Impact of Solvent Structuring in Water/ tert-Butanol Mixtures on the Assembly of Silica Nanoparticles to Aerogels

Hybrid Silver Nanowire-CMC Aerogels: From 1D Nanomaterials to 3D Electrically Conductive and Mechanically Resistant Lightweight Architectures

The directed assembly of nanomaterials into 3D architectures is a powerful tool to produce macroscopic materials with tailored physical properties. We show in this article that such a process can be advantageously performed for the fabrication of lightweight electrically conductive materials. Silver nanowire aerogels (AgNWAs) with very low densities (down to ∼6 mg cm^-3) were ice-templated and freeze-dried, leading to 3D shaped cellular materials based on one-dimensional nanoscopic building blocks. Due to their intrinsic moderate mechanical resistance, the potential use of pure AgNWAs in real life applications appears rather limited. We demonstrate that the addition of carboxymethylcellulose (CMC) in a 1:1 weight ratio leads to the fabrication of hybrid aerogels with highly improved mechanical properties. The molecular weight of the CMC is shown to be a critical parameter to ensure a good dispersion of the AgNWs, and thus to reach excellent performances such as a very low resistivity (0.9 ± 0.2 Ω·cm at 99.2 vol % porosity). The combination of silver nanowires with CMC-700k results in a gain higher than 7100% of the Young's modulus, from 10.4 ± 0.9 kPa (at very low density, i.e., 12 mg cm^-3) for the AgNWAs to 740 ± 40 kPa for the AgNW:CMC aerogel. Electromechanical characterizations allowed us to quantify the piezoelectric properties of these hybrid aerogels. The very good elasticity and the piezoelectric behavior stability up to 100 cycles of compression under high (50%) deformation were revealed, which may be of interest for various applications such as pressure sensors.

Camel back shaped Kirkwood-Buff Integrals

Some binary mixtures, such as specific alcohol-alkane mixtures, or even water-tbutanol, exhibit two humps "camel back" shaped KBI. This is in sharp contrast with usual KBI of binary mixtures having a single extremum. This extremum is interpreted as the region of maximum concentration fluctuations, and usually occurs in binary mixtures presenting appreciable micro-segregation, and corresponds to where the mixture exhibit a percolation of the two species domains. In this paper, it is shown that two extrema occur in binary mixtures when one species forms "meta-particle" aggregates, the latter which act as a meta-species, and have their own concentration fluctuations, hence their own KBI extremum. This "meta-extremum" occurs at low concentration of the aggregate-forming species (such as alcohol in alkane), and is independant of the other usual extremum observed at mid volume fraction occupancy. These systems are a good illustration of the concept of the duality between concentration fluctuations and micro-segregation.

False Morphology of Aerogels Caused by Gold Coating for SEM Imaging

The imaging of non-conducting materials by scanning electron microscopy (SEM) is most often performed after depositing few nanometers thick conductive layers on the samples. It is shown in this work, that even a 5 nm thick sputtered gold layer can dramatically alter the morphology and the surface structure of many different types of aerogels. Silica, polyimide, polyamide, calcium-alginate and cellulose aerogels were imaged in their pristine forms and after gold sputtering utilizing low voltage scanning electron microscopy (LVSEM) in order to reduce charging effects. The morphological features seen in the SEM images of the pristine samples are in excellent agreement with the structural parameters of the aerogels measured by nitrogen adsorption-desorption porosimetry. In contrast, the morphologies of the sputter coated samples are significantly distorted and feature nanostructured gold. These findings point out that extra care should be taken in order to ensure that gold sputtering does not cause morphological artifacts. Otherwise, the application of low voltage scanning electron microscopy even yields high resolution images of pristine non-conducting aerogels.