Ice/Water Phase Transitions In Carbon Nanotubes Studied By Molecular Dynamical Simulations

As the energy crisis comes up energy storage materials and technologies will boom rapidly. The phase transition material is one of many energy storage materials which are widely used. It has a big energy-storage density and the output of energy and temperature is relatively stable etc., therefore, the research of phase transition materials become the focus in the material science. There is an energy mutation when the phase transition of ice/water occurs in carbon nanotubes. In order to better use the ice/water confined in carbon nanotubes as the phase change material, we need to know more about its static structures and dynamic properties. This article uses the molecular dynamics method to simulate and calculate the ice/water phase transition in (14, 0) "zigzag" nanotube. The structural difference of the ice/water confined in carbon nanotubes before and after the phase transition and the changes of temperature and pressure are analyzed by the atomic density distribution function, orientation distribution function, the number of hydrogen bonds, the mean square displacement and so on.In this paper, the phase transition and principle theories are discussed, and some definition, classification and essence of the phase transition are clarified. Meanwhile, the molecular dynamics simulation is more comprehensively described, for example, the selection of force field, long-range force calculation, the calculation model simplification, the common force field of water molecules, the selection of integration step and the periodic boundary conditions. This simulation process consists of three aspects: Firstly, the temperature was settled to 240K during the pressurization process, and the pressure values was increased gradually from 500MPa to 1GPa in a step of 25MPa and from 1GPa to 11GPa in a step of 1GPa; Secondly, the pressure value was settled to 200MPa during the cooling process, and the temperature was decreased from 300K down to 260K with a step of 10K and down to 220K with a step of 5K; Thirdly, the pressure was settled to 500MPa in during second the same cooling process. The simulation results show: during the pressurization process, the water molecule structure transforms from the four-molecule rings to five-molecule rings at 575MPa, and the chain of water molecules begin to appear in the center of five-molecule rings at 5GPa; during the cooling process (500MPa), the crystal four-molecule rings occurred at 255K, and the four-molecule water rings transited to the five-molecule rings when the temperature reduced to 220K. The radial distribution function and the direction of dipole moment show that the five-molecule water ring tends to be more orderly than four-molecule ring. The number of water molecules in carbon nanotube can be examined from the axial density distribution function and it can be distinguished whether a similar phase transition forms in the pressurization process or in cooling process. The distributions of the orientation of the OH bond in the nanotube suggest that only one OH bond be involved in the formation of hydrogen bonds under this condition. During pressurization process, the number of hydrogen bonds will have a peak value; while the cooling process, the number is constantly increasing. During the pressurization process, the diffusion coefficient of water molecules become smaller, while the cooling process showing a regular pattern is required only under the specific pressure. The diffusion coefficient of the four-molecule water rings is bigger than the five-molecule rings.