556_559_23_G-O-05 Ersundu.indd
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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:miraycelikbilek@gmail.com, celikbilek@itu.edu.tr 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) 557 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 558 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. References [1] El-Mallawany R. H.: Tellurite Glasses Handbook: Physical Properties and Data, (2002), CRC Press LLC. [2] Öveçolu M.L., Kabalc I., Özen G., Öz B.: „Microstructural characterization of (1x)TeO2 – xPbF2 (x = 0.10, and 0.25 mol) glasses”, J. Eur. Ceram. Soc., 27, (2006), 1801. [3] Blanchandin, S., Marchet, P., Thomas, P., ChamparnaudMesjard, J.C., Frit, B., Chagraoui, A.: „New investigations within the TeO2–WO3 system: phase equilibrium diagram and glass crystallization”, J. Mater. Sci., 34, (1999), 4285. [4] Öveçolu M.L., Özen G., Cenk S.: „Microstructural characterization and crystallization behaviour of (1x) TeO2 – xWO3 (x = 0.15, 0.25, 0.3 mol) glasses”, J. Eur. Ceram. Soc.,26, (2005), 1149. [5] Kosuge, T., Benino, Y., Dimitrov, V., Sato, R., Komatsu, T.: „Thermal Stability and Heat Capacity Changes at the Glass Transition in K2O–WO3–TeO2 Glasses”, J Non-Cryst. Solids, 242, (1998), 154. [6] Shaltout, I., Tang, Y.I., Braunstein, R., Abu-Elazm, A.M.: „Structural Studies of Tungstate – Tellurite Glasses by Raman Spectroscopy and Differential Scanning Calorimetry”, J. Phys. Chem. Solids, 56, (1994), 141. [7] Eds. By Vlasov A.G., Florinskaya V.A.: „Structure and Physicalo-Chemical Properties of Inorganic Glasses”, Khimiya, Leningrad, 1974, quoted by Safonov V.V.: „Interactions in the TeO2-TiO2-WO3 System”, Russion Journal of Inorganic Chemistry, 53, (2008), 460. i Received 7 April 2010; accepted 5 May 2010 MATERIA£Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010) 559