A chemical route to large-scale preparation of spherical and monodisperse Ni powders

Article abstract of DOI:10.1111/j.1551-2916.2005.00582.x

A facile chemical route to prepare a well-defined Ni powder on a mass-production scale without external heating was suggested. A highly concentrated Ni complex was formed between NiCl2 and N2H4 solution, and subsequent addition of NaOH solution triggered serial reactions from the Ni complex decomposition via Ni(OH)2 formation to the reduction of a Ni(OH)2 gel into well-defined Ni spheres. The high temperature necessary for the reaction was attained not by external heating but by exothermic reactions such as complex formation and reduction. The preparation of monodisperse and spherical Ni powder was attributed to uniform distribution of the reducing agent (N2H4) within the Ni(OH)2 gel.

Full text of DOI:10.1111/j.1551-2916.2005.00582.x

J. Am. Ceram. Soc., 88 [11] 3020–3023 (2005)
DOI: 10.1111/j.1551-2916.2005.00582.x
r 2005 The American Ceramic Society
ournal
J
A Chemical Route to Large-Scale Preparation of Spherical and
Monodisperse Ni Powders
w
Jae-Young Choi, Yong-Kyun Lee, Seon-Mi Yoon, Hyun Chul Lee, Byung-Ki Kim, and Jong Min Kim
Materials Laboratory, Samsung Advanced Institute of Technology, Suwon 440-600, Korea
,w
Kang-Min Kim and Jong-Heun Lee*
Division of Materials Science & Engineering, Korea University, Seoul 136-701, Korea
A facile chemical route to prepare a well-defined Ni powder on a
mass-production scale without external heating was suggested.
A highly concentrated Ni complex was formed between NiCl2
and N H solution, and subsequent addition of NaOH solution
([NiCl ] 5 1.3–1.9M) and N H aqueous solutions, (b) the for-
2 2 4
mation of a condensed Ni(OH) gel with a uniform N H dis-
2 2 4
tribution by adding a NaOH solution, and (c) the reduction of a
Ni(OH) gel into well-defined Ni particles via a dissolution–
2
4
2
triggered serial reactions from the Ni complex decomposition
via Ni(OH) formation to the reduction of a Ni(OH) gel into
reduction–reprecipitation process. The high temperature for the
reaction was attained not by external heating but by exothermic
reactions such as complex formation and reduction.
2
2
well-defined Ni spheres. The high temperature necessary for the
reaction was attained not by external heating but by exothermic
reactions such as complex formation and reduction. The prep-
aration of monodisperse and spherical Ni powder was attributed
to uniform distribution of the reducing agent (N H ) within the
II. Experimental Procedure
2
4
NiCl
2
ꢁ 6H
2
O (Chemical Pure, Incheon Chemical Co., Ltd.,
Ni(OH) gel.
2
Incheon, Korea), 80% N H ꢁ H O (Extra pure, Dae Jung Chem-
2
4
2
ical & Metal Co., Ltd., Shiheung, Korea), and 50% NaOH
aqueous solution (Chemical Pure, Dae Jung Chemical & Metal
Co., Ltd.) were used as the source materials. During the entire
reaction, the temperature of the stock solution was measured at
short intervals (1 s) using a computer-interfaced thermocouple
data logger (TLOG 1000, Carvolab Co., Ltd., Bucheon, Korea).
For controlling the size of the Ni particles, three different
NiCl /N H /NaOH molar ratios, 1.35/4.5/3.6, 1/4/4, and 1/12/12,
I. Introduction
MULTILAYER ceramic capacitor (MLCC) is one of the most
representative passive components for highly integrated
A
1
–4
electronic and telecommunication devices. The key concept
is a large capacitance in a small volume, which is realized by the
alternate stacking of BaTiO -based dielectric and Ni-based elec-
3
2
2
4
trode layers up to several hundred layers. For the downsizing of
MLCC or for achieving an ultra high capacitance, a Ni layer as
well as an active layer need to be competitively being thinned
down to a submicrometer scale, which requires a well-defined
were used. The following is the procedure for preparing the NiCl
2
/
2 4
N H /NaOH 51.35/4.5/3.6. The other reactions showed a similar
behavior except for small variations in the temperature profile
of the solution. First, the solutions were prepared by dissolving
1
,5,6
nanoscale Ni powder sized from 50 to 200 nm.
The Ni powder for MLCC can be prepared via the vapor
phase method, spray pyrolysis, and solution chemical routes
3
21.26 g of NiCl
water. With vigorous stirring, 282.2 g of 80% N
(4.5 mol of N ) was poured into the solution instantaneously,
2
ꢁ 6H
2
O (1.35 mol of NiCl
2
) in 375 g of distilled
7
8,9
2
H
4
2
ꢁ H O
1
0,11
12–16
H
2 4
such as polyol processes
and reduction in an aqueous
or
1
7
which resulted in a pale violet precipitate. The precipitate on
the wall was washed with additional distilled water (50 g) for
B60 s. The solution temperature increased up to B651C by the
exothermic formation of a Ni complex between NiCl and N H .
non-aqueous solution. Among these methods, reduction in an
aqueous solution is a cost-effective and promising approach for
preparing crystalline, spherical, and monodisperse Ni powders
by the reduction of a complex between a Ni salt (NiCl , NiSO ,
2
2
4
2
4
1
2–16
When the solution temperature became 501C by cooling in an
ambient atmosphere, 288 g of a 50 wt% NaOH solution (3.6 mol
of NaOH, the temperature of NaOH solution 5201C) was
poured instantaneously into the stock solution. The solution tem-
perature initially decreased and then increased spontaneously up
to the second maximum (B541C), after which it decreased again
to room temperature. The reaction was complete 1 h after the
temperature reached the second maximum (541C). The black Ni
precipitate was washed six times with distilled water and dried at
room temperature for 16 h under constant ventilation. For all the
experimental conditions, approximately 100% of the Ni source
solution was successfully converted into Ni particles. In order to
investigate the reaction intermediates, the precipitate in the solu-
tion was sampled and analyzed by X-ray diffraction (X’pert, Phil-
lips, Eindhoven, the Netherlands). The size and morphology of Ni
particles were observed by a scanning electron microscope (SEM,
Model S-4300, Hitachi, Tokyo, Japan).
Ni(CH COO)
3
2
)
2 4
and a reducing agent (N H ) in the pres-
ꢀ
ence of OH ions. This process is expected to replace the current
vapor phase method. However, because a dilute solution
2
1
(
[Ni ]r0.1M) is generally used, not only is large-scale produc-
tion difficult but the stock solution needs to be heated up to 501–
2
complex for eventual reduction into Ni.
001C in order to induce the decomposition reaction of the Ni
12–16
Moreover, when
the reaction volume is large, the external heating of the stock
solution establishes a spatially heterogeneous temperature pro-
file, which tends to produce polydisperse particles.
This study proposes a new wet-chemical method for prepar-
ing spherical and monodisperse Ni powder on a large scale
without external heating in the following sequences: (a) the
Ni complex formation between highly concentrated NiCl2
N. Dudney—contributing editor
III. Results and Discussion
Manuscript No. 20216. Received February 24, 2005; approved May 9, 2005.
*Member, American Ceramic Society.
Authors to whom correspondence should be addressed. e-mail: jaeyoung88.choi@
samsung.com; jongheun@korea.ac.kr
w
The evolution of the solution temperature, solution color, and
the crystalline phase of the reaction intermediate during the re-
3
020

