Synthesis of Li4SiO4 by a modified combustion method

Article abstract of DOI:10.1111/j.1551-2916.2005.00262.x

We report for the first time the synthesis of Li₄SiO₄ by the modified combustion method, a rapid chemical process that takes 5 min for completion. This method uses nonoxidizer compounds instead of nitrate mixtures, which are not always commercially available. The effects of the following parameters on the production of Li₄SiO₄ were studied: (1) different lithium hydroxide:silicic acid:urea (LiOH:H ₂SiO₃:CH₄N₂O) molar ratios; (2) the presence of air flow in the furnace chamber; and (3) the furnace heating temperature. It was found that LiOH:H₂SiO₃:CH ₄N₂O molar ratios 6:1:3 heated at 1100°C in the presence of additional air in the muffle chamber formed the best precursors to produce Li₄SiO₄.

Full text of DOI:10.1111/j.1551-2916.2005.00262.x

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J. Am. Ceram. Soc., 88 [7] 1720–1724 (2005)
DOI: 10.1111/j.1551-2916.2005.00262.x
ournal
J
Synthesis of Li4SiO4 by a Modified Combustion Method
Ã
Daniel Cruz and Silvia Bulbulianw
Departamento de Quı
´
mica, Instituto Nacional de Investigaciones Nucleares, Me
noma del Estado de Mexico, Instituto Literario No 100 Col. Centro C.P.
50000, Toluca, Edo. de Mexico
xico 11801, D.F., Mexico
´ ´
Ã
Facultad de Ciencias, Universidad Auto
´
´
´
We report for the first time the synthesis of Li4SiO4 by the
modified combustion method, a rapid chemical process that
takes 5 min for completion. This method uses nonoxidizer com-
pounds instead of nitrate mixtures, which are not always com-
mercially available.
The effects of the following parameters on the production of
Li4SiO4 were studied: (1) different lithium hydroxide:silicic ac-
id:urea (LiOH:H2SiO3:CH4N2O) molar ratios; (2) the presence
of air flow in the furnace chamber; and (3) the furnace heating
temperature. It was found that LiOH:H2SiO3:CH4N2O molar
ratios 6:1:3 heated at 11001C in the presence of additional air
in the muffle chamber formed the best precursors to produce
Li4SiO4.
II. Experimental Procedure
Analytical grade reagents were used without further puri-
fication. H2SiO3, LiOH, and CH4N2O were purchased from
Mallinckrodt (New York) and Merck (Darmstadt, Germany).
The weight of precursors utilized varied from 1.699 to 2.365 g.
Based on the studies performed in the previous paper,8 the
amounts of CH4N2O and H2SiO3 were always maintained con-
stant, but in order to prepare four precursors with the following
molar ratios: LiOH:H2SiO3:CH4N2O 5 2:1:3, 5:1:3, 6:1:3, and
7:1:3, LiOH was increased from 0.266 to 0.932 g. In a typical
experiment, 0.266 g of LiOH, 0.434 g of H2SiO3, and 0.999 g of
CH4N2O were mixed and suspended in 5 mL of water to obtain
molar ratios LiOH:H2SiO3:CH4N2O of 2:1:3. The mixture was
heated until most of water evaporated. The resulting paste was
transferred into a 30 mL crucible that was introduced for 5 min
into a Muffle Furnace (Barnstead Themolyne Corporation,
Braunschweig, Germany) (20 cm ꢀ 20 cm ꢀ 17 cm). Heating
temperatures from 4501 to 11001C were applied. The entire
combustion process was complete in 5 min, producing mainly
lithium silicate powders. Some experiments were performed in
the presence of additional air in the furnace chamber through an
air flow of 2000 mL/min.
Samples are referred to in this paper as their LiOH:H2-
SiO3:CH4N2O molar ratio. For example, sample 2:1:3 is the
name given to the sample prepared with LiOH, H2SiO3, and
CH4N2O with the corresponding LiOH:H2SiO3:CH4N2O molar
ratios of 2:1:3. On occasion, they are also referred to according
to the heating temperature marked as a sub index, for example,
sample 2:1:3650 is the name given to sample 2:1:3 obtained by
heating the precursor at 6501C. Samples heated in the presence
of additional air in the furnace chamber are marked with a su-
perscript ‘‘a’’; accordingly, sample 2:1:3 heated in the presence
of additional air in the furnace chamber is designated as a2:1:3.
The dried powder was scanned using a Siemens (Oldburg,
Germany) D-500 diffractometer using CuKa radiation. The re-
sulting compounds were identified by the corresponding files
Joint Committee on Powder Diffraction Standards (JCPDS).
