Research Progress of High Thermal Conductivity Silicon Carbide Ceramics
Research on high thermal conductivity silicon carbide ceramics: By optimizing the purity of raw materials and the sintering process, the problem of low thermal conductivity of polycrystalline materials is addressed, promoting its application in the semiconductor and new energy fields.
Nov 11th,2025164 Views
Research Progress of High Thermal Conductivity Silicon Carbide Ceramics.
SiC ceramics possess excellent high-temperature resistance, corrosion resistance, radiation resistance, and inherent thermal conductivity, making them highly applicable in energy conservation, semiconductors, and nuclear industries. However, the thermal conductivity of SiC ceramics at room temperature typically ranges from 30 to 270 W/mK, significantly lower than the inherent value of 490 W/mK observed in SiC single crystals. The reduction in thermal conductivity is attributed to factors such as impurities within the SiC lattice, grain boundaries, residual second phases, and pores.
Microscopic factors influencing the thermal conductivity Lattice defect
The thermal conductivity of SiC ceramics is significantly influenced by intrinsic lattice defects, including point defects, line defects, and planar defects. The main defects of SiC ceramics are silicon vacancies, carbon vacancies, and interstitial atoms. They disrupt the periodicity of the lattice and enhance phonon scattering in SiC ceramics. Dislocations, as the most typical line defects, hinder the propagation of phonons along the SiC lattice. Additionally, two-dimensional planar defects, such as stacking faults, subgrain boundaries, and twin boundaries, significantly enhance phonon scattering in SiC ceramics. These intrinsic lattice defects reduce the average speed and average free path of phonons, thereby lowering the thermal conductivity of SiC ceramics. To improve the thermal conductivity of SiC ceramics, these intrinsic lattice defects must be eliminated as much as possible. Optimizing the sintering and annealing conditions, as well as adding additives that reduce these intrinsic lattice defects, can increase the average free path of phonons and enhance the thermal conductivity of SiC ceramics.
2) Lattice Oxygen The dissolved oxygen impurities within the SiC lattice are also widely regarded as one of the most detrimental defects affecting the thermal conductivity of SiC ceramics. Silicon carbide powder is the main raw material for preparing silicon carbide ceramics, and it inherently contains oxygen impurities. These impurities are classified as lattice oxygen, with a concentration range of approximately 5000 to 6500 ppm. During the high-temperature sintering process, the strong covalent bond characteristics of silicon carbide make it challenging to remove the lattice oxygen from the starting powder. Additionally, the silica impurities present on the surface may also dissolve into the SiC lattice, helping to form additional lattice oxygen. Therefore, reducing the content of lattice oxygen becomes a key issue in manufacturing SiC ceramics with enhanced thermal conductivity, which requires special attention. Selecting high-purity SiC starting powder and using aluminum-free sintering aids with strong deoxidation capabilities are effective methods to enhance the thermal conductivity of SiC ceramics.
3) Al dissolved in the SiC lattice Al compounds, such as Al2O3 and AlN, are commonly used as sintering aids for SiC ceramics. However, Al is regarded as one of the most harmful impurities that reduce the thermal conductivity of SiC ceramics. AlN and SiC are completely miscible and can form an infinite solid solution. The formation of solid solutions in SiC ceramics causes considerable lattice distortion. This distortion enhances the scattering of lattice waves (phonons), thereby leading to a decrease in the thermal conductivity of SiC ceramics. Although Al compounds are effective sintering aids for densifying SiC ceramics, the defects induced during the SiC ceramic manufacturing process can deteriorate the thermal conductivity. During the preparation process, the use of sintering aids containing Al must be avoided to eliminate the adverse effects of Al on thermal conductivity.
Granularity SiC ceramics, as a polycrystalline material, are mainly composed of grains and grain boundaries. The larger the grain size, the fewer the number of grain boundaries. The influence of grain size on the thermal conductivity of SiC ceramics is as shown in the following figure:
The larger the average grain size, the fewer the number of grain boundaries. In most cases, the reduction in the number of grain boundaries significantly reduces the scattering effect of these grain boundaries on phonons, thereby improving the thermal conductivity of SiC ceramics. However, in a few specific cases, an increase in grain size does not necessarily enhance the thermal conductivity of SiC ceramics. This requires considering the synergistic effect of other factors, such as the solubility of impurity elements in the SiC lattice.
Grain boundary phase The second phase remaining at the grain boundaries is called the grain boundary phase. The content and crystallinity of the grain boundary phase have a significant impact on the thermal conductivity of SiC ceramics. Reducing the number of grain boundary phases can enhance the adjacency between SiC grains, while increasing the crystallinity of the grain boundary phase helps improve the thermal conductivity of the grain boundary phase. Both of these factors contribute to improving the overall thermal conductivity of SiC ceramics.
