摘要:

AbstractThe morphological evolution of initially planar phase boundaries between ionic crystals due to ion fluxes which are driven by an externally applied electric field is investigated. In this particular study, the boundaries under investigation are interfaces between KC1 and AgCl. It is shown that the moving boundary is morphologically unstable if the flux of silver ions is directed towards the KCl‐crystal, whereas it is stable if the flux of potassium ions is driven towards AgCl. This behaviour can be explained by the rules which derive from a formal stability analysis. The geometrical evolution of the moving phase boundary in its later stages is mainly governed by the elastic properties of the contiguous phase

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990102

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractIn binary or quasi‐binary systems the superposition of chemical diffusion and field‐driven migration may lead to unstable diffusion fronts. This means that an initially one‐dimensional transport geometry may become three (two)‐dimensional. This concept is tested with the solid solution AgCl‐NaCl aboveT=200°C, which is above the critical point of the miscibility gap. Morphological instabilities of the concentration profile in the inter‐diffusion zone are found if an external electric field is applied. This field must exceed a critical value and the AgCl‐side must be anodic in order to induce an instability of the diffusion front. Numerical solutions of the transport equations and the given boundary conditions corroborate the experi

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990103

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractBy drawing a constant electric current through a symmetric electrochemical cell of the type Ag|a‐AgI|Ag we observed, under certain experimental conditions, strong periodic oscillations of the voltage across the cell. Depending on current density and applied mechanical force, these oscillations show either periodic or irregular behaviour.Further experiments show that these strong voltage oscillations are caused by the anodic interface at which Ag‐metal is dissolved electrochemically into the solid electrolyte. The phenomenon itself is reproducible, but the appearance of the oscillations is very sensitive to the experimental conditions. We present experimental results for different experimental conditions and cell types and discuss a possible mechanism for the observed phenomenon. An important element of the attempt to find a microscopic explanation is the basic process of forming lattice vacancies in the metal at the anodic interface. The behaviour of these vacancies is crucial for the kinetic properties of the metal‐solid electrolyte interface under non‐equilibrium con

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990104

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractWe have studied the reduction mechanism and electronic conduction during electrodeposition of tantalum in various TaCl5‐alkali chloride melts at temperatures up to 1100 K. This has been performed by simultaneous measurements of impedance and Raman spectra together with cyclic voltammograms. A detailed analysis of electrochemical and spectroscopic results yields the following predominant reduction steps during electrodeposition in pure alkali chloride melts: Ta(V)‐Ta(IV)‐Ta(0). The equilibrium potentials of the corresponding redox reactions are separated by 40 mV which is clearly resolved in impedance measurements. In LiCl/KCl‐eutectic melts a three step mechanism is found with Ta(III) being the intermediate species. In evaluating the impedance data we found indications for electronic contributions which are most prominent with added oxide impurities to the molten electrolyte. Direct evidence for an appreciable electronic conductivity has been obtained by electrochemical polarization measurements. These are reported here the first time for fused KCl‐TaCl5/Ta at 1050 K, where electronic conductivities up to 0.5ω−1cm−1have been measured, i.e. of same order of magnitude as ionic

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990105

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractThe photoelectrochemical investigation of l,3,3‐trimethylspiro(2H‐l‐benzopyran‐2,2′‐indoline) (SP‐O) and l,3,3‐trimethyl‐6‐nitrospiro(2H‐l‐benzopyran‐2,2′‐indoline) (SP‐1) has been performed in a dimethylformamide (DMF) solution at room temperature and at a low temperature (‐42°C). Nitro‐substituted SP‐1 is electroactive in the experimental potential range of ‐1.8V to +0.8 V (vs. Ag/Ag, while no nitro‐substituted parent spirobenzopyran SP‐0 is lacking in electroactivity. In the case of SP‐1, nearly identical electrochemical behavior was found for the closed‐ring (SP) and open‐ring (MC) isomer at room temperature or at ‐42°C, suggesting that the ring form, i.e. open or closed, has no influence on the redox reaction of SP‐1. Moreover, reversible color changes were observed in the DMF solution of SP‐1 depending on reduction and oxidation potentials, namely, a change to yellow in the reduction and to deep violet in the oxidation. Both voltammetric and spectroelectrochemical evidences suggest the production of a nitrobenzopyran radical anion of closed‐ring (yellow) in a reduction and an open‐ring isomer MC (deep violet) in an oxidation either from SP and from MC of SP‐1 in the DMF solution. This behavior leads to the discovery of a novel photochromism and electrochromism “three state” conjugated system which may potentially be applicable to the

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990106

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractCopper colloids have been prepared by y‐radiolysis of cupric ions in the presence of poly(ethyleneimine). The reduction takes place in two, distinct steps. Copper (II) is reduced quantitatively to copper (I), and then nucleation of the colloid occurs after an induction period. The nucleation of colloidal copper is catalysed by the presence of less noble Pb(II) or T1(I) ions, which is explained by the generation of Pb and Tl nuclei in the solution, and their subsequent reoxidization by Cu(I). The formation of copper colloids is also catalysed by the presence of colloidal silver or silver ions, and radiolysis results in the formation of copper coated silver colloids. The optical shifts observed during the deposition of copper on silver cannot be explained by Mie theory. The differences are suggested to be due to weaker interband transitions in the copper monolayers than in the bu

