1. RESEARCHARTICLE
Copyright © 2006 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 6, 558–561, 2006
Novel Growth of Aluminium Nitride Nanowires
M. Radwan∗
and M. Bahgat
Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87 Helwan, Cairo 11421, Egypt
This work describes novel growth of aluminium nitride (AlN) nanowires by nitridation of a mixture
consists of aluminium and ammonium chloride powders (Al:NH4Cl = 1.5:1 weight ratio) at 1000 C
for 1 h in flowing nitrogen gas (1 l/min). XRD analysis of the product showed the formation of pure
hexagonal AlN. SEM micrographs of as-synthesized product revealed the growth of homogeneous
AlN nanowires ( 40–150 nm). No droplets were observed at the tips of obtained nanowires which
suggests that they were grown mainly by a vapor-phase reactions mechanism. Thermodynamic
analysis of possible intermediate reactions in the operating temperatures range illustrates that these
nanowires could be grown via spontaneous vapor-phase chlorination-nitridation sequences.
Keywords: AlN, III-V Nitrides, Nanowires, Nitridation, Vapor-Phase Growth.
1. INTRODUCTION
Aluminium nitride (AlN) is a III–V nitride-based semi-
conductor with unique physical properties such as wide
bandgap, high thermal conductivity, high volume resistiv-
ity, low dielectric constant and a thermal expansion coeffi-
cient matches that of silicon.1–3
It is considered as an ideal
substrate and packaging material potential for advanced
electronic and optoelectronic devices. AlN is also known
as interesting technical ceramics with good chemical sta-
bility and high hardness and used in various structural and
refractory composites applications.4
Many efforts have been spent on the development of
fibrous materials (whiskers, wires, pillars, etc.) because of
their crystalline perfection and small dimensions which
offers superior physical and chemical properties funda-
mentally differs from their corresponding macro or bulk
materials.5
The fibers of AlN have attracted much atten-
tion and there are several methods reported in the literature
for their growth including (i) evaporation-condensation
using Al6
or AlN7–11
as source materials in high-purity
nitrogen gas flow, (ii) chemical vapor deposition using
aluminium halide-ammonia complexes at temperatures of
750–1000 C,12 13
(iii) carbothermic reduction nitridation
of alumina at about 1800 C in flowing nitrogen gas in
presence of catalysts14–16
or recently without catalyst,17–18
(iv) flux growth at lower temperatures,1 19
(vi) direct
nitridation of aluminium powder with aluminium chlo-
ride promoter20–22
or alumina template23
at temperatures
of 1100–1250 C in flowing N2/NH3 gas, and (v) the
self-propagating high-temperature synthesis of metallic Al
powder under pressurized nitrogen gas in presence of
∗
Author to whom correspondence should be addressed.
additives.24
However, it is still difficult to state the tech-
nique reasonable for the fabrication of homogeneous AlN
fibers. The current drawbacks are the application of high
temperatures (such as in methods i and iii), starting with
complex (method ii) or expensive (method iv) precursors,
or inconveniency for scaling up (methods ii and v). On
our experiments to prepare pure AlN nanopowders by the
simple direct nitridation method from metallic Al pow-
der (180 micron in size) in a tube furnace at a relatively
low temperature (1000 C) in flowing N2 gas, we acci-
dentally observed the growth of unique particles-free AlN
nanowires when an amount of ammonium chloride was
blended with Al reactant. It has been published previ-
ously that the addition of NH4Cl to starting Al powder
enhances the nitridation rate and can promote the forma-
tion of nanocrystalline AlN powders25 26
but it has not
been reported before to enhance the growth of fibrous
forms. The following sections describe the characteris-
tics of produced AlN nanowires. A proposed growth
mechanism supported by a thermodynamic analysis will
be stated.
