556_559_23_G-O-05 Ersundu.indd

MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010), 556-559
www.ptcer.pl/mccm
Microstructural and Thermal Characterization
of the TeO2–WO3 System
MIRAY ÇELIKBILEK*, GÜNKUT KARADUMAN, A. ERÇIN ERSUNDU, NURI SOLAK, DEMET TATAR, SÜHEYLA AYDIN
Istanbul Technical University, Department of Metallurgical and Materials Engineering, Istanbul, 34469, Turkey
*e-mail:[email protected], [email protected]
Abstract
In the present study, thermal behaviour and microstructure of the TeO2–WO3 system have been investigated. Different compositions of
(1x)TeO2 – xWO3 system (x = 0.02, 0.05, 0.15, 0.25 and 0.40 in molar ratio) were prepared. The samples waited at 750°C in a platinum
crucible for 30 min and then quenched in a water bath. DTA studies were performed on glassy samples. Afterwards, all samples were heattreated at 550°C for 24 h and furnace cooled to obtain phase equilibrium. XRD and SEM/EDS studies were performed on the crystallized
samples for microstructural analysis.
Keywords: Tellurite glasses, TeO2–WO3 system, Thermal properties, Microstructure - nal
CHARAKTERYSTYKA MIKROSTRUKTURALNA I CIEPLNA UKADU TeO2 – WO3
W prezentowanej pracy zbadano zachowanie cieplne i mikrostruktur tworzyw w ukadzie TeO2-WO3. Przygotowano róne skady
ukadu (1x)TeO2 – xWO3, gdzie x = 0.02, 0.05, 0.15, 0.25 i 0.40 w stosunku molowym. Próbki przebyway w 750°C w tyglu platynowym
przez 30 min a potem szybko chodziy si w wodzie. Badania DTA przeprowadzono na próbkach szklistych. Nastpnie, wszystkie
próbki zostay wygrzane w . 550°C przez 24 h i schodzone z piecem w celu osignicia równowagi fazowej. Badania XRD i SEM/EDS
przeprowadzono na krystalizowanych próbkach w przypadku analizy mikrostrukturalnej.
Sowa kluczowe: szka tellurytowe, ukad TeO2–WO3, waciwoci cieplne, mikrostruktura nalna
1. Introduction
Tellurite glasses have superior properties than silicate,
borate and phosphate glasses in ber optic and up-conversion laser applications; due to their low phonon energy
(750 cm-1), high refractive index (2.1-2.3), high dielectric
constant, thermal and chemical stability [1, 2].
TeO2 is the main but the conditional glass former; therefore, an addition of a network modier, such as heavy metal
oxides, increases the glass forming ability. The addition of
WO3 to tellurite glasses provides suitable properties, such as
doping in a wide range, modifying the composition by a third,
fourth, and even fth component, controlling the optical properties, enhancing the chemical stability and devitrication
resistance of the glass [1, 3]. In comparison to other tellurite
glasses, tungsten-tellurite glasses have higher phonon energy and higher glass transition temperature, therefore they
can be used at high optical intensities without exposure to
thermal damage. Due to these favorable properties, in the
present study WO3 was selected as a network modier.
Several studies exist on TeO2–WO3 binary system in the
literature, concerning thermal stability, crystallization behaviour, phase equilibria and microstructural characterization
[2-6]. However, the available data are contradictory and do
not cover a systematical investigation. As part of a system-
556
atic phase equilibria study of TeO2–WO3–CdO system, the
present study aims to investigate microstructural and thermal
characterization of the TeO2–WO3 binary system.
2. Experimental
Different samples of TeO2–WO3 binary system were
prepared with the compositions of (1x)TeO2 – xWO3, where
x = 0.02, 0.05, 0.15, 0.25 and 0.40 in molar ratio. All chemicals used in the experiments were reagent grade of TeO2
(99.99 % purity, Alfa Aesar Company) and WO3 (99.8 %
purity, Alfa Aesar Company). The powder batches of 5 g
size were melted in a platinum crucible with a closed lid at
750°C for 30 min. The molten samples quenched in a water
bath and thermal characterization experiments were realized
by using the differential thermal analysis (DTA) technique.
Afterwards, as-cast samples were heat-treated at 550°C for
24 h and thermal characterization experiments were repeated
to obtain thermal equilibrium of the system. DTA scans of
samples were carried out in a Perkin ElmerTM Diamond
TG/DTA to determine the glass transition, crystallization,
eutectic, liquidus and phase transformation temperatures.
The glass transition onset temperatures (Tg) were taken as
the inection point of the step change of the calorimetric
signal. Onset temperatures specied at the intersection of
MICROSTRUCTURAL AND THERMAL CHARACTERIZATION OF THE TeO2–WO3 SYSTEM
the extrapolated baseline and the extrapolation of the linear
part of the peaks. The DTA scans were recorded by using
25 mg powdered samples. All thermal analyses realized with
a heating rate of 10 K/min from room temperature to 750°C
in a platinum crucible.
