Deposition of nickel nanoparticles onto aluminum powders using a modified polyol process

Article abstract of DOI:10.1016/j.materresbull.2008.03.028

Aluminum is commonly used as fuel additive for propellants. The main limitation to its use lies in comparatively slow ignition and oxidation/combustion kinetics. Combustion performance of aluminized propellants can be improved through the use of Ni-coated

Full text of DOI:10.1016/j.materresbull.2008.03.028

Materials Research Bulletin 44 (2009) 95–99
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Deposition of nickel nanoparticles onto aluminum powders using
a modified polyol process
b
b
b
J.L. Cheng a, H.H. Hng a, , H.Y. Ng , P.C. Soon , Y.W. Lee
*
a School of Materials Science & Engineering, Division of Materials Science, Nanyang Technological University, Singapore 639798, Singapore
b DSO National Laboratories, 20 Science Park Drive, Singapore 118230, Singapore
A R T I C L E I N F O
A B S T R A C T
Article history:
Aluminum is commonly used as fuel additive for propellants. The main limitation to its use lies in
comparatively slow ignition and oxidation/combustion kinetics. Combustion performance of aluminized
propellants can be improved through the use of Ni-coated Al particles. Nanoparticles, with its increased
reactivity, also improve combustion performance. Hence, in this work, nano-sized Ni particles coated
onto commercially available micron-sized Al powders using a modified polyol process were synthesized
and evaluated. Ni-coated Al powders of various compositions produced by this method showed
significant improvement in the oxidation kinetics as compared to untreated Al powders. The onset
oxidation temperatures for the Ni-coated Al powders were found to be significantly reduced as compared
to pure untreated Al. Other than the oxides, the intermetallic compound, Al3Ni was also detected in the
10 wt% Ni–Al powders that were heated to temperature above 1050 8C.
Received 13 December 2007
Received in revised form 3 March 2008
Accepted 27 March 2008
Available online 8 April 2008
Keywords:
A. Metals
B. Chemical synthesis
C. Differential scanning calorimetry (DSC)
C. Thermogravimetric analysis (TGA)
ß 2008 Elsevier Ltd. All rights reserved.
1. Introduction
In this work, we attempt to study the effects of combining these
two factors to produce Al powders coated with nano-Ni particles.
Currently, aluminum is widely used as high-energy fuel in areas
such as propellants. However, including Al in fuel compositions is
accompanied by a number of undesired features. A critical problem
is the ignition of Al particles. Due to the protective oxide (alumina)
layer on the particle surface, Al typically ignites in oxidizing
atmosphere only after heating up to temperatures close to the
melting point of alumina at 2300 K [1]. In addition, agglomeration
of Al particles in the propellant surface layer also leads to
incomplete burning of Al particles and promotes slag formation
[2]. All these adverse effects result in the limited realization of the
energy potential of the fuel. Previous studies have shown that the
use of Ni-coated Al particles improves the combustion perfor-
mances of aluminized propellants. Coating Al particles with a thin
(10–100 nm) film of nickel was found to reduce particle
agglomeration during metallized propellant combustion [3,4]. In
addition, Ni-coated Al particles are observed to ignite easier than
original Al particles, and the ignition temperature in Ni–Al system
is close to the melting point of Al [5–7]. A recent report also found
that the addition of nano-Ni powders to Al-based composite
propellant effectively raised the propellant’s burning rate and
improve its combustion characteristics [8].
The nano-sized Ni particles will be synthesized using a modified
polyol process using hydrazine as reducing agent [9,10], and
deposited onto commercially available micron-sized Al particles at
the same time. The aim of this work is to determine whether the
combination of nano-particles and Ni coating will improve the
combustion behavior of Al powders through thermal analysis
studies.
2. Experimental procedures
Three sets of Al–Ni samples with bulk compositions of 5, 10 and
30 wt% Ni were prepared. The nominal composition of the powders
was determined from the starting precursor compositions.
