Urchin-like boron nitride hierarchical structure assembled by nanotubes-nanosheets for effective removal of heavy metal ions

Water pollution has become a serious global issue owing to the large amounts of contaminants generated from industrial and agricultural development. Recently, various boron nitride-based micro/nano-materials have exhibited efficient sorption capacity for contaminants from water. Herein, novel urchin-like boron nitride hierarchical structure assembled by free-growing boron nitride nanotubes and crapy boron nitride nanosheets is firstly fabricated via a sample two-step approach, including the synthesis of analogous "core-shell" structured boron-containing precursor and thermal catalytic chemical vapor deposition. A combined growth mechanism of vapor-liquid-solid and vapor-solid is proposed to control the formation of BN hierarchical structure. The unique structure exhibits superior removal capacity of 115.07?mg?g^?1 and 92.85?mg?g^?1 for Pb²⁺ and Cu²⁺ in water solution, respectively. The excellent adsorption performance of the product mainly derives from the vast lattice imperfections, the high-density edge active sites, the expanded interplanar spacing, as well as the unique structural characteristics. They are beneficial for structural stability and enough space for accommodating the adsorbed heavy metal ions. These results indicate that the urchin-like boron nitride hierarchical structure is a promising adsorption material for water treatment.     

Mechanism of growth of silicon nitride crystals in combustion of ferrosilicon in nitrogen

The process of nitride formation in burning of iron-silicon melt in gaseous nitrogen was investigated. It was found that silicon nitride is synthesized at temperatures (2100°C) which are much higher than the temperature of appearance of the liquid phase (1206°C). Silicon is in the liquid state during synthesis in the form of an iron-silicon melt and a gaseous melt. The electron-microscopic studies showed that silicon nitride crystals grow according to two mechanisms: vapor-liquid-crystal and crystallization from iron-silicon melt. The ratio of the contributions of these mechanisms to structural formation of silicon nitride is determined by the conditions of synthesis. __________ Translated from Steklo i Keramika, No. 8, pp. 18–21, August, 2007.     

The initial oxidation of beryllium by water vapor

The initial stage of adsorption and beryllium oxidation by water (clearly a nonadiabatic process) was studied for a wide temperature range, using AES, XPS, DRS, and CPD measurements. The mechanism of room temperature (RT) oxidation by water vapor was found to be by nucleation and growth of 3 monolayer oxide islands, laterally spreading until coalescence takes place. When a full oxide layer is achieved, a further slow oxidation takes place, virtually stopping at ∼6 monolayer depth. Exposure of the surface to water vapor at 150 K yielded dissociation to H and OH, chemisorbed on the surface, as detected by an XPS chemical shift. The lack of such a shift at RT indicates a full dissociation of the water molecule on the surface. A giant effect of Be electron-stimulated oxidation (ESO) by water vapor, as opposed to Be mild ESO by O₂, was observed, reaching the maximal possible oxidation rate for the ratio of ≥150 impinging electrons per water molecule. It is suggested that the mechanism is a Mott—Cabrera-like one, enabled by a combination of an electric field applied by negative OH and/or oxygen ions formed at the surface, probably by secondary electron attachment, and a very fast diffusion of Be²⁺ ions enabled by the presence of hydrogen in the oxide bulk. The water vapor ESO exhibits an inverse dependence on the substrate temperature, presumably due to the decrease with temperature of hydroxyl surface concentration, leading to the weakening of the electric field formed across the oxide.     

Features of the synthesis of carbon nitride oxide (g-C3N4)O at urea pyrolysis

First from urea was synthesized carbon nitride oxide (g-C₃N₄)O which is formed in a vapor gas reactionary space and is deposited outside of a place of precursor localization. The reaction of urea conversion was investigated in an interval of temperatures 350–450 °С, but carbon nitride oxide is detected in the products of pyrolysis only at temperatures above 430 °С. Carbon nitride oxide is dissolved in water and at a reducing it by hydroquinone is formed reduced carbon nitride oxide consisting, as we believe, from azagraphene sheets.     

