1. Chemical composition
The thermal conductivity of ordinary ceramics is (0.02~1.5) W/(m·K). In practical applications, ceramic materials with low thermal conductivity are far from meeting the requirements for energy saving, high efficiency and special performance. Component doping is an important method to improve the thermal conductivity of ceramic materials. This method can be divided into two types according to the nature of doping components: adding non-metallic materials or adding metallic materials. In the application field of electronic packaging materials, aluminum nitride (AlN)-based special ceramics are prepared from aluminum nitride ceramic composite powders, with a thermal conductivity of 210W/(mk), which is 5-8 times the thermal conductivity of alumina ceramics. High temperature above 2200℃. With high thermal conductivity, stable mechanical properties and excellent electrical insulation properties, with extremely high insulation resistance and low dielectric loss, it is an ideal thermally conductive ceramic material for a new generation of large-scale integrated circuits and power electronic products.
2. Raw material powder particle size
The particle size, purity and phase of the raw material powder will have an important impact on the thermal conductivity and mechanical properties of the ceramic material. The larger the particle size, the more obvious the delay in the densification process of the ceramic. During the preparation of the ceramic, pores will inevitably appear in the ceramic. Generally, the more pores, the higher the thermal conductivity of the material.
When the ceramic powder particle size drops to nanometer level, its thermal conductivity will decrease accordingly. The surface effect and small size effect of nanoparticles change the properties of the material itself. The size of the crystal grains is reduced, the distribution is narrowed, the separation area between the grain boundary and the pores is reduced, and the sintering temperature is reduced, so that the abnormal growth of the crystal grains is not easy to occur during the sintering process, and the grain distribution of the ceramic body is uniform. Therefore, when the particle size of the powder is smaller, the thermal conductivity of the ceramic will decrease as the particle size decreases. In actual production, proper control of the raw material powder particle size can improve the heat transfer performance of ceramic materials.
3. Stoma
Under the same porosity conditions, the larger the pore size, the greater the thermal conductivity. Interconnected pores have higher thermal conductivity than closed pores. The higher the closed porosity, the lower the thermal conductivity. The reason for this phenomenon is: the larger the size of the pores, the greater the gas convection in the pores and the greater the radiation heat transfer between the pore walls. This is the opposite of the effect of material density on thermal conductivity. Nevertheless, after coordinating the effects of the two methods, two measures can be taken at the same time to improve the thermal conductivity of the material. In the study of porous ceramics, it was also concluded that when the pore size is less than 4μm, the convective heat transfer inside the material can be ignored.
In addition, in porous ceramics, heat transfer methods such as convection, radiation, and heat conduction exist. Therefore, when analyzing the thermal conductivity of ceramics, the pore size, distribution and connection mode of the pores should be considered comprehensively.
4. Organizational structure
The internal structure of ceramic materials is an important factor affecting its thermal conductivity. The influence of the internal structure on the thermal conductivity of ceramics is mainly determined by the phonon heat conduction mechanism of the material. In order to increase the thermal conductivity of the material, some materials that can improve the thermal conductivity can be introduced into the material. Taking into account that these materials may have a series of physical and chemical reactions with the original materials during the ceramic preparation process, which will affect the internal structure of the ceramics, and the appearance of internal defects in the ceramics may be caused by the addition of these materials. Therefore, it is necessary to discuss the structural changes of ceramics with other phases. The thermal conductivity of ceramics largely depends on the thermal conductivity of the filler and the internal structure formed during the ceramic preparation process.
For example, in porous ceramics, impurities distributed along the surface of the pores, such as impurity atoms, lattice defects, etc., will migrate from the hot end to the cold end of the pores and precipitate on the surface of the pores as the internal heat of the ceramic is released. This process will have a significant impact on the heat absorption process of the ceramic material, thereby further affecting the heat transfer performance of the material.
5. Sintering process
In the ceramic manufacturing process, sintering is one of the most important processes. This process will affect a series of physical and chemical changes of the green body, and affect the microstructure and mineral composition of the finished product. During the sintering process, the composition of ceramics will undergo different changes. In the sintering process, the temperature, the length of time, the temperature rise and fall speed, the highest firing temperature and the holding time will all affect the thermal conductivity of the ceramic material.
Second, the method of improving the thermal conductivity of ceramic materials
The methods for improving the thermal conductivity of ceramic materials mainly include component doping, controlling the size of raw material powder and improving the internal structure of ceramic materials. In the actual application process, the improvement of ceramic thermal conductivity is the result of the combined effect of multiple variables.
1. Component doping
In order to improve the thermal conductivity of ceramic materials, it is necessary to improve the purity of the ceramic materials, and try not to add or add as little additives as possible. However, in order to increase the density of the material and control the grain size, it is necessary to add a certain amount of additives, and an appropriate amount can also be added. Doping some non-metals (Al2O3, Fe2O3, etc.), metals (such as Cu, etc.) with high thermal conductivity, and loading specific organics on the ceramic surface to form high thermal conductivity composite materials.
2. Control the particle size
When the particle size of the raw material drops to the nanometer level, the thermal conductivity of the ceramic material decreases, and proper control of the particle size can significantly increase the thermal conductivity. In addition, increasing the density of the ceramic material, reducing the pores and glass phase, making it as close to the theoretical density as possible, can also improve the thermal conductivity of the ceramic material.
3. Improve the internal structure of ceramic materials
The internal structure of a ceramic material has a complicated influence on its thermal conductivity. There are many ways of internal heat transfer according to different situations. For example, the interconnected pores in the internal structure of ceramic materials have higher thermal conductivity than closed pores. The higher the closed porosity, the lower the thermal conductivity. The addition of other components may change the internal structure of the ceramic during the firing process and affect the performance of the ceramic. Under certain circumstances, choose to take measures to change the internal structure of the ceramic to meet specific functional needs. The connection method of the pores, the size of the raw material particles, and the appearance of internal defects such as microcracks will have a significant impact on the thermal conductivity of the material.
The influence of internal defects and microstructure on the thermal conductivity of ceramics is mainly determined by the phonon thermal conductivity mechanism of the material. All kinds of defects are the centers that cause phonon scattering, and these defects will reduce the mean free path and thermal conductivity of phonons. The heat transfer mechanism of silicon nitride ceramics is phonon heat transfer. When the lattice is complete and defect-free, the greater the mean free path of phonons, the higher the thermal conductivity, and the oxygen in the lattice is often accompanied by vacancies and dislocations. Such structural defects significantly reduce the mean free path of phonons, resulting in a decrease in thermal conductivity. Therefore, reducing the lattice oxygen content is the key to improving the thermal conductivity of silicon nitride.