Study On Silver-coated (Hollow) Core-shell Composite Particles And Their Electromagneitc Shielding Materials

Electromagnetic shielding composite materials have been widely used in civil project and military engineering. It has become a research hotspot in civil control of electromagnetic radiation technology and military equipment shielding technology. According to Schelkunoff electromagnetic shielding theory, conventional electromagnetic shielding fillers had disadvantages of large specific density or weak shielding effectiveness, we have designed and prepared silver-coated glass microspheres core-shell composite particles, silver hollow microspheres and Fe@Ag core-shell composite particles several shielding fillers in this thesis. The influence of chemical preparation process parameters on micro-structure of silver-coated glass microspheres core-shell particles and silver hollow microspheres were investigated. Especially, the relationship between micro-structure, electromagnetic properties of Fe@Ag core-shell composite particles and shielding effectiveness of composite shielding material were systematically investigated, chemical preparation technology of conductive magnetic electromagnetic shielding fillers were established. In order to improve the compatibility on the interface of core and shell shielding fillers, further enhance electrical property and electromagnetic shielding effectiveness of Fe@Ag core-shell composite particles, we proposed firstly a low-temperature heat treatment on the composite particles. We have achieved the desired research goals, and obtained the following conclusions:Silver-coated glass microsphere core-shell particles were prepared by silver (Ag) electroless plating on the self-assembly monolayer technique. And chemical composite mechanisms of particles were analyzed in detail. Chemical preparation technologies of Silver-coated glass microsphere core-shell particles were established. The related properties of the electromagnetic shielding coatings based on the core-shell particles were studied. In the presence the glass microspheres modified with a self-assembled MPTMS monolayer, metallic Ag nuclei firstly forms on the surface of the glass microspheres via a heterogeneous nucleation process due to the low energy and the strong chemical interaction between the-SH groups anchored on the surface and Ag atoms. Then with prolonging t, the metallic Ag nuclei gradually grow into silver nanoparticles deposited on the surface of the glass microspheres, finally forming the silver-coated glass microspheres core-shell particles with even and compact silver shells. The silver shell of composite particles was even, compact and intact when the reaction temperature was 25℃, the pH value was 13.0, the concentrations of ammonia and PVP were 100mL/L,10 g/L, respectively, the loading capacity was 10 g/L. The dielectric constants of the core-shell particles are much bigger than before. The electric properties of the electromagnetic shielding coating increase with the increasing of the volume fraction of the shielding filler. When the volume fraction of the filler reached 25%, the resistivity of the shielding coating was 4.0×10-3Ω·cm. At the same time, shielding effectiveness of the coating varied from 40 dB to 65 dB when the frequency of electromagnetic wave ranged between 30 MHz and 1500 MHz. The coating material had good mechanical properties and durability. From the results, it could be concluded that the shielding materials based on Ag-coated glass microsphere core-shell particles described in this paper have good application prospects on the field of electromagnetic compatibility.A facile and large-scale synthesis method to fabricate silver hollow microspheres with controllable morphologies and shell thickness is described using silver-coated glass microspheres core-shell particles as material through wet chemical etching technology. The related properties of the electromagnetic shielding coatings based on hollow particles were studied in detail. The method mainly involves two steps of the preparation of silver-coated glass microsphere core-shell particles by a controllable liquid reduced reaction of Ag[(NH3)2]+ solution, which only produces silver nanoparticles anchored on the surface of the thiolated glass microsphere templates, and the removal of glass microspheres by wet chemical etching with HF solution. The as-prepared core-shell particles and hollow particles have even and compact silver shells. The electromagnetic shielding coatings based on the silver hollow microspheres are demonstrated to have high conductivity, excellent shielding effectiveness and long durability, suggesting that the silver hollow microspheres obtained here are a novel light-weight electromagnetic shielding filler and will have extensive applications in the electromagnetic compatibility fields.Fe@Ag core-shell composite particles with different surface microscopy and shape were prepared selecting carbonyl iron powders for the core particles and glucose or formaldehyde as reducing agent by electroless plating technology. By controlling the reaction conditions to regulate the reduction rate of the silver particles, silver heterogeneous nucleation may grow up and finally the silver shell with compact and uniform structure was formed on the surface of carbonyl iron powders. The electromagnetic properties of Fe@Ag core-shell composite particles were studied. The relationship between micro-structure and electromagnetic properties of Fe@Ag core-shell composite particles were systematically investigated. Using iron-silver core-shell composite particles as shielding filler, a novel kind of wide-frequency and high-performance electromagnetic shielding rubber materials were prepared. The microstructure of electromagnetic shielding fillers was characterized. Complex permeability, conductivity, magnetic properties and shielding effectiveness of composite shielding fillers were compared with those of carbonyl iron powders and silver powders, respectively. The results showed that iron-silver core-shell composite particles with intact core-shell microstructure had excellent high electric conductivity and magnetic properties. At the same time, the electromagnetic shielding rubbers based on iron-silver core-shell composite particles showed excellent electromagnetic wave shielding effectiveness due to strong absorbing loss and reflecting loss. Compared to traditional shielding rubbers, conductive magnetic electromagnetic shielding rubber materials show better shielding effectiveness. Shielding effectiveness of conductive magnetic shielding composite rubber material with the frequency of electromagnetic wave ranging from 30 MHz to 1500 MHz was-38～-43 dB. The electric property and magnetic property of membrane Fe@Ag core-shell composite particles were better than them of strawberry Fe@Ag composite particles, thus shielding effectiveness of membrane Fe@Ag core-shell composite particles was than that of the latter. Spheric and flaky Fe@Ag core-shell composite particles had same saturation magnetization, which shape of composite particles didn't influence their magnetic properties. Due to shape anisotropy of flaky composite particles, theirμ'was larger than them of spherical particles. We proved that the percolation threshold of electromagnetic shielding rubber containing flaky particles is smaller than spherical particles' by calculating its value using effective medium theory. And flaky particles more easily form a conductive network. The experiment results showed that volume resistivity of electromagnetic shielding rubber containing flaky Fe@Ag filler is 4.87×10-3 Q-cm, showing an increase of 78% compared to the rubber containing spherical filler. And the shielding effectiveness is-51dB～-55dB, an increase of-6dB～-19dB in comparison with the rubber containing spherical filler.In order to reduce the resistivity of Fe@Ag composite particles, increase the density of Ag shell and shielding effectiveness of Fe@Ag composite particles, we proposed a low-temperature heat treatment on the composite particles. The results showed that the temperature of heat treatment had nothing to do with the phase structure and saturation magnetization, but had great influence to volume resistivity and shell density. The Fe@Ag composite particles treated at 140℃have the highest shell density and a minimal volume resistivity, but as the temperature of heat treatment increasing, shell density and volume resistivity would be worse. The SE value of electromagnetic shielding rubber containing the Fe@Ag composite particles treated at 140℃is-28 dB～-53 dB, showing a decrease of-10 dB～-15 dB compared to the one without heat-treatment. The effect of heat treatment to the shell morphology and related property was reasonably explained by the interface wetting theory and crystal growth theory.