Isothermal oxidation behavior of electrospark deposited MCrAlX-type coatings on a Ni-based superalloy

A MCrAlX-type coating has been prepared by electrospark deposition (ESD) and its isothermal oxidation behavior studied. The results indicate that deposition rate and surface roughness of the coatings increase with increasing spark pulse energy. A splattered porous morphology was observed in the surface layer, and underneath this, a uniform superfine columnar γ phase structure with a column width of about 0.6 μm. When exposed at 1000 °C, θ-Al₂O₃ formed rapidly in the early oxidation stage. After 100 h oxidation, a large amount of θ-Al₂O₃ was still present, and a dense and adherent, thin α-Al₂O₃ scale had formed beneath it.     

Synthesis and characterization of graft copolymers of melamine: Thermal stability, electrical conductivity, and optical properties

In this study, novel three Schiff bases of melamine were synthesized via condensation reaction of melamine with salicylaldehyde, 3-hydroxybenzaldehyde, and 4-hydroxybenzaldehyde namely N,N′,N′′,-tris[(2-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (2-HPMTT), N,N′,N′′-tris[(3-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (3-HPMTT), N,N′,N′′-tris[(4-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (4-HPMTT), respectively. Then, oligo/polyphenol derivatives of these Schiff bases were obtained by grafting melamine onto oligosalicylaldehyde (OSA), oligo-3-hydroxybenzaldehyde, and oligo-4-hydroxybenzaldehyde that have generate names of poly-N,N′,N′′-tris[(2-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (P-2-HPMTT), poly-N, N′,N′′-tris[(3-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (P-3-HPMTT), and oligo-N,N′,N′′-tris[(4-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (O-4-HPMTT), respectively. The structures of the synthesized compounds were confirmed by FT-IR, UV–vis, ¹H NMR, and ¹³C NMR techniques. The characterization was made by TG-DTA, DSC, size exclusion chromatography (SEC), and solubility tests. Electrical conductivities of the synthesized materials were measured by four-point probe technique using a Keithley 2400 electrometer showing that the synthesized oligo/polyphenols have higher electrical conductivities than the monomeric Schiff bases. Also considerable increases in the conductivities were observed when they were doped with iodine as a doping agent. The order of increase rates of the conductivities were found as follows: 2-HPMTT > P-2-HPMTT > P-3-HPMTT > 3-HPMTT > 4-HPMTT > P-4-HPMTT. Additionally, the optical band gaps (E_g) were calculated by using the absorption spectra and found to be 2.78, 2.17, 3.58, 3.30, 4.03, and 2.82 eV for 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT, respectively.     

Synthesis of crystalline γ-Al2O3 with high purity

Crystalline γ-AlO(OH) was synthesized by the precipitation of sodium aluminate and oxalic acids in aqueous solution. And then γ-AlO(OH) was successfully transferred to γ-Al₂O₃ after subsequent high temperature heat treatment. The effects of reaction conditions on formation of γ-AlO(OH) and γ-Al₂O₃ were further investigated in detail. The XRD analysis shows that the complete formation of crystalline γ-Al₂O₃ is at pH 8–9, reaction temperature of 93–96 °C and calcination temperature of higher than 400 °C. The product of γ-Al₂O₃ contains impurity, including iron, calcium and silicon ion with a low content of about 0.01% and has large specific surface area and high pore volume of 269.9 m²/g and 0.57 mL/g, which can be applied in catalysts and catalyst supports.     

High-temperature oxidation of Fe3Al-based iron aluminides in oxygen

The isothermal high-temperature oxidation behavior of Fe₃Al-based iron aluminides in oxygen has been studied. Fe–25Al was oxidized at 1225, 1330, 1425 and 1530 K, while Fe–28Al, Fe–24Al–5Cr, Fe–24Al–5Ti, Fe–28Al–2Cr and Fe–30Al–4Cr (all compositions in atom percent) were oxidized at 1330 K. The weight gain data were analyzed and rate constants (k_p) determined by assuming a parabolic rate law. The variations of instantaneous parabolic rate constant with time reflected the complexity of the oxidation behavior. These have been attributed to the changes taking place in the nature and properties of the scale as a function of time. The values of k_p for oxidation of Fe₃Al were one to two orders of magnitude lower than those for Ti₃Al-based intermetallics. As revealed by X-ray diffraction, the scale formed on Fe–25Al was predominantly α-Al₂O₃ at higher temperatures, while θ-Al₂O₃ was observed after oxidation at lower temperatures. The observed kinetics matched with α-Al₂O₃-formation kinetics at higher temperatures and θ-Al₂O₃-formation kinetics at lower temperatures. For all the other intermetallics, only α-Al₂O₃ was identified at 1330 K. The whisker morphology of θ-Al₂O₃ and the ridged morphology of α-Al₂O₃ were confirmed by scanning electron microscopy. Alloying with Cr or Ti increased the oxidation rate of iron aluminides, especially during the initial stages. Addition of Ti changed the nature, color, and morphology of the scale, leading to improved adherence.     

