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Gels
Special Issues
Aerogels—Preparation and Properties
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Aerogels—Preparation and Properties

A special issue of Gels (ISSN 2310-2861). This special issue belongs to the section "Gel Processing and Engineering".

Deadline for manuscript submissions: 30 September 2024 | Viewed by 3821

Special Issue Editors

Prof. Dr. Stoyan Gutzov

E-Mail Website
Guest Editor
Department of Physical Chemistry, Faculty of Chemistry and Pharmacy, Sofia University “St. Kliment Ohridski”, 1164 Sofia, Bulgaria
Interests: aerogels; optical materials; sol-gel; luminescence; solid state
Special Issues, Collections and Topics in MDPI journals
Dr. Xiaodong Wang

E-Mail Website
Guest Editor
School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
Interests: aerogels; optical coatings; sol-gel; Raman spectroscopy; SERS; CDI; solid state physics

Special Issue Information

Dear Colleagues,

A new challenge in materials science is aerogels—a class of porous solids with extremely valuable material properties and different chemistry, including organic materials, carbon, ceramic oxides and even metals.

The valuable properties of aerogels based on their unique structure, such as low density, thermal insulation properties and transparency, make them attractive in many research areas. Aerogels are included as part of NASA's projects for the development of materials applicable in space, in aircrafts, insulation equipment, astronaut suits, blankets, etc.

A promising direction is the search for new modified aerogel materials that combine the advantages of the matrix and the properties of the new component embedded into their structure: for example, reinforced silica aerogels with a polymer component incorporated into the matrix. Another new research direction for hybrid composite-based aerogels is the incorporation of optical active components into aerogel granules and powders. In this way, aerogel composite materials, which combine the properties both of the aerogel matrix and the embedded complex, are developed.

The present Special Issue, “Aerogels—Preparation and Properties”, focuses on several topics, including but not limited to the following:

  1. The preparation–structure–property relationships of aerogels;
  2. Aerogel composites;
  3. Specific applications of aerogels and their composites, etc.

Original articles, reviews, short communications and perspectives are all suitable types of papers for submission.

Prof. Dr. Stoyan Gutzov
Dr. Xiaodong Wang
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Gels is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2100 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • aerogels
  • aerogel composites
  • aerogel hybrids
  • nano-aerogels
  • supercritical drying
  • subcritical drying
  • optical properties
  • electrical properties
  • magnetic properties
  • mechanical properties

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Published Papers (4 papers)

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Research

13 pages, 2102 KiB  
Article
Machine Learning Techniques to Analyze the Influence of Silica on the Physio-Chemical Properties of Aerogels
by Hamdi Chaouk, Emil Obeid, Jalal Halwani, Jack Arayro, Rabih Mezher, Omar Mouhtady, Eddie Gazo-Hanna, Semaan Amine and Khaled Younes
Gels 2024, 10(9), 554; https://doi.org/10.3390/gels10090554 - 27 Aug 2024
Viewed by 473
Abstract
This study explores the application of machine learning techniques, specifically principal component analysis (PCA), to analyze the influence of silica content on the physical and chemical properties of aerogels. Silica aerogels are renowned for their exceptional properties, including high porosity, large surface area, [...] Read more.
This study explores the application of machine learning techniques, specifically principal component analysis (PCA), to analyze the influence of silica content on the physical and chemical properties of aerogels. Silica aerogels are renowned for their exceptional properties, including high porosity, large surface area, and low thermal conductivity, but their mechanical brittleness poses significant challenges. The study initially utilized cross-correlation analysis to examine the relationships between key properties such as the Brunauer–Emmett–Teller (BET) surface area, pore volume, density, and thermal conductivity. However, weak correlations prompted the application of PCA to uncover deeper insights into the data. The PCA results demonstrated that silica content has a significant impact on aerogel properties, with the first principal component (PC1) showing a strong positive correlation (R2 = 94%) with silica content. This suggests that higher silica levels correspond to lower thermal conductivity, porosity, and BET surface area, while increasing the density and elastic modulus. Additionally, the analysis identified the critical role of thermal conductivity in the second principal component (PC2), particularly in samples with moderate to high silica content. Overall, this study highlights the effectiveness of machine learning techniques like PCA in optimizing and understanding the complex inter-relationships among the physio-chemical properties of silica aerogels. Full article
(This article belongs to the Special Issue Aerogels—Preparation and Properties)
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Figure 1

