or WO3 containing tellurite glasses

or WO3 containing tellurite glasses

Journal of Non-Crystalline Solids 358 (2012) 641–647 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal ho...

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Journal of Non-Crystalline Solids 358 (2012) 641–647

Contents lists available at SciVerse ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol

Characterization of B2O3 and/or WO3 containing tellurite glasses A.E. Ersundu, M. Çelikbilek, S. Aydin ⁎ Istanbul Technical University, Department of Metallurgical and Materials Engineering, Istanbul, 34469, Turkey

a r t i c l e

i n f o

Article history: Received 12 September 2011 Received in revised form 3 November 2011 Available online 3 December 2011 Keywords: Tellurite glasses; Thermal behavior; Fourier transform infrared spectroscopy; Crystallization behavior; Microstructure

a b s t r a c t Characterization of B2O3 and/or WO3 containing tellurite glasses was realized in the 0.80TeO2–(0.20 − x) WO3 − xB2O3 system (0 ≤ x ≤ 0.20 in molar ratio) by using differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectrometry techniques. Glasses were prepared with a conventional melt-quenching technique at 750 °C. To recognize the thermal behavior of the glasses, glass transition and crystallization temperatures, glass stability value, glass transition activation energy, fragility parameter were calculated from the thermal analyses. Density, molar volume, oxygen molar volume and oxygen packing density values were determined to investigate the physical properties of glasses. Fourier transform infrared spectra were interpreted in terms of the structural transformations on the glass network, according to the changing B2O3 and/or WO3 content. Crystallization behavior of the glasses was investigated by in situ X-ray diffraction measurements and microstructural characterization was realized by scanning electron microscopy and energy dispersive X-ray spectrometry analyses. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Tellurite glasses have become technologically and scientifically important due to their outstanding properties, such as relatively low phonon energy, high refractive index, high dielectric constant, good infrared transmittance, low glass transition and melting temperature, thermal and chemical stability and high crystallization resistance. These advantageous properties make tellurite glasses preferable host materials for some infrared and infrared to visible upconversion applications in optical data storage, lasers, sensors and spectroscopic devices [1–6]. Tellurium dioxide (TeO2) is a conditional glass former which does not transform to the glassy state without the addition of a secondary component under conventional cooling conditions. Therefore, glass forming agents such as alkalis, heavy metal oxides or halogens are used to obtain tellurite glasses [1–6]. It is known that the structure and properties of oxide glasses are strongly dependent on the nature and concentration of the constituent oxides. Addition of WO3, as a network modifier or intermediate oxide network, to tellurite glasses provides several advantageous properties, such as doping with rareearth elements in a wide range, modifying the composition by a third, fourth, and even fifth component, enhancing the chemical stability and devitrification resistance. Furthermore, compared to other

⁎ Corresponding author. Tel.: + 90 212 285 68 64; fax: + 90 212 285 34 27. E-mail address: [email protected] (S. Aydin). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.11.012

tellurite glasses, WO3 containing tellurite glasses have slightly higher phonon energy and higher glass transition temperature, therefore they can be used at high optical intensities without exposure to thermal damage [2–5]. Addition of B2O3 to tellurite glasses provides some unique features by enhancing the thermal and chemical stability and crystallization resistance [1,7,8]. Due to these favorable properties, a considerable number of publications have been published on WO3 and/or B2O3 tellurite glasses by different researchers [1–15]. Although there exist numerous studies on binary and ternary tellurite glasses containing B2O3 and/or WO3, thermal and structural behavior of tellurite based glasses need to be studied in detail to develop and use them in opto-electronic applications. Therefore, in the present study the authors aim to investigate the thermal and structural properties and crystallization behavior of B2O3 and/or WO3 containing tellurite glasses in the 0.80TeO2–(0.20 − x)WO3–xB2O3 system (0 ≤ x ≤ 0.20 in molar ratio) by applying differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy and energy dispersive X-ray spectrometry (SEM/EDS) analyses. 2. Experimental procedure In the experimental studies, different compositions of the 0.80TeO2–(0.20 − x)WO3–xB2O3 system (0 ≤ x ≤ 0.20 in molar ratio) were prepared with a conventional melt-quenching technique to characterize B2O3 and/or WO3 containing tellurite glasses. High purity powders of TeO2 (99.99% purity, Alfa Aesar Company), WO3 (99.8% purity, Alfa Aesar Company) and H3BO3 (99.5% purity, Sigma-Aldrich Company) were thoroughly mixed and 5 g size powder batches were

