The combustion synthesis of iron group metal fine powders

Article abstract of DOI:10.1016/j.jssc.2003.08.003

The new approach has been developed for the synthesis of nickel (Ni), cobalt (Co) and iron (Fe) powders from the appropriate oxides by the solid combustion method. The reduction was made by sodium azide (NaN₃) at the presence of carbon in the argon atmosphere. The variation of combustion temperature and velocity was performed by using alkali metal salt as an inert diluent. The values of combustion parameters were measured and also the temperature distribution in a combustion wave are obtained. The geometric sizes of reactionary zones and the activation energy of the process were estimated. The optimum conditions for single-phase metal powder synthesis were found. Powders fabricated in this way had cubic structure and particles size about 0.5-2.0μm for Ni, Co and 1-3μm for Fe. In a number of cases the formation of spherical particles with the average size about 5-15μm were observed.

Full text of DOI:10.1016/j.jssc.2003.08.003

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 251–256
The combustion synthesis of iron group metal fine powders
ꢀ
J.H. Lee, H.H. Nersisyan, and C.W. Won
Industries Lab, Rapidly Solidified Materials Research Center Sierra (RASOM), Chungnam National University, 220 Kungdong,
Yuseong, Daejeon 305764, South Korea
Received 25 May 2003; received in revised form 1 August 2003; accepted 2 August 2003
Abstract
The new approach has been developed for the synthesis of nickel (Ni), cobalt (Co) and iron (Fe) powders from the appropriate
oxides by the solid combustion method. The reduction was made by sodium azide (NaN ) at the presence of carbon in the argon
3
atmosphere. The variation of combustion temperature and velocity was performed by usingalkali metal salt as an inert diluent.
The values of combustion parameters were measured and also the temperature distribution in a combustion wave are obtained. The
geometric sizes of reactionary zones and the activation energy of the process were estimated. The optimum conditions for single-
phase metal powder synthesis were found. Powders fabricated in this way had cubic structure and particles size about 0.5–2.0 mm for
Ni, Co and 1–3 mm for Fe. In a number of cases the formation of spherical particles with the average size about 5–15 mm were
observed.
r 2003 Elsevier Inc. All rights reserved.
Keywords: Combustion synthesis; Iron group; Inert diluent
1
. Introduction
synthesis, the simplicity of equipment and high quality
of final products. Ultrafine powders of several metals
have been synthesized by combustion synthesis; tita-
nium [12], tantalum [13], tungsten [14,15]. In all cases
the synthesis was carried out from appropriate oxides
with use of magnesium as reduction agent. To our
knowledge the production of the powders of iron group
metals by combustion synthesis has not been reported in
the literature.
The purpose of this study is to describe a new simple
way for the preparation of metallic Ni, Co and Fe fine
powders by combustion processes usinginexpensive
metal resources (metal oxide) and as a reducinga ge nt
sodium azide combinated with carbon. Single phase
nickel, cobalt and iron powders have been obtained and
characterized herein.
Sodium azide as solid source of nitrogen is common in
the combustion field [16,17]. It has been also widely used
for the preparations of transition metal nitrides via
combustion like rapid metathesis reaction [18,19]. But
an alkali metal azide was not considered as reduction
agent in combustion synthesis, because it contains large
quantity of nitrogen. In the case of iron group metals
the formation of nitride phase can be excluded, thus the
combustion reaction leads to the formation of pure
metal.
Iron group metals (Fe, Co, Ni) are of great
importance in the industry. They have been widely used
in the pure state as well as in the composition of various
alloys. Nowdays ultrafine powders of these metals
having uniform shape and high purity are increasingly
required for specific uses in many technological areas,
such as Base Metal Electrode Multilayered Ceramic
Capacitor (BME-MLCC) industry for personal compu-
ters, cellular phones, and other electronic devices,
chemical industry—as catalyst and other. A variety of
techniques have been used to produce pure metals such
as, chemical reduction [1,2], gas evaporation [3,4], ball
milling [5], sol–gel process [6,7], sonochemical [8], etc.
