Fabrication of gradient density SiO<Subscript>2</Subscript> aerogel

With tetraethoxysilane as the organic precursor, gradient density aerogels were fabricated by three different methods: layer-by-layer gelation, sol-co-gelation and continuous formation technics. Through layer-by-layer method, a 5-layer graded density silica aerogel whose density ranges from 50 to 200 mg/cm³ was obtained, but it existed a dense skin between adjacent layers which could result in density mutation in the interface. In order to optimize its interface character, sol-co-gelation technique was created to improve the interdiffusion and smooth out the density mutation via a self-built device. Finally, on the base of the device and sol-co-gelation technics, a continuous formation process was developed to fabricate the completely gradient density silica aerogel. Optical microscope and X-ray phase contrast method were used to characterize the samples prepared by three different technics and comparatively research their interface feature.     

Boosting CO2 Electroreduction on N,P-Co-doped Carbon Aerogels

Electroreduction of CO₂ to CO powered by renewable electricity is a possible alternative to synthesizing CO from fossil fuel. However, it is very hard to achieve high current density at high faradaic efficiency (FE). Here, the first use of N,P-co-doped carbon aerogels (NPCA) to boost CO₂ reduction to CO is presented. The FE of CO could reach 99.1 % with a partial current density of −143.6 mA cm⁻², which is one of the highest current densities to date. NPCA has higher electrochemical active area and overall electronic conductivity than that of N- or P-doped carbon aerogels, which favors electron transfer from CO₂ to its radical anion or other key intermediates. By control experiments and theoretical calculations, it is found that the pyridinic N was very active for CO₂ reduction to CO, and co-doping of P with N hinder the hydrogen evolution reaction (HER) significantly, and thus the both current density and FE are very high.     

Boosting CO2 Electroreduction on N,P‐Co‐doped Carbon Aerogels

Electroreduction of CO₂ to CO powered by renewable electricity is a possible alternative to synthesizing CO from fossil fuel. However, it is very hard to achieve high current density at high faradaic efficiency (FE). Here, the first use of N,P‐co‐doped carbon aerogels (NPCA) to boost CO₂ reduction to CO is presented. The FE of CO could reach 99.1 % with a partial current density of ?143.6 mA cm^?2, which is one of the highest current densities to date. NPCA has higher electrochemical active area and overall electronic conductivity than that of N‐ or P‐doped carbon aerogels, which favors electron transfer from CO₂ to its radical anion or other key intermediates. By control experiments and theoretical calculations, it is found that the pyridinic N was very active for CO₂ reduction to CO, and co‐doping of P with N hinder the hydrogen evolution reaction (HER) significantly, and thus the both current density and FE are very high.     

Synthesis and characterization improvement of gradient density aerogels for hypervelocity particle capture through co-gelation of binary sols

In this paper, we reported the preparation technics and details of gradient density aerogels (GA) via continuous formation technics, and introduced an effective and facile method to analyze its physical properties and microstructure correctly, since these properties are density dependent. With tetramethoxysilane as precursor and two-step sol–gel process, high transparence and crack-free of GA were fabricated, the density ranges from 0.02 to 0.2 g/cm³. The linear shrinkage of GA increases continuously with density increases from top to bottom. Base on systematically analyze the formation mechanism of GA, the co-gelation of binary sols with different concentrations method was developed to prepare mixed aerogels, which possessed a similar internal skeleton structure compare with GA. The internal skeleton structure of these aerogels is cross-linked by distinct sizes silica particles from two concentrations of silica-sols. Four mixed aerogels corresponding to different density positions of GA (a—0.15, b—0.12, c—0.09, and d—0.05 g/cm³) were selected to test. X-ray phase contrast method was used to shown the density distribution of GA. BET observation, SEM analysis, and TEM study, DMA and UV–vis results shown that GA are with excellent performance in physical properties, such as high optical transmittance, and good mechanical strength, which are critical characteristics for practical applications of GA, particularly in capture particle areas, for easy observation and lossless capture.     

