Fragility and brittleness have always been flaws of delicate three-dimensional aerogel structures. Especially when exposed to harsh conditions (e.g. in aerospace applications), their frailty has limited the broad application of aerogels in such fields. A Chinese team of researchers from the Beijing Jiaotong University has now attended to this matter. By equipping a ZrO₂–SiO₂ aerogel with Polycrystalline ZrO₂ fibers (ZrO_2f), aerogels possessing high thermal and mechanical resilience have been devised. Astonishingly, these monolithic aerogels do not only excel in terms of stability, but also show low bulk densities and thermal conductivities.

The idea of adding a fibrous agent to the aerogel matrix is that, when dispersed evenly throughout the matrix, the fibers act as an additional mechanical backbone. This means that the fibers hinder fracturing and irreversible deformation through fiber-bridging and crack-deflection (see Figure below) when the monolithic structure is being strained. Furthermore, due to the even dispersion of the fibers in the aerogel matrix, the contribution of heat conduction through the fibers is minimized and hence the overall heat conductivity increases only marginally upon addition of the fibrous ZrO₂.

a) Image of ZrO2f/ZrO2-SiO2 aerogel monolith; b)–d) SEM images of fractures of the aerogel composite.

In summary, these effects lead to highly insulating aerogels possessing compressive strengths 3-10 times higher than previously reported. Therefore, applications in very demanding environments are facilitated, which might prove to be significant in aerospace engineering and other fields in which stable and highly insulating materials are essential.

More details: Xianbo Hou, Rubing Zhang and Daining Fang; An ultralight silica-modified ZrO₂–SiO₂ aerogel composite with ultra-low thermal conductivity and enhanced mechanical strength, Scripta Materialia Volume 143, 15 January 2018, Pages 113-116. http://doi.org/10.1016/j.scriptamat.2017.09.028

Researchers from the Kyoto University (Japan) successfully synthesized transparent, machinable, scalable, super-compressible, highly elastic and super-insulating polyvinylpolymethylsiloxane aerogels and xerogels. Remarkably, the study reports that these outstanding features were present not only in aerogels produced using supercritical drying, but in those produced using ambient pressure drying, too.

The sample preparation was achieved by mixing vinylmethyldimethoxysilane (VMDMS) or vinylmethyldiethoxysilane (VMDES) with 1-5 % of di-tert-butyl peroxide (DTBP), to initiate the radical polymerization at 120 °C, yielding a transparent viscous liquid mainly containing polyvinylmethyldimethoxysilane (PVMDMS) or polyvinylmethyldiethoxysilane (PVMDES). Thereafter, BzOH, H₂O and tetramethylammonium hydroxide (TMAOH) were added to the liquid and the mixture was heated to 80 °C for one hour to obtain transparent and flexible gels. Prior to solvent exchange with isopropanol (IPA), the gels were aged between four or five days at 80-100 °C. Removal of the liquid was subsequently accomplished in three different ways (see Figure below): (1) supercritical drying with CO₂; (2) solvent exchange into n-hexane followed by drying at ambient pressure; (3) direct drying from IPA at ambient pressure.

Comparison of different drying methods to obtain PVMDMS or PCMDES aerogels and xerogels.

It was established that key to the intriguing properties of the dried aerogels and xerogels are their homogeneous porous nanostructure, composed of flexible hydrocarbon chains chemically cross-linked with polymethylsiloxanes. Notably, this type of nanostructure structure exhibited low densities (0.16-0.22 g/cm³) and heat conductivities (15.0-15.4 mW/m K), as well as high specific surface areas (900-1000 m²/g), good transparency (>80 % light transmittance), and extraordinary flexibility (see attached video). Additionally, the flexible network structures allowed for a recovery of the evaporation-induced gel shrinkage through a “spring-back” effect (see Figure below), making the supercritical drying step dispensable.

Progression of gel volume during ambient pressure drying. “spring-back” effect yields xerogels of nearly the same volume as the parent gel.

