Foreign element doping and thermal stability of alumina aerogels

The addition of foreign elements is considered as an effective method to improve the thermal stability of alumina aerogels at a higher temperature. However, the location and stabilizing mechanism of the foreign elements in the alumina aerogel have not been carefully studied. In this work, Si or La was introduced into the network of alumina aerogels through a sol-gel strategy. The Si-doped alumina aerogel maintained high surface area (92 m²/g) and pore volume (0.572 cm³/g) even at 1300°C. The dopants prevented α-Al₂O₃ transformation at elevated temperatures (1200°C–1300°C). The distribution of foreign ions and their stabilizing mechanism were discussed in detail. The doped alumina aerogels reinforced by mullite fiber felt, with quite low density and thermal conductivity, can be used as high-temperature thermal insulations.     

Fabrication and characterization of the monolithic hydrophobic alumina aerogels

The monolithic hydrophobic mesoporous alumina aerogels were successfully synthesized by acid–base sol–gel polymerization of aluminium chloride hexahydrate (AlCl₃·6H₂O) in deionized water/alcohol solution (v/v = 3:7). To minimize shrinkage during drying, alumina hydrogels were aged in tetraethylorthosilicate (TEOS)/acetonitrile solution, and modified using trimethylchlorosilane (TMCS)/acetonitrile solution. Properties of the final product were examined by contact angle measurement, FTIR, FESEM, TEM and BET analyses. Surface modification was confirmed by FTIR spectroscopy. It was found that hydrophobic property of the alumina aerogels was affected by the contents of TMCS. When the molar ratio of TMCS to AlCl₃·6H₂O is 0.35, hydrophobic alumina aerogels shows lower bulk density (0.453 g/cm³) and higher surface area (495 m²/g) than those of unmodified alumina aerogels (0.933 g/cm³, 413 m²/g).     

Large size and low density SiOC aerogel monolith prepared from triethoxyvinylsilane/tetraethoxysilane

Crack-free silicon oxycarbide (SiOC) aerogel monolith was fabricated by pyrolysis of precursor aerogel prepared from triethoxyvinylsilane/tetraethoxysilane (VTES/TEOS) using sol-gel process and ambient drying. Effects of different precursors, the amount of base catalyst (NH₄OH) and the heating rate during pyrolysis on the properties such as monolithicity, bulk density, surface area and pore size distribution of aerogels were investigated. The results show that the crack-free SiOC aerogel can be easily obtained from VTES/TEOS as compared to that of methyltriethoxysilanes/tetraethoxysilane (MTES/TEOS) and phenyltriethoxysilanes/tetraethoxysilane (PhTES/TEOS) precursors. The influence of heating rate during pyrolysis process on shrinkage rate, ceramic yield and surface area of the SiOC aerogels could be ignored, while the variation in the amount of NH₄OH exerted a strong impact on the properties of SiOC aerogels. Increasing the amount of NH₄OH resulted in the decrease of bulk density and surface area of SiOC aerogels from 0.335 g/cm³ and 488 m²/g to 0.265 g/cm³ and 365 m²/g. The resultant SiOC aerogels exhibit high compressive strength (1.45–3.17 MPa). ²⁹Si MAS NMR spectra revealed the retention of Si-C bond in the SiOC aerogels after pyrolysis at 1000 °C. The present work demonstrates VTES/TEOS is a promising co-precursors to easily and low cost synthesize large size SiOC aerogel monolith.     

Organic/inorganic double-precursor cross-linked alumina aerogel with high specific surface area and high-temperature resistance

Alumina aerogel has drawn a great research interest in the aerogel community owing to its great high-temperature heat resistance. The typical preparation can be divided into two approaches depending on the type of precursors. However, the alumina aerogel derived from organic aluminum alkoxides suffers from poor monolithic integrity, whereas the one from inorganic aluminum salts presents unsatisfied thermal stability. In this work, we develop a novel organic/inorganic double-precursor cross-linking method to prepare alumina aerogels. Aluminum chloride hexahydrate is used to provide an acid condition for the hydrolysis and condensation reaction of aluminum sec-butoxide (ASB), and serves as a reactant to co-condensate with the hydrolyzed ASB to form an interpenetrating chain structure. The resulting alumina aerogel exhibits a high specific surface area (SSA) of 667 m²/g and good high-temperature thermal insulation performance. Moreover, it can still retain SSA of 224 and 105 m²/g after heating at 1000 and 1300°C, respectively.     

