Merging flexibility with superinsulation : machinable, nanofibrous pullulan-silica aerogel composites

Freeze-dried nanofibrous scaffolds are flexible, but typically have high thermal conductivities. Conversely, silica aerogel has an ultra-low thermal conductivity, but is brittle. Here, the impregnation of pullulan/PVA nanofiber scaffolds with hydrophobic silica aerogel decreased the thermal conductivity from 31.4 to 17.7 mW/(m·K). The compatibility between the silylated nanofibers and the silica aerogel promotes the overgrowth of silica particles onto the fiber surfaces and the fiber incorporation. The composites display improved compressive and tensile properties compared to the neat pullulan scaffold and silica aerogel. The composite's E-modulus is 234 kPa compared to 4 kPa for the pullulan scaffold and 102 kPa for the silica aerogel. The composite's tensile strength is five times higher than that of the silica aerogel. Because of its reduced brittleness, the pullulan-silica aerogel composites can be shaped using a sharp blade. The composites can sustain uniaxial compression up to 80% strain, but the decompressed composites display two times higher densities because the strain is partially irreversible. This densification reduces thermal conductivity to 16.3 mW/(m·K) and increases final compressive strength by a factor of seven. Both the as prepared and densified composites demonstrate unique material properties in terms of thermal conductivity, mechanical strength and machinability

Reinforced and superinsulating silica aerogel through in situ cross-linking with silane terminated prepolymers

Silica aerogels have only half the thermal conductivity of conventional insulation, but their application potential is limited by the poor mechanical properties. The fragility arises from the thin necks between the silica nanoparticle building blocks. Here, we produce strong silica aerogels through co-gelation of the polyethoxydisiloxane precursor with a variety of silane terminated prepolymers that reinforce the inter- particle necks, followed by hydrophobization and supercritical CO2 drying. All prepolymers enabled the synthesis of aerogels with excellent thermal and mechanical properties, but the shortest prepolymer (∼2–3 nm long) yielded the best results. The hybrid aerogels can sustain uniaxial compression without brittle rupture to at least 80% strain for all prepolymer concentrations (5–50 wt%), leading to a final strength of up to 21 MPa, an E modulus up to 3.4 MPa, and an up to 400 times lower dust release rate. In contrast to classical reinforcement strategies, the mechanical improvement does not come with a penalty in thermal conductivity, which remains between 14 and 17 mW m−1 K−1. The hybrid aerogels are a unique class of superinsulating materials with superior thermal and mechanical properties and a scalable production process

Energy in buildings: Efficiency, renewables and storage

This lecture summary provides a short but comprehensive overview on the “energy and buildings” topic. Buildings account for roughly 40% of the global energy demands. Thus, an increased adoption of existing and upcoming materials and solutions for the building sector represents an enormous potential to reduce building related energy demands and greenhouse gas emissions. The central question is how the building envelope (insulation, fenestration, construction style, solar control) affects building energy demands. Compared to conventional insulation materials, superinsulation materials such as vacuum insulation panels and silica aerogel achieve the same thermal performance with significantly thinner insulation layers. With low-emissivity coatings and appropriate filler gasses, double and triple glazing reduce thermal losses by up to an order of magnitude compared to old single pane windows, while vacuum insulation and aerogel filled glazing could reduce these even further. Electrochromic and other switchable glazing solutions maximize solar gains during wintertime and minimize illumination demands whilst avoiding overheating in summer. Upon integration of renewable energy systems into the building energy supply, buildings can become both producers and consumers of energy. Combined with dynamic user behavior, temporal variations in the production of renewable energy require appropriate storage solutions, both thermal and electrical, and the integration of buildings into smart grids and energy district networks. The combination of these measures allows a reduction of the existing building stock by roughly a factor of three —a promising, but cost intensive way, to prepare our buildings for the energy turnaround

Aerogels Handbook

Aerogels are the lightest solids known. Up to 1000 times lighter than glass and with a density as low as only four times that of air, they show very high thermal, electrical and acoustic insulation values and hold many entries in Guinness World Records. Originally based on silica, R&D efforts have extended this class of materials to non-silicate inorganic oxides, natural and synthetic organic polymers, carbon, metal and ceramic materials, etc. Composite systems involving polymer-crosslinked aerogels and interpenetrating hybrid networks have been developed and exhibit remarkable mechanical strength and flexibility. Even more exotic aerogels based on clays, chalcogenides, phosphides, quantum dots, and biopolymers such as chitosan are opening new applications for the construction, transportation, energy, defense and healthcare industries. Applications in electronics, chemistry, mechanics, engineering, energy production and storage, sensors, medicine, nanotechnology, military and aerospace, oil and gas recovery, thermal insulation and household uses are being developed with an estimated annual market growth rate of around 70% until 2015. The Aerogels Handbook summarizes state-of-the-art developments and processing of inorganic, organic, and composite aerogels, including the most important methods of synthesis, characterization as well as their typical applications and their possible market impact. Readers will find an exhaustive overview of all aerogel materials known today, their fabrication, upscaling aspects, physical and chemical properties, and most recent advances towards applications and commercial products, some of which are commercially available today. Key Features: •Edited and written by recognized worldwide leaders in the field •Appeals to a broad audience of materials scientists, chemists, and engineers in academic research and industrial R&D •Covers inorganic, organic, and composite aerogels •Describes military, aerospace, building industry, household, environmental, energy, and biomedical applications among other

