Mechanical properties of polymer-modified silica aerogels dried under ambient pressure

Silica gels prepared by copolymerizing tetraethylorthosilicate with 3-aminopropyltriethoxy-silane were modified using polymer derived from toluene diisocyanate and dried under ambient pressure. The successful preparation of silica aerogels depended on the effective control of shrinkage during drying. The resulting material, polymer-modified silica aerogel, was then characterized by thermogravimetric analysis and uniaxial compression tests. Results indicated that the apparent elastic modulus and compressive strength of the polymer-modified silica aerogels decreased with increasing amounts of incorporated polymer because of decreasing shrinkage and density, while the strains at the surface cracking point and the final failure point increased significantly during compression tests. The strength and modulus of the silica skeleton could be calculated from the apparent strength and modulus of the silica aerogels respectively. It was interestingly shown that the elastic modulus of the silica skeleton of the silica aerogels increased because of the incorporated polymers, while the polymers had no effects on the compressive strength of the silica skeleton. In addition, the relationships between the apparent elastic modulus or the apparent compressive strength of the polymer-modified silica aerogels and their shrinkage were quantitatively expressed.     

Comparison between flexural and uniaxial compression tests to measure the elastic modulus of silica aerogel

The Young’s moduli of a set of silica aerogels have been measured by two techniques: 3-point bending and uniaxial compression. The data found by the two methods differ strongly. The uniaxial compression test gives generally underestimated values of Young’s modulus, because of geometrical effects. The appropriate gauge lengths were estimated based on the discussion of Euler buckling and nonuniform stress distribution. The measured compressive moduli were analyzed to correct for machine compliance and possible misalignment under compression of the aerogels. Similarly, moduli obtained by 3-point bending depend on the length/thickness ratio of the sample, reaching equilibrium only for ratios above about 10. The corrected compressive moduli were comparable to those measured by 3-point bending on samples of sufficient length.     

SiO2气凝胶分子动力学模拟研究进展

二氧化硅(SiO₂)气凝胶是一种拥有三维骨架网络结构的纳米多孔材料,具有高孔隙率、低密度和低热导率等许多独特的性能。但是由于二氧化硅气凝胶本身的脆性及高温稳定性差等原因,限制了其大规模应用。二氧化硅气凝胶的热力学性能与其内部的三维骨架和孔结构紧密相关,掌握二氧化硅气凝胶内部微结构演化规律与宏观性能的关联,是改善其热力学性能的前提。分子动力学模拟可以从原子层面分析和探索气凝胶的结构并预测其热力学性能。本文对分子动力学模拟下二氧化硅气凝胶势函数、多孔结构建模、结构表征、力学性能和热性能方面进行了详细总结,有助于从原子层面解释二氧化硅气凝胶结构与性能之间的关系,为从成分和结构方面设计气凝胶提供一种理论指导方法。     

Rapid synthesis of amine cross-linked epoxy and methyl co-modified silica aerogels by ambient pressure drying

The silica aerogels were synthesized by sol–gel method via ambient pressure drying. Tetraethyl orthosilicate (TEOS) was used as a main silica source, methyltriethoxysilane (MTES) as a co-precursor silica source and (3-Glycidoxypropyl)trimethoxysilane (GPTMS) as a silane coupling agent. The silica aerogels obtained were further undergoing cross-linking epoxy from GPTMS with amine from diethylenetriamine (DETA) which played a dual role of base catalyst and reagent. The cumulative volumes for open pores of the cross-linked aerogels were evaluated to be 1.4 cm³/g. The Young's modulus and maximum compression strength were 25.4 MPa and 6.17 MPa, respectively. The addition of MTES accelerated the solvent exchange of alcohol within the pores with n-hexane and reduced the shrinkage of aerogels network during the ambient pressure drying. The formation of organic network enhanced the strength of the cross-linked aerogels to prevent the crack generation and the subsequent failure of the monolith during the ambient drying, therefore, protected the nanoporous structure of aerogels.     

金刚石涂层的纳米压痕力学性能研究

用HFCVD法在硬质合金刀具上制备了CVD金刚石涂层,利用纳米压痕仪研究了CVD金刚石涂层的硬度和弹性模量等力学性能.结果表明,反应室气压、衬底温度、反应气体中CH4含量、沉积时间等参数改变了CVD金刚石膜中sp2成分含量、晶界数量及晶界上缺陷,从而影响CVD金刚石涂层的纳米硬度和弹性模量.较高或较低的衬底温度都会导致硬质合金刀具上CVD金刚石涂层的纳米硬度、弹性模量降低;随着反应室气压、反应气体中CH4含量的增加,硬质合金刀具上CVD金刚石涂层的纳米硬度、弹性模量降低;沉积时间低于6 h时,沉积时间对硬质合金刀具上CVD金刚石涂层的纳米硬度、弹性模量影响显著,沉积时间超过6 h后,沉积时间对硬质合金刀具上CVD金刚石涂层的纳米硬度、弹性模量逐渐趋向稳定.     

