Synthesis of nanoporous silica aerogel by ambient pressure drying

A crack-free silica aerogel monolith was fabricated from a cheap water glass derived silicic acid solution by adding glycerol, which served as a drying control chemical additive (DCCA). The OH surfaces of the wet gel with glycerol were modified using a TMCS/n-hexane mixture followed by solvent exchange from water to n-hexane. The obtained surface modified wet gel was dried at 75 °C under ambient pressure. The addition of glycerol appears to give the wet gel a more homogeneous microstructure (larger pore size and uniform size distribution) as well as enhanced stiffness. However, glycerol also retards surface modification and solvent exchange. The aerogel synthesized with glycerol added to the silica sol maintained a relatively low bulk density compared with the aerogels aged in a mixed ethanol (EtOH)/TEOS solution. The reproducibility of aerogel production was further improved in the aerogel synthesized with glycerol added to the silica sol and aged in a 70%EtOH/30%TEOS solution.     

Alkoxy-derived multiscale porous TiO<Subscript>2</Subscript> gels probed by ultra-small-angle X-ray scattering and small-angle X-ray scattering

Titania (TiO₂) monoliths with well-defined bicontinuous macropores and gel skeletons were prepared through the alkoxy-derived sol–gel process accompanied by spinodal decomposition, and the structural evolution during evaporation drying and heat treatment was probed by a combination of ultra-small-angle X-ray scattering and small-angle X-ray scattering. X-ray scattering profiles of wet and dried gels revealed that microporous structures related to the existence of primary particles are present in the gel skeletons at the wet stage and are preserved during drying. Additionally, it is found that the primary particles swollen in the wet condition are dried to compact aggregates to produce the smooth surface of gel skeletons. Upon heating at 400 °C, the particle–particle correlation associated with regularity of mesostructures is enhanced. From nitrogen adsorption–desorption measurements, the average pore size is less than 1 nm in the dried gel and increases to 3.1 nm by the heat treatment. Homogeneous growth of primary particles due to interparticle-polycondensation reaction is responsible for the increased size and uniform distribution of mesopores in the heat-treated gel.     

DMF及热处理对常压制备Cu掺杂SiO2纳米复合气凝胶的影响

以正硅酸乙酯(TEOS)为硅源, 硝酸铜(Cu(NO3)2·3H2O)为铜源, 通过在复合溶胶体系中引入干燥控制化学添加剂(DCCA)N,N-二甲基甲酰胺(DMF)进行原位共溶胶-凝胶, 结合常压干燥工艺, 制备出具有高比表面积(560 m2·g-1)的Cu-SiO2纳米复合气凝胶(含铜质量分数为5%). 研究了DMF对凝胶时间、干燥过程和复合气凝胶形态结构的影响, 利用N2物理吸附, 全自动X射线衍射(XRD)仪, 傅立叶变换红外(FT-IR)光谱仪, 透射电子显微镜(TEM)等对样品的形貌结构进行了表征. 实验结果表明, DMF能有效防止凝胶的开裂, 抑制颗粒团簇的产生, 使所得复合气凝胶的粒径减小, 比表面积增加, 微观结构更趋完善. 高温热处理后, Cu-SiO2中的铜物种仍高度分散于骨架网络中, 复合气凝胶显示出良好的热稳定性.     

Synthesis of silica nanoparticles by modified sol–gel process: the effect of mixing modes of the reactants and drying techniques

A modified preparation of silica nanoparticles via sol–gel process was described. The ability to control the particle size and distribution was found highly dependent on mixing modes of the reactants and drying techniques. The mixture of tetraethoxysilane and ethanol followed by addition of water (Mode-A) produced monodispersed powder with an average particle size of 10.6 ± 1.40 nm with a narrow size distribution. The freeze drying technique (FD) further improved the quality of powder. In addition, the freeze dried samples have shown unique TGA decomposition steps which might be related to the well-defined structure of silica nanoparticles as compared to the heat dried samples. DSC analysis showed that FD preserved the silica surface with low shrinkage and generated remarkably well-order, narrow and bigger pore size and pore volume and also large endothermic enthalpies (ΔH _FD = −688 J g⁻¹ vs. ΔH _HD = −617 J g⁻¹) that lead to easy escape of physically adsorbed water from the pore at lower temperature.     

Characterization of Hydrophobic SiO2 Powders Prepared by Surface Modification on Wet Gel

Hydrophobic porous silica has been prepared by surface modification of TEOS (tetraethylorthosilicate) wet gel with 6 and 12 vol.% of TMCS (trimethylchlorosilane). We characterized the products by using FT-IR, TGA, DTA, N₂ adsorption/desorption, contact angle and SEM. Surface silanol groups of the gel were widely replaced by–Si(CH₃)₃ to result in a hydrophobic SiO₂ powder as confirmed by contact angle measurements with H₂O, 1-butanol and ethanol. The modified dried gels had a surface area of 950–1000 m²/g (average pore size 120 Å), compared to the non-modified surface which had a surface area of 690 m²/g (average pore size 36 Å). The adsorption/desorption isotherm curves indicated they had similar pore characteristics as aerogels prepared by the supercritical drying process.     

