International Journal of Applied Glass Science, 1–13 (2014) DOI:10.1111/ijag.12103
The TeO2-Na2O System: Thermal Behavior, Structural Properties, and Phase Equilibria Kutlu B. Kavaklıo g lu and S€ uheyla Aydin* Department of Metallurgical and Materials Engineering, Istanbul Technical University, Istanbul, 34469, Turkey
Miray C ß elikbilek and A. Ercßin Ersundu Department of Metallurgical and Materials Engineering, Yildiz Technical University, Istanbul, 34220, Turkey
Thermal behavior, structural properties, and phase equilibria of the (100x)TeO2-xNa2O system were studied in the 5 ≤ x ≤ 50 mol% composition range. Investigation of glass formation behavior in the binary system was realized, and the glass formation range was determined as 7.5 ≤ x ≤ 40 mol%. Differential thermal analysis (DTA) and Fourier transform infrared (FTIR) spectroscopy techniques were used for thermal and structural characterization of the glasses. Influence of Na2O content on glass transition temperature (Tg), glass stability (ΔT), density (q), molar volume (VM), oxygen molar volume (VO), and oxygen packing density (OPD) values of sodium tellurite glasses was evaluated considering the structural transformations in the glass network. For the phase equilibria studies, DTA, X-ray diffraction (XRD), and scanning electron microscopy/energy dispersive X-ray (SEM/EDS) techniques were utilized to characterize the heat-treated samples. According to the phase equilibria studies, three eutectic regions were detected in the 0 < x < 50 mol% composition range of the (100x)TeO2-xNa2O system. A new invariant endothermic reaction was detected for the compositions between 40 ≤ x ≤ 45 mol%. Na2O.8TeO2 (11.11 mol% Na2O) compound that was claimed to exist in the binary system in the literature was found to be the metastable d-TeO2 phase.
Introduction Tellurite glasses have received great attention of researchers in the last two decades.1 Studies realized so far have shown that tellurite glasses exhibit highly promising properties by meeting the requirements in optical applications and they have a great potential to be used in *
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amplifiers, switching devices, and laser technology. Compared with SiO2, P2O5, B2O3, and GeO2-based glass systems, relatively low-phonon energy, high refractive index, high dielectric constant, and good infrared transmissivity are some of the advantageous properties that make tellurite glasses ideal matrices for amplifier mediums and laser hosts. Moreover, their relatively low glass transition and melting temperature, thermal and chemical stability, and high devitrification resistance provide ease of manufacturing and durability.1–5
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It was mentioned in the literature that pure TeO2 glass can be obtained with rapid cooling but a closer inspection revealed that these pure bulk glasses indeed contain small amounts of impurities.6 Therefore, preparation of tellurite glasses requires addition of a secondary component as TeO2 acts as a conditional glass former under normal quenching conditions such as casting in metal molds/plates or dipping in water bath. Heavy metal oxides, halogens, or alkali oxides can be used as secondary components to make TeO2 form a network structure or in other words vitrify.1–5 It is known that properties of a glass are directly related to the quantity and the nature of its constituents. Therefore, it is important to determine the influence of different components to obtain a tellurite-based glass with desired thermal, physical, and structural features. In this study, Na2O was chosen as the secondary component to obtain tellurite glass. Na2O is known to enhance the solubility of rare-earth ions by modifying the network structure and also decrease the glass transition temperature and viscosity of tellurite glasses.3,5,7,8 To the best of our knowledge, first observations on the glass formation behavior of the (100x)TeO2xNa2O binary system were reported by Mochida et al.9 in the literature. In their study, the glass formation range was determined as 10 ≤ x ≤ 46.5 mol%.9 Heo et al.10 were the first to conduct thermal and FTIR analyses on sodium tellurite glasses with five different compositions. Sekiya et al.11 reported a detailed Raman investigation on different binary tellurite glasses along with sodium tellurite glasses. Other studies on glasses of this binary system were mostly concentrated on the investigation of the optical features. Apart from the investigations on glasses of the TeO2-Na2O