Editorial

3 pages, 192 KiB

Open AccessEditorial

Editorial on Special Issues “Aerogels” and “Aerogels 2018”

by Françoise Quignard and Nathalie Tanchoux

Gels 2020, 6(3), 19; https://doi.org/10.3390/gels6030019 - 29 Jun 2020

Viewed by 1994

Aerogels can be defined as ultralight materials with a 3D porous structure, similar to their parent wet gels, where the solvent has been replaced by a gas without a collapse of the gel structure, thanks to the drying process used (supercritical CO₂ [...] Read more.

Aerogels can be defined as ultralight materials with a 3D porous structure, similar to their parent wet gels, where the solvent has been replaced by a gas without a collapse of the gel structure, thanks to the drying process used (supercritical CO₂ drying, freeze drying, etc [...] Full article

(This article belongs to the Special Issue Aerogels 2018)

Research

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16 pages, 4240 KiB

Open AccessFeature PaperArticle

Influence of Structure-Directing Additives on the Properties of Poly(methylsilsesquioxane) Aerogel-Like Materials

by Marta Ochoa, Alyne Lamy-Mendes, Ana Maia, António Portugal and Luísa Durães

Gels 2019, 5(1), 6; https://doi.org/10.3390/gels5010006 - 28 Jan 2019

Cited by 9 | Viewed by 3270

Abstract

The effect of glycerol (GLY) and poly(ethylene glycol) (PEG) additives on the properties of silica aerogel-like monoliths obtained from methyltrimethoxysilane (MTMS) precursor was assessed. The tested molar ratios of additive/precursor were from 0 to 0.1 and the lowest bulk densities were obtained with [...] Read more.

The effect of glycerol (GLY) and poly(ethylene glycol) (PEG) additives on the properties of silica aerogel-like monoliths obtained from methyltrimethoxysilane (MTMS) precursor was assessed. The tested molar ratios of additive/precursor were from 0 to 0.1 and the lowest bulk densities were obtained with a ratio of 0.025. When a washing step was performed in the sample containing the optimum PEG ratio, the bulk density could be reduced even further. The analysis of the material’s microstructure allowed us to conclude that GLY, if added in an optimum amount, originates a narrower pore size distribution with a higher volume of mesopores and specific surface area. The PEG additive played a binder effect, leading to the filling of micropores and the appearance of large pores (macropores), which caused a reduction in the specific surface area. The reduction of the bulk density and the microstructural changes in the aerogels induced by adding a small amount of these additives confirm the possibility of fine control of properties of these lightweight materials. The achieved high porosity (97%) and low thermal conductivity (~35 mW·m⁻¹·K⁻¹) makes them suitable to be used as thermal insulators. Full article

(This article belongs to the Special Issue Aerogels 2018)

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11 pages, 3630 KiB

Open AccessArticle

Millimeter-Size Spherical Polyurea Aerogel Beads with Narrow Size Distribution

by Despoina Chriti, Grigorios Raptopoulos, Maria Papastergiou and Patrina Paraskevopoulou

Gels 2018, 4(3), 66; https://doi.org/10.3390/gels4030066 - 06 Aug 2018

Cited by 20 | Viewed by 5337

Abstract

We report the room temperature synthesis of spherical millimeter-size polyurea (PUA) aerogel beads. Wet-gels of said beads were obtained by dripping a propylene carbonate solution of an aliphatic triisocyanate based on isocyanurate nodes into a mixture of ethylenediamine and heavy mineral oil. Drying [...] Read more.

We report the room temperature synthesis of spherical millimeter-size polyurea (PUA) aerogel beads. Wet-gels of said beads were obtained by dripping a propylene carbonate solution of an aliphatic triisocyanate based on isocyanurate nodes into a mixture of ethylenediamine and heavy mineral oil. Drying the resulting wet spherical gels with supercritical fluid (SCF) CO₂ afforded spherical aerogel beads with a mean diameter of 2.7 mm, and a narrow size distribution (full width at half maximum: 0.4 mm). Spherical PUA aerogel beads had low density (0.166 ± 0.001 g cm^–3), high porosity (87% v/v) and high surface area (197 m² g^–1). IR, ¹H magic angle spinning (MAS) and ¹³C cross-polarization magic angle spinning (CPMAS) NMR showed the characteristic peaks of urea and the isocyanurate ring. Scanning electron microscopy (SEM) showed the presence of a thin, yet porous skin on the surface of the beads with a different (denser) morphology than their interior. The synthetic method shown here is simple, cost-efficient and suitable for large-scale production of PUA aerogel beads. Full article

