Elastic Properties of Porous Silica Derived from Colloidal Gels

The elastic properties were determined for gel-derived vitreous silica sintered from a relative density of 0.15 to full density. The gels were made from colloidal silica, potassium soluble silicate, and formamide. The soluble silicate was found to reinforce the gel network during gelation and its concentration relative to the colloidal silica affected Young's modulus for samples of relative density, 0.15 to 0.6 Young's modulus varied from 300 to 700 MPa at a relative density of 0.15 as a function of the soluble silicate concentration. Young's modulus at full density was measured to be 72.6 GPa. Poisson's ratio was found to be a function of relative density.     

Laws of the Formation and Diffusion Properties of Silica and Agarose Gels

The physicochemical and mass-transfer properties were studied for two gels that can be applied in the fabrication of bioreactors using additive manufacturing. The fundamental difference between these gels lies in the mechanisms of their formation from solutions. The first one is a silicic acid gel, which is formed in a chemical reaction and has an irreversible gelling process. The second one is an agarose gel, which is formed from a water solution of agarose upon cooling and turns back into liquid when heated. Studies were performed by optical methods that include the spectroscopy of gel samples during their formation and the visualization of the diffusion fronts of transferred substances. Being nonintrusive, these methods have no effect on the state of samples and do not distort the dynamics of processes. Characteristic patterns of changes in the spectral parameters during gelling were studied for both gels depending on the dispersed phase concentration, which was relevant to the practical application of the gels. New experimental data were obtained on the structure of the gels and the mechanisms of its change during gel aging. The main laws are determined for changes in the diffusion coefficients in the gels with increasing concentration of the dispersed phase.     

Structural Investigations of Alkali Silicate Gels

Investigation of a series of synthetic alkali silicate gels and gels produced by the alkali silica reaction (ASR) in field concrete using ²⁹Si NMR spectroscopy, X-ray diffraction, and bulk chemical analysis shows that local structures of the synthetic and field gels are quite similar. The most abundant Si sites for the field and synthetic gels with similar compositions have Q³ polymerization, and the number of non-bridging oxygens per Si is similar for these samples. These samples also yield a basal X-ray diffraction peak near 8–12 Å, suggesting that the structure is dominated by sheet-like units, consistent with the dominant Q³ polymerization. Calculations based on the relative site abundances of the sites observed by ²⁹Si NMR and the bulk chemical compositions indicate that there is insufficient alkali to charge-balance all the non-bridging oxygens and that there is a significant concentration of Si–OH linkages. The results provide strong support for the basic structural concepts of the so-called kanemite model for ASR gel proposed by Wieker and coworkers, although the overall gel structure is likely to be more complex.     

Drying Behavior of Colloidal Silica Gels

Observations of the drying behavior of thick-wall colloidal silica gel structures are reported. Various techniques are examined to prevent cracking during the drying of these highstrain viscoelastic materials. Experiments are described which illustrate the effect of relative humidity on the drying rate and on the shrinkage of various samples under isothermal conditions. Surface temperature measurements indicate that evaporation occurs at approximately the wet-bulb temperature of the surrounding atmosphere. Acoustic detection of the internal strain activity during drying leads to the conclusion that control of the sol-gel surface tension in the material preparation is crucial to reducing the interior stresses.     

非离子型凝胶球在水中的溶胀行为

以过硫酸铵和N,N,N',N',-四甲基乙二胺氧化-还原体系引发丙烯酰胺聚合,制备球状非离子型水凝胶.利用带刻度的显微镜记录了凝胶球在水中的溶胀扩散过程,采用Tanaka-Fillmore凝胶溶胀动力学模型进行数学处理,得到凝胶网络扩散系数.在此基础上,进一步考察了交联剂、引发剂、单体浓度对凝胶网络扩散系数的影响,结果表明:随着交联剂用量的提高,扩散系数先变小后增大;增加引发剂﹑单体浓度,扩散系数均增大.对凝胶网络扩散系数-温度理论模型进行修正,更加符合实验结果,可用于凝胶网络扩散系数的预测.     

Surface Area Study of Pretreated Silica Gels

Abstract

The surface areas of a series of pretreated silica gels were measured by a frontal-analysis chromatographic technique using benzene for which the cross-sectional area had been obtained by adsorption measurements on a sample of Sterling MT carbon black as a standard. Much smaller surface areas were found than when nitrogen was used. The chromatographic equilibrium constants for aromatic adsorbates, calculated using the surface areas measured with benzene, point to a change in the adsorption mechanism which was not apparent when nitrogen surface areas were used.     