system, several studies were also conducted on the crystallization behavior and phase equilibria of this system. The partial phase diagram of the (100x)TeO2-xNa2O binary system in the 0 ≤ x ≤ 50 mol% Na2O compositional range was reported for the first time in the literature by Troitskii et al.12 The pseudo-phase diagram consisted of three eutectic reactions and three compounds formulated as Na2Te4O9 (x = 20 mol% Na2O), Na2Te2O5 (x = 33 mol% Na2O), and Na2TeO3 (x = 50 mol% Na2O). However, Zhu et al.13 reported the existence of a new compound named as Na2O.8TeO2 (11.11 mol% Na2O) and modified the existing phase diagram. According to their study, Na2O.8TeO2 was stable up to 330°C and with increasing temperature irreversibly transformed into
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a-TeO2 and Na2Te4O9 phases.13 Later, NMR studies conducted by Holland et al.14 indicated that Na2O.8TeO2 phase was structurally similar to metastable d-TeO2 phase. Also, Santic et al.15 investigated the crystallization of sodium tellurite glasses (0 ≤ x ≤ 33.3 mol% Na2O) above the first crystallization temperatures. Consequently, some discrepancies still exist on the phase equilibria of the TeO2-Na2O binary system. Although numerous studies have been reported on sodium tellurite glasses in the literature, a systematical study covering the entire glass formation range is required to gain a better understanding of the effect of Na2O content on tellurite glasses. Therefore, in this study, it was aimed to investigate the compositional dependence of thermal and structural properties of sodium tellurite glasses. Authors of this study also aimed to realize a systematical research on the phase equilibria of the TeO2-Na2O binary system to remove the discrepancies exist in the literature by conducting detailed phase and microstructural analyses.
Experimental Procedure Experimental studies were conducted in the (100x)TeO2-xNa2O (5 ≤ x ≤ 50 mol%) binary system using 13 different compositions. Samples were prepared using conventional melt-quenching method where high purity powders of TeO2 (99.99% purity; Alfa Aesar, Karlsruhe, Germany) and Na2CO3 (99.9% purity; Alfa Aesar) were thoroughly mixed to prepare 5-g-size powder batches. Each batch was put into a platinum crucible and placed into an electric furnace for melting. It is reported in the literature that evaporation of TeO2 is possible at temperatures above 900°C.16 Therefore, in this work, melting temperature of the samples was chosen as 750°C in accordance with the present phase diagram of the binary system.12 Also, platinum crucible was closed with a platinum lid to prevent any possible evaporation during melting. Batches were kept at 750°C for 30 min and then quenched in water bath to obtain as-cast samples. X-ray diffraction (XRD) analyses were conducted after sample preparation to distinguish glass samples from crystalline samples. A BrukerTM (BRUKER AXS, Inc., Madison, WI) D8 Advanced Series powder diffractometer using Cu Ka radiation was utilized in the 2h range from 10° to 60° with a step size of 0.02°.
The TeO2-Na2O System
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Differential thermal analysis (DTA) technique was used for thermal characterization of the as-cast samples. DTA analyses were carried out with a Perkin ElmerTM, Shelton, CT. Diamond Thermogravimetric/Differential Thermal Analyzer (with an error estimate of 1°C), and approximately 25 mg of powdered samples was used with a heating rate of 10°C/min under flowing (100 mL/min) argon gas for each scan. The glass transition onset temperatures (Tg) were determined as the inflection point of the endothermic change of the calorimetric signal. 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. To estimate the devitrification resistance or stability of glasses against crystallization, ΔT = Tc1Tg values were calculated. Density (q) of glasses was measured, and molar volume (VM), oxygen molar volume (VO), and oxygen packing density (OPD) values were calculated to investigate the changes in the physical properties of glasses with changing composition. Densities, q, of the glasses were determined at room temperature by the Archimedes’ method using ethanol as an immersion liquid and a digital balance of sensitivity 104 g. Density values obtained by three repeated measurements showed an error of 0.1%. Density values of glasses, qglass, were calculated using the following relation: qglass ¼
Wair Wair Wliquid
ð1Þ
