Az alumínium-oxid por nem egyféle. A nátrium-hidroxid-tartalom és a felületi terület határozzák meg a tulajdonságait.

Buyers still send us RFQs that just say “alumina powder, 99%.” That number tells you it is Al₂O₃. It does not tell you whether the powder will sinter at 1550°C, polish a seal face, or sit in a castable matrix and refuse to densify. The useful conversation starts with four things: soda, specific surface area, particle size, and how far the calcination went toward alpha.

Most industrial powder starts as Bayer hydrate. We calcine it, usually somewhere in the 1100–1500°C range, to drive off water and convert toward α-Al₂O₃. How hard you fire it sets crystal size and BET. A lightly calcined powder can show several square meters per gram and still carry leftover transition phases. A hard-calcined grade drops toward 0.3–2 m²/g, grows coarser primary crystals, and reads mostly alpha. Reactive alumina is the ground version of that idea: D50 often around 1–2 μm, BET in the 1–5 m²/g band, high alpha, made to pack and sinter. That is a different tool from an unground calcine with a D50 of 60–80 μm.

Soda is the impurity people under-price. Normal soda calcines sit around 0.3–0.5% Na₂O. Low soda is closer to 0.05–0.15%. Ultra-low goes under 0.03%. Sodium acts like a flux. In a refractory brick that can be acceptable. In a high-alumina wear part, a substrate, or a spark-plug insulator, leftover soda grows grains, drops strength, and wrecks electrical performance. If the part has to be white, dense, and electrically quiet, we do not start with a commodity soda grade and hope the kiln fixes it.

Particle size distribution is the other half of packing. A monomodal fine reactive powder fills a matrix. A broader grind can cut water demand in a castable if you place it between tabular aggregate and the finest reactive cut. Put the wrong D90 into a slip and you get either a rock that will not de-air or a slurry that settles before it casts. Polishing powders care about crystal hardness and size more than sinterability. Coarse, hard-calcined crystals cut. Fine, soft-calcined material finishes. Same chemistry on the bag. Opposite jobs.

We also get asked to treat fused grain and tabular as if they were powder. They are not. Fused alumina is melted and crushed. Tabular is sintered near 1800–1900°C into dense plates, then crushed to aggregate. Those are skeletons in a mix. Calcined and reactive powders are the fines that have to bond.

If you are specifying alumina powder, put Na₂O, BET, D50/D90, alpha content, and LOI on the sheet before you argue price. A 99.5% Al₂O₃ with 0.4% soda and a 70 μm median will not behave like a low-soda reactive at 1.5 μm, no matter how similar the assay looks. The kiln does not erase a bad powder choice. It just makes the mistake permanent.