阐明碳纳米管的生长机理，实现碳纳米管的可控制备，是碳纳米管合成领域的最终目标，也是真正实现碳纳米管应用的必经之路。本论文以多壁碳纳米管阵列为研究对象，考察了生长过程中成核、生长和终止三个阶段的影响因素和作用机制，以实现多壁管阵列的可控制备。 成核阶段受到前期生长沉积在反应腔器壁上的积碳的影响。在无积碳的反应腔内，碳源气促使催化剂变成纳米颗粒并使碳管开始成核过程；但在有积碳的反应腔内，在碳源气通入之前，从积碳中释放出的某些活性成分就已经开始使催化剂变成颗粒，因而能够促进多壁管阵列的成核，使得在有积碳的反应腔体内生长的阵列具有更高的碳管密度和排列规整性。 生长阶段是阵列生长过程中历时最长的阶段，这一阶段可以通过一种简单的生长标记方法来表征，所做的标记可直接在扫描电镜甚至光学显微镜下观察到。这种标记方法可以用来判定阵列的生长模式，也可以用来测量生长阶段的生长速率。生长速率体现出的活化能反映出碳管生长可能并不是一个扩散限制过程，而是可能受限于碳源气在催化剂表面的反应。 终止阶段可以通过控制催化剂附近的局域气氛来实现可控。采用通常的切断碳源气使生长终止的“自然终止”方式，碳管在基底附近缠绕弯曲；如果在终止阶段辅以大流量的惰性气体使之“突然终止”，则碳管根部会与基底笔直规整地接触。突然终止方式产生的碳管阵列与基底的结合力要高于自然终止方式，这提供了一种通过可控终止生长增强结合力的方法。 对机理的理解可应用于可控制备超顺排碳纳米管阵列。通过调节催化剂薄膜厚度和生长时间，可以控制超顺排阵列中碳管的直径、壁数和长度，并进一步调控抽出的超顺排薄膜的电学和光学性质。超顺排薄膜的归一化的低温电阻性质与碳管直径分布有关，而表面电阻率、透过率和发光偏振度则与母体阵列高度有关。这些结果为未来实现超顺排薄膜的实际应用提供了借鉴。
The ultimate goal in studying the synthesis of carbon nanotubes (CNTs) is to clarify the growth mechanism and control the growth process as desired, which is indispensable for realizing applications of CNTs. In this thesis, the mechanisms as well as some influencing factors at the three stages (nucleation, growth and termination) during the growth process of multi-walled carbon nanotube (MWCNT) arrays were studied, aiming for controlled growth of MWCNT arrays. The nucleation stage is affected by carbon deposits on reactor innerwalls formed during previous growth cycles. In a reactor without these carbon deposits, carbon source gas makes continuous catalyst films into nanoparticles and starts the nucleation of CNTs. In a reactor with carbon deposits, however, some activated species released from carbon deposits have cracked continuous catalyst films into particles before the carbon source gas is led in, thus facilitating the nucleation of CNTs. Consequently, as-grown arrays synthesized in the reactor with carbon deposits have a larger surface density and a better alignment of CNTs. The growth stage, which lasts most of the time during the growth process, can be studied by a growth mark method. These marks can be directly observed under scanning electronic microscope or even optical microscope. This growth mark method can be used to clarify growth model and measure growth rates of MWCNT arrays. The activation energy, derived from the temperature dependence of growth rate, indicates that the growth process may be limited by the surface reaction of the carbon source gas on catalyst particles, rather than the diffusion of carbon atoms in the catalyst. The termination stage can be controlled by tuning local ambience around the catalyst. The most common way to terminate the growth of CNTs, called ‘natural termination’, is shutting off the carbon source gas, which gives rise to curved and tangled CNT roots. However, if an extra large flux of inert gas is led in during the termination stage, CNT roots will straightly contact with the substrate. This termination way is called ‘abrupt termination’. Direct force measurements show that, compared with natural termination, abrupt termination gives rise to a much larger adhesion force between the CNT array and the substrate. This termination-controlled growth provides a simple way to enhance the adhesion force, which facilitates many applications. Based on the understanding of growth mechanism, one kind of high-quality array, that is, super-aligned CNT array, can be controllably synthesized. By varying thicknesses of catalyst films and growth time, the tube-diameter, number of walls, and lengths of as-grown super-aligned arrays can be well controlled. As a result, electrical and optical properties of as-drawn super-aligned CNT films can be tuned. The experimental results show that, the temperature dependence of normalized resistance of super-aligned films is merely relevant to the distribution of tube-diameter of CNTs, but sheet resistivity, transmittance and polarization degree of light emission are relevant to the length of matrix arrays. These results provide referenced data for applications of super-aligned CNT films in the future.
