【作者】 王超；

【导师】 贺跃辉；

【作者基本信息】 中南大学 ， 材料科学与工程， 2013， 博士

【摘要】 钨(W)是重要的功能材料和结构材料,具有熔点高、高温性能好、强度硬度高、电阻率低、电子逸出功低、热膨胀系数和蒸气压低等特点,以及优良的抗腐蚀性能,被广泛应用于国防、航空航天、电子信息、能源化工等领域。一维W纳米材料兼具金属W和一维纳米结构的双重特点,使一维W纳米材料具有更新奇的物理和化学特性。但由于W的超高熔点及超低蒸汽压,其对应一维纳米材料很难采用常规制备方法获得,因此寻求一种简单、稳定、低成本、高可控性、高产量的一维W纳米材料制备方法成为研究的热点。目前,制备W纳米线的方法大多工序繁杂,反应条件苛刻,生产成本高,可控性差,从而限制了W纳米线的批量化制备及规模化应用。本文采用金属催化法制备一维W纳米材料,主要研究了一维W纳米材料的金属催化制备及其生长机理,研究的主要内容和结论如下：(1)通过磁控溅射法和化学法在SiO2基片表面制备得到大规模、分散性良好、粒径分布较为均匀的Ni纳米催化颗粒,采用金属催化低温气相沉积的方法在基片上成功合成了W单晶纳米线阵列。研究结果表明,金属催化法同样适合于一维金属纳米材料的制备,这为实现金属纳米线阵列的可控制备提供一种新方法。(2)对Ni催化气相沉积制备W纳米线的生长机理进行研究,研究发现在W纳米线的生长过程中顶端有Ni4W的固态催化颗粒,提出了顶端气-固-固(VSS)催化机制。这种VSS催化机制可以实现金属纳米线的可控制备,有效的控制纳米线的生长位置和尺寸大小,为实现纳米器件的大规模生产和应用奠定了基础。(3)对一维W纳米结构阵列的可控制备进行系统的研究,研究结果表明：W纳米线阵列的直径主要受到催化剂和N2流量的影响,随着催化剂颗粒尺寸的减小或者N2流量的减小,催化生长后W纳米线直径的尺寸也相应变小；W纳米线阵列的长度主要受到生长时间的影响,生长时间的延长会使W纳米线阵列的长度也相应的增加；W纳米线阵列的密度主要受到催化剂和WO3粉末粒径的影响,随着催化剂颗粒密度的增大或者WO3粒径的减小,W纳米线阵列的密度也相应增大；W纳米线阵列的形貌主要受到生长温度和基片材料的影响,温度升高会使得W纳米线在横向的生长速率不一样,在生长初期形成倒锥形的微观形貌,在生长后期形成有规则的六边形形貌的一维W微米管结构；同时,基片材料也是影响W纳米线阵列形貌的一个重要的因素,在相同条件下,Si02基片上生成的W纳米线阵列为六方结构形貌,而W基片上生成的W纳米线阵列为四方结构形貌。因此,通过对W纳米线阵列可控制备的研究,掌握了对W纳米线阵列的规则、整齐和均一性的全局调控的能力,从而有望实现通过外场控制全局的纳米器件设计思想。(4)采用一种基于金属Ni催化气相沉积过程的方法,成功在W基片上合成了具有四方形貌的W纳米线阵列。四方W纳米线顶端呈尖状形貌,底端的宽度要明显大于纳米线的平均宽度。这种具有独特几何形貌结构的W纳米线阵列,是理想的新型场致电子发射材料。论文对四方W纳米线阵列的生长条件进行研究,研究结果表明：四方W纳米线阵列的宽度直接受到Ni催化剂尺寸的影响,随着催化剂颗粒尺寸的减小,四方W纳米线阵列的宽度也逐渐减小；当生长温度在950℃时,四方W纳米线阵列的密度最大,有利于四方W纳米线的生长；随着生长时间的延长,不仅有利于四方W纳米线阵列长度的增加,还有利于形成有序稳定的四方W纳米线结构；N2流量太小或太大都不利于四方W纳米线阵列的生长,N2和H2气压会影响四方W纳米线气相生长基元的平均自由程λ,流速会对温度梯度以及热量散发等因素有影响,对四方W纳米线阵列生长的影响十分重要。另外,本论文首次提出底端VSS催化生长机理制备四方W纳米线阵列,同时认为四方W纳米线阵列的生长遵循空间竞争机制。(5)采用磁控溅射法在Si基片表面制备得到Cu纳米催化颗粒,通过气相沉积的方法,在950℃成功合成直径在100nm左右,长度达到10μm,具有极高长径比的W纳米线阵列。Cu-W合金属于固相不溶于液相的系统,在高温过程中不会发生成分的变化,属于假合金。Cu催化合成W纳米线阵列的机理可能是由于Cu纳米颗粒在催化过程中主要起诱导形核的作用,气体W源在H2气氛下还原生成的W原子会优先吸附在Cu纳米颗粒上。随着生长时间的增加,晶核在动力学的作用下会沿着一定的方向择优生长,从而形成W纳米线阵列。更多还原

【Abstract】 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.更多还原