Oxidation of pressureless sintered Si2N2O materials

The oxidation behaviour of pressureless sintered Si₂N₂O materials prepared from both amorphous and crystalline starting powders has been examined. The materials exhibited excellent oxidation resistance at temperatures of up to 1350°C. Thin protective oxide scales formed which had a duplex morphology after long exposures to air at high temperatures. Substantial crystallisation of the intergranular glass phase with formation of Y₂Si₂O₇ occurred during oxidation at 1200°C and 1350°C. Catastrophic oxidation occurred at temperatures ≥ 1400°C. This behaviour is enhanced by an oxidation-induced shift in the composition of the material to a liquid-forming region in the Y-Si-Al-O-N system.     

Strength degradation of Si3N4---SiC-based ceramics in salt environments

The mechanism and kinetics of molten salt corrosion of reaction-sintered Si₃N₄---SiC-based ceramics, as well as the effect of these salts on its strength and fracture toughness at 20°C and 1200°C, were investigated. It was found that this material is highly resistant to corrosion in NaCl and sea salt up to 1100°C but degrades easily in Na₂SO₄ being a strong oxidant. Molten salt corrosion and salt-assisted oxidation cause mechanical properties' deterioration. Thus, NaCl effects involve up to 25% and 30% strength decrease at room temperature and at 1200°C, respectively. The results are explained by the etching of grain boundaries and by the formation of sodium silicate glassy layer during oxidation with it further cracking after cooling.     

F2, H2O, and O2 etching rates of diamond and the effects of F2, HF and H2O on the molecular O2 etching of (110) diamond

Oxidation kinetics of natural (110) diamond by oxygen and water were investigated using in situ Fizeau interferometry. Apparent activation energies of 53 and 26 kcal mol⁻¹ were obtained for the etching of (110) type Ia diamond by O₂ and H₂O respectively. The etch rate was found to follow second-order kinetics with respect to O₂ pressure in the pressure range 0.04–10 Torr. For water over the vapour pressure range 0.1–2 Torr, the reaction has a reaction order near unity. The diamond (110) surface was impervious to etching by molecular fluorine at all temperatures up to 1300 °C. Fluorine, hydrogen fluoride and water were found to inhibit the molecular oxygen etching of diamond. Below 900 °C, oxidation is inhibited by the addition of F₂ and HF presumably by blocking reactive sites on the diamond surface through formation of C---F bonds. Above 900 °C, the fluorine is thought to desorb from the diamond (110) surface, rendering the surface susceptible to further oxidation. Addition of water below 800 °C was found to retard etching by molecular oxygen. This is attributed to the formation of C---OH bonds, analogous to C---F.     

Porous mullite–ZrO2 composites from reaction sintering of zircon and aluminum

Mullite–zirconia porous bodies were prepared by reaction sintering of zircon and alumina derived from oxidation reaction of Al at sintering temperatures between 1200 and 1600 °C. The results show that the incorporation of TiO₂ improves the oxidation reaction of Al, dissociation of zircon subsequently formation of mullite and zirconia. Composites containing TiO₂ obtain a high tetragonal concentration at 1500 °C, which reduces by increasing sintering temperature to 1600 °C. No tetragonal zirconia phase was detected at 1500 °C in TiO₂-free composites while tetragonal concentration was increased over this temperature. The major oxidation reaction of Al proceeds with a liquid–gas mechanism that is suitable for producing low dense ceramics. In spite of the higher porosity of the composites containing TiO₂, they possess almost the same flexural strength values as obtained from the TiO₂-free composites.     