Improved high-Q microwave dielectric resonator using ZnO-doped La(Co1/2Ti1/2)O3 ceramics

The microwave dielectric properties and the microstructures of ZnO-doped La(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. Doped with ZnO (up to 0.75 wt%) can effectively promote the densification of La(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. At 1320 °C, La(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt% ZnO addition possesses a dielectric constant (_r) of 30.2, a Q × f value of 73,000 GHz (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −35 ppm/°C.     

Stabilization of supported platinum nanoparticles on γ-alumina catalysts by addition of tungsten

The thermal stabilization of Al₂O₃ using W⁶⁺ ions has been found useful to the synthesis of Pt/Al₂O₃ catalysts. The simultaneous and sequential methods were used to study the effect of W⁶⁺ upon Pt/γ-Al₂O₃ reducibility, Pt dispersion, and benzene hydrogenation. The W/Pt atomic ratios were from 0.49 to 12.4. In the first method we found that the W⁶⁺ ions delayed reduction of a fraction of Pt⁴⁺ atoms beyond 773 K. At the same time, W⁶⁺inhibited sintering of the metallic crystallites once they were formed on the surface. For the sequential sample with a W/Pt atomic ratio of 3.28 W⁶⁺ did not inhibit the H₂ reduction of Pt oxides even below of 773 K, the Pt oxides were reduced completely. After reduction at 1073 K, sequential samples impregnating Pt on WO_x–γ-Al₂O₃ were more active and stable during benzene hydrogenation. TOF of the reaction did not change when the W/Pt atomic ratio, preparation technique and reduction temperature changed and its value was of 1.1 s⁻¹. W⁶⁺ ions promoted high thermal stability of Pt crystallites when sequential catalysts were reduced at 1073 K and decreased their Lewis acidity_.     

Structural evolution of alumina membrane prepared on an alumina support using a sol–gel method

We prepared supported and unsupported alumina membranes using a sol–gel method. The supported membrane system consisted of an alumina support, two intermediate α-Al₂O₃ layers, and a top alumina membrane. The θ- to α-Al₂O₃ transformation in supported and unsupported alumina membranes was investigated using X-ray diffraction (XRD) and scanning electron microscopy. XRD patterns showed that the supported membrane had a 100°C higher θ- to α-Al₂O₃ transformation temperature than the unsupported one. A similar effect was observed for microstructures of the membranes. We explained their transformation-temperature difference with a stress generated in the supported top membrane using a theoretical approach.     

Growth and microstructural stability of epitaxial Al films on (0001) α-Al2O3 substrates

Al films were grown epitaxially on single-crystal α-Al₂O₃ substrates by magnetron sputtering and molecular beam epitaxy, respectively. The microstructure and thermal stability of these films were analysed in detail using X-ray diffraction methods and electron microscopy techniques. The films consist of two twin-related growth variants, related by a 180° rotation around the <111> film normal resulting in a {111} Al || (0001) α-Al₂O₃, and ± < > Al || < > α-Al₂O₃ orientation relationship. The Al variants are separated by Σ3 { } Al twin boundaries possessing a rigid body translation of the {111} Al planes across the boundary plane in order to reduce their energy. Motion of the twin boundaries was observed by annealing plan-view samples in situ in a transmission electron microscope. The twin boundaries advance in jerky motion at velocities of several μm/s at temperatures of ˜400 °C, resulting in grain coarsening. In all cases, heat treatments resulted in increased area fraction of one twin variant, which finally will result in single-crystal films upon further annealing.     

