Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, USA.
J Phys Chem B. 2013 Jun 13;117(23):7095-105. doi: 10.1021/jp3128737. Epub 2013 Jun 3.
Structural and mechanical properties of silica aerogels are studied using a flexible coarse-grained model and a variety of simulation techniques. The model, introduced in a previous study (J. Phys. Chem. C 2007, 111, 15792-15802), consists of spherical "primary" gel particles that interact through weak nonbonded forces and through microscopically motivated interparticle bonds that may break and form during the simulations. Aerogel models are prepared using a three-stage protocol consisting of separate simulations of gelation, aging, and a final relaxation during which no further bond formation is permitted. Models of varying particle size, density, and size dispersity are considered. These are characterized in terms of fractal dimensions and pore size distributions, and generally good agreement with experimental data is obtained for these metrics. The bulk moduli of these materials are studied in detail. Two different techniques for obtaining the bulk modulus are considered, fluctuation analysis and direct compression/expansion simulations. We find that the fluctuation result can be subject to systematic error due to coupling with the simulation barostat but, if performed carefully, yields results equivalent with those of compression/expansion experiments. The dependence of the bulk modulus on density follows a power law with an exponent between 3.00 and 3.15, in agreement with reported experimental results. The best correlate for the bulk modulus appears to be the volumetric bond density, on which there is also a power law dependence. Polydisperse models exhibit lower bulk moduli than comparable monodisperse models, which is due to lower bond densities in the polydisperse materials.
使用灵活的粗粒模型和各种模拟技术研究了二氧化硅气凝胶的结构和力学性能。该模型在前一项研究（J. Phys. Chem. C 2007, 111, 15792-15802）中引入，由通过弱非键相互作用和微观上有动机的颗粒间键相互作用的球形“初级”凝胶颗粒组成，这些键在模拟过程中可能会断裂和形成。气凝胶模型是使用包括胶凝、老化和最后松弛三个阶段的协议制备的，在最后松弛阶段不允许进一步形成键。考虑了不同粒径、密度和粒径分散度的模型。这些模型的分形维数和孔径分布进行了特征描述，并且这些指标通常与实验数据吻合较好。详细研究了这些材料的体积模量。考虑了两种获得体积模量的不同技术，即涨落分析和直接压缩/扩展模拟。我们发现，涨落分析结果可能会由于与模拟压力计的耦合而出现系统误差，但如果仔细进行，则可以得到与压缩/扩展实验等效的结果。密度对体积模量的依赖性遵循幂律，指数在 3.00 到 3.15 之间，与报道的实验结果一致。体积键密度似乎是体积模量的最佳相关量，键密度也存在幂律依赖性。多分散模型的体积模量低于可比的单分散模型，这是由于多分散材料中的键密度较低。
