Thermal stability of second generation carbosilane dendrimers with peripheral ammonia groups

Spinel nickel zinc ferrite nanowires were successfully prepared in mesoporous silica SBA-15 as a host matrix. The powder was annealed at a range of temperatures (500–900 °C) with heating rate 0.5 °C/min. The required NiZnFe₂O₄ phase was obtained at 700 °C. The specific surface area S_BET data revealed that the surface area of the mesoporous silica after annealing was decreased from 821 to 90 m²/g which indicated that the spinal ferrite fills the channels of mesoporous materials. The one-dimensional spinel nanostructures were characterized by X-ray diffraction, infrared spectroscopy, vibrating sample magnetometer, and transmission electron microscopy before and after a selective removal of the silica template in aqueous solution of NaOH or HF. The presence of SBA-15 lowers the formation temperature of nickel zinc ferrite nanowires compared to the corresponding bulk material. The magnetic properties revealed a high saturation magnetization level (~43 emu/g) for the Ni–Zn nanowires at 900 °C.     

Controlled SnO2 nanocrystal growth in SiO2–SnO2 glass‐ceramic monoliths

Crack‐free (100–x) SiO₂–x SnO₂ glass‐ceramic monoliths have been prepared by the sol–gel method obtaining for the first time SnO₂ concentrations of 20% with annealing at 1100 °C. Heat‐treatment resulted in the formation and growth of SnO₂ nanocrystals within the silica matrices. Combined use of Fourier transform–Raman spectroscopy and in situ high‐temperature X‐Ray diffraction shows that SnO₂ particles begin to crystallize in the cassiterite‐type phase at 80 °C and that their average apparent size remains around 7 nm, even after annealing at 1100 °C. Nanocrystal sizes and size distributions determined by low‐wavenumber Raman are in good agreement with those obtained from transmission electron microscopy measurements. Results indicate that the formation and the growth of SnO₂ nanocrystals impose a residual porosity in the silica matrix. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of temperature on the synthesis of silver nanoparticles with polyethylene glycol: new insights into the reduction mechanism

Polyethylene glycol (PEG) molecules act as a reducing and stabilizing agent in the formation of silver nanoparticles. PEG undergoes thermal oxidative degradation at temperatures over 70 °C in the presence of oxygen. Here, we studied how the temperature and an oxidizing atmosphere could affect the synthesis of silver nanoparticles with PEG. We tested different AgNO₃ concentrations for nanoparticles syntheses using PEG of low molecular weight, at 60 and 100 °C. At the higher temperature, the reducing action of PEG increased and the effect of PEG/Ag⁺ ratio on nanoparticles aggregation changed. These results suggest that different synthesis mechanisms operate at 60 and 100 °C. Thus, at 60 °C the reduction of silver ions can occur through the oxidation of the hydroxyl groups of PEG, as has been previously reported. We propose that the thermal oxidative degradation of PEG at 100 °C increases the number of both, functional groups and molecules that can reduce silver ions and stabilize silver nanoparticles. This degradation process could explain the enhancement of PEG reducing action observed by other authors when they increase the reaction temperature or use a PEG of higher molecular weight     

In-situ and ex-situ study of crystallization behaviour of magnetron sputtered Ni63Zr37 amorphous thin film

ABSTRACT

The stages of crystallization of magnetron sputter-deposited Ni₆₃Zr₃₇ film with mostly amorphous structure have been investigated by differential scanning calorimetry (DSC) and in-situ annealing at 300°C by use of heating stage on a high-resolution transmission electron microscope (HRTEM). These results have been further confirmed by grazing incidence X-ray diffraction analyses of thin film specimens annealed ex-situ at 300°C for various durations. The temperature for crystallization found by DSC has been found to increase from 371°C to 434°C with an increase in heating rate from 3°C/min to 10°C/min, and the apparent activation energy for amorphous to crystalline transformation has been found as ～260.2?kJ/mol from the Kissinger plot. Studies on HRTEM using in-situ heating stage have shown the crystallization to occur on annealing at 300°C for ～10?min. Crystallization at a temperature lower than that found by DSC is attributed to structural relaxation with reduction of free volume due to thermal activation. It has been observed that Ni₃Zr forms first due to its large negative enthalpy of formation, and is followed by the formation of Ni-rich solid solution (Ni_ss) grains. HRTEM studies have shown grain rotation with the formation of partial dislocations at Ni₃Zr-Ni_ss interfaces as well as twinning followed by detwinning with dislocation formation in the Ni_ss matrix possibly to reduce the interfacial energy.     

