Silica Dissolution-Redeposition in Gels and Aerogels

Silica dissolution-redeposition phenomenon is investigated on a microscopic scale using Small Angle Xray Scattering (SAXS) method. The changes occurring in the microtexture are different according to the procedure used to dry the gels. A xerogel, a CO₂ supercritically dried aerogel and an alcohol supercritically dried aerogel were compared. Silica does not dissolve in CO₂ supercritical. The transformation of heat treated gels into CO₂ supercritically dried aerogels is demonstrated as having minor effect on the microtexture. SAXS data show oscillations around Porod law I(Q) Q ^–4 where Q is the wave vector. The oscillations are more pronounced for alcohol dried aerogel than for xerogel or CO₂ dried aerogel. These oscillations are related to curvatures of the surface i.e. the surface roughness. It was found that according to silica dissolution extent, the solid network surface was continuously smoothened in alcohol supercritically dried aerogel. The average chord length of the solid phase is shifted to larger values while the distribution seems to be more narrow. This effect does not give rise to the formation of closed pores.     

Development of Ferroelectric Aerogels

For ultrasonic applications in gases porous piezoelectric transducers with low acoustic impedances are required. Highly porous piezoceramics can be prepared by sol-gel processing, supercritical drying and subsequent firing. These PT and PZT aerogels are obtainable as crystalline monoliths with porosities up to 70 vol%. In this paper the influence of sol-gel-synthesis, stoichiometry, drying conditions and heat treatment on the material properties are reviewed.Lowering the coercive field strength by dopants is necessary in order to facilitate the polarization of highly porous PZT-ceramics. New results show that the introduction of 2 mol% neodymium is possible in the sol-gel synthesis of the aerogel precursor. Most structural properties of the resulting PbNd_0.02Zr_0.53Ti_0.47O₃ aerogels are similar to those of the undoped material.     

Plasticity in Aerogels

When gently stressed, aerogels show an elastic response. However it was found that under isostatic pressure aerogels display an irreversible shrinkage which may be attributed to plastic behaviour. As a consequence of this plastic shrinkage it is possible to densify and modify the elastic properties of aerogels at room temperature.The structural evolution is followed by Small Angle X ray Scattering and the increase of the connectivity is revealed by the evolution of the elastic properties of the material.The SAXS data show that the densification mechanism is different from that obtained by sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, conversely to a sintering effect. We also show that the elasticity of the material is strongly influenced by this structural transformation. The power law evolution of the elastic modulus as a function of the density, usually observed on as-prepared and sintered aerogels, is not valid for compressed material.     

Crystalline Organization in Syndiotactic Polystyrene Gels and Aerogels

The crystalline structure of syndiotactic polystyrene gels and aerogels has been investigated by using x-ray diffraction. Results show that, depending on the solvent, the crystalline structure of the junction zones of the gels is a clathrate phase or the solvent free orthorhombic β-form. For aerogels obtained from gels with a clathrate phase, the aerogel crystalline phase consists of the nanoporous δ-form while for aerogels obtained from gels with the β-form, the original crystalline structure is maintained.     

PZT Aerogels: Sol-Gel Derived Piezoelectric 3-3 Composites

Ultrasonic transducers with low acoustic impedances are required for applications in gaseous media. In porous piezoelectrics the pore volume acts as a second medium of a solid-gas composite and thus lowers the overall acoustic impedance. Lead zirconate titanate (PbZr_.53Ti_.47O₃, PZT) aerogels were prepared by sol-gel processing and supercritical drying in isopropanol. Prefiring at 400°C in air and subsequent thermal treatment in saturated PbO-atmosphere results in phase transformation to PZT (perovskite phase), grain growth and aerogel densification. In this paper the densification of this new class of piezoelectric material at temperatures above 600°C is described. Porosities were measured as a function of sintering time and temperature. Changes in the microstructure were characterized by scanning electron microscopy. Dielectric measurements are presented.     

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.     

Numerical Simulations of Sintering, Application to Partially Densified Aerogels

Following recent scaling theories, the sintering of silica aerogels in modeled starting from computer generated gel structures obtained by diffusion-limited cluster-cluster aggregation on a cubic lattice. Two steps compose the sintering process model: a dressing step in which all particles are replaced by overlapping larger particles and a contraction step in which an adequate length rescaling is performed in order to conserve the total mass. Several quantities are computed during sintering as a function of the aerogel density such as the two characteristic cut-off lengths (lower and upper limits of the fractal regime) and the specific interface area. Comparison is made with results of similar off-lattice calculations when available. Some of the numerical results are compared with experiments on partially densified base-catalysed aerogels.     

