Studies on thermal decomposition and oxidation of CoSb<Subscript>3</Subscript>

Thermal durability of CoSb₃ in vacuum, helium and air was investigated over the temperature range of 20–850 °C. A scanning electron microscope (SEM) and X-ray powder diffraction technique were used to investigate the microstructure and to carry out the phase analysis of the degradation products. The analysis of a non-isothermal and isothermal decomposition and oxidation of the CoSb₃ powders and polycrystalline samples were performed using simultaneous TG/DTA technique. More detailed studies were carried out on the oxidation in the temperature range 400–600 °C. It was established that the decomposition products are CoSb₂ and a volatile antimony. In case of oxidation a complex three-layered scale is formed, consisting of CoSb₂O₄, CoSb₂O₆ and Sb₂O₄ layers. Both the decomposition and the oxidation kinetics are determined by a diffusion through the growing layers of products. The electrical resistivity and Seebeck coefficient measurements have been also performed on pure and oxidized samples, which showed a large influence of the oxidation on the electrical properties.     

Chlorine contaminants poisoning of solid oxide fuel cells

Investigation has been conducted on the poisoning effect of various contaminants containing chlorine at ppm level (<10 ppm) on the performance of Ni-YSZ anode-supported solid oxide fuel cells. The results indicate that cell performance drops by exposure to 1 ppm Cl₂(g) at 750 °C, whereas the introduction of Cl₂(g) with concentration higher than 5 ppm causes only a slight degradation at 850 °C. The presence of 2–6 ppm CH₃Cl(g) and C₂H₃Cl(g) can also induce measurable cell performance decline at 750 and 850 °C and this deterioration cannot be completely removed after switching to pure fuel at 850 °C. No performance loss is found when the cell is operated in fuel containing 1–8 ppm HCl(g) at 750 and 850 °C. It is thus concluded that chlorine in the form of Cl₂(g) yields the largest poisoning effect at 750 °C, while the degradation rate caused by addition of C₂H₃Cl(g) increases with the increase of operation temperature. Agglomerations at anodic region are observed in the samples after poisoning test by Cl₂(g), CH₃Cl(g), and C₂H₃Cl(g), but the anode microstructure is uniform for the sample exposed to HCl(g) for poisoning test.     

Preparation and characterization of CuAlO2 transparent thin films prepared by chemical solution deposition method

CuAlO₂ thin films were prepared on quartz glass and sapphire substrates by chemical solution deposition method using copper acetate monohydrate, aluminum nitrate nonahydrate and 2-methoxyethanol as starting precursor and solvent. The effects of annealing temperature on the structural, morphological, electrical and optical properties have been studied. Via the optimized annealing treatment condition, CuAlO₂ film annealed at 850 °C in nitrogen flow of 400sccm under atmosphere pressure exhibits the best performance with the lowest room temperature resistivity of 3.6 × 10² Ω cm and the highest optical transmission in the visible region (>70% at around 600 nm wavelength). CuAl₂O₄ and CuO phases, not CuAlO₂ phase are obtained when annealing temperature is lower than 850 °C. However, a further increase of annealing temperature weakens the crystallization quality and deteriorates the surface morphology of CuAlO₂ films as the annealing temperature exceeds 850 °C, leading to an increase in the resistivity and a decrease of the optical transmission in the visible region of CuAlO₂ films.     

Synthesis of high surface area transitional alumina from Al(OPh)<Subscript>3</Subscript>

Al(OPh)₃ involving sterically hindered phenyl groups on ultrasonic assisted micro hydrolysis yielded a mixture of boehmite and bayerite as deduced from the FTIR and powder X-ray diffraction pattern. In the thermogravimetric trace, the complete removal of decomposable moieties of the hydrolyzed gel occurred around 530 °C. Calcining the gel at temperatures 600, 700, 800 and 900 °C showed crystalline tetragonal δ-Al₂O₃ to be the product at 900 °C as deduced from FTIR, ²⁷Al NMR and PXRD techniques. δ-Al₂O₃ showed a surface area of 135 m²/g with rectangular bar like morphology with the sizes below 50 nm in the TEM images.     