Large deformation of chlorotrimethylsilane treated silica aerogels

Silica aerogels were prepared through an acid-base process and surface modified with chlorotrimethylsilane. This novel application of a common non-crosslinking surface modification to improve mechanical properties allows the treated aerogels to deform plastically to compressive strains greater than 80% without macroscopic damage. This improvement in mechanical properties remains after heating in air at 500 °C for 3 h, as do residual organic groups. Heating at 700 °C for 1 h removes all organics and the aerogel behaves similar to the unmodified control. The treated aerogels also exhibit a greater resistance to sintering. Nitrogen adsorption measurements show a reduction in the number of micropores with surface modification. It is concluded that the organic monolayer increases the ductility of the silica network by filling and strengthening surface micropores that serve as crack initiators, and that these organics remain effective at elevated temperatures.     

Nanostructural damage associated with isostatic compression of silica aerogels

Isostatic compression of silica aerogels is known to allow densification of these highly porous materials. However, at the onset of compression, hydrophobic and consequently slightly reacting aerogels, exhibit a decrease in bulk modulus. This unusual behavior is associated with damage occurring at low pressures which recovers with further density increase. Damage development and healing are analyzed measuring elastic modulus and, for the first time, internal friction as a function of compression. It is proposed that the origin of damage and healing could be associated with the rupture of tenuous links between clusters of dense silica particles at low density levels, and with the creation of new links between the resulting arms and reacting species that are revealed at cluster interface under higher pressure.     

HNTs/SiO2复合气凝胶制备及其性能研究

采用正硅酸乙酯(TEOS)为硅原,以硅烷改性的埃洛石纳米管(HNTs)为增强相,利用CO2超临界干燥技术制备具有优良力学和隔热性能的HNTs/SiO2复合气凝胶.利用傅立叶红外光谱、扫描电镜、比表面积与孔径分析仪、万能试验机和导热率测量仪等手段对HNTs改性后的表面状态、HNTs/SiO2复合气凝胶的微观形貌、孔结构、力学和导热性能进行了测试分析.结果表明:改性后的HNTs均匀分散到二氧化硅气凝胶基体中,并与SiO2纳米颗粒实现良好的结合,HNTs/SiO2复合气凝胶呈三维网络结构,当HNTs含量为15wt;时,平均孔径为10.47 nm;随着HNTs含量的增加,复合气凝胶的力学性能不断增强,同时其导热系数也不断增大,当HNTs含量为15wt;时,HNTs/SiO2复合气凝胶的抗压强度为0.85 MPa,导热系数为0.024 W/mK.     

Polysaccharide-based aerogels as drug carriers

The application of aerogels as drug delivery system was successfully demonstrated for silica aerogels previously. However, being biocompatible silica matrices are not biodegradable, which is a certain disadvantage for a number of pharmaceutically oriented applications. For these purposes biodegradable materials are beneficial. Supercritical drying of polysaccharide gels results in highly porous biodegradable aerogel matrices with large surface areas. Structural properties of the polysaccharide aerogels depend on the preparation method and chemical nature of the gel phase. In this work different polysaccharide precursors (starch, alginate) were used to produce aerogels, which later on were loaded with the drugs ibuprofen and paracetamol. Furthermore release kinetics was studied in vitro. Thereby it has been shown that the release rate depends primary on the properties of the matrix. The presented results demonstrate for the first time the high potential of polysaccharide aerogels for pharmaceutical applications.     

Synthesis of ultrahigh-pore-volume carbon aerogels through a “reinforced-concrete” modified sol-gel process

Ultrahigh-pore-volume carbon aerogels were synthesized by adding rigid silica nanoparticles to resorcinol-formaldehyde sols, followed by supercritical drying, pyrolysis and HF leaching. The presence of silica nanoparticles in polymer gels dramatically inhibits volume shrinkage and framework collapse during the supercritical drying and pyrolysis processes, resulting in the obtained carbon aerogels exhibiting very low bulk density and high pore volume. By changing the mass ratio of silica nanoparticles/resorcinol-formaldehyde resin, pore volumes of carbon aerogels can be tuned in the range of 2.8-6.0 cm³/g.     

