Gasless Combustion of Ti–Al Bimetallic Multilayer Nanofoils

Ti–Al multilayer foils were produced magnetron vacuum deposition. The microstructure period varied in the range of 5–110 nm, the number of layers was 150–4700, and the total thickness of a multilayer foil reached 15–20 m. The gasless combustion of the foils was studied. Steadystate and pulsating combustion regimes were revealed; combustion temperatures were determined for both regimes. It was shown that the most probable mechanism of the selfpropagating reaction is the diffusion of Al in Ti at a temperature close to the temperature of the transition.     

Growth kinetics of bubbles electrogenerated at microelectrodes

The growth kinetics of electrogenerated hydrogen, oxygen and chlorine gas bubbles formed at microelectrodes, were determined photographically and fitted by regression analysis to the equation;r(t)=t ^x , wherer(t) is the bubble radius at timet after nucleation, the growth coefficient, andx the time coefficient. The coefficientx was found to decrease from a short time (< 10 ms) value near unity, typical of inertia controlled growth, through 0.5, characteristic of diffusional control, to 0.3, expected for Faradaic growth, at long times (\s> 100 ms). The current efficiency for bubble growth increased with bubble lifetime, reflecting the decrease in local dissolved gas supersaturation. The pH dependency of the bubble departure diameter indicated that, in surfactant-free electrolytes, double layer interaction forces between the negatively charged hydrogen evolving cathode or positively charged oxygen/chlorine evolving anode and positively (pH \s< 2) or negatively (pH \s> 3) charged bubbles, were the determining factor. The effect of addition of an increasing concentration of cationic (DoTAB) or anionic (SDoS) surfactant was to progressively reduce the pH effect on departure diameter, due to surfactant adsorption on the bubble and, to a lesser extent, on the electrode.Nomenclature C coefficient [3] - D diffusion coefficient (m² s^–1) - I current (A) - P pressure (kN m^–2) - R universal gas constant (8.314 J mol^–1 K^–1) - r bubble radius (m) - T absolute temperature (K) - t time (ms) - x time coefficient - zF molar charge (96 487z C mol^–1) - growth coefficient (m s^–0.33) - P Laplace excess pressure (kN m^–2) - surface tension (mN m^–1) - electrolyte density (kg m^–3) - contact angle () Paper presented at the International Meeting on Electrolytic Bubbles organized by the Electrochemical Technology Group of the Society of Chemical Industry, and held at Imperial College, London, 13–14 September 1984.     

Electrochemistry of anodic F2 evolution at carbon electrodes: Bubble adherence effects in the kinetics at rotating cone electrodes

Fluorine-evolving carbon anodes exhibit unusually high overvoltages characterized also by remarkably large Tafel slopes having values 0.4–0.8 V per decade of current density change. Also, at high current densities, a so-called anode effect associated with a type of passivation sets in. Experiments are described which aim to distinguish high polarization arising from an intrinsically large Tafel slope, generated by a non-ohmic charge transfer barrier layer effect due to CF film formation, from effects due to difficulties of F₂ bubble detechment and F₂ gas film formation at the CF film. Steady state polarization measurements have been made at a rotating carbon cone electrode from which F₂ bubbles, which otherwise remain attached to the electrode and block access to the electrolyte, can be spun away. At the rotated electrode, at low and intermediate current densities, linear Tafel behaviour is still observed but with high slopes associated with the barrier layer film effect. At higher current densities an anode effect, associated with the F₂ gas film, is developed, leading to a type of passivation of the electrode. The two sources of unusually high polarization in the F₂ evolution reaction at carbon are not independent as it is also the formation of the CF film that causes difficulties in gas bubble detachment owing to the lyophobic properties of the fluorinated C/F₂/KF·2HF interface. Polishing effects confirm this conclusion.     

Electrochemical reduction of nickel ions from diluteartificial solutions in a GBC reactor

The electrochemical reduction of nickel ions from a dilute solution has been carried out in a gas diffusion electrode packed bed electrode cell (GBC). Particle size and electrode configuration have been found to have a significant influence on the reduction process. Electrodes with a high porosity and large pores have been found to be the best for nickel deposition. The nickel current efficiency, Ni, is reported to be dependent on the current density, volumetric flow rate, nickel and boric acid concentration, and the pH. The fall in the nickel current efficiency is caused by an increase in electrode surface pH above a certain level, caused by either high bulk solution pH or high current density, leading to possibly formation of Ni(OH)2. It has been found that under conditions of exclusively metallic nickel deposition Ni/(1–Ni) is proportional to i0.69,Q10.52,CNi0.67,CBA–0.19 and pH1.0.     

