Colloidal chemistry

The dispersion of colloids in a liquid medium is a defining stage in the manufacture of an ink, a slurry or a ceramic paste. In fact, the state of dispersion of these particles in their matrix influences a large number of the final properties of the material generally.

Dispersion of inorganic particles in a liquid medium (organic or aqueous solvent) involves two very distinct stages:

1. wetting: air is displaced from the surface of particles and/or the interstices between the particles and is replaced by the liquid phase;

2. grinding: breaking down the agglomerates and/or aggregates into smaller units dispersed in the medium by mechanical energy;

In effect, obtaining stable colloidal suspensions containing finer and finer, even nanometric (less than 100 nm), particles entails using suitable grinding techniques.

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The industrial applications are numerous and are to be found in the area of preparation of ceramic inks (inkjet printing), dispersion of actual ceramic nanoparticles before they are incorporated into a host matrix (organic or inorganic coating etc), preparation of suspensions for micro-machining of silicon or even reactive grinding (figure x). The first two applications are also those in which the CRIBC has particularly invested over recent years and where it hopes to increase even further its competence for the future.

Figure x: Example of reactive grinding of nanoparticles (on the left: development as a function of grinding time and on the right: surface functionalisation)

3. stabilisation of separated particles by adsorption of polymers (dispersants and/or charged varieties) of the solvent to prevent flocculation of the dispersed particles

To quantify the stability of a colloidal suspension, the idea of x potential is used. This potential, introducing the idea of a double layer, is defined as the potential of the plane separating the fixed layer from the mobile layer. A high z potential, in absolute value, indicates that the system being studied is truly stabilised.

In the absence of stabilisation, maintaining the dispersed state will depend on the relative speeds of the mechanical separation and the flocculation of the particles. These 3 stages are not sequential, they occur simultaneously. However the third stage will become important when wetting and grinding are almost complete.

Project: CERATEX, CAPTINK

 
 
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