Compressive tests on supercritically dried monoliths

Large, low density silica monoliths with a composition of EGMS(Si)/P123/1 M HCl of 82(8)/30/70 (E802A) were dried supercritically with either CO2 or MeOH as supercritical fluid following the procedure described in Chapter 2.1.3.1. By this, low shrinkage as well as mesopore-template extraction was achieved. The resulting monoliths had a cylindrical shape with a height of approximately 25 mm and a diameter of 9-10 mm. The obtained monoliths were tested under compression on a Zwick Z050. Two different experimental setups, which are described in Chapter 2.4.2, were employed to minimize errors due to friction and start-up behaviour.

In Table 3.11 the relevant sample characteristics are listed, including the density, which is between 0.21–0.27 g/cm³ for the investigated material, the wall thickness of the mesopores, the pore diameter and the BET surface area. Samples that were dried with MeOH experienced a higher temperature (~250 °C) than samples dried with CO2 (only 40 °C). The higher temperature leads to a decrease of BET surface from 891 m²/g to 462 m²/g.

Table 3.11. Relevant data of the drying process and the resulting properties of the mesoporous gels. The d-spacing relates to hexagonally arranges mesopores inside the SiO2 network structure.

Supercritical

3.6 MECHANICAL PROPERTIES

Figure 3.6.9 shows the resulting stress-strain curves of supercritically dried EGMS-gels during compression tests performed with (a) setup A, (b) setup B (for experimental details see Chapter 2.4.2). Higher temperature in the drying process results in higher Young´s modulus and ultimate strength. SEM images depict that samples dried at ambient temperatures (CO2) buckle under the compression, whereas cracks are observed for the denser samples obtained with MeOH-drying. Schematic drawing of a monolith after failure.

Table 3.12. Result for Young´s modulus (E), ultimate strength (Fmax) and compression at Fmax (ε-Fmax) for cylindrical mesoporous SiO2-monoliths in compression mode.

CO2 MeOH

This is attributed to a loss in microporosity, in particular associated with increasing condensation of silanols (Si-OH) to siloxane (Si-O-Si) bonds. Thus, the pore diameter of the mesopores becomes larger, whereas the SiO₂-walls become more dense. Also a temperature of 250 °C enhances the macroscopic shrinkage of the monolithic gel bodies.

To summarize, the overall density as well as the skeleton density is enhanced in the MeOH-dried samples. No change in the mesostructure takes place as was proven by the SAXS-measurements. Also the mesoporous network-structure of the material is conserved by both drying procedures.

In Figure 3.6.9 the obtained stress-strain curves are shown. The Young´s modulus, E, was estimated to be in the range of 70 MPa for the CO₂-dried monoliths, gels and in the range of 300-400 MPa for the MeOH dried monoliths. Compared to the results obtained by instrumented hardness test (E_CO2=112 MPa, E_MeOH=310 MPa) the value for the modulus of the CO -dried material is lower in compression. This can be explained by the fact, that the

3.6 MECHANICAL PROPERTIES

viscous fraction plays a more important role in the compression test. Since the samples dried at lower temperature have a higher viscous fraction the difference in the results is more pronounced than for the measurements on the MeOH-dried samples. For the MeOH-samples an increase of compressive stress and at the same time a reduction of strain at failure could be observed. The denser material therefore is more brittle and has a higher strength at failure. The different failure modes are visible in the fractured surface investigated by SEM. For the more brittle MeOH-dried monoliths cracks were observed, whereas a more structured fracture surface, with characteristic buckling, was obtained for the CO2-samples. For all materials the monoliths flew in pieces after reaching the ultimate strength. One cone-shaped and one larger acuminated piece were left, schematically depicted in Figure 3.6.9.

Influence of the Porosity

As described in Table 3.11 the overall density of the investigated monolithic samples is between 0.21-0.27 g/cm³. The densities of the single samples can be correlated to the corresponding measured Young´s modulus. In Figure 3.6.10 the Young´s modulus of the compression tests is plotted against the density of the samples. The data is normalized by the Young´s modulus (ES=73 GPa) and the density (ρS=2.2 g/cm³) of fused silica. The lines depicted in the plot show fits according to Eq. 1.1.

The compact line takes E=ES, ρ=ρS and E=0, ρ=0 into account and leads to a scaling exponent n=2.6. Assuming that the Young´s modulus of the solid, ES, the density of the non-porous framework, ρS, and the structure of the differently dried samples are alike, the following scaling exponent was calculated according to Eq. 1.1: n ~ 5.9 (lower limit: n=4;

upper limit: n=8.8) for the samples measured with experimental setup A (dotted line).

Of course this result has to be handled with care. First, the density on the level of the mesopore walls is not identical for the two sample preparations, due to the loss of microporosity in the samples dried with MeOH. Second, an important geometric parameter for defining the stiffness and strength of a cellular material with given density is the connectivity, meaning the number of struts meeting in each node. Here also it is likely that samples dried at higher temperature exhibit stronger connections.

Third, the number and the thickness as well as the effective length of the struts, that is the distance between two nodes, play a significant role.Assuming that the principle morphology of the material is not changed during the drying procedure, the enhanced macroscopic shrinkage of the MeOH-dried samples suggests that the effective length of the struts building the macroscopic network has decreased.

3.6 MECHANICAL PROPERTIES

(b) Mesostructured SiO₂(É)

2μm 2μm

(c) SiO₂aerogel (∆) (a)

8x10^-2 10^-1 1,2x10^-1

10^-4 10^-3 10^-2

Relative Young´s modulus E/E S

Relative density ρ/ρ

S

(b) Mesostructured SiO₂(É)

2μm 2μm 2μm

(c) SiO₂aerogel (∆) (a)

8x10^-2 10^-1 1,2x10^-1

10^-4 10^-3 10^-2

Relative Young´s modulus E/E S

Relative density ρ/ρ

S

Figure 3.6.10. (a) The Young´s modulus of SiO2 with a bimodal pore structure consisting of macropores and periodically arranged mesopores (b) plotted against density. The data is normalized by the Young´s modulus (ES=73 GPa) and the density (ρ_S=2.2 g/cm³) of fused silica. Additionally two points for a SiO2 aerogel without macroporosity and ordered mesoporosity (c) but of similar overall density measured with the resonant beam method [102] are shown for comparison. The compact and the dotted line are the fitting curves according to Eq. 1.1. The compact line takes the points E=ES, ρ=ρS and E=0, ρ=0 into account and leads to an exponent of n=2.6.

Highly porous gels are not completely mechanically connected and densification causes a greater portion of the structure to take the load. Woignier et al [101] report an increase of the elastic moduli of highly porous aerogels with ρⁿ, where n lies between 3 and 4. They claim that the most important parameter is the connectivity which can be enhanced by temperature or isostatic pressure.

The determination of the elastic constants against temperature of a supercritically dried aerogel without ordered mesoporosity and a density of ~0.18 g/cm³ has been performed by the group of Herwig Peterlik [102] using the resonant beam method. This procedure gives values for the modulus of ~ 16 MPa at RT and 100 MPa after being heated up to 700 °C (density change to 0.242 g/cm³). The results obtained by compression tests on monolithic samples in this work agree quite well with the published data for the non-ordered material.

This suggests that a compression test is a quite suitable method to gain information on the macroscopic materials properties. The modulus proves to be of the same order of magnitude for the hierarchical (Figure 3.6.10(b)) and the unordered porous material (Figure 3.6.10(c)). As long as the overall density is comparable, Young´s modulus of the hierarchically material is even slightly less dependent on the apparent density.