Reaction Product Layer Research Articles

Initial formation of contact layers on Ni/SiC samples studied by XPS

AbstractThis study deals with the quantitative assessment of the coverage and thickness of Ni silicide films formed during annealing of SiC substrates with sputtered thin films of Ni. The analytical approach involves the use of XPS and depth profiling by means of successive ion etchings and XPS analyses. For either 3 or 6 nm initial Ni film thickness, a 10 nm Ni2Si product is formed. On top of this product, the C released is accumulated in a very thin (1–2 nm) film. In neither case, the Ni2Si covers the whole surface, although the coverage is almost complete (∼90%) in the latter case. For the greater initial Ni‐film thickness of 17 nm, the thickness of the Ni2Si product corresponds well to the value of 25 nm expected from the Ni/Ni2Si stoichiometric relationship. This thickness is significantly greater than a critical level and the film covers the whole surface. Carbon is similarly accumulated in a very thin layer on the top surface, although the major part of C (∼70%) is found inside the main reaction product layer. Copyright © 2008 John Wiley & Sons, Ltd.

REACTION PRODUCT LAYER ON CHALCOPYRITE IN CUPRIC CHLORIDE LEACHING

During the leaching of chalcopyrite in cupric chloride solutions, a reaction product layer forms on the mineral surface. In the present work, the formation and the composition of the reaction product layer on solid stationary chalcopyrite was studied in a chemical environment similar to that of the HydroCopper® process. The effect of the process parameters on the reaction product layer was investigated. Electrochemical measurements, including A.C. impedance spectroscopy, were carried out and an equivalent circuit was used to estimate the surface parameters of the reaction product layer, such as resistance, capacitance and porosity. Scanning electron microscopy was used to complement the electrochemical measurements. The results suggest that at pH = 1, a fragile more resistive elemental sulphur layer forms, whereas at pH = 3, a porous less resistive FeOOH layer is formed.Lors de la lixiviation de la chalcopyrite en solutions de chlorure cuivrique, une couche de produit de reaction se forme a la surface du mineral. Dans le present travail, on a etudie la formation et la composition de la couche du produit de reaction sur de la chalcopyrite solide stationnaire dans un environnement chimique similaire a celui du procede d’HydroCopper®. On a etudie l’effet des parametres du procede sur la couche de produit de reaction. On a effectue des mesures electrochimiques, incluant la spectroscopie a impedance A.C., et l’on a utilise un circuit equivalent pour estimer les parametres de surface de la couche de produit de reaction, comme la resistance, la capacitance et la porosite. On a utilise la microscopie electronique a balayage pour completer les mesures electrochimiques. Les resultats suggerent qu’a un pH = 1, une couche fragile de soufre elementaire plus resistive se forme, alors qu’a un pH = 3, une couche poreuse moins resistive de FeOOH est formee.

Diffusion-limited reactive wetting: effect of interfacial reaction behind the advancing triple line

Using the “dispensed drop” variant of the sessile drop technique, spreading kinetics of dilute Cu–Cr alloys on smooth vitreous carbon substrates are measured under helium microleak conditions. In this system, it is known that the drop spreading rate is controlled by diffusion of the reactive atom species (Cr) from the bulk liquid to the triple line, where wetting is induced by formation of an interfacial layer of chromium carbide. Microstructural characterization of rapidly cooled drops shows that growth of the interfacial reaction product layer continues behind the moving solid–liquid–vapor triple line. The spreading velocity is modeled by finite-difference numerical analysis of diffusion near the triple line in the presence of continued interfacial reaction, simplifying the growth rate as being constant and using realistic parameter values. We show that continued interfacial reaction explains the dependence of the triple line spreading rate on the instantaneous wetting angle that is observed in this system.