where Wair and Wliquid are the weight of the glass sample in air and in ethanol, respectively.17 Theoretical values of the density were estimated using the following relation:18 X qtheo ¼ qtheoi xi ð2Þ where qtheoi and xi are the density and fraction of the constituent oxides, respectively. Molar volume values were calculated as a function of the molar fraction of each component and the measured density values.3,5 Oxygen molar volume values were calculated using the following formula:3,5 X ðxi Mi Þ 1 P VO ¼ ð3Þ q ðxi ni Þ where xi is the molar fraction of each component i; Mi is the molecular weight and ni is the number of oxygen atoms in each constituent oxide. Oxygen packing density
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values were calculated using the density and composition values by applying the following formula:3,5 OPD ¼ 1000C ð
q Þ M
ð4Þ
where C is the number of oxygen atoms per each composition and M is the molecular weight. Fourier transform infrared (FTIR) spectroscopy analyses were conducted to monitor the changes in the glass structure with changing composition. A Perkin Elmer Frontier FTIR spectrometer was used in the wavenumber range from 400 to 1000 cm1 with a resolution of 1 cm1 at room temperature. To prepare samples for FTIR scans, the CsI pellet technique was used and each pellet was a ground mixture of powdered glass sample and CsI powder with the weight ratio of 1:100, respectively. Mixtures were pressed at 15 tons for 1 min in a stainless steel mold to obtain pellets as transparent disks in required dimensions. To investigate phase equilibria of the binary system, heat treatments were applied to the as-cast samples to achieve thermal equilibrium for each composition. Each sample was heat-treated for 48 h above the peak temperature of the last crystallization reaction recorded by DTA analyses. Thermal analyses were repeated on each heattreated sample to examine the thermal behavior of the samples in thermal equilibrium. XRD analyses were also repeated with the heat-treated samples, and the patterns obtained were compared with the International Centre for Diffraction Data (ICDD) files to identify the crystalline phases in thermal equilibrium. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDS) spectroscopy techniques were used before and after heat treatment to identify the final chemical composition of glasses and microstructural characterization of the existing phases in thermal equilibrium, respectively. A JEOLTM Model JSM 7000F SEM (Tokyo, Japan) operated at 15 kV and linked with an Oxford Inca EDS attachment was used with platinumcoated samples for microstructural investigations. The EDS measurements on as-cast and heat-treated samples were taken at four different points for each composition. Results Thermal and Structural Characterization of the Glasses Under the applied sample preparation conditions, as-cast samples in the (100x)TeO2-xNa2O system with
2014 The relative error in the density values determined from three measurements for each glass and in the calculated VM, VO, and OPD values does not exceed 0.1%. The error in Tg and Tc1 values is estimated 1°C.
66.12 65.19 63.01 60.92 58.82 56.82 55.60 54.69 52.99 15.42 15.75 16.54 17.38 18.31 19.30 19.98 20.45 21.57 29.11 29.14 29.36 29.55 29.75 29.92 29.97 30.17 30.19 5.42 5.33 5.16 4.99 4.82 4.65 4.54 4.48 4.31 5.23 5.14 4.94 4.74 4.54 4.36 4.24 4.16 3.99 38 41 49 12 72 43 52 53 33 321 318 312 352 304 269 263 268 254 283 277 263 250 232 222 211 215 221 6.9 9.2 15.3 19.0 24.1 29.4 32.3 34.0 38.8 93.1 90.8 85.7 81.0 75.9 70.6 67.7 66.0 61.2 7.5 10 15 20 25 30 33 35 40 92.5 90 85 80 75 70 67 65 60 7.5 10 15 20 25 30 33 35 40 = = = = = = = = = x x x x x x x x x
Final Composition (mol%) Original Composition (mol%)
the compositions 7.5 ≤ x ≤ 40 mol% were visually transparent and the samples with the compositions x = 5 mol% and x = 42.5 mol% were partially transparent, whereas the samples with the compositions x = 45 and 50 mol% were obtained opaque. Opaque samples were yellowish white, while colors of the transparent samples changed from dark yellow to pale yellow with increasing Na2O content. XRD analysis results of the transparent as-cast samples revealed no detectable peaks showing their amorphous structure. The chemical compositions of these amorphous samples calculated from EDS spectra are in very good agreement with their original batch compositions. The final compositions of glasses collected from EDS analysis in weight% with an error of 0.1% were converted to mol% and compared with their original compositions in Table I. Thermal behavior of each glass sample was determined by DTA analysis, and