Thermal stability and cathodoluminescence of potassium strontium vanadates

We have studied the thermal stability of five potassium strontium vanadates: KSr(VO₃)₃, K₂Sr(VO₃)₄, K₄Sr(VO₃)₆, K₆Sr(VO₃)₈, and KSrVO₄. The double orthovanadate undergoes a reversible polymorphic transformation at 1117°C and is stable up to 1500°C. The double metavanadates melt peritectically in the range 490–517°C to give Sr₂V₂O₇ crystals and peritectic melt. Pulsed cathodoluminescence studies have shown that the potassium: strontium ratio in the vanadates and their crystal structure have little effect on their optical emission properties. The performance parameters of new vanadate-based phosphors have been determined.     

UV-photoassisted etching of GaN in KOH

The etch rate of GaN under ultraviolet-assisted photoelectrochemical conditions in KOH solutions is found to be a strong function of illumination intensity, solution molarity, sample bias, and material doping level. At low e-h pair generation rates, grain boundaries are selectively etched, while at higher illumination intensities etch rates for unintentionally doped (n～ 3×10¹⁶cm^?3) GaN are ≥1000Å·min^?1. The etching is diffusion-limited under our conditions with an activation energy of ～ 0.8kCal·mol^?1. The etched surfaces are rough, but retain their stoichiometry.     

Binding energy of exciton-impurity complexes in semiconductors with diamond and zinc blende structure

The binding energies of four complexes — exciton + charged impurity, exciton + neutral impurity — were calculated by a variational method in semiconductors with diamond and zinc blende structure taking into account the degeneracy of the valence-band edge. The numerical calculations were performed for exciton-impurity complexes in a series of II–VI, III–V, and IV–IV crystals. Fiz. Tekh. Poluprovodn. 32, 583–587 (May 1998)     

Characterization of Au/MnO x /TiO2 for Photocatalytic Oxidation of Carbon Monoxide

The Au/MnO _x /TiO₂ catalyst was used for the photocatalytic oxidation of carbon monoxide. The catalytic activity of Au/MnO _x /TiO₂ with low concentration of manganese (3–7 mol%) was much higher than that of Au/TiO₂. The surface of Au/MnO _x /TiO₂ was characterized by XPS and Raman spectroscopy. While the main state of manganese in 13.8 mol% MnO _x /TiO₂ was Mn⁴⁺ species, Mn³⁺ was the dominant species in the samples with below 6.5 mol% manganese. Raman spectroscopy revealed that the interaction between the MnO _x and TiO₂ form Mn–O–Ti species in which the state of manganese was Mn³⁺. The Au particles also interacted with both MnO _x and TiO₂ to modify the surface of them. In the case of the Au species, low loading of manganese produced the metallic Au⁰ and perimeter interfacial Au^δ+, whereas high loading showed the coexistence of three components which were metallic Au⁰, perimeter interfacial Au^δ+, and oxidic Au³⁺. The catalytic active component was the metallic Au⁰ and perimeter interfacial Au^δ+ species, which were dispersed on TiO₂ and Mn³⁺/TiO₂.     

Role of complexation in forming composition of Co-W coatings electrodeposited from gluconate electrolyte

The purpose of the given research has been to specify the route of formation of Co-W coatings. To this end, the gel-chromatographic separation of gluconate electrolyte has been used, as well as the photo-colorimetric determination of the composition of the separation products, and the evaluation of the electro-chemical activity of these complexes in the pH range of 4.0 to 8.0. As a result, it has been found out that the composition of the Co-W coatings thus formed is defined by the relation of the rates of the reduction of those heterometallic complexes (with the Co-W ratio 1: 1) and Co complexes in solution, as well as the reaction of hydrogen reduction, dependent on both the electrolyte pH and hydrodynamic conditions.     

