Enhancing mechanical properties of silica aerogels

Low-density monolithic silica and hexylene-bridged polysilsesquioxane aerogels were chemical vapor deposition (CVD) treated with hexamethyldisilazane or hexachlorodisilane silylating agents producing TMS (trimethylsilane) or Si layers on the aerogel. Reinforcing the weak aerogels by controlled deposition process improved their compressive strength and preserved their properties characteristic of low-density aerogels. When the silica and hexylene-bridged polysilsesquioxane aerogels were CVD treated with hexamethyldisilazane, the compressive modulus more than doubled in some cases. However, when treating hexylene-bridged aerogels with hexachlorodisilane the compressive modulus increased six fold. Not only did CVD treatment of the aerogels improve the compressive modulus, but the low densities, high surface areas, high porosities, and the transparency of the aerogels were not significantly affected.     

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.     

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.     

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.     

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.     

Simulation of the microstructural evolution of a polymer crosslinked templated silica aerogel under high-strain-rate compression

Surfactant-templated mesoporous silica aerogels (or nanofoams) with their entire skeletal framework nanoencapsulated conformally by a thin polyurea layer are emerging as materials with high specific strength and high energy absorption. In this paper a modified split Hopkinson pressure bar was used to investigate their mechanical behavior under dynamic compression at high strain rates. The evolution of the mesoporous structure under such dynamic impact conditions was simulated using the Material Point Method (MPM). The material point model was generated from X-ray micro-computed tomography whereas each voxel was converted to a material point corresponding to the local skeletal density of the material. Simulation results agree well with the experimental data, indicating that the MPM can effectively model the compression of complex mesoporous structures. Simulations indicate a nearly uniform deformation at all three stages of compression: the elastic region, compaction and the final densification due to the low ratio of pore size to wall thickness and random distribution of the pores. Simulations have also indentified the function of the conformal polymer coating as a reinforcing factor, showing that different porosities, obtained by varying the skeletal wall thickness, affect the local stress distribution. Eventually, simulations confirm that the stress-strain behavior of aerogels under compression follows a power-law relationship with the initial bulk density, consistent with experimental results.     

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.     

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.     

Globally anisotropic high porosity silica aerogels

We discuss two methods by which high porosity silica aerogels can be engineered to exhibit global anisotropy. First, anisotropy can be introduced with axial strain (i.e. axial compression). In addition, intrinsic anisotropy can result during growth and drying stages and, suitably controlled, it can be correlated with preferential radial shrinkage in cylindrical samples. We have performed small angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We show that global anisotropy originating from either strain or shrinkage leads to optical birefringence and that optical cross-polarization studies are a useful characterization of the uniformity of the imposed global anisotropy.     

Mechanical strength of silica aerogels

Pure silica aerogels are obtained by hypercritical evacuation of the solvent. The strength is measured by the three-point flexural test on monolithic parallelepipedic samples and by a diametral compression test on cylindrical samples. The stress-strain curve shows a perfect elastic behaviour and the “conchoidal” fracture morphology indicates that the material is as brittle as a conventional glass. The mechanical properties are followed as a function of the bulk density. Aerogels with the highest porosity (P > 95%) reveal a maximum flexural strength lower than 10⁻² MPa. A model is proposed to account for the obtained mechanical properties.     

KDP晶体力学参数测试与分析

采用全自动、高精度RMT-150C力学试验系统开展了KDP晶体的力学参数测试,获得了KDP晶体[001]和[100]晶向的弹性模量、泊松比、抗压强度和抗拉强度.结果表明:[001]和[100]晶向的弹性模量分别为39.25 MPa和16.82 MPa,泊松比分别为0.24和0.16,KDP晶体为典型的弹脆性横观各向同性材料.KDP晶体在[001]晶向的抗压强度、抗拉强度均较[100]晶向的高,其在[100]晶向更容易发生脆性破坏.结合KDP晶体的生长受力状态,揭示了KDP生长过程中产生的破坏以拉破坏为主,为开展KDP晶体开裂的力学损伤分析、提出防止开裂的力学措施奠定了重要的基础.     

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.     

Nitridation of SiO2–B2O3 aerogels

SiO₂–B₂O₃ aerogels have been prepared by drying wet gels at a supercritical condition for ethanol in an autoclave. Aerogels have been nitrided for 6 h in flowing ammonia at the temperature of 1200 °C. It has been found that the amount of nitrogen incorporated in these aerogels always exceeds 20 wt%. This is a much higher value compared with the amount of nitrogen incorporated in a pure silica aerogel nitrided at the same conditions. The specific surface area of SiO₂–B₂O₃ aerogels has been between 312 and 359 m²/g. After nitridation some shrinkage of aerogels has been observed and the surface area decreases about 20%. In FTIR spectra of SiO₂–B₂O₃ aerogels a typical bands for SiO₂ are observed. After nitridation a shift and broadening of 1100 cm^?1 band to lower wavenumbers indicates that Si–N and B–N bonds are formed in nitrided aerogels.     