Deposition, microstructure and properties of texture-controlled CVD α-Al2O3 coatings

The influence of nucleation on the microstructure and properties of CVD Al₂O₃ was investigated. The experimental α-Al₂O₃ layers were deposited (a) without nucleation control and (b) with nucleation steps resulting in pronounced , and growth textures. The experimental layers were characterised using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Wear properties of the textured coatings were evaluated in turning. The chemistry of the nucleation surface appeared to be an important factor in pre-determining the phase content and growth textures of the Al₂O₃ layers. Optimised nucleation resulted in substantially improved wear properties and these kinds of α-Al₂O₃ layers were typically composed of relatively small, defect-free grains exhibiting no porosity. The textured α-Al₂O₃ layer showed the best wear resistance.     

Oxygen partial pressure dependence of memory effect of sputtered nc-Al/α-Al2O3 thin films

Nanocrystalline aluminum embedded in amorphous dielectric alumina matrix thin films (nc-Al/α-Al₂O₃) was synthesized via reactive magnetron sputtering. The nc-Al/α-Al₂O₃ films at different oxygen partial pressures were sputtered on p-type Si substrates from a pure Al target in the mixed ambient of Ar and O₂. Both deposition rate and aluminum concentration increase as the oxygen partial pressure decreases. X-ray photoelectron spectroscopy and high-resolution transmission electron microscope studies give the confirmation of nanocrystalline Al embedded in amorphous Al₂O₃ matrix. This nanocomposite thin film exhibits memory effect as a result of charge trapping. The flat band voltage value depends on the Al nanocrystal concentration which is related to oxygen partial pressure.     

Magnesium-assisted formation of metal carbides and nitrides from metal oxides

A series of carbides (TiC, V₂C, Mo₂C) were synthesized by the corresponding metal oxides (TiO₂, V₂O₅, MoO₃), CaC₂ and magnesium as starting materials in a stainless steel autoclave at 600 °C. Through similar processes, transition metal nitrides (TiN, VN and CrN) could also be produced by employing the corresponding metal oxides (TiO₂, V₂O₅, Cr₂O₃), NaNH₂, and magnesium as starting materials at 550 °C. The FE-SEM image showed that the TiC sample was mainly consisted of flower-like microstructures. TEM image showed the other carbides (V2C, Mo2C) and the obtained nitride (TiN, VN, CrN) were consisted of nanoparticles. The possible synthesis mechanism of TiC had been described.     

Oxidation Behavior of a Single-Crystal Ni-Base Superalloy in Air—II: At 1000, 1100, and 1150°C

The oxidation behavior of a single-crystal (SC) Ni-base superalloy was studied over the temperature range from 1000–1150°C and analysed by TGA, XRD, EDAX, and SEM. The results indicated that the SC Ni-base superalloy exhibited parabolic oxidation kinetics, which were controlled by the growth of the inner -Al₂O₃ layer. A mixed scale formed on the SC Ni-base superalloy after prolonged oxidation. The scale consisted of an outer layer of spinel, a sublayer of mainly -Al₂O₃ with small amount of spinel adjoined by a very thin and even discontinuous layer of CrTaO₄-rich oxide, and an inner -Al₂O₃ layer. The inner -Al₂O₃ layer provided good protection. No internal oxides or nitrides were observed below the inner -Al₂O₃ layer after 1000 hr at 1000°C, and after 200 hr at 1100 and 1150°C.     

Synthesis and supercapacitive behavior of carbon aerogel microbeads encapsulated by in situ Co3O4 nanoparticle

Co₃O₄/carbon aerogel microbead (Co₃O₄/CAMB) composites are prepared by in situ coating method and their supercapacitive behaviors are investigated. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectrum, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectrum are used to characterize the structure and morphology of Co₃O₄/CAMB. The results show that nano-sized Co₃O₄ particles are homogenously encapsulated on the surface of CAMB. Electrochemical performances of the composite electrodes with different Co₃O₄ amount are studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge measurements. It is found that Co₃O₄/CAMB composites show better electrochemical performance than CAMB, and the optimum content of Co₃O₄ in Co₃O₄/CAMB composites is 10 wt.%. The specific capacitance of 10%–Co₃O₄/CAMB composite in 6 mol L^?1 KOH electrolyte is about 350.18 F g^?1 at a current density of 1 A g^?1. Moreover, 10%–Co₃O₄/CAMB composite supercapacitor exhibits relatively good high-rate capability and excellent cycle life.     

