介孔材料低温氮气吸脱附的孔道网络模型

介孔结构的表征和分析对于介孔材料的开发至关重要,其中低温氮气吸脱附法是最常用的介孔表征方法之一。然而,目前氮气吸脱附法采用的分析模型仍基于平行孔假设,无法描述脱附过程堵孔现象,以及获取孔道连通性等重要孔结构信息。本文建立了低温氮气吸脱附的孔道网络模型,用于分析介孔结构对氮气等温吸脱附行为的影响。通过对比氧化铝材料的氮气吸脱附实验数据和模拟结果,证实了建立的孔道网络模型能很好地描述介孔材料中低温氮气吸脱附行为。模拟结果表明平均孔径较小时,毛细凝聚分压低,液氮堵孔效应显著,氮气吸脱附曲线回滞环的范围和面积较大;孔径分布较宽时,小孔和大孔数量均较多,毛细凝聚和堵孔效应显著,回滞环面积较大;孔道连通性不会影响吸附过程,但会通过改变堵孔效应显著影响脱附过程,连通性越差,堵孔效应越强。证实了堵孔效应对氮气脱附过程影响显著,因而氮气吸脱附法需要考虑堵孔效应,建立的孔道网络模型也可为介孔结构分析提供合理的模型工具。     

Pore structure and graphitic surface nature of ordered mesoporous carbons probed by low-pressure nitrogen adsorption

Ordered mesoporous carbons (OMCs) were produced by pyrolysis of sucrose adsorbed in two different silica matrices (MCM-48 and SBA-15), followed by dissolution of the matrix in hydrofluoric acid. Subsequently, some of these OMCs were heat-treated at temperatures of up to 1600 °C. The OMC pore structure was studied by low-pressure nitrogen adsorption. Information on the graphitic order of the surface of the mesopore walls was also obtained from the nitrogen adsorption data. These results were correlated to the order of the graphene layers at the outer surface, which was studied by X-ray photoelectron spectroscopy (XPS).

The OMCs were predominantly mesoporous, but they also contained micropores. For OMCs produced in an SBA-15 matrix, the micropore volume decreased upon heating. After heating to 1600 °C, nearly all micropores had disappeared. Furthermore, upon heating the width of the mesopores increased from 35 to 50 Å. All these changes can be explained by a shrinking of the OMC framework upon heating. A different behavior was found for OMCs derived from MCM-48. Upon heating these materials at increasingly high temperatures, the width of the mesopores first decreased, and for temperatures above 1100 °C it increased again. For all OMCs studied the graphitic order of the mesopores and the order of the graphene layers at the outer surface increased upon heating. For a given temperature, the graphitic surface order of OMCs derived from SBA-15 and MCM-48 was similar.     

Determination of total nitrogen content by different approaches in petroleum matrices

Two analytical methods are developed and validated for determination of low total nitrogen levels (below 0.0150%) in different petroleum matrices. The experimental and the instrumental conditions are optimized during the study. The oxidative combustion method with chemiluminescence’s detection is automatic with less handling while the Kjeldahl method involves more manual handling procedures and hence, has more potential for problems rise from contamination. The oxidative combustion with chemiluminescence’s detection method is more sensitive, faster (i.e. the determination requires few minutes compared to several hours with Kjeldahl method) and lees hazardous (i.e. no handling of dangerous boiling sulfuric acid). The estimated analytical characteristics suggest that two methods are reliable: the mean values of recovery for the Kjeldahl method and the other method are respectively 91.5% and 98.6%. The relative uncertainties are less than 2.5% and 2.8% respectively for the developed methods. The detection limits, based on the triple standard deviation, are 5 and 0.03 mgN kg⁻¹ respectively.     

Phase transformation in mesoporous silica films induced by the degradation of organic moiety

We demonstrate a phase transformation approach for selective control of mesophase in a silica film. Here, the phase transformation is induced by the degradation of organic moiety during calcination of the film. The films are prepared using Brij-30 [C₁₂ H₂₅ (OCH₂CH₂)₄OH] as a structure directing agent and TEOS (tetraethylorthosilicate) as a source of silica under acidic conditions. The degree of silica condensation of the SiO₂/Brij-30 mesophase is found to be a crucial factor enabling the phase transformation.     

Vapour induced ordering phenomena in syndiotactic polystyrene amorphous films

Summary Films of amorphous syndiotactic polystyrene were exposed to dichloromethane vapour at varying activity, up to a=1. Infrared analysis and wide angle X-ray scattering were used to follow the development of ordered structures, induced by the vapour. It has been found that characteristic bands in the infrared spectrum appear after activity a=0.45, whereas X-rays show that the sample is still amorphous. The appearance of the crystalline reflections occurs at higher activities and a continuous improvement up to a=1 in the X-ray pattern is observed.     

