Thermal and mechanical properties and micro-mechanism of SiO_(2)/epoxy nanodielectrics

In addition to electrical insulation properties,the thermal properties of nanodielectrics,such as glass transition temperature,thermal expansion coefficients,thermal conductivity,and mechanical properties,including Young's modulus,bulk modulus,and shear modulus,are also very important.This paper describes the molecular dynamics simulations of epoxy resin doped with SiO_(2)nanoparticles and with SiO_(2)nanoparticles that have been surface grafted with hexamethyldisilazane(HMDS)at 10%and 20%grafting rates.The results show that surface grafting can improve certain thermal and mechanical properties of the system.Our analysis indicates that the improved thermal performance occurs because the formation of thermal chains becomes easier after the surface grafting treatment.The improved mechanical properties originate from two causes.First,doping with SiO_(2)nanoparticles inhibits the degree of movement of molecular chains in the system.Second,the surface grafting treatment weakens the molecular repulsion between SiO_(2)and epoxy resin,and the van der Waals excluded region becomes thinner.Thus,the compatibility between SiO_(2)nanoparticles and polymers is improved by the grafting treatment.The analysis method and conclusions in this paper provide guidance and reference for the future studies of the thermal and mechanical properties of nanodielectrics.

A continuum theory of surface piezoelectricity for nanodielectrics

In this paper,a phenomenological continuum theory of surface piezoelectricity accounting for the linear superficial interplay between electricity and elasticity is formulated primarily for elastic dielectric materials.This theory is inspired by the physical idea that once completely relaxed,an insulating free dielectric surface will sustain a nontrivial spontaneous surface polarization in the normal direction together with a tangential self-equilibrated residual surface stress field.Under external loadings,the surface Helmholtz free energy density is identified as the characteristic function of such surfaces,with the in-plane strain tensor of surface and the surface free charge density as the independent state variables.New boundary conditions governing the surface piezoelectricity are derived through the variational method.The resulting concepts of charge-dependent surface stress and deformationdependent surface electric field reflect the linear electromechanical coupling behavior of nanodielectric surfaces.As an illustrative example,an infinite radially polarizable piezoelectric nanotube with both inner and outer surfaces grounded is investigated.The novel phenomenon of possible surface-induced polarity inversion is predicted for thin enough nanotubes.