Nickel(II) acetate tetrahydrate (Ni(Ac)2ꢀ4H2O) supplied from
Sigma–Aldrich was dehydrated in an oven at 120 8C for 5 h prior
to the experiment. The whole experiment consists of mainly two
stages. In the first stage, the required amount of Ni(Ac)2 and
sodium hydroxide (NaOH supplied from Sigma–Aldrich in pellets
form) were suspended in 250 ml of ethylene glycol. Ethylene glycol
acts both as a solvent and as a particle surface protective agent to
prevent severe agglomeration of the particles. The mixture was
then heated up to 60 8C for 25 min and stirred using a mechanical
stirrer. In the second stage, Al powders (99.5% purity supplied by
Alfa Aesar of 325 mesh; average particle size 44–50
mm) was
* Corresponding author. Tel.: +65 67904140; fax: +65 67909081.
E-mail address: ashhhng@ntu.edu.sg (H.H. Hng).
added into the mixture followed by the addition of hydrazine
0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2008.03.028

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J.L. Cheng et al. / Materials Research Bulletin 44 (2009) 95–99
hydrate (N2H4ꢀH2O supplied by Sigma–Aldrich equivalent to 64%
hydrazine). This stage involves the nucleation and growth of the
nickel particles from the ethylene glycol solution and deposited
onto the Al particles; or by nucleating from the activation sites of
the Al particles itself; or a combination of both. This process was
allowed to proceed for 120 min at 60 8C. At the end of all reactions,
the suspension was cooled to room temperature and the residue
was separated from the solution and washed with acetone and
ethanol several times. The collected powders were then dried in an
oven at 80 8C for 12 h. All the chemicals used throughout the study
were reagent grade in purity.
The modified polyol process involves the following steps: (1)
formation of Ni(OH)2; (2) dissolution of the Ni(OH)2; (3) reduction
of the dissolved Ni(OH)2 by hydrazine; (4) nucleation and growth
of the nickel particles from the ethylene glycol solution onto the
surfaces of the Al particles. The reactions took place according to
the following equations:
Fig. 1. SEI micrographs of as-prepared Ni particles.
NiðAcÞ2 þ 2NaOH ! NiðOHÞ2 þ 2NaAc
(1)
Al particles. The coating process is proposed to occur not only by
pure adhesion of Ni nanoparticles onto the Al surface but also
nucleation of Ni particles from the Al particles itself. Upon reaching
super saturation, precipitation of nickel particles occurred and the
presence of Al particles acts as sites for heterogeneous nucleation
of nickel to take place. The coating mechanism is proposed to occur
more favorably at the grain boundaries since thermodynamically,
lower activation energy is required. However, it is also possible for
homogenous precipitation of nickel to occur in the solution and to
subsequently physical adhere to the surface. During the process,
the solution was mechanically stirred at 400 rpm. This mechani-
cally stirring process can generate new grain boundaries and
defects which can enhance the nucleation of Ni particles from Al.
This can eventually lead to a more uniform coating. EDX elemental
mapping was also done to further confirm that a fairly uniform
coating was achieved. However, it should be noted that the Ni
coating is not in the form of a uniform shell. There will still be some
portion of Al particle that are exposed and an oxide layer will still
exist in regions where there is no Ni coating.
NiðOHÞ2 þ ð1=2ÞN2H4 ! Ni þ ð1=2ÞN2 þ 2H2O
(2)
This modified polyol method requires
a
low-processing
temperature of 60 8C. This is much lower as compared to
conventional polyol process which requires the reaction to occur
at the boiling temperature of polyol (ꢁ200 8C) [11,12]. Moreover,
the by-products (N2 and H2O) are environmentally friendly and can
be easily removed.
