Choosing calcined alumina is rarely a problem of finding a supplier; it is a problem of matching a grade to a forming process, a firing profile and a performance target. Buyers who compare only purity percentages usually end up with parts that crack, warp or lose dimensional tolerance after sintering. This checklist walks through the five parameters that actually decide whether a powder works in your line.
1. Alpha-Phase Content Comes Before Purity
Alpha-phase content determines how the powder behaves in the kiln. Below roughly 90% alpha phase, the transition to alpha alumina continues during sintering, and that transformation is accompanied by volume change and shrinkage that is difficult to control. Grades with alpha-phase content above 95% shrink predictably, which is why they are preferred for dry pressing and isostatic pressing of precision parts.
Check: request the alpha-phase ratio on the certificate of analysis, not just Al2O3 content. A 99.7% alumina with 85% alpha phase will behave worse than a 99.5% alumina with 96% alpha phase.
2. Sodium and Alkali Content Drive Microstructure
Sodium oxide is the most underrated specification in alumina purchasing. Alkali acts as a flux: at sintering temperature it forms a glass phase, accelerates grain growth and produces exaggerated grains that act as fracture initiation sites. For wear parts and seals, high sodium means unpredictable wear life.
Typical practice is to specify Na2O at or below 0.05% for fine technical ceramics, and 0.08% for less demanding wear components.
3. Particle Size and Crystal Size Are Two Different Numbers
Particle size (often D50) controls packing, flowability and green density. Primary crystal size controls the sintered microstructure. A powder with a 3 micron particle size made of 1 micron crystals behaves differently from a spray-dried granule of the same D50 built from sub-micron crystals.
- Dry pressing: granulated grades with good flow and 2 to 6 micron granules
- Isostatic pressing: finer, more uniform powders with narrow distribution
- Hot pressure casting: slurries formulated around specific surface area and settling behaviour
4. Silica and Iron Limits Depend on the Application
For electrical insulators and electronic substrates, iron and silica both degrade dielectric performance, so limits tighten to a few hundred ppm. For structural wear parts the tolerance is wider, and buyers can usually accept a cheaper grade without sacrificing service life. Specify by application, not by habit.
5. Sintering Behaviour Must Be Tested, Not Assumed
Ask for a dilatometer curve or at least a sintering shrinkage range at your target holding temperature. Two powders that meet identical chemical specifications can differ by several percent in fired shrinkage, which directly translates into tolerance stack-up and scrap rate.
| Application | Alpha-phase | Na2O max | Forming route |
|---|---|---|---|
| Mechanical seals and wear parts | above 95% | 0.08% | Dry pressing |
| Electronic substrates and insulators | above 95% | 0.05% | Tape casting, dry pressing |
| Semiconductor-grade ceramic rings | above 95% | 0.03% | Isostatic pressing |
| Grinding media and liners | above 94% | 0.10% | Rolling, pressing |
| Battery separator coating | n/a (high purity powder) | 200 ppm | Slurry coating |
A Practical Qualification Sequence
Rather than running a full trial on every candidate, most successful buyers qualify in three steps: first, screen certificates of analysis against the specification above; second, press and fire a small batch at production conditions and measure shrinkage, density and water absorption; third, machine a prototype and inspect for chipping and surface defects. Only then discuss price.
Common Costly Mistakes
- Buying on Al2O3 percentage alone and ignoring alpha phase and sodium.
- Switching powder lots without re-checking shrinkage, then scrapping a firing batch.
- Using a granulated pressing grade for isostatic pressing because it was cheaper.
- Over-specifying iron and silica for structural parts and paying a premium for no benefit.
Working With KeGu
KeGu supplies calcined alumina powders in grades ALC-11, ALC-12, ALC-13G and ALC-21G, all with Al2O3 content of 99.7% or above and alpha-phase content above 95%, together with 99.95% battery-grade high-purity alumina and sintered alumina ceramic components produced at up to 1700C. Technical datasheets, particle size reports and samples are available on request so buyers can qualify a grade before committing to volume.
