Tungsten outer electron configurations is sd46s, thus, five kinds of valence (+ 2, + 3, + 4, + 5 + 6) can present in the compounds. The valence of W in WO3 is highest in tungsten oxide. Usually the oxygen content of WO3 does not meet the strict stoichiometric ratio, it represented in the form of WO3-x.

 

WO3 formula is quite simple, but its physical properties is very complex.The melting point of WO3 reported in literature is 1473 ℃, but in 1100 ℃ WO3 starts sublimation, sublimation heat is 459.8 kJ/mol. Strictly meet the stoichiometric ratio of tungsten trioxide and without any impurities, WO3 should be transparent insulation. The forbidden band width is about 2.9 eV at room temperature.The WO3-x show n type semiconductor behavior, and forbidden band width is 2.4~2.8eV. Due to the phase change of the structure of WO3, with the change of temperature, resistivity of WO3 will appear jump phenomenon, its resistivity is commonly in 0.3~10 Ω·cm.

 

 

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Yellow tungsten oxide is a kind of tungsten oxide.The production of yellow tungsten oxide should be traced to the discovery of tungsten. Tungsten has a rich history dating back to its discovery during the 18th century. Peter Woulfe was the first to recognize a new element in the naturally occurring mineral, wolframite. Tungsten was originally known as wolfram, explaining the choice of "W" for its elemental symbol. Swedish chemist Carl Wilhelm Scheele contributed to its discovery as well with his studies on the mineral scheelite.

 

In 1841, a chemist named Robert Oxland gave the first procedures for preparing tungsten trioxide and sodium tungstate. He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry.

 

 

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Yellow tungsten oxide is one of the most important, highly pure intermediates for the production of other tungsten compounds including tungsten metal powder. Yellow tungsten oxide is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce yellow tungsten oxide. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal. Yellow tungsten oxide can be synthesized using various chemical methods, such as ‘wet’ chemical precipitation, hydrolysis of tungsten alkoxide or thermal decomposition of tungsten salts.

 

Scheelite is allowed to react with HCl to produce tungstic acid, which decomposes to yellow tungsten oxide and water at high temperatures. Another common way to synthesize yellow tungsten oxide is by calcination of ammonium paratungstate (APT) under oxidizing conditions.

 

 

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In recent years, tungsten trioxide has been employed in the production of electrochromic windows, or smart windows. These windows are electrically switchable glass that change light transmission properties with an applied voltage. This allows the user to tint their windows, changing the amount of heat or light passing through. Tungsten oxide is widely used for many purposes in life. It is frequently used in industry to manufacture tungstates for fireproofing fabrics, for x-ray screen phosphors and in gas sensors. Yellow tungsten oxide is used as a pigment in ceramics and paints due to its rich yellow color.

 

 

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Due to the outstanding electrochromic, optochromic, and gaschromic properties, tungsten oxides have attracted great attention and have been investigated extensively. Since one-dimensional tungsten oxide nano materials exhibit superior properties in some promising applications compared with bulk materials, much effort has been made to synthesize tungsten oxide nanowires by using dfferent methods, including a soft-chemistry approach, the chemical vapour deposition (CVD) method, hot-filament-assisted synthesis, flame synthesis and multistage field enhancement of tungsten oxide nanowires.

 

Tungsten oxide nanowires were prepared by a vapor transport method using yellow tungsten oxide powder as a raw material. The crystal structure and morphology of WO3 nanowires were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The obtained nanowires were hexagonal WO3.

 

 

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Compared with other transition metal oxides, tungsten oxides have attracted great attention and have been investigated extensively due to their outstanding electrochromic, optochromic, and gaschromic properties. With those distinctive properties, tungsten oxides have been used to make at panel displays, photoelectrochromic smart windows and gas sensors. Non-fully oxidized tungsten oxides (WO3-x), such as W20O58, W18O49 and WO2, are of particular interest because of their unique electronic characteristics, e.g. tungsten oxide has been found to be a semiconductor, a conductor or a superconductor in different oxidization states.

 

The W18O49 nanowires are synthesized on the tungsten rod substrate by the CVD reaction in which tin powders are used to control oxygen concentration in the furnace and to aid the nanowires to grow. An appropriate flow rate of Ar and an extremely low oxygen concentration are essential for the W18O49 nanowire formation, which is still in the framework of the VS mechanism. The W18O49 nanowires may have potential applications in electronic nanodevices.

 

 

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Although the arc method has a higher production rate than IGC system, the latter produces larger particles. Hence, a modified technique based on the IGC system was developed, in which blowing gases were introduced to obtain finer particles with better particle size distribution. To retain the benefits of both the arc discharge method and the IGC system, a plasma arc is used as the heat source and blowing gas is applied to quench the evaporated materials in this modified system. A simplified manufacturing process that enhances the purity of the as-prepared products is required. Therefore, an effective method, namely, the modified plasma arc gas condensation technique has been proposed.

 

A modified plasma arc gas condensation technique was successfully used to synthesize various nano-sized tungsten oxide nano materials with morphologies and structures that may be tuned by controlling the experimental parameters. Various non-stoichiometric WO3−x nano materials could be prepared by tuning the oxygen content during the process. W18O49nanotubes and nanorod bundles were also prepared by He plasma arc with different Ar/O2 ratios. In addition, W18O49/TiO2 core–shell nanoparticles were prepared by evaporating a dual target. In the present study, we addressed the feasibility of the plasma arc gas condensation technique and confirm its potential for nanomaterial fabrication.

 

 

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Nano-sized materials and products have been used widely in many applications because of their outstanding properties, different from those of the bulk materials. In early investigations into nanotechnology, the arc discharge technique was the first well-developed method used to manufacture nanoproducts.

 

However, owing to difficulties involved in controlling the manufacturing parameters, the purity and quality of nanoproducts synthesized by arc discharge called for improvement. The inert gas condensation (IGC) system was thus established by Gleiter. Because there are no catalysts or containments, it is considered the cleanest method of producing high quality products. In this system, metals are first placed in a tungsten or graphite boat and evaporated. The metal vapor is then cooled under an inert gas atmosphere (e.g. helium or argon) to condense into clusters or nanoparticles.

 

 

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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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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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