Pore Pores are a common defect in SiC ceramics. The porosity and pore size significantly affect the thermal conductivity. Generally, the thermal conductivity of SiC ceramics tends to increase as the porosity decreases. Due to the significant scattering effect of pores on phonons, combined with the inherently close-to-zero thermal conductivity of the pores, the pores have a significant impact on the thermal conductivity of SiC ceramics. As the porosity increases, the average free path of phonons decreases, resulting in a reduction in the thermal conductivity of SiC ceramics. At the same porosity level, a larger pore diameter means a lower specific surface area of the pores, which helps to weaken the phonon scattering effect and enhance the thermal conductivity coefficient. In summary, to obtain SiC ceramics with high thermal conductivity, it is crucial to minimize the porosity as much as possible.
The β/α phase composition of SiC The crystal phase composition of SiC has a significant impact on the thermal conductivity of SiC ceramics. This effect is mainly attributed to the enhanced phonon scattering at the β/α hetero-interface compared to the β/β or α/α homo-interface. When SiC ceramics are mainly composed of 100% β phase or 100% α phase, the absence of a β/α hetero-interface usually leads to relatively high thermal conductivity. Conversely, when SiC ceramics contain a considerable amount of β phase and α phase, the presence of many high thermal resistance β/α hetero-phase boundaries may result in lower thermal conductivity.
The influence of raw materials on thermal conductivity Silicon carbide raw material Using medium-sized (submicron) and high-purity β-SiC starting powders is the most promising method for obtaining high thermal conductivity SiC ceramics.
Auxiliary sintering agent The manufacture of SiC ceramics without a flux is crucial for maintaining their high purity, as it can prevent the introduction of external impurities and secondary phases. However, the absence of additives also means that lattice oxygen impurities cannot be effectively eliminated during the sintering process. SiC ceramics without additives have an original microstructure, characterized by the absence of low-conductivity grain boundary phases, thereby achieving high thermal conductivity. Moreover, the development of a textured structure with preferential grain orientation plays an important role in reducing phonon scattering caused by random grain arrangement, thereby further enhancing the thermal conductivity of these SiC ceramics without additives.
The influence of sintering conditions on the thermal conductivity Sintering temperature The sintering temperature has a significant impact on the densification, phase composition and microstructure of SiC ceramics, thereby significantly influencing their thermal conductivity. Appropriately increasing the sintering temperature can effectively enhance the densification of SiC ceramics, promote the β→α phase transformation, expand the grain size, and reduce the lattice oxygen content, thereby significantly increasing the thermal conductivity of SiC ceramics.
Sintering time When the sintering temperature is fixed, changing the holding time will also affect the thermal conductivity of SiC ceramics. Extending the sintering time can promote the growth of grains in SiC ceramics, reduce the lattice oxygen content, and thereby increase the thermal conductivity of SiC ceramics.
Sintering atmosphere SiC ceramics are usually sintered in an atmosphere of argon (Ar) or nitrogen (N2). The sintering atmosphere has a significant impact on the thermal conductivity of SiC ceramics. Argon, as a chemically inert gas, does not participate in chemical reactions with SiC ceramics during the sintering process. On the contrary, nitrogen interacts with SiC during sintering, thereby regulating its thermal conductivity. During the liquid-phase sintering of SiC ceramics, N2 dissolves at the grain boundaries of the liquid phase. As the dissolution-reprecipitation mass transfer process proceeds, N atoms are incorporated into the SiC lattice, forming lattice nitrogen. This phenomenon stabilizes the β-SiC phase and hinders the transformation of the β-SiC phase to the α-SiC phase. At the same time, it leads to an increase in the conductivity of silicon carbide ceramics, amplifying the contribution of electron heat transport. However, after long-term sintering at high temperatures, as the degree of β→α phase transformation increases, the content of lattice nitrogen decreases, resulting in a decrease in electrical conductivity and a reduction in the contribution of electron heat transport. Therefore, when determining the appropriate sintering atmosphere, it is necessary to comprehensively consider the characteristics of SiC powder, sintering additives, and sintering process conditions. These factors will jointly determine the final thermal conductivity of SiC ceramics. The above factors have a significant impact on the thermal conductivity of silicon carbide ceramics. By optimizing process parameters, high-performance silicon carbide ceramic materials can be obtained, further reducing costs and facilitating the large-scale application of silicon carbide ceramics. Common application scenarios include: 1) Due to the outstanding high-temperature strength, excellent high-temperature creep resistance, and thermal shock resistance of silicon carbide ceramics, they are one of the main materials for thermal components in rockets, aircraft, automotive engines, and gas turbines. 2) Silicon carbide ceramics have excellent high-temperature properties and high thermal conductivity. They are also widely used in heat exchange systems in the steel and metallurgy industries. In addition to utilizing the excellent thermal conductivity of silicon carbide materials, their high elasticity and high elastic modulus are used to prepare bulletproof armor materials. There are also related applications in the semiconductor field, such as fixtures and crucibles.
