Editorial

4 pages, 179 KiB

Open AccessEditorial

Editorial on the Special Issue Entitled “Recent Advances in Aerogels”

by Gabrijela Horvat and Uroš Maver

Gels 2024, 10(3), 154; https://doi.org/10.3390/gels10030154 - 20 Feb 2024

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Aerogels are unique solid materials that consist mainly of air and have an extremely low density, large open pores, and a large internal surface area [...] Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

Research

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23 pages, 4650 KiB

Open AccessArticle

Integrated Performance Evaluation of Aerogel-Based Fibre-Enhanced Thermal Renders Applied on Building Walls

by Marco Pedroso, José Dinis Silvestre, Maria da Glória Gomes, Ahmed Hawreen, Jéssica D. Bersch and Inês Flores-Colen

Gels 2023, 9(11), 898; https://doi.org/10.3390/gels9110898 - 13 Nov 2023

Cited by 1 | Viewed by 1028

Abstract

In this work, aerogel renders were enhanced with fibres for use in new building walls, emphasising a Mediterranean climate. The main novelty of the study relies on an integrated evaluation of the aerogel-based fibre-enhanced thermal renders from environmental, energy and economic approaches. Therefore, [...] Read more.

In this work, aerogel renders were enhanced with fibres for use in new building walls, emphasising a Mediterranean climate. The main novelty of the study relies on an integrated evaluation of the aerogel-based fibre-enhanced thermal renders from environmental, energy and economic approaches. Therefore, optimum insulation thicknesses, life cycle savings, payback periods, abiotic depletion potential from fossil fuels (ADP-ff) and global warming potential (GWP) impacts were quantified as a function of the energy consumption. The cost optimisation of aerogel-based renders enabled a reduction from 2477.4 to 1021.7 EUR∙m⁻³ for the reference formulation, and the sisal-optimised render led to the best-integrated performance. A higher DD* (degree-days equivalent) led to higher optimum thicknesses (the Azores required 0.02 m and 0.01 m and Bragança 0.06 m and 0.03 m for cost-optimised and non-optimised thermal renders with sisal fibre, respectively). The optimum thickness related to the ADP-ff and GWP impacts was higher, 0.04 m for the Azores and 0.09 m for Bragança. A steeper decrease in the annual energy consumption occurred for thermal renders up to 0.02 m in the Azores and 0.04 m in Bragança. Aerogel-based fibre-enhanced thermal renders had benefits, mainly from 600 DD* onwards. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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16 pages, 4013 KiB

Open AccessArticle

Polypyrrole Aerogels: Efficient Adsorbents of Cr(VI) Ions from Aqueous Solutions

by Patrycja Bober, Islam M. Minisy, Zuzana Morávková, Helena Hlídková, Jiří Hodan, Jiřina Hromádková and Udit Acharya

Gels 2023, 9(7), 582; https://doi.org/10.3390/gels9070582 - 17 Jul 2023

Cited by 1 | Viewed by 1090

Abstract

Three-dimensional and porous polypyrrole (PPy) aerogels were prepared using a facile two-step procedure in which cryogels were synthesized via the cryopolymerization of pyrrole with iron (III) chloride in the presence of supporting water-soluble polymers (poly(N-vinylpyrrolidone), poly(vinyl alcohol), gelatin, methylcellulose or hydroxypropylcellulose), [...] Read more.

Three-dimensional and porous polypyrrole (PPy) aerogels were prepared using a facile two-step procedure in which cryogels were synthesized via the cryopolymerization of pyrrole with iron (III) chloride in the presence of supporting water-soluble polymers (poly(N-vinylpyrrolidone), poly(vinyl alcohol), gelatin, methylcellulose or hydroxypropylcellulose), followed by freeze-drying to obtain aerogels. The choice of supporting polymers was found to affect the morphology, porosity, electrical conductivity, and mechanical properties of PPy aerogels. PPy aerogels were successfully used as adsorbents to remove toxic Cr(VI) ions from aqueous solutions. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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18 pages, 3471 KiB

Open AccessArticle

Improvement of the Mechanical Properties of Silica Aerogels for Thermal Insulation Applications through a Combination of Aramid Nanofibres and Microfibres

by Mariana Emilia Ghica, Jandira G. S. Mandinga, Teresa Linhares, Cláudio M. R. Almeida and Luisa Durães

Gels 2023, 9(7), 535; https://doi.org/10.3390/gels9070535 - 30 Jun 2023

Cited by 2 | Viewed by 1416

Abstract

Reinforcement of silica aerogels, remarkable lightweight mesoporous materials with outstanding insulation performance, is still a challenging research topic. Among the strategies used to overcome their brittleness, one of the most effective is the manufacturing of aerogel composites with embedded fibres. In this work, [...] Read more.