Ambient pressure dried tetrapropoxysilane-based silica aerogels with high specific surface area

In the present paper, we report the synthesis of tetrapropoxysilane (TPOS)-based silica aerogels with high surface area and large pore volume. The silica aerogels were prepared by a two-step sol-gel process followed by surface modification via a simple ambient pressure drying approach. In order to minimize drying shrinkage and obtain hydrophobic aerogels, the surface of the alcogels was modified using trichloromethylsilane as a silylating agent. The effect of the sol-gel compositional parameters on the polymerization of aerogels prepared by TPOS, one of the precursors belonging to the Si(OR)₄ family, was reported for the first time. The oxalic acid and NH₄OH concentrations were adjusted to achieve good-quality aerogels with high surface area, low density, and high transparency. Controlling the hydrolysis and condensation reactions of the TPOS precursor turned out to be the most important factor to determine the pore characteristics of the aerogel. Highly transparent aerogels with high specific surface area (938 m²/g) and low density (0.047 g/cm³) could be obtained using an optimized TPOS/MeOH molar ratio with appropriate concentrations of oxalic acid and NH₄OH.     

快速制备高掺杂CuO/SiO2复合气凝胶

通过环氧丙烷预反应法, 以乙腈为溶剂快速制备了高掺杂的氧化铜/二氧化硅复合气凝胶. 在典型的合成过程中, 将正硅酸甲酯(TMOS)、乙腈、去离子水和环氧丙烷混合进行预反应, 然后将该溶液与氯化铜的乙腈-水溶液混合并添加环氧丙烷, 在35℃烘箱中静置0.5 h 后转化为湿凝胶, 再经过CO₂超临界流体干燥和热处理即可获得黑色块状CuO/SiO₂复合气凝胶. 最终气凝胶样品密度约为180 mg·cm^-3, 比表面积高达625 m2·g^-1, 平均掺杂比为19.91%±2.42% (Cu:Si 摩尔比), 压缩模量为1.639 MPa, 具有成型性好、分散均匀等优点,是良好的背光源靶材料. 本论文还通过对比实验对凝胶化过程的机理进行分析, 结果表明, 通过改变溶剂和采用环氧丙烷预催化均衡了两种不同前驱体的反应速率, 实现了共凝胶的目的. 此外, 该方法还有望为其它金属氧化物/二氧化硅复合气凝胶的制备提供新思路.     

Synthesis of MWCNTs doped sodium silicate based aerogels by ambient pressure drying

The successful incorporation of multiwalled carbon nanotubes (MWCNTs) into silica aerogels prepared by sol–gel method is reported herein. Pure silica aerogels prepared using sodium silicate precursor by ambient pressure drying are so fragile that they cannot be used easily. MWCNTs were used as reinforcements to improve the mechanical properties of silica aerogels. Results show that inserting small amounts of MWCNTs in the gels causes enhanced dimensional stability of silica aerogels. The silica aerogels were prepared by doping MWCNTs in silica matrix before gelation. The influence of MWCNTs on some microstructural aspects of silica matrix has been studied using nitrogen adsorption–desorption isotherms. From SEM study it is confirmed that the silica particles get capped on the surface of MWCNTs suggesting an enhanced toughness. Further, FTIR, Raman, EDAX, thermal conductivity and hydrophobicity studies of these doped aerogels were carried out. By addition of MWCNTs, silica aerogels were formed with 706 m²/g BET and 1,200 m²/g Langmuir surface areas and 149^o contact angle. Low density (0.052 g/cc) and low thermal conductivity (0.067 W/m K) MWCNTs doped silica aerogels were obtained for the molar ratio of Na₂SiO₃::H₂O::MWCNTs::citric acid::TMCS at 1::146.67::2.5 × 10⁻³::0.54::9.46 respectively with improved mechanical strength.     

