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Dynamics of mobile ions in glass - What do conductivity spectra tell us ?
In glassy electrolytes, the diffusive motion of the mobile ions consists of thermally activated hops from site to site. Beyond this statement, little is known about the microscopic dynamics of ionic transport in glass, the problems originating mainly from the lack of long-range order. An important step forward has been made recently by employing the technique of conductivity spectroscopy in a frequency range that covers about fourteen decades, extending up to the far infrared. This particular experimental tool acts as a "microscope in time" resolving hopping processes down to the sub-picosecond time regime. The power of the method is exemplified for the case of a lithium-ion conducting lithium bromide-lithium borate glass, which is representative in many respects. Among the results obtained is the frequent oecurrence of correlated back-and-forth hopping processes as well as the finding that ions have preferences for optimally configured sites which play the role of stepping stones for translational diffusion
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
FC2 stabilizes POR and suppresses ALA formation in the tetrapyrrole biosynthesis pathway
During photoperiodic growth, the light-dependent nature of chlorophyll synthesis in angiosperms necessitates robust control of the production of 5-aminolevulinic acid (ALA), the rate-limiting step in the initial stage of tetrapyrrole biosynthesis (TBS). We are interested in dissecting the post-translational control of this process, which suppresses ALA synthesis for chlorophyll synthesis in dark-grown plants.
Using biochemical approaches for analysis of Arabidopsis wild-type (WT) and mutant lines as well as complementation lines, we show that the heme-synthesizing ferrochelatase 2 (FC2) interacts with protochlorophyllide oxidoreductase and the regulator FLU which both promote the feedback-controlled suppression of ALA synthesis by inactivation of glutamyl-tRNA reductase, thus preventing excessive accumulation of potentially deleterious tetrapyrrole intermediates.
Thereby, FC2 stabilizes POR by physical interaction. When the interaction between FC2 and POR is perturbed, suppression of ALA synthesis is attenuated and photoreactive protochlorophyllide accumulates. FC2 is anchored in the thylakoid membrane via its membrane-spanning CAB (chlorophyll-a-binding) domain.
FC2 is one of the two isoforms of ferrochelatase catalyzing the last step of heme synthesis. Although FC2 belongs to the heme-synthesizing branch of TBS, its interaction with POR potentiates the effects of the GluTR-inactivation complex on the chlorophyll-synthesizing branch and ensures reciprocal control of chlorophyll and heme synthesis.Chinese Scholarship CouncilDeutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Peer Reviewe
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
Fundamental questions relating to ion conduction in disordered solids
A number of basic scientific questions relating to ion conduction in
homogeneously disordered solids are discussed. The questions deal with how to
define the mobile ion density, what can be learned from electrode effects, what
is the ion transport mechanism, the role of dimensionality, and what are the
origins of the mixed-alkali effect, of time-temperature superposition, and of
the nearly-constant loss. Answers are suggested to some of these questions, but
the main purpose of the paper is to draw attention to the fact that this field
of research still presents several fundamental challenges.Comment: Reports on Progress in Physics, to appea
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
Deciphering the Nature of an Overlooked Rate‐Limiting Interphase in High‐Voltage LiNiMnO Cathodes: A Combined Electrochemical Impedance, Scanning Electron Microscopy and Secondary Ion Mass Spectrometry Study
High-voltage cathode active materials, such as LiNiMnO (LNMO), are of major interest for the development of high-energy lithium-ion batteries. However, it has been reported that composite cathodes based on high-voltage active materials suffer from high impedances and low rate capabilities. The origin of the high impedances has not yet been clarified. Here, we use a combination of electrochemical impedance spectroscopy (EIS), focused ion beam/scanning electron microscopy/energy-dispersive X-ray spectroscopy (FIB/SEM/EDX) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) for showing that in the case of LNMO-based cathodes, a major part of the cathode impedance is related to the formation of a passivating interphase on the Al current collector. Remarkably, the impedance of this interphase can be mitigated by the targeted formation of a distinct passivating interphase, namely on the surface of the LNMO particles. The interplay between these interphases is discussed
Specificity and sensitivity evaluation of novel and existing Bacteroidales and Bifidobacteria-specific PCR assays on feces and sewage samples and their application for microbial source tracking in Ireland
Three novel ruminant-specific PCR assays, an existing ruminant-specific PCR assay and five existing human-specific PCR assays, which target 16S rDNA from Bacteroidales or Bifidobacteria, were evaluated. The assays were tested on DNA extracted from ruminant (n = 74), human (n = 59) and non-ruminant animal (n = 44) sewage/fecal samples collected in Ireland. The three novel PCR assays compared favourably to the existing ruminant-specific assay, exhibiting sensitivities of 91 - 100% and specificities of 95 - 100% as compared to a sensitivity of 95% and specificity of 94%, for the existing ruminant-specific assay. Of the five human-specific PCR assays, the assay targeting the Bifidobacterium catenulatum group was the most promising, exhibiting a sensitivity of 100% (with human sewage samples) and a specificity of 87%. When tested on rural water samples that were naturally contaminated by ruminant feces, the three novel PCR assays tested positive with a much greater percentage (52 - 87%) of samples than the existing ruminant-specific assay (17%). These novel ruminant-specific assays show promise for microbial source tracking and merit further field testing and specificity evaluation.ERTD
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