Reinforcement of silica aerogels, remarkable lightweight mesoporous materials with outstanding insulation performance, is still a challenging research topic. Among the strategies used to overcome their brittleness, one of the most effective is the manufacturing of aerogel composites with embedded fibres. In this work, the incorporation of nanofibres together with microfibres in a tetraethoxysilane–vinyltrimethoxysilane matrix is investigated for the first time for the development of novel aerogel nanocomposites. The nanofibres, synthesized from different aramid fibres, including Kevlar^® pulp, Technora^®, Teijinconex^® and Twaron^® fibres, were used in different combinations with microaramids and the resulting nanocomposites were thoroughly investigated for their physicochemical and thermomechanical features. The properties depended on the type and amount of the nano/microfibre used. While the microfibres exhibited low interaction with the silica matrix, the higher surface of the nanofibres ensured increased contact with the gel matrix. A low bulk density of 161 kg m⁻³ and thermal conductivity of 38.3 mW m⁻¹ K⁻¹ (Hot Disk^®) was achieved when combining the nanofibres obtained from Kevlar^® pulp with the Technora^® or Teijinconex^® long fibres. The nanofibres showed higher dispersion and random orientation and in combination with microfibres led to the improvement by a factor of three regarding the mechanical properties of the aerogel nanocomposites reinforced only with microfibres. The scale-up process of the samples and simulated tests of thermal cycling and vacuum outgassing successfully conducted indicate good compliance with space applications. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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14 pages, 6354 KiB

Open AccessArticle

Thermomechanical Performance Assessment of Sustainable Buildings’ Insulating Materials under Accelerated Ageing Conditions

by Ana Dora Rodrigues Pontinha, Johanna Mäntyneva, Paulo Santos and Luísa Durães

Gels 2023, 9(3), 241; https://doi.org/10.3390/gels9030241 - 18 Mar 2023

Cited by 8 | Viewed by 1601

Abstract

The reliable characterization of insulation materials in relevant environmental conditions is crucial, since it strongly influences the performance (e.g., thermal) of building elements. In fact, their properties may vary with the moisture content, temperature, ageing degradation, etc. Therefore, in this work, the thermomechanical [...] Read more.

The reliable characterization of insulation materials in relevant environmental conditions is crucial, since it strongly influences the performance (e.g., thermal) of building elements. In fact, their properties may vary with the moisture content, temperature, ageing degradation, etc. Therefore, in this work, the thermomechanical behaviour of different materials was compared when subjected to accelerated ageing. Insulation materials that use recycled rubber in their composition were studied, along with others for comparison: heat-pressed rubber, rubber_cork composites, aerogel_rubber composite (developed by the authors), silica aerogel, and extruded polystyrene. The ageing cycles comprised dry-heat, humid-heat, and cold conditions as the stages, during cycles of 3 and 6 weeks. The materials’ properties after ageing were compared with the initial values. Aerogel-based materials showed superinsulation behaviour and good flexibility due to their very high porosity and reinforcement with fibres. Extruded polystyrene also had a low thermal conductivity but exhibited permanent deformation under compression. In general, the ageing conditions led to a very slight increase in the thermal conductivity, which vanished after drying of the samples in an oven, and to a decrease in Young’s moduli. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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24 pages, 5498 KiB

Open AccessArticle

Origin of the Springback Effect in Ambient-Pressure-Dried Silica Aerogels: The Effect of Surface Silylation

by Fabian Zemke, Julien Gonthier, Ernesto Scoppola, Ulla Simon, Maged F. Bekheet, Wolfgang Wagermaier and Aleksander Gurlo

Gels 2023, 9(2), 160; https://doi.org/10.3390/gels9020160 - 16 Feb 2023

Cited by 5 | Viewed by 1793

Abstract

Ambient pressure drying (APD) can prospectively reduce the costs of aerogel fabrication and processing. APD relies solely on preventing shrinkage or making it reversible. The latter, i.e., the aerogel re-expansion after drying (so-called springback effect—SBE), needs to be controlled for reproducible aerogel fabrication [...] Read more.

