Study On Phase Diagiam Of Ca?NO₃?₂-LiNO₃-H₂O System And Design Of Phase Change Materials ?PCM?

With the continuous development of society, the demand of people about the energy sources is increasing, but energy supply and demand have strong time dependence, in many cases, the energy sources can't be rational used, which leads to a large amount of waste. A new energy, which can take full advantage of the heat, is an important way to solve the energy crisis. Phase change material?PCM? has become a hot topic in the research at home and abroad, which can release and store latent heat during phase change process, maintain at a constant temperature and solve the unbalance between supply and demand of energy. An efficient way to search PCM is to chose a appropriate thermodynamic model for calculating the phase diagrams of multi-component salt hydrate system and to text heat storage performance for the a eutectic point. In this paper, thermodynamic model has been chosen for calculating the phase diagram of Ca?NO₃?₂-LiNO₃-H₂O system and a eutectic point used as a potential phase change material was obtained. This topic of work work is as follows:?1? A method for precise determination of calcium and lithium in the ternary system of Ca?NO₃?₂-LiNO₃-H₂O was discussed. The total content of calcium and lithium was elaborately determined by drying water for mixture solution containing calcium nitrate and lithium nitrate. The content of calcium was quantitatively separated from lithium using calcium oxalate gravimetric method. Then the content of lithium was calculated by difference subtraction. The results show that water in the ternary system of Ca?NO₃?₂-LiNO₃-H₂O can be thoroughly removed, when the temperature is controlled in 225²50 °C and time is controlled in 10¹2 h during drying water. Also, when the mass ratio of Ca?NO3?2 and Li NO3 is less than 10:1, the relative errors of analysis results can be controlled within ±0.9%. Compared with the classic method, this method meets requirement of solubility data with high accuracy.?2? Because of the lack of literature data, the solubility isotherms of the Ca?NO₃?₂-LiNO₃-H₂O ternary system at 273.15 K?298.15 K and 323.15 K were accurately determined by isothermal equilibrium method, and the phase diagrams were obtained. The results show that there are two solubility branches for Ca?NO3?2·4H2O and Li NO3·3H2O at 273.15 K. There are five solid phase, Ca?NO3?2·4H2O?Ca?NO3?2·3H2O? double salt? Li NO3 and Li NO3·3H2O, which are consistent with the literature data at 298.15 K. Because of the high viscosity of Ca?NO3?2, only the solubility branch corresponding to solid phase Li NO3 and solubility of Ca?NO3?2-H2 O system were determined at 323.15 K.?3? The modified BET modle was used to predict the solubility of the multi-temperature phase diagram of Ca?NO₃?₂-LiNO₃-H₂O.only with binary parameters. The results show that there is a certain deviation between calculated value and experimental data in this work. In order to obtain accurate information of the phase diagram of the Ca?NO₃?₂-LiNO₃-H₂O system, the empirical interaction parameter of salt-salt was fitted to the experimental data in this work. The isotherms of Ca?NO₃?₂-LiNO₃-H₂O system were recalculated by combining with binary parameters and empirical interaction parameter and were compared with literature data. The complete phase diagram of the system at 273.15³73.15 K was calculated, and then two eutectic points with appropriate temperature were found. The properties of the predicted eutectic material were measured, including the melting and crystallization behavior and DSC. The results show that the binary eutectic point Li NO3·3H2O?41.565%?-Ca?NO3?2·4H2O?58.435%? which maintains at 18 °C for more than 300 minutes in the environment of 11²1 °C, and the heat enthalpy is 106.5 J/g, could be taken as potential room temperature PCM with excellent heat storage property. The ternary eutectic point Li NO3·3H2O?11.74%?-Li NO3?25.8%?-Ca?NO3?2·4H2O?62.46%? which maintains at 14.5 °C for more than 250 minutes in the environment of 18 °C, and the eutectic material maintains at 11 °C for only 60 minutes in the environment of 11 °C, could also be taken as potential room temperature PCM. Meanwhile, the results also show that BET model can play an important role in the design of PCM.