Research On High Efficient And Economic Synthesis Of Carbon Nanocoils And Their Microwave Absorption Property

Carbon materials own various forms,among which carbon nanocoils(CNCs)possessing a special 3D helical structure have attracted much attention.Compared with carbon nanotubes(CNTs),CNCs exhibit more excellent mechanical and electromagnetic properties.Therefore,CNCs are hopeful to be applied in basic units of micro/nano systems,including microwave absorption materials,field emitters,induction coils,nanosprings,electromechanical sensors,near-infrared detectors,etc.In macro applications.CNCs are expected to be fabricated to be high strength elastic composites and electromagnetic wave absorption materials.Therefore,the method for high-yield and economic synthesis of CNCs becomes an important issue.In this thesis,the researches are focused on the low-cost substrates used to grow CNCs and the growth mechanism to lay a good foundation for practical applications.Furthermore,the microwave absorption properties of purified CNCs and CNCs/tissue are also studied.First of all,copy papers were applied to synthesize CNTs and CNCs,while only CNTs can be obtained on pure papers by the same method.It is found that,calcium carbonate particles can promote the copy papers to adsorb more catalyst particles,which inducing more suitable size particles for the growth of CNCs.Meanwhile,sulfur content was also increased,which was necessary for the helical growth of the CNTs and carbon fibers.Therefore,by adding CaCO3 particles on the pure papers,high productivity of CNCs also can be achieved on the pure papers.On the basis of the paper substrates,the sheets made of plant fibers,including tissue,cotton and bamboo fiber,were also can be used to synthesize high-yield CNCs.The growth results show that various morphologies and productions of CNCs can be obtained from three kinds of substrates.After the observation for the structure features of the carbon fibers consisting of plant fiber substrates.It is found that the carbon fibers consisting of tissue substrate own larger diameter and are tabular,and those in cotton substrate have tight organization,which can promote the catalyst aggregations to fabricate high yield of CNCs.By contrast,the carbon fibers in bamboo fiber substrate own loose arrangements and small diameter resulting in a relatively small number of catalyst particles deposited on the surface and tend to be isolated.Therefore,it is more likely to obtain the growth of a certain amount of the carbon nanofibers and carbon nanotubes.In addition,by coating calcium chloride particles on the bamboo fiber,the catalyst adsorption ability can be improved and then high production of the regular CNCs can also be achieved.The above substrates are all suitable for the growth of CNCs in the fixed bed method,and we also research the substrates adapted in the fluid bed method.In the thesis,the CNCs were synthesized on the alumina spheres deposited by Fe/Sn catalyst.Because of that suitable carbon layer thickness is needed by successful growth of CNCs,a too thin thickness cannot provide enough support for the helical growth of CNCs and an over thick layer can decrease the productivity of CNCs.Different catalyst precursor concentrations and molar ratios of Fe/Sn were used to control the thickness of carbon layer and production of CNCs on the alumina spheres.When the molar ratio of Fe/Sn are 60:1,30:1,and 10:1,high productivity of CNCs can be obtained.In addition,different diameters of alumina spheres were applied to synthesize CNCs.It is found that the yield of the CNCs will be decreased with the decreasing of diameter of alumina spheres.It is due to the fact that,at the same time the carbon layer was reduced,which has a negative effect on the helical growth of CNCs.The carbon layer can be increased by increasing the catalyst concentration,which induced the enhancement of the productivity of CNCs on the smaller alumina spheres.Excellent electromagnetic property of carbon nanocoils benefiting from their special structure can be applied to be electromagnetic microwave absorption materials.The CNCs/tissue obtained by the method in the third chapter and purified CNCs have been mixed respectively with paraffin and researched as the microwave absorption materials.The experimental results show that they both own the outstanding microwave absorption properties.For purified CNCs,the reflection loss can be-37.15 dB at 13.92 GHz,and the frequency width of reflection loss value under-10 dB can be 8.4 GHz with 8 wt.%of CNCs,which is beneficial for the application of broadband absorption.It is due to the fact that for purified CNCs,the helical structure can emerge cross polarization effect and the interconnections among CNCs and paraffin can form the LCR circuit to increase dielectric loss.For CNCs/tissue sample,as the carbon material they can better combine with paraffin to get a suitable impedance matching to increase the input of electromagnetic wave.The test results show that the maximum reflection loss value can reach up to-46.36 dB at 14.72 GHz with 10 wt.%of CNCs/tissue and a highly efficient absorption property can be achieved.