Formation and properties of BaxFe3−xO4 with spinel structure by mechanochemical reaction of α-Fe2O3 and BaCO3

Magnetic Ba_xFe_3−xO₄ (x  0.23) with spinel structure was fabricated by ball milling of mixture of BaCO₃ and nonmagnetic α-Fe₂O₃ powders, and the molar ratio of BaCO₃ and α-Fe₂O₃ is 1:6. In the milling process, a mechanochemical reaction took place between BaCO₃ and α-Fe₂O₃, and Ba cation incorporated into α-Fe₂O₃ with rhombohedral structure to form a α-(Fe,Ba)₂O₃ solid solution. The Ba content in the α-(Fe,Ba)₂O₃ increased with increasing milling time, when the Ba content exceeded a limited solubility, the α-(Fe,Ba)₂O₃ transformed into a phase of Ba_xFe_3−xO₄ with spinel structure, where the Ba cation occupied an octahedral site or tetrahedral site. The product obtained in the balling process was different from that prepared in the annealing process at atmospheric pressure, which was BaFe₂O₄ with orthorhombic structure. Accompanying the crystal structure transition from α-(Fe,Ba)₂O₃ to Ba_xFe_3−xO₄, the magnetic properties also changed from nonmagnetism into ferromagnetism. The saturation magnetization was 53.3 emu/g and coercivity was 113.7 Oe. The mechanism of transitions of the crystal structure was discussed in the present work.     

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media was monitored by X-ray diffraction (XRD), ⁵⁷Fe Mössbauer and Fourier transform infrared (FT-IR) spectroscopies and field emission scanning electron microscopy (FE SEM). Acicular and monodisperse α-FeOOH particles were precipitated at a very high pH by adding a tetramethylammonium hydroxide solution to an aqueous solution of FeCl₃. The XRD analysis of the samples precipitated in the presence of Zn²⁺ ions showed the formation of solid solutions of α-(Fe, Zn)OOH up to a concentration ratio r = [Zn]/([Zn] + [Fe]) = 0.0909. ZnFe₂O₄ was additionally formed in the precipitate for r = 0.1111, whereas the three phases α-FeOOH, α-Fe₂O₃ and ZnFe₂O₄ were formed for r = 0.1304. In the corresponding FT-IR spectra, the FeOH and FeO stretching bands were sensitive to the Zn²⁺ substitution, whereas the FeOH bending bands of α-FeOOH at 892 and 796 cm⁻¹ were almost insensitive. The Mössbauer spectra showed a high sensitivity to the formation of α-(Fe, Zn)OOH solid solutions which were monitored on the basis of a decrease in B_hf values in dependence on Zn-doping. A strictly linear decrease in B_hf for α-FeOOH doped with Zn²⁺ ions was measured up to r = 0.0291, whereas for r = 0.0476 and higher there was a deviation from linearity. The presence of α-(Fe, Zn)OOH, α-Fe₂O₃ and ZnFe₂O₄ phases in the samples was determined quantitatively by Mössbauer spectroscopy. Likewise, Mössbauer spectroscopy did not show any formation of the solid solutions of α-Fe₂O₃ with Zn²⁺ ions. FE SEM showed a strong effect of Zn-doping on the elongation of acicular α-FeOOH particles (500–700 nm in length) up to r = 0.1111. For r = 0.1304 the sizes of ZnFe₂O₄ particles were around 30–50 nm, and those of α-Fe₂O₃ particles were around 500 nm, whereas a relatively small number of very elongated α-(Fe, Zn)OOH particles was observed. A possible mechanism of the formation of α-(Fe, Zn)OOH, α-Fe₂O₃ and ZnFe₂O₄ particles was suggested.     

The high-temperature corrosion behavior of Nb-Al alloys in a H2/H2S/H2O gas mixture

The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H₂/H₂S/H₂O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of -Nb and Nb₃Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO₂ layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO₂ and minor amounts of Nb₂O₅, NbS2, and -Al₂O₃, while the scales formed on Nb-25Al consisted of mostly Nb₂O₅ and some -Al₂O₃. The scales formed on Nb-75Al consisted of mostly -Al₂O₃ and Nb₃S₄ atT 900°C, and mostly -Al₂O₃ , Nb₃S₄ and some AlNbO₄ at 1000°C. The formation of -Al₂O₃ and Nb₃S₄ resulted in a significant reduction of the corrosion rates.     