The relative content of silicates was estimated from the areas
under the diffraction peaks.9 As no internal standard was in-
troduced, the X-ray absorption for each compound was as-
sumed to be the same.
I. Introduction
ITHIUM ceramics compounds such as Li2SiO3 and Li4SiO4
have been proposed as solid breeder materials for D–T-
L
based fusion reactors by the following reaction:1
3
1
4
6Li þ n ! Heþ T
In particular, Li4SiO4 has the advantage of being more stable at
high temperature than Li2SiO3.2 Li4SiO4 has been prepared by
Pfeiffer et al.3 by several conventional powder synthesis tech-
niques, including solid-state synthesis, coprecipitation, and the
sol–gel process. The final products were Li2SiO3, Li2Si2O5,
Li4SiO4, and quartz. Although these methods have advantages,
high-temperature calcination for 4 h is required to obtain crys-
talline compounds. On the other hand, combustion synthesis is a
very rapid chemical process that is based on the principle of ex-
plosive decomposition of nitrate–fuel mixtures.4–7 In order to
study further the role of the oxidizers, the combustion method
was modified by Cruz and Bulbulian8 to apply it to a rather
simple system, utilizing nonoxidizer compounds. This method
was applied to prepare lithium silicates, mainly Li2SiO3, by uti-
lizing the heat provided by the urea (CH4N2O) exothermic com-
bustion. The precursors used were a mixture of silicic acid
(H2SiO3), lithium hydroxide (LiOH), and CH4N2O. CH4N2O
not only worked as complexing agent for the precursors mixture
but also provided combustion heat for calcinations.
A Magna-IR (Madison, WI) Spectrometer 550 Nicolet was
used to investigate the presence of carbonates in the prepared
powders. Samples were mixed with potassium bromide to form
tablets.
The objective of the present study was to explore the modified
combustion synthesis method to prepare Li4SiO4 powders and
to gain a better understanding of the significant process param-
eters: precursors molar ratio, furnace temperature, and the pres-
ence of additional air in the furnace chamber.
III. Results
Most samples obtained by the modified combustion method
with various precursors utilizing different LiOH:H2SiO3:
CH4N2O molar ratios and different heating temperatures were
white opaque powders. The color of sample 7:1:3 was grayish
when heated at 11001C. As determined by X-ray diffraction
(XRD) patterns, Figs. 1–4 show various crystalline compounds
present in the powders obtained. Standard deviations of the
measured samples varied from 1.5% to 3.0%.
R. F. Cooper—contributing editor
Manucript No. 10206. Received May 13, 2003; approved November 15, 2004.
D. Cruz thanks financial support from (CONACYT).
wAuthor to whom correspondence should be addressed. e-mail: sb@nuclear.inin.mx
1720

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Synthesis of Li4SiO4 by a Modified Combustion Method
1721
Fig. 1. (A) X-ray diffraction (XRD) patterns of lithium silicate samples
prepared by the modified combustion method at different temperatures
and 2:1:3 LiOH:H2SiO3:CH4N2O molar ratios. (B) Percentages of com-
pounds identified by XRD: m, Li2SiO3; %, Li2Si2O5. LiOH, lithium
hydroxide; H2SiO3, silicic acid; CH4N2O, urea.
Fig. 2. (A) X-ray diffraction (XRD) patterns of lithium silicate samples
prepared by the modified combustion method at 6501C. (B) Percentages
of compounds identified by XRD: m, Li2SiO3; ꢁ, Li4SiO4; ~, Li2CO3.
The products formed with various molar ratios were investi-
gated before the synthesis of Li4SiO4 by the combustion reaction
was attempted. The diffraction patterns of the prepared samples
2:1:3 in the muffle furnace heated at different temperatures are
shown in Fig. 1(A). The analysis of the powdered material re-
vealed that when the precursors were heated at 4501C, no crys-
talline compounds were found. However, at higher temperatures
(5501–7501C), the products obtained were crystalline. The com-
pounds identified by XRD patterns and the percentages ob-
tained are presented in Fig. 1(B). Sample 2:1:3550 resulted in the
formation of 42.1% of Li2SiO3 (JCPDS 29-828), along with
a considerable amount, 57.9%, of Li2Si2O5 (JCPDS 14-322). At
increasing temperatures, 6501 and 7501C, the percentages of
Li2SiO3 (43.0% and 43.5%) and Li2Si2O5 (57.0% and 56.5%)
remained almost constant, in the range of experimental error.
