Marina Schwan

Flexibilisation of resorcinol–formaldehyde aerogels

Ambient pressure dried resorcinol–formaldehyde (RF) organic aerogels are usually hard and brittle with Youngs moduli in the range of 1–2 MPa, strengths of about 100 kPa and densities in the range of 0.2 to 0.4 g cm−3. Modifications of the classical sol–gel synthesis route transform these brittle materials into rubber- or cork-like flexible aerogels. We observed that in a small window of process parameters, the aerogel density decreases by an order of magnitude as well as the Youngs moduli and the compression strengths. These new types of RF aerogels are elastically deformable by more than 40% in an almost reversible manner. In this paper we describe the effects of various sol–gel parameters on the flexibility, such as resorcinol to water and catalyst molar ratio and the pH of the initial solution. The aerogels are characterized with respect to the envelope density, stress–strain behavior and microstructure as observed from SEM fractographs. The chemical structure and structural differences arising between brittle and flexible RF aerogels were studied by recording 13C-NMR spectra.

From Fragile to Resilient Insulation: Synthesis and Characterization of Aramid-Honeycomb Reinforced Silica Aerogel Composite Materials

The production of a new composite material embedding aramid honeycomb materials into nano-porous silica aerogels is studied. Our aim is to improve the poor mechanical strength of silica aerogels by aramid honeycombs without losing the amazing properties of the aerogels like little density and low thermal conductivity. The composite materials were prepared using two formulations of silica aerogels in combination with aramid honeycomb materials of different cell sizes. The silica aerogels are prepared using silicon alkoxides methyltrimethoxysilane and tetraethylorthosilicate as precursors in a two-step acid–base sol–gel process. Shortly in advance of the gelation point, the aramid honeycombs were fluted by the sol, gelation occurred and, after the aging process, the gel bodies were supercritically dried. The properties of the received composite materials are satisfying. Even the thermal conductivities and the densities are a bit higher than for pure aerogels. Most importantly, the mechanical strength is improved by a factor of 2.3 compared to aramid honeycomb materials and by a factor of 10 compared to the two silica aerogels themselves. The composite materials have a good prospective to be used as an impressive insulation material.

The three-dimensional structure of flexible resorcinol-formaldehyde aerogels investigated by means of holotomography

Organic aerogels based on resorcinol-formaldehyde gels display remarkable properties due to their pronounced nanoporosity. Therefore, studies towards the understanding of their structure-property-relationship are of high value for the design of improved materials. X-ray tomography is a technique that has been used for the structural elucidation of porous materials, but so far no highly resolved three-dimensional structures of resorcinol-formaldehyde gels have been obtained under the classical absorption-based experimental X-ray setup. This paper reports on the successful analysis of a superflexible resorcinol-formaldehyde aerogel using zoom holotomography that yielded images with an unprecedented resolution in the sub-micrometer range. The preparation of suitable powder from monolithic superflexible resorcinol-formaldehyde, the experimental conditions for tomography, and data-processing to obtain a 3D-image of the dried gel sample are described. Macropores above ca. 75 nm could be identified and visualized. They were shown to adopt almost spherical shape and to display a low connectivity. A quantitative analysis of the pore space revealed that most of the identified pores are small macropores (diameter < 0.5 µm), yet most pore volume is located in larger macropores of 1–4 µm diameter.Graphical Abstract