交联大豆蛋白凝胶的溶胀性能的研究

制备了大豆蛋白凝胶与戊二醛交联大豆蛋白凝胶,研究了凝胶的溶胀性及其影响因素,以质构仪、扫描电镜对凝胶的质构和断面微观结构进行了表征,结果表明:大豆蛋白浓度为13%,其凝胶溶胀率为15.00g/g;当大豆蛋白浓度为12%,25%戊二醛用量为50μL,交联大豆蛋白凝胶溶胀率为25.1g/g,;氯化钠浓度和水的温度对交联大豆蛋白凝胶的溶胀率有影响,氯化钠浓度从0升至0.1M,交联凝胶溶胀率从25.1减小为5.34g/g;水的温度从20升至60℃,凝胶的溶胀率从21.81增至49.12g/g;质构分析和扫描电镜结果显示,戊二醛交联凝胶比大豆蛋白凝胶具有更高的硬度、弹性、内聚性、破裂强度和更有序的微观结构。     

Diffusion During the Supercritical Drying of Silica Gels

Drying is the most critical elaboration step of large monolithic and crack-free silica aerogel plates. In the present work, we are studying the supercritical CO₂ drying and more precisely the first step, here called the supercritical washing step. This phase consists of replacing the liquid phase contained in the nanopores with supercritical CO₂. Within this study, this step is governed by molecular diffusion through the gels. These phenomena were investigated experimentally in order to estimate the duration of the washing step. The experimental results were then fitted with an analytical mass transfer model to identify the effective diffusion coefficient.     

Diffusion During the Supercritical Drying of Silica Gels

Drying is the most critical elaboration step of large monolithic and crack-free silica aerogel plates. In the present work, we are studying the supercritical CO₂ drying and more precisely the first step, here called the supercritical washing step. This phase consists of replacing the liquid phase contained in the nanopores with supercritical CO₂. Within this study, this step is governed by molecular diffusion through the gels. These phenomena were investigated experimentally in order to estimate the duration of the washing step. The experimental results were then fitted with an analytical mass transfer model to identify the effective diffusion coefficient.     

Elastic Moduli of Silica Gels Prepared with Tetraethoxysilane

The elastic moduli of several organometallic silica gels were measured using pulse-superposition interferometry (PSP). A series of gels were prepared under acid-catalyzed conditions where the molar ratio of water to tetraethoxysilane (TEOS) in the sol was varied (4:1, 12:1, 16:1, and 24:1). It was found that the elastic moduli varied systematically with the hydrolysis conditions used to prepare the gels, as well as with the relative humidity used to equilibrate the gels before and during the PSP measurement. The experimental PSP data for the gels were evaluated using a composite model (matrix plus pores) and then the elastic moduli of the gel matrix were determined by applying a correction for porosity based on a self-consistent-scheme (SCS) approximation. The results show that the elastic moduli of the gel matrix also exhibit a systematic dependence upon the preparation conditions, but they always fall below the corresponding values for fused silica. Even when excess water Was used to hydrolyze the TEOS, this did not lead to a fully polymerized silica matrix structure. It is suggested that the lowered matrix moduli values are due to the presence of residual hydroxy and ethoxy groups which are internal to a polymeric structure.     

Structural Evolution of Colloidal Silica Gels to Glass

The gel-to-ceramic evolution of colloidal-derived silica gels was examined by thermal analysis, infrared photoacoustic spectroscopy, physical characterization, and high-temperature calorimetry. Oxygen in the firing atmosphere promoted sintering of colloidal gels relative to firing in a nitrogen or argon atmosphere. Water vapor in the atmosphere promoted consolidation and crystallization to cristobalite. Addition of PVA to the colloidal gel resulted in consolidation and crystallization of the gel at 1000°C, ∼150°C less than gels without the PVA binder. The enthalpy of consolidation of the gels to fused silica glass was approximately − 10 kJ/mol, primarily due to reduction of surface area.     

Diffusion of Alkali Ions in Vitreous Silica

The diffusion of Cs⁺, Rb⁺, and K⁺ ions was measured in three grades of vitreous SiO₂ by the radiotracer-sectioning technique or Rutherford, backscattering spectroscopy. The values of the diffusion coefficient, D , at 1000°C decrease strongly with increasing ionic radius, changing by about two and one-half orders of magnitude per row of the periodic table. The difference between D_Cs and D_Rb is largely in the preexponential factor D_o in the Arrhenius expression rather than in the activation enthalpy Q . The values of D are much smaller than the tracer D values for the alkali metal ion in homogeneous Rb or Cs silicate glasses. Residual metallic impurities in the SiO₂ decrease both Q and D_o for the diffusion of Rb. The results are analyzed in terms of the interstitial structure of vitreous SiO₂.     