Synthesis of boron nitride nanotubes by combining citrate-nitrate combustion reaction and catalytic chemical vapor deposition

High-quality boron nitride nanotubes were successfully synthesized via a novel two-step method, including citrate-nitrate combustion reaction and catalytic chemical vapor deposition. The composition, bonding features and microstructures of as-synthesized sample were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, transmission electron microscopy and selected area electron diffraction techniques. The results show that the as-synthesized boron nitride nanotubes with smooth surface are relatively pure. The diameter ranges between 20 and 80?nm, while the length is about dozens of micrometers. During the synthesis process of boron nitride nanotubes, citric acid chelates the cobalt ions and reacts with nitrate to form the cobalt oxide, depositing on the surface of boron powder homogeneously. The catalyst content and annealing temperature have a significant impact on the composition and microstructures of the final products. Based on the experimental results and thermodynamic analysis, the possible chemical reactions are listed, and vapor-liquid-solid mechanism is proposed to be dominant for the formation of boron nitride nanotubes.     

Plasma-Enhanced Chemical Vapor Deposition of Boron Nitride Using Polymeric Cyanoborane as Source

The plasma-enhanced chemical vapor deposition (PECVD) of boron nitride films was studied using polymeric cyanoborane, (CNBH₂) n , a material previously examined by thermally activated CVD. The PECVD procedure yields boron nitride coatings containing ≅20 wt% paracyanogen as a contaminant. This impurity can be removed by heat treatment under vacuum or in an ammonia atmosphere. The boron nitride coatings are hexagonal and appear to be boron deficient. The PECVD process takes place at 300°C, measured at the backside of the substrate, as compared with 600°C in the thermally activated CVD process.     

Nickel ions removal from water by two different morphologies of induced CNTs in mullite pore channels as adsorptive membrane

In this study, chemical vapor deposition (CVD) method (with two proposed synthesis processes) was used for inducting two different morphologies of CNTs in mullite pore channels as a novel adsorptive membrane for nickel ions (Ni²⁺) removal from water. Cyclohexanol and ferrocene were used as carbon source and catalyst, respectively. The first proposed synthesis process involves coevaporation and pyrolysis of a mixed solution composed of cyclohexanol and ferrocene in a neutral atmosphere and the second involves sublimation and decomposition of ferrocene in a reactor individually and subsequently introduction of cyclohexanol as vapor to the reactor by a carrier gas during the reaction. Effects of synthesis parameters such as reaction time, catalyst content and reactor pressure on growth process, and structure and properties of the induced CNTs in pore channels of the mullite substrate were also investigated. Finally the optimized CNTs growth conditions for achieving a uniform distribution of the CNTs in the mullite pore channels were reported. The CNTs–mullite composite membranes prepared under the optimum conditions were oxidized with nitric acid and then successfully used as adsorptive membranes for nickel ions removal from water. Moreover, Langmuir and Freundlich isotherm models were used to describe adsorption behavior of nickel ions by the prepared adsorptive membrane.     

Large-scale synthesis and growth mechanism of boron nitride nanocomposite assembled by nanosheets and nanotubes

Boron nitride nanocomposites assembled by nanosheets and nanotubes can exert multi-dimensional synergistic toughening and strengthening effects. This material is expected to be a high-efficiency reinforcement additive in advanced structural ceramics. In this study, we designed a universal method for synthesizing gram-scale boron nitride nanocomposites by annealing the precursor containing catalyst in chemical vapor deposition equipment under flowing ammonia, and a combined growth mechanism of surface-diffusion and solid-liquid-solid is proposed. The boron nitride nanosheets were initially formed by a surface-diffusion reaction between boron trioxide and ammonia at 1300°C. At elevated temperatures (1400°C-1500°C), the boron nitride nanotubes grew in-situ from the nanosheets in the presence of catalysts through a solid-liquid-solid mechanism, forming the desired boron nitride nanocomposite.     

Preparation and properties of SiC/SiC-SiYC with excellent water-oxygen corrosion resistance

In this study, the SiC/SiC-SiYC composites were fabricated via chemical vapor infiltration (CVI) combined with the reactive melt infiltration (RMI) process. The excellent infiltration of Si-Y alloy assisted in fabricating composites with a density of 2.94 g/cm³ and a porosity of only 2.0%. After 20 h of corrosion at 1300 °C in the water-oxygen environment, the generated oxide layer, consisting of a glass layer and a diffusion layer, effectively protected the composites, and the flexural strength retention is 114.2%. This study highlights the significant potential of Si-Y alloy as a modification phase that is resistant to water and oxygen. It also presents a novel approach for developing high-density ceramic matrix composites that are resistant to water-oxygen corrosion.     