Carbon black oxidation in the presence of Al2O3, CeO2, and Mn oxide catalysts: An EPR study

Carbon black oxidation in the presence of CeO₂, Al₂O₃ and manganese oxide catalysts has been studied in tight contact conditions. In the presence of manganese based catalysts, the temperature gain is about 275 °C compared to the non-catalysed carbon black oxidation. The contribution of the manganese species to enhance the reactivity of carbon black oxidation has been evaluated by EPR technique. For Mn/Ce + CB mixtures the Mn²⁺ content considerably increases consequently to tight milled treatment indicating the reduction of some manganese species with higher oxidation states into Mn²⁺ ions. This phenomenon can be considered as the first step in the carbon black oxidation mechanism in the presence of Mn/Ce catalysts.     

Formation of Y3Al5O12–Al2O3 eutectic microstructure with off-eutectic composition

Homogeneous-eutectic microstructure of Y₃Al₅O₁₂–Al₂O₃ system without coarse primary crystals was formed at an off-eutectic composition. This method utilizes a low migration rate in an amorphous phase. A mixture of Y₂O₃ and Al₂O₃ having the off-eutectic composition was melted and quenched rapidly to form an amorphous phase. A heat-treatment of the amorphous phase at 1000 °C and 1300 °C for 30 min formed Y₃Al₅O₁₂ and Al₂O₃ phases. SEM observation of this material, which was formed from the amorphous phase at 1300 °C for 30 min, showed homogeneous eutectic-like microstructure. The formation of the primary crystals (coarse Al₂O₃), which are always observed in the off-eutectic compositions by ordinary method, was completely suppressed.     

The reaction sequence and dielectric properties of BaSm2Ti4O12 ceramics

To establish the correct reaction sequence of BaO–Sm₂O₃–4TiO₂, phases present in different calcining temperatures are identified by X-ray diffraction patterns. When different calcining temperatures are used, the source phase BaO (BaCO₃) consumes below 850°C, the source phases TiO₂ and Sm₂O₃ consume at 1000 and 1150°C; the intermediate phases BaTiO₃, BaTi₄O₉, and Sm₂Ti₂O₇ consume at 1050, 1200, and 1250°C, respectively. The BaSm₂Ti₄O₁₂ phase starts to reveal at the 1100°C-calcined powder. The integrating intensity of BaSm₂Ti₄O₁₂ phase increases with the raising of calcining temperatures, accompanying with the decrease of integrating intensities of the source and intermediate phases. As the sintering temperature increases, the densities, quality values, and dielectric constants of BaSm₂Ti₄O₁₂ ceramics increase and saturate at 1325^oC. The BaSm₂Ti₄O₁₂ ceramics sintered at 1325°C have the properties of Q^*f=5180,_r=81.8, and τ_f=−19.2 ppm/°C.     

Corrosion of selected ceramic materials in hot gas environment

The temperature dependence of the hot gas corrosion behaviour of various ceramic materials (Al₂O₃, ZrO₂ (Y-TZP), mullite, ZrSiO₄ and YAG) was investigated. The tests were performed in a high temperature burner rig at temperatures between 1200 °C and 1500 °C, a total pressure of 1 atm with a water vapour partial pressure of 0.24 atm, a gas flow velocity of 100 m/s and test times of about 130–300 h.

ZrO₂ (Y-TZP) showed absolutely no corrosion, however, a very high susceptibility to thermal shock and phase transformation was observed.

The other materials suffered degradation above 1300 °C. This was the consequence of the formation and evaporation of volatile hydroxides (e.g. Si(OH)₄ and Al(OH)₃). YAG showed a low corrosion rate and the formation of a protective surface layer. The corrosion susceptibility of these materials was found to be higher with increasing temperature.

Thermochemical calculations of the partial pressure of volatile species formed in reaction with water vapour, affirm the observed differences in corrosion behaviour.     

