Particle size is the most requested figure and one of the most poorly specified. The request usually arrives as a single number — "10 micron powder" — and a number does not describe a distribution.
The three points, and what each means
A distribution is read at points on the cumulative curve. D50 is the median: half the material sits below that diameter. D10 and D90 mark the 10% and 90% percentiles, and they are what describe the tails — the fine fraction and the coarse fraction.
A three-point specification, with D10, D50 and D90, is considered complete and appropriate for most particulate materials. A single point is not.
The width of the distribution is summarised by the span, calculated as (D90 − D10) / D50.
Two powders with identical D50 and different spans pack differently, disperse differently and react differently. When behaviour changes between batches while the D50 has not, the span is usually the first place to look.
The method is part of the result
ISO 13320 is the laser diffraction standard, applicable from roughly 0.1 µm to 3 mm — the range that covers most technical ceramic powders. It also sets repeatability criteria: the coefficient of variation across the measurement set must stay below 3% at the D50 and below 5% at the D10 and D90.
Those limits are useful as a quality reference for the test itself. A spread wider than that points to a method or preparation problem, not to material variation.
Laser diffraction and sieving do not produce the same number for the same powder, because they measure different things. So the specification declares the method alongside the band: comparing values obtained by different techniques is not a comparison.
Agglomerate or primary particle?
This is the point that generates most disagreement between reports. Fine calcium phosphates tend to agglomerate, and laser diffraction measures whatever is in suspension at the moment of reading. If preparation dispersed well, the result approaches the primary particle; if not, the agglomerate is what gets measured.
Two measurements of the same powder at different dispersion energies can produce quite distinct distributions — and neither is wrong, as long as the preparation is declared. That is why the dispersion protocol (medium, dispersant, ultrasound, time) is part of the method, not an operational detail.
When the material is headed for a matrix where dispersion will be limited, in fact, the agglomerate figure may be the one most representative of real use.
What particle size does not tell you
It does not describe shape. Two particles of the same equivalent diameter can be a sphere and a needle, with completely different surface areas and flow behaviour. That is why particle size travels with SEM, for morphology, and with BET, when available surface matters.
At Triplet, particle size is part of the set that defines how the material is presented — alongside morphology and surface area — and it is specified as a band, with the method and the dispersion protocol declared, so that the number on the report means the same thing on every batch.
References
- ISO 13320:2020 — Particle size analysis — Laser diffraction methods
- ISO 13779-3:2018 — Implants for surgery — Hydroxyapatite — Part 3
- Guides on interpreting particle size distributions and setting three-point (D10/D50/D90) specifications and span
