Preparation process and mechanism of ultra-fine spherical cobalt powders by hydrogen reduction of calcium cobaltite

Article abstract of DOI:10.1016/j.jallcom.2017.08.070

For the improvement of the properties of cemented carbides, ultra-fine spherical cobalt powders as binder were prepared using calcium cobaltite. The effects of the ratio of CaO to CoO, the calcination temperature and the reduction temperature on particle

Full text of DOI:10.1016/j.jallcom.2017.08.070

Journal of Alloys and Compounds 726 (2017) 1119e1123
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Preparation process and mechanism of ultra-fine spherical cobalt
powders by hydrogen reduction of calcium cobaltite
*
Jiajing Wu, Jiancheng Tang , Xiaoxiao Wei, Nan Ye, Fangxin Yu
School of Materials Science and Engineering, Nanchang University, Nanchang 330031, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 January 2017
Received in revised form
For the improvement of the properties of cemented carbides, ultra-fine spherical cobalt powders as
binder were prepared using calcium cobaltite. The effects of the ratio of CaO to CoO, the calcination
temperature and the reduction temperature on particle size and morphology of cobalt powders were
studied. The results show that the average particle size of cobalt powders decreases with increasing the
weight ratio of CaO to CoO, but increases with the calcination temperature and reduction temperature.
The spherical cobalt powders with the mean particle size of 84 nm are obtained when the weight ratio of
CaO to CoO is 7:4 and the calcination temperature and reduction temperature are 1000 C and 700 C,
respectively. In addition, CaO regarded as the blocking agent can refine cobalt powders during the
hydrogen reduction process.
7
August 2017
Accepted 9 August 2017
Available online 12 August 2017
ꢀ
ꢀ
Keywords:
Cobalt powders
Calcium cobaltite
Hydrogen reduction process
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
ultra-fine spherical cobalt powders. During the hydrogen reduction
process, CaO plays an important role in controlling the shape and
Cobalt and its alloys have been widely used in superalloy,
magnetic materials [1,2], rechargeable alkaline batteries [3], het-
erogeneous catalysis [4], cemented carbide and diamond tools [5],
because of their high-density magnetic property, sintering reac-
tivity, hardness levels and excellent impact resistance property
size of Co powders. The effects of the ratio of CaO to CoO, the
calcination temperature and reduction temperature on the size and
morphology of cobalt powders were investigated. The role of CaO
during the preparation process of cobalt powders was discussed.
[1,2]. Cobalt powders have a strong influence on the formation of
2. Experimental
homogeneous microstructure and the improvement of mechanical
properties of WC-Co cemented carbides. Ultra-fine spherical cobalt
powders are favorable to the development of high-quality WC-Co
cemented carbides [6]. Physical and chemical properties of cobalt
powders strongly depend on their size and morphology [7,8],
therefore the improvement of fabrication methods may be used for
enhancing the properties of cobalt powders [9e11]. At the present,
cobalt powders may be prepared by several methods, such as hy-
drothermal reduction [12], vacuum pyrolytic decomposition [13],
ultraviolet-irradiation [14], chemical vapor condensation [15], ɤ-
irradiation [16], polyol process [17], and liquid phase reduction [18].
However, the disadvantages of these methods are apparent, such as
the high cost that retards industry applications, and the difficulty to
achieve ultra-fine spherical cobalt powders.
2
.1. Preparation of calcium cobaltite
CaO (purity ꢁ 98.0%) and CoO (purity ꢁ 99.5%) were used as raw
materials. The raw materials were uniformly mixed at room tem-
perature. The total mass and its weight ratio were summarized in
Table 1. These materials were placed into three agate grinding
cylinder with volume 500 ml. Agate ball was used as milling ball
(diameter 5e12 mm) and the weight ratio of ball to powders was
3
7
:1. The homogenous mixtures were obtained after milling for
h at 500 rpm, then placed into three corundum boats and,
ꢀ
ꢀ
ꢀ
respectively, calcinated at 900 C, 1000 C and 1100 C for 5 h in air
to obtain precursor powders. The 5 g precursor powders were
placed into three railbaots and, respectively, reduced in hydrogen
atmosphere at 600 C, 700 C, 800 C and 900 C for 2 h. To avoid
the influence of oxygen on the cobalt, the powders were cooled in
hydrogen atmosphere. Finally, the prepared mixtures were washed
In this paper, we exploit a two-stage procedure for preparing
ꢀ
ꢀ
ꢀ
ꢀ
4
by NH Cl solution to remove the excess CaO and obtain the cobalt
*
Corresponding author.
E-mail address: tangjiancheng@ncu.edu.cn (J. Tang).
powders.
http://dx.doi.org/10.1016/j.jallcom.2017.08.070
925-8388/© 2017 Elsevier B.V. All rights reserved.
0

