Synthesis of Highly Dispersed Zirconium Diboride for Fabrication of Special-Purpose Ceramic

Reduction of zirconium dioxide with boron carbide and nanofibrous carbon in argon yielded a highly dispersed powder of zirconium diboride. Characteristics of zirconium diboride powders were examined by various analytical methods. The material obtained is represented by a single phase, zirconium diboride. Powder particles are for the most part aggregated. The average size of particles and aggregates is 10.9–12.9 μm with a wide size distribution. The specific surface area of the samples is 1.8–3.6 m² g^–1. The oxidation of zirconium diboride begins at a temperature of 640°C The optimal synthesis parameters were determined: ZrO₂: B₄C: C molar ratio of 2: 1: 3 (in accordance with stoichiometry), process temperature 1600–1700°C, synthesis duration 20 min.     

One-step synthesis of VB2 powder by epoxy-assisted borothermal reduction of V2O5

Vanadium diboride was directly synthesized by borothermal reduction of V₂O₅ with the addition of epoxy resin as a reducing agent for the low-temperature reduction of vanadium(V) to vanadium(IV), which leads to the gradual removal of oxygen by the formation of CO gas. The slow rate of gas release prevents destruction of green body, which usually occurs during conventional borothermal reduction. This makes it possible to directly obtain VB₂ powder with an average particle size of 200–300 nm without need to prepare intermediate lower vanadium oxides.     

Preparation and microstructure evolution of diboride ultrafine powder by sol–gel and microwave carbothermal reduction method

Titanium diboride ultrafine powder was prepared from sucrose, tetrabutyl titanate and boric acid by the sol–gel and microwave carbothermal reduction method. The influence of reaction temperature, ratio of Ti to C and Ti to B on the synthesis of titanium diboride was studied. The results indicated that the carbothermal temperature, the content of carbon and the amount of H₃BO₃ show obvious effects on the formation of TiB₂. 1,300 °C was the optimum synthesis temperature and pure TiB₂ could be prepared. The microstructure of prepared TiB₂ was investigated by field emission-scanning electron microscopy (FE-SEM), which results showed that the crystalline size of the prepared titanium diboride at 1,300 °C was about 3–5 μm. The quantities of the crystalline phases of the powders prepared at different temperatures were analyzed by Rietveld refinement method. An erratum to this article can be found at     

Molten salt synthesis and localized surface plasmon resonance study of vanadium dioxide nanopowders

Rutile-type vanadium dioxide nanopowders with four different sizes were successfully synthesized by carbothermal reducing V₂O₅ in KCl-LiCl molten salt. XRD and TEM characterizations suggested that vanadium dioxide particles formed by a broken and reunited process of vanadium oxide. Molten salt and organic carbon sources are crucial to the size of final particles. In the presence of the molten salt, the organic carbon with a shorter chain length would induce smaller particles. The UV-VIS-IR spectral measurements for as-prepared vanadium dioxide announced an obvious localized surface plasmon resonance band in the near infrared region at 90 °C.     

Nanosized zirconium diboride: Synthesis and properties

The thermolysis of Zr(BH₄)₄ vapor at 573 and 623 K in a vacuum of 1.33 × 10⁻¹ Pa was studied. Nanosized zirconium diboride was produced as an X-ray amorphous powder and a crystalline film. According to electron microscopy data, the X-ray amorphous zirconium diboride powder obtained at 573 or 623 K consists of spherical particles 30–40 nm in diameter, which is in quite a good agreement with the equivalent particle diameter (∼35 nm) calculated from the specific surface area of ZrB₂. After annealing at 1273 K, the X-ray amorphous zirconium diboride powder crystallizes into a hexagonal lattice with the unit cell parameters a = 0.3159 nm and c = 0.3527 nm. The coherent scattering length D _hkl is ∼27 nm. The zirconium diboride film produced at 573 or 623 K crystallizes into a hexagonal lattice with the unit cell parameters a = 0.3163−0.3168 nm and c = 0.3524−0.3531 nm. The coherent scattering length D _hkl is ∼14 nm. The thickness of the ZrB₂ film on quartz, glass ceramics, and stainless steel is 5–7 μm. The microhardness of the film on a stainless steel substrate under a load of 20 g is 17.8 GPa.     

