A study of formation of nanometric W by room temperature mechanosynthesis

Article abstract of DOI:10.1016/S0925-8388(03)00125-7

Nanometric particles of elemental tungsten can be produced by solid state room temperature reduction of tungsten (VI) oxide with magnesium metal. The reaction is driven by high-energy ball milling powder mixtures of WO ₃ and Mg and involves a thermite route that allows for very short reaction times (as small as 8 min) under the proper milling conditions. The evolution of the reaction was followed as a function of time by X-ray diffraction, showing a gradual decrease in crystallite size of the reactant WO₃, with formation of intermediate reaction products, until a near-instantaneous reaction takes place leaving only MgO and elemental W as final products. MgO was removed by leaching in diluted HCl and high purity tungsten (purity >99%) was obtained with a typical molar yield of 84%. The W powders are formed of impact welded aggregates of small particles of homogeneous dimension of about 70-100 nm, the typical crystallite size is 19 nm.

Full text of DOI:10.1016/S0925-8388(03)00125-7

Journal of Alloys and Compounds 358 (2003) 71–75
L
www.elsevier.com/locate/jallcom
A study of formation of nanometric W by room temperature
mechanosynthesis
*
Riccardo Ricceri, Paolo Matteazzi
Department of Chemical Sciences and Technologies, Faculty of Engineering, University of Udine, via Cotonificio 108, I-33100 Udine, Italy
Received 12 November 2002; received in revised form 31 December 2002; accepted 6 January 2003
Abstract
Nanometric particles of elemental tungsten can be produced by solid state room temperature reduction of tungsten (VI) oxide with
magnesium metal. The reaction is driven by high-energy ball milling powder mixtures of WO3 and Mg and involves a thermite route that
allows for very short reaction times (as small as 8 min) under the proper milling conditions. The evolution of the reaction was followed as
a function of time by X-ray diffraction, showing a gradual decrease in crystallite size of the reactant WO3, with formation of intermediate
reaction products, until a near-instantaneous reaction takes place leaving only MgO and elemental W as final products. MgO was removed
by leaching in diluted HCl and high purity tungsten (purity .99%) was obtained with a typical molar yield of 84%. The W powders are
formed of impact welded aggregates of small particles of homogeneous dimension of about 70–100 nm, the typical crystallite size is 19
nm.
2003 Elsevier B.V. All rights reserved.
Keywords: Metals; Nanostructured powders; Mechanosynthesis; Scanning electron microscopy; X-ray diffraction
1. Introduction
its high melting point and low vapor pressure. The best
known use is for lamp filaments. Tungsten is also used as
an electron emitter because it can be used at very high
temperatures; it also improves arc stability and gives
longer life to welding electrodes; metallic tungsten is also
used in many other applications (furnace elements, heat
shields, etc.). Nonmetallurgical uses include brilliant or-
ganic tungsten dyes and pigments that can be used in a
variety of materials. Tungstates are used as phosphors in
fluorescent lights, cathode-ray tubes, and X-ray screens,
while tungsten compounds are used as catalysts in petro-
leum refining.
The most widely used process for producing tungsten is
based on a long route from either ore wolframite (FeWO4)
or from scheelite (CaWO4) [2,3]. Wolframite is dissolved
in acid and the tungsten separated from impurities by lime
precipitation as CaWO4. Scheelite is then dissolved in acid
and precipitated using ammonia to form ammonium tung-
state which is subsequently calcined to form WO3. The
trioxide is thermally reduced with hydrogen to form
metallic tungsten.
Tungsten is used in four main different applications
[1–3]: as tungsten carbide, as an alloy additive, as essen-
tially pure tungsten, and as tungsten chemicals. Tungsten
carbide is probably the most common hard material in use
today for cutting tools and other high strength applications
because of its high hardness at high temperatures. This
application accounts for about 65% of tungsten usage,
mostly in the form of cemented carbides. For carbide
formation the tungsten is blended with carbon and heated
at high temperatures (about 1600 8C) in a hydrogen
atmosphere. Scientific literature reports also the formation
of WC by reaction between W and C during extended ball
milling [4–6]. About 16% of tungsten usage is as an alloy
additive: tungsten is added to steels and forms a dispersed
tungsten carbide phase that imparts a finer-grain structure
and increases the high temperature hardness. The finer-
grain size improves toughness and produces a more
durable cutting edge. Metallic tungsten accounts for 16%
of tungsten consumption; tungsten is often used because of
Mechanosynthesis has been shown to be very effective
for preparation of many metals with high melting points
and ceramic materials [4–11]. In the case of elemental
*
Corresponding author. Fax: 139-0432-558-803.
E-mail address: matteazzi@dstc.uniud.it (P. Matteazzi).
0925-8388/03/$ – see front matter
doi:10.1016/S0925-8388(03)00125-7
2003 Elsevier B.V. All rights reserved.

