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Isostatic Pressing for Round Tungsten Bar

Details: Category: Tungsten Information; Published on Tuesday, 24 May 2016 11:05; Written by xuejiao; Hits: 250

wolfram round bars Tungsten round bars formed by isostatic pressing can improve the uniformity of the tungsten material structure that can avoid the problem of brittle fracture on tungsten wire to some extent, thereby improving its wire-wounded performance, and improve the uniformity of wire diameter and improve the finished product rate, which has good reproducibility. When use square bar molds for making tungsten bars, the binder should be added to the doped tungsten powder, so that the internal forming tungsten is easily to cause stratification and likely to cause uneven density, cracks and other phenomena. Especially during the processing of tungsten square bars, the amount of deformation the four corners of tungsten bars is large, on the other hand, its center deformation is small, so its deformation is very unevenly at a high temperature, it is easy to produce localized cracking and final affecting the quality of tungsten wire and the finished product rate.

In order to solve the problems of tungsten square bars, developing isostatic pressing way to form round tungsten bars. Hot isopressing (HIP) is a manufacturing process, theorized in the 1970s, used to reduce the porosity of metals and increase the density of many ceramic materials. This improves the material's mechanical properties and workability. During isostatic pressing, it is preferable to make certain provisions of the metal powder. Different shapes, particle size and distribution of particle size of metal powders have a significant impact on manufacturing process of tungsten bars. Therefore, average particle size of doped tungsten powder should be 2.5 ~ 5.0μm, the oxygen content is lower than 2500 × 10-6. At the time of preparation of tungsten powder, it is important to control the humidity. When the tungsten powder to be filled into a soft film sets, be sure to place the powder uniformly and intimately, do not leave any gaps, in order to ensure the quality of the final product. After the preparation is completed, the speed of the release should be slowly. But there are some problems in the process. In the high pressure liquid when pressed, the size and shape of the final product cannot be controlled. Therefore, it is better to use a metal powder automatic loading machine, so we can guarantee that the final tungsten bars are flat, smooth and uniform.

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IF Furnace for Highly Purity Tungsten Bar

Details: Category: Tungsten Information; Published on Tuesday, 24 May 2016 11:02; Written by xuejiao; Hits: 223

Most high-purity tungsten bars are formed by conventional vertical sintered manufacturing processes, this kind of traditional process can consume a lot of power. In addition, at the time of preparation, tungsten bar would be affected by hydrogen flow and voltage fluctuations. If the operators have insufficient professionalism or their operations have some differences in the mass-produced, which will result in different quality of products in the same batch. On the contrary, if sintering in the intermediate frequency (IF) furnace, it would not only cost the lower energy, but also has with high collection rate.

Chemical compositions of tungsten bars made by the two processes are the same. So, IF furnace sintering method can also produce highly purity tungsten bars. Compared to the traditional production method, the density and hardness of wolfram bars formed by medium frequency furnace are much higher, and distribution of its grain is uniform. Density distribution of each part of wolfram bar made by conventional sintering is parabolic state from the top down, the density of a tungsten bar has a difference of 0.37g / cm3. The density of a wolfram bar produced by IF firing only has a difference of 0.2 g / cm3, whose density distribution is more uniformity than the traditional one. Vertical furnace can only hold a tungsten bar, which would take away a lot of heat from surface because of the excessive hydrogen, resulting in the surface temperature of tungsten is lower than the core temperature. However, wolfram bar itself is heating element in medium frequency furnace, so that the core temperature will be higher than the surface temperature. Therefore, the grains of a sintered tungsten bar will be unevenly distributed. For IF sintering, it can be placed more tungsten bars which can be displayed closely, then the heat taken away by hydrogen will be relatively less. Therefore, the final products would have more uniform density distribution.

All in all, IF sintering not only has a capacity of producing a higher purity tungsten bar, but also ensures a stable quality for the manufacturing of goods in large quantities. Further, since the power consumption is low, the required cost is also less. IF sintering adopts eddy current sensor which can make crucible furnace heat and then radiation to the tungsten, so the outside and inside of tungsten bars are nearly the same, therefore the tungsten grains of cross-section are uniform.

tungsten round bars

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Zr Affects Tungsten Trioxide Denitration Catalyst

Details: Category: Tungsten Information; Published on Monday, 23 May 2016 17:09; Written by chunyan; Hits: 269

Recent years, with the vehicle fleet constantly increasing, the pollution of harmful emissions from motor vehicles, such as NOx and particulates has become increasingly prominent. SCR is the most mature and widely used denitration technology, which is mainly vanadium system, its main active substance is vanadium pentoxide, and titanium dioxide as the carrier; for increasing its anti-sulfur poisoning, you can add tungsten trioxide to this catalyst, thereby improving the denitration performance. Traditional vanadium SCR denitration catalyst has the high and narrow range (300~450°C) of activation temperature, thus limiting its use in diesel engines.

Studies have found that the introduction of some metals (such as Ni, Fe, Cu and Zr, etc.) will enhance the low-temperature activity of tungsten trioxide diesel engines denitration catalyst. Among them, in particular, the catalyst incorporated Zr has the best low-temperature activity with highest temperature window. Experiments showed that, after the Zr loaded, tungsten trioxide denitration catalyst had the denitration efficiency of 95% under relatively low temperatures (180°C), which shows that the addition of Zr help to improve the denitration performance at low temperatures, prompting load synergy between the metal oxide catalyst. In addition, with the increasing of Zr, the denitration catalyst performance better within the appropriate range.

