Synthesis and Characterization of Hydrophobic Silica Aerogels Using Trimethylethoxysilane as a Co-Precursor

The hydrophobic property is one of the most important requirements for the long-term use of silica aerogels for transparent or translucent window insulation and opaque thermal insulating systems. Therefore, the present paper deals with the synthesis and characterization of hydrophobic silica aerogels using trimethylethoxysilane (TMES) as a co-precursor. Silica sol was prepared by keeping the molar ratio of tetramethoxysilane (TMOS) precursor, methanol (MeOH) solvent, water (H₂O) and ammonia (NH₄OH) catalyst constant at 1:12:4:3.7 × 10^–3 respectively throughout the experiments and the TMES/TMOS molar ratio (A) was varied from 0 to 2.35. The resulting silica alcogels were dried supercritically by high-temperature alcohol solvent extraction. Hydrophobicity of the aerogels was tested by measuring the percentage of water adsorbed by the aerogels after putting them directly on the surface of water under humid conditions. Alternately, the hydrophobicity was also tested by contact angle measurements. It was found that as the A value increased, the hydrophobicity of the aerogels increased but the optical transmission decreased from 93% to less than 5% in the visible range. The thermal stability of the aerogels was studied in the temperature range from 25 to 400°C. The hydrophobic nature of the aerogels was maintained up to a temperature of 300°C. The aerogels were characterized by infrared spectroscopy, optical transmittance, Scanning electron microscopy (SEM) and contact angle measurements. The results have been discussed by taking into account the hydrolysis and condensation mechanisms.     

Porous structure of polybenzoxazine-based organic aerogel prepared by sol–gel process and their carbon aerogels

New organic aerogels were successfully prepared from a new class of phenolic resins called polybenzoxazines synthesized via conventional thermal curing reaction of a benzoxazine monomer using xylene as a solvent. Without the need for using supercritical conditions to remove the solvent during the process, the carbon aerogels were obtained with a much shortened time. From two different concentrations of benzoxazine solution, 20 and 40 wt%, the resulting polybenzoxazine aerogels, having densities of 260 and 590 kg/m³, respectively, were obtained after the curing process. The subsequent carbon aerogels were prepared by the carbonization of polybenzoxazine aerogels. The corresponding carbon aerogels exhibited a microporous structure with pore diameters less than 2 nm, the densities of 300 and 830 kg/m³, and surface area of 384 and 391 m²/g, respectively. The texture of the carbon aerogels was denser than that of their organic aerogel precursor, as evidenced by scanning electron microscopy. The transformation of the polybenzoxazine aerogel to the carbon aerogel was clearly observed using fourier transform infrared spectroscopy.     

炭气凝胶纳米颗粒固定葡萄糖氧化酶的直接电化学

利用间苯二酚和甲醛在碱性环境下制备炭气凝胶(CA), 通过扫描电镜(SEM)、透射电镜(TEM)、比表面积测试Brunauer-Emmett-Teller (BET)等方法分析载体的形貌结构; 以CA为载体通过吸附法固定葡萄糖氧化酶(GOD)并修饰玻碳(GC)电极, 得到GOD/CA/GC电极. 在0.1 mol·L^-1磷酸盐缓冲溶液中, 利用循环伏安法研究了GOD/CA/GC 电极的直接电化学行为和对葡萄糖的催化性能. 结果表明, 以CA为载体可以很好地固定GOD并保持其生物活性, 在无任何电子媒介体存在时, GOD在电极上实现了直接电子转移, GOD/CA/GC电极对葡萄糖具有很好的电催化性能.     

Polydopamine-Coated Poly-Lactic Acid Aerogels as Scaffolds for Tissue Engineering Applications

Poly-L-lactic acid (PLLA) aerogel-based scaffolds were obtained from physical PLLA gels containing cyclopentanone (CPO) or methyl benzoate (BzOMe) molecules. An innovative single step method of solvent extraction, using supercritical CO₂, was used to achieve cylindrical monolithic aerogels. The pore distribution and size, analyzed by SEM microscopy, were found to be related to the crystalline forms present in the physical nodes that hold the gels together, the stable α’-form and the metastable co-crystalline ε-form, detected in the PLLA/BzOMe and PLLA/CPO aerogels, respectively. A higher mechanical compressive strength was found for the PLLA/CPO aerogels, which exhibit a more homogenous porosity. In vitro biocompatibility tests also indicated that monolithic PLLA/CPO aerogels exhibited greater cell viability than PLLA/BzOMe aerogels. An improved biocompatibility of PLLA/CPO monolithic aerogels was finally observed by coating the surface of the aerogels with polydopamine (PDA) obtained by the in situ polymerization of dopamine (DA). The synergistic effect of biodegradable polyester (PLLA) and the biomimetic interface (PDA) makes this new 3D porous scaffold, with porosity and mechanical properties that are tunable based on the solvent used in the preparation process, attractive for tissue engineering applications.     

