**2.4.2.6 Surfactant effect**

252 Ceramic Coatings – Applications in Engineering

(a) (b)

By observing brightness and reflectance versus stirring time based on (Figures 2.12-a and 2.12-b) Taguchi method, it can be seen that the maximum of lightness and reflectance are

(a) (b) Fig. 2.12. Effect of stirring rate parameter R on the a) lightness and b) reflection at different

The stirring rate affects the densification of the membrane layer directly. If the stirring rate is too low, the reaction solution can not form sufficient turbulence [2.17], then the microscopic mixture is uneven, and the generated crystal particles of SnO2 are of various sizes. On the other hand, if the stirring rate is too high, it will affect the rate of growing of the crystal nucleus, with the result that some tiny colloidal micro-particles enter the solution through the filter paper rather than depositing on the surface of mica flakes, which will cause the light to scatter. Therefore, the stirring rate in this reaction should be

Fig. 2.11. SEM images of pigments synthesized at different levels of reaction time

**2.4.2.5 Stirring rate** 

levels

of level 2.

related to a stirring rate of about level 2.

Figure 2.13 shows the SEM images of optimized sample with surfactant and without it. As figure 2.13-a shows in the sample containing surfactant, SnO2 particles coat mica flakes uniformly without any agglomeration. In addition, these particles agglomerate on mica flakes only and are not formed freely among mica flakes, but in the sample without surfactant, SnO2 particles formed irregularly and are agglomerated on mica flakes and also freely among them (Figure 2.13-b).

Fig. 2.13. SEM images of coated mica flakes in optimized sample a) with surfactant in magnification of 1 KX and b) without surfactant in magnification of 500 X

Surfactant molecules are able to surround a small volume of suspension including seed and due to repulsive forces between electrical charged boundaries of mica and SnOH particles, prevent the over sticking of particles and growth of agglomerates [2.13, 2.18, 2.19]. In addition the mica boundary and hydrated metal oxide formed by hydrolysis is activated with increasing amount of surfactant making for a smooth surface and stable adsorption [2.20].