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990107

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractThe adsorption of the nonanoic acid at the electrified Hg/aqueous solution interface is studied by means of capacity and surface tension measurements. Comparison of the experimental rmaxvalue with the calculated more favoured solvated conformation suggests that the acid is adsorbed in a partially coiled conformation. The adsorption free energy here determined, together with previously reported values found in the literature concerning other saturated acids and alcohols, is compared with reactivity and calculated molecular descriptors. The collected evidence suggests that the hydrophobic nature of the acid is responsible for the adsorption, while the electric field/molecule interaction, occurring at the electrified interface, governs the desorption. Lastly, some experimental (capacitive transients, effect of the temperature on the tensammetric curves) and theoretical findings are discussed which indicate that the adsorbed acid monolayer should be regarded as a usual, in the field of adsorption at electrodes, disordered network. Thus excluding the occurrence of a two‐dimensional phase transition involving the formation of a 2D solid‐like f

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990108

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Fe(III)‐catalyzed oxidation of bisulfite in aqueous solution was studied at pH = 3 in the presence of benzene by monitoring the rise of hydrogen ion activity and phenol product concentration as a function of time, keeping [HS03−/[Fetot]>>l. After a brief initial period, during which product concentrations rose rapidly and Fe(III) was largely converted toward Fe(II), the reaction underwent a period of slower growth characterized by a quasistationary state and nearly constant [Fe(II)]/[Fe(III)]ratio until toward the end of the reaction, when most of the S(IV) was consumed, Fe(II) was reoxidized toward Fe(III). In two sets of experiments either the concentration of benzene or that of [Fe(III)]owas varied in ranges of 0.3–2.2 mmol dm−3and 8.5–51 μmol dm−3, respectively, while keeping the other parameters constant ([HSO3−]o= 2 mmol dm−3). The data were analyzed by means of a steady state expression developed for the time period of nearly constant [Fe(II)]/[Fe(III)]ratio on the basis of a radical‐driven chain oxidation mechanism presented previously. The data display the behavior predicted by the steady state equation; this allowed the determination of ratios of rate coefficients for reaction pairs: SO4−reacting with HSO3−and benzene,k11/k12= 0.31±0.04; S05−reacting with Fe2+and HSO3−,k3/k10= 54.1±1.7; HSO5−reacting with Fe2+and HSO3−,k4/k16= 2.2±0.2; and the branching ratio for the reaction of SO5−with HSO3−leading to (a) SO3−and (b) SO4−radicals,k10b/k10≤0.04. The experimental results larg

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990109

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractIgnition temperatures of gaseous mixtures of methane with oxygen or air at initial pressures between 50 and 1000 bar have been measured. Two cylindrical reaction vessels of 4 and 10 cm3and one spherical vessel with 21 cm3volume were applied which withstand dynamic pressures up to 6000 bar at 900 K (600°C). Most experiments used the “heat‐up” method with a high pressure gas mixture heated up inside and together with the reaction vessel until spontaneous rapid reaction began. Additional experiments were made with preheated empty vessels and with forced ignition by hot wires. Spontaneous ignition temperatures for stoichiometric methane‐air mixtures are around 870 K (600°C) at atmospheric pressure and decrease to 660 K (390°C) at 1100 bar ignition pressure. Semenov plots give straight lines for various reaction conditions. Effective activation energies could be derived. With the assumption of a global reaction order of 2, the activation energy for stoichiometric methane‐air is 359±4 kJ/mole, for stoichiometric methane‐oxygen it is 253±10 kJ/mol. Considering earlier experiments with “hydrothermal” supercritical flames, ignition with added high pres

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990110

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY

摘要:

AbstractThe viscosity of the melts of the system KF–K2MoO4–B2O3has been measured by means of the computerised torsional pendulum method.The viscosity of the melts increases both with the increased content of K2MoO4and B2O3. In the binary system KF–K2MoO4the simple additivity of the viscosity has been found. In the ternary system strong deviation from the additive behaviour was found which is possible to ascribe to the chemical reactions between the components.From the obtained results it follows, that the binary interaction in the KF‐B203system corresponds to the chemical reaction8KF+7B2O3= 3K2B4O7+ 2KBF4, while the interaction in the K2MoO4–B2O3system corresponds most probably to the formation of the heteropolyanion of the type [BMo6O24]9‐according to the chemical reaction: 6K2MoO4+ B2O3= K9[BMo6O24] + 3 KBO2.Statistically significant ternary interaction may be explained as the entrance of the fluorine atoms into the coordination sphere of the molybdenum in the heter

ISSN:0005-9021    

DOI:10.1002/bbpc.19950990111

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1995

数据来源: WILEY