| Recently, nano-carbon material has been a hot research topic and showed an attractive application perspective. In this thesis, I have investigated the electromagnetic property and thermo-physical performance of carbon nanotube (CNT), core-shell structured carbon encapsulated metal nanocapsules and graphite nanoplatelets (GNP) and their composites. The agglomeration of CNT has been regarded to be the bottleneck towards CNT application, a novel method was proposed in this thesis to directly and quantitatively characterize dispersion and stability of CNT suspensions. A liquid blending method was proposed to prepare bamboo like multi-walled carbon nanotube (BCNT) incorporated polymer composite for electromagnetic interference shielding application. Core-shell structured carbon encapsulated cobalt nanocapsule (Co(C)) which consisted of carbon shell with dielectric loss and cobalt core with magnetic loss was synthesized and exhibited excellent microwave absorption performance. A thermally conductive composite with silica-gel-matrix incorporated core-shell structured carbon encapsulated copper nanocapsule (Cu(C)) was investigated. The thermo-physical properties of GNP/silicone resin composites with improved thermal conductivity were also explored.1. The dispersion of CNTMulti walled carbon nanotubes (MWNTs) suspensions were prepared by using propylene glycol n-propyl ether (PnP), ethylene glycol monobutyl ether (EB) and propylene glycol monomethyl ether (PM) as solvents, copolymer Disperbyk-2012 (D-2012) and Disperbyk-198 (D-198) as surfactants associated with ultrasonication. The de-dispersion and sedimentation behaviours of suspensions were characterized by a step view centrifugation methodology which allows accelerating and quantifying the stability of dispersions in a direct way. During 1000 rpm accelerated centrifugation for 5600 s, PnP, EB and PM suspensions showed severe de-dispersion and sedimentation behaviours compared with that of surfactant suspensions, the sedimentation velocities of PnP, EB and PM dispersions were far more than that of surfactant suspensions. Both real time and statistic instability index analysis were performed and the results showed that PnP/D-198 and EB/D-198 were superior mediums for MWNTs suspensions, a plausible dispersing and stabilizing mechanism of the used surfactants and solvents for MWNTs was proposed.2. BCNT/silicone resin composite for electromagnetic shielding applicationMulti-walled, bamboo-like carbon nanotube (BCNT)/methyl vinyl silicone (MVQ) composites with different concentrations of BCNT were fabricated by liquid blending method with an aim to investigate the behavior of such composites as effective electromagnetic interference shielding materials in the frequency range of 1-6 GHz. The morphology and structure of BCNT were characterized and elucidated. Scanning electron microscopy examination showed that the BCNTs homogeneously dispersed in MVQ. The electrical conductivity (σ) and shielding effectiveness (SE) of the composite were measured and discussed. The results showed that the BCNTs/MVQ composites had a relatively low percolation threshold at 0.92 wt.% BCNT, the σ showed a decreasing linear relation with temperature which the σ slightly decreased with increasing temperature. The BCNTs/MVQ composites with SE of-(33-38) dB were obtained at 7 wt.% BCNT loading. Shielding mechanism was studied by resolving the total incident energy into absorbed, reflected and transmitted contributions and the result showed that the dominated shielding mechanism was reflection loss.3. The electromagnetic property of Co(C)The carbon encapsulated cobalt nanoparticles (Co(C)) were synthesized by an arc discharge method. The carbon shell consisted of both 15-20 layers ordered carbon and amorphous carbon, the inner crystal cobalt core was completely encapsulated by outer carbon shell. The electromagnetic characteristics of Co(C) embedded in paraffin with 10, 40,50,70 and 80 wt.% in 2-18 GHz were investigated. The special microstructure of Co(C) and the carbon layers were mainly responsible for high complex permittivity, the interfacial polarizations happened at the interface between cobalt cores and carbon shells dominantly determined the dielectric behaviour. Real part of complex permeability (μ) decreased and imaginary part of μ kept constant small with increasing frequency, which revealed good insulation between the metallic cobalt cores was achieved by carbon encapsulation. The major electromagnetic absorption mechanism was dielectric loss. The minimum calculated reflection loss (RL) was -52 dB at 7.54 GHz with 50 wt.%Co(C) for a 3 mm Co(C)/paraffin composite. The measured RL of a Co(C)/epoxy coating with 40 wt.% loading at 3 mm showed good agreement with that of calculated RL. The electromagnetic property of Co(C) can be tailored through both control Co(C) concentrations and coating thickness.4. Thermally conductive composite prepared by silica-gel-matrix incorporating Cu(C)A core-shell structured nanoparticle which copper core encapsulated in carbon shell (Cu(C)) was synthesized by a direct current arc discharge method. The morphological and microstructural characterization showed that the Cu(C)consisted of a nano sized Cu core and carbon shell, the carbon shells which contained 6-15 ordered graphitic layers and amorphous carbon effectively shield the metallic Cu core from oxidation. A thermally conductive composite was successfully fabricated using silica gel as matrix incorporated with Cu(C) as the filler. The Cu(C) nanoparticles homogeneously dispersed in silica gel. The effect of Cu(C) on thermal conductivity, electrical resistivity and coefficient of thermal expansion (CTE) of the composite were investigated. The thermal conductivities at 50℃ were 0.32-0.77 W/(m.K), the electrical resistivities were 1.98×109,3.48×107,302, and 1Ω. m, the CTEs at 200℃ were 3.79×10-4-3.44×10-4K-1 of composites with 6.1 vol.%,11.0 vol.%,16.7 vol.%, and 23.3 vol.% content of Cu(C), respectively. The result revealed that the ordered graphitic shells in Cu(C) increased both thermal and electrical conduction, but decreased CTE because they prevented the Cu core from expansion.5. Thermo-physical property of GNP/silicone resin thermally conductive composite with improved thermal conductivityThe graphite nanoplatelets (GNP)/silicone resin composites at various loadings were produced. The utilized GNPs were characterized by two dimensional structure with high aspect ratio (-1810), the GNP with approximately 10-30 nm thickness and 10-50 μm in length evenly dispersed throughout the resin matrix, which enables GNPs effectively act as thermally conductive medium, thus contributed considerably to the formation of an efficient three dimensional network for heat flow. The thermal conductivities of 5,10,15 and 20 wt.% GNP composites were 0.35,1.02,1.32, and 2.01 W/(m.K), is ca.0.9,4.7, 6.3 and 10.2 times higher than that of silicone resin at room temperature, respectively. The thermal conductivity decreased with elevated temperature in 25-200℃, which was reminiscent at higher loading. Differential scanning calorimeter analysis showed that GNP addition increased the curing temperature of silicone resin from 90℃ to 119℃, probably by hindering the free movement (mobility) of the silicone chains. The result showed that the GNP not only reduced the CTE but also improved the thermal stability of composite simultaneously. |
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