To determine the crystalline phases, X-ray diffraction
(XRD) analyses were carried out on heat-treated samples
and scanning electron microscopy (SEM) studies were conducted for microstructural characterization. The X-ray diffraction investigations were carried out with powdered samples
in a BrukerTM D8 Advanced Series powder diffractometer
using CuKD radiation in the 2T range from 10 to 90°. The
Joint Committee on Powder Diffraction Standards (JCPDS)
data les were used to determine the crystallized phases by
comparing the peak positions and intensities. SEM investigations were conducted with gold-coated bulk samples in
a JEOLTM Model JSM 5410, operated at 15 kV and linked with
Noran 2100 Freedom energy dispersive spectrometer (EDS)
attachment. For all samples, surface SEM micrographs were
taken in the secondary electron imaging (SEM/SEI) mode.
For x = 0.05 composition two exothermic peaks were
observed and the onset values were determined at 399
and 462°C. However, Blanchandin et al. [3] reported four
exothermic peaks, while Shaltout et al. [6] determined three
exothermic peaks for this composition.
Three exothermic peaks existed for x = 0.15 composition.
For the rst two peaks, the onset values were found at 406
and 475°C, respectively; however the third exothermic onset
value was not detected since it overlaps with the previous
peak. Blanchandin et al. [3] also detected three exothermic
peaks, while Öveçolu et al. [4] and Shaltout et al. [6] reported two exothermic peaks for x = 0.15 composition.
For x = 0.25 composition, the exothermic onset peak
temperature was found at 471°C. In the literature, the
same composition was studied by Blanchandin et al. [3]
and Öveçolu et al. [4]. Blanchandin et al. [3] found one
exothermic peak, while Öveçolu et al. [4] did not observe
any exothermic reaction.
Tp1
3. Thermal analyses
Te
Tpt
Tp1
Tg
Te
Heat Flow (a.u)
(d) x= 0.25
Tg
Tp1
Tp2
Tp3
Te
(c) x= 0.15
Tg
Tlp
Tp2
Tp1
Te
(b) x= 0.05
Endo
T h e D TA c u r v e s o f 0 . 9 8 Te O 2 – 0 . 0 2 W O 3 ,
0 . 9 5 Te O 2 – 0 . 0 5 W O 3 , 0 . 8 5 Te O 2 – 0 . 1 5 W O 3 ,
0.75TeO2 – 0.25WO3 and 0.60TeO2 – 0.40WO3 samples
scanned at a heating rate of 10 K/min up to 750°C are
shown in Fig. 1.
DTA scans show glass transition, exothermic peaks
corresponding to the crystallization or transformation of the
crystalline phases and endothermic peaks related to the
eutectic, liquidus and phase transformation temperatures.
The glass transition onset (Tg), crystallization onset and peak
(Tc/Tp), eutectic onset and peak (Te/Tm), liquidus onset and
peak (Tlo/Tlp) and phase transformation (Tpt) temperature
values were listed in Table 1.
For low contents of WO3 (x = 0.02 and 0.05), glass transition was not detected and the rst exothermic peaks showed
very low intensities on DTA curves.
The x = 0.02 composition showed only one weak exothermic peak, while the onset value was observed around
480°C. In the literature, glass formation was not observed
for compositions where x 0.05 in the TeO2–WO3 binary
system. However, in the present study an exothermic peak
was observed for x = 0.02 composition indicating the crystallization from the glass matrix. Therefore, ongoing studies are
currently in progress for this composition.
Exo
(e) x= 0.40
Te
(a) x= 0.02
Tlp
Tlp
250
350
450
550
650
750
Temperature (°C)
Fig. 1. DTA curves of (1x)TeO2 – xWO3 as-cast samples, where
x = a) 0.02, b) 0.05, c) 0.15, d) 0.25, and e) 0.40 in molar ratio.
Table 1. Values of glass transition onset, Tg, crystallization onset, Tc, crystallization peak, Tp, eutectic onset, Te, eutectic peak, Tm, liquidus
onset, Tlo liquidus peak Tlp and phase transformation, Tpt, temperatures of the (1-x)TeO2 + xWO3 samples.
Sample
[°C]
Tg
Tc1 / Tp1
Tc2 / Tp2
x = 0.02
-/-
x = 0.05
399 / 410
462 / 487
475 / 487
Tc3 / Tp3
- / 497
Te / Tm
Tlo / Tlp
619 / 623
711 / 725
618 / 623
690 / 711
618 / 624
- / 663
x = 0.15
334
406 / 431
x = 0.25
350
471 / 493
618 / 623
x = 0.40
343
458 / 485
620 / 625
Tpt
743 / 747
- : undetermined values
MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010)
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M. ÇELIKBILEK, G. KARADUMAN, A.E. ERSUNDU, N. SOLAK, D. TATAR, S. AYDIN
For x = 0.40 composition, one exothermic peak was
observed with the onset value at 458°C. Shaltout et al. [6]
also found one exothermic peak for the same composition.