Preparation and Characterization of Porous Titania by Modified Sol-Gel Method

Mesoporous titania, especially anatase, is attractive due to its potential applications. A novel method to control pore structure of titania, surfactant- or polymer modification, is proposed. The wet gels and gel films, prepared from Ti(O-nC₄H₉)₄ were dried at 90°C and annealed at 500°C after immersion in surfactant or polymer solutions, and mesoporous anatase was obtained. The pore size, pore volume and specific surface area of the surfactant-modified bulk gels, estimated from N₂ absorption-desorption curves, are more than twice larger than those of the gels without modification. The pore size of the surfactant-modified gel films, observed by SEM, are similar to that of the bulk gels. The pore size obviously depended on the size of micelles. The pore size of the gels modified with hydrophilic polymers hardly increased, but the pore volume and the specific surface area increased.     

块状TiO2/SiO2气凝胶的非超临界干燥法制备及其表征

分别通过TiO2和SiO2的单独溶胶和TiO2/SiO2复合凝胶,并添加干燥控制化学添加剂甲酰胺,形成比较完善的凝胶网络结构,同时通过正硅酸乙酯的乙醇溶液浸泡,低表面张力溶剂替换和分级陈化以及干燥等步骤,实现了块状TiO2/SiO2复合气凝胶的非超临界干燥制备.所得TiO2/SiO2气凝胶为无色或乳白色轻质块状多孔固体,表观密度约0.4～0.9g/cm3,孔隙率约80%～95%.它由直径约10nm的TiO2和SiO2微粒相互分散复合而成,孔洞直径约几十纳米.其相态SiO2为无定形,TiO2为锐钛矿晶型.随着焙烧温度的升高,直到800℃不发生相变化.     

常压干燥法制备Al2O3块状气凝胶

以无机铝盐Al(H2O)9(NO3)3为前驱体，甲酰胺作为干燥控制化学添加剂(DCCA)，1,2-环氧丙烷作为凝胶网络诱导剂，通过溶胶-凝胶法制备氧化铝凝胶；在常压条件下对凝胶进行干燥，得到乳白色、半透明、轻质、块状氧化铝气凝胶.初步探讨了在凝胶制备和陈化过程中增强凝胶网络结构强度的途径和机理.     

Drying of alkoxide gels—Observation of an alternate phenomenology

The process of drying of a porous material as per the current phenomenological theory can be divided into two stages. At first the body shrinks by an amount equal to the volume of liquid that evaporates, and the liquid-vapor interface remains at the exterior surface of the body. The second stage begins when the body becomes too stiff to shrink and the liquid recedes into the interior, leaving air filled pores near the surface. We shall refer to this phenomenology as the drying front model. In our investigation of drying of alkoxide silica gels of less than 50 Angstroms pore radius, we have observed a different drying pattern, in which even after the gel body stops shrinking, drying continues to occur by evaporation on the exterior surface of the gel body, causing spontaneous nucleation of partially or fully dried opaque clusters, randomly distributed in the interior parts of the gel. These clusters than increase in number and size till they coalesce to form an opaque body. Upon further drying, the gel returns to its transparent form. We postulate that this is possible only if the rate of fluid flow in the pores by diffusion is faster than that by Darcy's flow, as well as the evaporation rate at the surface of the gel body. We shall refer to this as the cluster drying model. We shall present results of pin-hole drying experiments on cylindrical alkoxide gels showing that for identical gels the evaporation rate can be increased to change the phenomenology from cluster drying to one that exhibits both phenomenology simultaneously and finally to that of the drying front phenomenology. We shall also show the effect of gel pore size distribution on the phenomenology of drying under identical drying conditions. Finally, we will present evidence that for successful drying of large cylindrical alkoxide gels, drying conditions favoring cluster drying phenomenology is desirable.     

Effect of Aging on the Stability of Ceramic Foams Prepared by Thermostimulated Sol-Gel Process

This work presents a new route of preparation of zirconium ceramic foams based on the thermostimulated sol-gel process. This method produces gelled bodies with up to 90% of porosity in the wet gel and can be used to make complex-shaped components. Unfortunately, the shrinkage during the drying step allows to a catastrophic reduction (>50%) of the foam porosity. To improve the foam stability we carried out a systematic study of the effect of gel foam aging on the drying process. Samples were aged in closed vessel at 25°C during different time period (from 6 to 240 h). The shrinkage and the mass loss during drying at 50°C were measured in situ, using a non-contact technique performed with a special apparatus. The results show that the total linear shrinkage decreases from 46% to 8% as the aging period increase from 6 to 240 h. This behavior is followed by a small change of total mass loss, from 42 to 54%. It indicates that by aging the structural stiffness of the foams increases due to secondary condensation reactions. Thus, by controlling the aging period, the porosity can be increased from 67 to 75% and the average size of mesopores of dried foams can be screened from 0.3 to 0.9 m. Finally, these results demonstrate that the thermostimulated sol-gel transition provides a potential route to ceramic foams manufacture.     

