Interfacial reactions of a MAX phase/superalloy hybrid

Oxidation resistant, strain tolerant MAX phase coatings are of general interest for high temperature applications. Accordingly, Cr₂AlC MAX phase coupons were vacuum diffusion bonded to an advanced turbine disk alloy at 1100 °C for compatibility studies. The interface revealed an inner diffusion zone consisting of ~10 µm of β‐Ni(Co)Al, decorated with various γ′ (Ni,Co)₃Al, Ta(Ti,Nb)C, and W(Cr,Mo)₃B₂ precipitates. On the Cr₂AlC side, an additional ~40‐µm Al‐depletion zone of Cr₇C₃ formed an interconnected network with the β‐Ni(Co)Al. On the superalloy side, enhanced carbide precipitation developed over a depth of ~80 µm. Subsequent annealing for 100 h and 1000 h at 800 °C coarsened some features, enhanced TCP precipitation in the superalloy, but only enlarged the diffusion layers by ~5 µm at most. Because of Al depletion from the MAX phase and corresponding Al enrichment of the alloy, the reaction zone displayed similarities to an oxidized Cr₂AlC surface and an aluminized superalloy, respectively. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Cross-section TEM for examinations of selective tungsten CVD for via hole filling

The reaction between tungsten layers [deposited by chemical vapor deposition (CVD) as well as by physical vapor deposition (PVD)] and an aluminium alloy (AlSi) has been investigated. For CVD tungsten layers deposited on AlSi by silane reduction of WF₆ the formation of an aluminium fluoride interlayer has been established by cross-section transmission electron microscopy (XTEM). At the interface between PVD-W and AlSi, an intermetallic compound may be formed depending on the thermal treatment. A crystalline intermediate layer has been found after annealing at 400°C (twice for 30 min), whereas an interface between as deposited double layers showed no interlayer. For all samples investigated, diffusion of aluminium into the tungsten layer was observed. The diffusion depth depends on the heat treatment and the tungsten morphology. Intermediate layers — if they exist — limit the diffusion but do not act as a diffusion barrier.     

Kinetics of solid state reactions with a positive feedback between the reaction and fracture

A model describing the steady-state kinetics of thermal decomposition in the solid phase occurring with a decrease in the solid phase volume and accompanied by fracture in the reaction zone is suggested. The model is based on the concept of positive feedback between the reaction and fracture. The rate of the fracture front and the size of the product fragments formed were calculated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 626–631, April, 1998.     

Preparation and characterization of novel hydroxyapatite/copper assemblies with well-defined morphologies

Micrometer-sized powders with small crystallite sizes, high dispersion, and high copper contents, like Cu and/or Cu₂O assembled on core particles would find potential applications in fields of catalysts, transparent conducting films and plasmonic-based technologies. Novel hydroxyapatite (HA)/copper assemblies were synthesized via a facile glucose reduction route. During hydrothermal treatment, copper ions were firstly released after dissolution of copper-modified HA, then reduced by glucose and finally assembled as shell on HA aggregates. After 12 h, cuprous oxide grew with truncated-octahedron morphology. When the reduction time was prolonged to 24 and 36 h, Cu phase was formed in situ via glucose reduction of Cu₂O. Interestingly, HA/copper assemblies with well-defined morphologies were prepared under different reaction conditions. With presence of more Na₂CO₃, the reduction of copper ion occurred at a fast rate, which resulted in formation of spherical assemblies. Contrastingly, reduction reaction hardly occurred without Na₂CO₃ addition and assemblies with irregular morphology were prepared. Additionally, copper fibers with length of millimeters were prepared without Na₃Cit addition. The UV–Vis absorbance peak of assemblies showed a blue or red shift due to the effect of crystalline size and/or hollowing process.     

Investigation of electrochemical reactions on copper electrode in Cu4RbCl3I2 solid electrolyte

Investigations are carried out by potentiostatic method. It is found that at potentials ϕ less than 100 mV, a reaction rate of copper deposition is limited by the formation and the three-dimensional growth of copper nuclei and the rate of copper dissolution is limited by a two-dimensional growth of holes in the metal. The rate of nucleus growth was evaluated at 10⁻⁹...10⁻⁶ μm s⁻¹ depending on the potential. At ϕ>120 mV, the reaction rate is limited by charge transport at the exchange current density of 2.7 mA cm⁻² and the anodic transfer coefficient α_a≈ 0.45. The accumulation of a divalent copper phase on Cu/Cu₄RbCl₃I₂ interface at anodic polarization is explained by a parallel course of Cu⁺ − e → Cu²⁺ reaction.     