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at the chosen temperatures, in the 2θ range from 10° to 90°. The International Centre for Diffraction Data (ICDD) files were used to determine the crystalline phases by comparing the peak positions and intensities with the reference patterns. Microstructural characterization experiments was conducted with gold/platinum coated bulk samples in JEOL™ Model JSM 7000F scanning electron microscope (SEM) linked with Oxford Inca energy dispersive X-ray spectrometer (EDS) attachment.

melted in a platinum crucible with a closed lid at 750 °C for 30 minutes in an electrical furnace and quenched in water bath. Original and final compositions of the samples are given in Table 1. The final compositions were calculated by performing chemical analysis using a Perkin Elmer AAnalyst 800 atomic absorption spectrometer with an error estimate of ± 2%. X-ray diffraction analyses (XRD) were realized to check the amorphous nature of the as-cast samples. XRD investigations were carried out with powdered samples in a Bruker™ D8 Advanced Series powder diffractometer using Cu Kα radiation in the 2θ range from 10° to 90°. Thermal behavior of the glass samples were investigated by applying differential scanning calorimetry analyses (DSC) in a Netzsch DSC 204 F1 (limit of detection: b0.1 μW, with an error estimate of ± 1 °C) using a constant sample weight of 20 ± 1 mg in aluminum pans, under flowing (25 ml/min) argon gas with a heating rate of 10 °C/min. The glass transition onset (Tg), crystallization onset and peak (Tc/Tp) temperatures were determined from the DSC scans. The glass transition onset temperatures (Tg) were determined as the inflection point of the endothermic change of the calorimetric signal. Crystallization onset temperatures were specified as the beginning of the reaction where the crystallization first starts and peak temperatures represent the maximum value of the exotherm. The temperature difference between the glass transition (Tg) and the first exothermic peak onset (Tc1), ΔT = Tc1 − Tg, indicating the value of glass stability was calculated. The activation energy of the glass transition, Eg, was determined by running non-isothermal DSC scans of the glass samples at seven different heating rates, B, (5, 10, 15, 20, 25, 30 and 40 °C/min) from room temperature to 550 °C. Densities, ρ, of the glass samples were determined at room temperature by the Archimedes principle using distilled water as the immersion liquid and a digital balance of sensitivity 10− 4 g. The density values obtained by repeated measurements showed an error of ±0.2%. The molar volume, VM, oxygen molar volume, VO and oxygen packing density, OPD, values were calculated to investigate the physical properties of glasses. Fourier transform infrared (FTIR) spectroscopy analyses were realized at room temperature, using the KBr pellet technique, in the wave number range from 400 to 1600 cm − 1 with a resolution of 1 cm − 1 using a Perkin Elmer Spectrum 100 FTIR spectrometer. For FTIR experiments 0.005 g of glass samples were weighed, mixed and ground with 0.300 g KBr. After which the mixture was pressed at 10 tons for 1 min, to yield transparent disks suitable for mounting in the spectrometer. The crystallization behavior of the glasses was investigated by running in situ X-ray powder diffraction (in situ XRD) measurements with a Philips X'pert MRD (Cu Kα radiation) fitted with a high temperature furnace (Anton-Parr DHS900). The heating rate was 10 °C/min and each pattern was recorded after an annealing time of 10 minutes

3. Theoretical basis The activation energy for the glass transition reaction can be calculated from the modified Kissinger equation [16]. Kissinger equation was originally introduced to determine the kinetics of chemical reactions and it is widely used to calculate the activation energy of crystallization [16,17]. The activation energy was calculated from the slopes of the linear fits to the experimental data from a plot of ln(Tg2/ B) versus 1/Tg, by using the following equation: ln

