The influences of heat treatment on the structural properties of lithium aluminates

Article abstract of DOI:10.1016/S0022-3697(00)00239-0

Two synthesis routes, sol precipitation and sol gelation, using aluminum isopropoxide and lithium nitrate as precursors have been used to prepare lithium aluminate. The sol precipitation method (LISOL) produced a mixture of γ-LiAlO₂ and LiAl₅O_8, and sol gelation (LIGEL) produced pure γ-LiAlO₂, both at 750°C. The density and specific surface area of the ceramic powders calcined at 1150°C were 2.72 g cm^-3 and 6 m² g^-1, respectively, for the powders obtained by sol precipitation and 2.47 g cm^-3 and 2 m² g^-1, respectively, for the powders obtained by sol gelation. The synthesized powders presented thermal stability over 600°C and the sintering process occurred in the temperature range of 600-1400°C. The absorbed heat by the material in this temperature range during DTA/TG experiment showed that the sintering process was more efficient when the powders were previously calcined over 550°C.

Full text of DOI:10.1016/S0022-3697(00)00239-0

Journal of Physics and Chemistry of Solids 62 (2001) 857±864
www.elsevier.nl/locate/jpcs
The in¯uences of heat treatment on the structural properties of
lithium aluminates
*
R.A. Ribeiro, G.G. Silva, N.D.S. Mohallem
 Â
Laboratorio de Materiais, Depto de Quõmica, ICEx, UFMG, 31270-901, Belo Horizonte, MG, Brazil
Received 12 May 2000; accepted 11 August 2000
Abstract
Two synthesis routes, sol precipitation and sol gelation, using aluminum isopropoxide and lithium nitrate as precursors have
been used to prepare lithium aluminate. The sol precipitation method (LISOL) produced a mixture of g-LiAlO2 and LiAl5O8,
and sol gelation (LIGEL) produced pure g-LiAlO2, both at 7508C. The density and speci®c surface area of the ceramic powders
calcined at 11508C were 2.72 g cm23 and 6 m2 g21, respectively, for the powders obtained by sol precipitation and 2.47 g cm23
and 2 m2 g21, respectively, for the powders obtained by sol gelation. The synthesized powders presented thermal stability over
6008C and the sintering process occurred in the temperature range of 600±14008C. The absorbed heat by the material in this
temperature range during DTA/TG experiment showed that the sintering process was more ef®cient when the powders were
previously calcined over 5508C. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
alkoxides [11] at temperatures as low as 5508C. Hirano
et al. [12] prepared this aluminate by hydrolysis of
lithium and aluminum ethoxides, producing crystalline
material at 10008C. Sol-gel synthesis has been used to
prepare g-LiAlO2 from aluminum sec-butoxide. The
sources of lithium can be LiOH, lithium methoxide or
lithium acetate which produce g-LiAlO2 at 750 [13],
1000 [13] or 12008C [14], respectively. Lithium alumi-
nate can be also obtained at 12008C by combustion
process [15] and reaction of LiOH and metallic alumi-
num [16] at 11738C. Recently, g-LiAlO2 has been
prepared by combustion synthesis from LiNO3 and
Al(NO3)3.´9H2O [17] and synthesis from bulky lithium
and aluminum alkoxides [18].
Lithium aluminate presents three crystalline phases: a-,
b- and g-LiAlO2 with hexagonal, monoclinic and tetragonal
structures, respectively [1]. The a phase (d ˆ 3.40 g cm23
)
is denser than the g phase (d ˆ 2.61 g cm23) [2], which has
been studied because of its application as a solid tritium
breeder material in fusion reactors [3±8]. This ceramic
phase has good thermochemical and irradiation stability,
low hydroscopicity and chemical compatibility with coolant
renders. g-LiAlO2 is also used as ceramic matrix in molten
carbonate fuel cells [9] and mixed with polymer electrolytes
to improve their mechanical stability at elevated tempera-
tures [10]. For these kinds of applications, this ceramic
needs a highly pure phase with well-de®ned stoichiometric
composition and homogeneous grain size, as well as homo-
geneous porosity.
Another aluminate, LiAl5O8, has been prepared using
intercalation compounds of aluminum hydroxide with
lithium salts [19], through a wet chemical route of gel to
crystallite conversion and hydrothermal synthesis [20]. A
mixture of g-LiAlO2 and LiAl5O8 was obtained when
lithium hydrotalcite, [Al4Li2(OH)12]CO3´4H2O, was heated
at high temperatures (,10008C) [21].
The purpose of this investigation was to study the
structural and thermal changes during the preparation
of lithium aluminates by two different synthesis routes
from aluminum isopropoxide, We also aimed to obtain
The preparation of lithium aluminates was initially
carried out by conventional solid state method [9].
Later, other new wet-chemical processes, such as synth-
esis from metallic alkoxides, were introduced. g-LiAlO2
was prepared by the hydrolysis of lithium and aluminum
* Corresponding author. Fax: 155-31-4995700.
E-mail address: nelcy@apolo.qui.ufmg.br (N.D.S. Mohallem).
0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0022-3697(00)00239-0

