SYNTHESIS OF α-ALUMINA FINE POWDERS WITH SEEDING BY AN EGG
WHITE SIMPLE ROUTE
R. E. P. Salem1,3*; E. M. Silva Jr.2; A. L. Chinelatto2; A. S. A. Chinelatto2;
E. M. J. A. Pallone3
1
Department of Materials Engineering, Paraná Federal University of Technology, Av. Dos
Pioneiros, 3131, Londrina, Paraná, Brazil, 86036-360
2
Department of Materials Engineering, Ponta Grossa State University, Av. Gal. Carlos
Cavalcanti, 4748, Ponta Grossa, Paraná, Brazil, 84030-900
3
FZEA – São Paulo University, Av. Duque de Caxias Norte, 225, Pirassununga, São Paulo,
Brazil, 13635-900
Alumina ceramics are used in many areas of industry because of their excellent mechanical, thermal and optical
properties. The synthesis of alumina powders in the stable α-alumina phase generally requires calcination
temperatures of above 1200ºC, in order to promote the complete transformation from metastable alumina
phases. Many studies have been done trying to reduce temperature of nucleation and growth of α-alumina,
intending to produce a stable powder with smaller particle size. In this work, a simplified route using egg white
and aluminium nitrate in aqueous medium was used to synthesize powders of ultrafine alumina. A small quantity
of submicrometric α-alumina seeds (1% in weight) was added to the gel as an attempt to decrease the energy of
activation and, consequently, the temperature of formation of α-alumina. The egg white-based precursor was
characterized by differential thermal analysis associated with thermogravimetry and calcined at 700-1200ºC
under air. The resulting powders were characterized by X-ray diffraction, scanning electron microscopy and
infrared spectroscopy. Results showed that the egg white route was effective to synthesize submicrometric
alumina powders, and the seeded powders promoted formation of α-alumina particles at lower calcination
temperatures than non-seeded powders.
Keywords: α-alumina, synthesis, egg white, seeds, calcination.
Introduction
Alumina is a ceramic oxide with wide applicability due to its good mechanical properties and
high refractoriness. The most stable alumina phase, the α phase, is used based on these
properties. The α-alumina produced by conventional process (Bayer process) forms at
temperatures near 1200oC, having the morphology of micrometric and submicrometric
particle aggregates. However, various chemical routes have been widely studied in order to
synthesize α-alumina and other ceramic oxides, aiming to produce a more homogeneous
material at lower temperatures, thus preventing particle growth and aggregation and
controlling more rigorously the properties of final powders.
Recent works [1,2] have shown synthesis of oxides using egg white as precursor medium.
Egg white is predominantly composed of water (>80%) and the remainder is composed of
proteins and a small amount of sugars and mineral salts. Among the proteins, the one in
largest quantity is ovalbumin, which has a molecule with charged sites, in which ions are able
to solvate in solution [2].
The main motivation to use egg white is that it has been confirmed as very suitable matrix to
uptake of metallic ions. After thermal treatment in relatively low temperatures, it allows the
formation of amorphous and crystalline powders, nanometric and submicrometric, in a
simple, economical and enviromentally sustainable way. In order to provide low-energy sites
for the formation of α-phase at lower temperatures than in the conventional process, seeding
of α-alumina particles into the precursor on chemical synthesis has been reported with success
[3-4]. The purpose of this work is to study the synthesis of α-alumina with seeding by the egg
white route, evaluating phase evolution, morphology and microstructure of the powders, and
consequently, the viability of this method for this material.