T 2g B

! ¼

Eg þ const RT g

ð1Þ

where Tg is the glass transition onset temperature for a given heating rate B, Eg is the glass transition activation energy, R is the gas constant. The fragility parameter, m, of the glass samples can be calculated (with an error estimate of ±1) from the following expression [18,19]: ! Eg m¼ ð2Þ RT g where m is the fragility parameter, Eg is the activation energy for glass transition, R is the gas constant and Tg is the glass transition onset temperature. The molar volume, VM, can be calculated as a function of the molar fraction of each of the three components and the oxygen molar volume, VO, can be calculated by using the following expression [20]: VO ¼

   xM 1 ∑ i i ρ ∑xi ni

ð3Þ

where xi is the molar fraction of each component i; Mi is the molecular weight; ρ is the glass density and ni is the number of oxygen atoms in each oxide. Oxygen packing density, OPD, can be calculated from the density and composition using the following formula: OPD ¼ 1000C ðρ=M Þ

ð4Þ

where C is the number of oxygen atoms per each composition, ρ is the calculated glass density, M is the molecular weight.

Table 1 Values of glass transition onset (Tg), crystallization onset and peak (Tc/Tp), glass stability (ΔT), activation energy of glass transition (Eg), fragility parameter (m), density (ρ), molar volume (VM), oxygen molar volume (VO), oxygen packing density (OPD) of 0.80TeO2–(0.20 − x)WO3–xB2O3 glasses. Sample Original ID Compositions (mol %)

Final Compositions (mol %)

Tg Tc1 / Tp1 (°C) (°C)

Tc2/ Tp2 Tc3 / Tp3 (°C) (°C)

349 345 344 343 340

–/488 –/497 –/459 –/460 –/488

ΔT Eg m (°C) (kJ mol− 1)

ρ VM ρ VO OPD theoretical (cm3 mol− 1) (cm3 mol− 1) (mol L− 1) at 25 °C −3 −3 (g cm ) (g cm )

TeO2 WO3 B2O3 TeO2 WO3 B2O3 TWB0 TWB5 TWB10 TWB15 TWB20

80 80 80 80 80

20 15 10 5 0

0 5 10 15 20

– : undetermined values.

