Melamine - Formaldehyde (mf) Aerogels And Its Modification Research

This thesis brief summarized the fundamental properties, preparation methods, characterization techniques and recent achievements of melamine-formaldehyde (MF) aerogels. Based on a series of experiments, the preparation process of MF aerogel was improved, and we also developed the preparation methods of phenol and metal oxide doped MF aerogels which have potential application in inertia constraint fusion (ICF) experiments. The investigation and corresponding results mainly includes:MF aerogels with nano network structure were prepared by sol-gel process, solvent exchange and supercritical drying. The effect of reactant concentration and gelation temperature to gelation time has been analyzed. The development of particles or clusters derived from reactants was continuously observed to verify the sol-gel theory. The alkali was added to sol to improve the preparation process of MF aerogel. The results indicated that the gelation time of MF sol was shortened and the density of MF aerogel was decreased. The Dmol3 and semi-empirical Vamp modules of Materials-Studio software were employed to simulate the optimal preparation conditions. The simulation result revealed that the increase of temperature can accelerate the gelation reaction rate and the gelation time can be significantly shortened by the substitution of water with dimethyl sulfoxide (DMSO) as a solvent in gelation process. The results of Fourier transform infrared spectroscopy (FTIR) spectrums showed that there is no obvious difference in chemical structure of MF aerogels prepared by different methods, except the sample prepared in DMSO. Due to the effect of higher temperature, this sample presents a stronger stretching vibration peak of C-H bond. The porosities of the MF aerogels were determined by high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscope (FESEM) and Brunmaner-Emmett-Teller (BET) methods. The results indicated that the aerogel prepared by traditional method was superior to samples prepared by other two methods in structure homogeneity.Phenol modified MF aerogels with nano network structure were prepared by adding resorcin to MF sol system, followed by CO₂ supercritical drying process. The results indicated that phenol modified MF aerogels had shorter gelation time than that of pure MF aerogel. Phenol modified MF aerogels with the density of 55mg/cm³ were obtained from the system with the total reactant concentration of 5%, and the effect of the mole ratio of melamine and resorcin (M/R) was investigated. It was found that the modified MF aerogel changes to opaque as M/R increased, while its apparent color was close to that of pure MF aerogel. The results of FT-IR spectrums showed that the modified aerogels with different mole ratio of M/R had the similar chemical structure. The results of HRTEM, FESEM and BET indicated that the network structure of the modified aerogels depended on the mole ratio of the M/R. As M/R increased, the network structure of modified MF aerogel gradually transformed to branching structure, which increases the pore size and reduces the BET surface area (SBET) and pore volume. The thermogravimeric analysis (TGA) suggested that the thermal stability of the phenol modified MF areogels was improved with M/R increasing.Titanium dioxide (TiO₂) doped MF aerogels were prepared by introducing TiO₂ sol to MF sol. It was found that TiO₂ sol had a saturation capacity. Beyond the value, TiO₂ colloid particles were easy to agglomerate, which leads to the formation of unstable and non-uniform TiO₂-doped MF gel. The results of HRTEM, FESEM and BET indicated that TiO₂-doped MF aerogels had a nano three-dimensional network structure. From the HRTEM micrograph, it was obvious that there were tiny crystalline dispersed in MF aerogel framework, and its EDS spectra revealed that the crystalline was TiO₂. The XRD analysis results showed that titanium doped MF aerogel possessed the same diffraction angle of MF. Apparently, only the XRD result can not provide the tangible evidence to confirm the crystal structure of TiO₂, further testing experiment is necessary to characterize this TiO₂-doped MF system.