Catalytic Applications of Aerogels

This paper is a review of studies on aerogel synthesis and its application for catalytic uses, which have been conducted for the past decade at the Clean Technology Research Center, Korea Institute of Science and Technology. Various organic and inorganic aerogels employing sol–gel modifications exhibited catalytically favorable textural and chemical properties such as extremely high porosity, high surface area, well-developed mesoporosity, good thermal stability, and high dispersion, which eventually led to excellent performance in catalytic reactions. Reactions observed in these studies include hydrogenation, photodecomposition, selective oxidation, complete oxidation, ammoxidation, reforming, and electrooxidation. The specific catalytic behaviors can be explained in terms of the strong active sites-support interactions, high thermal stability, and extremely high dispersion.     

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.     

高性能聚酰亚胺气凝胶的制备进展

气凝胶是轻质开孔的介孔材料,由于其特殊的性质,如低密度(0.003~0.5g/cm3)、高孔隙率(70%~99.8%)、低介电常数(~1.1)、低热导率(最低为0.012W/(m·K))和高比表面积(100~1600m2/g),因此可应用于隔热材料、隔音材料、催化剂载体、药物缓释材料、低介电材料、吸附剂等。聚酰亚胺是一类重要的高性能聚合物,近些年,聚酰亚胺气凝胶备受重视。本文综述了现有多种聚酰亚胺气凝胶的制备方法及其优缺点,并对今后的研究工作进行了展望。     

Frontispiece: An All-Natural Wood-Inspired Aerogel

The oriented pore structure of wood endows it with a variety of outstanding properties, among which the low thermal conductivity has attracted researchers to develop wood-like aerogels as excellent thermal insulation materials. However, the increasing demands of environmental protection have put forward new and strict requirements for the sustainability of aerogels. Here, we report an all-natural wood-inspired aerogel consisting of all-natural ingredients and develop a method to activate the surface-inert wood particles to construct the aerogel. The obtained wood-inspired aerogel has channel structure similar to that of natural wood, endowing it with superior thermal insulation properties to most existing commercial sponges. In addition, remarkable fire retardancy and complete biodegradability are integrated. With the above outstanding performances, this sustainable wood-inspired aerogel will be an ideal substitute for the existing commercial thermal insulation materials.     

利用离子液体制备纤维素基气凝胶

纤维素基气凝胶材料的研究在近年来吸引了人们极大的关注,这是因为这类新型材料具有通常无机气凝胶的典型结构特点,如超轻、高孔率、高比表面积等,同时具有天然生物质材料的原料丰富、可再生、可生物降解的优点。本文首先简要介绍了纤维素基气凝胶材料及其发展概况,进而主要介绍了以离子液体为溶剂制备再生纤维素基气凝胶的研究进展,包括纤维素基气凝胶的制备方法、结构及其功能性。最后对离子液体法制备的纤维素基气凝胶材料的前景进行了简要的展望。     

石墨烯气凝胶的可控组装

石墨烯气凝胶一般是由石墨烯片层经过湿法化学组装或气相化学生长获得的一种具有连通多孔网络结构的石墨烯三维宏观体材料,表现出极高的比表面积、良好的导电性以及优异的机械性能等,在电化学储能、吸附、催化以及传感等领域有着极为重要的应用。本文从石墨烯气凝胶的结构设计与组装策略出发,综述了近年来石墨烯纳米结构单元在石墨烯气凝胶材料（氧化石墨烯、还原氧化石墨烯、化学气相沉积（CVD）石墨烯、以及复合气凝胶等）中的组装行为,并对石墨烯气凝胶目前的现状及今后发展方向做了简要评述。     

常压干燥法制备气凝胶的研究进展

气凝胶是一类轻质、低密度的三维纳米多孔固态材料,因其独特的高孔隙率、高比表面积和低导热系数等特性,使其在吸附、催化、保温隔热和隔音等诸多领域具有广泛的用途,目前其相关研究在材料科学领域受到了广泛的关注。气凝胶的制备主要包括溶胶-凝胶过程和湿凝胶干燥两个步骤,湿凝胶的干燥是制备气凝胶过程中至关重要而又较为困难的一步。传统的气凝胶通过超临界干燥制备,工艺复杂、成本高,而且由于干燥过程在高温高压条件下进行,有一定的危险性并且不适宜大规模生产,因此如何通过常压干燥获得高比表面积、高孔隙率、低密度的性能优异的气凝胶是其研究的重要方向之一。本文简要介绍了湿凝胶的制备以及凝胶干燥理论,详细介绍了近年来常压干燥方法气凝胶制备的研究进展,并对其未来发展前景做出了展望。     

