Effects of ultrasound on the preparation of zeolite A coatings

Preparation of zeolite 4A coatings on stainless steel substrates by direct synthesis at 60 °C from a clear aluminosilicate solution was carried out in the presence of ultrasound of 35 kHz at different times and the results were compared to those obtained by performing conventional static synthesis of zeolite 4A coatings under similar conditions. XRD and SEM analyses revealed that more nuclei formed at earlier times in the presence of ultrasound, allowing the preparation of thinner and continuous zeolite coatings in significantly shorter synthesis times. When zeolite 4A coatings were prepared by using conventional static synthesis at 60 °C, a closed layer with a thickness of about 1.5 μm could be obtained on stainless steel after a synthesis time of 10 h. Under the same experimental conditions, when sonication was applied, the layer thickness and the closure time were decreased to about 0.6 μm and 3 h, respectively. The surface roughness, which decreased initially, later started increasing upon continued nucleation and crystal growth, and showed a final decrease towards the closure of the layers. The final roughness value was smaller for the coating prepared in the presence of ultrasound.