Proprietes electroniques et electrogalvaniques du tellurure d'argent α domaine d'existence

A study of the electronic and electrogalvanic properties of α silver telluride has been performed on samples whose non-stoiehiometry has been determined by coulometric titration with the cell Ag/AgI/Ag₂Te/Pt. The data lead to the adoption of a model of Frenkel defects on the silver sublattice that are fully ionised. The homogeneity range is reported from 162 to 508°C: it extends essentially on the tellurium side of the stoichiometric composition. The free energies of formation of the α and β forms of the tellurides whose compositions are near Ag_1,6Te, Ag_1,9Te and Ag_2,0Te have been determined between 23°C and 360°C.     

In situ growth of silver nanoparticles in mesoporous silica by spray pyrolysis

Silver-dispersed mesoporous silica was in situ synthesized in aerosol phase. The solidification of silica was catalyzed in the presence of the silver, which increased the order and d values of the mesophase at given reactor temperature. Silver nanoparticles grew confined in the pores when the atomic % of silver to Si was below 10 and the pore wall turned to be impermeable above 400 °C. Silver permeated through the pore wall below 400 °C to grow freely in the carrier gas. The mesophase deteriorated using spray pyrolysis above 800 °C due to the further densification of silica, or above 10 at.% of silver due to the excessive growth within the phase. The highest dispersion of silver ~4 nm in diameter with the highest order of the mesoporosity was obtained at 600 °C with 5 at.% silver. Calcination following the spray pyrolysis further densified the silica phase to freeze the growth of silver particles as well as lower the d value of the mesophase.     

Effect of thermal treatments on sputtered silver nanocluster/silica composite coatings on soda-lime glasses: ionic exchange and antibacterial activity

Silver nanocluster/silica composite coatings were deposited on both soda-lime and silica glasses by radio frequency (RF) co-sputtering. The effect of thermal treatments on the microstructure in the range of 150?C450?°C were examined by UV?Cvisible spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Time of Flight-Elastic Recoil Detection Analysis. Sodium/silver ionic exchange was evidenced for coatings sputtered on soda-lime substrates after heating at 450?°C; presence of silver ions and/or silver nanoclusters, nanocluster size and their position inside the sputtered layers will be discussed for as-deposited and heated coatings on both substrates. The antibacterial activity of all coatings was determined against Staphylococcus aureus and Candida albicans by disk diffusion method and colonies forming units count; in agreement with microstructural results, the antibacterial activity present on all coatings was slightly reduced after heating at 450?°C. All coatings have been submitted to humidity plus UV ageing and sterilization by autoclave, gamma ray and ethylene oxide gas. Tape resistance (ASTM D3359-97) tests have been done on each coating before and after ageing and sterilizations, revealing a good adhesion on soda-lime substrates, except for those aged in humidity plus UV and sterilized by autoclave. Scratch tests and nanoindentation tests have been done on each coating, as-deposited and after heating at 450?°C. The coating hardness was improved by heating only when coatings were deposited on silica. The heating of coatings deposited on soda-lime substrates gave opposite effect on their hardness.     

Surface-enhanced absorption by self-organized silver films with aciniform-like nanoaggregates at elevated temperatures

Morphological evolution of silver nanocomposite films prepared by the wet colloidal route and surface-enhanced phenomena on aggregate nanostructures evolved during annealing were investigated. Dramatic changes in morphologies of particles and pores incurred by rearragement, coarsening, premelting, and dewetting of the silver clusters at different concentrations (i.e., mass thicknesses). At a higher mass thickness, the morphological transitions from self-organized nanoaggregates with aciniform pattern at 300 °C to elongated and coarsened particles with circular holes at 400 °C to island clusters at 500 °C occurred in the films. The peculiar absorption with a much redder and broader surface plasmon feature, which gone far beyond the theoretical prediction, induced by the formation of aciniform nanoaggregates embedded in the porous polymer matrix at a critical mass thickness of 9.6 nm during partial degradation of the PVP polymer and rearrangement of silver clusters at 300 °C. The surface-enhanced absorption was dramatically reduced by the elemination of the aggregate nanostructures and the spontaneous formation of the silver nanoisland film at the dewetting temperature of 500 °C.     

