Research

12 pages, 3211 KiB

Open AccessArticle

Experimental Characterization of the Thermal Conductivity and Microstructure of Opacifier-Fiber-Aerogel Composite

by Hu Zhang, Chao Zhang, Wentao Ji, Xian Wang, Yueming Li and Wenquan Tao

Molecules 2018, 23(9), 2198; https://doi.org/10.3390/molecules23092198 - 30 Aug 2018

Cited by 29 | Viewed by 4691

Due to their high-porosity, nanoporous structure and pores, aerogel materials possess extremely low thermal conductivity and have broad potential in the thermal insulation field. Silica aerogel materials are widely used because of their low thermal conductivity and high temperature resistance. Pure silica aerogel [...] Read more.

Due to their high-porosity, nanoporous structure and pores, aerogel materials possess extremely low thermal conductivity and have broad potential in the thermal insulation field. Silica aerogel materials are widely used because of their low thermal conductivity and high temperature resistance. Pure silica aerogel is very fragile and nearly transparent to the infrared spectrum within 3–8 μm. Doping fibers and opacifiers can overcome these drawbacks. In this paper, the influences of opacifier type and content on the thermal conductivity of silica fiber mat-aerogel composite are experimentally studied using the transient plane source method. The thermal insulation performances are compared from 100 to 750 °C at constant pressure in nitrogen atmosphere among pure fiber mat, fiber mat-aerogel, 20% SiC-fiber mat-aerogel, 30% ZrO₂-fiber mat-aerogel and 20% SiC + 30% ZrO₂-fiber mat-aerogel. Fiber mat-aerogel doped with 20% SiC has the lowest thermal conductivity, 0.0792 W/m·K at 750 °C, which proves that the proper type and moderate content of opacifier dominates the low thermal conductivity. The pore size distribution indicates that the volume fraction of the micropore and mesopore is also the key factor for reducing the thermal conductivity of porous materials. Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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12 pages, 3059 KiB

Open AccessArticle

Preparation of Silica Aerogels by Ambient Pressure Drying without Causing Equipment Corrosion

by Lixiao Zhu, Yali Wang, Suping Cui, Feihua Yang, Zuoren Nie, Qunyan Li and Qi Wei

Molecules 2018, 23(8), 1935; https://doi.org/10.3390/molecules23081935 - 02 Aug 2018

Cited by 24 | Viewed by 4017

Abstract

The silica aerogels were prepared via a sol-gel technique and ambient pressure drying by using industrial solid wastes, dislodged sludges, as raw materials. A strategy was put forward to reduce the corrosion of equipment during the drying procedure. The pore structure, hydrophobicity, and [...] Read more.

The silica aerogels were prepared via a sol-gel technique and ambient pressure drying by using industrial solid wastes, dislodged sludges, as raw materials. A strategy was put forward to reduce the corrosion of equipment during the drying procedure. The pore structure, hydrophobicity, and thermal insulation property of the obtained samples were investigated in detail. The results show that the corrosion can be effectively avoided by using an equimolar mixture of trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDS) as silylation agents. At a Si:TMCS:HMDS molar ratio of 1:0.375:0.375, the silica aerogels possess a desirable pore structure with a pore volume of 3.3 ± 0.1 cm³/g and a most probable pore size of 18.5 nm, a high hydrophobicity with a water contact angle of 144.2 ± 1.1°, and a low thermal conductivity of 0.031 ± 0.001 W/(m∙K). Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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14 pages, 3219 KiB

Open AccessArticle

Salt-Mediated Au-Cu Nanofoam and Au-Cu-Pd Porous Macrobeam Synthesis

by Fred J. Burpo, Enoch A. Nagelli, Lauren A. Morris, Kamil Woronowicz and Alexander N. Mitropoulos

Molecules 2018, 23(7), 1701; https://doi.org/10.3390/molecules23071701 - 12 Jul 2018

Cited by 9 | Viewed by 3878

Abstract

Multi-metallic and alloy nanomaterials enable a broad range of catalytic applications with high surface area and tuning reaction specificity through the variation of metal composition. The ability to synthesize these materials as three-dimensional nanostructures enables control of surface area, pore size and mass [...] Read more.