Bimetallic AuRu aerogel with enzyme-like activity for colorimetric detection of Fe2+ and glucose

Aerogels have become a hot topic of research due to their extremely low density and special interconnected structure as well as their enzyme-like activity. The development of new multifunctional nano-enzyme aerogels with high activity and good stability is still a considerable challenge. In this paper, AuRu aerogels with peroxidase and oxidase activities were synthesized using a simple one-step method and successfully used to construct colorimetric sensors for the detection of Fe²⁺ and glucose based on their enzyme-like activities. Furthermore, we are fortunate to find that AuRu aerogels have good photothermal properties. This suggests that AuRu aerogels can be used not only for in vitro testing but also for promising applications such as disease treatment.     

Aerocellulose: Aerogels and Aerogel-like Materials made from Cellulose

Summary: Cellulose aerogels have been prepared starting from cellulose-NMMO solutions via the classical aerogel-path. Different cellulosic materials have been tested and their influence on the properties of the product aerogels has been studied. Other parameters that have been varied include solution composition as well as the way of cellulose regeneration (solvent and temperature). More than 300 different samples were prepared and analysed. Their density is in a typical range from 0.02 g/cm³ to 0.2 g/cm³ and their internal surface area ranges from 100 m²/g to 400 m²/g. Another property investigated in detail beside density and internal surface area was the shrinkage of the cellulosic bodies during the production process.     

空间高速粒子捕获用密度梯度气凝胶的热学与力学特性

采用正硅酸四乙酯(TEOS)的乙醇-水溶液为前驱体,氢氟酸为催化剂,结合溶胶-凝胶过程与CO2超临界流体干燥工艺,一步反应获得了密度为40-175mg·cm-3的单元气凝胶.以上述工艺为基础,通过逐层凝胶法、溶胶共凝法和梯度溶胶共凝胶法分别制备了三种密度梯度气凝胶样品,并研究了其功能梯度特性.结果表明:不同密度的气凝胶均具有粒径约为40-90nm球形颗粒构成的三维骨架结构,密度越低,骨架越疏松,峰值孔径越大,孔径分布也更为分散;三种方法制备的样品均具有明显的密度梯度,梯度特性由不连续到连续.动态热机械性能测试表明,随着密度的降低,气凝胶在低温(-100℃)和常温(25℃)下杨氏模量均有减小的趋势,其范围分别约为4.6×105-1.9×105Pa和5.0×105-2.1×105Pa.热学测试表明,随着密度的降低,气凝胶的热扩散系数增高,单位体积热容降低,而热导率则不成单调变化.     

Biopolymer-Based Aerogels for Electromagnetic Wave Shielding and Absorbing

The rapid development of communication technology and electronic industry has brought unprecedented serious electromagnetic interference (EMI) and electromagnetic wave (EMW) pollution. Although EMI shields and EMW absorbers based on metal or magnetic materials were used to solve these problems, they have long been criticized for their high price, high density and easy corrosion. In order to achieve low density and efficient dissipation of electromagnetic energy, aerogels stand out among manifold materials. However, constructing aerogels with good EMI shielding or EMW absorption performance and acceptable mechanical properties is not an easy task. Burgeoning biopolymers, such as cellulose, lignin, chitin/chitosan and alginate, breathe new life into aerogels for high-efficiency EMW shielding and absorbing. Here, we reviewed the contributions of biopolymers in the fields of aerogels for EMW shielding and absorbing. At the same time, some challenges and outlook were also pointed out, aiming to promote the advance of aerogel-based EMI shields and EMW absorbers as well as the rational utilization of biopolymers.     

