Tungsten Oxide Hydrates Crystallites
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- Category: Tungsten Information
- Published on Monday, 09 March 2015 17:49
- Written by zsq
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The past couple of decades have witnessed an exponential growth of activities in this field worldwide, driven both by the excitement of understanding new science and by the potential hope for applications and economic impacts. The largest activity in this field at this time has been in the synthesis of new nanoparticles of different sizes and new shapes. Besides giving us new properties, creating novel nanostructures requires new understanding of the properties of their surfaces, which has been in the center of attentions from many scientific researchers. As one kind of well-known semiconductor materials, tungsten oxide and its hydrates (WO3•nH2O, n=0~2) have been extensively researched and employed in various application fields. Because many of their nanostructures and morphologies are inextricably bound to properties in several fields, scientists have been concentrating on the research of the preparation of micro/nanostructures of these materials. However, there still remains further investigations on the exploration of developing facile, green and economical techniques when controlling these interesting promising structures, which are believed to be very necessary for expanding the application fields of tungsten oxide hydrates materials. In this dissertation, a facile hydrothermal technique has been developed to prepare micro/nanostructured tungsten oxide hydrates materials (particles and films) with/without the assistant of templates and microwave heating. The following are the main results and conclusions:
Tungsten oxide hydrates crystallites with a diversity of phases and morphologies were successfully controlled by using tungsten powders as starting materials via a facile and additive-free hydrothermal process. Various micro/nanostructures of products, including cubic blocks, “snowflakes”, round angular blocks, hexagonal “gears”, cuboid rods, hexagonal plates and nanoflakes,were obtained by simply changing the reaction conditions. Results show that the nucleation, crystallization and diffusion of the growth units were found to play key roles in different conditions to vary the products phases and morphologies.
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Hydrothermal Synthesis of Tungsten Oxide Nanostructures
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- Category: Tungsten Information
- Published on Monday, 09 March 2015 17:46
- Written by zsq
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Tungsten oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals.Tungsten ores are treated with alkalis to produce WO3. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal.
Tungsten oxides inorganic semiconductor materials have received considerable attention in recent years because of their unique physico-chemical properties and widespread applications in various areas，such as electrochromic ( EC ) devices，gas sensors，photocatalytic systems，photoelectrochemical devices，and so on．Recently hydrothermal method has been exploited for the preparation of tungsten oxide micro / nanostructures with different sizes and shapes． Combining with our group' s work on the preparation of tungsten oxide micro /nanomaterials，the progress of preparation of tungsten oxide micro / nanomaterials by hydrothermal method is presented． The key influencing factors，such as the choice of reagents，the reaction time and temperature，are discussed in detail． The development trends of the tungsten oxide micro / nanomaterials fabricated by hydrothermal method are also proposed．
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Tungsten Oxide Film
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- Category: Tungsten Information
- Published on Monday, 09 March 2015 17:37
- Written by zsq
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In the past few years, an increasing interest has been put on the tungsten oxide films due to their potential applications in smart windows, gas sensors, photocatalytic reactions, and optoelectronic devices etc. The unique properties of the tungsten oxide films were usually determined by the oxygen defects and the valence states of tungsten ions. Besides, it is very important to improve the properties of tungsten oxide films and other oxide films by controlling their morphologies and crystalline phases, which mainly depend on the preparation technologies and thermal treating temperatures and atmospheres.
Tungsten oxide thin films were deposited on glass substrates by the magnetron sputtering of WO3 bulk at room temperature. The deposited films were annealed at different temperatures in air. The structural measurements indicate that the films annealed below 300 C were amorphous, while the films annealed at 400 C were mixed crystalline with hexagonal and triclinic phases of WO3. It was observed that the crystallization of the annealed films becomes more and more distinct with an increase in the annealing temperature. At 400C, nanorod-like structures were observed on the film surface when the annealing time was increased from 60 min to 180 min. The presence of W=O stretching, W–O–W stretching, W–O–W bending and various lattice vibration modes were observed in Raman measurements. The optical absorption behaviors of the films in the range of 450–800 nm are very different with changing annealing temperatures from the room temperature to 400C. After annealing at 400C, the film becomes almost transparent. Increasing annealing time at 400C can lead to a small blue shift of the optical gap of the film.
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Synergistic Effect Between Ceria and Tungsten Oxide
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- Category: Tungsten Information
- Published on Monday, 09 March 2015 17:42
- Written by zsq
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WO3–CeO2–TiO2 catalysts for NO (nitrogen monoxide) reduction by ammonia were prepared by a sol–gel method. The catalysts were characterized by BET, XRD, Raman, NH3/NO adsorption and H2-TPR to investigate the relationships among the catalyst composition, structure, redox property, acidity and deNOx activity. WO3–CeO2–TiO2 catalysts show a high activity in a broad temperature range of 200–480 1C. The low-temperature activity of catalysts is sensitive to the catalyst composition especially under low-O2-content atmospheres. It may be related to the synergistic effect between CeOx and WOx in the catalysts. On one hand, the interaction betweenceria and tungsten oxide promotes the activation of gaseous oxygen to compensate the lattice oxygen consumed in NH3-SCR (selective catalytic reduction) reaction at low temperatures. Meanwhile, the Br nsted acid sites mainly arise from tungsten oxides, Lewis acid sites mainly arise from ceria. Both of the Br nsted and Lewis acid sites facilitate the adsorption of NH3 on catalysts and improve the stability of the adsorbed ammonia species, which are beneficial to the NH3-SCR reaction.
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Scandia Effect Tungsten Oxide Powder Reduction Process
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- Category: Tungsten Information
- Published on Monday, 09 March 2015 17:34
- Written by zsq
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Vacuum electronic devices have a wide application in civil and military fields such as communication, radar and industrial heating. Cathode is an important component in the device since it provides the required electronic beam for the device. Among all the electron emitters, scandate cathodes have aroused great attention among all the cathodes due to their copious emission property.
Scandia doped tungsten powders were prepared by spray drying combined with two-step hydrogen reduction. The particle size of doped tungsten powder, powder morphology and doped tungsten matrix were characterized by scanning electron microscope, X-ray diffraction and laser diffraction particle size analyzer, respectively. The reduction behavior of Sc2O3 doped tungsten oxide and the effect of Sc2O3 on the property of tungsten powder were studied by the temperature programmed reduction. The experimental results showed that the precursor powders prepared by spray drying had spherical shape. The addition of Sc2O3 could decrease the reduction temperature of tungsten oxide. The scandia doped tungsten powder had sub-micrometer size in the range of 0.1 to1 μm and scandium distributed evenly in the powder. By using this kind powder, sub-microstructure cathode matrices with semispherical grains and homogenous distribution of scandium were obtained.
The addition of Sc2O3 shifted the hydrogen consumption peak to the lower temperature side. Namely, the reduction temperature of tungsten oxide decreased with Sc2O3 addition. Adding scandia could decrease the particle size of tungsten. The scandia doped tungsten powder prepared by spray drying method had sub-micrometer size in the range of 0.1μm to1μm in semispherical shape and scandium distributed evenly in the powder. Using scandia doped tungsten powder, sub-microstructure cathode matrices with semispherical grains and homogenous distribution of scandium was obtained.
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