Influences of precursor and additive on the morphology of nanocrystalline α-alumina

Article abstract of DOI:10.1016/j.jpcs.2007.10.024

The temperature for complete transformation to α-Al₂O₃ changed with precursor and the transformation temperature is lower in the case of using aluminum nitrate precursor as compared to aluminum sulfate precursor. The size of alumina particles obtained using aluminum nitrate precursor was smaller than that obtained using aluminum sulfate precursor, and this may be related to the ability of precursor anion to provide colloid stability. The particles obtained from aluminum nitrate precursor have elongated shape, while those obtained from aluminum sulfate precursor have spherical shape, and it could be attributed to the different characteristics of precipitate formed in the initial stage of reaction. The addition of AlF₃ results in decrease of the phase transition temperature to α-alumina. The effect of AlF₃ additive on modification of morphology of particles is different depending on the precursor. The change of morphology of α-alumina particles to the plate-like shape was observed in the case of using aluminum nitrate precursor, but not in the case of using aluminum sulfate precursor.

Full text of DOI:10.1016/j.jpcs.2007.10.024

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Journal of Physics and Chemistry of Solids 69 (2008) 1521–1524
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Influences of precursor and additive on the morphology
of nanocrystalline a-alumina
Hae Jeong Kim, Tae Gun Kim, Jeong Ja Kim, Seong Soo Park,
Ã
Seong Soo Hong, Gun Dae Lee
Division of Chemical Engineering, Pukyong National University, Busan 608-739, Republic of Korea
Received 13 July 2007; received in revised form 12 October 2007; accepted 30 October 2007
Abstract
The temperature for complete transformation to a-Al2O3 changed with precursor and the transformation temperature is lower in the case
of using aluminum nitrate precursor as compared to aluminum sulfate precursor. The size of alumina particles obtained using aluminum
nitrate precursor was smaller than that obtained using aluminum sulfate precursor, and this may be related to the ability of precursor anion
to provide colloid stability. The particles obtained from aluminum nitrate precursor have elongated shape, while those obtained from
aluminum sulfate precursor have spherical shape, and it could be attributed to the different characteristics of precipitate formed in the initial
stage of reaction. The addition of AlF3 results in decrease of the phase transition temperature to a-alumina. The effect of AlF3 additive on
modification of morphology of particles is different depending on the precursor. The change of morphology of a-alumina particles to the
plate-like shape was observed in the case of using aluminum nitrate precursor, but not in the case of using aluminum sulfate precursor.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics; B. Chemical synthesis; C. Electron microscopy; D. Microstructure
1. Introduction
fracture toughness more significantly than the ball-like
grains because the plate-like grains can form crack bridging
easily in ceramics matrix [4,5]. Thus, it can be said that
the alumina ceramic components required for advanced
engineering and structural applications need particles
having desired morphology. It was reported that the
precursor could affect the morphology and particle size
of micro-sized alumina [6]. It was also demonstrated that
some additive, especially fluorides, could decrease phase
transformation temperature to a-alumina and change the
morphology of alumina [4,7].
In the present work, nanocrystalline a-alumina particles
were prepared using different precursors and additive. The
influences of precursor (aluminum sulfate or aluminum
nitrate) and additive (metal fluorides) on the morphology
of alumina particles were studied.
Recently much attention has been paid to the synthesis
of nanocrystalline ceramics. It is well known that the
mechanical properties of ceramics strongly depend on their
microstructure, which closely relates with the shape and
size of the ceramic particles. It has been shown that the
mechanical properties of ceramics can be improved by the
formation of nanocrystalline structure [1]. Among various
ceramics, alumina, especially nano-sized a-alumina has
proven to be one of the most important materials because it
has many applications both as structural and functional
materials due to its excellent properties [2]. It was reported
that nano-sized ball-like alumina could improve some
mechanical properties, such as the fracture strength and
toughness [1]. In addition, it was demonstrated that the
morphology of alumina particles affects the mechanical
properties [3]. The nano-sized plate-like grains can increase
2. Experimental
Ã
The nano-sized a-alumina particles were prepared using
a method reported in literature [4]. NH4OH solution
Corresponding author. Tel.: +82 51 620 1566; fax: +82 51 625 4055.
E-mail address: gdlee@pknu.ac.kr (G.D. Lee).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.10.024

