Synthesis of W-Cu-Ag Nanopowders Produced by A Co-Precipitation Process

Abstract

A homogeneous precipitation process was employed to prepare nanosized W-10%wtCu-10%wtAg powders using ammonium meta tungstate, copper nitrate and silver nitrate as precursors. The initial precipitates were obtained by reacting ammonium meta tungstate, copper nitrate and silver nitrate solutions under certain PH and temperature. In order to synthesis W-Cu-Ag composite powders, the initial precipitates washed, dried, and then calcined in air in order to prepare CuWO 4-x , Ag 2 W 4 O 13 and WO 3 oxide powders for the next step reduction. The reduction was carried out in a hydrogen atmosphere to form the final W-Cu-Ag nanocomposite powders. The powders were characterized by X-ray diffraction (XRD) technique. The morphologies of the powders were observed by scanning electron microscopy (SEM).

Full text

Synthesis of W- Cu- Ag Nanopowders Produced by
A Co-Precipitation Process
Golnaz Taghavi
1,a
, Hamid Reza Rezaie
1,b
, Hekmat Razavizadeh
1,c
1
Department of Metallurgical and Materials Engineering, Iran University of Science and Technology
(IUST), Tehran, Iran
a
golnaz_tpa@yahoo.com,
b
hrezaie@iust.ac.ir,
c
hrazavizadeh@iust.ac.ir
Keywords: Nano Materials, W-Cu-Ag Nanopowders, Co-Precipitation
Abstract. A homogeneous precipitation process was employed to prepare nanosized W-10%wtCu-
10%wtAg powders using ammonium meta tungstate, copper nitrate and silver nitrate as precursors.
The initial precipitates were obtained by reacting ammonium meta tungstate, copper nitrate and
silver nitrate solutions under certain PH and temperature. In order to synthesis W-Cu-Ag composite
powders, the initial precipitates washed, dried, and then calcined in air in order to prepare CuWO
4-x
,
Ag
2
W
4
O
13
and WO
3
oxide powders for the next step reduction. The reduction was carried out in a
hydrogen atmosphere to form the final W-Cu-Ag nanocomposite powders. The powders were
characterized by X-ray diffraction (XRD) technique. The morphologies of the powders were
observed by scanning electron microscopy (SEM).
Introduction
Tungsten-Copper composites have been widely used for heavy-duty electrical contacts, resistance
welding electrodes, thermal management devices such as heat sinks and spreaders, and microwave
materials. Use of these composites in the mentioned applications is based on a combinition of high
hardness, hot strength, wear resistance, high arc erosion and low thermal expansion coefficient of
tungsten with high electrical and thermal conductivity of copper [1-4]. Particularly, a W-Cu alloy
containing 5-20wt% Cu is used as a heat sink material due to suitable thermal conductivity and
thermal expansion coefficient of W-Cu alloy that is similar to that of ceramics. Moreover, the W-Cu
alloy containing 20-40wt% Cu is used in industry as electric contact materials and as high-density
shape charge liner materials for ammunition [5].
The conventional method for fabrication of W-Cu composites is infiltration of copper to tungsten
preforms. Since copper and tungsten are mutually insoluble, alloying does not happen in the
conditions (i.e. pressures and temperatures) usually employed in the infiltration process. Following
infiltration, parts are mechanically machined to the final dimensions. However, the infiltration
process does not result in a homogeneous microstructure and it is not a net shape process and also it
is a time consuming process. Hence, it is caused to high production costs [6]. Furthermore, without
adding small amounts of other elements such as Ni,Co and Fe, full-densification of these materials
by liquid phase sintering is difficult, that is due to mutual insolubility and low wetability between
W and Cu (high contact angle of liquid copper on tungsten). In addition, these sintering aids have a
negative influence on the electrical and thermal conductivity of W-Cu composites [7-9]. On the
other hand, it is usually noted that the ultra-fine size
and the well-mixed state of component
powders improve the sinterability of powder compact, especially in a liquid phase sintering system
such as W-Cu system in which the influential sintering mechanism is particle rearrangement [10].
Recently, use of nanocomposite powders with very fine dispersion of W and Cu has solved the
problem of low sinterability without any need to adding additives. There are several methods for
synthesis of these powders such as mechanothermochemial [11], mechanical alloying [12],
mechanochemical [13]. However, these techniques, especially mechanical alloying and
Defect and Diffusion Forum Vols. 312-315 (2011) pp 312-318
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mechanochemical processes because of long time ball milling have the problem of contamination
which decreases conductivity after sintering of powders compacts [14,15].
Newly, a new process
was applied to synthesis W-Cu composite powders with nanoscale dispersion of the constituents
(i.e. tungsten and copper) and high sinterability at relatively low sintering temperatures. The process
included initial precipitation of W-Cu compounds from the solution containing the corresponding
W and Cu ions, calcination the precipitates and reduction the calcined compounds [16].
This paper aims to survey the effect of silver on W-Cu composites. A co-precipitation process was
applied to produce W-Cu-Ag nanocomposite powders from ammonium meta tugstate-AMT, copper
nitrate and silver nitrate as precursors. This present work showed that this process results in
nanosized W-Cu-Ag powders with high dispersion of tungsten,copper and silver phases which will
be likely benefitical to the achievement of high dense W-Cu-Ag composites with good physical and
mechanical properties.
Experimental
Synthesis Procedure
White colored ammonium meta tugstate-AMT (H
26
N
6
O
40
W
12
.aq, > 99.0% purity), blue colored
copper nitrate (Cu(NO
3
)
2
.3H
2
O, > 99.o% purity) and white colored silver nitrate (AgNO
3
, > 99.0%
purity) were used as raw materials. In order to synthesis W-10%wtCu-10%wtAg composites,
AMT,copper nitrate and silver nitrate solutions with desired weight proportion were made
separately in distilled water. Subsequently, for adjusting PH, aqueos ammonia (NH
3
.H
2
O, > 99%,
Merck, Germany) was added to the copper nitrate solution. Then this mixture and also the silver
nitrate solution were added to the AMT solution. By stirring and heating the solution up to 75-85°C,
ammonia evaporated and light green precipitates formed. The process was continued for 4 h. In the
end of 4 hours, the solution became compeletely colorless. In this situation, PH was in the range of
5-6. Then the obtained precipitates washed and dried.
In next step, the precipitates calcined at 600°C for 2 h in an electrical muffle furnace with a heating
rate of 10
o
C/min. The calcined precipitates reduced at 850°C for 2 h in hydrogen atmosphere (dew
point -76°C) in a tube type electrical furnace with a heating rate of 10
o
C/min.
The procedure for preparing W-Cu-Ag nanopowders is shown in Fig.1.
Characterization of the Synthesized Powders
Initial, calcined and reduced powders were all analyzed by X-ray diffraction (Jeol, JDX-8030) in
order to detect present phases. The microstructure, morphology and also distribution of the powders
were studied by scanning electron microscope (SEM, Cambridge S360).
Defect and Diffusion Forum Vols. 312-315 313