Hydrothermal synthesis of nanocrystalline ZnSe: An in situ synchrotron radiation X-ray powder diffraction study

The hydrothermal synthesis of nanocrystalline ZnSe has been studied by in situ X-ray powder diffraction using synchrotron radiation. The formation of ZnSe was studied using the following starting mixtures: Zn+Se+H₂O (route A) and ZnCl₂+Se+H₂O+Na₂SO₃ (route B). The route A experiment showed that Zn powder starts reacting with water at 134 °C giving ZnO and H₂ followed by the formation of ZnSe which takes place in temperature range from 167 to 195 °C. The route B experiment shows a considerably more complex reaction path with several intermediate phases and in this case the formation of ZnSe starts at 141 °C and ZnSe and Se were the only crystalline phases observed at the end of the experiment where the temperature was 195 °C. The sizes of the nanocrystalline particles were determined to 18 and 9 nm in the route A and B experiments, respectively. Nanocrystalline ZnSe was also synthesized ex situ using the route A and B methods and characterized by conventional X-ray powder diffraction and transmission electron microscopy. An average crystalline domain size of ca. 8 nm was determined by X-ray powder diffraction in fair agreement with TEM images, which showed larger aggregates of nanoparticles having approximate diameters of 10 nm. Furthermore, a method for purification of the ZnSe nanoparticles was developed and the prepared particles showed signs of anisotropic size broadening of the diffraction peaks.     

Synthesis of nano-sized titanium diboride in a melt of anhydrous sodium tetraborate

X-ray powder diffraction, scanning electron microscopy, infrared spectroscopy, and elemental analysis were used to study the interaction of titanium powder with finely powdered boron of particle size 10?C20 ??m in Na₂B₄O₇ ionic melt, in the temperature range 973?C1088 K, at the 5?C10 h contact duration. The TiB₂ formation was shown to occur at the temperatures 1018 K or above, that is, at the borax melting temperature. According to the scanning electron microscopy, theTiB₂ powder consists of the 70?C75 nm particles, and its coherent scattering region calculated from the XRD data amounts to 55 nm.     

Vanadium-doped hafnia: elaboration and structural characterization

Powders of HfO₂ doped with various amounts of vanadium were prepared by pyrolysis of oxalic precursors at relatively low temperature (700 °C). This pyrolysis was carried out under different atmospheres in order to insert vanadium under different oxidation states into hafnia. The various amounts of inserted vanadium into HfO₂ were determined by energy dispersive spectroscopy. The powders were structurally characterized by X-ray diffraction, and electron diffraction. Nano particles of monoclinic HfO₂ were obtained when incorporating V⁵⁺. The solubility limit in that case was found to be less than 10 at %. The insertion of vanadium with a lower oxidation state than 5+ led to the stabilization of the cubic phase, with a solubility limit higher than 30 % at.     

Nanostructure of Dispersed and Compact Nonstoichiometric Vanadium Carbide

Structure and properties of dispersed and compact nanocrystalline vanadium carbide VC0.875 were studied. Dispersed vanadium nanocarbide was prepared by prolonged aging of a coarse-grained VC0.875 powder. It has a unique nanostructure: nanocrystallites of dispersed vanadium carbide have a shape of bent lobes up to 600 nm in diameter and 15-20 nm thick. Annealing and quenching of compact samples of coarsegrained VC0.875 carbide result in the formation of a nanodomain structure. The nanostructured carbide VC0.875 corresponds in composition to the higher boundary of a homogeneity area of a cubic phase and mainly contains ordered V8C7 phase, which belongs to the P4₃32 space group. Cold pressing and subsequent vacuum sintering of vanadium carbide nanopowder at 2000 K gave sintered samples approaching diamond in microhardness.     

Synthesis,crystal structures and magnetic properties of Co particles encapsulated in carbon nanocapsules

Carbon-coated cobalt particles were produced by arc-evaporation of a Co-packed graphite rod in a modified fullerene generator. Cobalt particles grown were in an fcc phase, with a trace amount of hcp-Co. The particles were nominally spherical in shape, and typically 10–100 nm in diameter. Thickness and structure of outer carbon layers could be controlled by varying the relative area of a Co-packed hole drilled in the graphite rod. Saturation magnetization (M _s) and the coercive force (H _c) was measured between room temperature and 800°C for powder samples: Temperature dependence of the measured M _s was consistent with that for fcc-Co. The highest value of M _s of 160 emu/g at room temperature, nearly the same value for bulk fcc-Co, was obtained for Co particles covered with thin carbon layers. H _c varied from 600 Oe to 300 Oe, depending on the size of Co particles.     

