| Ultra fine copper powder is widely known as the 21 st century material due to its excellent conductivity, low-cost and availability and high chemical stability. Evaporati on-condensation method has shown high efficiency with better control of the particle size and powder collection and the copper powder produced has a high purity, narrow particle size distribution. It is shown in the experimental study that the evaporation te mperature and vacuum are two key factors in the preparation of ultrafine copper powd er in the evaporation-condensation method. In order to improve the parameters of vacuum evaporation condensation process for copper powder, we used both molecular dynamics simulation and experiments to study the melting, evaporation and condensation process of copper powder.Materials Explorer6.0 molecular dynamics software package was used to simulate the system under vacuum (lOPa), with 72,108,256 or 500 and 864 copper atoms and in the temperature range 473-2073K. It is shown in the simulation that: Under vacuum conditions, with the increase of the number of atoms in the system, the melting temperature increases and when the atom number is above 500, it reaches a stable range from 1357-1373K,which is consistent with experimental melting temperature,1357K. Based on the relationship between system energy and the temperature, it can be predicted that the temperature range in which the transitions occur at standard atmospheric pressure (latm) was higher than that under vacuum. In other words, the melting temperature increases in the system.The CASTEP (Cambridge Sequential Total Energy Package) module in Material Studio package was used to simulate the changes of copper atoms in the range of 273 1573K under vacuum (10Pa). It’s shown that the system undergoes significant change in the temperature range of 1273K～1373K and the system transits from solid to liquid phase. It is known that the electronic density of copper atoms is provided by the 4s,4p and 3d orbital electrons and the energy of 3d orbital is higher than the sum of 4s and 4p orbits. Therefore, the hybridization between the copper atoms is mainly attributed to the 3d orbital.The Forcite module in Material Studio package was used to simulate the changes of copper atoms in the range of 1473-2073K under vacuum (10Pa) and 100Kpa. It is found that the thickness of the transition zone is roughly linear to the temperature and it increases with temperature. At constant temperature, the thickness of the transition zone under the vacuum is greater than that under atmospheric pressure. Based on the trend between concentration and temperature under various pressures, it is concluded that when the temperature increases from 1773K to 1873K, the z-axis direction dimension difference becomes particularly large under vacuum (10Pa) and at 100KPa; therefore, the system undergoes phase. In other words, boiling point of copper under vacuum (10Pa) is between 1773-1873K, which is about 1000K lower than that at atmospheric pressure 2833K.The Forcite module in Material Studio package was used to simulate the changes of the copper nano droplet evaporation-condensation process in the range of 1473-2073K under vacuum (10Pa). It is shown in the simulation that with temperature increase, more molecules enter the gas phase and the droplets become nearly spherical due to the fluid isotropic. When the heating temperature increases from 1773K to 1873K, the diameter of the droplet occurs reduces significantly, which is attributed to the reduced boiling point of copper under vacuum. During the temperature increase, the system has transformed from a liquid phase to a vapor phase and a large number of molecules evaporate from liquid to gas phase, which results in smaller droplet.Based on the analysis of the experimental results, it is found that the produced copper has reached 1# copper standard In addition, the copper powder possess a face-centered cubic structure (fcc), The contains of CuO, Cu2O is small in copper powder. |
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