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.     

High Purity Glass Forms by a Colloidal Sol-Gel Process

Novara Technology (NTECH) has developed its own process for bulk silica glass based on Sol-Gel chemistry. The process is characterized by its thermomechanical ideal isotropy of the aerogel and the glass it forms. Remarkable glass properties are: ultrapurity up to 1 ppb or better for single metal contaminant, optical compatibility with high-temperature treatments up to 2000°C, precision mouldability without intrinsic dimensional limitations.The process, that makes large use of the fumed silica Aerosil® in combination with TEOS, is characterized by the synthesis of monolithic material, i.e. a chemically manufactured body with shape and dimensions defined at reaction time, when the whole material is generated as a unit. Process implementation was achieved with the operation of a pilot plant. The road to industrialization besides volume scale-up, readily achieved for the chemical and hypercritical drying sector of the plant, was focused on getting a cost effective procedure.     

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.     

Nuclear Waste Storage in Gel-Derived Materials

For long life nuclear wastes (essentially actinides) research is in progress to propose new matrices with an improvement of the chemical durability. High silica content glasses present high chemical durability, and we describe a process to prepare silica glass embedding the nuclear waste. Porous silica (gel) is used as a host matrix for nuclear waste. Neodymium oxide and cerium oxide are used to simulate the actinide oxides. The gel is soaked in a solution containing the simulant in nitrate salt form.We investigate the effect of the porous network features (pore size distribution) on the ability of the material to soak up the simulating salts. A new kind of gel (composite aerogel) is proposed owing to its large permeability. After drying and nitrate decomposition, the composite material is fully sintered trapping the nuclear waste.The glass-ceramic materials (silica glass + simulating oxide) have been synthesized with simulant content close to 20 weight percent. We show that the 2 simulant oxides behave differently because of their ability to form silicate phases. The chemical durability of the glass-ceramics is investigated. The large improvement of the chemical durability (60 times) compared to the classical borosilicate glasses shows that this containment process can be a suitable way to confine actinides and long life nuclear wastes.     

Convectional, sedimentation, and drying dissipative structures of black tea in the presence and absence of cream

Convectional, sedimentation, and drying dissipative structures of black tea with and without cream were studied in a tea cup, a cover glass, a watch glass and a glass dish on macroscopic and microscopic scales. The convectional patterns were vigorous and irregular at the initial stage but soon highly distorted Bernard cells grew. The global integrated flows of the tea particles coated with cream at the air–suspension interface were observed vaguely from the central area toward outside edge at the initial stage in a tea cup and a large watch glass, but the flow direction turned oppositely from the outside to the central area. At the similar time, the short and few spoke lines appeared at the outside edge and grew long toward the central area. Then, the cooperative formation of clusters and bundles of the spoke lines took place at the middle and final convectional stages, and then the dynamic sedimentation patterns appeared. The drying patterns of tea with and without cream were composed of the broad ring at the outside edge and a round hill accompanied sometimes with the bundles of spoke lines. These features are consistent with those of suspensions of non-spherical particles. The pinning effect is not always supported by this work, but importance of the gravitational and Marangoni convectional flows is proposed instead. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

基板界面对PS/PMMA共混物薄膜相逆转组成比的影响

近年来高分子共混体系中的界面、表面效应逐渐引起了越来越多研究者的兴趣 .人们发现 ,当共混物薄膜厚度减至一定程度时 ,聚合物共混物薄膜中的相形态、相容性及相分离动力学与本体中有较大的不同[1～ 3] .基板界面作用对共混薄膜体系的热力学、动力学行为产生很大的影响 .我们以往的研究 [4 ,5]也发现 ,PP/EVAc(70 /30 )共混体系退火过程中 ,基板界面 (如玻璃 )作用可大大加速分散相(EVAc)粒子的粗化凝聚过程 .本研究用聚甲基丙烯酸甲酯和聚苯乙烯共混物的四氢呋喃溶液在不同基板介质 (如玻璃基板 ,PP基板 )上成膜 ,用相差显微镜观测了…     