超临界干燥法制备Fe2O3-SiO2气凝胶

以正硅酸乙酯（TEOS）和硝酸铁为原料,采用溶胶-凝胶法和超临界干燥工艺制备了Fe2O3-SiO2气凝胶,研究了Fe2O3-SiO2醇凝胶的形成条件的影响,并对所得气凝胶样品结构特性进行了初步表征．所得气凝胶样品是由直径约8nm的胶体粒子构成的低密度、高孔隙率的块状非晶固态材料．     

Elastic piezoelectric aerogels from isotropic and directionally ice-templated cellulose nanocrystals: comparison of structure and energy harvesting

We report the preparation of highly compressible and elastic piezoelectric aerogels of carboxylated cellulose nanocrystals (CNCs). Aqueous CNC dispersions containing polyethyleneimine and crosslinker were frozen isotropically to yield isotropic aerogels, while oriented aerogels were prepared by directional freezing. These aerogels were highly flexible and porous (~?85% void fraction), exhibiting greater than 90% recovery at 50% compressive strain even after 100 compression–decompression cycles. Since such aerogels with low bulk modulus and high anisotropy would be an ideal platform for leveraging the piezoelectric properties of CNCs, we used them to prepare piezoelectric nanogenerator devices and determined their energy transduction behavior. Anisotropic aerogels led to an enhanced open-circuit voltage of 840 mV (at ~?8 N applied force), which is over 2.6 times higher than isotropic aerogels (320 mV). The energy density of anisotropic aerogels was around 52 nW/cm², representing outstanding piezoelectric performance for cellulose-based aerogels. Such aerogels with high compressibility, elastic recovery and exceptional piezoelectric performance could have potential applications in sensors, wearable electronics, etc.

     

环氧化物法制备锑掺杂氧化锡气凝胶

以无机金属盐为前驱体,采用环氧丙烷添加法结合CO2超临界流体干燥和热处理工艺,制备了不同锑掺杂浓度的二氧化锡(ATO)气凝胶.所得气凝胶为深蓝色块体,平均密度约为600 mg?cm-3,锑掺杂浓度在5%到20%(x)之间.电子显微镜图片显示ATO气凝胶的骨架由粒径约为数十纳米的颗粒堆积而成,而这些颗粒又由数纳米的初级球形颗粒构成.X射线衍射谱表明,样品的主要晶相为SnO2四方相金红石结构,锑的掺杂仅引起微小的晶格畸变.X射线光电子谱显示锡元素以+4价态存在,而锑则具有+3和+5的混合价态.四探针电阻率测试仪的结果表明,ATO气凝胶的电阻率在2.7-40Ω?cm之间变化,其中在锑掺杂浓度(x)为12%时具有最低电阻率.     

Effect of Processing Temperature on Gelation and Physical Properties of Low Density TEOS Based Silica Aerogels

In the present paper the experimental results of the effect of sol-gel processing temperature on the physical properties of the TEOS based silica aerogels are reported and discussed. The aerogels were produced by the two step sol-gel process at various temperatures in the range of 26–70^∘;C followed by supercritical drying using methanol solvent extraction. A remarkable reduction in the gelation time was observed from three and a half days at room temperature to a mere 18 hours at 50^∘;C. The best quality aerogels in terms of low density and high optical transmission were obtained for 6 hours hydrolysis time. The aerogels were characterized by the measurements of bulk density, volume shrinkage, porosity, refractive index and optical transmission. Monolithic aerogels with ultra low density (∼0.018 g/cm³), extremely high porosity (∼99%) and optimum optical transmission at 700 nm (∼75%) were obtained for the molar ratio of TEOS:MeOH:acidic water:basic water at 1:99:10.42:14.58 respectively.     

Optically transparent silica aerogels based on sodium silicate by a two step sol–gel process and ambient pressure drying

Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol–gel process. To produce optically transparent silica aerogel granules, NH₄F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol–gel process-based aerogels is higher than the conventional single step aerogels. The N₂ adsorption–desorption analysis depicts that the two step sol–gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of ∼0.125 g/cc, low thermal conductivity of ∼0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of ∼425 m²/g were obtained by using NH₄F (1 M), HCl (4 M), and a molar ratio of Na₂SiO₃::H₂O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 °C.     

