Metal nitrate/fuel mixture reactivity and its influence on the solution combustion synthesis of γ-LiAlO2

Article abstract of DOI:10.1007/s10973-009-0078-4

The reactivity of LiNO₃ and Al(NO₃)₃ with respect to urea and β-alanine was investigated. Experimental results proved that β-alanine is a more suitable fuel for LiNO₃, whereas urea seems to be more adequate for

Full text of DOI:10.1007/s10973-009-0078-4

J Therm Anal Calorim (2009) 97:209–214
DOI 10.1007/s10973-009-0078-4
Metal nitrate/fuel mixture reactivity and its influence
on the solution combustion synthesis of c-LiAlO2
˘
˘
Robert Ianos¸ Æ Ioan Lazau Æ Cornelia Pacurariu
Published online: 3 June 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract The reactivity of LiNO3 and Al(NO3)3 with
respect to urea and b-alanine was investigated. Experi-
mental results proved that b-alanine is a more suitable fuel
for LiNO3, whereas urea seems to be more adequate for
Al(NO3)3. Based on the different metal nitrate/fuel mixture
reactivity, nanocrystalline c-LiAlO2 powders were pre-
pared by solution combustion synthesis using a fuel mix-
ture of urea and b-alanine. This fuel mixture yielded
single-phase nanocrystalline c-LiAlO2 (32.6 nm) directly
from the combustion reaction. The resulted powder had a
specific surface area of 3.2 m2/g and no supplementary
annealing was required. On the other hand, pure c-LiAlO2
could not be obtained by using a single fuel (urea, b-ala-
nine) unless annealing at 900 °C for 1 h was performed.
the applications of lithium aluminate, such as tritium
breeder for fusion nuclear reactor [3, 4] or electrolyte
matrix for molten carbonate fuel cells [5, 6] rely on the
excellent thermo-mechanical, chemical and low radiation
damage susceptibility of c-LiAlO2. Other applications of
LiAlO2 aim the synthesis of various phosphor materials
[7, 8] and substrates for microelectronics [9].
The traditional method of lithium aluminate preparation
is based on solid-state reactions between different poly-
morph forms of Al2O3 and various lithium containing
compounds, such as Li2CO3 or LiOH [10]. In this case,
elevated temperatures and long soaking times are required
to ensure the formation of pure c-LiAlO2. The situation is
even more complicated due to the high volatility of lithium
component. In order to overcome the problems related to
the preparation of single-phase c-LiAlO2 powders, several
synthesis methods have been reported: coprecipitation [11],
mechanochemical synthesis [12], gel to crystallite con-
version [13], Pechini [2], spray pyrolysis [6], hydrothermal
method [14], sol–gel [15] etc.
Keywords Combustion synthesis ꢀ Fuel mixture ꢀ
Lithium aluminate ꢀ Metal nitrate ꢀ Urea ꢀ b-alanine
Introduction
Although these methods are able to ensure the formation
of pure c-LiAlO2, many sophisticated techniques and time-
consuming procedures are required, which are not cost
effective and represent obstacles to reproducibility and
reliability of the final powder.
Lithium aluminate, LiAlO2, possesses a number of valu-
able properties, which makes it a first choice material for
many applications [1]. Lithium aluminate exhibits three
allotropic forms, namely: a-LiAlO2 (hexagonal), b-LiAlO2
(monoclinic) and c-LiAlO2 (tetragonal). Among all poly-
morph modifications of LiAlO2, c-LiAlO2 is the most
stable one, so that at elevated temperature (above 750 °C)
a-LiAlO2 and b-LiAlO2 turn into c-LiAlO2 [2]. Some of
In the past 20 years, combustion synthesis merged as an
innovative and promising powder preparation method [16,
17]. The method relies on the strong exothermic self-
propagating reaction, which takes place during the rapid
heating up to 500 °C of an aqueous solution of metal
nitrates and a suitable fuel. The temperature reached during
combustion reaction enables the formation and crystalli-
zation of the designed oxide compound without any addi-
tional annealing [18]. However, we have shown in our
previous papers [19–21] that in the case of combustion
˘
˘
R. Ianos¸ (&) ꢀ I. Lazau ꢀ C. Pacurariu
Faculty of Industrial Chemistry and Environmental Engineering,
‘‘Politehnica’’ University of Timis¸oara, P-¸ta Victoriei no. 2,
Timis¸oara 300006, Romania
e-mail: robert_ianos@yahoo.com
123

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R. Ianos¸ et al.