(This article belongs to the Special Issue Aerogels 2018)

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9 pages, 1569 KiB

Open AccessArticle

Prevention of the Aggregation of Nanoparticles during the Synthesis of Nanogold-Containing Silica Aerogels

by István Lázár and Hanna Judit Szabó

Gels 2018, 4(2), 55; https://doi.org/10.3390/gels4020055 - 19 Jun 2018

Cited by 15 | Viewed by 4821

Abstract

Nanogold is widely used in many areas of physics and chemistry due to its environment-sensitive plasmon resonance absorption. The immobilization of gold nanoparticles in highly porous silica aerogel offers an attractive alternative to liquid gold solutions as they show a mechanically stable structure, [...] Read more.

Nanogold is widely used in many areas of physics and chemistry due to its environment-sensitive plasmon resonance absorption. The immobilization of gold nanoparticles in highly porous silica aerogel offers an attractive alternative to liquid gold solutions as they show a mechanically stable structure, are permeable to gases, and can even be used at elevated temperatures. We have found that the commercially available citrate-stabilized 10 nm gold nanoparticles may suffer from aggregation prior to or under the base-catalyzed gelation process of tetramethoxy silane. In the wet gels, Au particles increased in size, changed shape, and demonstrated the loss of plasmon resonance absorption, due to the formation of larger aggregates. We have studied a range of water-miscible organic solvents, stabilizing agents, and the gelation conditions to minimize changes from occurring in the aerogel setting and the supercritical drying process. It has been found that atmospheric carbon dioxide has a significant effect on aggregation, and it cannot be entirely excluded under normal synthetic conditions. Methanol resulted in an increase in the particle size only, while dimethyl sulfoxide, dimethylformamide, and urea changed the shape of nanoparticles to rod-like shapes, and diols led to an increase in both size and shape. However, using the polymeric stabilizer poly(vinyl pyrrolidone) efficiently prevented the aggregation of the particles, even in the presence of high concentrations of carbon dioxide, and allowed the production of nanoAu containing silica aerogels in a single step, without the modification of technology. Full article

(This article belongs to the Special Issue Aerogels 2018)

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13 pages, 3422 KiB

Open AccessArticle

Advanced Recycled Polyethylene Terephthalate Aerogels from Plastic Waste for Acoustic and Thermal Insulation Applications

by Hong Wei Koh, Duyen K. Le, Gek Nian Ng, Xiwen Zhang, Nhan Phan-Thien, Umeyr Kureemun and Hai M. Duong

Gels 2018, 4(2), 43; https://doi.org/10.3390/gels4020043 - 17 May 2018

Cited by 63 | Viewed by 9230

Abstract

This work presents for the first time, a simple, practical and scalable approach to fabricating recycled polyethylene terephthalate (rPET) aerogels for thermal and acoustic insulation applications. The rPET aerogels were successfully developed from recycled PET fibers and polyvinyl alcohol (PVA) and glutaraldehyde (GA) [...] Read more.

This work presents for the first time, a simple, practical and scalable approach to fabricating recycled polyethylene terephthalate (rPET) aerogels for thermal and acoustic insulation applications. The rPET aerogels were successfully developed from recycled PET fibers and polyvinyl alcohol (PVA) and glutaraldehyde (GA) cross-linkers using a freeze-drying process. The effects of various PET fiber concentrations (0.5, 1.0 and 2.0 by wt.%), fiber deniers (3D, 7D and 15D) and fiber lengths (32 mm and 64 mm) on the rPET aerogel structures and multi-properties were comprehensively investigated. The developed rPET aerogels showed a highly porous network structure (98.3–99.5%), ultra-low densities (0.007–0.026 g/cm³), hydrophobicity with water contact angles of 120.7–149.8°, and high elasticity with low compressive Young’s modulus (1.16–2.87 kPa). They exhibited superior thermal insulation capability with low thermal conductivities of 0.035–0.038 W/m.K, which are highly competitive with recycled cellulose and silica-cellulose aerogels and better than mineral wool and polystyrene. The acoustic absorption results were also found to outperform a commercial acoustic foam absorber across a range of frequencies. Full article

(This article belongs to the Special Issue Aerogels 2018)

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10 pages, 42133 KiB

Open AccessArticle

The Relation between the Rheological Properties of Gels and the Mechanical Properties of Their Corresponding Aerogels

by Mingze Sun, Hua Sun, Yutao Wang, Miguel Sánchez-Soto and David A. Schiraldi

Gels 2018, 4(2), 33; https://doi.org/10.3390/gels4020033 - 09 Apr 2018

Cited by 32 | Viewed by 5882

Abstract

A series of low density, highly porous clay/poly(vinyl alcohol) composite aerogels, incorporating ammonium alginate, were fabricated via a convenient and eco-friendly freeze drying method. It is significant to understand rheological properties of precursor gels because they directly affect the form of aerogels and [...] Read more.

