A Preparation Method of Nanocrystalline Tungsten Powder by Ball-Milling

Tungsten is the most promising plasma material to be used in fusion reactors due to its high melting point, low coefficient of thermal expansion, low sputter corrosion rate, high thermal conductivity, low hydrogen retention, and high strength at high temperatures. These excellent properties make tungsten has the ability to stand high doses of particle irradiation (such as neutron irradiation) and high heat flow in a fusion reactor.

Tungsten has many excellent properties, but there are still many problems as a tungsten divertor: brittleness at low temperature; low ductile-brittle transition temperature of pure tungsten; in the high temperature environment of the fusion reactor, the grain is prone to grow up, which reduces the strength and hardness of the material. The neutron irradiation effect causes degradation of the material, such as nuclear enthalpy to form a metamorphic product, which causes the material to swell and the processing is difficult. Studies have shown that refining tungsten grains and doping second phase particles in tungsten can significantly improve the mechanical properties and anti-irradiation properties of tungsten. tungsten-based materials.

Using coarse-grained tungsten as the original ball-milling material, a planetary ball mill is used, with nitrogen (purity 99.999%) as the shielding gas and absolute ethanol (chromatographic purity) as the process control agent, by adjusting the ball-material ratio (4:1, 10: 1 and 15:1), and add different mass fractions (7% to 9%) of anhydrous ethanol in ball-milled tungsten powder with different ball-material ratios, making the ball milling process faster and more efficient, using a certain ball milling time. Finally a small-grained nanocrystalline tungsten powder is obtained. The procedures are as follows:

1) Mix zirconia grinding balls (diameter 1 mm) and initial tungsten powder into a ball mill tank at a mass ratio of 4:1, 10:1, and 15:1, and add anhydrous ethanol having a mass fraction of 7% to 9%. Make sure the total volume of powder, grinding ball and absolute ethanol not more than 2/3 of the volume of the ball mill tank, add a sealing ring at the mouth of the tank, cover the lid of the ball mill tank, and pass high-purity nitrogen as a shielding gas;

2) Turn on the power switch, put the ball mill tank filled with sample into the ball mill, set the speed to 500rpm, set 10 cycles each time, one cycle is 20 minutes, ball mill 10 minutes / pause 10 minutes, total ball grinding 36 times The total ball milling time is 120 hours, wherein the net ball milling time is 60 hours;

3) Remove the ball mill tank from the ball mill, taking out the sample in a vacuum glove box, dry it, separate the powder sample and the grinding ball with a mesh sieve to obtain a nanocrystalline tungsten powder;

4) The ball-milled tungsten powder was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the particle size and grain size of the tungsten powder were visually observed.

Nanocrystalline tungsten having a tungsten grain size of about 5 to 30 nm was visually observed by a transmission electron microscope and a scanning electron microscope.

The preparation method of the nanocrystalline tungsten and the nanocrystalline tungsten-based powder proposed by the above method, the high energy ball milling method, is easier to implement than the other powder preparation methods and the obtained tungsten and tungsten-based powders have smaller grain size characteristics. By selecting the ball mill process control agent, adjusting the parameters of the high energy ball milling process, combined with transmission electron microscopy and scanning electron microscopy characterization techniques, nanocrystalline tungsten and nanocrystals with grain sizes below 30 nm (minimum grain size of 3 nm) were visually observed. 

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