Effect of alumina incorporation on restricting grain growth of nanocrystalline tin(IV) oxide

In this project, nanocrystalline SnO₂ powders were successfully prepared by (a) citrate sol-gel and (b) direct precipitation methods. Powders were characterized using thermal analysis techniques (DTA-TG-DSC), X-ray powder Diffraction (XRD), surface area (BET) and electrical conductivity measurements. XRD patterns showed the presence of the cassiterite structure. SnO₂ particles, prepared through sol-gel method exhibit crystallite sizes in the range from 3.1 to 22.3 nm when the gel is heat treated at different temperatures up to 900°C. SnO₂ nanocrystallites prepared by the precipitation method are comparatively larger in size. The higher specific surface area was obtained for the powder prepared using sol-gel method and the obtained average grain size (d) is relatively large compared with that of the average crystallite size. The powders show a semiconducting behavior with increasing temperature. The higher conductivity obtained for SnO₂ prepared by sol-gel method can be attributed to their smaller average crystallite size. XRD of alumina doped powder exhibits finer particles than pure SnO₂. TEM images showed that the particles are spherical in shape and consist of a core of SnO₂ surrounded by a coating of alumina. The calculated surface area was found to decrease with temperature increases. Due to the effective role of Al₂O₃ additive as a grain growth inhibitor for the matrix grains, the observed surface area for the coated materials are predominantly higher than for the uncoated materials.      

Effects of vacuum annealing on the particle size and phase composition of nanocrystalline tungsten carbide powders

The effects of the vacuum annealing temperature (400–1400°C) on the phase and chemical composition, particle size, and microstresses of the nanocrystalline powders of tungsten carbide WC with 20–60 nm particles were studied by X-ray diffraction and electron microscopy. Vacuum annealing of WC nano-powders at T _ann ≤ 1400°C was accompanied by decarbonization, resulting from the interaction of carbon with the oxygen impurity. Changes in the chemical composition of the nanocrystalline powder of tungsten carbide led to changes in its phase composition. The annealing was accompanied by growth of powder particles due to the aggregation of nanoparticles and by a decrease of microstresses.     

Mathematical Modeling of Aluminum Diboride Combustion in an Air Flow

A mathematical model and the results of calculating the ignition and combustion of energetic condensed systems based on mono- and polydispersed aluminum diboride particles in air flows in constant-cross-section channels are reported. The kinetic characteristics of the transformations that separate aluminum diboride particles formed by gasification of energetic condensed systems undergo in a high-temperature oxidizing medium were determined using the dependences of the ignition induction period and combustion time on the air temperature and diameter and initial temperature of the particles. These dependences, in turn, were calculated using the model of parallel chemical reactions. The range of combustion conditions corresponding to the initial air temperatures from 300 to 2000 K and Mach numbers in the channel from 0.1 to 1.5 was considered. The influence of the aluminum diboride particle size and of the rate and initial temperature of the air flow on the combustion efficiency was demonstrated. The relationships between the combustion completeness factor of aluminum diboride particles at various initial parameters of the air flow and gasification products of energetic condensed systems at various fuel mixture equivalence ratios, corresponding to the diffusion and kinetic combustion, were determined. The conditions of the transition between the diffusion and kinetic control modes were found.

     

A facile preparation of nanocrystalline Mo2C from graphite or carbon nanotubes

Nanocrystalline Mo₂C powders were successfully synthesized at 500 °C by reacting molybdenum chloride (MoCl₅) with C (graphite or carbon nanotube) in metallic sodium medium. X-ray powder diffractometer (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscope (XPS) and surface area analyzer (BET method) were used to characterize the samples. Experiments reveal that the carbon source used for the carbide synthesis has a great effect on the particle size and the surface area of the samples. When micro-sized graphite was used as C source the obtained nanocrystalline Mo₂C powder consists of particles of 30∼100 nm, with a surface area of 2.311 m²/g. When carbon nanotubes were used as C source, the as-synthesized Mo₂C sample is composed of particles of 20∼50 nm, with a surface area of 23.458 m²/g, which is an order of magnitude larger than that of the carbide prepared from the graphite.     

Synthesis and luminescent properties of SnO2:Eu nanopowder via polyacrylamide gel method

Nanocrystalline europium-doped tine oxide (SnO₂:Eu) were synthesized by a polyacrylamide gel method. The effects of heat treatment on structure, grain size and luminescence properties of SnO₂:Eu were studied with X-ray powder diffraction, transmission electron microscopy and photoluminescence measurements. The results indicate that high heat treatments can enhance greatly luminescence intensity of the samples. Furthermore, the presence of carbon network/cages in the polyacrylamide gel can effectively prevent particles agglomeration, so even when sintered at higher temperatures (1000 °C), the grain size is still below 20 nm.     