November 2005
Large-Scale Preparation of Spherical and Monodisperse Ni Powders
3021
D
E
F
G
100
A
Pouring
of 50%
NaOH
8
6
4
2
0
0
0
0
0
Solution
(
Tsol=20°C)
A
@
50°C
F
G
B
E
C
N H H O
solution
2
4
2
D
0
1000
2000
3000
Time (sec.)
Fig. 1. Evolution of the solution temperature, solution color, and the crystalline phase of the reaction intermediate during the reaction for the solution
2
2 4
with the molar ratio of NiCl /N H /NaOH 51.35/4.5/3.6.
action was investigated when a source solution with a molar
ratio of NiCl /N /NaOH 5 1.35/4.5/3.6 was used (Fig. 1). By
pouring 80% N O solution into a 1.9M NiCl aqueous
ꢁ H
solution, a Ni complex between NiCl and N (pale violet
precipitate) was formed actively and the solution temperature
increased because of an exothermic reaction. After B2 min the
During the initial 1.5 min after a 50% NaOH solution
(T 5 201C) was poured into the stock solution (T 5 501C) (point
B), the color of the solution changed rapidly from violet via blue
to green. Note that the rather abrupt temperature decrease just
after B is because of the introduction of the cold NaOH solu-
tion. (T 5 201C) Subsequently, until point D, the solution
temperature decreased gradually to B401C for 10.5 min, and
the color of the solution increasingly darkened to black. From
point D, the solution temperature increased again spontaneous-
ly until the second maximum (point F, B541C) for 14.5 min,
after which it decreased as a result of heat dissipation.
2
2 4
H
H
2 4
2
2
2
2 4
H
pouring of N
2
H
4
ꢁ H
2
O, the temperature reached a maximum
(
B651C) (point A), and then decreased gradually as a result
of the dissipation of the heat generated to the ambient. The
z
3 6 2
complex phase was identified as a mixture of [Ni(NH ) ]Cl
and [Ni(N H ) ]Cl . (inset A in Fig. 1) Considering the molar
2 4 2 2
y
2
1
ratio ([N H ]/[Ni ] 5 4.5/1.35 5 3.33) used, Ni[(N H ) ]Cl is a
2
The detailed reaction mechanism from B to F is currently un-
der investigation. However, the decrease in NH or N H radicals
3 2 4
4
2
4 3
2
1
8
probable complex species.
in the Ni complex can be deduced from the change in the color of
the solution from a short to long wavelength (from pale violet
NiCl2 þ 3N2H4 ! ½NiðN2H4Þ ꢂCl2
(1)
3
8
ꢀ
via blue to green). This suggests that the supply of OH pro-
motes the decomposition of the Ni complex. Another factor that
needs to be considered is the temperature of the stock solution at
the point of NaOH addition. The reaction took B24 h to com-
plete when a 50% NaOH solution (T 5201C) was poured into
the low-temperature stock solution (To301C). At a higher tem-
perature the stock solution resulted in a shorter reaction time.
For the reaction to be complete within 6 h, the temperature of
stock solution at the point of NaOH addition needs to be
4401C. The high solution temperature (601–701C) can be at-
tained without external heating only when the Ni complex be-
tween the highly concentrated NiCl and N H is actively formed
1
9
However, it is known to be less stable than Ni[(N
2
H
4
)
2
2
]Cl .
Therefore, it can decompose into Ni[(N
release of N H .
2
H
4
)
2
]Cl
2
with the
2
4
½
NiðN2H4Þ ꢂCl2 ! ½NiðN2H4Þ ꢂCl2 þ N2H4
(2)
3
2
The [Ni(NH ) ]Cl was reported to appear when [Ni(N H ) ]Cl
3
6
2
2
4 2
2
2
0
is heated to a high temperature. Accordingly, Ni[(N
is believed to be transformed partly into [Ni(NH ]Cl
temperature conditions (B651C) according to the following re-
2
4 2
H ) ]Cl
2
3
)
6
2
at high-
20
action :
2
2
4
in a short time. Therefore, high concentrations of the source ma-
terials are an important driving force for auto-thermal reactions.
The decomposition of a Ni complex in a highly alkalic
environment (pH 5 B13) will result in the formation of
½
NiðN2H4Þ ꢂCl2 þ 5=2N2H4
2
(
3)
!
½NiðNH3Þ ꢂCl2 þ 3=2N2ðgÞ "
6
1
3,15
z
Ni(OH)
2 4
and it can be reduced by N H from the follow-
Joint Committee on Powder Diffraction Standards (JCPDS), Swarthmore, PA
now International Centre for Diffraction Data (ICDD), Newtown Square, PA)
Card No. 76-1842).
2
1
2,21
(
(
ing reaction
:
yJoint Committee on Powder Diffraction Standards (JCPDS), Swarthmore, PA
now International Centre for Diffraction Data (ICDD), Newtown Square, PA)
Card No. 28-0695).
(
(
2þ
Ni þ N2H4 þ 4OHꢀ ! 2Ni þ N2 þ 4H2O
2
(4)

