Synthesis of nanocrystalline aluminum nitride by nitridation of δ-Al2O3 nanoparticles in flowing ammonia

Article abstract of DOI:10.1111/j.1551-2916.2005.00715.x

Nanocrystalline aluminum nitride (AlN) with surface area more than 30 m²/g was synthesized by nitridation of nanosized δ-Al ₂O₃ particles using NH₃ as a reacting gas. The resulting powders were characterized by CHN elemental analysis, X-ray diffraction (XRD), Fourier transform infrared spectra, X-ray photoelectron spectra, field-emission scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller surface area techniques. It was found that nanocrystalline δ-Al₂O₃ was converted into AlN completely (by XRD) at 1350°-1400°C within 5.0 h in a single-step synthesis process. The complete nitridation of nanosized alumina at relatively lower temperatures was attributed to the lack of coarsening of the initial δ-Al₂O₃ powder. The effect of precursor powder types on the conversion was also investigated, and it was found that α-Al ₂O₃ was hard to convert to AlN under the same conditions.

Full text of DOI:10.1111/j.1551-2916.2005.00715.x

J. Am. Ceram. Soc., 89 [2] 415–421 (2006)
DOI: 10.1111/j.1551-2916.2005.00715.x
r 2005 The American Ceramic Society
ournal
J
Synthesis of Nanocrystalline Aluminum Nitride by Nitridation of
d-Al O Nanoparticles in Flowing Ammonia
2
3
,w
Qinghong Zhang and Lian Gao*
State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,
Chinese Academy of Sciences, Shanghai 200050, China
1
1
Nanocrystalline aluminum nitride (AlN) with surface area more
than 30 m /g was synthesized by nitridation of nanosized
ature nitridation of metallic aluminum,
precursors, and vapor synthesis were reported to prepare
nanocrystalline AlN.
organo-metallic
2
1
2–15
d-Al O particles using NH as a reacting gas. The resulting
Other routes such as self-propagating
2
3
3
powders were characterized by CHN elemental analysis, X-ray
diffraction (XRD), Fourier transform infrared spectra, X-ray
photoelectron spectra, field-emission scanning electron micros-
copy, transmission electron microscopy, and Brunauer–Em-
mett–Teller surface area techniques. It was found that
high-temperature synthesis or combustion synthesis generally
produced micrometer-sized aluminum nitride particles and
16–19
whiskers.
AlN in flowing ammonia or a mixture of N
time periods, and the conversion could be carried out at 14001C
In addition, alumina could be transformed into
2
/NH over long
3
2
0
21
nanocrystalline d-Al O3 was converted into AlN completely
2
in the presence of CaF as additives. Hoch and Nair reported
2
(
by XRD) at 13501–14001C within 5.0 h in a single-step syn-
that up to 80% of the amorphous alumina was converted to
AlN at 12001C for 24 h, and the rest of the starting material
was transformed to a-Al O . Rocherulle et al. demonstrated
2 3
that pure aluminum nitride powder was produced by reacting
alumina (B1 mm) with ammonia at 13001C for 48 h. For car-
bothermal synthesis of AlN, Keramont Corporation observed
that nitridation parameters were very dependent on the type of
alumina/carbon used: a temperature difference as much as
thesis process. The complete nitridation of nanosized alumina
at relatively lower temperatures was attributed to the lack of
coarsening of the initial d-Al O powder. The effect of precursor
powder types on the conversion was also investigated, and it was
found that a-Al O was hard to convert to AlN under the same
2
2
2
3
2
3
conditions.
2
001C might be observed in requirements for nitridation using
1
I. Introduction
2 3 3
different types of alumina. So, for the Al O –NH system, us-
ing alumina nanoparticles instead of alumina of 1 mm size for
the nitridation reaction may shorten the reaction time, and also
much finer AlN powders are expected. A modified carbothermal
HE importance of nanocrystalline powders has been known
for many years in powder metallurgy, ceramics, catalysis,
T
and other fields. It is common knowledge that ultrafine powders
have better sinterability, and some possess higher catalytic
activity, or higher dispersability. Unfortunately, unlike well-
investigated nanocrystalline oxides, the preparation of nano-
crystalline nitrides is more difficult as the preparation generally
proceeds at high temperatures, which also undergoes dynamic
growth and agglomeration of the resulting nitride particles.