Figure 1
<p>Cross-correlations among the physio-chemical properties of silica aerogel.</p> Full article ">Figure 2
<p>(<b>a</b>) PCA biplot representation of datasets for the properties of silica aerogel composites that possess TEOS as a precursor and glass fiber as a reinforcing agent (data were obtained from the previous findings of Jadhav and Sarawade [<a href="#B14-gels-10-00554" class="html-bibr">14</a>]; <a href="#gels-10-00554-t001" class="html-table">Table 1</a>). Black circular bullets indicate silica aerogels following the wt.% of silica; square grey bullets indicate the physical properties involved. (<b>b</b>) Contribution of the variables (%) following the first two PCs.</p> Full article ">Figure 3
<p>(<b>a</b>) PCA biplot representation of the dataset of <a href="#gels-10-00554-t001" class="html-table">Table 1</a>, with the exclusion of aerogels dried by APD (3.5% and 10% of the wt.% of silica). Black circular bullets indicate silica aerogels following the wt.% of silica; square grey bullets indicate the physical properties involved. (<b>b</b>) Contribution of the variables (%) following the first two PCs.</p> Full article ">Figure 4
<p>Principal components PC1 (<b>a</b>) and PC2 (<b>b</b>), with respect to the wt.% of silica in the investigated aerogel composite.</p> Full article ">Figure 5
<p>PC2 with respect to the wt.% of silica in the investigated aerogel composite, with the exclusion of samples with a low amount of silica (0% and 1%).</p> Full article ">
13 pages, 4147 KiB  
Article
Synthesis of Flexible Polyamide Aerogels Cross-Linked with a Tri-Isocyanate
by Daniel A. Scheiman, Haiquan Guo, Katherine J. Oosterbaan, Linda McCorkle and Baochau N. Nguyen
Gels 2024, 10(8), 519; https://doi.org/10.3390/gels10080519 - 7 Aug 2024
Viewed by 546
Abstract
A new series of flexible polyamide (PA) aerogels was synthesized using terephthaloyl chloride (TPC), 2,2′-dimethylbenzidine (DMBZ) and cross-linked with an inexpensive, commercially available tri-isocyanate (Desmodur N3300A) at polymer concentrations of 6–8 wt.% total solids and repeating units, n, from 30 to 60. [...] Read more.
A new series of flexible polyamide (PA) aerogels was synthesized using terephthaloyl chloride (TPC), 2,2′-dimethylbenzidine (DMBZ) and cross-linked with an inexpensive, commercially available tri-isocyanate (Desmodur N3300A) at polymer concentrations of 6–8 wt.% total solids and repeating units, n, from 30 to 60. The cross-linked DMBZ-based polyamide aerogels obtained, after supercritically drying using liquid CO2, had shrinkages of 19–27% with densities ranging from 0.12 g/cm3 to 0.22 g/cm3, porosity and surface areas up to 91% and 309 m2/g, respectively, and modulus values ranging from 20.6 to 109 MPa. Evidence suggests that a higher flexibility could be achieved using DMBZ in the polyamide backbone with N3300A as a cross-linker, when compared to previously reported TPC-mPDA-BTC PA aerogels, N3300A-polyimide aerogels, and N3300-reinforced silica aerogels. Full article
(This article belongs to the Special Issue Aerogels—Preparation and Properties)
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Figure 1

