The Modification Research Of Ternary Nitrates

The heat storage technology is an effective method to solve the energy problem,the research of heat storage material is the key of heat storage technology. Molten saltis widely used as thermal transfer and storage medium because of its stable chemicalproperties, large heat density and low cost. In practical application, it’s important tobroaden molten salt’s range of application and increase its latent heat of phase change.In this article HITEC（53％KNO3-7％NaNO3-40％NaNO2）was taken as base salt,carbonate and nitrate were taken as additive. Through testing methods like differentialthermal analysis, TGA and so on, the influences of carbonate and nitrate on thermalphysical properties of HITEC were researched, including melting point, latent heat ofphase change, decomposition temperature, cycling stability and corrosivity.Through analyzing samples’ DSC curves, it was found that the species ofadditive had influence on melting point, but the amount of additive had less influence.Aimed at this experimental phenomenon, the model of molten salt’s melting processwas put forward on the basis of existing research in the article, it was concluded thatthis phenomenon was related to crystal lattice’s energy change caused by flaw. Inaddition, through analyzing samples’ DSC curves, from the point of the structuraldifference of carbonate and nitrate, and the interaction of anion and cation, the majorfactor affecting material’s latent heat of phase change was explained.To avoid NaNO2being oxidized by oxygen in the air, before TGA experiment,experimental apparatus had been improved. Through analyzing thermogravimetriccurves, sample’s decomposition temperature’s range had been put forward. Afteranalyzing the decomposition process of molten salt and calculating gibbs free energyof possible chemical reaction of carbonate additive at high temperature, combinedwith existing research result, the main influence of molten salt’s instability at hightemperature was analyzed.Add two types and three types of carbonate to HITEC as additive respectivelyand compare the influence of unary additive and multiple additive on material.Salt-mixture with maximal latent heat value was selected as sample of cycle test, theDSC curves after20,50,100,200cycles were tested respectively, change of melting point and latent heat was compared, and the stability of this sample in cyclic processwas analyzed. Then the compatibility of molten salt with these two stainless steel wasresearched through analyzing dynamiting curve and kinetic curve of304and310Sstainless steel after dipped in molten salt for24h,72h,168h,264h and360h.In the article, it was found that the influence of nitrate on HITEC mainlyreflected in reducing the melting point,80%HITEC-20%LiNO3had the lowestmelting point(70.173℃). The influence of unary salt and multiple salt as additivemainly reflected in raising latent heat and decomposition temperature. Molten salt’sdecomposition temperature was above600℃, the major factor of decomposition wasthe transformation of NaNO2.95%HITEC-5%Na2CO3had highest latent heat ofphase change(110.542J/g), better thermal cycling stability, and better compatibilitywith310S stainless steel.