Because all the above applications require fine metallic
powders, an alternative inexpensive and simple method
for producingsuch powders is of considerable interest.
One possible approach is to prepare the powders by
combustion synthesis [9–11].
Advantages of the combustion synthesis in compar-
ison with another ones are small energy consumption
for the achievement of high temperatures, high rates of
ꢀ
Correspondingauthor. Fax: +82-42-822-9401.
E-mail address: jong-lee@cnu.ac.kr (J.H. Lee).
0022-4596/$ - see front matter r 2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2003.08.003

ARTICLE IN PRESS
J.H. Lee et al. / Journal of Solid State Chemistry 177 (2004) 251–256
252
2
. Experimental
3.1. Thermodynamics of process
The raw materials used in this study were: NiO,
Co O , Fe O , NaN , NaCl, granulated black carbon
The preliminary thermodynamic analyses of the NiO–
0.67NaN –0.335C, Fe O –2.7NaN –1.35C and Co O –
2.7NaN –1.35C stoichiometric mixtures in the presence
of NaCl as an inert diluent were performed by the
program ‘‘THERMO’’ [20]. Calculation of equilibrium
characteristics is based on minimizingthe thermody-
namic potential which expression accounts for the
contributions of thermodynamic potentials for all the
components present in the system.
3
4
3
4
3
3
3
4
3
3
4
(o0.1 mm) (99.5% purity, Samchun Pure Chemical Co.
3
LTD, Korea), and argon gas (99.9% purity). Appro-
priate amounts of metal oxide, sodium azide, black
carbon and sodium chloride were mixed in the ceramic
mortar at room temperature. Preliminary the initial
mixture was stamped into the cylindrical metallic
mould from stainless steel with the 2 mm thickness,
3
0 mm diameter and 40 mm in height. Experimental
As a typical example the diagram of adiabatic
combustion temperature and equilibrium composition
of combustion products in NiO–0.67NaN –0.335C–
kNaCl (where k is mol-number of NaCl) system is
given in Fig. 1. As one can see thermodynamically
possible products of combustion reaction are metallic
nickel and sodium carbonate. The adiabatic combustion
3
densities of samples were 1.5–1.7 g/cm . Duringa
typical experiment the pellet is placed into a constant
pressure reactor and the reaction is initiated by means of
a nickel-chromium wire connected to the power supply,
which was programmed to produce an energy pulse by
setting15–20 V for about 1.0–1.5 s. The combustion
was carried out in argon atmosphere at the pressure of
3
temperature (T ) is maximum at k ¼ 0 and makes
ad
0
.5 MPa.
The maximum combustion temperature (Tc) and
1352 K. It is obvious that Tad can be lower by the
addition of NaCl in the initial mixture as an inert diluent
that creates milder conditions for combustion and phase
formation. In this figure, the changes of enthalpy (DH)
and entropy (DS) of final products are also presented,
and they increase with increase of k: The similar
thermodynamic situation was recorded for the
cobalt and iron containingsystems. The values of Tad
obtained from the thermodynamic calculation are given
in Table 1.
temperature distribution were measured by tungsten-
rhenium thermocouples (W/Re-5 vs. W/Re-20) with 100
and 50 mm in diameter previously covered with alumina
thin layer and embedded in the center of the sample at
predefined distances from each other.
The input signals of thermocouples were transformed
by data logger (DASTC) at the rate of 10 Hz, and were
recorded by computer. The average values of combus-
tion velocity are as follows: Uc ¼ l=t; where Uc is the
combustion velocity; l is the distance between thermo-
couples; and t is the temporary time between signals of
thermocouples. The standard error of measurement for
3.2. The combustion laws of Me O –NaN –C–NaCl
x
y
3
system
ꢁ
Tc and Uc were 725 and 5%, respectively.
