Structure And Thermal Property Of Nitrate-Nitrite-Carbonate Molten Salts System

60%NaNO3-40%KNO3 and 7%NaNO3-53%KNO3-40%NaNO2 molten salts are two of the most important heat medium for the solar thermal power generation.These salts are very important in the field of solar thermal power,and they are also used in the design for the new kinds of heat-transfer aluminum reduction cell as the heat medium.For this special use,the thermal stability of the salt at high temperature needs to be improved.In this thesis,a method was developed by adding certain proportions of Na2CO3 and K2CO3 to the system,and then the ion structures as well as the relative thermal properties of the molten salt were studied to investigate the influences of carbonate addition.A large amount of work was carried out to optimize and find an applicable composition of the heat transfer medium for the new designed heat-transfer aluminum cell which sets higher demands to the operating temperature of the medium.Firstly,the Raman spectrums of molten NaNO3,KNO3,NaNO2,Na2CO3 and K2CO3 were measured respectively,and the Raman spectrum was then analyzed with corresponding simulation and calculation to obtain the bond-length of N-O/C-O in the molten salt at various temperatures.Raman analysis shows that the carbonate could inhibit the decomposition of NO3-ion in molten NaNO3-KNO3 system.The addition of 5%of Na2CO3?K2CO3 or equivalent Na2CO3-K2CO3 would significantly improve the thermal stability of the original NaNO3-KNO3-NaNO2 system.Secondly,the density of the molten salt was measured using Archimedes method.An empirical formula between the density of the molten salt and the temperature was obtained.After introducing carbonate into the nitrate system,the density of the molten salt showed an increasing trend which suggests more heat could be stored for every certain volume unit.Under the same temperature and additive concentration,the Na2CO3 content showed remarkable contribution to the density.Thirdly,the thermal stability of the molten salt under two compositions was tested by thermogravimetric analysis.The results show that with the increasing concentration of the carbonates,the thermal stability of the salt gets better.Contrastively,Na2CO3 additive have better effect on the thermal stability of the molten salt than K2CO3 additive.After that,the differential scanning calorimeter(DSC)method was used to investigate the melting point and the phase enthalpy of the melts.The results shows that for the binary NaNO3-KNO3 system,the extra addition of carbonate does not have much influence on the melting point of the molten salt.While the carbonate addition have significant effect on the phase enthalpy,the phase enthalpy decrease with the increasing concentration of the carbonate.The maximum enthalpy occurred at 5%Na2CO3 addition to reach 117.68J/g.However,for the ternary system,the melting point did not change a lot with carbonate addition and the phase enthalpy decreased significantly with the increase of carbonate concentration.Melts with 5%of Na2CO3-K2CO3 addition have the maximum enthalpy of 49.13J/g.