Characterization methods
synthesis of mixed metal oxides, the use of a single fuel
yields amorphous powders or mixtures of crystalline pha-
ses. As a result, the formation of the desired compound
demands additional thermal treatments.
Heating behaviour of the precursor mixtures (samples 1–3)
were monitored by thermal analysis, using a NETZSCH
STA 449 C instrument. The investigated temperature range
was 30–900 °C, with a 10 °C min-1 heating rate, in N2
atmosphere (20 mL min-1) and alumina crucibles. Losses
on ignition (L.O.I.) were determined after annealing the
combustion residue at 900 °C for 1 h. The evolution of the
crystalline phases was monitored by XRD, using a Bruker
D8 Advance System, CuKa monochromated radiation. The
crystallite size was determined based on the XRD patters
using the Sherrer’s Eq. 1. The peaks used for the crystallite
size determination were the ones corresponding to 101
(22.246°), 102 (33.321°) and 200 (34.645°) hkl planes.
Considering the individual behaviour of metal nitrate/
fuel binary mixtures as well as the particular case of
c-LiAlO2 preparation, an explanation is given for the dif-
ficulty related to the formation of pure c-LiAlO2 by using a
single fuel. The solution of using fuel mixtures instead of a
single fuel is suggested, showing that in this case, pure
c-LiAlO2 powders can be obtained directly from the
combustion reaction.
Experimental
0:9 ꢀ k
D ¼
ð1Þ
LiNO3 (Merck), Al(NO3)3ꢀ9H2O (Merck), urea (CO (NH2)2)
(Merck) and b-alanine (NH2(CH2)2COOH) (Merck) were
used as starting materials.
b ꢀ cos h
where D is the crystallites size in nm, k is the radiation
wavelength (CuKa, 0.15406 nm), b is the full width at half
of the maximum in radians and h is the Bragg-angle.
BET (Brunauer, Emmett and Teller) surface area mea-
surements were performed using a Micromeritics ASAP
2020 instrument and nitrogen as adsorption gas. The
powder morphology was investigated by scanning electron
microscopy (SEM) using a FEI Inspect S electron
microscope.
The reactivity of LiNO3 and Al(NO3)3 with respect to
urea and b-alanine
In the first part of the experiments, the combustion reaction
between each metal nitrate and each fuel was investigated.
For this purpose stoichiometric metal nitrate/fuel combi-
nations were designed and the phase composition of the
resulted powders was determined. Recipes were calculated
in order of obtain 0.150 moles of Li2O and 0.150 moles of
Al2O3, respectively. It was assumed that the gaseous by-
products of the combustion reactions are: CO2, H2O and
N2.
Results and discussion
The individual reactivity of LiNO3 and Al(NO3)3 with
respect to urea and b-alanine
Combustion reactions assumed to take place in the case of
metal nitrate/fuel binary mixtures are represented by the
Eqs. 2–5
Preparation of LiAlO2 powders
After determining the most suitable fuel for each metal
nitrate, the preparation of c-LiAlO2 was aimed. The first
two samples were designed using a single fuel, urea
(sample 1) or b-alanine (sample 2), whereas sample 3
represents the innovative concept, involving the use of urea
and b-alanine fuel mixture. Batches were calculated for
0.150 moles of LiAlO2. Stoichiometric metal nitrate/fuel
molar ratios were also used in all samples. Appropriate
amounts of LiNO3 and Al(NO3)3ꢀ9H2O were dissolved in
distilled water in a porcelain-evaporating dish. Then stoi-
chiometric amount of fuel was added according to each
recipe. The resulting solution was rapidly heated to 300 °C
in a preheated electric nest causing water evaporation and
the ignition of a self-sustaining combustion reaction,
except for sample 1 which failed to ignite. The space of
time between the initiation of the combustion reaction and
its finalization was measured.
