Structure and Permeability of Porous Silicon Investigated by Self-Diffusion NMR Measurements of Ethanol and Heptane

J Puibasset,A Grosman,E Rolley,P Porion

doi:10.2516/ogst/2015045

Abstract

The adsorption and phase transitions of confined fluids in nanoporous materials have been studied intensely because of both their fundamental interest and their crucial role in many technologies. Questions relating to the influence of the confinement of fluids, and the disorder or elastic deformation of porous solids on the liquid-gas phase transition are still under debate. Model systems are needed to understand the adsorption phenomenon. In this context, Porous Silicon (PoSi), which is a single crystal obtained by etching a (100) silicon wafer is an excellent candidate. Indeed, it consists of non-connected tubular pores running parallel to the [100] axis perpendicular to the wafer surface, with transverse sections with a polygonal shape of nanometric size whose areas are widely distributed. Once detached from the wafer, free PoSi membranes can be considered a nanoscale disordered honeycomb. Adsorption/desorption experiments have been performed to characterize the structure: they have shown that evaporation occurs collectively, an intriguing observation generally associated with a disordered pore structure with many interconnections through narrow necks. The characterization of fluid mobility inside the pores should give complementary information about the pore structure and topology. This paper focuses on the dynamics of a fluid confined inside the structure of porous silicon, and in particular the self-diffusion measurements (pulsed field gradient spin echo Nuclear Magnetic Resonance (NMR)). The results show a strong anisotropy of the self-diffusion tensor, as expected in this highly anisotropic structure. However, a non-zero self-diffusion in the directions perpendicular to the pore axis is observed. In order to interpret these puzzling results, molecular and Brownian dynamics calculations are underway.

Highlights

Information about the morphology, structure and topology of the void space of a porous material is important in understanding and predicting the dynamics of confined fluids
The former is associated with random motion, and takes place without any driving force. The latter is associated with a coherent motion of molecules due to a chemical potential gradient. Both are influenced by the structure and topology of the porous material, as well as the interactions between the fluid molecules and the solid surface
It is important to note that self-diffusion and convection are quite different processes which are not sensitive to the same morphological and topological characteristics of the porous medium [1, 2]

Summary

Introduction

Information about the morphology, structure and topology of the void space of a porous material is important in understanding and predicting the dynamics of confined fluids. Dynamics of confined fluids includes self-diffusion and convection. The former is associated with random motion, and takes place without any driving force. The latter is associated with a coherent motion of molecules due to a chemical potential gradient (driving force). Both are influenced by the structure and topology of the porous material, as well as the interactions between the fluid molecules and the solid surface. It is important to note that self-diffusion and convection are quite different processes which are not sensitive to the same morphological and topological characteristics of the porous medium [1, 2]

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Conclusion