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.     

Polyvinyl alcohol hydrogels I. Microscopic structure by freeze-etching and critical point drying techniques

Froeze-etching (FE) and critical point drying (CPD) techniques were employed to prepare samples for investigating surface and bulk structures of polyvinyl alcohol (PVA) hydrogels by scanning electron microscopy. The hydrogels were obtained by freezing homogeneous solutions containing PVA polymer in either water or an aqueous solution of dimethyl sulfoxide (DMSO). An oriented porous structure was observed in the PVA hydrogel prepared without DMSO. The structure on the surface was found to be more porous than in the bulk for PVA hydrogels prepared from aqueous DMSO solutions. For given compositions of the hydrogels, samples prepared by FE technique showed a highly porous fibrillar structure on the surface, while those prepared by CPD technique showed a collapsed fibrillar structure with much less porosity. This marked difference indicates a collapse of the surface structure caused by the CPD technique. The CPD technique also led to significant reduction in porosity and loss of fibrillar structure in the bulk. Volume shrinkage of hydrogels caused by dehydration in ethanol may be responsible for the surface collapse as well as alteration of bulk structure. The FE technique reveals a more native structure of hydrogels than the commonly used CPD technique. However, it suffers from disadvantages such as charging and structural damage at high magnifications.     

Effect of drying on porous silicon

The phase transition of a fluid - in particular water - confined in the pores of silicon during drying is studied. The influence of this process on surface size and porosity is discussed. Methods of air drying, supercritical drying and freeze drying are considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal expansion studies on some gamma-irradiated binary borate glasses

The thermal expansion, density and molar volume of some binary borate glasses were measured before and after exposure to a gamma-ray dose of 10³ kGy. The expansion curves for all glasses, which were measured from room temperature to above the softening temperature, displayed similar characteristics. Increase of the lead oxide content decreased the thermal coefficient of expansion, but the effects of different alkali metal cations were shown to depend on their ionic radii. The various proposed mechanisms of thermal expansion are dealt with. The experimental results could be explained by considering the bond strengths, the polarizing powers of the different cations and the damage produced by radiation. The possible compaction of the structure due to irradiation is also discussed.     

生物质微波干燥及其对热解的影响

通过与常规热风干燥方式比较,研究生物质微波干燥过程及其对热解的影响,以探索在生物质快速热解液化工艺中采用微波干燥技术进行原料预处理的可行性。干燥实验表明,微波炉的干燥速率明显大于烘箱（5倍以上）,同时在微波快速干燥过程中,原料内部的孔隙结构得到了改善。热天平上干燥样品的热解表明,微波干燥处理有利于生物质的热解,特别是纤维素和半纤维素的热解,并且能在一定程度上抑制生物油蒸汽的二次裂解反应,从而使实际流化床热解液化装置中的生物油产率有所提高。研究表明,将微波干燥技术用于生物质热解液化的原料预处理过程在技术上和经济上均具有可行性。     

DSC studies on sodium phosphate glasses doped with chlorides of Cd,Co and Ag

A number of samples of sodium phosphate glasses doped with Cd/Co or Ag chlorides were prepared and characterized by X-ray diffraction, IR spectral, ion transport and DSC studies. It was found from DSC studies that the glass transition temperature (T _g) and crystallization temperature (T _c) values increased with the increasing concentrations of the dopants Cd/ or Co chlorides. However, the T _g and T _c values were found to decrease when the AgCl was taken as the dopant and the following sequence is observed: T _g(CoCl₂)>T _g(CdCl₂)> T _g(AgCl) T _c(CoCl₂)>T _c(CdCl₂)>T _c(AgCl) These results have been discussed and explained on the basis of changes in the structure of sodium phosphate glassy matrix by the addition of different cations as dopants.The authors are thankful to Prof. M. L. Srivastava, Head, Department of Chemistry, D. D. U. Gorakhpur University, Gorakhpur, and Prof. Suresh Chandra, Department of Physics, B. H. U. Varanasi for providing necessary laboratory facilities and help in carrying out the present work. The financial support from the Department of Science and Technology and University Grants Commission, New Delhi is also gratefully acknowledged.     