High surface area methyltriethoxysilane-derived aerogels by ambient pressure drying

In this paper we report the synthesis of methyltriethoxysilane (MTES) based aerogels by non-supercritical/ambient pressure drying. The alcogels have been aged in different concentrations of silane precursor solutions before drying and aerogels with low density and high porosity were obtained. The 60% vol silane aged aerogel shows a surface area of 416 m²/g with a pore volume of 0.99 cm³/g and a maximum surface area of 727 m²/g was obtained for 80% vol silane aged aerogel. The non-silane aged sample possess a surface area of 471 m²/g with a total pore volume of 0.83 cm³/g. The aerogels show broad pore-size distribution. The FT-IR studies reveal the retention of Si–C bond in the network and the formation of a hydrophobic gel. The ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS-NMR) studies were also employed to characterize the local environment around the silicon atoms and to obtain information on the condensation degree of the gel network. By varying the hydrolysis pH, highly flexible aerogels have also been successfully prepared. The porosity studies on the flexible aerogels are also presented here.     

Preparation of silica aerogel from oil shale ash by fluidized bed drying

The silica aerogel with high specific surface area and large pore volume was successfully synthesized using oil shale ash (OSA) via ambient pressure drying. The oil shale ash was burned and leached by sulfuric acid solution, and then was extracted using sodium hydroxide solution to produce a sodium silicate solution. The solution was neutralized with sulfuric acid solution to form a silica gel. After washing with water, the solvent exchange with n-hexane, and the surface modification with hexamethyldisilazane (HMDZ), the aged gel was dried by fluidization technique and also using a furnace to yield silica aerogels. The physical and textural properties of the resultant silica aerogels were investigated and discussed. The results have been compared with silica aerogel powders dried in a furnace. From the results, it is clear that the properties of silica powders obtained in fluidized bed are superior to that of powders dried in the furnace. Using fluidization technique, it could produce silica aerogel powders with low tapping density of 0.0775 g/cm³, high specific surface area (789 m²/g) and cumulative pore volume of 2.77 cm³/g.     

Enhanced thermal shrinkage behavior of phenolic-derived carbon aerogel-reinforced by HNTs with superior compressive strength performance

A novel carbon/m-HNTs composite aerogel was synthesized by introducing the modified halloysite nanotubes (m-HNTs) into phenolic (PR) aerogels through chemical grafting, followed with carbonization treatment. In order to explore the best proportion of HNTs to phenolic, the micromorphology of PR/m-HNTs were investigated by SEM before carbonization, confirming 10 wt% of m-HNTs is most beneficial to the porous network of aerogels. The interaction between PR and HNTs was studied by FTIR spectra, and microstructure evolution of the target product-carbon/m-HNTs composite aerogel were illustrated by SEM and TEM techniques. SEM patterns indicated that the carbon/m-HNTs aerogels maintain a stable porous structure at 1000 °C (carbonization temperature), while a ~20 nm carbon layer was formed around m-HNTs generating an integral unit through TEM analysis. Specific surface area and pore size distribution of composite aerogels were analyzed based on mercury intrusion porosimetry and N₂ adsorption–desorption method, the obtained results stayed around 500 m²g^?1 and 1.00 cm³g^?1 (pore volume) without significant discrepancy, compared with pure aerogel, showing the uniformity of pore size. The weight loss rate (26.76%) decreased greatly compared with pure aerogel, at the same time, the best volumetric shrinkage rate was only 30.83%, contributed by the existence of HNTs supporting the neighbor structure to avoid over-shrinking. The highest compressive strength reached to 4.43 MPa, while the data of pure aerogel was only 1.52 MPa, demonstrating the excellent mechanical property of carbon/m-HNTs aerogels.     