Interphase effects and macrokinetics of chemisorption in the absorption of CO2 by aqueous solutions of alkalis and amines

Interference laser microscopy was used to study the macrokinetics of absorption of carbon dioxide by aqueous solutions of chemisorbents. Chemisorption was shown to be a complex and multistage process. Its evolution included the occurrence of a diffusion-controlled chemical reaction close to the interphase boundary, the appearance of interphase instability, and the development of spontaneous small-and large-scale convection in the liquid phase. The characteristic dimensions of small-scale convective structures and their dependence on the concentration and chemical nature of absorbents were determined. The brim meniscus of wetting was shown to play a fundamental role in the appearance of large-scale convective flows. The main physicochemical rules governing interphase boundary stability loss by the chemicapillary mechanism during chemisorption were considered. A description of a large-scale spontaneous convection in the liquid phase appearing under the influence of the brim meniscus of wetting and resulting in the self-emulsification of a reaction product layer was suggested. Changes in surface tension at the reaction product-chemisorbent boundary resulting in the appearance of an elastic component of product layer deformation were estimated. It was shown that, because of the Weissenberg effect, tangent stresses in a product layer flowing down the brim meniscus of wetting can cause the appearance of tensile strain, nucleation, and formation of desorption microbubbles earlier recorded in several experiments by laser spectroscopy.

Joining of SiC Ceramic by Hot Pressing Reaction Joining Process Using Ni-51Cr Powders as Filler

Joining of ceramics is of importance from both technical and economical points of view. Joining of recrystallized SiC ceramic to itself has been realized by hot pressing reaction joining process using Ni-51Cr powders (consisting of Ni+51wt%Cr powders) as filler. The optimized technological parameters have been obtained by orthogonal experiment, which are joining temperature of 1150°C, holding time of 20min, joining pressure of 10MPa and cooling rate of 0.25°C/s. The most effective factor to influence the weld strength is joining pressure within the range of testing. Under the optimized conditions, the maximum relative bending strength of joint, 70.7%, is achieved. The microstructure and phase composition of the weld zone were examined by SEM, EDX and XRD. The results show that interdiffusions and chemical reactions take place in the joining process. A reaction layer, which is mainly composed of Ni2Si and Ni3C with a little Cr, exists between the welding base material SiC ceramic and the filler reaction product layer called as interlayer, which is mainly composed of Ni2Si and Cr.

Joining of SiC Ceramic by High Temperature Brazing Using Ni-Cr-SiC Powders as Filler

Joining of ceramics is of importance from both technical and economical points of view. Brazing is a widely used process to join ceramics. In order to increase the working temperature and weld strength of joints, a high temperature brazing process using Ni-Cr-SiC powders (consisting of Ni, Cr and SiC powders) as filler to join recrystallized SiC ceramic has been investigated. The obtained optimized technological parameters are joining temperature of 1360°C, holding time of 5min and filler mass of 280mg. Under these conditions the maximum relative bending strength of joints, 70.5%, is achieved. Microstructure and phase analysis reveals that interdiffusions and chemical reactions take place in the weld zone. A reaction layer, of which the major phase is Ni2Si, exists between the welding base material SiC ceramic and the filler reaction product layer, called as interlayer, of which the major phase is Cr23C6.

