Some Features of Sintering of Tungsten Powders

A method of activating the sintering process for tungsten powders using a closed reaction space and hydrogen, steam-saturated water was observed. This sintering process is allowed to activate super coarse-grained (1000 μm) tungsten powder sat relatively low temperatures (1000 1200 °C).


Introduction
Tungsten and its alloys because of their unique characteristics have wide application in many sectors of the industrial production [1][2][3].One of the topical applications of tungsten materials is the thermonuclear energy, where tungsten can be used for protection against plasma of parts of the first wall and the diverter [4,5], because of its operational strength to 20 times higher than the resistance of iron, copper, beryllium and 10 times-molybdenum.The most important is creation of porous tungsten materials, through which molten lithium must be delivered to the work surface.
While tungsten is highly Refractory metal, sintering of porous products has some difficulties: the need to achieve temperatures of 0.7-0.8melting point, applying a protective environment (vacuum or inert gas), using of special equipment.So the current problem is to find methods to simplify the process of sintering.One way to do this is activated sintering of powder tungsten products with alloying additives, which enter stored by deposition on the tungsten particles surface layer of metal salts (nickel) in the evaporation of the solvent from a liquid solution of metal salts or metal layer by electrolysis [6][7][8].In the paper [9] an attempt was made to activate the sintering of tungsten with using as an activator of tungsten nanopowder.Applying the methods described in [6][7][8][9], made possible to reduce sintering temperature of tungsten up to 0.4-0.5 melting point.However, the disadvantage of these methods was using of additional materials for activation of sintering process.This makes impossible to have a clean, without impurities, tungsten materials.
In the works [10][11][12] there are the results of research of super coarse-grained tungsten powders producing with particle sizes up to 1000 microns, which can be used as starting components to produce products with a given porosity.It should also be noted that studies on activation of the sintering process carried out on samples manufactured from tungsten powder with particle sizes up to 1 micron.However, the results of studies on sintering activation for more coarse-grained powders of tungsten in the literature are not given.
To intensify the growth of particles of tungsten powders in the works [10][11][12] was applied the technique of closed reactor space, providing hydrogen gas environment, saturated with water vapours, which are formed due to reduction of oxygen-containing compounds of tungsten, with strong growth in the tungsten particles [1,11].Using of a closed reactor space with high humidity gas environment during sintering of porous tungsten materials can be helpful to activate the process of sintering of pure, without any addition, tungsten powders of different grain sizes.

Experimental Studies
The purpose of this work is to determine the impact of gas environment on the sintering behaviour of large (up to 1000 microns) particles of tungsten using as an activator of sintering the hydrogen gas environment with high content of water vapour.Tungsten powders, obtained in a closed reactor, were divided into fractions 3-5, 5-100, 40-100, 100-300, 300-500 and 500-1000 µm.Samples with a diameter of 9 mm and a height of 9 mm (fig. 1) were pressed.The powders were previously handled by the softener to improve compressibility.Preparation of basic powders to form (granulation) and pressing were done on common in powder metallurgy technique.Sintering was done in hydrogen environment at a temperature of 1200° c in the classic running system and in a closed reactor [10].During sintering in a closed reactor, after removal plasticizer from the samples, water was introduced to create water steam-hydrogen gas environment.After sintering microstructure of these samples was studied by using raster electronic microscope ZEISS EVO 50XVP and compressive strength was determined on a test machine.Speed of loading of sintered samples in these tests was 1 mm/min.

Results and Discussion
The results of the experiment are presented in fig.From fig. 2, a it can be seen that the most super coarse-grained molded samples with particle sizes of tungsten more than 300 microns after removal of the plasticizer in the flowing reactor did not retain its shape and collapsed.
The using of hydrogen gas environment, saturated steam water in a closed reactor (fig.2, b), allowed to sinter samples even from tungsten particles with sizes up to 1000 microns.The effect is caused by activation of redox process and the formation of gaseous tungsten-contained connections WO 2 (OH) 2 [1] in gas environment during the process of sintering, due to the partial oxidation of the surface of the particles of tungsten with subsequent formation of hydroxyl WO 2 (OH) 2 in reactions: W+3Н 2 О→WO 3 +3Н 2 ; (1) WO 3 +Н 2 О→WO 2 (ОН) 2 . ( Partial pressureof WO 2 (OH) 2 on tungsten particles will be higher than in their contacts.As a result WO 2 (OH) 2 will diffuse into the particle tungsten contacts, creating around them pressure WO 2 (OH) 2 more equilibrium.Excess WO 2 (OH) 2 will be reduced by hydrogen after the reaction: Tungsten, formed after reduction of WO 2 (OH) 2 in reaction (3), will condense in places of contact of source particles of tungsten, fastening them.Above-described processes of oxidation-reduction should go more actively in the samples with small particles of tungsten, if at the same time, as the "inhibitory" process, does not manifest deterioration of permeability during sintering.
From fig. 2, b can see on the samples sintered with the use of a closed space, that the effect of the "inhibition" of access of hydroxyl WO 2 (OH) 2 to the internal volume of the samples does not appear because there has been no deterioration in the sintering process of samples with the decrease of the original tungsten particles from which the samples were formed.On the contrary, the state of the sintered samples with the decrease of the original particles is improving, as evidenced by the fixed linear shrinkage formolded samples of tungsten powders with particle sizes 3-5 µm sintered in the closed reactor space.The shrinkage was 8%.
Also effect of hydrogen gas environment, saturated with water vapor, in the process of sintering of tungsten powdersclearly manifestedin the microstructure of sintered samples (fig.3).So in fig. 3 can see that in case of using a closed reactor space the processes, associated with the effect of migrating tungsten-contained substances via the gas phase, are activated.That contributes to the process of sintering even large particles of tungsten, despite a very small number of specific surface of powder.Although, judging from fig. 3, e, f there is no formation of contacts between the most coarse particles of tungsten, their presence shows the fig. 1, b.From fig 1, b, can see that after sintering of samples in the closed reaction space with hydrogen gas environment, saturated with water vapor, the existence of contacts between particles is indirectly confirmed by retention of the geometrical shape of sintered samples.
Test of sintered samples of tungsten materials in compression showed promising of As can be seen from the data, the tungsten material sintered in closed space reactor at a temperature of 1200° Chas value of compressive strength R cm more than in 2 times more than values of this characteristic in samples from powders W the same fraction, sintered in the flowing reactor.It should also be noted that in our studies we did not apply any activators of sintering, so chemical composition of porous tungsten material conforms to the chemical composition of the original pure tungsten powder.

Conclusions
The use of a closed reactor with hydrogen saturated steam water allows to activate the process of tungsten powders sintering at relatively low temperatures (1000-1200 °C), providing the wide range of pore size in sintered material, depending on the size of the original particles of tungsten powder.
Application of above activation method of tungsten powders sintering will produce new radiation-resistant porous tungsten materials for use in fusion reactors to protect working fusion reactor wall and diverter parts, as well as in the metallurgical, chemical processes.

2 .Fig. 2 .
Particle size of tungsten, used in the experiment, increases from left to right and is listed above.a b Samples sintered in the flowing (and) and closed (b) reactor.
Tab.I The results of the tests of sintered samples in compression.