The hierarchical structure and mechanical property of shell nacre are experimentally investigated from the new aspects of chemical stability and chemistry-mechanics coupling. Through chemical deproteinization or demineralization methods together with characterization techniques at micro/nano scales,it is found that the nacre of abalone,haliotis discus hannai,contains a hierarchical structure stacked with irregular aragonite platelets and interplatelet organic matrix thin layers. Yet the aragonite platelet itself is a nanocomposite consisting of nanoparticles and intraplatelet organic matrix framework. The mean diameter of the nanoparticles and the distribution of framework are quite different for different platelets. Though the interplatelet and in-traplatelet organic matrix can be both decomposed by sodium hydroxide solution,the chemical stability of individual aragonite platelets is much higher than that of the microstructure stacked with them. Further,macroscopic bending test or nanoindentation experiment is performed on the micro/nanostructure of nacre after sodium hydroxide treatment. It is found that the Young's modulus of both the stacked microstructure and nanocomposite platelet reduced. The reduction of the microstructure is more remark than that of the platelet. Therefore the chemical-mechanical stability of the nanocomposite platelet itself is much higher than that of the stacked microstructure of nacre.
SUN JinMei & GUO WanLin Institute of Nanoscience,Nanjing University of Aeronautics and Astronautics,Nanjing 210016,China
Based on molecular force fields,a new finite element model is constructed for multi-walled carbon nanotubes where the interlayer interactions and C--C bonds are simulated by the elements of piece-wise linear spring and rectangular cross section beam,respectively.For high computation efficiency and atomic reification,the radial breathing modes of multi-walled carbon nanotubes are studied systemically using this model.The results show the correspondence between carbon nanotube structures and vibrational modes,which provide unequivocal data for the experimental characterization of carbon nanotubes.An empirical relationship of radial breathing modes frequencies with the nanotube radius are also obtained for two-layer carbon nanotubes.
G-band mode is one of the most important Raman modes of single-walled carbon nanotubes （SWCNTs）. The vibrational frequency of the mode can be used to characterize SWC- NTs. However, analytical expression that can link the frequency to the geometrical parameters of a SWCNT is to date not reported. Based on a molecular mechanics model, the analytical solution is obtained for G-band mode frequency of SWCNTs. The result calculated from the present solutions is in good agreement with the existing experimental and numerical data.