Effect of LaF<Subscript>3</Subscript> admixture on thermal stability of borosilicate glasses

Oxyfluoride glass-ceramics based on the aluminosilicate glass-matrix with the nano-phase of fluoride is an interesting material for optoelectronics. A new glass from the SiO₂–B₂O₃–Na₂O–LaF₃ system in which nanocrystallization of LaF₃ could be obtained as well is presented. Thermal stability of glass and the crystalline phases formed upon heat treatment were determined by DTA/DSC and XRD methods, respectively. The effect of the glass composition on thermal stability was investigated by the SEM method. It has been found that the addition of LaF₃ increases the tendency to decomposition of the borosilicate glass. In glasses with the ratio B₂O₃/(Na₂O+3La₂F₆)<1 it is possible to obtain the immersed crystallization of LaF₃ in transparent glassy matrix. The process is preceded by LaOF formation. Glasses with the composition B₂O₃/(Na₂O+3La₂F₆)≥1 revealed the tendency to La(BSiO₅) crystallization.     

柳树河油页岩微波干燥及其对热解特性的影响

在对家用微波炉改造基础上搭建了微波干燥实验台,研究了柳树河油页岩微波干燥特性及对热解特性的影响。结果表明,微波干燥所需的时间为传统干燥所需时间的20%;微波干燥速率要明显大于传统干燥速率;Page模型适用于描写柳树河油页岩微波干燥过程。微波干燥的油页岩同热风干燥后及原样油页岩的热解活化能随转化率的变化曲线基本一致,整体呈先上升后下降的趋势,在转化率为0.7时达到最大值;热解活化能在80~200 kJ/mol变动;微波干燥油页岩热解反应有机质分解段的活化能增加。     

Effects of spray drying on physicochemical properties of milk protein-stabilised emulsions

The effect of spray drying and reconstitution has been studied for oil-in-water emulsions (20.6% maltodextrin, 20% soybean oil, 2.4% protein, 0.13 M NaCl, pH 6.7) with varying ratios of sodium caseinate and whey protein, but with equal size distribution (d₃₂=0.77 μm). When the concentration of sodium caseinate in the emulsion was high enough to entirely cover the oil–water interface, the particle size distribution was hardly affected by spray drying and reconstitution. However, for emulsions of which the total protein consisted of more than 70% whey protein, spray drying resulted in a strong increase of the droplet size distribution. The adsorbed amount of protein ranged from 3 mg m⁻² for casein-stabilised emulsions to 4 mg m⁻² for whey protein-stabilised emulsions with a maximum of 4.2 mg m⁻² for emulsions containing 80% whey protein on total protein, which means that for all these emulsions about one quarter of the available protein was adsorbed at the oil–water interface. The adsorbed amount of protein was hardly affected by spray drying. After emulsion preparation casein proteins adsorbed preferentially at the oil–water interface. As a result of spray drying, the relative amount of β-lactoglobulin in the adsorbed layer increased strongly at the expense of _s1-casein and β-casein. Percentages of _s2-casein and κ-casein in the adsorbed layer remained largely unchanged. The changes in the protein composition of the adsorbed layer as a result of spray drying and reconstitution were the largest when beforehand hardly any whey protein was present in the adsorbed layer and hardly any sodium caseinate in the aqueous phase. Apparently, during spray drying conditions have been such that β-lactoglobulin could unfold, aggregate, and react with other cystein-containing proteins changing the particle size distribution of the emulsions and the composition of the adsorbed layer. It seemed, however, that non-adsorbed sodium caseinate in some way was able to protect the adsorbed casein proteins from being displaced by aggregating whey protein.     

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.     

Anarchy in the solid state: structural dependence on glass-forming ability in triazine-based molecular glasses

We have recently shown that molecular glasses, small molecules capable of readily forming glassy solids as opposed to crystals, can be designed by exploiting molecular association through strong and directional intermolecular interactions, as exemplified by several members of the bis(mexylamino)triazine family. Herein, 43 new bis(mexylamino)triazine derivatives were synthesized, 31 of which have been found to spontaneously form glassy phases and did not crystallize upon heating.     

