Effect Of In-situ Synthesized Nanoparticles On Thermal Properties Of NaNO₃-KNO₃

Nitrate molten salt has been proved promising heat storage materials with good thermophysical properties in concentrating solar power station.Increasing specific heat capacity of molten salt can help enhance its thermal storage capacity.In this paper,the effect of nanoparticles on specific heat capacity of NaNO₃-KNO₃ is investigated.Nanoparticles were synthesized in molten salt by three different synthetic methods.They are the molten salt method,the sol-gel method,and the solution combustion method,respectively.MgO nanoparticles are synthesized by using in-situ method based on the high temperature decomposition reaction of Mg?OH?₂ in solar salt.We adopted the Sol-gel combustion method and solution combustion,Mg?NO₃?₂ as raw material,citric acid and urea as fuel,and finally we synthesized MgO nanoparticles into the low melting point binary nitrate mixture of NaNO₃-KNO₃,and ignite at different temperatures.The specific heat capacity of solar salt was significantly enhanced while the latent heat was slightly decreased with addition of precursor.The specific heat capacity,phase transition temperature and latent heat of modified solar salt are discussed.we tested the materials'thermal physical properties in order to explored the effects of adding nanoparticles on nitrate specific heat and the best preparation process parameters optimization.The specific heat capacity,latent heat and phase transition temperature of composites with different contents and different processes were tested by differential scanning calorimetry?DSC?.The results show that the specific heat capacity of composites prepared by molten salt method and sol-gel method are all improved.In the molten salt method,addition of Mg?OH?₂ from 0%to 14.8 wt.%treating at400?,500?and 600?.After the samples calcined at 600?for 1h,the specific heat capacity is increased by 118.6%in solid state when adding 14.8wt.%Mg?OH?₂,and increased by 168.1%in liquid state when adding 10.4wt.%Mg?OH?₂,respectively.The mechanism of specific heat capacity of nanoparticles is discussed with the application of thermodynamics theory and Fourier transform infrared spectroscopy?FT-IR?.In the sol-gel combustion synthesis method,heat capacity of the composite materials,which were synthesized under different ratio of Mg?NO₃?₂ to fuel,has varying degrees of increase.MgO nanoparticles have the best dispersion and the smallest particle size when Mg?NO₃?₂ and fuel reacted completely.Correspondingly,the increase of heat capacity of composite materials is the largest.The nanoparticles prepared under different conditions have almost no effect on the latent heat value of the material.The specific heat capacity of composites prepared by solution combustion synthesis was tested by microcalorimeter?C80?.The results showed that the specific heat capacity of citric acid system did not increase much compared with that of the original molten salt.In the urea system,the specific heat capacity increased.The phase changes before and after the reaction were investigated by X-ray diffraction?XRD?,energy dispersive spectrometer?EDS?and X-ray photoelectron spectroscopy?XPS?.In the molten salt method,the synthesis of nano-MgO particles was proved;in the sol-gel combustion synthesis,the formation of nano-MgO and MgCO₃ was proved.The molten salt did not change significantly.The microstructure of the composites was observed by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?,and the changes of the structure before and after the reaction were analyzed.In the molten salt method,Mg?OH?₂ pyrolyzes into spherical and dendritic nanoparticles.In the sol-gel combustion synthesis,the dendritic structure is formed by the reaction of citric acid and the spherical particles are formed by the reaction in urea system.In the solution combustion synthesis method,the presence of nanoparticles was observed.