Methane oxidation over PdO x : on the mechanism for the hysteresis in activity and oxygen content

A conceptual picture is developed to explain the peculiar kinetic features of methane oxidation over supported Pd catalysts (observed by several investigators), notably the hysteresis in activity accompanying temperature cycles. Experiments were performed with supported Pdcatalysts to illustrate these features. The activity hysteresis is closely coupled with a hysteresis in oxygen content. The latter is in turn attributed to the properties of the PCT-diagram of the involved three-phase system; gas phase O₂ and the two solid phases, Pd and PdO_x. The two main ingredients in the mechanism are: (i) the so-called absorption and decomposition plateau pressures for the O₂-Pd-PdO _x system are different, i.e., show a hysteresis, (ii) these pressures are not independent ofx, but increase with increasing oxygen content. Both features are deviations from the ideal three-phase system and are frequently observed for H₂-metalmetal hydride systems.     

Pore Propagation Directions in P+ Porous Silicon

A comparative study is presented on the pore propagation directions of porous silicon layers (PSL) formed on p⁺-type substrates of different orientations. PSLs were formed on plain (0 0 1) and (1 1 1) silicon wafers as well as on structured (0 0 1) wafers containing facets of various orientations. During anodization, regular pores follow the 0 0 1 direction on the (0 0 1) planes. While on the (1 1 1) planes fewer regular pores develop and seemingly propagate closely to the 1 1 1 direction. These results indicate that the pores propagate perpendicular to the surface i.e. along the field lines when the surface orientation is either (0 0 1) or (1 1 1).When the silicon surface provided (1 1 0) orientation (Chuang, Collins, and Smith, 1989), or its position is in between the (0 0 1) and (1 1 1) planes then the pores do not propagate perpendicular to the surface but along the 0 0 1 direction.All the phenomena exhibited might be explained by presuming that during formation, the pores propagate along the 1 0 0 directions, and that those 1 0 0 directions are preferred which are closely to the field lines. In PSLs formed on (0 0 1) surfaces the field lines and the 0 0 1 crystallographic direction are coincident. However, in the (1 1 1) oriented wafer where three equally probable 1 0 0 directions exist around the field lines, more irregular structure of PSLs will develop.     

Mechanisms of electroless metal plating. III. Mixed potential theory and the interdependence of partial reactions

Electroless plating reactions are classified according to four overall reaction schemes in which each partial reaction is either under diffusion control or electrochemical control. The theory of a technique based on the observation of the mixed potential as a function of agitation, concentration of the reducing agent and concentration of metal ions is presented. Using this technique it is shown that in electroless copper plating the copper deposition reaction is diffusion-controlled while the formaldehyde decomposition reaction is activation-controlled. Values of the kinetic and mechanistic parameters for the partial reactions obtained by this method and by other electrochemical methods indicate that the two partial reactions are not independent of each other.Nomenclature a Tafel slope intercept - A electrode area - b _M Tafel slope for cathodic partial reaction - b _R Tafel slope for anodic partial reaction - B _M diffusion parameter for CuEDTA^2– complex - diffusion parameter for dissolved oxygen - B _R diffusion parameter for HCHO - C _M bulk concentration of copper ions - bulk concentration of dissolved oxygen - C _R ^a surface concentration of HCHO - C _R bulk concentration of HCHO - D _R diffusion coefficient of HCHO - E electrode potential - E _M thermodynamic reversible potential for the metal deposition reaction - E _M ⁰ standard electrode potential for copper deposition - E _MP mixed potential - E _R thermodynamic reversible potential for reducing agent reaction - E _R ⁰ standard electrode potential for HCHO - F Faraday constant - i _M current density for metal deposition - i _M total cathodic current density - i _M ^k kinetic controlled current density for metal deposition - i _M ⁰ exchange current density for metal deposition - i _M ^D diffusion-limited current density for metal deposition - i _M ^D diffusion-limited current density for total cathodic reactions - current density for oxygen reduction - i _plat plating current density - i _R current density for HCHO oxidation - i _R ⁰ exchange current density for HCHO oxidation - i _R ^D diffusion-limited current density for HCHO oxidation - n _M number of electrons transferred in metal deposition reaction - n _R number of electrons transferred in the HCHO oxidation reaction - R gas constant - T absolute temperature - stoichiometric number - _M transfer coefficient for metal deposition - _R transfer coefficient for HCHO oxidation - _M symmetry factor - number of steps prior to rate determining step - _M overpotential for metal deposition - _R overpotential for HCHO oxidation - v kinematic viscosity - rotation rate of electrode     