Figure 2(A) shows the diffraction patterns of various samples
heated at 6501C. The products and percentages displayed in
Fig. 2(B) revealed that Li4SiO4 (JCPDS 37-1472) was already
present in the powders; Li2SiO3, which was the main compound
found (53.6%) in sample 5:1:3650, decreased to 40.1% and
28.2%, respectively, in samples 6:1:3650 and 7:1:3650. Excess of
LiOH induced the formation of Li4SiO4, which was present in
small proportions in sample 5:1:3650 (11.7%), and increased in
samples 6:1:3650 and 7:1:3650 to 19.1% and 18.1%. Additional
air in the furnace chamber increased even more the presence of
Li4SiO4 to 31.0%, 17.6%, and 21.9% and decreased the pres-
ence of Li2SiO3 to 49.5%, 43.4%, and 26.1% in samples
6:1:3650 and 7:1:3650 to 40.8% and 53.8% and in the presence
of additional air, it decreased in sample 5:1:3650 (19.5%) but
a
remained almost constant in samples a6:1:3650 (39.0%) and
a7:1:3650 (52.0%).
Figure 3(A) exhibits the diffraction patterns of various sam-
ples heated at 9001C. The products and percentages displayed in
Fig. 3(B) show that Li4SiO4 is already the main crystalline com-
pound formed, 73.5%, 83.3%, and 82.0% in samples 5:1:3900
,
6:1:3900, and 7:1:3900, but Li2SiO3 was still present in samples
5:1:3900 (26.5%) and 6:1:3900 (5.0%). The presence of additional
air in the samples increased the presence of Li4SiO4 slightly in
these two samples to 80.0% and 86.2% and reduced Li2SiO3 to
20.0% and 8.6% in samples a5:1:3900 and a6:1:3900. Li2SiO3 was
a
not present at all in samples 7:1:3900 and 7:1:3900, and Li2CO3
was not found in XRD patterns of samples 5:1:3900 and
a5:1:3900; however, excess of LiOH induced the formation of
11.7% and 18.0% of Li2CO3 in samples 6:1:3900 and 7:1:3900
.
,
a
and 5.2% and 18.0% in samples 6:1:3900 and a7:1:3900
Figure 4(A) displays the diffraction patterns of samples heat-
ed at 11001C. The products and percentages shown in Fig. 4(B)
indicate that the main product, Li4SiO4, increases to 92.6%,
92.6%, and 89.3% in samples 5:1:31100, 6:1:31100, and 7:1:31100,
and Li2SiO3 decreased to 7.4%, 3.7%, and 0.0%. Additional air
considerably affected Li4SiO4 presence in samples a5:1:31100
,
a6:1:31100, and a7:1:31100 (89.3%, 98.0%, and 79.4%, respective-
ly) and of Li2SiO3 (10.7%, 0.0%, and 8.7%). Li2CO3 was not
found in XRD patterns of samples 5:1:31100 and a5:1:31100, but it
was present in 6:1:31100 (3.7%) and 7:1:31100 (10.7%). It de-
creased in the presence of additional air (2.0%) in sample
a
a
a5:1:3650, 6:1:3650, and 7:1:3650, respectively. Li2CO3 (JCPDS
22-1141) present in sample 5:1:3650 (34.8%) increased in samples

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1722
Journal of the American Ceramic Society—Cruz and Bulbulian
Vol. 88, No. 7
Fig. 3. (A) X-ray diffraction (XRD) patterns of lithium silicate samples
prepared by the modified combustion method at 9001C. (B) Percentages
of compounds identified by XRD: m, Li2SiO3; ꢁ, Li4SiO4; ~, Li2CO3.
Fig. 4. (A) X-ray diffraction (XRD) patterns of lithium silicate samples
prepared by the modified combustion method at 11001C. (B) Percent-
ages of compounds identified by XRD: m, Li2SiO3; ꢁ, Li4SiO4; ~,
Li2CO3.
a6:1:31100 but remained constant in sample a7:1:31100 (11.9%). In
general, 5:1:3 samples heated at 9001 and 11001C produced
Li2SiO3 (7.4%–26.5%) and 7:1:3 samples produced mainly lith-
ium orthosilicate along with Li2CO3 (10.7%–18.0%). a6:1:3
samples produced, in general, higher amounts of Li4SiO4.