ガス窒化処理による生体用 Ti-29Nb-13Ta-4.6Zr 合金および Ti-6Al-4V ELI 合金表面の硬質皮膜生成に及ぼす合金元素の影響

Formation of the reaction product layer on the surface of biomedical titanium alloys, Ti-29Nb-13Ta-4.6Zr (TNTZ) and Ti-6Al-4V ELI (Ti64), during gas nitriding was investigated. These alloys were exposed to nitrogen atmosphere at 1023, 1073, 1123 and 1223 K. After the gas nitriding, a reaction product layer was observed on the surface of both alloys, and was analyzed using an X-ray diffraction (XRD), Auger electron spectroscopy (AES) and X-ray Photoelectron spectroscopy (XPS). The layer was comprised of two phases, which were outer oxide layer (mainly TiO2) and inner nitride layer (mainly TiN or Ti2N). In these layers, the thickness of the oxide layer particularly depended on the kinds of alloys. According to the thermodynamics and point defect theory, the growth rate of oxide layer is expected to be increased by the presence of Al in TiO2. Namely, the dissolution of Al into TiO2 may increase the number of oxygen vacancies, resulting in acceleration of oxygen diffusion inward. On the other hand, the elements that accelerate the growth of the oxide layer are not contained in TNTZ. Thus, the oxide layer formed on Ti64 was thicker than that of TNTZ. In a similar way, the elements that accelerate the growth of the nitride layer are not contained in both TNTZ and Ti64. Thus, the nitride layers with similar thicknesses may be formed on TNTZ and Ti64 during gas nitriding.

Microstructure of tricalcium silicate and Portland cement systems at middle periods of hydration-development of Hadley grains

The development of the microstructure of C 3S paste and a Portland cement paste was studied between 7 and 24 h by means of backscattered electrons in a field-emission SEM. The course of hydration was measured by isothermal calorimetry. While the abundant occurrence of Hadley grains (hollow-shells) in Portland cement systems is well documented from a number of SEM and other microscopy studies, some earlier reports have noted that Hadley grains do not form in C 3S or alite paste alone. This report shows evidence of Hadley grains in C 3S paste, and follows their development from middle to late hydration stages. At around 10 h the microstructure with respect to Hadley grains were seen to develop in a very similar manner in C 3S and cement. In both systems, a narrow gap often developed between the receding anhydrous cores and layer of reaction product enveloping the cores. By 1 day, Hadley grains had continued to develop only in the cement paste, where they became a prominent feature. Only small ‘hollowed-out’ hydration shells were observed in the C 3S paste by 1 day. These were presumably reminiscences of the small gapped Hadley grains seen at the earlier hydration stages.

Epitaxial B2-NiAl layers formed by nanosecond laser irradiation of thin Al/Ni bilayers

The results of experiments on the synthesis of epitaxial B2-NiAl layers by means of nanosecond laser irradiation of sequentially deposited thin nickel and aluminum layers (Al/Ni bilayers) on a MgO(001) substrate surface are presented. Features of the phase formation under the laser action and during the combustion wave initiation are considered. The rapid formation of an epitaxial B2-NiAl layer is explained in terms of a martensitic-like mechanism of the transfer of reacting atoms via a layer of reaction products. It is suggested that this mechanism can compete with diffusion via grain boundaries and dislocation, thus explaining the ultrafast transfer of reacting atoms via a layer of reaction products for various methods of initiation of the solid-state synthesis.

High Temperature Chemical Interaction Between SiO<sub>2</sub> Substrates and Ag-Cu Based Liquid Alloys in Vacuo

The interfaces formed between vitreous or thermally devitrified fused quartz substrates and silver alloys after 90 min at 850 °C in vacuum have been characterized. Three silver alloys have been used: Cusil (Ag–28 wt % Cu), Cusil-ABA (Ag–35 wt % Cu–1.5 wt % Ti), and Incusil-ABA (Ag–27 wt % Cu–12 wt % In–2 wt % Ti). A non wetting condition is found for the Cusil alloy in both substrates. In contrast, the formation of Ti5Si3, Cu3Ti3O and Ti2O3, following the sequence SiO2 → Ti2O3 → Ti5Si3 → Cu3Ti3O, is observed at the metal/ceramic interface for the two titaniumcontaining alloys on both substrates. Ti2O3 is commonly found as small particles dispersed in a silver-rich matrix. During the experiments, the reaction product layers detach from the ceramic surface and float away from the ceramic/metal interface due to their relatively low density with respect to the liquid alloy. The formation of the phases detected at the ceramic/metal interface can be explained in terms of their relative thermodynamic stability.