72
R. Ricceri, P. Matteazzi / Journal of Alloys and Compounds 358 (2003) 71–75
tungsten, another route to form the metal is by reaction of
scheelite with metallic magnesium during extended ball
milling in a tumbling mill for 100 h in an inert atmosphere
[4]: the final products are tungsten, CaO and MgO; the
oxides can be easily removed by leaching in acid, leaving a
fine powder composed of 99% pure tungsten metal.
Another interesting route for producing elemental tung-
sten is based on a mechanochemical synthesis using
magnesium and tungsten trioxide as starting materials [12].
The reaction takes place during a room temperature, high
energy milling process lasting only 60 s, to form metallic
tungsten and magnesium oxide; subsequent leaching by
dilute nitric acid of the unwanted oxide leaves elemental
tungsten. That process is substantially a combustive re-
action that takes place once the ignition temperature is
reached during ball impacts: the conditions reported in
literature allow for no control of the reaction, and an
abrupt increase in temperature occurs; moreover, the final
powders are polydisperse and of irregular shape, because
the reaction takes place almost instantaneously, preventing
homogeneous mixing and pulverization of the reactants, an
important condition in order to get a homogeneous and
finely dispersed powder product. In this work, we show
how the mechanochemical route already reported in litera-
ture could be improved thanks to a very fast process based
on the thermite reaction of Mg with tungsten trioxide with
different milling conditions: the reaction pathway has been
studied in order to be able to control the process in view of
a possible scaling-up for commercial use. A higher ball-to-
powder weight ratio was employed in order to obtain very
fine pulverization and very large contact surfaces of
reagents, with longer (of the order of minutes) reaction
times and with a more gradual release of the heat of
reaction. The final product is thus a more finely dispersed
powder, of regular, rounded shape, and fairly homoge-
neous nanometric (70–100 nm) dimensions, with crys-
tallite size of the order of a few tens of nanometers (typical
crystal size: 19 nm); all the reported properties are of great
importance in view of possible use of these powders in
powder metallurgy technology.
WO3 1 3Mg → W 1 3 MgO
Milling was performed with a ball-to-powder weight ratio
of 24:1. The vial used for milling was sealed in a glove-
box under a pure argon atmosphere; Mg was employed in
excess (about 10% w/w more than stoichiometric amount)
in order to account for surface oxidation of the metal and
to residual oxygen that could be adsorbed onto reactant
surfaces and onto vial walls. Milling was performed at
room temperature and different reaction times were in-
vestigated ranging from a few minutes up to several hours.
Milled products were then leached using 2.0 M HCl
under magnetic stirring for 2 h (solid content of the
dispersion: 1% w/w) in order to remove MgO, Fe released
by the vial and milling balls, and unreacted Mg. The
residual solid was then centrifuged and washed first with
2.0 M HCl and then several times with distilled water in
order to remove traces of acid. Powders were finally dried
at 120 8C in air.
Scanning electron micrographs of the products were
obtained through a JEOL 5600 instrument.
XRD patterns were collected on milled products and
leached products using an INEL diffractometer equipped
with a CPS-120 position-sensitive detector and a ger-
manium monochromator using Co Ka (l50.1789 nm)
radiation. The mean crystallite size was calculated from the
method described elsewhere [9,13] using the full width at
half maximum (FWHM) of the X-ray peaks corrected for
instrumental broadening and taking into account strain
effects.
3. Results and discussion
Milling of tungsten (VI) oxide and magnesium has been
investigated at different reaction times in order to under-
stand the mechanism of the reaction. In Fig. 1 the XRD
diffraction data for milled powders at different milling
times are reported: it is evident that reaction is near-
instantaneous and takes place after only 8 min of milling.
This trend is quite similar to that found for reaction of
boron oxide and magnesium [14–15]. Further milling of
the mixture results in a widening of W(0) diffraction peaks
(see Fig. 2). After a few hours milling, W–Fe compounds
begin to form (Fe is present as a pollutant due to ball and
vial wall consumption during prolonged milling), thus
decreasing the overall final yield of metallic tungsten (Fig.
2). Interesting information on the reaction path can be
obtained by analysis of XRD data at different milling times
before reaction has taken place. The crystal sizes of
elemental Mg and W(VI) oxide against milling time are
reported in Fig. 3. It is clear that the crystal size of Mg is
quite large, between about 40 and 65 nm, in comparison
with W(VI) oxide. The starting powders show a gradual
decrease in crystal size for Mg down to about 40 nm after
4 min milling, then crystal size increases again: this could
The process could thus be extended to tungsten minerals
like scheelite and thus could find potential commercial
applications, having lower costs and requiring relatively
simple equipment in comparison with the industrial pro-
cesses now in use.
2. Materials and methods
Tungsten (VI) oxide WO3 (purity .99%, particles size
about 20 microns) was provided by Aldrich. Commercial
Mg (purity .99.5%, particles size between 350 and 125
microns) was bought from Pometon S.p.A. (Italy). The
synthesis of elemental tungsten was performed in a vibrat-
ory ball mill (Spex 8000 mixer-mill) using carbon steel
balls (diameter: about 8 mm) according to the reaction:

