16 research outputs found
Application of Nonlinear Conductivity Spectroscopy to Ion Transport in Solid Electrolytes
The field-dependent ion transport in thin samples of different glasses is
characterised by means of nonlinear conductivity spectroscopy. AC electric
fields with strengths up to 77 kV/cm are applied to the samples, and the
Fourier components of the current spectra are analysed. In the dc conductivity
regime and in the transition region to the dispersive conductivity, higher
harmonics in the current spectra are detected, which provide information about
higher--order conductivity coefficients. Our method ensures that these
higher--order conductivity coefficients are exclusively governed by
field--dependent ion transport and are not influenced by Joule heating effects.
We use the low-field dc conductivity and the higher--order dc
conductivity coefficient to calculate apparent jump distances
for the mobile ions, . Over a temperature range from 283 K to 353
K, we obtain values for between 39 \AA and 55 \AA . For all
glasses, we find a weak decrease of with increasing temperature.
Remarkably, the apparent jump distances calculated from our data are
considerably larger than typical values published in the literature for various
ion conducting glasses. These values were obtained by applying dc electric
fields. Our results provide clear evidence that the equation used in the
literature to calculate the apparent jump distances does not provide an
adequate physical description of field-dependent ion transport.Comment: 6 pages, 5 figure
Nonlinear Ionic Conductivity of Thin Solid Electrolyte Samples: Comparison between Theory and Experiment
Nonlinear conductivity effects are studied experimentally and theoretically
for thin samples of disordered ionic conductors. Following previous work in
this field the {\it experimental nonlinear conductivity} of sodium ion
conducting glasses is analyzed in terms of apparent hopping distances. Values
up to 43 \AA are obtained. Due to higher-order harmonic current density
detection, any undesired effects arising from Joule heating can be excluded.
Additionally, the influence of temperature and sample thickness on the
nonlinearity is explored. From the {\it theoretical side} the nonlinear
conductivity in a disordered hopping model is analyzed numerically. For the 1D
case the nonlinearity can be even handled analytically. Surprisingly, for this
model the apparent hopping distance scales with the system size. This result
shows that in general the nonlinear conductivity cannot be interpreted in terms
of apparent hopping distances. Possible extensions of the model are discussed.Comment: 7 pages, 6 figure
Nanoscopic Study of the Ion Dynamics in a LiAlSiO Glass Ceramic by means of Electrostatic Force Spectroscopy
We use time-domain electrostatic force spectroscopy (TD-EFS) for
characterising the dynamics of mobile ions in a partially crystallised
LiAlSiO glass ceramic, and we compare the results of the TD-EFS
measurements to macroscopic electrical conductivity measurements. While the
macroscopic conductivity spectra are determined by a single dynamic process
with an activation energy of 0.72 eV, the TD-EFS measurements provide
information about two distinct relaxation processes with different activation
energies. Our results indicate that the faster process is due to ionic
movements in the glassy phase and at the glass-crystal interfaces, while the
slower process is caused by ionic movements in the crystallites. The spatially
varying electrical relaxation strengths of the fast and of the slow process
provide information about the nano- and mesoscale structure of the glass
ceramic.Comment: 5 pages, 4 figure
The role of atomic vacancies on phonon confinement in α-GeTe
Atomic defects and their dynamics play a vital role in controlling the behavior of non-volatile phase change memory materials used in advanced optical storage devices. Synthesis and structural analysis by XRD and Raman spectroscopy on α-GeTe single crystal with different sizes are reported. The spectroscopic measurements on micron and nano sized α-GeTe single crystal reveal the evolution of phonon confinement with crystal sizes of few hundred nanometers. The characteristic vibrational modes of bulk α-GeTe structure are found to downshift and asymmetrically broaden to lower frequency with decreasing the single crystal size. We attribute the observed downshift of Raman lines in α-GeTe is largely due to the presence of high concentration of atomic vacancies. The crystal size and temperature dependent Raman spectra provide explicitly the dynamics of vacancies on optical phonon confinement in α-GeTe structure. Thus, the observed large concentration of vacancies and their size dependency might influence the phase change phenomenon in GeTe based alloys
Polaron transport mechanism in maricite NaFePO4: A combined experimental and simulation study