6LiNO3 þ 5CH4N2O ! 6Li2O þ 5CO2 þ 10H2O þ 8N2
ð2Þ
6LiNO3 þ 2C3H7NO2 ! 3Li2O þ 6CO2 þ 7H2O þ 4N2
ð3Þ
6AlðNO3Þ3 þ 15CH4N2O ! 3Al2O3 þ 15CO2 þ 30H2O
þ 24N2
ð4Þ
6AlðNO3Þ3 þ 6C3H7NO2 ! 3Al2O3 þ 18CO2 þ 21H2O
þ 12N2
ð5Þ
As seen in Table 1, LiNO3 and Al(NO3)3 react very dif-
ferent with b-alanine and urea. In the case of LiNO3/urea
binary mixture (Eq. 2) no combustion reaction was
123

Metal nitrate/fuel mixture reactivity and its influence
211
Table 1 The individual reactivity of LiNO3 and Al(NO3)3 with respect to urea and b-alanine
Metal nitrate
Combustion reaction duration/s
Urea
Phase composition of the resulted powder
b-alanine
Urea
b-alanine
LiNO3
No combustion
10
\1
240
LiNO3
Li2O
Al(NO3)3
a-Al2O3
Amorphous
Table 2 Characteristics of the combustion-synthesized powders
Sample
Fuel
Combustion reaction duration/s
Gase velocitya/L/s
Powder colour
L.O.I./%
1
2
3
a
Urea
No visible combustion
–
Yellowish
Grey
47.0
8.4
b-alanine
35
60
1.09
0.80
Urea ? b-alanine
White
0.1
The velocity of the combustion generated gases calculated under normal conditions for 0.150 moles of LiAlO2
observed. At the same time, the presence of LiNO3 peaks on
the XRD pattern of the resulted powder indicates that
reaction (2) did not take place. On the other hand, LiNO3
reacted extremely fast (explosively) with b-alanine (Eq. 3)
leading to the formation of a white crystalline Li2O powder.
In the case of Al(NO3)3 the situation is quite opposite.
The reaction of Al(NO3)3 with urea (Eq. 4) was an intense
and rather fast flaming combustion process. XRD investi-
gations evidenced that the resulted powder consisted of
single-phase a-Al2O3. On the other hand, the reaction
between Al(NO3)3 and b-alanine (Eq. 5) was a slow and
smouldering combustion process, which yield an amor-
phous black powder.
concept does not take into consideration that the two metal
nitrates react quite different with urea and b-alanine, as it
was already shown.
The different reactivity LiNO3 and Al(NO3)3 manifest
with respect to urea and b-alanine leads to the rational
conclusion that in the case of c-LiAlO2 a mixture of fuels
should be used (Eq. 8). In this case urea was assumed to
react with Al(NO3)3, whereas b-alanine was assumed to
react with LiNO3.
As seen in Table 2, in the case of sample 1, the combus-
tion reaction failed to occur. The absence of the combustion
reaction, which is confirmed by the elevated loss on ignition
of the resulted powder, can be correlated with the absence of
the combustion reaction in the LiNO3/urea system.
Thermal analysis investigation of the precursor mixture
consisting of LiNO3, Al(NO3)3 and CH4N2O (sample 1)
suggests that LiNO3 does not participate in the combustion
reaction, but it acts as an inert material, hindering the
development of the combustion reaction (Fig. 1).
Considering the individual reactivity of LiNO3 and
Al(NO3)3 with respect to urea and b-alanine (Table 1) the
conclusion can be drawn that there is a predilection of these
metal nitrate with respect to urea and b-alanine. While
b-alanine seems to be an excellent fuel for LiNO3, urea
seems to react better with Al(NO3)3.