A temperature and magnetic field dependence Mössbauer study of ɛ-Fe2O3

?-Fe₂O₃ was synthesized as nanoparticles by a pre-vacuum heat treatment of yttrium iron garnet (Y₃Fe₅O₁₂) in a silica matrix at 300°C followed by sintering in air at 1,000°C for up to 10 h. It displays complex magnetic properties that are characterized by two transitions, one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 120 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2). CAF2 has a smaller resultant magnetic moment (i.e. smaller canting angle) than CAF1. Analysis of the zero-field Mössbauer spectra at different temperatures shows an associated discontinuity of the hyperfine field around 120 K. In an applied field, the different magnetic sublattices were identified and the directions of their moments were assigned. The moments of the two sublattices are antiparallel and collinear at 160 K but are at right angle to each other at 4.2 K.     

Beobachtungen von Diffusionsprozessen im Schichtensystem Silber-Zinksulfid mit dem Photoemissions-Elektronenmikroskop

By means of the photoemission electron microscope Balzers Metioscope KE 3 the diffusion processes were investigated which occur at elevated temperatures in a system of silver and zincsulfide coatings. At 150 °C the formation of Ag₂S at the surface of the silver film can be seen (high emitting round spots on bright background) and at 240 °C one can observe the diffusion of silver through the zincsulfide film (high emitting small spots on dark background). On heating coagulation process of the silver particles takes place in the boundary zone near the silver layer. With increasing temperatures the mobility of these particles becames high enough to allow them to migrate into the silver layer.     

Synthesis of CoFe2O4 nanoparticles embedded in a silica matrix by the citrate precursor technique

A metals–citrate–silica gel was prepared from metallic salts, citric acid and tetraethylorthosilicate by sol–gel method (citrate precursor technique) and it was further used to prepare magnetic nanocomposites. The gel was dried at 100 °C and then calcined at temperatures between 600 and 1000 °C to obtain powder samples. The nanocomposites were characterized by XRD, IR, VSM and TEM techniques. The diffraction patterns show the formation of a single magnetic phase identified as CoFe₂O₄. Magnetic nanoparticles with average size less than 50 nm were obtained which are well dispersed in the silica matrix. The combination of different metals concentrations and calcining temperatures allowed obtaining samples with magnetization ranging from 3.6 to 25.3 emu/g.     

An application of oxide and silver electrode on the BaO-doped Bi2O3 electrolyte

Oxide and silver paste were applied on the BaO-doped Bi₂O₃ electrolyte and their behavior was studied as a function of temperature and oxygen partial pressure. Interface resistance of most oxide/electrolyte were of the same order of magnitude with those of Ag paste/electrolyte in air (300–500°C). A high electrode capacitance of (0.8–1.7)×10^?2 F/cm² was observed for the silver electrode at 450°C in the P_O2 region of 1–10^?5 atm.     

Behavior of Volatile Elements in the Graphite Furnace in the Presence of Silver as Matrix Modifier

The effect of silver, as an aqueous solution of AgNO₃, on the pretreatment and atomization behaviour of As, Cd, Bi, Hg, Pb, Sb, Se, Sn and Tl during electrothermal atomic absorption spectrometry has been investigated. The presence of silver in the graphite furnace leads to thermal stabilisation of all investigated volatile elements to allow higher pyrolysis temperatures. The maximum, loss‐free, pretreatment temperatures (°C) in the presence of 100 µg Ag by atomization from the wall or from a platform are respectively: As (1500°C, –); Cd (800°C, 800°C); Bi (700°C, 700°C); Hg (250°C, –); Pb (600°C, 900°C); Sb (1200°C, 1200°C); Se (1400°C, 1400°C); Sn (1100°C, 1100°C) and Tl (1000°C, 1100°C). Also, silver facilitates a relatively low atomization temperature (°C) from the wall for Cd (1300°C), Bi (1700°C), Pb (1400°C), Se (1900°C) and Tl (1400°C). In addition, silver enhances the measurement sensitivity by a factor of 1.2–1.8.     