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1522
(0.2 mol lÀ1) was added slowly into Al(NO3)3 Á 9H2O or
Al(SO4)3 Á 18H2O solution (4.5 mol lÀ1) under stirring.
Polyethylenegylcol (PEG) solution was used as a disper-
sant. The pH of slurry was adjusted to 9.0 and then
the precipitate was aged with constant stirring for 1 h. The
slurry was filtered and dried at 70 1C for 24 h. After that the
dried gel was milled with alcohol in high purity alumina
medium for 24 h. In some cases, AlF3 (2 wt%) was added in
this stage. Then gel was dried at 55 1C for 24 h and calcined
at different temperatures for 1 h with heating rate of
10 1C minÀ1. The particle phase was identified using an
X-ray diffractometer (XRD; PHILIPS, X’Pert-MPD Sys-
tem) with Ni filtered Cu Ka radiation. The particle
morphology was characterized by scanning electron
microscopy (FE-SEM; JEOL, JSM-6700F).
(Fig. 1(a)), small diffraction peaks corresponding y-Al2O3
have been found for the sample calcined at 1000 1C and the
phase transformation of y-Al2O3 to a-Al2O3 completed at
1100 1C. On the contrary, in the case of using aluminum
sulfate precursor (Fig. 1(b)), y-Al2O3 and a-Al2O3 coex-
isted for the sample calcined at 1100 1C and high intensities
of a-Al2O3 peaks were observed after increasing calcination
temperature up to 1200 1C. Normally, the phase transfor-
mation temperature of y-Al2O3 to a-Al2O3 is as high as
1200 1C [8]. It can be seen that the temperature for
complete transformation to a-Al2O3 changed with pre-
cursor and the transformation temperature is lower in the
case of aluminum nitrate precursor as compared to
aluminum sulfate precursor. It was reported that NH4NO3
is formed during the preparation of alumina from
aluminum nitrate and ammonia solution [8]. The NH4NO3
decomposes to release a large amount of energy during
calcination, leading to the decrease of temperature for the
complete transformation of y-Al2O3 to a-Al2O3 [4]. In the
present work, it seems that the presence of NH4NO3 results
in decrease of the complete transformation temperature for
the sample obtained from aluminum nitrate since the
NH4NO3 can be formed only in the case of using aluminum
nitrate precursor.
3. Results and discussion
Fig. 1 shows the XRD patterns of the particles obtained
from the aluminum nitrate or aluminum sulfate precursor
without AlF3 additive and calcined at different temp-
eratures. In the case of using aluminum nitrate precursor
Fig. 2 shows SEM micrographs of the a-alumina
particles obtained without AlF3 additive. The particles
shown in Fig. 2(a) were obtained using aluminum nitrate as
a precursor and calcined at 1100 1C and those in Fig. 1(b)
were obtained aluminum sulfate as a precursor and
calcined at 1200 1C. It can be seen in Fig. 2 that the
particles obtained from aluminum nitrate precursor are
smaller and have a wider size distribution as compared
with those obtained from aluminum sulfate precursor. The
average particle sizes of a-alumina obtained using alumi-
num and nitrate aluminum sulfate precursor were found to
be 118 and 200 nm, respectively. This could be attributed
to the poorer ability of sulfate, compared with nitrate, to
provide colloid stability that results in termination of
agglomeration of the primary particles formed at a larger
size [6]. It can be also observed that the particles obtained
from aluminum nitrate precursor have elongated shape,
while those obtained from aluminum sulfate precursor
have spherical shape. It was observed in the initial stage of
reaction that aluminum nitrate precursor formed volumi-
nous gelatinous precipitate, whereas aluminum sulfate
precursor produced compact granular powders. Similar
phenomena were observed by Ramanathan et al. [6] who
synthesized alumina powder by the reaction of aluminum
nitrate or aluminum sulfate with urea. They claimed that
Al(OH)3 gel and amorphous powder of Al4(OH)10SO4
were formed in the initial stage of reaction from aluminum
nitrate and aluminum sulfate precursor, respectively. Thus
the change of shape of resultant a-alumina particles with
precursor could be attributed to the different character-
istics of precipitate formed in the initial stage of reaction.
The open structure of gelatinous precipitate formed from
aluminum nitrate precursor allowed compact crystallites of
α-Al2O3
θ-Al2O3
1100°C
1000°C
10
20
30
40
50
60
70
80
α-Al2O3
θ-Al2O3
1200°C
1100°C
10
20
30
40
50
60
70
80
2θ (degree)
Fig. 1. XRD patterns of the particles obtained without AlF3 additive
(a) aluminum nitrate precursor and (b) aluminum sulfate precursor.

