Among them, high thermal conductivity is the "signboard" of aluminum nitride ceramics. The theoretical thermal conductivity of AlN ceramics is as high as 320W·m-1·K-1, but it is far from the theoretical thermal conductivity value in practical applications. We know that aluminum nitride is a covalent compound, and its thermal conduction is achieved by lattice vibration. The energy of lattice vibrations is Machinable Ceramic quantized, and thermal conduction can be viewed as a continuous non-resonant elastic wave propagating through phonons or quanta of thermal energy interacting with phonons. Oxygen has a strong affinity with AlN, and it will dissolve into the lattice of AlN to form aluminum vacancies. The lattice exhibits anharmonicity, which affects scattered phonons and induces thermal conductivity degradation. In addition, the melting point of AlN is higher than 2200 °C, the atomic self-diffusion coefficient is small, it is difficult to sinter densely, and the pores will reduce the phonon mean free path. Therefore, although the theoretical thermal conductivity of aluminum nitride is ideal, the thermal conductivity of the actual product is far from the theoretical value due to the existence of impurities and defects.
Therefore, in order to improve thermal conductivity, two Macor Ceramics key problems must be solved: one is to reduce the presence of oxygen impurity atoms; the other is to achieve dense sintering.
In addition to improving the purity of aluminum nitride powder, the main solution is to pay attention to two key points in the sintering process, that is, adding sintering aids and selecting a suitable sintering method.
The choice of sintering method
Commonly used sintering techniques for aluminum nitride ceramics include pressureless sintering, hot pressing sintering, spark plasma sintering, and microwave sintering. Among them, pressureless sintering is the simplest and most common sintering method in ceramic sintering, but it must be combined with sintering aids to complete the sintering process. Hot-pressing sintering facilitates the contact diffusion of particles and the flow and mass transfer process, thereby reducing the sintering temperature and porosity. The uniform self-heating of each particle in the spark plasma sintered body activates the surface of the particle, so it has a high thermal conductivity and can densify the sintered body in a short time. Microwave sintering is a new and efficient sintering technology with unparalleled advantages over traditional sintering technology. The microwave sintering method without adding any sintering aids is considered as a very promising low-cost technological route to obtain AlN transparent ceramics.
02
Add sintering aid
The sintering aids are compounds of some rare earth metals, alkaline earth metals and alkali metals, for example: Y2O3, CaO, CaF2, Li2O, etc. After the sintering aid is introduced into the sintering system, on the one hand, it can react with the alumina on the surface of the AlN powder to form a low melt, generate a liquid phase, and use the liquid phase mass transfer to promote sintering and improve the density of the material; on the other hand, The sintering aid reacts with oxygen impurities and precipitates in the form of Y-Al2O3 and Ca-Al2O3 compounds at the grain boundary, reducing the oxygen content of the AlN lattice, and purifying the lattice, thereby improving the thermal conductivity of the AlN sintered body.
1. Selection principle of sintering aids
The selection of AlN ceramic sintering aids should follow the following principles:
2. Commonly used low temperature composite sintering aid system
(1) Y2O3-CaO system
(2) Y2O3-CaO-Li2O
(3) CaF2-Y2O3 series
With CaF2-Y2O3 as sintering aid, the following reactions will occur:
During the sintering process, AlF3 sublimes, and the Ca-N compound is discharged from the AlN ceramic in gaseous form. Finally, the grain boundary phase is dominated by the aluminate phase.
The research shows that when CaF2-Y2O3 is used as sintering aid and Advanced Ceramic Parts the total amount of additives is constant (4wt%), AlN ceramics containing 3wt% CaF2 have the highest thermal conductivity.
(4) CaF2-YF3 series
YF3 does not introduce oxygen and has a lower melting point than Y2O3, so it can be used as a sintering aid.
In the CaF2-YF3 system, the (Ca, Y)F2 solid solution is formed at a low temperature of 1200 °C, so that CaYAl3O7 and CaYAlO4 are still difficult to form at 1650 °C due to the lack of Y2O3.
Therefore, at high temperature, the liquid (Ca, Y)F2 and Ca-Al-O compounds flow and redistribute among the AlN particles, so that the YF3 in it has sufficient opportunities to interact with the oxygen on the surface of the AlN particles, thereby effectively reducing the The oxygen content on the surface of AlN particles reduces the formation of oxygen defects in the AlN lattice at high temperature.