Among nitrides, aluminum nitride (AlN) has attracted much
interest in recent years, particularly in the electronics industry as
a substrate material, because of its high intrinsic thermal con-
reduction method was also developed, and it focused on more
23,24
homogeneous mixing of alumina with carbon. More recent-
ly, Komeya et al. reported a novel process for synthesizing
spherical or fibrous AlN powder by reduction–nitridation of
Al
2
O
3
of particle diameter 0.91 and 5.8 mm using NH
3
–C
3 8
H as
25–27
reactant gases,
and a small amount of crystalline aluminum
25,27
oxide still remained in their products.
source derived from the pyrolysis of C
ridation reaction to take place at a lower temperature compared
with the conventional carbothermal reaction. Haussonne
et al. reported an interesting process by heating the mixture
of aluminum powder with lithium salts in nitrogen at temper-
The gaseous carbon
8
H stimulated the nit-
3
25
ꢁ1
ductivity (320 W ꢀ (mꢀ K) ), high electrical insulation, and ther-
28
1
mal expansion coefficient close to that of silicon. In addition,
AlN has been added to polymers as a filler in a powder or fiber
form to modify the thermal properties of electronic-packaging
materials.
atures ranging from 8001 to 12001C, and they obtained pure al-
uminum nitride with a specific surface area up to 17.51 m /g at
2
2
,3
9401C.
Commercial AlN powder is currently produced via the di-
rect nitridation of aluminum metal or the carbothermal reduc-
tion–nitridation of alumina. Aluminum metal melts at a
For the gas–solid reaction, it is expected that the temperature
may decrease and the time for complete reaction may be short-
ened significantly using finer solid particles as a precursor. Re-
cently, alumina nanoparticles in crystalline phase or amorphous
state have been commercially available, and their conversion to
aluminum nitride may be carried out at a relatively low tem-
perature within a shorter time. The present work aims to estab-
lish a method to obtain a pure and ultrafine AlN powder via a
carbon-free and cost-effective synthesis technique. Ultrafine and
pure (by X-ray diffraction (XRD)) AlN powder with a specific
4
temperature much lower than its nitridation temperature with
N . The AlN crust on liquid droplets of Al prevents the unre-
2
acted Al core from further nitridation, and thus a complete
conversion to AlN powder is difficult to achieve in this process.
For the carbothermal reduction–nitridation of alumina, there
are also some disadvantages such as a relatively high nitriding
temperature (higher than 16001C) as well as an additional post-
5
–7
oxidation step necessary to remove the excessive carbon.
2
8
surface area of more than 30 m /g was obtained by this one step
Recently, some new methods such as pulsed laser ablation,
carbothermal synthesis, electrochemical method, low-temper-
9
10
process. The complete nitridation of nanosized Al
ried out in flowing NH at temperatures lower than the sintering
temperature of fine alumina powder.
2 3
O was car-
3
N. Jacobson—contributing editor
II. Experimental Procedure
Three kinds of Al powder were used for the nitridation re-
action: One is the laboratory synthesized amorphous Al , and
Manuscript No. 20732. Received July 1, 2005; approved July 27, 2005.
*Member, American Ceramic Society.
