Enhancing aerogel mechanical properties with incorporation of POSS

Silica aerogels consisting of nanoparticles and numerous nano-pores have many attractive attributes. However, the weak mechanical properties severely limited the practical application of the silica aerogel. In this work, a facile approach was employed to strengthen the silica aerogel while the multi-alkoxy polyhedral oligomeric silsesquioxane (POSS) joined with methyltriethoxysilane (MTES) as the co-precursor. Three kinds of stiff core POSS with different amount of alkoxy groups were chosen to prepare aerogels. The result attributes showed all aerogels owned mesoporous structure (10–20 nm), high specific surface area (760–877 m² g⁻¹) and good thermal stability. Moreover, with the introduction of POSS, the mechanical properties had been apparently enhanced. The suitable addition of functional groups and the adjustment of cross-linking density made the aerogel own more room to deform and the skeleton still strong enough to endure large deformation. In addition, new peaks appeared in the XRD patterns at the same time. The preparation strategy of composite aerogels may guide a facile way to regulate the aerogel attributes. Furthermore, the aerogel potential application in oil-water separation has also been investigated.     

环氧化合物法制备金属氧化物气凝胶研究进展

环氧化合物法作为一种新的溶胶-凝胶制备金属氧化物气凝胶的方法近年来得到了长足发展。综述了环氧化合物法制备不同价态金属氧化物气凝胶的研究进展,并阐述了其在溶胶-凝胶过程中的作用机理     

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.     

细菌纤维素/热塑性聚氨酯复合气凝胶纤维制备及性能

使用绿色有机材料细菌纤维素(BC),并掺杂增强材料热塑性聚氨酯弹性体(TPU)经过湿法纺丝制备复合气凝胶纤维,通过傅里叶变换红外光谱(FTIR)、X射线衍射光谱(XRD)、热重分析(TG)、扫描电子显微镜(SEM)、全自动比表面孔隙度分析仪和单丝强力仪对制备的气凝胶纤维进行结构分析和性能表征,结果表明复合气凝胶纤维具有多孔结构,良好的力学性能和隔热性能,断裂强度达到24.69Mpa,断裂伸长为38.54%。     

常压干燥法制备SiO2气凝胶粉体

以工业硅溶胶为原料,通过凝胶过程与干燥条件的控制,采用常压干燥法制备了SiO2 气凝胶粉体,并考察了老化液中正硅酸乙脂(TEOS)含量和干燥控制化学添加荆甲酰胺的添加对气凝胶粉体堆积密度、比表面积和孔径分布的影响.结果表明:所得气凝胶粉体具有纳米多孔结构,组成气凝胶结构的基本粒子呈圆球形,粒径为10～25 nm,由基本粒子连接而成的网络结构具有5～50 nm的孔径分布;随着热处理温度从常温升至1 100℃,SiO2气凝胶从最初的无定形态转化为方石英;在,TEOS的醇溶液中老化,有利于增强凝胶骨架的强度;添加甲酰胺可以改善气凝胶粉体的孔径分布,提高其比表面积.     

In situ production of spherical aerogel microparticles

Due to their high surface area, low density, open pore structure and excellent insulation properties aerogels are intensively investigated since the past decades for a diverse range of applications. The current methods of silica aerogel production by supercritical extraction produce monolithic aerogels, where the sol is aged in molds and dried by extraction with supercritical CO₂. Aerogels in the form of spherical microparticles would be beneficial for many applications, for instance, drug delivery for respiratory route; or as insulating materials. However, because of aerogel's mechanical properties, it is difficult, rather impossible, to obtain spherical microparticles by milling or crushing of the monolithic aerogels. This work presents a new method to produce biocompatible spherical aerogel microparticles using an emulsion technique (in situ production) followed by supercritical extraction of the resulted dispersion (gel-oil). Water in oil emulsion was produced by mixing the sol (dispersed phase) with a vegetable oil (continuous phase) followed by the gelation of the dispersed phase. The size distribution of the final gel particles was found to be influenced by agitation, surfactant concentration and sol:oil volume ratios. The gel-oil dispersion was subsequently extracted with supercritical CO₂, Silica aerogel spherical microparticles with a surface area of 1100 m²g⁻¹, pore volume of 3.5 cm³/g and different mean particle diameters ranging from 200 μm to a few millimeters were produced using the presented method.     