In summary, these findings imply that an ultra-low cost pathway to manufacture aerogels by ambient pressure drying while still preserving extraordinary properties required for applications as superinsulators has been established. This means that one of the main obstacles for the broad application of aerogels — their high manufacturing costs — has been overcome, which might pave the way for their large scale deployment.

More details: Zu et al.; Transparent, Superflexible Doubly Cross-Linked Polyvinylpolymethylsiloxane Aerogel Superinsulators via Ambient Pressure Drying , ACS Nano, January 8, 2018. https://doi.org/10.1021/acsnano.7b07117

Video of aerogel bending test: Click Here

The search for active yet economical water purification strategies is in full swing as increasing industrial activity results in sharp surges in wastewater production, and the ever-growing global population increases demand for clean drinking water.

Commonly, separated, sophisticated absorption processes are deployed to remove either organic or inorganic contaminants from sewage water due to their high efficiency and moderate cost. However, it remains a challenge to devise robust, efficient and economical absorbents for the wide range of trace elements occurring in wastewater. Ideally, novel absorption materials should be able to remove inorganic compounds such as dyes or heavy metals and also be active against harmful viral or bacterial pathogens.

In pursuit of such a material, researchers from Jinan University (China) have synthesized an aerogel structure exhibiting extraordinary dye and heavy metal absorbing properties, by using graphene oxide (GO) and a type of abundant mineral called montmorillonite (MMT). The desired anti-pathogenic activity was realized through equipping the aerogel matrix with a common anti-bacterial agent, resulting in absorbents displaying excellent antibacterial activity against Gram-positive and Gram-negative bacteria.

The aerogel material exhibiting these intriguing properties was manufactured through mixing GO powder, ascorbic acid, and a MMT solution, then inducing gelation through heat treatment at 95 °C. After aging of the hydrogel in a PVA solution for two days, the gel was then freeze dried at -55 °C, resulting in a monolithic aerogel structure, which is shown in the Figure below.

Image of black GO-MMT aerogel placed on top of kapok tree fiber.

Absorption experiments showed that the aerogel absorbents were not only able to remove more than 95 % of methyl orange and methylene blue dyes from aqueous solutions, but also exhibited great properties for the removal of heavy metals from water (e.g. >90 % removal efficiency for chromium ion removal). This activity was found to be stable over numerous absorption/desorption cycles, with sample regeneration being achieved by vigorous shaking. Furthermore, the addition of antibacterial dodecyl dimethyl benzyl ammonium chloride (1227) to the initial precursor solution was found to provide the aerogel with antibacterial activity, which was shown using E. coli and S. aureus bacteria cultures, each losing over 90 % of their cell viability in the presence of the GO-MMT-1227 aerogel material.

Due to these extraordinary findings, the researchers are confident that they have found an efficient, versatile, recyclable, and robust absorbent material, which has the potential to revolutionize water purification. If economical large scale manufacturing and long term stability can be achieved, the novel material might indeed replace state-of-the-art sorbents in wastewater treatment systems.

More details: Yunyun Zhang et al.; The utilization of a three-dimensional reduced graphene oxide and montmorillonite composite aerogel as a multifunctional agent for wastewater treatment, RSC Adv., 2018,8, 4239-4248. https://doi.org/10.1039/C7RA13103H

The enhancing effects of polyaniline (PANi) coatings on ion and electron conductivity are well known (see also here), which is why researchers from the Sun Yat-sen University (China) devised PANi-coated activated carbon/sulfur aerogels (ACA-500-S) in order to improve the electrochemical properties of the un-coated composite. By synthesizing this novel electrode material, the research team hopes to address drawbacks of Li-S batteries (e.g. low ionic conductivity, dissolution of polysulfides in electrolytes).

Carbon/sulfur composite aerogels were produced by heat-melting state-of-the-art activated carbon aerogels (ACA-500) in the presence of elemental sulfur. As shown in the Figure below, the PANi coating was subsequently applied by in-situ chemical oxidative polymerization at low temperatures, yielding the final ACA-500-S@PANi electrode material. Investigation of the sample morphology via FESEM and TEM revealed that the manufacturing technique led to a homogeneous distribution of sulfur and PANi on the aerogel surface. Because of the even dispersion of the active agents on the aerogel matrix, enhanced electrochemical performance was expected from the ACA-500-S@PANi composite.