Author to whom correspondence should be addressed. e-mail: liangaoc@online.sh.ch
2 3
O
w
2 3
O
4
15

4
16
Journal of the American Ceramic Society—Zhang and Gao
Vol. 89, No. 2
the others are the commercial nanocrystalline d-Al O3 and
III. Results and Discussion
1) Effect of Precursor Oxides on Conversion of
Al O to AlN
2
a-Al
2
O
3
. For the preparation of amorphous Al
2 3
O powder,
(
0
.1M Al (SO solution was neutralized with 0.4M NaAlO
2
)
4 3
2
2
3
solution isovolumetrically at room temperature, and then the
precipitate was separated from the mother solution by filtration
and washed with distilled water several times. The thoroughly
washed precipitate was dried at 1101C for 24 h and subsequently
calcined at 8001C for 2 h to remove the undesired water for the
nitridation reaction. The XRD result showed that the powder
after calcination at 8001C was amorphous, having a Brunauer–
When amorphous Al
1
2
O
3
and a-Al
2 3
O were nitrided both at
4001C for 2 h, the amorphous alumina with a specific surface
2
area of 82 m /g predominantly converted in aluminum nitride,
and the remaining amorphous parts crystallized to a-Al
Fig. 1(d)). The conversion of a-Al O powder with a surface
2
O
3
(
2
3
2
area of 25.0 m /g was much lower than that of amorphous
2
Al O , and only very weak peaks of hexagonal AlN (100) were
2
3
Emmett–Teller (BET) specific surface area of 86 m /g. Assum-
detected in its XRD patterns as shown in Fig. 1(b). Hoch and
ing all particles are spherical, the value of the specific surface
area corresponds to a primary particle size of 16 nm. A com-
2
1
Nair also reported difficulty in the conversion of a-Al O to
2
3
AlN. They found that increasing the nitridation temperatures of
amorphous alumina generally crystallized more amorphous
2 3
mercially available spherical d-Al O powder (Aluminium Oxide
C, Degussa, Frankfurt, Germany) with a surface area of
2
Al O to a-Al O rather than forming higher concentrations
21
9
5.6 m /g and an average primary particle size of 13 nm was
used as a nanocrystalline precursor. For comparison, the com-
mercial a-Al powder (Al Type-HFF25, Shanghai Zhon-
gyuan Chem. Ltd. Co., Shanghai, China) with a BET specific
2
3
2
3
of AlN. By comparison, the nanocrystalline d-Al O converted
2
3
into AlN almost completely and a very small peak correspond-
ing to the residual a-Al was detected by XRD (Fig. 1(c)).
The presence of a-Al O is because of d–a transformation of
O
2 3
2 3
O
2 3
O
2
surface area of 25.0 m /g was also used as a precursor for
synthesizing AlN. It was found that nanocrystalline d-Al
powder was converted into AlN more easily than the others, so
2
3
alumina, which also indicates that the reaction time of 2 h is not
enough for complete conversion to AlN. For a longer nitridat-
ion (such as 5 h), no a-Al O was detected by XRD in the sam-
2 3
ples (refer to Fig. 2). The commercial nanocrystalline d-Al O
3
powder consists of very fine (13 nm) nanoparticles and does not
coarsen in NH gas, which should promote the formation of
3
2 3
O
d-Al O powder was used as a precursor to investigate the
2
3
effects of the processing parameters on the conversion of
Al into AlN and the crystallite size of the resultant AlN.
Two grams of alumina was placed in a high-purity alumina
2
2 3
O
AlN at relatively low temperatures for a shorter time. Coales-
cence of Al O particles not only limits the ammonia availability
boat and set in an alumina tube furnace (inner diameter of 90
mm) with air-tight end gaskets. The reactor was flushed with
argon to eliminate oxygen in the system during the heating-up
2
3
but also reduces the gas–solid interface area significantly, thus
greatly retarding the nitridation reaction. Besides the effect of
3
period. As the temperature reached 9001C, a gas of NH (99.9%
2 3
Al O particle size on nitridation reaction, the relative stability
purity) was introduced from the extremity of the reactor, at a
flow rate of 1 L/min (stp). The pressure in the alumina tube was
slightly higher than 1 atm, which permitted removal of the re-
actant and product gases from the exit side of the furnace tube.
of different phases of alumina should also be taken into account.
2
1
Hoch and Nair calculated the free energies of reaction and the
equilibrium water vapor pressure for different types of alumina
(
alumina can react with NH at a higher partial pressure of water
amorphous and a-Al O ), and they concluded that amorphous
2 3
The NH
3
was adsorbed with a water pool before the waste gas
was released into air. The furnace was heated to the experimen-
tal reaction temperature (12001–15001C) at a rate of 81C/min,
and the reaction temperature was held subsequently for 2–5 h.
Then, the sample was cooled to room temperature at a rate of
3
(
0.027 vs 0.0075 atm at 14001C). This means that the amorphous
alumina may be nitrided with a higher yield. d-Al is a me-
sostable phase compared with a-Al O , and a more stable phase
2
O
3
2
3
than amorphous alumina. Owing to these considerations, amor-
phous alumina may display a higher nitridation rate than the
others. However, amorphous alumina crystallizes and the par-
ticles grow at very high temperatures (Fig. 1(d)). The crystalli-
zation rate is relatively fast, and crystallization into a-Al O
2 3
reduces the speed of the subsequent nitridation. So, we conclude
that the absence of coalescence and the mesostable phase of
6
1C/min in an ammonia atmosphere.
