Study On Crystal Evolution Mechanism Of Cordierite Glass-ceramics

Glass ceramic refers to a basic glass composed of a certain proportion.Under the action of appropriate heat,light and electricity,nano-crystallizing treatment forms a nano-material in which the glass phase and the nano-sized crystal are tightly combined.The morphology and size of the nano-sized crystals in the matrix are closely related to the properties of the glass-ceramics.In this paper,MgO-Al2O3-SiO2 cordierite(Mg2Al4Si5O18)glass ceramics is used as the research object,using Differential Scanning Calorimeter(DSC),Scanning Electron Microscopy(SEM),X-ray Diffraction(XRD),Transmission Electron Microscopy.(TEM)and other testing methods to carry out related research.The heat treatment system with appropriate shape and crystal size change rate was determined,and the change of crystal morphology and evolution mechanism were analyzed.According to the observed phenomenon and Molecular Dynamics simulation,the major factors affecting crystal size were determined.Finally,under the thermal-electric field coupling,the size of the crystals was changed,and the density and Vickers hardness of the sample were analyzed.The results show that the traditional "two-step" heat treatment has a complete spherical shape and a moderate dimensional change rate.The cordierite phase precipitates as a main crystal phase,and spinel phase containing low iron is appeared with the introduction of TiO2.The nucleating agent will play a role in inhibiting precipitation and will not affect the experiment.In the crystallization process,the formation of "spherulites" is caused by the aggregation of "dendritic" crystals.The migration and diffusion of Mg2+ is also a major factor in crystal spheroidization.As the heat treatment time changes,Mg2+aggregates around the crystal to form a diffusion barrier layer,which prevents the crystal from growing further.After Molecular Diffusion simulation,it is found that after the diffusion barrier layer is formed,Aggregation of aluminum occurs around the crystal,causing the crystals cannot grow further.The applied electric field at different stages will affect the size or distribution of the crystal.When the electric field is added in the nucleation stage of the heat treatment,the distribution of crystals in the glass-ceramics will be changed,so that most of the crystals are distributed close to the sides.The position of the crystal is changed when the electric field is applied in the crystallization stage of the heat treatment because the crystal nucleus has completely precipitated at this time,so that the crystal distribution cannot be changed.Pressurization at different times during the crystallization stage can change the size of the crystal,where the largest size has reached 40 ?m,much larger than the sample without the added electric field.Therefore,it has been judged that the addition of the electric field has an effect on the size of the crystal.At the same time,it is found that under the action of the electric field,the growth orientation of the crystal changes regularly,and the guess may affect the morphology.For the physical properties of the samples at different stages of pressurization and at different times,it was found that the crystallinity of the latter changed,and the coupling between the heat and the electric field in the crystallization process increased the crystallinity,so the density and Vickers hardness changed.Where the density is increased and the Vickers hardness is relatively reduced.As the pressurization time increases,the density of the sample gradually increases,and the Vickers hardness gradually decreases.