WOx phase growth on SiO2/Si by decomposition of tungsten hexacarbonyl: Influence of potassium on supported tungsten oxide phases

Synchrotron based photoemission spectroscopy was used to study the adsorption of tungsten hexacarbonyl on SiO₂ surfaces modified by potassium. Results were compared with the ones obtained when no potassium was present. Experiments using W4f and Si2p intensities variations show that, at 140 K, the tungsten hexacarbonyl growth proceeds via a simultaneous multilayer mode for the two kinds of surfaces but with differences in compositions of growing layers. Indeed, it is evidenced that, even at cryogenic temperatures, the presence of potassium induces decomposition of a significant part of tungsten hexacarbonyl molecules through a strong interaction between tungsten and potassium atoms in opposition to potassium-free surface cases where W(CO)₆ molecules are simply physisorbed. Additional irradiation of adsorbed molecules with photons coming from 0-order synchrotron radiation, subsequent going back to room temperature and additional thermal treatments up to about 700 K were then used to induce further decomposition of the adsorbed precursor. It allows as well to get rid of carbon and, finally, to stabilize different W-based species on the surface. The state of tungsten remaining on the surface is then strongly influenced by presence of potassium. When potassium is present, highly oxidized tungsten species are observed, whereas reduced species are mainly detected for potassium-free surfaces. Moreover, as diffusion of potassium is revealed during formation of tungsten phase, one should guess that potassium plays a crucial role in tungsten oxidation mechanism.     

Synthesis of tungsten carbide nanoparticles in WO3 pyrolysis in a plasma arc

Synthesis of tungsten carbides is investigated in the process of WO₃ pyrolysis in a dc electric arc during dispersion of a composite electrode (graphite-WO₃) in a helium medium at 25 Torr. The synthesized material is analyzed by high-resolution transmission electron microscopy. The interplanar spacings of nanocrystallites are measured directly by using both TEM images and electron diffractometry. The size distribution functions of nanoparticles containing tungsten are measured. It is shown that W, WC, W₂C, and WO₃ nanoparticles can be obtained in the conditions of the present experiments. The content of these compounds in the synthesized material was quantitatively assessed. It is shown that the W₂C content forms the dominant portion of nanoparticles in the synthesized material.     

Effect of adding W to Fe catalyst on the synthesis of SWCNTs by arc discharge

Combining iron (Fe) and tungsten (W) as a bimetallic catalyst, we synthesized high-yield single-wall carbon nanotubes (SWCNTs) of narrow diameter distribution by a hydrogen–argon arc discharge method. Raman spectra indicate that the diameters of SWCNTs prepared using the Fe–W catalysts are about 0.5 nm smaller than those using Fe catalyst alone. The transmission electron microscopy and X-ray diffraction studies show that the SWCNTs prepared by the bimetallic catalyst coexist with few graphite flakes and other amorphous carbon. At the W content of 2–4 at%, tungsten cannot be found in the SWCNT samples. Thus by using a simple two-step purification process, high-purity SWCNT samples can be obtained. We have demonstrated the growth mechanism for the high melting metal (such as W, Mo)–Fe catalyst synthesis of SWCNTs by the arc discharge method.     

Gallium nitride nanoparticle synthesis using nonthermal plasma with gallium vapor

Gallium nitride (GaN) nanoparticles are synthesized by the gallium particle trapping effect in a N₂ nonthermal plasma with metallic Ga vapor. A proposed method has an advantage of synthesized GaN nanoparticle purity because the gallium vapor from the inductively heated tungsten boat does not contain any impurity source. The synthesized particle size can be controlled by the amount of Ga vapor, which is adjusted using the plasma emission ratio of nitrogen to gallium, owing to the particle trapping effect. The synthesized nanoparticles are investigated by electron microscopy studies. High-resolution transmission electron microscopy (HRTEM) studies confirm that the synthesized GaN nanoparticles (10–40 nm) crystallize in a single-phase wurtzite structure. Room-temperature photoluminescence (PL) measurements indicate the band-edge emission of GaN at around 378 nm without yellow emission, which implies that the synthesized GaN nanoparticles have high crystallinity.     

Microstructure and thermostability of a W–Cu nanocomposite produced via high-pressure torsion

A coarse-grained W–25% Cu alloy is subjected to high-pressure torsion (HPT) at room temperature to different strains. Evolution of the microstructure during HPT processing is studied using X-ray diffraction analysis, scanning and transmission electron microscopy. It is demonstrated that HPT processing results in fragmentation of the tungsten particles and the formation of a 5–15?nm grain size nanostructure at equivalent strains of ≥256 (saturation). It is shown that the nanostructured W–25% Cu is thermostable up to 500°C, with grain growth up to 50?nm at 720°C. During HPT processing, the lattice parameter of the copper and tungsten was found to increase and decrease, respectively, with increased level of equivalent strain. This is proposed to occur through the interdiffusion of copper atoms into tungsten grains and tungsten atoms into copper grains, as suggested by energy-dispersive X-ray analysis of the individual grains. The formation of a limited solid solution is considered and possible mechanisms for this effect discussed.     

