Ultrasonic and static compression experiments were performed in the frequency range 50 kHz to 1 MHz in order to investigate the mechanical behaviour of silica aerogels as a function of internal gas pressure pG and external stress Pext. The measurement of longitudinal and transverse sound velocities allows the Young's modulus and the Poisson's ratio for aerogels of different densities to be determined upon variation of pG and Pext. For low-density aerogels ( rho approximately=100 kgm-3) the authors found a decrease in sound velocity upon evacuation. Surprisingly, the sound velocity decreases upon uniaxial compression with small loads. The most important finding from the static compression experiments is that low-density aerogels display creeping with a time constant of about 45 min.
The
goal of this work is to understand adsorption-induced deformation
of hierarchically structured porous silica exhibiting well-defined
cylindrical mesopores. For this purpose, we performed an in situ dilatometry
measurement on a calcined and sintered monolithic silica sample during
the adsorption of N2 at 77 K. To analyze the experimental
data, we extended the adsorption stress model to account for the anisotropy
of cylindrical mesopores, i.e., we explicitly derived the adsorption
stress tensor components in the axial and radial direction of the
pore. For quantitative predictions of stresses and strains, we applied
the theoretical framework of Derjaguin, Broekhoff, and de Boer for
adsorption in mesopores and two mechanical models of silica rods with
axially aligned pore channels: an idealized cylindrical tube model,
which can be described analytically, and an ordered hexagonal array
of cylindrical mesopores, whose mechanical response to adsorption
stress was evaluated by 3D finite element calculations. The adsorption-induced
strains predicted by both mechanical models are in good quantitative
agreement making the cylindrical tube the preferable model for adsorption-induced
strains due to its simple analytical nature. The theoretical results
are compared with the in situ dilatometry data on a hierarchically
structured silica monolith composed by a network of mesoporous struts
of MCM-41 type morphology. Analyzing the experimental adsorption and
strain data with the proposed theoretical framework, we find the adsorption-induced
deformation of the monolithic sample being reasonably described by
a superposition of axial and radial strains calculated on the mesopore
level. The structural and mechanical parameters obtained from the
model are in good agreement with expectations from independent measurements
and literature, respectively.
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