the particle size

Particle size is simply how big the individual particles of a powder are, usually reported as a diameter. It is the single most quoted number about any powder, because so much else follows from it. As a rough map: sand grains are around 100 to 1000 microns, a fine ceramic powder is below 1 micron, and a nanopowder is below 0.1 micron, or 100 nanometres. Smaller particles are harder to see, dustier to handle, and, crucially for a ceramist, they sinter far more eagerly, because being small means having a lot of surface for their volume.

There is a subtlety that trips up beginners: a powder particle is rarely a neat sphere, so its size is not a single unambiguous length. A rod or a plate has different widths in different directions, and measuring devices report an equivalent diameter, the diameter of a sphere that would behave the same way in the test, whether by settling speed, laser light scattering, or surface area. Different methods therefore give slightly different numbers for the same powder, which is normal. A useful shortcut connects size to surface: for smooth spheres the specific surface area equals 6 divided by the product of density and diameter, so halving the particle size doubles the surface area, which is why fineness and sinterability march together.

Particle size matters because it sets the driving force and the pace of sintering. Fine particles have high surface energy, which is the fuel sintering spends, and short diffusion distances, so a fine powder densifies at a lower temperature and gives a finer final grain structure and higher strength. That is why makers push for fine powders. The honest counterpoint is that a single average size never tells the whole story. What also matters is how spread out the sizes are, the distribution, and whether the particles are clumped, so particle size is best read alongside the size distribution and the surface area, not on its own.