Thin Reaction Layer Research Articles

The effect of interfacial reaction layer thickness on fracture of titanium-SiC particulate composites

Composites of commercial-purity Ti reinforced with 10 vol pct of SiC particles have been produced by cospraying and by powder blending and extrusion. Interfacial reaction layers have been studied by electron and optical microscopy and by Auger electron spectroscopy (AES) of fracture surfaces. The work of fracture has been measured as a function of reaction layer thickness for extruded and heat-treated composites. Material with very thin reaction layers (<∼0.1 μn) can be produced by cospraying, but porosity levels are relatively high (∼5 to 10 pct). Extruded material has been produced with a thin reaction layer (∼0.2 μm) and low porosity (<1 pct). It appears that the rate of reaction conforms with published parabolic rate constant data over a wide range of time and temperature. The reaction layer always consists of TiC and Ti5Si3, but the TiC grains tend to be larger than those of Ti5Si3. As the reaction layer thickness becomes greater than about 1 μm, the work of fracture falls sharply and the cracking pattern changes from one involving fracture of SiC particles to one in which cracking between the particles and adjacent reaction zones becomes predominant. It is suggested that the volume contraction accompanying this reaction, calculated at about 4.6 pct from density data, has a significant effect in promoting crack formation in these locations by generating radial tensile stresses across the interface. Thus, for this particular composite system, the important effect of a thicker reaction layer may be that it promotes the formation of an interfacial crackvia an effect on the local stress state, rather than itself constituting a larger flaw in the form of a through-thickness crack assumed to be present.

The wetting, reaction and bonding of silicon nitride by Cu-Ti alloys

The wettability and reactivity of pressureless sintered Si3N4 by powdered Cu-Ti alloy were investigated using sessile drop tests conducted in a vacuum. Bonding of Si3N4 to itself was also carried out and joint strength was evaluated by compressive shear testing. The correlation of wetting behaviour with reaction and bond strength was interpreted. The wettability of Cu-Ti alloys on Si3N4 was improved greatly by addition of titanium up to 50 wt%. However, the reaction-layer thickness was increased up to 10 wt% and thereafter decreased up to 50 wt%. We propose the dovetail model which describes the reaction-layer growth behaviour with titanium. As the titanium content was increased, it tended to form a continuous thin reaction layer which greatly improved the wettability. From metallographic and XRD analyses, TiN and Ti suicide were found in the reaction layer. The thermodynamic reaction for TiN formation was suggested to be Si3N4(s) + 4Ti (1 − sol) = 4TiN(s) + 3Si(s). Ti-silicide might be formed during cooling by the reaction with Ti and Si which had been decomposed from Si3N4, diffused to and dissolved in the liquid Cu-rich alloy. The reaction layer growth was controlled by diffusion of nitrogen or titanium in the reaction layer according to the titanium concentration. The shear strength of Si3N4 to Si3N4 was affected by the morphology and thickness of the reaction layer rather than the wettability. As the titanium content increased, shear strength also increased rapidly up to 5 wt% and then slowly up to 50 wt%. As the reaction temperature and time were increased, shear strength was lowered due to the greater thickness of the reaction layer despite improved wettability.

Improvement of lubrication breakdown behaviour of isogeometrical phosphorus compounds by antioxidants

The effect on the transition from low to high wear of the presence of extreme pressure-antiwear (EP-AW) additives and of antioxidants added separately and in combination to a base lubricant has been investigated with a ball-on-plate test device. The chemical composition of the worn surfaces was studied by Auger electron spectroscopy. Well-characterized isogeometrical phosphorus compounds were employed as EP-AW additives. The transition from low to high wear was found to coincide with the breakdown and disappearance of a thin reaction layer mostly consisting of phosphate and its replacement by oxides. The lubrication breakdown behaviour in the presence of EP-AW additives was found to be affected by their molecular structure and thermal stability. In the absence of EP-AW additives the antioxidants tested did not improve the wear behaviour. However, the presence of both an EP-AW additive and an antioxidant led to a distinct improvement of the wear behaviour, indicating a synergistic effect. A possible explanation of the observed effect is given.