Effect of stoichiometry and diffusion on an epoxy/amine reaction mechanism

The bulk phase kinetics of an epoxy (DGEBA) /amine (DDS) thermoset have been studied using DSC, FTIR, and ¹³C-NMR. In the absence of catalyst, the reaction was found to involve a main exothermic reaction between epoxide and amine hydrogen and a side reaction between tertiary amine formed in the main reaction and epoxide. The main reaction was exothermic while the side reaction had no discernable exotherm. Etherification did not occur to any significant extent. Since only the main reaction is exothermic, DSC was very useful for studying the main reaction kinetics. FTIR was used for determining whether epoxide and amine hydrogen were consumed at different rates as a way of following the side reaction. An IR band previously unused by other investigators was used to monitor the amine hydrogen concentration. NMR confirmed the above mechanism by identifying the formation of a quaternary ammonium ion/alkoxide ion pair as a reaction product of tertiary amine and epoxide. This mechanism has been successfully fit to a rate law valid over the entire extent of reaction. The rate constant for the epoxy/amine addition reaction was found to depend on hydroxide concentration (extent), reaction temperature, and glass transition temperature and included contributions from uncatalyzed and autocatalyzed parts. The side reaction (quaternary ammonium ion formation) formed weak bonds which did not affect the overall system T_g. Both reactions were second order. The rate constants for the main reaction first increase with increasing extent due to autocatalysis by hydroxide before decreasing due to the diffusion limit caused by gelation and vitrification. © 1995 John Wiley & Sons, Inc.     

Regularities of Phosphides Formation on Phosphorus Interaction with Elements

Abstract

During phosphide synthesis from elements in the evacuated tubes the reaction rate is controlled, as a rule, by the diffusion of element ions through solid phosphide layers. Such a synthesis takes a number of hours. During interaction under combustion regime (1) the reaction region spreads over the powder mixture of metal and phosphorus, the phosphorus evaporates and interacts with the heated metal. Then a liquid phase with composition close to the eutectic is formed, which dissolves metal particles and is simultaneously enriched with phosphorus with heat liberation. The liquid phase provides high reaction rates and the synthesis is finished after some seconds. During interaction in ternary systems, e.g. Ni-Fe, Cu, Mo, Ti, Cr, B-P, Fe-Mo-P, Cu-Zr-P, as a rule, solid solutions or mixtures of binary and ternary phosphides, seldom individual phosphides, are formed. A study of the interaction reactions under combustion regime between the red phosphorus and oxides of some metals (Cu, Fe, Co, Ni) resulting in phosphides and phosphates formation was carried out. In aqueous medium phosphorus interacts with Cu, Ag, Au ions, and those of platinoids Interaction of copper sulphate with yellow phosphorus aqueous dispertion was studied in detail. As a result of P°) phosphorus disproportioning copper phosphide Cu₃P (P_3-) and orto-phosphoric acid (P⁵⁺) are formed.     

Change of the roginski “control band” in the case of surface diffusion of adsorbed particles

The effect of surface particle migration between different surface patches on the distribution of the catalytic activity of patches of a linearly inhomogenous surface has been studied. It has been shown that the diffusion can result in an equilibrium distribution of particles A_ads on the surface patches and, therefore, shift of the “control band” and significant change of the reaction rate.     

Interfacial reactions of duplex stainless steels with molten aluminum

The morphology and constitution of the intermetallic layers formed on the surface of duplex stainless steels (DSSs) immersed in molten aluminum at 750 °C for 30 min have been studied in detail by scanning electron microscopy and electron probe micro‐analyzer. Compared with H13 steel, the DSSs exhibited a better corrosion resistance. The weight loss rates, as expressed in terms of weight loss per square centimeter of the specimen per minute, of DSSs are smaller than that of H13. The thickness of the intermetallic layers of DSSs is comparatively thinner. And the interface between the intermetallic layers and DSSs substrate is much flatter. The intermetallic layers of duplex stainless steels are consisted of inner continuous (Fe,Cr)₂Al₅ phase and outer porous (Fe,Cr)Al₃ phases. Microstructure observations suggest that the retarded interfacial reaction between DSSs and molten aluminum is associated with a continuous Al‐Fe‐Cr intermetallic phases layer formed on the solid/liquid interface, which acts as an effective diffusion barrier. The precipitation phase particles distributed along the austenite/ferrite and ferrite/ferrite interfaces also had a good effect on the corrosion resistance properties of DSSs to molten aluminum. Copyright © 2015 John Wiley & Sons, Ltd.     