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J. Wu et al. / Journal of Alloys and Compounds 726 (2017) 1119e1123
Table 1
The quantity and weight ratio of raw materials used in the experiment.
1
2
3
Quantity(CaO þ CoO)/g
100
6:4
100
7:4
100
8:4
Weight ratio(CaO/CoO)
2.2. Characterization of powders
The phases of calcium cobaltite powders were determined by X-
ray diffraction (XRD, Bruker D8 X-ray diffractometer Focus) using
Cu K radiation. The morphology and average size of cobalt pow-
a
ders were observed using scanning electron microscopy (Nova
Nano SEM450).
3
. Results
Calcium cobaltite as the precursor was prepared by calcining the
ꢀ
CaO and CoO at 1000 C and its patterns are shown in Fig. 1. As can
3 2 6
be seen in Fig.1(a), the precursor consists of CoO, CaO and Ca Co O
phases. The phase of CoO appears in the precursor, indicating that
CaO and CoO have not been reacted completely. When the weight
ratio of CaO to CoO increases to 7:4, the phase of CoO disappears, as
shown in Fig. 1(b). With the ratio further increases to 8:4, the phase
ꢀ
Fig. 2. XRD patterns of precursor produced at different temperatures: (a) 900 C; (b)
1
ꢀ ꢀ
000 C; (c) 1100 C.
of Ca
ularly shape of Ca
for obtaining spherical cobalt powders in the process of reduction
5,19]. Therefore, it can be concluded that the optimum weight ratio
3
Co
2
O
6
transforms into Ca
2
Co
2
O
5
. However, due to the irreg-
temperature can lead to the grains of calcium cobaltite growth and
aggregation and accelerate the reaction. As mentioned above, the
product consists of three phases (Co O , Ca Co O and CaO). This
2
Co , it is difficult to use Ca
2
O
5
2
Co as precursor
2 5
O
3
4
2
2 5
[
leads to the cobalt powders non-uniform in the hydrogen reduction
process in low temperature. Therefore, it can be concluded that the
of CaO to CoO is 7:4.
ꢀ
Fig. 2 shows the XRD patterns of the calcined powders at
optimum calcination temperature is 1000 C.
different temperatures. From Fig. 2(a), it can be seen that the
Fig. 3 shows the XRD patterns of the reduced powders at
different temperatures. As seen in Fig. 3(a), the reduced powders at
ꢀ
calcined powders at 900 C are consisted of Ca
3
Co
phase indicates that the CoO phase
, and the reaction between CaO and
2 6 3 4
O and Co O
ꢀ
phases. The presence of Co
just transformed into Co
3
O
4
600 C still consist of Ca Co O phase, indicating that Ca Co O is
3 2 6 3 2 6
3
O
4
not converted into CaO and Co completely. With the temperature
increasing, the patterns only contain peaks corresponding to Co,
CoO is not completed. When the calcination temperature reaches
ꢀ
1
000 C, the powders are identified to be Ca
3
Co
2
O
6
, as shown in
CaO and Ca(OH) phases, as shown in Fig. 3(bed), and the peaks of
2
Fig. 2(b). With further increase of the calcination temperatures, the
cobalt shows sharp, which indicates that the cobalt powders have a
Ca
3
Co
2
O
6
phase transit into Ca
9
Co12
O
28 phase (Fig. 2(c)). It is
good crystallinity state.
ꢀ
indicated that when the calcination temperature is above 1000 C,
CaO and CoO have been reacted completely. In view of that the high
SEM images of cobalt powders are shown in Fig. 4(aec). It is
ꢀ
Fig. 1. XRD patterns of precursors produced at different ratio of CaO and CoO: (a) 6:4;
Fig. 3. XRD patterns of reduced cobalt powders at different temperatures: (a) 600 C;
ꢀ ꢀ ꢀ
(b) 700 C; (c) 800 C; (d) 900 C.
(
b) 7:4; (c) 8:4.