Generating Parahydrogen-Induced Polarization Using Immobilized Iridium Complexes in the Gas-Phase Hydrogenation of Carbon–Carbon Double and Triple Bonds

Immobilized iridium complexes synthesized using [Ir(COD)Cl]₂ by anchoring on hydrous and anhydrous silica gels were studied in terms of generating parahydrogen-induced polarization (PHIP) in the gas-phase hydrogenation of propylene and propyne. Distinguishing differences in the hydrogenations of carbon–carbon double and triple bonds were found. It has been shown that in the double bond hydrogenation both catalysts are very active even at 25 °C. The reaction yield in continuous flow experiments is more than 70 %, whereas the obtained PHIP degrees are very low. In the case of the triple bond hydrogenation, a more or less active hydrogenation reaction was observed at relatively high temperatures (≈70–80 °C) for the catalyst immobilized on anhydrous silica, while the catalyst immobilized on hydrous silica was inactive at these temperatures. Contrary to the double bond hydrogenation, the triple bond hydrogenation provided significant signal enhancements observed in ¹H nuclear magnetic resonance spectra for the signals corresponding to protons of vinyl fragments of product propylene in both PASADENA and ALTADENA experiments. The catalyst, however, is not stable under the triple bond hydrogenation reaction conditions, and deactivates within several minutes. It was also found that at higher temperatures (100–120 °C), this catalyst demonstrates a reactivation most likely associated with the reduction of Ir(I) that results in the formation of Ir(0) surface metal nanoparticles.     

Carbon nanostructures produced by pyrolysis under high pressure inside a nanosize silica matrix

In this work, the pyrolysis under high pressure of hydrocarbons dispersed inside a nanosized silica matrix (Aerosil) was investigated. The samples consisted of hydrophobic nanometric silica powder terminated by methyl groups with carbon contents ranging from 0.7 to 4 wt%. The pyrolysis was carried out in the temperature range from 1000 to 1600 °C under high pressure (1.25 up to 7.7 GPa) to keep the two‐dimensional distribution of carbon atoms originally at the silica grain boundaries. Evidences from Raman spectroscopy and transmission electron microscopy suggested that the resulting carbon nanostructures were actually graphene‐like nanoflakes. The size of the nanostructures calculated from the I_D/I_G ratio increased from 6 to 30 nm for processing temperatures increasing from 1000 to 1600 °C under pressure, respectively. The results revealed that the very good dispersion of the methyl groups inside the nanosize silica matrix, and the confinement under high pressure during the pyrolysis, played both a relevant role in the resulting carbon nanostructures. Copyright © 2012 John Wiley & Sons, Ltd.     

Ambient effect on optical absorption of Au nanoparticles dispersed within pores of mesoporous SiO2

A gold nanoparticles/monolithic mesoporous silica assembly was synthesized by ultrasonic irradiation of monolithic porous silica presoaked with precursor solution. Subsequent exposure to ambient air (ageing) and then drying at 120°C induce a new optical absorption peak around 470?nm (falling into the range from 460 to 475?nm) which is stable at room temperature, in addition to the normal surface plasmon resonance (SPR) of Au nanoparticles. Further drying results in the decline and disappearance of this peak, accompanied by increase of the normal SPR. If the sample, in which the new peak has disappeared due to long drying at 120°C, is exposed to the ambience once more, this peak will appear again after subsequent drying at 120°C, showing reversibility. Further experiments indicate that ambient ageing for a certain time plays a crucial role in the appearance of the new peak after subsequent drying at 120°C. Increased ageing time increases this peak. In addition, the ambient relative humidity and temperature during exposure are also important to the appearance of this peak. This peak may be associated with Au clusters with a size less than 1.5?nm. Based on the porous structure of the assembly and hydrophilicity of its pore wall, a nanodroplet formation and evaporation model is presented which can well explain all evolution behaviours of this peak. The model predicts the existence of the peak at 470?nm in the Au/silica assembly prepared by methods other than ultrasonic irradiation, which has also been confirmed by further experiments.     