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R.A. Ribeiro et al. / Journal of Physics and Chemistry of Solids 62 (2001) 857±864
Fig. 1. X-ray diffraction pattern of the ceramic powder prepared by LISOL route at different temperatures: (a) 5508C; (b) 7508C; (c) 9508C; (d)
11508C ( £ ) a-LiAlO2, (1) h-Al2O3, (A) g-LiAlO2, (K) LiAl5O8, (W) NaCl.
crystalline lithium aluminate at lower calcination
temperatures.
ometer, using CuKa radiation and NaCl as internal standard,
and infrared spectroscopy techniques. Vibrational spectra in
the region of infrared were obtained in a Galaxy 3030 Matt-
son spectrometer using the KBr disk technique in the trans-
mission mode. Speci®c surface area and porosity were
analyzed by BET method using a nitrogen gas adsorption
equipment type Autosorb (Quantachrome Nova-1200).
Density measures were obtained with a helium picnometer
(Micrometrics). Chemical analysis was made by atomic
absorption and emission spectroscopy (Hitachi, model Z-
8200). Lithium was determined in air-acetylene ¯ame
mode and aluminum in nitrous oxide-acetylene mode.
TGA/DTA analysis was performed in a TA Instrument
SDT 2960 equipment. Samples were heated from room
temperature to 14008C at 108C/min under 110 ml N2 and
air ¯ow.
2. Experimental procedure
a) Sol precipitation (LISOL route): distilled water was
mixed with aluminum isopropoxide (Al(OC3H7)3) and
isopropyl alcohol to produce an opaque solution. Solid
LiNO3 was added to this mixture, followed by concentrated
HNO3, giving a clear solution which was stirred for 2 h. The
Al:Li:H2O:alcohol molar ratio was 1:1.1:100:26, respec-
tively. After that, concentrated ammonium hydroxide was
added to the solution producing the precipitation of the sol
by the increase of pH (,10). The precipitate was washed
with distilled water under centrifugation to remove excess
reagent and with ethanol to avoid particle aggregation.
b) Sol gelation (LIGEL route): distilled water was mixed
with solid aluminum isopropoxide to produce a white solu-
tion, which was stirred at 808C until complete hydrolysis.
LiNO3 and concentrated HNO3 were added to the solution,
which immediately turned into an opaque gel. The Al:Li:-
H2O:HNO3 molar ratio was 1:1.1:100:0.3.
The ceramic powders calcined at 11508C were pressed
uniaxially into pellets (around 2.1 mm in height and
11 mm in diameter) at p ˆ 490 MPa, without binder, and
sintered in air at 12508C for 6 h. The sintered samples
were polished and their morphology were observed by
SEM (Jeol JXA, model 8900RL).
The precipitate (LISOL) and the gel (LIGEL) obtained by
the two routes were dried at 808C for 12 h and calcined at
550, 750, 950 and 11508C for 3 h, producing a white
powder.
3. Results and discussion
3.1. Structural analysis
The crystalline phases of the samples were identi®ed by
X-ray diffraction in a Rigaku Geiger¯ex 3034 diffract-
Fig. 1 shows XRD patterns of LISOL powders. a-LiAlO2