Preparation of uniform rhodamine B-doped SiO2/TiO2 composite microspheres

Uniform rhodamine B (RB)-doped SiO₂/TiO₂ composite microspheres with catalytic and fluorescent properties were prepared by an easy and economical method in this paper. The composite microspheres were built up with well-dispersed silica particles as the cores, RB as both the doped agent and stabilizer, and the TiO₂ shells were obtained through the hydrolysis of TiCl₄ in water bath. The morphology and structure of the particles were characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). The characterization results indicate that composite particles are all in spherical shape and have a narrow size distribution. The composite particles calcined above 500 °C reveal clear crystalline reflection peaks of the rutile TiO₂ which exhibits well catalytic property. The photocatalytic experiment was carried out in order to examine the catalytic property of composite microspheres. The fluorescent property of particles was also investigated. Dye-leakage test indicates that RB molecules entrapped in the composite particles by this method are stable inside the particles.     

Structural and optical characterization of Ag-doped TiO2 nanoparticles prepared by a sol–gel method

Undoped and silver-doped TiO₂ nanoparticles (Ti_1?x Ag _x O₂, where x?=?0.00?C0.10) were synthesized by a sol?Cgel method. The synthesized products were characterized by X-ray diffraction (XRD), particle size analyzer (PSA), scanning electron microscope (SEM), and UV?CVisible spectrophotometer. XRD pattern confirmed the tetragonal structure of synthesized samples. Average crystallite size of synthesized nanoparticles was determined from X-ray line broadening using the Debye?CScherrer formula. The crystallite size was varied from 8 to 33?nm as the calcination temperature was increased from 300 to 800?°C. The incorporation of 3 to 5% Ag⁺ in place of Ti⁴⁺ provoked a decrease in the size of nanocrystals as compared to undoped TiO₂. The SEM micrographs revealed the agglomerated spherical-like morphology of particles. SEM, PSA, and XRD measurements show that the particles size of the powder is in nanoscale. Optical absorption measurements indicated a red shift in the absorption band edge upon silver doping. Direct allowed band gap of undoped and Ag-doped TiO₂ nanoparticles measured by UV?CVis spectrometer were 3.00 and 2.80?eV, respectively, at 500?°C.     

Synthesis of Zirconium Diboride Nanoparticles by the Reaction of ZrCl4 with NaBH4 in an Ionic Potassium Bromide Melt

Nanosized particles of zirconium diboride are synthesized by the reaction of ZrCl₄ with NaBH₄ in an ionic KBr melt.

     

Preparation of Disperse Silver Particles by Chemical Reduction

Mastery over the microscopic shape and size of a nanoparticle enables accurate control of its properties for some strict application. The mechanism of shape-controlled synthesis was discussed by investigating the formation of silver nanospheres prepared by chemical reduction method using Ag(NH₃)₂⁺ as metal source, ascorbic acid as reducing agent and polyvinylpyrrolidone (K-30) as dispersant. The effects of temperature, PVP/AgNO₃ mass ratio, pH value and the interaction between PVP and silver on the shape and particle size were studied by XRD and SEM. The results show that the morphology of silver particles could transform from branched to spherical and the particle size gradually decrease with the increase of PVP/AgNO₃ mass ratio. The particles size can also be significantly influenced by pH value and temperature. The key point for preparing high dispersity spherical silver powder is that the growth rate of each plane of the particle must be uniform and synchronous. Silver powders with spherical particles with mean size of 0.2 μm were synthesized under the optimum conditions (PVP/AgNO₃ mass ratio 0.6, pH 7, reaction temperature of 40°C).     

Polyol-mediated synthesis of Li4Ti5O12 nanoparticle and its electrochemical properties

Li₄Ti₅O₁₂ nanoparticles were precipitated from ethylene glycol solution of titanium tetra isopropoxide (Ti(O-iPr)₄) and Li₂O₂ by refluxing at 197 °C for 12 h. The obtained particles were filtered and dried at 100 °C for 12 h, and the dried powder samples were heated at 320, 500 and 800 °C for 3 h. The X-ray diffraction patterns of the obtained samples exhibited a good fit with the spinel phase. The field emission-SEM images of the dried powder sample and the samples heated at 320, 500 and 800 °C for 3 h showed a uniform spherical morphology with a particle size of 5, 8, 10 and 400 nm, respectively. According to the results of electrochemical testing, the dried powder sample and the samples heated at 320, 500, and 800 °C for 3 h showed initial capacities of 200, 310, 320, and 260 mA h/g, respectively, at a current density of 0.05 mA/cm². Nanosized (6–8 nm) particles with good crystallinity were obtained by controlling the synthesis conditions. The sample heated at 500 °C for 3 h exhibited a high capacity and an excellent rate capability over 60 cycles.