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A novel core-shell structure Ag@Al2O3 nanoparticles were synthesized and doped into polyimide as conductive fillers to prepare the composite films with high dielectric properties and low dielectric loss.The morphology and structure of the Ag@Al2O3 nanoparticles were characterized by transmission electron microscopy (TEM),X-ray diffraction (XRD) and UV-visible spectroscopy.All the results proved that the Ag@Al2O3 nano-particles had a typical core-shell structure,for Al2O3 coated Ag particles and the average sizes of Ag@Al2O3 particles were between 30nm to 150nm.The obtained Ag@Al2O3 nano-particles were doped into the polyimide with different weight contents to fabricate the Ag@Al2O3/PI composite films via in-situ polymerization process.SEM analysis on composite films showed that the Ag@Al2O3 nano-particles had a homogeneous dispersion in polyimide matrix with nanoscale.As dielectric materials for electronic packaging systems,the Ag@Al2O3/PI composites exhibited appropriate mechanical properties and enhanced dielectric properties,included the greatly enhanced dielectric constant and just slightly increase on dielectric loss.These improvements were attributed to the core-shell structure of fillers and their fine dispersion in the PI matrix.