M. ARDESTANI: CHEMICAL SYNTHESIS AND DENSIFICATION BEHAVIOR OF Ag/ZnO METAL-MATRIX COMPOSITES
281–286
CHEMICAL SYNTHESIS AND DENSIFICATION BEHAVIOR OF
Ag/ZnO METAL-MATRIX COMPOSITES
OBNA[ANJE Ag/ZnO KOMPOZITA S KOVINSKO OSNOVO PRI
KEMIJSKI SINTEZI IN ZGO[^EVANJU
Mohammad Ardestani
Islamic Azad University, Science and Research Branch, Department of Materials Engineering, Tehran, Iran
ardestani80@gmail.com
Prejem rokopisa – received: 2014-09-24; sprejem za objavo – accepted for publication: 2015-04-07
doi:10.17222/mit.2014.243
In this research, the chemical synthesis and densification behavior of Ag/ZnO metal-matrix composites were investigated. The
initial precipitates were obtained by adding ammonium carbonate solution to silver and zinc nitrate solutions. The precipitates
consisted of silver and zinc carbonates. The thermal behavior of the precipitates was studied using thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC). The TGA curve showed three distinguishable mass-loss stages, related to
the evaporation of physically adsorbed water and also the decomposition of silver and zinc carbonates into silver and zinc
oxides, respectively. The mass-loss stages were associated with three endothermic reactions. According to the thermal-analysis
results, the precipitates were calcined at 600 °C. The compressibility behavior of the synthesized Ag/ZnO powders was
evaluated using the Heckel, Panelli and Ge models. The compaction data of the powders was best fitted to the Heckel equation.
The synthesized powders were sintered at 930 °C. The results showed that by increasing the cold-compaction magnitude prior to
sintering, the sinterability of the powders was enhanced. The microstructural evaluation of the synthesized composites carried
out with a scanning electron microscope (SEM) confirmed a fine and homogenous dispersion of ZnO within the silver matrix.
Keywords: Ag/ZnO composites, densification, thermal analysis, sintering
V raziskavi je bilo preiskovano obna{anje Ag/ZnO kompozita na kovinski osnovi pri kemijski sintezi in zgo{~evanju. Za~etni
izlo~ki so bili dobljeni z dodatkom raztopine srebrovega in cinkovega karbonata v raztopino amonijevega nitrata. Izlo~ki so bili
sestavljeni iz srebrovega in cinkovega karbonata. Toplotno obna{anje izlo~kov je bilo posneto s termogravimetri~no analizo
(TGA) in z diferen~no vrsti~no kalorimetrijo (DSC). TGA krivulja je pokazala stopnje ob~utne izgube mase, ki so bile povezane
z uplinjanjem fizi~no adsorbirane vode in tudi z razgradnjo srebrovega in cinkovega karbonata v srebro in v cinkov oksid.
Stopnje izgube mase so bile povezane s tremi endotermnimi reakcijami. Na podlagi rezultatov termoanalize so bili izlo~ki
kalcinirani pri 600 °C. Stisljivost sintetiziranega Ag/ZnO prahu je bila dolo~ena z uporabo Heckel, Panelli in Ge modelov.
Podatki za kompaktiranje prahov so se najbolje ujemali s Heckel ena~bo. Sintetizirani prahovi so bili sintrani pri 930 °C. Rezul-
tati so pokazali, da pove~evanje obsega zgo{~evanja pred sintranjem pospe{uje sposobnost prahu za sintranje. Ocena mikro-
strukture sintetiziranega kompozita z vrsti~no elektronsko mikroskopijo (SEM) je potrdila drobno in homogeno razporeditev
ZnO v osnovi iz srebra.
Klju~ne besede: Ag/ZnO kompoziti, zgo{~evanje, termi~na analiza, sintranje
1 INTRODUCTION
Silver-matrix particle-reinforced composites are used
in several types of electrical contacts with different
applications such as starting switches, oil- and air-circuit
breakers, relays and thermostat controls. The properties
of the contact depend on the characteristics and volume
fraction of the reinforcement(s). Various non-refractory
and refractory metals and hard materials like iron, tungs-
ten, molybdenum, tungsten carbide and zinc oxide are
used as the reinforcement for this group of metal-matrix
composites. Silver-matrix composites are made with the
melt-cast method or manufactured with powder-metall-
urgy (P/M) processes.1 The conventional P/M method,
applied as the manufacturing process includes cold com-
paction and sintering of silver and reinforcing consti-
tuent powder blends.1,2
The magnitude of the cold-compaction pressure and
sintering temperature significantly affects the final sint-
ering density and other physical properties of the powder
compacts. Recently, it has been shown that a fine and
homogenous dispersion of the reinforcement within the
matrix phase leads to the desired electrical advantages
like the arc stability and also a reduction of the arc-ero-
sion rate of a contact.3 Different methods like mechani-
cal milling4,5, mechanochemical6 and chemical precipita-
tion7 were applied to synthesize the particle-reinforced
silver-matrix composites with a fine dispersion of the
reinforcing phase.
