Synthesis And Characterization Of Copper Oxide/fluorinated Silicon Oligomers Nanocomposite Multi-functionalized Heat Transfer Surfaces

The rapidly increasing needs for multi-functionalized heat transfer surfaces with good corrosion resistance,anti-fouling and high thermal conductivity are expected in many industrial fields,such as chemical industry,food fabrication,electric industry,desalination, nuclear,energy and so on,where the traditional coatings based on epoxy and phenolic resins can not satisfy all requirements owing to complex coated technology and the difficulty for repairing.In this dissertation,the work is focused on the synthesis and characterization of metal oxide/ceramic nanocomposite material by sol-gel technique and fluorinated organic-inorganic hybrid materials by free-radical random copolymerization.We attempted to fabricate a multi-functionalized nanocomposite material with good corrosion resistance,anti-fouling and high thermal conductivity by combining the useful properties of metal oxide/ceramic nanocomposite material and fluorinated organic-inorganic hybrid material.This composite material could be self-layered on heat transfer surface in one step.Homogeneous CuO/SiO₂ and NiO/SiO₂ nanocomposite coatings containing CuO and NiO nanoparticles in silica matrix were successfully synthesized by sol-gel process, respectively.The results indicated that the dispersed second phase in the silica matrix can cause crack bridging,crack pinning,and crack deflection to control the size and density of processing flaws,reduce the residual stress of the silica matrix and produce more compact surface.The thermal conductivity as well as corrosion resistance of nanocomposite coatings was significantly improved by the introduction of metal oxide particles.In comparison with NiO/SiO₂ nanocomposite coatings,CuO/SiO₂ composite coatings displayed lower corrosion protective behavior but higher thermal conductivity.Experimental results revealed that the nature of embedded nanoparticles had a vital rule on the improvement of corrosion resistance and thermal conductivity of the composite coating.Two series of CuO_x/SiO₂ samples with various Cu/Si molar ratios,have been synthesized by sol-gel technique from two different copper metal precursors,cupric nitrate hydrate(Cu(NO₃)₂·3H₂O) and cupric acetylacetonate(Cu(acac)₂) respectively.The results indicated that the structures and chemical states of CuO_x nanoparticles in the silica matrix were sensitive to the nature of chemical precursors and Cu content.At low Cu content,the primary copper oxide nanoparticles self-aggregated into normally known spherical aggregates. With the amounts of copper oxide increasing,different structure transitions of CuO_x have been found,i.e.,from nanosphere to uniform nanorod using Cu(NO₃)₂·3H₂O as chemical precursor and,to square-like frames starting from Cu(acac)₂.It has also been found that,with the increase of the CuO content,more and enlarged flaws were formed due to the formation of larger copper oxide aggregates through the Ostwald ripening process,which led to a gradually declining corrosion protection.Consequently,it could be concluded that minimum agglomeration and uniform distribution of the second phase particles in the matrix are preferred for enhancing the corrosion protection ability of CuO/SiO₂ nanocomposite coatings. Moreover,the thermal conductivity of the composite coatings could be significantly improved by adding CuO nanoparticles.But it showed a fluctuant tendency with increasing CuO content.Fluorinated silicon oligomers were prepared by free-radical random copolymerization and sol-gel process from dodecafluoroheptyl methyl acrylate(FA),vinyltriethoxy silane (VTES),and tetraethyl orthosilicate(TEOS).FTIR and XPS results indicated that, hydrophobic perfluoroalkyl groups were preferentially enriched to the outermost layer at the coating film-air interface.The fluorinated silicon oligomers had a good water repellency property,governed by hydrophobic fluorocarbon groups of the outermost layer.The nature and concentration of solvent had a significant influence on the structure of molecular assemblies of the oligomers and the mobility of perfluoroalkyl side chain.Compared with ETOH and Butanol,THF was a good solvent for fluorinated silicon oligomers prepared by free-radical random copolymerization.The fluorinated silicon oligomers showed a looser random coil-like shape in THF solution.Hydrophobic perfluoroalkyl groups of the random coil-like shape fluorinated silicon oligomers can move more easily and enrich at the coating film-air interface.With the increase of the concentration of solvent,the structure of molecular assemblies of the oligomers would be rather small and loose,which made the movement of perfluoroalkyl side chain at the coating film-air interface more easily.An important role of TEOS was to increase the density of the reaction sites of the-OH groups and the thickness of the bottom layer composed of mainly silica network originating from hydrolysis and polycondensation of TEOS.The useful properties of hybrid coatings,like better wear resistance and corrosion resistance,could be significantly enhanced by the increase of TEOS, due to the improvement of thickness of the bottom SiO₂ layer and bonding between SiO₂ layer and aluminum substrates,while it had a slight influence on the surface wettability of hybrid coatings.Copper oxide/fluorinated silicon oligomers nanocomposite hybrid materials,with high corrosion resistance,anti-fouling and high thermal conductivity,have been synthesized by co-hydrolyzed and co-condensed using the as-prepared fluorinated silicon oligomers,TEOS and cupric nitrate hydrate as chemical precursor.The dispersed second phase embedded in the silica matrix have caused particle bridging,crack pinning,crack deflection and stress induced micro-cracking effects to improve corrosion resistance behavior and thermal conductivity, while it had a slight influence on the surface wettability of the hybrid coatings.The characteristics of CaCO₃ crystallization fouling on heat transfer surface were investigated in convective heat transfer test rig.It has been demonstrated that these metal oxide/fluorinated silicon oligomers nanocomposite hybrid coatings with very low surface energy could reduce the formation of deposits on heat transfer surfaces significantly.The heat transfer coefficients remained almost constant during the running,impling that almost no CaCO₃ scale formed on the treated surfaces.Obviously,this is a most promising result which indicates substantial benefits in practice.