May 15, 2026 · viseray

Understanding Alumina Particle Size: D50, D90, and BET Explained

Particle size distribution and specific surface area are the most important parameters when specifying alumina. Here's what D50, D90, and BET actually mean and how they affect your process.

Three Numbers That Define Your Powder

When you look at an alumina spec sheet, three numbers dominate: D50, D90, and BET. They’re not just lab trivia — they directly control how your powder flows, packs, disperses, and sinters.

D50: The Median

D50 is the particle diameter at which 50% of the sample (by volume) is finer and 50% is coarser. If D50 = 5 μm, half the particles are below 5 μm, half above.

In practice, D50 determines:

  • Packing density: A broad distribution around a well-chosen D50 packs better than a narrow one.
  • Sintering rate: Finer D50 = higher driving force for sintering = lower sintering temperature.
  • Flowability: Very fine D50 (submicron) powders are cohesive and don’t flow freely; coarser ones do.

For refractory castables, D50 in the 10–30 μm range provides good packing between tabular aggregate particles. For advanced ceramics, D50 below 1 μm is common to achieve high green density and low sintering temperature.

D90: The Coarse Tail

D90 is the diameter at which 90% of particles are finer. It tells you about the largest particles in the distribution.

Why D90 matters:

  • Surface finish: In polishing applications, a single 80 μm particle in a powder rated at D50 = 5 μm will scratch the surface. A tight D90/D50 ratio means fewer oversize particles.
  • Dispersion: A long coarse tail means the powder contains agglomerates. These may not break down during mixing, leading to defects in the fired body.
  • Nozzle wear: In spray-dried powders for pressing, oversize granules cause die wear and density variations.

A well-controlled alumina grade has D90/D50 ≤ 3. Ratios above 5 indicate a wide or poorly controlled distribution.

BET: Surface Area, Not Size

BET specific surface area (m²/g) measures the total surface area of one gram of powder, including internal porosity. It’s measured by nitrogen adsorption.

BET is not the same as particle size. Two powders with identical D50 can have BET values that differ by a factor of 5, because BET picks up surface roughness and internal porosity that laser diffraction misses.

Why BET matters:

  • Water demand in castables: Higher BET = more water needed to wet the particle surfaces. This is the single biggest lever for reducing mix water.
  • Reactivity: High BET means more surface area for solid-state diffusion, so the powder sinters faster and at lower temperatures.
  • Dispersant dosage: Higher BET powders need more dispersant to achieve the same flow.

Reading a Spec Sheet: An Example

A spec sheet says: D50 = 3.5 μm, D90 = 10 μm, BET = 6 m²/g. What does this tell you?

  1. D90/D50 = 2.9 — tight distribution, well-controlled.
  2. BET of 6 m²/g at D50 of 3.5 μm — this is a reactive alumina. The high surface area relative to particle size means it will sinter actively and demand more water than a calcined grade with similar D50.
  3. Best application — low-cement castable matrix component or ceramic sintering powder.

The Full Picture

Never specify a powder by D50 alone. Always request D10, D50, D90, and BET. Two 5 μm powders can behave completely differently in your process if their distribution widths or surface areas differ.

For help interpreting your alumina specifications, see our calcined alumina and reactive alumina product pages with full particle size data, or contact us with your application details.