Enhancement of thermal shock resistance in β-Si3N4 coating with in situ synthesized β-Si3N4 nanowires/nanobelts on porous Si3N4 ceramics

A dense β-Si₃N₄ coating toughened by β-Si₃N₄ nanowires/nanobelts was prepared by a combined technique involving chemical vapor deposition and reactive melt infiltration to protect porous Si₃N₄ ceramics in this work. A porous β-Si₃N₄ nanowires/nanobelts layer was synthesized in situ on porous Si₃N₄ ceramics by chemical vapor deposition, and then Y–Si–Al–O–N silicate liquid was infiltrated into the porous layer by reactive melt infiltration to form a dense composite coating. The coating consisted of well-dispersion β-Si₃N₄ nanowires/nanobelts, fine β-Si₃N₄ particles and small amount of silicate glass. The testing results revealed that as-prepared coating displayed a relatively high fracture toughness, which was up to 7.9 ± 0.05 MPa m^1/2, and it is of great significance to improve thermal shock resistance of the coating. After thermal cycling for 15 times at ΔT = 1200 °C, the coated porous Si₃N₄ ceramics still had a high residual strength ratio of 82.2%, and its water absorption increased only to 6.21% from 3.47%. The results will be a solid foundation for the application of the coating in long-period extreme high temperature environment.     

Combustion synthesis of porous ceramic β- Si3N4 -based composites with the use of ferroalloys

A method for manufacturing silicon nitride-based composites with a desired porosity using a self-propagating high-temperature synthesis has been developed. The effects of the composition of starting specimens, pressure and diameter on the combustion parameters of pre-structured specimens based on ferro-silicoaluminum with addition of aqueous-alkali solution were studied. The phase composition of composites was determined. The obtained composites showed porosities of 25.11%–55.08% with a predominance of open pores of average size in the range of 21–305 μm.     

Heterogeneous nucleation of β-type precipitates on nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy

Zirconium (Zr) is an important alloying element to Mg-Zn-based alloy system. In this paper, we report the formation of the β-type precipitates on the nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy during ageing at 180°C. Scanning transmission electron microscopy examinations revealed that the nanoscale Zr-rich [0001]_α rods/laths are dominant in the Zr-rich core regions of the as-quenched sample after a solution treatment at 430°C. More significantly, these Zr-rich particles served as favourable sites for heterogeneous nucleation of the Zn-rich β-type phase during subsequent isothermal ageing at 180°C. This research provides a potential route to engineer precipitate microstructure for better strengthening effect in the Zr-containing Mg alloys.     

Enhanced corrosion resistance of layered double hydroxides/steam coating composite coating prepared by the hydrothermal method on LA43M magnesium–lithium alloy substrates

An excellent anticorrosion Mg–Al layered double hydroxide (LDH) composite coating was successfully fabricated on LA43M magnesium alloy substrates via an in situ steam coating (SC) process and a subsequent hydrothermal treatment at different temperatures. The microstructure, composition and phase formation of the composite coatings were studied via X-ray diffractometer, energy disperse spectroscopy, and scanning electron microscope, respectively. The corrosion resistance of composite coatings was further investigated using electrochemical measurements and corrosion test. The results showed that LDH/SC composite coating has typical nanosheets microstructure, which effectively seal the defects of SC. As the hydrothermal temperature increases, the thickness and density of nanosheets increases, and the corrosion resistance was significantly improved. Especially, the Mg–Al LDH/SC composite coating prepared at 100°C was the most dense and thickness, and exhibited the optimal and long-term anticorrosion resistance in 3.5 wt.% NaCl soultion. It has the lowest I_corr (1.767 × 10⁻⁸ A/cm²), which decreased by three and two orders of magnitude compared with the bare substrate and SC. Furthermore, it can maintain good chemical stability after immersion in the corrosion medium for 192 h and its hydrogen evolution rate (0.00416 mL·cm⁻²·h⁻¹) and weight lost rate (0.00266 mg·cm⁻²·h⁻¹) were the lowest compared with other samples.     