In this research, a synthesis of Ag/ZnO powders via
the chemical-precipitation process was investigated.
Also, the effect of the cold-compaction pressure magni-
tude prior to the sintering on the solid-state sinterability
of the synthesized powders was studied. Furthermore,
the compressibility behavior of the synthesized powders
was evaluated using the Heckel, Panelli-Ambrosio and
Ge models:8,9
ln
1
1−
⎛
⎝
⎜ ⎞
⎠
⎟
D
= KP + B1 (Heckel equation) (1)
Materiali in tehnologije / Materials and technology 50 (2016) 2, 281–286 281
UDK 620.168:621.762.5:669.018.95 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(2)281(2016)
ln
1
1−
⎛
⎝
⎜ ⎞
⎠
⎟ =
D
KP + B2 (Panelli-Ambrosio equation) (2)
lg ln
1
1−
⎛
⎝
⎜ ⎞
⎠
⎟⎡
⎣⎢
⎤
⎦⎥D
= K ln P + B3 (Ge equation) (3)
where D is the relative density, P is the cold pressure
and B1, B2 and B3 are the constants.
2 EXPERIMENTAL WORK
Silver and zinc nitrate were used as the precursors. In
order to synthesize the 8 % of mass fractions of Ag, ZnO
powders, silver and zinc nitrate solutions were made in
distilled water separately. A 0.5 molar solution of ammo-
nium carbonate was added to the prepared solutions drop
by drop. Subsequently, the prepared solutions containing
the precipitates were mixed and the prepared mixture
was stirred vigorously for 3 h at 50 °C. In order to
determine the appropriate calcination temperature of the
obtained precipitates, the thermal behavior of the
precipitates was investigated using simultaneous thermal
analysis (STA) up to 800 °C with a heating rate of 10
°C/min in air atmosphere. On the basis of the STA result,
the precipitates were calcined at 600 °C for 2 h. The
calcined powders were characterized with the X-ray
diffraction (XRD) technique. The calcined powders were
cold compacted with (100, 200 and 300) MPa in a 10
mm diameter cylindrical die and sintered at 930 °C for 2
h in air atmosphere. The microstructures of the powders
and the sintered composites were evaluated with
scanning electron microscopy (SEM).
3 RESULTS AND DISCUSSION
Figure 1 shows the microstructure of the initial
precipitates. According to this figure, the particles have
cylindrical and polygonal shapes. One of the advantages
of the applied precipitation process was the use of
chemical precursors that did not contain sodium. This
agent was used due to the agglomeration of the synthe-
sized precipitates. However, in order to decrease the
agglomeration degree of the particles, the synthesized
compounds should be washed for several times, which is
a time consuming process. The XRD pattern of the
precipitates is shown in Figure 2. According to the XRD
pattern, the precipitates consisted of silver and zinc
carbonates.
The TGA, DTG and DSC curves of the initial pre-
cipitates are shown in Figures 3a and 3b. According to
the TG and DTG curves, the mass loss of the precipitates
occurred up to 600 °C and above this temperature no
detectable mass change is seen in the TGA curve. This
M. ARDESTANI: CHEMICAL SYNTHESIS AND DENSIFICATION BEHAVIOR OF Ag/ZnO METAL-MATRIX COMPOSITES
282 Materiali in tehnologije / Materials and technology 50 (2016) 2, 281–286
Figure 1: SEM image of the initial precipitates
Slika 1: SEM-posnetek za~etnih izlo~kov
Figure 3: a) TGA and DTG curves of the initial precipitates, b) DSC
curve of the initial precipitates
Slika 3: a) TGA- in DTG-krivulji za~etnih izlo~kov, b) DSC-krivulja
za~etnih izlo~kov
Figure 2: XRD pattern of the initial precipitates
Slika 2: Rentgenogram za~etnih izlo~kov
observation shows that the volatile constituents, such as
carbon dioxide, evaporated completely due to the heating
of the initial precipitates at 600 °C. Also, according to
the DSC curve, the decomposition of the precipitates was
due to three endothermic reactions at (197, 226 and 346)
°C. So, the appropriate calcination temperature of the
initial precipitates was 600 °C.