All samples show a similar endothermic peak, indicating
the eutectic reaction of the TeO2–WO3 binary phase diagram.
The eutectic reaction was detected at 630°C for 16.3 mol.% in
the literature [7], which was later presented by Blanchandin et
al. [3] as liquid D-TeO2 (paratellurite) + orthorhombic WO3
taking place at 622 ± 5°C for 22 ± 1 mol% WO3. The eutectic
temperature was also conrmed by Öveçolu et al. [4]. In the
present study, according to the results obtained from a wide
range of compositions, eutectic reaction onset temperature
was determined at 618 ± 2°C (Tm = 623 ± 2°C). With increasing
WO3 content, the liquidus peak approaches to the eutectic
peak temperature and disappears at the eutectic composition. In the present study, for 0.60 TeO2 – 0.40 WO3 sample,
in the hyper-eutectic region, the liquidus temperature was
not detected, which was, however, observed by Blanchandin
et al [3]. A secondary endothermic peak was determined
at 743°C for x = 0.40 composition, representing the phase
transformation from orthorhombic WO3 to tetragonal WO3.
a)
4. Microstructural characterization
On the basis of the DTA results, XRD and SEM analyses
were carried out on heat-treated samples for phase and
microstructural characterization. The XRD patterns of the
fully crystallized samples are given in Fig. 2.
b)
c)
Fig. 2. X-ray diffraction patterns of (1x)TeO2 – xWO3 samples
heat treated at 550°C for 24 h , where x = a) 0.02, b) 0.05, c) 0.15,
d) 0.25, and e) 0.40 in molar ratio.
According to the XRD analyses, when the fully crystallization was achieved at 550°C, the whole structure was
composed of D-TeO2 (paratellurite) and orthorhombic WO3
crystalline phases. With increasing WO3 content, DTeO2
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MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010)
d)
Fig. 3. SEM Micrographs of (1x)TeO 2 – xWO 3 samples heat-treated at 550°C for 24 h, where x = a) 0.02, b) 0.05,
c) 0.15, and d) 0.40.
MICROSTRUCTURAL AND THERMAL CHARACTERIZATION OF THE TeO2–WO3 SYSTEM
(paratellurite) peak intensities decrease, while orthorhombic
WO3 becomes more pronounced in the structure.
SEM investigations were conducted on the fully crystallized samples in order to identify the morphology of crystalline phases.
Figs. 3a and 3b represent the SEM micrographs of
0.98TeO2 – 0.02WO3 and 0.95TeO2 – 0.05 WO3 samples
heat treated at 550°C for 24 h respectively, which reveal the
presence of grains in the whole structure with a secondary
phase precipitated along the grain boundaries. For x = 0.02
composition EDS spectra taken from the grains and the grain
boundaries, showed that the WO3 content is higher along the
grain boundaries. SEM micrographs of 0.85TeO2 – 0.15WO3
sample shown in Fig. 3c, revealed the degradation of grains
and the formation of dendritic rod like crystallites. Öveçolu
et al. [4] also determined lamellar crystals in shape of long
rods oriented in various directions. Fig. 3d represents the
SEM micrograph of 0.60TeO2 – 0.40WO3 sample where the
morphology reveals small polygonal crystallites along the
whole structure.
5. Conclusions
Different samples of TeO2–WO3 binary system, with the
compositions of (1x)TeO2 – xWO3, where x = 0.02, 0.05,
0.15, 0.25 and 0.40 in molar ratio were studied by DTA, XRD
and SEM techniques in order to investigate the thermal and
microstructural behaviour of the system. Glass transition,
crystallization, eutectic, liquidus and phase transformation
temperatures were determined by means of differential thermal analysis technique. A binary eutectic was determined at
618 ± 2°C, which is in agreement with literature. According to
the DTA crystallization peak temperatures, all samples were
heat-treated at 550°C for 24 h for fully crystallization and
XRD analyses were performed. Basing on the determined
XRD patterns, D-TeO2 (paratellurite) and orthorhombic WO3
crystalline phases were found when the nal crystallization
was achieved at 550°C. SEM investigations revealed different microstructures, which are grains and precipitates along
the grain boundaries, dendritic rod like crystallites and small
polygonal crystallites.
Acknowledgements
The authors wish to express their gratitude to The
Scientic & Technological Research Council of Turkey (TUBITAK) for the nancial support under the project numbered
108M077.
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i
Received 7 April 2010; accepted 5 May 2010
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