Diffusion Transfer in the Course of Benzene Alkylation with Propylene over a β-Zeolite Catalyst

By simulating benzene alkylation with propylene over -zeolite in a fixed-bed bubble reactor, suitable external mass-transfer correlation equations were chosen from the literature to calculate the coefficients and the external mass-transfer resistance. The internal mass-transfer resistance was derived from the experimental efficiency factor of catalyst pellets. When the catalyst pellet is 3 mm in diameter and consists of 0.2-m zeolite crystals, the mass-transfer resistance in the alkylation process decreases in the following order: gas–liquid film > liquid–solid film > macropore diffusion > intrinsic kinetics > intracrystalline diffusion.     

Electron paramagnetic resonance of color centers in nanoporous glasses impregnated with copper β-diketonatewith the use of supercritical carbon dioxide

The spectral properties of nanoporous glasses containing Cu²⁺ ions, which are introduced into glass pores in the form of organometallic compound of copper -diketonate dissolved in supercritical CO₂, are investigated for the first time. The analysis of the EPR and optical absorption spectra demonstrates that Cu²⁺ ions are located in octahedral sites with symmetry D_4h. It is revealed that the intensity of the EPR spectrum decreases by one order of magnitude after annealing of the glass at a temperature of 200°C, which correlates with the temperature of decomposition of copper -diketonate molecules (250°C) with the formation of CuO or metallic copper. This process is accompanied by a change in the spectrum shape and the spin-Hamiltonian parameters, even though these parameters also correspond to the octahedral environment with symmetry D^4hIt is assumed that the remaining Cu²⁺ ions form new complexes with fragments of decomposed molecules. According to the optical data, the inference is made that high-temperature annealing leads to the formation of copper metal nanoparticles in the glass.Original Russian Text Copyright © 2004 by Fizika i Khimiya Stekla, Bagratashvili, Bogomolova, Jachkin, Krasilnikova, Rybaltovskii, Tsypina, Chutko.     

Effects of a silane coupling agent on the exfoliation of organoclay layers in polyurethane/organoclay nanocomposite foams?

An attempt was made to synthesize polyurethane (PU)/organoclay nanocomposite foams with high thermal insulation properties. The organoclay was modified by polymeric 4,4′‐diphenylmethane diisocyanate (PMDI) with a silane coupling agent. The structure of the organoclay‐modified PMDI with the silane coupling agent was determined by Fourier transform infrared spectroscopy and nuclear magnetic resonance. Transmission electron micrographs and wide‐angle X‐ray diffraction patterns showed that the interlayer distance increased for the PU/organoclay nanocomposites with the addition of the silane coupling agent. It was expected that the distance between the organoclay layers would increase and that the organoclay would be dispersed on a nanoscale in the PU matrix because of the organic/inorganic hybrid bond formation between the organoclay and silane coupling agent. Compressive and flexural strengths of the PU/silane coupling agent/organoclay nanocomposite foams were similar to those of the PU/organoclay nanocomposite foams. However, the thermal conductivity appreciably decreased from 0.0250 to 0.0230 W/m h °C in the PU/silane coupling agent/organoclay nanocomposite foams. Scanning electron micrographs showed that the cell size of the PU/silane coupling agent/clay nanocomposite foams also decreased. On the basis of these results, it is suggested that the smaller cell size and lower thermal conductivity of the PU/silane coupling agent/organoclay nanocomposite foams were mainly due to enhanced exfoliation of the organoclay layers by the silane coupling reaction. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Role of ceria in the improvement of SO2 resistance of LaxCe1 − xFeO3 catalysts for catalytic reduction of NO with CO

Perovskite-type catalysts with LaFeO₃ and substituted La_xCe_1 − xFeO₃ compositions were prepared by sol–gel method. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed reduction (CO-TPR), and SO₂ temperature-programmed desorption (SO₂-TPD). Catalytic reaction for NO reduction with CO in the presence of SO₂ has been investigated in this study. LaFeO₃ exhibited an excellent catalytic activity without SO₂, but decreased sharply when SO₂ gas was added to the CO + NO reaction system. In order to inhibit the effect of SO₂, substitution of Ce in the structure of LaFeO₃ perovskite has been investigated. It was found that La_0.6Ce_0.4FeO₃ showed the maximum SO₂ resistance among a series of La_xCe_1 − xFeO3 composite oxides.     

