Synthesis And Growth Mechanism Of One-dimensional Tungsten Nanomaterials

Tungsten (W) is an important function and structural materials. Tungsten having a high melting point, good high temperature performance, high strength and hardness, low resistivity, low electron work function, low thermal expansion coefficient and vapor pressure, and excellent corrosion resistance, is widely used in the defense, aerospace, electronic information, energy, chemical, and other fields. One-dimensional W nanomaterials with novel physical and chemical properties have the dual characteristics of both metal W and one-dimensional nanostructures. However, the one-dimensional W nanomaterial is difficult to synthesize with conventional preparation methods due to its ultra-high melting point and low vapor pressure. Therefore, seeking a simple, stable, low-cost, high controllability and high yield method to synthesize one-dimensional W nanomaterials become hot. Currently, most of the synthesis methods of W nanowires have the following disadvantages, such as the complicated steps, harsh reaction conditions, high production costs, poor controllability, thereby limiting the batch preparation and large-scale applications of W nanowires. In this thesis, a metal catalyzed method was used to prepare one-dimensional W nanomaterials. We will focus on the research of the metal catalyzed preparation and growth mechanism of one-dimensional W nanomaterials. The main contents and conclusions of the thesis are as follows:(1) Large-scale, well dispersed and uniform Ni nano-catalytic particles were prepared by magnetron sputtering and chemical methods on SiO2substrate first, and then the single crystal W nanowires were successfully synthesized on the substrate by metal catalyzed vapor deposition method at low temperature. The research results show that the metal catalytic method is also suitable for the preparation of one-dimensional metal nanomaterials, which provides a new method to prepare metal nanowires controllably.(2) Growth mechanism of W nanowires synthesized by Ni catalytic vapor deposition was studied. It’s found that there were Ni4W solid catalytic particles on the top of the W nanowire during growth process. Therefore the catalytic mechanism of top vapor-solid-solid (VSS) was proposed. The VSS mechanism can achieve the controllable preparation of metal nanowires, which laid the foundation for the large-scale production and application of nanodevices.(3) Controllable preparation of W nanowire arrays was studied in detail. The results show that the diameter of the W nanowire arrays is mainly decided by the influence of the catalyst and N2flow. The diameter of the W nanowires is correspondingly decreased with the reduction in size of the catalyst particles, or the reduction of the N2flow rate; The length of the W nanowire arrays is mainly influenced by the growth time. Extension of the growth time will correspondingly increase the length of W nanowire arrays, which is beneficial to synthesis extra long W nanowire arrays; The density of the W nanowire arrays is mainly decided by the influence of the catalyst and N2flow. The density of the W nanowires correspondingly increases with the increasing of the density of the catalyst particles or decreasing of WO3particle size; The morphology of W nanowire arrays is mainly influenced by the growth temperature and the substrate material. The temperature rise will cause that the growth rate in the transverse direction of W nanowire is not the same, which lead to morphology of forming an inverted conical in the initial growth and microns tube in the late growth. The substrate material is also an important factor. Hexagonal structure was generated on SiO2substrate, while tetragonal structure was generated on W substrate under the same conditions. Therefore, by the studying of the controllable preparation of the W nanowire arrays, we master the ability of global regulation, which is expected to achieve design ideas of nanodevices through controlling of the situation through the outfield.(4) Tetragonal W nanowire arrays were successfully fabricated on tungsten substrate using Ni catalysts by chemical vapor deposition. The top of the tetragonal W nanowire is tip-shaped morphology, and the bottom of the diameter is significantly larger than the average diameter of the nanowires. The W nanowires with such unique geometry morphology are ideal new field electron emission materials. The study results show that:the diameter of the tetragonal W nanowire is directly affected by the size of the Ni catalyst, the size of the diameter is gradually reduced with the reduction of the thickness of the catalyst; the optimum growth temperature is950℃. With the extension of the growth time, not only conducive to the increase of the length, but also conducive to the formation of an orderly and stable tetragonal W nanowire; Too much or too little N2flow is not conducive the growth of tetragonal W nanowire. The pressure of N2and H2will affect the mean free path λ of the vapor growth primitives. The flow will affect temperature gradient and heat distribution, which is very important to the growth of tetragonal W nanowire arrays. In addition, the bottom VSS catalytic growth mechanism of tetragonal W nanowire was first proposed in the thesis, and growth of tetragonal W nanowire follows the space competition mechanism.(5) Cu nano-catalytic particles were prepared by magnetron sputtering on the Si substrate first, and then the W nanowire arrays with diameter of about100nm and length of10μm were successful synthesis by Cu-catalyzed vapor deposition method at950℃. Cu-W alloy is a pseudo-alloy, which belong to the system that the solid phase does not dissolve in the liquid phase and the phase component does not change in a high-temperature process. We believe that the mechanism of the catalytic synthesis could be the induced nucleation of the Cu nano-catalytic particles, and the W atoms in H2atmosphere preferentially adsorbed on Cu nanoparticles. With the increase in the growth time, crystal nucleus will preferential grow along a certain direction under the action of kinetics, thereby forming W nanowire arrays.