Thermal properties of fluoride glasses and their gel precursors

Article abstract of DOI:10.1023/B:JTAN.0000017331.26326.54

Xerogels were prepared from zirconium, barium, aluminum, lanthanum and lithium acetates, corresponding to a Li containing ZBLA composition. The study of their thermal properties (DSC, TG/DTG, FT-IR) showed that they might be used as chemically stable precursors in the preparation of fluoride glasses. Hydrofluoric acid in solution was chosen as a mild fluorinating agent. This newly proposed technique of fluorinating allowed to obtain high quality ZBLALi glass which presents the advantage of higher thermal stability and homogeneity in comparison with the glass obtained using individual commercial fluorides.

Full text of DOI:10.1023/B:JTAN.0000017331.26326.54

Journal of Thermal Analysis and Calorimetry, Vol. 75 (2004) 87–93
THERMAL PROPERTIES OF FLUORIDE GLASSES
AND THEIR GEL PRECURSORS
1
*
1
P. Melnikov , R. Rolim , A. Delben , J. R. Delben , A. C. Souza and
A. E. Job
1
1
1
1
2
Physics Department/CCET/UFMS, Caixa Postal 549, Campo Grande/MS, Brazil
Faculty of Science and Technology/UNESP, Rua Roberto Simonsen 305,
Presidente Prudente/SP, Brazil
2
(
Received September 23, 2002; in revised form October 2, 2003)
Abstract
Xerogels were prepared from zirconium, barium, aluminum, lanthanum and lithium acetates, corre-
sponding to a Li containing ZBLA composition. The study of their thermal properties (DSC,
TG/DTG, FT-IR) showed that they might be used as chemically stable precursors in the preparation
of fluoride glasses. Hydrofluoric acid in solution was chosen as a mild fluorinating agent. This
newly proposed technique of fluorinating allowed to obtain high quality ZBLALi glass which pres-
ents the advantage of higher thermal stability and homogeneity in comparison with the glass ob-
tained using individual commercial fluorides.
Keywords: fluoride glasses, glass precursors, xerogels
Introduction
The extended transmission window, low phonon energy and high solubility of rare
earth compounds in fluoride glasses make them suitable for creating passive as well
as active optical systems such as fiber lasers, optical amplifiers, ultra-low loss optical
fibers, planar waveguide and upconversion devices [1–3]. Nonetheless, conventional
melt-quenching techniques have certain limitations as to the fabrication of glasses
with the required high purity and specific geometry. In the case of oxide materials,
the sol–gel method provides a simple way to prepare high purity glasses with a wide
assortment of shapes, from thin films and layers to large monolithic blocks [4]. Hy-
brid materials in the form of xerogels are used for obtaining coatings with special op-
tical and mechanical properties [5]. In the case of fluoride glasses, however, the
sol–gel technique is far from perfect as any perceptible change of precursor, compo-
sition, solvent or fluorinating agent alters significantly not only the properties of the
final glass but also influences the parameters of the chemical process itself. Glasses
*
Author for correspondence: E-mail: petrmelnikov@yahoo.com
1
388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
©
2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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MELNIKOV et al.: PROPERTIES OF FLUORIDE GLASSES
and thin films prepared using alkoxides, oxychlorides, crown esters and acetates are
generally used in the preparation of fluoride glasses by this method. Soon after ob-
taining xerogels, fluorination by NH F⋅HF, gaseous HF, trifluoroacetic acid or NbF5
4
is carried out [6–9]. All these agents have certain disadvantages: NH F⋅HF always
4
leaves ammonium groups trapped in the network, gaseous HF is a poisonous and haz-
ardous material, CF COOH does not permit a complete fluorination and NbF con-
3
5
taminates the final product with undesirable niobium. The aim of the present work is
to study thermal stability and some properties of fluoride glass ZBLALi and its gel
precursors. The latter were prepared using acetates as starting materials in order to
compare a newly proposed fluorination method, using hydrofluoric acid solution,
with a conventional one using NH F⋅HF. Thermal analysis (DSC and TG/DTG) pro-
4
vide a simple technique of characterizing gels-precursors and glasses within the re-
quired interval of temperatures.
Experimental
Starting materials and methods
The gels were prepared using the following acetates: Ba(CH COO) (Fluka),
3
2
La(CH COO) (Fluka), Li(CH COO) (Fluka), Zr(CH COO) was used in the form
3
3
3
3
4
of 14% solution in acetic acid (Aldrich). All reagents were of analytical grade purity. Hy-
drofluoric and acetic acids were purchased from Merck. NH F⋅HF was purchased from
4
Nuclear. Both LaF and BaF were of fiber glass purity purchased from BDH. Aluminum
3
2
acetate loses its solubility quickly after preparation owing to its ageing, so a commercial
reagent might not be used. Fresh Al(CH COO) solution was prepared precipitating
Al(OH) from the solution of double aluminum-ammonium sulfate; the hydroxide was
3
3
3
filtered out and quickly dissolved in a calculated volume of glacial CH COOH. The com-
3
position of gel matrix was calculated in order to produce the final glass with the follow-
ing proportions of constituents: 47ZrF , 14BaF , 4LaF , 5AlF and 30LiF, in mol%. They
4
2
3
3
are represented as fluorides in order to make comparisons. An identical composition was
also prepared directly by mixing up pure commercial fluorides.
The starting components were combined and resulting solutions were left for
evaporation under the flux of air. Gelation came into being during 24 h, the resultant
gels were homogeneous and optically clear. X-ray diffraction investigation showed no
reflections. After drying at 60°C for 72 h, two methods of gel treatment were tested:
a) fluorination by adding hydrofluoric acid to the gels at room temperature with conse-
quent drying and heating at ca. 200°C;
b) fluorination by adding NH
elimination of ammonia and excessive NH HF).
4
F⋅HF heating firstly at 300°C and then to 500°C (for
4
Melting of fluorinated materials, as well as the reference sample containing
commercial fluorides, was carried out in a platinum crucible at 850°C for 20 min. All
operations were carried out in a dry box under nitrogen atmosphere. Samples were
quenched by pouring into a brass mould and then annealed at 260°C for 30 min.
J. Therm. Anal. Cal., 75, 2004