Synthesis of tungsten carbide nanopowder via submerged discharge method

Tungsten carbide nanopowder was prepared via pulsed discharge of bulk tungsten and graphite rods immersed in pure ethanol. The effect of discharge parameters on the characteristics of final products was investigated. Structural and morphological characterization of nanopowder was performed by means of X-ray diffraction analysis and transmission electron microscopy. In order to determine the feasibility of using synthesized material as an electrocatalyst, tungsten carbide nanopowder was tested for hydrogen evolution. A correlation was found between morphology of nanoparticles, their phase composition and electrocatalytic activity.     

Atomic imaging of oxygen desorption from tungsten trioxide

We report direct observations by high-resolution electron microscopy of oxygen desorption from tungsten trioxide. Clear evidence is found for layer epitaxial growth of metallic tungsten with (110)W|(100)WO₃ and [001]W and [001]WO₃ parallel to the electron beam, consistent with low-energy electron diffraction data on the low-temperature epitaxy of WO₃ on W. W[001] was always observed parallel to the electron beam independent of the surface normal. The results suggest that bulk and surface damage, and displacement processes, are similar.     

Ion Radiation Impact on Microstructure and Mechanical Properties of W–6Re Alloy at 500°С

Physics of Atomic Nuclei - The paper reports the results of the ion irradiation impact on a monocrystalline tungsten alloy, W–6Re. This material is considered for use in the design of fusion...     

Effect of doping and annealing on the physical properties of ZnO:Mg nanoparticles

Well-dispersed undoped and Mg-doped ZnO nanoparticles with different doping concentrations at various annealing temperatures are synthesized using basic chemical solution method without any capping agent. To understand the effect of Mg doping and heat treatment on the structure and optical response of the prepared nanoparticles, the samples are characterized using X-ray diffraction (XRD), energy-dispersive X-ray (EDX), UV–Vis optical absorption, photoluminescence (PL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements. The UV–Vis absorbance and PL emission show a blue shift with increasing Mg doping concentration with respect to bulk value. UV–Vis spectroscopy is also used to calculate the band-gap energy of nanoparticles. X-ray diffraction results clearly show that the Mg-doped nanoparticles have hexagonal phase similar to ZnO nanoparticles. TEM image as well as XRD study confirm the estimated average size of the samples to be between 6 and 12 nm. Furthermore, it is seen that there was an increase in the grain size of the particles when the annealing temperature is increased.     

Thermal effects on the structural properties of tungsten oxide nanoparticles

Tungsten oxide nanoparticles are prepared by evaporating and oxidizing the tungsten boat in helium and oxygen atmosphere and then quenched to the liquid nitrogen temperature. The as-prepared tungsten oxide nanoparticles are porous-free with uniform size. The morphology and particle size distribution of the as-prepared and after sinter treatments tungsten oxide nanoparticles are revealed by TEM and AFM. The long-range order of these nanoparticles can be examined by X-ray diffraction technique. The as-prepared nanoparticles exhibit a mixture structure of monoclinic and hexagonal crystals. Preliminary X-ray diffraction results indicate that the hexagonal structure is transformed to monoclinic structure after annealing to above 600°C. In order to better distinguish the structural properties of the tungsten oxide (WO_{3− x}) nanoparticles before and after annealing, the X-ray absorption spectrum technique is utilized; thus, the detailed local atomic arrangement of oxygen and/or tungsten can be determined. According to the XAS result, the shape of the W L₃-edge undergoes no considerable changes. This infers that structural transformation of tungsten oxide nanoparticle may be caused by the migration of oxygen after sintering. From the O K-edge of absorption spectrum, it suggests that a mixture phase structure is obtained when sintered below 300°C. And this result indicates that heat treatment to approximately 600°C produces a stable structure of a monoclinic crystal of WO₃.     

Study of the Microstructure and Phase-Composition of a WC-Alloy with a Hadfield Steel Binder

The phase composition and structure of a tungsten carbide alloy with a Hadfield steel binder are studied using metallography, x-ray diffraction analysis, and transmission electron microscopy. It is found that tungsten carbide is dissolved in the binder during sintering. This gives rise to a nonuniform distribution of the alloying additions in the -solid solution, which results in precipitation of complex carbides of the (W, Fe) _x C type having different crystal-lattice parameters. The volume fraction of these carbides is as high as 9–10%.     