J. Wu et al. / Journal of Alloys and Compounds 726 (2017) 1119e1123
1121
Fig. 5. XRD pattern of cobalt powders.
the fact that the hydrogen temperature determines the rate for both
nucleation and growth of the cobalt powders. The possible mech-
anism is as follows. In low temperature,
Gk represent dynamic barrier and thermal barrier, respectively),
beneficial for the nucleation of Co powders during the process.
However, in high temperature, G < 0 is favored for the
DG
v
ꢂ
k
v
DG > 0 (DG and
D
DG
v
ꢂ
D
k
grain growth of Co powders, resulting in the formation of large
particles. Based on such an assumption, it can be concluded that the
ꢀ
optimum temperature of the hydrogen reduction is 700 C.
XRD pattern of Co powders is shown in Fig. 5. The pattern only
contains the peaks corresponding to Co without other impurities.
This indicates that Ca
ders completely. Additional, CaO and Ca(OH)
washing by NH Cl solution.
3
Co
2
O
6
have been transformed into Co pow-
2
disappear after
4
In order to understand the effects of CaO on Co powders growth,
two processes for preparing cobalt powders are compared. One is
that Co powders are prepared by reducing cobaltous oxide (CoO)
with hydrogen directly, and the other is that CaO and CoO are
3 2 6
calcined into calcium cobaltite (Ca Co O ) and then reduced under
hydrogen atmosphere. It is observed that the cobalt powders are of
irregular shape with the average particle size of 390 nm, as shown
in Fig. 6 (a). While in Fig. 6 (b), the morphology of cobalt powders is
nearly spherical with the average particle size of 84 nm. CaO plays
an important role in refining cobalt powders. The specific mecha-
nism will be explained in the next section.
4. Discussion
4.1. Thermodynamics analysis of the reaction for preparing calcium
cobaltite
The basic condition for solid-phase reactions is that the Gibbs
free energy changes of the reaction is smaller zero, i.e., < 0. In
DG
m
this experiment, the possible reaction equation during the process
for preparing calcium cobaltite is [21].
6
CaOðsÞ þ 4CoOðsÞ þ O ðgÞ ¼ 2Ca Co O ðsÞ
(1)
2
3
2 6
According to thermodynamic law, Gibbs free energy of the re-
action can be calculated by
ꢀ
Fig. 4. SEM images for reduced cobalt powder at different temperatures: (a) 700 C;
(
ꢀ ꢀ
b) 800 C; (c) 900 C.
q
m;1
q
q
D
r
G
¼ 2Df
G
ðCa Co O Þ ꢂ 6Df
G
ðCaOÞ
m;T
1
3
2
6
m;T
1
q
q
seen that the increase in the hydrogen temperature leads to the
grain growth and slight aggregation. The reason for this is due to
ꢂ 4Df
G
ðCoOÞ ꢂ Df
G
ðO2Þ
(2)
m;T
1
m;T
1

