Immobilized 13C-labeled polyether chain ends confined to the crystallite surface detected by advanced NMR

A comprehensive 13C nuclear magnetic resonance (NMR) approach for characterizing the location of chain ends of polyethers and polyesters, at the crystallite surface or in the amorphous layers, is presented. The OH chain ends of polyoxymethylene are labeled with 13COO-acetyl groups and their dynamics probed by 13C NMR with chemical shift anisotropy (CSA) recoupling. At least three-quarters of the chain ends are not mobile dangling cilia but are immobilized, exhibiting a powder pattern characteristic of the crystalline environment and fast CSA dephasing. The location and clustering of the immobilized chain ends are analyzed by spin diffusion. Fast 1H spin diffusion from the amorphous regions shows confinement of chain ends to the crystallite surface, corroborated by fast 13C spin exchange between chain ends. These observations confirm the principle of avoidance of density anomalies, which requires that chains terminate at the crystallite surface to stay out of the crowded interfacial layer.

Влияние микропримесей d-элементов на электронную структуру кристаллитов оксида бария в катодах СВЧ-приборов

Using the electron spectroscopy method for chemical analysis, it was experimentally established for the first time that microimpurities of d elements (W, Re, Ni, Pd, Os) in BaO crystallites from the composition of cathodes of microwave devices lead to the formation of donor-type surface states on the crystallite surface, which can partially compensate acceptor-type surface states due to surface oxygen vacancies, which leads to a decrease in the potential barrier on the crystallite surface and, accordingly, to a decrease the magnitude of the work function of the cathode. Key words: cathodes of microwave devices, crystallites of barium oxide, trace elements of d-elements, surface states of donor and acceptor type.

EVALUATION OF BOUND WATER FORMS IN CLAYS

Clays during lithogenesis change the composition, structure and properties, including physicochemical. The result of the thermal analysis was thermograms, on which four peaks of loss of bound water mass were established. These peaks in the author's interpretation are presented as different types of bound water in clays. The obtained experimental data, from the position of the size of the structural elements, allowed to distinguish the following types of water: loosely bound and strongly bound water of the surface of colloids, water surface of crystallites (minerals) and the crystal lattice of minerals. It has been revealed that when the montmorillonite and kaolinite clays are heated, endo-effects are observed in the temperature range up to 900°C caused by the release of H2O and OH-, the source of which is adsorption water and crystal lattice water of minerals. For montmorillonite in the high-temperature range, endo-effects due to the release of CO and CO2 were also noted, which is due to the presence of calcite in its composition. The energy activity of the surfaces of the particles of montmorillonite and kaolinite clays is distributed in the following sequence: the crystallite surface > of the surface of the colloid > of the surface of the strongly bound water of the colloid, which does not contradict the physical nature of the formation of the clay particle. It has been revealed that in montmorillonite clay, the water of the surface of crystallites has the greatest strength of bonds between molecules, the smaller is the strongly bound water of the colloid and the least is loosely bound water of the colloid. In kaolinite clay, the greatest strength of the bonds between molecules is the water of the surface of crystallites, the smaller is the loosely bound water of the colloid and the least strongly bound water of the colloid. In kaolinite and montmorillonite clays, the temperature of the effect and the heat of dehydration of the studied forms of bound water determine the change in the loss of bound water mass in different directions. The results of the studies allowed to quantify various forms of water in clay minerals.

A transition metal oxofluoride offering advantages in electrocatalysis and potential use in applications

The recently described solid solution (Co,Ni,Mn)3Sb4O6F6has proved stable and efficient as a catalyst for electrocatalytic water oxidation. The end component Co3Sb4O6F6was found to be most efficient, maintaining a current density ofj= 10 mA cm−2at an overpotential of 443 mV with good capability. At this current density, O2and H2were produced in the ratio 1 : 2 without loss of faradaic current against a Pt-cathode. A morphological change in the crystallite surface was observed after 0.5 h, however, even after 64.5 h, the overall shape and size of the small crystallites were unaffected and the electrolyte contained only 0.02 at% Co. It was also possible to conclude fromin situEXAFS measurements that the coordination around Co did not change. The oxofluorides express both hydrophilic and hydrophobic surface sites, incorporate a flexible metalloid element and offer the possibility of a mechanism that differs from other inorganic catalytic pathways previously described.

Self-cleaning and superhydrophobic CuO coating by jet-nebulizer spray pyrolysis technique

The pocket-sized nebulizer equipped jet-spray coating of a monoclinic CuO crystallite surface showed excellent superhydrophobic self-cleaning properties owing to its compact crystallite texture and high surface roughness.

Selective epoxidation of propylene to propylene oxide with H2 and O2 over Au/Ti-MWW catalysts

Direct epoxidation of propylene to propylene oxide (PO) with H2 and O2 has been performed on bifunctional catalysts, Au nanoparticles supported on novel Ti-MWW titanosilicate (Au/Ti-MWW). In comparison to conventional Au/TS-1 catalysts, Au/Ti-MWW exhibited a similar phenomenon with respect to PO formation, that is, the PO selectivity increased with increasing Si/Ti ratio of titanosilicate. However, at optimized Ti contents corresponding to Si/Ti ratio >140, the PO selectivity of Au/Ti-MWW catalysts was lower than 60 % in comparison to ca. 90 % achieved on Au/TS-1. A large number of boron species and defect-site-related hydroxyl groups contained in Ti-MWW were assumed to retard the desorption of PO from the channels or crystallite surface of zeolite, which favors side reactions such as over-oxidation and decomposition of PO. Poststructural rearrangement was then carried out on Ti-MWW with piperidine (PI) solution to improve effectively its hydrophobicity, leading to defect-less Re-Ti-MWW. This enhanced significantly the PO selectivity of Au/Re-Ti-MWW thus prepared, which reached as high as 92 % at Si/Ti ratio of 135. Au/Re-Ti-MWW(135) then gave the highest PO formation rate of 22.0 gPO kg–1 h–1.

