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Stress and strain during supercritical drying

  • Original Paper: Modeling, computational tools and theoretical studies of sol-gel and hybrid materials
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Abstract

Supercritical drying avoids capillary forces, but damaging stresses can be generated during solvent exchange, pressurization, and depressurization. Each of those steps has been analyzed, so the duration of the process can be minimized without risk to the sample. This work, which was initiated in collaboration with Professor Jean Phalippou’s group, is reviewed. Poromechanics is the study of stresses and strains resulting from interaction of the solid and liquid phases in a porous material. For example, if a gel is immersed in another liquid for solvent exchange, the gel network may undergo a transient contraction if the original pore liquid diffuses out faster than the replacement liquid diffuses in; poromechanical analysis quantifies the resulting stresses and indicates whether there is a risk of cracking. In the preparation of aerogels, when the pressure in the autoclave is decreased, the higher pressure inside the aerogel causes it to expand, which may result in cracking. Poromechanics reveals the rate of depressurization that can be imposed without creating damaging stresses. We also show that the sensitivity of gels to pore pressure can be exploited to measure their permeability to gases and liquids by observing the response to small loads or changes in temperature.

A dilatometer incorporated into the autoclave measures dilatation of the gel during the supercritical drying cycle, and poromechanical analysis allows extraction of the permeability from the measured strain. Similar analyses are used to minimize the cycle time while avoiding damaging stresses during the solvent exchange, pressurization, and heating, and depressurization steps.

Highlights

  • Poromechanics quantifies supercritical drying stresses during solvent exchange, pressurization and heating, and depressurization.

  • Supercritical drying duration is minimized by accounting for the pressure dependence of permeability, viscosity and compressibility.

  • By monitoring strain of the aerogel during depressurization, permeability can be measured as a function of pressure.

  • The permeability of gels is found from their response to small loads or changes in temperature.

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Acknowledgements

In 1989, the author had the pleasure of spending 3 months as a visitor in the group of Jean Phalippou. That began a long and stimulating collaboration, much of the fruit of which is reviewed in the present paper. Jean became more than a colleague: he was a friend and a role model as a teacher and scientist. Jean will be missed, but he will be remembered with great affection.

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  1. Department Civil & Env. Eng./PRISM, Princeton University, Princeton, NJ, 08540, USA

    George W. Scherer

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Correspondence to George W. Scherer.

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Scherer, G.W. Stress and strain during supercritical drying. J Sol-Gel Sci Technol 90, 8–19 (2019). https://doi.org/10.1007/s10971-018-4808-6

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  • DOI: https://doi.org/10.1007/s10971-018-4808-6

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