Chlorine evolution during the fusion of chlorofluoroindate glasses

Article abstract of DOI:10.1007/s10973-005-0086-y

Samples with a composition of 40InF₃-20ZnF₂-5MCl- xBaF₂-ySrF₂, where M=Na, Li and x+y=35 mol%, were prepared. The thermal properties related to the Ba/Sr ratio and to the remaining chlorine content in the glasses were studied. Thermal stability is improved with the addition of chlorine. However, chlorine concentration is regulated by the sublimation of indium fluorides which takes place at about 600°C. Indium fluorides arc formed during glass fusion. The mechanisms of chlorine sublimation were studied.

Full text of DOI:10.1007/s10973-005-0086-y

Journal of Thermal Analysis and Calorimetry, Vol. 79 (2005) 469–472
CHLORINE EVOLUTION DURING THE FUSION OF
CHLOROFLUOROINDATE GLASSES
J. R. J. Delben1*, S. S. Rojas1, K. Miazato1, P. Melnikov1, A. A. S. T. Delben1, C. X. Cardoso2
and A. Job2
1DFI-CCET, Universidade Federal de Mato Grosso do Sul, C.P. 549 - 79070-900 Campo Grande, MS - Brazil
2DFQB-FCT, Unesp, cx.p 957- 19060-080 Presidente Prudente, SP - Brazil
Samples with a composition of 40InF3–20ZnF2–5MCl–xBaF2–ySrF2, where M=Na, Li and x+y=35 mol%, were prepared. The ther-
mal properties related to the Ba/Sr ratio and to the remaining chlorine content in the glasses were studied. Thermal stability is im-
proved with the addition of chlorine. However, chlorine concentration is regulated by the sublimation of indium fluorides which
takes place at about 600°C. Indium fluorides are formed during glass fusion. The mechanisms of chlorine sublimation were studied.
Keywords: fusion process, intermediate reactions, sublimated compounds
Introduction
convert infrared beam to visible light [9–15]. Lasing
effect occurs at many different wavelengths for rare
earth doped fluoride glasses, including the three tele-
communications windows at 0.8, 1.3 and 1.55 mm, de-
pending on the rare earth doping [16–19]. The chal-
lenge is to enhance lasing efficiency, which can be
achieved either by obtaining better fluoride glass com-
positions or by using alternative preparation methods,
such as sol–gel [20] to provide samples with better pu-
rity and homogeneity. The present paper reports about
glasses in chlorofluoroindate system. The addition of
chlorides to fluoride glasses extended the infrared
transmission up to 10 mm [19], reducing rare earth
emission losses and focuses attention to this mixed-ha-
lide system [5, 21–24]. The chlorine content influences
the physical and optical glass characteristics, but upon
fusion sublimation takes place, where the evolved
compound is reported as InCl3 [25]. Further researches
indicated that the quantity of the remaining chlorine
depended on the sample composition [26]. The aim of
the present work is to study the process of fusion, to
determine the sublimation temperatures, the composi-
tion of the sublimated compounds, the influence of the
remaining chlorine on the glass samples and the ther-
mal characteristics of the obtained glasses.
Fluoride glasses have been studied since the middle of
the seventies because of their potential application in
optical telecommunications [1, 2]. The special spectro-
scopic properties of fluoride glasses, such as low re-
fraction index, extended optical transmission window
and high lifetime in the excited states of rare earth ions
in fluoride host, are determined by the specific charac-
teristics of fluorine ion, such as high electronegativity,
low polarizability, weak metal–fluor ionic bonding and
small metal–fluor distances. Due to the extended trans-
parency in the IR range, fluoride glass fibers present
lower theoretical optical losses compared to the silica
fibers [3]. So the first proposed application of the fluo-
ride glass fibers was as long-haul telecommunication
fibers. Unfortunately, the promised ultra-low-loss level
for fluoride fiber has never been achieved experimen-
tally, mainly due to the fusion preparation method,
where the required high purity level for optical fibers
was not produced. Besides their application as passive
optical fibers, fluoride glasses can also be used in ac-
tive optical devices. In comparison with silica glasses,
they are better hosts for rare earth ions, which are re-
sponsible for the luminescent emission, since the low
phonon energy of fluoride glasses admits longer life-
times in metastable state. Fluoride glasses allow very
broad range of lasing wavelengths: it covers
the 0.35–3.5 mm interval, whereas silica glasses lase in
the region of 0.6–2 mm [4]. So, fluoride glasses are ex-
cellent for optical fiber lasers and optical fiber amplifi-
ers [5–8], replacing electronic regenerators in optical
telecommunication, as well as in special devices which
Materials and methods
Samples with the following composition 40InF3
–20ZnF2–5MCl–xBaF2–ySrF2, where M=Na, Li and
x+y=35 mol%, have been prepared. The initial com-
pounds were BaF2, InF3 and SrF2 Fluortran-grade (fiber
*
Author for correspondence: delbenbr@yahoo.com.br
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

