Influence of the purity of the starting materials on the recording of dynamic holograms in Bi12TiO20 crystals

The parameters of Bi₁₂TiO₂₀ crystals grown with bismuth oxide of different degrees of purity were measured by a holographic recording technique in an external ac electric field and were compared with a crystal specially doped with chromium. Various crystal parameters such as the diffusion length of the photoexcited carriers and the Debye screening length were determined. It was found that the absorption spectrum of the material and the holographic recording efficiency were strongly influenced by light chromium doping and by insufficient purity of the starting mixture. Pis’ma Zh. Tekh. Fiz. 23, 27–32 (March 12, 1997)     

Nonequilibrium nonstoichiometry in ionic compounds

Ionic substances under normal conditions are stoichiometric compounds with phase diagrams featuring no homogeneity regions. Crystals grown under nonequilibrium conditions, in particular, under significant centrifugal acceleration, are characterized by nonstoichiometric compositions (e.g., K_2−x Br_1.07). In this way, it is possible to obtain nonequilibrium nonstoichiometric ionic compounds possessing certain homogeneity regions. Nonstoichiometric crystals of KCl, KBr, and KI grown under various accelerations ranging within (1.3–100.0)×10³ g exhibit insignificant variations in the lattice constants and a significant increase in micro-hardness with acceleration.     

A method for determining the charge state of chromium in Cr-doped Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> and Bi<Subscript>12</Subscript>TiO<Subscript>20</Subscript> materials

A chemical method for determination of the charge state of chromium in B₂O₃-based materials (Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀ single crystals) in a wide Cr concentration range (1 × 10^–5 – 2 × 10^–2 wt.%)is proposed. The method is based on a color reaction between Cr⁶⁺ and diphenylcarbazide in an acid medium.Translated from Novye Ogneupory, No. 8, pp. 61 – 63, August, 2004.     

Specific features of operation of a membrane-electrode assembly of an air-hydrogen fuel cell

Specific features of the operation of the membrane-electrode assembly with high catalytic activity that are a part of the simplified design of a low-temperature air-hydrogen fuel cell under conditions of forced and natural convection of air on the cathode are studied. The governing effect of water balance on the specific power of the fuel cell in the stationary mode (～1 h) is shown, and the range of the operating conditions of the cell with self-control is determined. The power of the fuel cell at an efficiency of ～50% and the surface density of platinum on a cathode of ≈0.2 mg/cm² is 200–250 and 100 mW/cm² in the forced and natural air-convection modes, respectively, which is comparable with the advanced results.     

Specific features of oxygen electroreduction on platinized carbon black-functionalized carbon nanotube composite

The electrochemical behavior of multiwalled carbon nanotubes (MWCNTs) containing functional surface groups has been studied by the methods of cyclic voltammetry and direct-current polarography with stationary and rotating disk electrodes. The surface catalytic activity of supported platinum in the reaction of molecular oxygen electroreduction and the influence of functionalized MWCNTs on this process were determined.     

Impurity defects in Cr-doped Bi12SiO20 and Bi12TiO20 crystals

Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀ crystals doped with l0^-5 to 2 x 10^-2 wt % Cr were grown by the Czochralski technique. The Cr content of the crystals and the valence state of Cr ions were determined by a new analytical technique with the use of 1,5-diphenylcarbazide. Correlations were revealed between the segregation coefficient of the dopant, the charge state of Cr ions, and the absorption coefficient of the crystals. Doping with Cr was found to shift the intrinsic edge to longer wavelengths and to give rise to a new absorption band in the near-IR region. The results are interpreted as indicating that the Cr ions in the sillenite crystals studied are in at least two oxidation states and occupy inequivalent sites in the sillenite cell.     

Nuclear magnetic resonance and photomagnetic phenomena in oxygen-deficient ferric borate

Single crystals of ferric borate (FeBO₃), annealed in vacuum so as to provide for the oxygen deficiency and the corresponding excess Fe²⁺ ion concentration, have been studied by ⁵⁷Fe NMR. The NMR signal intensity and the transverse nuclear relaxation rate in oxygen-deficient ferric borate are significantly increased compared to those in the initial state. The annealed samples also exhibit a giant shift of the resonance frequency under the action of light, which is several times as large as the analogous shift in samples with the equilibrium concentration of Fe²⁺ ions. The obtained experimental data confirm the model of photomagnetic phenomena in antiferromagnetic materials that is based on the notion of Fe²⁺-containing complexes playing the role of photosensitive centers.     

Nonuniformity of the macropore etching front in thin <Emphasis Type="Italic">n</Emphasis>-Si wafers

A factor responsible for the appearance of a dome-shaped front of macropore etching in thin silicon wafers during their photoelectrochemical etching in a cell with a ring anode has been established. It is shown that the formation of a dome-shaped front is related to a radial voltage drop in the substrate, which leads to a decrease in the current density in the central part of the etched wafer. The effect of applied voltage on the shape of the front has been studied. We suggest monitoring of the development of nonuniformity in the course of etching using a change (i) in the temporal variation of the adjusted intensity of illumination and (ii) in the current-voltage characteristic. Various methods intended to provide deep uniform macropores are considered.