| Benefiting from their helical characteristics in morphology, helical materials display remarkable elasticity, mechanical strength, chirality and electro-magnetic properties. With the development of nanotechnology, materials with helical structure in molecular, nano-or macro-scale have been prepared in recent years. As to helical fibers, commonly, they are prepared and controlled by varying reaction temperature, catalyst type, gas composition and so on. The alteration of these variables will result in a significant change in helical structure and yield of the obtained helical fibers. To realize this control, a deeply understanding of the growth mechanism and the effects of the adjustable condition are essential. To date, precise control over the structure of helical fibers has been met with only limited success. In this thesis, by applying nano Cu and nano Ni catalysts to prepare helical fibers, we selectively obtained the helical fibers with desired morphology, and explored the underlying growth mechanism of corresponding helical materials. The main achievements and conclusions are summarized as follows.(1) As a catalyst precursor, cupric (Ⅱ) tartrate was prepared by a precipitation method. From the results of dynamics analysis using characterization by TG and DSC, we found the decomposition of cupric (Ⅱ) tartrate existed two different stages:loss of its crystalline water (Ⅰ) and the main decomposition of Cu(C4H4O6). The kinetic model and the parameters of the decomposition processes were also determined using Flynn-Wall-Ozawa method. The apparent activation energy of the two decomposition stages (Ⅰ) and (Ⅱ) were94.24and177.33kJ/mol, respectively. The probable integral form of kinetic mechanism function were G(α)=1-(1-α)1/3at stage (Ⅰ) and G(α)=lna2(α<0.5),(1-α)-1(α>0.5) at stage (II). Meanwhile, influence factors on the growth of Cu nanocrystal were also systematically investigated. The nano Cu crystals obtained at271℃under Ar or N2at the heating rate of less than5℃/min, had relatively large size and spheroidal shape, while Cu nanocrystals prepared under H2had a wide size distribution.(2) Helical nanofibers were synthesized using acetylene as the reactant and nanocopper crystals, produced by in situ decomposition of cupric (Ⅱ) tartrate, as the catalyst. Their chemical structures were confirmed to be organic compounds including the main polymer chain of-CH=CH-, with a few other groups such as CH3, C=O, etc. according to FT-IR,1H-NMR and elemental analyses. The morphologies of the catalyst before, during and after the fiber growth were observed by SEM and TEM, and the results revealed that the shape of the nanocopper particles changed from quasi-spherical to polyhedral during the adsorption of acetylene. Besides, density functional theory (DFT) calculations of adsorption behavior of four kinds of gases (Ar, N2, H2and C2H2) on Cug cluster and Cu facets were carried out to clarify the interaction between the catalyst and the absorbed gases. The four gases had different values of adsorption energy, so do different adsorbing sites on Cug and surfaces of nano Cu crystals, revealing that the adsorption is gas-and site-selective for Cu nanoparticles (NPs). The atmosphere of Ar and N2had very little effect on the Cu NPs growth, while H2was adsorbed dissociatively on the Cu surfaces and C2H2was either molecularly or dissociatively adsorbed. Based on the experimental and theoretical evidence, a growth mechanism of coordination polymerization and asymmetric growth on distinctive crystal planes was proposed to interpret the structural and morphological variations of the helical nanofibers. We also proposed for the first time a modified gas-induced technique to realize the in situ preparation of high-purity straight or helical carbon nanofibers (CNFs) on the formed nano Cu catalysts from the decomposition of cupric (II) tartrate. Interplay among gas-inducing, thermal and size effects on the formation of carbon fibers was also put forward. Thin and straight CNFs grow when "nano effect" was dominant, helical fiber grew under gas-inducing effect, some abnormal fiber with Y-shape and microscaled helix fromed by thermal effect.(3) Straight CNFs and three types of carbon coils (single-helix carbon nanocoils, single-helix carbon microcoils and twinning double-helix carbon microcoils) were prepared at the reaction temperature of660-750℃, by using the Ni catalyst obtained by liquid phase reduction with hydrazine hydrate as catalyst. A simple approach of controlling the gas composition to control the growth of carbon fibers was developed based on the bottom-up regulation adjusting the particles size of Ni at the scale range of90-500nm. Twinning structure existed in each fiber of the double-helix carbon microcoils regardless of circular or flat shape, which might be separated by tips of a catalyst particle due to the different rates of carbon deposition on edge and vertex, respectively. A mechanism was proposed based on different adsorptive capacity, decomposition and growth rates of carbon nanoparticles on the facet, edge and vertex of catalyst grain. (4) Two types of supported nanocopper catalysts for helical nanofibers have been prepared by decomposing cupric (Ⅱ) tartrate that grown on the carriers of MgO and T-ZnO respectively. In the case of MgO carried n-Cu, the helical CNFs could only be prepared at the MgO/cupric (Ⅱ) tartrate mass ratio of3:1; otherwise, straight CNFs co-existed in the helical fibers. By combining the co-deposition technology with gas-induced method, the cob-like tetrapod-ZnO, helical CNFs warpping T-ZnO and mixed fibers warpping T-ZnO were prepared at the molar ratio of less than0.4mol%,0.6mol%and more than0.8mol%, respectively. The formed "helical CNFs/T-ZnO" materials became "carbon coil with tetrapod-hollow" after heat treatment under Ar at900℃. This novel material, named "carbon coil with T-hollow", displays a lot of hollows with tetrapod-shape in micro-scale.(5) Addtionally, comparative researches on electromagnetic properties were conducted for several kinds of helical materials:the as-prepared helical polymer fibers, carbon coils obtained from carbonization under Ar, and carbon coil with T-hollow. The relative permeability and permittivity values of the samples were determined with vector network analyzer by using coaxial line method, and reflection loss curves of the products were calculated by reflection loss simulation soft. In the frequency of2～18GHz, carbon coils had only dielectric loss and the reflection loss values were higher than-10dB, while the helical polymer fibers exhibited neither dielectric loss nor magnetic loss. Interestingly, carbon coil with T-hollow exhibited remarkably improvement in electromagnetic wave loss compared with the pure helical nanofibers. The enhanced loss ability might be arised from the efficient dielectric friction, eddy current impedance, interface resonate in the complex nanostructures and the micro-scaled tetrapod-hollow structure. |
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