Multi-metallic and alloy nanomaterials enable a broad range of catalytic applications with high surface area and tuning reaction specificity through the variation of metal composition. The ability to synthesize these materials as three-dimensional nanostructures enables control of surface area, pore size and mass transfer properties, electronic conductivity, and ultimately device integration. Au-Cu nanomaterials offer tunable optical and catalytic properties at reduced material cost. The synthesis methods for Au-Cu nanostructures, especially three-dimensional materials, has been limited. Here, we present Au-Cu nanofoams and Au-Cu-Pd macrobeams synthesized from salt precursors. Salt precursors formed from the precipitation of square planar ions resulted in short- and long-range ordered crystals that, when reduced in solution, form nanofoams or macrobeams that can be dried or pressed into freestanding monoliths or films. Metal composition was determined with X-ray diffraction and energy dispersive X-ray spectroscopy. Nitrogen gas adsorption indicated an Au-Cu nanofoam specific surface area of 19.4 m²/g. Specific capacitance determined with electrochemical impedance spectroscopy was 46.0 F/g and 52.5 F/g for Au-Cu nanofoams and Au-Cu-Pd macrobeams, respectively. The use of salt precursors is envisioned as a synthesis route to numerous metal and multi-metallic nanostructures for catalytic, energy storage, and sensing applications. Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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12 pages, 4067 KiB

Open AccessArticle

Thermal Failure Analysis of Fiber-Reinforced Silica Aerogels under Liquid Nitrogen Thermal Shock

by Ai Du, Mingfang Liu, Shangming Huang, Conghang Li and Bin Zhou

Molecules 2018, 23(7), 1522; https://doi.org/10.3390/molecules23071522 - 24 Jun 2018

Cited by 2 | Viewed by 3485

Abstract

Aerogel materials are recognized as promising candidates for the thermal insulator and have achieved great successes for the aerospace applications. However, the harsh environment on the exoplanet, especially for the tremendous temperature difference, tends to affect the tenuous skeleton and performances of the [...] Read more.

Aerogel materials are recognized as promising candidates for the thermal insulator and have achieved great successes for the aerospace applications. However, the harsh environment on the exoplanet, especially for the tremendous temperature difference, tends to affect the tenuous skeleton and performances of the aerogels. In this paper, an evaluation method was proposed to simulate the environment of exoplanet and study the influence on the fiber-reinforced silica aerogels with different supercritical point drying (SPD) technology. Thermal conductivity, mechanical property and the microstructure were characterized for understanding the thermal failure mechanism. It was found that structure and thermal property were significantly influenced by the adsorbed water in the aerogels under the thermal shocks. The thermal conductivity of CO₂-SPD aerogel increased 35.5% after the first shock and kept in a high value, while that of the ethanol-SPD aerogel increased only 19.5% and kept in a relatively low value. Pore size distribution results showed that after the first shock the peak pore size of the CO₂-SPD aerogel increased from 18 nm to 25 nm due to the shrinkage of the skeleton, while the peak pore size of the ethanol-SPD aerogel kept at ~9 nm probably induced by the spring-back effect. An 80 °C treatment under vacuum was demonstrated to be an effective way for retaining the good performance of ethanol-SPD aerogels under the thermal shock. The thermal conductivity increases of the ethanol-SPD aerogels after 5 shocks decreased from ~30 to ~0% via vacuum drying, while the increase of the CO₂-SPD aerogels via the same treatments remains ~28%. The high-strain hardening and low-strain soften behaviors further demonstrated the skeleton shrinkage of the CO₂-SPD aerogel. Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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10 pages, 7230 KiB

Open AccessArticle

One-Dimension Diffusion Preparation of Concentration-Gradient Fe₂O₃/SiO₂ Aerogel

by Ting Zhang, Haoran Wang, Bin Zhou, Xiujie Ji, Hongqiang Wang and Ai Du

Molecules 2018, 23(7), 1502; https://doi.org/10.3390/molecules23071502 - 21 Jun 2018

Cited by 6 | Viewed by 3363

Abstract

Concentration-gradient Fe₂O₃/SiO₂ aerogels were prepared by placing an MTMS (methyltrimethoxysilane)-derived SiO₂ aerogel on an iron gauze with an HCl atmosphere via one-dimensional diffusion, ammonia-atmosphere fixing, supercritical fluid drying and thermal treatment. The energy dispersive spectra show that [...] Read more.