Phase transformation in mesoporous silica films induced by the degradation of organic moiety

We demonstrate a phase transformation approach for selective control of mesophase in a silica film. Here, the phase transformation is induced by the degradation of organic moiety during calcination of the film. The films are prepared using Brij-30 [C₁₂ H₂₅ (OCH₂CH₂)₄OH] as a structure directing agent and TEOS (tetraethylorthosilicate) as a source of silica under acidic conditions. The degree of silica condensation of the SiO₂/Brij-30 mesophase is found to be a crucial factor enabling the phase transformation.     

Vacancy ordering in zirconium carbide with different carbon contents

Zirconium carbide (ZrC_x) ceramics with different carbon contents were prepared by reactive hot-pressing. The rock-salt structure of ZrC_x was the only phase detected by x-ray diffraction of the hot pressed ceramics. The relative densities of ZrC_x decreased as carbon content increased, in general. The actual carbon contents were measured by completely oxidizing the ZrC_x ceramics to ZrO₂. For most compositions, the actual carbon contents were higher than nominal batched compositions, presumably due to carbon uptake from the graphite furnace and hot press dies. Selected area electron diffraction and neutron powder diffraction revealed the presence of carbon vacancy ordering in ZrC_x for 0.6 < x < 0.75. Rietveld refinement of the neutron diffraction patterns determined that the crystal structure of the ordered phase was hexagonal, and the carbon site occupancies were higher than nominal batched carbon stoichiometry.     

Protection by different agents against inactivation of lipoxygenase by hydrogen peroxide

H₂O₂ is a potent inactivator of lipoxygenase. In his paper, the ability of different agents [mannitol, oleic, stearic and linoleic acid,n-butanol, and hydroperoxy octadecadienoic acid (HPOD)] to prevent the inactivation of tomato lipoxygenase by hydrogen peroxide has been studied. The involvement of OH^· in the inactivation process is suggested by the ability of mannitol to prevent the loss of activity. This radical would be produced by reaction of H₂O₂ with the Fe(II) lipoxygenase. The most effective protection was displayed by HPOD, the product of the reaction of lipoxygenase with linoleic acid. This result could be explained by the conversion of the native enzyme into the Fe(III) lipoxygenase in the presence of HPOD; the Fe(III) enzyme is not able to react with H₂O₂, and no OH^· will be produced. The protective effect obtained with oleic and stearic acid could be explained by an occupation of the active center by these inhibitors. The enzyme would not transform them, but their presence would hamper the conversion of H₂O₂ in OH^· and limit the damage in the active center.     

Solid-state phase transition induced by ordering of flexible chain alkylamine in N-ethyl-n-butylammonium 3, 5-dinitrobenzoate monohydrate

A new molecule-based compound, N-ethyl-n-butylammonium 3, 5-dinitrobenzoate monohydrate, which undergoes a reversible solid-state phase transition at 197 K (T_c), has been successfully constructed based on long-chain alkylamine. Specific heat capacity and differential scanning calorimetry (DSC) measurements confirm that it is first-order phase transition with a 5 K thermal hysteresis between heating and cooling scans. Dielectric constants exhibit distinct anomalies around T_c accompanied with step-like responses between low and high dielectric states, which further confirm the phase transition. Variable-temperature single-crystal X-ray diffraction analyses illustrate that the phase transition was elucidated from ordering of the flexible N-ethyl-n-butylammonium cations coupled with reorientational motion of the alkylamine chain.     

Stretch‐induced molecular ordering in amorphous/crystalline polylactide blends

The effect of amorphous poly(D ,L ‐lactide) (PDLLA) on the molecular orientation and crystallization of crystalline poly(L ‐lactide) (PLLA) on stretching is reported in this study. It is indicated that the presence of PDLLA in its miscible blends with PLLA is not favorable for either cohesive mesophase formation below T_g or strain‐induced crystallization above T_g at strains beyond the segmental extensibility limit. Because of lack of constraints imposed by cohesive mesophase or crystals, oriented segments are liable to slide each other or recoil, responsible for low‐molecular orientation in the stretched blends. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Pore formation in p-type silicon in solutions containing different types of alcohol

ABSTRACT: Macroporous structure of silicon can be obtained with anodization in HF solution. The macropore formation in the presence of alcohol was studied. Macroporous layer formation in a low-concentration HF solution is stabilized with increasing the number of carbon in alcohol. Dissolution at the topmost part of the porous layer is observed though the behavior depends upon the type of alcohol. Meanwhile the total mass loss of dissolved silicon is almost constant. Such dissolution at the top surface occurs only when the concentration of HF is low. Adding organic solvents to the HF solution also leads to the suppression of the pore wall dissolution. The type of alcohol and HF concentration in solution affect the formation of porous silicon.     

