RSiC also known as recrystallized silicon carbide, is a high-temperature structural material with a high SiC content (above 98% by weight). It retains many excellent properties of SiC, such as high strength at high temperatures, strong corrosion resistance, excellent oxidation resistance, and good thermal shock resistance. It is widely used in high-temperature kiln furniture, diesel vehicle exhaust purifiers, and other fields. Additionally, due to its good thermal conductivity and semiconductor properties, it can also be used as a heat exchange material and ignition component, among other functional materials.
This article reviews the preparation process, structural and performance characteristics, usage performance studies, and material applications of recrystallized silicon carbide materials in recent years.
1. Preparation Process of Recrystallized Silicon Carbide
Recrystallized silicon carbide materials are prepared using two different particle sizes of high-purity silicon carbide as raw materials, with the addition of an appropriate amount of temporary binder (without adding sintering aids). After mixing evenly in a specific proportion, they are shaped using methods such as casting or gel casting. Under high temperatures (2200–2450°C) and protective atmospheres, evaporation-condensation recrystallization occurs, and particle coalescence takes place at the particle contact points to form a sintered body. Therefore, recrystallized silicon carbide does not shrink or undergo liquid phase sintering during the sintering process, resulting in a porous network structure with interconnected pores.
Many factors affect the performance of recrystallized silicon carbide materials, including the purity and particle shape of the raw materials, forming methods, sintering conditions (temperature and atmosphere), and so on.
1) Purity of Silicon Carbide Raw Materials and Particle Shape
Recrystallized silicon carbide materials require high purity in the silicon carbide raw materials, with SiC content above 99% by weight. Ordinary silicon carbide materials, with high impurity content, may decompose at sintering temperatures (2700 K), causing silicon volatilization and significantly reducing the performance of the products. Moreover, the shape of ordinary silicon carbide micro powders is irregular, and the long edges can reduce the bulk density of the molded body. After surface shaping treatment, most silicon carbide micro powders become spherical or nearly spherical, which helps increase the bulk density of the molded body.
2) Shaping Methods
The main shaping methods for recrystallized silicon carbide are casting and extrusion molding.
- Casting can produce complex shapes and large, thin-walled parts with uniform structure. Many factors influence the casting process, such as the performance of surface-modified silicon carbide powders, the amount of defoamer, the size, shape, and distribution of coarse particles, and the grading of coarse and fine silicon carbide particles, among others.
Currently, the density of green bodies prepared by casting molding methods abroad can reach 2.75 g/cm³, with industrial production already realized.
- Extrusion molding has advantages such as continuous production, high efficiency, and a wide range of applications. Factors affecting extrusion molding include the shape, size, and distribution of silicon carbide particles, particle grading, and the type and amount of plasticizers added.
- In recent years, research on isostatic pressing molding methods for recrystallized silicon carbide materials has emerged. Isostatic pressing has the characteristic that the pressure transmitted through the liquid medium is equal in all directions, and it has a wide range of applicable raw material particle sizes, especially for difficult-to-mold powder materials. The force applied to the green body is uniform, and the density distribution is even, resulting in significant improvements in the properties of the sintered product.
Factors affecting the isostatic pressing molding of recrystallized silicon carbide include the grading of coarse and fine silicon carbide, types and amounts of binders, molding pressure, and holding time. Currently, no research has been found on machine-pressing molding methods in the preparation of recrystallized silicon carbide materials.
| Item | Casting Molding | Extrusion Molding | Isostatic Pressing Molding |
| Adaptable particle size range | Particle size should not be too large | Particle size should not be too large | Wide particle size range |
| Green body forming density | Lower | Lower | Higher |
| Green body shrinkage rate | Larger | Larger | Smaller |
| Green body shape | Adaptable to various shapes | Suitable for tubular shapes | Suitable for part shapes and block forms |
| Production efficiency | Lower | Higher | Lower |
| Adaptability to automated operations | Not suitable | More suitable | Not suitable |
The table shows the comparison of the three shaping methods for recrystallized silicon carbide materials. From it can be seen that extrusion molding has significant advantages in production efficiency and adaptability to automated operations, and may become the main direction of future research in the shaping of recrystallized silicon carbide materials.
