Alumina, or aluminum oxide (Al2O3) is one of the most academically studied ceramics in ceramic science due to its ease of manufacturing, resistance to corrosion, creep, and sagging, low cost, and high-temperature refractory nature. It is the most widely used ceramic electronic substrate, and is commonly found in milling media, spark plugs, and wear-resistant applications such as pump seals and welding nozzles.
Classifications of Alumina
There are many different forms into which alumina occurs and can be synthesized. Of particular interest are:
- Corundum - Single crystal alumina that occurs naturally in the form of sapphire or, when a small amount of chromium oxide is included in the crystal, ruby. Single crystal alumina can also be made by drawing crystals from a melt.
- Large-grained alumina - Alumina with grains up to 100-200 microns in size. Typically used for its great creep resistance, and ranges in color from white (due to its large band gap) to an array of colors dependent on any sintering aids or colorants added.
- Small-grained alumina - Used in applications that demands high strength.
- High-purity alumina - Used for its corrosion resistance, as acids tend to attack impurities which may accumulate at grain boundaries.
- Low-alkali alumina - Common for electronic applications.
- Coarser-grained alumina is often a raw material in the reduction of aluminum metal (metallization).
Alumina also has a number of phases, possibly the most proliferous being the α phase. Almost all sintered, densified alumina is this alpha phase, which forms hexagonal close-packed structures in which aluminum ions fill two-thirds of the octahedral sites due to their small ionic radii (roughly 0.5 Å).
The γ phase is a very fine-grained alumina that occurs in a cubic spinel structure but converts readily to the α-Al2O3 in sintering temperatures (specifically, γ-Al2O3 inverts at 1150°C), making it very difficult to impossible to sinter into a dense body. However, gamma powder has a very high specific surface area of about 100 square meters per gram, while by comparison, α-Al2O3 has about 5 square meters per gram. This dramatic difference illustrates how quickly alumina grains coarsen when they undergo the transformation γ- to α-Al2O3 phase at 1150°C. The phase transformation is also an exothermic one (Q=20 to 40 kJ/mol), which drives up the temperature of the system, resulting in coarsening and further phase transformation which perpetuates and makes temperature stabilization in this transformation regime very difficult.
Beta “phases” of alumina also exist, but are actually alumina with the incorporation of sodium ions in the lattice. Particularly, β-alumina is Na2O·11Al2O3 and the β″ phase is Na2O·5Al2O3. These high concentrations of sodium are common artifacts of the Bayer process which uses caustic soda as a reaction precursor and contribute to ionic conductivity of these aluminas. Thus, they are good electrode materials for batteries and can also be used as a solid electrolyte for load-leveling batteries.
Various additives are added to alumina to promote high densification. Talc may be added to promote the formation of a liquid phase, with a particularly ideal additive being the silica-rich eutectic composition in the MgO-SiO2-Al2O3 system. However, it is important to note that the amount of this liquid phase is dynamic, so the sintering rates (which show Arrhenius behavior) are also dynamic and depend on the relative amount of liquid phase present.
MgO added in quantities between 300 and 500 ppm act as inhibitors of grain growth during alumina sintering which can reduce the relative rate of coarsening to densification. Reducing grain boundary mobility during sintering also greatly reduces the occurrence of pores becoming entrapped within grains due to effects related to pore drag. While this small addition of MgO significantly inhibits grain boundary movement (by a factor of 35), it also slightly increases pore mobility at the grain boundary, allowing them to diffuse to the surface faster. Without MgO, though, a typical alumina sample will have grains between 50 and 60 microns and considerable intragranular porosity; for example, Alcoa A16G has a starting size of 400 nanometers which grow to 40 microns during sintering.
High densification in sintering also can be achieved by eliminating γ-Al2O3 from the starting powder. Because γ-Al2O3 is water soluble, it will dissolve and reprecipitate at particle necks, causing coarsening without densification. It also promotes the formation of agglomerates which inhibit the efficient pressing of green bodies. Of course, the normal problems associated with the sintering of gamma alumina also work against the densification of α-Al2O3 if present, namely, runaway coarsening due to the exothermic phase transformation.
Other sintering additives include CuO or TiO2 and work by causing more vacancies in the alumina lattice. These vacancies promote diffusivity and increase diffusion rates; however, they produce dark coloration to the final ceramic.