A study on mechanism of charging/discharging at amorphous manganese oxide electrode in 0.1 M Na2SO4 solution

In the present work, the mechanism of charging/discharging at the amorphous manganese oxide electrode was investigated in 0.1 M Na₂SO₄ solution with respect to amount of hydrates and valence (oxidation) states of manganese using a.c.-impedance spectroscopy, anodic current transient technique and cyclic voltammetry. For this purpose, first the amorphous manganese oxide film was potentiostatically electrodeposited, followed by heat-treatment at 25–400 °C to prepare the electrode specimen with different amounts of hydrates and oxidation states of manganese. For as-electrodeposited electrode with the most hydrates, the anodic current transient clearly exhibited a linear relationship between the logarithm of current density and the logarithm of time, with a slope of −0.5, indicating that the charging/discharging is purely limited by Na⁺/H⁺ ion diffusion. From the analyses of the impedance spectra combined with anodic current transients measured on the hydrated electrode heat-treated at 25–150 °C, it was found that as the amount of hydrates decreases, the depth of cation diffusion in the electrode becomes shallower and the ratio of charge-transfer resistance to diffusion resistance also increases. This suggests that a transition occurs of pure diffusion control to a mixed diffusion and charge-transfer reaction control. For the dehydrated electrode heat-treated at 200–400 °C, the charging/discharging purely proceeds by the charge-transfer reaction. The reversibility of the redox reaction increases with increasing amount of hydrates and oxidation states of manganese, which provides us the higher power density. On the other hand, the pseudocapacitance decreases in value with increasing heat-treatment temperature, thus causing the lower energy density.     

H4SiW12O40-catalyzed oxidation of nitrobenzene in supercritical water: kinetic and mechanistic aspects

The catalytic effect of a heteropolyacid, H₄SiW₁₂O_40, on nitrobenzene (20 and 30 μM) oxidation in supercritical water was investigated. A capillary flow-through reactor was operated at varying temperatures (T=400–500 °C; P=30.7 MPa) and H₄SiW₁₂O₄₀ concentrations (3.5–34.8 μM) in an attempt to establish global power-law rate expressions for homogenous H₄SiW₁₂O₄₀-catalyzed and uncatalyzed supercritical water oxidation. Oxidation pathways and reaction mechanisms were further examined via primary oxidation product identification and the addition of various hydroxyl radical scavengers (2-propanol, acetone, acetone-d₆, bromide and iodide) to the reaction medium. Under our experimental conditions, nitrobenzene degradation rates were significantly enhanced in the presence of H₄SiW₁₂O₄₀. The major differences in temperature dependence observed between catalyzed and uncatalyzed nitrobenzene oxidation kinetics strongly suggest that the reaction path of H₄SiW₁₂O₄₀-catalyzed supercritical water oxidation (average activation E_a=218 kJ/mol; k=0.015–0.806 s⁻¹ energy for T=440–500 °C; E_a=134 kJ/mol for the temperature range T=470–490 °C) apparently differs from that of uncatalyzed supercritical water oxidation (E_a=212 kJ/mol; k=0.37–6.6 μM s⁻¹). Similar primary oxidation products (i.e. phenol and 2-, 3-, and 4-nitrophenol) were identified for both treatment systems. H₄SiW₁₂O₄₀-catalyzed homogenous nitrobenzene oxidation kinetics was not sensitive to the presence of OH√ scavengers.     

Effects of deposition temperature on the crystallinity of Ba0.5Sr0.5TiO3 epitaxial thin films integrated on Si substrates by pulsed laser deposition method

Epitaxial Ba_0.5Sr_0.5TiO₃ (BSTO) thin films were grown on TiN buffered Si (0 0 1) substrates by PLD method and the effects of deposition temperature on their crystallinity and microstructure were studied. BSTO thin films were prepared with substrate temperature ranging from 350 to 650 °C. The BSTO films grown at below 400 °C showed amorphous phase and the film grown at 450 °C showed mixed phase of crystalline and amorphous, where crystalline phase was observed only at the top surface portion of the film. The BSTO films with fully crystalline phase were obtained in the samples deposited at above 500 °C. The (0 0 l) preferred orientation and the crystallinity of the BSTO films were improved with increasing the temperature. The dielectric constant, measured at 100 kHz and at room temperature, of the BSTO film grown at 650 °C was measured to be as high as 1129.     