Development of Na+-sensitive membranes based on sputtered Na-Al-Si glasses

Na⁺-sensitive microdevices are of increasing interest for integration in microanalytical systems e.g. for biomedical applications or for industrial process control. In order to produce ultra thin Na⁺-sensitive layers with fixed and reproducible composition and, in particular, defined Na concentration by means of RF sputtering, an off-axis geometry of a magnetron with cylindrical target was chosen for minimizing back-sputtering effects from the already deposited material. With this inverted cylindrical magnetron (ICM) it was possible to obtain reproducible and controllable sodium aluminosilicate glass layers on semiconductor substrates. Several surface and thin layer analytical techniques were applied for characterization of the membranes and for stoichiometry control. Especially by the non-destructive nuclear reaction analysis method a constant Na profile throughout the glass layer and — together with AES depth profiles — the diffusion barrier effect of an Si₃N₄ interface layer could be verified. Electrochemical measurements proved Nernstian sensitivity down to 10^–4 M Na⁺ in solutions of pH 7, supporting sufficient stability and reproducibility of the sputtered Na⁺-sensitive layers.     

Effect of air drying on speciation of heavy metals in flooded rice paddies

Flooded soil samples were collected in the typical area of the Yangtze Delta Region;fractions of heavy metals in flooded and air dried samples were measured with BCR sequential extraction method and atomic absorption spectrometry.In flooded soils, fractions of heavy metals increased in the order of acid soluble < oxidizable < reducible < residual.The acid soluble and reducible fractions significantly decreased but residual fraction significantly increased when the samples were air dried.The data obtained from air dried soil samples could not accurately represent the speciation of heavy metals in flooded field conditions.     

用电子鼻研究不同烘丝过程对卷烟挥发性成分的影响

采用电子鼻对卷烟烟丝挥发性组分进行分析,分别考察了前处理制样与电子鼻顶空平衡时间对结果的影响。确定采用三包卷烟的烟丝作为大样本,混合均匀后称取适量样品直接电子鼻分析,电子鼻顶空平衡温度为55℃。应用电子鼻在该参数下对不同烘丝方式和温度参数处理的卷烟烟丝挥发性成分进行气味指纹图谱和PCA分析,同连续流动法测定的常规化学成分结果作了比较,结果表明电子鼻能够反映规常规化学成分检测手段不能区分的不同烘丝过程带来的挥发性组分改变。     

The effect of heat-treatment on the physico-chemical properties of silica aerogel prepared by sub-critical drying technique

Synthesis of transparent and crack-free monoliths of silica aerogel by sub-critical drying technique is reported in the present article. Silane ageing with 50% tetraethylorthosilicate:ethanol followed by solvent exchange using ethanol was adopted. The effect of heat-treatment on the textural and physical characteristics of silica aerogel was evaluated. The chosen composition resulted in a high surface area silica aerogel of 1,000 m² g⁻¹ and a pore volume of 1.4 cm³ g⁻¹ at room temperature. The aerogel heat-treated at 900 °C possessed a surface area of 450 m² g⁻¹ with a pore volume of 0.4 cm³ g⁻¹. The decrease in surface area and pore volume was associated with the sintering process. The present technique seems advantageous in preserving the high surface area of the material at high temperatures. The XRD studies showed that the amorphous nature of aerogel matrix was retained till 1,400 °C, beyond which it crystallized to phase pure crystoballite.     

Effect of drying process on structure and property of polyaspartic acid resin

The effect of drying temperature on the physical and chemical structure and water absorbent behavior of polyaspartic acid resin were investigated via methods of infrared spectrum analysis and the electron microscope. The results show that: the heat treatment could result in a damage of the cross-linked bonds and a decrease of the cross-linked degree to the cross-linked polyaspartic acid resin. When the drying temperature exceeded 180°C, both backbone basic skeleton and side chain portions were also damaged heavily and gave off NH₃ in addition to the damage of the cross-linking portions. A relatively fast dehydrating rate at 40°C was in favor of the product with loose, uniform and nondestructive porous network structure to be obtained, which has big porosity and pore size. This kind of product has excellent water-absorbency and gel strength. The gel strength of PASP resin decreased with the increase of the drying temperature. The drying temperature has a greater effect on the swelling ratio and gel strength of the polyaspartic acid resin products with lower cross-linking degree than on that of the polyaspartic acid resin products with higher cross-linking degree.     