Microstructure of SiC / TiAl Interface Formed by Solid-State Diffusion Bonding

The microstructure of the solid-state diffusion bonded interfaces of silicon carbide (SiC) and titanium aluminide (TiAl) were investigated. A 100-µm-thick Ti-48at%Al foil was inserted between two SiC specimens and then heat-treated in vacuum. The interfacial microstructure has been analyzed by scanning electron microscopy, electron probe microanalysis and X-ray diffractometry. Four layers of reaction products are formed at the interface by diffusion bonding: a layer of TiC adjacent to SiC followed by a diphase layer of TiC+Ti2AlC, a layer of Ti5Si3CX containing Ti2AlC particles and a layer of TiAl2. However, the TiAl2 layer is formed during cooling. The actual phase sequence at the bonding temperatures of 1573 K and 1673 K are described as SiC/TiC/(TiC+ Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti1-XAl1+X/TiAl and SiC/TiC/(TiC+Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti5Al11 /Ti1-XAl1+X/TiAl, respectively. The phase sequences are successfully expressed on the basis of the Ti-Al-Si-C quaternary chemical potential diagram.

Reaction-induced weakening of plagioclase–olivine composites

The localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions. In order to study the effects of solid–solid mineral reactions on plastic deformation of rocks, we have investigated the shear deformation of plagioclase–olivine composites during the reaction plagioclase + olivine → orthopyroxene + clinopyroxene + spinel (± garnet). Microstructures of plagioclase–olivine composites were studied after shear deformation experiments in a Griggs apparatus. Experiments were performed on anorthite–forsterite (An–Fo) and labradorite–forsterite (Lab–Fo) composites at 900 °C, confining pressures between 1000–1600 MPa and with constant shear strain rates of ∼5 × 10 −5 s −1. In absence of reaction, Lab–Fo composites are stronger than pure olivine and labradorite end-members that deform with a high temperature plasticity mechanism. Lab–Fo composites strain–harden due to the inhibition of extensive recrystallisation by interphase boundaries. In An–Fo composites, the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine-grained (size < 0.5 μm) polyphase reaction products. Interconnecting layers of reaction products accommodate most of the applied strain by grain size sensitive creep. Recovery processes are pronounced during syndeformational reaction: original anorthite and olivine dynamically recrystallise by subgrain rotation and bulging recrystallisation. Presumably, the dynamic recrystallisation is caused by reduced stress conditions and partitioning of strain and strain rates between the new reaction products and the relict An–Fo grains. The results of our experiments are in good agreement with natural observations of shear localisation in the lower crust and upper mantle, and imply that anhydrous mineral reactions can be important causes for localisation of deformation.

Transmission electron microscopic studies on crystallization of YBa2Cu3O7−y films deposited by advanced TFA-MOD method

We have prepared Y123 quenched and fully crystallized films by the conventional and the advanced TFA-MOD processes. Then, we have investigated microstructures of the films by means of transmission electron microscopy, in order to understand the initial stage of the Y123 growth. As a result, it is found that Y–Ba–O–F grains firstly nucleate on LaAlO3 single crystals as well as on CeO2 buffered metal substrates both in the TFA-MOD processes. The Y–Ba–O–F is grown with epitaxially orientated relationship to the substrate. A BaCeO3 reaction product layer is formed due to Ba diffusion from Y123 to CeO2 mainly through the grain boundaries. At the crystallization temperature of 760°C, the diffusion rate of Ba is slow. Therefore, it was found that the thickness of the BaCeO3 layer does not increase much further with increasing the holding time until about 1000min.