Figure 1
<p>Physical state of cross-linked PA aerogels made with (<b>a</b>) mPDA/TPC/N3300A (10 wt.%, <span class="html-italic">n</span> = 30), (<b>b</b>) ODA/TPC/BTC (10 wt.%, <span class="html-italic">n</span> = 30), and (<b>c</b>) DMBZ/TPC/N300A (8 wt.%, <span class="html-italic">n</span> = 30).</p> Full article ">Figure 2
<p>A solid <sup>13</sup>C NMR of a polyamide aerogel formulated at 6 wt.% total polymer concentration with <span class="html-italic">n</span> of 30.</p> Full article ">Figure 3
<p>FTIR spectrum of a polyamide aerogel formulated at 6 wt.% total polymer concentration with <span class="html-italic">n</span> of 30; (<b>a</b>) full scale from 4000 cm<sup>−1</sup> to 800 cm<sup>−1</sup> and (<b>b</b>) enlarged scale from 2000 cm<sup>−1</sup> to 800 cm<sup>−1</sup>.</p> Full article ">Figure 4
<p>Empirical model for (<b>a</b>) shrinkage (%), (<b>b</b>) density, and (<b>c</b>) porosity (%) vs. polymer concentration and total <span class="html-italic">n</span>.</p> Full article ">Figure 5
<p>SEM images of the N3300A cross-linked PA aerogels at 6 wt.% with (<b>a</b>) <span class="html-italic">n</span> = 30 (ρ = 12.1 mg/cm<sup>3</sup>), (<b>b</b>) <span class="html-italic">n</span> = 45 (ρ = 12.9 mg/cm<sup>3</sup>), (<b>c</b>) <span class="html-italic">n</span> = 60 (ρ = 13.1 mg/cm<sup>3</sup>), and at 8 wt.% with (<b>d</b>) <span class="html-italic">n</span> = 30 (ρ = 17.1 mg/cm<sup>3</sup>) (<b>e</b>) <span class="html-italic">n</span> = 45 (ρ = 16.5 mg/cm<sup>3</sup>) and (<b>f</b>) <span class="html-italic">n</span> = 60 (ρ = 22.1 mg/cm<sup>3</sup>).</p> Full article ">Figure 6
<p>Pore volume vs. pore diameter of the cross-linked PA aerogels at different n values and polymer concentrations.</p> Full article ">Figure 7
<p>Adsorption and desorption isotherms of the cross-linked PA aerogels at different n values and polymer concentrations.</p> Full article ">Figure 8
<p>BET surface area of the cross-linked PA aerogels.</p> Full article ">Figure 9
<p>(<b>a</b>) Typical stress–strain curves from compression test of N3300A-cross-linked PA aerogel; (<b>b</b>) log–log plot of density vs. Young’s modulus of N3300A-DMBZ-TPC (closed symbol), and BTC-mPDA-TPC (open symbol) [<a href="#B15-gels-10-00519" class="html-bibr">15</a>] polyamide aerogels, N3300A-DMBZ-BPDA polyimide aerogels [<a href="#B19-gels-10-00519" class="html-bibr">19</a>], and N3300A-reinforced silica aerogels [<a href="#B30-gels-10-00519" class="html-bibr">30</a>].</p> Full article ">Figure 10
<p>TGA curves of N3300A cross-linked PA aerogels at <span class="html-italic">n</span> = 30 and <span class="html-italic">n</span> = 60.</p> Full article ">Figure 11
<p>Empirical model of final onset decomposition temperature of N-3300A cross-linked polyamide aerogel.</p> Full article ">Scheme 1
<p>Chemical reaction of a N3300A cross-linked polyamide aerogels.</p> Full article ">Scheme 2
<p>Schematically synthetic steps in fabricating N3300A cross-linked polyamide (or x-linked PA) aerogels.</p> Full article ">
14 pages, 8775 KiB  
Article
Facile Synthesis of Surface-Modified Hollow-Silica (SiO2) Aerogel Particles via Oil–Water–Oil Double Emulsion Method
by Pratik S. Kapadnis, Ki-Sun Nam, Hyun-Young Kim, Hyung-Ho Park and Haejin Hwang
Gels 2024, 10(6), 380; https://doi.org/10.3390/gels10060380 - 2 Jun 2024
Viewed by 991
Abstract
Due to their high surface area and low weight, silica aerogels are ideally suited for several uses, including drug delivery, catalysis, and insulation. Oil–water–oil (OWO) double emulsion is a simple and regulated technique for encasing a volatile oil phase in a [...] Read more.
Due to their high surface area and low weight, silica aerogels are ideally suited for several uses, including drug delivery, catalysis, and insulation. Oil–water–oil (OWO) double emulsion is a simple and regulated technique for encasing a volatile oil phase in a silica shell to produce hollow silica (SiO2) aerogel particles by using hydrophilic and hydrophobic emulsifiers. In this study, the oil–water–oil (OWO) double emulsion method was implemented to synthesize surface-modified hollow silica (SiO2) aerogel particles in a facile and effective way. This investigation mainly focused on the influence of the N-hexane-to-water glass (OW) ratio (r) in the first emulsion, silica (water glass) content concentration (x), and surfactant concentration (s) variations. Furthermore, surface modification techniques were utilized to customize the aerogel’s characteristics. The X-ray diffraction (XRD) patterns showed no imprints of impurities except SiO2. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images highlight the hollow microstructure of silica particles. Zeta potential was used to determine particle size analysis of hollow silica aerogel particles. The oil–water–oil (OWO) double emulsion approach was successfully employed to synthesize surface-modified hollow silica (SiO2) aerogel particles, providing precise control over the particle characteristics. By the influence of the optimization condition, this approach improves the aerogel’s potential applications in drug delivery, catalysis, and insulation by enabling surface modifications. Full article