Evaluation of thermodynamic and kinetic stability of CuAlO<Subscript>2</Subscript> and CuGaO<Subscript>2</Subscript>

Thermodynamic and kinetic stabilities of CuAlO₂ and CuGaO₂ have been evaluated by using thermogravimetry and thermodynamic calculations. It has been revealed that CuAlO₂ and CuGaO₂ are not thermodynamically stable in air below 800 °C and 1,200 °C, respectively, and that the oxidation reaction, 4CuMO₂ + O₂ → 2CuO + 2CuM₂O₄ (M = Al, Ga), should occur if the reaction kinetics are high enough. However, rate constants and activation energies indicated slow kinetics of the oxidation reaction, showing kinetic stability of CuMO₂ even under some thermodynamically unstable temperatures and atmospheres. It was also concluded that CuAlO₂ showed higher thermodynamic and kinetic stability than CuGaO₂.     

Oxidation kinetics of nickel nano crystallites obtained by controlled thermolysis of diaquabis(ethylenediamine)nickel(II) nitrate

The metal complex, [Ni(en)₂(H₂O)₂](NO₃)₂ (en = ethylenediamine), was decomposed in a static furnace at 200 °C by autogenous decomposition to obtain phase pure metallic nickel nanocrystallites. The nickel metal thus obtained was studied by XRD, IR spectra, SEM and CHN analysis. The nickel crystallites are in the nanometer range as indicated by XRD studies. The IR spectral studies and CHN analyses show that the surface is covered with a nitrogen containing species. Thermogravimetric mass gain shows that the product purity is high (93%). The formed nickel is stable and resistant to oxidation up to 350 °C probably due to the coverage of nitrogen containing species. Activation energy for the oxidation of the prepared nickel nanocrystallites was determined by non-isothermal methods and was found to depend on the conversion ratio. The oxidation kinetics of the nickel crystallites obeyed a Johnson–Mehl–Avrami mechanism probably due to the special morphology and crystallite strain present on the metal.     

Oxidation behavior of rare earth oxide-coated Fe20Cr and Fe20Cr5Al alloys

This article presents the influence of surface additions of nanocrystalline rare earth (RE) oxides CeO₂, La₂O₃, and CeO₂ + La₂O₃ on the isothermal oxidation behavior of Fe20Cr and Fe20Cr5Al at 1000 °C. Thermogravimetric studies revealed parabolic kinetics in all cases and the scale thickness on specimen surfaces varied with the nature of RE oxide. The oxidation resistance of specimens coated with two RE oxides was significantly higher than those coated with either one of the two oxides. The marked increase in the oxidation resistance of the alloys coated with two RE oxides is due to optimization of RE ion radius and RE oxide grain size/shape.     

SrSnO<Subscript>3</Subscript>:Nd obtained by the polymeric precursor method

In this work, the synthesis of Nd-doped SrSnO₃ by the polymeric precursor method, with calcination between 250 and 700 °C is reported. The powder precursors were characterized by TG/DTA and high temperature X-ray diffraction (HTXRD). After heat treatment, the material was characterized by XRD and infrared spectroscopy. Ester and carbonate amounts were strictly related to Nd-doping. According to XRD patterns, the orthorhombic perovskite was obtained at 700 °C for SrSnO₃ and SrSn_0.99Nd_0.01O₃. For Sr_0.99Nd_0.01SnO₃, the kinetics displayed an important hole in the crystallization process, as no peak was observed in HTXRD up to 700 °C, while a XRD patterns showed a crystalline material after calcination at 250 °C.     

In-situ electrochemical study of chalcopyrite pressure oxidation leaching from 110 °C to 150 °C under saturated vapor pressure

Pressure oxidation leaching behavior of chalcopyrite in sulfuric acid solution from 110 °C to 150 °C were investigated by in-situ electrochemical methods. Leaching experiments under saturated vapor pressure conditions were used to simulate the anoxic environment that may be encountered in industrial applications. Scanning electron microscope and X-ray photoelectron spectroscopy were used to characterize the morphology and the chemical status of chalcopyrite surface. Results show that the copper extraction was increased with the increase of leaching temperature. Under the optimal leaching conditions under saturated vapor pressure, the copper and iron extraction are 8.3% and 29.8%, respectively. When the temperature increased from 110 °C to 150 °C, the self-corrosion potential and electrochemical reaction resistance firstly increased and then decreased. In contrast, the resistance of the passive film was always increased with the increase of temperature. The electrochemical study results indicated that the increase in temperature affected the oxidation of chalcopyrite by altering the kinetics of the cathodic reaction and the anodic passivation. Both the self-corrosion current density (i_corr) and rate constant were affected by the reduction of Fe(III). The XPS results show that elemental sulfur and H₃O(Fe₃(SO₄)₂(OH)₆) were the main leaching solid products. The formation of H₃O(Fe₃(SO₄)₂(OH)₆) not only caused a decrease in cathodic reaction kinetics, but also increased the resistance of mass transfer process. Due to the faster release of iron, copper-rich sulphides were formed, which mixed with the elemental sulfur and/or H₃O(Fe₃(SO₄)₂(OH)₆) led to coverage of the chalcopyrite surface.     