The powder phase composition was identified by XRD
Model D/max 2550V, Rigaku Co., Tokyo, Japan), operated
(
˚
at 40 kV and 100 mA using CuKa (l 5 1.5406 A) radiation. El-
emental analysis for carbon (C), hydrogen (H), and nitrogen
(
N) contents in the nitrided powder was carried out using a
Perkin Elmer CHN analyzer (Model 2400-II, Boston, MA), and
oxygen was introduced for complete combustion of the nano-
crystalline nitride powder. CHN elemental analysis was very ef-
fective in quantitatively determining the nitrogen content in the
d-Al O are responsible for the complete conversion to AlN in a
2
3
much shorter time.
2
9
nanocrystalline InN powder. The morphology and the crys-
tallite size of the resulting nitrides were observed using a trans-
mission electron microscope (TEM) (Model 200CX, JEOL,
Tokyo, Japan) as well as a field-emission scanning electron mi-
croscope (FE-SEM) (Model JSM-6700F, JEOL). The BET spe-
cific surface area measurement was performed using a nitrogen
adsorption apparatus (Model ASAP2010, Micromeritics Instru-
ments, Norcross, GA). Simultaneous thermogravimetry (TG)
and differential scanning calorimetry (DSC) analyses of the
sample were performed in air using a thermal analyzer (Model
STA 449C, Netzsch Inc., Selb, Germany) at the rate of 51C/min.
Using a Noclet Series NEXUS Fourier transform infrared
(
FTIR) spectrometer (NEXUS, Nicolet, Madison, WI), absorp-
tion spectra were collected on the samples in the spectral range
ꢁ
1
of 400–4000 cm using the KBr pellet method. The X-ray pho-
toemission spectroscopy (XPS) analysis was performed using a
Microlab MK II XPS surface analyzer (VG Scientific Ltd., East
Grinstead, U.K.) using MgKa X-ray irradiation. The peak po-
sitions were calibrated with respect to the position of C 1s car-
bon contamination peak usually located at about 284.6 eV,
when observed without charging effects. The chemical compo-
sition after ionic cleaning of the surface was determined by
quantification of the corresponding photoelectron peaks.
2 3 2 3
Fig. 1. X-ray diffraction patterns of (a) a-Al O powder, (b) a-Al O
powder after nitridation, (c) d-Al O after nitridation, and (d) amor-
2
3
2 3 3
phous Al O after nitridation at 14001C for 2 h in flowing NH gas.

February 2006
Synthesis of Nanocrystalline Aluminum Nitride
417
Fig. 3. Specific surface area of d-Al
O
2 3
powder calcined at various
temperatures in (ꢂ) NH gas and (m) air.
3
2 3
Fig. 2. X-ray diffraction patterns of nanosized d-Al O after nitridation
at (a) 12001C, (b) 13001C, (c) 13501C, and (d) 14001C for 5 h, using a
2 3
commercial nanosized d-Al O (aluminum oxide C, Degussa) as starting
material.
2 3
to a-Al O and exaggregated growth may occur, which reduces
the speed of the nitriding reaction and results in an incomplete
nitridation at 15001C.
To consider gas–solid reaction kinetics, the whole reaction will
be considered:
(
2) Influence of Reaction Temperature on Nitridation of
Al2O3ðsÞ þ 2NH3ðgÞ ! 2AlNðsÞÞ þ 3H OðgÞ
(1)
2
Nanocrystalline d-Al
Figure 2 shows the powder XRD patterns of nanocrystalline
d-Al O powder after nitridation at different temperatures for
5 h. When d-Al O powder was calcined in NH at 12001C for
5 h, it neither transformed into a-Al O nor converted into
O
2 3
However, ammonia readily decomposes at high temperatures
and consequently, the reaction may be better represented as fol-
lows
2
3
2
3
3
7
2 3
crystalline AlN. However, some nitrogen (5.09 wt%) was de-
tected in this sample (see Table I), suggesting adsorption of ni-
trogen species and/or formation of Al–N layers on the d-Al O
Al2O3ðsÞ þ N2ðgÞ þ 3H2ðgÞ ! 2AlNðsÞ þ 3H2OðgÞ
(2)
2
3
3
surfaces. The adsorption of both NH and the Al–N layers may
Thermodynamically, it is quite unlikely that reaction (2) will
proceed to the right as a very low equilibrium partial pressure of
H O is needed to be maintained for the above reaction. How-
inhibit d–a transformation and prevent exaggerated growth dur-
ing further nitridation at a high temperature, which is similar
to that observed for oxide nanoparticles with a higher thermal
2
1
2
3
0,31
ever, ammonia may reach reaction sites and react as NH
it decomposes, especially when very fine alumina is used (such as
3 nm d-Al and amorphous alumina) as starting materials.