Oxidation and cracking/spallation resistance of ZrB2–SiC–TaSi2–Si coating on siliconized graphite at 1500 °C in air

A ZrB₂–SiC–TaSi₂–Si coating on siliconized graphite substrate was prepared by a combination process of slurry brushing and vapor silicon infiltration. The high-temperature oxidation behavior and cracking/spallation resistance of the as-prepared coating were investigated in detail. It was revealed that the oxidation kinetics at 1500 °C in static air followed a parabolic law with a relatively low oxidation rate constant down to 0.27 mg/(cm²·h^0.5). The crack area ratio of the as-prepared coating was determined as 3.8 × 10⁻³ after severe thermal cycling from 1500 °C to room temperature for 20 times. Apart from the formation of ZrO₂ as skeleton phase with SiO₂ as infilling species, the good oxidation and cracking/spallation resistance of the coating also could be attributed to its unique duplex-layered structure, i.e., a dense ZrB₂–SiC–TaSi₂ major layer filled with Si and an outermost Si cladding top layer. Meanwhile, the strong adhesion strength of the SiC transition layer with the graphite substrate and the outer ZrB₂–SiC–TaSi₂–Si layer was a vital factor as well.     

Structural investigation on the stoichiometry of β-PdHx in Pd/SiO2 catalysts as a function of metal dispersion

Structural investigations of dispersed Pd/SiO₂ catalysts were carried out with XRD and SAXS techniques, supported by TPR and CO chemisorption measurements. The stoichiometry of Pd hydride, -PdH_x, was determined by measuring the shift of Pd 111 XRD reflection in the presence of hydrogen. It was confirmed thatx decreases when the metal dispersion increases. This behaviour could be quantified up to about 0.45 of Pd dispersion since the angular position of XRD peaks cannot be determined with the necessary precision at higher dispersions. TPR data, obtained up to dispersions of about 0.6, confirms such behaviour. The -PdH_x stoichiometry versus Pd dispersion relationship substantially agrees with that found with other techniques by Boudart and Hwang.     

The influence of hydrogen pressure on the heterogeneous hydrogenation of β-myrcene in a CO2-expanded liquid

This paper presents the second part of work on the effect of hydrogen partial pressure on the hydrogenation of a terpene in a CO₂-expanded liquid. The effect of hydrogen partial pressure on the hydrogenation of β-myrcene possessing three CC bonds catalysed by alumina-supported ruthenium and rhodium was studied. Experiments were performed at various hydrogen pressures in the range from 2.0 up to 4.5 MPa at a fixed total pressure of 12.5 MPa. In all the conditions the reaction proceeded in two phases (liquid + gas), that is, the total pressure was below the critical pressure of the CO₂ + β-myrcene + H₂ system. The liquid phase volume is expanded in relation to the initial volume of β-myrcene in a fashion that is strongly dependent on the hydrogen and carbon dioxide pressures. An increase of H₂ pressure concomitantly diminishes carbon dioxide pressure, which leads to the enhancement of the liquid phase in hydrogen and a terpene. It does not direct to straightforward higher reaction rate, but surprisingly the effect of higher concentrations either hydrogen or β-myrcene is opposite. It is attributed to the fact that the hydrogenation of β-myrcene rate-controlling factor turns out to be the hydrogen to β-myrcene ratio which decreases as the hydrogen pressure increases. These unexpected appealing results present that lower pressures of hydrogen guide to higher hydrogen/β-myrcene ratios in the liquid phase, but on the other hand they also amplify the initial reaction rate constant. The obtained results are opposite to the results achieved for effect of hydrogen pressure on the Pd-catalysed hydrogenation of limonene consisting of two CC double bonds.     

