Laser granulometry

Real powders almost always possess high polydispersity, e.g. they consist of particles of different sizes. Particle size distribution (for powder, granules, or particles dispersed in a liquid) presents the relative quantities of particles having particular size
Equipment measuring this size distribution is represented by particle size analysers. Information on the particle size distribution is important for understanding physical and chemical properties of the material. Particle size influences the reactivity of solids in chemical reactions. In many industrial products such as printer toners or cosmetics it is important to monitor these characteristics thoroughly.

Vibrational table for sieve analysis

The most evident method to determine the particle size distribution is the sieve analysis. This method allows one to determine the weight fraction of particles having particular size, e.g. from 45 to 53 µm if the corresponding sieves are used. The data obtained are easily represented in cumulative form: the weight fraction of the particles that pass the sieve and the weight fraction of particles remaining on the sieve are registered (e.g. -45 µm and +45 µm). The measurement range is limited by the smallest mesh size in the sieves available.
In practice, sieves with mesh size less than 45-50 µm are rarely used. Sieves having smaller mesh size are undurable and their cleaning is inconvenient. In a simplified consideration, the particle shape is assumed to be close to spherical while the particles pass the square sieve meshes. Nevertheless real particles often have irregular shape. For instance the particles of fiber materials have shape which is far from spherical. Therefore the characteristics of a powder will considerably depend on the measurement method chosen.
The probe prepared for analysis must be representative; it is important to avoid the partition of particles according to their sizes and the loss of the fine fraction.

It is possible to study particle size distribution by microphotograph analysis, by Coulter counter (which is based on the measurement of the electroconductivity of a liquid with non-conductive particles in it), by sedimentation (precipitation), by acoustic spectroscopy.

The method most widely used in industry is laser granulometry based on the laser beam diffraction.

In our projects, the granulometric analysis of powders is carried out using the laser-based analyser of particle size distribution Horiba LA-950 (or Analysette 22). According to the information from Horiba LA-950 equipment producers the particle size detected ranges from 100 nm to 3000 µm. The corresponding range for Analysette is from 0.3 to 300 µm.
In the case of fine powders the well-known problem is the aggregation of fine particles. As a result of such aggregation, the granulometry data reflect the aggregate size instead of the particle size. To solve the problem the powders under investigation are dispersed in water by ultrasonic treatment. To reduce possible aggregation of powders different surfactants can be added to the sample. The choice of conditions (pH, particular surfactant) for the best possible deagglomeration of nano-particles is an issue for a special independent investigation.
The data of granulometric analysis can be represented in terms of the sample volume or in terms of the number of particles in the sample.

Example. Results of the aluminium alloy AA6061 powders granulometric analysis. Measurements were made on the Analysette 22 laser-based granulometer (EADS data). Y-axis represents the fraction of particles having particular size denoted by X-axis. Here the particle size distribution is represented in terms of the particles volume.

The particle size distribution for the initial Al alloy powder. Particle size ranges from 40 to 100 µm.

The particle size distribution for the milled Al alloy powder. The average particle size is 8 µm. Milling was carried out in a planetary mill MPP-1 at acceleration 28 G (in the TTD company in the scope of the collaborate project “Activation”).

The particle size distribution for the sample of a layered silicate. Milling was performed in the planetary mill MPP-1 for 8 minutes. Measurements were carried out using laser granulometer Horiba LA-950 (with ultrasonic treatment). The particle size distribution is shown in terms of the number of particles. The average particle size in this case is 210 nm.

Laser granulometer Horiba LA-950.