Stress or Strain Does Not Impact Sorption in Stiff Mesoporous Materials

Abstract

The present paper investigates strain-induced sorption in mesoporous silicon. Contrarily to a previous report based on indirect evidence, we find that external mechanical strain or stress has no measurable impact on sorption isotherms, down to a relative accuracy of 10 −3 . This conclusion is in agreement with the analysis of the sorption-induced strain of porous silicon and holds for other stiff mesoporous materials such as porous silicas.

Evaporation Process in Porous Silicon: Cavitation vs Pore Blocking

“…While the simplest explanation is the existence of connections between neighboring pores, another possibility, put forward by Grosman and Ortega, 35 would be an indirect mechanical coupling. However, in view of our recent experiments on strain-and stress-induced adsorption, 19 this mechanical coupling is negligible. Moreover, the existence of connections between pores is also suggested by the moderate anisotropy of the diffusion constant measured by NMR.…”

“…For instance, as shown in Figure 1, the evaporation pressure increases with the pore length l. This can be explained by an increase in the diameter of the interconnections with the etching time, a reasonable hypothesis in view of an earlier work where we observed that the isotherms change with the etching time. 19 In this picture, the observation in the same samples of a small cavitation event (about 1% of the total volume for the 20 μm/70% sample at 77 K) implies that some pores or some sections of the pores are connected to their neighbors only through some very narrow constrictions.…”

“…23,24 Nitrogen Optical Isotherms. The porous silicon layer being transparent above a wavelength ≃500 nm, the condensed mass could be determined using white light interferometry (WLI) 19,25 rather than volumetric measurements. The key advantage of the method is its better sensitivity, which enables measurement of thin samples with very small porous volumes.…”

“…Optical sorption isotherms ΔL = f(P/P sat ) thus have a shape very similar to that of ordinary sorption isotherms obtained through volumetry. 19 This method is extremely sensitive almost up to the critical bulk point as, in contrast to volumetry, the interference does not depend on the contents of dead volumes outside the membrane. In practice, using a specially designed optical cryostat, it allows us to measure sorption isotherms for the thinnest porous layers (1 μm) with a very small porous volume (less than ∼10 −9 cm 3 ) up to 124 K and 30 bar.…”

“…Second, by studying the temperature dependence of the evaporation pressure, both for native and ink-bottle samples, we evidenced a crossover between homogeneous cavitation at a high temperature and meniscus recession at a low temperature. Third, in the latter case, an optical interferometric technique 19 allowed us to reveal longrange spatial correlations between ink-bottle pores and show the collective nature of evaporation by meniscus recession. Finally, putting together the results of these different experiments, we could propose a network geometry for thick porous silicon.…”