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Why does tungsten oxide come in different colors

2018-08-23
Nano-tungsten oxide is a powder metallurgy raw material for producing hard alloys and tungsten products. It can be used for powder metallurgy to manufacture tungsten carbide, cemented carbide, superhard molds, tungsten rods, tungsten wires, etc., and can also be used for X-ray screens and fireproof fabrics, as well as colorants and analytical reagents for ceramics.
 
      The color of the nano-tungsten oxide is in the W-O system, and there are tungsten oxides such as WO3, WO 2.9, WO 2.72, and WO2. WO3 (yellow) - WO 2.90 (blue) - WO 2.72, (purple) - WO2 (brown) - W (gray black). That is, tungsten has four stable oxides: yellow oxide (WO3), blue oxide (WO 2.90), purple oxide (WO 2.72), and tan oxide (WO 2 ).
 
      Blue tungsten oxide refers to a series of non-ratio oxides of tungsten. The chemical formula is represented by WOx, mainly WO2.9. Blue tungsten oxide is used to produce tungsten powder, doped tungsten powder, tungsten rod and hard alloy, UV resistance, photocatalysis, etc. Blue tungsten oxide will change color and will turn into a faint blue color. If you refill it with a black opaque bag, it will turn into the original color.
 
      Nano-tungsten trioxide is a lemon-yellow powder with a density of 7.2-7.4g/cm3, a melting point of about 1470°C, a boiling point of between 1700 and 2000°C, a significant sublimation above 800°C, and a tungsten dioxide generating heat of 202.8. /mol, tungstic anhydride can be slightly soluble in water (0.2g / L), insoluble in all inorganic acids except hydrofluoric acid. The tungstic acid is easily dissolved in a caustic solution (NaOH or KOH) and aqueous ammonia to form a tungstate [NaWO4, K2WO4 and (NH4)2WO4]. The tungsten trioxide dissolves slowly in the ammonia solution, and dissolves more slowly when heated to a high temperature. Tungsten trioxide is easily reduced by various reducing agents. At room temperature, even a small amount of organic matter can be reduced and its color changed. However, when the air is heated, the original color is restored. At 700-900 ° C, tungsten trioxide is easily reduced to metal tungsten by hydrogen, carbon monoxide and carbon.
 
Nano-tungsten dioxide is a tan chocolate-like powder that is sensitive to moisture. When heated, the color changes from shallow to dark, and becomes dark orange. After cooling, the original color is restored. The density is 10.9~11.1g/cm3, the boiling point is about 1700 °C, the heat is 134 kcal/mol, and the tungsten trioxide is reduced to form tungsten dioxide at 575-600 °C. Tungsten dioxide is not dissolved in water, alkali solution, hydrochloric acid and dilute sulfuric acid. Nitric acid oxidizes tungsten dioxide to high valence oxides. The tungsten dioxide is quickly oxidized into tungsten trioxide in air and becomes a blue oxide when heated to 500 ° C in nitrogen oxide. At 1020 ° C, tungsten dioxide can be reduced to metal tungsten by carbon. At 250 to 300 ° C, powdered purple tungsten oxide (WO 2.72) can be obtained by reducing tungsten trioxide with hydrogen or carbon monoxide and heating tungsten trioxide to 200 to 250 ° C in a vacuum.
 
Nano-tungsten oxide specific use:
 
1) Gas sensing, catalysis, especially photocatalysis.
2) Solar photosensitive film.
3) Advanced pigments, oil paint watercolor.
4) X-ray shielding and fireproof fabric.
5) Doping with tungsten-doped materials.
6) Gas sensing material.
7) Primary or auxiliary catalyst for petrochemical industry. Hydrogen dehydrogenation, oxidation, hydrocarbon isomerization, alkylation and many other reactions have good catalytic performance and are commonly used catalysts such as petrochemicals.