Microstructure of α-Al base matrix and SiC particulate composites

The microstructures of a high temperature monolithic AlFeVSi alloy and a metal matrix and SiC particulate composite (MMC) based on this alloy matrix were characterized by conventional and high resolution transmission electron microscopy, microdiffraction and X-ray spectroscopy techniques. Silicide particles of average composition Al 3(Fe,V) 3Si were present in both of these materials. These particles were unstable under the electron beam at voltages above 200 kV, and exhibited radiation-induced disordering. SiC particulates present in the MMC structure were predominantly of the hexagonal 6H polytype, but the rhombohedral 87R-VII polytype was also observed. A very thin reaction layer was present between the matrix and SiC particles. No segregation of alloying elements such as iron, vanadium, or silicon at thee matrix-SiC interface was observed. The second phase present at the α-AlSiC interface was probably a disordered silicide.

Surface Alteration of Pollucite under Hydrothermal Conditions

In connection with studies for evaluating the leachability of Cs from solidified radio-active waste, the surface alteration of pollucite was studied under hydrothermal conditions. Pollucite exposed to NaCl or KCl at 300°C formed on its surface a precipitate phase of analcime in the case of NaCl or leucite in the case of KCl. Cubo-octahedral analcime built up not only on the outermost surface of sample but also in the altered layer underneath. This crystal penetration was attributed to precipitation and isomorphous substitution of Na by Cs in the pollucite. The leachability of Cs proved to increase in keeping with salt concentration. Exposure to CaCl2 or to MgCl2 formed a thick precipitate layer of anorthite in the case of CaCl2 or clinochlore in the case of MgCl2 over a thin reaction layer. This precipitate layer had the effect of inhibiting the migration of Cs. Si and Al to restrain Cs leaching into solution, which caused the amount of Cs leached to level off beyond 0.01 mol·dm−3 salt concentration in the case of CaCl2, and beyond 0.01 mol·dm−3 in the case of MgCl2.

Effect of additional elements in Ag-Cu based filler metal on brazing of aluminum nitride to metals.

Brazing of aluminum nitride to copper was performed using Ag-Cu-Ti, Ag-Cu-Ti-Co and Ag-Cu-Ti-Nb brazing filler metals in an argon atmosphere. The reaction layer formed at the interface between AlN and braze layer in AlN-Cu joints was found to increase by increasing brazing time for all brazing filler metals used. In the case of the fillen metal Ag-Cu-Ti-Nb, a thinner reaction layer was formed in comparison with the cases of using other filler metals. Form an EPMA analysis, it was found that not only Ti, but also Nb were concentrated in this thin reaction layer. From XRD analysis, it was found that TiN lattice was distorted by Nb addition. Shear strengths were measured for AlN-W joints at room temperature. The joint brazed at 1 173 K for 0.3 ks using Ag-Cu-Ti filler metal showed a strength of 118 MPa. In the case of using Ag-Cu-Ti-Co filler metal, almost the same shear strength, 116 MPa, was obtained after brazing at 1 173 K for 0.6 ks. The joints brazed at 1 173 K for 0.3 ks using Ag-Cu-Ti-Nb filler metal exhibited the maximum shear strength 147 MPa. The shear strength of the joints was discussed in relation to the thickness of the reaction layer.

Surface alteration of pollucite under hydrothermal conditions.

In connection with studies for evaluating the leachability of Cs from solidified radio-active waste, the surface alteration of pollucite was studied under hydrothermal conditions. Pollucite exposed to NaCl or KCl at 300°C formed on its surface a precipitate phase of analcime in the case of NaCl or leucite in the case of KCl. Cubo-octahedral analcime built up not only on the outermost surface of sample but also in the altered layer underneath. This crystal penetration was attributed to precipitation and isomorphous substitution of Na by Cs in the pollucite. The leachability of Cs proved to increase in keeping with salt concentration. Exposure to CaCl2 or to MgCl2 formed a thick precipitate layer of anorthite in the case of CaCl2 or clinochlore in the case of MgCl2 over a thin reaction layer. This precipitate layer had the effect of inhibiting the migration of Cs. Si and Al to restrain Cs leaching into solution, which caused the amount of Cs leached to level off beyond 0.01 mol·dm−3 salt concentration in the case of CaCl2, and beyond 0.01 mol·dm−3 in the case of MgCl2.

