Understanding the diffusional tortuosity of porous materials: An effective medium theory perspective
Understanding the diffusional tortuosity of porous materials: An effective medium theory perspective
复制标题
DOI：
10.1016/j.ces.2013.09.050
复制
发表时间：
2014-05
影响因子：
4.7
通讯作者：
Xuechao Gao;J. D. Costa;S. Bhatia
中科院分区：
文献类型：
--
作者：
Xuechao Gao;J. D. Costa;S. Bhatia
对多孔材料中输运实验数据的解释通常基于使用单一代表性孔径，忽略了孔径分布（PSD）和孔隙网络连通性的影响，并拟合了所有这些不确定性都被纳入其中的曲折度。利用有关 N2、Xe 和 i-C4H10 在介孔 Shell 二氧化硅球中扩散的文献数据，我们证明弯曲度取决于代表性孔隙半径以及气体种类的选择。该实验室开发的考虑分散流固相互作用的克努森模型和振荡器模型都通过拟合网络配位数而不是曲折度，结合有效介质理论 (EMT) 充分解释了数据。这种对模型不敏感的原因是这种二氧化硅的平均介孔半径较大，为 7.4 nm；然而，我们发现振荡器模型产生的配位数值更接近该材料的预期值范围。使用 EMT，我们证明了弯曲度取决于温度和扩散物质，因为代表性孔隙半径处的电导与取决于网络连通性和 PSD 的真实电导之间的温度依赖性存在差异。在大孔径下获得的滑流流态中，我们表明努森和粘性机制的叠加导致了弯曲度的温度和物种依赖性，因为这两种贡献的孔径依赖性不同。这导致努森流态和粘性流态中的限制曲折和 PSD 依赖性不同。这些关键方面在文献中很大程度上没有得到重视，甚至曲折度随温度或扩散物种的系统变化也通常被忽视，常常导致对潜在机制的错误表述。
The interpretation of experimental data on transport in porous materials is often based on the use of a single representative pore size, overlooking effects of the pore size distribution (PSD) and pore network connectivity, and fitting a tortuosity into which all such uncertainties are consigned. Using literature data on the diffusion of N2, Xe and i-C4H10in mesoporous Shell silica spheres, we demonstrate that the tortuosity depends on the choice of the representative pore radius as well as gas species. Both the Knudsen model and the Oscillator model considering dispersive fluid–solid interactions, developed in this laboratory, are found to adequately interpret the data in conjunction with effective medium theory (EMT) by fitting a network coordination number instead of tortuosity. This insensitivity to model is due to the large mean mesopore radius of 7.4 nm for this silica; however, the Oscillator model is found to yield a value of the coordination number closer to the range of values expected for this material. Using the EMT we demonstrate that the tortuosity is dependent on temperature and diffusing species, because of differences in temperature dependence between the conductance at the representative pore radius and the true conductance which depends on the network connectivity and PSD. In the slip flow regime, which is obtained at large pore size, we show that the superposition of Knudsen and viscous mechanisms leads to temperature and species dependence of tortuosity, because of the different pore size dependence of the two contributions. This leads to different limiting tortuosities and PSD dependence in the Knudsen and viscous flow regimes. These critical aspects are largely unappreciated in the literature, and even systematic variations of tortuosity with temperature or diffusing species usually overlooked, often leading to misrepresentation of the underlying mechanism.