Size control of tungsten powder synthesized by self-propagating high temperature synthesis process

Article abstract of DOI:10.1016/S0025-5408(00)00171-9

Tungsten powder was prepared by self-propagating high temperature synthesis (SHS) from a mixture of WO₃ and Mg. The MgO in the product was leached with an HCl solution. The complete reduction of WO₃ required a 33% excess of magnesium over the stoichiometric molar ratio Mg/WO₃ of 3. The tungsten product had a purity of 99.98%, which was higher than that of the reactants. The high purity resulted because the impurities in the reactants were volatilized during the highly exothermic reaction and dissolved during leaching of the product. Size distribution and the shape of the tungsten particles produced was affected by compaction pressure on the green pellet.

Full text of DOI:10.1016/S0025-5408(00)00171-9

Materials Research Bulletin, Vol. 34, Nos. 14/15, pp. 2239–2245, 1999
Copyright © 2000 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/99/$–see front matter
PII S0025-5408(00)00171-9
SIZE CONTROL OF TUNGSTEN POWDER SYNTHESIZED BY
SELF-PROPAGATING HIGH TEMPERATURE SYNTHESIS PROCESS
Chang-Whan Won*, Joong-Chai Jung, Seog-Gueon Ko, and Jong-Hyeon Lee
Rapidly Solidified Materials Research Center, Chungnam National University,
Taejon City, 305-764, Korea
(Refereed)
(Received October 1, 1998; Accepted March 8, 1999)
ABSTRACT
Tungsten powder was prepared by self-propagating high temperature synthe-
sis (SHS) from a mixture of WO3 and Mg. The MgO in the product was
leached with an HCl solution. The complete reduction of WO3 required a 33%
excess of magnesium over the stoichiometric molar ratio Mg/WO3 of 3. The
tungsten product had a purity of 99.98%, which was higher than that of the
reactants. The high purity resulted because the impurities in the reactants were
volatilized during the highly exothermic reaction and dissolved during leach-
ing of the product. Size distribution and the shape of the tungsten particles
produced was affected by compaction pressure on the green pellet. © 2000
Elsevier Science Ltd
KEYWORDS: A. metals, A. oxides, B. chemical synthesis, C. X-ray diffrac-
tion, D. thermodynamic properties
INTRODUCTION
Tungsten has the useful characteristics of high melting point (3410°C), high-temperature
strength, and electrical conductivity. There are many applications in the fields of illumina-
tion, electronics, electrical contact, and heat-resistant structure using its good properties. It is
also the main raw material for the ultra-hard alloys used in cutting tools, anti-friction tools,
and high-speed steels [1]. Currently, tungsten is produced by the reduction of WO3 at 1000
to 1100°C with carbon, H2, or calcium, or by the hydrogen reduction of WF6 at high
*To whom correspondence should be addressed.
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temperature [1]. Many pure substances have been prepared by self-propagating high tem-
perature synthesis (SHS) [2–4]. This process can be used to prepare a fine powder of
high-temperature materials at 2000 to 4000°C, using their high exothermic heats of reaction.
High-purity W powder has been produced by SHS, using Mg powder as a reducing agent [5].
This process is relatively simple and rapid without requiring a large external heat supply
[6,7]. However, it is necessary to control the particle size in order to commercialize the
product by the SHS process. The SHS reaction is very difficult to control because the
combustion rate is too fast and the reaction mechanism is not yet completely understood.
In this study, as a preliminary work to commercializing W powder by SHS, the effects of
the cooling rate, compaction pressure, and molar ratio of reducing agent to WO3 on the
microstructure of W powder were investigated.
EXPERIMENTAL
Predetermined amounts of the reactants were mixed in an alumina ball mill and pressed into
pellets of 30 mm diameter and about 30–40 mm height under various compaction pressures.
The pellets were placed in a SHS reactor under argon and ignited by a tungsten wire
connected to a power supply. Temperature profiles were measured by C-type thermocouples
(W–5%Re vs. W–26%Re, 0.5␾) connected to a data logger (DASTC). The product was very
porous, and a substantial amount of magnesium and zinc oxide dust was generated. The
product mixture, which was easily broken in a mortar, was leached with 10 to 6.5 M HCl
solution at 80°C for 40 min to remove MgO and ZnO, and then dried in a drying oven at 70°C
for 24 h. The powder thus produced was analyzed by X-ray diffraction (XRD) to determine
its crystal structure and by scanning electron microscopy (SEM) to determine its microstruc-
ture. Its chemical composition was determined by inductively coupled plasma spectrometry
(ICP).
FIG. 1
Temperature profiles of combustion reaction (Ar atmosphere).

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TUNGSTEN POWDER SYNTHESES
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FIG. 2
X-ray diffraction patterns of reaction products with different molar ratios (before leaching).
FIG. 3
Effect of leaching with 6.5 M HCl solution at 80°C for 40 min (WO3:2.4Zn:0.9Mg system).

