Mechanical alloying method is an effective method to compound fine crystalline alloy powder material. For example, titanium aluminum alloys was required to use this method to form crystals, its most important feature is easier to obtain nano-crystalline organization, but its compactness is not good and its forming is more difficult, besides, the rapid cooling method can not get titanium aluminum alloy products.

Tungsten contacts prepared by mechanical alloying method have the following features:

1. With dispersion strengthening phase and fine, uniform microstructure;
2. The production process is simpler and more economical;
3. Improve the electrical and mechanical properties of tungsten contacts;
4. Prepare supersaturated solid solution, so immiscible tungsten contact material can be an alloying system;
5. Prepare nano contact materials with good performance;
6. Prepare refractory metal, metal oxide-based and other series of contact materials.



 

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The interest in CaWO₄ and CaMoO₄ has been driven by the fact that these compounds have especially attractive properties for a variety of optical applications (phosphors, laser hosts, scintillators). Recently, CaWO₄ was implemented in the CRESST dark matter search, enabling discrimination between electron and nuclei recoils. Different types of interaction events can be identified by simultaneously measuring a combination of phonon and scintillation signals from cryogenic phonon-scintillation detectors (CPSD) operating at milli-kelvin temperatures. This technique offers high sensitivity and the unique advantage of background identification, which is crucially important for experimental searches for rare events.

 

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For their approved use in scintillating applications, solid-state optoelectronic devices, lasers and optical fibers-like components, scheelite-type photonic crystals have attracted much interest and materials like scheelite CaWO4 have been synthesized in the last years by a diversity of methods including Pechini, polymeric precursor, co-precipitation, solvothermal, Czochralski, and microemulsion-mediated hydrothermal. However, scheelites obtained by these routes show serious problems like off-stoichiometry in any case and undesirable optical quality and scintillating properties in single crystals.

In this work, powders and single crystal minirods of scheelite CaWO4 were obtained by the methods of solid state reaction and laser heated pedestal growth, respectively, and an investigation has been made of their composition, scintillating and optical properties as revealed by X-ray diffraction (XRD), radioluminescence (RL) and optical absorption (OA) measurements.

We have found single phase CaWO4 in both as-obtained powders and single crystals, and wellbehaved optical and scintillating properties. Enhanced scintillating properties in single crystal samples were evidenced, with single or double RL peaks between 400 nm and 500 nm, with sources of β-ray or X-ray, respectively. Also, a decrease of the RL light yield with the temperature increase was observed in all the samples. A shift of the RL peak was also observed with the temperature increase.

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Powders and high optical quality single crystal minirods of stoichiometric scheelite CaWO4 have been prepared by solid state reaction and floating zone methods, respectively, and their single phase formation, optical and scintillating properties have been evaluated by X-ray diffraction, optical absorption and radioluminescence measurements. All obtained results indicate that every sample is stoichiometric, while scintillation measurements reveal radioluminescence peaks at about 420 nm, 440 nm and 490 nm depending on the source of irradiation used. The single crystal samples exhibit higher radioluminescence light yield.

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The rate of decay of a very phosphorescent sample of calcium tungstate was determined photographically and found to fit the empirical equation of Becquerel, <i>I</i>^-x = <i>a</i>+<i>bt</i>, within the experimental error, from thirty seconds to five minutes after the exposure to x-rays. The phosphorescence of this sample could not be detected photographically more than 50 hours after the exposure of 300 milliampere-seconds at a distance of 30 cm from the target. Under the same conditions the phosphorescence of normal calcium tungstate disappeared in one minute. The intensity of the phosphorescence was found to be closely proportional to the product of the intensity of the x-rays and the time of exposure up to a limiting value, beyond which increased exposure caused a negligible increase in the phosphorescence. In general, these properties are quite similar to those of the “phosphors” of Lenard and others.