Monolithic resorcinol–formaldehyde alcogels and their corresponding nitrogen-doped activated carbons

Here we report the adaptation of formaldehyde crosslinked phenolic resin-based aerogel and xerogel synthesis to ethanol-based solvent systems. Three specific formulations, namely one resorcinol–formaldehyde (RF) and two resorcinol– melamine–formaldehyde (RMF) systems were studied. As-prepared resins were characterized in terms of envelope and skeletal density. Furthermore, resin samples were pyrolyzed and activated in a CO2 gas atmosphere using a single-step protocol. The corresponding carbon materials featured high surface areas, moderate water uptake capacity and thermal conductivities in the 0.1 W.m−1K−1 range, in line with comparable activated carbons. The amount of formaldehyde in the synthesis of the RMF derived carbons proved to be a critical parameter in terms of both structural features and amount of N dopant in the carbonaceous matrix. Furthermore, a high formaldehyde concentration also has a drastic effect on the pore structure of the corresponding RMF carbons, leading primarily to mesopore formation without almost any macropore formation. Perhaps more importantly, the effect of the ammonia curing catalyst concentration on the material microstructure showed the opposite effect as observed in classical, water-based phenolic resin preparations. The ethanol-based synthesis clearly affects the pore structure of the resulting materials but also opens up the possibility to create inorganic/organic hybrid materials by simple combination with classical alkoxide-based silica sol–gel chemistry

Biopolymer Aerogels and Foams: Chemistry, Properties, and Applications

ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

Merging flexibility with superinsulation : machinable, nanofibrous pullulan-silica aerogel composites

Freeze-dried nanofibrous scaffolds are flexible, but typically have high thermal conductivities. Conversely, silica aerogel has an ultra-low thermal conductivity, but is brittle. Here, the impregnation of pullulan/PVA nanofiber scaffolds with hydrophobic silica aerogel decreased the thermal conductivity from 31.4 to 17.7 mW/(m·K). The compatibility between the silylated nanofibers and the silica aerogel promotes the overgrowth of silica particles onto the fiber surfaces and the fiber incorporation. The composites display improved compressive and tensile properties compared to the neat pullulan scaffold and silica aerogel. The composite's E-modulus is 234 kPa compared to 4 kPa for the pullulan scaffold and 102 kPa for the silica aerogel. The composite's tensile strength is five times higher than that of the silica aerogel. Because of its reduced brittleness, the pullulan-silica aerogel composites can be shaped using a sharp blade. The composites can sustain uniaxial compression up to 80% strain, but the decompressed composites display two times higher densities because the strain is partially irreversible. This densification reduces thermal conductivity to 16.3 mW/(m·K) and increases final compressive strength by a factor of seven. Both the as prepared and densified composites demonstrate unique material properties in terms of thermal conductivity, mechanical strength and machinability

Strong, machinable, and insulating chitosan–urea aerogels: toward ambient pressure drying of biopolymer aerogel monoliths

Biopolymer aerogels are an emerging class of materials with potential applications in drug delivery, thermal insulation, separation, and filtration. Chitosan is of particular interest as a sustainable, biocompatible, and abundant raw material. Here, we present urea-modified chitosan aerogels with a high surface area and excellent thermal and mechanical properties. The irreversible gelation of an acidic chitosan solution is triggered by the thermal decomposition of urea at 80 °C through an increase in pH and, more importantly, the formation of abundant ureido terminal groups. The hydrogels are dried using either supercritical CO2 drying (SCD) or ambient pressure drying (APD) methods to elucidate the influence of the drying process on the final aerogel properties. The hydrogels are exchanged into ethanol prior to SCD, and into ethanol and then heptane prior to APD. The surface chemistry and microstructure are monitored by solid-state NMR and Fourier transform infrared spectroscopy, scanning electron microscopy, and nitrogen sorption. Surprisingly, large monolithic aerogel plates (70 × 70 mm2) can be produced by APD, albeit at a somewhat higher density (0.17–0.42 g/cm3). The as prepared aerogels have thermal conductivities of ∼24 and ∼31 mW/(m·K) and surface areas of 160–170 and 85–230 m2/g, for SCD and APD, respectively. For a primarily biopolymer-based material, these aerogels are exceptionally stable at elevated temperature (TGA) and char and self-extinguish after direct flame exposure. The urea-modified chitosan aerogels display superior mechanical properties compared to traditional silica aerogels, with no brittle rupture up to at least 80% strain, and depending on the chitosan concentration, relatively high E- moduli (1.0–11.6 MPa), and stress at 80% strain values (σ80 of 3.5–17.9 MPa). Remarkably, the aerogel monoliths can be shaped and machined with standard tools, for example, drilling and sawing. This first demonstration to produce monolithic and machinable, mesoporous aerogels from bio-sourced, renewable, and nontoxic precursors, combined with the potential for reduced production cost by means of simple APD, opens up new opportunities for biopolymer aerogel applications and marks an important step toward commercialization of biopolymer aerogels