Selective oxidation of H2S in Claus tail-gas over SiC supported NiS2 catalyst

Very high activity and selectivity could be achieved for the direct oxidation of H₂S into elemental sulfur at low reaction temperature (40–60°C), on nickel sulfide supported SiC catalyst. The heterogeneous nature of the support surface (hydrophilic and hydrophobic areas) could explain the important role played by water to maintain a high and stable H₂S conversion level. The formation of a very active superficial nickel oxysulfide phase was proposed in order to explain the activation period necessary at reaction temperatures <60°C. Total selectivity for sulfur was attributed to the very low reaction temperature and the absence of any microporosity in the support.     

Internal friction behavior of an unidirectionally solidified Al2O3/YAG eutectic at high temperature

The internal friction behavior of a unidirectionally solidified Al₂O₃/YAG eutectic was examined between room temperature and 1400 °C. No internal friction was observed up to 1200 °C. Above 1200 °C, the internal friction drastically increased with increasing temperature and the number of torsional loading cycles. For the 1400 °C test, the internal friction gradually increased with the number of loading cycles and then saturated after 10³ cycles.     

DTA characterisation of three types of Al2O3SiO2 gels made from TEOS-Al (OBu)3 mixture with variation of water

Mixtures of TEOS and Al (OBu)₃ had been hydrolysed by varying the water contents at a fixed acid concentration. This led to the formation of three different kinds of gels. Diphasic gel formed when excess of water was used, which transformed to mullite at 1320 °C in exothermic reaction. When the amount of water content was reduced, the resultant monophasic gel completed its mullitization at lower temperatures from an amorphous aluminosilicate phase and cubic mullite by two paths at 1150 °C and 1250 °C exothermic reactions, respectively. When less water is used, the monophasic gel converted directly to tetragonal mullite at 980 °C in exothermic reaction. Thus, the variation of water contents during the gelation caused great variations in the course of mullitization processes.     

Thermal conductivity of AlN ceramics sintered with CaF2 and YF3

Dense AlN ceramics with a thermal conductivity of 180W/m·K were obtained at the sintering temperature of 1750 °C using CaF₂ and YF₃ as additives. At temperatures below 1650 °C, the shrinkage of AlN ceramics is promoted by liquid (Ca,Y)F₂ and Ca₁₂Al₁₄O₃₂F₂. Liquid CaYAlO₄ mainly improves the densification of the sample when the sintering temperature increases to 1750 °C. The formation of liquid (Ca,Y)F₂ at a relatively low temperature results in homogeneous YF₃ distribution around the AlN particles, which benefits the removal of oxygen impurity in the AlN lattice, and thus a higher thermal conductivity.     

Selective catalytic reduction of NOx with NH3 over Cu-ZSM-5—The effect of changing the gas composition

The selective catalytic reduction of nitrogen oxides (NO_x) with ammonia over ZSM-5 catalysts was studied with and without water vapor. The activity of H-, Na- and Cu-ZSM-5 was compared and the result showed that the activity was greatly enhanced by the introduction of copper ions. A comparison between Cu-ZSM-5 of different silica to alumina ratios was also performed. The highest NO conversion was observed over the sample with the lowest silica to alumina ratio and the highest copper content. Further studies were performed with the Cu-ZSM-5-27 (silica/alumina = 27) sample to investigate the effect of changes in the feed gas. Oxygen improves the activity at temperatures below 250 °C, but at higher temperatures O₂ decreases the activity. The presence of water enhances the NO reduction, especially at high temperature. It is important to use about equal amounts of nitrogen oxides and ammonia at 175 °C to avoid ammonia slip and a blocking effect, but also to have high enough concentration to reduce the NO_x. At high temperature higher NH₃ concentrations result in additional NO_x reduction since more NH₃ becomes available for the NO reduction. At these higher temperatures ammonia oxidation increases so that there is no ammonia slip. Exposing the catalyst to equimolecular amounts of NO and NO₂ increases the conversion of NO_x, but causes an increased formation of N₂O.     

