Adjusting the porous and structural properties of mesoporous silica by the addition of organic modifiers

The effects of the organic modifiers 2-cyanoethyltriethoxysilane and 3-aminopropyltriethoxysilane on the porous properties of silicates synthesised by co-condensation with tetraethyl orthosilicate were investigated. Materials were synthesised in aqueous solutions of cetyltrimethylammonium bromide or n-dodecylamine surfactants. Preparations using cetyltrimethylammonium bromide and sodium hydroxide gave MCM-48, with an average pore diameter of 25 Å, while addition of 10 mol% (of total silica) 2-cyanoethyltriethoxysilane improved the mesoporosity characteristics; the modified preparation gave a more intense X-ray diffraction pattern confirming a more ordered structure with a greater volume of regular mesopores. Disordered materials with pores 145 Å in diameter were formed using 2-cyanoethyltriethoxysilane modifier and ammonia as base. All preparations incorporating 3-aminopropyltriethoxysilane modifier were disordered and microporous. For silicates synthesised using n-dodecylamine surfactant, mesopores with diameter 118 Å were formed using 2-cyanoethyltriethoxysilane while preparations with 3-aminopropyltriethoxysilane and in the absence of a modifier were microporous. The particle sizes of the silicates displayed a clear correlation with the pore diameters: microporous silicates were 0.5–1 μm in particle diameter, MCM-48 phase (25/27 Å) were 1–2 μm, while relatively large mesoporous materials (>100 Å) had particle dimensions 10–100 μm.     

Ag-doped antibacterial porous materials with slow release of silver ions

The inorganic–organic hybrids of polyethyleneglycol (PEG), tetraethoxysilane (TEOS) and triethylphosphate (TEP) doped by silver ions were prepared by sol–gel method. After molding and heating at 600 °C to remove organic components, porous Ag–P₂O₅–SiO₂ monoliths were obtained. Thermogravimetry (TG), differential thermal analysis (DTA), infrared spectra, ultraviolet–visible (UV–vis) spectra and pore structure of the samples were measured to show that organic components and residual water could be removed by a heat-treatment up to 600 °C and the mesopores with 6 nm pore diameter were formed. Specific surface area and pore volume of the samples were adjusted with different contents of TEP in the starting composition. Ag⁺ ions could be stably released into water at 30 °C up to 28 days. Antibacterial experiment showed that such materials treated at 600 °C could restrain Escherichia coli effectively.     

A novel process to prepare a hollow silica sphere via chitosan-polyacrylic acid (CS-PAA) template

Hollow silica spheres were synthesized by using a chitosan-polyacrylic acid (CS-PAA) template. Polyacrylic acid (PAA) was titrated into the dissolved chitosan solution to form CS-PAA particles. For forming the core-shell particle, colloidal silica which was prepared from homogeneous nucleation was mixed with a solution of CS-PAA particles. Colloidal silica was adsorbed on to the surface of the CS-PAA particles to form a shell structure. CS-PAA could be removed from the core of the core-shell particles by adjusting with HCl to pH < 1 for forming the hollow silica structure. The outside particle size diameter, shell thickness, specific surface, pore diameter and pore volume of the final hollow silica sphere are about 200 nm, 20 nm, 350.95 m²/g, 5.4078 nm and 0.516768 cm³/g, respectively.     

Silica-doped alumina cryogels with high thermal stability

Alumina cryogels with a dopant of silica in the content range of 0–10 wt% were synthesized from aqueous boehmite sol through the sol-gel technique and subsequent freeze drying. The higher thermal stability was achieved by the addition of 10 wt% silica; a γ-Al₂O₃ phase still remained after heating at 1200 °C for 5 h, and the surface area and pore volume were 47 m² g⁻¹ and 105 mm³ g⁻¹, respectively. The marked stability was ascribed to the synergetic effect of the very low bulk density (0.05 g cm⁻³) and the dopant. The thermal stability was lower for the cryogels than for the corresponding aerogels; however, it was also suggested that cryogel was highly durable in water in contrast to aerogel.     

Hydrophobic and thermal insulation properties of silica aerogel/epoxy composite

Silica aerogel/epoxy composite was prepared by dry mixing hydrophobic aerogels with epoxy powders and heat pressing method. The composite materials show a serviceability temperature up to 250 °C with low thermal conductivity (0.11–0.044 W/m k) and hydrophobic property (water contact angle of 117–140°). Transmission electron microscope photos proved that part of silica aerogels nanopores had been immersed by epoxy. Based on this phenomenon, an immersion model was build up to study the effect of immersion on the thermal insulation and hydrophobic properties. In addition a thermal conductivity prediction equation of aerogel/polymer system was obtained and confirmed by comparing the experimental data.     