The XRD pattern of the calcined precipitates is
shown in Figure 4. This pattern confirms the formation
of Ag and ZnO during the calcination that was due to the
decomposition of silver and zinc carbonates, respec-
tively. The SEM image of the calcined powders is shown
in Figure 5. As it is observed, the particles were agglo-
merated due to calcination at 600 °C. Furthermore, it
seems that the heat treatment of the initial precipitates
and also the evaporation of the volatile compounds led to
considerable microstructural changes of the precipitates
during the calcination.
The result of the XRD analysis is in good agreement
with the theoretical weight-loss measurements of the
precipitates during the calcination that can be calculated
using the thermogravimetric curve. According to the
TGA curve, the total weight loss during the calcination is
24 %. The theoretical weight loss of the sample can be
calculated considering the XRD analysis of the initial
and calcined precipitates as shown below:
Ag2CO3, ZnCO3  Ag, ZnO
% total weight loss =
2
3
m m
m m
Ag ZnO
Ag CO ZnCO2 3
+
+
	74 % (4)
100 – 74 = 26 % (theoretical weight loss)
where m is the molar weight of the constituent.
The difference between the theoretical calculations
and experimental data (2 %) may be due to the evapo-
ration of the physically adsorbed water during the first
weight-loss stage of the sample. According to the TGA
curve, the weight loss of the precipitates during the first
stage is 1.5 %, which is too close to the difference value.
The second stage of the weight loss in the TGA curve
corresponds to the decomposition of zinc carbonate to
zinc oxide. The corresponding weight loss of the sample
during the second stage is about 10.5 %, which is in
good agreement with the theoretical calculation:
Ag2CO3, ZnCO3  Ag2CO3, ZnO
% weight loss of the second stage =
=
2
3
m m
m m
Ag CO ZnO
Ag CO ZnCO
2 3
2 3
+
+
	12 % (5)
The third weight-loss stage in the TGA curve corres-
ponds to the decomposition of silver carbonate to silver.
The cold-compressibility behavior of the synthesized
powders was investigated using the Heckel, Panelli-Am-
brosio and Ge models. Three plots were drawn: a plot
was drawn with ln(1/1–D) along the y-axis and P along
the x-axis (the Panelli-Ambrosio model – Figure 6); a plot
was drawn with ln(1/1–D) along the y-axis and P along
the x-axis (the Heckel model – Figure 7); and a plot was
drawn with lg(ln(1/1–D)) along the y-axis and lg P along
the x-axis (the Ge model – Figure 8). Based on the R2
values, it can be declared that the compaction data of the
M. ARDESTANI: CHEMICAL SYNTHESIS AND DENSIFICATION BEHAVIOR OF Ag/ZnO METAL-MATRIX COMPOSITES
Materiali in tehnologije / Materials and technology 50 (2016) 2, 281–286 283
Figure 6: ln(1/1–D) versus the square root of uniaxial-pressure com-
paction magnitude P (the Panelli-Ambrosio equation)
Slika 6: Odvisnost ln (1/1–D) od kvadratnega korena nihanja tlaka pri
enoosnem stiskanju P (ena~ba Panelli-Ambrosio)
Figure 4: XRD pattern of the calcined precipitates
Slika 4: Rentgenogram kalciniranih izlo~kov
Figure 5: SEM image of the calcined powder. The powder particles
are agglomerated.
Slika 5: SEM-posnetek kalciniranega prahu. Delci prahu so aglome-
rirani.
synthesized powders was best fitted to the Heckel
equation.
As mentioned before, the synthesized powders were
sintered at 930 °C, which is below the melting points of
silver and zinc oxide. In other words, the powder com-
pacts were sintered in the solid state. As it is known,
during the solid-state sintering, the main mechanism for
the densification and elimination of microstructural
porosities is the diffusion of atoms. However, it can be
declared that in the case of powder blends, the con-
stituent with a relatively low melting point has a larger
effect on the overall densification, which is due to the
relatively low activation energy for the diffusion of
atoms. So, in the case of the Ag/ZnO powders, the
dominant mechanism for the consolidation of the powder
compacts is the diffusion of silver atoms during the
sintering process. It is worthy to note that the melting
points of zinc oxide and silver are 961.8 °C and 1975 °C,
respectively.