Two typical parameters of combustion reaction were
measured duringthe experiment: velocity of wave
propagation and combustion temperature. The curves
reflectingthe relation between these two parameters are
shown in Fig. 2. The changes of combustion parameters
The phase composition of the combustion products
was determined by X-ray diffractometer (XRD; Siemens
D5000, Germany). The morphology of products and
mean particle sizes were determined by scanning
electron microscopy (SEM) (JSM 5410, Japan). Ther-
mogravimetric and differential thermal analyses (TG-
DTA) were performed under air or argon atmosphere,
2000
1600
1200
1.5
∆
∆
H, KJ
S, J/K
ꢁ
respectively, at heatingrate of ran ge 10–20 C/min.
C
Ni
1
3
. Results and discussion
The investigation was carried out on the Me O –
T
ad
800
x
y
−∆Hprod.
0.5
NaN –C–NaCl system, where Me O is NiO, Co O
4
∆Sprod.
3
x
y
3
and Fe O . The reduction of metal oxides was
3
400
0
CNa2CO3
4
performed by sodium azide combinated with carbon.
Total process of reduction can be described by the
followingequation:
0
0
0.2
0.4
0.6
0.8
1
KNaCl, mol
Fig. 1. Diagram of adiabatic combustion temperature and equilibrium
Me O þ 2y=3NaN þ y=3C
x
y
3
3
composition of combustion products in NiO–0.67NaN –0.335C–
¼
xMeþy=3Na CO3 þ yN2:
ð1Þ
kNaCl system, T ¼ 298 K, P ¼ 0:5 MPa.
2
o
o

ARTICLE IN PRESS
J.H. Lee et al. / Journal of Solid State Chemistry 177 (2004) 251–256
253
Table 1
Combustion characteristics of NiO+NaN
3
+C+inorganic salt systems
No.
Green mixture
Tc (K)
Tad (K)
Uc (cm/s)
Phase composition
1
2
3
4
5
6
7
8
9
NiO–0.67NaN
NiO–0.67NaN
NiO–0.67NaN
NiO–0.67NaN
3
3
3
3
–0.335C
1390
1195
1080
990
1352
1183
1122
1073
1023
1466
1314
1185
1130
1130
1090
1060
1042
0.4
Ni(f.c.c.)
–0.335C–0.3NaCl
–0.335C–0.5NaCl
–0.335C–0.7NaCl
0.13
0.08
0.048
0.17
0.34
0.12
0.073
0.042
0.11
0.08
0.065
0.04
Ni(f.c.c.), NiO (5 wt%)
Ni(f.c.c.), NiO (o10 wt%)
Ni(f.c.c.), NiO (o15 wt%)
Ni(f.c.c.), NiO (o20 wt%)
b-Co(f.c.c.)
NiO–0.4NaN
3
–0.4C
1070
1770
1270
1070
950
Co
Co
Co
Co
3
3
3
3
O
4
O
4
O
4
O
4
–2.7NaN
3
3
3
3
–1.35C
–2.7NaN
–2.7NaN
–2.7NaN
–1.35C–1NaCl
–1.35C–2NaCl
–1.35C–3NaCl
b-Co(f.c.c.)
b-Co(f.c.c.), Co (h.c.p.)
b-Co(f.c.c.), a-Co (h.c.p.), CoO (o10 wt%)
1
1
1
1
0
1
2
3
Fe
Fe
Fe
Fe
3
4
O –2.7NaN
4
O –2.7NaN
4
O –2.7NaN
4
O –2.7NaN
3
3
3
3
–1.35C
1170
1090
1000
900
a-Fe(b.c.c.)
3
3
3
–1.35C–0.5NaCl
–1.35C–NaCl
–1.35C–1.5NaCl
a-Fe(b.c.c.)
a-Fe(b.c.c.), FeO (o10 wt%)
a-Fe(b.c.c.), FeO (o15 wt%)
0
0
0
0
0
.5
.4
.3
.2
.1
0
♦
♦
•
◊
⊗
Ni
Fe
Co(f.c.c.)
Co(h.c.p.)