超临界干燥法制备SiO2气凝胶的研究

通过溶胶－凝胶法和超临界干燥技术制得了ＳｉＯ２气凝胶，研究了溶液的ｐＨ值对凝胶时间及气凝胶密度的影响，应用ＸＲＤ、ＳＡＸＳ、ＴＥＭ和等方面对气凝胶样品的结构进行了初步研究，结果表明，ＳｉＯ２气凝胶是连续网络的非晶态纳米多孔固体，其基本粒子的平均直径约１０ｎｍ．     

非超临界干燥法制备SiO2气凝胶

应用廉价的国产硅溶胶为原料，通过凝胶过程和干燥过程条件的选择，以非超临界干燥技术最终获得了块状SiO2气凝胶.该气凝胶外观状态与应用正硅酸乙酯为原料制得的完全一致，其微观结构也相当良好，其直径和孔分布均匀.溶液的配比和pH对凝胶过程和气凝胶样品的密度有比较明显的影响，同时pH值与SiO2的粒径之间也有一定的关系.依据制备条件的变化，所得SiO2气凝胶的密度约在200～400 kg•m－3，比表面在250～300 m2•g－1之间变化，平均孔径约为11～20 nm，而孔隙率在91％左右.     

相变气凝胶的设计合成及其微环境调控性能研究

人体微环境调控可以通过材料本身性能的调控实现穿着凉爽舒适,对提高生活品质、减少碳排放具有重要价值.然而传统材料仅能单向地实现致冷或保温,同一材料实现致冷、保温双向功能,依然是本领域重要挑战和值得及时探索的方向.本文采用冷冻-解冻制备水凝胶与冷冻干燥技术,设计合成系列以聚乙烯醇和相变微胶囊为骨架的相变气凝胶.此类相变气凝...     

添加环氧丙烷法常压干燥制备ZrO2气凝胶

以无机锆盐硝酸氧锆(ZrO(NO3)2·5H2O)为前驱体,1,2-环氧丙烷(PO)为凝胶促进剂,甲酰胺(FA)为干燥控制化学添加剂(DCCA),采用溶胶-凝胶法并通过常压干燥制备了轻质二氧化锆(ZrO2)气凝胶,利用差热分析(DTA)、扫描电镜(SEM)、N2吸附/脱附等测试技术对所制得的ZrO2气凝胶进行了表征.结果表明:添加环氧丙烷常压干燥制得的ZrO2气凝胶具有纳米尺寸的多孔网络结构,表观密度可低至202.08 kg·m-3,比表面积可高达645.0 m2·g-1,与超临界干燥效果相当.环氧丙烷因其环氧原子的强亲核性和不可逆的开环反应,可促进凝胶化,并通过环氧丙烷的量控制反应过程和凝胶状态.     

柔性交联型聚酰亚胺气凝胶的制备及性能

利用溶胶-凝胶反应制备了聚酰亚胺凝胶, 经过超临界干燥得到了聚酰亚胺气凝胶. 研究了固含量和交联剂比例对气凝胶性能的影响规律. 结果表明, 聚酰亚胺气凝胶的密度和线收缩率都随着固含量和交联剂比例的增加而增加; 随着固含量的增加, 气凝胶的室温热导率呈现出先降低再增加的趋势(0.026~0.033 W·m^-1·K^-1), 气凝胶的力学刚度和强度明显提升; 交联剂的加入, 可以提高材料的韧性, 断裂应变最高达21.7%; 制得的柔性聚酰亚胺气凝胶具有良好的热稳定性, 是满足尖端武器以及空间飞行器对于轻质、 柔性热防护要求的理想材料之一.     

