Application of dilatometric analysis to the study of autoclaved calcium silicate materials

The process of shrinkage of calcium silicate hydrate was investigated by dilatometry up to 350 °C. The properties of this material are based on the formation of C–S–H phases during the reaction at temperatures between 180 and 205 °C and water vapor pressure lower than 16 bars. The main C–S–H phases are 11.3 Å tobermorite and xonotlite. 11.3 Å tobermorite converts to 9.3 Å tobermorite on air at temperatures around 300 °C. The hydrosilicate materials were prepared from quicklime and finely ground sand with different CaO/SiO₂ ratios under different hydrothermal conditions. The reaction time was 24 h. Materials based on xonotlite and tobermorite were produced, and the calcium silicate phases were characterized by XRD and TG/DTA methods. Dilatometry measurements were used to study the effect of heating conditions on sample shrinkage. Dehydration of hydrated calcium silicate minerals occurred during heating. The results show that sample shrinkage is dependent on the type and amount of C–S–H phases, the amount of bound water and formation of 9.3 Å tobermorite. All samples showed shrinkage after heating up to 350 °C, but this change was not irreversible for all samples after cooling to room temperature.

     

Characterization of the electronic structure and thermal stability of HfO2/SiO2/Si gate dielectric stack

X‐ray photoelectron spectroscopy was used to investigate thermal stability of HfO₂ on SiO₂/Si substrate prepared by atomic layer deposition, followed by annealing at different temperature. Hf silicate and Hf silicide are formed at the interface of HfO₂ and SiO₂ during deposition. The Hf silicide disappears, while the amount of the Hf silicate is intensified after post‐deposition annealing treatment at 400 °C. Phase separation of the Hf silicate layer occurs when the annealing temperature is over 400 °C, resulting in the Hf silicate decomposition into Si and Hf oxides. Moreover, crystallization at high temperature leads to grain boundaries formation, which deteriorates the gate leakage current, as observed by the electrical measurements. The similar annealing temperature dependence of both internal electric field and the amount of Hf silicate implies that the Hf silicate plays a key role in building up the internal electric field, which is attributed to generation of oxygen vacancies (V_o) in the Hf silicate layer. Copyright © 2017 John Wiley & Sons, Ltd.     

Shape- and Size-Selective Preparation of Hectorite-Supported Ruthenium Nanoparticles for the Catalytic Hydrogenation of Benzene

The cationic organometallic aqua complexes formed by hydrolysis of [(C₆H₆)₂RuCl₂]₂ in water, mainly [(C₆H₆)Ru(H₂O)₃]²⁺, intercalate into white sodium hectorite, replacing the sodium cations between the anionic silicate layers. The yellow hectorite thus obtained reacts in water with molecular hydrogen (50 bar, 100 °C) to give a dark suspension containing a black hectorite in which large hexagonally shaped ruthenium nanoparticles (20–50 nm) are intercalated between the anionic silicate layers, the charges of which being balanced by hydronium cations. If the reduction with molecular hydrogen (50 bar, 100 °C) is carried out in various alcohols, spherical ruthenium nanoparticles of smaller size (3–38 nm depending on the alcohol) are obtained. In alcohols other than methanol, the reduction also works without H₂ under reflux conditions, the alcohol itself being the reducing agent; the ruthenium nanoparticles obtained in this case are spherical and small (2–9 nm) but tend to aggregate to form clusters of nanoparticles. Whereas the ruthenium nanoparticles prepared by reduction of the yellow hectorite in refluxing alcohols without hydrogen pressure are almost inactive, the nanoparticles formed by hydrogen reduction catalyze the hydrogenation of benzene to give cyclohexane under mild conditions (50 °C) with turnover frequencies up to 6500 catalytic cycles per hour, the best solvent being ethanol. Dedicated to Professor C. N. R. Rao, pioneer of nanocluster chemistry, on the occasion of his 75th birthday.     

