Role And Mechanism Of Diffusion Field And Surface Energy In The Formation Of Dendritic Structures

The dendritic structure is common in nature,from snowflakes to branches,from river beds to lightning,from solidification process to material preparation,from cell proliferation to crack development.The similarity of morphology means the internal connection of its formation mechanism.However,the research on the mechanism of dendrite growth is decentralized and specific in different disciplines,and the lack of related common theoretical research has led to the formation of isolated theories in a single discipline,which cannot be expanded and promoted.Therefore,the generation mechanism of dendrite structure is one of the common problems to be solved in scientific community.Controversy of the theory of dendrite growth has also caused the problem of dendrites in engineering applications.The growth process of the material structure includes the diffusion transfer and reaction combination process of the material constituent units.By adjusting the reaction rate and the diffusion rate,our group can control the preparation of nanomaterials with different morphologies,including dendritic structures,and we found the interface concentration gradient is the root reason for dendrite formation.In order to obtain a more universal model of dendrite growth,in this thesis,we expand the interface concentration field to the interface temperature field,and introduce the crystallographic characteristics of the material itself,and propose a dendrite growth model based on the joint control of the interface field and surface energy.The interface field is the driving force for dendrite growth,the surface energy is the resistance for dendrite growth,and they are constantly changing during the growth of the crystal structure.Based on the scientific thinking,the coordination between the two factor is the stability condition of dendrite growth.Therefore,in this thesis,from the perspective of material physics and chemistry,the coordination laws of thermodynamics(surface energy)and kinetics(diffusion field)in the evolution of crystal morphology are explored in order to establish a universal mesoscale mechanism of dendrite growth.First,the effect of the interface concentration field and surface energy on the structure of the material was verified.Taking sodium chloride particles prepared by the solution evaporation method as an example,the crystallization process of sodium chloride was observed and recorded in situ by a laser confocal microscope.By adding gelatin to the sodium chloride solution,the viscosity of the solution is changed,and the concentration of sodium chloride is kept constant(the reaction rate is constant).Sodium chloride particles are obtained by solvent evaporation,and the product is characterized.The results show that when the viscosity of the solution is low,that is,when the diffusion coefficient of the particles in the solution is large,the formation of sodium chloride particles is controlled by the reaction rate,and the slow nucleation and growth rate result in the formation of large polyhedral particles,namely the most common cubic structure of sodium chloride.When the viscosity of the solution gradually increases,the diffusion transmission rate of ions in the solution decreases,a diffusion-limited condition with a certain concentration gradient is formed,and sodium chloride dendrites are formed.When the viscosity of the solution continues to increase,the diffusion coefficient of ions in the solution decreases exponentially,resulting in extreme diffusion-limited conditions.A large number of crystal nuclei are formed in the solution,and the supply of monomer ions around the crystal nuclei is insufficient,and finally sodium chloride nanocrystals are formed.By adding a surfactant to the sodium chloride solution,the surface energy of the sodium chloride crystals is changed,and the regulation effect of the surface energy on the material structure is revealed.Gelatin,agar,and polyvinyl alcohol(PVA)were selected as surfactants,and a certain amount of the above-mentioned active agents were added under the condition of ensuring viscosity one.The gas-liquid interfacial tension of the saturated solution of sodium chloride and its relationship with chlorination,and contact angle of sodium single crystal were experimentally measured.The liquid-solid surface energies and anisotropic coefficients of(100)and(110)crystal planes of different sodium chloride saturated solutions and sodium chloride single crystals were calculated.The conclusion is that under the same diffusion field conditions,the difference in surface energy of different systems such as gelatin,agar,and PVA has a decisive influence on the final crystalline product of sodium chloride.The phase field simulation calculation was used to explore the change of the crystal morphology of sodium chloride with the diffusion coefficient and surface energy anisotropy.The near-perfect fusion of phase field simulation is consistent with experimental results,which further proves that the diffusion field and surface energy can regulate the crystal morphology of sodium chloride.The diffusion field and surface energy don't affect the morphology of the crystal.The competition between the two factor determines the morphology of the final product of sodium chloride.In order to verify the universality of the regulation effect of diffusion field and surface energy on particle morphology,the ice crystal process is used as an example to explore the regulation effect of supercooling and surface energy on material structure.Inspired by the concept of mesoscience,this paper raises the scientific hypothesis that surface energy can regulate the growth morphology of ice crystals.Different additives(sucrose,sodium chloride,and surfactant SDS)were added to the aqueous solution to change the surface tension of the aqueous solution,and the icing process with different surface tensions was observed by a laser confocal microscope with in-situ device.It was found that the morphologies of ice crystals transfer from dendritic with good symmetry to disordered seaweed crystals as the surface tension of the solution decreased at the same undercooling conditions,and this change was verified in different kinds of solutions.Further investigations show that the lower surface tension of the solution resulted in an increased resistance to ice growth in the solution and an reduced ice growth rate.At higher growth rate,the growth surface of ice crystals is unstable,and the crystal plane grows anisotropically,resulting in the formation of dendrite structures.However,at a lower growth rate,the growth surface of ice crystals is covered by the additive molecules.The anisotropy of the crystal plane disappears,and hence a disordered dendrite structure is formed.The above findings verify the dominant role of the compromise of the competition between supercooling and surface energy in the growth of the material structure.This provides an experimental basis for the development of materials mesoscience.Based on the study of the crystallization process of sodium chloride and ice crystals,the regulation of the material structure by the diffusion field and surface energy was verified.A dendrite structure was generated when the diffusion field and surface energy controlled the growth of the crystal plane.The generation of dendrite structure is a typical mesoscale problem,which is the result of coordination between diffusion field and surface energy competition.However,in-depth mesoscale research on this issue requires the use of hightemporal resolution instruments and simulation methods,in situ Record the diffusion field and crystal plane growth process,reveal the stability conditions of material structure evolution,and then guide the design and synthesis of materials.