77.7 79.2 79.8 80.6 81.8

22.3 16.1 10.7 5.8 0

0 4.7 9.5 13.6 18.2

447/492 435/459 434/453 420/439 415/431

98 90 –/480 90 475/486 77 75

598 552 507 512 469

115 107 98 99 92

5.85 5.66 5.40 5.14 4.92

5.97 5.74 5.51 5.28 5.05

29.75 29.32 29.23 29.13 28.78

13.52 13.8 14.26 14.75 15.15

73.95 75.04 75.28 75.53 76.44

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4. Results In the present study, as-cast samples in the 0.80TeO2–(0.20 − x) WO3–xB2O3 system (0 ≤ x ≤ 0.20 in molar ratio) were obtained transparent and homogeneous at macro size under the applied sample preparation conditions. XRD analyses were carried out with the as-cast samples in order to identify their amorphous nature and the results are shown in Fig. 1. DSC analyses were performed to determine the thermal behavior of the glasses. DSC thermograms are shown in the temperature range of 300–550 °C in Fig. 2 and the thermal analysis details are given in Table 1. A shallow endothermic change between 340 and 349 °C corresponding to the glass transition temperature (Tg) was observed for all glasses. According to the DSC results, different exothermic peaks, indicating different crystallization reactions were detected for all samples. The temperature difference between Tg and the first exothermic peak onset Tc1, ΔT, indicating the glass stability showed a decrease from 98 to 75 °C by the substitution of WO3 with B2O3 (see Fig. 3). The onset of the endothermic change is commonly used to define the Tg of the glass transition. The heating/cooling rate dependence of the glass transition temperature can be used to determine the activation energy of the transition from glassy to liquid state. Using the Kissinger equation (Eq. (1)), glass transition activation energy, Eg, of the 0.80TeO2–(0.20 − x)WO3–xB2O3 glasses were determined from the linear fits of ln(Tg2/ B) versus 1/Tg plots (see Fig. 4). As shown in Fig. 4, the glass transition activation energy decreased with increasing B2O3 content from 598 kJ/mol to 469 kJ/mol (see Table 1). The values of fragility parameter, m, which is often used to determine the strong-fragile characters of glass forming liquids, are given in Table 1. In the present study, the fragility parameters, m, showed a decrease from 115 to 92 with increasing B2O3 content. The measured density, ρ, molar volume, VM, oxygen molar volume, VO and oxygen packing density, OPD, of B2O3 and/or WO3 containing tellurite glasses are listed in Table 1. The density values of the glasses regularly decreased from 5.85 to 4.92 g/cm 3 with increasing B2O3 content. The VM values were calculated by taking the measured densities into account. The highest molar volume corresponds to the TWB0 glass sample. The oxygen packing density values of the glasses increased from 73.95 to 76.44 mol/L with increasing B2O3 content.

Fig. 1. X-ray diffraction patterns of the 0.80TeO2–(0.20 − x)WO3–xB2O3 glasses.

Fig. 2. DSC curves of the 0.80TeO2–(0.20 − x)WO3–xB2O3 glasses, scanned at a heating rate of 10 °C/min.

The oxygen molar volume also showed an increase from 13.52 to 15.15 cm 3/mol with the substitution of WO3 by B2O3. FTIR spectroscopy analyses were realized to investigate the structure of B2O3 and/or WO3 containing tellurite glasses. Fig. 5 shows the FTIR spectra in the spectral range of 400–1600 cm − 1. The vibrational properties are interpreted by taking into account the FTIR spectra assignments for different tellurite based glasses reported in the literature (see Table 2) [13,20–22]. In general, FTIR spectra of the glasses showed broad peaks and shoulders and the broadening of the peaks are attributed to the disorderness. In situ XRD patterns of the TWB10 sample obtained at different temperatures to investigate the crystallization behavior of boro– tungsten–tellurite glasses are shown in Fig. 6. As can be seen from Fig. 6, XRD pattern of the as-cast TWB10 sample revealed no detectable peaks confirming the vitreous structure. The XRD patterns obtained at 445, 460 and 480 °C revealed the presence of γ-TeO2, α-TeO2, WO3 and B2O3 crystalline phases. According to the DSC analyses, the XRD scan temperature was increased to 550 °C to obtain the thermal equilibrium of the system and the XRD analysis realized at this temperature showed that only α-TeO2 and WO3 crystalline phases exist in the system when the thermal equilibrium was achieved.

Fig. 3. Change in glass transition onset, Tg, and glass stability, ΔT, as a function of B2O3 content in the 0.80TeO2–(0.20 − x)WO3–xB2O3 glasses.

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A.E. Ersundu et al. / Journal of Non-Crystalline Solids 358 (2012) 641–647 Table 2 FTIR band assignments for 0.80TeO2–(0.20 − x)WO3–xB2O3 glasses. Wavenumber Assignment (cm− 1) 480–505 612–629 668–674 742–768 864–853 931–942 915–924 995–1018 1230–1243 1343–1350

Fig. 4. The Kissinger plots for determining glass transition activation energy, Eg, of the 0.80TeO2–(0.20 − x)WO3–xB2O3 glasses (with an estimate accuracy of 98%).

SEM/EDS analyses were conducted on the TWB10 sample heattreated at 445, 460, 480 and 550 °C for 24 hours, to identify the morphology of the crystallized phases in boro–tungsten–tellurite glasses.