1
♦
(
a)
b)
c)
♦
3
2
•
(
(
•
◊
7
00
900
1100
1300
, K
1500
1700
1900
◊
◊
⊗
T
c
⊗
20
30
40
50
60
70
80
Fig. 2. The relation between Tc and Uc in Me
x y 3
O –NaN –C–kNaCl
2
θ, CuKα
system, To ¼ 298 K, Po ¼ 0:5 MPa, 1-Hi, 2-Fe, 3-Co.
Fig. 3. Typical XRD patterns of nickel (a), iron (b) and cobalt
(
c) powders.
were achieved by dilution of the initial mixture. The
compositions of green mixtures are given in Table 1.
Accordingto the results obtained, the dilution of gr een
mixtures leads to simultaneous decreases of Tc and Uc:
At the high degree of dilution the combustion limit
observed, and the combustion temperature is below
SEM studies gave a clear indication of a good state of
particle dispersion. The typical microstructures of metal
powders received by SEM analysis are shown in Fig. 4.
The sizes of these powders estimated from the micro-
graphs are in the range of 0.5–2 mm for Ni and Co and
1–3 mm for Fe. The typical image (Figs. 4a and b)
demonstrates that the nickel and cobalt particles are
partially spheroidal in shape and the degree of particle
agglomeration is relatively low. The average sizes of the
agglomerates are less than 5 mm. In comparison with
the Ni and Co the particles of Fe have the form of
well-defined crystals and low degree of agglomeration
(Fig. 4c).
It is worth also to note that under a certain condition,
the combustion process is able to lead to the formation
of metal particles havingthe ideal spherical form
(Fig. 5). In particular in Fig. 5a the micrograph of the
nickel powder containinga spherical particle with the
particle sizes about 5–15 mm is shown. The mechanism
of these particle formations remains unclear, but from
8
50 K.
After combustion samples were kept up in the reactor
up to complete cooling. Usual operations of washing by
warm water and dryingin vacuum dryingfurnace at the
temperature 330–340 K of final products were made.
Accordingto X-ray analysis sin gl e-phase powders of
nickel, cobalt and iron were obtained in the interval of
the temperature about 1200–1300 K (Fig. 3). At lower
temperatures the combustion reaction, as a rule, does
not complete and a certain quantity of metal oxide
remains in a final product (see Table 1). It should be
noted that as distinct from Ni and Fe the main b-Co
(
f.c.c) phase with a small quantity of a-Co (h.c.p) always
is observed at the reduction of cobalt oxide. The
quantity of a-Co phase considerably increases with the
decreasingof the combustion temperature.

ARTICLE IN PRESS
J.H. Lee et al. / Journal of Solid State Chemistry 177 (2004) 251–256
254
(a)
(b)
(c)
(a)
(b)
(c)
Fig. 4. SEM micrographs of nickel (a), cobalt (b) and iron
c) powders.
Fig. 5. Dynamics of the spherical particle formation: (a) nickel powder
with spherical particles; (b) external shape of nickel particle at large
magnification and (c) spherical cobalt particles with a melted shell.
(
an external structure of separate particles (Fig. 5b) it is
possible to suppose that they are formed by consolida-
tion of small ones. Approximately similar mechanism of
spherical particle formation takes place also in the
cobalt containingsystem ( Fig. 5c).
high temperatures. A lot of useful information on the
process come from the measured temperature distribu-
tion in a combustion wave. In particular with their help
it is possible to estimate the sizes of preheatingand
reactionary zones, the meltingzones, leadingsta ge s of
reaction and other.
3.3. Thermal structure of a combustion wave and kinetics
of reaction
The most informative and accurate technique to
obtained temperature distributions is microthermocou-
ple technique which is widely used for the description of
Reduction of metals from the correspondingoxides is
multistage and complex chemical process proceeding at

ARTICLE IN PRESS
J.H. Lee et al. / Journal of Solid State Chemistry 177 (2004) 251–256
255
a combustion processes in the condensed systems
21,22].
