【作者】 王世良；

【导师】 贺跃辉； Zou Jin；

【作者基本信息】 中南大学 ， 材料学， 2008， 博士

【摘要】 自上世纪五十年代开始,微米晶须材料,因为具有独特的准一维结构和极其优异的力学性能,而成为材料学界的一个经久不衰的研究热点。目前,晶须已经发展成为各种新型复合材料的主要补强增韧材料之一,而性能优异的晶须材料的合成和生长机理,以及显微结构分析等相关的基础应用研究越来越受到人们的关注。最近十多年间,直径在100 nm以下的纳米晶须材料,如纳米管、纳米线、纳米带和纳米同轴缆等,是纳米科学与技术的研究和发展中最为前沿的材料。它们一方面可以作为纳米科技中制作微/纳器件的基本构件,另一方面还作为人们探究材料在纳米尺度下的一些新特性和新功能提供一个巨大的平台。难熔金属W的晶须材料,具有非常优异的物理化学和力学性能,已有近半个世纪的研究历史。但目前人们仍然没有找到一种能进行低成本和大批量合成W晶须的方法。氧化钨准一维微/纳米结构具有非常优异的场电子发射性能和气敏特性,在场电子发射器件和高性能敏感器件等方面具有广泛的应用前景。本文主要研究了难熔金属W及其氧化物的准一维微/纳米结构的气相合成和生长机理,主要的创新结果包括:(1)采用一种成本低廉、过程简单的气相沉积方法,大批量地合成出具有各种特殊形貌的氧化钨准一维微/纳米结构,包括具有纳米锥尖的微/纳米针,纳米线,纳米带和微米管等。合成的氧化钨微米针具有非常独特的几何形貌与结构特征(锥尖直径小于10 nm,底部直径大于1μm,长度大于100μm,单晶结构),有望能克服通常的准一维纳米结构(如纳米线和纳米管)径向机械强度较低和单操作性较差的弱点,是用作扫描探针针尖,纳米机械压头和钻头,以及新型的场电子发射针尖的理想材料。(2)提出一种新的生长机制—“基于CVD过程的聚合机制”—解释氧化钨微米管的形成过程。这种新的机制能被用来解释其它多种材料的微米管状结构的形成过程,可能是一种形成微米管状结构的普适机制(3)分别采用Ni、Fe-Ni和Co-Ni作为催化剂,通过气相方法在相对较低的850℃的温度下(相对于难熔金属W的熔点和W晶须的高温合成方法),成功地合成出难熔金属W的纳米和亚微米晶须。本项研究首次从实验上,将开创于半个世纪以前的金属催化合成方法成功拓展到金属晶须的生长。这不仅为金属W晶须的可控、大批量和低成本合成,及其物性研究打下了良好的基础。同时,也为人们梦寐以求的金属晶须的可控、大批量和低成本合成提供了一条新的思路。(4)结合实验结果和理论计算,提出“表面扩散增强的气-固-固(VSS)”模型来解释金属W晶须的催化生长过程。为目前仍然存在争议的VSS机制提供了一系列的新的实验证据,并首次从实验上直接证明,通过VSS机制不仅能合成出直径在50 nm以下的纳米晶须;还能合成出直径远在100 nm以上的亚微米晶须,此外,还从理论上揭示了VSS模型中晶须生长的动力学过程所:具有的某些重要特征。(5)采用无催化剂无模板辅助的气相传输方法,在相对较低的900～1000℃下(相对于难熔金属W的熔点和目前的W晶须的高温合成方法)成功地合成出金属W亚微米晶须及其阵列。合成的W晶须具有纳米尺度的锥尖(＜50 nm)和高长径比(＞100),可能是一种非常理想的补强增韧剂和场电子发射材料。同时,本研究中所开发的这种简单、低成本和大批量合成W晶须方法,不仅有望能延伸到其他金属晶须(如Mo和Ta等)的合成,还可能进一步拓展到金属纳米晶须阵列的合成。(6)采用无催化剂无模板辅助的气相传输方法合成出单晶W微米管,并提出了一种新的生长机制—“表面扩散诱导的自模板机制”—解释所合成的单晶W微米管的形成过程。本研究中所采用的方法和提出的生长机理可能同样适用于其它单晶微米管状结构的生长。更多还原