Densification Behaviour of Silica Aerogels upon Isothermal Sintering

Isothermal sintering has been studied for a homologous series of supercritically dried base-catalyzed silica aerogels with different initial densities. The change in density and macroscopic viscosity has been recorded in situ using a dilatometer and a beam-bending viscosimeter. Changes of the nanostructural features of the aerogel network have been monitored by means of small angle X-ray scattering. The cylinder model introduced by Scherer is generally accepted to describe the sintering behaviour of bodies with moderate porosity, such as xerogels. In case of highly porous aerogels, however, preferential densification of large pores and little loss of specific surface area is observed, at least during the initial stages. For the samples under investigtion the increase in density and the corresponding variation of specific inner surface and viscosity are compared to the analytical predictions by Scherer, the scaling approach introduced by Sempéré et al. and the results obtained from numerical simulation of viscous flow.     

Structural Efficiency and Microstructural Modeling of Wet Gels and Aerogels

We present a microstructural model of aerogels that includes the effect of particle necks, tortuosity and dangling ends on the scaling of elastic moduli with density. Relative neck radii can be determined for sintering series of silica aerogels and for Resorcinol Formaldehyde (RF) aerogels produced with different catalyst concentrations. The density of elastically ineffective dangling ends and the tortuosity can be estimated using information from thermal conductivity and elastic modulus measurements in silica aerogels. Typical values for the load bearing mass range from >50% for high density and heat treated aerogels to <10% of the total mass for low density wet-gels.     

Biopolymer Aerogels and Foams: Chemistry,Properties, and Applications

Biopolymer aerogels were among the first aerogels produced, but only in the last decade has research on biopolymer and biopolymer–composite aerogels become popular, motivated by sustainability arguments, their unique and tunable properties, and ease of functionalization. Biopolymer aerogels and open‐cell foams have great potential for classical aerogel applications such as thermal insulation, as well as emerging applications in filtration, oil–water separation, CO₂ capture, catalysis, and medicine. The biopolymer aerogel field today is driven forward by empirical materials discovery at the laboratory scale, but requires a firmer theoretical basis and pilot studies to close the gap to market. This Review includes a database with over 3800 biopolymer aerogel properties, evaluates the state of the biopolymer aerogel field, and critically discusses the scientific, technological, and commercial barriers to the commercialization of these exciting materials.     

Scaling Properties and Structure of Aerogels

Young’s modulus as well as solid thermal and electrical conductivity of aerogels have been observed to scale with density. No quantitative explanations were available up to now for these experimental findings. To establish a quantitive relationship between morphological and topological features of fractal gel networks, a simulation procedure is introduced that allows to produce three-dimensional gel structures, from which two important parameters can be extracted: i) the fraction α of interconnected mass of the gel network and ii) the ratio γ of Pythagorean distance to minimum path length on the gel backbone. Surprisingly the product αγ, which enters important macroscopic parameters such as elasticity or solid thermal (and electrical) conductivity, was found to scale with an exponent that is only a function of the mass fractal dimensionD. Also, an analytical relation between modulus and conductivity can be derived.     

Effect of Aging on Alumina Gels Rheology and Aerogels Surface Area

Alumina gels were synthesized by catalyzed gelation of aluminum sec-butoxide (ASB) via the Yoldas process. The gels were aged for up to 6 months and then supercritically dried (SCD) with CO2. The molar ratio of acid to ASB was in the range of 0.01–0.6. Viscosity measurements of the gels showed a shear thinning and plastic behavior with no response up to a limiting yield stress. The gel rheology obeys the Casson model. Analysis of the viscosity as a function of the acid to alkoxide molar ratio, showed that the average molecular weight of the gels is inversely proportional to the acid to alkoxide molar ratio.The viscosity of all the gels increased with aging time for a period of about 6 months reaching an asymptotic value after 1–2 weeks. The viscosity is shown to correlate with the microstructure of these nanomaterials during aging. Aging gives rise to a nearly constant surface area of 350 m2/g regardless of acid to alkoxide ratio in an aging period of about 6 months.     

Recent Advances in the Synthesis and Application of Three-Dimensional Graphene-Based Aerogels

Three-dimensional graphene-based aerogels (3D GAs), combining the intrinsic properties of graphene and 3D porous structure, have attracted increasing research interest in varied fields with potential application. Some related reviews focusing on applications in photoredox catalysis, biomedicine, energy storage, supercapacitor or other single aspect have provided valuable insights into the current status of Gas. However, systematic reviews concentrating on the diverse applications of 3D GAs are still scarce. Herein, we intend to afford a comprehensive summary to the recent progress in the preparation method (template-free and template-directed method) summarized in Preparation Strategies and the application fields (absorbent, anode material, mechanical device, fire-warning material and catalyst) illustrated in Application of 3D GAs with varied morphologies, structures, and properties. Meanwhile, some unsettled issues, existing challenges, and potential opportunities have also been proposed in Future Perspectives to spur further research interest into synthesizing finer 3D GAs and exploring wider and closer practical applications.     

