People's expectations regarding the performance of various materials are becoming increasingly stringent to meet the challenges posed by the rapid development of science and technology. High thermal conductivity, low dielectric constant, thermal expansion coefficient that matches silicon, high resistance, low density, good chemical stability, good mechanical properties, and non-toxicity are all benefits of AlN ceramic materials. Important fields such as aerospace and large-scale integrated circuits benefit greatly from the application of aluminum nitride ceramic materials, which have won increasing interest from domestic and international researchers and manufacturers.
Low Temperature Sintering of Aluminum Nitride Ceramics
Sintering and densifying AlN is extremely challenging due to the fact that it is a covalent compound that has a low self-diffusion coefficient. To promote sintering, it is usually necessary to use rare earth metal oxides and alkaline earth metal oxides as sintering aids, but a sintering temperature above 1800°C is still required.
In recent years, people have paid attention to the research of AlN low-temperature sintering technology in consideration of factors such as reducing energy consumption, lowering costs, and realizing co-sintering of AlN and metal paste. The term "low-temperature sintering" is a relative concept that refers to lowering the sintering temperature of AlN to between 1600°C and 1700°C in order to achieve high-density sintering. It is universally believed that at high temperatures, the oxygen on the surface of AlN begins to diffuse into the lattice. Because of this, low-temperature sintering may also have the positive effect of delaying the diffusion of surface oxygen into the AlN lattice during high-temperature sintering and improving the oxygen exhaust effect during the subsequent heat treatment process, which is advantageous for the production of ceramic materials with high thermal conductivity.
Effects of Sintering Aids
Generally speaking, to sinter and densify aluminum nitride ceramics through liquid phase sintering, alkaline earth metal compounds and rare earth lanthanide compounds are added. AlN ceramic materials and products can be prepared with high purity and densification by using the sintering aid, which can significantly speed up the sintering of AlN ceramics in the early and middle stages of sintering and partially volatilize from the ceramic material in the later stage of sintering. In this process, the type, adding method and quantity of sintering aids all have a significant degree of influence on the structure and performance of AlN ceramic materials and products.
The following is a diagram of the action process of the sintering aid:
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Selection Principle of Sintering Aids
(1) It should be able to eutectically melt with the alumina on the surface of AlN particles at a lower temperature to produce a liquid phase, which allows the sintering temperature to be reduced;
(2) The generated liquid phase has a high level of wettability with AlN particles, allowing it to act as an effective sintering aid;
(3) The sintering aid has a strong binding ability with alumina to remove impurity oxygen and purify AlN grain boundaries.
(4) The liquid phase has good fluidity and flows to the triangular grain boundary during the growth of AlN grains in the later stage of sintering without forming a thermal resistance layer between AlN grains.
(5) It is preferable for the sintering aid not to react with AlN; otherwise, lattice defects are easily created and complete polyhedral AlN crystal form is challenging to form.
Commonly-Used Low Temperature Composite Sintering Aid System
Y2O3-CaO System
When the Y2O3-CaO system is used as a sintering aid, the two processes of atmospheric pressure sintering and hot pressing sintering are frequently used to prepare AlN ceramics. The second phase of AlN ceramics produced by atmospheric pressure sintering is primarily composed of Y3Al5O12 and Ca3Y2O6. These compounds are found in the grain boundaries of AlN ceramics and have the ability to lower the amount of oxygen present in the sintered body. On the other hand, under N2, hot pressing sintering of AlN ceramics is performed in a graphite mold. The content of the second phase of the hot-pressing sintered AlN ceramics decreases as some of the product volatilizes as gas.
The following are the physical characteristics of AlN ceramic materials sintered with various sintering aid systems:
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Y2O3-CaO-Li2O System
Y2O3, CaO, and Al2O3 combine to form Y4Al2O9 and Ca3Al2O6 aluminate liquid phases when sintering aids of Y2O3-CaO-Li2O system are added. The longer the holding time, the more liquid phase is formed. The distribution of the liquid phase in the AlN grain boundary prompts sintering densification and impurity accumulation. It also binds oxygen atoms in the second phase of the grain boundary, which gradually raises the thermal conductivity of AlN ceramics. In the sintering aid system, the function of Li2O is to decrease the reaction temperature of CaO, Y2O3, and Al2O3 by a significant amount, improve the wettability of the liquid phase and AlN grains, promote the densification of low-temperature sintered AlN ceramics, and purify the crystal lattice.
CaF2-Y2O3 System
Using CaF2-Y2O3 system as the sintering aid will cause the following reaction: 36CaF2+22AlN+2Al2O3+N2→24AlF3+Ca3Al2O6+3Ca11N8
AlF3 is sublimated and Ca-N compounds are released from AlN ceramics in gaseous form during the sintering process. Eventually, the aluminate phase will dominate the grain boundary phase. AlN ceramics containing 3wt% CaF2 have the highest thermal conductivity when CaF2-Y2O3 is used as a sintering aid and the total amount of additives (4wt%) is kept constant.
CaF2-YF3 System
YF3 can be used as a sintering aid because it does not introduce oxygen and has a lower melting point than Y2O3. (Ca, Y)F2 solid solution forms in the CaF2-YF3 system at a low temperature of 1200 °C, which makes it difficult for CaYAl3O7 and CaYAlO4 to form at a higher temperature of 1650 °C due to the absence of Y2O3. At high temperatures, liquid (Ca,Y)F2 and Ca-Al-O compounds flow and redistribute between AlN particles, which can effectively reduce the oxygen content on the surface of AlN particles and reduce the formation of oxygen defects in the AlN lattice.
Conclusion
Adding rare earth multi-phase composite sintering aids during the process of AlN sintering is one of the important ways to achieve low-temperature and atmospheric-pressure sintering of AlN ceramic materials. These aids are conducive to the formation of low-temperature liquid phase, which in turn lowers the sintering temperature, increases the sintering density, and purifies the AlN grain boundaries. As a result, a higher thermal conductivity can be obtained. In the future, if densified sintering at lower temperatures can be successfully realized, continuous production can be achieved, and costs can be reduced, the application of AlN ceramics in the electronics industry and other fields will become more widespread.