Phase composition of Al2O3 nanopowders prepared by plasma synthesis and heat-treated

Article abstract of DOI:10.1134/S0020168512040152

We present X-ray diffraction data for Al₂O₃ nanopowders prepared by oxidizing aluminum powder in an air plasma, followed by size separation via centrifugation and heat treatment. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0020168512040152

ISSN 0020ꢀ1685, Inorganic Materials, 2012, Vol. 48, No. 4, pp. 342–349. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.P. Sirotinkin, V.F. Shamrai, A.V. Samokhin, N.V. Alekseev, M.A. Sinaiskii, 2012, published in Neorganicheskie Materialy, 2012, Vol. 48, No. 4,
pp. 409–416.
Phase Composition of Al2O3 Nanopowders Prepared by Plasma
Synthesis and HeatꢀTreated
V. P. Sirotinkin, V. F. Shamrai, A. V. Samokhin, N. V. Alekseev, and M. A. Sinaiskii
Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, 119991 Russia
eꢀmail: sir@imet.ac.ru
Received October 12, 2011
Abstract—We present Xꢀray diffraction data for Al2O3 nanopowders prepared by oxidizing aluminum powder
in an air plasma, followed by size separation via centrifugation and heat treatment.
DOI: 10.1134/S0020168512040152
INTRODUCTION
samples obtained through size classification and heat
treatment of the nanopowders. The plasmaꢀprepared
Al2O3 nanopowder had a specific surface area of
26.6 m2/g and consisted of spherical particles.
The nanopowders were classified into size ranges
by centrifuging aqueous suspensions (Sigma 2ꢀ16P
centrifuge). The suspensions were prepared by sonicaꢀ
tion (Bandelin SONOPULS HD 3100 ultrasonic
homogenizer). The samples obtained by size separaꢀ
tion had specific surface areas of 38.4, 30, 24.4, 19.4,
15.3, and 6.7 m2/g.
Alumina is widely used in the production of refracꢀ
tory ceramics, catalysts, protective coatings, etc. Al2O3
preparation and processing techniques have been the
subject of extensive studies for more than hundred
years now. Al2O3 can be prepared by many processes,
including plasma synthesis, which yields Al2O3 nanopꢀ
owders consisting of spherical particles. Despite the
intense research in this area, there is currently insuffiꢀ
cient information about the phase composition of
Al2O3 powders prepared by different techniques and
the polymorphic transformations induced by subseꢀ
quent processing. The reason for this is that there are
many metastable Al2O3 polymorphs, which form
depending on the preparation technique and condiꢀ
tions and the nature of the starting materials. Most
previous studies employed samples prepared from
boehmite or gibbsite. The phase composition of Al2O3
prepared by plasma synthesis has been determined in a
limited number of studies [1–3]. In addition, new data
on Al2O3 polymorphism and various plasma synthesis
apparatuses and procedures proposed to date have led
us to redetermine the phase composition of Al2O3
powders prepared by this method.
The plasmaꢀprepared Al2O3 was heatꢀtreated in an
electric furnace in air for 2 h at 600, 800, 1000, 1150,
1230, and 1300°С. The specific surface areas of the
heatꢀtreated powders were 25.4, 25.3, 23.4, 22.2, 10.1,
and 6.1 m2/g, respectively.
The phase composition of the powders was deterꢀ
mined by Xꢀray diffraction (XRD) on a Rigaku
Ultimaꢀ4 Xꢀray diffractometer (Japan) with filtered
Cu Kα radiation (D/teX highꢀspeed detector, PDXL
software package, PDFꢀ2 database).
RESULTS AND DISCUSSION
The purpose of this work was to study the phase
composition of Al2O3 nanopowders prepared by oxiꢀ
dizing aluminum powder in an air plasma of an arc
plasma generator in a reactor with a restricted jet flow
[4]. To gain insight into possible relationships between
the phase composition and average particle size of aluꢀ
mina, the nanopowders obtained in the plasma reactor
were investigated after size separation by centrifugaꢀ
tion. In addition, we examined the effect of subseꢀ
quent heat treatment on the phase composition of the
synthesized Al2O3 nanopowders.