Ambient pressure drying (APD) can prospectively reduce the costs of aerogel fabrication and processing. APD relies solely on preventing shrinkage or making it reversible. The latter, i.e., the aerogel re-expansion after drying (so-called springback effect—SBE), needs to be controlled for reproducible aerogel fabrication by APD. This can be achieved by an appropriate surface functionalization of aerogel materials (e.g., SiO₂). This work addresses the fabrication of monolithic SiO₂ aerogels and xerogels by APD. The effect of several silylation agents, i.e., trimethylchlorosilane, triethylchlorosilane, and hexamethyldisilazane on the SBE is studied in detail, applying several complementary experimental techniques, allowing the evaluation of the macroscopic and microscopic morphology as well as the composition of SiO₂ aerogels. Here, we show that some physical properties, e.g., the bulk density, the macroscopic structure, and pore sizes/volumes, were significantly affected by the re-expansion. However, silylation did not necessarily lead to full re-expansion. Therefore, similarities in the molecular composition could not be equated to similarities in the SBE. The influences of steric hindrance and reactivity are discussed. The impact of silylation is crucial in tailoring the SBE and, as a result, the APD of monolithic aerogels. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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16 pages, 5626 KiB

Open AccessArticle

Preparation of Bio-Based Aerogel and Its Adsorption Properties for Organic Dyes

by Penghui Li, Chi Yang, Xuewen Xu, Chen Miao, Tianjiao He, Bo Jiang and Wenjuan Wu

Gels 2022, 8(11), 755; https://doi.org/10.3390/gels8110755 - 21 Nov 2022

Cited by 10 | Viewed by 2254

Abstract

The effective utilization of biomass and the purification of dye wastewater are urgent problems. In this study, a biomass aerogel (CaCO₃@starch/polyacrylamide/TEMPO-oxidized nanocellulose, CaCO₃@STA/PAM/TOCN) was prepared by combining nanocellulose with starch and introducing calcium carbonate nanoparticles, which exhibited a rich [...] Read more.

The effective utilization of biomass and the purification of dye wastewater are urgent problems. In this study, a biomass aerogel (CaCO₃@starch/polyacrylamide/TEMPO-oxidized nanocellulose, CaCO₃@STA/PAM/TOCN) was prepared by combining nanocellulose with starch and introducing calcium carbonate nanoparticles, which exhibited a rich three-dimensional layered porous structure with a very light mass. Starch and nanocellulose can be grafted onto the molecular chain of acrylamide, while calcium carbonate nanopores can make the gel pore size uniform and have excellent swelling properties. Here, various factors affecting the adsorption behavior of this aerogel, such as pH, contact time, ambient temperature, and initial concentration, are investigated. From the kinetic data, it can be obtained that the adsorption process fits well with the pseudo-second-order. The Langmuir isotherm model can fit the equilibrium data well. The thermodynamic data also demonstrated the spontaneous and heat-absorbing properties of anionic and cationic dyes on CaCO₃@STA/PAM/TOCN aerogels. The adsorption capacity of Congo red (CR) and methylene blue (MB) by CaCO₃@STA/PAM/TOCN was 277.76 mg/g and 101.01 mg/g, respectively. Therefore, cellulose and starch-based aerogels can be considered promising adsorbents for the treatment of dye wastewater. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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17 pages, 6230 KiB

Open AccessArticle

Polyvinylidene Fluoride Aerogels with Tailorable Crystalline Phase Composition

by Jorge Torres-Rodriguez, Diana E. Bedolla, Francesco D’Amico, Ann-Kathrin Koopmann, Lisa Vaccari, Giulia Saccomano, Richard Kohns and Nicola Huesing

Gels 2022, 8(11), 727; https://doi.org/10.3390/gels8110727 - 09 Nov 2022

Cited by 1 | Viewed by 1745

Abstract

In this work, polyvinylidene fluoride (PVDF) aerogels with a tailorable phase composition were prepared by following the crystallization-induced gelation principle. A series of PVDF wet gels (5 to 12 wt.%) were prepared from either PVDF–DMF solutions or a mixture of DMF and ethanol [...] Read more.

In this work, polyvinylidene fluoride (PVDF) aerogels with a tailorable phase composition were prepared by following the crystallization-induced gelation principle. A series of PVDF wet gels (5 to 12 wt.%) were prepared from either PVDF–DMF solutions or a mixture of DMF and ethanol as non-solvent. The effects of the non-solvent concentration on the crystalline composition of the PVDF aerogels were thoroughly investigated. It was found that the nucleating role of ethanol can be adjusted to produce low-density PVDF aerogels, whereas the changes in composition by the addition of small amounts of water to the solution promote the stabilization of the valuable β and γ phases. These phases of the aerogels were monitored by FTIR and Raman spectroscopies. Furthermore, the crystallization process was followed by in-time and in situ ATR–FTIR spectroscopy. The obtained aerogels displayed specific surface areas > 150 m² g⁻¹, with variable particle morphologies that are dependent on the non-solvent composition, as observed by using SEM and Synchrotron Radiation Computed micro-Tomography (SR-μCT). Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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14 pages, 7586 KiB