Wettability study of surface modified silica aerogels with different silylating agents

In wettability study, surface free energy interactions are of crucial importance for silica aerogels in which absorption of organic liquids and transportation of chemicals carried out for chemical and biotechnological applications. In present study, we have used Lifshitz–van der Waals/acid–base approach for calculation of surface free energy of aerogel sample. We have investigated that the surface free energy values of aerogels are 45.95, 51.42 and 45.69 mJ/m² by modifying their surfaces using 7 % chlorotrimethylsilane (TMCS), dimethyldichlorosilane (DMDCS) and hexamethyldisilazane (HMDZ) silylating reagents with solvent, respectively. The alcogels were prepared by two step acid–base catalyzed process where the molar ratio of precursors tetraethoxysilane:methanol:oxalic acid:NH₄OH:NH₄F was kept at optimal value of 1:16.5:0.71:0.58:0.60:0.98, respectively. To modify gel surfaces, TMCS, DMDCS and HMDZ concentration have been varied from 5 to 12 % and such alcogels were dried at ambient pressure. The aerogels have been characterized by fourier transform infrared spectroscopy, scanning electron microscopy, thermo-gravimetric and differential thermal analysis and Wetting properties of silica aerogel surfaces was studied by contact angle measurements. The surface chemical composition of DMDCS modified silica aerogels was studied by using X-ray photoelectron spectroscopy. As there is not any direct method, we have used Lifshitz–van der Waals/acid–base approach which gives, polar and non-polar components of aerogels surface free energy.     

Nanofibrillar cellulose aerogels

Highly porous aerogels consisting of cellulose nanofibrils were prepared by dissolution/regeneration of cellulose in aq. calcium thiocyanate followed by regeneration and carefully controlled drying. The influence of drying method (regular freeze drying, rapid freeze drying, and solvent exchange drying) on resulting porosity was studied by electron microscopy and nitrogen adsorption. While regular freeze drying caused significant coalescence of microfibrillar units, solvent exchange drying gave highly porous aerogel composed of approx. 50 nm-wide cellulose microfibrils. Correspondingly, specific surface area of the solvent-exchange-dried aerogels ranged 160–190 m²/g, in contrast to 70–120 m²/g of regular freeze-dried materials. Rapid freeze technique using liquid nitrogen-cooled metal plate gave aerogel sheets with asymmetrical porosity, with the face contacted by copper having porous structure similar to those of solvent-exchange dried material.     

Transparent and flame retardant cellulose/aluminum hydroxide nanocomposite aerogels

Cellulose-based nanocomposite aerogels were prepared by incorporation of aluminum hydroxide (AH) nanoparticles into cellulose gels via in-situ sol-gel synthesis and following supercritical CO₂ drying. The structure and properties of cellulose/AH nanocomposite aerogels were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, ultraviolet-visible spectrometry, N₂ adsorption, thermogravimetric analysis, and micro-scale combustion calorimetry. The results indicated that the AH nanoparticles were homogeneously distributed within matrix, and the presence of AH nanoparticles did not affect the homogeneous nanoporous structure and morphology of regenerated cellulose aerogels prepared from 1-allyl-3-methylimidazolium chloride solution. The resultant nanocomposite aerogels exhibited good transparency and excellent mechanical properties. Moreover, the incorporation of AH was found to significantly decrease the flammability of cellulose aerogels. Therefore, this work provides a facile method to prepare transparent and flame retardant cellulose-based nanocomposite aerogels, which may have great potential in the application of building materials.     