Combustion synthesis of c-LiAlO2 powders
For the preparation of c-LiAlO2 powders, the fallowing
reactions (6–8) were considered:
6LiNO3 þ 6AlðNO3Þ3 þ 20CH4N2O
! 6LiAlO2 þ 20CO2 þ 40H2O þ 32N2
ð6Þ
ð7Þ
ð8Þ
6LiNO3 þ 6AlðNO3Þ3 þ 8C3H7NO2
! 6LiAlO2 þ 24CO2 þ 28H2O þ 16N2
6LiNO3 þ 6AlðNO3Þ3 þ 2C3H7NO2 þ 15CH4N2O
! 6LiAlO2 þ 21CO2 þ 37H2O þ 28N2
In the first two cases (Eqs. 6, 7) the traditional version of
the combustion synthesis was used. This means that a
single fuel, either urea (Eq. 6) or b-alanine (Eq. 7), is
expected to react with both metal nitrates. Obviously, this
Fig. 1 TG and DTA curves of precursor mixture 1
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212
R. Ianos¸ et al.
Below 200 °C dehydration, hydrolysis and destruction
LiNO3 with b-alanine (Table 1). Such behaviour points out
that LiNO3 accelerates the combustion reaction of
Al(NO3)3 with b-alanine, or, Al(NO3)3 decelerates the
combustion reaction of LiNO3 with b-alanine.
The colour of the sample indicates that the temperature
reached during combustion was not very high, so that some
carbon impurities originating from the incomplete oxida-
tion of b-alanine contaminated the powder. As shown in
Fig. 3, the DTA curve of the precursor mixture containing
LiNO3, Al(NO3)3 and C3H7NO2 (sample 2) exhibits an
endothermic effect at 170 °C, which is accompanied by a
mass loss.
of nitrate groups from aluminium nitrate take place as well
as the partial decomposition of urea [22]. The endothermic
effect from 222 °C accompanied by a mass loss is asso-
ciated with the dehydroxylation of aluminium hydroxy
salts (resulted from the hydrolytic processes) as well as the
LiNO3 melting. The very weak exothermic effect from
285 °C might be related with the combustion reaction
between Al(NO3)3 and urea in the presence of inert LiNO3
melt. The mass loss on the TG curve between 500 °C and
600 °C is due to the thermal decomposition of unreacted
LiNO3. The presence of unreacted LiNO3 in the final
powder was confirmed by the XRD analysis (Fig. 2).
Due to the retarding effect of LiNO3 the combustion
reaction between urea and aluminium nitrate is so much
downgraded that instead of a-Al2O3 (which is formed in
the case of Al(NO3)3/urea binary system), poor crystalline
c-Al2O3 could be identified on the XRD pattern (Fig. 2).
b-alanine (sample 2) gave a fast combustion reaction
(Table 2), leading to the formation of a very voluminous
grey powder, with a surface area of 32.0 m2/g (Table 3).
The combustion reaction duration is in agreement with the
individual reactivity of metal nitrate/fuel combinations.
This reaction is faster than the reaction of Al(NO3)3 with
b-alanine but at the same time is slower than the reaction of
As in the case of sample 1, partial aluminium nitrate
decomposition might be responsible for this effect. The
development of the combustion reaction is evidenced on
the DTA curve by a strong exothermic effect (245 °C)
corroborated with a very rapid mass loss on the TG curve.
At the same time one can see that the sample still losses
about 8 mass% up to 600 °C, which agrees very well with
the loss on ignition determination (Table 2). This process
might be due to the burning out of carbon impurities. The
crystalline phases identified on the XRD pattern of the
powder prepared by using b-alanine are b-LiAlO2 and
c-LiAlO2 (Fig. 2). The formation of LiAlO2 proves that
combustion reaction (7) occurred; however, the tempera-
ture was not high enough to ensure the complete conver-
sion of b-LiAlO2 to c-LiAlO2 and carbon oxidation.
