Fabrication And Electromagnetic Properties Of Morph-Genetic Functional Composites

Electromagnetic interference (EMI) shielding is in increasing demand recently due to the significant increase of the use of portable wireless devices; particularly cell phones, radios, etc. which interfere with increasingly sensitive and important electronic devices. EMI shielding is also important for underground vaults containing transformers and other electric and electronic equipments that are used in electric power distribution and telecommunication. In addition, electromagnetic waves do harm to health of human being due to heat effect, non-heat effect and population effect. Therefore, there is growing interest in fabricating cost-efficient, light-weight, and effective EMI shielding and absorbing materials, which shield electromagnetic energy by component effect coupled with structural effect. Developing novel EMI shielding materials requires basing on principles of interactions between electromagnetic waves and materials, and conceiving ideas of shielding by multi-mechanisms. Research and development of nano-materials and porous materials recently provide new ideas and ways for fabrication of novel EMI shielding and absorbing materials. Natural materials have fine hierarchical porous structures on micro and nano scales. It is proposed to fabricate porous EMI shielding materials with self-assembly interconnected carbon nano-ribbon networks using low-cost natural materials as carbonaceous sources and transitional metals as catalysts.Morph-genetic functional composites with coupling structures of hierarchical pores and carbon/metal nanostructures were fabricated by a sol-gel method using three plant fibres as raw materials. The phases and constituents of the nanocomposites were determined by using XRD and thermogravimetic analysis. The microstructures of the samples were characterized using SEM and TEM. The porous structures were investigated with nitrogen-absorption method. Direct current (DC) volume electrical conductivity was measured and calculated using a four-probe method. The electromagnetic shielding effectiveness (SE) of the samples were measured with the waveguide method using a Vector Network Analyzer and it is given as the total loss of surface reflection, absorption and multi-path reflection over a range of 8.2-12.4 GHz. The electromagnetic parameters were determined using the coaxial cable method in the frequency range of 2-18 GHz. The study investigated effects of processing parameters on microstructures, electrical conductivity, electromagnetic parameters and SE of the composites. The main results are summarized as follows.Morph-genetic functional composites were characterized with coupling structures of hierarchical pores and carbon/metal nanostructures. The critical temperature for catalytic effect of Fe nanoparticles on amorphous carbon was 700°C, while those for Ni and Co nanoparticles were 800°C. As the treatment temperature were above 1000°C, self-assembly interconnected carbon nano-ribbon networks formed throughout the amorphous carbon matrix. The growth mechanism of the carbon nano-ribbons can be explained using the solution-precipitation model.The SE of the morph-genetic functional composites was almost independent of frequency in the measured region. As the treatment temperature increased from 500°C to 1000°C, the SE of composites with and without metal-impregnation increased from less than 5 dB to 15 and 40 dB, respectively. It illustrated that the formation of carbon/metal nanostructures improved extremely the SE of composites. The shielding mechanisms for the morph-genetic functional composites were component effect coupled with structural effect. The graphitization of the amorphous carbon matrix and the formation of the carbon/metal nanostructures increased the reflection loss and absorption loss of the composites, while the existence of the hierarchical pores multi-reflected the electromagnetic waves and improved the multi-reflection loss.The electromagnetic parameters of the morph-genetic functional composites were related to the porous structures and the carbon/metal nanostructures. The porous structures decreased effective permittivity and benefited impedance match. The carbon/metal nanostructures increased both the real and imaginary permittivity of the composites, but had little effect on the permeability. As the treatment temperature was above 600°C, the real and imaginary permittivity increased with the increase of the temperature. However, the permeability changed little with the temperature. The mainly absorbing mechanism of the morph-genetic functional composites was dielectric loss.According to theoretical analysis, it is possible using the morph-genetic functional composites as a low-reflection, broad-band and thin layer microwave absorption material. The real and imaginary permittivity increased with the increase of the treatment temperature. As a result, the frequency band corresponding to low reflection broadened and the maximum absorbing loss increased. The absorbing loss of a single-layer material was attributed to"characteristic loss"and"geometrical effect". The maximum reflection loss obviously shifted to lower frequency with increasing thickness and reached 40 dB at 4.2 GHz with 5 mm thickness layer.The paper investigated EMI shielding of the morph-genetic functional composites with coupling structures of hierarchical pores and carbon/metal nanostructures, and particularly analyzed the effects of processing parameters on the microstructures and electromagnetic properties of the composites. This study can provide theoretical and experimental reference for research and development of novel EMI shielding and absorbing materials.