MELNIKOV et al.: PROPERTIES OF FLUORIDE GLASSES
89
Differential scanning calorimetry (DSC), was performed using a DSC/50-
Shimadzu instrument under the following conditions: sample mass ~15 mg, gas
–
flux 100 mL min of both synthetic air and nitrogen. Heating rate varied: for the
1
–
1
–1
study of gels it was 4°C min , while for the glass samples it was 10°C min .
Thermogravimetric analysis (TG) of the gels was performed under synthetic air at-
–
1
mosphere, with a heating rate of 4°C min . A TGA/50H-Shimadzu instrument was
employed. FT-IR spectra of gaseous products which were released during heating
were registered using a Brucker instrument, model Vector 22, HgCdT detector with
–
1
resolution of 4 cm and 32 scans. In order to evaluate crystallinity and identify the
compounds resulting from thermal events, X-ray diffraction study was performed us-
ing a Kristalloflex-Siemens diffractometer, CuK radiation, Ni filter. The identifica-
α
tion was achieved with a set of DIFFRACT programs and ICSD (International Centre
for Diffraction Data) data files.
Results and discussion
DSC curves of gel (Fig. 1) presented three endothermic effects due to successive dehy-
dration. The third one is partially superposed with the first of the two following exother-
mic peaks, at approximately 323 and 380°C. This pattern was observed when synthetic
air was used as a gaseous carrier (Fig. 1a). On the contrary, they disappear when the ex-
periment is held using nitrogen atmosphere (Fig. 1b), proving the fact that both effects re-
flect only the oxidation and do not indicate any crystallization processes. In this case, the
incineration of organic components would occur around 323°C and the carbon particles
formed would be burnt at 380°C. Meanwhile, nitrogen acts only as a neutral carrier.
Fig. 1 DSC curves of gel ZBLALi prepared by using a – synthetic air and b – nitrogen
TG curves reported in Fig. 2 show four significant mass losses. That around 70°C
corresponds to the elimination of absorbed water and that at 150°C reflects the release
of water retained within gel pores. The mass loss observed at 320°C makes manifest gel
network decomposition. The small peak at 389°C is due to the oxidation of the afore-
J. Therm. Anal. Cal., 75, 2004