DELBEN et al.
optic grade), from BDH-Merck, ZnF2 – 99% purity –
fication of remaining chloride ions of these glass sam-
ples was performed by using of ion selective electrode.
from Aldrich, NaCl and LiCl – 99% purity – from
Synth and In2O3 – 99% purity – from Merck. Indium
oxide was fluorinated in air with the excess of ammo-
nium bifluoride, at 380°C for 30 min in a Pt crucible,
according to the following reaction:
Results
Infrared spectra of A1 samples (A1-powder and
A1-remelted sample) containing 5% of NaCl, 10% of
BaF2 and 25% of SrF2 are presented in Fig. 1.
In2O3+6NH4F×HF®2InF3+6NH4F+3H2 O (1)
After then, this material was submitted to an-
other heat treatment for 30 min at 500°C, to eliminate
reaction residues.
Two different series of samples were prepared,
as it can be seen in the Table 1.
For series
A
five batches with various
BaF2/SrF2 ratios, without NaCl and with fluorinated
In2O3 were treated at 900°C for 90 min in air in Pt cru-
cible. The batches were cooled down spontaneously
out of the furnace and then were milled. Each composi-
tion was divided into two parts. A first part of them
was melted again at 850°C for 30 min in dry box under
N2, with the addition of NaCl in the last 3 min of the
experiment, in order to reduce chlorine sublimation
(series A-remelted). In case of the other part, NaCl was
just added without any further heating (series A-pow-
der). For series A-powder combined thermogravi-
metry – FTIR spectroscopy was done using a
Netzsch TG-209 coupled to a Bruker FTIR-Vector 22
FTIR apparatus, to follow the reactions that occurred
during the fusion of glass. The applied experimental
conditions were: dynamic nitrogen atmosphere and
10 K min–1 heating rate up to 900°C. The effect of
chlorine inclusion, the intermediate reactions and the
sublimation could be followed at any temperature.
FTIR was performed on the A1-remelted sample
and on A1-powder.
The glasses of series B were prepared with similar
compositions, containing LiCl, however commercial
InF3 was used to avoid the N–H residues of the fluorina-
tion process. Since the interest was on chlorine sublima-
tion, quantification of remaining chloride ions on these
glasses has been performed. The batches were melted
at 850°C for 30 min in Pt crucible, in a dry box under N2
atmosphere. LiCl was also added in the last 3 min of the
experiment. Samples were poured in brass mold and an-
nealed at 240°C. Glass transition temperatures were de-
termined using a Shimadzu DSC-50 equipment. Quanti-
Fig. 1 Infrared spectra of samples containing 5% NaCl, 10%
BaF2 and 25% SrF2; — – A1-powder (without any heat
treatment) and --- – A1-remelted sample
The same absorption frequencies, with different
relative intensities were observed in both samples.
The assignments of bands are listed in Table 2.
The band around 497 cm–1, exhibiting the highest
absorption of the sample of the second series, corre-
sponds to [InF6]3– which belongs to the pre-crystalline
structure in the glasses containing indium fluoride [27].
The other lines correspond to O–H and N–H bonds as-
sociated to the reaction atmosphere and to the residues
of indium fluorination process. These lines were stron-
ger on the A1-powder because the O–H and N–H
groups were reduced under the previous heating of the
A1-remelted sample. Even after heat treatments at
500°C for 30 min and at 850°C for 30 min small amount
of ammonium residue was observed, although its inten-
sity was reduced compared to the In–F absorption line.
Consequently, ammonium residue elimination during
fluorination by NH4F×HF requires high temperature and
time to be completed.
Table 1 Compositions of samples (in mol%) with different contents of BaF2 and SrF2. Series A are not glasses vs. series B
Sample
InF3
ZnF2
BaF2
SrF2
NaCl
Sample
InF3
ZnF2
BaF2
SrF2
LiCl
A1
A2
A3
A4
A5
40
40
40
40
40
20
20
20
20
20
10
15
20
25
30
25
20
15
10
5
5
5
5
5
5
B1
B2
B3
B4
B5
40
40
40
40
40
20
20
20
20
20
10
15
20
25
30
25
20
15
10
5
5
5
5
5
5
470
J. Therm. Anal. Cal., 79, 2005