Water and oxide ions concentration dependence of the specific mass and conductance of the molten NaOH-KOH eutectic mixture

The conductivity of the molten NaOH-KOH eutectic mixture was measured as a function of the oxoacidobasicity of the melt at concentrations ranging from a mole fraction of sodium oxide of 0.16 to a mole fraction of water of 0.09.The conductivity of the melt is enhanced when the water concentration increases while it decreases when oxide ions are dissolved into the liquid.Relationships between the conductivity and the concentrations of water or oxide ions are proposed which are in perfect agreement with the experimental results. A proton transfer conduction mechanism is suggested.The water concentration dependence of the specific mass of the melts has been measured in the concentration range extending from the dry mixture to a mole fraction of water of 0.09.     

Electrochemical formation of AlN in molten LiCl-KCl-Li3N systems

Electrochemical formation of aluminum nitride was investigated in molten LiCl-KCl-Li₃N systems at 723 K. When Al was anodically polarized at 1.0 V (versus Li⁺/Li), oxidation of nitride ions proceeded to form adsorbed nitrogen atoms, which reacted with the surface to form AlN film. The obtained nitrided film had a thickness of sub-micron order. The obtained nitrided layer consisted of two regions; the outer layer involving AlN and aluminum oxynitride and the inner layer involving metallic Al and AlN. When Al electrode was anodically polarized at 2.0 V, anodic dissolution of Al electrode occurred to give aluminum ions, which reacted with nitride ions in the melt to produce AlN particles (1-5 μm of diameter) of wurtzite structure.     

炉水氢电导率偏高的原因分析及解决措施

某330 MW机组炉水氢电导率偏高,导致锅炉排污量大,造成水汽损失和热量损失,影响了机组运行的经济性。通过对机组的水汽品质和水处理药品进行检测分析,发现水汽系统加药用的氨水质量不合格,其中氯离子和硫酸根离子明显超标。此质量不合格氨水进入系统导致炉水氢电导率偏高,更换质量合格的氨水后炉水水质恢复正常。     

The electrochemistry of nitrate-amide melts: Reactions of nickel and copper in nitrate-amide melts

Copper and nickel may be electrodeposited from their ions in solution in nitrate-amide melts at room temperature. In the ammonium nitrate-acetamide-urea melt at 23°C, the reduction to the metal competes with the corrosion reaction at low rates and with the reduction of the ammonium and nitrate ions of the melt at high current densities. Two distinct types of nickel complexes are found in solution. The nickel complex formed by the corrosion reaction is bound by at least one ammonia ligand. Nickel complexes formed by dissolving the halide in the melt show evidence of coordination by less strongly bounding ligands, probably by amides. Similarly, the visible spectra of copper chloride in solution suggest that the cupric ions are coordinated primarily by amides. The copper corrosion reaction produces a complex with a spectra distinctly different from that of cupric chloride in solution. The shift in absorption maxima suggests that the copper complex formed by the corrosion reaction has at least one ammonia ligand in the coordination sphere.     

Influence of the thermal process of carbon template removal in the mesoporous boron nitride synthesis

Influence of the thermal process involved in the carbon template elimination during the synthesis of mesoporous boron nitride by using nanocasting process of a mesoporous CMK-3 carbon with a borazinic precursor is presented. The borazinic precursor, the tri(methylamino)borazine (MAB), is converted to boron nitride (BN) inside the mesopores of a CMK-3 mesoporous carbon template by ceramization under nitrogen or under ammonia. The carbon template elimination is carried out by thermal treatment under air or under ammonia. The X-ray diffraction, TEM and pore size analysis are used to study the texture of the boron nitride synthesized from the carbon template. A template elimination performed by hydrogenation with an ammonia treatment allows to obtain an organized porous structure, which is not possible by using an oxidation treatment. In order to preserve the mesoporous organization of boron nitride, a two steps procedure (ceramization followed with template elimination by hydrogenation) is more efficient than a one step procedure (ceramization and template hydrogenation simultaneously).     

Preparation of high-molecular-weight polycaproamide by the method of removing water from the melt with nitrogen in the stage of polycondensation at normal pressure

Conclusions A necessary condition for obtaining high-quality PCA is carrying out the synthesis process, and, especially, the polycondensation reactions, at a water content in the reaction system which is close to the equilibrium figure. Thereupon, it is preferable to maintain a reduced melt temperature (513–528°K) in the finishing stage of the polycondensation.It is possible to fulfill the indicated conditions at reasonably high process rates by performing the first stage under water vapor pressure until monomer polymer equilibrium is attained, and performing the polycondensation stage with repeated renewal of a well-developed melt surface, which assists in intensive water removal without local drying out of the polymer.Translated from Khimicheskie Volokna, No. 5, pp. 20–22, September–October, 1983.     