Synthesis of hybrid mesoporous spheres using the chitosan as template

Hybrid mesoporous spheres of Al and Si oxides were synthesized for the mixture of organic material (chitosan) with inorganic material (aluminum and silicon hydroxide). It was observed that chitosan with larger polymerization degree, resulted in a larger mechanical resistance of the spheres. The oxides were characterized by the following: Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), differential scanning calorimetry (DSC), as well as, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and adsorption isotherms of N₂ (BET). Highly uniform oxide sphere diameters were obtained (average of 1.0 mm). The results of the adsorption isotherms indicated that the material is mesoporous. The surface area of the materials ranged between 620 and 245 m²/g, and the pore volume varied between 0.82 and 0.28 cm³/g, depending on the molar ratio of the organic and inorganic materials.     

Morphological and mechanical properties of hybrid matrices of polysiloxane–polyvinyl alcohol prepared by sol–gel technique and their potential for immobilizing enzyme

Hybrid matrices of polysiloxane–polyvinyl alcohol (POS–PVA) were prepared by sol–gel technique using different concentrations of the organic component (polyvinyl alcohol, PVA) in the synthesis medium. The goal was to prepare carriers for immobilizing enzyme by taking into consideration properties as hardness, mean pore diameter, specific surface area and pore size distribution. The matrices were activated with sodium metaperiodate to render functional groups for binding the lipase from Candida rugosa, used here as a study model. Results showed that low proportion of PVA gave POS–PVA with low surface area and pore volume, although with higher hardness. The chemical activation decreased the pore volume and increased the pore size with a decrease on the surface area of about 60–75%. The matrices for enzyme immobilization were chosen considering the best combination of high surface area and hardness. Thus, the POS–PVA prepared with 5.56 × 10^?5 M of PVA with a surface area of 123 m²/g and hardness of 71 HV (50 gf 30 s) was shown to be suitable to immobilize the lipase, with an immobilization yield of about 40%.     

Experimental investigation and modeling of the creep behavior of ceramic fiber-reinforced SiO2 aerogel

Ambient creep tests at constant stresses of 0.05 and 0.1 MPa were conducted on a ceramic fiber-reinforced SiO₂ aerogel for more than 240 h. Experimental result shows that there are creep deformations even at very low stress level, and the creep strains finally reach constant values. High temperature creep experiments were also investigated at constant stresses of 0.05 and 0.2 MPa for 1800 s. The creep strains in 1800 s at 900 °C are 20–69 times higher than that at 300 °C. The creep behavior could be well described by the power-law creep model. Scanning Electron Microscope (SEM) analysis was also conducted to understand the micro mechanism of the creep behavior. Thermal softening of aerogel nanoparticles to generate larger ones was found to be the main reason for the change in mechanical properties at high temperature.     

Positron annihilation study of pore size in ordered SBA-15

Ordered mesoporous materials such as SBA-15 possess a network of channels and pores with a well-defined size in the nanoscale range (2–50 nm). This particular pore architecture makes them suitable candidates for a variety of applications. Different techniques have been used to measure pore diameters. PALS (positron annihilation lifetime spectroscopy) nanoprobe has been used to investigate free volume in several materials, including mesoporous silica. PALS can be used to find out if the micropore and mesopore structures of samples prepared under different experimental conditions are different. Indeed, considering that the pores present a cylindrical shape, an equation was developed that uses specific pore volumes, theoretical density, and specific surface measurements to evaluate structural connectivity. Our goal is to determine the influence of aging temperature on the porous structure of SBA-15 samples. The structural evolution was studied by PALS, small-angle X-ray (SAXS), N₂ adsorption desorption isotherms and computational modeling to evaluate connectivity. The variation of aging temperature changes the pore structure, indicating the presence of micropores and connections between mesopores. Materials aged at high temperatures present the lowest microporosity.     

Preparation and characterization of agglomerated porous hollow silica supports for olefin polymerization catalyst

Self-prepared porous hollow silica (PHS) nanoparticles were agglomerated with an oil (kerosene)–ammonia method and used as a novel support of metallocene catalysts for olefin polymerization. Transmission electron microscopy (TEM), scanning electron microcopy (SEM), small-angle X-ray diffraction (SA-XRD) and nitrogen adsorption–desorption were employed to characterize the morphologies and mesostructures of the PHS particles and agglomerated PHS particles. It was found that the agglomerated PHS particles had an average diameter between 15 and 30 μm, a specific surface area of 214 m²/g, a pore diameter of 11–18 nm and a pore volume of 1.41 mL/g. Metallocene catalysts prepared with the agglomerated PHS as support demonstrated a very high activity of 8350 g PE/gCata. for ethylene polymerization due to their particular morphology and mesostructures. Analysis with Differential Scanning Calorimetry (DSC), SEM, TEM and molecular weight analysis indicated that the polyethylene product was composed of tiny spherical particles with good crystalline and narrow molecular weight distribution, and the morphology and structure of the individual PHS support particle remained unchanged in polyethylene following the polymerization.     