Inhibited enzyme electrodes. Part 2: The kinetics of the cytochrome oxidase system

An inhibition enzyme electrode to measure toxic gases can be constructed using the respiratory enzyme cytochrome oxidase. The rate of enzyme turnover is followed by reducing cytochrome c on a gold electrode modified with the mediator bis(4-pyridyl) disulphate. The kinetics and mechanism of the system have been measured. The electrochemical kinetics for the oxidation of cytochrome c have been studied by rotating disc voltammetry and are shown to obey the Koutecky—Levich equation. The standard electrochemical rate constant is found to be 3 × 10 −3 cm s −1. At ambient oxygen concentration the orders of the current with respect to the concentration of cytochrome oxidase, cytochrome c and oxygen are found to be 1 2 , 1 2 and zero respectively. These orders are consistent with the rate limiting step being the turnover of the enzyme under saturated conditions in a thin reaction layer close to the electrode. At lower oxygen concentrations a good fit between the experimental results and a theoretical model further confirms the assignation of the mechanism. The rate constants describing the oxidation and reduction of the enzyme have been measured. The pH dependence of the current has been studied.

The wetting and bonding of Si 3N 4 by copper-titanium alloys with other elements

The wettability of pressureless sintered Si 3N 4 by ternary alloys of copper, titanium and aluminium, silicon and yttrium has been investigated in vacuum at 1160°C. The bond strength of Si 3N 4 to itself was also evaluated by compressive shear test. While the wettability is influenced by the titanium activity, it may be overcome by the surface tension of the alloy. Titanium activity is lowered when aluminium and silicon, which have high interactions with titanium, are added. On the other hand, titanium activity is enhanced when yttrium, which has a low interaction with titanium, is added. It was found that TiN and various Ti-silicides formed in the reaction layer. The type and position of the reaction products are changed by the titanium activity, according to the alloying additions and their contents. Higher strength was obtained with the addition of aluminium and thin reaction layer formation. As the reaction temperature and time were increased, shear strength decreased due to the increased thickness of the reaction layer and the formation of a brittle phase in spite of improved wettability. Aluminium appeared to be the most effective element for the control of interfacial reaction. Yttrium addition (less than about 5 %wt) was also very effective in increasing strength under very low concentrations of titanium (less than about 5 %wt).

Large-eddy simulation of turbulent diffusion with chemical reactions in the convective boundary layer

Bottom-up and top-down diffusion of two reacting air constituents in a dry homogeneous convective boundary layer with zero mean wind are simulated using large-eddy simulations. The model includes a simple binary, irreversible reaction without heat release. Subgrid-scale contributions of reactions are neglected as justified by spectral analysis of the results. The results depend upon the ratio R of time scales for convection relative to that of the chemical reaction, on the concentration ratio of the two reacting components, and on the initial conditions. Cases with zero, finite and infinite reaction rate are considered. Without reactions, the bottom-up diffusivity is about twice as large as top-down diffusivity due to buoyancy effects. For infinite reaction rate, the thin reaction layer becomes highly convoluted. For R > 0.1, i.e. for parameter values which are typical for the reaction between ozone and nitrogen oxide in the atmosphere, the reaction rate is strongly influenced by turbulence. The effective eddy diffusivities may be enhanced or reduced by chemical reactions, depending on the importance of gradient forcings of mass fluxes relative to other source and sink terms. A simple ‘box-model’ describes the time evolution of the mean concentrations in the mixed layer fairly well for R ⩽0.1 and for infinite reaction rate.