Figure 4 reveals some large differences in the relative intensi-
ties of Li4SiO4 reflections (JCPDS file 37-1472); the intensity of
reflection (001) increased in comparison with other Li4SiO4 re-
flections. In particular, (001) and (100) have been chosen to
study this effect (Table I), which was peculiar to samples heated
at 11001C. The ratio of the integrated intensities (001)/(100) de-
creased from 0.87 in sample 5:1:31100 to 0.02 in sample 7:1:31100
the missing lithium could be attributed to very small particles of
Li2CO3. All spectra presented an intense band between 3500 and
3000 cmꢂ1 because of O–H vibrations. The bands reported
by Nakamoto10 for [SiO4]ꢂ2 tetrahedral arrangements, n1, 819
cmꢂ1; n3, 956 cmꢂ1; and n4, 527 cmꢂ1, were found in the present
work at 789, 940–943, and 519–525 cmꢂ1, respectively. The vi-
bration bands at 1060–1070 and 623–625 cmꢂ1 were attributed to
Si–O–Si and O–Si—O,11 respectively. The band at 984–986
cmꢂ1 can be attributed to Si–O vibration.11 Other peaks at
488–496 and 735–743 cmꢂ1 may be attributed to silicon (IV)
oxide12 and O–Li–O structure,10 respectively. The principal two
bands assigned to carbonate ions10 1350–1380 cmꢂ1 (u3) and
856–866 cmꢂ1 (u2) were found to be shifted to 1440 cmꢂ1 (u3) in
the present work. It was shown that although CO23ꢂ ions were
a
a
and from 0.73 in sample 5:1:31100 to 0.11 in sample 7:1:31100
,
revealing nonuniform intensities, which, in turn, showed that the
structure of the solid was modified. Sample 7:1:31100 was mixed
properly with quartz to observe whether the differences in the
relative intensities could be annihilated. However, the XRD
pattern (not shown) of the product obtained revealed that there
was no considerable change in the ratios of the integrated in-
tensities (001)/(100) of sample 7:1:31100 (0.02) and sample
7:1:31100 mixed with quartz (0.05).
Li2CO3 was shown only in some XRD measurements, as very
small particles and low percentages are not revealed by this
method. Figure 1 shows only the presence of Li2SiO3 and
Li2Si2O5 in samples 2:1:3, indicating that some Li is missing in
the XRD patterns. The same peculiarity was observed in sam-
ples 5:1:3 heated at 9001 and 11001C (Figs. 3 and 4). Therefore,
infrared (IR) analyses were performed in samples 2:1:3 (Fig. 5)
and 5:1:3 (not shown in the paper) in order to find out whether
Table I. Ratio of the Integrated Intensities (001)/(100)
of Li4SiO4 X-Ray Diffraction (XRD) Reflections Produced
During the Combustion of Urea
Samples
Ratio (001)/(100)
5:1:3
6:1:3
7:1:3
a5:1:3
a6:1:3
a7:1:3
0.87
0.29
0.02
0.73
0.15
0.11

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Synthesis of Li4SiO4 by a Modified Combustion Method
1723
increased in the precursor and also the presence of oxygen in the
furnace chamber through an air flow of 2000 mL/min.
Li-rich precursors and additional air in the furnace favored
Li4SiO4 production, which was already formed in sample
5:1:3650 and increased in sample a5:1:3650. It was found that
percentages of Li4SiO4 present in 5:1:3650 and 5:1:3900 were in-
cremented when additional air was introduced in the muffle fur-
nace. On the other hand, the increase of LiOH in the precursors
also favored the formation of Li2CO3 because of the CO2 ad-
sorption characteristics of Li compounds.13
Samples 5:1:3 and 6:1:3 heated at 9001C resulted in the for-
mation of Li4SiO4 as the main compound along with small
amounts of Li2SiO3. The presence of additional air in samples
a5:1:3 and a6:1:3 slightly increased the presence of Li4SiO4. Sam-
ples 5:1:3 and 6:1:3 heated at 11001C produced high percentages
of Li4SiO4. Finally, it was found that the best precursor to pro-
duce Li4SiO4 (98.0%) as the main compound along with minor
amounts of Li2CO3 was LiOH:H2SiO3:CH4N2O 5 6:1:3, heated
at 11001C in the presence of additional air in the muffle cham-
ber. Higher molar ratios, 7:1:3, produced an increase of Li2CO3
as excess of LiOH increases the formation of Li2CO3. XRD
patterns and IR spectra evidenced that all samples studied con-
tained carbonates. When precursors were ignited at a low tem-
perature, the energy generated by the combustion reaction was
not sufficient to facilitate the mobility of the ions and to allow
the combination of four Li atoms with one Si atom to form
Li4SiO4. Higher temperatures affected the products as addition-
al energy was imparted to Li ions, increasing the possibility of
forming bonds with Si ions in higher proportions and conse-
quently forming Li-rich Li4SiO4. This effect, evidenced by XRD,
showed that at a higher furnace temperature, the Li4SiO4 pro-
duced was more abundant than when the furnace was heated at
a lower temperature.