Schematic of synthesizing process of ACA-500-S@PANi aerogels from state-of-the-art activated carbon aerogels (ACA-500)

This expectation was confirmed in ensuing experiments, which compared the specific capacity, capacity discharge, and capacity retention of the ACA-500-S and ACA-500-S@PANi materials. It was shown that the latter material outperformed the carbon/sulfur aerogel in all regards, which was attributed to the synergistic effect of the carbon aerogel matrix and the PANi coating on ion and electron conductivity as well as electrode stability. More specifically, the researchers discovered that the ACA-500-S@PANi structure presented high reversible capacities at low (at 0.2 C: 1208 mAh g⁻¹) and high (at 3.0 C: 542 mAh g⁻¹) discharge rates. Furthermore, long term cycling at 1.0 C demonstrated that it exhibits a notable initial discharge capacity of 926 mAh g^-1 and outstanding capacity retention of over 65 % at a low capacity decay rate of 0.48 ‰ per cycle.

Because of these findings, the researchers are confident that the novel aerogel composite will be able to boost the performance of lithium-sulfur batteries, meeting the increasing demand for batteries exhibiting a high energy density and long life span at low cost.

More details: Tang et al.; Polyaniline-Coated Activated Carbon Aerogel/Sulfur Composite for High- performance Lithium-Sulfur Battery, Nanoscale Research Letters 2017. https://doi.org/10.1186/s11671-017-2372-6

Although we might not recognize it, electronic sensors have developed into essential components in our everyday life. They provide our smartphones with necessary information, guarantee safe travels in our cars and ensure that our homes are adequately air conditioned. With increasing automatization in the industrial and private sector (e.g. internet of things, smart home, autonomous driving, etc.), the importance of those tiny helpers for our prosperity, well-being and safety will only increase. Therefore, it will be essential to develop more efficient, powerful and complex sensors, which are able to process information in a reliable fashion.

Researchers from the Linköping University (Sweden), were now able to manufacture a type of thermoelectric aerogel, which is able to simultaneously sense pressure and temperature. Moreover, the pressure and temperature signals are decoupled, allowing for a reliable synchronous measuring of both parameters.

The dual-parameter PNG aerogels sensors were synthesized from a mixture of poly3,4-ethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS), nano-fibrillated cellulose (NFC) and glycidoxypropyl trimethoxysilane (GOPS). This precursor selection led to an elastic, yet robust thermoelectric PNG aerogel, being able to sense pressure changes (NFC & GOPS) and temperature variations (PEDOT:PSS). Post-treatment in dimethylsulfoxide (DMSO) vapor not only increased the pressure sensitivity of the aerogel, but also resulted in a decoupling of the temperature and pressure signal by changing the internal charge carrier transport through the material.

Testing of the aerogel composite in an experimental setup, schematically depicted below, showed that pressure and temperature gradient could be obtained from two independent parameters of the current-voltage curve. The pressure signal is indicated by the slope, whereas the y-intercept denotes the temperature gradient across the aerogel (see also Figure below).

a) Schematic of experimental setup to test the dual-parameter sensor made of PNG aerogel. b) I–V curves obtained for a constant pressure (230 Pa) and different temperatures after 10 min DMSO treatment.

These revolutionary findings could pave the way for further research on multi parameter sensing aerogels, facilitating innovations which rely on powerful sensors. The Swedish research team already anticipates the utilization of their novel dual scale PNG aerogel in e-skin applications.

More details: Han et al.; Thermoelectric Polymer Aerogels for Pressure–Temperature Sensing Applications, Advanced Functional Materials, Volume 27, Issue 44 November 24, 2017. https://doi.org/10.1002/adfm.201703549,

Annually, the Swiss Federal Office of Energy awards the Watt d’Or Award to people, companies and organizations that “develop the energy technologies for the future, bring innovative products onto the market and set new standards for practical solutions that unite energy and environment awareness with comfort requirements, aesthetics and economic interests”.
This year, a prototypical aerogel-insulated apartment building, devised by the Zurich-based architectural office Dietrich Schwarz has been awarded the Watt d’Or in the category Buildings and Space.