Stretching vibrations of Te–O–Te or O–Te–O linkages Stretching vibrations of TeO4 tbp units Stretching vibrations of TeO3/TeO3+1 units Stretching vibrations of Te–O– Stretching vibrations of W–O–W in WO4 or WO6 units Stretching vibrations of W–O– and W O bonds inWO4 or WO6 units B–O stretching vibrations in BO4 units B–O stretching vibrations in BO4 units B–O stretching vibrations in BO3 units B–O– stretching vibrations in BO3 units

The secondary (SEI) and back-scatterred (BEI) electron micrographs of the TWB10 sample heat-treated at different temperatures are shown in Fig. 7.

5. Discussion XRD patterns of the as-cast samples revealed no detectable peaks, proving their vitreous structure (see Fig. 1). As seen in Figs. 2 and 3, the glass transition temperatures shifted to lower temperature values with increasing B2O3 content. It was observed that the first crystallization reaction onset temperatures shifted to lower values with the increase in B2O3 content. In his study on the effect of B2O3 on the structure and properties of tungstentellurite glasses, Saddeek [13] reported a similar change in the glass transition and first crystallization onset temperatures with increasing B2O3 content. As reported by El-Mallawany [1], a decrease in Tg reflects an increase in the looseness of packing in the structure. Therefore, the decrease observed in both glass transition and crystallization temperatures was probably due to the decrease in number of bonds per unit volume [23]. The ΔT values obtained in the present study were found to be close to the values reported in the literature by Saddeek [13], however lower than the reported ΔT values by Xudong et al. [15] since the glass compositions contain Er2O3 and Yb2O3 as doping agents.

Fig. 5. FTIR spectra of the 0.80TeO2–(0.20 − x)WO3–xB2O3 glasses.

Fig. 6. In situ XRD patterns of the TWB10 sample obtained at different temperatures.

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Fig. 7. SEM/SEI-BEI micrographs of the TWB10 sample heat treated 24 h at (a)–(b) 445 °C, (c)–(d) 460 °C, (e)–(f) 480 °C, (g)-(h) 550 °C.

Similar to the change observed in Tg values, the highest Eg value was calculated for the tungsten-tellurite glass (TWB0), whereas the lowest Eg value was determined for the boro-tellurite glass (TWB20). From the obtained data, it can be concluded that the glass transition activation energy showed a decrease with increasing B2O3 content. As it was reported in the literature, the decrease in Tg, and hence Eg, with the increase of B2O3 at the expense of WO3 is attributed to the decrease in the coordination number in the ratio 3/6 [14]. Furthermore, since the bond strength of B–O is weaker than that of W–O, the

increase in B2O3 content decreases the cross-link density, which reflects a decrease in the required glass transition activation energy [13,23,24]. There is no sharp limit to characterize strong–fragile characters of liquids on the basis of their fragility parameters. However, it was mentioned that a fragility value less than 90 can be attributed for strong liquids, whereas a value greater than 135 is typical for fragile liquids [18]. Therefore, B2O3 and/or WO3 containing tellurite glasses have a fragility character between strong and fragile and with increasing B2O3 content they tend to have a strong character. Fragile