Before discussing the present results, it is worth
analyzing available information and ICDD (JCPDS)
Powder Diffraction File data for the Al2O3 polymorꢀ
phs. Since this study deals with Al2O3 powders preꢀ
pared by plasma synthesis from aluminum, we will not
consider phases obtained by dehydrating boehmite,
gibbsite, or other oxyhydroxides.
The Al2O3 polymorphs (usually denoted by differꢀ
ent Greek letters) can be divided into several groups.
One group comprises the lowꢀtemperature metastable
phases γ, η, and χ [5]. According to Tsybulya and
Kryukova [5], these polymorphs have spinelꢀrelated
structures with different octahedral and tetrahedral
site occupancies. They contain stacking faults and
EXPERIMENTAL
We studied Al2O3 nanopowders prepared by oxidizꢀ have a cubic unit cell with an identity period of about
ing ASDꢀ4 aluminum powder in an air plasma and also 7.9 Å. Characteristically, their powder XRD patterns
342

PHASE COMPOSITION OF Al2O3 NANOPOWDERS PREPARED BY PLASMA
343
280000
260000
240000
220000
200000
180000
160000
140000
120000
100000
80000
60000
40000
20000
0
100
50
0
46ꢀ1212
100
50
23ꢀ1009
0
20
30
40
50
60
70
2θ, deg
Fig. 1. XRD pattern of the Al O powder prepared from aluminum by plasma synthesis and annealed at 1300°С for 2 h, and scheꢀ
2
3
matic XRD patterns of αꢀ and θꢀAl O (cards 46ꢀ1212 and 23ꢀ1009, respectively).
2 3
b
= 7.956
as . Card 46ꢀ1215 presents data for a sample
obtained by plasmaꢀspraying ꢀAl2O3 into water.
Tsybulya and Kryukova [5] described a tetragonal
ꢀphase obtained by annealing the ꢀphase at 1513 K
(card 56ꢀ1186, = 7.9631 = 23.3975 Å).
The stable highꢀtemperature phase is
Å, c = 11.711 Å [3]). This phase is designated
show broad diffraction peaks. The γꢀphase is comꢀ
monly identified using the JCPDS PDF card 10ꢀ425
(see, e.g., Rozita et al. [6]).
δ*
α
Another group is constituted by the highꢀtemperaꢀ
δ
γ
ture metastable phases
for the ꢀphase are presented in card 23ꢀ1009 (sp. gr.
2/ a = 11.813 = 2.906 = 5.625
β = 104.10 [7]). The ꢀphase has been the subject of
δ, θ, and κ. Reliable XRD data
a
Å, c
θ
α
ꢀAl2O3
C
m,
Å
δ
,
b
Å,
c
Å,
(corundum). The XRD data for this polymorph are
presented in card 46ꢀ1212.
°
many studies, but its structure is still open to question.
According to Levin and Brandon [8] and Jayaram and
The transformation sequence reported by many
researchers (see, e.g., Refs. [8–10]) for metastable
phases obtained by heating amorphous or liquidꢀ
Levi [9], the
ture (sp. gr.
Card 46ꢀ1215 describes metastable orthorhombic
Al2O3 with smaller unitꢀcell parameters ( = 7.934 Å,
δꢀpolymorph has an orthorhombic strucꢀ
P
222 or 212121, aγ aγ 1.5aγ).
P
a
,
b
2 , c
quenched Al2O3 has the form γ → δ/θ → α. The γ to
α
phase transition occurs in the temperature range
a
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344
SIROTINKIN et al.
140000
130000
120000
110000
100000
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
20
30
40
50
60
70
46ꢀ1212
100
50
0
100
50
23ꢀ1009
46ꢀ1215
0
100
50
0
20
30
40
50
60
70
2θ, deg
Fig. 2. XRD pattern of the Al O powder prepared from aluminum by plasma synthesis and annealed at 1230°С for 2 h, and scheꢀ
2
3
matic XRD patterns of αꢀ, θꢀ, and δ*ꢀAl O (cards 46ꢀ1212, 23ꢀ1009, and 46ꢀ1215, respectively).