Characterization of nickel–alumina aerogels with high thermal stability

The characterization of NiO–Al₂O₃ aerogels prepared from nickel nitrate and aluminum iso-propoxide through a sol–gel processing and subsequent supercritical drying was performed. The UV–visible, XPS and FT–IR investigations revealed that nickel ions were incorporated into alumina spinel structure as nickel aluminate form, not as nickel oxide, after calcination. TEM observations after the subsequent reduction exhibited uniform and fine nickel particles less than 6 nm diameter throughout the alumina aerogel support with high dispersion, by which not only high thermal stability of the metal at elevated temperatures but also high reforming activity and stability should be brought about. The large surface area and pore volume also provided the catalyst stability through the improvement of the thermal stability of alumina support.     

Studies of hydrogen interaction with carbon deposit containing carbon nanotubes

Nanocarbon materials were obtained by catalytic decomposition of ethylene on nanocrystalline iron at 500 and 550 °C. The increase in the carbon mass was controlled using the thermogravimetric method. After synthesis the samples were analyzed using the HRTEM and X-ray diffraction methods and contained iron carbide (cementite) and nanocarbon materials (carbon nanofibers, carbon nanotubes and amorphous carbon). Cementite crystallites were observed at the end of filamentous structures. In order to remove amorphous carbon and thick carbon nanofibers the samples were reduced under hydrogen atmosphere. As a result of the hydrogenation treatment, the cementite was decomposed into iron and carbon. The samples contained mainly multi-walled carbon nanotubes with open ends and had diameters in the range of approx. 10–30 nm.     

Magnetostatic properties of amorphous and nanostructured Fe73.5Si13.5B9Cu1Nb3 wires

We present the results of experimental investigations of magnetostatic properties of Fe-based amorphous wires with diameter about 100 μm. The samples were annealed at different conditions. The as-quenched sample had a weak increase (about 5%) of the magnetization with magnetic field decrease – the negative differential magnetic permeability could be observed in its hysteresis loops. The coercive force was about 0.2 Oe. The conventional annealing decreased the value of the saturation field down to 5 Oe without change of the coercive force value. When the annealing was accompanied with torsion stress the coercive force became less than 50 mOe. After the current annealing any peculiarities disappeared in magnetization in a weak magnetic field.     

Multiferroic behavior in silicate glass nanocomposite having a core–shell microstructure

Nanosized iron core and barium titanate shell microstructure was generated within a silicate glass of composition 23.1 Na₂O, 23.1 BaO, 23.0 TiO₂, 7.6 B₂O₃, 5.8 Fe₂O₃, 17.4 SiO₂ by first reducing it at 893 K for ½ h and then subjecting it to heat treatment at 759 K for 4 h. Transmission electron microscopy showed the composite particles to have a mean diameter of 3.9 nm. The nanocomposite exhibited both ferroelectric and ferromagnetic behavior. The dielectric constant peak was not prominent because of a small thickness of the barium titanate phase. The magnetic hysteresis loop showed an asymmetric behavior giving rise to a small exchange bias field. This is believed to arise due to exchange interaction between the ferromagnetic iron core and the thin layer of Fe₃O₄ on the core surface with a spin glass-like behavior. The magnetization under zero-field cooled (ZFC) and field cooled (FC) conditions indicated superparamagnetic behavior at temperatures higher than 300 K. The optical absorption spectra exhibited a peak at around 325 nm. This was analyzed satisfactorily on the basis of a metal core–oxide shell nanoconfiguration. The extracted values of metal core conductivity showed a metal insulator transition for iron core diameters less than 2.4 nm. The present synthesis approach will lead to newer multiferroic nanocomposites and glasses with multifunctionalities.     

Correlated surface perturbation in stressed foil made of amorphous alloy

A foil strip made of amorphous alloy Fe₇₀Cr₁₅B₁₅ was loaded with the tensile stress ～500 MPa and remained with the ends kept fixed during 2 h. The statistical characterization of the temporal transformation of the sample surface relief was performed using the autocorrelation functions computed for one-dimensional profiles measured along the axis of loading. The profiles were recorded with the help of the scanning tunneling profilometer. The acquisition time of a single scan was about 0.5 min. The calculated autocorrelation function of the profile depths in the original foil shows a long-range (～2 × 10³ nm) correlation between relief components. The application of the tensile stress causes an immediate, significant increase of the roughness depth from ～40–50 nm to 100–200 nm with a corresponding drop of the correlation length to 0.7 × 10³ nm. The surface relief does not remain stable during loading but varies as a result of the competitive processes, such as microcrack growth, stress-induced flattening, and shearing. The variability of the deformation process in the ‘structureless’ amorphous alloy is due to the dynamical non-uniformity of the multi-focal damage nucleation.     