DTA analysis of initial mixtures was carried out in an
only single stage temperature distributions are revealed
ꢀ
[
in the combustion wave. Here T is the starting
temperature of reaction; LR the thickness of the
reactionary zone; and lmix the thickness of the preheat-
ingzone which corresponds to the chan ge s of tempera-
argon atmosphere to specify starting temperature of the
ꢀ
reaction (T ). The DTA result thus received for NiO–
0
ꢀ
.67NaN –0.335C mixture as a typical exemple is
3
ture from To up to T : As one can see sharp increasing
presented in Fig. 6. As it is visible up to 670 K initial
mixture is stable and does not undergo chemical
changes. At 670 K DTA analysis registers sharp
exothermic peak significative, that is ignition of a
of the temperature takes place near to the ordinate. The
difference between the sizes of preheatingand reac-
tionary zones is not high (lmix ¼ 0:7570:25 mm,
LR ¼ 1:2570:25 mm), thus indicatingabout classical
narrow reaction zone the concept of which were
developed by Zeldovith and Frank-Kamenetsky [23].
Accordingto this concept the combustion velocity is
connected with the combustion temperature by the
followingequation:
ꢀ
mixture (T ). This temperature corresponds to the
decomposition temperature of NaN . There are also
3
one endothermic area in the DTA at 1150 K which most
likely corresponds to the meltingof Na CO .
Temperature distributions in the combustion wave of
2
3
NiO–0.67NaN –0.335C–kNaCl mixture at different k
3
are given in Fig. 7. As a result of experimental research,
2
c
ꢂE=RTc
:
c
U ¼ AT e
ð2Þ
It is obvious that the activation energy for the
combustion process can be derived from the experimental
values Uc and Tc in Eq. (2). For it the values of Tc and Uc
were processed in coordinates ln Uc=Tc ꢂ 1=Tc: The
results are presented graphically in Fig. 8. The activation
energies which can be extracted from the plots are: E ¼
22; 30 and 30kcal/mol for NiO–NaN –C, Co O –NaN –
250
200
150
100
3
3
4
3
C and Fe O –NaN –C system, respectively.
3
4
3
3.4. Oxidation behavior of nickel, cobalt and iron powder
50
The stability of the resultant Ni, Co, Fe powders in air
were characterized by thermogravimetry (TG). The
heatingrate of the samples was 10 C/min. The weight
of the nickel powder synthesized by SHS process began
to increase from around 820 K, which means the starting
of oxidation as shown in Fig. 9. It seems that the Ni
powder was completely oxidized to NiO at around
0
ꢁ
2
00
400
600
800
1000
1200
1400
T, K
3
Fig. 6. DTA of NiO–0.67NaN –0.335C system.
1270 K. The final weight gain is around 122.5% by TG
that is little lower weight gain than theoretical (127.3%)
value for perfect conversion of pure Ni to NiO. This
datum indicates that the particles consist of pure nickel
and small amount of unreduced nickel oxide.
4
1
/Tc.10 K
-6.5
5
6
7
1
8
9
10
11
12
-
7
-
7.5
E1=22 kcal/mol
-8
E2=E3=30 kcal/mol
-8.5
-9
2, 3
-
9.5
10
10.5
-
-
Ni
-
-Co
Fig. 7. Average temperature distribution in the combustion zone for
NiO–0.67NaN –0.335C–kNaCl system: (1) k=0; (2) k=0.3; (3) k=0.5
and (4) k=0.7.
-Fe
3
Fig. 8. Dependencies of ln Uc=Tc from 1=Tc:

ARTICLE IN PRESS
J.H. Lee et al. / Journal of Solid State Chemistry 177 (2004) 251–256
256
2
. It is shown that the received powders as a rule have
135
130
125
120
115
110
105
100
Fe
cubic structure and only cobalt powder contains
small amount of hexagonal phase. The average sizes
of particles were made: Ni—0.5–2 mm, Co—0.5–2 mm
and Fe—1–3 mm. The shapes of particles are near to
spherical (Ni, Co) or well defined crystals (Fe).
. It was established that received metals powders have
a good stability to oxidation. Further work includes
decreasingof unreduced oxide phase while keeping
the stability to the oxidation.