3
before
stability.
a specific surface area as high as 82.7 m /g as presented in
Fig. 3, which suggests its high thermal stability in NH . For the
sample nitrided at 13001C for 5 h, it transformed into a-Al O
After nitridation at 12001C, d-Al
2 3
O also retained
2
1
O
2 3
3
Al O powder with a high specific surface area may absorb more
2
3
2
3
NH
3
molecules, and some of them may react with the oxide be-
. Moreover, the ultrafine crys-
partially (Fig. 2(b)), accompanied by a significant decrease of
surface area and increase of nitrogen content (7.03 wt%) com-
pared with the sample nitrided at 12001C. All XRD peaks for
fore the decomposition of NH
tallite size of initial precursor aids in mass transfer in the
boundary layer and mass transfer by diffusion in the product
3
the powder obtained by nitridation of commercial d-Al
2 3
O pow-
2 3
layer. At higher temperatures, d-Al O powder may transform
der at 1350 or 14001C for 5 h may be attributed to the hexagonal
Table I. Conversion of Nanocrystalline Al O Powders to AlN at Different Temperatures in Flowing NH Gas
2
3
3
Crystallite size (nm)w
N (wt%)
z
H (wt%)
z
O (wt%)y
Sample
Starting materials
T (1C)
Time (h)
Phase
A
B
d-Al O3
1200
1300
5
5
d-Al O3
13
14 (d-Al O )
5.09
7.03
o 0.3
40.04
37.36
2
2
d-Al O3
d-Al O3
0.41
2
2
2
3
a-Al O3
95 (a-Al O )
2
2
3
C
D
E
F
d-Al O3
1350
1400
1400
1500
5
2
5
5
AlN
AlN
AlN
AlN,
39
40
43
28.88
29.07
30.44
23.97
0.49
0.50
0.45
7.26
6.99
5.11
14.02
2
d-Al O3
2
d-Al O3
2
d-Al O3
54 (AlN)
4100 (Al O )
o 0.3
2
a-Al O3
2
2
3
G
H
I
a-Al O3
1400
1400
1400
2
5
2
a-Al O3
91 (Al O )
2
1.23
4.75
o 0.3
o 0.3
o 0.3
45.36
40.51
13.04
2
2
3
AlN(trace)
a-Al O3
a-Al O3
65 (AlN)
4100 (Al O )
2
2
AlN
AlN
2
3
Amor.
Al O3
39 (AlN)
72 (Al O )
24.34
a-Al O3
2
2
2
3
wThe crystallite size is calculated from the broadening of XRD peaks at d104 5 2.552 A for a-Al
˚
˚
O and d100 5 2.695 A for AlN, respectively. The nitrogen and hydrogen
z
2 3
contents were analyzed by a CHN elemental analyzer, H contents in samples C–E are close to 0.5 wt%, indicating that the surfaces of AlN particles were hydroxylated after
y
its exposure to air. Balanced concentration by assuming that the oxidation state of O, N, and Al is ꢁ2, ꢁ3, and 13, respectively. AlN, aluminum nitride; XRD, X-ray
diffraction.

4
18
Journal of the American Ceramic Society—Zhang and Gao
Vol. 89, No. 2
2 3
Fig. 4. X-ray diffraction patterns of nanosized d-Al O after nitridation
at 15001C for (a) 2 h and (b) 5 h.
