Institute of Physics, Kazan Federal University, 420008Kazan, Russia.
Tatarstan Academy of Sciences, 420111Kazan, Russia.
J Phys Chem B. 2023 Feb 16;127(6):1459-1470. doi: 10.1021/acs.jpcb.2c08251. Epub 2023 Feb 5.
We report on the first observation of diffusion anisotropy of gaseous helium-3 entrapped in ordered aerogels at 4.2 K. The origins of He diffusion anisotropy in aerogels of different porosity are discussed. The correlations between gas diffusion coefficient and basic parameters of aerogels, such as porosity, fiber diameter, and fiber's degree of alignment, are inspected using simple diffusion simulations within the framework of classical diffusion model in both oriented and chaotic aerogels under conditions of diffuse (Knudsen diffusion) and specular reflections of atoms from the walls. The failure of the two-phase and Knudsen diffusion models at low temperature in isotropic and anisotropic aerogels is observed. The effect of a wall attractive potential on the gas dynamics is suspected to play a crucial role in the gas diffusion and its anisotropy. The rough theoretical estimates of that effect at low temperatures in aerogel space confirm this assumption. The observed peculiar diffusion is universal and is expected to occur with other probe gases at higher temperatures.
我们首次观察到 4.2 K 下有序气凝胶中被捕获的气态氦-3 的扩散各向异性。讨论了不同孔隙率气凝胶中氦扩散各向异性的起源。使用经典扩散模型在定向和无规气凝胶中的简单扩散模拟，检查了气体扩散系数与气凝胶基本参数（如孔隙率、纤维直径和纤维取向度）之间的相关性，在漫反射（Knudsen 扩散）和原子从壁面镜面反射的条件下。在各向同性和各向异性气凝胶中，低温下两相和 Knudsen 扩散模型失效。怀疑壁面吸引力对气体动力学的影响在气体扩散及其各向异性中起着关键作用。在气凝胶空间中低温下对该效应的粗略理论估计证实了这一假设。观察到的特殊扩散是普遍的，预计在较高温度下其他探针气体也会出现这种扩散。