Blistering and Helium Retention of Tungsten and 5% Chromium Doped Tungsten Exposed to 60 keV Helium Ions Irradiation

Pure tungsten(W) and chromium doped W(W-5%Cr) are prepared by powder metallurgy. The microstructure,blistering and helium retention are investigated by x-ray diffraction,scanning electron microscopy,transmission electron microscopy and thermal desorption spectroscopy(TDS). These results show that the average size and density of helium blisters on the surface of pure W are much larger than those on the W-5%Cr alloy. Vacancyimpurity pairs can reduce the migration coefficients of vacancy and vacancy-helium complexes, and Cr may play a role of such an impurity. Moreover, the TDS result shows that the highest desorption peak moves to higher temperature, which is attributed to the He_mCr_kV_n complexes in the W-Cr alloy. In addition,the helium retention is found to be higher in W than in W-5%Cr.     

Controllable Formation of Niobium Nitride/Nitrogen‐Doped Graphene Nanocomposites as Anode Materials for Lithium‐Ion Capacitors

Niobium nitride/nitrogen‐doped graphene nanosheet hybrid materials are prepared by a simple hydrothermal method combined with ammonia annealing and their electrochemical performance is reported. It is found by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) that the as‐obtained niobium nitride nanoparticles are about 10–15 nm in size and homogeneously anchored on graphene. A non‐aqueous lithium‐ion capacitor is fabricated with an optimized mass loading of activated carbon cathode and the niobium nitride/nitrogen‐doped graphene nanosheet anode, which delivers high energy densities of 122.7–98.4 W h kg^?1 at power densities of 100–2000 W kg^?1, respectively. The capacity retention is 81.7% after 1000 cycles at a current density of 500 mA g^?1. The high energy and power of this hybrid capacitor bridges the gap between conventional high specific energy lithium‐ion batteries and high specific power electrochemical capacitors, which holds great potential applications in energy storage for hybrid electric vehicles.     

Preparation and Investigation of the Properties of W/CeB<Subscript>6</Subscript>/W Heterostructure as a Sensitive Element of Single-Photon Thermoelectric Detector

The possibility of W/CeB₆/W heterostructure preparation on Al₂O₃, AlN, Si, and W substrates by electron-beam evaporation method was investigated. The conditions for preparation of W thin films on dielectric substrates and CeB₆ films, as well as of stoichiometric CeB₆ films on W films, dielectric and tungsten substrates are determined. The reflection spectra of W films, the results of X-ray diffractometry, X-ray microanalysis, and electron microscopy of W and CeB₆ films are presented. W/CeB₆/W heterostructures of various configurations and sizes are produced. It was shown by means of computer simulation that at the detection of 6–50 eV photons, a detector with W/CeB₆/W heterostructure-based sensitive element may provide microvolt level signal at terahertz count rate. The obtained results serve as the basis for creation of a prototype of a sensitive element of single-photon thermoelectric detector.     

Synthesis and room-temperature NO_2 gas sensing properties of a WO_3 nanowires/porous silicon hybrid structure

We report on the fabrication and performance of a room-temperature NO2 gas sensor based on a WO3 nanowires/porous silicon hybrid structure. The W18O49 nanowires are synthesized directly from a sputtered tungsten film on a porous silicon （PS） layer under heating in an argon atmosphere. After a carefully controlled annealing treatment, WO3 nanowires are obtained on the PS layer without losing the morphology. The morphology, phase structure, and crystallinity of the nanowires are investigated by using field emission scanning electron microscopy （FESEM）, X-ray diffractometer （XRD）, and high-resolution transmission electron microscopy （HRTEM）. Comparative gas sensing results indicate that the sensor based on the WO3 nanowires exhibits a much higher sensitivity than that based on the PS and pure WO3 nanowires in detecting NO2 gas at room temperature. The mechanism of the WO3 nanowires/PS hybrid structure in the NO2 sensing is explained in detail.     

On the factors determining the pyrophoric stability of tungsten nanopowder obtained by plasma-chemical pyrolysis of W(CO)6

Nonpyrophoric tungsten powders with an average particle size of about 30 nm were obtained by pyrolysis of tungsten hexacarbonyl in a flow of microwave discharge nitrogen plasma. It is found that these powders are stable in air up to 300°C. The reason for such stability is that the structure of powder particles is of the core-double shell type, in which the metal core is covered with an oxide film approximately 1 nm in thickness, coated in turn with roentgenoamorphous layer consisting of carbon, oxygen, and nitrogen atoms. It is also established that the powders under investigation mainly release carbon oxides (CO and CO₂) and water into the gas phase upon heating in vacuum. Among the molecules present in the gas phase in small concentrations, nitrogen monoxide (NO) and formaldehyde (H₂CO) are worth mentioning apart from C1–C3 hydrocarbons.     