3
022
Journal of the American Ceramic Society—Choi et al.
Vol. 88, No. 11
Fig. 2. Scanning electron micrographs of the reaction intermediates (a) at point D in Fig. 1, (b) at point E in Fig. 1, and (c) at point F in Fig. 1, and (d),
e), and (f) their schematic diagrams illustrating the formation of spherical Ni particles from uniform nano-scale Ni(OH) precipitates.
(
2
2 2 4
Fig. 3. Scanning electron micrographs of the spherical Ni particles prepared at (a) NiCl /N H /NaOH5 1.35/4.5/3.6 (average diameter 5203 nm), (b)
2 2 4 2 2 4
NiCl /N H /NaOH5 1/4/4 (average diameter 5150 nm), and (c) NiCl /N H /NaOH 51/12/12 (average diameter 5 87 nm).
The evolution of the phase and morphology was verified by
sampling the precipitate during the reaction (insets D, E, F, and
G in Fig. 1, and SEM photos in Fig. 2). At point D, the Ni
complex was completely transformed into a condensed Ni(OH)2
gel composed of uniform nanoscale precipitates (inset D in
Fig. 1, and Fig. 2(d)), which was confirmed by TEM. The ir-
matrix of Ni(OH) precipitates. This is analogous to the process
2
known as the ‘‘gel–sol’’ method to prepare monodisperse oxide
powders from a condensed metal hydroxide gel, which was sug-
2
2
23
gested by Sugimoto et al. and Muramatsu and Sugimoto The
core concept is that the solutes in a condensed metal hydroxide
gel are dissolved and subsequently recrystallize as a monodis-
perse metal or metal oxide sol. Agglomeration can be avoided
effectively because the precursory solid network plays the role of
a protective matrix against the coagulation of subsequently
regular morphology of the Ni(OH)
Fig. 2(a)) was caused by agglomeration during the drying proc-
ess. When Ni becomes predominant over Ni(OH) (inset E in
Fig. 1), spherical particles with a uniform size (B200 nm) and a
number of nanoscale Ni(OH) precipitates (Fig. 2(e)) appear,
2
shown in the SEM photo
(
2
21
formed particles. In this study, the Ni ions, which were dis-
solved from a condensed Ni(OH) gel, are thought to recrystal-
2
2
which could be observed as the small non-spherical particles af-
ter the drying process (Fig. 2(b)) decreased significantly. This
suggests that the spherical Ni particles form at the cost of a
uniform Ni(OH) matrix. Finally, the remaining Ni(OH) pre-
cipitates had completely disappeared as a result of further re-
duction into Ni (insets F and G in Fig. 1 and Figs. 2(c) and (f)),
which resulted in spherical and monodisperse Ni particles. X-ray
photoelectron spectroscopy (XPS) analysis on sputtering of the
lize into spherical and non-agglomerated Ni particles via the
reduction reaction (4). This gel–sol conversion mechanism in
this study strongly suggests that a well-defined and non-agglom-
erated Ni powder can be prepared on a mass-production scale.
Indeed, this idea was confirmed by the preparation of B700 g of
monodisperse Ni powders using a 20 L reactor via a one-pot
reaction.
2
2
The manipulation of the Ni particle size is another important
concern in the fabrication of a highly integrated MLCC. There-
surface of Ni particles confirmed that the small amount of
(
ꢀ
OH) ion presents only at the surface. The key reasons why
fore, the Ni powders were prepared from various NiCl /N
2
NaOH ratios. Figure 3 shows the Ni powders prepared at NiCl
2
H
4
/
/
the Ni(OH)
but by the decomposition of a Ni complex could be the uni-
form distribution of the reducing agent (N ) throughout
the Ni(OH) matrix as a result of complex decomposition and
2
phase was prepared not by simple precipitation
2
N H /NaOH5 1.35/4.5/3.6, 1/4/4, and 1/12/12. The average
4
2
H
2 4
particle sizes were 203, 150, and 87 nm, respectively. The par-
ticle sizes tended to decrease with increasing N /NiCl (3.3, 4,
12 in molar ratio), which can be explained by the promotion of
2
H
2 4
2
the molecular-scale uniformity of the reaction temperature as a
result of the chemical reaction-induced self-heating. This will
lead to uniform and homogenous nucleation of spherical Ni
particles.
9
Ni nucleation as a result of the enhanced reduction rate. This
shows that the particle size can be controlled easily by changing
the reaction conditions.
As stated above, the monodisperse, spherical, and non-
agglomerated Ni particles were prepared from a homogenous
The serial reactions from Ni-complex decomposition via
Ni(OH) formation to the reduction of Ni(OH) into spherical
2
2