Co
3
Ni
95
200 300 400 500 600 700 800 900 1000 1100 1200 1300
References
Temperature(K)
[
[
1] A. Degen, J. Macek, Nanostruct. Mater. 12 (1999) 225–228.
2] S. Ayyappan, R.S. Gopalan, G.N. Subbanna, C.N.R. Rao,
J. Mater. Res. 12 (1997) 398–401.
Fig. 9. TGA curves of Fe, Co and Ni powders in the air atmosphere.
[3] L. Dong, Z. Zhang, S. Jin, W. Sun, Y. Chuang, Nanostruct.
Mater. 10 (1998) 585–592.
The stabilities of cobalt and iron powder were found
to be comparatively lower than that of nickel powder.
The oxidation started from 550 to 600 K for iron and
cobalt powder, respectively. Also, complete oxidation
was found at around 970 K.
[
[
[
4] G.A. Niklasson, J. Appl. Phys. 62 (1987) 258–265.
5] J. Huang, Y. Wu, H. Ye, Acta Mater. 44 (1996) 1201–1209.
6] S. Roy, A. Chatterjee, D. Chakraborty, J. Mater. Res. 8 (1993)
6
89–692.
[7] Syukri, B. Takayuki, O. Yutaka, T. Yasataka, Mater Chem. Phys.
8 (2003) 383–391.
7
[8] Y. Koltypin, G. Katabi, X. Cao, R. Prozorov, A. Gedanken,
J. Non-Cryst. Solids 201 (1996) 159–162.
4
. Conclusion
[9] A.G. Merzhanov, Int. J. SHS 4 (1995) 323–330.
10] Z.A. Munir, U. Anselmi-Tamburini, Mater. Sci. Rep. 69 (1989)
[
The most important result of this work is the
277–283.
11] Y. Miyamoto, J. Mineral. Soc. Jpn. 18 (1988) 383–391.
12] H.H. Nersisyan, J.H. Lee, C.W. Won, Mater. Res. Bull. 38 (2002)
[
[
development of the simple approach allowingsynthesis
of fine iron group metal powders by the combustion
synthesis. This approach is not only limited to synthesize
these metals but also to make various alloys on the basis
of these metals. Also large opportunities are present for
the synthesis of metals from other groups of the table of
elements.
1
135–1146.
13] H.H. Nersisyan, J.H. Lee, S.I. Lee, C.W. Won, Combustion
[
and flame, 2003, in print.
[14] P. Zivanovich, R. Curcich, G. Djurkovich, V. Jokanovich,
D. Uskokovich, Int. J. SHS 10 (2001) 91–99.
[15] J. Zhang, J.H. Lee, D.Y. Maeng, C.W. Won, J. Mater. Sci. 36
(2001) 3233–3238.
16] J.B. Holt, US patent number 4,944,930, July 31, 1990.
The major conclusions of this study are as follows:
[
[
17] V.V. Zakorzhevskii, I.P. Borovinskaya, Int. J. SHS 9 (2000)
1
1
. The synthesis of iron group metals by combustion
method was realized. The interrelation between the
combustion temperature and wave velocity were
established. It was shown that the combustion
process begins after decomposition of NaN3 at
71–191.
18] E.G. Gilan, R.B. Kaner, Chem. Mater. 8 (1996) 333–343.
19] I.P. Parkin, Chem. Soc. Rev. 25 (1996) 199–208.
[
[
[20] A.A. Shiryaev, Int. J. SHS 4 (1995) 351–355.
[
[
[
21] A.A. Zenin, A.G. Merzhanov, H.H. Nersisyan, Dokl. Akad.
Nauk SSSR 250 (1980) 880–884 (in Russian).
6
70 K and is characterized by narrow zones of
22] A.A. Zenin, A.G. Merzhanov, H.H. Nersisyan, Combust.,
Explos., Shock Waves 1 (1981) 79–86.
reaction. The temperature distributions on a combus-
tion wave were constructed and activation energies of
combustion processes were calculated.
23] J.B. Zeldovich, D.A. Frank-Kamenetsky, J. Phys. Chem
12 (1938) 100–105 (in Russian).