With new challenges in terms of energy efficiency and space requirements arising, architects are faced with a fundamental conflict — providing highly effective insulation at constant or even slimmer wall thicknesses. The only escape from this dilemma are advancements in insulation materials, yielding scalable structures of extremely low thermal conductivity.
Aerogels are one type of material promising exactly those required characteristics and therefore are generally considered to possess great merit for the future building insulation market.

Award-winning six-floor apartment building concept by the architectural office Dietrich Schwarz, located in Hohlstrasse 100, Zurich (Switzerland)

Due to these intriguing insulating properties of aerogels, the architects of Dietrich Schwarz (Switzerland) have selected aerogel-equipped wood elements to insulate the exterior facade of their latest award-winning project in Zurich (see image above). Thereby, the building floor space was maximized without jeopardizing energetic requirements placed on modern architecture. Additionally, vacuum-insulated windows, phase-change materials in the facades reducing the required cooling and heating demands, rooftop PV panels and a thermal heat pump complete the holistic approach to reduce the energy intensity of modern housing.
Another eye-catching, futuristic feature of the apartment block are its convex oriels, providing sound insulation from the noisy street. Through these elements regular room ventilation via opened windows can be achieved without experiencing excessive noise pollution.

In light of the abundance of novel architectural elements, providing a high level of comfort at vastly reduced energetic footprint, found in this building concept, the selection of the jury of the Swiss Federal Office of Energy does not come as a surprise.
If building concepts such as the one by Dietrich Schwarz will prove their worth, it will only be a matter of time until aerogel-based insulation materials will become a market standard.

Read more at:
https://www.tagesanzeiger.ch/zuerich/stadt/zuercher-architekten-ueberzeugen-mit-ultraduenner-daemmung/story/21704599
https://www.baublatt.ch/verschiedenes/watt-dor-2018-ein-intelligentes-licht-fuer-tier-und-mensch

Announcing the Fourth International Seminar on AEROGELS-2018.

Click here for further details.

Finding non-cytotoxic carrier materials that ensure a controlled release of an active hydrophobic drug over a long period of time is considered to be the “holy grail” in wound dressing applications. A team of researchers from Brazil and the USA report to have found a material that combines all of those desired characteristics.
The novel silica-polymer hybrid (SPH) aerogels, synthesized from silica nanoparticles, polyvinyl alcohol (PVA), polyacrylic acid (PAA), and water, were manufactured via freeze-drying and subsequent thermal treatment (see Figure below). While the freeze-drying ensured a complete removal of water, the ensuing thermal treatment at 160 °C facilitated the cross-linking between PVA and PAA, guaranteeing aerogel stability in aqueous media.

Schematic of manufacturing process of silica-polymer hybrid (SPH) aerogels.

Experiments conducted with dexamethasone (DEX), an agent used for the treatment of skin diseases, allergies, and rheumatic problems, showed that the SPH aerogels exhibit high drug encapsulation efficiencies, taking up around 75 % of DEX from an ethanol/water mixture within 24 hours. This feature was ascribed to the trapping of DEX molecules within the mesoporous silica nanoparticles.
The release rate of DEX from the aerogels was investigated by placing the loaded samples into a stirred phosphate buffered saline (PBS) solution at 37 °C and measuring the progression of the DEX concentration with time. These measurements showed that after an initial rapid discharge of DEX, the release rate leveled off, so that even after two months a steady drug discharge was obtained. This slow and prolonged release of DEX molecules was attributed to the polymer pore structure, limiting the mass transfer from the drug encapsulation site (silica nanoparticles) to the solution. Supporting this finding were results for SPH aerogels of a different constitution, which showed that a change in PVA:PAA ratio, yielding morphological modifications of the polymer pore network, leads to a significant change in drug release behavior.
In order to assess the biocompatibility and cytotoxicity of the synthesized material, the cell viability of vero cells and L929 fibroblasts was investigated in the presence of the SPH aerogels. This line of experiment showed that there is virtually no decrease in cell viability for either cell type after 72 hours, regardless of precursor selection.
The authors see potential applications of SPH aerogels in the treatment of skin burns or melanoma, which require wound dressing over extended periods of time. Furthermore, they see potential in tailoring the aerogel structures (e.g. with antibodies) to further extend their potential biological and medical applications.