The effect of a magnetic tape coating process on the structure and properties of a semi-crystalline polymer

A combination of structure identifying and bulk property experiments were combined with a two-phase analytical methodology to elucidate the influence of a magnetic tape coating process on the substrate polymer film. Employing a typical tape manufacturing process that utilizes coating, drying, and calendering stages, polyethylenete-rephthalate polymer film samples with and without the magnetic coating were prepared.Experiments and modeling studies performed on these samples demonstrate that the drying and calendering processes may increase the crystallinity and noncrystalline orientation of the substrate film. In addition, dynamic mechanical experiments identified a viscoelastic transition at 50 °C for the PET substrate film sample which is near the glass transition of the magnetic coating utilized. Overall, the results of this investigation provide a basis for evaluating structure property interrelations of polymer-based magnetic tapes.     

CaCO_3颗粒模板法制备聚合物超亲水/超疏水表面

以碳酸钙(CaCO3)颗粒层为模板,运用简单的热压和酸蚀刻相结合的方法制备聚合物超亲水/超疏水表面.首先在玻璃基底上均匀铺撒一层CaCO3颗粒,以此作为模板,通过热压线性低密度聚乙烯(LLDPE)使CaCO3颗粒均匀镶嵌在聚合物表面,获得了超亲水性质;进一步经酸蚀得到了具有微米和亚微米多孔结构的表面,其水滴静态接触角(WCA)可达(152.7±0.8)°,滚动角小于3°,具备超疏水性质.表面浸润性能和耐水压冲击性能研究表明该超疏水表面具有良好的稳定性和持久性.用同样工艺微模塑/酸蚀刻其它疏水性聚合物,得到类似结果.     

Preparation of P(AN–MMA) microporous membrane for Li-ion batteries by phase inversion

A novel process was proposed for preparation of microporous poly(acrylonitrile–methyl methacrylate) (P(AN–MMA)) membranes by phase inversion techniques using ultrasonic humidifier. Being prepared by dissolving the polymer (PAN–MMA) in the N,N-dimethylformamide (DMF) solution with mechanical stirring, the homogenous casting solution was cast onto a clean glass plate. Successively, the glass plate was exposed to the water vapor produced by ultrasonic humidifier, inducing the phase inversion. It is found the pore size is much more uniform across the cross-section of the membrane than that of the porous membrane prepared by conventional water bath coagulation technique. The microporous membranes were directly obtained after the washing and drying. It had about 1–5 μm of pores and presented an ionic conductivity of 2.52 × 10⁻³ S/cm at room temperature when gelled with 1 M LiPF₆/EC-DMC (1:1 vol.%) electrolyte solution. The test cells with the gel electrolytes prepared from as-prepared microporous membranes showed stable cycling capacities, indicating that the microporous membrane, which was prepared from cheap starting materials acrylonitrile and methyl methacrylate, can be used for the gel electrolyte of lithium batteries.     

Wettability of proteins adsorbed on silica glasses treated with fluorosurfactants

Generally, the apolar/polar surface is probed by water-wetting, which is measured using a method such as the sessile liquid drop method. However, when one tries to measure the wetting of a surface where biological macromolecules are adsorbed, there is the problem of a change in conformation due to drying the surface; hence, using this method in situ information cannot be obtained. We have developed a new method that can be used to measure the wettability of the adsorbed protein surface without drying. This method, the dropping time method, which is based on measuring the dropping time of a film of liquid along a protein-covered surface when this surface is instantaneously vertically removed from the protein solution. The adsorption behavior of four proteins (albumin, lysozyme, β-lactoglobulin, ovalbumin) on the surface of silica glass that has been treated with various fluorosurfactants is studied using this method. At a high concentration of protein, the surfaces of adsorbed proteins of any kind are fairly hydrophilic on glass treated with all fluorosurfactants. At a lower concentration of protein, the hydrophilicity of the protein layer depends on the kind of fluorosurfactant and also on the protein adsorption process. The apolar glass surface becomes more hydrophilic with increasing dipping time in the protein solution. On the other hand, the hydrophilic glass surface shows a complex change in the hydrophilicity with elapsed time after dipping it into a solution of albumin or lysozyme, i.e., the hydrophilicity decreases in the early stage of the adsorption and then increases with proceeding adsorption. Received: 19 March 1999 Accepted in revised form: 10 June 1999     

