Synthesis of α-Al2O3 Nanopowder by
Microwave Heating of Boehmite Gel
T. Ebadzadeh and L. Sharifi*
Abstract. Alumina nanopowders were prepared by a sol-gel process from the
mixture of boehmite gel and carbon black followed by microwave heating (2.45
GHz and 900 W) at different times. The average size of boehmite particles in sol
was 25 nm. After heating, the products were characterized by powder x-ray diffraction. The results show that the main phase is γ-Al2O3 at low heating times (4
and 6 min, 610° and 790°C, respectively), while after 8 min heat-treatment
(960°C), besides the strong peaks of γ-Al2O3, the weak peaks of α- Al2O3 were
also appeared. α- Al2O3 was the only crystalline phase after 10 min heating
(1050°C). Synthesized particles were observed by TEM and the average size of
particles was measured to be 35 nm. The surface area of powder heated for 10
min was 51 m2/g.
1 Introduction
It is generally desirable that the transformation of boehmite (AlOOH) to αalumina occurs as fast as possible and at as a low temperature as possible to
minimize the energy that is used in the process [1]. Microwave at the 2.45 GHZ
frequently are used almost universally for industrial and scientific applications.
Some reasons for the growing interest in using microwave energy in ceramic materials processing are the rapid kinetics (saving of time and energy) and synthesis
of new materials. Compared to conventional heating, microwave leads to the more
uniform and rapid heating followed by lowering process temperatures which are
suitable to give nano-sized powders [2,3].
In this study, high specific surface area carbon black was used to form an obstacle during the thermal processing to produce alumina nanopowders from microwave heating of boehmite precursor.
2 Experimental Procedure
Aluminum nitrate (Al(NO3)3.9H2O, MERCK) was used as a starting material for
preparation of boehmite gel. In this manner aluminum salt was dissolved in
T. Ebadzadeh and L. Sharifi
Ceramic Division, Materials & Energy Research Centre, Tehran, Iran
416
T. Ebadzadeh and L. Sharifi
deionized water in the ratios of 1 to 20 at 80 °C. The pH of the solution was
reached to 8 by adding of NH4OH. The evaporation of water from the solution was
performed on a hot plate at 80 °C while stirred severely and dried at 100 °C. The
precursor powder was heated in a microwave oven (900 W and 2.45 GHz) for 10,
15 and 20 minutes. The equivalent temperature of each heating time is shown in
Table 1. The progress of formation of α-alumina was examined by x-ray analysis
with a Philips' Diffractometer Simens D-500 system, using CuKα radiation. The
average crystallite size (t) was calculated using Scherrer's equation:
t = −0.9
λ
cos θ
β
(1)
where λ is the wavelength of X-rays, β is the half-width corrected using the (111)
line of the silicon as the standard. The morphology of synthesized powders was
observed by TEM (CM 200 FEG-Philips). Surface area analysis of the alumina
powders was performed by a BET technique with N2 adsorption (Micrometrics,
Gemini 2375).
600
B
Intensity(a.u)
500
400
B
300
B
B
B
200
100
0
5
15
25
35
45
55
65
75
2teta (Degree)
Fig. 1 XRD pattern of synthesized boehmite
3 Results and Discussion
The x-ray pattern of synthesized boehmite is shown in Figure 1. The broad peaks
of synthesized boehmite reveal the small crystallite size. Figure 2 shows phase development of the boehmite precursor during microwave heating for different
times. As observed, γ-alumina is the only crystalline phase after heating of precursor powder for 10 min. By increasing the heating time to 15 min the sharp peaks
Synthesis of α- Al2O3 Nanopowder by Microwave Heating of Boehmite Gel
417
Intensity(arbitrary unit)
of α-alumina are appeared while the weak peaks of γ-alumina are still distinguishable. α-alumina is the only crystalline phase after 20 min heating. The crystallite
size measured on x-ray peaks was about 35 nm which has in high correspondence
with size of particles shown by TEM (Fig. 3).
20
α
α
α
α
α
α
α
α
α
20 min
15 min
24 28
32 36
γ
γ
γ
40 44
48 52
56 60
64 68
10 min
72
Two Theta
Fig. 2 XRD patterns of the alumina precursor heated for different times (α: α-alumina and γ: γalumina)
Fig. 3 TEM micrograph of alumina precursor heated for 20 min
418
T. Ebadzadeh and L. Sharifi
Table 1 Temperature-heating time values
Time (min)
Temperature (°C)
10
784
15
20
1030
1205
References
1. Kumagai, M., Messing, G.L.: Controlled transformation and sintering of a boehmite solgel by α-alumina seeding. J. Am. Ceram. Soc. 68, 500–505 (1985)
2. Binner, J.G.P., Hassine, N.A., Cross, T.E.: Microwave synthesis of nanoxide ceramic
powders. Ceramic Transactions 59, 565–572 (1995)
3. Gedye, R., Westaway, K., Smith, F.: The application of microwave to rapid chemical
synthesis. Ceramic Transactions 59, 525–531 (1995)