NaOH plays an important role in this process. The first role of
NaOH is to allow Ni(OH)2 to precipitate from the Ni(Ac)2. The
second role of the alkali is to provide sufficient alkalinity for the use
of the hydrazine as an effective reducing agent. Lastly, NaOH also
serves to remove some of the thin alumina layer present on the Al
surface before depositing the Ni nano-particles. NaOH may act as a
strong alkali and can help to ‘‘dissolves’’ away the alumina oxide
layer. The synthesis process was done in a flask reactor flushed
with a continuous flow of nitrogen. It was not done in an inert
condition thus aluminum is consistently being oxidized through-
out the process. The reaction involved is as shown:
XRD spectra of the coated powders of various bulk compositions
are shown in Fig. 3. All the diffraction peaks correspond to the pure
elemental Al and Ni phases. No additional peaks corresponding to
secondaryphaseswereobserved.Thisindicatesthatonlythecoating
of Ni onto Al particles took place, and no Al–Ni intermetallic phases
were formed during the synthesis process. It is also noted that the
diffraction peaks corresponding to Ni are broader than those
corresponding toAl. Thenickelparticles havean average diameterof
Al2O3ðsÞ þ 2NaOHðaqÞ þ 3H2OðlÞ ! 2NaþðaqÞ þ 2½AlðOHÞ4ꢂꢃðaqÞ
(3)
The powders obtained were characterized using various
techniques. A scanning electron microscope (SEM) (JEOL JSM-
6360A) was used to analyze the size and morphology of the
powders. The SEM is equipped with energy dispersive X-ray
spectrometer (EDX) that allows elemental analysis. The X-ray
diffraction patterns of the powders were obtained with a Shimadzu
ꢁ100 nm compared to Al of ꢁ50
mm. Hence, the difference in peak
broadness indicates the crystalline size difference between the
micron-sized Al particles and nano-sized Ni coating.
6000 X-ray Diffractometer (XRD) using a Cu target (lCu K
a
˚
= 1.5405 A). Differential scanning calorimetric (DSC) and thermo-
Thermal analysis was performed on the Ni-coated Al powders
using TGA and DSC. The study of the thermal behavior of the
powders in air provides a good indication of the combustion
behavior of the Al powders, since combustion is an oxidation
process. The onset temperature, defined as the temperature at
which the first oxidation commenced, can be taken to be the
temperature where a first sharp increase in weight gain is
registered in the TGA curve. This onset TG temperature can be
used to indicate the ease of ignition of the Al-based powders. TG
onset can serve as one of the parameters of measurement for the
reactivity in air as reported by Ilyin et al. [13]. It was also
mentioned that the TG onset could be used to reflect the reactivity
of Al-metal system. Hahma et al. [14] also made a study on the
ignitability of different Al-metal/organic coatings and concluded
that the lower the temperature of the first strong exotherm, the
better is the ignitability.
gravimeteric analysis (TGA) were done in purified air using the TA
instruments SDT 2960 Simultaneous DSC-TGA to study the
thermal properties of the samples. The sample was ramped from
room temperature to 1400 8C at a heating rate of 20 8C/min in
purified air.
3. Results and discussion
Ni particles synthesized in a blank run, without the addition of
the Al powders, is shown in Fig. 1. Nano-sized (ꢁ100–200 nm) Ni
particles, which were spherical and uniform in shape, were
obtained using the modified polyol process. With the addition of
the Al powders, Ni-coated Al powders were obtained, and the
morphology of the coated powders is shown in Fig. 2(a)–(c). The
nano-sized Ni particles are clearly seen deposited onto individual

J.L. Cheng et al. / Materials Research Bulletin 44 (2009) 95–99
97
Fig. 2. SEI and EDX images of (a) 5 wt% Ni–Al; (b) 10 wt% Ni–Al; (c) 30 wt% Ni–Al.
A comparison of the TGA curves obtained for the as-synthesized
Ni nanoparticles and a commercially available Ni powders (Alfa
The TGA curves, showing the respective sample mass increase
as a function of temperature, obtained for the pure untreated Al
and Ni-coated Al powders at a heating rate of 20 8C/min are shown
in Fig. 5(a) and (b). For pure Al powders, four distinct stages were
observed, and the oxidation mechanism is in accordance to that
proposed by Trunov et al. [15]. It was proposed that during stage 1,
from 600 to 650 8C, the process first involves the formation of the
Aesar, 100 mesh, particle size of ꢁ150
mm) is provided in Fig. 4. It is
obvious that the oxidation of the Ni particles synthesized using the
modified polyol process took place at a much lower temperature
(ꢁ317 8C) than that of the pure commercial Ni powders (ꢁ567 8C).
The early TG onset was due to the nano-sized powders having a
larger specific surface area and thus hasten the oxidation process.