Ultralow density silica aerogels prepared with PEDS

This paper deals with the synthesis of ultralow density silica aerogels using polyethoxydisiloxanes (PEDS) as the precursor via sol-gel process followed by supercritical drying using ethanol solvent extraction. Ultralow density silica aerogels with 5 mg/cc of density were made for the molar ratio by this method. A remarkable reduction in the gelation time was observed by the effect of the catalyst NH₄OH at room temperature. The microstructure and morphology of the ultralow density silica aerogels were characterized by the specific surface area, S_BET, SEM, TEM and the pore size distribution techniques. The results show that the diameter of the silica particles is about 13 nm and the pore size of the silica aerogels is about several nm. The specific surface area of the silica aerogel is 339 m²/g and the specific surface area, pore volume and average pore diameter decrease with increasing density of the silica aerogel.     

High resolution patterning of silica aerogels

Three-dimensional metallic structures are fabricated with high spatial resolution in silica aerogels. In our method, silica hydrogels are prepared with a standard base-catalyzed route, and exchanged with an aqueous solution typically containing Ag⁺ ions (1 M) and 2-propanol (0.2 M). The metal ions are reduced photolytically with a table-top ultraviolet lamp, or radiolytically, with a focused X-ray beam. We fabricated dots and lines as small as 30 × 70 μm, protruding for several mm into the bulk of the materials. The hydrogels are eventually supercritically dried to yield aerogels, without any measurable change in the shape and spatial resolution of the lithographed structures. Transmission electron microscopy shows that illuminated regions are composed by Ag clusters with a size of several μm, separated by thin layers of silica.     

Synthesis and Characterization of Silica Aerogel-Polymer Hybrid Materials

Abstract

Silica aerogel-based hybrid composites containing three different polymers such as poly(styrene) (PS), poly(methyl methacrylate) (PMMA), and PS-co-PMMA were synthesized by two steps: sol-gel reaction to form vinyl silica aerogels and radical polymerization to combine the silica aerogels with the polymers. The reactions were confirmed using FTIR and FE-SEM, showing successful polymerization in the surface of the silica aerogel network. Incorporation of the polymers into the silica aerogel allows for the enhancement of thermal stabilities. From dielectric measurement, the polymer hybridization leads to an increase in the static dielectric constant, compared to bare silica aerogel.     

Cost-effective synthesis of silica aerogels from fly ash via ambient pressure drying

Silica aerogels were synthesized from the industrial fly ash by ambient pressure drying method. The process consists of two stages, preparation of sodium silicate solution from fly ash by hydrothermal reaction with sodium hydroxide, and synthesis of porous silica aerogels from the obtained sodium silicate solution. Silica wet gels were formed by vitriol-catalysis or resin-exchange-alkali-catalysis of the obtained sodium silicate solution. The trimethylchlorosilane(TMCS)/ethanol(EtOH)/hexane mixed solution was used for solvent exchange/surface modification of the wet gel so as to obtain porous silica aerogels via ambient pressure drying. The results indicated that the synthesized silica aerogels were lightweight and hydrophobic. The BET specific surface area, pore volume and average pore diameter were 362.2-907.9 m² g^− 1, 0.738-4.875 cm³ g^− 1, and 7.69-24.09 nm respectively. Particularly, the synthesized silica aerogels by resin-exchange-alkali-catalysis method showed uniform mesoporous structure, and had much higher specific surface area (907.9 m² g^− 1) and pore volume (4.875 cm³ g^− 1) than that of by vitriol-catalysis process.     

Microstructural and physical properties of the ambient pressure dried hydrophobic silica aerogels with various solvent mixtures

The experimental results on the microstructural and physical properties of the ambient pressure dried hydrophobic silica aerogels with various solvent mixtures have been reported. The aerogels were prepared with sodium silicate precursor, ammonium hydroxide catalyst, trimethylchlorosilane (TMCS) silylating agent, solvent mixture of methanol-isopropanol (MeOH/IPA) and various aprotic solvent mixtures namely, hexane and benzene (HB), hexane and toluene (HT), hexane and xylene (HX), heptane and benzene (HpB), heptane and toluene (HpT), heptane and xylene (HpX). The physical properties of the aerogels such as % of volume shrinkage, density, % of optical transmission, surface area, % of porosity, pore volume, thermal conductivity and heat capacities of the aerogels were studied. The hydrophobicity of the aerogels was studied by contact angle measurements. The HX and HpX aerogels have been found to be more hydrophobic (contact angle, θ > 155°) than the other aerogels. It has been observed that the % of weight increase is highest (1%) for the HT aerogels and lowest (0.25%) for HpX aerogels by keeping them at 70% humidity for 350 h. Further, the aerogels have been characterized by pore size distribution (PSD), Fourier transform infra red spectroscopy (FTIR) and thermogravimetric and differential thermal (TG-DGA) analysis and transmission electron microscopy (TEM) techniques. The results have been discussed by taking into account the surface tension, vapor pressure, molecular weight and chain length of the solvents. Low density (0.051 g/cc), hydrophobic (165°), transparent (85%), low thermal conductive (0.059 W/m K), low heat capacity (180 kJ/m³ K) and highly porous (97.38%) silica aerogels were obtained with HpX solvent mixture.     