Galvanic synthesis of copper selenides Cu2 − x Se and CuSe in alkaline sodium selenosulfate aqueous solution

Spherical copper selenide nanoparticles (NPs) were prepared by a simple reaction of sodium selenosulfate with metal copper at room temperature in alkaline Na₂SeSO₃ aqueous solution. It is a galvanic process that operates on a coupled anodic copper oxidation and selenosulfate reduction. 1-Thioglycerol is found to catalyze this reaction. With gold and graphite as the positive electrodes, nanocrystallites of nonstoichiometric copper selenide (Cu_2 − x Se) and stoichiometric copper selenides (CuSe) were produced, respectively. The XRD study shows that the produced CuSe and Cu_2 − x Se are in the pure hexagonal phase and clausthalite phase, respectively. Transmission electron microscopy images show that the diameters of the produced CuSe and Cu_2 − x Se NPs are in the range of 10∼20 and 5∼15 nm, respectively.     

Heterogeneous reaction of NO2 on the surface of NaCl particles

The heterogeneous reaction of NO₂ on NaCl particles has been investigated with the new sample preparation and the mode of gas-solid free diffusion. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is used to characterize the adsorbed products in situ, combined with Ion Chromatographic (IC), X-ray Photoelectron Spectroscopy (XPS) and Scan Electron Microscopy (SEM). Our results indicate that the reaction is not limited to the surface of the NaCl particles, but penetrated into the upper layers. Surface reactive sites determine the reaction. Kinetic measurements show that nitrate formation on sodium chloride is second order in NO₂ concentration and reactive uptake coefficient is (1.54 ± 0.70) × 10⁻⁵.     

Thin-layer chromatographic separation and identification of some anions on copper sulphate impregnated silica gel layers

Summary Thin-layer chromatographic behaviour of thirteen anions on plain silica gel and silica gel impregnated with copper sulphate solution has been investigated in aqueous-organic solvents containing acetone, some of which have achieved reliable and reproducible separations. The effect of copper sulphate concentration on the mobility of anions has been examined. The results obtained on plain silica gel have been compared with those obtained on copper sulphate impregnated layers. The impregnated layers dramatically change the selectivity and permit separations not possible on untreated silica. Aqueous sodium chloride-acetone (9∶1) and ammonium hydroxide-acetone (9∶1) were the most effective solvent systems for differential migration of anions. Better results in terms of clarity of detection and compactness of spots were found with HCOOH-acetone as compared to HCl-acetone. The effect of anion loading on R_F values has been investigated and identification limits on impregnated layers determined.     

Interface microstructure and diffusion kinetics in diffusion bonded Mg/Al joint

The interface microstructure, formation of diffusion bonded joint and regulation of atom diffusion were studied by means of scanning electron microscope (SEM), energy dispersion spectroscopy (EDS) and electron probe microanalyser (EPMA). The experimental results indicated that an obvious interfacial transition zone was formed between Mg and Al, and there are three intermetallic layers Mg₁₇Al₁₂, MgAl and Mg₂Al₃ in this zone. Diffusion activation energy of Mg and Al in the above layers was lower than that in the Mg and Al base metals. The thickness (x) of each layer can be expressed as x ² = 4.14exp(−28780/RT)(t−t ₀), x ² = 31.4exp(−25550/RT)(t−t ₀) and x ² = 0.6exp(−22600/RT)(t−t ₀) corresponding to Mg₁₇Al₁₂, MgAl and Mg₂Al₃ with heating temperature (T) and holding time (t).     