Selective oxidation of H2S to elemental sulfur over chromium oxide catalysts

CrO_x and CrO_x supported on SiO₂ have been found to be active for the selective oxidation of hydrogen sulfide to elemental sulfur. The catalysts show maximum sulfur yield at a stoichiometric ratio of O₂/H₂S, 0.5. Amorphous Cr₂O₃ exhibits higher yield of sulfur and has stronger resistance against water than supported Cr/SiO₂, especially at low temperatures. At high temperatures above 300°C, the sulfur yield over the supported catalyst becomes similar to amorphous Cr₂O₃ because the Claus reaction occurring on the silica support removes SO₂ to increase the sulfur yield. Active sites are the amorphous monochromate species that can be detected as a strong temperature programmed reduction (TPR) peak at 470°C. Catalytic activity can be correlated with the amount of labile lattice oxygen and the strength of Cr–O bonding. The reaction proceeds via the redox mechanism with participation of lattice oxygen.     

Thermal evolution of Fe2O3-TiO2 sol-gel thin films

Fe₂O₃-TiO₂ thin films were deposited on silica glass slides using three sol-gel solutions containing Ti-butoxide and different iron oxide precursors. The thermal evolution of the coatings was followed by DTA-TGA and XRD from 200 to 1000°C. All the iron-containing samples were amorphous up to 700°C, when a phase separation between iron and titanium oxides was evidenced by the presence of crystalline Fe₂O₃. Above that temperature a titanate compound (pseudobrookite) formed according to the equilibrium phase diagram. The use of different precursors did not affect the thermal evolution and all the different samples exhibited the same trend.     

The NO selective reduction on the La1−xSrxMnO3 catalysts

La_1−xSr_xMnO₃ (x=0, 0.1, 0.3, 0.5, 0.7) perovskite-type oxides (PTOs) were prepared by coprecipitation under various calcination temperature, and their performances for the NO reduction were evaluated under a simulated exhaust gas mixture. The X-ray diffraction (XRD) and thermogravimetric analysis were carried out to find the formation process of the perovskite. The NO reduction rate under different reaction temperature, the concentration of oxygen and the presence of hydrocarbon were observed by the input/output analysis. In the presence of 10% excess oxygen, the catalyst La_0.7Sr_0.3MnO₃ calcined at 900 °C showed a NO reduction rate of 61% at 380 °C. The study of the reaction curves showed that C₃H₈ could act as the reducer for the NO reduction below 400 °C. The NO reduction is highly affected by increasing the O₂ concentration from 0.5 to 10%, especially at high temperatures when oxygen becomes more competitive than NO on the oxidation of C₃H₈, leading to a decrease of the NO reduction from 100% to zero at 560 °C.     

Mechanochemical synthesis and thermal evolution of La–ZrO2 cubic solid solutions

The formation of cubic solid solutions in the system La₂O₃–ZrO₂ by mechanochemical activation of a mixture of the oxides (molar ratio ZrO₂ 82%–La₂O₃ 18%) is studied. After 6 h of activation at room temperature, a poorly crystalline cubic solid solution is formed, with ultimate crystallite sizes in the nanometer range. The mixtures activated during 1–3 h form the solid solution on subsequent heating at 1000 °C, while the non-activated mixture does not react, even after thermal treatment at 1200 °C. The solid solution obtained at room temperature undergoes partial structural ordering at temperatures between 800 and 1000 °C. Long time heating at temperatures of 1000 °C and above results in the formation of La₂Zr₂O₇ and rejection of the excess ZrO₂. Mechanochemical activation offers interesting possibilities for the synthesis of these materials at temperatures lower than those used in conventional processing, and for the control of their physicochemical and microstructural properties.