We report, for the first time, systematic investigations on electronic properties of maricite NaFePO4 with different crystallite sizes by a combined experimental and theoretical approach. Ac impedance spectroscopy has been used to study the polaron transport behaviour in maricite NaFePO4 structure with different crystallite sizes over a wide range of temperatures. With the decrease in crystallite size, we observe a polaronic conductivity enhancement of approximately an order of magnitude at the nanoscale level as compared with its bulk counterpart. The temperature dependent dc conductivity has been analysed within the framework of the Mott model of polaron hopping and various physical parameters relevant for the polaron hopping process were extracted. Additionally, by introducing an approximated Mott model with calculated hole polaron migration barrier from density functional theory, we evaluated the polaronic conductivity as function of crystallite size in fair agreement with experimental data. The enhanced polaronic conductivity with crystallite size reduction is found to be due to the combined effect of increased polaron concentration, reduced hopping length, and lowered migration barrier
Size induced structural changes in maricite-NaFePO 4 : an in-depth study by experiment and simulations
Rechargeable batteries based on the most abundant elements, such as sodium and iron, have a great potential in the development of cost effective sodium ion batteries for large scale energy storage devices. We report, for the first time, crystallite size dependent structural investigations on maricite-NaFePO4 through X-ray diffraction, X-ray absorption spectroscopy and theoretical simulations. Rietveld refinement analysis on the X-ray diffraction data reveals that a decrease in the unit cell parameters leads to volume contraction upon reduction in the crystallite size. Further, the atomic multiplet simulations on X-ray absorption spectra provide unequivocally a change in the site symmetry of transition metal ions. The high resolution oxygen K-edge spectra reveal a substantial change in the bonding character with the reduction of crystallite size, which is the fundamental cause for the change in the unit cell parameters of maricite-NaFePO4. In parallel, we performed first-principles density functional theory (DFT) calculations on maricite-NaFePO4 with different sodium ion vacancy concentrations. The obtained structural parameters are in excellent agreement with the experimental observations on the mesostructured maricite-NaFePO4. The volumetric changes with respect to crystallite size are related to the compressive strain, resulting in the improvement in the electronic diffusivity. The nano-crystalline maricite-NaFePO4 with improved kinetics will open a new avenue for its usage as a cathode material in sodium ion batteries
Direct correlation between non-random distribution of cations and ion transport mechanism in soda-lime silicate glasses
We report a direct correlation between dissimilar ion pair formation and alkali ion transport in soda-lime silicate glasses established via broad band conductivity spectroscopy and local structural probe techniques. The combined Raman and Nuclear Magnetic Resonance (NMR) spectroscopy techniques on these glasses reveal the coexistence of different anionic species and the prevalence of Na+-Ca2+ dissimilar pairs as well as their distributions. The spectroscopic results further confirm the formation of dissimilar pairs atomistically, where it increases with increasing alkaline-earth oxide content These results, are the manifestation of local structural changes in the silicate network with composition which give rise to different environments into which the alkali ions hop. The Na+ ion mobility varies inversely with dissimilar pair formation, i.e. it decreases with increase of non-random formation of dissimilar pairs. Remarkably, we found that increased degree of non-randomness leads to temperature dependent variation in number density of sodium ions. Furthermore, the present study provides the strong link between the dynamics of the alkali ions and different sites associated with it in soda-lime silicate glasses. (C) 2014 Elsevier B.V. All rights reserved
Mesoporous bioactive glass and glass-ceramics: Influence of the local structure on in vitro bioactivity
Mesoporous quaternary bioactive glasses and glass-ceramic with alkali-alkaline-earth oxide were successfully synthesized by using non-ionic block copolymer P123 and evaporation induced self assembly (EISA) process followed by acid treatment assisted sal-gel method. As prepared samples has been characterized for the structural, morphological and textural properties with the various analytical techniques. Glass dissolution/ion release rate in simulated body fluid (SBF) was monitored by inductively coupled plasma (ICP) emission spectroscopy, whereas the formation of apatite phase and its crystallization at the glass and glass-ceramic surface was examined by structural, textural and microscopic probes. The influence of alkaline-earth oxide content on the glass structure followed by textural property has become more evident. The pristine glass samples exhibit a wormhole-like mesoporous structure, whereas the glass-ceramic composition is found to be in three different phases, namely crystalline hydroxyapatite, wollastonite and a residual glassy phase as observed in Cerabone (R) A/W. The existence of calcium orthophosphate phase is closely associated with the pore walls comprising nanometric-sized ``inclusions''. The observed high surface area in conjunction with the structural features provides the possible explanation for experimentally observed enhanced bioactivity through the easy access of ions to the fluid. On the other hand, presence of multiple phases in glass-ceramic sample inhibits or delays the kinetics of apatite formation. (C) 2013 Elsevier Inc. All rights reserved