Abstract: Based on Zeng et al's model and in combination with simplified unit cell structure of cubic array of nano-spheres, the influencing factors, mechanism, and variation process of mean free path in nano-porous space are analyzed in this paper. The results show that the nano-porous structure features of nano-porous materials are directly related with specific surface area and density. The mean free path of gas molecules in nano-porous space decreases apparently with the rising of specific surface area and density. The nano-porous materials with a relative higher specific surface area and a larger density are more favorable for confining the gaseous conductivity in nano pores space. It is shown that p=10⁴ Pa and p=4×10⁵ Pa are two inflection pressures. When p<10⁴ Pa, the mean free path of gas molecules almost does not change with vary of pressure. When p>4×10⁵ Pa, the limiting effect of nano-porous structures on movement of gas molecules can be ignored. With the increase of temperature, the mean free path of gas molecules in nano-porous space is increasing, while, the increasing rate decreases gradually.

Key words: nanostructure, porous media, kinetics, mean free path, aerogel, thermal conductivity

摘要： 以Zeng等提出的纳米孔隙内气体分子平均自由程模型为基础，考虑材料的微观结构特点，结合纳米小球堆积构成的杆状立方阵列结构，对纳米孔隙空间内的气体分子平均自由程的影响因素、影响机理及变化规律进行了分析。研究结果表明纳米孔材料的微观结构特征与材料的密度和比表面积直接相关，纳米孔隙空间内的气体分子平均自由程随着比表面积和密度的增加而降低，具有相对高的比表面积和密度更有利于进一步限制纳米孔中的气相导热。p=10⁴ Pa和p=4×10⁵ Pa 是两个拐点压力，当p<10⁴ Pa时，气体分子平均自由程就不再随着压力的降低而发生变化，当p>4×10⁵ Pa时，纳米孔隙对气体分子自由运动的限制作用就可忽略。随着温度的升高，纳米孔隙空间中的气体分子平均自由程在升高，但升高幅度逐渐降低。

关键词: 纳米结构, 多孔介质, 动力学, 平均自由程, 气凝胶, 热导率