Nano-particulate Silica Hydrogels Imaged in 2D and 3D using TEM: Effects of local pore structure on mass transport and applications in aggregation dynamics
Sammanfattning: Porous materials are frequently used in everyday applications including food science, pharmaceuticals, fuel cell membranes, batteries and solar cells. A key aspect in these porous materials is how their micro- and nano-structure affect the internal transport of for example water, particles or charges. The transport is a three-dimensional process. Therefore, structure information in three dimensions is especially important when analysing and further improving as well as developing new types of porous materials. We used Electron Tomography (ET) to directly reveal and quantify the three-dimensional structure of one type of soft porous material on the nano-scale.
Three different nanoscale Particulate Silica Hydrogel (PSH) samples were synthesised and examined regarding the relationship of material nanostructure to both mass transport and particle aggregation dynamics. The PSH samples had equal percentage of silica, but had particle sizes ranging from 3.6 nm to 22 nm, and different aggregation parameters which rendered structural differences between the samples.
The samples were imaged in two dimensions using Scanning Transmission Electron Microscopy (STEM) and in three dimensions using ET. The two-dimensional STEM micrographs were examined using stereological and geometrical image analysis. The water transport properties of the PSHs were studied by direct measurements, Nuclear Magnetic Resonance (NMR) diffusometry and by in situ Lattice-Boltzmann Modelling (LBM).
In both two and three dimensions, we determined the local structure of the different PSHs. We quantified the characteristics of the PSHs concerning primary particle shape in two dimensions and geometrical pore size distribution in two and three dimensions. Here, we highlight both the nanostructure of the particle network and the connectivity of the pore network, seeing indications that the finer two gels had more narrow pore throats than the coarser gel. The pore size distribution had a minimal impact on water permeability below a pore size of 120 nm. Despite the almost identical flow properties of the two finer gels, they showed large differences concerning the accessible pore volume fraction in relation to their mean pore size. The LBM simulations showed higher values than the experiments for the permeability of the gels. Hence, a comparative study of simulated PSH sections was performed, from which it was clear that the simulation templates needed to be thicker in order to provide values more coherent to the experimental data.
We have proposed and demonstrated an approach for estimating the three-dimensional structure from two-dimensional STEM micrographs, using the intensity profile as a structure parameter in the third dimension. From the 2D data, we concluded that the Reaction Limited Cluster Aggregation process (RLCA) explains the structure of the particle network in the hydrogels better than the Diffusion Limited Cluster Aggregation process (DLCA) does. However, there is not a perfect match. Our preliminary results show that the acquired three-dimensional data agrees significantly better to the RLCA model compared to the two-dimensional data.
Here, the liquid flow properties of PSHs and the relation between liquid flow and nanoscale structure, including the interconnectivity of the pore network, is addressed. Thus, contributing to the field of mass transport in nanoscale porous materials. We also predict ET to be applied more frequently in the field of particulate hydrogels, including e.g. controlled release, battery and biomedical applications.
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