the obtained thermograms and the thermal analysis data are given in Fig. 1 and Table I, respectively. A broad and shallow endothermic change was detected for each glass sample corresponding to the glass transition reaction. As can be seen in Fig. 2, glass transition temperature, Tg, values were observed to change between 211°C and 283°C with changing Na2O composition. DTA results of the glass samples showed exothermic peaks representing the crystallization and/or phase transformation reactions of different phases. First crystallization onset temperature, Tc1, values were determined from the thermograms and found to vary between 254°C and 352°C. In accordance with the Tg and Tc1 values, glass stability, ΔT, values are calculated and listed in Table I. Changes in physical properties of the glasses according to the changing Na2O were investigated by measuring the density and calculating VM, VO, and OPD values, and the obtained values are plotted in Figs. 3 and 4 and listed in Table I. Measured density values of the glasses decreased from 5.23 to 3.99 g/cm3 with increasing Na2O content. VM of the glasses were found to increase from 29.11 to 30.19 cm3/mole with increasing Na2O content. VO values increased from 15.42 to 21.57 cm3/mole, whereas OPD values decreased from 66.12 to 52.99 mole/l as the amount of Na2O increases the glass composition. Effect of the changing composition on the glass structure was investigated using FTIR spectroscopy analysis. Samples with the compositions x = 7.5, 15, 25, 35, and 40 mol% were chosen to represent the changes in glass structure according to the increasing Na2O content. FTIR spectra of the raw materials and reported data in the
(100x) TeO2-xNa2O qtheo VM VO (x in mol%) TeO2 Na2O TeO2 Na2O Tg (°C) Tc1 (°C) DT (°C) q (g/cm3) (g/cm3) (cm3/mole) (cm3/mole) OPD (mole/L)
International Journal of Applied Glass Science—Kavaklıo g lu, et al.
Table I. Values of Glass Transition (Tg) and First Crystallization Onset (Tc1) Temperature, Glass Stability (ΔT), Density (q), Molar Volume (VM), Oxygen Molar Volume (VO), and Oxygen Packing Density (OPD) of TeO2-Na2O Glasses
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The TeO2-Na2O System
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Fig. 3. Variation of density, q, and molar volume, VM, values with increasing Na2O content.
Fig. 1. DTA thermograms of the as-cast samples.
Fig. 4. Variation of oxygen molar volume, VO, and oxygen packing density, OPD, values with increasing Na2O content.
Fig. 2. Change in glass transition temperature, Tg, values with increasing Na2O content.
literature were taken into account for the interpretation of the obtained spectra given in Fig. 5.3,6,19 It was observed that each spectrum showed broad peaks and shoulders revealing the disordered nature of the glasses. It was observed from the spectra that sodium tellurite glasses presented four main peaks with the peak positions determined as 610 cm1, 662 cm1, 754 cm1, and 795 cm1. Thermal, Phase, and Microstructural Characterization of the Heat-Treated Samples Phase equilibria of the (100x)TeO2-xNa2O (5 ≤ x ≤ 50 mol%) binary system was investigated with
heat-treated samples. Based on the as-cast DTA results, heat treatment temperatures were selected as follows: 425°C for 0 < x ≤ 20 mol%, 365°C for 20 < x < 33 mol%, 400°C for 33 ≤ x ≤ 45 mol%, and 500°C for the x = 50 mol% samples (as shown in Table II). DTA analysis results of the samples heat-treated above the peak temperature of the last crystallization reactions are given in Fig. 6. These samples exhibited no exothermic peaks, whereas they showed at least one endothermic peak. The determined onset and peak temperature values (Teo/Tep) of the endothermic reactions are listed in Table II. XRD analysis results of these samples are given in Fig. 7, and the identified equilibrium phases for each sample are summarized in Table II. It was determined that the heat-treated samples with the compositions between 0 < x < 20 mol% showed the crystallization of tetragonal a-TeO2 (0760679) and triclinic Na2Te4O9 (032-1166) phases while Na2Te4O9 was the only phase identified for the
a-TeO2, Na2Te4O9 a-TeO2, Na2Te4O9 a-TeO2, Na2Te4O9 a-TeO2, Na2Te4O9 Na2Te4O9 Na2Te4O9, Na2Te2O5 Na2Te4O9, Na2Te2O5 Na2Te2O5 Na2Te2O5, Na2TeO3 Na2Te2O5, Na2TeO3 Na2Te2O5, Na2TeO3 Na2TeO3, Na2TeO4 –/593 –/638 529/539 524/531 –/730
–/471
–/692 –/670 –/625
475/480 477/483 478/484 476/484 487/491 420/424 422/429 432/434 –/432 –/430 –/430 668/677 48 48 48 48 48 48 48 48 48 48 48 48 The error in Teo and Tep values is estimated 1°C.