Conductivity of a new pyrophosphate Sn0.9Sc0.1(P2O7)1−δ prepared by an aqueous solution method

New pyrophosphate Sn_0.9Sc_0.1(P₂O₇)_1−δ was prepared by an aqueous solution method. The structure and conductivity of Sn_0.9Sc_0.1(P₂O₇)_1−δ have been investigated. XRD analysis indicates that Sn_0.9Sc_0.1(P₂O₇)_1−δ exhibits a 3 × 3 × 3 super structure. It was found that Sn_0.9Sc_0.1(P₂O₇)_1−δ prepared by an aqueous method is not conductive. The total conductivity of Sn_0.9Sc_0.1(P₂O₇)_1−δ in open air is 2.35 × 10⁻⁶ and 2.82 × 10⁻⁹ S/cm at 900 and 400 °C respectively. In wet air, the total conductivity is about two orders of magnitude higher (8.1 × 10⁻⁷ S/cm at 400 °C) than in open air indicating some proton conduction. SnP₂O₇ and Sn_0.92In_0.08(P₂O₇)_1−δ prepared by an acidic method were reported fairly conductive but prepared by similar solution methods are not conductive. Therefore, the conductivity of SnP₂O₇-based materials might be related to the synthetic history. The possible conduction mechanism of SnP₂O₇-based materials has been discussed in detail.     

Abnormal High Growth Rates of Metastable Aluminas on FeCrAl Alloys

Experimental evidence in high temperature oxidation of alumina-forming alloys has accumulated that the overall growth kinetics of the oxide scale are slower for 1000°C, where the stable α-Al₂O₃ phase predominates, than for 900°C where metastable γ-Al₂O₃ and/or θ-Al₂O₃ polymorphs predominate. This intriguing behaviour has been unanimously related to the substantial presence of twin boundaries and the cation vacancy network intrinsic to the metastable aluminas allowing faster diffusion than in the nearly close packed corundum structure. This paper shows that this abnormal growth rate accompanying the presence of stable alumina polymorphs in platelets or needle-like morphology is rather due to the formation of a corundum-alumina-rich compact layer from an outer metastable layer by the concomitant sintering at the intersection vertices of the platelets and secondary recrystallization in these platelets. These phenomena are illustrated from oxidation tests performed on thin FeCrAl foils in both a conventional muffle furnace (designed by AET) and thermogravimetric analysis furnace (TGA) over the temperature range of 800–1300°C using field emission scanning electron microscope (FEG-SEM), transmission electron microscope (TEM), electron probe microanalysis (EPMA), atomic force microscope (AFM), grazing incidence X-ray diffraction (GIXRD) and image analysis (IA) techniques.     

Corrosion properties of Co43Fe20Ta5.5B31.5 bulk glassy alloy

The corrosion properties of Co₄₃Fe₂₀Ta_5.5B_31.5 bulk glassy alloy with a diameter of 2 mm were investigated. The passive current density of the glassy alloy rod in 1N HCl and 1N H₂SO₄ solutions is in the order between 10⁻¹ and 10¹ A/m², respectively, indicating that except the ultrahigh strength and excellent soft-magnetic properties which have been reported before, this bulk glassy alloy also exhibits rather high corrosion resistance even without any corrosion-resistant elements.     

The oxidation behavior of Ti–46.5Al–5Nb at 900 °C

The early stages of Ti–46.5Al–5Nb (at%) oxidation at 900 °C have been investigated combined with TEM and STEM. The results reveal that a layer composed of polycrystalline TiO₂ and amorphous Al₂O₃ phase formed firstly after 5-min oxidation. The base alloy connected with the oxide scale has some deformation compared with the inner full lamellar TiAl structure. After 30-min oxidation, the phases of γ-Al₂O₃, κ-Al₂O₃, titanium nitrides and Ti₅Al₃O₂ were formed in the area from the nitride layer to base alloy. After 50 h of oxidation, Ti₅Al₃O₂ vanishes at the interface of oxide scale/base alloy in Ti–46.5Al–5Nb, contrary to the continuous formation of Ti₅Al₃O₂ in γ-TiAl at the interface of oxide scale/base alloy, Al₃Nb phase formed in this zone, which hinders the continuous formation of Ti₅Al₃O₂.