造纸污泥薄层干燥实验及动力学模型分析

通过造纸污泥薄层干燥实验,考察了温度对造纸污泥干燥特性的影响,引入薄层干燥模型对污泥干燥动力过程进行模拟。结果表明,泥层越薄和干燥温度越高,污泥干燥的速率就越快,当干燥温度从80℃上升到160℃时,相对应的最大干燥速率从0.008g/(g·min)上升到0.030g/(g·min);Modified page模型描述了薄层造纸污泥在烘箱中的干燥过程,应用Fick扩散模型,薄层造纸污泥在干燥温度为80℃～160℃时,有效扩散系数为2.2×10^-10m²/s~3.96×10^-10m²/s。通过Arrhenius方程建立温度和有效扩散系数之间的关系,得出水分扩散的活化能为9.435kJ/mol。     

Thermal study of the influence of chemical bond ionicity on the glass transformation in (Na2O, CaO, MgO)-Al2O3-SiO2 glasses

The relation between change of the specific heat (ΔC _p) accompanying the glass transformation and the chemical composition of glasses (Na₂O, CaO, MgO)-Al₂O₃-SiO₂ system has been studied. The exchange of modifiers in the glass structure causes the ΔC _p increase in the sequence Na>Ca>Mg. Change the glass network composition by introducing Al into it makes smaller increase of the ΔC _p values. It has been shown that degree of ΔC _p value changes is dependent on the iconicity/covalence of chemical bonds of cations with oxygen of glass structure network. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of anisotropic deformation on the differential scanning calorimetry response of polymer glasses

Large anisotropic deformation affects the physical state of a polymer glass, where the changes in the state of material are revealed by performing a differential scanning calorimetry (DSC) experiment. Previously, the deformation was applied to polymers well below their glass transition temperatures, and it was found that uniaxial compressive loading–unloading resulted in a broad exothermic peak on the DSC trace. Here we report on the effect on the subsequent DSC response of a deformation experiment performed in uniaxial extension on a ductile 50:50 co-polymer poly(BMA-co-MMA) (PBMA/MMA). The deformation of up to 80% strain was applied at T_g − 30°C and T_g − 40°C, that is, closer to T_g than in the previous work. Unlike in the well below T_g deformation case, the DSC trace contains an endothermic peak followed by an exothermic peak. The magnitude of the endothermic peak as well as the asymptotic glassy heat capacity increase with the amount of mechanical work performed during the deformation cycle.     

Laser-ablation ICP-MS analysis of siliceous rock glasses fused on an iridium strip heater using MgO dilution

Trace element determination in rocks by fusion on an iridium strip heater followed by LA-ICP-MS analysis of the glass beads is extended here to SiO₂-rich rocks; rapid fusion of samples with >55 wt% SiO₂ is facilitated by dilution by high purity MgO. The method developed here can rapidly and accurately determine numerous trace elements in a large range of rock compositions in a short time (about 50 samples/day). Systematic evaluation for a large range of rock compositions (natural rocks and reference materials AGV-2, GSP-2, JG-1a) with SiO₂ contents between 45 and 80 wt% shows that reproducibility and accuracy within 10% can be routinely achieved for most of the 28 trace elements investigated (Rb, Sr, Cs, Ba, Ti, Zr, Hf, Nb, Ta, Sc, V, Cr, Ni, Pb, Th, U, REE). The 40 mg sample size is smaller than for XRF, INAA or solution-ICP-MS, detection limits are lower, and trace element palettes more complete than XRF and INAA. This microchemical method is thus attractive for the analysis of all natural geological materials as well as for experimental applications with small samples. Samples with SiO₂-contents >55 wt% require hot and long melting to achieve homogeneous glasses and eliminate all residual minerals, particularly refractory accessory phases. Melting conditions of 1600 °C and 20 s for samples are recommended for SiO₂ contents between 55 and 70 wt%, whereas 1800 °C and 20–30 s are often required for samples with >70 wt% SiO₂. Problems are encountered for Pb and Cs due to volatilization on the Ir strip, for Sc due to interferences, and Zr and Hf due to their sequestration in refractory accessory minerals. Correspondence: Franziska Nehring, Department of Geosciences, University of Mainz, Becherweg 21, 55099 Mainz, Germany