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FIG. 4
SEM photomicrographs of combustion-synthesized W with different molar ratios of Mg/
WO3 (in Ar atmosphere, at 480 MPa): (a) 3.0, (b) 3.5, and (c) 4.0.
RESULTS AND DISCUSSION
The reactions involved in the experiments are as follows:
WO3 ϩ 3Mg 3 W ϩ 3MgO
(1)
(2)
WO3 ϩ xZn ϩ xMg 3 W ϩ xZnO ϩ xMgO
Zn powder was added to control the combustion temperature and rate. The combustion
reactions of eqs. 1 and 2 are governed by the molar ratio of Mg/Zn. Hence, experiments to
study the effects of the stoichiometric molar ratio of MgZn/WO3 and compaction pressure on
particle size were carried out. Figure 1 shows the combustion temperature of products with
different Mg concentrations. As the molar ratio of Mg/WO3 increased, the combustion
temperature was increased. The cooling rate of the specimen with 4.0 mole of Mg was slower
than that of the other samples. The maximum combustion temperature, 1227°C, was recorded

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TUNGSTEN POWDER SYNTHESES
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FIG. 5
X-ray diffraction patterns of the reaction products with various concentrations of Mg in the
WO3:2.1Zn:xMg system.
for the sample with 4.0 mole of Mg. However, there was a large deviation in the measured
combustion temperature from a theoretical adiabatic temperature, approximately 14,000°C.
This may be because the reaction was explosive, so the instantaneous reaction could not be
measured exactly.
Figure 2 shows the XRD patterns of products with different Mg concentrations before
leaching. W and a small quantity of MgO were dominant phases. As the Mg concentration
increased, the amount of unreacted WO3 decreased. All WO3 was eliminated at 4.0 mole of
Mg. In the case of a stoichiometric molar ratio of Mg/WO3 (i.e., 3), some WO3 remained
unreduced because of incomplete contact and Mg loss during the rapid SHS reaction, which
generates a high temperature. The concurrently formed MgO in the product was easily
removed by leaching with 100 ml of 6.4 M HCl solution at 80°C for 40 min (Fig. 3).
Figure 4 shows SEM photographs of combustion-synthesized W with different Mg con-
centrations after leaching. The W particles consist of irregular and spherical types from under

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FIG. 6
SEM photomicrographs of combustion-synthesized W under different compaction pressures
for WO3:3.5Mg (spherical 70 mesh, Ar atmosphere): (a) 160, (b) 400, and (c) 640 MPa.
1 to 2 ␮m at the molar ratio Mg/WO3 of 3.0, but at a higher molar ratio, 3.5–4.0, the tungsten
particles produced had a spherical shape with a narrow size distribution.
Figure 5 shows XRD patterns of the products with various concentrations of Mg in the
WO3:2.1Zn:xMg system (x ϭ 0.9, 1.2 and 1.5) before leaching. Complete reductions of WO3
were achieved with Mg concentrations greater than 1.2; whereas unreacted WO3 was
detected with Mg concentration of 0.9, due to the relatively high free-energy of ZnO. It is
believed that the Zn and Mg powders were lost during the explosive combustion reaction. As
in the case of various Mg concentrations in the WO3:2.1Zn:xMg system, similar results were
obtained in the WO3:xMg system. The W powder produced had irregular and spherical
shaped particles with a mean particle size of 1–2 ␮m.
As shown in Figure 6, compaction pressure affected particle size. As the compaction
pressure increased from 160 to 640 MPa, the W particle size increased from 1 to 5 ␮m. This
may mean that the higher compaction pressure caused higher density of the green pellets and
larger contact area among the reactants. In addition, better contact of the reactants can cause
higher reactivity, which increases combustion temperature. Thus, we believe that the particle
growth of the products under higher compaction pressure resulted from the higher combus-
tion temperature.
The chemical composition of the W powder produced, as analyzed by ICP, is as follows
(in wt%): 99.98 W, 0.0007 K, 0.003 Fe, 0.006 Na, 0.005 Mg, and 0.006 Al. Ca was not

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detected. The overall purity of 99.98% is greater than that of the starting materials. This is
due to the vaporization of impurities at the high combustion temperature, which results from
the high exothermic reaction as well as their dissolution in the leaching acid solution.
CONCLUSIONS
For the complete reduction of WO3, an excess addition of Mg was necessary in the WO3:xMg
and WO3:2.1Zn:xMg systems. The concurrently formed MgO and ZnO were easily elimi-
nated by leaching the products in 100 ml 6.5 M HCl solution at 80°C for 40 min. Increased
compaction pressure caused better contact among reactants and maintained the reaction heat
in the samples for longer times and at higher temperatures. This made the products rounder
and larger. It was possible to control the W particle size from 1 to 5 ␮m by varying the
compaction pressure. The W powder produced had a higher purity than the reactant, because
of the vaporization of impurities during SHS reaction and the dissolution by the leaching
solution.
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