Formation of salt crystal whiskers on nanoporous coatings and coating onto open celled foam.

Abstract

Salt crystal whiskers were grown from salt solution saturated nanoporous silica coatings. Coated substrates were partially immersed into an aqueous potassium chloride solution and then kept in a controlled relative humidity chamber for whisker growth. The salt solution was first wicked into the coating by capillary action, and then evaporation ensued and a supersaturated condition was reached. Crystals grew from the surface by a base growth mechanism in which salt ions were added to the surface of the crystal that was in contact with the nanoporous coating. Optical microscopy and SEM results demonstrated this mechanism. Crystals with whisker morphologies, typically 2 - 50 µm in lateral dimension and up to ~1 cm in length, emerged from the coating surface at a position above the original liquid level. Sheet-like crystals also formed from whiskers that had fallen flat onto the porous coating surface. Inspired by the sheet formation mechanism and liquid transportation phenomenon, a seeding technique was developed to reduce whisker width. Attritor ground salt particles were placed on the nanoporous coating surface to initiate simultaneous whiskers growth and salt nano-whiskers with lateral dimension as small as 50 nm were obtained on the surface of the coating. This crystal growth method can be applied to different materials, namely water soluble materials, and creates whisker crystals with controllable size and location on the nanoporous coating. Open celled foam is a three dimensional structure. In some applications, other materials are coated on internal surface of the foam to provide desired final product functionality. Because of their complicated 3D structures, coating onto foam is challenging. A new coating process that combines dip coating and spin coating was developed. Dip coating step was used to load the solution into the foam and a spin treatment step was added to remove the trapped liquid and redistribute the liquid to obtain uniform coating. The dip and spin process was also used to create -alumina and zeolite coatings, which are of interest for catalysis applications.