Optimization Design And Controllable Preparation Of Glass Microspheres Coated By Thin Compact Silver Shell And Their Based Conductive Composite Materials

The conductivity of polymer-based conductive composite materials filled withparticles depends on not only the material of conductive fillers, but also themorphology, particle size, size distribution and filling modes. Therefore, forcore-shell composite particles, optimizing the morphology，regulating the thickness ofshells and creating reasonable design for particle size and size distributions areparticularly important. This thesis aiming at the problems of existing glassmicrospheres coated by silver shell (GM@Ag), such as incompact shell and highshell thickness, proposed and developed binary reductants method to synthesizeGM@Ag composite particles with compact shell and controllable shell thicknessbased on heterogeneous nucleation and growth theory, and discussed the mechanismof controlling the compactness and thickness of silver shell. Due to the uncertainparticle grading principle of filler particles in conductive composite materials, thisthesis also introduced fractal theory to study the influence of fractal dimension ofGM@Ag CPs particle-size distributions on their conductivity, and illuminated therules of conductive composite materials depending on. The major results are asfollows:Firstly, GM@Ag CPs with thin and compact silver shell were synthesized bybinary reductants method. The influence of nucleation and growth on the thicknessand morphology of silver shell and the conductivity of GM@Ag CPs were studiedsystematically. It is found that the nucleation and growth process of the silver grainsincludes three parts: a large number of nuclei were formed; the nuclei isotropicallygrew into grains; grains grew to contact each other and to higher grains to fill the gapsand formed a compact shell. The strong reductant, NaBH4, could accelerate thenucleation rate and led to a high number density of nuclei to get thin compact silvershell. The weak reductant, glucose, made grains slowly grow to uniform size andobtained compact shell. The silver content of the GM@Ag CPs prepared by binaryreductants could be as low as5.1%. The percolation threshold of ECAs with theseparticles was about37%, and the volume resistivity of the ECAs with GM@Ag CPsof45%volume fraction was7.02×10-3·cm. These GM@Ag CPs could be widely used as conductive composite materials due to their compact thin shell, low silvercontent and high conductivity.Secondly, the fractal dimension of GM@Ag CPs particle-size distributions wasintroduced based on fractal theory to guide the particle grading of GM@Ag CPs withdifferent diameter. The influence of particles size and fractal dimension of GM@AgCPs particle-size distributions on their conductivity were investigated. It is found that,according with the increasing of particle diameter, the percolation threshold ofGM@Ag/epoxy ECAs increased; the conductivity of ECAs decreased; and the silvercontent decreased. According with the increasing of the fractal dimension, theconductivity of ECAs increased firstly and then decreased a little. The volumeresistivity of the ECAs got its minimum at D=2.8. The percolation of the ECAs withGM@Ag CPs of D=2.8was1.2%less than the particles of18-25μm diameter, andthe total surface area of the particles in a unit volume ECAs at percolation thresholddecreased20%. The GM@Ag/epoxy ECAs with particles of proper fractal dimensionhad good conductivity and low percolation threshold, and solved the problem ofparticle sedimentation to some extent.