Hematites Precipitated in Alkaline Precursors: Comparison of Structural and Textural Properties for Methane Oxidation

Hematite (α-Fe₂O₃) catalysts prepared using the precipitation methods was found to be highly effective, and therefore, it was studied with methane (CH₄), showing an excellent stable performance below 500 °C. This study investigates hematite nanoparticles (NPs) obtained by precipitation in water from the precursor of ferric chloride hexahydrate using precipitating agents NaOH or NH₄OH at maintained pH 11 and calcined up to 500 °C for the catalytic oxidation of low concentrations of CH₄ (5% by volume in air) at 500 °C to compare their structural state in a CH₄ reducing environment. The conversion (%) of CH₄ values decreasing with time was discussed according to the course of different transformation of goethite and hydrohematites NPs precursors to magnetite and the structural state of the calcined hydrohematites. The phase composition, the size and morphology of nanocrystallites, thermal transformation of precipitates and the specific surface area of the NPs were characterized in detail by X-ray powder diffraction, transmission electron microscopy, infrared spectroscopy, thermal TG/DTA analysis and nitrogen physisorption measurements. The results support the finding that after goethite dehydration, transformation to hydrohematite due to structurally incorporated water and vacancies is different from hydrohematite α-Fe₂O₃. The surface area SBET of Fe₂O₃_NH-70 precipitate composed of protohematite was larger by about 53 m²/g in comparison with Fe₂O₃_Na-70 precipitate composed of goethite. The oxidation of methane was positively influenced by the hydrohematites of the smaller particle size and the largest lattice volume containing structurally incorporated water and vacancies.     

Exfoliated CrPS4 with Promising Photoconductivity

Layered semiconductors have attracted significant attention due to their diverse physical properties controlled by composition and the number of stacked layers. Herein, large crystals of the ternary layered semiconductor chromium thiophosphate (CrPS₄) are prepared by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. A simple, one‐step protocol for mechanical exfoliation onto a transmission electron microscope grid is developed, and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements' atoms. CrPS₄ is also liquid exfoliated, and in combination with colloidal graphene, an ink‐jet‐printed photodetector is created. This all‐printed graphene/CrPS₄/graphene heterostructure detector demonstrates a specific detectivity of 8.3 × 10⁸ (D*). This study shows a potential application of both bulk crystal and individual flakes of CrPS₄ as active components in light detection, when introduced as ink‐printable moieties with a large benefit for manufacturing.     

Influence of Cr doping on the phase composition of Cr,Ca:YAG ceramics by solid state reaction sintering

In the present work, the influence of Cr and Ca co-additives on the phase formation under conditions emulated the real sintering process of Cr⁴⁺:YAG ceramics is studied. The XRD analysis of the treated samples revealed the difference in formation rates of intermediate phases between the samples with and without the Cr₂O₃ additive. The formation of intermediate phases in the solid-state reaction between Y₂O₃ and Al₂O₃ is observed to shift toward higher temperatures (ie, toward the stage of fast shrinkage) if the mixture of Cr₂O₃ and CaO is added. The reason for such shift is the appearance of new intermediate, which contains Cr⁴⁺ ions in perovskite structure, as has been established by optical absorption and luminescent investigations. It is found that the Cr,Ca:YAG ceramics prepared by vacuum solid state reaction sintering at 1750°C, 10 hours possesses better optical transparency than Ca:YAG ceramics prepared under the same conditions.     

Effect of Gamma Irradiation on Dynamics of Charge Exchange Processes between Single Trap and Nanowire Channel

In the present study, transport properties and single trap phenomena in silicon nanowire (NW) field‐effect transistors (FETs) are reported. The dynamic behavior of drain current in NW FETs studied before and after gamma radiation treatment deviates from the predictions of the Shockley–Read–Hall model and is explained by the concept taking into account an additional energy barrier in the accumulation regime. It is revealed that dynamics of charge exchange processes between single trap and nanowire channel strongly depend on gamma radiation treatment. The results represent potential for utilizing single trap phenomena in a number of advanced devices.     

A Study of the influence of plasma treatment on the working surface of superabrasive grinding wheels

The paper addresses the effect of plasma treatment on the working surface of polymer- and metal-bonded superabrasive grinding wheels. The investigation has shown a change in hardness of the wheel surface treated with a plasma jet. The paper provides some data on the performance of plasmatreated grinding tools.     

Possibilities of using inhomogeneity in light energy distribution for estimating the degree of coherence of superposing waves

The paper presents a new method for determining the degree of coherence of superposing plane linearly polarized waves converging at the angle of 90°. The spatial modulation of polarization, which causes the spatial modulation of the averaged values of the Poynting vector, presets the modulation of the volume energy density. Such an inhomogeneous optical field can affect nano-sized particles randomly caught in this field. The paper shows that the maximum velocity of "trapping" the particles into the regions of maximum averaged values of the Poynting vector determines the degree of coherence of interacting waves.     

Nonlinear metasurfaces: a paradigm shift in nonlinear optics

1. Download high-res image (132KB)
2. Download full-size image

     

The effects of nanoparticle shape and orientation on the low frequency dielectric properties of nanocomposites

In this paper, we examine the effects of aspect ratio and orientational order of nanoparticles on the dielectric properties of nanocomposites. The motivation is to clearly establish the effects of orientational order, since ambiguities exist in the literature. We focus on metallic nanoparticles, and show that, in the dilute concentration limit, theory, experiments and numerical simulations all unequivocally indicate that the effective dielectric constant increases with increasing aspect ratio and increasing degree of alignment of rod-like nanoparticles when they orient in direction of the electric field.     

Multiphase Biomineralization: Enigmatic Invasive Siliceous Diatoms Produce Crystalline Calcite

Diatoms are considered unicellular eukaryotic organisms exclusively depositing biogenic silica. Heretofore there has been no report of calcification by these algae. Here it is shown that calcium carbonate within the stalks of Didymosphenia geminata, a nuisance species that has prolifically colonized streams and rivers globally, is biogenic in origin and occurs as a network of calcite nanofibers. The nanofibrous framework in the mineralized polysaccharide matrix imparts mechanical support to the stalks, providing stability in variable flow conditions. The results demonstrate that D. geminata possesses cellular and periplasmic carbonic‐anhydrases that contribute to carbon fixation and biomineralization, respectively. The activity of external carbonic‐anhydrase was more than 50% of the total activity, which points to its role in anchoring this bioeroding diatom on hard surfaces. The first evidence of multiphase biomineralization by diatoms that deposit both biogenic silica and crystalline biogenic calcite which are imparting distinct functional advantage to the organism is provided.