Impinging-Jets Micromixers and Microreactors: State of the Art and Prospects for Use in the Chemical Technology of Nanomaterials (Review)

Theoretical Foundations of Chemical Engineering - The review considers the micromixing processes and the existing designs, applications, and advantages of impinging-jets micromixers and...     

Field-Emission Cathodes Based on Microchannel Plates

Semiconductors - The existing methods of fabricating low-field cathodes do not permit the development of device structures that comply with the requirements of developers of systems. In this work,...     

Oxygen-induced Restructuring of a Pd/Fe3O4 Model Catalyst

We have studied the influence of oxygen on the structure and morphology of a Pd/Fe₃O₄ model catalyst using molecular beam (MB) methods, IR reflection absorption spectroscopy (IRAS) and scanning tunneling microcopy (STM). The model catalyst was prepared under ultrahigh vacuum (UHV) conditions by physical vapor deposition (PVD) and growth of Pd nanoparticles on an ordered Fe₃O₄ thin film on Pt(111). It is found that surface oxides are formed on the Pd nanoparticles even under mild oxidation conditions (temperatures of 500 K and effective oxygen partial pressures of around 10⁻⁶ mbar). These surface oxides are initially generated at the Pd/Fe₃O₄ interface and, subsequently, are formed at the Pd/gas interface. The process of formation and reduction of surface and interface oxides on the Pd particles is fully reversible in that all oxides formed can be fully reduced. As a result, the oxide phase acts like a storage medium for oxygen during oxidation reactions, as probed via CO oxidation. The process of surface and interface oxidation is directly connected with the onset of a non-reversible sintering process of the Pd particles. It is suggested that this sintering process occurs via a mobile Pd oxide species, which is stabilized by interaction with the Fe₃O₄ support. The restructuring is monitored via STM and IRAS using CO as a probe molecule. In addition to a decrease in particle density and Pd surface area, a reshaping of the particles occurs, which is characterized by the formation of well-ordered crystallites and with a relatively large fraction of (100) facets. After a few oxidation/reduction cycles at 500 K, the sintering process becomes very slow and the system shows a stable behavior under conditions of CO oxidation.     

Fractal generalization of Thomas–Fermi model

The Thomas–Fermi model is developed for a multielectron neutral atom at an arbitrary metric dimension of the electron cloud. It has been shown that the electron cloud with the reduced dimension should be located in the close vicinity of the nucleus. At a metric dimension of the electron cloud of 2, the differential equation of the model admits an analytical solution. In this case, the screening parameter does not depend on the charge of the nucleus.     

深埋地下洞室群动力时程分析中人工边界的设置

针对深埋地下洞室群动力时程分析中建模范围和人工边界设置问题,基于地下洞室群地震波动场的分布特征,提出一种动力时程分析中洞室群模型人工边界的设置方法。该方法允许模型不建至地表,而采用解析法求解地表反射波动场,并通过人工边界入射至模型。基于波场分离原理,使模型四周边界仅透射外行散射波,保障模型四周边界的计算精度。理论试验和实际工程计算表明,该方法计算精度较高,能满足地下洞室群动力时程计算对人工边界的精度要求,同时有效减少模型单元量,缩短动力时程的求解耗时。     

Enhancement of Ca3Co4O9 thermoelectric properties by Cr for Co substitution

Ca₃Co_4−xCr_xO₉ polycristalline thermoelectric ceramics with small amounts of Cr have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Cr has been incorporated into the Ca₃Co₄O₉ structure and no Cr-containing secondary phases have been produced for Cr contents≤0.05. Apparent density measurements have shown that all samples are very similar, with densities around 75% of the theoretical one. Electrical resistivity decreases and Seebeck coefficient slightly raises when Cr content increases until 0.05 Cr addition. The improvement in both parameters leads to higher power factor values than the usually obtained by conventional solid state routes.     

Effect of promotion with cobalt or zinc on the hydrogenating and oligomerizing activities of the Pd/Al2O3 catalyst in the hydrogenation of the BTX fraction

The promoter nature and content effects on the catalytic activity and stability of Pd-Co/δ-Al₂O₃ and Pd-Zn/δ-Al₂O₃ bimetallic catalysts in the hydrogenation of dienic and vinyl aromatic hydrocarbons in the BTX fraction have been investigated by IR spectroscopy and temperature-programmed reduction. The Pd : Co (Zn) molar ratio in the catalysts prepared is 1.0 : 0.5, 1.0 : 1.0, or 1.0 : 1.5, and their Pd content is 0.5 wt %. The support is δ-Al₂O₃ doped with sodium (0.5 wt %). Promotion of the palladium catalyst with zinc and cobalt causes the disappearance of cationic palladium species, thereby reducing the oligomerizing capacity of the active component, and, as was demonstrated by 100-h-long catalytic tests, enhances the stability of the catalyst. The Pd-Co/δ-Al₂O₃(Na) catalyst with Pd : Co = 1.0 : 1.0 mol/mol is recommended for the hydrogenation of the BTX fraction under industrial conditions. The expected service life of this catalyst between regenerations is 16 months.     

Coal from Kazakhstan for the direct reduction of iron

The characteristics of lignite and long-flame coal from Kazakhstan are studied: its overall composition, the composition of its organic and mineral components, its petrographic composition, and its physicochemical properties (density, porosity, reactivity, structural strength, and thermal stability). The use of such coal for the direct reduction of iron is recommended.     

SOLUBILITY OF ACETAMINOPHEN AND IBUPROFEN IN BINARY AND TERNARY MIXTURES OF POLYETHYLENE GLYCOLS 200 AND 400, PROPYLENE GLYCOL,AND WATER AT 25°C

The solubilities of acetaminophen and ibuprofen in the mixtures of propylene glycol-water, polyethylene glycols 200- and 400-propylene glycol, and polyethylene glycols 200- and 400-propylene glycol-water (122 data points) at 25°C were determined and mathematically represented by the Jouyban-Acree model. The solubilities were measured using the shake flask method, and the model was used to fit the solubility data of each drug in the solvent mixtures. The density of the solute-free solvent mixtures was measured and the density of the drug-saturated solutions was predicted. The obtained overall mean relative deviations (OMRDs) for fitting the solubility data of acetaminophen and ibuprofen in binary mixtures are 1.5% and 11.7%, respectively. The OMRDs for fitting the solubilities in ternary solvent mixtures for acetaminophen and ibuprofen are 16.3% and 42.0%, respectively, and the OMRD values for predicting all solubilities of acetaminophen and ibuprofen by these trained versions of the Jouyban-Acree model were 5.7% and 20.4%, respectively. The prediction OMRD for the density of saturated solutions was 2.5%.     

Improvement of textured Bi1.6Pb0.4Sr2Co1.8Ox thermoelectric performances by metallic Ag additions

Bi_1.6Pb_0.4Sr₂Co_1.8O_x thermoelectric ceramics with small Ag additions (0, 1, and 3 wt%) have been successfully grown from the melt, using the laser floating zone method. Microstructure has shown a reduction in the amount of secondary phases and a better grain alignment with respect to the growth direction for an Ag content of 3 wt%. The microstructural evolution, as a function of Ag content, is confirmed with the electrical resistivity values, which show an important decrease for the 3 wt% Ag samples, leading to maximum power factor values of about 0.42 mW/K² m at 650 °C, which are among the best results obtained in this type of material.