1. Growth of Quasi-Aligned AlN Nanofibers by Nitriding
Combustion Synthesis
Mohamed Radwanw
and Yoshinari Miyamoto*,
**
Department of Nano/Micro Structure Control, Smart Processing Research Center, Joining and Welding Research
Institute, Osaka University, Osaka 567-0047, Japan
Quasi-aligned AlN nanofibers were formed by the nitriding
combustion synthesis according to a unique micro-reactor
model. A charge composed of aluminum and aluminum nitride
diluent powders (40/60 mol%) with a mixture of yttria and
ammonium chloride as additives (5 wt% each) was combusted
at low nitrogen gas pressures of 0.25 MPa. The FE-SEM
images of as-synthesized AlN product showed the formation of
ball-like grains (same shape and size as the original Al reactant)
that consisted of a thin surface nitride layer or crust cover
quasi-aligned AlN nanofibers grown in the interior. The cross-
sectional view is sea anemone like. Formation of this novel mor-
phology is believed to occur through a two-stage process. The
first one occurs at the preliminary stage of the combustion out-
side Al particles. After the ignition, the heat generated causes
the sublimation and dissociation of ammonium chloride into
various gaseous species. This effectively interrupts the combus-
tion and slows down the increase of reaction temperature. In
addition, yttria interacts with the native oxide layer present on
the surface of Al particles and forms a stable Al–N–Y–O crust.
The second stage begins by the infiltration of various gaseous
species such as HCl(g), NH3(g), and N2(g) through the crust into
the molten Al cores. The ‘‘crust–core’’ systems function as ‘‘mi-
cro-reactors’’ where both the nitridation and growth processes
occur inside. The molten Al cores are spontaneously halogenated
to AlCl3 vapors and the nitridation proceeds by the gas–gas
reaction of AlCl3 and NH3/N2 vapors. The AlN nanofibers
are then grown from the vapor phase quasi-aligned inside the
micro-reactors by VLS and VS mechanisms.
I. Introduction
DURING the past two decades, aluminum nitride (AlN) has
attracted considerable interest in the electronics industry
because it possesses an excellent combination of material
properties, including high intrinsic thermal conductivity (B320
W/mK), wide band gap (6.2 eV), high electrical resistivity
(41010
O Á cm), low dielectric constant (8.6), high mechanical
strength, chemical stability, and low thermal expansion coeffi-
cient (4.2Â 10À6
1C)À1
matches to both Si and GaN semicon-
ductors. AlN components and substrates are used for various
applications in power electronics (electrical engines), microelec-
tronics (LSI circuits, sensor carriers), naval radio and defense
systems, railway systems, aeronautical systems, and environ-
mental systems.1–3
Recently, the growth of one-dimensional (1-D) AlN nano-
structures such as nanowhiskers,4
nanowires,5–8
nanofibers,9
nanobelts,10
nano-pillars,11
nanoribbons,12
nanotubes,13–15
nanocones,16,17
nanotips,18
and nanorods19
has become the
focus of scientific research and proposed promising applications
such as field emitters, flexible pulse-wave sensors, and nanome-
chanical resonators.16,18
Several methods have been developed
for the preparation of 1-D AlN nanostructures, mostly by the
nitridation at elevated temperatures of 10001–13501C in con-
ventional horizontal-type tube furnaces. These methods can be
categorized as follows:
(i) A halide-assisted direct nitridation of starting charges
contains halide compounds such as Al/AlX3,4,12
AlX3,14
or
Al/NH4X7,9
where X: Cl, F, or I. The Al halide acts as a trans-
port agent and supports the nitridation in the vapor phase and
unidirectional growth by a vapor–solid (VS) mechanism.
(ii) A catalyst-assisted direct reaction of metallic Al
and NH3/N2. In general, a small amount of a metallic catalyst
(0.5–1.0 mmol/1 g Al) such as Ni,6
Co,10,13
or Au18,19
is used. It
forms eutectic quasi-molten droplets that act as active nucle-
ation sites and promote the vapor–liquid–solid (VLS) growth
mechanism of 1-D AlN nanostructures.6
(iii) A template-assisted carbothermic reduction and nitr-
idation of Al2O3/Al mixtures.5
AlN nanowires have been syn-
thesized in bulk from carbon nanotubes (CNTs), which act as a
carbon source and template for confined carbothermic reduc-
tion and nitridation reactions. The synthesis mechanism was
described by space-limited nucleation and the growth occurred
from the vapor phase along the template channels.
