【作者】 简贤；

【导师】 周祚万；

【作者基本信息】 西南交通大学 ， 材料学， 2013， 博士

【摘要】 螺旋结构赋予材料独特的物理和化学性能,如超弹性、高比强度、手征性和电磁交叉极化特性等。随着纳米科学技术的发展,多种尺度的螺旋结构被发现或人工合成出来,其中螺旋纤维是最早被发现并受到持续高度关注的一类新材料。尽管人们可以通过合理设计实验温度、催化剂成份和原料配比等工艺条件,获得不同尺度和多种形貌的螺旋纤维,但要实现其精确调控还需要进行更多深入系统的研究,特别是螺旋纤维的生长机理和多层次结构调控机制等基础性学术问题,亟待从理论基础和实验验证方面取得实质性突破。本论文针对这两方面的问题,以微纳尺度螺旋纤维为研究对象,通过理论上的模拟计算和实验方法上的系统研究,在纳米Cu和纳米Ni这两种有代表性的纳米催化剂晶体生长、可控制备、催化机理与活性调控等方面开展了系统研究,深入探究和分析了微纳尺度螺旋纤维的生长机理,为实现螺旋纤维的化学结构、微观形貌、尺度分布、理化性能等本征特性调控,以及螺旋结构的功能化设计和应用开发奠定学术基础。(1)采用沉淀法制备了酒石酸铜纳米颗粒,通过热失重(TG)和示差扫描量热仪(DSC)表征了作为纳米铜催化剂前驱体的酒石酸铜,发现其分解历程包括脱结晶水阶段(Ⅰ)和酒石酸铜分解阶段(Ⅱ)。通过设置系列升温速度,研究了酒石酸铜的热分解动力学过程,采用Flynn-Wall-Ozawa法进行数据处理,得出上述两个阶段的热分解机理函数分别为过程(Ⅰ)：G(α)=1-(1-α)1/3和过程(Ⅱ)：G(α)=lnα2(α<0.5)；(1-α)-1(α>0.5),其热分解活化能分别为94.24和177.33kJ/mol。在此基础上,系统研究了反应条件对纳米铜催化剂的影响,发现调控升温速率小于5℃/min,分解温度在271℃时,在惰性气氛Ar或者N2中,酒石酸铜粉末分解得到的纳米铜相对较大,基本为椭球体形；而在H：气氛中制备的纳米铜存在大尺寸及小尺寸两种Cu颗粒。(2)系统研究了酒石酸铜分解生成纳米铜及其催化乙炔原位生长制备纳米螺旋纤维的过程和条件,采用多种现代仪器分析技术研究了纳米铜催化剂在螺旋纤维生长前后的晶体结构和微观形貌演变规律及其催化制备的纳米纤维多层次结构。结果表明,在271℃、乙炔气氛条件下,酒石酸铜分解生成纳米铜催化乙炔可以得到形态和尺寸均匀的高纯度螺旋纤维,其化学结构类似聚乙炔,并含有少量CH3和C=O等基团。通过系统的实验研究和针对纳米铜催化剂的模拟计算,发现在乙炔气氛中,纳米铜催化剂颗粒从准球形被诱导转变为规则形貌的多面体晶体；提出了基于乙炔分子在纳米铜表面发生配位聚合,并通过纳米催化剂的晶面活性差异,发生不对称生长成纳米螺旋聚合物纤维的新机理。基于第一性原理开展模拟计算,采用Materials Studio软件中的DMol3模块,研究了Ar,N2,H2和C2H2四种气体分子在Cu8团簇和纳米Cu晶体不同晶面上的吸附行为,发现不同气体表现出不同的吸附行为,同一种气体的化学吸附也因吸附位和吸附结构的不同,而在纳米铜表面表现出差异性吸附；乙炔与Cu表面的相互作用最强,H2次之,Ar和N2最小；乙炔和H2均存在化学吸附,而Ar和N2仅存在物理吸附。