Dielectric and magnetic properties of multiferroic BiFeO3 ceramics sintered with the powders prepared by hydrothermal method

BiFeO₃ ceramics were sintered in the temperature range of 700–900 °C by using the pure BiFeO₃ powders hydrothermally synthesized at 250 °C. The low reaction temperature and low sintering temperature prevent the element volatilization and phase decomposition. The ceramics sintered at 800 and 850 °C exhibit much dense microstructure with clear grains and grain boundaries. They also show high dielectric constant, dielectric dispersion and low loss tangent. At room temperature, the dielectric behaviors of BiFeO₃ ceramics are mainly attributed to the transition of localized charge carriers and the microstructure of grains and grain boundaries. The temperature dependence of dielectric constant and loss tangent confirms that the localized charge carriers are a main contribution to the dielectric permittivity. Activation energy E_α of relaxation process for the BiFeO₃ ceramic sintered at 850 °C is 0.397 eV. The obtained BiFeO₃ ceramics show magnetic responses, which are relative to the grain size.     

Zinc coatings for oxidation protection of ferrous substrates

A common phenomenon in the process industries is the oxidation of the exterior surface of steel pipes used in superheated steam or hot oils networks. For their protection different coatings could be used. In the present work the performance of zinc coatings deposited with hot-dip galvanizing, pack cementation and thermal spraying was considered, in order to protect industrial equipment up to 400°C. For that purpose coated carbon steel coupons were exposed at 400°C and their behavior was examined with light microscopy, scanning electron microscopy and X-ray diffraction. Thermogravimetric analysis was also used in order to observe in situ the oxidation phenomena. From this investigation it was deduced that in every coating a scale is formed that is mainly composed of ZnO, while Fe oxides were also detected in galvanized and pack coatings. The growth of this scale took place at the metal/scale interface. Moreover, as far as it regards the kinetics of the oxidation, it was concluded that the increase of the mass of the specimens is a function of the square root of the exposure time, which means that the scale formed is rather protective for the underlying zinc. From the above observation it seems that the behavior of zinc coatings would be excellent at 400°C. However, the presence of the Fe/Zn phases inside the galvanized and pack coatings led to the formation of cracks, which could expose the substrate and thus destabilize the coating. This phenomenon does not take place in the thermal sprayed coatings, where the Fe/Zn phases are absent.     

Green micellar stability-indicating high-performance liquid chromatography method for determination of rupatadine fumarate in the presence of its main impurity desloratadine: Oxidative degradation kinetics study

A green micellar stability-indicating high-performance liquid chromatography method was developed for rupatadine fumarate determination in existence with its main impurity desloratadine. Separation was attained using Hypersil ODS column (150 × 4.6 mm, 5 μm), the micellar mobile phase consisted of 0.13 M sodium dodecyl sulfate, 0.1 M disodium hydrogen phosphate adjusted by phosphoric acid to pH 2.8 and 10% n-butanol. The column was maintained at 45^◦C and detection was carried out at 267 nm. A linear response was achieved over the range of 2–160 μg/ml for rupatadine and 0.4–8 μg/ml for desloratadine. The method was applied for rupatadine determination in alergoliber tablets and alergoliber syrup without the interference of methyl paraben and propyl paraben present as main excipients. Rupatadine fumarate revealed pronounced susceptibility to oxidation; further study of oxidative degradation kinetics was carried out. Rupatadine was found to follow pseudo-first-order kinetics when exposed to 10% H₂O₂ at 60 and 80°C and the activation energy was found to be 15.69 Kcal/mol. At a lower temperature (40°C), degradation kinetics regression was best fitted as a polynomial quadratic relationship, thus rupatadine oxidation at a lower temperature tends to adopt a second-order kinetics rate. Oxidative degradation product structure was revealed using infrared and found to be rupatadine N-oxide at all temperature values.     