Alumina aerogels prepared via rapid supercritical extraction

Alumina aerogels with surface areas from 460 to 840 m²/g and bulk densities from 0.025 to 0.079 g/cm³ were successfully fabricated using variations of an aluminum isopropoxide-based recipe developed by Armor and Carlson and the rapid supercritical extraction (RSCE) process developed at Union College. By utilizing the Union College RSCE method, it is possible to convert an alumina aerogel precursor mixture into aerogel monoliths in as little as 7.5 h. This process is safer than methanol extraction in an autoclave and faster and simpler than liquid CO₂ solvent exchange and extraction. By increasing the concentration of aqueous HNO₃ used in the precursor mixture, we were able to fabricate aerogels with significantly increased surface area, decreased bulk density, and altered microstructure. We attribute the observed variation in these aerogel properties at a given HNO₃ concentration to environmental factors such as humidity. The ability to more easily fabricate alumina aerogels with desirable properties will assist in making them a viable option for catalytic and other applications.     

Microstructure and catalytic properties of Rh and Ni dispersed on TiO2-SiO2 aerogels

The influence of the preparation method on the microstructure and catalytic behavior of Rh and Ni dispersed on TiO₂-SiO₂ aerogels is investigated.The autoclave method has been followed to prepare titania-silica aerogels with TiO₂ contents ranging between 0 and 10 mole %. These aerogels have been used as matrices to disperse catalytically active metals: Rh and Ni. The metals can be deposited by impregnation of aerogels, or alternatively, can be added into the hydrolysis water used in the synthesis of gels. The resulting catalysts present surface areas higher than 550 m²·g^–1.The percentage of titania, the method followed for the introduction of the metal, and the nature of the metal itself affect both the activities and selectivities of the catalysts in the hydrogenolysis of n-butane. Thus, the presence of titania in Rh catalysts increases the activity values, and the samples prepared by impregnation present selectivities towards ethane higher than 80%. Whereas, the rhodium catalysts in which the metal has been introduced before gelling, do not orientate the reaction in favor of a definite product. For the case of Ni, it is quite frequent to obtain high selectivities towards the breakdown of the C-C terminal bonds. In summary, the preparation methods allow to modulate into very broad limits the catalytic behavior of the samples.     

Preparation and characterization of silica aerogels from diatomite via ambient pressure drying

The silica aerogels were successfully fabricated under ambient pressure from diatomite. The influence of different dilution ratios of diatomite filtrate on physical properties of aerogels were studied. The microstructure, surface functional groups, thermal stability, morphology and mechanical properties of silica aerogels based on diatomite were investigated by BET adsorption, FT-IR, DTA-TG, FESEM, TEM, and nanoindentation methods. The results indicate that the filtrate diluted with distilled water in a proportion of 1: 2 could give silica aerogels in the largest size with highest transparency. The obtained aerogels with density of 0.122–0.203 g/m³ and specific surface area of 655.5–790.7 m²/g are crack free amorphous solids and exhibited a sponge-like structure. Moreover, the peak pore size resided at 9 nm. The initial aerogels were hydrophobic, when being heat-treated around 400°C, the aerogels were transformed into hydrophilic ones. The obtained aerogel has good mechanical properties.     

混甲酚甲醛炭气凝胶的制备及表征

以混甲酚和甲醛为原料,经溶胶－凝胶民、酸洗老化、超临界干燥得有机气凝胶,密度０．１５０７ｇ／ｃｍ＾３,进一步炭化得炭气凝胶。采用正交试验方法重点考察了炭化工艺条件对炭气凝胶结构和性能的影响,并用ＴＥＭ、ＴＧ、低及附等手段进行了表征。结果表明,炭化工艺条件影响因素顺序为：升温速率〉炭化终温〉维温时间,最佳炭化条件下炭气凝胶密度为０２６０ｇ／ｃｍ＾３,幽静面积人１０２２ｍ＾２／ｇ,平均戏５．６ｍｍ。Ｔ