| Due to the superior properties of metakaolinite,the related in-depth development is of great significance for expanding its application fields and accelerating our country’s economic development.Molecular Dynamics simulation has been recognized as an efficient tool to investigate complex physical and chemical processes under nanoscale.It can not only overcome the limitations of experimental conditions,but also provide directional guidance for macroscopic experiments.This paper aims at revealing the microscopic mechanism of metakaolinite physical and chemical properties by MD simulation,which provides theoretical basis and guidance for promoting the fine modification of metakaolinite and its wide application in various fields.The results of this study are mainly summarized as follows:(1)Rationality and structure of metakaolinite model.According to the metakaolinite configuration described by early scholars,we verify and compare the rationality of the five anhydrous system from the perspective of the minimization strategy by molecular dynamics.The results indicate that Brindley-Nakahira model is the most suitable for subsequent simulation.The particle size,morphology and specific surface area of kaolinite before and after calcination were characterized by SEM and BET.It was found that the dehydroxylation of kaolinite results in the decrease of interlayer hydrogen bond force,causing the subsequent breakup of particle and the increase of specific surface area.When the calcination temperature is too high,the pores of kaolinite are blocked and the specific surface area is reduced.(2)Wetting behavior of metakaolinite on the basal surfaces.A contact angle could quantify wettability in a gas-fluid-solid or a fluid-fluid-solid system.The wetting process was captured and the time-average density profiles of the equilibrated configuration were analyzed to determine the contact angle of different basal surfaces(silicon surface and aluminum surface)of metakaolinite.The results indicate that the structure of silicon surface of kaolinite remains unchanged after calcination,and the contact angle of silicon surface of metakaolinite is about 105°which is consistent with that of kaolinite,and both are identified as hydrophobic.Unlike the superhydrophilic aluminum surface of kaolinite,the contact angle of metakaolinite is 17°.Its hydrophilicity is weakened due to the loss of hydroxyls and protons.The results of experiments and simulation indicate that the calcination of kaolinite results in the structural change on the aluminum sheets and different macroscopic wettability.(3)Adsorption of sodium ions on calcined kaolinite.The trajectories of MD modeling were analyzed to obtain the density distribution,coordination number,mean square displacement and radial distribution function.The results show that(Ⅳ)-Al metakaolinite has the best adsorption effect on sodium ion.Different from the electrostatic bonding of kaolinite,Na~+mainly shows irreversible chemical bonding adsorption on the aluminium-oxygen surface of metakaolinite.A chemical bond length of 2.3(?)is formed between Na~+and bridged oxygen,which indicates that sodium ion reacts with metakaolinite to form alkali aluminosilicate.At the same time,the self-diffusion coefficient of the adsorption zone is smaller than that of the free zone,and both of them are much smaller than the diffusion coefficient in solution.(4)Mechanical behavior of metakaolinite under nanoindentation and nanoscratching.A rigid indenter was pressed on two basal planes to explore the mechanical properties of metakaolinite,so as to obtain the relationship of force,hardness and Young’s modulus with the indentation depth.In the nanoscratching process,the influences of indentation depth on normal force,scratch force and friction coefficient are mainly explored.The results show that the hardness and Young’s modulus of metakaolinite in different basal surfaces are anisotropic,which is determined essentially by the arrangement of atoms.The silica facet possesses a hexagonal-close-packed plane with higher strength than that of gibbsite facet.During the scratching,the normal and scratch forces that the substrate exerted on the indenter both increased as the scratch processed.The depth dependence is more conspicuous on the scratch force than for the normal force and the friction coefficient increases with the increase of depth.As the scratching depth increases,the material appears to soften slightly,which can be proved by measuring the average normal hardness.The pile-up of atoms and formation of chip ahead of the indenter only observed at scratching process in high scratching depth.While the chip volume was found to increase with increasing the scratching depth.(5)Combined with the research content and results of this paper,some deficiencies of related research and the prospect of future research have been put forward from the aspects of of model construction,wettability,adsorption,and mechanical properties. |
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