Thermal and hydrothermal stability of flame hydrolytically synthesized SiO2/TiO2 mixed oxides

The phase stability of the two TiO₂ modifications (anatase and rutile) in fumed SiO₂/TiO₂ nano-composites (0–24.8 wt-% silica) under thermal and hydrothermal conditions was investigated by X-ray powder diffraction, transmission electron microscopy (TEM) and gas adsorption methods (BET). The results show that the phase transformation from anatase to rutile type of structure and the growth of anatase crystallites are significantly retarded by mixing small amounts of SiO₂ into TiO₂, while the specific surface area is maintained. The SiO₂/TiO₂-composites reveal a remarkable shift in the anatase to rutile transformation temperature from approx. 500 °C (pure TiO₂) to approx. 1000 °C (samples with SiO₂ contents of more than 10%). The rate of phase transformation from anatase to rutile is enhanced under hydrothermal conditions compared to conventional thermal treatment, e.g. pure titania (AEROXIDE^® TiO₂ P25) annealed under hydrothermal conditions (100 g/m³ absolute humidity, 4 h at 600 °C) had a rutile content of 85%, while the same specimens annealed in absence of humidity contained only 46% rutile. However, the difference in rate of phase transformation became less pronounced when the silica content in SiO₂/TiO₂-composites was further increased.TEM results showed that the surface of the anatase crystallites was covered with silica. This averts coalescence of anatase crystallites and keeps them under a critical size during the annealing process. When the crystal domains grew larger, a rapid conversion to rutile took place. The critical size of anatase crystallites for the phase transformation was estimated to be 15–20 nm.     

Neue Verbindungen im System CaO/SiO2/CaCl2/H2O

New Compounds in the System CaO/SiO₂/CaCl₂/H₂O The hydrothermal formation of novel calcium silicate hydrates of compositions 5 CaO · 2 SiO₂ · CaCl₂ · 4 H₂O, 5 CaO · 2 SiO₂ · CaCl₂ · 2 H₂O and 4 CaO · 2 SiO₂ · CaCl₂ · H₂O from Ca₃SiO₅ and mixtures of CaO and SiO₂, respectively, in presence of calciumchloride at 200°–350 °C is described. From molybdate-reaction, ²⁹Si MAS NMR, DTA and TG measurements it is concluded that these compounds are based on disilicate anions and are to be interpreted as calcium hydroxide disilicate chlorides.     

Low temperature high-pressure synthesis of LnNiO3 (Ln = Eu,Gd) in molten salts

We report a new low temperature method for the synthesis of LnNiO₃ (Ln = Eu, Gd) at 400 °C under 180 bar oxygen pressure with the flux method. Utilization of the LiCl/KCl flux allowed for a decrease of the reaction temperature from 1000 °C and resulted in the synthesis of pure phase compounds. These materials have been characterized by powder X-ray diffraction and thermogravimetric analysis. LnNiO₃ (Ln = Eu and Gd) compounds crystallize in the orthorhombic GdFeO₃-type perovskite structure (space group: Pbnm). Both materials decompose to Ln₂O₃ and NiO at 775 °C under a nitrogen atmosphere and undergo reduction to Ln₂O₃ and Ni metal (at 385 °C and 340 °C for Eu and Gd, respectively) under a hydrogen atmosphere (10% H₂/N₂). Attempts to prepare the first T′-type infinite layer compound with Ni²⁺, EuNiO₂, by low temperature reduction of EuNiO₃ were unsuccessful.     

Self-assembled Nafion–silica nanoparticles for elevated-high temperature polymer electrolyte membrane fuel cells

In this paper, a novel Nafion/SiO₂ nanocomposite membrane based on the self-assembled Nafion–SiO₂ nanoparticles was developed. The average particle size of Nafion–SiO₂ nanoparticles prepared by self-assembly process was 2.8 ± 0.5 nm. The self-assembled Nafion–SiO₂ nanoparticles significantly enhance the durability of the Nafion/silica nanocomposite membrane as compared to that of conventional Nafion/silica composite and Nafion 212 membranes under wet/dry cyclic tests at 90 °C. With an addition of 5 wt% self-assembled Nafion–SiO₂ nanoparticles, the Nafion/SiO₂ nanocomposite membrane shows a significantly improved performance stability at cell/humidifying temperatures of 100 °C/60 °C under a current density of 600 mA/cm², and the degradation rate is 0.12 mV/min, almost 20 times lower than 2.33 mV/min measured on the pristine Nafion 212 membrane under the same conditions. The present results demonstrate the promises of the self-assembled Nafion/SiO₂ nanocomposite membrane for elevated-high temperature PEM fuel cells applications.     