不同干燥过程对超细TiO2粉体性质的影响

考察了采用不同干燥工艺制备的TiO2粉体在粒子形貌、颗粒大小与分布、晶相组成以及比表面积和孔结构等织构和结构性质方面的差异。结果表明，利用常规的干燥方法，由水凝胶脱水所得的颗粒，颗粒间严重团聚，颗粒粒径大且分布不均匀，比表面积和孔体积最小；由醇凝胶直接脱水，则可以显著提高粉体的织构性能．而采用超临界流体干燥法则可以进一步提高粉体的性能，比表面积由水凝胶的4．88m2·g－1增大到113．8m2·g－1，提高了近30倍；孔体积由0．027cm3·g－1增大到0．41cm3·g-1．大约提高了15倍；而且其能够有效地防止粒子间的团聚，较好地保持了湿凝胶的网络结构，使颗粒尺寸降低且分布均匀，可重复性好．     

Stress During Drying of Two Stone Consolidants Applied in Monumental Conservation

The object of this paper is to evaluate behaviour during drying of two stone consolidants: Wacker OH and Tegovakon V, containing tetraethoxysilane. During drying, the gel network contracts due to capillary pressure generated by solvent evaporation. When the consolidant dries inside the stone porous structure, the shrinkage is constrained in all three dimensions. In these conditions, the dried gel suffers a high stress that could cause it to crack. When there is a free surface, as for a consolidant layer on the surface of a pore, the stress can relax in the direction normal to the surface. In this case, the stress is controlled by network rigidity.The rigidity of the gel network has been evaluated by mercury porosimetry, while pore size, which controls capillary pressure, has been determined by nitrogen adsorption. The shrinkage of gels under mercury pressure is characterised by high moduli. This fact suggests a high rigidity of the networks. The small pore radii found in the network (<3 nm) indicate that high capillary pressures are generated within the gel network.     

Thermal and Temporal Aging of Two Step Acid-Base Catalyzed Silica Gels in Water/Ethanol Solutions

Dissolution and reprecipitation of silica during aging in water improve the wet gels mechanical stiffness and strength, and hence shrinkage during supercritical drying is reduced. We have investigated how the strength and stiffness of a 2-step TEOS acid-base catalyzed wet gel can be improved by aging in a solution of water/ethanol (20–40 vol%) at various temperatures (20–70°C) and time (2 h and 24 h) and how this influences the aerogels properties. The linear shrinkage during supercritical drying was reduced from 29% to 2% by introducing the aging step in the 20 vol% water/ethanol solution for 24 h at 60°C.We have also in previous works introduced the idea of preparing ambient pressure dried silica aerogels by increasing the wet gels stiffness by aging in a TEOS solution until shrinkage during drying is almost eliminated. The gels aged in the water/ethanol solutions were further aged in a TEOS/ethanol solution and the effect of the increasing water content in the pore liquid was studied. A xerogel density of 0.20 g/cm³ is reported for gels with a shear modulus (G) of 30 MPa.     

Mechanical strengthening of TMOS-based alcogels by aging in silane solutions

It has been shown that aging of tetramethoxysilane (TMOS)-based alcogels in solutions of tetraethoxysilane (TEOS)/methanol (MeOH) provides new monomers to the alcogel and favorably increases the strength and stiffness of the alcogel and hence reduces the shrinkage during the subsequent drying. Load relaxation experiments have been performed to determine the shear modulus (G), Poisson's ratio (), and the permeability of wet gel rods as a function of aging time in the TEOS/MeOH solution. The modulus of rupture (MOR) and G have also been obtained from 3-point bending tests. Aging the gels in 70 vol% TEOS/MeOH causes an increase in G from 0.48 MPa to 1.8 MPa and 7.4 MPa after aging for 24 hours and 144 hours, respectively.It is shown that the drying stress is actually increased by the aging treatment, but the increase in strength of the gel is even greater; hence, strengthening of the alcogels dramatically reduces the probability of cracking during drying. Unaged gels with higher TMOS concentrations corresponding to the silica content of gels aged in TEOS solution, however, showed large shrinkage and severe cracking.     

