Effect Of SiO₂ Nanoparticles On The Crystallization Of KNO₃ Aqueous Solution And Its Mechanism

The morphology of micro/nano crystals have important influence on the physicochemical properties such as light,electricity,magnetism,mechanics and catalysis of materials.Thus,the controllable synthesis of micro/nano crystals with special morphology is helpful to explore the new properties of materials.Especially for some inorganic phase-change materials of energy storage,the special size effect caused by the micro/nano crystallites leads to a better thermophysical properties.In this paper,submicron potassium nitrate crystals were prepared successfully by SiO2 nanoparticles assisted crystallization method,and the formation mechanism of submicron potassium nitrate crystals was further explored.The potassium nitrate crystals with uniform fibrous morphology were induced by SiO2 nanoparticles.The effect of SiO2 nanoparticles concentration,dispersion stabilization of SiO2 nanoparticles,temperature and vessel on the crystal morphology of potassium nitrate were also studied.When the mass ratio of SiO2/KNO3 was 5%,the ultrasonic dispersion condition was 30%+ 30 min and the evaporation crystallization temperature was 10℃,it’s proved that the potassium nitrate crystals with a minimum diameter of 0.5 μn could be obtained.Using a DSC analysis,compared to KNO3 crystal,it presented that the specific heat capacity of solid phase and liquid phase of submicron KNO3 crystals increased by 20.9%and 39.8%In order to further understand the formation mechanism of submicron KNO3 crystals,the changes of nucleation induction,solubility,metastable zone and surface tension of aqueous KNO3 solution under SiO2 nanoparticles were studied.The results showed that the nucleation induction period of KNO3 solution was shortened from 109 s to 36 s with the addition of 5 wt%SiO2 nanoparticles in a supersaturated KNO3 solution under the conditions of a stirring rate of 100 r/min and a supersaturation of 0.13 s.The decrease of induction time leads to the increase of KNO3 nucleation rate.At the same time,the addition of SiO2 nanoparticles to KNO3 solution can reduce its solubility,and with the increase of the concentration of SiO2 nanoparticles,the solubility decreases more significantly.In addition,adding SiO2 nanoparticles narrowed the metastable zone width,decreased the stability of the metastable solution and promoted the nucleation of KNO3 solution.The effect of SiO2 nanoparticles on the surface tension of KNO3 solution was discussed.The results showed that the surface tension decreased 0.73%when the concentration of KNO3 solution was 0.9 mol/L and the concentration of SiO2 nanoparticles was 0.15 wt%.The addition of SiO2 nanoparticles into the KNO3 solution can reduce the solution surface tension and the surface energy,resulting in a low energy barrier for nucleation and heterogeneous nucleation of KNO3 crystals at the gas-liquid interface.The formation of fibrous submicron KNO3 crystals is the result of the interaction of SiO2 nanoparticles,the glass wall and the anisotropic growth of KNO3.On the one hand,the addition of SiO2 nanoparticles changed the surface tension and metastable zone to promote the formation of gas-liquid interfacial nuclei.On the other hand,the interaction of SiO2 nanoparticles on the glass wall and the wall made it possible to KNO3 crystal nuclei,and the presence of SiO2 nanoparticles strengthens the anisotropic growth of KNO3 crystals,resulting in the formation of fibrous nitrite potassium crystals.