A series of low density, highly porous clay/poly(vinyl alcohol) composite aerogels, incorporating ammonium alginate, were fabricated via a convenient and eco-friendly freeze drying method. It is significant to understand rheological properties of precursor gels because they directly affect the form of aerogels and their processing behaviors. The introduction of ammonium alginate impacted the rheological properties of colloidal gels and improved the mechanical performance of the subject aerogels. The specific compositions and processing conditions applied to those colloidal gel systems brought about different aerogel morphologies, which in turn translated into the observed mechanical properties. The bridge between gel rheologies and aerogel structures are established in the present work. Full article

(This article belongs to the Special Issue Aerogels 2018)

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9 pages, 775 KiB

Open AccessArticle

Densification and Strengthening of Aerogels by Sintering Heat Treatments or Plastic Compression

by Thierry Woignier and Laurent Duffours

Gels 2018, 4(1), 12; https://doi.org/10.3390/gels4010012 - 31 Jan 2018

Cited by 13 | Viewed by 3615

Abstract

Due to their broad range of porosity, aerogels are suited to various applications. The advantages of a broad range of porosity are used directly, for example, in thermal and acoustic insulation, as materials for space applications or in catalysers. However, an overly high [...] Read more.

Due to their broad range of porosity, aerogels are suited to various applications. The advantages of a broad range of porosity are used directly, for example, in thermal and acoustic insulation, as materials for space applications or in catalysers. However, an overly high pore volume can also be a drawback, for example, in a glass precursor and host matrix. Fortunately, aerogel porosity can be tailored using sintering or isostatic compression. Sets of silica aerogels—sintered and compressed aerogels—have been studied with the objective of comparing these different densification mechanisms. We focus on the mechanical changes during the two processes of densification. Full article

(This article belongs to the Special Issue Aerogels 2018)

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15 pages, 8712 KiB

Open AccessArticle

Clay-Facilitated Aqueous Dispersion of Graphite and Poly(vinyl alcohol) Aerogels Filled with Binary Nanofillers

by Lei Liu, Yuxin Wang, Saeed M. Alhassan, Hua Sun, Kyungwho Choi, Choongho Yu and David A. Schiraldi

Gels 2018, 4(1), 8; https://doi.org/10.3390/gels4010008 - 12 Jan 2018

Cited by 8 | Viewed by 4144

Abstract

Dispersion of graphite in water was achieved using clay as dispersing aid. In the absence of polymer, the clay/graphite suspensions were sufficiently stable to produce aerogels composed of very thin layers of uniformly dispersed nanoparticles. Poly(vinyl alcohol) (PVOH) aerogels containing binary nanofillers (clay [...] Read more.

Dispersion of graphite in water was achieved using clay as dispersing aid. In the absence of polymer, the clay/graphite suspensions were sufficiently stable to produce aerogels composed of very thin layers of uniformly dispersed nanoparticles. Poly(vinyl alcohol) (PVOH) aerogels containing binary nanofillers (clay plus graphite) were then fabricated and tested. These composites were found to maintain low thermal and electrical conductivities even with high loading of graphite. A unique compressive stress-strain behavior was observed for the aerogel, exhibiting a plateau in the densification region, likely due to sliding between clay and graphite layers within the PVOH matrix. The aerogels containing only graphite exhibited higher compressive modulus, yield stress and toughness values than the samples filled with binary nanofillers. X-ray diffraction (XRD) spectra for the same composite aerogel before and after compression testing illustrated the compression-induced dispersion changes of nanofillers. Composites containing 50 wt % graphite demonstrated a downshift of its 2D Raman peak implying graphite exfoliation to graphene with less than 5 layers. Full article

(This article belongs to the Special Issue Aerogels 2018)

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13 pages, 3673 KiB

Open AccessArticle

Enhancement Studies on Manufacturing and Properties of Novel Silica Aerogel Composites

by Sunil Chandrakant Joshi, Periyasamy Manikandan and Yogeswari Jothi

Gels 2018, 4(1), 5; https://doi.org/10.3390/gels4010005 - 07 Jan 2018

Cited by 7 | Viewed by 5196

Abstract

Silica Aerogel composites are ultra-low density, highly porous foam-like materials that exhibit excellent thermal insulation and high strain recovery characteristics. In the present work, environment-friendly silica aerogel composites are fabricated using silica aerogel granules with bio based porcine-gelatin as the binding agent dissolved [...] Read more.