Zirconia has the acidic, alkaline, oxidizing and reducing, widely used as a co-catalyst and crystal transformation inhibitors. When the Traditional vanadium-tungsten-titanium denitration catalyst loaded 3% of vanadium, and 6% of tungsten, it shows the best activity under the high temperature of 350°C, and the 93%conversion rate of NOx. However, after doped zirconia, it exhibits a highly efficient and stable denitration activity under the flue gas temperature among 200~350℃, with the optimum activity temperature point of 300℃, and 96% of conversion rate of NOx.

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WO3/TiO2-ZrO2 SCR Denitration Catalyst

Details: Category: Tungsten Information; Published on Monday, 23 May 2016 17:13; Written by chunyan; Hits: 255

Denitration catalyst is the core part of the SCR process, the denitration catalyst applied in industrial mostly is vanadium/titanium based, with the temperature window of 370~430℃, and the denitration efficiency of about 80%. However, the vanadium/titanium (anatase) system is an unstable system, titanium dioxide is the metastable crystal with a smaller surface area; besides, when heated, the surface is easily shrank, and converted into stable thermodynamic state of rutile at a certain temperature and pressure, resulting in the declining of irreversible denitration efficiency. Experiments showed that the adding zirconium oxide can improve the tungsten trioxide denitration catalyst surface acidity, thermal stability and support denitrification efficiency.

It is well known that the first step of SCR denitration reaction is ammonia adsorption, the longer residence time benefits the reaction at the catalyst surface. The doped of tungsten oxide and zirconium oxide makes catalyst having superior storage capacity of ammonia, thus to conducive to the denitration reaction. Experimental prepared tungsten trioxide denitration catalyst doped zirconium, which showed as follows:

1. Prepare titanium oxide-zirconium oxide solid-solution vector (the molar ratio of Ti and Zr is 1) by coprecipitation method

Dropwise the titanium tetrachloride into deionized water containing zirconium oxychloride under the condition of ice-cooling with stirring;

2. After completing the addition, adjust the pH value to 9~10 with concentrated aqueous ammonia, stand for 12h after the white precipitate producing, then discard the upper-layer of serum;

3. Wash the precipitate with deionized water and go through vacuum filtration, until no Cl- exists;

4. Dry the washed the precipitate in an oven at 110℃ for 12 hours, grinding, and calcined in a muffle furnace at 450℃ for 4 hours to titanium-zirconium solid solution;

5. Preparation of WO₃/TiO₂-ZrO₂ catalyst, add ammonium tungstate into titanium-zirconium solid solution, then going through a oil bath at 25 ℃ for 4 hours and evaporating off all of the moisture at 85℃;

6. Dip the ammonium paratungstate under the condition of magnetic stirring with stirring, and then evaporated to get the dryness solid, transferred to an oven at 110℃ for 12 hours; calcined in a muffle furnace at 450 ℃ for 4 hours after milled to give the desired WO₃/TiO₂-ZrO₂ denitration catalyst.

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Ammonium Tungsten Bronze Nanoparticles Preparation

Details: Category: Tungsten Information; Published on Monday, 23 May 2016 16:04; Written by xinyi; Hits: 267

Tungsten bronze compounds are a series of important inorganic compounds, tungsten ions exist as mixed valence state of W^{6 +}, W^{5 +} and W^{4 +} in such compounds to make a balance in the overall charge. Rich crystal structure, the tunnel structure and this particular valence state lead to its excellent properties, such as electronic and ionic conductivity, superconductivity, optical properties, which has caught widespread research interest in the aspects of the secondary battery, electrochromism, near-infrared absorption and application of chemical sensors.

Currently, methods to synthesize tungsten bronze compounds mainly are the wet chemical method, heat reduction method and thermal decomposition method. Wet chemical method to synthesize ammonium tungsten bronze is to put the starting material in the reducing solvent refluxing for several days, size of the sample obtained by this method is too large, it’s usually between a few to ten micron, and the preparation process requires a long time and high energy consumption. Thermal reduction method is to uniformly mix tungsten oxide, tungsten powder and metal tungstates in proper proportions, then heated in a vacuum or under an inert atmosphere, the reaction temperature is usually about 1000 ℃, and remove unreacted impurities after the reaction is completed. Since the thermal stability of ammonium tungsten bronze difference is poor and decomposition temperature (300 ℃) is lower than the synthesis temperature, the thermal reduction method can not be used to synthesize ammonium tungsten bronze. The thermal decomposition method to synthesize ammonium tungsten bronze is to heat and decompose ammonium paratungstate in a reducing atmosphere (H₂ or a mixed gas of H₂, Ar, etc.), the size of the resulting sample is too large, and this method can not obtain completely pure phase ammonium tungsten bronze, ammonium content in sample is too low and easy to excessive decomposed into tungsten oxide.

The pure phase ammonium tungsten bronze nano-powder can not be directly obtained in current study, so usually break the large micron-sized particles obtained into small particles by milling, but these compounds are easily to be oxidized and lost live and decompose in the milling process, also accompanied by crystallization performance degradation. For the above problems, some scholars have proposed a synthesis method to directly synthesize ammonium tungsten bronze powder with controllable particle size.

Preparation of reduced state ammonium tungsten bronze nanoparticles: dissolve 0.01~1g tungsten hexachloride or tungsten tetrachloride in 20~40mL oleic acid solution, and stirred to obtain homogeneous solution, then added 4~30mL oleylamine, and mix evenly, move to supercritical reaction kettle, crystallization reaction at 150~350 ℃ for 0.5~48 hours, the powder samples were centrifuged and washed after reaction, dry under vacuum at 40~250 ℃ for 1~12 hours, and the reduced state ammonium tungsten bronze nanoparticles are obtained, the mole fraction of ammonium group in the composition is between 0.2~0.3. In addition, samples obtained by this method have strong near-infrared absorption ability, the film containing nanoparticles can effectively shield the near infrared rays of 780 ~ 2500nm and maintain high visible light transmittance.

ammonium tungsten bronze structure

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