Product distribution in preparative-scale electrolysis: Part VIII. Transition between fast and slow chemical kinetics. Application to mechanisms involving one or two first-order consecutive chemical steps

The variations of the product distribution in preparative-scale electrolysis with the various experimental parameters are an important tool for both mechanistic studies and yield optimization in cases where competing chemical reactions are coupled with the initial electron transfer steps. Two limiting situations have been treated so far, involving respectively fast chemical reactions taking place in a thin reaction layer, thinner than the diffusion layer, or, conversely, slow chemical reactions taking place exclusively outside the diffusion layer. The intermediate case of moderately fast reactions and thus the passage from one situation to the other is investigated for two reaction schemes involving one and two irreversible follow-up chemical steps. The variations of the product concentrations with time are discussed as functions of a minimal number of dimensionless rate parameters.

Interfacial reaction between silicon nitride and aluminium

Abstract The microstructure of a silicon nitride/aluminium interface has been identified by analytical transmission electron microscopy. HIPed silicon nitride without any additive was bonded by pure aluminium at 800°C for 15 min. A thin (∼ 5000 A) reaction layer was recognized at the interface. It consisted of two regions. One an amorphous layer facing the aluminium; it contained aluminium, silicon and oxygen. The other was on the silicon nitride side and consisted of fine particles which were less than 100 A in diameter. These particles were considered to have β'-sialon type structure.

Corrosion of Fused Silica Refractories by Molten CaO-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-SiO&lt;sub&gt;2&lt;/sub&gt; Slag

The corrosion of fused silica refractories in molten CaO-Al2O3-SiO2 slag was studied. The experiment was carried out with a rotating cylindrical specimen. The retults are as follows:(1) The diameter of specimens decreased linearly with time. The increase both in temperature and rotating speed promoted the corrosion.(2) A thin reaction layer was observed on the surface of the specimen. The change in SiO2 concentration in the slag was best described by a rate equation (which was derived) for the mass transport controlled mechanism. The apparent activation energy of corrosion was 64kcal/mol, implying that the corrosion is controlled by diffusion of ion in the slag.(3) A non-dimensional equation κ=αU0.67 was obtained, the exponent of which agreed with the literature values.

The mechanism of faradaic reactions at the thionine coated electrode

Results are presented for the reduction of Fe(III) and Fe(CN) 3− 6 by leucothionine in a thionine modified electrode. Systematic analysis of the variation of the reaction rate for Fe(III) with rotation speed, film thickness and electrode potential proves that the reaction is a “surface” reaction between the outermost layer of leucothionine and Fe(III) in solution. A general equation is derived which includes both the “surface” reaction and the case where the reaction takes place in a thin reaction layer in the firm. These two parallel processes are shown to be distinct and special cases of the general equation. The value of the heterogeneous rate constant for the surface reaction is in reasonable agreement with a value estimated from the second order rate constant for the homogeneous reaction. The Fe(III) reaction is located in a block diagram which shows all the possible mechanisms for this type of modified electrode. The rate of the Fe(CN) 3− 6 reaction is five orders of magnitude faster so that the rate is controlled either by the transport of Fe(CN) 3− 6 through the electrolyte or by the transport of electrons through the film. This change in the rate limiting process agrees with the position of the system in the block diagram.

Separation ofthe partial currents for reduction of hydronium ion and water in dilute strong acid solutions using the sinusoidal hydrodynamic modulation method

The simultaneous mass-transport controlled reduction of hydrogen ions and charge-transfer controlled reduction of water has been studied in 1 M KCl solutions of dilute strong acid. Using the sinusoidal hydrodynamic modulation technique, SHM, at a rotating disk electrode, RDE, these studies confirm the existence of a thin reaction layer at the electrode surface where the neutralization reaction between H+ and OH− produced from water reduction causes a decrease in the limiting flux of hydrogen ions at the electrode. An approximate solution to the convective-diffusion problem has been obtained, as has the decrease in modulation current response and the thickness of the reaction boundary layer, δr. The negligible distortion in the conventional convective-diffusion limiting RDE current accompanying the parallel processes is explained by the magnitude of δr. Separability of the mass-transfer and non-mass transfer controlled components by the SHM method is demonstrated with studies at various applied potentials, modulation amplitudes and frequencies.

