Studies On Vibrational Characters And Thermal Properties Of Nanocrystalline Materials

Nanomaterials are new materials which were developed in the mid 1980s. Because of their particular characters different from normal materials, nanomaterials have become hot research problems in physics, chemistry and material science. The aim of this thesis is to explore particular characters about vibration and thermodynamic for one-dimensional (ID) nanomaterials. On the basis of investigation and study of a lot of literatures, we find that there are problems or some untouched regions in the research on vibrational and thermodynamical characters of nanomaterials.After discussing the research headway about thermodynamical characters of nanomaterials at length in the part of introduction, we established the one-dimensional random model for nanomaterials according to their especial structure and concision of one-dimensional problem. In this model, size of crystal grain and force constant between nearest atoms in crystal grain distribute ramdomly. Considering second-neighbor interaction, the vibrational characters and some thermodynamical parameters of ID nanomaterials are calculated by means of lattice dynamics, negative eigenvalue theory and algorithm for five diagonal matrix. And the result is analyzed systemly. As comparison, the corresponding characters of ID crystal and disordered system are also calculated. The result shows that, at low frequencies, phonon density of state of nanomaterals is higher than that of single crystal and it increases with decreasing of crystal grain. And the shape of phonon density of state of nanomaterals is different from that of single crystal and disordered system. These results fit well with experimental and theoretical results we already have. Like electronic eigenvectors, phonon eigenvectors of nanomaterials have characters of localization. The degree of localization is relation with size of crystal grain size of system and eigenfrequency and it increases with increasing of eigenfrequency. This research has never been reported in literatures. Comparing to single crystal and disordered system, nanomaterials have distinctive thermodynamical characters such as high heat capacity and high entropy. These parameters have direct relation with temperature, size of grain and so on. The research also showsthat not only variety percent of atoms in crystal boundary but also distortion of crystal grain have great effect on characters of nanomaterials. This has been seldom noticed by researchers.Finally, the investigations are summarized and further study is prospected to get improvements.