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The configuration coordinate model has been used to calculate the luminescence band of CaWO₄ at various temperatures. The force constant k_g of the harmonic vibration in the ground state has been computed from the vibrational Raman spectrum of WO₄/²~; the result is k_g = 5.8 × 10⁵dyne/cm. The force constant k_e for the excited state, deduced from the temperature dependence of the width at half the maximum intensity of the emission band, isk_e = 1.5 × 10⁵ dyne/cm. In order to obtain satisfactory agreement between theory and experiment we have to assume the variation in interatomic distance on excitation to be 0.20 Å which is in accordance with the analogous quantity of OsO⁴ where a calculation from spectroscopical data yields an expansion of 0.15 Å.

 

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Coarse and spherical tungsten powders are prepared by wet hydrogen reduction. Ammonium tungstate solutions with the additives of Li, Na and K alkali metal salts are used as raw materials. The effects of the type and content of the alkali metal salts as well as reduction time on the microstructure, chemical composition, phase composition and particle size of ammonium paratungstate and coarse tungsten powders are investigated by scanning electronic microscopy, energy dispersive spectroscopy, X-ray diffraction, and test sieves.

It is shown that ideal tungsten powders with good fluidity, sphericity, integrity and even distribution can be obtained by using the ammonium tungstate solution with 3g.L^-1 NaCl, KCl and Li₂CO₃ and reducing at 1000℃ for 180 min in wet hydrogen atmosphere. The average particle size of the obtained tungsten powders is 67um, the maximum particle size is 150 um, the biggest loose density is 13.41g.cm^-3, and the best powder flowability is 9s for 50g tungsten powders.




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Tungsten is being widely applied in the modern science and industry, especially tungsten alloy, tungsten carbide and other tungsten heavy alloy which is of high wear resistance and high corrosion resistance. Among which coarse spherical tungsten powder is an important material for preparing coarse tungsten carbide powder. Coarse spherical tungsten powder is of higher density and hardness which is widely applied in mining tools, oil drills, cutting tools, etc.

Nowadays the main tungsten powder preparing method is calcinating ammonium paratungstate into blue tungsten oxide or yellow tungsten oxide and then process through hydrogen reduction. Tungsten powder prepared by this method is of regular multi crystal and grain size is between 2-5um. In 1980s, adding alkali metal salts in tungsten oxide and process mid-temperature reduction method will enlarge the grain size of tungsten powder. Adding Na and Li together will maximize the property of tungsten powder.


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Ammonium paratungstate preparing coarse spherical tungsten powder affecting factors are listed as below.

1.Adding NaCl and Li₂CO₃ will affect the grain size of ammonium paratungstate. Adding NaCl and LiCO₃ will enlarge the grain size of ammonium paratungstate. But the effect of Li2CO3 is more apparent. The grain size can reach 60um.

2.As the reduction time prolonged, grain size of tungsten powder increases. This is because the fine tungsten oxide particles volatilize and deposit on the tungsten macro tungsten powder which increase grain size of tungsten powder gradually.

3.Adding additives. Adding alkali metal salt will enlarge the grain size of tungsten powder. When the grain size is between 45-61um, tungsten powder grain size grows rapidly. It turns out that Li will maximize the growing of tungsten powder. When the particle size reaches 100um, the growing speed slows down, the average size of tungsten powder reaches 67um which can meet the requirement in industry.




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Tungsten powder is not only the raw material of tungsten carbide, but also for tungsten alloy. It can be applied in machining, mining explore, geological drilling, construction tools, military area, etc. In order to promote the property of materials, prepare submicron spherical tungsten powder is a tendency.

Submicron spherical tungsten powder after sintering can promote the hardness and density of metal materials which improve its property. If the distribution of submicron tungsten powder is closed, materials after sintering will still be fine particles, it will affect mechanical property of materials. Methods for preparing submicron tungsten powder include ball milling, reduction under dry atmosphere, hydrogen reduction, tungsten acid reduction, etc. These methods have a lot of disadvantages such as it is of high cost and low producing efficiency which is not fit for industrial production.

Ammonium paratungstate preparing submicron spherical tungsten powder is more environmental friendly, easy to operate, the parameter is under easy control, the equipment is accessible and it is more suitable for massive production.




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