R.A. Ribeiro et al. / Journal of Physics and Chemistry of Solids 62 (2001) 857±864
725 cm21
859
,
which are characteristic of the LiAl5O8
phase [22]. The spectra obtained at 550, 750 and
9508C also show a band at 1430 cm21 due to the carbo-
nate [23] from organic compound pyrolysis, which disap-
pears at 11508C.
Fig. 3 shows the X-ray diffraction patterns of LIGEL
powders. Differently from the results presented by
LISOL powders, LIGEL powders present a pure crystal-
line phase. The a-LiAlO2 phase was identi®ed at 5508C.
This phase changes to g-LiAlO2 at 7508C. Infrared spec-
tra (Fig. 4) at 5508C present characteristic bands due to
AlO6 vibrations and the LiO6 1 AlO6 combination at 520
and 740 cm21, respectively [22]. These spectra also show
bands at 1370, 1420 and 1475 cm21 [23] at 550, 750 and
9508C due to the starting reagents residues (lithium
nitrate and aluminum isopropoxide) which disappears at
11508C.
Quantitative determination by X-ray diffraction (inter-
nal standard method) [24] of the crystalline phases of
ceramic powder prepared by LISOL route and calcined
at 11508C revealed 45 and 55% (in weight) of g-LiAlO2
and LiAl5O8 phases, respectively. The chemical analysis
of these samples showed that the stoichiometry
Al:Li ˆ 1:1 is kept only for the LIGEL route. The de®-
ciency of lithium (488/w) observed in samples prepared
by LISOL route, probably due to the loss of lithium
during washing, stimulated the formation of LiAl5O8
phase.
3.2. BET analysis
Table 1 presents some characteristics of the LISOL
and LIGEL samples calcined at 11508C for 3 h. The
speci®c surface area of the ceramic powder prepared
by LISOL route was higher than those obtained by
LIGEL route. However, its density is high because this
ceramic powder is a mixture of g-LiAlO2 and LiAl5O8
phases with theoretic densities 2.61 [2] and 3.60 g cm21
[5], respectively.
After sintering, at 12508C for 6 h, the porosities of the
sintered bodies were 35 and 26% for the samples
prepared by LISOL and LIGEL routes, respectively.
Fig. 5(a) presents the fracture surface of LISOL sintered
body with spherical grains and Fig. 5(b), the fracture
surface of LIGEL sintered body with grains without
de®ned form and different sizes. These shapes are
Fig. 2. Infrared spectra of the ceramic powder prepared by LISOL
route at different temperatures.
and g-Al2O3 phases were identi®ed at 5508C and the a-/
g-LiAlO2 and LiAl5O8 phases at 7508C. The g-LiAlO2
phase is well de®ned in the infrared spectrum (Fig. 2)
at 7508C, with strong bands at 810, 650 cm21 and weak
bands at 550, 520, 450 cm21. The 810 and 650 cm21
bands are related to the stretching frequencies of AlO4
tetrahedra [1,22]. At 550, 520 and 450 cm21, the bands
refer to the AlO4±LiO4 lattice, LiO4 and to a mixture of
frequencies in the AlO4±LiO4 lattice, respectively. At
9508C, infrared spectrum shows characteristic bands of
the g-LiAlO2 phase and bands at 490, 610, 680 and
Table 1
Characteristics of samples calcined at 11508C
LISOL
LIGEL
Speci®c surface area (m2g21
)
6
21
2
22
Average pore radius (Angstrom)
Grain diameter (nm)a
175
2.72
551
2.47
Real density (g cm23
)
a
Determined by BET method [25].

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R.A. Ribeiro et al. / Journal of Physics and Chemistry of Solids 62 (2001) 857±864
Fig. 3. X-ray diffraction pattern of the ceramic powder prepared by LIGEL route at different temperatures: (a) 5508CÐ a-LiAlO2; (b) 7508C;
(c) 9508C; (d) 11508CÐg-LiAlO2, (W) NaCl.
different from the described in literature and reported by
Valenzuela et al. [13].
Fig. 6(c) shows a 26% weight loss up to 6008C due only to
water loss and organic residue pyrolysis.
TGA and DTA curves of LIGEL samples previously
calcined at 80, 300 and 5508C are shown in Fig. 7(a)±(c),
respectively. Fig. 7(a) shows an endothermic peak at 1208C
with a weight loss of 30% attributed to sample dehydration.
A second weight loss (9%) near 2558C (endothermic peak)
is attributed to organic residue pyrolysis and the melting
process of LiNO3. Two small endothermic peaks and 6%
total weight loss, attributed to organic residue pyrolysis
were observed at 340 and 4258C. An endothermic peak
with 24% weight loss is observed at 5858C, involving ther-
mal decomposition of lithium nitrate and the formation of
oxide.
3.3. Thermal analysisÐTGA/DTA simultaneous
TGA and DTA curves of LISOL samples previously
calcined at 80, 300 and 5508C are shown in Fig. 6(a)±(c),
respectively. Powders calcined at temperatures over 5508C
had similar behavior as the calcined at 5508C. In Fig. 6(a),
an endothermic peak at 1008C and 13% weight loss are
observed, attributed to dehydration of the sample. The two
endothermic peaks at 230 and 2708C are attributed to
organic residue pyrolysis with 61% weight loss and melting
of LiNO3 [26], respectively. The endothermic peak at 5608C
with 10% weight loss is related to thermal decomposition of
LiNO3 and oxide formation.
Fig. 7(b) shows an endothermic peak at 1108C with a
weight loss of 30%, which is attributed to sample dehydra-
tion. The endothermic peak at 2408C, in this case without
weight loss, is attributed exclusively to the melting of LiNO3.
At 5758C, an endothermic peak with a 32% of weight loss is
attributed to the thermal decomposition of LiNO3 and oxide
formation. The chemical reaction proposed in this case for
the formation of the aluminate, consistent with the weight
loss, is presented below. The Li2O phase proposed in this
reaction was not detected by XRD probably because it has
substantial solubility in Al2O3 [14].
In Fig. 6(b), an endothermic peak is observed at
1008C with 28% weight loss, attributed to dehydration
of the sample. When the material is calcined at 3008C,
it is amorphous and adsorbs large quantity of water
before thermal analysis. The endothermic peak due to
melting of LiNO3 is observed at 2508C. At 5558C, the
slight endothermic peak, with 25% weight loss, is attrib-
uted to thermal decomposition of LiNO3 and oxide
formation. A proposition for the decomposition reaction
corresponding quantitatively to a 25% weight loss is
presented below:
Al2O311=2Li2O 1 LiNO3 ! 2LiAlO21NO211=4O2
Fig. 7(c) shows an endothermic peak at 808C with little
weight loss (2.5%) due to absorbed water on the material
surface.
3Al2O312LiNO3 ! LiAlO21LiAl5O812NO211=2O2