More details: Follmann et al.; Multifunctional Hybrid Aerogels: Hyperbranched Polymer- Trapped Mesoporous Silica Nanoparticles for Sustained and Prolonged Drug Release, Nanoscale, December 2017. http://doi.org/10.1039/C7NR08464A

Coal fired power plants will play a substantial role in energy production for the majority of industrialized nations until at least the middle of this century. This is an issue since the large-scale combustion of carbon-based resources is a major contributor to the rising atmospheric CO₂ levels. In order to achieve the ambitious multilateral goals to lessen the detrimental effects of global warming by stabilizing the global CO₂ levels outlined in the Paris Agreement, the greenhouse gas emissions from coal fired power plants will need to be reduced drastically.

One way to achieve this reduction in emitted greenhouse gases without abandoning coal-based power production is the capturing and sequestration of emitted CO₂ (see graphic below). This can be done by retro-fitting existing power plants with carbon capture and storage (CCS) units, which remove carbon dioxide from flue gas, before storing it in a designated location. Most commonly, temperature-swing adsorption (TSA) processes, utilizing aqueous amine solutions as sorbents, are proposed for CO₂ capturing. However, the regeneration of the amine solution results in an energy penalty which drastically reduces the thermal efficiency of the power plant. Therefore, alternative sorbents requiring less energy for regeneration are under investigation.

Diagram of carbon capture and storage life cycle.
From: Scottish carbon capture and storage

In a joint effort, a project team consisting of Aspen Aerogels, the University of Akron, ADA-ES, and Longtail Consulting have synthesized and tested solid amine functionalized aerogels (AFA) in a bench scale fluidized bed reactor, in order to assess their potential for future application in CCS.

It was found that the AFAs synthesized by Aspen Aerogels showed promising CO₂ adsorption behavior and good stability over numerous adsorption-desorption cycles. On top of that, the novel solid sorbents possess significantly lower heats of reaction with CO₂ than commonly deployed liquid and solid sorbents. In a subsequent step, the AFAs were coated at the University of Akron, yielding pellets possessing a good cyclic stability in the presence of SO₂. The novel AFA pellets were then tested in a bench scale fluidized bed reactor to determine their physical properties (e.g. fluidizing gas velocities, void fraction, etc.) in such a reactor setup.

Based on those findings, Longtail Consulting modeled the hydrodynamic and heat transfer properties of the solid aerogel sorbent in the fluidized bed reactor and finalized the process requirements. Subsequently, a techno-economic analysis (TEA) of the entire CO₂ capturing unit (assuming 90 % capturing efficiency) fitted to a coal fired, supercritical steam cycle power plant producing 550 MW_e was performed. Despite offering a promising behavior from a technological perspective, the TEA revealed that utilizing novel AFA sorbents results in approx. 20 % higher levelized electricity costs. This finding was mainly attributed to the fact that the attrition of the material during fluidization was unknown, yielding high variable costs for the selected scenario. Additionally, a relatively high apparent particle density inside the fluidized be reactor led to a  massive pressure drop, causing a surge in auxiliary electricity input.

In light of these findings, the project team concluded that further testing under practical conditions and simultaneous optimization of the AFAs will be essential to make solid sorbents an economical alternative to state-of-the-art aqueous amines.

As the need for creative solutions to reduce greenhouse gas emissions is only increasing, it is promising to see that the vast potential of aerogels is being explored to address climate change. Whether or not solid sorbents can be successfully utilized in TSA in an economically reasonable way may prove to be one of the key factors determining the success of CCS.

Full Report: https://doi.org/10.2172/1349123

Read more at: https://www.osti.gov/scitech/biblio/1349123