Open AccessArticle

Structurally Stable, High-Strength Graphene Oxide/Carbon Nanotube/Epoxy Resin Aerogels as Three-Dimensional Skeletal Precursors for Wave-Absorbing Materials

by Lina Zhang, Guojun Song, Zetian Zhao, Lichun Ma, Hui Xu, Guanglei Wu, Yinghu Song, Yinuo Liu, Lihan Qiu and Xiaoru Li

Gels 2022, 8(10), 618; https://doi.org/10.3390/gels8100618 - 28 Sep 2022

Cited by 5 | Viewed by 1934

Abstract

Three-dimensional (3D) graphene oxide aerogel (GOA) is one of the best fillers for composites for microwave absorption. However, its further development has been hindered by the poor mechanical properties. Methodology to improve the mechanical properties of the aerogel remains an urgent challenge. Herein, [...] Read more.

Three-dimensional (3D) graphene oxide aerogel (GOA) is one of the best fillers for composites for microwave absorption. However, its further development has been hindered by the poor mechanical properties. Methodology to improve the mechanical properties of the aerogel remains an urgent challenge. Herein, graphene oxide/carbon nanotube/epoxy resin composite aerogel (GCEA) was successfully prepared by a facile method. The results showed that the prepared GCEA with the hierarchical and 3D cross-linked structures exhibited excellent compression performance, structural and thermal stability, high hydrophilicity, and microwave absorption. The prepared GCEA recovered from multiple large strain cycles without significant permanent deformation. The minimum reflection loss (RL) was −39.60 dB and the maximum effective absorption bandwidth (EAB) was 2.48 GHz. The development of the enhanced GO aerogels will offer a new approach to the preparation of 3D microwave-absorbing skeletal materials with good mechanical properties. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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11 pages, 4422 KiB

Open AccessArticle

Characterization of Phase Change Materials Fabricated with Cross-Linked Graphene Aerogels

by Chengbin Yu and Young Seok Song

Gels 2022, 8(9), 572; https://doi.org/10.3390/gels8090572 - 08 Sep 2022

Cited by 8 | Viewed by 1585

Abstract

3D porous graphene aerogel exhibits a high surface area which can hold plenty of pure phase change material (PCM) into the internal space. In order to maintain the flexibility of PCM without volume shrinkage under the external force, cross-linked graphene aerogel was prepared [...] Read more.

3D porous graphene aerogel exhibits a high surface area which can hold plenty of pure phase change material (PCM) into the internal space. In order to maintain the flexibility of PCM without volume shrinkage under the external force, cross-linked graphene aerogel was prepared by the cysteamine vapor method. The cross-linked graphene aerogel had a high stress–strain durability and chemical stability for infiltrating PCM to produce a form-stable PCM composite. The latent heat of PCM is one of the elements to estimate the capacity of PCM thermal energy storage (TES) during the phase transition process. The cross-linked graphene aerogel-supported PCM composite showed a great TES to be utilized in thermal-to-electrical energy harvesting. The cross-linked graphene aerogel also had an excellent mechanical property of preventing damage at a high temperature. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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18 pages, 7041 KiB

Open AccessArticle

Biological Thermal Performance of Organic and Inorganic Aerogels as Patches for Photothermal Therapy

by Tânia Ferreira-Gonçalves, Ana Iglesias-Mejuto, Teresa Linhares, João M. P. Coelho, Pedro Vieira, Pedro Faísca, José Catarino, Pedro Pinto, David Ferreira, Hugo A. Ferreira, Maria Manuela Gaspar, Luísa Durães, Carlos A. García-González and Catarina Pinto Reis

Gels 2022, 8(8), 485; https://doi.org/10.3390/gels8080485 - 03 Aug 2022

Cited by 9 | Viewed by 2540

Abstract

Aerogels are materials with unique properties, among which are low density and thermal conductivity. They are also known for their exquisite biocompatibility and biodegradability. All these features make them attractive for biomedical applications, such as their potential use in photothermal therapy (PTT). This [...] Read more.