Study on the effect of process parameters on CO2/CH4 binary gas separation performance over NH2-MIL-53(Al)/cellulose acetate hollow fiber mixed matrix membrane

The aim of current work is to study the interaction of process parameters including, temperature, CO₂ feed composition and feed pressure were towards CO₂ separation from CO₂/CH₄ binary gas mixture over hollow fiber mixed matrix membrane using design of experiment (DoE) approach. The hollow fiber mixed matrix membrane (HFMMM) containing NH₂-MIL-53(Al) filler and cellulose acetate polymer was successfully spun and fibers with outer diameter of approximately 250–290 nm were obtained. The separation results revealed that the increment of temperature from 30 °C to 50 °C reduced the CO₂/CH₄ separation factor while, increasing feed pressure from 3 bar to 15 and increment of CO₂ feed composition from 15 to 42.5 vol% increased the separation factor of HFMMM. The DoE results showed that the feed pressure was the most significant process parameter that intensely affected the CH₄ permeance, CO₂ permeance and CO₂/CH₄ separation factor. Based on the experimental results obtained, maximum CO₂ permeance of 3.82 GPU was achieved at feed pressure of 3 bar, temperature of 50 °C and CO₂ feed composition of 70 vol%. Meanwhile, minimum CH₄ permeance of 0.01 GPU was obtained at feed pressure of 15 bar and temperature of 30 °C and CO₂ feed composition of 70 vol%. Besides, maximum CO₂/CH₄ separation factor of 14.4 was achieved at feed pressure of 15 bar and temperature of 30 °C and CO₂ feed composition of 15 vol%. Overall, the study on the interaction between separation processes parameters using central composite design (CCD) coupled with response surface methodology (RSM) possesses significant importance prior to the application of NH₂-MIL-53(Al)/Cellulose Acetate HFMMM at industrial scale of natural gas purification.     

Formation and characterization of phosphate-modified silicate materials derived from sol–gel process

Phosphate-containing silicate materials prepared using sol–gel method from Si(OC₂H₅) were investigated at the variation of the amount of phosphate modifier from 5 to 50 wt% in term of P₂O₅. Chemical composition, textural and structural properties of these materials were characterized by FTIR-spectroscopy, TEM, X-ray diffraction and nitrogen adsorption. It was shown that the materials posse monomodal pore size distribution of 5–20 nm for the samples dried at 100 °C and 40–60 nm for the specimens calcined at 600 °C. The mean pore size and surface area depended on the amount of phosphoric acid. Before the stage of high temperature treatment phosphoric acid, introduced into the structure of the materials as a modifying agent, was uniformly distributed inside a porous space of the material and was not chemically bonded with silicate. After high temperature treatment both chemical interaction of silicate with phosphate, providing the formation of silicate-phosphate structures, as well as redistribution of free modifier from the bulk of granules to their surface took place. The polyphosphate layer is formed on the material surface closing the internal porous space. However, in this case a part of the phosphate modifier remains chemically unbound to SiO₂ structure.     

Size effect of silica nanoparticles on thermal decomposition of PMMA

The size effect of silica nanoparticles (SiO₂) on thermal decomposition of poly(methylmethacrylate) (PMMA) was investigated by the controlled rate thermogravimetry. Thermal degradation temperature of PMMA–SiO₂ composites depended on both fraction and size of SiO₂, the thermal degradation temperature of 23 nm (diameter) SiO₂–PMMA (6.1 wt%) was 13.5 °C higher than that of PMMA. The thermal stabilities of 17 nm SiO₂–PMMA (3.2 wt%) and 13 nm SiO₂–PMMA (4.8 wt%) were 21 and 23 °C, respectively, higher than that of PMMA without SiO₂. The degree of degradation improvement was increased linearly with the surface area of SiO₂. The number of surface hydroxyl group in unit volume of SiO₂ particle increased with increasing the specific surface area of SiO₂, and the interaction between hydroxide group of SiO₂ and carbonyl group of PMMA had an important role to improve the thermal stability of PMMA.     