NO abatement with CH4 in the presence of O2 on H-ZSM5 with Si/Al=15–200: the dependence of activity on the proton concentration

The abatement of NO with methane in the presence of oxygen was studied on commercial H-ZSM5 samples with Si/Al = 15–200 in a conventional flow apparatus. H-ZSM5 samples were used in the acid form or after exchanging protons with sodium ions to various extents. Their catalytic activity was compared with that of commercial H-mordenite and H-Y. On all H-ZSM5 catalysts, reaction rates R _NO and (molecules s⁻¹ g⁻¹) increased proportionally to the proton concentration, showing that either all protons or a constant fraction of them were equally active. On sodium-exchanged H-ZSM5 samples with Si/Al = 15–17, both R _NO and nearly exponentially increased with the proton concentration. Conversely, on sodium-exchanged H-ZSM5 with Si/Al = 50, both R _NO and linearly increased with the proton concentration. At lower Si/Al ratios, replacing the H-ZSM5 protons with sodium ions partly inactivated the remaining protons. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microwave-assisted reduction of levulinic acid with alcohols producing γ-valerolactone in the presence of a Ru/C catalyst

γ-Valerolactone can be synthesized by reduction of levulinic acid and its esters in the presence of secondary alcohols as hydrogen donors and Ru/C as catalyst. The reaction rate increases when using microwave heating. Quantitative formation of γ-valerolactone was observed within 25 min at 160 °C under microwave heating based on levulinic acid and i-propanol. The reaction appears to proceed via a dehydrogenation–hydrogenation sequence.     

Stereo-Preference in the degradation of the erythro and threo isomers of β-O-4-type lignin model compounds in oxidation processes. part 2: In the reaction with hydroxyl and oxyl anion radicals under hydrogen peroxide bleaching conditions

We examined which isomer, the erythro or threo, of non-phenolic β-O-4-type lignin model compounds is stereo-preferentially attacked by hydroxyl radical and its conjugate base, oxyl anion radical, generated by the decomposition of hydrogen peroxide in the presence of ferric ion and its precipitates under hydrogen peroxide bleaching conditions. The intrinsic stereo-preference of oxyl anion radical was slightly toward the erythro isomer, while hydroxyl radical had a further smaller stereo-preference. These stereo-preferences can be explained by our prior knowledge that oxyl anion radical preferentially attacks the side-chain of the lignin model compounds rather than the aromatic nucleus. The amount of the degraded lignin model compounds became less great with decreasing pH, but reversely and intensively greater in the pH range below 10. This phenomenon can be attributed to the change in the decomposition mechanism of hydrogen peroxide accompanying the pH variation.     

A low-cost counter electrode of ITO glass coated with a graphene/Nafion® composite film for use in dye-sensitized solar cells

Carbon black (CB), multi-walled carbon nanotube (MWCNT), and graphene (GN) were examined as catalytic films on the counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). GN exhibits the best performance among these materials for the corresponding cell. A composite film of GN/Nafion® was next used as the catalytic film on the CE of a DSSC. Nafion® is demonstrated to be an excellent dispersant for inhibiting the aggregation of GN. A solar-to-electricity conversion efficiency (η) of 8.19% was achieved for the DSSC sensitized by TBA(Ru[(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(4,4′-bis(5-(hexylthio)-2,2′-bithien-5′-yl)-2,2′-bipyridine)(NCS)₂]), i.e., CYC-B11 dye, which was synthesized by our group, after the optimization of the composition of GN/Nafion®, and an η of 8.89% was exhibited for the cell with a sputtered-Pt (s-Pt) film on its CE under the same conditions. The significance of the cell with the composite film of GN/Nafion® lies in the fact that the expensive Pt is avoided in this case, thereby the cost of the pertinent DSSC was greatly reduced, even though its η (8.19%) is slightly smaller than that of the cell with the s-Pt film (8.89%).     

