Calcined alumina, a high-purity form of aluminium oxide (Al₂O₃), occupies a position that few industrial materials can match. Its resistance to high temperatures, hardness and chemical stability make it a core input for products ranging from precision ceramics to refractory linings. For engineers and material selectors, an understanding of its physical and chemical behaviour remains central to optimising product performance.
From powder to structure: the nature of calcined alumina
Calcined alumina is produced by heating aluminium hydroxide to between 1,200°C and 1,300°C, converting it into the thermodynamically most stable crystalline phase, alpha-alumina. That phase transition gives the material its high density and stability.
Raising the calcination temperature further, to between 1,900°C and 2,000°C, yields tabular alumina with coarse, plate-like crystals, widely used as a refractory aggregate. Temperature control, particle size distribution and sodium content during calcination directly determine the final sintering behaviour.
Selection guide: sodium content and applications
Sodium content is the decisive specification for industrial use. Grades differ markedly in performance across electronic components, insulation materials and glass enamels.
| Grade type | Main applications | Performance focus |
|---|---|---|
| Low-sodium alumina | Electronic ceramics, high-performance substrates | High insulation, electrical stability |
| Medium-sodium alumina | Electrical insulators, porcelain manufacturing | Mechanical strength, thermal stability |
| High-sodium alumina | Glass, enamels, fibreglass | Melt control, chemical stability |
Core function in ceramic processes
In traditional ceramics, calcined alumina is often used as a refractory filler. It markedly reduces drying and firing shrinkage, raises whiteness and relieves the stress generated by quartz conversion. Because alumina is considerably more refractory than silica, however, direct substitution tends to lower body maturity, so systematic formulation with fluxes such as feldspar is normally required.
In glaze technology, particle size is critical. Micron-scale alumina dissolves relatively well in the glaze melt, whereas coarser particles tend to remain undissolved, stabilising the melt or imparting a matte finish to the surface.
Technical parameters and performance limits
Calcined alumina has a melting point of 2,050°C, and dense ceramic parts made from it retain about 90% of their room-temperature strength above 1,100°C. Those properties make it a preferred material for grinding media, cutting tools, kiln furniture and wear-resistant engineering components.
Selection considerations
- Thermal expansion: the coefficient of thermal expansion is higher than that of zirconia-based ceramics, requiring additional assessment in applications exposed to severe thermal shock.
- Dispersion and forming: through pH adjustment or polyelectrolyte dispersants, high-density bodies can be produced by slip casting even in the absence of plasticity.