It is also noted that there is an initial decrease in weight for the
polyol synthesized Ni sample. This is due mainly to the
vaporization of absorbed species such as moisture or other organic
species.
amorphous alumina and then conversion into the
g-Al2O3. The
thickness of the natural amorphous alumina layer increases and
when it exceeds the critical thickness of amorphous alumina layer
of 4 nm, it transforms into
increases rapidly at the onset and decreases as the
g
-Al2O3. The rate of this process
-Al2O3
g
Fig. 3. X-ray diffraction patterns of different wt% Ni-coated Al samples. Si was added
Fig. 4. TGA graphs of Ni nanoparticles prepared by modified polyol process and
as an internal reference.
commercially available Ni micron-sized particles.

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J.L. Cheng et al. / Materials Research Bulletin 44 (2009) 95–99
Table 1
Comparison of the TG onset of untreated Al powders, the different wt% Ni-coated Al
powders and pure Ni nanoparticles synthesized using the modified polyol process
Sample
TG onset (8C)
Untreated pure Al
607
417
403
347
317
5 wt% Ni-coated Al
10 wt% Ni-coated Al
30 wt% Ni-coated Al
As-synthesized Ni nanoparticles
For the Ni-coated Al powders, four distinct stages similar to
pure Al were still observed, except that the onset temperature of
the first oxidation process (stage 1) occurs at a much lower
temperature. For stage 1, it is the oxidation of nickel that is taking
place, and this occurs at temperatures ranging from 350 to 420 8C
for the Ni-coated Al powder samples. The onset is close to the value
observed for the pure Ni particles obtained in the blank run (Fig. 4).
The presence of the nano-sized nickel particles lowered the overall
onset of oxidation to a much lower temperature (stage 1) as
compared to pure untreated Al. Upon reaching stage 2, the Ni
particles were more or less being completely oxidized. This is
supported by the TGA graph for pure Ni particles in Fig. 4, showing
a plateau upon reaching ꢁ600 8C.
The onset TG temperatures obtained from the TGA curves for all
the samples are summarized in Table 1. It is observed that the TG
onset for Ni-rich samples (30 wt% Ni) is lower than the Al-rich
samples (5 and 10 wt% Ni). Oxidation of nano-sized Ni particles
occurred at a much lower temperature as compared to the micron-
sized Al particles. With increasing Ni content, oxidation of Ni
becomes the dominating process compared to the conversion of
amorphous Al2O3 to
onset.
g-Al2O3 during this stage thus lowering the TG
Fig. 5(c) shows the DSC graphs of the different composition of
Ni-coated Al powders together with the untreated Al powders. It is
observed that the powders first undergo a solid-state energy
released which corresponds to the stage 1 oxidation shown in the
TGA graphs. It is noted that the energy released for the Ni-rich
sample (30 wt% Ni) is much higher than the Al-rich samples (5 and
10 wt% Ni) for this exotherm. The larger enthalpy produced in the
first exotherm is mainly due to the oxidation of Ni to NiO which is
the main oxidation step as compared to the oxidation of Al in Al-
rich samples. This is in good agreement with a higher rate of
oxidation for Ni-rich samples in the stage 1 observed in the TGA
graphs. Oxidation of Ni occurs at a faster rate compared to Al due to
its nano-size, which contributes to larger effective surface area.
Moreover, oxidation of Al is hindered by the formation of the non-
porous oxide layer. Thus, the ease of Ni oxidation contributed to
the larger enthalpy released shown in the first exotherm.
The second exotherm corresponds to stage 3 shown in the TGA
curves. This stage mainly involves the breakage of the alumina
layer followed by the rapid oxidation of the exposed active Al. The
general oxidation enthalpy of the second exotherm is observed to
be larger in the Al-rich powders (5 and 10 wt% Ni) than the Ni-rich
(30 wt% Ni) powders. This is mainly due to the fact that Al has a
higher specific enthalpy than Ni. However, it is interesting to note
that the 10 wt% Ni-coated Al sample has the largest enthalpy
during this stage, although the Al content is lower as compared to
the 5 wt% Ni sample. This is also consistent with the TGA results
shown in Fig. 5(b) whereby the 10 wt% Ni sample has the fastest
rate of oxidation during this stage. A possible explanation for this is
that the 10 wt% Ni-coated Al powders have the most optimum
amount of coating among the samples. This may reduce
agglomeration of the melted Al, and thus contributing to more
active Al for combustion. Hence, it can be concluded that the
Fig. 5. TGA graphs of (a) pure untreated Al powders and (b) different wt% Ni-coated
Al powder samples; (c) DSC curves of different wt% Ni-coated Al powder samples.