Absorption spectrum of Ge-doped silica samples and fiber preforms in the vacuum ultraviolet region

We report absorption measurements in the vacuum ultraviolet (VUV) spectral region on Ge-doped silica samples grown through chemical vapor deposition (CVD) and fiber preforms grown through modified chemical vapor deposition (MCVD) with GeO₂ doping concentration from 0.3 to 4.0 mol%. We observed in all spectra an absorption band at 5.15 eV and a structure at about 7.0 eV. We observed differences in the absorption spectrum between Ge-doped silica samples and fiber preforms in the shape and relative amplitude of optical absorption bands. The intensity of defect-related bands depends on the Ge concentration and distribution. Differences in Ge concentration and distribution are due to the preparation methods and growing conditions.     

SiO2气凝胶纳米多孔结构的自组装过程模拟

基于分子动力学理论,模拟并计算了纳米多孔SiO2气凝胶的原子尺度模型和力学性能.SiO2气凝胶网络结构的自组装形成过程表明,当密度为0.078 g/cm3时,形成的结构以纳米团簇为主,难以形成连通的骨架结构;当密度为0.172 g/cm3及以上时,硅氧元素分布已扩展形成了连通的无定形骨架结构.通过对不同密度体系模型单轴施加应变并计算相应的应力值,得到应力-应变关系曲线,并依据弹性范围求得弹性模量.模拟结果表明,弹性模量与密度成一次线性关系,当气凝胶密度在0.078～0.443 g/cm3时,弹性模量为0.1265～0.7889 MPa.     

A new route to aerogels: Monolithic silica cryogels

We have explored several different gel syntheses and drying procedures for producing silica cryogels with similar properties to those of silica aerogels, particularly in terms of monolithicity, density, porosity and surface area. These materials could be a suitable alternative to silica aerogels and ambigels. Some successful preparation methods are presented and properties of the corresponding cryogels are discussed, including comparison of these materials to supercritically dried products and an assessment of the effects of the experimental variables in the preparation process on the properties of the resultant cryogels. Two routes for the preparation of cryogels are highlighted, one of which is especially attractive as it has the advantage (compared to the known syntheses of APD aerogels) of not requiring any solvent exchange step.     

直流电弧等离子体喷射化学气相沉积高质量金刚石膜残余应力分布的垃曼谱分析

不同工艺条件下在钼衬底(φ60mm)上用100 kW直流电弧等离子体喷射化学气相沉积设备进行金刚石膜的制备.金刚石膜用扫描电镜(SEM)、拉曼谱(激光激发波长为488nm)和X射线衍射来表征.研究结果表明,在直流电弧等离子体喷射化学气相沉积金刚石膜的过程中,内应力大小从金刚石膜的中央到边缘是增加的,并且应力形式是压应力.这说明了在金刚石膜中存在明显的应力不均.甲烷浓度和衬底温度都影响金刚石膜中的内应力.随着甲烷浓度和衬底温度的提高,金刚石膜中的内应力呈增加的趋势.     

Textural characterization of high temperature silica aerogels

Silica alcogels were synthetized by the sol–gel polymerization of tetraethylorthosilicate in acid media. Conventional and supercritical drying was performed in order to obtain xerogels and aerogels. Different process parameters of the supercritical drying were altered in order to control the texture of the resulting gel. The texture and the structural evolution of xero- and aerogels were studied by thermogravimetric-differential thermal analysis, Fourier-transform infrared spectroscopy, transmission electron microscopy and N₂ physisorption at 77 K. ²⁹Si magic angle spinning nuclear magnetic resonance experiments on silica samples were used to resolve various silicon local environments. Hydrophilic microporous xerogels and hydrophobic micro- or mesoporous silica aerogels were obtained, whose microscopic structure is very similar. However, the samples obtained by different drying procedures exhibit a different structural evolution with temperature.