Low-temperature Vapor Phase Synthesis And Growth Mechanism Of Tungsten And Tungsten Oxide Quasi-one-dimensional Micro-/nano-structures

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.