纳米银负载的炭气凝胶制备及抗菌性能研究

以常压干燥法制备的酚醛有机气凝胶为原料,通过在硝酸银溶液中浸渍使银吸附或沉积在有机气凝胶上,在常压条件下干燥并炭化制得纳米银负载的炭气凝胶。利用扫描电镜(SEM)、透射电镜(TEM)、X-射线衍射(XRD),比表面积及孔径分析等方法研究了浸渍前后凝胶密度和结构的变化,以及制备条件对银负载炭气凝胶的载银量和凝胶结构的影响;研究了载银炭气凝胶对大肠埃氏杆菌(ATCC25922)和金黄色葡萄球菌(ATCC25923)的抗菌效果。结果表明,这类银负载炭气凝胶对大肠杆菌和金黄色葡萄球菌均有很强的杀灭能力,预期炭气凝胶在作为催化剂或吸附杀菌材料方面有良好的应用前景。     

High surface area alumina-supported nickel (II) oxide aerogels using epoxide addition method

Nickel (II) oxide aerogels with an amorphous alumina support were synthesized by the expoxide addition method. The monoliths were obtained by adding propylene oxide to an alcoholic solution of hydrated metal nitrate salts. The wet gels were dried by supercritical extraction to produce porous monolithic aerogels. The as-synthesized aerogels were amorphous containing aluminum and nickel hydroxides. Annealing of the as-synthesized aerogels at 400 °C yields crystalline nickel oxide materials which retain the high surface areas (>160 m²/g) and porosities of the original aerogels. The resultant aerogel materials were characterized using powder X-ray diffraction, thermo-gravimetric analysis, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, and nitrogen adsorption/desorption analysis.     

Early Stages of Crystallization in Gel Derived ZrO2 Precursors

Three different precursor materials giving rise to contrasted nanostructures, xerogels, aerogels and precipitates, are prepared by a sol gel route in the Zrn -propoxide—acetylacetone—water—n-propanol system. Clear homogeneous gels are made by using a proper amount of acetylacetone and water. The gels are dried either by conventional processing (xerogels) or by supercritical evacuation of alcoholic solvent (aerogels). The complexation ratio (R = [acetylacetone]/[Zr(OR)₄]) is the main parameter controlling the size of ZrO₂ primary particles. WhenR = 0 , precipitates are obtained.Xerogels, aerogels and precipitates are characterized and their textures are compared through small angle X-ray scattering measurements. The fractal structure of gels is destroyed by conventional drying and is preserved in aerogels. On the other hand precipitates are described as homogeneous agglomerates of very small primary units.The first crystallization steps are studied by transmission electron microscopy and X-ray diffraction experiments. The contribution of crystallite size and microstrain effects are investigated by Rietveld whole pattern fitting. The crystallization of precipitate powders starts at the agglomerate scale with large crystal like distorted lattices.     

碳气凝胶的制备研究

气凝胶是由纳米量级超细粒子或高聚物分子聚结构成的多孔性轻质固态材料 ,它在声学、光学、电学、动力学和低温热学等方面具有独特性质 ,因此受到多方面研究者的重视 [1～ 4 ] .碳气凝胶最先是由 Pekala等 [5] 在 80年代末研制成功的 ,其突出特点是网络连续 ,电导率高 ,孔洞微小且相互贯通 ,比表面大 ,密度变化范围大 ,是制造高性能电容器和电池的新一代理想材料[6 ,7] .这种材料的低温电导率随温度连续单调变化 ,在一定温区范围内 ( <1 0 0 K)其电阻温度敏感度远大于传统的掺碳玻璃、锗以及其它金属电阻温度计材料 ,可望成为一种理想的低…     

Chitosan-based aerogels with high adsorption performance

New natural polymer-based aerogels, cross-linked chitosan aerogels, were prepared by the sol-gel route with glutaradehyde, glyoxal, and formaldehyde as cross-linkers. The alcogels were dried by supercritical carbon dioxide (CO2) fluid extraction. The resulting materials were characterized using scanning electron microscopy (SEM), nitrogen adsorption/desorption analysis, and Fourier transform infrared (FT-IR) spectroscopy. Furthermore, the adsorption of the anionic surfactant sodium dodecylbenzene-sulfonate (SDBS) from aqueous solution by the materials was investigated. The aerogels exhibit high adsorption capability, can remove SDBS from acidic aqueous solutions, and have potential applications in controlling SDBS pollution.     

