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Gas Sensing Characteristics of WO3 (II)

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 17:58; Written by cq; Hits: 1725

(2) Gas sensing characteristics to H2S

Hydrogen sulfide (H2S) is a malodorous toxic gas, as the one of representative reducing inorganic gases, which draws researcher’s interests. Studies showed the nanowires and platelets performed more quickly response to the same concentration of H2S gas in the same working temperature compared to the nanoparticles. This is because the larger surface area provides more adsorption desorption sites and results in a higher sensitivity. The sensing mechanism of WO3 to H2S is also provided.

First, the atmospheric oxygen adsorbs electrons from the conduction band (CB) of WO3 and occurs on the oxide surface in the form of O2-and O-

O2(air) + e-→O2(ads)-

O2(ads)-+e-→2O(ads)-

In this way, oxygen adsorption reaction creates a thin electron-depleted layer at the surface of WO3. Then H2S reacted with the adsorbed oxygen species according to the following formula

H2S + 3O( ads)-→SO2+H2O + 3e-

Thus, the resistance of WO3 decreases. As the detection of H2S was usually based on the reaction between H2S molecules and surface oxygen of WO3.

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Gas Sensing Characteristics of WO3 (I)

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 17:52; Written by cq; Hits: 1654

Electrical properties of oxides semiconductor depend on the composition of the surrounding gas atmosphere. The following is the latest progress on gas sensing characteristics of WO3 with three aspects of oxidizing inorganic gases NO2, reducing inorganic gases H2S and organic vapors.

(1) Gas sensing characteristics to NO2

To our best knowledge, much effort on gas sensing characteristics of WO3 prepared by hydrothermal method is devoted to highly toxic NO2. The detection of NO2 is usually based on the formation of absorbed surface-trap states NO2-ads. The resistance of WO3 was increased with the increasing barrier in the conduction band (CB). The resistance increased with the increasing concentration of NO2 because of capturing more electrons from the CB.

It is well known that the gas sensitivity of the WO3 sensing materials to NO2 is much dependent on the annealing temperatures, the additives, the operating temperatures, as well as electrode materials. For example, WO3 nanopowders annealed at high temperature showed better response to NO2 in dry air. This was attributed to the improvement of crystalline quality at higher temperature.

Improving the crystalline quality, reducing the size of product, adding optimal active catalyst and dopant amount as well as optimizing the design of electrode are all good for the gas sensing characteristics of WO3. However, the working temperature is commonly above the room temperature. Even the sensor shows response, the sensitivity value is still low. This drawback limited application of WO3-based gas sensors in a certain extent.

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Filter Initial Ammonium Paratungstate Precipitate

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 17:38; Written by zsq; Hits: 516

Since the solubility of ammonium paratungstate increases rapidly at the high pH necessary to avoid molybdenum contamination, when very small molybdenum levels are desired, it may be desirable to leave some tungsten in solution, filter out the initial ammonium paratungstate precipitate, and recycle the filtrate. For example, if the precipitation is performed at a pH slightly above 8.5, 85 percent to percent of the tungsten will be precipitated as pure ammonium paratungstate. Further precipitation of tungsten may be obtained by boiling the filtrate in an open vessel. During this evaporation step ammonia and water are removed and the pH of the filtrate drops progressively to approximately 7.0. The molybdenum'content of the ammonium paratungstatethat is precipitated between the'pl-l values of 8.5 and 7.0'will rise from approximately 0.02 percentv t percent. This secondary precipitate will contain as high as 13 percent of the initial molybdenum. This material may then be recycled to a new batch of concentrated digester solution where it will raise the tungsten concentration. The filtrate resulting from this secondary precipitation step will contain approximately 2 percent or the initial tungsten and 92 percent of the initial molybdenum. If molybdenum is a desired byproduct, it may be obtained by any of a number f methods from this resulting filtrate.

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Hydrothermal Synthesis of WO3

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 17:45; Written by cq; Hits: 1870

The adsorption of gases basically occurs at the surface level of sensing materials, and an increase in the active surface area of the semiconductor oxide would enhance the properties of the materials used for gas sensors. Therefore，surface morphology of WO3 influences the gas sensitivity in a great degree. It has been verified that, in the hydrothermal process, the morphology of the WO3 can be tuned by some assisting agents such as surfactant, inorganic salt, complex agent and some dissoluble organic acid. The tungstate is widely used as tungsten sources.

Tungsten trioxide can be produced by different chemical reactions. The addition of assistant reagents affects the structural morphologies and grain sizes of WO3. Most final products are nanostructures and possess higher surface-to-volume ratio so that the sensor signal caused by the reaction with the target gas is stronger than the larger scale ones. Therefore, adopting hydrothermal method for the synthesis of WO3 is conducive to improve the gas sensing characteristics of WO3.

In addition, the synthesized WO3 exhibits uniform size and morphology. Once the WO3 is widely applied in the gas sensor filed, the hydrothermal method will be used for a mass synthesis of WO3.

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The Applications of Ammonium Paratungstate

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 17:28; Written by zsq; Hits: 509

Ammonium paratungstate is a very desirable form of tungsten for many industrial applications. Much effort has been expended by industry in order to arrive at economical methods of producin this product. Tungsten is derived primarily ll'QIl'l scheelite ore which is substantially calcium tungstate. This is normally treated by dissolution under pressure in a sodium carbonate solution. In this manner an aqueous solution of sodium tungstate is formed which may be used as a starting point for the preparation of ammonium paratungstate. it will be obvious that this sodium paratungstate solution contains not only tungsten values but also impurities such as sodium carbonate, sodium molybdate, silica, sodium fluoride, phosphorous, and molybdenum. Of these contaminants, molybdenum is one of the most difficult to control. At the present time a complex multi-step process is needed for producing ammonium paratungstate from such industrial solutions while substantially avoiding serious contermination of the product. A large number of pH changes are required which entail th use of large quantities of various acids and alkali Even with this complex procedure, however, impurities are almost always present in the final product.

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