Aminic epoxy resin hardeners as reactive solvents for conjugated polymers: polyaniline base/epoxy composites for anticorrosion coatings

Polyaniline (PANI) has much been studied in the context of corrosion prevention, particularly on steel and aluminium. To prepare epoxy coatings consisting of PANI has turned to be nontrivial, due to its relatively rigid conformation and poor solubility. Therefore, as the aim has typically been first to dissolve PANI in the epoxy component before curing, auxiliary solvents have been required, and less attractive Lewis-type hardeners have been required if the conducting salt form has been used. In this work, we describe a particularly simple concept where emeraldine base (EB) form of PANI is first dissolved in specific aminic hardeners which are observed to be solvents for EB at low concentrations, and the mixtures are unconventionally cross-linked upon adding epoxy resin, diglycidyl ether of bisphenol-A (DGEBA). Suitable hardeners are N,N,N′ ,N′-tetrakis(3-aminopropyl)-1,4-butanediamine (DAB-AM-4) and trimethylhexanediamine (TMDA). Even if the subsequent cross-linking promotes phase separation, the forming cross-link sites may also control the phase separation. As a result, sufficiently homogeneous coatings are identified which contain only 1 wt% EB in the cured EB/DGEBA/TMDA composites where in aqueous 3.5% NaCl solution the corrosion front propagation is suppressed, and electrochemical impedance studies indicate the formation of a charged interface or reaction product layer between EB and steel. For reference, similar net EB/DGEBA/TMDA-compositions were prepared, where EB was first mixed in DGEBA without any solubility and which were cured by added TMDA, and they gave essentially no anticorrosion effect. We expect that the present concept opens new ways to prepare cured epoxy composites also with other conjugated or nonconjugated polymers for anticorrosion and other functional purposes.

Metal to ceramic joining via a metallic interlayer bonding technique

The lack of suitable joining techniques for dissimilar materials such as metal and ceramic for high-temperature applications and devices is a major hindrance to the utilization of their outstanding properties and lower costs. This paper presents the results of joining between the high-temperature and corrosion resistant iron–chromium–aluminium alloy (Fecralloy) with silicon nitride by use of metallic interlayer. The primary objectives were to produce joints for high-temperature applications (above 500 °C) and to observe the effects of processing parameters with regards to joint strength. Joining by use of a thin (12.5 μm) and non-remaining copper foil produced joints with an average maximum shear strength of 67.5 MPa (±5) at 1100 °C, 30 min dwell time at 9.5 MPa, under vacuum. The processing conditions were found to play a very important role in the joining process and effected joint strength. The results suggest that a thin aluminium nitride, AlN, reaction product layer was present on the silicon nitride interface in the joined samples. This is an indication that reactive wetting was occurring. Iron, chromium, aluminium and copper were found to infiltrate the silicon nitride, whilst silicon and nitrogen diffused into the Fecralloy. Joining was also attempted using a Ti/Cu/Ti multilayer and its variations. Joining was not possible due to the formation of a brittle reaction product layer, highlighting that wetting and joining are not synonymous.

Carburization performance of Incoloy 800HT in CH4/H2 gas mixtures

Abstract Carburization performance of Incoloy 800HT has been studied after cyclic and isothermal exposures to CH 4 /H 2 carburizing gas mixtures at high temperatures for 500 h. At 800 °C in 2% CH 4 /H 2 , Incoloy 800HT suffered external oxidation and carburization, the external continuous layer of reaction products consists primarily of Cr 7 C 3 , Mn 1.5 Cr 1.5 O 4 , and FeCr 2 O 4 with Fe(Cr, Al) 2 O 4 as a minor phase. At 1100 °C in 10% CH 4 /H 2 , external carburization did not occur likely due to high carbon dissolution in the alloy substrate at this temperature. A thermodynamic analysis indicated that 1000 °C was an approximate critical temperature, below which the environment should result in mixed oxidizing/carburizing behavior, while above this temperature reducing carburizing behavior should occur. The experimental results approximately agree with the thermodynamic analysis. Metal dusting was not observed under highly carburizing conditions ( a C >1). The size and morphology of Cr-rich phases (or Cr-carbides) are both temperature- and time-dependent, while the external continuity is more temperature-dependent rather than time-dependent.