Experimental part
Aluminium nitrate nona-hydrate (Al(NO3)3.9H2O, Vetec, Brazil) was dissolved in a small
amount of water and dropped in a fresh extracted egg white aqueous solution (60% vol) under
magnetic stirring. After 10 minutes, α-alumina seeds (AKP-53, Sumitomo Chemical, Japan)
were added to the mixture. The weight of seeds added was 1% of the estimated mass of
alumina to be produced. After vigorous stirring for 30 minutes, the mix was kept on hot plate
(110oC) for 24 hours and the dry precursor was crushed in agate mortar and calcinated at
500oC in air for 2 hours. The resulting powder was divided in equal parts and calcined in
temperatures varying between 700-1200oC, with intervals of 50oC. The precursors and the
calcined powders were characterized by X-ray diffraction, differential thermal analysis with
thermogravimetry, scanning electron microscopy with elemental analysis by EDX and
infrared spectroscopy.
Results and discussion
Fig. 1 shows DTA/TG curve of seeded egg white/aluminium nitrate precursor after drying on
hot plate. These curves were similar to the curves obtained from non-seeded precursor. In
thermogravimetry it is observed a continuous mass loss, attaining about 80% at 1300oC. The
mass loss was most vigorous under 500oC, corresponding to the elimination of most of the
organic matter from the egg white precursor and hydration water of the aluminium salt. The
subsequent mass loss is related to residual organic matter slowly released due to the complex
morphology of the powder and its changes with phase transformations. The DTA peak at
790.5oC corresponds to the transformation of amorphous alumina to γ-alumina transition
phase. In the range of 950-1200oC it is observed a band in the DTA curve, which corresponds
to the continuous process of phase transformation from transition aluminas to α-alumina,
since this transformation is reconstructive, involving breakage and rearrangement of chemical
bonds in order to form octahedral Al – O bonds.
Fig. 1 – Differential thermal analysis / thermogravimetry of seeded precursor to 1300 oC.
Fig. 2 shows X-ray diffraction of calcined powders. It can be seen the evolution from
transition phases into α-phase with increase of calcination temperature. Non-seeded powders
(Fig. 2 (a)) showed traces of α-phase from 1050oC, while seeded powders (Fig. 2 (b)) formed
α-alumina at temperatures as low as 900oC. This results show that alumina seeds played an
important role on the reduction of phase transformation temperatures, although at all
calcination temperatures there can be found traces of transition aluminas (mostly γ-alumina).
Powders calcinated at 1150 and 1200oC also showed peaks of β-alumina (Na2O.11Al2O3),
possibly with Na+ cations partially substituted by K+ cations. This phase has variations on
stoichiometry in function of temperature and crystallizes only at temperatures above 1100 oC.
The formation of this phase was unexpected, however it was possible due to the residual
sodium and potassium present after calcination of egg white. EDS results showed presence of
small amounts of these elements in the calcinated powders, confirming DRX results.
(a)
(b)
Fig. 2 – X-ray diffractometry of calcined alumina powders for 2 hours. (a) non-seeded (b) seeded.
Conclusions
Changes on the surface of titanium by plasma nitriding can promote changes in the properties of its
surface. These properties are related to the biological response of the surface. In this work, the
evaluation of the biologic response relative to the surface of the new biomaterial showed that the
behavior of osteoblasts, was influenced by the roughness of the surface wettability and topography of
titanium subjected to this treatment. The roughness values obtained were significantly different
between the smooth samples and treated samples and in competition between the two surfaces, the cell
not only had preference for the treated part, but this was more intense than in the case of exposure of
the surface to surface completely treated.
References
[1] Dhara, S., 2005. Synthesis of nanocrystalline alumina using egg white. J. Am. Cer. Soc.,
88, 2003-2004.
[2] Al-Angari, Y. M., 2011. Magnetic properties of La-substituted NiFe2O4 via egg-white
precursor route. J. Mag. Mag. Mat., 323, 1835-1839.
[3] Kumagai, M.; Messing, G. L., 1984. Enhanced densification of boehmite sol-gels by αalumina seeding. J. Am. Cer. Soc., 67, 230-231.
[4] Shiau, F. S.; Fang, T. T., 1999. Low temperature synthesis of α-alumina using citrate
process with α-alumina seeding. Mat. Chem. Phys., 60, 91-94.
Acknowledgement:
CAPES, Fundação Araucária, CNPq.