Sedimentation and drying dissipative patterns of ternary mixtures of colloidal silica spheres having different sizes

The sedimentation and drying dissipative structural patterns were formed during the course of drying ternary mixtures of colloidal silica spheres of 183 nm, 305 nm, and 1.205 μm in diameter in aqueous suspension on a watch glass, a glass dish, and a cover glass. The patterns were observed by closed-up pictures, metallurgical optical microscopy, 3D profile microscopy, reflection spectroscopy and AFM images. The concentrations of the three spheres ranged from 0.0023 to 0.0128 keeping the same concentrations for each spheres. Broad ring-like sedimentation patterns were formed within a short time in suspension state especially in a glass dish. In a watch glass, colorful three layered ring-like drying patterns were observed and composed of the outer, middle and inner layers of small, medium, and large spheres, respectively. The three colored segregated layers were formed by the balancing between the outward convectional flow and the inward sedimentation of spheres. In a glass dish, wave-like macroscopic drying patterns were observed in the intermediate areas between the outside edge of the broad ring at the central area and the inner wall of the cell especially at low sphere concentrations. The size of the broad ring at the central area increased as sphere concentration increased. On a cover glass, size segregation also took place, i.e., small, medium, and large spheres located at the outer, medium, and central areas, though these segregations were not so complete compared with those on a watch glass.     

Drying dissipative structures of thermo-sensitive gel spheres of poly(N-isopropylacrylamide). Influence of degree of cross-linking of the gels

Drying dissipative patterns of de-ionized suspensions (colloidal crystal state above the critical concentrations of crystallization) of the thermo-sensitive gels of poly(N-isopropylacrylamide) with degrees of cross-linking of 10% and 2% (pNIPAm(200–10) and pNIPAm(200–2)) were observed at 20?°C and 45?°C on a cover glass, a watch glass, and a Petri glass dish. The broad rings were observed, and their size decreased as micro-gel concentration decreased. Formation of the monodispersed agglomerated particles and their ordered arrays were observed. Microscopic drying structures of (a) flickering ordered spoke-lines, (b) ordered rings, (c) net structure, and (d) lattice-like ordered structures of the agglomerated particles are observed. The net and lattice structures formed more favorably at high degrees of cross-linking, at high concentrations of the gels, and/or high temperatures. By the addition of sodium chloride, very large dendrite-like and net structures of the large agglomerated particles formed at 20?°C and 45?°C, respectively. Importance of the cooperated convectional flow of the agglomerated particles during the drying processes is supported for the ordered array formation. The role of the electrical double layers around the agglomerated particles and the interaction of the particles with the substrate surfaces during dryness are also important for the ordering. The microscopic drying patterns of gel spheres were different from those of linear type polymers and also from typical colloidal spheres, though the macroscopic patterns of gel system such as broad ring formation at the edges of the dried film were similar to other two systems.     

Sedimentation and drying dissipative patterns of binary suspensions of colloidal silica spheres having different sizes