Effect of nano-dispersed silica on the ion-conducting behavior of PMMA-based polymer gel electrolytes containing LiPF<Subscript>6</Subscript>

Nano-sized silica poly(methylmethacrylate)-based gel electrolyte containing lithium hexafluorophosphate (LiPF₆) was synthesized by using different binary solvent mixture (propylene carbonate(PC) and dimethylformamide (DMF) in different volume ratio). Role of DMF in PC: Higher DMF content in PC-based electrolyte shows higher ionic conductivity at all polymer content and at wide temperature regions (10-70 °C). A small increment in ionic conductivity at lower content of polymer in liquid/gel electrolyte was observed and having maximum conductivity of 13.12 mS/cm at 25 °C. Stability (mechanically and electrically), viscosity and ionic conductivity of gel electrolytes were improved with the addition of nano-sized silica at ambient temperature. Ionic conductivity of nano-sized silica-based gel electrolyte does not change much over 5^o–70 °C temperature range and is factor-wise only which make indispensable in different electrochemical devices. Also polymer gel electrolyte membranes as such and with dispersed silica nano-particles were characterized through scanning electron microscope to study the morphology of gel matrix.     

Complex dielectric permittivities of the Ag2O-Ag2CO3 system at microwave frequencies and temperatures between 22 °C and 189 °C

The silver oxide-silver carbonate system comprises a thermally regenerable chemisorbent for carbon dioxide, which has been employed to revitalize CO₂-laden air within space suits. To evaluate the compatibility of these materials with thermal regeneration via dielectric heating, complex permittivities were studied over the microwave-frequency span between 4 and 26 GHz using a vector network analyzer based measurement system. Complex permittivities were determined over temperatures from 22 °C to 189 °C. The resulting values for the real and imaginary components of the complex permittivity, corresponding to polarization and energy-loss terms, respectively, were uniformly low for both the oxide and the carbonate under all test conditions. These results suggest that efficient dielectric heating of these materials is improbable. Received: 29 October 2001 / Accepted: 7 January 2002 / Published online: 3 May 2002     

Influence of sintering temperature and pH on the phase transformation,particle size and anti-reflective properties of RHA nano silica powders

Nano silica powders were synthesized from rice husk ash, the most silica-rich raw material, using alkaline extraction followed by acid precipitation. The phase transformation during sintering, the influence of sintering temperature and pH on the particle size and anti-reflective properties of nano silica were investigated. The results showed that the amorphous SiO₂ sintered at 600°C were transformed to a cristobalite structures completely during the sintering process at 800°C and 1100°C. With the increasing sintering temperature and pH, the particle size distributions (d₅₀) were increased respectively in the range of 62–84, 192–240, and 283–329?nm at sintering temperatures of 600°C, 800°C, and 1100°C. When the sintering temperatures were increased at 1100°C, 98.15% and 96.84% of transmittances were obtained respectively at the highest and lowest points of the anti-reflection band and could be used for anti-reflective applications.     

Intercalation of [Pt(NH3)4]2+ ions into layered sodium silicate magadiite: a useful method to enhance their stabilisation in a highly dispersed state

Al-free layered sodium silicate magadiite has been used as the host material for the stabilisation of [Pt(NH₃)₄]²⁺ ions via intercalation and/or ion-exchange reactions. The stabilisation of Pt(NH₃)₄]²⁺ ions in between the layers of Na-magadiite was confirmed by thermogravimetric analysis (TGA), where increased decomposition temperatures were observed for the intercalated materials. The intercalation behaviour of Na-magadiite was evident from the significant uptake of Pt ions (22.2 wt%). When silica gel was used as the host matrix, negligible uptake of Pt ions (1.3 wt%) was noticed. The X-ray diffraction (XRD) measurements revealed no appreciable change in the basal spacing of the intercalated materials. Nevertheless, the decrease in the intensity of the 001 peak with increasing Pt loadings (from 13.0 to 22.2 wt%) substantiated the intercalation of [Pt(NH₃)₄]²⁺ ions within the interlayer spaces of Na-magadiite. The transmission electron microscopy (TEM) studies of the intercalated materials revealed that [Pt(NH₃)₄]²⁺ ions were homogeneously intercalated in the magadiite matrix, ranging from 2 to 3 nm. Subsequent calcination of the intercalated materials at 600 °C in air led to the formation of Pt nanoparticles supported on silica. The results of XRD and TEM indicated that Pt nanoparticles were highly dispersed on the silica support and were in the range of 5–12 nm. Moreover, chemical analyses confirmed the high loading of Pt on silica in agreement with the TGA results.