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Tungsten Oxide Nanorods Growth from Heated Tungsten Foils

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 18:35; Written by cq; Hits: 1504

Tungsten oxides nanostructures have exhibited application perspectives in such fields as electrochromic (EC) devices, photocatalysis, gas sensing, adsorption of organic dyes, etc. So far, in most high-temperature-reaction fabrications of the tungsten oxide nanostructures, the reactant sources and the substrates are separated. Occasionally, tungsten oxide nanostructures are grown directly from the metallic tungsten, which serves as both the reactant source and the substrate. The advant ages of direct heating method include short fabrication time, convenient manipulation, less expensive instrumentation and easy accession to high reaction temperatures. Most importantly, tungsten ox-ide nanostructures can grow directly from, instead of depos-iting on, the substrates. Therefore, a number of desired properties, such as good adhesion and low contact interface resistance, can be expected.

As previously reported, a fast heating chemical vapor deposition (CVD) system was developed, in which the temperature of the carbon substrate could be raised to 1800°C in 15 seconds with the passage of large electric current. Combined structures of conical carbon fiber and carbon nanotube (CCF/CNT), which had large length, good straightness and perfect crystallinity, were fabricated using this system. Recently, tungsten oxide nanostructures were also attained with similar instrumentation.

Nanorods of W18O49, an oxygen-deficient nonstoichiometric tungsten oxide, were fabricated by directly heating W foils. In the fabrication, the W foils functioned as both the sub-strates and the reactant sources. Other necessary conditions included the coverage of the substrates with KBr and the introduction of low pressure wet oxygen. The oxidized W surface and the KBr on it combined into eutectic droplets and tungsten oxide segregated from them due to supersatu-ration. This growth is referred to as an SLS process. Field emission was obtained from the such-fabricated W18O49 nanorods and the UPS analysis suggested that the emitted electrons were likely to have come from the conduction band, impurity levels and/or surface states of these nanorods.

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Tungsten Crystal Growth during Hydrogen Reduction of Tungsten Oxide (II)

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 18:29; Written by cq; Hits: 1559

In order to obtain coarser powder, the normal method is reducing and carburizing treatment of tungsten oxide in the presence of alkali metal compounds at high temperature. Thus, the obtained powder particle size can be above 50 µ m (Fsss). But its defects and micro-strain are relatively more than those of the powder without addition of dopant. Some researches show that the crystalline perfection of tungsten carbid e grain in the alloy can be impaired by tungsten starting material which undergoes the adulteration of alkali metal.

At high temperature, yellow tungsten oxide (WO3) is reduced to metal tungsten under hydrogen in the stepwise reduction sequence of WO3→WO2.9→W 18O 49→WO2→W. The overall reduction rate is limited mainly by the WO2→W transition. The nucleation and growth of

W grain occur in the WO2→W transition.

The technical conditions of WO2→W transition greatly affect the grain size, uniformity and crystalline of W powder.

In order to obtain coarse-grain tungsten powder, the transition of WO2→W must proceed under relatively high pH2O/pH2 in hydrogen atmosphere throughout.

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Research Status and Prospects of WO3-based Gas Sensor

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 18:17; Written by cq; Hits: 1576

Tungsten oxide is usually used to fabricate gas sensor to monitor the toxic gases or some exhaust gases in the air. The gas sensing properties of WO3 may be improved by optimizing itself characteristics (size, surface morphology, shape and crystalline structure) . Thus the hydrothermal method for preparation of WO3 attracts considerable attentions. At present, most of work has been devoted to the improvement of sensitivity，the reducing of concentration of target gas and the increase of gas types.

In fact, these aspects all gain well advancement by optimizing the grow method, dopant and electrode. However, the WO3-based gas sensor just usually performed well above 200℃. This temperature range limits the wide application of WO3-based gas sensor. Therefore, seeking approaches to reduce the working temperature may be the next goal. On the one hand, seeking new dopant material to change the gas sensing characteristics of WO3 is a common way. On the other hand, controlling the preparation conditions accurately and making use of the very assisting agents to optimize the relative characteristics of WO3 are the technique measures.

In addition, change the manner of WO3-based sensors for gases detection. Do not monitor the changes of electrical properties of WO3-based sensors. Recording the changes of optical properties of WO3-based sensors is also used to analyze the gas sensing response. This is because the optical properties will change when the WO3-based sensors exposure under the target gases.

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Tungsten Crystal Growth during Hydrogen Reduction of Tungsten Oxide (I)

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 18:26; Written by cq; Hits: 1472

In these years, significantly incremental enhancements and specialized developments in the area of hardmetal continued shift towards ultra-fine and ultra-coarse grades. Compared with conventional cemented carbide (middle grain or fine grain carbides), ultra-coarse grain cemented carbides(＞5µm) demonstrate good fracture toughness and thermal fatigue resistance. Thus, they attract much attention from different departments in the area of industry. Currently, it is widely applied in many fields and found increasing usage in areas such as milling tools, punching dies, and boring drills. Coarse grain W and WC powders are key materials for ultra-coarse grain cemented carbides producing. The industrially established method of preparing tungsten and carbide is the hydrogen reduction of tungsten oxide, tungstic acid or ammonium para tungstate at 900-1 200℃ or even higher temperature, initially to the metal powder followed by carburization. The average particle size of the powder obtained in this way is below 12 µm (Fsss).

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Dartboard with Magnetic Rubber Sheet

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 18:15; Written by xsh; Hits: 431

Disclosed herein is a dartboard with a magnetic rubber sheet. The dartboard includes a target board, a holding pole and two closure members. The target board includes a magnetic rubber sheet having a rectangular shape and containing iron, the two sheets of cotton flannel being attached to both surfaces of the magnetic rubber sheet and having raisings on their outer surfaces, and an upper fitted piece attached to the upper end of the front surface of one of said two sheets of flannel. The holding pole includes a hollow for holding one end of the target board, a slit formed along the entire length of the holding pole to pass through the inner and outer circumferential surfaces of the holding pole, and holes for allowing a hanging string to be fastened to the holding pole. The two closure members are each provided with an inserting recess into which one end of the holding pole is inserted.

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