2. EXPERIMENTAL DETAILS
The direct nitridation experiments were carried out in a
porcelain boat (8 cm long) set in the center of an alu-
mina tube (3 cm inner diameter and 100 cm long) mounted
in a horizontal electric-resistance furnace. The starting
materials were commercially available aluminium pow-
der with about 99% purity and an average particle size
of 180 micron, and a chemical-grade ammonium chloride
powder with minimum assay of 99%. They were mixed
(Al/NH4Cl = 1.5/1 weight ratio) manually in agate mortar.
About 1 g of loose powder mixture was put into the boat
558 J. Nanosci. Nanotechnol. 2006, Vol. 6, No. 2 1533-4880/2006/6/558/004 doi:10.1166/jnn.2006.102

2. RESEARCHARTICLE
Radwan and Bahgat Novel Growth of Aluminium Nitride Nanowires
and placed in the alumina tube. The system was flushed
with nitrogen gas for several minutes to remove any oxy-
gen and moisture. The nitrogen gas used was purified from
moisture by passing it through a silica gel tower. The fur-
nace was heated to 1000 C with a rate of 15 C/min
under nitrogen gas flow of 1 l/min and maintained for 1 h.
Then, the boat was drawn to the end of the tube outside
the heating furnace and kept for cooling down to room
temperature under the nitrogen atmosphere. The product
was observed visually and analyzed by X-ray diffraction
(XRD, BRUKER axc–D8 Advance) using Cu-K radia-
tion (40 kV/40 mA). Morphology of as-synthesized AlN
product was examined with scanning electron microscope
(SEM, JEOL-JSM-5410). Samples for SEM analyses were
coated with thin film of sputtered gold.
3. RESULTS AND DISCUSSION
The synthesized AlN product consists of loose powder
of white color. Figure 1 shows the XRD pattern of the
nitride product. It is seen that the product is a pure
hexagonal AlN phase with traces of aluminium metal.
The morphology of the as-synthesized AlN was investi-
gated by SEM. The product consists of particles-free AlN
nanowires homogeneously distributed allover the sample
as in Figure 2(a). Most of the nanowires are straight
although a variety of shapes such as kinks, branches and
twining-plant-like has been observed in the same sample
as shown in Figure 2(b–d). These wires have diameters
of 40–150 nm and large aspect ratios (length/diameter).
Some larger wires with a complicated shape have also
been found, Figure 2(e). The tips of all wires did not show
droplets which may suggest that these nanowires were
grown probably by a vapor-phase reactions mechanism.
2θ (degrees)
Intensity(arbitraryunits)
20 25 30 35 4540 50 55 60 65 70 75 80
AlN (≥ 96%)
Al (≤ 4%)
Fig. 1. X-ray diffraction pattern of the as-synthesized product.
(a) (b)
(c) (d)
(e)
Fig. 2. SEM micrographs of the as-synthesized AlN nanowires with
various shapes: (a) particles-free homogeneous nanowires, (b) kinks,
(c) branches, (d) twining-plant-like and (e) complicated structures.
The summary reaction of the direct nitridation of the
Al–NH4Cl mixture under flowing nitrogen gas can be
expressed as follows:
Al+NH4Cl+
1
2
N2 = AlN +NH3 +HCl
In which, the ammonium chloride plays a critical role on
the growing of AlN nanowires. The synthesis of these AlN
nanowires implies that vapor-phase spontaneous reactions
and intermediate volatile species should be involved.
During the nitridation experiments, we observed that
large white vapors were evolved after the temperature rea-
ched about 300 C. This suggests that the beginning reac-
tion will be the dissociation of ammonium chloride at a
low temperature into volatile ammonia and hydrogen chlo-
ride according to this reaction:
NH4Cl s = NH3 g +HCl g
Gaseous hydrogen chloride is very active and spontaneo-
usly reacts with the aluminium particles and the following
intermediate reaction may occur:
Al s l +3HCl g = AlCl3 g +
3
2
H2 g
The reaction system will contain many gaseous species
(NH3 g , HCl g , H2 g , and AlCl3 g ) and when the AlCl3 g
intermediate is mixed with the flowing nitrogen gas the
possible spontaneous vapor-phase nitridation reaction is:
AlCl3 g +
1
2
N2 g +
3
2
H2 g = AlN s +3HCl g
Under a critical (low) supersaturation condition the AlN
molecules will be condensed in the form of nanowires.