Ultralight,high-surface-area,multifunctional graphene-based aerogels from self-assembly of graphene oxide and resol

The self-assembly between graphene oxide sheets and resol-type phenolic prepolymers was investigated as a method to form three-dimensional porous carbon objects with high surface areas and low densities. After freeze-drying and subsequent pyrolysis of the assembled hydrogels, ultralight graphene/carbon composite aerogels with high surface areas and porosity, good conductivity, and well-defined bulk shape were obtained. By adjusting the amount of graphene oxide and resol in the precursor mixture, aerogels with a density as low as 3.2 mg/cm³ or a surface area as high as 1019 m²/g could be prepared. It is proposed that resol molecules are first adsorbed on the surface of graphene oxide sheets, and then the surface-coated sheets are crosslinked by the polymerization of resol prepolymers. The absorption performance was evaluated for the aerogel with the lowest density. Due to the high porosity, the aerogel displayed fast absorption rates for organic solvents as well as high absorption efficiencies. The high conductivity of the aerogels permits good performance as binderless monolithic electrodes for supercapacitors.     

Synthesis and thermal evolution of polysilazane-derived SiCN(O) aerogels with variable C content stable at 1600 °C

Ceramic aerogels possess intriguing thermophysical properties which make them excellent candidates for high temperature thermal insulators. However, their properties can degrade at high temperature because of crystallization phenomena or because of densification (causing a sensible reduction of their specific surface area and porosity).The polymer derived ceramic (PDC) route is a relatively new way of developing ceramic aerogels. Several aspects influence the properties of the final product when dealing with preceramic polymers, among them their chemical composition and molecular architecture.In this work, we investigated the possibility of producing aerogels belonging to the SiCN system from polysilazanes mixtures, namely perhydropolysilazane (PHPS) and a methyl/vinyl-containing polysilazane, namely Durazane 1800®, thus changing the C/Si ratio of the amorphous pyrolyzed products. It is shown that the chemical composition of the ceramic aerogel affects the main properties of the porous materials, such as thermal stability and specific surface area (SSA). Results show that the presence of carbon in the aerogels inhibits crystallization of Si₃N₄ up to 1600 °C in N₂ and allows to maintain a SSA of ~90 m²/g up to this temperature.     

Ultralight conducting polymer/carbon nanotube composite aerogels

By embedding carbon nanotubes into poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) supermolecular hydrogels in the presence of a very small amount of polyvinyl alcohol (PVA), we have presented the fabrication of ultralight conducting polymer/carbon nanotube composite aerogels with the apparent density of 0.04–0.07 g/cm³ made by supercritical CO₂ drying of as-made composite hydrogel precursors. The carbon nanotubes employed here are directly applicable to pristine (MWCNTs) or acid treated (c-MWCNTs) multi-wall nanotubes. Infra-red spectroscopy is used to confirm that PVA used for stabilizing nanotubes during the synthesis of hydrogel precursors has been completely removed by solvent exchange before supercritical CO₂ drying. The morphology and textural properties of the resultant composite aerogels are investigated by scanning electron microscopy, nitrogen adsorption/desorption, and X-ray powder diffraction tests. The thermal stability, together with electrical conductivities, of the resulting composite aerogels is revealed by the thermal gravitational analysis as well as conductivity tests. The results show that embedding of either MWCNTs or c-MWCNTs into PEDOT–PSS aerogel matrix can significantly enhance the specific surface areas (280–400 m²/g), the thermal stability and electrical conductivities (1.2–6.9 × 10⁻² S/cm) of the resulting composite aerogels.     

Improvement of thermal stability of ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> aerogels by an inorganic–organic synergetic surface modification

The thermal stability of ZrO₂–SiO₂ aerogels was significantly improved by inorganic–organic synergetic surface modifications: inorganic ions [Fe(III)] surface modification and hexamethyldisilazane gas phase modification. The replacement of Hs from surface hydroxyl groups on the aerogel by Fe(III) ions and silyl groups played a critical role in isolating the hydrous particles of ZrO₂–SiO₂ aerogels. So the particle growth caused by the condensation of hydroxyl groups upon firing was inhibited. Meanwhile, the decomposition of the silyl groups upon heat treatment produced SiO₂ particles, which could serve as pining particle to inhibit the crystallization of ZrO₂. Hence, the porous microstructure of the modified aerogels was still well preserved up to 1000 °C, with a high specific surface area of 203.5 m²/g, and a considerable pore volume of 0.721 cc/g. These characteristics of the modified aerogels suggest that it has great potential on ultrahigh-temperature applications in the fields of thermal insulation, catalysis, and catalyst support, etc.     