FUSION OF CHLOROFLUOROINDATE GLASSES
Table 2 Absorption frequencies taken from Fig. 1
Absorption/cm–1 Assignment
3100–3600
H2O band
Characteristic frequency of N–H bond
or
coupling of H2O molecules
3223
2891
2323
N–H bond
N–H bond
Amine group
or
fundamental vibration mode of H2O
1626
Fig. 3 TG curves of samples containing different amount of BaF2
1433
1260
1124
500
N–H bond
Frequencies of amine vibration
NH+3 cations
Cluster [InF6]3–
mately at 600°C for the ones containing of 25
and 30% BaF2. The glass composition influences the
chlorine sublimation, the elimination of the fluorina-
tion residue and the temperature of these events.
The remaining chlorine content of glasses with
LiCl (series B) support this observation. Samples con-
taining 5% LiCl were prepared with different amounts
of BaF2/SrF2. The glass transition temperatures of
these glasses are in the 253–263°C temperature range.
A1-powder, containing 5% NaCl, 10% BaF2 and
25% SrF2 (series A) was studied by simultaneous
TG-FTIR, in order to follow the reaction mechanism
of chlorine inclusion and sublimation. The corre-
sponding three-dimensional FTIR spectrum can be
seen in Fig. 2. FTIR patterns for all other samples are
practically identical to this one.
Fig. 4 Remaining chlorine content in glasses containing
various quantities of BaF2 and SrF2
Fig. 2 FTIR spectra of gases evolved from A1-powder sample
containing 5% NaCl, 10% BaF2 and 25% SrF2 (without
any heat treatment)
The chlorine content was determined by using
ion selective electrodes. The results are indicated in
Fig. 4. The remaining chlorine content corresponding
to 15% of BaF2 is almost three times higher than the
chlorine content of sample with 30% BaF2.
The absorption band around 500 cm–1 corre-
sponds to In–F bonding. Its presence above 600°C is
unusual, since InF3 melts at 1170°C. Consequently,
the outcoming gas with In–F bonding should be an in-
dium fluorochloride compound e.g. InF2Cl, or a
mixed metal–indium fluorochloride.
Discussion
TG curves of the A-powder (series A), in which
NaCl was just mixed, containing different proportion
of BaF2/SrF2 are collected in Fig. 3. Mass loss steps
can be seen between 100–800°C, in a wide tempera-
ture range. However, the numerical mass loss values
strongly depend on the BaF2/SrF2 ratio. Mass losses
started at 400 and at 800°C, for samples containing 10
to 20% BaF2, and the process has begun approxi-
The obtained results can be explained by the occurrence
of intermediate reactions during heating and fusion, as-
sociated to the formation of volatile compounds at the
given preparation temperatures. The sublimation of
InCl3 has been already reported, however, according to
the present results, sublimation of other complex phases
can be also observed. IR absorption at 500 cm–1 is asso-
J. Therm. Anal. Cal., 79, 2005
471

DELBEN et al.
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