Activation energy for ionic conduction

The activation energy for ionic conductivity is derived from a plot of ... 

Estimate the activation energy for ionic conductivity in the Lu2Ti207 phase illustrated in Figure 6.5b. 

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. 

Figure 18 shows the influence of caustic concentration upon conductivity of the membrane. The decrease of conductivity with the increase in caustic concentration is ascribed to the decrease in mobility of sodium ions caused by the dehydration of the membrane. The increase of apparent activation energy for ionic conductance along with caustic concentration as is given in Table IE reflects the existance of increasing interaction between sodium ion and the fixed ion in the membrane. 

Table III Apparent activation energy for ionic conductance in the membrane...

Extensive experiments have been carried out on the effect of impurity ions on the kinetics of decomposition, the optical properties, and the temperature dependence of ionic conductivity of several azides in an attempt to determine the nature and concentration of the species in the material. Torkar and colleagues studied the kinetics and conductivity of pure and doped sodium azide [97] and observed that cationic impurities and anionic vacancies speed up decomposition by acting as electron traps which facilitate the formation of nitrogen from N3. They also found that the activation energy for ionic conductivity was close to that for decomposition, implying a diffusion-controlled mechanism of decomposition. These results are qualitatively in accord with the microscopic observations of decomposition made by Secco [25] and Walker et al. [26]. 

The temperature dependence of the conductivity can be described by the classical Arrhenius equation a = a"cxp(-E7RT), where E is the activation energy for the conduction process. According to the Arrhenius equation the lna versus 1/T plot should be linear. However, in numerous ionic liquids a non-linearity of the Arrhenius plot has been reported in such a case the temperature dependence of the conductivity can be expressed by the Vogel-Tammann-Fuller (VTF) relationship a = a°cxp -B/(T-T0), ... 

Since the activation energy for ionic recombination is mainly due to viscosity we use the activation energy for viscous flow (10kJ.mol l). AH ] and 3 were determined from conductance as 44.2kJ.mol and 11,4kJ.mol From the data presented in Table III it is clear that the temperature dependence of the slope is very satisfactorily described by A% +l/2(AHd-AH3). Another, and rather critical, test for the applicability of eq. 14b is the effect of pressure since the slope of eq. 14b is largely pressure independent so that we ask here for a compensation of rather large effects. From Table III we Indeed see an excellent accordance between the experimental value and the pressure-dependence calculated from the activation volume of viscous flow (+20.3 ctPmol ), AVd (-57.3 cnAnol" ) and (-13.9 cnAnol ) the difference between the small experimental and calculated values is entirely with the uncertainties of compressibility - corrections and experimental errors. 

It is an interesting fact that the activation energy for electrolytic conductance is almost identical with that for the viscous flow of water, viz., 3.8 kcal. at 25 hence, it is probable that ionic conductance is related to the viscosity of the medium. Quite apart from any question of mechanism, however, equality of the so-called activation energies means that the positive temperature coefficient of ion conductance is roughly equal to the negative temperature coefficient of viscosity. In other words, the product of the conductance of a given ion and the viscosity of water at a series of temperatures should be approximately constant. The results in Table XVI give the product of the conductance of the acetate ion at... 

The data in Table 1 also illustrate several additional facets of solid state ionic conduction. First the fast (or super ) ion conductors are charaeterized by very low activation energies for conduction (12-36 kj/mol) compared to the normal ionic conductors (> 60 kJ/mol). The low activation energy for superionic conduction is a major contributing factor to the high ionic conductivity at relatively low temperatures. 

Interestingly increases with increasing Pma, denoting an increase in the apparent activation energy for proton conduction which varies from 13-24 kJ/mol. Nevertheless the increase in proton conductivity is due to the increase in the preexponential factor A, which must be directly related to the increase of the ionic charge carriers concentration in the membrane. 

Conductivity measurements have been carried out on powder samples of NaEu2Cl6 by means of impedance spectroscopy (fig. 12). The activation energy for the conduction process was calculated to 0.23 eV The value fits into the range one would expect for semiconductors on the other hand, there should be contributions of the Na ions to the overall conductivity. With the partial filling of the sodium sites and the channel along the c-axis the structure certainly fiilfills the requirements for ionic conduction. Measurements on powder pellets of NaSrEuClg and AgSrEuClg indeed proved ionic motion but the... 

Such solid solutions are solids with chemical disorder because the ionic defects are generated by chemical composition and distributed statistically. Their concentration is independent on temperature and oxygen partial pressure. The activation energy for ion conduction is much higher ( 100 kJ/mol) as compared to that for solids with structural disorder ( 20-40 kJ/mol). 

---