| 其他摘要 | Carbon microcoils (CMCs), which have a peculiar micro coil or helix shape, attract constant scientific and application interest for their unique morphologies and special properties such as high elasticity, high stability and specific surface area, and unique electrical and electromagnetic (EM) wave absorption properties. They are expected to be widely used for the EM wave absorber, tactile micro-sensors, actuators, tunable micro-devices and chiral catalysts, etc. CMCs prepared by CVD method usually have two types in terms of fiber cross-section: circular CMCs and flat CMCs. In this paper, CMCs were obtained by CVD method at the optimized reaction conditions: C2H2 of 30ml/min, H2 of 90ml/min, temperature at 760℃, and thiophene content at 0.8-1.0mol%.
We compared the differences between circular and flat CMCs in morphologies, microstructures, electrical and electromagnetic properties, by SEM, TEM, Raman, X-ray, etc. The results revealed that there was little difference in carbon grain size that composed CMCs: circular CMC was composed of graphite sheet in the size of 3.44nm, the other was of 3.17nm. The dependence of CMCs’ electrical resistance on extension ratio of coils was studied. We found that flat CMCs had much more obvious such dependence than flat one: electrical resistance increased by 2-4 times with coil extension ratio of 1-2 times than original length. The electromagnetic property was examined in the frequency range of 2-18GHz by measuring relative permittivity (ε) and permeability (μ). The results indicated that circular CMCs may have better performance as microwave absorbent in low frequency band, and flat CMCs may have better performance as microwave absorbent in high frequency band.
During the growth of CMCs, acetylene flow was intensively stopped for 30 seconds at interval of 10 minutes. It turns out that the fiber cross-section changes correspondingly: flat CMC with coil diameter of 4.2μm changes successfully to circular CMC with coil diameter of 6.0μm in about 10μm along coil axis. With the acetylene flow recovering, the circular CMC gradually changes back to flat CMC. This experimental result indicates the reversibility of changing fiber cross-section.
Based on phenomenon and regularities in our experiment and three-dimensional growth mechanism proposed by S. Motojiam, we give a new growth model: dynamic-three-dimensional (D3D) growth model. It is believed that catalytic anisotropy of catalyst affects both coil diameter and fiber cross-section, and the change of anisotropy caused by changing reaction conditions will lead to the changes of coil diameter, and fiber cross-section. D3D growth model explains our experimental results successfully, and can explain most of results reported in aspect of fiber cross-section very well. |
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