(This article belongs to the Special Issue Aerogels—Preparation and Properties)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>Optical microscopy images: (<b>a</b>) <span class="html-italic">r<sub>1</sub></span> = 20:60 (mL), (<b>b</b>) <span class="html-italic">r<sub>2</sub></span> = 30:50 (mL), (<b>c</b>) <span class="html-italic">r<sub>3</sub></span> = 40:40 (mL) volume ratio for the primary (<span class="html-italic">OW</span>) emulsion.</p> Full article ">Figure 2
<p>SEM and TEM analysis of silica aerogel particles prepared from (<b>a<sub>1</sub></b>,<b>a<sub>2</sub></b>) <span class="html-italic">r<sub>1</sub></span> = 20:60 (mL), (<b>b<sub>1</sub></b>,<b>b<sub>2</sub></b>) <span class="html-italic">r<sub>2</sub></span> = 30:50 (mL), or (<b>c<sub>1</sub></b>,<b>c<sub>2</sub></b>) <span class="html-italic">r<sub>3</sub></span> = 40:40 (mL) volume ratio (<span class="html-italic">r</span>) for the primary (<span class="html-italic">OW</span>) emulsion.</p> Full article ">Figure 3
<p>X-ray diffraction (XRD) analysis of surface-modified hollow silica powder.</p> Full article ">Figure 4
<p>Optical micrographs of different surfactant variations: (<b>a</b>) <span class="html-italic">S<sub>1</sub></span> = 0.5 g, (<b>b</b>) <span class="html-italic">S<sub>2</sub></span> = 1 g, (<b>c</b>) <span class="html-italic">S<sub>3</sub></span> = 1.5 g.</p> Full article ">Figure 5
<p>Particle size distribution of different surfactant variations: <span class="html-italic">S<sub>1</sub></span> = 0.5 g, <span class="html-italic">S<sub>2</sub></span> = 1 g, and <span class="html-italic">S<sub>3</sub></span> = 1.5 g.</p> Full article ">Figure 6
<p>Schematic representation of formation mechanism of hollow silica (SiO<sub>2</sub>) aerogel by varying silica content concentration (<span class="html-italic">x</span>).</p> Full article ">Figure 7
<p>Optical micrographs of silica concentration variations (<span class="html-italic">x</span>): (<b>a</b>) <span class="html-italic">x<sub>1</sub></span> = 4.92%, (<b>b</b>) <span class="html-italic">x</span><sub>2</sub> = 6.09%, (<b>c</b>) <span class="html-italic">x</span><sub>3</sub> = 8.68%.</p> Full article ">Figure 8
<p>SEM and TEM analysis of silica aerogel particles with silica concentration variations (<span class="html-italic">x</span>): (<b>a<sub>1</sub></b>,<b>a<sub>2</sub></b>) <span class="html-italic">x</span><sub>1</sub> = 4.92%, (<b>b<sub>1</sub></b>,<b>b<sub>2</sub></b>) <span class="html-italic">x</span><sub>2</sub> = 6.09%, (<b>c<sub>1</sub></b>,<b>c<sub>2</sub></b>) <span class="html-italic">x</span><sub>3</sub> = 8.68%.</p> Full article ">Figure 9
<p>Experimental flow chart for the synthesis of hollow silica aerogel particles by <span class="html-italic">OWO</span> emulsion.</p> Full article ">
14 pages, 4795 KiB  
Article
Enhancing Water Resistance in Foam Cement through MTES-Based Aerogel Impregnation
by Zhi Li, Shengjie Yao, Guichao Wang, Xi Deng, Fang Zhou, Xiaoxu Wu and Qiong Liu
Gels 2024, 10(2), 118; https://doi.org/10.3390/gels10020118 - 1 Feb 2024
Cited by 1 | Viewed by 1373
Abstract
The propensity of foamed concrete to absorb water results in a consequential degradation of its performance attributes. Addressing this issue, the integration of aerogels presents a viable solution; however, their direct incorporation has been observed to compromise mechanical properties, attributable to the effects [...] Read more.
The propensity of foamed concrete to absorb water results in a consequential degradation of its performance attributes. Addressing this issue, the integration of aerogels presents a viable solution; however, their direct incorporation has been observed to compromise mechanical properties, attributable to the effects of the interface transition zone. This study explores the incorporation of MTES-based aerogels into foamed cement via an impregnation technique, examining variations in water–cement ratios. A comprehensive analysis was conducted, evaluating the influences of MTES-based aerogels on the thermal conductivity, compressive strength, density, chemical composition, and microstructure of the resultant composites across different water–cement ratios. Our findings elucidate that an increment in the water–cement ratio engenders a gradual regularization of the pore structure in foamed concrete, culminating in augmented porosity and diminished density. Notably, aerogel-enhanced foamed concrete (AEFC) exhibited a significant reduction in water absorption, quantified at 86% lower than its conventional foamed concrete (FC) counterpart. Furthermore, the softening coefficient of AEFC was observed to surpass 0.75, with peak values reaching approximately 0.9. These results substantiate that the impregnation of MTES-based aerogels into cementitious materials not only circumvents the decline in strength but also bolsters their hydrophobicity and water resistance, indirectly enhancing the serviceability and longevity of foamed concrete. In light of these findings, the impregnation method manifests promising potential for broadening the applications of aerogels in cement-based materials. Full article
(This article belongs to the Special Issue Aerogels—Preparation and Properties)
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Figure 1