Facile synthesis of hydrophobic,thermally stable,and insulative organically modified silica aerogels using co-precursor method

Silica aerogels have low density and high specific surface area, but there are restrictions regarding their durability and commercialization owing to their fragile nature and the strong moisture absorbing behavior of the siloxane network. To overcome these restrictions, this study evaluated hybrid organically modified silica (ORMOSIL) aerogels by employing 3-(trimethoxysilylpropyl) methacrylate (TMSPM) in tetraethyl orthosilicate (TEOS) through a two-step sol-gel co-precursor method. The methacrylate organic groups were incorporated into the silica networks via reactions between the Si-OH moieties in silica aerogels, resulting in ORMOSIL aerogels. The properties of the ORMOSIL aerogels were strongly affected by the amount of TMSPM co-precursor. The highest concentration of TMSPM (30 wt%) resulted in ORMOSIL aerogels with improved characteristics when compared with the pristine TEOS-based silica aerogels, such as hardness (0.15 GPa), Young's modulus (1.26 GPa), low thermal conductivity (0.038 W/m K), high water contact angle (140°), and high thermal stability (350 °C).     

The unusually formation of porous silica nano-stalactite structure by high temperature heat treatment of SiO2 aerogel synthesized from rice hull

The SiO₂ aerogel has attracted the attention of many researchers in recent years in terms of energy saving due to its properties such as very low density, high porosity, and superior insulation. The primary purpose of this study was to synthesize SiO₂ aerogel. For this purpose, the sodium silicate obtained from rice hull ash was used as pre-initiator and dried at ambient pressure and SiO₂ aerogel was synthesized. BET surface area of the obtained SiO₂ aerogel was found as 241 m²/g. The difference of this study is the structures obtained after this stage. After obtaining SiO₂ aerogel, it was subjected to heat treatment at a high temperature to increase the BET area of aerogel and after examining under SEM, it was noticed that some uncommon structures that have not seen before formed. After observing nano-stalactite type structures in the sample, the study was expanded on the causes of the formation of these structures. It was seen that this structure forming after heat treatment was a stalactite composed of SiO₂ nanoparticles having a crystal structure. This study examined the formation mechanism and some properties of this structure which was not encountered in previous studies. Brunauer-Emmett-Teller (BET), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis were conducted in order to determine the physical and chemical properties of both synthesized SiO₂ aerogel and nano-stalactite type structure.     

Carbon aerogel spheres prepared via alcohol supercritical drying

We have developed a method that would allow for the fabrication of carbon aerogel (CA) spheres. The inverse phase suspension polymerization of resorcinol and formaldehyde monomers with Na₂CO₃ as a catalyst followed by supercritical drying was explored. The effects of the chemical formulation and processing procedures and the conditions of the structures of organic and related carbon aerogels were studied. The experimental results indicated that it was easy to avoid the accumulation of polymerization heat during gelation, and easy to take out the products from the reaction container, through this fabrication method. Sol-gel microspheres with diameters ranging from about 30-1000μm could be obtained. After drying the sol-gel spheres under alcohol supercritical drying conditions, aerogel spheres with a bulk density of 0.8-1.0 g/cm³were prepared, and by subsequently pyrolyzing them, CA spheres with surface areas of 250-650 m²/g were obtained. The resultant CA spheres could be used as the electrode materials of supercapacitors. The specific capacitance of the CA spheres was as high as 215 F/g, and the equivalent series resistance at 48 Hz was about 1 Ω.     

Improvement of thermal insulation performance of silica aerogels by Al2O3 powders doping

Silica aerogel is deemed as a kind of high-performance thermal insulation materials. However, the existence of macropores in the structure is always ignored in the research and application of aerogels. Thus the thermal insulation performance of silica aerogels could be further improved if the macropores are reduced. In this work, nano-sized Al₂O₃ powders are explored as nano fillers to reduce the macropore volume fraction in silica aerogels by filling the big voids among the silica aggregates, and further lower the thermal conductivity. The experimental results showed that the macropore volume fraction (${\mathrm{V}}_{\mathrm{MAC}}$) was dramatically reduced from 63.05% to 23.12% with the addition of Al₂O₃ powders ranging from 0.0 g to 1.0 g. This trend was also verified by the variation of (V_T*-V_BET) and (V_BET/V_T*). Accordingly, the thermal insulation performance was improved due to the reduction of macropores in aerogels. The lowest thermal conductivity of Al₂O₃-doped aerogels reached 7.41 mW/(m K) in contrast with that of pure silica aerogels (9.00 mW/(m K)), which was a significant decline for aerogel-based materials due to the gaseous heat transfer being further weakened. Moreover, the increment of thermal conductivity from 7.41 to 9.71 mW/(m K) with the Al₂O₃ powders increasing could be attributed to the enhancement of solid heat transfer in the system. The variation of experimental thermal conductivity was in good agreement with the result of theoretical calculation. This study proposed an innovative idea to improve the thermal insulation of aerogel under ambient conditions.     