425 425 425 425 425 365 365 400 400 400 400 500 5 7.5 10 15 20 25 30 33 35 40 45 50
Temp. (°C)
Time (h)
Teo1/Tep1 (°C)
Teo2/Tep2 (°C)
Teo3/Tep3 (°C)
= = = = = = = = = = = =
In the literature, Na2O.8TeO2 phase was reported to crystallize in the 0 < x < 20 mol% interval.13 In this study, existence of this phase was investigated by applying additional heat treatments to the x = 5, 7.5, 10, and 15 mol% samples above the peak temperature values of the first exothermic reactions. For this
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x x x x x x x x x x x x
Characterization of the d-TeO2 Phase
(100x)TeO2-xNa2O (x in molar ratio)
x = 20 mol% sample (Fig. 7a). When Na2O content was between 20 < x < 33 mol% (Fig. 7b), Na2Te4O9 phase was found to coexist with the monoclinic Na2Te2O5 (051-1829) and its hydrated compound, Na2Te2O5.2H2O (074-0859). The x = 33 mol% sample showed the formation of Na2Te2O5 and Na2Te2O5.2H2O phases. Structure of the samples between 33 < x < 50 mol% consisted of Na2Te2O5, Na2Te2O5.2H2O, monoclinic Na2TeO3 (083-1778), and Na2TeO3.5H2O (070-2181) phases, as can be seen in Fig. 7c. For the x = 50 mol% sample, observed diffraction peaks matched with the Na2TeO3 and Na2TeO4 (044-0854) phases. Secondary electron images that can be seen in Fig. 8 revealed the morphologies of the equilibrium phases and the changes in the final microstructure in accordance with the changing composition.
Heat treatment
Fig. 5. Fourier transform infrared (FTIR) spectra of the TeO2Na2O glasses.
Equilibrium phases
International Journal of Applied Glass Science—Kavaklıo g lu, et al.
Table II. Heat Treatment Parameters (temperature and time) and Onset (Teo) and Peak (Tep) Temperature Values of the Detected Endothermic Peaks and Identified Equilibrium Phases for the Heat-Treated Samples
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The TeO2-Na2O System
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(a)
(a)
(b)
(b)
(c) (c)
Fig. 6. Differential thermal analysis (DTA) thermograms of the heat-treated samples (a) x = 5, 7.5, 10, 15, and 20 at 425°C, (b) x = 25 and 30 at 365°C, (c) x = 33, 35, 40, and 45 at 400°C and x = 50 at 500°C.
Fig. 7. XRD patterns of the heat-treated samples (a) x = 5, 7.5, 10, 15, and 20 at 425°C for 48 h, (b) x = 25 and 30 at 365°C for 48 h, (c) x = 33, 35, 40, and 45 at 400°C for 48 h and x = 50 at 500°C for 48 h.
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(a)
(b)
(c)
(d)
Fig. 8. SEM/SEI micrographs the heat-treated samples (a) x = 15 at 425°C for 48 h, (b) x = 25 at 365°C for 48 h, (c) x = 30 at 365°C for 48 h, (d) x = 35 at 400°C for 48 h.