Generalized analytical expressions for Tafel slope,reaction order and a.c. impedance for the hydrogen evolution reaction (HER): mechanism of HER on platinum in alkaline media

Following the generally accepted mechanism of the HER involving the initial proton discharge step to form the adsorbed hydrogen intermediate, which is desorbed either chemically or electrochemically, generalized expressions for the Tafel slope, reaction order and the a.c. impedance for the hydrogen evolution reaction are derived using the steady-state approach, taking into account the forward and backward rates of the three constituent paths and the lateral interactions between the chemisorbed intermediates. Limiting relationships for the Tafel slope and the reaction order, previously published, are deduced from these general equations as special cases. These relationships, used to decipher the mechanistic aspects by examining the kinetic data for the HER on platinum in alkaline media, showed that the experimental observations can be consistently rationalized by the discharge-electrochemical desorption mechanism, the rate of the discharge step being retarded on inactive platinum compared to the same on active platinum.Nomenclature C _d double-layer capacity (µF cm^–2) - E _rev reversible electrode potential (V) - F Faraday number (96 487 C mol^–1 ) - R gas constant - T temperature (K) - Y _f Faradaic admittance (^–1 cm^–2) - Y _t Total admittance (^–1 cm^–2) - Z _f Faradaic impedance ( cm²) - i _f total current density (A cm^–2) - i _nf nonfaradaic current density (A cm^–2) - j - k ₀ ¹ rate constant of the steps described in Equations 1 to 3 (mol cm^–2 s^–1 ) - j - qmax saturation charge (µC cm^–2) - Laplace transformed expressions for i, and E - _{1 3} symmetry factors for the Equations 1 and 3 - saturation value of adsorbed intermediates (mol cm^–2) - overpotential - coverage by adsorbed intermediates - angular frequency This paper is dedicated to Professor Brian E. Conway on the occasion of his 65th birthday, and in recognition of his outstanding contribution to electrochemistry.     

The electrochemical properties of polyaniline derivatives: poly(4,4′-diphenylamine methylenes) and poly(4,4′-diphenylimine methines)

Summary The electrochemical properties of poly(4,4-diphenylamine methylenes) and poly(4,4-diphenylimine methines) were investigated by cyclic voltammetry. A dehydrogenation reaction occured when poly(4,4-diphenylamine methylenes) underwent a electrochemical reaction and transformed to poly(4,4-diphenylimine methines). The fully oxidized poly(4,4-diphenylimine methines) had the electrochemical band gaps of 1.60–1.72 eV, which were found to significantly smaller than those of the fully reduced poly(4,4-diphenylamine methylenes).     

Textural and structural studies on nickel hydroxide electrodes. II. Turbostratic nickel (II) hydroxide submitted to electrochemical redox cycling

The textural and structural modifications involved in electrochemical redox cycling of turbostratic nickel (II) hydroxide has been investigated using X-ray diffraction and electron microscopy methods. It was found that during the first cycles, different phenomena compete: redox reactions which occur in the solid state, and ageing reactions via the solution. For the first galvanostatic charge performed at the C/5 rate in 4.5 N KOH, the direct oxidation of (II) to (III) and the ageing of (II) to (II) via the solution followed by the oxidation to (III) are in competition. The study of the discharge mechanism shows that the direct reduction (III)(II) is parallel to the reduction (III)(II) and the ageing of the turbostratic hydroxide via the solution. After the first cycle it was established that the alpha-generated (II) active phase consisted of a mixture of two kinds of particles, the oxidation of which follows two paths: (II)/(III) for the thicker particles and for the thinner (II)/(III), but these latter (II) particles aged via the solution by Oswald ripening and the (II)/(III) couples swung to (II)/(III).     

Corrosion inhibition of copper in near neutral aqueous solutions by gamma-pyrodiphenone

The inhibition effect of gamma-pyrodiphenone (PD) on copper corrosion in near neutral sulphate solutions was investigated. Weight loss and polarization measurements show a high inhibition efficiency (>96%) of PD on copper corrosion at extremely low concentrations. PD suppresses active copper dissolution and oxygen reduction. The potentiodynamic cathodic reduction measurements showed an inhibition of oxide formation and stabilization of the Cu2O oxide in the presence of PD. XPS measurements suggested that the high inhibition effect of PD on copper corrosion is due to the adsorption of PD molecules, as a first stage, and the formation of a film with a polymer-like structure by coordination of PD with Cu2O on prolonged exposure.     