Fig. 1. Flow chart of preparing W-Cu-Ag metallic powders
Results and Discussion
Powder Characterization
As precursors, ammonium meta tugstate-AMT, copper nitrate and silver nitrate were used. AMT is
a material with amorphous structure and good water soluble tungsten chemicals (at 25
o
C, the
solubility of it is 30 g per 100 g water) [17]. The reaction between aqueous solutions of
H
26
N
6
O
40
W
12
, Cu(NO
3
)
2
.3H
2
O and AgNO
3
in the presence of aqueous NH
3
resulted to form the
light green precipitates. Addition of the ammonia to the copper nitrate solution caused the formation
of complex Cu(NH
3
)
42+
ions in the solution [18]. Subsequent addition of AMT and silver nitrate
solutions to the copper containing solution initiated the precipitation, but the existence of ammonia
restrained the precipitation and a complex compound solution resulted. However, when the complex
solution was heated to moderate temperatures, the ammonia evaporated. This resulted in the
seperation of the precipitates out of the solution. The XRD pattern of initial precipitates is shown in
Fig.2. According to this pattern, the precipitates consisted of chemical compounds which could not
be accurately identified by XRD. Based on the applied chemical process, these compounds are
likely composed of W, Cu, Ag, NH
4
and H
2
O.
314 Diffusion in Solids and Liquids VI