Size-controllable synthesis of functional heterostructured TiO<Subscript>2</Subscript>–WO<Subscript>3</Subscript> core–shell nanoparticles

Mono and bicomponent TiO₂ and WO₃ nanoparticles were synthesized inside Vycor^® glass pores, by cycles of impregnation of the glass with the respective oxide precursor followed by its thermal decomposition. The impregnation-decomposition cycle (IDC) methodology promoted a linear mass increase of the glass matrix, and allowed tuning the nanoparticle size. X-ray diffraction and Raman spectroscopy data allowed identifying the formation of TiO₂ as anatase phase, while WO₃ is a mixture of the γ-WO₃ (monoclinic) and δ-WO₃ (triclinic) phases. High resolution transmission electron microscopy images revealed that for 3, 5, and 7 IDC, the TiO₂ nanoparticles obtained presented average diameters of 3.4, 4.3, and 5.1 nm, and the WO₃ nanoparticles have 2.9, 4.6, and 5.7 nm sizes. These TiO₂ and WO₃ monocomponent nanoparticles were submitted to IDC with the other oxide precursor, resulting in bicomponent nanoparticles. The broadening and shift of the Raman bands related to titanium and tungsten oxides suggest the formation of hetero-structure core–shell nanoparticles with tunable core sizes and shell thicknesses.     

Structure and orientation of an intermetallic phase in a W-Ni-Co alloy

The aim of this investigation is crystal structure determination of an intermetallic phase formed in a W-Ni-Co alloy during a heat-treatment carried out at a temperature of 800°C. This intermetallic phase is expected to play a critical role on the final microstructure (fine tungsten particles in an FCC matrix that is present in between large tungsten grains) and thereby, on the properties of the alloy. 92W-5.3Ni-2.7Co alloy was prepared through powder metallurgy route (liquid phase sintering) followed by heat-treatment at 800°C for 5?h. The intermetallic phase formed at this temperature was characterised using transmission and scanning electron microscopes. The intermetallic phase was found to have orthorhombic crystal structure with Pnam (62) space group as determined using automated diffraction tomography along with precession electron diffraction. Chemical analysis in TEM suggested that the intermetallic phase is based on stoichiometry (Co,Ni)₂W. Orientation imaging of the phase was also carried out in TEM and EBSD to understand its evolution. Equiaxed or lath morphology of the intermetallic phase was found to depend on the crystallographic orientation relationship of the phase with the tungsten grains and the matrix phase.     

Synthesis of graphene oxide sheets decorated by silver nanoparticles in organic phase and their catalytic activity

The graphene oxide(GO) sheets decorated by Ag nanoparticles were prepared using a liquid–liquid two-phase method at the room temperature. The synthesized samples existed in the organic phase and were characterized by X-ray diffraction, transmission electron microscopy, UV–vis spectroscopy and Raman spectra. The results demonstrate that these silver-nanoparticles with diameter of about 10 nm assembled on graphene oxide sheets are flexible and can form stable suspensions in organic phase. Raman signals of graphene oxide sheets are increased by the attached silver nanoparticles, displaying higher surface-enhanced Raman scattering activity. Furthermore, Ag/GO are found to serve as effective catalysts to activate the reduction of 4-nitrophenol (4NP) in the presence of NaBH₄.     

Highly sensitive and selective trimethylamine sensors based on WO3 nanorods decorated with Au nanoparticles

One-dimensional tungsten oxide (WO₃) gas sensing materials have been widely used for the detection of trimethylamine (TMA) gas. Furthermore, it is believed that an effective method to improve the gas sensing performance is to introduce noble metals into sensing materials. In this work, a novel gas sensing material was prepared by decorating Au nanoparticles on WO₃ nanorods. Based on field emission scanning electron microscopy (FESEM/EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM), the morphology and microstructure of as-prepared samples were characterized. Results show that Au nanoparticles with diameter of 13–15 nm are loaded on the surface of WO₃ nanorods with length of about 1–2 µm and width of 50–80 nm. Gas sensing tests reveal that the Au@WO₃ sensor has remarkably enhanced response to TMA gas compared with pure WO₃ nanorods. In addition, and the gas sensing mechanism has been investigated based on the experimental results. The superior sensing features indicate the present Au@WO₃ nanocomposites are promising for gas sensors, which can be used in the detection of the trimethylamine gas and this work provides insights and strategies for the fabrication of sensing materials.