776 research outputs found
Hot compaction of nanocrystalline TiO2 (anatase) ceramics. Mechanisms of densification: Grain size and doping effects
The hot compaction of nanocrystalline TiO2 anatase powders is investigated using dilatometry. The constant rate of heating (CRH) method is applied to determine effective activation energies of the processes involved during sintering. Grain size and doping effects are studied, using dopant cations of different radius and charge: Zn2+, Al3+, Si4+, Nb5+. The results are interpreted by a mechanism including superplastic deformation and boundary diffusion. The former is predominant for small particles and low temperature, whereas the latter is more important for larger particles and higher temperature. Dopant effects on densification kinetics are discussed in view of defect chemistry
Design of organic-inorganic hybrid membranes based on sulfonated polyaryl-ether-ketones: correlation between water uptake and electrical conductivity.
Cu/Ni/Au multilayers by electrochemistry: A crucial system in electronics - A critical review
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The Interstellar N/O Abundance Ratio: Evidence for Local Infall?
Sensitive measurements of the interstellar gas-phase oxygen abundance have
revealed a slight oxygen deficiency ( 15%) toward stars within 500 pc of
the Sun as compared to more distant sightlines. Recent observations of
the interstellar gas-phase nitrogen abundance indicate larger variations, but
no trends with distance were reported due to the significant measurement
uncertainties for many sightlines. By considering only the highest quality
( 5 ) N/O abundance measurements, we find an intriguing trend in
the interstellar N/O ratio with distance. Toward the seven stars within
500 pc of the Sun, the weighted mean N/O ratio is 0.217 0.011, while for
the six stars further away the weighted mean value (N/O = 0.142 0.008) is
curiously consistent with the current Solar value (N/O =
0.138). It is difficult to imagine a scenario invoking
environmental (e.g., dust depletion, ionization, etc.) variations alone that
explains this abundance anomaly. Is the enhanced nitrogen abundance localized
to the Solar neighborhood or evidence of a more widespread phenomenon? If it is
localized, then recent infall of low metallicity gas in the Solar neighborhood
may be the best explanation. Otherwise, the N/O variations may be best
explained by large-scale differences in the interstellar mixing processes for
AGB stars and Type II supernovae.Comment: accepted for publication in the Astrophysical Journal Letter
Sol–gel synthesis, X-ray photoelectron spectroscopy and electrical conductivity of Co-doped (La, Sr)(Ga, Mg)O3−δ perovskites
La0.8Sr0.2Ga0.8Mg0.2−xCoxO3−δ (LSGMC) powders containing different amounts of Co (x = 0.05 and 0.085) were prepared by a citrate sol–gel method. The powders were used to prepare highly phase-pure LSGMC sintered pellets with controlled composition and fractional densities larger than 95%. For the first time, LSGMC materials were subjected to X-ray photoelectron spectroscopy (XPS) characterization. XPS data confirmed the presence of the dopants in the material and allowed to identify two different chemical states for Sr2+ and oxygen, both related to the oxygen-deficient perovskite structure of LSGMC. The conductivity of LSGMC sintered pellets containing different amounts of Co ions in the B sites of the perovskite lattice was assessed by electrochemical impedance spectroscopy (EIS) in the 250–750 °C temperature range. Conductivity values and apparent activation energies were in good agreement with previously published data referring to materials with same composition, but prepared by solid-state route. Therefore, the physicochemical and electrochemical characterization clearly demonstrated the ability of sol–gel methods to produce high-purity Co-doped LSGM perovskites, which represent promising solid electrolytes for intermediate-temperature SOFCs
Slow, Steady-State Transport with "Loading" and Bulk Reactions: the Mixed Ionic Conductor LaCuO
We consider slow, steady transport for the normal state of the superconductor
LaCuO in a one-dimensional geometry, with surface fluxes
sufficiently general to permit oxygen to be driven into the sample (``loaded'')
either by electrochemical means or by high oxygen partial pressure. We include
the bulk reaction OO, where neutral atoms () go into ions
() and holes (). For slow, steady transport, the transport equations
simplify because the bulk reaction rate density and the bulk loading rates
then are uniform in space and time. All three fluxes must be
specified at each surface, which for a uniform current density corresponds
to five independent fluxes. These fluxes generate two types of static modes at
each surface and a bulk response with a voltage profile that varies
quadratically in space, characterized by and the total oxygen flux
(neutral plus ion) at each surface. One type of surface mode is associated with
electrical screening; the other type is associated both with diffusion and
drift, and with chemical reaction (the {\it diffusion-reaction mode}). The
diffusion-reaction mode is accompanied by changes in the chemical potentials
, and by reactions and fluxes, but it neither carries current (J=0) nor
loads the system chemically (). Generation of the diffusion-reaction
mode may explain the phenomenon of ``turbulence in the voltage'' often observed
near the electrodes of other mixed ionic electronic conductors (MIECs).Comment: 11 pages, 1 figur
Silica containing composite anion exchange membranes by sol–gel synthesis: a short review
This short review summarizes the literature on composite anion exchange membranes (AEM) containing an organo-silica network formed by sol-gel chemistry. The article covers AEM for diffusion dialysis (DD), for electrochemical energy technologies including fuel cells and redox flow batteries, and for electrodialysis. By applying a vast variety of organically modified silica compounds (ORMOSIL), many composite AEM reported in the last 15 years are based on poly (vinylalcohol) (PVA) or poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) used as polymer matrix. The most stringent requirements are high permselectivity and water flux for DD membranes, while high ionic conductivity is essential for electrochemical applications. Furthermore, the alkaline stability of AEM for fuel cell applications remains a challenging problem that is not yet solved. Possible future topics of investigation on composite AEM containing an organo-silica network are also discussed
Single-step electrodeposition of superhydrophobic black NiO thin films
International audienceBlack finished surfaces have extensive applications in many domains, such as optics, solar cells, and aerospace. The single step electrodeposition of superhydrophobic black NiO films from a dimethyl sulfoxide based electrolyte is described in this paper. The physicochemical properties of the obtained film were characterized using Scanning Electron Microscopy, X-ray Diffraction, and electrochemical tests (Electrochemical Impedance Spectroscopy and potentiodynamic polarization). A rough surface with a low reflection of light was formed after the deposition process that increased the contact angle of water from about 87º (for bare Cu) to 163º (in presence of the black coating), which improved the corrosion resistance of the Cu substrate by about 30%. The formed black NiO film revealed a notably high stability and kept its appearance even after corrosion tests
Analysis of Temperature-Promoted and Solvent-Assisted Cross-Linking in Sulfonated Poly(ether ether ketone) (SPEEK) Proton-Conducting Membranes
Crosslinked SPEEK membranes: Mechanical, thermal, and hydrothermal properties
The thermal and mechanical behavior, the water uptake (WU), and water diffusion coefficient of sulfonated poly(ether ether ketone) (SPEEK)membranes annealed at 180 degrees C for different times were explored by high-resolution thermogravimetric analysis, mechanical tensile tests, dynamic mechanical analysis, and WU measurements. The mechanical and thermal stability increased with the thermal treatment time, i.e., with the degree of crosslinking. The effect of residual casting solvent, dimethyl sulfoxide (DMSO), on the WU within SPEEK was probed. In presence of residual DMSO, crosslinked SPEEK exhibited higher water sorption at low and medium relative humidity (RH), and lower water sorption at high RH. These membranes have properties well adapted to fuel cell applications
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