Ranganathan Raghavan, Rokkam Srujan, Desai Tapan, Keblinski Pawel, Cross Peter, Burnes Richard
Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
Advanced Cooling Technologies, Inc., 1046 New Holland Ave., Lancaster, Pennsylvania 17601, USA.
J Chem Phys. 2015 Aug 28;143(8):084701. doi: 10.1063/1.4928633.
In this work, we study diffusion of gases in porous amorphous carbon at high temperatures using equilibrium molecular dynamics simulations. Microporous and mesoporous carbon structures are computationally generated using liquid quench method and reactive force fields. Motivated by the need to understand high temperature diffusivity of light weight gases like H2, O2, H2O, and CO in amorphous carbon, we investigate the diffusion behavior as function of two important parameters: (a) the pore size and (b) the concentration of diffusing gases. The effect of pore size on diffusion is studied by employing multiple realizations of the amorphous carbon structures in microporous and mesoporous regimes, corresponding to densities of 1 g/cm(3) and 0.5 g/cm(3), respectively. A detailed analysis of the effect of gas concentration on diffusion in the context of these two porosity regimes is presented. For the microporous structure, we observe that predominantly, a high diffusivity results when the structure is highly anisotropic and contains wide channels between the pores. On the other hand, when the structure is highly homogeneous, significant molecule-wall scattering leads to a nearly concentration-independent behavior of diffusion (reminiscent of Knudsen diffusion). The mesoporous regime is similar in behavior to the highly diffusive microporous carbon case in that diffusion at high concentration is governed by gas-gas collisions (reminiscent of Fickian diffusion), which transitions to a Knudsen-like diffusion at lower concentration.
在这项工作中，我们使用平衡分子动力学模拟研究了高温下气体在多孔非晶碳中的扩散。通过液体淬火法和反应力场在计算机上生成微孔和介孔碳结构。出于理解诸如H2、O2、H2O和CO等轻质气体在非晶碳中的高温扩散率的需要，我们研究了扩散行为与两个重要参数的函数关系：（a）孔径和（b）扩散气体的浓度。通过采用分别对应于密度为1 g/cm³和0.5 g/cm³的微孔和介孔状态下的非晶碳结构的多种实现方式，研究了孔径对扩散的影响。本文详细分析了在这两种孔隙率状态下气体浓度对扩散的影响。对于微孔结构，我们观察到，主要是当结构高度各向异性且孔之间包含宽通道时，会产生高扩散率。另一方面，当结构高度均匀时，显著的分子-壁散射导致扩散行为几乎与浓度无关（类似于克努森扩散）。介孔状态的行为与高扩散性的微孔碳情况相似，即高浓度下的扩散受气体-气体碰撞控制（类似于菲克扩散），在较低浓度下转变为类似克努森的扩散。