Nimesulide adsorbed on silica aerogel using supercritical carbon dioxide

Silica aerogel (SA) was loaded with nimesulide, a drug model compound, to demonstrate the potentiality of adsorption processes based on the usage of supercritical carbon dioxide to treat poorly water-soluble drugs, forming new kinds of drug delivery systems. Adsorption isotherms and kinetics were measured and described by models. The effect of pressure, temperature and solution concentration on loaded SA were also studied. Modelling of kinetic data showed that the sorption process was best described by a pseudo-second-order model. The adsorption isotherm data were best fitted by the Freundlich isotherm. The drug/SA composites were characterized using scanning electron microscopy, X-ray microanalysis, and FT-IR. Release kinetics of the adsorbed drug were also evaluated by in vitro dissolution tests. Results showed that nimesulide can be uniformly dispersed into the aerogel and that the release rate of nimesulide from the composite, constituted by drug and silica aerogel, is much faster than that of the crystalline drug.     

Flexible Electrospun strawberry-like structure SiO2 aerogel nanofibers for thermal insulation

Herein, a novel flexible SiO₂ aerogel composite nanofiber membrane with strawberry-like structure and excellent thermal insulation properties, in which SiO₂ aerogel particles act as thermal insulation filler, was prepared by electrospinning technology. With the addition of nano-pore structure SiO₂ aerogel particles, the heat transfer path of the fibers inside the membrane became discontinuous, endowing the as-prepared membrane an ultra-low thermal conductivity of 30.3 mW/(m?K) and large surface area of 240 m²/g. Moreover, the nanofibers membrane also possesses the combined merits of excellent fire resistance, high-temperature stability, and temperature-invariant flexibility, rendering it a promising in the application of insulation and gas adsorption. The successful preparation of this flexible nanofiber membrane paves a new way to design materials with excellent thermal insulation and adsorption properties.     

盐酸沉淀法制备纳米白炭黑

采用盐酸沉淀法制备纳米白炭黑,通过实验调整盐酸的浓度、添加剂的加入、反应体系的温度、pH值的大小。制得粒径小、品质好的纳米白炭黑。利用X ray衍射仪和扫描电镜分析,研究白炭黑的结构。     

Self-assembled flower-like FeS2/graphene aerogel composite with enhanced electrochemical properties

Graphene aerogel (GA) supported flower-like ferrous disulfide (FeS₂) composite was synthesis by a two-step self-assembly method using eco-friendly and low-cost precursors. The formation of well-crystallized pyrite FeS₂ and the reduction of graphene oxide (GO) was demonstrated by X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. According to the scanning electron microscopy images, the flower-like FeS₂ distributes uniformly on the inter-linking GA networks. The electrochemical tests indicate that the as-prepared GA-FeS₂ exhibits enhanced specific capacitance (313.6 F/g at the current density of 0.5 A/g), which is almost twice as high as that of bare FeS₂ (163.5 F/g). It is noticed that this composite also has excellent cyclability (88.2% retention after 2000 cycles at 10 A/g) and low transfer resistance. A symmetric supercapacitor device with wide potential range was assembled using GA-FeS₂, while its energy density could reach 22.86 Wh/kg. The excellent specific capacitance, good rate capability, and high energy density make it a promising candidate for next generation supercapacitors.     

Capacitive deionization of NaCl solution with carbon aerogel-silicagel composite electrodes

Carbon aerogel-silica gel composite materials with different kinds of ratios were used as the electrode-activematerials for a capacitive deionization (CDI) process, which were prepared by a paste rolling method. The electrochemical parameters such as current values, coulombs, specific coulombs and coulombic efficiencies were investigated using a unit cell. The composite electrodes showed good wettability and high mechanical strengths with adequate durability under 1,000 ppm NaCl solution. Also, the composite electrodes showed an effective cycle ability without destroying active materials from the composite electrode, which decreased the manufacturing time by 50%. Additions of silica gel obviously improved coulombic results and led to effective performance of the CDI process.     

用废弃煤矸石制备高比表面积的SiO2-Al2O3二元复合气凝胶

以兖州煤矸石为原料,经过低温焙烧、酸浸除杂、碱熔活化,采用溶胶-凝胶法和真空干燥法制备完整的块状SiO₂-Al₂O₃二元复合气凝胶。通过X射线衍射、扫描电镜、傅里叶变换红外光谱、氮气吸附等检测手段对原料、SiO₂-Al₂O₃气凝胶及其制备过程中得到的中间产物的物理、化学特性进行表征,研究SiO₂-Al₂O₃凝胶的形成机制。实验结果表明,煤矸石经过酸浸除去了大部分的铁、钾、碱土金属等杂质。在经过碱熔活化后,煤矸石主要组分石英和高岭石均转化为非晶态,反应活性提高。制备得到的SiO₂-Al₂O₃气凝胶是以Si-O-Si、Si-O-Al网络结构为骨架的非晶态纳米颗粒聚集体,其堆积密度0.37 g·cm^-3,比表面积483 m²·g^-1,比孔容1.87 cm³·g^-1,平均孔径10.29 nm,最可几孔径9.32 nm,具有较好的介孔特征。     