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liquids are determined with a fast increase in their viscosity and a large change in their heat capacity as the glass transition temperature is approached; while the viscosity and heat capacity of strong liquids show a small change in the glass transition region [18,19,25]. The decrease in the density values is attributed to the lower molecular weight of B2O3 (69.6 g/mol) compared to WO3 (231.84 g/mol). The measured density values of the glasses are in agreement with the calculated theoretical density values. The VM values were found to decrease with the substitution of WO3 by B2O3 as the glass structure became less dense. The oxygen packing density and oxygen molar volume showed an increase with the substitution of WO3 by B2O3 due to the formation of more B–O linkages and fewer W–O–W linkages with increasing B2O3 content. As seen in Fig. 5, the FTIR spectrum of the tungsten–tellurite glass (TWB0) showed six peaks at 480, 612, 668, 742, 864 and 942 cm − 1. The peak at 480 cm − 1 is attributed to the stretching vibrations of Te–O–Te or O–Te–O linkages in between two TeO4 four-coordinate atoms. The envelope of the three characteristic absorption bands at 612, 668 and 742 cm − 1 corresponds to the stretching vibrations of Te–O bond in TeO4 and TeO3 units. The peak observed at 864 cm − 1 was found to belong to the W–O–W in WO4 or WO6 units and the peak at 942 cm− 1 was assigned to the stretching vibrations of W–O − and W O bonds associated with WO4 and WO6 units. With the introduction of B2O3 to tellurite glasses new absorption bands corresponding to the stretching vibrations of B–O units was observed in the FTIR spectra in the wavenumber range 915– 1350 cm − 1. As it was determined for the tungsten-tellurite glass (TWB0), the first four peaks of B2O3 containing tellurite glasses (TWB5, TWB10, TWB15, TWB20) were related to the stretching vibrations of Te–O units and they shifted to the higher frequencies with increasing B2O3 content. This behavior can be explained due to the transformation of TeO4 units into TeO3 units with the increase in B2O3 content. For the TWB5 sample, two bands corresponding to the W–O units were detected at lower wavelengths comparing to the TWB0 glass and two new absorption bands related to the stretching vibrations of B–O units were observed in the spectra. By increasing the B2O3 content to 10 mol%, another band representing B–O stretching vibrations in BO4 units was detected at 995 cm − 1 for TWB10 sample. For TWB15 glass, only one peak corresponding to the W–O units was observed at 853 cm − 1 since the FTIR peak related to the stretching vibrations of W–O – and W O bonds in WO4 or WO6 units disappeared and another FTIR band corresponding to the stretching vibrations of B–O units was detected at 924 cm − 1. For boro-tellurite glass (TWB20), apart from the four absorption bands corresponding to the Te–O units, four absorption bands representing the B–O units were detected. The FTIR bands at 915 and 1018 cm − 1 were assigned to the stretching vibrations in BO4 units, while the bands at 1230 and 1350 cm − 1 were found to be related to the B–O and B–O– stretching vibrations in BO3 units, respectively. As can be seen from the in-situ XRD patterns of TWB10 sample (Fig. 6), increasing the temperature from 445 to 480 °C resulted in a decrease in the peak intensities of γ-TeO2 and B2O3 phases, while the peak intensities of α-TeO2 and WO3 crystalline phases increased. The reason for detecting only α-TeO2 and WO3 crystalline phases at 550 °C, when the thermal equilibrium was achieved, can be explained due to the transformation of the metastable γ-TeO2 phase into stable α-TeO2 phase with increasing temperature, which was also observed in our previous studies on different binary and ternary tellurite systems [2,4]. The reason for not detecting the B2O3 phase in the XRD scan at 550 °C is thought to be due to its potential melting behavior in the structure. Bürger et al. reported the transformation of B2O3 into the liquid phase at around 709 K (436 °C) in the TeO2–B2O3 binary system. Therefore, in our study it is considered that B2O3 phase transforms into the liquid state above 480 °C. However, this melting behavior could not be clearly detected from the DSC measurements