Magnetic Properties of the La0.50Ba0.50MnO3 Nanomanganites

Nanocrystalline La0.50Ba0.50MnO3 manganite was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size increases with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously with the average crystallite size. The most significant lattice compression occurs in the sample with an average crystallite size of ~ 30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curie temperature and in the spontaneous magnetic moment. The anomalous behavior of the magnetic properties of the La0.50Ba0.50MnO3 manganites obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+ – O – Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

Identification of Possible Bonding Sites for Post Deposition Oxygen Absorption in Microcrystalline Silicon

ABSTRACTUsing infrared (IR) spectroscopy and x-ray diffraction, the nature of the grain boundaries in twom c-Si films deposited by hot wire CVD, displaying similar crystalline volume fractionsbut very different behavior upon exposure to atmospheric contaminants, is analyzed. For the film exhibiting significant post deposition oxidation, the IR spectrum in the 2100 cm−1 Si-H stretch mode region contains two sharp and very narrow peaks, suggesting that the crystallites have been incorporated into them c-Si films with their hydrogenated surfaces relatively intact.By comparing these peak frequencies to those in the literature for Si-H bonding on c-Si surfaces, we identify certain crystallite orientations which, when comprising the c-Si grain boundaries, are particularly susceptible to oxidation. We further suggest that the distribution of H in this grain boundary/crystallite surface region is crucial for depositing c-Si films with good electronic properties and minimal post deposition oxidation.

H Evolution from Nano-Crystalline Silicon- Comparison of Simulation and Experiment

ABSTRACTThe temperature dependent H evolution from a-Si:H provides unique information on the H-bonding and microstructure. Traditional undiluted a-Si:H films show a high temperature H-evolution peak near 600°C. However device-quality compact nanocrystalline silicon films grown near the phase boundary of amorphous and microcrystalline growth show a new low temperature H- evolution peak near 400°C in addition to a second high temperature peak near 600°C. The origin of this peak cannot be attributed to microvoids or a substantial density of dihydride species typical of porous low-temperature films. We have simulated the H evolution using a molecular dynamics generated model of nanocrystalline silicon, where nano-crystallites reside in a background amorphous matrix. An excess density of H occurs at the crystallite surface. We find a low temperature evolution peak at 250-400°C, where the H-evolution starts from the surface of the nano-crystallite. In addition there is a higher temperature peak at 700-800°C providing good agreement with H-evolution measurements. The mobile H is found to exist in both the bond-centered type of species and H2 molecules – which has implications for H-diffusion models.

Estudio fundamental del sistema ZrO₂-WOx

In this study, a series of ZrO2-WOx samples was prepared by the following methods: i) Precipitation with impregnation, ii) Coprecipitation and iii) Hydrothermal. The amorphous material obtained by coprecipitation method crystallized into tetragonal phase of ZrO2 when calcined at high temperature. This structure remains as a sole phase until 700oC, indicating high interaction between zirconia and tungsten oxide phases. However, the Rietveld analysis of XRD spectrum showed the presence of two tetragonal phases (T1 and T2). The difference between these phases was the oxygen position along the c axis. In T2 phase a flat crystallite surface perpendicular to the c axis was observed, while in T1 phase this crystallite surface was a rough. When the samples were annealed at high temperature (700 and 800oC), the highly dispersed tungsten species present in the T2 phase were segregated, forming the WO3 orthorhombic phase. The as-synthesized zirconia samples prepared by hydrothermal method were crystalline at temperatures as low as 145ºC. When tungsten was added to the synthesis mixture only an amorphous phase of t-ZrO2 was detected In this way a microcrystalline ZrO2-WOx of 309 m2g-1 was prepared. The material becomes progressively more crystalline with increasing synthesis temperature. At 225 oC a well a well-crystallized material was obtained (79% of t-ZrO2 and 21% of m-ZrO2 ). The crystallite sizes of both phases were not too different (17.5 and 14.5 nm for t-ZrO2 and 11.5 for m-ZrO2). In all the samples, crystalline phase associated to WOx species were not identified by XRD. FT-IR spectroscopy of W-promoted zirconia showed a band at 943cm-1 which was attributed to the W=O stretching mode of octahedrally coordinated tungsten species. The WOx species must be highly dispersed and strongly bound to the ZrO2 surface. Post-synthesis calcination treatment at higher temperatures (560, 700 and 800ºC) brought about sintering of these dispersed tungsten species and formation of a more bulky tungsten oxide species identified by two FT-IR vibration bands at 970 and 1100cm-1. Considering the decrease of surface area, the calcination process brings about an increase in the density of tungsten species by unit area. At these higher annealed temperatures, FT-IR of adsorbed pyridine showed that these bulky tungsten species favored the Brønsted acidity formation on ZrO2-WOx materials which increases the catalytc activity for the isomerization of n-hexane.