3) Sintering Temperature and Atmosphere
The sintering temperature range for recrystallized silicon carbide materials is generally between 2200 and 2450°C. Since recrystallized silicon carbide materials are primarily used as high-temperature structural materials, especially as load-bearing components in an oxidizing atmosphere above 1500°C, high-temperature bending strength is one of the key indicators for studying sintering temperature and atmosphere.
The sintering atmosphere for recrystallized silicon carbide materials is usually vacuum or argon-protected. Nitrogen atmosphere cannot be used in the sintering process of recrystallized silicon carbide materials because it can disrupt the evaporation-condensation recrystallization process, leading to decomposition of the silicon carbide.
2. Composition, Structure, and Performance Characteristics of Recrystallized Silicon Carbide Materials
Recrystallized silicon carbide materials have distinctive structural and performance characteristics, significantly different from reaction-sintered silicon carbide(SiSiC), non-pressure-sintered silicon carbide(SSiC), and self-bonded silicon carbide materials(OSiC). By combining the preparation processes of these different silicon carbide materials , the composition, microstructure, performance, and application characteristics of these four materials are compared and introduced, as shown in below table.
3. Application Overview of Recrystallized Silicon Carbide Materials
Due to its excellent comprehensive performance, recrystallized silicon carbide materials are widely used in high-temperature kiln furniture, diesel vehicle exhaust purifiers, and other areas.
Advantages:
1）Used as High-Temperature Kiln Furniture
Recrystallized silicon carbide kiln furniture products include shelves, beams, rollers, etc., and offer significant advantages in energy savings and reducing labor intensity. Compared to traditional silicon carbide kiln furniture (such as clay-bonded silicon carbide kiln furniture and oxide-bonded silicon carbide kiln furniture), recrystallized silicon carbide kiln furniture has the characteristics of high-temperature resistance, high strength, and high thermal conductivity. It can be made into thin-walled materials, greatly reducing the loading area, improving loading efficiency and labor efficiency, and shortening the firing cycle.
Additionally, recrystallized silicon carbide, non-pressure-sintered silicon carbide, reaction-sintered silicon carbide, and self-bonded silicon carbide all have higher high-temperature bending strength at 1200°C than at room temperature. This may be due to the SiO₂ film formed by SiC oxidation at high temperatures, which helps to bridge cracks and defects in the material, inhibiting crack propagation. At temperatures above 1400°C, the high-temperature bending strength of reaction-sintered silicon carbide and self-bonded silicon carbide drops sharply due to the presence of trace silicon inside, which forms a liquid phase at 1400°C. Therefore, reaction-sintered silicon carbide can be used at 1350°C but cannot be used above this temperature, requiring the use of recrystallized silicon carbide or other materials.
2) Used as Diesel Vehicle Exhaust Purifiers
Compared to cordierite materials used in diesel vehicle particulate filters, recrystallized silicon carbide has significant advantages. Due to its high strength, high-temperature resistance, and corrosion resistance, RSiC is more resistant to the chemical erosion of particulate matter in diesel exhaust filters. It can handle higher temperatures and higher concentrations of particulate matter (almost unaffected by the removed components), making it suitable for high-power exhaust emissions and offering a lower regeneration treatment temperature, resulting in higher thermal shock resistance, operating temperature, and service life. Moreover, recrystallized silicon carbide materials have high mechanical strength, are easy to install and handle, and are more suitable for withstanding severe vibrations during the operation of heavy-duty diesel vehicles.
5. Conclusion
Recrystallized silicon carbide(RSiC) materials are expanding in their application areas due to their excellent comprehensive performance. Currently, there are two main development directions for recrystallized silicon carbide materials: one is to solve the high porosity problem of the material and develop high-performance dense silicon carbide materials to expand their application range in high-temperature structural materials; the other is to improve the process and reduce costs to produce high-performance cost-effective materials.
In conclusion, the types of recrystallized silicon carbide products will continue to increase, and their application range will continue to expand, representing the future direction of development.