The impedance of the Ledanché cell. I. The treatment of experimental data and the behaviour of a typical undischarged cell

The impedance spectrum of an undischarged commercial Leclanché cell (Ever Ready type SP11) is presented in the forms of the Sluyters plot and the modified Randies plot. The decomposition of the experimental cell impedances into the component parts has been achieved using a computer. The decomposition process and the component processes representing the overall cell behaviour are described.List of symbols R _s in-phase component of (experimental) electrode impedance - R _t charge transfer resistance referred to nominal area of Zn ( cm²) - 1/(C _s) out-of-phase component of (experimental) electrode impedance - angular frequency (= 2f) - R resistance of electrolyte solution - charge transfer resistance - C _L double layer capacitance - C _DL double layer capacitance of electrode referred to nominal area of Zn (F cm^–2) - j –1 - Warburg coefficient - D factor in Equations 1 and 2 - C _s R _s calculated values ofC _s andR _s (first approximation) - C _s R _s calculated values ofC _s andR _s (refined values taking into account the additional network) - C _s R _s calculated values of C_s andR _s (refined values taking into account porosity) - _x resistive part of additional series component (parallel connection) - C _x capacitance part of additional series component (parallel connection) - D factor in Equations 6 and 7     

Chemical modification of copoly(methylsilylene/1,4-phenylene) by Pt-catalyzed hydrosilation reactions with functionally substituted alkenes

Copoly(methyl-3-biphenoxypropylsilylene 1,4-phenylene), copoly(methyl-3-cyanopropylsilylene 1,4-phenylene), copoly(methyl-3-ethoxypropylsilylene 1,4-phenylene), copoly(methyl-3-phenoxypropylsilylene 1,4-phenylene), and copoly(methyl-4,7,10,13-tetraoxatetradecanylsilylene 1,4-phenylene) have been prepared by platinum-catalyzed hydrosilation graft reactions between poly (methylsilylene 1,4-phenylene) and appropriate functionally substituted alkenes. These polymers have been characterized by¹H,¹³C, and²⁹Si NMR as well as by FT-IR and UV-vis spectroscopy. The molecular weight distribution of these polymers has been determined by gel permeation chromatography (GPC), and their glass transition temperatures (T _e) by DSC.     

Properties ofSrMO 3-δ(M=Fe,Co) as oxygen electrodes in alkaline solution

Strontium ferrates and cobaltates with compositions SrFeO_3- (0.060.40) and SrCoO_3- (0.040.30) were synthesized. The dependence of the oxygen electrode properties on the value was examined in 1 mol dm^–3 KOH solution. In the SrFeO_3- series, the samples with 0.24<<0.29, showed the highest activity in both oxygen evolution and reduction reactions. In contrast, no strong dependence on the value was observed in SrCoO_3-, which also showed a high catalytic activity for oxygen evolution.     

Experimental Methods for Studying the Combustion of Granular Powders in a Broad Range of Process Parameters

The combustion of granular powders is studied in a broad range of loading densities. The nontraditonal design of manometric bombs and the model ballistic installations on which the conditions close to a real detonation were simulated were used. It was established experimentally that the instability of a shot from a light–gas gun is caused by poor reproducibility of the ignition and combustion of powders fired at a loading density of 0.5 g/cm³. It is shown that the differences between the specific features of gas formation upon combustion of fine tubular and multiperforated powders under conditions of a classical manometric bomb ( 0.3 g/cm³) and in a real shot ( 0.6 g/cm³) are connected not only with the nonstationary character of combustion, but also with the dependence of the shape of a burning grain (area of the burning surface) on experimental conditions and, in particular, on the loadng density. It is shown that the use of a stepwise dependence of the factor in a geometrical law of combustion allows one to obtain a good agreement between the experimental and calculated pressure curves, including the ignition period, which makes it possible to describe the combustion of combined charges more correctly.     

