摘要:

AbstractIn investigations of the isothermal transport properties of mixed conductors the dynamical coupling between ionic and electronic partial fluxes is usually neglected. Within the formalism of (linear) irreversible thermodynamics this assumption is expressed by a vanishing cross coefficientL12in the matrix of the phenomenological transport coefficients. By analysing the phenomenological flux equations for cation vacancies and electron holes, experiments for the determination of the cross coefficient are defined. Experimental results for the coupling effect in Co1‐δO are presented, which have been obtained from three independent techniques. The results are discussed in the light of previous theoretical considerations. It is shown that a strong coupling between cation vacancies and electron holes exists in the case of Co1‐δO. To account for this coupling, an effective charge number of ions has to be introduced for use in the formulation of ionic f

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981002

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

年代:1994

数据来源: WILEY

摘要:

AbstractDiffusion of carbon in titanium carbide was investigated over the entire phase field between 1179 and 2413 K using suitably designed diffusion couples.Concentration‐depth profiles of carbon were taken by SNMS and EMPA. The diffusion coefficients of carbon obtained by the two analytical techniques in different temperature ranges agree remarkably well. Above 1468 K, they exhibit a strong concentration dependence and can be expressed by\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm In}\,D_{\rm c} = 26.3 - 148.5x_{\rm c} + 150.3x_{\rm c}^2 - \frac{{310\,{\rm kJ}/{\rm mol}}}{{{\rm RT}}}. $$\end{document}The thermodynamic factor obtained from a thermodynamic analysis of the TiC system was used to describe the concentration dependence. Tracer diffusion coefficients calculated from the chemical diffusion coefficients with the help of the thermodynamic factor are in accordance with literature data. Below 1468 K concentration‐independent diffusion coefficients were obtained which yielded a lower energy of activation:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm In}\,D_{\rm c} = - 21.2\frac{{174\,{\rm kJ}/{\rm mol}}}{{{\rm RT}}}. $$\end{document}This behavior is attributed to the preponderance of grain boundary diffusion over volume diffusion at low temperatures in the samples at h

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981003

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

年代:1994

数据来源: WILEY

摘要:

AbstractWe present a theoretical study concerning the effect of the electric field poling process on the molecular angular distribution of polar nonlinear‐optical molecules in a polymeric environment. This is required for investigating the contribution of the molecular order to the anisotropy and nonlinear‐optical processes (second‐harmonic generation (SHG), electro‐optic Pockels (EOPE) and Kerr (EOKE) effects). We demonstrate mathematically that the order parameters at thermodynamical equilibrium are defined exactly by the spherical modified Bessel functions (An(μE/kT) =in(μE/kT)/io(μE/kT)). They can be related to Langevin functions but with complicated relations for high orders ofn. Their evolution withμE/kTconfirms that the second order nonlinear‐optical processes (SHG and EOPE) are more sensitive to the strength of the poling field than are the linear (birefringence) and third‐order nonlinear‐optical effects (EOKE). We show that it is possible to measure the molecular anisotropy of nonlinear‐optical molecules by quadratic electro‐optic experiments. The transients of the poling are studied with the rotational diffusion equation, characterized by a rotational diffusion constantD. This is solved using important recurrence relations of Legendre polynomials. An almost exact analytical solution is found for the physical and practical case whereμE/kTis close to 1, and the nonvanishing order parameters are almost equal to the rigourous expressions given by the spherical modified Bessel functions. It is shown that the onset of birefringence and nonlinear‐optical processes (SHG, EOPE and EOKE) involves two time constants τ1,2= [2D(2 ± √1–(μE/kT)2/5)]−1which depend onDandμE/kT. After the removal of the poling field, the relaxation of the linear and nonlinear‐

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981004

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe passive state of Ni–Cr (80/20) coatings prepared by a plasma beam sputter deposition process was studied using cyclic voltammetry and impedance spectroscopy techniques. The highly protective quality of the film formed in acetate buffer, pH = 5.7, is probably based on chromic species. The nickel content in the film was very low and influenced only the electronic properties of the film.The appearance of a distinct transition in the capacitance potential characteristic may be correlated with the presence of higher valent chromium species. Cyclic voltammetry demonstrated that Cr(VI) was incorporated into the outer part of the oxide film, or was tenaciously adsorbed on the electrode surface. Its formation and reduction were reversibl

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981005

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

年代:1994

数据来源: WILEY

摘要:

AbstractSpin‐coated Thin films of peroxopolytungstic acid (PTA) showed pronounced photocoloration under UV irradiation only when the films contained a reducing reagent having OH groups. Decoloration occurred when the colored films were exposed to oxidizing reagents. The origin of the photocoloration is likely a reaction between excited triplet state of PTA and OH groups of the reducing reagents. Accumulation of reducing power in the films, and competition between the photocoloration and known photohardening process were discussed; an application of these characteristics was also show

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981006

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

年代:1994

数据来源: WILEY

摘要:

AbstractPulse radiolysis study of liquid benzyl chloride has been carried out at room temperature. In order to identify transient species that absorb in the UV‐vis region, various scavengers were used including oxygen, ethanol, pyrrole, biphenyl, and mesitylene (1,3,5‐trimethylbenzene). The spectrum of the benzyl cation was obtained by subtracting the transient spectrum for the 0.6 M solutions of ethanol in O2‐saturated benzyl chloride from that for O2‐saturated benzyl chloride. The difference spectrum obtained has a strong absorption band at 303±2 nm and a weak broad band near 500 nm. The analyses of end products formed in theγradiolysis of benzyl chloride were done in the presence and absence of scavengers. The formation of benzyl ethyl ether with aGvalue of 0.9 from a 0.6 M solution of ethanol in O2‐saturated benzyl chloride was taken as evidence that added ethanol reacts with benzyl cations to give benzyl ethyl ether. Some pulse radiolysis experiments were performed with benzyl bromide. Transient spectra observed in benzyl bromide are similar to those observed in benzyl chloride. It is, however, inferred from the product analysis that the radiolytic mechanisms of benzyl bromide appreciably differ from those of benz

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981007

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

年代:1994

数据来源: WILEY

摘要:

AbstractOptical fluorescence spectra of 9,9′‐bianthryl (BA) in polyisobutene (PIB) were measured as a function of the excitation wavelength at various temperatures between 210 and 293 K. Irradiation at the red edge of the absorption spectra selectively excites distinct conformers with respect to the torsional angle. This leads to a strong dependence of the vibronic band shape of the fluorescence spectra on the excitation wavelength. The marked temperature dependence of the band shape may be attributed to the viscosity‐dependent deceleration of the torsional relaxation of BA in the highly viscous polymer which prevents the excited state ensemble from reaching thermal equilibrium. The predominant broadening mechanism affecting the band shape is given by a combination of the S1torsional distribution and the torsional angle‐dependent transition frequency. These effects are interpreted quantitatively within a torsional relaxation model based on the description of the torsional dynamics according to a Smoluchowski equation which is adapted to the steady state case. The band shape analysis of the fluorescence spectra enables the determination of the effective S1torsional potential as well as the friction coefficient related to the internal torsional motion of BA in the polymer matrix. A value of ΔV= 325 cm−1is deduced for the local barrier of the S1double minimum potential at ϕ = 90° while the minimum torsional angle is found to be ϕmin= 61°. Additionally, model calculations on time‐resolved fluorescence spectra of BA in apolar solvents are carried out as a function of excitation wavelength and temperature showing the importance of conformational relaxation for the interpretation of transient spectra of fl

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981008

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

年代:1994

数据来源: WILEY

摘要:

AbstractThe viscositiesηof homogeneous binary mixtures of liquids are usually approximated as Inη=ϕ1Inη1+ϕ2Inη2whereϕiandηiare the volume fractions and the viscosities, resp., of the i‐th pure substance; the behavior of real systems is then discussed in terms of Δ Inη, the deviations from the above reference behavior. Here a semi‐empirical approach is presented according to which volume fractions are replaced by the surface fractionsΩito create a more realistic reference state, and the thermodynamic interaction parametergis taken into account. The new equation reads (expressing it for practical purposes still in the terms ofϕi)\documentclass{article}\pagestyle{empty}\begin{document}$$ \Delta \,{\rm In}\,\eta = \frac{{\gamma \delta }}{{1 + \gamma \varphi _2 }}\varphi _2 (1 - \varphi _2) + 2g\frac{{(1 + \gamma)}}{{(1 + \gamma \varphi _2)^2 }}\varphi _2 (1 - \varphi _2); $$\end{document}γis a geometric factor, measuring the difference betweenΩiandϕi, andδ In (η2/η1). The comparison of calculations according to the above equation with experimental data demonstrates that the first term is decisive in the majority of cases and only with systems of large interaction does the second become important. For all nine systems under investigation quantitative agreement between experiment and theory can be achieved using a theoretical value for eitherγorg, and adjusting the other within physically reasonable limits. Possible explanations for the necessity to fit one of

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981009

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

年代:1994

数据来源: WILEY

摘要:

AbstractA relationship between the Raoultian and Henrian activity of any component in any given mixture is derived which proves to be a real alternative to the relationships known so far. The derivation reveals that the Henrian saturation activity of the respective component represents the link between the two reference systems which has to date been overlooked in the literature.

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981010

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

年代:1994

数据来源: WILEY

摘要:

AbstractAqueous dispersions of 1,2‐dioleoyl‐sn‐glycero‐3‐phosphatidylethanolamine (DOPE) exhibit a well‐known phase transition from a lamellar liquid‐crystalline (Lα) to an inverted hexagonal (HII) phase as the temperature is raised above 13°C. Recently, it has been reported [1, 2] that a bicontinuous cubic phase can be induced in this system, when the lipid is rapidly cycled between these two phases. Their X‐ray diffraction results indicated that the cubic lattice is most consistent with either the Pn3m or Pn3 space group. In studies on similar systems [3, 4], two cubic phases could be detected in the Lα‐HIItransition region, however. One aim of the present study was to investigate, if still further bicontinuous cubic phases can be induced in simple DOPE dispersions by changing the experimental conditions. Indeed, we found that two cubic lattices may be induced when the lipid system is cycled between –5 and 15°C more than thousand times. The second goal of the present investigation was to study the temperature dependence of the lattice parameters, the stability and transformations of these cubic phases over a wide temperature range. It has been discussed that the presence of cubic phases may be a general feature of HII‐forming lipids in the region of their lamellar to hexagonal phase transition region and that non‐lamellar lipid structures might play an important role as transient intermediat

ISSN:0005-9021    

DOI:10.1002/bbpc.19940981011

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

年代:1994

数据来源: WILEY