Fig. 5. Infrared spectra of 2:1:3 lithium silicate samples prepared by the
modified combustion method at different muffle temperatures.
not observed in samples 2:1:3 and 5:1:3, as analyzed by XRD,
they were evidenced by IR studies, probably in the form of very
small particles of Li2CO3.
The ratio of the integrated intensities (001)/(100) of the
Li4SiO4 reflections decreased from sample 5:1:31100 to sample
7:1:31100 and from sample a5:1:31100 to sample a7:1:31100, reveal-
ing nonuniform intensities, which in turn show that the structure
of the solid was modified. There are two possibilities that can
explain these modifications: the first one is to consider that on
heating at high temperatures, precursors with high content of
LiOH develop orientation texture produced during the heating
process, i.e., the material may attain a preferential crystallite
orientation rather than a purely random one, changing the solid
from a specimen whose crystallites are randomly oriented to
asymmetric crystallites, oriented preferentially. These effects are
similar to those observed in the crystalline rocks and minerals
that develop orientation texture in crystallizing from the melt
and/or from mechanical operations such as stretching, which
also increases the orientation effects.14 The second possibility
that can explain the modification in the solids is that high tem-
peratures promote the mobility of Li ions in excess, changing its
population in some planes, developing new locations. In order
to find out which one of these possibilities is the correct one,
sample 7:1:31100 was properly mixed with quartz to observe
whether the nonuniform intensities effect was annihilated. How-
ever, the XRD pattern of the product obtained revealed that
there was no significant change in the ratios of the integrated
intensities (001)/(100) in sample 7:1:31100 (0.02) and in sample
7:1:31100 mixed with quartz (0.05). Therefore, the orientation
texture produced during the heating process is very small, if any.
It followed that nonuniform intensities resulted, mainly from the
mobility of Li ions in excess, changing its population in some
planes and developing new locations of Li ions in highly heated
samples.
IV. Discussion
A novel ceramic synthesis technique, the modified combustion
method, was explored to produce Li4SiO4 utilizing precursors
formed by different molar ratios of LiOH, H2SiO3, and an or-
ganic fuel, CH4N2O. The mixtures were heated for 5 min at
different temperatures, on occasion, in the presence of addition-
al air in the furnace chamber. The process involved the exother-
mic reaction of an organic fuel, CH4N2O, with oxygen present in
the air in the furnace chamber. CH4N2O not only worked as a
complexing agent but also provided combustion heat for calci-
nations. The residual carbonate content was investigated by tai-
loring the different powders formed.
The precursors formed by the different LiOH:H2-
SiO3:CH4N2O molar ratios can strongly influence the composi-
tion of the silicates produced. The products were investigated
before the synthesis of Li4SiO4 by the combustion reaction was
undertaken. LiOH:H2SiO3 molar ratios 2:1 and 4:1 were ex-
pected to produce Li2SiO3 and Li4SiO4, respectively; however,
the diffraction patterns of the prepared samples 2:1:3 in the
muffle furnace heated from 5501 to 7501C revealed that two dif-
ferent crystalline lithium silicates were produced, and Li2Si2O5
was the predominant phase along with a considerable amount
of Li2SiO3.
As the main purpose of this paper was to obtain Li4SiO4, a
second series of experiments was performed with different pre-
cursor molar ratios, different heating conditions, and additional
air in the furnace chamber. Li4SiO4 is both rich in lithium and
oxygen; however, in the modified combustion method, oxygen
in the air was utilized as the only oxidizer of CH4N2O combus-
tion. Therefore, an oxygen-poor atmosphere in the interior
chamber of the furnace was expected. Deficiency of lithium or
oxygen during heating could prevent Li4SiO4 formation. Hence,
in order to increase Li4SiO4 production, LiOH proportion was
Figures 1–4 show that the distribution of the compounds
identified by XRD differ notably from the stoichiometric com-
position of lithium silicates, indicating that some Li is missing in
the XRD patterns. Nevertheless, no other phase containing Li
was seen in XRD patterns. In samples 2:1:3 and 5:1:3, these
differences may be explained by the presence of Li2CO3 evi-
denced by IR spectra, as very small particles and low percent-
ages are not exhibited by XRD. IR spectroscopy revealed