Dielectric relaxation protonic conduction

Fig. 13. Hydration dependence of protonic conduction. The dielectric relaxation time, Ts, is shown versus hydration, h, for lysozyme powders. The relaxation time is proportional to the reciprocal of the conductivity. (A) H20-hydrated samples solid curve, lysozyme without substrate , lysozyme with equimolar (GlcNAc)< at pH 7.0 , with 3x molar (G1cNAc)4 at pH 6.5. The relaxation time is nearly constant between pH 5.0 and 7.0. (B) HjO-hydrated samples solid curve, lysozyme without substrate 9, lysozyme with equimolar (GlcNAcb at pH 7.0. From Careri etal. (1985). 

Frequency dependent conductivity, microwave dielectric relaxation and proton dynamics... 

Progress in the understanding of superionic conduction is due to the use of various advanced techniques (X-ray (neutron) diffuse scattering, Raman spectroscopy and a.c.-impedance spectroscopy) and-in the particular case of protons - neutron scattering, nuclear magnetic resonance, infrared spectroscopy and microwave dielectric relaxation appear to be the most powerful methods. A number of books about solid electrolytes published since 1976 hardly mention proton conductors and relatively few review papers, limited in scope, have appeared on this subject. Proton transfer across biological membranes has received considerable attention but is not considered here (see references for more details). 

To conduct proton conductivity measurements, Buchi et al. [3] designed a current interruption device that used an auxiliary current pulse method and an instrument for generating fast current pulses (i.e. currents > 10 A), and determined the time resolution for the appropriate required voltage acquisition by considering the relaxation processes in the membrane of a PEM fuel cell [3]. They estimated that the dielectric relaxation time, or the time constant for the spontaneous discharge of the double-layer capacitor, t, is about 1.4 x 10 ° s. They found that the potential of a dielectric relaxation process decreased to <1% of the initial value after 4.6r (6.4 x 10 s) and that the ohmic losses almost vanished about half a nanosecond after the current changes. Because there is presently no theory about the fastest electrochemical relaxation processes in PEM fuel cells, the authors assumed a conservative limit of 10 s, based on observations of water electrolysis membranes. They concluded that the time window for accurate current interruption measurements on a membrane is between 0.5 and 10 ns. Another typical application of the current interruption method was demonstrated by Mennola et al. [1], who used a PEM fuel cell stack and identified a poorly performing individual cell in the stack. 

Chief among the interfacial properties of aqueous systems that suggest the occurrence of thermal anomalies are the following index of refraction, density, activation energy for ionic conductance, rates of surface reactions, surface tension, surface potentials, membrane potentials, heats of immersion, zeta potentials, rate of nucleation, viscous flow, ion activities, proton spin lattice relaxation times, optical rotation, ultrasonic velocity and absorption, sedimentation rates, coagulation rates, and dielectric properties. 

From (9.32), which shows that is determined by the least effective conductivity mechanism, it is evident that this static conductivity may be quite critically dependent upon the presence of proton-donor or proton-acceptor impurities and, in this sense, the behaviour of ice is quite analogous to that of electronic semiconductors like germanium or silicon. Mole fractions of impurity which are significant are of the order of io . The high-frequency conductivity and relaxation time t are, from (9.30) and (9.35), rather less sensitive to impurity content than is doping levels. The static dielectric constant depends on purity in a rather complex way, as we shall discuss presently. 

The incorporation of tunicin whiskers induced an approximately threefold decrease in conductivity. Possible explanations include (i) the low dielectric constant of ceUulosic fillers, (ii) interactions between cellulose and PEO, and (iii) the effect of whiskers on salt dissociation and ion mobility. Indeed, the relaxation time of H protons was significantly reduced with the addition of whiskers, and the diffusion coefficients for both cation and anion decreased by almost a factor of 3." The decrease in ionic mobility is in... 

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