R. Ricceri, P. Matteazzi / Journal of Alloys and Compounds 358 (2003) 71–75
73
Fig. 3. The crystal sizes of elemental Mg (h) and WO3 (s) against
milling time.
Fig. 1. XRD traces of mixture Mg1WO3 at different milling times
(milling conditions: 0.87 g powders, balls-to-powders ratio 24:1, 8 balls)
and powder products after leaching (elemental W). (s) Mg; (1) WO3;
the XRD spectra of milled powders even after only 4 min
milling: this shows that a very small amount of W(VI)
oxide begins to react with Mg as soon as a critical value of
excess surface energy is reached: actually, this could be
due to a statistical distribution of structural defects and to
the extended network of intergranular boundaries (crystal
size) for the powder and could take place even after a very
short milling time, much shorter than the average one, as
shown by experimental data. The free energy of the
reaction between W(VI) oxide and Mg is highly negative,
and the reaction is also strongly exothermic; moreover, the
very high adiabatic temperature for the reaction (about
3710 K), calculated according to the procedure reported
elsewhere [16] accounts for a thermite reaction: thus, once
a critical crystal size for W(VI) oxide is reached, reaction
can be self-propagating, with Mg reducing the mechanical-
ly activated W(VI) oxide (the kinetics of this reaction is
clearly favored by increasing the surface energy of W(VI)
oxide and favoring the intimate contact by reactants during
ball milling). A typical molar yield of 84% for metallic W
has been obtained: the crystal size of W(0) after 8 min
milling has been found to be 19 nm; in this case, the
crystal size after leaching with acid did not show any
significant increase. Thus, no substantial dissolution of
W(0) had taken place (massive metallic tungsten is slowly
attacked by concentrated mineral acids only at high
temperatures). The crystal size for W(0) after prolonged
milling showed a significant decrease after 3 h milling (9
nm) but not after 40 min milling (19 nm, the same result
obtained after 8 min milling). Leaching with acid of
powders after 3 h milling resulted in an increase of crystal
size for elemental W; no substantial changes were found in
the other case (40 min milling). This means that prolonged
milling for 3 h favored the formation of a large fraction of
small crystallites of W(0), as confirmed by the small
average crystal size; these crystallites are more reactive
and more easily leachable even employing a diluted
mineral acid like HCl.
(*) W; (3) MgO; ([) W20O58
.
be due to local melting of Mg metal because of tempera-
ture increase with local reactions between Mg and WO3
after a few minutes milling (melting point for Mg is about
922 K, thus this temperature can be easily reached locally).
On the other side, the crystal size of W(VI) oxide
gradually decreases down to about 14 nm, when the
reaction takes place in a near-instantaneous way, between
the seventh and the eighth minute of milling, when the
amount of excess surface energy is such that reaction can
be self-propagating. After 7 min milling, a diffraction peak
due to an intermediate reaction product is present that can
be attributed to W20O58, thus showing that the reduction of
W(VI) oxide is a very fast but multi-step reaction. More-
over, diffraction peaks of elemental tungsten are present in
Fig. 2. XRD traces of mixture Mg1WO3 (milling conditions: 0.87 g
powders, balls-to-powders ratio 24:1, 8 balls) after completion of the
reaction at different milling times (3 h and 40 min); and powder products
after leaching (purified powders after 3 h milling, and after 40 min
milling). (*) W; (3) MgO; (§) Iron–tungsten compounds.
SEM micrographs (Fig. 4) for elemental W powders