2 3
phase of AlN (Figs. 2(c) and (d)). The conversion of d-Al O
into AlN was very sensitive to the nitridation temperature and
reaction time, and pure AlN powder could be prepared only in a
relatively narrow temperature range. As shown in Fig. 4, when
the nitridation temperature was as high as 15001C, the conver-
sion of Al O into AlN was not complete, and some a-Al O
2
3
2
3
remained in the product. This temperature was higher than the
32
sintering temperature (13501C) of fine alumina powder, and
sintered powder had low reactivity and also as encountered dif-
fusion limitations with respect to ammonia’s to make inability
contact with unreacted core of alumina. Figure 5 shows bright-
field TEM images and electron diffraction (ED) patterns of
nanocrystalline d-Al O after nitridation at 15001C for 5 h. The
2
3
sample consisted of primary particles 40–100 nm in size
(
nm, Fig. 5(b)). The ED patterns in Fig. 5(b) indicate that the
Fig. 5(a)) and spherical particles of much larger size (4500
Fig. 5. Transmission electron microscope micrograph of nanocrystal-
larger particles consist of a-Al O . The result suggests that the
line d-Al
aluminum nitride particles and (b) the sintered a-Al
sample. X-ray diffraction result shows that it was nitrided partially and
some amount of a-Al existed in the nitrided powder. The electron
diffraction (ED) patterns labeled a, b, and c are assigned (1 ꢁ1 ꢁ1),
0 1 ꢁ1) and (1 0 ꢁ2) crystallographic planes of a-Al , respectively.
O
2 3
after nitridation at 15001C for 5 h in flowing NH
3
gas: (a)
2
3
2 3
O
particles in this
temperature of 15001C is too high to prepare nanocrystalline
AlN because of the sintering and exaggeration of a-Al par-
ticles. As given in Table I, the calculated crystallite size of AlN
2 3
O
2 3
O
based on the broadening of XRD peaks is also much finer than
(
2 3
O
that of a-Al
a-Al
It is notable that the growth of Al O nanoparticles in the
2 3
O , also suggesting larger particles attributable to
O .
2 3
Fig. 6. The particle size of AlN obtained by nitridation of d-
Al O nanoparticles at 13501C for 5 h is finer than that obtained
2
3
flowing NH gas is much slower than that in air (see Fig. 3). The
3
2
3
specific areas of samples calcined at 12001C both in the flowing
at 14001C for 5 h. The AlN powders prepared by our method
are uniform and very fine. In contrast, Sheppard found that the
commercial AlN was 800 nm or above in size, and Kim et al.
reported that they produced AlN 400 nm in size (by TEM) by
1
NH gas and in air show a slight decrease. However, after cal-
cination at 13001C, the specific surface area decreased signifi-
cantly, especially in air. As shown in Fig. 2(b), after calcination
3
33
at 13001C in NH
3
gas, the major d-Al
2
O
3
transformed into a-
3
nitridation of aluminum alkoxide hydrazide with NH at
13001C.
Al at this temperature. The d–a transformation as well as the
O
2 3
discontinuous growth of particles during the transformation
may be responsible for the lowering of the specific surface
Figure 7 shows the characteristic TEM images corresponding
to representative samples. For the AlN powder obtained by
ammonolysis of an oxide precursor at 13501C for 5 h, spherical
crystalline particles with 30–50 nm in size were observed, which
is much larger than that of d-Al O nanoparticles, because of the
area. AlN powders obtained by nitridation of d-Al
2
O
3
have
specific surface areas of around 30 m /g, which is higher than
2
2
22
the value of 11 m /g reported by Rocherulle et al., who used
Al of 1 mm size as starting materials. As-prepared nanocrys-
2
3
O
2 3
growth of nanoparticles at high temperatures. AlN obtained at
higher temperatures generally had a larger particle size. For ex-
ample, the AlN prepared by the nitridation of d-Al O nano-
talline aluminum nitride powders with high surface areas have
potential, and have promising applications in low-temperature
sinterable ceramics and as additives for electron packaging ma-
terials. Moreover, this nitridation reaction takes place under at-
mospheric pressure for a remarkably reduced time (less than 5 h
2
3
particles at 14001C for 5 h had a diameter of 40–80 nm. The
calculated diffusion rate inside the reacting particles was in the
range 8–20 nm/h at 13501–14001C. This means that for one
Al O particle having a radius of 500 nm, a nitridation time as
2
2
in the present work versus 48 h in the previous work ). Both
methods make this route attractive for industrial production of
ultrafine AlN on a large scale, although a more effective gas–
solid reactor is necessary for economic considerations.
