The rapid development of science and technology has led to increasingly high demands on material performance. Aluminium Nitride (AlN) ceramic material is widely recognized and valued in various fields such as large-scale integrated circuits and aerospace due to its high thermal conductivity, low dielectric constant, thermal expansion coefficient matching silicon, excellent thermal and chemical stability, good mechanical properties, and non-toxicity.
Why is an AlN ceramic sintering aid needed?
AlN is a covalent compound with strong atomic bonding and a low self-diffusion coefficient. According to sintering theory, the relationship between the sintering temperature (Ts) and the melting point (Tm) of a salt can be approximated as:
Ts ≈ 0.57 Tm
Since the melting point of AlN is 3300°C, the sintering temperature of AlN ceramic is above 1900°C, which severely limits its industrial applications. Adding appropriate sintering aids is an important method to lower the sintering temperature of AlN ceramic.
Principle of sintering aids:
React with aluminum oxide on the surface of AlN powder to form low-melting compounds, creating a liquid phase that enhances material density during sintering.
React with oxygen impurities and form Y-Al2O3 and Ca-Al2O3 compounds in the grain boundaries. This reduces oxygen content in the AlN lattice, purifies the lattice, and improves thermal conductivity in the sintered AlN body.
The selection of sintering aids for AlN ceramics should consider the following principles:
- Co-melting with aluminum oxide on AlN particle surfaces at lower temperatures creates a liquid phase and reduces the sintering temperature.
- The generated liquid phase should effectively wet AlN particles and act as a sintering aid.
- Strong binding ability with aluminum oxide to remove impurity oxygen and purify AlN grain boundaries.
- Good flowability of the liquid phase during later-stage AlN grain growth after sintering, without forming thermal barrier layers between AlN grains.
- Ideally, the sintering aid should not react with AlN to avoid lattice defects and promote the formation of well-formed, polyhedral-shaped AlN crystals.
Common Low-Temperature Composite Sintering Aid Systems
Y2O3-CaO system
AlN ceramics are prepared using the Y2O3-CaO sintering aid system through atmospheric pressure sintering and hot pressing. This reduces oxygen content and eliminates second phases at grain boundaries. Hot pressing in a graphite mold under an N2 atmosphere further reduces oxygen content and second phases in low-temperature sintered AlN. The second phase content is reduced in hot-pressed AlN ceramics through partial volatilization of certain gaseous products.
Y2O3-CaO-Li2O system
The addition of Y2O3-CaO-Li2O sintering aids in the sintering process of AlN ceramics resulting in the formation of Y4Al2O9 and Ca3Al2O6 aluminum salt liquid phase. This liquid phase, formed by the combination of Y2O3, CaO, and Al2O3, promotes the densification of sintering and the aggregation of impurities at the grain boundaries of AlN. It also binds oxygen atoms in the second phase at the grain boundaries, gradually increasing the thermal conductivity of the AlN ceramic. The sintering aid Li2O in this system reduces the reaction temperature of CaO, Y2O3, and Al2O3, improves the wetting properties of the liquid phase with AlN grains, promotes the densification of low-temperature sintered AlN ceramics, and purifies the lattice.
CaF2-Y2O3 System
The following reaction occurs when CaF2-Y2O3 is used as a sintering aid:
36CaF2+22AlN+2Al2O3+N2→24AlF3+Ca3Al2O6+3Ca11N8
During sintering, AlF3 sublimes and Ca-N compounds are released as gases from AlN ceramics. Aluminum salt phases dominate the grain boundaries. Studies show that AlN ceramics with 3wt% CaF2, using CaF2-Y2O3 as a sintering aid, exhibit the highest thermal conductivity with a fixed total additive amount of 4wt%.
CaF2-YF3 System
YF3, with its lower melting point and absence of oxygen introduction, is suitable as a sintering aid. In the CaF2-YF3 system, (Ca, Y)F2 solid solution forms at low temperatures (1200℃). However, the formation of CaYAl3O7 and CaYAlO4 is challenging at 1650℃ due to the absence of Y2O3. At high temperatures, the liquid (Ca, Y)F2 and Ca-Al-O compounds redistribute between AlN particles. This allows YF3 to react with surface oxygen, reducing oxygen content and minimizing oxygen defects in the AlN lattice.
During AlN sintering, rare earth multiphase composite sintering aids are added to create a low-temperature liquid phase, reducing sintering temperature and improving density. This enhances thermal conductivity and enables low-temperature atmospheric sintering of AlN ceramics. Achieving successful densification sintering at lower temperatures in future research would enable continuous production and cost reduction, leading to expanded applications of AlN ceramics in industries such as electronics.
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