Figure 1
<p>Pore structure changes before and after immersion of FC and AEFC samples with different water–cement ratios.</p> Full article ">Figure 2
<p>Pore structures of (<b>a</b>) FC and (<b>b</b>) AEFC; and (<b>c</b>) SEM images of MTES-based aerogels in AEFC.</p> Full article ">Figure 3
<p>The synthetic procedure employed in this study involves hydrolysis (<b>a</b>) and condensation reactions (<b>b</b>).</p> Full article ">Figure 4
<p>(<b>a</b>) FTIR spectra of MTES-based aerogel, FC, and AEFC; and (<b>b</b>) hydrophobicity of FC and AEFC.</p> Full article ">Figure 5
<p>(<b>a</b>) Water absorption of FC at different times; (<b>b</b>) water absorption of AEFC at different times.</p> Full article ">Figure 6
<p>(<b>a</b>) Softening coefficient of FC and AEFC under different water–cement ratios; (<b>b</b>) relationship between water absorption and softening coefficient.</p> Full article ">Figure 7
<p>(<b>a</b>) Density and porosity of FC and AEFC; (<b>b</b>) compressive strength of FC and AEFC.</p> Full article ">Figure 8
<p>(<b>a</b>) Variation trend of thermal conductivity of FC and AEFC with water–cement ratios; and (<b>b</b>) comparison between experimental data and effective thermal conductivity calculated by different heat transfer models.</p> Full article ">Figure 9
<p>Schematic diagram of the experimental flow.</p> Full article ">
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