Synthesis and characterization of a xonotlite fibers–silica aerogel composite by ambient pressure drying

Xonotlite fibers (XFs) reinforced silica aerogel composites were prepared by a sol–gel method under ambient pressure drying. XFs were synthesized through a dynamic hydrothermal route and had a noodle-like structure with length of 5–10 μm and average diameter of 150–200 nm. The microstructure analysis showed that XFs were inlaid in silica aerogel matrix by physical combination which contributed to restrict the volume shrinkage of alcogels and maintain the integrality aerogels during drying process. The physical, naonporous and thermal properties of the as prepared aerogel composites were investigated and discussed in detail. The new aerogel composites possessed porous nanostructure, which exhibited typical properties of 0.126 g/cm³ density, 4.132 cm³/g pore volume, and thermal conductivity of 0.0285 W/(m K). The results indicated that the introduced XFs didn’t significantly alter the porosity, hydrophobicity or thermal conductivity of aerogel matrix. It was also found that the aerogel composites had much more outstanding porosity than that of pure aerogel upon calcinations at 800 °C. This study fabricated XFs–silica aerogel composites and explored a new way for silica aerogels to endure and remain monolithic under ambient pressure drying.     

Preparation of monolith SiC aerogel with high surface area and large pore volume and the structural evolution during the preparation

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogel was synthesized by sol–gel process followed by supercritical drying (SCD). Monolithic SiC aerogel was obtained from RF/SiO₂ aerogel after carbothermal reduction. The evolution of physical property, crystal structure, morphology and pore structure from RF/SiO₂ to SiC aerogel was investigated by different methods, such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and N₂ adsorption/desorption. The as-synthesized SiC aerogel presented typical mesoporous structure and possessed high porosity (91.8%), high surface area (328 m²/g) and large pore volume (2.28 cm³/g). Carbothermal reduction mechanism was also discussed based on the experiment and characterization results.     

Facile preparation for gelatin/hydroxyethyl cellulose-SiO2 composite aerogel with good mechanical strength,heat insulation,and water resistance

The advanced thermal insulation materials with low cost and high mechanical properties play an important role in transport packaging and thermal protection fields. An inorganic/organic composite aerogel was prepared through hydrogen bonds and chemical crosslinking among silica aerogel particles, gelatin (GA), and hydroxyethyl cellulose (HEC). The as-prepared GA/HEC-SiO₂ composite aerogels were characterized by compression tests, scanning electron microscopy, Fourier transform infrared, thermogravimetric analyzer, and contact angle tests to investigate the chemical composition and physical structure. The GA/HEC-SiO₂ composite aerogels exhibited a strong mechanical strength (0.53–4.01 MPa), a high compression modulus (1.33–11.52 MPa), a lower volume density (0.035–0.081 g/cm³), thermal conductivity as low as 0.035 W/[m K]), a porosity of more than 93%, and hydrophobic angle as high as 150.01° after hydrophobic modification. These results indicate that biopolymer composite aerogels embedded with SiO₂ aerogel particles display a bright future in thermal insulation.     

Effects of Solvent Exchange Period and Heat Treatment on Physical and Chemical Properties of Rice Husk Derived Silica Aerogels

In this study, hydrophobic silica aerogels were synthesized from rice husk ash-derived sodium silicate through sol-gel processing, solvent exchange, surface modification and ambient pressure drying. By volume, 10% of trimethylchlorosilane (TMCS) in 90% of n-hexane was used as a hydrophobic solution in the surface modification process. The physical and chemical properties of silica aerogels were characterized by density and porosity measurements, scanning electron microscopy (SEM), Fourier transforms infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller theory (BET) and dynamic scanning calorimetry (DSC). The hydrogels prepared were in the form of 2.5 ± 0.5 mm beads and then converted into alcogels through solvent exchange with ethanol for repetition of 3, 6 and 9 days. It is found that the optimal quality of silica aerogels with the BET surface area as high as 668.82 m²/g was obtained from the alcogels of the solvent exchange period of 9 days. Depending on the size of the gel’s block, a longer solvent exchange period will ensure adequate removal of pore water. Post heat treatment on silica aerogels obtained from the 9 days of solvent exchange at 200, 300 and 400 °C for 2 h results in slight decreased of aerogel’s density from 0.048 g/cm³ to 0.039 g/cm³ and the hydrophobicity of the aerogels is decreased above 380 °C as confirmed by DSC analysis.

     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.     