9
0
MELNIKOV et al.: PROPERTIES OF FLUORIDE GLASSES
mentioned carbon particles. Another small mass loss after it seems to be due to the de-
composition of trace carbonates.
The FT-IR spectra of gaseous products released during the gel thermal treatment
–
1
(Fig. 3) shows intensive bands around 3400–3800 and 1645 cm corresponding to
water desorption from differently bonded H O populations (absorbed water and wa-
ter contained in the pores), as it occurs in the case of silica gel and coprecipitated fer-
2
–
rite powders [10, 11]. An intensive band at 2450 cm is attributed on the bases of
1
classical data [12] to antisymmetric vibrations in gaseous CO . Carbon dioxide starts
2
to form above 300°C, exactly corresponding to the onset of third exothermic effect in
Fig. 2 TG and DTG curves of gel ZBLALi
Fig. 3 Gaseous FT-IR specter of gel ZBLALi
J. Therm. Anal. Cal., 75, 2004

MELNIKOV et al.: PROPERTIES OF FLUORIDE GLASSES
91
Figs 1 and 2. We must assume that we are dealing with two different sources of CO2:
the first one, according to the temperature interval corresponds to oxidation of the or-
ganic ligand while the second confirms the decomposition of metal carbonates, prin-
cipally those of lanthanum and barium.
According to X-ray patterns, the samples of gels treated up to 200°C are amor-
phous. They commence to crystallize after heating at 300–400°C showing the pres-
ence of ZrO (ICSD file 37-1413) (Fig. 4). For treatments above 500°C the presence
2
of barium carbonate could also be observed. These crystalline phases have high melt-
ing temperatures representing serious impediments to fluorides fusion at 850–900°C.
Hence, the temperature of gels treatment should not go above 200–280°C.
Fig. 4 X-ray diffractogram of gel ZBLALi thermally treated at 400ºC
The first fluorination method tested was the addition of NH F⋅HF to the gels. All
4
attempts to melt the samples failed. The impossibility of fluoride glass preparation by
this technique must be related to the necessity of working at temperatures above 300°C,
which leads to the formation of zirconia, as showed above. This stable phase cannot be
melted at the temperature at which the fluoride glasses are usually prepared, as even
low ZrO contents induce further nucleation and crystallization.
2
On the contrary, the second method, e. g. fluorination of gels dried at 200°C us-
ing the solution of hydrofluoric acid, could have been easily carried out at room tem-
perature, later melting resulting in homogeneous, transparent glass samples. Thermal
characteristics of this glass were compared with the characteristics of a reference
sample obtained directly by fusion of commercial fluorides.
The reference curve (Fig. 5a) presented T –T =72°C and three crystallization
x
g
peaks: the first (medium) at 340°C, the second at 382°C (strong) and the last (shoul-
der) at 405°C accompanied by endothermic melting peaks of 522 and 542°C, respec-
tively, with comparable intensity (1:1.1).
The curve registered with the sample prepared by sol–gel (Fig. 5b) had greater
thermal stability (T –T =86°C) presenting a single crystallization peak at 380°C.
x
g
Meanwhile, the relative intensity of melting peaks at 507 and 528°C was drastically
changed (1.0:3.0).
J. Therm. Anal. Cal., 75, 2004

9
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MELNIKOV et al.: PROPERTIES OF FLUORIDE GLASSES
Fig. 5 DSC curves of ZBLALi glasses obtained by two different routes
The lack of compositional homogeneity within the vitreous phase favors the for-
mation of various crystalline species to which correspond different temperatures of
crystallization and melting. Hence, the presence of only one crystallization peak and
the reduction of the first melting peak strongly confirm an increase in homogeneity of
the glass prepared from the gel precursor.
Conclusions
Gels synthesized from acetates corresponding to ZBLALi composition proved to be
proper precursors of fluoride glasses. The investigation of their thermal properties al-
lowed the choice of operational conditions and a suitable fluorinating agent: hydro-
fluoric acid in solution. The ZBLALi glass successfully obtained by the newly pro-
posed technique is similar to the glass obtained from commercial fluoride but
presents the advantage of higher thermal stability and homogeneity.
*
* *
Financial support by CAPES, PADCT - 620535/98 and UFMS (Brazilian agencies).
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