In the case of sample 3, where a mixture of urea and
b-alanine was used, the combustion reaction was extremely
intense, resulting in a white powder, with a surface area of
3.2 m2/g (Table 3). XRD analysis confirmed that in this
case the only crystalline phase present on the diffraction
patterns was c-LiAlO2 (Fig. 2), with an average crystallite
size of 32.6 nm. SEM investigations of the c-LiAlO2
powder prepared by using the urea and b-alanine fuel
Fig. 2 XRD patterns of the powders resulted from the combustion
reactions
Table 3 Characteristics of the combustion-synthesized powders
before and after annealing
Sample Before annealing
After annealing at 900 °C/1 h
DXRD/nm SBET/m2/g DXRD/nm
SBET/m2/g
1
2
3
–
–
32.3
27.5
33.1
2.1
16.0
1.8
12.4
32.6
32.0
3.2
Fig. 3 TG and DTA curves of precursor mixture 2
123

Metal nitrate/fuel mixture reactivity and its influence
213
mixture (Fig. 4) revealed that grains have an irregular
shape and are smaller than 50 lm.
Sample 3 exhibits lower surface area and higher crys-
tallite size than sample 2, which indicate that the temper-
ature reached during the combustion reaction (8) was
considerably higher than in the case of reaction (7). On the
other hand, the lower surface area and larger crystallites
size in the case of powder 3 could be also explained by the
lower velocity of the combustion generated gases
(Table 2).
Thermal analysis of the precursor mixture consisting of
LiNO3, Al(NO3)3, CH4N2O and C3H7NO2 (Fig. 5) shows
that below 200 °C the TG and DTA curves exhibit a
similar profile with the other two precursors (Figs. 1, 3).
The exothermic effect associated with the combustion
Fig. 6 XRD patterns of the powders after annealing at 900 °C for 1 h
process occurs at 273 °C and is also accompanied by a
rapid mass loss. After combustion reaction ends, the
resulting powder has no more mass loss, meaning that the
reaction (8) was completed. Loss on ignition of powder 3
agrees with the results obtained by thermal analysis; in
both cases the obtained value is practically negligible
(Table 2).
In the case of sample 1, the formation of pure c-LiAlO2
with an average crystallite size of 32.3 nm requires
annealing at 900 °C for 1 h (Fig. 6). In the case of sample
2 one can observe that after annealing c-LiAlO2 is the only
crystalline phase identified on the XRD pattern. Moreover,
a significant increase of the crystallites as well as the
reduction of the surface area of the powder may be noticed
(Table 3).
In the case of sample 3, where pure c-LiAlO2 was
obtained directly from the combustion reaction no addi-
tional thermal treatment is needed. In this case, after
annealing at 900 °C for 1 h the average crystallite size of
the powder exhibits only a slight increase, whereas the
surface area of the powder decreases below 2 m2/g.
Fig. 4 SEM image of the c-LiAlO2 powder prepared by using the
urea and b-alanine fuel mixture
Conclusions
The reactivity of LiNO3 and Al(NO3)3 with respect to urea
and b-alanine was investigated. Experimental results
proved that the above-mentioned metal nitrates have dif-
ferent affinities with respect to urea and b-alanine. LiNO3
does not react with urea but gives an explosive combustion
reaction with b-alanine. Al(NO3)3 gives a smouldering
combustion reaction with b-alanine and a vigorous com-
bustion reaction with urea.
Because of different behaviour of these metal nitrate/
fuel binary combinations, in the case of c-LiAlO2 prepa-
ration, a mixture of urea and b-alanine is recommended,
Fig. 5 TG and DTA curves of precursor mixture 3
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R. Ianos¸ et al.
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instead of using a single fuel. The use of urea and b-alanine
fuel mixture allowed the formation of pure nanocrystalline
c-LiAlO2 powders directly from the combustion reaction.
In this case no supplementary annealing was required.
On the other hand, pure c-LiAlO2 could not be obtained
in the case of using only urea or only b-alanine as fuel,
unless annealing at 900 °C for 1 h was performed. Urea
failed to produce a combustion reaction; the resulting
powder was a phase mixture of unreacted LiNO3 and
poorly crystalline c-Al2O3. b-alanine led to the formation
of a mixture of b-LiAlO2 and c-LiAlO2.
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