Aerogels—Recent Progress in Production Techniques and Novel Applications

Aerogels are sol-gel derived nanostructured materials with extraordinary properties according to their high porosity. Though first prepared more than 60 years ago, silica aerogels became widely known only in the late 1980s when they were used in Cerenkov detectors and their potential was recognized as high performance thermal insulants. Nowadays, aerogel research has attracted many scientists from different fields, resulting in some 100 publications per year and the fifth aerogel symposium (ISA 5) in Montpellier/France in September 1997. This review will focus on recent developments in fast supercritical and ambient pressure drying processes. The state of the art with respect to structural characterization and measuring the material properties is reported including nondestructive techniques and alterations induced by invasive methods. A brief survey is given on modeling the aerogel structure and simulating properties. Special attention will be given to carbon aerogels and their organic precursors. Due to the high electrical conductivity of their graphitic backbone and the large specific inner surface areas, carbon aerogels can be considered ideal electrodes in supercapacitors and fuel cells.     

常压干燥制备二氧化硅气凝胶*

二氧化硅气凝胶是典型的纳米多孔轻质材料,由于具有独特的性能并在许多领域存在潜在的应用价值而受到广泛关注。二氧化硅气凝胶的制备传统上采用超临界干燥工艺,但此工艺成本高、工艺复杂而且具有一定的危险性。为了实现二氧化硅气凝胶的大批量生产和商品化应用,研究低成本常压干燥制备技术非常必要。目前常压干燥制备工艺已取得了较大进展,本文主要介绍了二氧化硅气凝胶的常压干燥制备方法及其特点,并概述了二氧化硅气凝胶复合材料制备的最新研究进展。以纤维和聚和物为增强体的二氧化硅气凝胶复合材料改善了气凝胶的力学性能,进一步扩宽了其应用范围。     

Doped-ZrO2 Aerogels: Catalysts of Controlled Structure and Properties

Doped ZrO₂ aerogels (characterised by TEM, DTA and N₂ adsorption) have been prepared and catalytically tested in CO/CO₂ hydrogenation [1] and CH₄ oxidation [2]. The primary aerogels showed cross-linked clusters of (X-ray) amorphous particles smaller than 5 nm which led to well-developed mesoporous solids with an average pore size of about 10 nm and high surface area (up to 250 m²g⁻¹) [1]. Cu/ZrO₂ aerogels (known to be very active and selective towards methanol synthesis in CO hydrogenation without predominant formation of alkanes even at higher temperatures [1]) are now seen to show these effects even more clearly in CO₂ hydrogenation. In methane oxidation, both Rh/ZrO₂ and Y₂O₃/ZrO₂ were very active. Consideration is given to the nature of the active sites, the role of CO₂ and metal/oxide interfaces and how an understanding of this reactivity can lead to better dispersed ZrO₂.     

Preparation of SiO2-TiO2 Aerogels Using Supercritical Impregnation

The preparation of SiO₂-TiO₂ aerogels by supercritical impregnation of titanium alkoxides into silica alcogels was investigated. A mixture of CO₂ and 2-propanol with dissolved titanium tetraisopropoxide modified with acetylacetone was used as the impregnation medium. Prior to the experiments, the supercritical behaviour of the impregnation solution was investigated. The microstructure and properties of aerogels prepared by the supercritical impregnation method were almost identical to those generated by the liquid impregnation. However, the time for impregnation was substantially decreased and the homogeneity of the impregnated titanium distribution on the aerogel increased.     

Synthesis and Photocatalytic Properties of Titanium-Porphyrinic Aerogels

We present novel titanium-porphyrinic gels (TPGs) and titanium-porphyrinic aerogels (TPAs), in which porphyrinic ligand tetrakis(4-carboxyphenyl)porphyrin is coordinated to Ti-oxo clusters. These hierarchically porous TPAs, with micro-, meso-, and macropores and reactant-concentration-dependent Brunauer-Emmett-Teller surface areas of 407–738 m² g⁻¹, are prepared by CO₂ critical point drying of TPGs. Although the Ti⁴⁺ → Ti³⁺ photoreduction of TPAs is less efficient than that of crystalline microporous Ti-porphyrinic framework DGIST-1, prompt diffusion of O₂ and spin-trapping agents into the TPA pores causes the rapid generation of reactive oxygen species (ROS), as observed by EPR spectroscopy. When used as an ROS scavenger, large 1,3-diphenylisobenzofuran is degraded by the best-performing TPA 10 times faster than by DGIST-1, suggesting that the accessibility of molecules (reactants) to pores (reactive centers) strongly influences photocatalytic activity.     

Sorption of Pollutant Traces by Nanoporous Crystalline Aerogels: Visualization by a Dye

Summary: The influence of the crystalline structure on the pollutant sorption properties of high porosity syndiotactic polystyrene (sPS) aerogels has been made visible by using azulene, a blue dye, capable to form a clathrate phase with sPS. The bluish coloration observed for aerogels with the nanoporous δ-form and the absence of coloration for aerogels with the densely-packed γ-form clearly establish that the crystalline nanopores play a key role for the removal of pollutant traces from water or air.