2 3
amounts of
patterns of the Al2O3 powders annealed at 1150, 1000,
800, and 600 for 2 h are very similar to that of the asꢀ
prepared powder (Fig. 3): there are a large number of
diffraction peaks, many of which overlap one another.
At the same time, the XRD intensities depend on
annealing temperature and there is a prominent
“hump” (increased background) in the angular range
θꢀAl2O3 and δ*ꢀAl2O3 (Fig. 2). The XRD
from 670 to 1200°С [10]. For alumina prepared by
chemical vapor deposition, the transformation
sequence has the form κ → α [8]. For all of the metaꢀ
stable Al2O3 polymorphs, the exact phase transition
temperatures are unknown.
°С
According to our results, the sample annealed at
1300
small amount of
heat treatment at 1300
obtaining phaseꢀpure corundum. The sample
annealed at 1230 contained ꢀAl2O3 and significant
°С
contained
ꢀAl2O3 (Fig. 1). This indicates that
for 2 h is insufficient for
αꢀAl2O3 (corundum) and a very
θ
2
θ = 30
above 600
patterns in Fig. 4 illustrate the annealing effect on the
°
–40
° for the powders that were not annealed
°С
°С. The characteristic portions of the XRD
°С
α
INORGANIC MATERIALS Vol. 48
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2012

PHASE COMPOSITION OF Al2O3 NANOPOWDERS PREPARED BY PLASMA
345
120000
1150
°
C
C
80000
40000
0
1000
°
800
°
°
C
C
600
20
40
60
2θ, deg
Fig. 3. XRD patterns of Al O powders prepared by plasma synthesis and then annealed for 2 h at different temperatures.
2
3
θ
δ
*
(а)
δ
*
δ
*
θ
120000
100000
80000
60000
40000
20000
1
2
3
4
5
0
43
44
45
46
, deg
47
48
49
2θ
δ
*
δ*
(b)
35000
30000
25000
20000
15000
10000
5000
δ
*
θ
θ
5
4
3
2
1
0
43
44
45
46
, deg
47
48
49
2θ
Fig. 4. Portions of the XRD patterns of Al O powders: (a) after (
1
) plasma synthesis and subsequent annealing at ( ) 1150,
2
2
3
(3
) 1000, (
4
) 800, and (
5
) 600
°
C; (b) after separation by centrifugation into size fractions with specific surface areas of (1) 38.4,
2
(2
) 30, ( ) 24.4, (
3
4) 19.4, and (
5
) 15.3 m /g.
INORGANIC MATERIALS Vol. 48
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346
SIROTINKIN et al.
40000
30000
20000
10000
0
5
4
3
2
1
10
20
30
40
50
θ, deg
60
70
80
2
Fig. 5. XRD patterns of the synthesized Al O powder after separation by centrifugation into size fractions with specific surface
2
3
2
areas of (1) 38.4, (2) 30, (3) 24.4, (4) 19.4, and (5) 15.3 m /g.
phase composition of the Al2O3 powders. The major and causes an additional peak to emerge near 43.5
The powder with a specific surface area of 6.7 m2/g
contains, in addition to ꢀAl2O3, the *ꢀ and ꢀphases.
°.
phase is
δ*ꢀAl2O3, which is characterized by two overꢀ
δ
δ
θ
lapping diffraction peaks, similar in intensity, near
45.5
the
°
and a weaker peak near 46.5
°. The percentage of
The table presents XRD data for the phases identiꢀ
fied in the Al2O3 powder after size separation by cenꢀ
trifugation (size fraction with a specific surface area of
38.4 m2/g). The phases present were identified based
on the data in card 46ꢀ1215 (and, accordingly, in Ref.
θꢀphase increases with annealing temperature.
The higher intensity of the smaller angle component
of the peak around 45.5 , the peak near 37.3 , and the
aboveꢀmentioned hump in the XRD pattern of the asꢀ
°
°
prepared Al2O3 powder can be interpreted as evidence
[3]) for
were refined in space group
ware [11]. We obtained = 7.933(2)
and = 11.715(3) Å, in good agreement with those
δ
*ꢀAl2O3. The orthorhombic cell parameters
222 using CELREF softꢀ
= 7.965(3) Å,
for the presence of the
parameter of about 7.96 Å. The peak near 37.3
2.41 , unrelated to *ꢀAl2O3, is then the strongest
reflection, 311, from a cubic spinel phase. The other
peaks from ꢀAl2O3 overlap peaks from the ꢀphase.