74
R. Ricceri, P. Matteazzi / Journal of Alloys and Compounds 358 (2003) 71–75
Fig. 4. SEM micrographs at different magnifications of elemental tungsten powder after leaching.
after purification (the powders have shown a W content
higher than 99%, as shown by XRD analysis and by
microanalysis; a quantitative determination of the residual
Mg content with a volumetric titration method with EDTA
solution confirmed the other analytical data) clearly show
the presence of a submicrometer size population, with a
very large amount of particles below 200 nm, although
particles tend to aggregate. At higher magnification, pow-
ders look like aggregates of very small particles with
dimensions lower than 100 nm.
volves a technology that requires higher costs. A further
milling of powders after completion of the redox reaction
for producing W can favor the production of finer powders.
On the other side, it also favors formation of unwanted
compounds (e.g. iron–tungsten compounds, as it has been
shown previously) thus decreasing the final yield of
elemental W, with pollution of the product with com-
pounds that in some cases cannot be easily removed with a
simple leaching with acid: for this reason, no investiga-
tions were performed with long milling times.
Submicrometer particles have been obtained thanks to
the very fine pulverization of powders during high energy
ball milling: the presence of MgO as co-product has acted
as a dispersant for the metallic W, avoiding agglomeration
of W (this kind of problem is often met in high energy
milling).
A simple leaching with acid can remove the unwanted
MgO leaving pure W as a finely dispersed powder.
The route that has been described here is very important
because elemental W powder can be obtained on a
submicrometer scale with a one-step process followed by
leaching with acid: the traditional industrial process in-
4. Conclusions
A new method for obtaining metallic tungsten by
reduction of tungsten (VI) oxide with magnesium thanks to
mechanosynthesis has been shown to be accomplished
with high yields and short reaction times (as low as 8 min).
The level of pollution from grinding media, in the limit of
the investigation techniques that have been employed, does
not seem to be significant, and no other reaction products
seem to be present in appreciable amounts, thanks to the

R. Ricceri, P. Matteazzi / Journal of Alloys and Compounds 358 (2003) 71–75
75
[2] E. Lassner, W.D. Schubert, E. Luederitz, H.U. Wolf, in: 5th Edition,
Ullmann’s Encyclopedia of Industrial Chemistry, Vol. A27, WCH
Verlagsgeselleschaft mbH, Weinheim, Germany, 1996.
[3] T.W. Penrice, in: 4th Edition, Kirk-Othmer Encyclopedia of Chemi-
cal Technology, Vol. 24, Wiley, New York, USA, 1992.
[4] N.J. Welham, J. Mater. Res. 14 (1999) 619.
very large contact surface between reactants and the self-
propagating nature of the reaction that allows for very
short reaction time. This new process for producing
elemental tungsten could thus be very interesting in view
of industrial applications. In fact, the final product is a
nanostructured powder (crystallite size less than 20 nm)
formed by nanometric particles with regular rounded shape
that are relatively monodispersed in the 70–100 nm range.
¨
[5] P. Matteazzi, G. Le Caer, J. Am. Ceram. Soc. 74 (1991) 1382.
[6] G.M. Wang, S.J. Campbell, A. Calka, W.A. Kaczmarek, J. Mater.
Sci. 32 (1997) 1461.
[7] A. Kerr, N.J. Welham, P.E. Willis, Nanostructured Materials 11
(1999) 233.
[8] N.J. Welham, J. Mater. Sci. 34 (1999) 21.
¨
[9] P. Matteazzi, G. Le Caer, J. Am. Ceram. Soc. 75 (1992) 2749.
Acknowledgements
¨
[10] P. Matteazzi, G. Le Caer, Mater. Sci. Eng. A156 (1992) 229.
¨
[11] P. Matteazzi, G. Le Caer, Mater. Sci. Eng. A149 (1991) 135.
Financial support from M.U.R.S.T. and C.S.G.I. is
gratefully acknowledged.
[12] D.K. Mukhopadhyay, K.A. Prisbrey, C. Suryanarayana, F.H. Froes,
in: Proceedings of the Third International Conference on Tungsten
and Refractory Metals, McLean, VA, USA, 1996, p. 239.
[13] L. Lanotte, P. Matteazzi, V. Tagliaferri, Mater. Sci. Technol. 6
(1990) 146.
[14] R. Ricceri, P. Matteazzi, Int. J. Powder Met., in press.
[15] V. Sundaram, K.V. Logan, R.F. Speyer, J. Mater. Res. 112 (1997)
2657.
References
[1] Properties and Selection: Stainless Steels, Tool Materials and
Special-Purpose Metals, 9th Edition, Metals Handbook, Vol. 3,
American Society for Metals, Metals Park, Ohio, USA, 1980.
`
[16] P. Matteazzi, M. Alcala, Mater. Sci. Eng. A230 (1997) 161.