R.A. Ribeiro et al. / Journal of Physics and Chemistry of Solids 62 (2001) 857±864
861
Fig. 5. Morphologies of the fractures of the sintered bodies from
ceramic powders obtained by (a) LISOL and (b) LIGEL routes.
values obtained for samples previously calcined at
5508C were higher than others values, because these
samples sintered with more ef®ciency. This fact was
visually con®rmed.
Table 2
Absorbed heat/sample weight ratio of the materials previously
calcined at 80, 300 and 5508C
Fig. 4. Infrared spectra of the ceramic powder prepared by LIGEL
route at different temperatures.
Sample
LISOL
Previous calcination
temperature (8C)
DTA curve areaa
(106´8C´s´kg21
)
80
300
550
80
90
120
660
60
A broad endothermic event is observed in every DTA
curve between 600 and 14008C, without weight loss,
which is attributed to the sintering of the ceramic mate-
rial. The absorbed heat in this process is proportional to
the mass corrected area in the DTA curve. Table 2
shows the absorbed heat/mass ratio determined between
600 and 14008C of the synthesized materials. The
LIGEL
300
550
120
660
a
Determined between 600 and 14008C with mass correction.

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R.A. Ribeiro et al. / Journal of Physics and Chemistry of Solids 62 (2001) 857±864
Fig. 6. TGA/DTA curves of powders obtained by LISOL route, dried previously at: (a) 808C; (b) 3008C; (c) 5508C, under N2 dynamics
atmosphere 110 ml/min, 108C/min.
4. Conclusion
Pure g-LiAlO2 was obtained by LIGEL route at 7508C.
This result shows the advantage of the sol±gel synthesis
to prepare pure ceramic powders with the desired
stoichiometry. Two crystalline aluminates, g-LiAlO2
Two methods of synthesis were used to prepare
aluminate ceramic powders: LIGEL and LISOL routes.

R.A. Ribeiro et al. / Journal of Physics and Chemistry of Solids 62 (2001) 857±864
863
Fig. 7. TGA/DTA curves of powders obtained by LIGEL route, dried previously at: (a) 808C; (b) 3008C; (c) 5508C, under N2 dynamics
atmosphere 110 ml/min, 108C/min.
and LiAl5O8, were obtained by LISOL route at 7508C:
Lithium loss was observed during the preparation of
ceramic powder, probably due to washing. This results
in a molar disproportion Al/Li, and in the formation of
the two oxides.
All samples reached constant weight at 6008C and
the sintering process was observed in the range of
600±14008C. The sintering process occurred more ef®-
ciently in the samples previously calcined at higher
temperatures.

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R.A. Ribeiro et al. / Journal of Physics and Chemistry of Solids 62 (2001) 857±864
powder by hydrolysis of metal alkoxides, J. Am. Ceram.
Soc. 70 (3) (1987) 171±174.
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