Kinetic method for determination of nanogram amounts of copper(II) by its catalytic effect on hexacynoferrate(III)-citric acid indicator reaction

This paper describes a highly sensitive, selective catalytic-kinetic-spectrophotometric method for the determination of copper(II) concentration as low as 6 ng ml⁻¹. The method is based on the catalytic effect of copper(II) on the oxidation of citric acid by alkaline hexacyanoferrate(III). The reaction was followed by measuring the decrease in absorbance of hexacyanoferrate(III) at 420 nm (λ_max of [Fe(CN)₆]³⁻, ∈ = 1020 dm³ mol⁻¹ cm⁻¹). The dependence of rate of the indicator reaction on the reaction variables has been studied and discussed to propose a suitable mechanism to get a relation between the reaction rate and [Cu²⁺]. A fixed time procedure has been used to obtain a linear calibration curve between the initial rate and lower [Cu²⁺] or log[Cu²⁺] in the range 1 × 10⁻⁷ to 4 × 10⁻⁴ mol l⁻¹ (6.35-25,400 ng ml⁻¹). The detection limit has been calculated to be 4 ng ml⁻¹. The maximum average error is 3.5%. The effect of the presence of various cations, commonly associated with copper(II) and some anions has also been investigated and discussed. The proposed method is sensitive, accurate, rapid and inexpensive compared to other techniques available for determination of copper(II) in such a large range of concentration. The new method has been successfully applied for the determination of copper(II) in various samples.     

Kinetic and thermodynamic studies of the nonisothermal decomposition of anhydrous copper(II) formate in different gas atmospheres

The thermal decomposition of anhydrous (orthorhombic) copper(II) formate was studied by programmed rising-temperature methods (TG, DTG, DTA and DSC) to about 250 °C in flowing gas atmospheres of nitrogen (inert), hydrogen (reducing) and air (oxidizing). The degradation reaction, anion breakdown, proceeded to completion in two distinct, but partially overlapping, rate processes and apparent Arrhenius parameters, calculated by the Ozawa nonisothermal kinetic method, agreed satisfactorily with the literature results. It was concluded that the two consecutive processes, contributing to the overall reaction, involved stepwise cation reduction: Cu²⁺→Cu⁺→Cu⁰, with copper(I) formate as intermediate. This mechanism is similar to that proposed in previous studies of the decompositions of copper(II) oxalate, malonate, maleate, fumarate, mellitate and squarate. For all of these reactants, the Cu⁺ salt has been identified as an intermediate, exhibiting a (slightly) lower relative reactivity than the corresponding Cu²⁺ salt. For copper(II) formate the response curves in the three different gaseous atmospheres were generally similar, showing that neither oxidizing nor reducing conditions caused a marked change in reactivity. The temperature of reaction initiation in H₂ was slightly diminished and the temperature of the second stage of reaction in O₂ was raised appreciably. It is believed that electron transfer contributed to the control of reactivity and that the gases present appreciably influence the rates of the contributory reactions occurring.     

Reaction and Morphology Development Influenced by Diffusion in a Thermoplastic/Thermoset Blend

Diglycidyl ether of bisphenol A (DGEBA) and 4,4′-methylenebis [2,6-diethylaniline] (MDEA) are miscible in polystyrene at 177 °C. We have studied how their diffusion rate in a molten polystyrene matrix influences their polymerization rate and the morphology of the thermoset particles formed at the end of the reaction. The global composition of the blend was 60 wt% of PS and 40 wt% of epoxy-amine. The diffusional control of the reaction was evidenced by comparing the time of reaction of an initially homogeneous mixture with that of different bi-layer samples. The reaction was controled by the diffusion for relatively thick layers (>0.3. mm). A gradient of morphology was obtained due to the diffusionnal control of the reaction. The asymetricity of this gradient may be explained by three factors: differences in diffusion coefficients, in thermodynamic interactions and in viscosity.     

Comparison of the efficacy of sulfurization agents in decreasing the sheet resistance of polyamide and polyethylene with copper sulfide layers and influence of their compositions

The process of obtaining semiconductive and electrical conductive layers of copper sulfides by the sorption — diffusion method on polymers (polyamide 6 and low density polyethylene) using solutions of potassium pentathionate, K₂S₅O₆, and higher polythionic acids, H₂S _n O₆ (n = 21, 33), was investigated. The layers were characterized for compositional and electrical properties by X-ray diffraction (XRD) analysis and sheet resistance measurements. The thickness of copper sulfides layers on polyamide and polyethylene increased with increasing time of polymer sulfurization and varied from 10 to 43 μm. The variations of the sheet resistance of copper sulfides layers formed on the surface of polymers on sulfurization agent used, the conditions of sulfurization, chemical and phase composition of the obtained layers were established. Sheet resistance of copper sulfides layers decreases with increasing time of the duration of sulfurization and the number of sulfur atoms in the polythionate anion. The sheet resistance values for copper sulfide layers formed on the polyamide surface are much lower than those of Cu _x S formed on the polyethylene surface. XRD showed the predomination of Cu _x S phases with low x values.      