Hydrogen bromide plasma–copper reaction in a new copper etching process

The hydrogen bromide (HBr) plasma–copper reaction, which is the base of a new copper etching process, has been studied. A high etch rate has been obtained with the reaction. Influences of process parameters, such as plasma exposure time, pressure, plasma power and substrate temperature, to the reaction process were explored. In addition to the reaction rate, we also investigated the relationship between the morphology and structure of the reaction product layer and the process condition. The results show that both the plasma-phase chemistry and the ion bombardment energy are important to the reaction. Mechanism for the HBr plasma–Cu reaction process was compared with that for the HCl or Cl 2 plasma–Cu reaction. The reaction product was characterized with X-ray photoemission spectroscopy and X-ray diffraction. These results are critical to the understanding of this new copper dry etching process.

Influence of Nanothick Layers of DLC on the Rates of Electrochemical Reduction Reactions on Magnetic Hard Disks

Potentiodynamic cathodic polarization tests and electrochemical impedance spectra were obtained for magnetic hard disks coated with a layer of diamond-like carbon (DLC) and immersed in aqueous borate buffer (pH 8.4) containing 0.1 M NaCl. The log of the cathodic current was linearly related to for disks coated with a layer of DLC and polarized between 0.2 and 0.4 V below the corrosion potential. In addition, the low frequency impedance of the disks coated with a layer of DLC decreased by a factor of ∼50 as the potential was decreased from 0.2 to 0.4 V below the corrosion potential. The results were independent of the DLC’s thickness (2-5 nm), dopant (H and and method of deposition (sputtering and ion-beam deposition). The results suggest the presence of a thin (≈12-19 Å) charge-denuded layer that forms by a reaction between the DLC and either air or the aqueous solution. At potentials within −0.4 V of the corrosion potential, the kinetics of the electrochemical reduction reactions on DLC-coated hard disks are explained equally well by Schottky emission or quantum mechanical tunneling of electrons through the charge-denuded, reaction product layer. Both charge-transport mechanisms suggest similar remedial actions for decreasing corrosion of the magnetic layer. © 2004 The Electrochemical Society. All rights reserved.

Interfacial Microstructure of Silicon Carbide and Titanium Aluminide Joints Produced by Solid-State Diffusion Bonding

This paper describes the microstructure and the formation mechanism of solid-state diffusion bonded interfaces of silicon carbide (SiC) and titanium aluminide (TiAl). Two SiC specimens were diffusion bonded using a Ti-48 at%Al foil in vacuum. The interfacial microstructure has been investigated by means of scanning electron microscopy, electron probe microanalysis and X-ray diffraction. Four layers of reaction products are formed at the interface by diffusion bonding: a layer of TiC adjacent to SiC followed by a diphase layer of TiC+Ti2AlC, a layer of Ti5Si3CX containing Ti2AlC particles and a layer of TiAl2. However, the TiAl2 layer is formed during cooling. The actual phase sequence at the bonding temperatures of 1573 K and 1673 K are SiC/TiC/(TiC+Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti1� XAl1þX/TiAl and SiC/TiC/(TiC+ Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti5Al11/Ti1� XAl1þX/TiAl, respectively. Ti5Al11 and Ti1� XAl1þX rapidly transform to TiAl2 during cooling. The layers grow obeying the parabolic law. The growth rate of the reaction layers change sensitively depending on the bonding temperature.

Oxidation Resistance of B, W2B5, and WC Powders

The oxidation of B, W2B5, and WC powders in air has been examined under isothermal conditions in the temperature range 800-1300 K (measurement times at each temperature 30, 60, 120, and 180 min) as well as under nonisothermal conditions on programmed heating with a Q-1500 derivative recorder with the simultaneous conduct of differential thermal analysis. All these specimens give parabolic curves in the interval 800-1300 K. The rectilinear character of the curves is due to the chemical interaction of the compound with atmospheric oxygen, while the parabolic form is due to diffusion of oxygen through the resulting layer of reaction products.