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

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）18:26に公開; 参照数: 1460

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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Research Status and Prospects of WO3-based Gas Sensor

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）18:17に公開; 参照数: 1558

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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Gas Sensing Characteristics of WO3 (III)

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）18:04に公開; 参照数: 1359

(3) Gas sensing characteristics to organic gases ( acetone, ethanol etc.)

The morphology of the materials could be designed in the stage of hydrothermal reaction. Some scholars employed a novel hydrothermal process to synthesize plateshaped WO3 nanostructures. The addition of structure-directing agent p-nitrobenzoic acid results in the formation of WO3 nanoplates. The pH value in the reaction system may cause the nano-plate etched partially so that some nanoplates become incomplete with rough edge regions or a hole in the middle. The voltage levels up quickly in presence of ethanol or acetone and restores soon after removing the gases. It demonstrates that the as-prepared WO3 nanoplates have good sensitivity and reversibility to ethanol or acetone. However, the working temperatures are relatively high (340℃ and 370℃) . These temperature values bring lot of difficulties for the actual application of WO3-based gas sensors.

Besides, Another scholars have successfully synthesized large-scale Co-doped h-WO3 nanorods by a facile hydrothermal method with Na2WO4·2H2O and Co(NO3)2·6H2O. They found the nanorods grown vertically from the center towards two opposite directions and had a uniform thickness of about 10 μm. Sensing responses of samples towards a series of typical organic solvents and fuels had been investigate. It is well known that WO3 is less sensitive to hydrocarbons. Whereas, it is noticed that Co-doped WO3 nanostructure is highly sensitive to these flammable organic gases. This illustrates that dopant endows WO3 with better sensing performance towards hydrocarbon gases.

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Gas Sensing Characteristics of WO3 (II)

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）17:58に公開; 参照数: 1649

(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)

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）17:52に公開; 参照数: 1595

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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Hydrothermal Synthesis of WO3

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）17:45に公開; 参照数: 1809

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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Gas Sensing Response Principle of WO3-based Gas Sensors (II)

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）17:23に公開; 参照数: 1687

The sensitivity (S) could be defined as the ratio between the respective variation of resistance in the air (ΔRair) and in the target gases (ΔRgas) according to

S = ΔRair / ΔRgas

ΔR represents the difference between the maximum and minimum values of the resistance during one complete period of aerating cycling. However, we prefer to express the sensitivity with a more simple calculation route like this

S = Rgas / Rair

or S = Rair / Rgas

Rair and Rgas are the resistance of sensor in dry air and target gases, respectively. Usually, in oxidizing atmosphere, the oxide surface is covered by negatively charged oxygen adsorbate and the adjacent space charge region is electron-depleted: the oxide layer presents therefore a high resistance (Rgas＞Rair). Under reducing conditions, the oxygen adsorbate is removed by the reaction with reducing gas species and the electrons are re-injected into the space charge layers: as a result, the oxide layer resistance decreases (Rgas＜Rair). Besides, the voltage change of oxide may be used to illuminate the gas sensitivity at times. Therefore, the change of resistance, conductivity or voltage could be used as the parameters to scale the sensitivity.

In addition, the response time is often defined as the time required for the conductance to reach 90% of the equilibrium value after the test gas is injected, and the recovery time is the time necessary for the sensor to attain a conductance 10% above the original value in air.

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Gas Sensing Response Principle of WO3-based Gas Sensors (I)

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）17:18に公開; 参照数: 1514

The WO3-based gas sensors operate on the principle that the sensor’s resistance or conductivity changes in the presence of target gases. When the gas sensing devices exposure under target gases, the gas molecules are adsorbed on the surface of WO3 and electron transfer occurs between WO3 and the adsorbent. As a result, a depletion or accumulation of charges occurs on the surface. Following that variation of surface potential barrier induces a change in the resistivity or conductivity. Thus, this change suggests that WO3 produces response to the target gases.

It is common to evaluate gas sensing characteristics by the parameters like sensitivity, response time, recover time, gas concentration and working temperature. The ideal sensor materials should be of high sensitivity to target gases, short response and recover time, low gas concentration and working temperature.

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Violet Tungsten Oxide

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）11:13に公開; 参照数: 1539

Violet tungsten oxide (VTO) is a kind of tungsten oxide. Violet tungsten oxide is a finely divided violet crystalline powder. It is produced by rotary calcining ammonium paratungstate at closely controlled temperatures in a reducing atmosphere. Violet tungsten oxide is used primarily for the production of tungsten metal powder and tungsten carbide.

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Tungsten Blue Oxide Uses

詳細: カテゴリ: Tungsten Information; 2015年2月05日（木曜）11:10に公開; 参照数: 1587

Blue tungsten oxide (BTO) is also called tungsten blue oxide (TBO). Tungsten blue oxide is one of the most important, highly pure intermediates for the production of other tungsten compounds including tungsten metal powder.

Tungsten blue oxides are finely divided blue-violet powders.They are produced by heating ammonium paratungstate at closely controlled temperatures in a reducing atmosphere. GTP produces two standard grades differing mainly in oxygen content. Tungsten blue oxide is used primarily for the production of tungsten metal powder and fine tungsten carbide as well as wire products.

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