The Effect Of Microstructure Evolution For SiO₂/Polymer On Dielectric And Thermal Propertiesunder Electric And Temperature Field

Nano-dielectric materials of polymer matrix have been widely used in electrical insulation,frequency conversion motor,dc cable and other fields due to their excellent electrical properties and thermal properties.The interface layer is the key factor of the thermal and electronic performance fornano-dielectric materials,which has already become a focus in science research.In this thesis,PI,LDPE and XLPE are adopted as matrix,SiO₂,gas phase SiO₂（V-SiO₂）and coated gas phase Si O₂ with coupling reagent（K-SiO₂）are adopted as nano-fillers,prepared polymer nano-dielectric materials.The effects of particle size,component,surface state and substrate properties on microstructure of composite material were discussed.Microstructure evolution law in composite materials and the effect of the dielectric properties on composite film were studied under different applied electric field and temperature field.The mainly work in thesis are as follow.SiO₂/PI、V-SiO₂/PI and K-SiO₂/PI were prepared though in-situ polymerization.SAXS and TEM were adoped to characterize the microstructure of composite films.The dispersity of SiO₂ particles with different size,component and surface state in PI matrix were studied,as the relationship between SiO₂ particles and interface layer.The results show that there is a threshold relationship between size,component of SiO₂ particles and interface layer.Interface layer appears in SiO₂/PI doped with subsize SiO₂（5nm and 10nm）,and the interface layer thickness increases with the size of particle decreases.When the particle radius is greater than 25 nm,no interface layer exits in composite films.The mass fractal exits in composite films with low component（lower than 15wt%）SiO₂,there is no interface layerin it.When SiO₂ component is higher than 15wt%,the composite films show both mass fractal and surface fractal characteristics,and an interface layer appear.The experiment results show that mass and surface fractal characteristics,interface layer exist in 1-20wt%K-SiO₂/PI composite films.V-SiO₂ particles disperse in PI matrix in the form of cluster and the diameter range from 100 to 500 nm.It is well distribution of K-SiO₂ in PI,scince the chemical bonds is strong between coupling agent and PI matrix.Focusing on practical application of PI material,short-term polarization and long-term corona were combined with dielectric performance test of composite films to research V-SiO₂/PI microstructure evolution under applied electric field.The results showed that,interface layer thickness is constant after short-term polarization,the electron density of interface layer increased.When temperature rise,the electron density of matrix and interface layer thickness are constant,electron density of matrix interface layer decrease.After long term electric field treated,the microstructure of composite films obviously changed.At the beginning of the corona,the thickness of interface layer increased rapidly.In the mid to late of corona,the thickness stay stable.The electron density of PI matrix decreases during corona process,interface layer thickness and its electron density increases.Corona resistant performance of composite films increases due to the formation of inorganic particles blocking layer on the film surface.V-SiO₂/LDPE、K-SiO₂/LDPE、V-SiO₂/XLPE、K-SiO₂/XLPE were prepared through melt-blending process.The microstructure and property of composite films were measured by SAXS、SEM、DSC、DMA.The relationship between V-SiO₂ component and interface layer in LDPE matrix were discussed,as the relationship between V-SiO₂/LDPE composite film microstructure evolution and conductivity.The theory of interface layer limited conductivity increasing in composite film was put forward and confirmed.The results show that,“Interface layer increase phenomenon” presented in V-SiO₂/LDPE composite films.When temperature rise,the interface layer thickness increases,lead to the scattering effect and inhibiting ability to carrier of composite film enhanced accordingly.The conductivity of composite film is less than pure LDPE two orders of magnitude.Space charge accumulation was restrained,and internal electric field of composite film was smooth.The sensitivity of the conductivity on temperature was reduced.The structure evolution and the mechanism of V-SiO₂ and K-SiO₂ in PI,LDPE,XLPE matrix were discussed under temperature field."The interface layer memory effect" model was put forward and confirmed by experiments.The study shows that,“positive interface layer memory effect” appears in V-SiO₂/LDPE,V-SiO₂/ XLPE,K-SiO₂/LDPE,K-SiO₂/XLPE films.With temperature rising,the interface layer thickness increases.When temperature drop,the interface layer thickness increases.When temperature drop,the interface layer thickness decreases and restored.“Negative interface layer memory effect” appears in K-SiO₂/PI.When temperature rises,interface layer thickness decreases.When temperature drops,the interface layer thickness increases and restored.The coated layer and matrix density decide the “interface layer memory effect” being positive or negative.The morphology of spherocrystaland lamellar crystal were changed in LDPE and XLPE by nano-particle doping,the influenceof the microstructure composite film evolution on thermal performance were studied.The “nano-particles induced lamellar deformation effect” model was put forward.The results show that,the introduction of SiO₂ lead to LDPE lamellar thickness increasing and length decreasing,which affect the growth orientation of lamellar.The thickness and length of XLPE lamellar have no obvious changed.The diameter of spherocrystal decreases and the number of spherocrystal increase in composite films by SiO₂ doped.Diameter and number of lamellar affect melting and crystallization temperature of composite films directly.According to the experimental results and discussion in this thesis,four kinds of microstructure evolution model（V-SiO₂/LDPE 、 K-SiO₂/LDPE 、 V-SiO₂/XLPE 、K-SiO₂/XLPE）were put forward.The results show that the collaborative relationship between particles,matrix and the interface layer are the main factors of microstructure evolution in composite films,as the change of lamellar size and shape in semicrystalline matrix film.