Silica Aerogel composites are ultra-low density, highly porous foam-like materials that exhibit excellent thermal insulation and high strain recovery characteristics. In the present work, environment-friendly silica aerogel composites are fabricated using silica aerogel granules with bio based porcine-gelatin as the binding agent dissolved in water and by further drying the mix at sub-zero condition. This article focuses on improvement studies carried on the mold design and the manufacturing process to achieve better geometric compliance for the silica aerogel composites. It also presents contact angle measurements, compressive behavior under different cycles of loading, time dependent behavior and flexural response of the composites. The influence of additives, such as fumed silica and carbon nanotubes on mechanical properties of the composites is also deliberated. Water droplet contact angle experiments confirmed the ultra-hydrophobic nature of the composites. The mechanical properties were characterized under cyclic loading-unloading compression and three-point flexure tests. On successive compression in three consecutive load cycles, the strain and thickness recovery were found to decrease by around 30%. The flexural properties of the aerogel composites were investigated using it as the core covered by thin carbon composite face sheets. It was found that the flexural strength and the failure strain of this aerogel sandwich composites is approximately half of the conventional nomex honeycomb sandwich equivalent. Full article

(This article belongs to the Special Issue Aerogels 2018)

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Review

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14 pages, 5179 KiB

Open AccessReview

Non-Conventional Methods for Gelation of Alginate

by Pavel Gurikov and Irina Smirnova

Gels 2018, 4(1), 14; https://doi.org/10.3390/gels4010014 - 01 Feb 2018

Cited by 89 | Viewed by 11185

Abstract

This review presents and critically evaluates recent advances in non-conventional gelation method of native alginate. A special focus is given to the following three methods: cryotropic gelation, non-solvent induced phase separation and carbon dioxide induced gelation. A few other gelation approaches are also [...] Read more.

This review presents and critically evaluates recent advances in non-conventional gelation method of native alginate. A special focus is given to the following three methods: cryotropic gelation, non-solvent induced phase separation and carbon dioxide induced gelation. A few other gelation approaches are also briefly reviewed. Results are discussed in the context of subsequent freeze and supercritical drying. The methods are selected so as to provide the readers with a range of novel tools and tactics of pore engineering for alginate and other anionic polysaccharides. Full article

(This article belongs to the Special Issue Aerogels 2018)

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5083 KiB

Open AccessReview

Kinetics of Supercritical Drying of Gels

by İbrahim Şahin, Yaprak Özbakır, Zeynep İnönü, Zeynep Ulker and Can Erkey

Gels 2018, 4(1), 3; https://doi.org/10.3390/gels4010003 - 29 Dec 2017

Cited by 86 | Viewed by 11622

Abstract

Supercritical drying of gels is considered as the most important step of aerogel production since it enables preservation of the three-dimensional pore structure which lead to unique material properties such as high porosity, low density, and large surface area. An understanding of the [...] Read more.

Supercritical drying of gels is considered as the most important step of aerogel production since it enables preservation of the three-dimensional pore structure which lead to unique material properties such as high porosity, low density, and large surface area. An understanding of the kinetics of supercritical drying is necessary to provide insight into material development, scale-up, and optimization of the aerogel manufacturing process. Thus, investigation of supercritical drying is gaining increased attention in recent years. This review paper covers the experimental considerations and techniques to study the kinetics of supercritical drying, fundamental mass transfer mechanisms during the drying process and modeling efforts to predict the drying kinetics for varying operating conditions and gel properties. Transport phenomena involving diffusion, convection, spillage by volume expansion, and axial dispersion are discussed by providing the fundamental equations and empirical correlations to predict transfer coefficients. A detailed review of literature covering experimental and theoretical studies on kinetics of supercritical drying is presented. Full article

(This article belongs to the Special Issue Aerogels 2018)

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