Untersuchungen im System SrO?SiO2?H2O

Investigation on the System SrO? SiO₂? H₂O On addition sodium silicate solutions to solutions of Sr(OH)₂, at room temperature strontium hydrogensilicates are precipitated which are always amorphous and contain silicate anions of various condensation degrees. At about 100°C at first also amorphous products are formed containing lower- and higher-molecular silicate anions. On standing of these precipitates at about 80°C under the mother liquor, however, cristallization occurs under complete degradation of the higher-molecular anions to monomeric resp. dimeric silicate anions. In dependence on the Na₂O: SiO₂ ratio of the sodium silicate solutions and on the Sr(OH)₂ concentrations the following crystalline compounds are formed: 1.25 SrO · 1 SiO₂ · 2 H₂O, 3 SrO · 2 SiO₂ · 3 H₂O and 3 SrO · 2 SiO₂ · 4 H₂O, with monomeric silicate anions; 2 SrO · 2 SiO₂ · 1.5 H₂O; 2 SrO · 2 SiO₂ · 2 H₂O, and 2 SrO · 2 SiO₂ · 3 H₂O, with dimeric anions.     

A facile and efficient method for synthesis of xanthone derivatives catalyzed by HBF4/SiO2 under solvent-free conditions

Abstract  

HBF₄/SiO₂ was used as an efficient, green, and inexpensive catalytic system for synthesis of 12-aryl or 12-alkyl-8,9,10,12-tetrahydro-11H-benzo[a]xanthen-11-one derivatives via a one-pot three-component reaction of aldehydes, 2-naphthol, and cyclic 1,3-dicarbonyl compounds. The reactions proceeded rapidly at 80 °C under solvent-free conditions and the desired products were obtained in good to excellent yields.     

The chemistry and thermal stability of HfTaO/Si interface by x‐ray photoelectron spectroscopy

The chemistry and thermal stability of HfTaO/Si interface as a function of annealing temperature have been investigated by x‐ray photoelectron spectroscopy. For the as‐deposited sample, the formation of Hf‐silicate bond is observed on Hf 4f core‐level spectra, which contributes to bulk HfO₂ and SiO₂. Besides, the suboxide of tantalum (Ta⁺¹) is formed at the interface at room temperature because of oxygen‐deficient conditions. HfSi₂, Hf_xSi_yO_4, and HfO₂ coexists in interfacial region at 850 °C, meanwhile, an evidence for transforming from the Ta¹⁺ to tantalum oxide (Ta⁵⁺) is verified. The peaks of Hf–O–Si and Hf–O have disappeared, only one peak of Hf silicide remained after the annealing at 950 °C. A stable SiO₂ phase in HfTaO/Si is formed under different annealing conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of the formation of the apatite-type phases La9.33+x(SiO4)6O2+3x/2 synthesized from a lanthanum oxycarbonate La2O2CO3

Lanthanum silicated apatites with nominal composition La_9.33+x(SiO₄)₆O_2+3x/2 (−0.2 < x < 0.27) have been successfully synthesized by solid state reaction using a new reagent La₂O₂CO₃ and amorphous SiO₂ precursors. The formation mechanism of La₂O₂CO₃ reagent, which cannot be purchased, has been followed by in-situ temperature depend XRD of La₂O₃ under CO₂ atmosphere. The stability of this reagent during the synthesis step allowed to limit the formation of secondary phase La₂Si₂O₇ and made the weighting of the reagent easier. High purity powders could be synthesized at the temperature of 1400 °C. Dense pellets (more than 98.5%) were obtained by isostatic pressing of powders calcined at 1200 °C and then sintered at 1550 °C. Traces of La₂SiO₅ secondary phase present in synthesized powder disappeared after densification and pure oxyapatite materials were obtained for all the compositions. Electrical measurements confirmed that conductivity behaviors of the sintered pellets were dependent to the oxygen over-stoichiometry. Indeed, a relatively high conductivity of 1 × 10⁻² S cm⁻¹ was exhibited at 800 °C for the nominal composition La_9.60(SiO₄)₆O_2.405 with low activation energy around 0.79 eV. The ionic conductivity properties were comparable with that of the earlier obtained materials.     

Influence of iron on the synthesis and stability of yttrium silicate apatite

The stability of yttrium silicate apatite has been investigated by studying the influence of iron as a “stabilising cation” and also by using different synthesis routes. The formation of apatite in samples has been followed by X-ray diffraction and by ²⁹Si MAS NMR spectroscopy. The apatite phase appears to be stable at high temperatures (≈1700 °C) especially when heated in a nitrogen atmosphere; it can also occur in a metastable state when heated in air at lower temperatures; ≈1600 °C if prepared from a Y₂O₃SiO₂ mixture or in the range 950 °C <T< 1150 °C if synthesised by the sol–gel process. Longer heat-treatments result in its decomposition into Y₂Si₂O₇ and Y₂SiO₅. Iron appears to have two roles depending on the temperature; it stabilises the apatite phase at high temperatures when produced by the sol–gel route and catalyses the decomposition of sol–gel derived apatite at low temperatures.     