Preparation and characterization of ZrCO/C composite aerogels

Zr-containing organic aerogels were synthesized by ligand substitution reaction of polyzirconoxone and 2, 4-dihydroxybenzoic acid, followed by polymerization with formaldehyde, and then supercritical drying using CO₂. After carbonization and carbothermal reduction under an argon atmosphere, ZrCO/C composite aerogels with controllable zirconium content (47.8–78.6 wt%) were obtained. The carbothermal reduction was substantially completed at 1,500 °C, and the obtained ZrCO/C composite aerogels exhibit low oxygen contents (9.4–6.7 wt%) and high surface areas (589–147 m²/g). Pore morphologies of the ZrCO/C composite aerogels were investigated in detail by nitrogen sorption measurements, scanning electron microscopy and its associated energy-dispersive X-ray microanalysis measurements. The results show that the aerogels are composed of carbon framework and Zr-conglomerations, and the surface area of aerogel is severely affected by its zirconium content. The presence of reductive ZrC crystals can greatly enhance the oxidation resistance ability of amorphous carbon framework and prevent collapse.     

Inter- and intra-primary-particle structure of monolithic carbon aerogels obtained with varying solvents

Different carbon aerogels were obtained by carbonization of organic aerogels prepared from the polymerization of resorcinol and formaldehyde using potassium carbonate as catalyst. Various solvents were added to the initial mixture to study their effects on the inter- and intra-primary-particle structure of the carbon aerogels. To carry out this study, various techniques were used, including high-resolution transmission and scanning electron microscopy, mercury porosimetry, mechanical tests, N2 and CO2 adsorption at -196 and 0 degrees C, respectively, and immersion calorimetry into benzene. Variation of the solvent used produced changes in the gelation time of the organic aerogels, which gave rise to variations in the inter- and intra-primary-particle structure of the carbon aerogels obtained. The monolith density of the carbon aerogels ranged from 0.37 to 0.87 g/cm3. Samples with a density higher than 0.61 g/cm3 had micropores and mesopores but no macropores. Macro- and mesoporosity had a monomodal pore size distribution. The elastic modulus showed a scaling relationship with density. In all samples studied, which had a mean micropore width of 0.62-1.06 nm, the surface area obtained by enthalpy of immersion into benzene yielded a realistic value of their total surface area.     

纳米纤维素基多层级孔道结构碳气凝胶的制备及在锂电池中的应用

采用纳米精磨法对商品桉木浆进行纳米纤丝化处理,得到了高长径比、尺寸均一的纳米纤丝化纤维素(NFC),平均直径为230.10 nm,长度达数十微米.将其组装、干燥后制得具有大量介孔的纳米纤丝化纤维素气凝胶(NFCA).将NFCA在氮气氛围下高温碳化制得碳气凝胶(CNFA),或在氢氧化钾条件下辅助碳化制得具有多层级孔道结构的碳气凝胶(CNFA-A),在保留的碳气凝胶骨架结构上进行孔洞构建.通过扫描电子显微镜(SEM)、透射电子显微镜(TEM)表征及Nanomeasure统计分析,发现NFC的平均直径经碳化后减小到53.16 nm.利用X射线衍射(XRD)、BET比表面积测试和拉曼光谱揭示了碳化处理对纳米纤维素结构、比表面积、石墨化程度和缺陷的影响.结果表明,KOH辅助碳化处理后的碳气凝胶不仅保留了纤维素气凝胶前驱体的网络结构,还在其骨架上二次构建了更多的微孔和介孔,其比表面积高达488.92 m2/g,总孔容为0.404 cm3/g,所得的碳骨架被部分石墨化,具有良好的导电性.这类源于生物质的高比表面积碳气凝胶在被用作锂离子电池(LIB)负极材料时表现出优异的电化学性能,在电流密度1 A/g下连续充放电1000次后比容量达到409 m A·h/g,在电流密度高达20 A/g下,比容量还能维持在219 m A·h/g.     

