacuum sintering furnace provided by SIMUWU is an excellent product for processing this kind of process. It has the characteristics of good temperature uniformity and high temperature control accuracy. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to giving customers the most convenient and efficient experience.