19 research outputs found
Impurity Deposition on a First Mirror Surface during Hydrogen Discharges in LHD
First mirrors in fusion devices have to retain the reflectivity and the degradation mechanism and dominant plasma operations that affect the reflectivity have to be understood. The reflectivity of visible laser light (635 nm) of a corner cube mirror is almost entirely determined by the initial hydrogen glow discharges for wall conditioning before conducting a main plasma experiment in the Large Helical Devices (LHD). The hydrogen glow discharge forms a carbon deposition layer on the mirror surface, which degrades its reflectivity. A neon glow discharge and subsequent main discharges had less effect
New Insight into Structural Evolution in Layered NaCrO<sub>2</sub> during Electrochemical Sodium Extraction
Electrochemical
properties and structural changes during charge for NaCrO<sub>2</sub>, whose structure is classified as α-NaFeO<sub>2</sub> type
layered polymorph (also O3-type following the Delmas’ notation),
are examined as a positive electrode material for nonaqueous Na-ion
batteries. NaCrO<sub>2</sub> delivers initial discharge capacity of
110 mAh g<sup>–1</sup> at 1/20C rate in the voltage range of
2.5–3.6 V based on reversible Cr<sup>3+</sup>/Cr<sup>4+</sup> redox without oxidation to hexavalent chromium ions, while the initial
discharge capacity is only 9 mAh g<sup>–1</sup> when cutoff
voltage is set to 4.5 V. Results from <i>ex-situ</i> X-ray
diffraction, X-ray absorption spectroscopy, and DFT calculations reveal
that the irreversible phase transition occurs after sodium extraction
by charging over a voltage plateau at 3.8 V associated with the lattice
shrinkage along the <i>c</i>-axis in the case of <i>x</i> > 0.5 in Na<sub>1–<i>x</i></sub>CrO<sub>2</sub>, which originates from the migration of chromium ions from
octahedral sites in CrO<sub>2</sub> slabs to both tetrahedral and
octahedral sites in interslab layer. The irreversible structural change
would disturb sodium insertion into the damaged layer structure during
discharge, resulting in the loss of reversibility as electrode materials.
Reversible cycle range with stable capacity retention is, therefore,
limited to the compositional range of 0.0 ≤ <i>x</i> ≤ 0.5 in Na<sub>1–<i>x</i></sub>CrO<sub>2</sub>