【Abstract】 Microsized whiskers with the unique quasi-one-dimensional structures and excellent mechanical properties have been an unfailing research hot since the fifties of the twentieth century. At present, whiskers, on the one hand, have been one of most useful strengthening and toughening agents in all kinds of novel composite materials, on the other hand, the related fundamental and applied researches, such as synthesis techniques, growth mechanism, and microstructure analyses of whisker materials with excellent properties have attracted more and more attentions. In the recent teen-years, the nanowhiskers with diameters less than 100 nm, such as nanotubes, nanowires, nanobelts and nanocables, have been the most front materials in field of nano-scale science and technology. They, one the one hand, can be used as the fundamental building blocks for micro-/nano-sized devices, one the other hand, provide a excellent model system for the investigating the novel properties and functions related to the size and/or dimension confinedment.The whiskers of refractory metallic W, showing excellent physical, chemical and mechanical properties, have been investigated for nearly half a century. However, a low-cost and large-scale method for the synthesis of W whiskers is still absent. W oxide quasi-one-dimensional nano-/micro- structures have quite excellent field emission and gas sensitivity properties, and show great potential applications in use as field emission materials and high sensitive gas sensor. In this thesis, we will focus on the investigation of the vapor-phase synthesis and growth mechanism of metallic W and W oxides. The innovative results of the thesis are as follows:(1) A low-cost and simple vapor phase method was developed to synthesize on a large scale a series of W oxide quasi-one-dimensional micro-/nano- structures, such as micro-/nano- needles with nanotips, nanobelts, nanowires, and microtubes. The synthesized tungsten oxide microneedles with quite unique geometrical and structural characteristics (tip diameters: <10 nm, root diameter: >1μm, length: >100μm, single-crystalline structure) are expected to be able to overcome the weakness of the low lateral strength, and poor individual manipulability of the quasi-one-dimensional nanostructures (such as nanowires and nanotubes) with uniform nanosized diameter along the axial direction, and are ideal materials to use as the probe tips in scanning probe microscopy, nanosized indentors and aiguilles, and the field electron emission tips.(2) A novel "aggregation mechanism based on CVD process" was proposed to describe the growth process of the synthesized W oxide microtubes. The proposed mechanism can be used to explain the formation process of some other microtubules synthesized by different methods, and may be a general growth model for the formation of microtubes.(3) The nano-/submicro- whiskers of refractory metallic W have been successfully synthesized by Ni, Fe-Ni, and Co-Ni catalyzed vapor phase methods. The study, for the first time, experimentally extended the powerful metal-catalyzed method developed nearly half a century ago to synthesize metallic whiskers. The research provided not only a novel route for the synthesis of metallic whiskers, but also a well base for the future investigation of the controlled and low-cost synthesis, and the properties and applications of metallic whiskers. At the same time, the research, for the first time, experimentally demonstrated that the vapor-solid-solid (VSS) mechanism is valid not only for nanowires with diameter below 50 nm, but also for submicro-whiskers with diameters significantly above 100 nm.(4) A "surface-diffusion-enhanced vapor-solid-solid mechanism" was proposed to explain the catalytic growth process of the synthesized W whiskers, based on the experimental and theoretical calculation results. The investigation not only provided a series of new experimental evidences to support the controversial VSS model, but also theoretically discloses some important characteristics of the kinetic process of VSS model.(5) Submicro-whiskers of refractory metallic W have been successfully synthesized by a non-catalyst and non-template assisted vapor transport method. The synthesized W whiskers with nanosized tips (< 50 nm) and high aspect ratio (>100) have a great potential applications as strengthening and toughening agents and field emission materials. In addition, the simple, low-cost and large-scale synthesis technique developed here is expected to be also suitable for other metallic whiskers (such as Mo and Ta), and might be further extended to metallic nanowire arrays.(6) Single-crystalline microtubes of refractory metallic W have been successfully synthesized by a non-catalyst and non-template assisted vapor transport method. A novel "surface-diffusion induced self-template mechanism" was proposed to explain the synthesized W microtubes. The developed synthesis method and the proposed mechanism may be extended to the formation of other metallic microtubes.更多还原