Effects of neutralizing or antioxidant agents on the consequences induced by enamel bleaching agents in immediate resin composite restorations

Purpose: Bleaching agents are claimed to impair the bonding to the tooth structure when resin composite restorations are immediately performed. This study aimed to evaluate the effect of a neutralizing solution (10% sodium bicarbonate) or an antioxidant agent (10% sodium ascorbate) on the immediate or delayed (15 days) shear bond strength (SBS) of composite restorations performed on enamel. Seventy flat buccal enamel surfaces obtained from bovine incisors were divided into seven groups (n = 10): control group, unbleached enamel, restored (3M ESPE/Adper Single Bond 2/Filtek Z350XT) (G1); bleached, immediately restored (G2); bleached, delayed restoration (G3); bleached, antioxidant (sodium ascorbate), immediately restored (G4); bleached, antioxidant, delayed restoration (G5); bleached, neutralizing (sodium bicarbonate), immediate restoration (G6); bleached, neutralizing, delayed restoration (G7). Specimens were submitted to SBS test and examined after failure using scanning electron microscopy (SEM). Results were statistically analyzed with ANOVA/Tukey’s tests (5%). Bonding to enamel immediately restored after bleaching (G2) was significantly lower than G1 (unbleached enamel; p < 0.05). Applying the antioxidant or neutralizing agent significantly improved the bonding to enamel compared with G2 (bleached, immediate restored), irrespective of the restoration time (immediate or delayed) (p < 0.05). No significance was found between the two agents when applied after bleaching, and compared with the control group, regardless of evaluation time (p > 0.05). SEM images demonstrated adhesive failures in the bleached, immediately restored group (G2). G3–G7 exhibited majority of cohesive and mixed failure patterns. 10% sodium bicarbonate or 10% sodium ascorbate neutralizes the negative immediate and delayed effects of bleaching on bond strength of enamel bleached enamel.     

Adsorption behaviors of organic vapors using mesoporous silica particles made by evaporation induced self-assembly method

Spherical mesoporous silica materials with controllable surface area and uniform pore size were synthesized via evaporation induced self-assembly (EISA) method in this study. Both well-ordered and less-ordered mesoporous silica particle (MSP) adsorbents were made via adjusting the surfactant/silica precursor molar ratio. And the relationships between the physical characteristics of MSP adsorbents and the acetone adsorption behaviors were examined for the first time. The results indicated that an increase in the specific surface area results in an increase in the acetone adsorption capacity. But if a further increase in the surface area causes a less structured adsorbent then the acetone adsorption capacity could become less even though the specific surface area is of the highest value of . The acetone adsorption capacity of well-ordered MSP adsorbent is more than twice of that of the commercial ZSM-5 zeolite adsorbent due to its relatively higher surface area and uniform pores. The well-ordered structure of MSP also leads to higher acetone adsorption efficiency and a sharper breakthrough curve due to fast pore diffusion.     

蒸发诱导自组装法合成介孔铝酸钴尖晶石

采用嵌段聚合物F127为表面活性剂,以硝酸铝和硝酸钴为无机先驱物,通过蒸发诱导自组装的方法合成了介孔结构的铝酸钴尖晶石。X射线衍射（XRD）表明样品具有单一的尖晶石型结构。利用氮气吸附-脱附比表面测定仪测得不同焙烧温度样品的比表面积和孔径,发现650 ℃焙烧的样品比表面积最大,为89.1 m2/g,孔径为 8.7 nm。通过紫外-可见光谱测定表明介孔铝酸钴样品为明亮的蓝色,在545、585、625 nm处有3重吸收峰。     

Investigation of the effect of different silane coupling agents on mechanical performance of basalt fiber composite laminates with biobased epoxy matrices

In recent years, it has been detected an increased interest in the development of materials from renewable resources. This trend has been intensified in the industrial sector where significant efforts have been made in this field in order to adapt these natural fibers to conventional industrial processes and applications. As a result, research has been done into developing new thermoplastic matrices which are compatible with this type of reinforcing fibers. This study evaluates the influence of different coupling agents based on silanes, on the mechanical properties of composite laminates made from a biobased epoxy resin matrix and basalt fabric by using vacuum assisted resin transfer moulding. The curing behavior of the biobased epoxy resin was evaluated by differential scanning calorimetry (DSC), gel point determination, and ionic conductivity. The evaluation of mechanical properties was done by tensile, flexural, impact, and hardness tests. Compatibility between basalt fibers and epoxy resin generally has managed to increase through the addition of silanes, after the addition of these, their mechanical properties are substantially improved compared to the sample without silane treatment, obtaining this way an easily processable material, with good properties and capable of competing with materials with petroleum‐based epoxy resins. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Characterization and catalytic properties of mesoporous CuO/SBA-16 prepared by different impregnation methods

CuO/SBA-16 catalysts were prepared by two different routes – the conventional impregnation method and the modified impregnation method with pH adjustment. These catalysts were characterized by X-ray diffraction (XRD), atomic absorption spectrometry (AAS), N₂ physisorption and hydrogen temperature programmed reduction (H₂-TPR) measurements which reveal that the cubic cage-like (Im3m) pore structure of the parent SBA-16 molecule sieves was well maintained throughout the synthesis. After introduction of Cu, a different CuO dispersion exists on these catalysts. The CuO/SBA-16 prepared by modified impregnation method has a single highly dispersed CuO which is considered as a highly efficient species for hydroxylation of phenol with H₂O₂. CuO/SBA-16 prepared by the conventional impregnation method shows the presence of bulk CuO species which is undesirable for this reaction.