Bioinspired Gradient Stretchable Aerogels for Ultrabroad-Range-Response Pressure-Sensitive Wearable Electronics and High-Efficient Separators

Broad-range-response pressure-sensitive wearable electronics are urgently needed but their preparation remains a challenge. Herein, we report unprecedented bioinspired wearable electronics based on stretchable and superelastic reduced graphene oxide/polyurethane nanocomposite aerogels with gradient porous structures by a sol-gel/hot pressing/freeze casting/ambient pressure drying strategy. The gradient structure with a hot-pressed layer promotes strain transfer and resistance variation under high pressures, leading to an ultrabroad detection range of 1 Pa–12.6 MPa, one of the broadest ranges ever reported. They can withstand 10 000 compression cycles under 1 MPa, which can't be achieved by traditional flexible pressure sensors. They can be applied for broad-range-response electronic skins and monitoring various physical signals/motions and ultrahigh pressures of automobile tires. Moreover, the gradient aerogels can be used as high-efficient gradient separators for water purification.     

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.     

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．     

Effects of silphenylene units on the thermal stability of silicone resins

A series of silicone resins containing silphenylene units were synthesized by a hydrolysis-polycondensation method, with methyltriethoxysilane, dimethyldiethoxysilane and 1,4-bis(ethoxydimethylsilyl)benzene. Their thermal degradation behaviours were studied by thermogravimetric analysis (TGA), differential thermogravimetry (DTG) and Fourier-transform infrared (FTIR) spectroscopy, and the effect of silphenylene units on the thermal stability of silicone resins was also investigated. Results showed that the thermal stability of silicone resins was improved by the introduction of silphenylene units into the backbone. Under nitrogen atmosphere, the temperature for maximum degradation rate of silicone resins with silphenylene units was lower compared to the pure methylsilicone resin. With the increase of silphenylene units, the amount of degradation residues increased under nitrogen atmosphere while it decreased under air atmosphere. Additionally, the short-term and long-term stability of silicone resins were also improved by the introduction of silphenylene units.     

Foam-like materials based on whey protein isolate

Low density materials from sustainable whey protein were fabricated through a simple, environmentally-friendly freeze-drying process. Aerogels produced solely from whey protein show poor mechanical properties, consistent with those of films produced from that biopolymer. The compressive moduli of these lamellar materials were increased by more than an order of magnitude by crosslinking, and further increased with increasing aerogel densities. Blending whey protein with alginate allowed for the production of bio-based aerogels with higher mechanical properties than those produced with whey alone, though thermal properties were slightly decreased by blending.     

Purification assay to prepared ultrapure carboxymethyl-chitosan

Carboxymethyl Chitosan (CMCh) is a semi-synthetic derivative of chitosan (a natural biopolymer) with increasing biomedical applications as a matrix or scaffold material for tissue engineering applications. Since, the presence of impurities can cause immunological reactions in vivo where ultimately pure materials are needed. To this end, purity of commercial-grade CMCh samples was investigated here along with their purification by a solvent/nonsolvent technique. The resulting polymer was characterized by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and energy dispersive X-ray spectroscopy (EDX) to confirm the validity of the purification process.     

Thermally stable and flame-retardant poly(vinyl alcohol)/montmorillonite aerogel via a facile heat treatment

The PVA/MMT aerogels prepared by a simple freeze-drying and heat treatment process exhibited a significant improvement of thermal stability and flame retardant property.     

Thermal and thermo-mechanical degradation of poly(3-hydroxybutyrate)-based multiphase systems

The influence of fermentation residues and quaternary ammonium salts on the thermal and thermo-mechanical degradation of a biodegradable bacterial poly(3-hydroxybutyrate), PHB, was studied. The results obtained from DSC, SEC and TG analyses performed on blends reveal that ammonium cations greatly enhance the degradation leading to a dramatic decrease in PHB molecular weight. These results are confirmed by the thermo-mechanical study. Besides, we show that the presence of fermentation residues does not affect significantly the PHB thermal stability in comparison to the ammonium cations. A kinetic analysis based on the Coats and Redfern model was applied to the non-isothermal TGA data. This method completed by NMR characterizations led us to determine the most probable mechanism for PHB degradation in the presence of the ammonium salts. The results demonstrate that ammonium surfactants commonly found in commercial nanoclays (for nanocomposites' production) effectively have a catalytic effect on the PHB degradation.