Showing papers on "Tungsten published in 2004"

Intelligent window coatings: Atmospheric pressure chemical vapor deposition of tungsten-doped vanadium dioxide

Abstract: Thin films of tungsten-doped vanadium(IV) oxide were prepared on glass substrates from the atmospheric pressure chemical vapor deposition of vanadium(IV) chloride, tungsten(VI) ethoxide, and water at 500−600 °C. The films were characterized by Raman microscopy, glancing angle X-ray diffraction (GAXRD), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering (RBS), scanning electron microscopy (SEM), and vis/IR reflectance−transmittance. The films showed a reduction in thermochromic transition temperatures from 68 °C in VO2 to 42 °C in V0.99W0.01O2approaching that required for commercial use as an intelligent window coating.

Methods for the reduction and elimination of particulate contamination with cvd of amorphous carbon

Abstract: A method is provided for forming an amorphous carbon layer, deposited on a dielectric material such as oxide, nitride, silicon carbide, carbon doped oxide, etc., or a metal layer such as tungsten, aluminum or poly-silicon. The method includes the use of chamber seasoning, variable thickness of seasoning film, wider spacing, variable process gas flows, post-deposition purge with inert gas, and post-deposition plasma purge, among others, to make the deposition of an amorphous carbon film at low deposition temperatures possible without any defects or particle contamination.

Method of forming low-resistivity tungsten interconnects

Abstract: Methods and apparatus for preparing a low-resistivity tungsten film on a substrate are provided. Methods involve the formation of a tungsten nucleation layer on a substrate using pulsed nucleation layer (PNL) techniques and depositing a bulk tungsten layer thereon. Methods for forming the tungsten nucleation layer involve the use of a boron-containing species, a tungsten-containing precursor, and optionally, a silane. The methods described are particularly useful for applications where thin, low resistivity films are desired, such as interconnect applications.

Effect of vanadia and tungsten loadings on the physical and chemical characteristics of V2O5-WO3/TiO2 catalysts

Abstract: Titania-supported tungsten and vanadia oxides with different W and V loadings and calcined at different temperatures have been prepared by the sol–gel method and characterized. Large amount of tungsten (9%; w/w) provides thermal stability to WO3/TiO2 systems upon addition of vanadia. It is found that WO3 and V2O5 crystallites are formed when their concentrations are higher than those corresponding to three monolayers. DTA analyses have shown that the presence of V2O5 crystallites is not essential for the transformation of titania anatase into rutile. High reactivity in limited temperature range (225–350 °C) has been observed for the catalyst with 8% (w/w) V2O5 with a high formation of N2O amount during the SCR reaction. The catalyst with 3% (w/w) V2O5 exhibits a slight lower reactivity but with a high selectivity to N2 preserved in all working temperature.

Esterification of palmitic acid with methanol over tungsten oxide supported on zirconia solid acid catalysts: effect of method of preparation of the catalyst on its structural stability and reactivity ☆

Abstract: Esterification of palmitic acid with methanol was studied on zirconia supported tungsten oxide solid acid catalysts prepared by both impregnation and co-precipitation methods; the tungsten loading and calcination temperature were varied. These catalysts were characterized by X-ray diffraction for structural elucidation and by temperature-programmed desorption of ammonia for their surface acidity. Whereas the impregnated catalysts exhibited both tetragonal and monoclinic phases of zirconia, at calcination temperatures beyond 500 °C their co-precipitated analogues showed stability towards the tetragonal phase. The formation of monoclinic phases of zirconia and crystalline tungsten oxide decreased the esterification activity, as a consequence of decreased surface acidity. Coexistence of tetragonal zirconia with the amorphous tungsten oxide offered maximum esterification activity.

Force microscopy with light-atom probes.

Abstract: The charge distribution in atoms with closed electron shells is spherically symmetric, whereas atoms with partially filled shells can form covalent bonds with pointed lobes of increased charge density. Covalent bonding in the bulk can also affect surface atoms, leading to four tiny humps spaced by less than 100 picometers in the charge density of adatoms on a (001) tungsten surface. We imaged these charge distributions by means of atomic force microscopy with the use of a light-atom probe (a graphite atom), which directly measured high-order force derivatives of its interaction with a tungsten tip. This process revealed features with a lateral distance of only 77 picometers.