基于实验研究和理论计算结果,提出通过气体诱导可控制备线形或螺旋形纳米纤维的新方法,并阐明了气体诱导效应与催化剂的纳米效应和热效应之间的竞争关系以及对纳米纤维形貌的影响规律,即：纳米效应有利于直线形纤维生长,气体诱导促进催化剂形成规则多面体而有利于螺旋结构生长,热效应则主导诱发“Y”形结构或微米螺旋纤维形成。(3)通过水合肼还原硫酸镍制备了微观形貌为准球形颗粒状的纳米镍催化剂,发现当反应温度控制在660～750℃,纳米Ni的粒径可以通过气氛控制,调控在90～500nm之间。采用纳米Ni催化乙炔合成了多种形态的螺旋碳纤维,发现通过对反应气氛的调整和控制,可分别得到线形纳米碳纤维、单螺旋纳米碳纤维、单螺旋微米碳纤维、双螺旋微米碳纤维。系统的表征和分析认为,这是由于气氛条件影响了纳米镍晶体的形貌、尺寸和分散性,并由于不同晶面的催化活性差异导致了碳纤维各向异性生长。此外,我们还发现,传统的双螺旋微米碳纤维具有“孪生”螺旋结构,并认为其形成是由于规则形貌催化剂晶体的棱边和顶点等部位与表平面对乙炔气体的吸附活性、以及催化分解和碳原子沉积形成纤维结构的能力差异导致的。(4)制备了以MgO和四针状氧化锌晶须(T-ZnO)为载体的负载型催化剂前驱体,并通过原位热分解得到负载型纳米催化剂催化乙炔发生配位聚合制备了纳米螺旋聚合物纤维。MgO载体使得纳米铜催化剂的粒径降低,当MgO/酒石酸铜质量比为3：1时,有利于制备高纯纳米螺旋纤维；偏离该配比时,所得纤维为线形和螺旋形混合纤维,或者不利于纳米聚合物纤维生长。在T-ZnO负载纳米Cu催化剂体系中,Cu与ZnO之间形成了有结合力的杂化结构；含铜原料的加入量对T-ZnO负载纳米Cu催化剂的粒径及均匀性有显著影响；原料配比中,Cu的含量低于0.4mo1%时,所得催化聚合产物的微观形貌呈颗粒状,粘附于T-ZnO表面；随着Cu加入量的增加,颗粒尺寸增大,当Cu加入量为0.6mo1%时,纳米铜尺寸和分布都比较均匀,有利于制备出高纯螺旋纤维围绕在T-ZnO表面；当Cu加入量大于0.8mo1%时,由于催化剂均匀性较差,得到线形和螺旋形混合纤维。高纯螺旋纤维/T-ZnO在Ar气氛中900℃下热处理,可得到四针状多孔螺旋碳纤维,该新型结构包含微米尺度的孔穴和纳米尺度的螺旋碳纤维。(5)通过对纳米螺旋聚合物纤维、纳米螺旋碳纤维、四针状多孔螺旋碳纤维等新型结构进行电磁学性能研究,采用矢量网络分析系统,测试了同轴波导样品的复介电常数和复磁导率,并通过反射率模拟软件得到各样品的反射率曲线,对比分析了样品的电磁波损耗特性。发现在2-18GHz频段内,纳米螺旋聚合物纤维的介电损耗和磁损耗值均很小,对电磁波的衰减性能很差；纳米螺旋碳纤维具有一定的介电损耗,电磁波响应频带较宽,但反射率衰减值不高；四针状多孔螺旋碳纤维表现出优异的电磁波损耗性能,当涂层厚度为2mm时,在3.2～18GHz频率范围内,反射衰减值均优于-4dB,峰值达到-17.57dB。分析认为,四针状多孔螺旋碳纤维由于具有独特的微纳复合多级结构和多层次界面效应,有利于满足入射电磁波的阻抗匹配,并通过多层次介电极化、界面共振和涡流损耗等形式实现对电磁波能量的损耗。更多还原

【Abstract】 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.更多还原