Photoinduced Formation of Small Gold Particles in Silica Gels

AuCl⁻₄ ions in sol-gel-derived silica gels (Au/silica gels) were photoreduced by ultraviolet light irradiation at room temperature. In the presence of ethylene glycol in the silica gels, small Au particles were effectively formed by the irradiation. When the storage temperature of the gels was increased in the dark after the irradiation, the surface plas-mon resonance absorption peak of the Au particles was shifted to longer wavelengths, indicating that the Au particles became large. At a storage temperature below 9°C, Au particles could not be observed in the silica gels. The apparent activation energy of Au particle formation was estimated to be 17. 3 kcal·tmol⁻¹.     

电解质影响凝胶溶胀行为的研究进展--水的状态和凝胶的溶胀收缩

综述了水凝胶研究的一些新进展。简要介绍了水凝胶中水的存在状态和水合氢键模型,分析了电解质影响水凝胶中水的分子状态和凝胶溶胀收缩的现象,并利用水合氢键模型加以解释,指出有待解决的相关问题。     

碱渗透溶胀法制备中空结构聚合物微球

采用预乳化饥饿型滴加方法结合碱溶胀处理制备了聚(甲基丙烯酸-甲基丙烯酸甲酯-丙烯酸正丁酯)/聚(甲基丙烯酸甲酯-苯乙烯-甲基丙烯酸)中空聚合物微球。试验集中讨论了种子阶段粒子大小,种子阶段引发剂用量、扩径时不同核壳比及不同碱进行碱处理时对形成中空聚合物微球的影响。同时采用TEM观察聚合物微球结构。试验所得的聚合物微球外径约为400 nm,内径约为220 nm。     

Influence of Diluent on Swelling Behavior,Compressive Properties,and Network Parameters of EPDM/4-tert-Butylstyrene Oil Gels

A series of oil gels based on 4-tert-butylstyrene (tBS) and ethylene-10 propylene-diene (EPDM) cross-linked with determined amount of divinylbenzene (DVB) was synthesized by suspension polymerization. Toluene (Tol), cyclohexane (Cyclohex), and their mixtures with different compositions were used as pore-forming diluents. The influence of diluent type, dosage, and composition on swelling 15 behaviors, compressive properties, effective cross-link density (ne), average molecular weight between cross-links (Mc), and polymer-solvent interaction parameter (v1) of oil gels was studied in this work.     

Pore Structure Control of Silica Gels Based on Phase Separation

In the alkoxy-derived sol-gel system, various macroporous morphologies can be obtained by inducing the phase separation parallel to the sol-gel transition. This principle of macroporous morphology control can be best applied to pure silica and silica-based multicomponent oxide systems. The earlier the phase separation takes place than the sol-gel transition, the larger the characteristic sizes of pores and gel skeletons become. The time resolved light scattering measurements revealed that the morphology formation process exhibits the features of spinodal decomposition and that the final gel morphology is determined by the competitive kinetics between the domain coarsening and the structure freezing by sol-gel transition. The mesopore structure of such macroporous gel skeletons could be easily tailored by the solvent exchange procedures. Silica gels with controlled macropores and mesopores were successfully applied as a material for the continuous rod type column for high performance liquid chromatography.     

Synthesis of Silica Gels and Organic-Inorganic Hybrids on Their Base

The main regularities of the synthesis of monolithic SiO₂ gels (nonfractured upon drying) are considered briefly. Most attention is focused on the xerogels prepared through hydrolysis and polycondensation of silicon alkoxides. The synthesis and structure of organic-inorganic hybrids are outlined using recent data available in the literature. The main fields of application of the materials prepared on their base are discussed.     

Synthesis of High Surface Area Silica Gels Using Porous Carbon Matrices

Porous silica was prepared using the sol-gel synthesis with porous carbon matrices as a pore-forming support. Tetraethoxysilane (TEOS) was hydrolyzed in an acid medium in the presence of a substoichiometric amount of water. Various carbon materials were used, among them Sibunit and catalytic filamentous carbon. Carbon matrices were impregnated with hydrolyzed TEOS and dried, then carbon was removed by burning out in air at 873 K. The obtained porous silica samples were studied by adsorption and electron microscopic methods. The specific surface area as high as 1267 m²g and pore volume as high as 5.7 cm³/g were determined for some silica samples. Thus deposited SiO₂ was found to cover the carbon surface copying its surface. With CFC used as carbon matrix, silica nanotubes were obtained. Thermostability of such silica is significantly greater as compared to silica gels reported earlier.