Oriented growth of 1-D AlN nanostructures has been report-
ed by halide-assisted nitridation and deposition on catalyzed
substrates (halide-CVD on catalyst-coated silicon or quartz
wafers) at moderate temperatures of 7001–8501C,11,16,17
and the
direct nitridation of Al in NH3/N2 at 11001C under confinement
of a Ni-catalyzed alumina template.8
The combustion synthesis (CS) method (also called the self-
propagating high-temperature synthesis or SHS) is considered
to be an economical approach for the production of pure AlN
powder.20–23
It involves the direct nitridation of metal alumi-
num, which is a typical exothermic reaction as follows:
Al þ 1
2N2 ! AlNðDH
¼ À318 kJ=molÞ (1)
The heat evolved sustains the reaction so that no extra energy
is needed except the small amount used for initiating the reac-
tion and the reaction is completed very quickly within seconds.
High conversions can be attained by careful selection of the
combustion parameters such as nitrogen pressure, amount and
type of diluents, etc. However, the literature of CS of AlN shows
that the product morphology is complex and often consists of
various grain morphologies such as agglomerated particles,
whiskers, faceted particles, rods, pyramids, etc.24–26
Although
a uniform morphology is very important to engineer the
properties of AlN-based materials or devices, a morphology-
controlled synthesis condition has not yet been realized. In this
paper, we report novel observation of the growth of quasi-
aligned AlN nanofibers inside the reacting Al particles during
the combustion under a low nitrogen pressure. A new growth
B. Derby—contributing editor
Radwan is deeply grateful to the Japan Society for the Promotion of Science (JSPS) for
the postdoctoral fellowship grant.
*Member, The American Ceramic Society.
**Fellow, The American Ceramic Society.
w
Author to whom correspondence should be addressed. e-mail: radwan@jwri.
osaka-u.ac.jp
Manuscript No. 22004. Received July 12, 2006; approved January 25, 2007.
Journal
J. Am. Ceram. Soc., 90 [8] 2347–2351 (2007)
DOI: 10.1111/j.1551-2916.2007.01747.x
r 2007 The American Ceramic Society
2347

2. model is proposed to explain the formation of this unique
morphology. As far as we know, this mode of growth has not
been reported in the literature before.
II. Experimental Procedure
High-purity Al powder (99.9%, Toyo Aluminum KK, Tokyo,
Japan), AlN diluent powder (type H, 99.9%, Tokuyama
K.K., Hino, Tokyo, Japan), NH4Cl (99%, Nacalai Tesque,
Inc., Kyoto, Japan), and Y2O3 (99.9%, Shin-Etsu Chemical Co.,
Tokyo, Japan) were used as the starting materials. Figure 1
shows the morphological characteristics of Al and AlN powders;
the average particle size is 23 and 0.5 mm, respectively. The re-
action charge was composed of Al, AlN (Al/AlN 5 40/60
mol%), and promoting additives of NH4Cl and Y2O3 (5 wt%
each). The reaction powders were mixed using mortar and pestle
for 10 min and then sieved through a 212-mm sieve to disperse
any large agglomerates. Fifty grams of the powder mixture was
poured into a porous graphite container (f42 mm  90 mm H)
and packed by tapping to a relative density B60% of the the-
oretical density. The density was estimated by measuring the
weight of the powder mixture in a fixed volume of the container.
The graphite container was then placed in the combustion
chamber. Two W–Re thermocouples (connected to a data ac-
quisition system) were inserted into the center of the charge (one
at the middle and another near the top surface) at a fixed dis-
tance of 30 mm and used to record the temperature–time pattern
of the combustion and determine the combustion speed by mea-
suring the time lapsed for the wave passage between the two
thermocouples. The chamber was evacuated and subsequently
filled with high-purity N2 gas (99.999%) up to 0.25 MPa pres-
sure. The combustion was initiated from the bottom by igniting
a 2-g ignition pellet (Al1AlN, 1:1) placed at the bottom of the
packed powder by passing an electric current (60 A, 20 V) for 10
s through a carbon ribbon under the pellet. The combustion re-
action was completed in about 5 min and the chamber was then
cooled to room temperature in B30 min. The reaction product
was visually observed. The product phases were identified by X-
ray powder diffraction (XRD; JEOL, JDX-3530, Tokyo, Japan)
using CuKa radiation. The morphology of as-synthesized pow-
der was observed by field emission scanning electron microscopy
(FE-SEM; ERA-8800, ELIONIX, Tokyo, Japan). Samples for
SEM observation were coated with thin films of sputtered gold
to reduce electrical charge-up.
III. Results and Discussion
The content of diluent (60 mol%) in the reaction mixture was
chosen according to a previous study for the relationship be-
tween Al molar ratio and nitrogen pressure on the yield and
properties of AlN product.27,28
It is used to reduce the reaction
temperature and prevent coagulation of melted aluminum
particles. The as-synthesized AlN cake was very fragile with
only a white color.