November 2005
Large-Scale Preparation of Spherical and Monodisperse Ni Powders
3023
5
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3
Ni particles were triggered by the addition of 50% NaOH
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J.-Y. Lee, S.-H. Hong, J.-H. Lee, Y. K. Lee, and J.-Y. Choi, ‘‘Uniform Coating
(
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tion at T 5 501C was poured instantaneously but there was no
significant change in the size and morphology. This means that
the temperature of the NaOH solution is not a key parameter.
However, the Ni powder showed a polydisperse size distribution
when the 50% NaOH solution (T 5 201C) was not poured
within a few seconds was dripped for B60 min. This could be
attributed to the overlapping of three sequential reactions by the
gradual addition of the NaOH addition. Therefore, the abrupt
addition of the NaOH solution is effective in separating the nu-
cleation and growth process, which is a prerequisite for prepar-
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6
3
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7
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8
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9
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0
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´ ´
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IV. Conclusion
12
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A new and simple chemical route for preparing a well-defined Ni
powder on a mass-production scale without external heating
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13
Nickel Ultrafine Powder and Crystalline Film by Chemical Control Reduction,’’
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14
highly concentrated NiCl
2
and N H
2 4
solutions not only provid-
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15
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‘
16
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2
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`
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2
H
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) within a Ni(OH)
2
17
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2 4
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18
21
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