Fabrication and Characteristic of Nextel 720 Fiber‐Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration Process

Most current processes for fiber‐reinforced silicon nitride composites are conducted at very high temperature, which is not possible to use oxide fiber as reinforcement. Here, low‐temperature process of chemical vapor infiltration (CVI) was utilized to fabricate Nextel 720 oxide fiber tow‐reinforced silicon nitride matrix composite with PyC as interphase. The tensile strength was analyzed by Weibull distribution. The microstructure showed that there were two types of interface bonding. The strong interface bonding determined the unexpected low strength of the composites. This indicated that the suitable interface design is the urgent issue for oxide fiber‐reinforced silicon nitride composite by CVI.     

Interfacial diffusion-limited vapor-liquid-solid mechanism for the growth of tadpole-shaped boron nitride nanostructures

Here, tadpole-shaped boron nitride (BN) nanostructures, with a length of ∼10 µm and a diameter ranging from 0.05 µm (tail) to 1.0 µm (head), were prepared by a facile two-step process, involving the synthesis of a cobalt carbonate-boron precursor, and its annealing using chemical vapor deposition in an ammonia atmosphere. Based on phase composition changes and microstructural evolution during annealing, and thermodynamic analysis, an interfacial diffusion-limited vapor-liquid-solid mechanism was proposed for the growth of the tadpole-shaped BN nanostructures. An understanding of the growth mechanism of the nano-tadpoles supplements the knowledge base of the BN nanostructure family and provides a new opportunity for the synthesis of analogous inorganic nanomaterials.     

Quantitative model for the viscous flow and composition of two-phase silicon nitride-based particles in plasma-spray deposition

Silicon nitride does not melt but decomposes at 1900 °C and so thermal spraying of pure silicon nitride powder is impracticable. However, the use of silicon nitride and other non-oxide ceramics as thick, thermally sprayed coatings has considerable engineering potential owing to their unique combination of properties. This research shows that embedding fine silicon nitride particles within an oxide matrix to form composite feedstock particles enables the formation of silicon nitride composite coatings with little decomposition of the silicon nitride. Successful deposition of the coatings depends critically on the flow of the feedstock particles on impact with the substrate. This paper concerns the design of oxide matrix systems for the deposition of silicon nitride composite coatings by thermal spraying. A quantitative model is developed for the viscous flow of two-phase feedstock particles at impact. A number of matrix systems are investigated, including a series of yttria–alumina and yttria–alumina–silica compositions. The research shows that certain oxide matrices can provide the required viscous flow and protect the silicon nitride from decomposition.     

Template-directed synthesis of boron nitride nanotube arrays by microwave plasma chemical reaction

Highly-ordered boron nitride (BN) nanotube arrays have been synthesized by microwave plasma-enhanced chemical vapor deposition (MW-PECVD) below 520 °C under the confinement of anodic aluminum oxide (AAO) template with borane/argon and ammonia/nitrogen as precursors. The low growth temperature and aligned arrangement of the BN nanotubes are beneficial to practical applications despite of the amorphous nature of the product. Novel morphology of Y-branching and dendriform BN nanotubes were also observed when the branching AAO template was used.     

Synthesis of Eu-doped gahnite in water and water–ammoniac fluids

The effect of ammonia contained in water fluid on the synthesis and properties of gahnite (ZnAl₂O₄) doped with (0.4–4 mol%) europium was investigated. Gahnite synthesized in water–ammoniac fluid has a smaller crystal size than does the one synthesized in water fluid, and bound nitrogen is detected in its structure. However, europium ions more effectively incorporate into the structure of nascent gahnite during synthesis in water fluid and generate formation there of complexes with oxygen vacancies. The excitation efficiency of luminescence of Eu³⁺ ions in the absorption band of oxygen vacancies in gahnite increases with the concentration of Eu³⁺. After annealing in air of gahnite samples synthesized in fluids of various compositions, the changes of Eu³⁺ ions luminescence differ. We conclude that ammonia not only participates in the processes of gahnite formation, causing an increased nucleation rate but it also remains in the gahnite structure.