Figure 9 shows the relative green and sintered
densities versus the cold compaction prior to the sinter-
ing. On the basis of the obtained results, the samples that
were cold compacted at 300 MPa had the highest final
sintered relative density among the specimens. Accord-
ing to Figure 9, by increasing the cold-compaction
pressure, the difference between the green- and final
sintered-density values of the powder compacts was
increased. However, the slope of line (1) is higher than
that of line (2) in Figure 9. This observation implies that
the cold-compaction pressure has a significant effect on
the densification of the powder compacts during the
solid-state sintering.
The difference between the relative sintered- and
green-density values at each compression is shown in
Figure 10. It seems that there is a logarithmic relation-
ship between the difference value and the cold-compac-
tion pressure. The value of correlation coefficient R2 for
the corresponding logarithmic curve is very close to
unity. Based on the derived equation (y = 0.0622 ln x –
0.2447), it can be concluded that if the cold-compaction
pressure of the powders is below 38 MPa, no detectable
densification will occur during the solid-state sintering
(y = 0). More investigations should be done in this
regard.
M. ARDESTANI: CHEMICAL SYNTHESIS AND DENSIFICATION BEHAVIOR OF Ag/ZnO METAL-MATRIX COMPOSITES
284 Materiali in tehnologije / Materials and technology 50 (2016) 2, 281–286
Figure 9: Green and sintered relative densities of the powders versus
the magnitude of cold compaction prior to sintering
Slika 9: Zelene in sintrane gostote prahov v odvisnosti od hladnega
stiskanja pred sintranjem
Figure 7: ln(1/1–D) versus uniaxial-pressure compaction magnitude
(P) (the Heckel equation)
Slika 7: Odvisnost ln(1/1–D) od nihanja enoosnega tlaka pri stiskanju
(P) (ena~ba Heckel)
Figure 10: Difference between the relative sintered- and green-density
values at each compression
Slika 10: Razlika vrednosti relativne sintrane in zelene gostote pri
vsakem stiskanju
Figure 8: lg(ln(1/1–D)) versus uniaxial-pressure compaction mag-
nitude (lg P) (the Ge equation)
Slika 8: Odvisnost lg(ln(1/1–D)) od nihanja enoosnega tlaka pri
stiskanju (lg P) (ena~ba Ge)
The sinterability () of the synthesized powders is
determined with the following Equation9:
 = (ds – dg) / (dth – dg) (6)
where dth, dg and ds are the theoretical, green and sin-
tered densities, respectively.
Figure 11 shows the sinterability of the powder
compacts at different cold-compaction pressures. As it is
observed, by increasing the cold-compaction pressure
prior to sintering, the sinterability of the powder com-
pacts was enhanced. So, it can be concluded that the
magnitude of the cold-compaction pressure has a signi-
ficant influence on the atomic diffusion during sintering.
By increasing the cold-compaction pressure of the
powder compacts, the plastic deformation of the powder
particles led to an increase in the dislocation density
within the microstructures of the particles.
Increasing the number of dislocations, which are one
of the main diffusion paths for the diffusion of atoms,
may facilitate atomic diffusion during the solid-state
sintering. However, the annealing of the cold-worked
powder particles during the solid-state sintering process
leads to the annihilation of dislocations within the micro-
structures of the particles. So, it can be concluded that
the positive effect of dislocations on the atomic diffusion
is more important at the initial stage of the sintering
process than at the intermediate or final stages.
SEM micrographs of the synthesized composites are
shown in Figure 12. These figures also show an elemen-
tal-map analysis of the composites that were cold
compacted at 300 MPa prior to the sintering. The micro-
structures of the composites consist of the silver matrix,
the zinc-oxide reinforcement and also porosities (Fig-
ures 12a to 12c). Figures 12d and 12e show the silver
and zinc map analyses of Figure 12c. The white regions
in Figures 12d and 12e are the silver and zinc oxide
regions, respectively. It can be stated that the porosities
mainly exist at the interface of silver and zinc oxide.
However, by increasing the cold-compaction pressure,
the size and volume fraction of the porosities were
reduced. According to Figure 12, the samples that were
cold compacted at 200 MPa and 300 MPa prior to the
sintering contain silver regions within the bulk of the
ZnO reinforcements. This fine dispersion of the consti-
tuents within the microstructures of the sintered speci-
mens enhances the homogeneity of the synthesized
composites.