Hydrodynamic effects on the efficiency of porous flow-through electrodes

Hydrodynamic conditions in porous flow-through electrodes are discussed with special emphasis on radial diffusion effects on the efficiency of reactant conversion. The effect of porosity and tortuosity on the conversion efficiency are also considered. It is shown experimentally that radial diffusion limits the electrode efficiency for(L)=vr ²/2DL>0.5 and normal porosity and tortuosity values; q1. For(L)<0.5, the electrode works with 100% efficiency.A porous flow-through electrode is divided, in the most general case, into three regions: (a) velocity entrance length h0.2vr²/v in which a steady velocity profile is developing; (b) diffusional entrance lengthHvr ²/2D for which(x)=vr ²/2Dx1; in this region a radial diffusional concentration profile is developing andh is usually much smaller thanH; (c) the region where the velocity and concentration profiles are fully developed. Only in region (c) does the electrode operate with 100% efficiency. In regions (a) and (b) radial diffusion limits the electrode efficiency.     

Silphinene Sesquiterpenes as Model Insect Antifeedants

Silphinene sesquiterpenes are established chrysomelid antifeedants. In this work, nine silphinene analogs, 11-acetoxy-5-angeloyloxysilphinen-3-one (1), 11-acetoxy-5-tigloyloxysilphinen-3-one (2), 11-acetoxy-5-iso- butyryloxysilphinen-3-one (3), 11-hydroxy-5-angeloyloxysilphinen-3-one (4), 11,5-dihydroxysilphinen-3-one (5), 11,5-diacetoxysilphinen-3-one (6), 5,11-diisobutyryloxysilphinen-3-one (7), silphinen-3,5,11-trione (8), and O-methyl-5-epicantabrenolic acid methyl ester (10), and a presilphiperfolane sesquiterpene (9) were tested against several divergent insect species, including the lepidopteran Spodoptera littoralis, the chrysomelid Leptinotarsa decemlineata, and five aphid species, and their antifeedant effects were compared with those of picrotoxinin, a GABA-antagonist, and thymol, an allosteric modulator for insect GABA receptors. All insects tested responded to at least one silphinene analog and/or GABA antagonist. Compound 3 and thymol were effective antifeedants against all species tested except S. littoralis, with varying potencies according to their feeding ecologies. The toxicity of these compounds was species-dependent and did not correlate with their antifeedant effect.     

Anisotropy in the strength of cellulosic fibres

Conclusions It is proposed to use the equation of a lemniscate, or the Cassini curve, to determine the destructive stress at break for cellulose films in any direction. Thereupon, to determine , it is necessary to know the destructive stress along two basic directions (₀ and ₉₀).To obtain cellulosic films which are isotropic in plane, a necessary and sufficient condition is ₀=₉₀.Translated from Khimicheskie Volokna, No. 6, pp. 34–35, November–December, 1987.     

Mass transfer studies at iron felts

Mass transfer has been studied at flow-through iron felts using the reduction of ferricyanide or copper cementation on iron as test reactions. Empirical correlations between a modified Sherwood number and the Reynolds number are proposed. Comparisons of the mass-transfer performance of iron felts with other three-dimensional structures are made.List of symbols a ₃ specific surface area per unit felt volume (m^–1) - A empty cross-section of the reactor (m²) - C concentration (mol m^–3) - C ₀ inlet concentration (mol m^–3) - d _h hydraulic diameter (m) - e fibre thickness (m) - E electrode potential (V) - D diffusion coefficient (m²s^–1) - F Faraday constant (A s mol^–1) - i current density (A m^–2) - I total current (A) - I _L limiting current (A) - J _m mass transfer j-factor=(k/v)Sc ^2/3 - K mass transfer coefficient (m s^–1) - l fibre width (m) - L electrode thickness (m) - Re Reynolds number - vd _h/ - Re modified Reynolds number - vl/ - Sc Schmidt number = /D - Sh modified sherwood number = ka _e l ²/D - t time (s) - T Temperature (K) - superficial liquid flow velocity (m s^–1) Greek characters void fraction - dynamic viscosity (kg m^–1 s^–1) - kinematic viscosity (m² s^–1) - ₃ charge number of the electrode reaction - iron density (kg m^—) - _a apparent density of the felt (kg m^–3) - _m residence time of the reservoir (s)     

Solutions of the general equations for the radiation dose dependence of the molecular weights of irradiated polymers with an initial Schulz-Zimm distribution

Summary The general equations for the radiation dose dependence of irradiated polymer molecular weights have been solved exactly. For an initial most probable molecular weight distribution (=1), the solutions are analytical and exact. For the general case (1) the solutions are numerical and exact. The present approach has resulted in the solutions for both =1 and 1 being incorporated into a group of FORTRAN computer programs which will solve experimental data for scission and crosslinking yields by both minimization and exact treatments. Simulated data treated using these FORTRAN programs are give. The FORTRAN programs are available from the authors.