2
3
long as 25–62.5 h is needed. The reported value of 48 h at
2
2
13001C fell in this range. As demonstrated above, when the
temperature was elevated to 15001C, was incomplete nitridation
of alumina was observed. This temperature is close to the sin-
tering temperature of fine alumina powder, and some extremely
coarse particles (1 mm) were also found in its TEM micrograph.
The sintering as well as the associated lower reactivity and the
retarded diffusion of reactive gas to the core of larger particles
(
3) Morphology of the Synthesized AlN
The resulting AlN particles are predominantly spherical, having
a diameter range of 60–150 nm, as shown by the SEM images in

February 2006
Synthesis of Nanocrystalline Aluminum Nitride
419
Fig. 6. Scanning electron microscope images of phase-pure aluminum
nitride obtained by nitridation of nanosized d-Al calcined at (a)
3501C for 5 h and (b) 14001C for 5 h in a NH gas flow, respectively.
2 3
O
1
3
were responsible for incomplete nitridation of fine alumina at
higher temperature (15001C). The employment of finer alumina
particles instead of larger particles lowers the nitridation tem-
perature and shortens the nitridation time, which is beneficial
for formation of the nanocrystalline nitrides.
(4) Nitrogen Contents and Crystallite Size in the
Nanocrystalline AlN Powders and the Thermal
Stability of Nanocrystalline AlN
Fig. 7. Transmission electron microscope micrographs of (a) nanocrys-
talline d-Al
a nitridation oxide precursor at (b) 13501C and (c) 14001C for 5 h in a
NH gas flow.
2 3
O and the resulting aluminum nitride powders prepared by
The problem encountered in the nitridation of oxide particles to
nitride is oxygen contamination. In the present work, the CHN
elemental analysis method was used to determine the nitrogen
content in nanocrystalline nitride. Only about 5 mg was needed
for this CHN analysis. Table I summarizes the results with re-
spect to nitrogen contents, phase composition, and crystallite
3
2 3
of AlN to Al O in air. The TG curve shows a strong weight
increase (19.7 wt%) in the range 8851–11651C, which is associ-
ated with the oxidation of AlN in air. The value of weight
increase from TG analysis is also slightly lower than the theo-
retical weight increase of pure AlN (24.3 wt%) and consistent
with the nitrogen content listed in Table I.
2 3
size of the nitrided powders obtained by nitridation of d-Al O
powder at different temperatures for 2–5 h. All samples were
exposed to air, and no special passivation for the fresh surface of
nanocrystalline AlN was carried out prior to CHN element
analysis and thermogravimetry (TG)–DSC analysis. The nitro-
gen content of AlN powder was 30.44 wt% (detected by CHN
elemental analysis), slightly lower than the theoretical value of
Infrared absorption spectra obtained on the d-Al O and the
2 3
nitrided samples are shown in Fig. 9. Overall, the IR spectra
collected on these samples were very different. An intense vi-
ꢁ
1
2 3
bration at 3600 cm was observed in d-Al O powder, which is
3
4.15 wt% for AlN. As shown in Fig. 8, TG and DSC were used
because of the stretching vibration of the water molecules, sug-
gesting the presence of adsorbed water on the surface of
to investigate the thermal stability of the resulting nanocrystal-
line AlN powder in air and to calibrate the result of the CHN
elemental analysis. A strong exothermal peak at 11031C is de-
tected in its DSC curve, which was attributable to the oxidation
d-Al
2 3
O . After heat treatment at high temperatures, the inten-
ꢁ1
sity of this vibration reduced, and shifted to 3440 cm , which
This result
3
3–35
is attributable to N–H stretching vibration.

4
20
Journal of the American Ceramic Society—Zhang and Gao
Vol. 89, No. 2
Fig. 8. Thermogravimetry–differential scanning calorimetry curves (in
air) of the aluminum nitride powder prepared by nitridation of nano-
Fig. 10. X-ray photoemission spectroscopy survey spectrum of alumi-
crystalline d-Al
O
2 3
at 14001C for 5 h.
num nitride powder prepared by nitridation of nanocrystalline d-Al
at 14001C for 5 h.
2 3
O
indicates the formation of N–H bands on the resulting AlN
nanoparticles. For d-Al O nanoparticles, the absorption peaks
For
2
3
gen is higher than the amount for the chemical equilibrium for
Al by assuming that aluminum exists as Al O and AlN, sug-
ꢁ
1
at 820, 740, and 600 cm correspond to Al–O linkages.
