化学气相沉积法生长石墨烯的CFD模拟研究

发布时间：2018-07-07 11:45

本文选题：计算流体力学 + 有限体积法　；参考：《中国科学技术大学》2015年硕士论文

【摘要】：石墨烯是碳材料家族中一种明星材料,因其独特的电子结构和优异的物理特性一直备受关注。石墨烯有望在高性能纳米电子器件、复合材料、场发射器、传感器和能量储存等领域获得广泛应用。现今主要制备石墨烯的方法有：微机械剥离法,外延生长法,氧化还原法和化学气相沉积法。其中在金属表面化学气相沉积法有望成为一种理想的大规模制备大面积,价格低廉的,高质量的石墨烯。铜金属由于其具有低碳溶解度,是一种很优良的生长衬底材料,而且在生长过程中比较容易控制石墨烯样品的层数。近来,对于在铜衬底上制备石墨烯的生长机制由很多理论研究工作。但是研究主要集中在铜衬底上的生长反应机制。关于气相动作用也只是从热力学角度进行简单分析。很少有关于气相动力学因素对石墨烯在铜衬底上生长影响的研究。但是在实验上已经表明气相动力学也是石果烯生长的一个很重要的影响因素。以甲烷作为碳源在铜衬底上CVD生长石墨烯实验中,反应室压强是影响石墨烯样品平整性的重要因素。理论沦分析甲烷的气相热分解反应导致反应室中各种活性组分处于非平衡状态,从气体流动上游到下游方向,活性组分浓度逐渐升高,因此在实验上观察到铜衬底处于下游相比铜衬底处于上游获得石墨烯样品的厚度较厚。基于这些结果,我们利用计算流体力学方法来研究气相动力学对在卧式管式炉中CVD生长石墨烯的影响。本工作利用计算流体力学软件包FLUENT仿真模拟以甲烷作为碳源在铜衬底上CVD生长石墨烯过程。从模拟结果可以发现,反应室的压强为低压(83Pa)时,甲烷的组分输运系数远大于表面化学反应常数,反应面上纵向没有浓度梯度,甲烷的表面浓度和主气流浓度差不多,表面沉积速率随温度变大呈指数增长；反应室的压强为常压(101325Pa)时,甲烷的化学反应常数大于组分输运系数,反应而上纵方向有明显浓度梯度,表面浓度最低,表而沉积速率随温度变大只是稍微增长。因此分析推出,反应室压强为低压时,石墨烯生长受限于表面化学反应；反应室压强为常压时,石墨烯生长受限于组分输运过程程。在常压(101325Pa)、中等压强(2666Pa)和低压(83Pa)三种不同压强条件下,并且在没有考虑甲烷气相分解反应情况下,我们模拟计算将铜衬底表面放置在加热区不同位置时表面沉积速率的变化。模拟获得的结果显示表面沉积速率从气体流动上游到下游方向是逐渐变小,与试验结果不相符,因此侧面说明了甲烷的热分解反应在整个石墨烯CVD生长过程中的重要性。
[Abstract]:Graphene is a kind of star material in carbon material family, which has attracted much attention because of its unique electronic structure and excellent physical properties. Graphene is expected to be widely used in high performance nanoelectronic devices, composites, field emitters, sensors and energy storage. At present, the main preparation methods of graphene are: micromechanical stripping, epitaxial growth, redox and chemical vapor deposition. Among them, chemical vapor deposition on metal surface is expected to be an ideal large-scale preparation of large area, low cost, high quality graphene. Because of its low carbon solubility, copper is a very good substrate material, and it is easy to control the layers of graphene samples during the growth process. Recently, there have been many theoretical studies on the growth mechanism of graphene on copper substrates. However, the mechanism of growth reaction is mainly focused on copper substrates. The use of gas phase action is also simply analyzed from the point of view of thermodynamics. There are few studies on the effect of gas kinetic factors on the growth of graphene on copper substrates. However, it has been shown experimentally that gas phase dynamics is also an important factor in the growth of iriodene. The pressure of the reaction chamber is an important factor affecting the flatness of graphene samples in the experiment of CVD growth of graphene on copper substrate with methane as the carbon source. It is theoretically analyzed that the gas phase thermal decomposition of methane results in the non-equilibrium of various active components in the reaction chamber, and the concentration of the active components increases gradually from the upstream to the downstream of the gas flow. Therefore, it is observed that the thickness of graphene sample is thicker than that of copper substrate downstream. Based on these results, the effect of gas dynamics on the growth of graphene in horizontal tube furnace is studied by computational fluid dynamics (CFD). In this work, the growth of graphene on copper substrate using methane as carbon source was simulated by computational fluid dynamics software package fluent. From the simulation results, it can be found that when the pressure of the reaction chamber is low pressure (83 Pa), the transport coefficient of methane component is much larger than the surface chemical reaction constant, there is no concentration gradient in the longitudinal direction of the reaction surface, and the surface concentration of methane is about the same as that of the main gas flow. The surface deposition rate increases exponentially with the increase of temperature, when the pressure of the reaction chamber is 101325Pa, the chemical reaction constant of methane is larger than the transport coefficient of the component, and there are obvious concentration gradients in the longitudinal direction of the reaction, and the surface concentration is the lowest. The deposition rate increases only slightly with the increase of temperature. Therefore, the growth of graphene is limited by the surface chemical reaction when the pressure of the reaction chamber is low, and the growth of graphene is limited by the transport process of the component when the pressure of the reaction chamber is normal. Under three different pressures: atmospheric pressure (101325Pa), moderate pressure (2666Pa) and low pressure (83Pa), and without considering the gas phase decomposition of methane, we simulated and calculated the surface deposition rate when copper substrate was placed in different positions in the heating region. The simulated results show that the surface deposition rate decreases gradually from upstream to downstream of the gas flow, which is inconsistent with the experimental results. Therefore, the importance of the thermal decomposition of methane in the whole growth process of graphene CVD is explained.
【学位授予单位】：中国科学技术大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TN304.05

【共引文献】