4 CONCLUSION
In this study, Ag/ZnO composites were synthesized
via a chemical precipitation process. Silver and zinc
carbonates were used as the precursors. The initial pre-
cipitates contained silver and zinc carbonates. The calci-
nation of the precipitates at 600 °C occurred due to the
decomposition of the carbonates and the formation of
desired powder constituents, i.e., silver and zinc oxide.
The compressibility-behavior investigations confirmed
that the most accurate model for describing the com-
pressibility behavior of the synthesized powders was the
Heckel equation. Furthermore, the results showed that by
increasing the cold-compaction pressure prior to sinter-
ing, the density of the synthesized composites was
increased.
M. ARDESTANI: CHEMICAL SYNTHESIS AND DENSIFICATION BEHAVIOR OF Ag/ZnO METAL-MATRIX COMPOSITES
Materiali in tehnologije / Materials and technology 50 (2016) 2, 281–286 285
Figure 12: SEM images of the sintered composites cold compacted at:
a) 100 MPa, b) 200 MPa and c) 300 MPa prior to sintering, d) silver
map analysis of Figure 12c, e) zinc map analysis of Figure 12c
Slika 12: SEM-posnetki sintranih kompozitov, ki so bili hladno
stisnjeni pri: a) 100 MPa, b) 200 MPa in c) 300 MPa pred sintranjem,
d) analiza razporeditve srebra na Sliki 12c, e) analiza razporeditve
cinka na Sliki 12c
Figure 11: Sinterability () of the powder compacts at different
cold-compaction pressures
Slika 11: Sposobnost sintranja () vzorcev, stisnjenih pri razli~nih
tlakih v hladnem
5 REFERENCES
1 ASM Handbook, Powder Metal Technologies and Applications, Vol.
7, ASM International, 1998
2 E. Lassner, W. Schubert, Tungsten: Properties, Chemistry, Technol-
ogy of the Element, Alloys and Chemical Compounds, Kluwer
Academic Publishers, New York 1999, doi:10.1007/978-1-4615-
4907-9
3 W. G. Chen, Z. Y Kang, H. F. Shen, B. J. Ding, Arc Erosion Beha-
vior of a Nanocomposite W-Cu Electrical Contact Material, Rare
Metals, 25 (2006) 1, 37–42, doi:10.1016/S1001-0521(06)60011-9
4 F. A Costa, A. G. P. Silva, F. A. Filho, U. U. Gomes, Solid state
sintering of a W–25 wt% Ag powder prepared by high energy
milling, International Journal of Refractory Metals and Hard
Materials, 26 (2008), 318–323, doi:10.1016/j.ijrmhm.2007.08.003
5 E. R. Rangel, E. R. Garcia, J. M. Hernandez, E. T. Rojas, Alu-
mina-based composites reinforced with silver particles, Advances in
Materials, 2 (2013) 6, 62–65, doi:10.11648/j.am.20130206.11
6 P. B. Joshi, V. J. Rao, B. R. Rehani, A. Pratap, Comparison of
properties of silver-tin oxide electrical contact materials through
different processing routes, Indian Journal of Engineering and
Materials Science, 15 (2008), 236–240
7 F. S. Jazi, N. Parvin, M. R. Rabiei, M. R. Tahriri, Z. M. Shabestari,
A. R. Azadmehr, The effect of the synthesis route on the grain size
and morphology of ZnO/Ag nanocomposite, Journal of Ceramic
Processing Research, 13 (2012) 5, 523–526
8 M. Ardestani, Compaction and solid state sintering behavior of
Cu-20wt. % ZnO powders, Kovove Materialy – Metallic Materials,
51 (2013), 367–371, doi:10.4149/km_2013_6_367
9 D. Jeyasimman, K. Sivaprasad, S. Sivasankaran, R. Narayanasamy,
Fabrication and consolidation behavior of Al 6061 nanocomposite
powders reinforced by multi-walled carbon nanotubes, Powder
Technology, 258 (2014), 189–197, doi:10.1016/j.powtec.2014.03.039
M. ARDESTANI: CHEMICAL SYNTHESIS AND DENSIFICATION BEHAVIOR OF Ag/ZnO METAL-MATRIX COMPOSITES
286 Materiali in tehnologije / Materials and technology 50 (2016) 2, 281–286