Investigation of the anodic decomposition of the solid electrolyte RbCu4Cl3I2

The process of electrochemical decomposition of the solid electrolyte RbCu₄Cl₃I₂ at a vitreous carbon electrode has been investigated. The anodic decomposition of the electrolyte occurs in two steps. At first, the oxidizing electrode reaction of Cu⁺ ions to Cu²⁺ ions takes place at potentials higher than 0.57 V and a layer of decomposition products is formed on the electrode surface, including the divalent copper compound RbCuCl₃. Then the oxidizing reaction of I^– ions occurs at potentials higher than approximately 0.67 V, with deposition of an iodine layer onto the electrode surface. The deposition rate of the layers of decomposition products is controlled by instantaneous nucleation and two-dimensional growth of the deposit. The total thickness of the passivating layer of decomposition products on the anode is equal to ca. 1 μm. Electronic Publication     

Diffusion and reactivity of ZnO-MnOx system

The solid state interaction between ZnO and MnO_x in air was investigated at different temperatures by means of the diffusion couple technique. No diffusion is observed at temperatures below 973 K. Above this temperature, Mn(IV) is already reduced to Mn(III) and the subsequent formation of Mn₂O₃ impels the diffusion of manganese into the ZnO pellet. However, it never enters the wurtzite lattice, so no homogeneous Mn:ZnO solid solution is formed. Simultaneously, Zn greatly diffuses in the manganese pellet, and as a consequence, a new phase layer develops at MnO_x/reaction zone interface. A mixture of cubic and tetragonal spinel-type phases initially comprises this layer. However at higher temperatures, the tetragonal ZnMn₂O₄ spinel is the unique phase present in the interface, and it forms a physical barrier for further diffusion of both zinc and manganese species in the respective pellets of the couple. Differences arising between ZnO, MnO₂ and Mn₂O₃ crystal structures are behind these diffusion behaviors.     

Non‐catalytic,gas‐solid reversible reaction mechanism implementing diffuse interface model

Gas‐solid reactions in chemical and metallurgical industries often involve solid pellets and a gaseous reactant. The progress of chemical reaction is measured by the movement of zones within the pellet and has been explained in terms of diffusion and chemical reaction processes. Earlier models identified a single reaction zone, in addition to product layer and unreacted core. In the present article, two reaction zones are envisaged as a more plausible explanation of the movement of the zones as the reaction proceeds. Earlier models for reversible reactions have assumed that conditions at the interface between the reaction zone and the unreacted core correspond to equilibrium at the prevailing temperature. The gaseous concentrations were assumed to permeate the core at the interfacial values so that no reaction occured in the core. More realistically, the present article envisages an additional zone within which the gaseous concentrations fall from the equilibrium values to zero. It is assumed that in the reaction zone proper, referred to as zone I, having thickness z_I, the concentration profile is sigmoidal. This agrees with the earlier work of Khan and Bowen [1] and Prasannan and Doraswamy. [2] In zone I and the concentration of the reactant gas varies between [A_i] and [A*]. In the zone II, having thickness z₂, concentration varies linearly between [A*] and zero. This model has been applied successfully to the data of the reduction of hematite [3] at different temperatures. The contribution of different forms of resistance, diffusion in product layer, chemical reaction and diffusion in the reactant core, is assessed as function of time (start to the end of reaction). The thickness of the zones remain almost constant as the reaction progresses. In particular, the influences of the product and core diffusion coefficients and chemical equilibrium constant on the extant reaction are evaluated. The dependence of concentration profile and zone thickness on equilibrium constant, K, velocity constant, k, diffusional coefficients D_C and D_P has been investigated thoroughly. The thickness of both zones has been evaluated for leading variables. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 559–570, 1999