Interaction of Co nanoparticles with SiO2: silicide formation

The interaction of 1 and 4 nm thick Co films with SiO₂ support in vacuum at high temperature has been investigated by TEM, SAED and HRTEM methods. It was found that annealing in vacuum at 800 °C caused the transformation of the smallest Co particles into Co₂Si silicide.     

Thermogravimetric and FTIR studies of chitosan blends

Results of spectrophotometric and thermogravimetric studies of chitosan (CH) blends with polyvinyl alcohol (PVAL), starch (S) and hydroxypropylcellulose (HPC) obtained by casting from solutions in the form of transparent films containing 0–1.0 weight fraction of CH were discussed. Blends containing S are homogeneous only in the case of low-weight fraction of S (to 0.3).On the basis of results of thermodegradation in dynamic and isothermal conditions, thermal stability of the tested systems was estimated. Thermogravimetric measurements in dynamic conditions were carried out in the temperature range of 100–450 °C at constant heating rate 15 °C/min. From thermogravimetry (TG) and DTG curves the activation energy and characteristic parameters of degradation of the tested blends were determined. The observed growth of activation energy and T_p—temperature of initial weight loss, T_max—temperature of maximal rate and C_e—degree of conversion at the end of the measurement (at temperature 450 °C) along with the increase of polymer fraction (HPC and S) in the CH blend provides an evidence of improved thermal stability of the systems tested.Investigations in isothermal conditions in air at temperature from 100 to 200 °C confirmed appreciable improvement of CH thermal stability in the blends being tested.Infrared spectroscopic analysis of the blends showed a distinct stabilization of the process of chain scission. In the band at 1080 cm⁻¹ associated with absorption in –C–O–C– group during degradation of the blends at temperature 200 °C much smaller decrease due to molecular scission were observed than in the case of pure CH.     

Effect of pressure on the low-temperature reaction of ethylene with N2O4

Calorimetric monitoring of the autoclave reaction N₂O₄ + C₂H₄ at –85 to +10 °C under argon pressure 10–30 bar revealed that the exothermic chemical reaction started at temperatures above –52 °C at 10 bar, whereas an intensive exothermic reaction started at –85 °C and pressure of 30 bar. IR study showed that oligo/polynitroethylene was formed at 30 bar, while carbonyl and hydroxy compound as well as nitrate R–ONO₂ formation occurred upon processing at 10 bar.     

Hydrogen sensing properties of a Pd/SiO2/AlGaN-based MOS diode

Hydrogen sensing properties of a Pd/AlGaN-based Schottky diode are improved by the deposition of SiO₂ at the metal/semiconductor (MS) interface. The wide Schottky barrier height variation of the MOS diode could be attributed to the large electric field across the SiO₂ layer. This leads to the presence of more hydrogen dipoles caused by the polarization effect. The sensing response of the MOS diode at room temperature (1.3 × 10⁵) is comparable to that of the MS one at 150 °C (2.04 × 10⁵). Thus, the MOS-type sensing device shows the benefit of low-temperature operation. Kinetic analyses confirm that the short response times of the MOS diode are attributed to high reaction rate at the Pd/SiO₂ interface.     

Characteristics of lime-silica fume mixtures

The hydration of two calcium hydroxide-silica fume mixtures was studied at 25°C. The mixtures were prepared at lime/silica molar ratios of 1.0 and 1.7. The free lime, free silica and chemically combined water contents were determined after various periods of hydration (0.5 h-90 days). Thus, the molar ratios CaO/SiO₂ and H₂O/SiO₂ molar in the calcium silicate hydrates (C-S-H) formed could be derived. The hydrates formed were identified by using differential thermal analysis. The mechanism of the hydration-gardening thermal analysis. The mechanism of the hydration-hardening reaction between lime and silica fumes was suggested. The changes in the molar ratios CaO/SiO₂ and H₂O/SiO₂ in the C-S-H formed with the time of hydration were found to follow the same trends as observed during the hydration course for the suggested mechanism.