Fig. 2. XRD pattern of initial precipitates after drying process
Figure 3 and Fig. 4 show XRD patterns of the calcined precipitates and the reduced powders,
respectively.
It is shown that the primary precipitates were converted into a mixture of tungsten oxide (WO
3
),
copper tungsten oxide (CuWO
4-x
) and silver tungsten oxid (Ag
2
W
4
O
13
) completely after calcination
at 600°C for 2 h (Fig. 3). Calcination of the initial precipitates occured due to evaporation of the
volatile components such as H
2
O and NH
3
and formation of oxide phases [17].
After 2 hours reduction process under hydrogen atmosphere at 850°C, the oxide mixture was
changed into metallic tungsten, copper and silver (W-Cu-Ag) composite powders (Fig. 4). There is
not any detailed information about the process of Ag
2
W
4
O
13
reduction, nevertheless, some
investigations have been done into the procedure for reduction of CuWO
4-x
and WO
3
oxide phases.
Non-isothermal reduction of CuWO
4-x
follows three steps below [15,19]:
1. CuWO
4-x
→ Cu + WO
3-x
. (1)
2. WO
3-x
→ WO
2
. (2)
3. WO
2
→ W. (3)
The presence of copper on the one hand helps (WO
2
→ W) reduction to start at lowered
temperatures, but on the other hand the activation energy of this reaction goes up by copper, since
copper acts as a barrier for formation and transportation of volatile WO
2
(OH)
2
compond which is
the main agent responsible for tungsten oxide reduction [20].
Generally, copper oxide phases can be reduced by hydrogen at relatively low temperatures [21].
Although, reduction of tungsten oxides, especially WO
3
needs high temperatures and is more
complicated, since the intermediate phases are formed throughout the reduction process [2,22,23]:
WO
3
→ WO
3-x
→ WO
2
→ W. On the basis of morphological investigations [2], reduction of
tungsten oxides can be hypothesized by oxygen and tungsten transport mechanisms. In the case of
oxygen transport mechanism, the direct oxygen removal from the solid oxides is responsible for the
reduction process, however, this mechanism is characterized for low reduction temperatures (below
1023 K) and is particularly for reduction of WO
3
to WO
2.9
. The other mechanism which is the main
one for the reduction of tungsten oxides is based on chemical vapor transport of tungsten by the
volatile WO
2
(OH)
2
[2,24,25]. Also, it was noted that if exess of WO
3
was present in the starting
oxide mixture, then metallic powders with lower copper contents could be obtained after the high
temperature reduction step [26].
Defect and Diffusion Forum Vols. 312-315 315

Fig. 3. XRD pattern of the precipitates calcined at 600°C for 2 h
Fig. 4. XRD pattern of the reduced powders in hydrogen at 850
°
C for 2 h
Investigation of morphology
Figure 5 indicates the SEM micrograph of the initial precipitates of W-10wt% Cu-10wt% Ag
nanocomposite powders. As Fig. 5 shows, the initial precipitates have been partially agglomerated.
Fig. 5. Typical SEM micrograph of the initial precipitates
316 Diffusion in Solids and Liquids VI

A SEM micrograph of the calcined powders is shown in Fig. 6. As seen in this figure, due to
calcination treatment and evaporation of the volatile compounds, the calcined powders have a
different morphology compared to initial precipitates. Also, according to this figure, the calcined
powders have been agglomerated. Also calcination at high temperatures may result to make
aggregation.
Fig. 6. Typical SEM micrograph of the calcined powders
SEM micrograph of the reduced powders is presented in Fig.7. This figure shows that the particles
are approximately worm shape and have submicrone size.
Fig.7. Typical SEM micrograph of the reduced powders
Conclusion
W-10%wtCu-10%wtAg composite powders synthesized via a precipitation method using
ammonium meta tungstate, copper nitrate and silver nitrate as raw materials. The process included
precipitation of W-Cu-Ag compounds, calcination of the initial precipitates in air, and reduction of
the calcined powders in hydrogen atmosphere. The calcined precipitates consisted of CuWO
4-x
,
WO
3
and Ag
2
W
4
O
13
phases and the reduced powders consisted of W, Cu and Ag phases.
Defect and Diffusion Forum Vols. 312-315 317

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Diffusion in Solids and Liquids VI
10.4028/www.scientific.net/DDF.312-315
Synthesis of W- Cu- Ag Nanopowders Produced by a Co-Precipitation Process
10.4028/www.scientific.net/DDF.312-315.312
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