Direct computation of the performance of sequential procedures based on a sequence of t-statistics,with application to a confidence interval for μ/σ in a normal population

Formulas are derived for the direct computation — as opposed to simulation or approximation — of sequential procedures that are based on a sequence of t-statistics. This is applied to the numerical computation of the performance of a one-sided confidence interval for γ = μ/σ in a normal population, when the probability of covering the true value γ as well as the probability of covering the false value γ ? d, for given d > 0,is specified.     

The crystallization and rheological behaviors of in situ microfibrillar isotactic polypropylene/polyamide 66 composites with a selective β-nucleating agent distribution

In situ microfibrillar composites (MFCs) with isotactic polypropylene (iPP) as matrix and β-nucleating agents filled polyamide 66 (β-PA66) as dispersed microfibrils were fabricated through multistage stretching extrusion technology. The simultaneous effects of β-nucleating agents, shear-flow and microfibrils on the crystallization and rheological behaviors of iPP were explored. Crystallization analysis demonstrated that the counteraction of shear flow and β-nucleating agents resulted in the absence of β-crystal in the as-extruded MFCs. When the MFCs were annealed at 210°C for 10 min, the flow field was erased accompanied with the migration of β-nucleating agents from PA66 phase to surface, which in turn inducing the formation of β crystal in the recrystallized samples. Shear rheological investigations suggested that the β-NAs at interface mechanically integrated the PA66 and iPP phase thus improving the interfacial adhesion. These findings provided a potential way to solve the interfacial problems of MFCs.     

Is there a limit for the passivation of Si surfaces during anodic oxidation in acidic NH4F solutions?

The interface state densities of wet anodic oxide layers on Si are normally very high and, therefore, there is need for special post-treatments. Repetitive oxidation/hydrogenation cycles in the current oscillating regime lead to improved passivation of the SiO₂/Si interface. The atomic force microscopy (AFM) images reveal the formation of macroscopically rough surfaces (holes with a diameter of at least 100 nm and a depth of up to 10 nm). This kind of surface structure is more favourable to decrease the strain induced by the Si---O---Si bond angle mismatch. This peculiarity of Si surfaces conditioning in acidic NH₄F solutions leads to a reduction of the defect concentration at the SiO₂/Si interface without further processing.     

The effect of phase formation during use on the chemical corrosion of magnesia–chromite refractories in contact with a non-ferrous PbO–SiO2 based slag

One of the main factors limiting the lining lifetime in pyrometallurgical smelters is continuous refractory oxides dissolution in the slag bath. The overall wear is accelerated when the slag infiltrates the porous brick and the dissolution thus occurs in a larger part of the lining. This work investigates the possibility of preventing deep infiltration by sealing off the pores with newly formed phases. Static finger tests at constant temperature (1200 °C) were performed in contact with a synthetic non-ferrous PbO–SiO₂–MgO slag, showing the formation of forsterite (Mg₂SiO₄) throughout the refractory sample by the reaction between SiO₂ (slag) and MgO (refractory). This phase grows with time, eventually sealing off the pores near the interface with the bath. The phase grows too slow to prevent full infiltration of the refractory but creates an equilibrium state in the sealed off part of the sample, ceasing the chemical corrosion in that part of the sample.     

Nitrogen donor-controlled chemoselectivity of reaction in oxidation of sulfides with tetra-n-butylammonium hydrogen monopersulfate catalyzed by a partially β-brominated meso-tetraphenylporphyrinatomanganese(III) acetate: a clue to the nature of active oxidant

Highly efficient and rapid oxidation of different sulfides to the corresponding sulfones with tetra-n- butylammonium hydrogen monopersulfate (TBAO) in the presence of catalytic amounts of Mn(TPPBr₂)OAc at room temperature is reported. Contrary to other nitrogen donors, using 4-cyanopyridine as co-catalyst leads to an increase in the ratio of sulfoxide to sulfone in the products. Comparison of the chemoselectivity of reaction in the presence of different nitrogen donors as co-catalyst shows the involvement of a high-valent Mn-oxo species as well as the six-coordinate Mn(TPPBr₂)(HSO₅)(B) (B = nitrogen donors) complex in sulfide oxidation reactions with TBAO.