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In the context of the search for fundamental size effects regarding electrical conduction, we analyze the transition from semi-infinite or isolated to overlapped depletion layers in appreciably dense nanocrystalline SrTiO3 ceramics occurring if the grain size becomes smaller than ∼ 100 nm. At these small grain sizes, the bulk impedance signal that is seen clearly for microcrystalline samples disappears, leaving only one impedance response, which can be attributed to the space-charge zones. Space-charge potentials and Mott—Schottky lengths are calculated and shown to be consistent with the assumption of the mesoscopic condition. In accordance with the increased homogeneity of the mesoscopic situation in an 80 nm sample, and unlike for microcrystalline samples, the impedance response is characterized by almost an ideal capacitance that no longer originates from space-charge polarization. These conclusions are corroborated by detailed numerical calculations using linear irreversible thermodynamics. [ABSTRACT FROM AUTHOR]

Abstract: The validity of impedance response measurements within the plateau region of galvanostatic charge–discharge curve of LiFePO4 cathode material was studied in detail. Based on (i) measurement of dependence of impedance response on the mode (voltage, current) and amplitude of excitation, (ii) analysis of higher harmonics of the response signal, and (iii) comparison of impedance response with time-domain measurements, we conclude that above a certain critical frequency that lies in the range 1–100mHz, the impedance response of LiFePO4 satisfies the criteria of linearity and time invariance (LTI). At lower frequencies the impedance response becomes scattered and does not satisfy the LTI criteria anymore. The critical frequency at which the transition from LTI to non-LTI behaviour occurs, depends on the amplitude of current excitation signal. [Copyright &y& Elsevier]

Abstract: Based on careful analysis of nine papers by different research groups, we show, for the first time, that in LiFePO4-based cathode materials the electrode resistance depends solely on the mean particle size. The effect of carbon coating is marginal, it suffices that each LiFePO4 particle is point-contacted with a reasonable number of carbon black particles usually added in the course of electrode preparation. We present a simple but general theoretical model that consistently explains this unexpected result. The main reason for the relatively small importance of carbon coatings is the fact that the ionic conductivity (ca. 10−11–10−10 Scm−1 at RT) is much smaller than the electronic (>10−9 Scm−1 at RT). The present finding could be of significant importance not only for further optimization of LiFePO4 cathodes, but also for preparation of other cathode materials in which the ionic conductivity is much lower than the electronic. [Copyright &y& Elsevier]

Abstract: An alternative route to lower the flammability of viscose fibres is presented. Instead of adding a flame retardant to the viscose dope chemically, we have grown a layer of silica (SiO2) on the surface of a regenerated cellulose fibre via a sol–gel process. One set of samples was used as-received, while the other was pre-treated in an 18% NaOH solution, giving a rough, etched surface to the fibre. The different surface morphology of both fibre types triggered a different growth of silica layers. On an untreated fibre, silica formed a 300–400nm thick surface layer containing a high density of cracks and holes. Conversely, on a NaOH pre-treated fibre, the silica layer intruded into fibre interior, adhered more tightly to the fibre structure and formed an almost defect-free, thin (100nm) layer on the outer fibre surface. This type of silica layer increased the temperature at which the fibre started to decompose by 20°C. It also hindered significantly the flow of oxygen to the generated volatiles during the thermal decomposition, and increased the temperature of glowing combustion of the residual char; the temperature of the corresponding exothermic peaks increased by ca. 20°C and 40°C. In contrast, the thermal effects of silica coatings that grew on an untreated fibre were much smaller. [Copyright &y& Elsevier]

We try to identify the rate-determining step of electrochemical kinetics of a LiFePO(4)-carbon composite electrode by varying the mass of electrode and, additionally, by varying the charge-discharge current in a wide range. It is shown that the reversible capacity is almost independent of electrode mass at currents lower than ca. 1 C (170 mAh g(-1)). At higher currents, however, the reversible capacity starts to drop significantly. The electrode resistance determined from the corresponding polarization voltage shows inverse proportionality with mass at currents smaller than 1 C. At higher currents the electrode resistance is almost independent of electrode mass. We conclude that at lower currents (below 1 C) the main transport step is related to the active particles themselves (either to incorporation reaction or solid state diffusion of Li). At higher currents the contribution of electronic and ionic transport towards the active particles becomes substantial and should be taken into account when designing high-rate insertion electrodes.