Concentration-gradient Fe₂O₃/SiO₂ aerogels were prepared by placing an MTMS (methyltrimethoxysilane)-derived SiO₂ aerogel on an iron gauze with an HCl atmosphere via one-dimensional diffusion, ammonia-atmosphere fixing, supercritical fluid drying and thermal treatment. The energy dispersive spectra show that the Fe/Si molar ratios change gradually from 2.14% to 18.48% with a height of 40 mm. Pore-size distribution results show that the average pore size of the sample decreases from 15.8 nm to 3.1 nm after diffusion. This corresponds well with TEM results, indicating a pore-filling effect of the Fe compound. In order to precisely control the gradient, diffusion kinetics are further studied by analyzing the influence of time and position on the concentration of the wet gel. At last, it is found that the diffusion process could be fitted well with the one-dimensional model of Fick’s second law, demonstrating the feasibility of the precise design and control of the concentration gradient. Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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13 pages, 4114 KiB

Open AccessArticle

Cellulose Nanofiber Biotemplated Palladium Composite Aerogels

by Fred J. Burpo, Alexander N. Mitropoulos, Enoch A. Nagelli, Jesse L. Palmer, Lauren A. Morris, Madeline Y. Ryu and J. Kenneth Wickiser

Molecules 2018, 23(6), 1405; https://doi.org/10.3390/molecules23061405 - 09 Jun 2018

Cited by 16 | Viewed by 5335

Abstract

Noble metal aerogels offer a wide range of catalytic applications due to their high surface area and tunable porosity. Control over monolith shape, pore size, and nanofiber diameter is desired in order to optimize electronic conductivity and mechanical integrity for device applications. However, [...] Read more.

Noble metal aerogels offer a wide range of catalytic applications due to their high surface area and tunable porosity. Control over monolith shape, pore size, and nanofiber diameter is desired in order to optimize electronic conductivity and mechanical integrity for device applications. However, common aerogel synthesis techniques such as solvent mediated aggregation, linker molecules, sol–gel, hydrothermal, and carbothermal reduction are limited when using noble metal salts. Here, we present the synthesis of palladium aerogels using carboxymethyl cellulose nanofiber (CNF) biotemplates that provide control over aerogel shape, pore size, and conductivity. Biotemplate hydrogels were formed via covalent cross linking using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) with a diamine linker between carboxymethylated cellulose nanofibers. Biotemplate CNF hydrogels were equilibrated in precursor palladium salt solutions, reduced with sodium borohydride, and rinsed with water followed by ethanol dehydration, and supercritical drying to produce freestanding aerogels. Scanning electron microscopy indicated three-dimensional nanowire structures, and X-ray diffractometry confirmed palladium and palladium hydride phases. Gas adsorption, impedance spectroscopy, and cyclic voltammetry were correlated to determine aerogel surface area. These self-supporting CNF-palladium aerogels demonstrate a simple synthesis scheme to control porosity, electrical conductivity, and mechanical robustness for catalytic, sensing, and energy applications. Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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14 pages, 5817 KiB

Open AccessArticle

Facile Synthesis of Flexible Methylsilsesquioxane Aerogels with Surface Modifications for Sound- Absorbance, Fast Dye Adsorption and Oil/Water Separation

by Xingzhong Guo, Jiaqi Shan, Zhongzhang Lai, Wei Lei, Ronghua Ding, Yun Zhang and Hui Yang

Molecules 2018, 23(4), 945; https://doi.org/10.3390/molecules23040945 - 18 Apr 2018

Cited by 32 | Viewed by 5888

Abstract

New flexible methylsilsesquioxane (MSQ) aerogels have been facilely prepared by a sol–gel process with methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) as co-precursors, followed by surface modification and ambient pressure drying. The microstructure, mechanical properties and hydrophobicity of these MSQ aerogels after surface modifications of [...] Read more.

New flexible methylsilsesquioxane (MSQ) aerogels have been facilely prepared by a sol–gel process with methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) as co-precursors, followed by surface modification and ambient pressure drying. The microstructure, mechanical properties and hydrophobicity of these MSQ aerogels after surface modifications of hexamethyldisiloxane (HMDSO) and/or hexamethyldisilazane (HMDS) were investigated in detail, and the applications of surface-modified MSQ aerogels in sound-absorbance, fast dye adsorption and oil/water separation were evaluated, respectively. The MSQ aerogels surface-modified by HMDS possess flexibility, elasticity and superhydrophobicity, and demonstrate good performance in the mentioned applications. The resultant MSQ aerogel used in sound-absorbance has high frequency (about 6 kHz) acoustic absorptivity of up to 80%, benefiting from its macroporous structure and porosity of 94%, and it also possesses intermediate frequency acoustic absorptivity (about 1 kHz) up to 80% owing to its elasticity. This MSQ aerogel can selectively separate oil from oil/water mixtures with high efficiency due to its superhydrophobicity and superlipophilicity, resulting from a lot of methyl groups, density as low as 0.12 cm³·g⁻¹ and a water contact angle as high as 157°. This MSQ aerogel can be assembled to be a monolithic column applied for fast dye adsorption, and shows selective adsorption for anionic dyes and removal efficiency of methyl orange of up to 95%. Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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10 pages, 15267 KiB