Methane-fueled SOFC with traditional nickel-based anode by applying Ni/Al2O3 as a dual-functional layer

Novel γ-Al₂O₃ supported nickel (Ni/Al₂O₃) catalyst was developed as a functional layer for Ni–ScSZ cermet anode operating on methane fuel. Catalytic tests demonstrated Ni/Al₂O₃ had high and comparable activity to Ru–CeO₂ and much higher activity than the Ni–ScSZ cermet anode for partial oxidation, steam and CO₂ reforming of methane to syngas between 750 and 850 °C. By adopting Ni/Al₂O₃ as a catalyst layer, the fuel cell demonstrated a peak power density of 382 mW cm^?2 at 850 °C, more than two times that without the catalyst layer. The Ni/Al₂O₃ also functioned as a diffusion barrier layer to reduce the methane concentration within the anode; consequently, the operation stability was also greatly improved without coke deposition.     

Oxidation behaviour of precipitation hardened steel TG,X-Ray,XRD and SEM study

In the present work the oxidation behavior of ageing treated steel was examined up to 1000°C in different environments (O₂ and CO₂) and with different heating rates. The examination was conducted by means of thermogravimetric analysis, scanning electron microscopy and X-ray diffraction. In this study it was deduced that in the case of O₂ an oxide scale is formed on top of the steel. The oxidation is uniform and the growth of the scale is more intent at low heating rate. It consist of different Fe, Mn, Mo and Cr oxides which are adjusted in the form of layers. This phenomenon was explained by the different diffusion coefficients of each metal in the already formed scale. Regarding the oxidation in CO₂, the scale formation takes place at a lower temperature than in the case of O₂. Hence the examined substrate is more vulnerable in CO₂.     

Effect of supercritical carbon dioxide treatment on structure and mechanical properties of β-nucleated polypropylene processed at different temperatures

Polypropylene (PP) was compounded with β-nucleating agent and injection-molded at 180 °C or 220 °C. The samples were subsequently treated by supercritical carbon dioxide (scCO₂) at different temperatures. Results show that processing temperature and scCO₂ treatment could strongly influence the tensile and impact properties of β-nucleated PP. In particular, the sample processed at 220 °C and treated at 120 °C exhibit much enhanced impact strength (4.8 times that of its untreated counterpart). FTIR, WAXD, SEM and DMA were performed to explore the effects of processing temperature and scCO₂ treatment on structure of the samples. Deformation-induced plastic flow and micro-voids were also evaluated to construct structure-property relations. It was found that the influence of processing temperature on mechanical properties is mainly associated with the β-form content and β-crystalline morphology, while the structural changes in the crystalline lamellar scale may be responsible for the toughening effect of scCO₂ treatment.     

Reactivity of a Ti–45.9Al–8Nb alloy in air at 700–900°C

A Ti–45.9Al–8Nb (at%) alloy with a lamellar structure (γ+α₂) was oxidised in air at 700, 800, 850 and 900°C in isothermal and thermal cycling conditions. The reaction progress was followed by thermogravimetric measurements. In isothermal conditions the oxidation kinetics followed approximately a parabolic rate law and the rate constants ranged from about 10^–12 kg² m^–4 s^–1 at 700°C to 10^–10 kg² m^–4 s^–1 at 900°C. The oxide scales were built of Al₂O₃ and TiO₂, the former being the main component of the outermost layer. The oxidation behaviour of Ti–45.9Al–8Nb was referred to a commercial titanium alloy, WT4 (Ti–6Al–1Mn), and selected oxidation-resistant alloys.     

Synthesis,characterization and kinetic computations of fullerene (C60)–CuO on the mechanism decomposition of ammonium perchlorate

CuO, C₆₀–CuO, and Al/C₆₀–CuO nanostructures were synthesized and characterized by scanning electron microscope (SEM)/energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and fourier transform infrared spectroscopy (FTIR). differential scanning calorimetry (DSC)/thermogravimetric analysis (TGA) measurements were performed to study the influence of these additives on ammonium percolate (AP) thermal decomposition. From the comparison of DSC and TGA plots, the catalytic effect of CuO and C₆₀–CuO has been clearly noticed in which the lower temperature decomposition of AP was decreased from 331 °C to 315 °C, 310 °C, and 303 °C (in the presence of CuO, C₆₀–CuO, and Al/C₆₀–CuO, respectively) and the HTD was dropped from 430 °C (pure AP) to 352 °C, 335 °C, and 317 °C (for the compounds AP/CuO, AP/C₆₀–CuO, and AP/Al/C₆₀–CuO, respectively). The kinetics of the samples were investigated by isoconversional models and compared with an iterative procedure. The results of pure AP indicated a complex decomposition process involving three decomposition steps with specific reaction mechanism. The nanocatalysts incorporated in the AP have clearly affected its decomposition process in which the reaction mechanism and the number of stages were changed.     