33,36
2
3
the sample after nitridation at 12001C for 5 h, both the bands
gesting that some AlOOH also occurs in the surface and sub-
surface regions. The photoelectron escape depth value for AlN is
ꢁ
ꢁ
1
20 and 600 cm are reduced to the shoulder peaks, and a new
8
1
band at 700 cm is observed. The change of IR spectra suggests
the structural evolution of d-Al nanoparticles although few
changes are observed in its XRD patterns as well as the specific
37
l 5 2.45 nm ; if oxygen is homogeneously distributed through-
2 3
O
out the XPS escape depth with a concentration of 30.12 at.%,
one can calculate mean oxygen concentrations as high as 10.23
wt% for AlN nanocrystals of 40 nm diameter. The calculated
value from XPS is much higher than the oxygen content in some
samples listed in Table I, which suggests that the oxygen exists in
ultrathin thickness at the surface rather than in AlN lattices or
ꢁ
1
surface area. The band at 700 cm is assigned to Al–N linkag-
es, demonstrating the formation of Al–N linkages, which is
3
3
also supported by the nitrogen content result (Table I). When d-
Al
2
O
3
nanoparticles were nitrided at 13501–14001C, the peak at
00 cm was much stronger and sharper, and the shoulder at
ꢁ
ꢁ
1
7
6
2 3 2 3
as unreacted Al O cores. The surface layers of Al O and
1
00 cm disappeared completely for the sample after nitridat-
AlOOH aware from the surface hydrolysis of AlN (especially
its nanocrystals) after the ultrafine AlN powders were exposed
ion at 14001C. The IR spectra are very similar to those of AlN
nanocrystals reported by Panchula and Ying. However, the
shoulder band centered at 740 cm is observed for both phase
3
5
22,35
to air, because of its high reactivity to air.
So, we conclude
ꢁ1
the oxygen exists as ultrathin layers on the surface of as-pre-
pared AlN, and special precautions should be taken to prevent it
from hydrolyzing as the oxygen contamination in AlN powder
3
6
pure (by XRD) AlN, indicating the existence of Al–O linkages.
Figure 10 shows typical survey scans of nanocrystalline AlN
prepared by nitridation of nanocrystalline d-Al at 14001C for
h. The XPS survey scans were recorded after Ar bombard-
ment for 3 min. The XPS spectra show the appearance of the O
s photoelectron peak with a strong intensity, indicating the
35
2 3
O
and ceramics may decrease the thermal conductivity of alumi-
num nitride materials.
1
5
35,38
The sintering behavior of as-prepared
ultrafine AlN powders and the effect of oxygen content on the
thermal conductivity of the sintered AlN ceramics are being in-
vestigated, and some sintering parameters should be optimized
1
predominance of aluminum oxide in the surface and sub-surface
regions. Based on the XPS spectra, the estimated atomic per-
centages are as follows: C 24.59 at.% (internal standard for the
normalization of binding energy), O 30.12 atom%, N 13.32
at.%, and Al 31.97 at.%. The atomic sum of oxygen and nitro-
39
according to Jackson’s published work.
IV. Conclusions
A novel route for the preparation of nanocrystalline AlN pow-
der via nitridation reaction of nanocrystalline d-Al O is pre-
2 3
sented. Nanocrystalline AlN powder of 30–60 nm crystallite size
and with a specific surface area of 30 m /g was obtained by nit-
2
2 3
ridation of the commercially available nanocrystalline d-Al O
at temperatures ranging from 13501 to 14001C for 2–5 h. The
remarkably reduced nitriding time (48 h were necessary in a
previous work) makes this method attractive for the industrial
production of ultrafine AlN on a large scale. The degree of con-
version to nitride was very sensitive to the preparative param-
eters, especially the nitriding temperature. The resultant AlN
was less agglomerated and uniform in particle size but some
oxygen was found to exist on the surfaces, which may have a
detrimental effect on the thermal conductivity of sintered AlN
ceramics. Both the finer crystallite size and the higher thermal
stability of d-Al
temperatures lower than the sintering temperature of fine Al
2 3
O are beneficial for the formation of AlN at
O .
2 3
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Fig. 9. Comparison of infrared absorption spectra collected on d-Al
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