The microstructure of as-synthesized AlN powder was
observed by FE-SEM using representative samples from three
different locations in the product cake: top surface, side surface,
and middle center. The grain morphologies of as-synthesized
AlN particles are given in Fig. 2. The microstructure consists of
two major types: aggregates of irregular particles (B0.5 mm,
same as original AlN diluent) and ball-like grains (same size and
shape as original Al particles) consist of thin crust (r150 nm)
covers unique quasi-aligned AlN nanofibers grown in the inte-
rior. Their cross-sectional view is similar to oval disk of the sea
anemone. Figure 3 shows the XRD pattern of the as-synthesized
product. The diffraction lines are assigned to a hexagonal AlN
structure similar to the bulk AlN powder reported (JCPDS-file
25–1133). Residual metallic Al has not been detected.
From the SEM observations, one can conclude that:
(1) The AlN diluent did not participate in the formation of
AlN particles and played only a passive role in controlling the
combustion temperature and dispersing the Al particles.
(2) There was no grain growth or sintering for both the
formed and original AlN diluent particles.
(3) The quasi-aligned AlN nanofibers were formed inside
the reacting Al particles.
As far as we know, this mode of growth, encapsulation in
reacting particles, has never been reported before. The question
is how were these balls of quasi-aligned AlN nanofibers formed?
The formation of a shell–core system during the course of
nitridation of Al metal is known in the direct nitridation (DN)
method and reported as a ‘‘core–shell’’ model.29–34
In this meth-
od, the nitridation takes place via three steps: nitridation at the
surface of the particles with the formation of a crystalline nitride
shell, breakaway or flowout of molten or vaporized Al core, and
volume nitridation outside the shell with a remaining hole or an
empty core. The final morphology of the AlN product is hon-
eycomb like (a clear SEM photograph of this morphology can
be seen in our recent paper by Radwan and Bahgat34
). In the
combustion synthesis method, both the surface nitridation and
breakaway were also observed when moderate combustions
(low Tmax) were promoted by using small amounts of additives
such as C and NH4Cl35–38
and/or ignition under an appropriate
nitrogen pressure.39–41
The nitride skins were formed at the pre-
heating stage of the combustion and then molten Al flowed out
with the formation of an eggshell-type AlN morphology. Nei-
ther of the previous observations can account for the present
SEM observations, which requires a new growth model without
a breakaway.
The typical temperature–time history of the nitridation reac-
tion (Fig. 4) sheds light on the behavior of the combustion. The
temperature was measured in the middle center of the charge.
The pattern shows a mild combustion with no explosive mode
and has a relatively low rate of temperature increase. It took
B53s to reach 6001C (below melting of Al), B73s to reach
50 µm
(a)
2 µm
(b)
Fig. 1. Scanning electron microscopy micrographs of starting powders: (a) Al and (b) aluminum nitride diluent.
2348 Journal of the American Ceramic Society—Radwan and Miyamoto Vol. 90, No. 8

3. 10001C, B160 s to Tmax (16201C), and lasted B27 s in the
afterburning stage. We noticed that in the temperature–time
histories of other combustion experiments without NH4Cl and
Y2O3 additions, once the combustion wave begins, the temper-
ature increases rapidly close to its maximum value. Owing to the
low combustion temperature, the grain growth and sintering of
AlN particles were avoided. The speed of the combustion reac-
tion was determined by measuring the time lapsed for the wave
passage between two thermocouples inserted into the center of
(e)
1 µm
(b)
10 µm
(d)
5 µm
50 µm
(a)
(c)
10 µm
Fig. 2. Field emission scanning electron microscopy images of the as-synthesized aluminum nitride product.
0
1000
2000
3000
4000
5000
20 30 40 50 60 70 80
2θ (°)
Intensity(a.u.)
·AlN
·
·
·
·
· ·
·
·
·
Fig. 3. X-ray diffraction pattern of the as-synthesized aluminum nitride
product.
Time, s
Combustiontemperature,°C
500
1000
1500
0 50 100 150 200 250 300 350
Fig.4. Temperature–time variation at the combustion front.
August 2007 Growth of Quasi-Aligned AlN Nanofibers 2349

4. the reaction bed at a (vertical) distance of 30 mm. The combus-
tion had a slow speed (0.26 mm/s).
In Fig. 5, we propose a new growth model to explain the
formation of quasi-aligned AlN nanofibers by the CS based on a
‘‘micro-reactors’’ model. The formation of nanofibers occurred
through two stages.