γꢀphase with a cubic cell
P
° (d
а
Å,
b
Å
)
δ
c
reported by Fargeot et al. [3]. As seen in the table, the
γ
δ*
Al2O3 powder with a specific surface area of 38.4 m2/g
The XRD patterns of the Al2O3 size fractions with
specific surface areas of 38.4, 30, 24.4, 19.4, and
15.3 m2/g (Fig. 5) differ little. The major phase is
contains a small amount of the θꢀphase. Note also that
3 of the 46 diffraction peaks indicated in the table canꢀ
not be assigned to any of the phases identified. These
peaks may be due to small amounts of other phases or
δ*ꢀAl2O3. With decreasing specific surface area, the
percentage of the θꢀphase increases. The XRD pattern
to an increase in the unitꢀcell parameters of δ*ꢀAl2O3
of the powder with a specific surface area of 6.7 m2/g
(Fig. 6) differs from those of the other size fractions,
especially in the angular range represented in Fig. 6b.
[8, 9]. The peak corresponding to an interplanar spacꢀ
ing of 2.41 Å was assumed to have the Miller indices
302 and 032, which are missing in Ref. [3] and card
46ꢀ1215. As mentioned above, this peak may arise
In this case, the tetragonal
leads to a marked increase in the intensity of the higher
angle component of the doublet peak around 45.5
δꢀphase prevails, which
°
from the γꢀphase (cubic spinel).
INORGANIC MATERIALS Vol. 48
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2012

PHASE COMPOSITION OF Al2O3 NANOPOWDERS PREPARED BY PLASMA
347
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
(а)
20
40
60
80
100
56ꢀ1186
46ꢀ1215
23ꢀ1009
50
0
100
50
0
100
50
0
20
40
60
80
2θ, deg
(b)
20000
δ
δ
*
δ
*
δ
*
δ
δ
δ
δ
θ
10000
θ
*
δ
0
43
44
45
46
, deg
47
48
49
2θ
2
Fig. 6. (a) XRD pattern of the synthesized Al O powder with an average specific surface area of 6.7 m /g and (b) characteristic
portion of the XRD pattern. Also shown are schematic XRD patterns of
23ꢀ1009, respectively).
2
3
δꢀ, δ*ꢀ, and θꢀAl O (cards 56ꢀ1186, 46ꢀ1215, and
2 3
INORGANIC MATERIALS Vol. 48
No. 4 2012

348
Phases identified in the Al2O3 powder with a specific surface area of 38.4 m2/g
, % , Å , % hkl , Å , % , Å , % , Å
refined data
SIROTINKIN et al.