Adsorption of benzene, toluene, and xylenes on monolithic carbon aerogels from dry air flows

Monolithic carbon aerogels were obtained by carbonization of organic aerogels prepared by polymerization of resorcinol and formaldehyde under different conditions. Some carbon aerogels obtained were further CO2-activated. Samples were characterized by gas adsorption, scanning electron microscopy, high-resolution transmission electron microscopy, and mechanical tests. Benzene, toluene and xylenes were adsorbed from dry air by using carbon bed columns, obtaining the breakthrough curves. There was no correlation between the amount adsorbed at the breakthrough point and the volume of micropores narrower than 0.7 nm. Conversely, a good linear relationship between the amount adsorbed at the breakthrough point and the total micropore volume up to a mean micropore width of around 1.05 nm was found. In addition, the height of the mass transfer zone decreased with the mean width of the total micropores up to a value of around 1.05-1.10 nm. One of the best adsorbents obtained showed the lowest height of the mass transfer zone and one of the highest amounts adsorbed at the breakthrough point, either per mass or volume unit. However, it had a lower elastic modulus and compressive strength than other monolithic carbon aerogels, although its compressive strength (3 MPa) was still high enough to use it in carbon bed columns. The sample with the best mechanical properties was a poorer adsorbent. Regeneration of the exhausted adsorbents allowed the recovery of the hydrocarbons adsorbed without any appreciable loss of adsorption capacity of the carbon bed.     

Improvement of the bioactivity of organic–inorganic hybrid aerogels/wollastonite composites with TiO2

Organic–inorganic hybrid aerogels containing P and Ti have been synthesized by supercritical drying of alkogels prepared by hydrolysis and poly-condensation of metalo-organic precursors under high-power ultrasound. These materials become bioactive when doped with Ca. Wollastonite particles (CaSiO₃) were added as an active phase, instead of incorporating Ca into the aerogel atomic network. These particles had previously been precipitated and were then added to the sol. The aerogels were studied by Fourier transform infrared analysis, scanning electron microscopy coupled with energy dispersive spectroscopy and X-ray diffraction and N₂ adsorption. The stress–strain behaviours were evaluated under compression to obtain the Young’s modulus. It was found that the incorporation of TiO₂ into wollastonite-P₂O₅ hybrid aerogels increased their capacity to form apatite and, consequently, improving their bioactive response.     

Activated carbon aerogels with high bimodal porosity for lithium/sulfur batteries

Activated carbon aerogels (ACAs) with high bimodal porosity were obtained for lithium/sulfur batteries by potassium hydroxide (KOH) activation. Then sulfur–activated carbon aerogels (S–ACAs) composites were synthesized by chemical deposition strategy. The S–ACAs composites were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy, and N₂ adsorption/desorption measurements. It is found that the activated carbon aerogels treated by KOH activation presents a porous structure, and sulfur is embedded into the pores of the ACAs network-like matrix after a chemical deposition process. The Li/S–ACAs (with 69.1 wt% active material) composite cathode exhibits discharge capacities of 1,493 mAh g^?1 in the first cycle and 528 mAh g^?1 after 100 cycles at a higher rate of C/5 (335 mA g^?1). The S–ACAs composite cathode exhibits better electrochemical reversibility, higher active material utilization, and less severe polysulfide shuttle than S–CAs composite cathode because of high bimodal porosity structure of the ACAs matrix.     

Hydrophobic Silica Aerogels Strengthened with Nonwoven Fibers

Hydrophobic silica aerogels were prepared via a sol‐gel process by surface modification at ambient pressure. Nonwoven fibers were distributed inside the silica aerogels as a composite to act as a supporting skeleton which increased the mechanical property of the silica aerogels. The morphology and pore structure of the composites were characterized by scanning electron microscopy (SEM) and N₂ adsorption analyzer. The contact angle and the adsorption capacities of the composites were also determined. The results show that silica aerogels dispersed uniformly and maintained high porosity in the aerogel‐fiber composites. They have excellent hydrophobic properties and are excellent adsorptive materials.     

The synthesis and characterization of zinc ferrite aerogels prepared by epoxide addition

Over the past decade sol–gel methods have become increasingly popular alternatives to the solid state synthesis of metal oxides. In many cases sol–gel synthesis is preferred due to desirable physical properties such as high surface area, high porosity, and small crystallite size. Monolithic zinc ferrite aerogels were produced by the epoxide addition sol–gel method. It was observed that addition of propylene oxide to 2-propanol solution of either the hydrated metal nitrate salts or the hydrated metal chloride salts resulted in the formation of stable red–brown gels. Aerogels were characterized using powder X-ray diffraction, high resolution scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption/desorption analysis. The metal salt used in the synthesis was found to significantly influence the properties of the aerogel. All aerogels synthesized exhibited low densities and high surface areas (>340 m²/g). Annealing of the aerogel at relatively low temperatures (below 450 °C) yielded a highly crystalline porous material which is composed of nanometer sized particles.