Open AccessArticle

A Co-Precursor Approach Coupled with a Supercritical Modification Method for Constructing Highly Transparent and Superhydrophobic Polymethylsilsesquioxane Aerogels

by Chaoshuai Lei, Junning Li, Chencheng Sun, Hailong Yang, Tao Xia, Zijun Hu and Yue Zhang

Molecules 2018, 23(4), 797; https://doi.org/10.3390/molecules23040797 - 30 Mar 2018

Cited by 9 | Viewed by 4231

Abstract

Polymethylsilsesquioxane (PMSQ) aerogels obtained from methyltrimethoxysilane (MTMS) are well-known high-performance porous materials. Highly transparent and hydrophobic PMSQ aerogel would play an important role in transparent vacuum insulation panels. Herein, the co-precursor approach and supercritical modification method were developed to prepare the PMSQ aerogels [...] Read more.

Polymethylsilsesquioxane (PMSQ) aerogels obtained from methyltrimethoxysilane (MTMS) are well-known high-performance porous materials. Highly transparent and hydrophobic PMSQ aerogel would play an important role in transparent vacuum insulation panels. Herein, the co-precursor approach and supercritical modification method were developed to prepare the PMSQ aerogels with high transparency and superhydrophobicity. Firstly, benefiting from the introduction of tetramethoxysilane (TMOS) in the precursor, the pore structure became more uniform and the particle size was decreased. As the TMOS content increased, the light transmittance increased gradually from 54.0% to 81.2%, whereas the contact angle of water droplet decreased from 141° to 99.9°, ascribed to the increase of hydroxyl groups on the skeleton surface. Hence, the supercritical modification method utilizing hexamethyldisilazane was also introduced to enhance the hydrophobic methyl groups on the aerogel’s surface. As a result, the obtained aerogels revealed superhydrophobicity with a contact angle of 155°. Meanwhile, the developed surface modification method did not lead to any significant changes in the pore structure resulting in the superhydrophobic aerogel with a high transparency of 77.2%. The proposed co-precursor approach and supercritical modification method provide a new horizon in the fabrication of highly transparent and superhydrophobic PMSQ aerogels. Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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11 pages, 4337 KiB

Open AccessArticle

The Effect of Annealing Treatment and Atom Layer Deposition to Au/Pt Nanoparticles-Decorated TiO₂ Nanorods as Photocatalysts

by Shuang Shuang and Zhengjun Zhang

Molecules 2018, 23(3), 525; https://doi.org/10.3390/molecules23030525 - 09 Feb 2018

Cited by 21 | Viewed by 5261

Abstract

The wide band gap of TiO₂ hinders the utilization of visible light in high-performance photocatalysis. Herein, vertically aligned Ti nanopillar arrays (NPAs) were grown by the glancing angle deposition method (GLAD) and then thermally oxidized into TiO₂ NPAs. The metallic nanoparticles [...] Read more.

The wide band gap of TiO₂ hinders the utilization of visible light in high-performance photocatalysis. Herein, vertically aligned Ti nanopillar arrays (NPAs) were grown by the glancing angle deposition method (GLAD) and then thermally oxidized into TiO₂ NPAs. The metallic nanoparticles (NPs) were fabricated by successive ion layer adsorption and reaction (SILAR) method. And we covered ultrathin TiO₂ layer on Au/Pt NPs decorated NPA using atomic layer deposition (ALD) method and did annealing process in the end. The photoelectrochemical (PEC) performance and dye degradation have been studied. We find the dye degradation efficiency of best combination reaches up to 1.5 times higher than that of original Au/Pt-TiO₂ sample under visible light irradiation. The TiO₂ ALD layer effectively protects the nanostructure from corrosion and helps the transmission of electrons to the electrolyte. By controlling the annealing temperature we could achieve a matched band gap due to change in noble metal particle size. Our work demonstrates that rational design of composite nanostructures enhances the usage of broader wavelength range light and optimizes photocatalytic degradation of organic pollutants in practical applications. Full article

(This article belongs to the Special Issue Chemistry of Aerogels and Their Applications)

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