The sedimentation and drying dissipative structural patterns were formed during the course of drying binary mixtures among colloidal silica spheres of 183 nm, 305 nm, and 1.205 μm in diameter in aqueous suspension on a watch glass, a glass dish, and a cover glass, respectively. The broad ring-like sedimentation patterns were formed within several hours in suspension state for all the substrates used. Colorful macroscopic broad ring-like drying patterns were formed for the three substrates. In a watch glass, macroscopic drying patterns were composed of the outer and inner layers of small and large spheres, respectively. The two colored layers were ascribed to the Bragg diffractions of light by the dried colloidal crystals of the corresponding spheres. The width ratio of the layers changed in proportion to the mixing ratio of each spheres. In a glass dish, wave-like macroscopic drying patterns were observed in the intermediate areas between the outside edges of the broad ring and the inner wall of the cell. On a cover glass, the sphere mixing ratios were analyzed from the widths of the drying broad rings of the small spheres at the outside edge. High and distinct broad rings of small spheres and the low and vague broad one formed at the outer edges and in the inner area, respectively. Drying dissipative pattern was clarified to be one of the novel analysis techniques of colloidal size in binary colloidal mixtures.     

Drying dissipative structures of thermosensitive gel spheres of poly (N-isopropylacrylamide). Influence of gel size

Drying dissipative patterns of de-ionized suspensions (colloidal crystal-state at high concentrations) of the thermosensitive gels of poly (N-isopropylacrylamide) with various sizes (ca. 400–1,500?nm in diameter at 20?°C) were observed at 20 and 45?°C on a cover glass, a watch glass, and a Petri glass dish. The broad rings were observed and their size decreased as gel concentration decreased. Formation of the monodispersed agglomerated particles and their ordered arrays were observed irrespective of gel size. The macroscopic flickering spoke-like patterns were observed for the gel spheres from 70 to 600?nm in diameter at 20?°C, but almost disappeared for extremely large spheres, poly(N-isopropylacrylamide)(1500-5). This work clarified the formation of the drying microscopic structures of (a) ordered rings, (b) flickering ordered spoke lines, (c) net structure, and (d) lattice-like ordered structures of the agglomerated particles. The ordered rings became rather vague as gel size increased. The large net structures formed so often for large gels. Size effect on the lattice patterns was not recognized so clearly. The role of the electrical double layers around the agglomerated particles and the interaction of the particles with the substrate surfaces during dryness are important for the ordering. The microscopic drying patterns of gel spheres were quite different from those of linear type polymers and also from typical colloidal hard spheres, though the macroscopic patterns such as broad ring formation at the edges of the dried film were similar to each other.     

Thermal degradation of both latex and latex cast films forming membranes

Latex collected from natural rubber trees forming membranes can be used as biomaterials in several fields being the temperature a key parameter. Thermogravimetry (TG) coupled to Fourier transform infrared spectroscopy (FTIR) is a useful technique to investigate the thermal degradation of both latex and cast films (membranes), wich were obtained from Hevea brasiliensis (RRIM 600 clone) and used without stabilization. The membranes were prepared by casting the latex onto a glass substrate at 65°C for 6 h. The thermal degradation was followed by FTIR spectra acquisition along the process, allowing the identification of the gaseous components evolved upon the thermal treatment. According to TG measurements, the main processes of thermal degradation of the latex and membranes occur at three temperature intervals for both.     

A cold silanization method for preparation of medium polarity capillary columns

Silane-coupling agents, commonly used for fiberglass reinforced plastics (FRP), were applied for deactivation of silanol sites in glass capillary columns prior to coating with mediumpolarity phases such as Carbowax and Superox. The columns, prepared in a two-stage process in the case of glass (acidic leaching and drying, dynamic cold silanization followed by static coating with the phase) or a one stage process in the case of fused silica, gave the best results when deactivation and hence wettability were induced by glycidoxypropyltrimethoxy silane.     