due to several exothermic reactions occurring at around the same temperature with the melting reaction of B2O3. The secondary electron micrograph of the TWB10 sample heattreated at 445 °C showed the presence of a network-like structure constituting the general matrix and dendritic rod-like crystallites in various orientations on the general matrix (see Fig. 7a). It was detected that irregular dark crystallites were also found to be present on the surface. By taking the in situ XRD results into account and considering the BEI micrograph (see Fig. 7b) and EDS analysis, it is thought that the network-like structure forming the general matrix correspond to the formation of α-TeO2 and WO3 crystalline phases (82.12 at.% Te and 17.88 at.% W). The rod-like crystallites formed on the general matrix were thought to be related to the γ-TeO2 phase and they are rich in TeO2 content (95.97 at.% Te), while the irregular dark crystallites were found to be rich in B2O3 (77.75 at.% B) and correspond to the B2O3 phase present on the surface. Similar to the sample heat treated at 445 °C, the SEI and BEI micrographs taken from the TWB10 sample heat-treated at 460 °C (see Fig. 7c and d) revealed the presence of the network-like structure on the background forming the general matrix which corresponds to the α-TeO2 and WO3 crystalline phases and the dendritic rod-like crystallites on the general matrix were related to the existing γ-TeO2 phase in the structure. It was also observed that the dark crystallites corresponding to the B2O3 phase were still present on the surface. Fig. 7e and f are the representative SEI and BEI micrographs of the TWB10 sample heat-treated at 480 °C revealing the network-like structure on the general matrix related to the formation of α-TeO2 and WO3 crystallites and dendritic rod-like crystallites corresponding to the γ-TeO2 phase in the structure. It can be seen that the dark crystallites related to the B2O3 phase became smaller in size but still present in the structure. The SEI and BEI micrographs of the TWB10 sample heat-treated at 550 °C are shown in Fig. 7g and h, respectively. It was observed that with increasing temperature, the dendritic rod-like crystallites corresponding to the γ-TeO2 phase and dark crystallites related to the B2O3 phase are no longer present in the structure. It was detected that the grain-like crystallites constituted the general matrix with small white crystallites precipitated along the grain boundaries. The EDS spectra taken from the general matrix showed that the grain-like crystallites are rich in TeO2 content (89.77 at.% Te and 10.23 at.% W) and it is thought that they belong to the α-TeO2 phase. However, the EDS spectra taken from the grain boundaries showed that the WO3 content is almost three times higher along the grain boundaries (71.79 at.% Te and 28.21 at.% W) than the grains, which means that the small white crystallites are related to the WO3 phase. 6. Conclusion Characterization of B2O3 and/or WO3 containing tellurite glasses was realized in the 0.80TeO2 – (0.20-x)WO3 – xB2O3 system (0 ≤ x ≤ 0.20 in molar ratio) through DSC, FTIR, XRD and SEM/EDS techniques. Thermal analysis results revealed that the glass transition and first crystallization reaction onset temperatures shifted to lower temperature with increasing B2O3 content and the glass stability values showed a decrease from 98 to 75 °C. The glass transition activation energy calculated from the Kissinger equation and found to be decreased from 598 kJ/mol to 469 kJ/mol with increasing B2O3 content. The fragility parameters showed a decrease from 115 to 92 with increasing B2O3 content and it was determined that B2O3 and/or WO3 containing tellurite glasses have a fragility character between strong and fragile and they tend to have a strong character with increasing B2O3 content. The density and molar volume values decreased with the increase in B2O3 content. With the substitution of WO3 by B2O3, the oxygen packing density and oxygen molar volume showed an increase, as the glass structure became less dense. FTIR spectroscopy analyses realized to investigate the structure of B2O3 and/or WO3 containing

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tellurite glasses revealed that Te–O, W–O and B–O units are present in the structure. In situ XRD and SEM/EDS analyses realized to investigate the crystallization behavior of boro–tungsten–tellurite glasses showed that γ-TeO2, α-TeO2, WO3 and B2O3 crystalline phases were present at 445, 460 and 480 °C in the structure. When the thermal equilibrium was achieved at 550 °C, only α-TeO2 and WO3 crystalline phases were found to exist in the structure due to the transformation of the metastable γ-TeO2 phase into stable α-TeO2 phase and the transformation of B2O3 into the liquid phase. Acknowledgement The authors of this study gratefully acknowledge The Scientific & Technological Research Council of Turkey (TUBITAK) for the financial support under the project numbered 108M077. In situ X-ray powder diffraction measurements were carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois. References [1] R.A.H. El-Mallawany, Tellurite Glasses Handbook, CRC Press, Boca Raton/London/ New York/Washington, DC, 2002. [2] A.E. Ersundu, G. Karaduman, M. Çelikbilek, N. Solak, S. Aydin, J. Eur. Ceram. Soc. 30 (2010) 3087–3092. [3] A.E. Ersundu, G. Karaduman, M. Çelikbilek, N. Solak, S. Aydin, J. Alloys Compd. 508 (2010) 266–272.

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