J. Nanosci. Nanotechnol. 6, 558–561, 2006 559

3. RESEARCHARTICLE
Novel Growth of Aluminium Nitride Nanowires Radwan and Bahgat
The above nitridation reaction regenerates gaseous
hydrogen chloride which can be seen as the key interme-
diate product essential to produce the volatile aluminium
chloride species and progress this chlorination-nitridation
growth mechanism of AlN nanowires.
There is another probable vapor-phase nitridation reac-
tion which is thermodynamically spontaneous:
AlCl3 g +NH3 g = AlN s +3HCl g
Although the nitridation by ammonia is much spontaneous
than by nitrogen, the major nitridation takes place by the
later one because this system yields nearly complete con-
version of the aluminium powder into AlN wires while the
amount of NH4Cl in the starting mixture is not enough
for that. Besides, large ammonia-based white vapors were
observed during the nitridation evolved from the system.
In the work of Lu et al. they utilized large amount of
ammonium chloride (Al:NH4Cl ≈1:6 weight ratio) and
heated the mixture in closed system which resulted in the
synthesis of nanocrystalline AlN powder (6 nm).27
Based
on our growth model we think that their condition had pro-
vided a high supersaturation environment which emerged
homogeneous nucleation in the vapor-phase and the con-
densation of their AlN nanopowders.
Figure 3 gives the Gibbs energy change of those inter-
mediate reactions in the operating temperature range, cal-
culated from the NIST–JANAF thermochemical data. It
shows that the thermodynamic calculations are consis-
tent with the above growth model in which the AlN
nanowires were grown by the ammonium chloride assisted
direct nitridation of an aluminium powder under a
–400
–300
–200
–100
0
100
200
300
400
0 200 600 1000
Temperature (˚C)
Gibbschange(kJ/mol)
h
g
f
e
d
c
b
a
400 800
Fig. 3. Gibbs energy change of possible intermediate reactions.
a. Al+3HCl=AlCl3+3/2H2; b. Al+1/2N2 =AlN; c. Al+NH4Cl+
1/2N2 =AlN+NH3+HCl; d. AlCl3+1/2N2+3/2H2 =AlN+3HCl;
e. AlCl3+NH3 =AlN+3 HCl; f. NH4Cl=NH3+HCl; g. AlCl3+
NH4Cl=AlN+4 HCl; h. AlCl3+1/2N2 =AlN+3/2Cl2.
flowing stream of nitrogen through spontaneous vapor-
phase chlorination-nitridation sequences. Growing of these
unique nanowires by this novel strategy will be of great
advantageous because it enables the fabrication of fine
wires from cheap reactants by a much reasonable nitrida-
tion condition compared to previous published reports.20 21
4. CONCLUSIONS
These results offer a new route for growing unique AlN
nanowires by the direct nitridation of aluminium powder
mixed with ammonium chloride (in 1.5:1 wt. ratio) under
isothermal heating at 1000 C for 1 h in flowing nitro-
gen gas stream (1 l/min). The grown wires are mostly
particles-free with nanometer dimensions (40–150 nm).
The growth model consists of sequences of chlorination-
nitridation intermediate reactions in the vapor-phase. At
a critical low supersaturation condition nanowires of AlN
were deposited. The summary reaction can be described
by the following reactions:
NH4Cl s = NH3 g +HCl g
Al s l +3HCl g = AlCl3 g +
3
2
H2 g
AlCl3 g +
1
2
N2 g +
3
2
H2 g = AlN s +3HCl g
Acknowledgments: M. Radwan wishes to thank
Professor Y. Miyamoto (JWRI, Osaka Univ.) for his
invaluable advice during the progress of this work.
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Received: 1 August 2005. Revised/Accepted: 4 August 2005.
J. Nanosci. Nanotechnol. 6, 558–561, 2006 561