Browsing by Subject "Spin coating"
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Item Coating of zeolite nanosheets(2014-05) Zhang, WanluZeolites are microporous crystalline aluminosilicates that are widely used as catalysts in petrochemistry and fine-chemical synthesis. While bulk zeolites can be used as catalysts and adsorption materials, thin zeolite films are suitable for applications such as catalytic membrane reactors, molecular sieve membranes and low-dielectric-constant materials. Zeolite nanosheets are silicate or aluminosilicate crystals with thicknesses on the order of one layer of the crystal structure (i.e., ~2 nm) and much larger lateral dimensions (i.e., ~10-100 nm). Nanosheets contain ordered molecular scale pores that are aligned through the sheet thickness. Compared with isotropic zeolites, synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets has more advantages with packing and processing. The overall goal of this research is to make coatings of zeolite nanosheets. To prepare the nanosheets, multilamellar MFI is synthesized as a precursor. Melt blending is applied to exfoliate the layered zeolite to achieve a polystyrene nanocomposite. A density gradient centrifugation process followed to purify the exfoliated zeolite nanosheets is able to remove both the polystyrene and the unexfoliated zeolite completely. After a suspension of zeolite nanosheets is produced, drop coating and spin coating are explored as the coating methods. Comparisons between these two coating methods are made after characterization of these films. For the drop coating method, drying temperature is varied and controlled to study its influence on the quality of zeolite films since it is a key factor for alignment of plate-like particles during sedimentation. For the spin coating method, spin rate is one of the most important operating parameters. Therefore, different spin rates accompanied with different dwell times are chosen for study when other parameters stayed the same. To ensure the removal of polystyrene and unexfoliated zeolite nanosheets, the purified MFI zeolite nanosheets are imaged by transmission electron microscopy. Surface information of the zeolite nanosheets films is characterized by scanning electron microscopy and optical microscope. The degree of particle orientation, close packing and surface coverage are determined from their images. Out-of-plane and in-plane X-ray diffraction data are recorded and analyzed to give more quantitative information about the orientation of the coatings.Item Formation of salt crystal whiskers on nanoporous coatings and coating onto open celled foam.(2012-02) Zhang, HengSalt 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.