Ruthenium as an underlayer for tungsten film deposition

Abstract: In one embodiment, a method for depositing a tungsten-containing film on a substrate is provided which includes depositing a barrier layer on the substrate, such as a titanium or tantalum containing barrier layer and depositing a ruthenium layer on the barrier layer. The method further includes depositing a tungsten nucleation layer on the ruthenium layer and depositing a tungsten bulk layer on the tungsten nucleation layer. The barrier layer, the ruthenium layer, the tungsten nucleation layer and the tungsten bulk layer are independently deposited by an ALD process, a CVD process or a PVD process, preferably by an ALD process. In some examples, the substrate is exposed to a soak process prior to depositing a subsequent layer, such as between the deposition of the barrier layer and the ruthenium layer, the ruthenium layer and the tungsten nucleation layer or the tungsten nucleation layer and the tungsten bulk layer.

Atmospheric pressure chemical vapour deposition of tungsten doped vanadium(IV) oxide from VOCl3, water and WCl6

Abstract: Thin films of thermochromic tungsten doped VO2 on glass substrates were prepared from the atmospheric pressure chemical vapour deposition (APCVD) reaction of VOCl3,·H2O and WCl6. X-Ray diffraction (XRD) and Raman spectroscopy indicated a solid solution V1−xWxO2 (x = 0.003–0.032). XPS studies indicated the tungsten was present as W4+. The thermochromic properties of the films were investigated by Raman, XRD and reflectance/transmission measurements. These indicated that incorporation of tungsten caused a reduction in the VO2 thermochromic switching temperature of 19 °C per W atom%. The thermochromic properties of the thin films show great potential for use as an intelligent window coating.

Appearance of bulk properties in small tungsten oxide clusters

Abstract: Contrary to the conventional understanding that atomic clusters usually differ in properties and structure from the bulk constituents of which they are comprised, we show that even a dimer of tungsten oxide (WO3)2 possesses bulklike features and the geometry of a small cluster containing only 4 tungsten and 12 oxygen atoms bears the hallmarks of crystalline tungsten oxide, WO3. This observation, based on a synergistic approach involving mass distributions under quasisteady state conditions, photoelectron spectroscopy, and first principles molecular orbital theory, not only illustrates the existence of a class of strongly covalent or ionic materials whose embryonic forms are tiny clusters but also lends the possibility that a fundamental understanding of complex processes such as catalytic reactions on surfaces may be achieved on an atomic scale with clusters as model systems.

Influence of proton insertion on the conductivity, structural and optical properties of amorphous and crystalline electrochromic WO3 films

Abstract: Amorphous and crystalline tungsten oxide films were RF-sputtered from a metallic target in oxygen/argon atmosphere onto un-heated and heated substrates. Upon cyclic electrochemical treatment in 0.1 M H2SO4 aqueous solution, both types of films showed good electrochromic reversibility beyond 1000 cycles. The crystallinity changes of both types of films were studied by XRD. For c-WO3, the results showed that hydrogen insertion enhance the degree of crystallinity with the clear appearance of tungsten bronze diffraction peaks due to the formation of H0.1WO3, with tetragonal crystalline structure. For a-WO3, coloration weakens the amorphous features and increases the crystallinity properties. The DC electrical conductivity increases, in a reversible way, with four order of magnitudes namely from 10−7 to 10−3 (Ω cm)−1 for bleached and colored films, respectively. Both types of films exhibit remarkable solar transmission modulation, about 51%, which is adequate for smart windows applications. In contrast to amorphous films, crystalline tungsten oxide films show reflection modulation in the NIR reaching 25% upon charge insertion. The Drude reflectance edge arises for colored films suggesting that coloration in c-WO3 is attributed to a scattering mechanism of Drude-like free electron, while coloration of a-WO3 is attributed to a hopping mechanism of small polaron. The visible, infrared and solar modulations are given for both types of films. Coloration/bleaching were found to be faster for amorphous films. Self-bleaching process in different oxidative environments involves a redox cycle aided by water polarity. Crystalline WO3 showed pronounced coloring persistence relative to amorphous films which is attributed to different binding energy related to large and small polarons.

Layered tungsten oxide-based organic/inorganic hybrid materials I: Infrared and Raman study

Abstract: Tungsten oxide-organic layered hybrid materials have been studied by infrared and Raman spectroscopy, and demonstrate a difference in bonding nature as the length of the interlayer organic `spacer' molecule is increased. Ethylenediamine-tungsten oxide clearly displays a lack of terminal -NH3+ ammonium groups which appear in hybrids with longer alkane molecules, thus indicating that the longer chains are bound by electrostatic interactions as well as or in place of the hydrogen bonding that must be present in the shorter chain ethylenediamine hybrids. The presence of organic molecules between the tungsten oxide layers, compared with the layered tungstic acid H2WO4, shows a decrease in the apical W=O bond strength, as might be expected from the aforementioned electrostatic interaction.

