Necessity to Lower Sintering Temperature of Alumina Ceramics
The application of alumina ceramics is somewhat constrained due to the high energy consumption associated with its high sintering temperature and high demands placed on thermal equipment. The low-temperature sintering technology of alumina ceramics becomes a significant research direction from the perspectives of energy conservation, emissions reduction, and environmental protection.
Improving the Fineness and Activity of Alumina Powder
The most crucial step in raw material preparation is controlling the raw material powder's particle size. Because sintering is accomplished by the migration of materials through surface tension, the particle size should be small. Because they have a higher lattice energy and a more stable structural state, high temperature oxides are more challenging to sinter. Higher activation energy, or lower activity, is needed for particle migration. Due to the short diffusion distance between the particles, monodisperse ultrafine Al2O3 powder with small crystal grains, a large specific surface area, and a high surface activity can be used with only a low sintering temperature and sintering activation energy.
The following chart shows the relationship between powder particle size and sintering temperature:
Currently, there are two categories of methods for making ultrafine activated, easily sintered Al2O3 powders: mechanical methods and chemical methods. The Al2O3 powder particles are refined using mechanical external force using the mechanical method. Ball milling, vibration milling, sand milling, airflow milling, and other frequently used crushing techniques are among them; sand milling is one of the most efficient ways to create ultrafine ceramic powder.
The ability to create ultrafine, high-purity powders using wet chemical techniques has advanced rapidly in recent years. The most developed method is the sol-gel method, which can create materials that are inaccessible to conventional techniques. The sol is extremely stable and has the ability to evenly and steadily distribute different metal ions throughout the colloid. A uniform gel (amorphous body) is created after further drying, and with the proper processing, a highly active ultrafine powder mixed oxide or a solid solution that is uniform can be obtained.
Adding Sintering Aids to Alumina Ceramics
The sintering aid can either form a liquid phase between the grains or a solid solution with the matrix. The intergranular liquid phase created by sintering aids is conducive to the migration of grain boundaries and can speed up the transport rate of materials during sintering. In order to activate the lattice and boost densification, the sintering aid and matrix should form a solid solution that encourages an increase in the number of defects.
Al2O3 is given additives, and lamellar crystals form naturally in Al2O3 ceramics, strengthening and toughening the material in a manner akin to whiskers. The material's mechanical properties are significantly enhanced by the uniform distribution of lamellar crystals and good physical and chemical compatibility with the matrix; furthermore, the addition of additives and embedded seed crystals can produce a structured microstructure.
Classification of Sintering Aids
(1) Additives used to produce low-temperature liquid phases with Al2O3.
Oxides of alkaline earth metals, such as SiO2, MgO, CaO, BaO, SrO, etc., have crystal structures that are mainly cubic close-packed and of the NaCl type.
(2) Additives that combine with Al2O3 crystals to form solid solutions
TiO2, Cr2O4, Fe2O3, MnO2, etc., their lattice constants are not significantly different from Al2O3, and the majority of them contain valence-changing elements, which can form various types of solid solutions with Al2O3, and the valence-changing effect strengthens the lattice defects, activates the crystal, and makes the matrix simple to sinter.
Using Special Sintering Process for Alumina Ceramics
Hot Pressing Sintering
The green body is heated and pressurized during the hot press sintering process. Hot pressing sintering is one of the key technologies to lower the sintering temperature of ceramics because sintering is not only completed by diffusion and mass transfer, but also by plastic flow at this time. The sintering temperature of the green body is significantly lower than that of normal pressure sintering.
Hot Isostatic Pressing Sintering
A technique for simultaneously forming and firing is hot isostatic pressing. Its main advantage is that it can quickly obtain a variety of materials at a lower firing temperature (only between 50% and 60% of the melting point). It is challenging to achieve industrialization with isotropic, almost entirely dense fine-grained ceramic products because hot isostatic pressing sintering necessitates encapsulating the green body, and the equipment and process control are complex, the product cost is high, and the production efficiency is low. Only new materials that cannot be solved by conventional processes will use this method.
Microwave Heating Sintering
Due to dielectric loss, the ceramic body's surface and interior are simultaneously heated and sintered using microwaves to interact with the medium. The microwave also makes the particles more active and makes migration easier, which helps the process of densification. The ceramic microwave sintering method can speed up heating and sintering when compared to the traditional sintering method. The temperature field is uniform, the thermal stress is small, and it has the benefits of high energy efficiency and no pollution because of the integral heating inside and outside.
Microwave Plasma Sintering
Microwave plasma sintering results in a ceramic microstructure refinement because the rapid heating minimizes grain coarsening brought on by surface diffusion and provides a stronger driving force and shorter diffusion path for grain boundary diffusion and volume diffusion.
Conclusion
Because of the strong ionic bond, alumina ceramics have a high sintering temperature, and a suitable increase in sintering temperature is helpful for improving ceramic properties. However, an excessively high temperature will hinder the performance of the product and the manufacturing process. Therefore, it is crucial to lower the sintering temperature of alumina ceramics because doing so can cut down on energy consumption, shorten the firing process, reduce the loss of kiln and furniture, and consequently lower production costs.