High-temperature oxidation behaviour of low-entropy alloy to medium- and high-entropy alloys

The high-temperature oxidation behaviour of CoCrNi, CoCrNiMn, and CoCrNiMnFe equimolar alloys was investigated. All three alloys have a single-phase face-centred cubic structure. Thermogravimetric analyses (TGA) were conducted at temperatures ranging from 800 to 1000 °C for 24 h in dry air. The kinetic curves of the oxidation were measured by TGA, and the microstructure and chemical element distribution in different regions of the specimens were analysed. The oxidation kinetics of the three alloys followed the two-stage parabolic rate law, with rate constants generally increasing with increasing temperature. CoCrNi displayed the highest resistance to oxidation, followed by CoCrNiMnFe and CoCrNiMn exhibiting the least resistance to oxidation. The addition of Mn to CoCrNi increased the oxidation rate. The oxidation resistance of CoCrNiMn was enhanced by the addition of Fe. Less Mn Content and the formation of more Cr₂O₃ were responsible for the reduction in the oxidation rates of CoCrNiMnFe. The calculated activation energies of CoCrNiMn and CoCrNiMnFe at 800, 850 and 900 °C were 108 and 137 kJ mol^?1, respectively, and are comparable to that of Mn diffusion in Mn oxides. The diffusion of Mn through the oxides at 800–900 °C is considered to be the rate-limiting process. The intense diffusion of Cr at 1000 °C contributed to the formation of CrMn_1.5O₄ spinel with Mn in the outer layer of CoCrNiMn and Cr₂O₃ in the outer layer of CoCrNiMn.     

On the oxidation behaviour of monolithic TiB2 and Al2O3-TiB2 and Si3N4-TiB2 composites

In view of the susceptibility of TiB₂ to oxidation, the thermal stability of monolithic TiB₂ and of Al₂O₃-30 vol% TiB₂ and Si₃N₄-20 vol% TiB₂ composites was investigated. The temperature at which TiB₂ ceramic starts to oxidize is about 400°C, oxidation kinetics being controlled by diffusion up toT≈900°C and in the first stage of the oxidation at 1000°C and 1100°C (up to 800 min and 500 min respectively), and by a linear law at higher temperatures and for longer periods. Weight gains in the Al₂O₃-TiB₂ composite can be detected only at temperatures above ≈700°C and the rate governing step of the oxidation reaction is characterized by a one-dimensional diffusion mechanism atT=700°C andT=800°C and by two-dimensional diffusion at higher temperatures. Concerning the Si₃N₄-TiB₂ composite, three different oxidation behaviours related to the temperature were observed, i.e. up to ≈1000°C the reaction detected regards only the second phase; at ≈1000<T<≈1200°C, the diffusion of O₂ or N₂ through an oxide layer is proposed as the rate-governing step; atT〉=1200°C, a linear kinetic indicates the formation of a non protective scale.     

TEM and SNMS studies on the oxidation behaviour of NiCrAlY-based coatings

The effect of minor Ti additions content (0% Ti, 0.4% Ti, 1% Ti, 2% Ti) on the oxidation behaviour of Ni-20Cr-10Al-0.4Y (in weight-%) model alloys was investigated in the temperature range 950° C to 1100° C up to 200 h in Ar – 20% O₂. Alloy microstructure, oxide scale morphology and microstructure of the scale were characterized by SEM/EDX and TEM. The growth mechanisms of the alumina scales formed on the model alloys were studied by two-stage oxidation experiments with ¹⁸O₂-tracer and subsequent SNMS-analyses. The microstructural observations were correlated with the oxide scale properties in respect to growth rates and spalling resistance, which was tested during cyclic oxidation. Received: 24 June 1996 / Accepted: 18 November 1996