(1) Formation of Micro-Reactors
This stage started in the preliminary stage of combustion and
occurred outside the Al particles through two steps. In the
first step (below 6001C), NH4Cl dissociated into HCl and NH3
vapors (NH4Cl sublimes at 3501C and dissociates at 5201C):
NH4Clðs; vÞ ! HClðgÞ þ NH3ðgÞ (2)
The sublimation and decomposition of NH4Cl are endother-
mic and produce several gaseous species. These reactions absorb
sufficient heat and disturb the direct nitridation of Al particles
with N2 gas, which retards the wave propagation. It also pro-
vides enough time for another endothermic reaction between
yttria and the surface alumina layer (at T ! melting point of Al)
with the creation of a thin crust on the surface of the Al
particles:
Y2O3ðsÞ þ Al2O3ðsÞ þ N2ðgÞ ! Al À Y À N
À OðsÞ stable thin crust
(3)
The energy dispersive X-ray spectroscopy microanalysis
showed the presence of yttrium in the composition of the thin
crust (Fig. 6). The formation of these new crusts also suppresses
the Al–N interaction and slows down the combustion propaga-
tion. Because of the low heat evolution and the presence of new
protective crusts, there was neither breakaway nor explosion of
molten cores.
(2) Nitridation and Growth
The crust–core systems function as unique ‘‘micro-reactors.’’
The various gaseous species present (HCl(g), NH3(g), N2(g))
diffuse through the crust (through pores or cracks) into the
molten AlN cores. Both the nitridation and growth steps then
occur inside the developed micro-reactors. The nitridation pro-
ceeds via spontaneous chlorination–nitridation sequences simi-
lar to that reported in our previous results of direct nitridation
of an Al/NH4Cl mixture.42
Gaseous hydrogen chloride is
very active and spontaneously reacts with molten Al cores to
produce AlCl3 vapors, which are nitrided by a gas–gas reaction
as follows:
AlðlÞ þ 3HClðgÞ ! AlCl3ðvÞ þ 3
2H2ðgÞ (4)
AlCl3ðvÞ þ 1
2N2ðgÞ þ 3
2H2ðgÞ ! AlNðsÞ þ 3HClðgÞ (5)
Ammonium chloride plays a critical role because it produces
hydrogen chloride, which can be considered to be a key inter-
mediate product, acting as a ‘‘catalyst.’’ HCl(v) promotes the
vaporization of molten aluminum cores into volatile aluminum
chloride species and facilitates progress of nitridation through
sequence of spontaneous chlorination and nitridation interme-
diate reactions.
The semi-molten Al–Y–N–O crusts seemed to function as
catalyzed self-substrates and provided active sites that promoted
homogenous nucleation of AlN embryos on the inner surface of
the crusts from the vapor phase by a VLS mechanism. The AlN
nanofibers might condense from the vapor phase, after a critical
(low) degree of supersaturation inside the ‘‘micro-reactors’’ is
attained, and grow on the preceding embryos by a VS growth
mechanism in an epitaxial way according to the classical crystal
growth theory.43
No droplets could be observed at the tips of
these nanofibers. This results in a unique oriented growth in
the interior of the reacting particles normal to the inner crust
surface.
The first stage of the combustion, formation of micro-reac-
tors, was the essential step for the formation of AlN nanofibers
inside reactant particles. The postulation of nucleation of AlN
by VLS in molten droplets or layer with a subsequent fiber
growth through VS mechanism is in close agreement with the
observations of Moya and colleagues.24,44,45
To the best of our knowledge, this mode of growth inside
reactant Al particles has not been observed before and it was
not expected in combustion reactions due to the high increase
Al–N–Y–O
crust
molten
Al core
HCl(g), N2(g),
NH3(g)
Stage I.
1. dissociation of NH4Cl
2. formation of Al-N-Y-O
3. infiltration of gaseous species into core
Stage II.
4. chlorination of molten core
5. gas-gas nitridation reaction
6. nucleation and growth of quasi-aligned AlN nanofibers
quasi-aligned
AlN nanofibers
AlCl3(g)/N2(g)
↓
AlN(s)
native oxide
layer
solid Al
core
Fig. 5. Schematic illustration of the growth model of quasi-aligned aluminum nitride nanofibers by the combustion synthesis method.
Fig.6. Energy dispersive X-ray spectroscopy pattern of the thin nitride
crust.
2350 Journal of the American Ceramic Society—Radwan and Miyamoto Vol. 90, No. 8

5. in nitridation temperature and fast speed of combustion. The
current combustion condition was successful creating new
micro-reactors during the combustion reaction, which promot-
ed this mode of oriented growth inside.
IV. Conclusion
Quasi-aligned AlN nanofibers were formed by the combustion
synthesis according to a new micro-reactor model. This route
might be a possible method for economical growth of AlN
nanofibers, which are obtainable only by complicated nitridat-
ion reactions at elevated temperatures. AlN with this unique
morphology can be used not only for AlN ceramics and
composites but also in nanotechnology applications.
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