Composites And Controlling Morphology Of Micro/Nano-silica

This manuscript was discussed in three aspects:Firstly, the transition of amorphous SiO2to crystalline state was investigated. Secondly, the mastoid-like and the branch-like micro/nano SiO2were prepared and characterized, the growth mechanisms were researched in detail, and the factors that affecting the morphologies of micro/nano-SiO2were analyzed more in-depth. Thirdly, hydrothermal method, chemical vapor deposition and blending method were developed to prepare the micro/nano SiO2core-shell composite materials and composite films. Both the micron technology frontier and reality are concerned but also the forward-looking and foundation of nanotechnology are focused in this study. Therefore, this research is valuable in theory and application.In this present study, the micro/nano SiO2was prepared from rice hull. The results of SEM, TEM and HRTEM show that the SiO2is amorphous when the roasting temperature is lower than570℃; the columnar-like SiO2appeared when the temperature rise to1080℃. The roasting temperature is critical factor for phase transformation of SiO2, it also decides the purity, morphology and structure of SiO2. These phenomena were explained by the change of deposition model of the ordered atomic clusters.Then, the micro/nano SiO2with different morphologies were prepared by two different methods. The native in-situ mastoid SiO2can be purified by acid and alkali pretreatment from rice husk. Studies have shown that the mastoid SiO2is amorphous, is place in an orderly arrangement on the outer surface of the rice husk, and is divided into three types of bimodal mastoid, unimodal mastoid and tumor peak mastoid. Si content is increasing from the outer surface to the inner surface of the husk. In other side, three branch-like micro/nano SiO2with different morphologies (including centipede-like, arborization and coral-like) were prepared by hydrothermal method, and was constructed by the main body and dendrtics, The growth rate and length of these dendrtics are different from each other. The raw materials, pressure, temperature and the temperature keeping time have great effects on the morphologies of SiO2. The growth mechanism of branch-like micro/nano SiO2can be explained by the vapor deposition and dangling bonds-assisted growth. At last, three kinds of micro/nano SiO2composites were prepared by different methods. C@SiO2@SiO2composites with core-shell-branch structure were prepared by hydrothermal method. The carbon nanowires are single crystal structure, and two SiO2shells are amorphous, the outer SiO2shell is branch-like structure. The formation of C@SiO2@SiO2can be explained by dangling bonds-assisted growth and the vapor deposition. SiC@SiO2composites with core-shell structure were prepared by chemistry vapor deposition. Its diameter is25-50nm, while its length is about hundreds of micrometers, even up to several centimeters. The defects of’convex knot’and’concave hole’exist on the SiC nanowires. There is a PL peak in438nm and464.5nm. The matching of deposition rate of the gaseous SiO2and the growth rate of SiC nanowires can control the morphology of SiC@SiO2. Three kinds of composite films, including SiO2/PI, CNT@SiO2/PI and SiC@SiO2/PI, were prepared by blending method. The heat stabilities of SiO2/PI were greatly increased and the dielectric constant was reduced by adding micro/nano SiO2which has better dispersion and compatibility in the PI matrix. The heat stability, rigidity, wear resistance, fracture strength, and electricity properties of CNT@SiO2/PI were improved by adding CNT@SiO2, especially the wear resistance property. By adding SiC@SiO2, the heat stabilities of SiC@SiO2/PI were greatly increased, and its volume resistivity was reduced to improve its conductivity. The mechanism properties, including fracture strength, rigidity, and wear resistance, were improved effectively. SiC@SiO2can enhance the strength and toughness by the crack deflexion, SiC@SiO2pulled out and bridged.When the carbon nanotubes (CNT) and SiC nanowires were coated by the micro/nano SiO2to modify its surface, they can not only be improved its dispersion but also be integrated and optimized its performance. Micro/nano SiO2, the core-shell CNT@SiO2and SiC@SiO2dispersed in the PI matrix, can improve the performances of the composite film. A dielectric constant of SiO2/PI was reduced, the improving wear resistance of CNT@SiO2/PI is more distinguished, and the increasing fracture strength and toughness of SiC@SiO2/PI are more prominent.