Conditions and delay time of ignition of iron microparticles in oxygen

Experiments on autoignition and combustion of iron microparticles in oxygen have been conducted. The parameters of the gas medium were created using a rapid-compression machine. Investigations have been carried out on powders with particle size from 20 to 125 μm at an oxygen pressure of to 28 MPa and a temperature from 650 to 1100 K. The ignition delay was measured by recording radiation from the lateral wall of the combustion chamber and determining the pressure on the end wall. Autoignition conditions have been represented as the dependences of the oxygen density and pressure at the end of the compression stroke on the temperature and the particle size. A point semiempirical mathematical model making it possible to satisfactorily describe experimental data on the dependence of the delay time of ignition of a microparticle fill on the oxygen temperature has been proposed.     

Stability and durability of PtRu catalysts supported on carbon nanofibers for direct methanol fuel cells

The chemical stability and durability of PtRu catalysts supported on carbon nanofibers (CNFs) for the anode electrode of a direct methanol fuel cell (DMFC) are investigated by Pt and Ru dissolution tests in sulfuric acid and long-term performance tests of a single cell discharging at a constant current density of 150 mA cm⁻² for approximately 2000 h. A CNF with a herringbone-type structure, which is characterized by the alignment of graphene symmetrically angled to the fiber axis, was selected as the catalyst support because it has an edge-rich surface and a high surface area. In the metal dissolution test, the PtRu/CNF catalysts showed 1.5–2 times lower Ru leaching than a tested commercial catalyst (supported on activated carbon). The results of long-term performance tests also prove the higher durability of the anode catalyst compared with the commercial catalyst, when the anode polarization is compared before and after operation for 2000 h. Some analytical measurements, including X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy were conducted to study the degradation of the catalyst activity.     

Sr0.92Y0.08TiO3−δ/Sm0.2Ce0.8O2−δ anode for solid oxide fuel cells running on methane

Yttria-doped strontium titanium oxide (Sr_0.92Y_0.08TiO_3−δ; SYT) was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The SYT synthesized by the Pechini method exhibits excellent phase stability during the cell fabrication processes and SOFC operation and good electrical conductivity (about 0.85 S/cm, porosity 30%) in reducing atmosphere. The performance of SYT anode is characterized by slow electrochemical reactions except for the gas-phase diffusion reactions. The cell performance with the SYT anode running on methane fuel was improved about 5 times by SDC film coating, which increased the number of reaction sites and also accelerated electrochemical reaction kinetics of the anode. In addition, the SDC-coated SYT anode cell was stably operated for 900 h with methane. These results show that the SDC-coated SYT anode can be a promising anode material for high temperature SOFCs running directly on hydrocarbon fuels.     

Combinatorial investigation of Pt–Ru–Sn alloys as an anode electrocatalysts for direct alcohol fuel cells

Low-temperature direct alcohol fuel cells fed with different kinds of alcohol (methanol, ethanol and 2-propanol) have been investigated by employing ternary electrocatalysts (Pt–Ru–Sn) as anode catalysts. Combinatorial chemistry has been applied to screen the 66-PtRuSn-anode arrays at the same time to reduce cost, time, and effort when we select the optimum composition of electrocatalysts for DAFCs (Direct Alcohol Fuel Cells). PtRuSn (80:20:0) showed the lowest onset potential for methanol electro-oxidation, PtRuSn (50:0:50) for ethanol, and PtRuSn (20:70:10) for 2-propanol in CV results respectively, and single cell performance test indicated that Ru is more suitable for direct methanol fuel cell system, Sn for direct ethanol fuel cell system, and 2-propanol could be applied as fuel with low platinum composition anode electrocatalyst. The single cell performance results and electrochemical results (CV) were well matched with the combinatorial electrochemical results. As a result, we could verify the availability of combinatorial chemistry by comparing the results of each extreme electrocatalysts compositions as follows: PtRuSn (80:20:0) for methanol, PtRuSn (50:0:50) for ethanol and PtRuSn (20:70:10) for 2-propanol.     

Peak-switching currents of power transformers

The switching conditions of nonloaded transformers have been investigated to understand the effect of current inrushes and limitations by considering the connecting circuit of the windings, magnetization performances, contact parameters, type of switcher drive, and the value and sign of residual magnetization. The peak-switching currents have been calculated, simulated, and measured and the results have been compared. The obtained results make it possible to increase the power efficiency of the facility by decreasing the losses and maintainign the quality factor of electric power in transient modes.