Time resolved thermal lens measurements of the thermo-optical properties of Nd2O3-doped low silica calcium aluminosilicate glasses down to 4.3 K

In this work, the thermal lens spectrometry was applied to measure the thermo-optical properties of Nd₂O₃-doped low silica calcium aluminosilicate glasses as a function of temperature, between 4.3 and 300 K. The thermal relaxation calorimetry was used to determine the specific heat, c_p. The results showed a decrease of the thermal diffusivity of about one order of magnitude from 4.3 K up to 300 K, with a T^?1 dependence in the interval between 20 and 70 K and a T^?0.35 between 4.3 and 20 K. The fluorescence quantum efficiencies of the doped samples were calculated down to 50 K, showing a variation of the order of 12% and 25% for the samples with 0.6 and 1.04 mol% of Nd₂O₃, respectively. In addition, the temperature corresponding to the maximum in c_p/T³, the so-called boson peak, was observed at about 17 K for the undoped sample and at lower temperatures for the doped glasses. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at temperatures lower than 300 K, bringing new possibilities for experiments in a wide range of optical materials.     

Silver nanoclusters formation in ion-exchanged glasses by thermal annealing,UV-laser and X-ray irradiation

The Ag-exchanged commercial soda-lime silicate glasses were treated by three methods: thermal annealing, UV-laser irradiation, and X-ray irradiation, in order to promote the silver nanoclusters formation. Absorption spectrometry and electron spin resonance measurement results indicated that the silver ions transferred to silver atoms after the above three treatments. The silver atoms diffused and then aggregated to become nanoclusters after thermal annealing in air, or after UV-laser irradiation. However, X-ray irradiation, which induced defects and reduction of Ag⁰ atoms, would not promote the silver nanocluster formation. After annealing at 600 °C for 45 h, the spherical nanoclusters with a diameter of 3–8 nm were formed. The nanoclusters with a diameter of about 2 nm were formed after 30 min UV-laser irradiation without subsequent heating. The surface plasmon resonance peak position of silver nanoclusters changed from 411 nm after thermal annealing to 425 nm after UV-laser irradiation. The peak position shift was due to the nanoclusters size difference.     

Regenerator performance below 4 K in Tm-based bulk metallic glasses

The main obstacle for reaching low temperatures below 4 K using cryocoolers was the inefficiency of the regenerator materials. We report that large volumetric specific heat below 4 K in a wide temperature range has been obtained in Tm-based bulk metallic glasses, and the peak temperature of specific heat peak is tunable. We show that the amorphization of rare-earth based crystalline alloys and the lower magnetic transition temperature of spin glass behavior in the glasses result in the specific heat anomaly below 4 K. The large and tunable specific heat anomaly below 4 K indicates the potential regenerator performance of the glassy materials for sub-4 K cryocoolers.     

A simple synthesis of catalytically active,high surface area ceria aerogels

A simple synthetic route for preparing high surface area, structurally stable ceria aerogels is presented. Ceria aerogels were doped with 1% palladium: a water-gas shift (WGS) active metal. A reduced Pd/ceria aerogel and an as-prepared Pd/ceria aerogel are compared to determine the effect of pretreatment conditions on WGS activity. The BET surface area of the as-prepared ceria aerogel was 345 m²/g. CO chemisorption data and high-resolution TEM images indicate that the particle size of palladium on the reduced Pd/ceria sample was approximately 15 nm. X-ray photoelectron spectroscopy (XPS) data establish that the fraction of Ce³⁺ from Ce₂O₃ was approximately the same for both reduced and as-prepared samples. XPS data also indicate that approximately 44% of the palladium on the reduced Pd/ceria sample was converted to the metallic form. Each catalyst was tested for WGS activity. The reduced Pd/ceria sample exhibited the greatest WGS activity as compared with the other samples used in this study.     

Hydrothermal synthesis and characterization of VO2 (B) nanorods array

Highly ordered nanorods array of B phase vanadium dioxide was firstly synthesized with n-butanol as the reducing agent via a simple hydrothermal method without using template. The samples have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FE-SEM). The size of VO₂ (B) nanorods has the dimension of 50–100 nm in diameter and about 1–5 μm in length. The samples were measured as electrode materials by charge–discharge technique and the VO₂ (B) nanorods array demonstrated a high specific capacity of 520 mAh/g at 0.2 C. The influence of reaction temperature on fabricating nanorods array has been studied. The possible growth mechanisms of formation of nanorods and assembly of array were discussed.