becomes thicker and more continuous. This process corresponds to
the oxide skin on the particles thickening and provides a kinetic
barrier to further weight gain. Stage 2 (650 8C to about 800 8C) is
mainly dominated by the oxidation of g-Al2O3 and is distinguished
by a flat plateau with minimal weight change as oxidation is
relatively slow and limited by the diffusion of oxygen atoms
through the thick Al2O3 layer. The oxidation rate increases rapidly
at the onset of stage 3 due to the eventual cracking of the thick
alumina layer leading to rapid diffusion of oxygen through the
cracks towards the Al/Al2O3 interface. A sudden increase in
oxidation of Al leads to a rapid weight change during this stage.
This layer of Al2O3 is characterized as the
Finally, stage 4 is observed for the transition from
stable -Al2O3. This process results in an abrupt reduction in the
oxidation rate due to the formation of significantly denser and
coarser crystallites of -Al2O3, which reduces the rate of oxygen
diffusion and resulting in slower alumina growth rate.
u
-Al2O3 polymorph.
u-Al2O3 to the
a
a

J.L. Cheng et al. / Materials Research Bulletin 44 (2009) 95–99
99
solid-state diffusion of Ni atoms in Al according to the following
equation:
3Ni þ Al ! NiAl3
However, this temperature can only be used as a reference as it
uses a different synthesis method; combustion synthesis to
produce the intermetallic.
Rietveld analysis was done on the XRD pattern for the sample
heatedto1200 8C. Thefinalcombustionproductconsistsof13.7 wt%
Al, 72.4 wt%Al2O3, 7.0 wt%NiOand6.9 wt%Al3Ni. Thebulkweightof
the combustion product was mainly contributed by the
a-phase
alumina with the intermetallic; NiAl3 forming the minor phase.
4. Conclusions
This work investigates the deposition of nanocrystalline nickel
particles on micron-sized Al powders using a modified polyol
process. Different compositions of Ni-coated Al powders were
successfully synthesized. The oxidation kinetics of the coated
powders was significantly improved as compared to untreated Al
powders. Other than the oxides, the intermetallic Al3Ni phase was
also detected in the combustion products heated to 1050 8C and
above.
Fig. 6. X-ray diffraction patterns for the 10 wt% Ni–Al post-TGA samples at different
temperature demonstrating the formation of the intermetallic product, NiAl3 from
1050 8C onwards.
enthalpy of the second exotherm depends not only on the
composition but also the amount of Ni coating onto the Al
powders.
In order to get a better understanding on the phase changes
upon heating, XRD analyses were also performed on the samples
heated at various temperatures to determine the combustion
products of the reaction. The temperatures were chosen based on
the TGA results. The temperatures chosen are the onset of mass
increase; point of inflection and the point whereby the final
plateau has reached (where oxidation was more or less com-
pleted). The samples at the selected temperatures were prepared
by heating the powders at room temperature to the temperature
required at a heating rate of 20 8C/min in the DSC followed by
subsequent cooling to room temperature and analysis using the
XRD. Fig. 6 shows the X-ray diffraction patterns of the 10 wt% Ni–Al
sample obtained at the various different temperatures. Prior to
reaching 600 8C, all the peaks correspond to the as-synthesized
powder. No additional phases were detected Diffraction peaks
corresponding to NiO phase were only detected in samples heated
to 600 8C and above. However, the onset of oxidation as
determined from TGA occurred at a much lower temperature.
This difference may be due to the formation of the amorphous
phase NiO first prior to the formation of the crystalline NiO phase.
Further thermal treatment to 1050 8C and above resulted in the
Acknowledgements
The authors thank the Defence Science and Technology Agency
(Singapore) and DSO National Laboratories for funding and support
given to this project.
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a-phase aluminum
oxide and an intermetallic compound, Al3Ni. According to Philpot
et al. [16], the reaction proceeds at around 640 8C and involves the