1122
J. Wu et al. / Journal of Alloys and Compounds 726 (2017) 1119e1123
the laws of thermodynamics,
DGm;1 < 0, the solid reaction of syn-
thesizing calcium cobaltite will occur, as described in Eq. (1).
4.2. Thermodynamics analysis of the reaction for preparing cobalt
powders
During the hydrogen reduction process, the thermodynamic
behavior of Ca Co and CaO is analyzed. The possible reduction
Co into Co and CaO into Ca can be generalized as
3
2 6
O
process of Ca
follows
3
2 6
O
Ca Co O ðsÞ þ 3H ðgÞ ¼ 3CaOðsÞ þ 2CoðsÞ þ 3H OðgÞ
(4)
(5)
3
2
6
2
2
CaOðsÞ þ H ðgÞ ¼ CaðsÞ þ H OðgÞ
2
2
According to thermodynamic laws, the Gibbs free energy
changes ( ) of the reaction (4) and (5) can be calculated ac-
cording to Eq. (6) and Eq. (7), respectively.
DG
m
3
2
3
a
$a $a
q
m;2
CaO Co
H
2
O
DGm;2
¼
¼
D
r
r
G
þ RT ln
2
aCa3Co2O6 $a3
H
2
3
3
½
CaOꢃ $P
q
m;2
H
2
O
D
G
þ RT ln
(6)
2
3
P
H
2
aCa$aH2
O
½Caꢃ$P
q
m;3
q
m;3
2
H O
DGm;3
¼
D
r
G
þ RT ln
¼
D
r
G
þ RT ln
2
2
aCaO$aH2
P
H
2
(
7)
q
m
where
DG
m
and
DG
represent the actual change values of Gibbs
free energy and the changes value of standard Gibbs free energy at
reaction temperature (T ), respectively, R is ideal gas law constant,
H2O and PH2 are partial pressure of water and hydrogen, respec-
2
P
tively. In order to simplify the calculation, we assume that most of
the water vapor are moved out of the camber by the hydrogen flow
ꢀ
and PH20/PH2 is 0.01. T
2
¼ 700 C is taken for calculation.
Based on Eqs. (6) and (7), the actual change values of Gibbs free
ꢂ1
ꢂ1
,
Fig. 6. SEM images of cobalt powders reduced of: (a) CoO; (b) Ca
3
Co
2
O
6
.
energy for
D
Gm;2 and
D
Gm;3 is ꢂ94.79 kJ mol and 345.95 kJ mol
3 2 6
respectively. Therefore Ca Co O are reduced to Co powders by
hydrogen as Eq. (4) and the reaction of Eq. (5) will not be occurred.
6
4
½
CaOꢃ ½CoOꢃ
q
m;1
DGm;1
¼
D
r
G
þ RT ln
(3)
1
P
O
4.3. Refinement mechanisms
2
q
q
where
D
G
m
,
D
r
G
and Df
G
represent the actual change values of
m
In the solid-phase reaction, two reactions are included, namely
contact reaction and diffusion reaction. Oxygen (O) ions are difficult
to diffuse because of its larger atomic radius than that of Co and Ca.
The charge balance is remained through the transmission of oxygen
at high oxygen partial pressure. At high temperature, diffusion re-
action is the primary mechanism, due to the mutual diffusion of Co
and Ca ions. This process can lead to numerous of defects, and is
beneficial for accelerating reaction rate.
m
Gibbs free energy, the changes value of standard Gibbs free energy
and standard Gibbs free energy of formation at reaction tempera-
ture (T
pressure of oxygen.
At T temperature (1000 C), the standard Gibbs free energy of
formation of each compounds in Eq. (2) are listed in Table 2. By
1
), respectively, R is ideal gas law constant, PO2 is partial
ꢀ
1
substituting
these
data
into
Eq.
(2),
we
have
q
m;T
ꢂ1
D
r
G
¼ ꢂ33.941 kJ mol . Because the activity of solid is 1 and
The corresponding reactions are analyzed as follows: At the
beginning CoO is oxidized to Co O with the calcination tempera-
2 3
1
the partial pressure of the gas is very small during the solid reac-
tion, the actual change values of Gibbs free energy (
lated from Eq. (3) is approximately ꢂ33.941 kJ mol . According to
D
Gm;1) calcu-
ture increasing, according to Eq. (8). The further increase in the
temperature can promote the homogeneous mixing of the Ca and
ꢂ1
3 2 6 2 3
Co ions. As a result, Ca Co O are obtained at the interface of Co O /
CaO, and Ca Co grains grow with the holding time. The corre-
sponding reaction equation is described as follows
3
2 6
O
Table 2
Standard Gibbs free energies of formation of compounds [20].
Compounds
CaO(s)
CoO(s)
Ca
3
CoO
6
(s)
4CoO þ O ¼ 2Co O
(8)
2
2 3
q
ꢂ1
)
Df
G
(KJ mol
ꢂ679.7
ꢂ296.51
ꢂ2615.18
m
2 3 3 2 6
Diffusion occurs at interface of Co O /Ca Co O by

J. Wu et al. / Journal of Alloys and Compounds 726 (2017) 1119e1123
1123
National Natural Science Foundation of China (Nos. 51471083,
51271090 and 51364036).
Co O þ 3Ca þ 3O2ꢂ ¼ Ca Co O
2
þ
(9)
2
3
3
2 6
3 2 6
Diffusion occurs at interface of Ca Co O /CaO by
Co3 þ 3CaO þ 3O2ꢂ ¼ Ca Co O
þ
(10)
2
3
2 6
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Co
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O
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decomposes
into CaO and Co O with the increase in temperature. The Co O
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3
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Acknowledgment
[
[
The authors are grateful to the financial support from the