Production of silica aerogel microparticles loaded with ammonia borane by batch and semicontinuous supercritical drying techniques

Silica aerogel microparticles were prepared by supercritical drying and used as support for hydrogen-storing ammonia borane (AB). The formation of aerogel microparticles was done using two different processes: batch supercritical fluid extraction and a semicontinuous drying process. Silica aerogel microparticles with a surface area ranging from 400 to 800 m²/g, a volume of pores of 1 cm³/g, and a mean particle diameter ranging from 12 to 27 μm were produced using the two drying techniques. The particle size distribution (PSD) of the microparticles was influenced by shear rate, amount of catalyst, hydrophilic–hydrophobic solvent ratio and hydrophobic surface modification. In particular, irregular aerogel particles were obtained from hydrophilic gels, while regular, spherical particles with smooth surfaces were obtained from hydrophobic gels. AB was loaded into silica aerogel microparticles in concentrations ranging from 1% till 5% wt. Hydrogen release kinetics from the hydride-loaded aerogel was analyzed with a volumetric cell at 80 °C. By stabilization of AB into the silica aerogel microparticles, an improvement of the release rate of hydrogen from AB was observed.     

High-temperature resistant ambient pressure-dried aluminum doped silica aerogel from inorganic silicon and aluminum sources

Aluminum doped silica aerogel (ASA) exhibiting improved high-temperature resistance is usually prepared via supercritical drying from organic silicon and/or aluminum precursors, which propels the production cost significantly. Herein we demonstrate a simple and effective method to prepare highly thermal resistant ASA via the sol-gel and ambient pressure drying route by using water glass and aluminum chloride as precursors. Effects of the Al/Si molar ratio in precursor, the calcination temperature and the modifier type on the crystallinity, morphology, pore structure of ASA are investigated. Results show that the Al/Si molar ratio and the calcination temperature have significant effects on the structure and heat resistance performance of ASA at temperature of 600–1000 °C. The sample with Al/Si molar ratio of 0.15 shows the highest specific surface area of 805.9 m²/g and pore volume of 5.038 cm³/g after heated to 600 °C, and retains 179.5 m²/g and 1.295 cm³/g respectively after heated to 1000 °C. Mechanism analysis indicates that, though the actual aluminum content is extremely low (0.18%, wt%), the high-temperature resistance of ASA is greatly improved owing to the effective doping of aluminum in the lattice of SiO₂ and the corresponding electrostatic repulsion between neighboring nanoparticles induced by the replacement of Si⁴⁺ by Al³⁺ ions.     

A machinable carbon aerogel composite with a low thermal conductivity and enhanced mechanical properties

ABSTRACT

Carbon aerogels are prepared via the sol–gel polymerisation of resorcinol with formaldehyde, followed by supercritical drying and carbonisation. The fabricated carbon aerogels have low densities in the range 0.028–0.196?g?cm^?3, ultra-low thermal conductivities in the range 0.0259–0.0707?W?(m?K)^?1 and high specific surface areas (>520?m²?g^?1). The carbon aerogel composites are reinforced with short carbon fibres by adding the carbon fibres to the resorcinol–formaldehyde solution to reduce their brittle nature and improve their machinability. The compressive strength of the composites containing 2?wt-% carbon fibres is 1.75?MPa, which is 56% higher than that of pure carbon aerogel. Both fracture toughness and compressive strength of these composites are improved. These composites also have good machinability, with the ability to maintain their shape after being machined with traditional steel tools. Furthermore, the composites with nanoporous structure exhibit ultra-low thermal conductivity up to 1400°C.     

Opacifier embedded and fiber reinforced alumina-based aerogel composites for ultra-high temperature thermal insulation

A novel method was developed to uniformly disperse sub-micron TiO₂ opacifier into fiber reinforcements using agar and silica as binders via freeze drying. TiO₂ opacifier/ fiber/ alumina-based aerogel ternary (TFA) composites with high strength and excellent high-temperature thermal insulation were successfully prepared by sol-gel route, impregnation process and supercritical fluid drying. The microstructure, mechanical and thermal insulation properties of TFA composites were investigated comprehensively. The results show that the mechanical property of TFA composites can be significantly enhanced by mullite fiber felt and the incorporation of SiO₂ binder. The effect of TiO₂ opacifier on the high-temperature thermal conductivity was studied by adjusting the content of TiO₂ from 0 to 15 wt%. The obtained TFA composites exhibit high Young's modulus of up to 3.58 MPa and low high-temperature thermal conductivities of 0.129 W/m·K at 800 °C and 0.168 W/m·K at 1000 °C, respectively. The mechanism of heat transfer in TFA composites at high-temperature was also analyzed.