Separation of the partial currents for reduction of hydronium ion and water in dilute strong acid solutions using the sinusoidal hydrodynamic modulation method

The simultaneous mass-transport controlled reduction of hydrogen ions and charge-transfer controlled reduction of water has been studied in 1 M KCl solutions of dilute strong acid. Using the sinusoidal hydrodynamic modulation technique, SHM, at a rotating disk electrode, RDE, these studies confirm the existence of a thin reaction layer at the electrode surface where the neutralization reaction between H + and OH − produced from water reduction causes a decrease in the limiting flux of hydrogen ions at the electrode. An approximate solution to the convective-diffusion problem has been obtained, as has the decrease in modulation current response and the thickness of the reaction boundary layer, δ r. The negligible distortion in the conventional convective-diffusion limiting RDE current accompanying the parallel processes is explained by the magnitude of δ r. Separability of the mass-transfer and non-mass transfer controlled components by the SHM method is demonstrated with studies at various applied potentials, modulation amplitudes and frequencies.

Agglomeration and ignition mechanism of aluminum particles in solid propellants

Aluminum powders added to conventional rocket propellants burn either as single particles or agglomerates which contain hundreds or even thousands of the original particles. Combustion efficiency and acoustic stability characteristics are very dependent on the final Al/Al2O3 particle size injected into the chamber flowfield. High-speed photographs of burning homogeneous propellants provided data on agglomeration size and visualization of the flow processes as a function of pressure (1 to 10 MPa), initial particle size (5 to 100 μm), and aluminum mass fraction (0.1 to 13%). Photomicrographs of extinguished surfaces revealed the importance of particle accumulation in a thin mobile reaction layer adjacent to the burning surface. A model was developed that interpreted data and observations from several sources. The model accounts for accumulation of aluminum particles in the mobile reaction layer, retention of particles by surface tension forces, melting, and ignition at the surface. The following agglomeration and particle behavior items are categorized: decreasing agglomerate size with increasing pressure, minimum mass loading required for agglomeration, prominent agglomeration for particles with diameters less than the reaction layer thickness, and sharply reduced agglomeration for larger particles. The model provides an approach for controlling and interpreting agglomerate size behavior.

Hydrodynamic voltammetry at channel electrodes Part III. Theory of kinetic currents

A theory of kinetic currents at a channel electrode is developed without any assumption on the magnitude of the rate constant of the preceding chemical reaction. An approximate equation with reasonable accuracy which relates the limiting current to the rate constant, flow velocity and the electrode geometries, is presented. The validity of the equation derived from the concept of the thin reaction layer is also discussed.

Hydrodynamic voltammetry at channel electrodes: Part III. Theory of kinetic currents

A theory of kinetic currents at a channel electrode is developed without any assumption on the magnitude of the rate constant of the preceding chemical reaction. An approximate equation with reasonable accuracy which relates the limiting current to the rate constant, flow velocity and the electrode geometries, is presented. The validity of the equation derived from the concept of the thin reaction layer is also discussed.

Messung der Stärke einer dünnen reaktionsschicht unter einer deckschicht mit der röntgenfluoreszenz

A method is described to measure the thickness Δ x of a thin reaction layer AB v betwen a substrate A and a thin overlayer B by X-ray fluorescence. Formulae are given for the calculation of Δ x from X-ray intensity measurements; Δ x can also be determined by means of calibration procedures. Calculation of Δ x from X-ray fluorescence measurements and determination of Δ x by weighing after dissolving the overlayer B give results that agree well, provided the absorption of the fluorescence beam is strong enough and the thickness of the overlayer B, before the reaction, is several thousand Ångströms.