I
d
I
d
I
d
I
d
I
, %
hkl
d
, Å
I
, %
d, Å
refined data
of card 46ꢀ1215
for *ꢀAl2O3
data of card 23ꢀ experimental
data of card 23ꢀ
experimental data
of card 46ꢀ1215
for *ꢀAl2O3
1009 for
θ
ꢀAl2O3
data
1009 for
37
θ
ꢀAl2O3
δ
δ
4
3
7.9564
6.5668
10
0 1 0 7.9702
1 0 0 7.9339
1 0 1 6.5694
0 1 1 6.5899
1 3 2 2.3130
26
2
9
71
5
43
4
2
1
3
2.2810
50 2 2 3 2.2810
2.257
10
2.2208
2.1615
1.9907
1.9900
1.9536
1.8012
1.7584
1.7320
1.7004
10 1 1 5 2.1625
80 0 4 0 1.9912
80 4 0 0 1.9832
50 0 0 6 1.9524
10 2 2 5 1.7995
1
11
2
5.4830
5.0610
4.7090
9
5.45
45
2.019
50
10
1 1 1 5.0675
1 0 2 4.7119
0 1 2 4.7186
4
7
2
2
3
4.5300
4.0570
3.7461
3.6460
3.5510
17
4.535
10
5
1 1 2 4.0554
2 0 1 3.7569
1 4 3 1.7311
4 2 2 1.6989
2 4 2 1.7027
5
5
2 1 0 3.5505
1 2 0 3.5592
2 1 1 3.3979
1 2 1 3.4054
2 0 2 3.2842
1 1 3 3.2068
2 1 2 3.0362
0 0 4 2.9285
9
6
1.6156
1.6041
10 1 4 4 1.6123
10 1 1 7 1.6039
2 2 6 1.6034
10 5 1 1 1.5426
0 2 7 1.5428
5
3.3960
10
4
1.5442
23
1.5426
6
4
4
2
6
24
18
34
2
3.2770
3.2100
3.0310
2.9200
2.8520
2.7864
2.7288
2.5938
2.5154
10
10
10
5
2 0 7 1.5418
3 1 6 1.5410
5
13
9
2
2
1.5405
1.5114
1.5036
1.4872
1.4615
10 1 5 2 1.5090
10 5 1 2 1.5039
4 1 5 1.4871
10 3 3 5 1.4633
3 2 6 1.4612
80
64
2.837
2.73
50
50
80
0 2 3 2.7881
2 2 1 2.7328
1 1 4 2.5973
1 3 0 2.5176
3 1 0 2.5096
3 1 1 2.4539
1 3 1 2.4614
3 0 2 2.4101
0 3 2 2.4181
0 2 4 2.3593
2 0 4 2.3560
3 1 2 2.3068
26
26
1.4883
1.4526
2 3 6 1.4621
2 5 2 1.4332
1 1 8 1.4171
3 0 7 1.4141
0 3 7 1.4157
1
12
1.4347
1.4147
55
11
5
2.4574
2.4106
2.3609
2.3079
80
62
2.444
5
10
1.4264
1.3883
68
100
1.4043
80 4 4 0 1.4052
5
1.3921 100 4 0 6 1.3913 100
100 0 4 6 1.3942
25
50
46
2.315
The phase composition of the Al2O3 powders preꢀ major metastable phase in the mixture was
δ
*ꢀAl2O3
The synthesized powder contained, in addition to the
ꢀphase, the ꢀphase, tetragonal ꢀphase, and, possiꢀ
bly, ꢀphase (cubic symmetry). The percentage of
*ꢀAl2O3 in different size fractions was found to
.
pared by plasma synthesis can probably be determined
more reliably by the Rietveld profile analysis method.
δ*
θ
δ
γ
δ
CONCLUSIONS
decrease with decreasing specific surface area. The
powder with a specific surface area of 38.4 m2/g conꢀ
The present results on the whole agree with previꢀ
ous data for the metastable Al2O3 polymorphs forming
during plasma synthesis. The synthesis conditions
employed in this study and the use of aluminum as a
starting material led to the formation of a mixture of
tained only trace levels of
powder with a specific surface area of 6.7 m2/g, the tetꢀ
ragonal ꢀphase prevailed. The annealingꢀinduced
transition to ꢀAl2O3 occurred through the ꢀpolyꢀ
θꢀAl2O3 and γꢀAl2O3. In the
δ
α
θ
metastable Al2O3 polymorphs, without αꢀAl2O3. The morph. A marked increase in the percentage of
INORGANIC MATERIALS Vol. 48
No. 4
2012

PHASE COMPOSITION OF Al2O3 NANOPOWDERS PREPARED BY PLASMA
XꢀRay Powder Diffraction Patterns of LowꢀTemperature
ꢀAl2O3 was observed even just above 600°С. The phase
349
θ
Polymorphs, Phys. Rev. B: Condens. Matter Mater. Phys.
,
formation process is probably also influenced by the
2008, vol. 77, paper 024 112.
highꢀtemperature annealing time.
6. Rozita, Y., Brydson, R., and Scott, A.J., An Investigation
of Commercial GammaꢀAl2O3 Nanoparticles, J. Phys.:
Conf. Ser., 2010, vol. 241, paper 012 096.
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