Facile preparation of ultralight polymer-derived SiOCN ceramic aerogels with hierarchical pore structure

SiOCN ceramic aerogels with lightweight, high surface areas, and macro-meso pores have been synthesized by a facile method combining freeze-drying technique and polymer-derived ceramic route. The wet gels are synthesized via the hydrosilylation reaction between polysilazane and divinylbenzene with cyclohexane as solvent. The solvent is then removed by a freeze-drying process to form pre-ceramic aerogels. The SiOCN ceramic aerogels are finally obtained by pyrolyzing the pre-ceramic aerogels at 1000°C in ultrahigh purity N₂. The thermogravimetric and mass spectrometry system (TG/DSC-MS) is used to investigate the polymer-to-ceramic conversion process during pyrolysis. The phase composition, structure, and morphology of the SiOCN ceramic aerogels are investigated by XRD, FT-IR, XPS, and SEM. The results show that SiOCN ceramic aerogels are composed of amorphous matrix phase and “free carbon” phase. The SiOCN aerogels possess three-dimensional (3D) network porous structure with low density (0.19 g/cm³), high specific surface area (134 m²/g), large pore volume (0.49 cm³/g), and hierarchical pore structures of both macro and meso pores. The formation mechanism and evolution process of SiOCN ceramic aerogels are discussed.     

Preparation and high-temperature service performance of hierarchically pore-structured BN fiber aerogels

Multifunctional aerogels with high porosity and good thermal insulation have attracted much attention in the field of energy and aerospace engineering. In this work, a three-dimensional BN fiber aerogel with hierarchically porous structure was prepared through a freeze-drying combined with in-situ carbothermal reduction nitridation route. The synthesized BN fiber aerogel exhibits a specific surface area of 154.3 m²/g, a high porosity of 96.8% and hydrophobicity. Moreover, the BN fiber aerogel shows a low thermal conductivity of 0.051 W/(m·K) and excellent thermal insulation properties owing to its hierarchical porous structure. Particularly, the BN fiber aerogel still maintains its low thermal conductivity and a rather high mechanical strength after re-heated at 1473 K for 3 h in Ar atmosphere, suggesting excellent high-temperature service performance. The successfully developed multifunctional BN fiber aerogel holds promising potential in high-temperature thermal insulation fields.     

High surface area SiC/silicon oxycarbide glasses prepared from phenyltrimethoxysilane-tetramethoxysilane gels

Phenyltrimethoxysilane (PhTMS) was hydrolyzed or cohydrolyzed with tetramethoxysilane (TMOS) to make aerogels and xerogels. Porous SiC/silicon oxycarbide glasses were prepared by further pyrolyzing these gels in inert atmosphere up to 1500°C. The pore structure and chemical nature were studied by nitrogen and water sorption measurement, chemical analysis, X-ray diffraction and scanning electron microscopy. It has been found that the addition of PhTMS into TMOS gels decreased the surface area and porosity of TMOS gels, but enhanced their hydrophobicity and thermal stability. Pyrolyzing 25 mole% PhTMS-75 mole% TMOS aerogel in argon resulted in porous silicon oxycarbide glass which has a surface area of 581 m²/g at 1000°C. Pyrolyzing pure PhTMS gels at 1400°–1500°C produced porous SiC/C/silicon oxycarbide composites having surface areas in the range of 400–500 m²/g.Also with the Department of Agronomy.     

Functionalisation and chemical characterisation of cellulose-derived carbon aerogels

A series of nitrogen- and oxygen-functionalised carbon aerogels was produced from a carbon aerogel derived from cellulose acetate. Samples were oxidised by H₂O₂ or HNO₃ and/or enriched in nitrogen by reaction with gaseous ammonia or co-heating of the carbon aerogel and melamine. Porosity variations and morphology were monitored using N₂ adsorption and helium pycnometry. The surface chemistry was characterised by elemental analysis, FTIR and XPS spectroscopy, pH of the point of zero charge and acid/base titration. The prepared carbons are mainly mesoporous and show a moderate porosity development (S_BET between 160 and 300 m²/g). The applied chemical methods allow producing a wide range of functionalised carbon aerogels differing in terms of oxygen and nitrogen groups, their distribution and basicity. Both oxidation methods introduce a similar amount of oxygen, while the produced carbons differ in term of their acid/base character. Treatment with ammonia produces the most basic materials, which is partly due to the introduction of basic nitrogen groups, but also to the reduction of the acidic oxygen functionalities.