Polyaniline nanofiber–silica composite aerogels

Strong, electrically conducting aerogels were prepared by introducing polyaniline nanofibers to a silica sol just prior to gelation and drying through supercritical carbon dioxide processing. The addition of a few milligrams of polyaniline per cm³ increased the flexural strength of the cylindrical monoliths by 200%. Using preformed polymeric nanofibers avoided filling of microporosity often observed with polymer reinforcement of aerogels and allowed preparation of polyaniline–silica composite aerogels with surface areas over 900 m²/g. Despite the small amount of polyaniline nanofibers (1.3–16.5 wt.%), the composite aerogels were electrically conducting (8.0 × 10^? 8–1.83 × 10^? 5 S/cm) and it was possible to prepare chemiresistor sensors for detection of acidic (HCl) and basic (ammonia) gaseous molecules with response times similar to thin film sensors containing orders of magnitude more polyaniline.     

Changing poisson’s ratio of mesoporous silica monoliths with high temperature treatment

The elastic moduli of mesoporous (fractal pore structure with a statistical pore size distribution with a maximum in the mesopore region) silica monoliths (silica aerogels) were measured in situ during calcination using the resonant beam technique. Above a temperature of approximately 573 K, a significant increase of the elastic moduli with increasing heat treatment temperature was observed, which was attributed to the rearrangement and completion of the network. Poisson’s ratio is close to zero up to this temperature and then increases to a positive value typical for isotropic bodies.     

Aggregation model for the gelation of a sol starting from the processing conditions

A stochastic computational model for the gelation of a sol is explained and tested for the case of neutral silica aerogels. The computational model produces the final structure of the sol after gelation, using two of the several physical phenomena occurring during gelation of sols. Diffusion, represented by Brownian motion, is modeled by a random walk, and chemical reactions are incorporated through a stochastic aggregation model using a probability function; the latter determined in terms of the processing conditions based on the knowledge of the cluster formation energies. The two phenomena are coupled by a Monte Carlo simulation. The analysis of the connected structure and its functionality is demonstrated for neutral silica aerogels. It is shown how the gelation process can be controlled to obtain different structures for different application requirements. The only parameters required by the model are the density and the processing conditions. The results of the model show that those parameters strongly affect the structure of the generated samples. Therefore, processing conditions could be selected to produce aerogels with structures tailored to specific applications, which would constitute a major achievement in aerogel fabrication.     

Molecular design of polymer precursors for controlling microstructure of organic and carbon aerogels

Organic and carbon aerogels were prepared by sol–gel polymerization of phenol, melamine and formaldehyde, followed by supercritical drying and pyrolysis. The effect of the mole ratio of melamine/phenol (M/P) on microstructure of organic and carbon aerogels was investigated by N₂ adsorption, SEM and TEM. Coordination M/P could change the hydrophilicity and cross-linking density of polymer framework, thereby affecting polymer colloid nanoparticle nucleation and growth, and ultimately determine the 3-dimensional network of the gels. The bulk densities of organic and carbon aerogels have maxima at M/P of 0.1, which are inversely proportional to volume shrinkage of gels during drying and pyrolysis. The size of the nanoparticles could be adjusted by varying M/P in the range from 10 to 22 nm. The mesopore volumes of organic and carbon aerogels are tailored in the range of 1.4–2.9 and 0.8–2.5 cm³/g, respectively. The average mesopore diameter has experienced a decreasing first and increasing afterward tendency with the increase of M/P, and exhibit a minimum at M/P of 0.1.     

Interactions between silica sand and sodium silicate solution during consolidation process

This work is based on the knowledge of the consolidation of silica sand with an alkaline solution in order to determine the mechanisms that occur during the drying of sand and various alkaline solution mixtures. The investigations concern effects of sand distribution size, dilution of sodium silicate solutions and drying temperature of the mixtures on consolidation behaviour. The thermal analysis performed on fresh mixtures show a release of free-water from diluted silicate solution during the consolidation. SEM observations and compressive strength tests results indicate that interactions between sodium silicate binder and silica sand depend on drying temperature. Consequently two consolidation mechanisms are proposed: at low drying temperature (70 °C), sodium silicate acts as a thin layer of glue covering sand grains and bind them to each other, while at high temperature (150 °C), dissolution-precipitation reaction occurred in the mixture consolidating more strongly the granular system. The increase of Si/Na molar ratio in a sodium silicate solution containing silica sand is in accordance with the proposed mechanism.     

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。     

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.