Aerogels are materials with unique properties, among which are low density and thermal conductivity. They are also known for their exquisite biocompatibility and biodegradability. All these features make them attractive for biomedical applications, such as their potential use in photothermal therapy (PTT). This technique is, yet, still associated with undesirable effects on surrounding tissues which emphasizes the need to minimize the exposure of healthy regions. One way to do so relies on the use of materials able to block the radiation and the heat generated. Aerogels might be potentially useful for this purpose by acting as insulators. Silica- and pectin-based aerogels are reported as the best inorganic and organic thermal insulators, respectively; thus, the aim of this work relies on assessing the possibility of using these materials as light and thermal insulators and delimiters for PTT. Silica- and pectin-based aerogels were prepared and fully characterized. The thermal protection efficacy of the aerogels when irradiated with a near-infrared laser was assessed using phantoms and ex vivo grafts. Lastly, safety was assessed in human volunteers. Both types presented good textural properties and safe profiles. Moreover, thermal activation unveils the better performance of silica-based aerogels, confirming the potential of this material for PTT. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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16 pages, 4486 KiB

Open AccessArticle

The Influence of the Precursor’s Nature and Drying Conditions on the Structure, Morphology, and Thermal Properties of TiO₂ Aerogels

by Jolanta Donėlienė, Eglė Fataraitė-Urbonienė, Nina Danchova, Stoyan Gutzov and Juras Ulbikas

Gels 2022, 8(7), 422; https://doi.org/10.3390/gels8070422 - 06 Jul 2022

Cited by 2 | Viewed by 1479

Abstract

A cost-effective solution for the synthesis of high-porosity TiO₂ aerogels, which can be used as a mesoporous perovskite network charge-carrier material during the manufacture of solar cells, is described. The effects of the synthesis parameters (precursor (titanium (IV) isopropoxide (TIP) and tetrabutyl [...] Read more.

A cost-effective solution for the synthesis of high-porosity TiO₂ aerogels, which can be used as a mesoporous perovskite network charge-carrier material during the manufacture of solar cells, is described. The effects of the synthesis parameters (precursor (titanium (IV) isopropoxide (TIP) and tetrabutyl orthotitanate (TBOT)), additional solvent exchange (n-hexane (nH), cyclohexane (CH), and diethyl ether (DE)), subcritical drying (800 mbar vacuum, 70 °C, 8 h), aging, and calcination on the aerogel’s structure have been investigated. Methods of XRD, FT-IR, BET, Raman, STA, SEM, UV–vis, and thermal conductivity measurements were applied to find out the relation between the synthesis conditions and the properties of the synthesized aerogels. Amorphous aerogels are polydispersed systems with the highest probability of pore diameter from 0.5 to 15 nm. An nH-exchanged, aged aerogel synthesized from the precursor TIP shows the highest diameter of pores. After calcination, the aerogels tend to crystallize into an anatase phase and the size of the crystallites depends on the precursor’s nature. Calcination leads to a significant increase in both the apparent and true density of the aerogels, and it also results in an increase in porosity and thermal conductivity. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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Review

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18 pages, 2138 KiB

Open AccessReview

A Brief Evaluation of Pore Structure Determination for Bioaerogels

by Gabrijela Horvat, Milica Pantić, Željko Knez and Zoran Novak

Gels 2022, 8(7), 438; https://doi.org/10.3390/gels8070438 - 13 Jul 2022

Cited by 29 | Viewed by 3396

Abstract

This review discusses the most commonly employed methods for determining pore size and pore size distribution in bioaerogels. Aerogels are materials with high porosity and large surface areas. Most of their pores are in the range of mesopores, between 2 and 50 nm. [...] Read more.

This review discusses the most commonly employed methods for determining pore size and pore size distribution in bioaerogels. Aerogels are materials with high porosity and large surface areas. Most of their pores are in the range of mesopores, between 2 and 50 nm. They often have smaller or larger pores, which presents a significant challenge in determining the exact mean pore size and pore size distribution in such materials. The precision and actual value of the pore size are of considerable importance since pore size and pore size distribution are among the main properties of aerogels and are often directly connected with the final application of those materials. However, many recently published papers discuss or present pore size as one of the essential achievements despite the misinterpretation or the wrong assignments of pore size determination. This review will help future research and publications evaluate the pore size of aerogels more precisely and discuss it correctly. The study covers methods such as gas adsorption, from which BJH and DFT models are often used, SEM, mercury porosimetry, and thermoporometry. The methods are described, and the results obtained are discussed. The following paper shows that there is still no precise method for determining pore size distribution or mean pore size in aerogels until now. Knowing that, it is expected that this field will evolve in the future. Full article

(This article belongs to the Special Issue Recent Advances in Aerogels)

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