Evaporation Process in Porous Silicon: Cavitation vs Pore Blocking

Abstract

We have measured sorption isotherms for helium and nitrogen in wide temperature ranges and for a series of porous silicon samples, both native samples and samples with reduced pore mouth so that the pores have an ink-bottle shape. Combining volumetric measurements and sensitive optical techniques, we show that, at high temperature, homogeneous cavitation is the relevant evaporation mechanism for all samples. At low temperature, the evaporation is controlled by meniscus recession, the detailed mechanism being dependent on the pore length and on the mouth reduction. Native samples and samples with ink-bottle pores shorter than one micrometer behave as an array of independent pores. In contrast, samples with long ink-bottle pores exhibit long-range correlations between pores. In this latter case, evaporation takes place by a collective percolation process and not by heterogeneous cavitation as previously proposed. The variety of evaporation mechanisms points to porous silicon being an anisotropic three dimensional pore network rather than an array of straight independent pores.

How nanoporous silicon-polypyrrole hybrids flex their muscles in aqueous electrolytes: In operando high-resolution x-ray diffraction and electron tomography-based micromechanical computer simulations

“…The segmentation is calibrated for the cubic slice of 300 × 300 × 300 voxel at position z = 900 to a relative density of Φ = 45.7 %. Moreover, it can be seen in the single segments, that the main pores exhibit interconnections, which would confirm recent results in literature [15,16,57]. The main pores in the tomography are in a range of 7 − 25 nm, but are not further evaluated here.…”

“…Recent experiments on self-diffusion [15] and cavitation events [16] in this porous medium hinted towards the importance of those pore interconnections and questioned the simple picture of nanoporous silicon as an array of parallel independent nanochannels. Also our study evidences that a consideration of nanoporous silicon as a parallel array of independent pores on a random hexagonal array is insufficient.…”

“…Since the discovery of self-organized nanoporosity in silicon during electrochemical etching the resulting material has been attracting much attention from both fundamental and applied sciences owing to its exceptional biocompatible [1,2], optical, electrical and thermal properties compared to the bulk material [3][4][5][6][7][8][9][10]. Nanoporous silicon (np-Si) can be prepared in monolithic form with parallel, channel-like pores and has been of particular interest for studies on the influence of geometrical confinement on condensed-matter systems [11][12][13][14][15][16]. It also allows one to synthesize biocompatible composite materials with finely tuned optical and electrical properties by liquid adsorption or infiltration [7,17,18].…”

A capillary-induced negative pressure is able to initiate heterogeneous cavitation

“…This model is closely similar to porous media emptying (desorption) of ink-bottle pores, which has long been studied experimentally, theoretically, and with simulations. 38–48…”

Wafer‐Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle‐Assisted Chemical Etching and Pseudomorphic Thermal Oxidation

“…[23,24] Nanoporous Si as a nanostructured scaffold offers a wide range of applications for the exploration of the fundamentals of spatially confined condensed-matter systems. [25][26][27][28][29][30] As a nanoporous mainstream semiconductor np-Si is interesting for sensorics, actuorics, (bio-)photonics, and drug delivery [16,[31][32][33][34][35][36][37][38] as well as energy storage and conversion. [39][40][41][42][43] Filling the pore space with soft or hard materials, for example, electrically conductive polymers can improve properties such as electrical conductivity and mechanical robustness.…”