Effect of drying methods on the structure and catalytic combustion activity of Mn-substituted hexaaluminate catalysts

Conventional oven drying (COD) and supercritical drying (SCD) methods were applied to the preparation of Mn-substituted hexaaluminate (BaMnAl₁₁O_19-α) catalysts. The effect of drying methods on phase composition, specific surface area, pore structure and combustion activity of the samples was investigated. The samples obtained by SCD have higher surface area, narrower pore size distribution, and higher combustion activity than those obtained by COD. This revised version was published online in August 2006 with corrections to the Cover Date.     

Drying and sintering of sol-gel derived large SiO2 monoliths

This review article summarizes the development of drying and sintering techniques for the production of sol-gel derived, large silica glass components. Gels may be synthesized using particulate or metal alkoxide precursors, or both in combination. Rapid fracture-free drying has been achieved easily with particulate gels because of their large pore size (100–6000 Å). Alkoxide gels, which generally have small pores (<200 Å), were initially difficult to dry without cracking. However, recent studies have shown that large alkoxide gel monoliths can also be dried in reasonably short times (<10 days). During subsequent heat treatment, alkoxide gels tend to have high shrinkage rates, which may cause trapping of hydroxyl ions or organic groups remaining on the gel surface. Although the removal of these species is easier for particulate gels, their large pore size necessitates heating above 1400°C to achieve full consolidation. Sintering at such temperatures was observed to deteriorate glass quality, through crystallization, warping, and/or sagging. Extensive optimization of the entire process has shown that on a laboratory scale, high-optical-quality glass can be produced from both alkoxide and particulate gels. It remains to be seen whether sol-gel process will be feasible for the manufacture of high-quality glass products on a commercial scale.     

Drying and Sintering of Bulk Silica Gels

Over the last few years, the feasibility of fabricating near net shape silica glass components, using a sub-critical drying process for pure alkoxide gels, has been demonstrated in our laboratories. Cracking during drying, due to capillary forces generated in the gel body, was overcome through two particular innovations. The first was the development and optimization of a dual-catalyzed high strength gel. The second was a controlled atmosphere drying process that allowed the gel to dry utilizing a newly observed phenomenology, postulated to be due to cavitation of the pore fluid. Contrary to conventional wisdom, in this drying approach the smallest pore size gels are the easiest to dry. Details of the types of gels and the drying process are reported. Gels of small size were sintered into crack-free glasses, utilizing conventional sintering approaches. However, large size gels always developed visible surface cracks that formed above 800°C. To successfully dry and sinter large monolithic gels then required re-optimization of the entire process. A great number of micron-range defects were initially detected in these sintered bodies. After analyzing the defects, further steps were taken to improve glass quality to the level of optical glass produced by vapor deposition processes. This included mixing and filtering of sols in a clean room, varying the pore size distribution, and optimizing the pre-sintering and sintering processes. Data of relevant glass quality parameters attained so far in the laboratory are reported.     

Study of the effect of formamide and N,N-dimethylformamide on the synthesis of CdS nanoparticles in a SiO2 matrix by sol-gel method

The influence of formamide (F) and N,N-dimethylformamide (DMF) in drying and firing of CdS nanocrystals in SiO₂ glasses by a sol-gel method has been studied. It has been established that pore size distribution and surface area are different with the use of DMF and that influences the quality of the final product. The presence of CdS semiconductor nanoparticles has been demonstrated by transmission electronic microscopy (TEM), and the quantum confinement effect due to the size of nanoparticles is denoted by a blue shift in the visible absorption spectra. From the results obtained in this paper, it is clear that DMF has an influence on final material microstructure.     

Sorption equilibria of CO2 on silica-gels in the presence of water

The sorption equilibria of carbon dioxide on three types of silica gel (SG) with different pore size distributions in the presence of water were studied experimentally using a volumetric method at 275?K with pressures from 0 to 3.7?MPa. Both the pore size distribution of the silica gel and the quantity of pre-sorbed water impact the formation of the CO₂ hydrates. For wet silicon gel A(SG-A) with water loading ratio of 0.75, the highest CO₂ sorption was about 2.5?mmol of CO₂ per gram of dry sorbent at 275?K. Similarly, the highest sorption was about 2.7?mmol for wet SG-B with R _w =0.81. However, CO₂ hydrate did not form on the wet surface of SG-C due to its large pore sizes.     

SiO2气凝胶的制备及微孔分布

本文以正硅酸乙酯(TEOS)为原料，水和乙醇为溶剂，盐酸和氨水为催化剂，采用溶胶-凝胶，水热合成和低温冷冻干燥等工艺，制备了SiO₂气凝胶，研究了pH值、TEOS浓度及焙烧温度对气凝胶的比表面积、孔径分布的影响。结果表明：在碱性条件下，制得的气凝胶比表面积较大，孔径较窄。气凝胶在较高温度下，比表面积有所下降，孔径略微变粗。