Thermodynamic Phase Stability And Solid Structure Transition Of Nanocrystals

As D drops into nanoscale of 1-100 nm, which lie in the realm between dissolved molecular cluster and macrosolid, their properties and even structures as well thermodynamic stabilities could differ from those of the corresponding bulk counterparts. With particles size decreasing, the surface/volume ratio increased, which increases the total internal energy of nanophases due to the appearance of the surface energy and surface stress. Since surface energy and surface stress of different phases are distinct, the relative energetic level of different phases could change. To drop the total Gibbs free energy of the nanocrystals G, structural transitions or orientation changes of solids may occur at different transition temperatures. Thermodynamic stability of nanophase and size-dependent thermodynamic solid phase transition thus becomes technologically important.Much of the interest in nanomaterials originates from the challenge to find useful properties that substantially differ from those of the same bulk materials. Formation of nanocrystals is also a potential tool for the preparation of metastable phases of bulk, which otherwise would not be attainable. These phenomena have been found in CdSe, ZnO, ZrO2, ZnS, Al2O3, TiO2, Fe2O3, Fe and C. The above phenomena bring out the requirement of the corresponding thermodynamic description to understanding their theoretical background due to scientific and technique reasons. This thesis summarizes our developed quantitative thermodynamic model to illustrate these kinds of solid-state nanophase transition. The essential basement of the model is the establishment of quantitative analytic solution for surface stress. Since the surface stress could be quantitatively estimated, thermodynamic description for these transitions become possible. The results show that our model could correctly predict and estimate the thermodynamic nanosolid transition conditions. In addition, the basic thermodynamic amounts of the related metastable phases are also determined. The success of our model implies that the classic thermodynamics can indeed be utilized in nanoscale although the thermodynamics has a statistic nature.The corresponding continuous thermodynamic size function is a powerful means in the applications of nanotechnology including understanding structure, fabrication and properties of materials. The concrete contents are list as follow:(1) A thermodynamic model is established by considering the surface energy Gs induced by surface energyγand the elastic energy induced by surface stress f and external pressure Pex, which is used to calculate the thermodynamic stability and solid structural phase transition in nanocrystals. Thermodynamics stability of some important oxides has been researched, and the phase diagrams of phase transition temperature T vs size D were established, the contribution of surface energyγand surface stress f on the total Gibbs free energy were discussed. Dimension and shape also have effects on the phase stability and transition temperature. Transition temperatures between wurtzite and sphalerite polymorphs of ZnS nanoparticles and nanobelts are calculated.(2) Based on the thermodynamics model, the size-dependent solid transition pressures of CdSe and ZnO have been determined thermodynamically and quantitatively. It is found that the transition pressure increases as size drops for CdSe and ZnO. The transition pressure change is dominated by surface stress for CdSe while it is done by surface energy for ZnO. (3) According to this model, thermodynamics stability of nanocrystalline Fe was studied, the contribution of surface, interface and crystalline boundary on the phase transition betweem bcc and fcc was discussed,(4) According to this model, size dependent transition temperature fuction of nanocarbon was established, and the results show that surface stress is the major diving force for the phase transition between nanodiamond and nanocarbon. Based on the Bundy′s carbon diagram and size dependent melting temperature model, size dependet temperature vs pressure digrams of nanocarbon were established. It is found that our model predixtions are consistent with available experimental results for the phase transition between nanodiamond and nanocarbon.