Factors influencing the swelling and elution properties of poly(vinyl alcohol) cast gels

Swelling and elution properties of physically crosslinked poly(vinyl alcohol) (PVA) cast gels depend on the network structure of the PVA and crosslink, which is characterized by the size, number, and distribution of microcrystallites. Therefore, the swelling and elution ratios can be manipulated by adept control of the conditions adopted for the preparation of gels. Among the various factors that influence the formation of microcrystallites, the temperature and relative humidity at gelation play an important role. In addition, the size of gel is also a key factor that determines the network structure of gels. To this end, this study quantitatively evaluates the macroscopic properties of swelling and elution, and the microscopic properties of the network structures in disk‐shaped PVA cast gels of the same diameter prepared by casting different weights of PVA solution in the same dish. Although the drying speed can be controlled by adjusting the three processing parameters, namely, drying temperature, humidity, and cast weight, the changes in swelling and elution ratios, microcrystallite size, and crystallinity independently depended on each parameter. Regardless of the three factors, the swelling ratio was found to correlate strongly with the elution ratio. Optimum factors to minimize the elution ratio are discussed on the basis of the change in the network structures obtained by varying the preparatory conditions. Based on the results of the systematic analyses, this study proposes a method to control the elution ratio while retaining high water‐absorbance ability. Copyright © 2015 John Wiley & Sons, Ltd.     

Cracking in drying latex films

Thin films of latex dispersions containing particles of high glass transition temperature generally crack while drying under ambient conditions. Experiments with particles of varying radii focused on conditions for which capillary stresses normal to the film deform the particles elastically and generate tensile stresses in the plane of the film. Irrespective of the particle size, the drying film contained, simultaneously, domains consisting of a fluid dispersion, a fully dried packing of deformed spheres, and a close packed array saturated with water. Interestingly, films cast from dispersions containing 95-nm sized particles developed tensile stresses and ultimately became transparent even in the absence of water, indicating that van der Waals forces can deform the particles. Employing the stress-strain relation for a drying latex film along with the well-known Griffith's energy balance concept, we calculate the critical stress at cracking and the accompanying crack spacing, in general agreement with the observed values.     

Drying dissipative structure of similar sized aggregates (1.5 μm in diameter) of nano-sized diamond particles (4 nm in diameter)

Macroscopic and microscopic drying patterns were observed on a cover glass, a watch glass, and a Petri glass dish during the course of dryness of aqueous suspensions of similar sized aggregates of diamonds (CD1), which formed from the deionization of the pre-particles of diamond 4 nm in diameter. Two kinds of macroscopic patterns, i.e., outer and inner broad rings, and spoke lines were formed. Cooperative drying processes of the convection, sedimentation, and solidification were clarified. Microscopic drying patterns showing the formation of very large dendritic aggregates from the CD1 particles were observed only when the excess amount of sodium chloride higher than 2 mM coexisted in the initial suspensions before dryness and further initial CD1 concentration is lower than 0.17 wt%.     

Drying dissipative structures of cationic gel spheres of lightly cross-linked poly(2-vinyl pyridine) (170 ∼ 180 nm in diameter) in the deionized aqueous suspension

Drying dissipative patterns of cationic gel crystals of lightly cross-linked poly(2-vinyl pyridine) spheres (AIBA-P2VP, 170?～?180 nm in diameter) were observed on a cover glass, a watch glass, and a Petri glass dish. Convectional patterns were recognized with the naked eyes. Two kinds of the broad rings were observed at the outside edge and inner region in the macroscopic drying pattern, and their size at the inner regions first decreased and then turned to increase as gel concentration decreased. Formation of the similar-sized aggregates, i.e., hierarchical aggregation and their ordered arrays were observed. This work supported strongly the formation of the microscopic drying structures of (a) ordered rings, (b) flickering ordered spoke-lines, (c) net structure, and (d) lattice-like ordered structures of the aggregated particles. The ordering of the similar-sized aggregates of the cationic gel spheres (AIBA-P2VP) in this work is similar to that of the large cationic gel spheres of poly(2-vinyl pyridine) (385?～?400 nm in diameter) and further to that of the anionic thermosensitive gel spheres of poly(N-isopropyl acrylamide). Role of the electrical double layers around the aggregates and their interaction with the substrates during dryness are important for the ordering. The microscopic drying patterns of gel spheres were different from those of linear-type polymers and also from typical colloidal hard spheres, though the macroscopic patterns such as broad ring formation were similar to each other.