Catalytic Effects of Metals on Direct Nitridation of Silicon

Abstract: Catalytic effects were investigated on the direct nitridation of silicon granules, impregnated with 0.125–2.0% by mass of calcium, yttrium, iron, copper, silver, chromium, or tungsten, in a stream of nitrogen with 10% hydrogen, using a tubular flow reactor operated at temperatures ranging from 1200° to 1390°C. Calcium and yttrium suppressed the formation of β-silicon nitride while iron enhanced the formation of β-silicon nitride over the temperature range investigated. An addition of 0.125% calcium resulted in about 99% overall conversion with 100%α-phase and a 2.0% yttrium addition yielded an overall conversion over 98% with an α-phase content above 97%. Copper promoted not only the nitridation but the formation of α-silicon nitride at 1200°C, but enhanced the β-phase formation at higher temperatures. The role of liquid phases on the formation of α-/β-silicon nitride was also discussed based on the nitridation of silicon impregnated with copper, calcium, silver, chromium, and tungsten.

Nano‐Scale Tungsten Oxides for Metastable Intermolecular Composites

Abstract: Tungsten oxides are of interest as an oxidant for metals in metastable intermolecular composites, a nano-scale powder useful for such applications as electric matches and gun primers. Smaller particles typically lead to faster reactions and we have synthesized nano-scale WO3 using wet chemistry. Ammonium paratungstate was dissolved in acid. Tungstic acid was precipitated by addition to distilled water and the resulting powder was dried. Analysis by X-ray diffraction, Raman spectroscopy and electron microscopy revealed hydrated WO3, having approximately 100 nm×7 nm platelet morphology. Thermal treatment in air at 200 °C or 400 °C removed water, forming cubic or monoclinic WO3, respectively. Tungsten forms other stable oxides and we accessed WO2 by reducing WO3 in dilute H2 at 650 °C. Reaction performance of these WOx-materials, mixed with nano-Al at various stoichiometries, is reported. Open-tray burn velocity reached 250 m/s. Pressure cell experiments found a maximum pressure of 1.45 MPa and maximum pressurization rate of 500 GPa/s. These maximum pressures and rates occurred very near a stoichiometric mixture of the Al and WO3.

Tungsten inverse opals: The influence of absorption on the photonic band structure in the visible spectral region

Abstract: We report on the fabrication and characterization of tungsten inverse opals for the visible and near-infrared spectral region. The crucial influence of the strong absorption in this spectral region is experimentally investigated by means of a gradient deposition technique and characterization with reflectance and transmittance spectroscopy. With increasing metal infiltration, we observe the breakdown of the photonic band structure, resulting first in a sphere-cavity-like behavior and finally in a behavior similar to that of a periodically structured metal surface.

Simple Approaches to Quality Large-Scale Tungsten Oxide Nanoneedles

Abstract: In this paper, a systematic study of large-scale production of highly crystalline W18O49 nanoneedles with high aspect ratios using a simple tungsten metal reacting with water at 800−1000 °C has been described. By altering the W source and other experimental conditions, we have generated W18O49 crystals with diverse morphologies, and needles with desired dimensions are achieved. The quality nanoneedles provide ideal samples for further property investigations. Possible growth mechanisms are discussed.

Gas phase reaction products during tungsten atomic layer deposition using WF6 and Si2H6

Abstract: The gas phase reaction products during tungsten (W) atomic layer deposition (ALD) using WF6 and Si2H6 were studied using quadrupole mass spectrometry. The gas phase reactions products were different for the WF6 and Si2H6 reactions. No surface reactions were observed for WF6 exposures at room temperature. The WF6 reaction produced H2, HF and SiF4 at a reaction temperature of 473 K. Mass spectrometer cracking patterns established that SiF4 is the silicon reaction product instead of SiHF3. Auger electron spectroscopy (AES) measurements confirmed that the H2, HF and SiF4 gas phase reaction products during WF6 exposure coincided with the loss of silicon surface species. The Si2H6 reaction showed two separate reaction channels depending on reaction temperature. At room temperature, a temperature insensitive reaction produced SiHF3 and H2 reaction products. A second reaction produced H2 as the reaction product at 473 K. AES measurements confirmed that the SiHF3 and H2 reaction products during Si2H6 exposure were...