Abstract: In order to reduce environmental pollution and build a green economy, efficient use of electricity is becoming more and more important. Power electronic equipment is the key technology to achieve this goal, which has been widely used in wind power generation, hybrid vehicles, LED lighting and other fields. This also puts forward higher requirements for heat dissipation substrates in electronic devices. The shortcomings of traditional ceramic substrates, such as AlN, Al2O3 and BeO, are increasingly prominent, such as low theoretical thermal conductivity and poor mechanical properties, which seriously hinder their development. Compared with traditional ceramic substrate materials, silicon nitride ceramics gra-dually become the main heat dissipation materials for electronic devices due to its excellent theoretical thermal conductivity and good mechanical properties. However, at present, the actual thermal conductivity of silicon nitride ceramics is still far lower than the theoretical value, and some high thermal conductivity silicon nitride ceramics in the world (>150 W/(m·K)) are still in the laboratory stage. The factors affecting the thermal conductivity of Si3N4 ceramics include lattice oxygen, crystal phase and grain boundary equality. Among them, oxygen atom becomes the main factor because the solid solution reaction will occur in the lattice to generate silicon vacancy and cause lattice distortion, which will cause phonon scattering and reduce the thermal conductivity of Si3N4 ceramics. In addition, the thermal conductivity of Si3N4 can also be affected to some extent by crystal phase transformation and crystal axis orientation. And how to realize the mass production of silicon nitride ceramic substrate is also a big problem. At the present stage, with the continuous optimization of the preparation process, the actual thermal conductivity of silicon nitride ceramics is also continuously improved. In order to reduce the oxygen content of lattice, firstly, the oxygen content is reduced in the selection of raw materials. On the one hand, the Si powder with relatively low oxygen content is selected as the starting material, but the introduction of oxygen impurities in the process of ball-milling should be avoided. On the other hand, using high-purity α-Si3N4 or β-Si3N4 as a starting material can also reduce oxygen content. Secondly, selecting appropriate sintering additives can also improve the thermal conductivity by reducing the oxygen content. At present, more sintering additives are Y2O3-MgO, but oxygen impurities are inevitably introduced, so non-oxide sintering additives can be used to replace oxide sintering additives, such as YF3-MgO, MgF2-Y2O3, Y2Si4N6C-MgO, MgSiN2-YbF3, etc., which have also achieved very good results in improving the thermal conductivity. It is found that adding carbon to reduce the oxygen content can also achieve a good effect. By doping some carbon into the raw material powder, the raw material powder is placed in a reductive environment during nitriding and sintering, which promotes the elimination of oxygen. In addition, the thermal conductivity can be improved to some extent by adding seed and increasing sintering temperature to promote crystal phase transformation and by applying magnetic field to make grain grow directionally. In order to meet the size requirements of electronic devices, casting molding has become a key technology for large-scale preparation of silicon nitride ceramic substrates. Starting from the main factors affecting the thermal conductivity, this paper mainly introduces three methods to improve the actual thermal conductivity, such as reducing the oxygen content in the crystal lattice, promoting the crystal phase transition and realizing the orientation of crystal axis. Then, it is pointed out that the casting molding is the key to prepare high thermal conductivity silicon nitride ceramics on a large scale. The sintering process of preparing high thermal conductivity silicon nitride ceramics is reviewed. Finally, the future research direction of silicon nitride high thermal conductivity ceramics is prospected.
廖圣俊, 周立娟, 尹凯俐, 王建军, 姜常玺. 高导热氮化硅陶瓷基板研究现状[J]. 材料导报, 2020, 34(21): 21105-21114.
LIAO Shengjun, ZHOU Lijuan, YIN Kaili, WANG Jianjun, JIANG Changxi. Research Status of β-Si3N4 Ceramics Based on High Thermal Conductivity. Materials Reports, 2020, 34(21): 21105-21114.
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2020, Vol. 34 
