Scheelite was first described in 1781 for an occurrence in Mount Bispbergs klack, Säter, Dalarna, Sweden, and named for Carl Wilhelm Scheele (1742–1786).Owing to its unusual heaviness, it had been given the name tungsten by the Swedes, meaning “heavy stone.” The name was later used to describe the metal, while the ore itself was given the name scheelerz or scheelite.



 

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Scheelite is a calcium tungstate mineral with the chemical formula CaWO₄. It is an important ore of tungsten. Well-formed crystals are sought by collectors and are occasionally fashioned into gemstones when suitably free of flaws. Scheelite has been synthesized via the Czochralski process; the material produced may be used to imitate diamond, as a scintillator, or as a solid state lasing medium. It was also used in radium paint in the same fashion as was zinc sulphide, and Thomas Edison invented a fluoroscope with a calcium tungstate-coated screen, making the images six times brighter that those with barium platinocyanide; the latter chemical allowed Röntgen to discover X-rays in early November 1895.

 

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There are several factors that influence the selection and use of X-ray shielding materials. Considerations such as attenuation effectiveness, strength, resistance to damage, thermal properties and cost efficiency can affect radiation protection in numerous ways. For example, metals are strong and resistant to radiation damage, but they undergo changes in their mechanical properties and degrade in certain ways from radiation exposure. Likewise, concretes are strong, durable and relatively inexpensive to produce, but become weaker at elevated temperatures and less effective at blocking neutrons.

In most cases, high-density materials are more effective than low-density alternatives for blocking or reducing the intensity of radiation. However, low-density materials can compensate for the disparity with increased thickness, which is as significant as density in shielding applications. Due to tungsten material has high density, excellent radiation attenuation properties and environmental friendly. Tungsten can be good choice to produced tungsten X-ray shielding.

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Tungsten X-ray shielding is the process of preventing radiation produced from X-rays reaching an unwanted place or person. Tungsten X-ray shielding or tungsten radiation protection is the science and practice of protecting people and the environment from the harmful effects of ionizing radiation.

Tungsten X-ray shielding is based on the principle of attenuation, which is the ability to reduce a wave’s or ray’s effect by blocking or bouncing particles through a barrier material. Charged particles may be attenuated by losing energy to reactions with electrons in the barrier, while X-ray and gamma radiation are attenuated through photoemission, scattering, or pair production. Neutrons can be made less harmful through a combination of elastic and inelastic scattering, and most neutron barriers are constructed with materials that encourage these processes.

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One of the important parts of examine the microstructure is analyzing the defect of tungsten points. to You can check the quality of products on the basis of the standards and its material microstructure of tungsten points. The determination of its defect type and size could be available according to the distribution of its microstructure and check whether they satisfy the requirements of tungsten points’ specification.

The defect types of tungsten points are listing as follows: pores, inclusions, porous holes, bubbling, squeezing cracks, and unevenly distributed organization, purity of tungsten points’ material, delaminating cracks or the particle size and morphology of tungsten powder... There are problems of the process, because the reason is complicated, you should comprehensively analyze and resolve them.

One way to test the quality of metal materials is microstructure analysis; it adopts the principle of quantitative metallography.

Defect types and sizes of tungsten points’ materials may also be relevant to production processes and other causes; therefore, the analysis of the causes of the defect phenomenon can be as a reference to improve product quality.



 

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Tungsten and molybdenum possess wear resistance, high boiling point, high hardness, anti-welding resistance, high melting point and other excellent properties. Silver and copper posses good electrical conductivity and good thermal conductivity and other properties. Tungsten carbide can improve the arc-erosion resistance and anti-welding properties of tungsten points’ material and others. So the performance of tungsten points can be improves greatly by adding an appropriate amount of copper, silver or tungsten carbide and other elements.
The copper tungsten points, silver tungsten points and silver tungsten carbide points can usually be prepared by infiltration sintering process. The processes can be seen as bellow:
press tungsten or tungsten carbide powder, and then mold it, then sinter it, therefore, a porous skeleton of tungsten or tungsten carbide can be formed. And then immerse molten silver or copper in porous skeleton, so due to the capillary action of tungsten or tungsten carbide, they will soak in skeleton pores. After that copper tungsten, silver tungsten or silver tungsten carbide points materials which own high density can be available. For copper tungsten, silver tungsten or silver tungsten carbide points material which own less tungsten or tungsten carbide content can use mixing and sintering process.

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The elements of silver tungsten carbide points’ material are silver and tungsten carbide. Silver owns good oxidation resistance, good electrical conductivity, nice thermal conductivity and good processing performance, etc., and tungsten carbide having the advantages of improving their anti-welding resistance and arc erosion resistance performance and so on.

After galvanic corrosion, the surfaces of silver tungsten carbide points will have cracks, its composition is not evenly distributed, you can see the loose structure of their melt layer surface, and compared to the previous experiment, the silver content is significantly reduced, contact surfaces also did not find holes and gas eruptions pits, what suggesting that silver drip splash does not occur in the high-temperature smelting process, among which is the reason for the cracks that the lack of silver result in the loose structure. Certainly, this does not explain that the higher of the silver content can prevent cracks. Studies have shown that if the silver content is too high, it is more easily lead to large silver shedding phenomenon. Therefore, the silver content should be controlled in a certain proportion, which can reduce drip splashes and prevent cracks.



 

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Mechanical alloying method is a method which is applied to alloy fine crystalline alloy powder material. For example, titanium-aluminum-based alloys should adopt this method to form crystals, whose most significant property is that it is easier to obtain nano-crystalline organization whereas is with bad compactness and difficult form shape. What is more, the rapid cooling method is not available to titanium-aluminum-based alloy products.

Tungsten points prepared by mechanical alloying possess the following properties:
1. Possess uniform microstructure and dispersed strengthening phase and fine;
2. Own simpler production process and cost-effective advantages;
3. The mechanical and electrical properties of tungsten points can be improved;
4. Supersaturated solid solution can be prepared, and alloying system of immiscible tungsten contact material can be prepared;
5. Nano contact materials with good properties also can be prepared;
6. Metal oxide-based, refractory metal and other series of contact materials can be prepared as well.



 

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