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Fig. 2. SEM micrographs of the a-alumina particles obtained without AlF3 additive (a) aluminum nitrate precursor calcined at 1100 1C and (b) aluminum
sulfate precursor calcined at 1200 1C.
bonded agglomerates formed initially from aluminum
sulfate precursor retained their structure during crystal-
lization into boehmite, resulting in spherical structure of
α-Al2O3
θ-Al2O3
final a-alumina particles [6].
Fig. 3 shows the XRD patterns of the particles obtained
from the aluminum salt precursors with addition of AlF3
and calcined at different temperatures. It is clear that the
addition of AlF3 reduces the transformation temperature
significantly. The phase transformation to a-Al2O3 com-
pleted at 800 and 900 1C in the case of using aluminum
nitrate and aluminum sulfate precursor, respectively. It was
claimed that an intermediate compound, AlOF, can be
800 °C
formed by addition of metal fluoride in the case of phase
transformation and AlOF accelerates the mass transporta-
700 °C
tion from transition alumina to a-alumina [9].
10
20
30
40
50
60
70
80
Furthermore, it was also reported that the addition of
AlF3 could modify the morphology of a-alumina particles
into plate-like shape [7]. Fig. 4 shows SEM micrographs of
the a-alumina particles obtained with addition of AlF3. As
can be seen Fig. 4, some particles with plate-like shape
were found in the case of using aluminum nitrate precursor
(Fig. 4(a)), while spherical particles were formed in the case
of using aluminum sulfate precursor (Fig. 4(b)). It could be
said that the effect of AlF3 additive on modification of
morphology of particles is different depending on the
precursor. It may be attributed to the different character-
istics of precipitate formed in the initial stage of reaction.
As mentioned above, the rearrangement of crystallites of
boehmite formed from aluminum nitrate precursor can
take place easily, whereas the compact and strongly
bonded agglomerates formed from aluminum sulfate
precursor retained their structure during crystallization.
α-Al2O3
θ-Al2O3
900 °C
800 °C
10
20
30
40
50
60
70
80
2θ (degree)
Fig. 3. XRD patterns of the particles obtained with AlF3 additive (a)
aluminum nitrate precursor and (b) aluminum sulfate precursor.
4. Conclusions
The temperature for complete transformation to a-Al2O3
is lower in the case of using aluminum nitrate precursor as
compared to aluminum sulfate precursor. The size of
alumina particles obtained using aluminum nitrate pre-
cursor was smaller than that obtained using aluminum
boehmite formed to undergo rearrangement and agglom-
eration freely, resulting in the evolution of the typical
morphology characteristics of boehmite, namely elongated
structure [6]. On the other hand, the compact and strongly

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H.J. Kim et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1521–1524
1524
Fig. 4. SEM micrographs of the a-alumina particles obtained with AlF3 additive (a) aluminum nitrate precursor calcined at 800 1C and (b) aluminum
sulfate precursor calcined at 900 1C.
sulfate precursor, and it may be due to the poorer ability of
sulfate to provide colloid stability. It was also observed
that the particles obtained from aluminum nitrate pre-
cursor have elongated shape, while those obtained from
aluminum sulfate precursor have spherical shape, and it
could be attributed to the different characteristics of
precipitate formed in the initial stage of reaction. The
AlF3 additive decreased the phase transition temperature
to a-alumina. The effect of AlF3 additive on modification
of morphology of particles is different depending on the
precursor. It was observed that, only in the case of using
aluminum nitrate precursor, the addition of AlF3 changed
the morphology of a-alumina particles to the plate-like
shape.
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This research was supported by the Program for the
Training of Graduate Students in Regional Innovation
which was conducted by the Ministry of Commerce
Industry and Energy of the Korean Government.