Structure Study On The Ternary NaNO₃-KNO₃-NaNO₂ Molten Salts

As the main heat transfer and storage medium of solar thermal power generation system,nitrate molten salt has many advantages,such as:low melting point,low vapor pressure,low corrosiveness,low cost,high thermal stability,wide temperature range,etc.However,due to the low freezing point of nitrate molten salt,it is easy to agglomerate when it is transported in the pipeline,resulting in the problem of"frozen pipe",which limits the development of solar thermal power to a certain extent.The micro-structure of substance determines its macro-properties,so the research on the structure of nitrate molten salts may shide some light on sowing the problems encountered in its application,improve the working efficiency of molten nitrate,and improve the theory of molten salt structure.In the present work,we choose mixed molten salt of NaNO₃,NaNO₂ and KNO₃as research system.Firstly,the thermophysical properties of the nitrate molten salt system were studied by XRD and TG-DSC.Then,the structural changes of the nitrate molten salt system in the process of heating were studied by high temperature Raman.Hitec,which is widely used in the field of solar thermal power generation.The density functional theory is used to optimize the micro-structure of Hitec and explore its bonding mode.Finally,the change of micro-structure of Hitec with temperature was determined by X-ray scattering and model fitting.The main research contents and conclusions are summiarized as follows:?1?The thermophysical properties of nitrate molten salt system were studied by XRD and TG-DSC.Results show that:the main body of the nitrate molten salt does not change nor produce new substance after mixing;it is found that there are three peaks of thermal change in the curve,corresponding to the endothermic peak caused by a small amount of water gasification,which lies between 100?¹50?;the peak of sample melting phase change,which lies between 150?³00?;the peak of NO₃?/NO₂?occur reaction,which lies near 500?.It is also found that the content of NaNO₂ has an obvious effect on the melting point and decomposition temperature of the mixed molten salt.When the content of NaNO₂ is relatively high,it is beneficial to reduce the melting point,increase the decomposition temperature.The melting point of binary and ternary mixed molten salt is 81.1?¹32.4?much lower than that of single salt,and the decomposition temperature is 25.34?⁴3.4?higher.?2?The effect of temperature on the structure of nitrate molten salt system were studied by high temperature Raman.The results show that the half peak width of the Raman characteristic peak increases by temperature increase,which is due to the more violent thermal movement of the anion and cation in the anions,resulting in the increase of the half peak width of the characteristic peak.The reason for the red shift of the Raman characteristic peaks is the thermal movement between atoms increases with the increase of temperature,the force between atoms weakens,the distance between atoms increases,the bond angle distribution widens,and the"cation effect"exists for the Raman frequency shift of NO₃?.?3?Density functional theory is employed to explore the bonding mode of Hitec ternary molten salt.By DFT calculation,the possible configurations of NaNO₃,KNO₃,NaNO₂,KNO₂ and Hitec are obtained,and the corresponding bond parameters and energy parameters are obtained.By comparing the interaction of various configurations,it is found that:in the above substances,Na⁺/K⁺and NO₃?/NO₂?form a configuration with monodentate as the main and bidentate as the subordinate,and the structure is relatively stable when the monodentate content is relatively high;the bidentate content is relatively higher,the structure is more unstable.?4?The influence of temperature on the microstructure of Hitec ternary molten salt was studied by combining X-ray diffraction experiment with model fitting.With the increase of temperature,the distance between the atoms and the coordination number of each atom in the molten salt are slightly reduced;the connection mode of Na/K-N changes from monodentate ligand to bidentate ligand,with the temperature increasing,for Na⁺,the proportion of monodentate ligand decreased from 78.4 to73.9%,and the proportion of bidentate ligand increased from 21.6 to 26.1%.For K⁺,the ratio of monodentate ligand decreased from 65.6 to 61.4%,and the ratio of bidentate ligand increased from 34.4 to 38.6%,the structure of Hitec molten salt is more and more unstable.