Table III. Heat Treatment Parameters (temperature and time) and the Identified Phases for the Samples Heat-treated Above the First Crystallization Peak Temperatures Heat treatment (100x)TeO2-xNa2O (x in molar ratio) x x x x
= = = =
5 7.5 10 15
Temp. (°C)
Time (h)
Phases
336 336 336 325
5 5 5 5
c-TeO2, a-TeO2, Na2Te4O9 d-TeO2, a-TeO2 d-TeO2 d-TeO2, c-TeO2, Na2Te4O9
purpose, heat treatment temperatures were selected as 336°C for the x = 5, 7.5, and 10 mol% samples and 325°C for the x = 15 mol% sample, and the heat treatment details are given in Table III. As the as-cast x = 5 mol% sample was partially glass after sample preparation, a glassy part of the sample was used for this investigation. XRD analysis results of the samples heat-treated above the first crystallization peak temperatures are given in Fig. 10, and the identified phases are listed in Table III. Crystallization of Na2Te4O9, a-TeO2, and orthorhombic c-TeO2 (052-0795) phases was observed for the x = 5 mol% sample. The x = 7.5 mol% sample
showed the peaks of a-TeO2 and d-TeO2 crystalline phases, while d-TeO2 (052-0796) was the only phase crystallized for the x = 10 mol% sample. The pattern obtained from the x = 15 mol% sample matched with c-TeO2, d-TeO2, and Na2Te4O9 phases. Discussion Thermal and Structural Behavior of the Glasses Amorphous structure of the transparent samples were confirmed by XRD analysis, and the glass formation range of the (100x)TeO2-xNa2O binary system
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The TeO2-Na2O System
was determined as 7.5 ≤ x ≤ 40 mol% under the applied sample preparation conditions. Compared with reported data in the literature, the glass formation range shifted and expanded toward the TeO2-rich region.1,14 This may have resulted given the influence of sample preparation conditions on glass formation behavior.3 As can be seen in Fig. 1 and Table I, the glass transition temperatures, Tg, decreased from 283°C to 211°C with increasing Na2O content up to x = 33 mol%. The decrease in Tg values of sodium tellurite glasses with increasing Na2O content between 7.5 ≤ x ≤ 35 mol% was also reported in the literature.9,14,15,20–22 The decrease observed in glass transition temperatures with increasing Na2O was probably due to the decrease in number of bonds per unit volume.3,5 However, as can also be seen from Fig. 2, a slight increase in Tg to 221°C was observed in this study, when Na2O was increased from x = 33–40 mol%. This anomalous variation of Tg values with changing composition was not reported before for sodium tellurite glasses. However, similar behavior was reported for other glass systems in the literature and claimed to be related to the decrease in the glass-forming ability of the system with decreasing glass former content.23 In addition, Kalampounias et al.19 stated in their study on TeO2-M2O (M2O: alkali oxides) binary glasses that excessive addition of a network modifier decreases vitrification behavior due to the decreasing stability of TeO3 (tp) units. Therefore, the increase in Tg values observed in this study in 33 < x ≤ 40 mol% Na2O interval is thought to be related to the decreasing glass-forming ability. First crystallization onset temperature, Tc1, values were found to change irregularly with increasing sodium oxide content. This behavior was a result of crystallization of different phases from the amorphous structure for different compositions. In accordance with the Tg and Tc1 values, glass stability, ΔT, values were also observed to show an irregular trend with increasing Na2O content and the maximum ΔT value was found as 102°C for the x = 20 mol% sample. ΔT values found in this study were found to be lower than the values reported by Heo et al.10 and slightly higher than the values reported by Holland et al.14; however, the x = 20 mol% glass composition was reported to have the highest glass stability value in all studies. As can be seen in Table I, the measured density values were lower but consistent with the theoretical
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density values (qtheo) which were determined using the theoretical densities of the constituent oxides according to their appropriate crystalline compositions. This difference in density values may be due to the variation in atomic arrangement between the structure of glass and molecules of the constituent oxides. The decrease in the density values with increasing Na2O content was attributed to the lower molecular weight of Na2O comparing to TeO2. Increasing Na2O content increased the VM values (see Fig. 3) and led the formation of excess free volume by opening up the glass structure. As shown in Fig. 4, oxygen molar volume and oxygen packing density values showed opposite behavior to each other. Substitution of higher field intensity Te4+ ions (0.71) with Na+ ions (0.19) led to an increase in oxygen molar volume and a decrease in oxygen packing density values which also resulted less tightly packing of the glass network in TeO2-Na2O glasses. The decrease in oxygen packing density values with increasing Na2O content can also be explained with the decrease in number of oxygen atoms per unit composition in the ratio 1:2, which results in fewer linkages in the glass network. Fourier transform infrared (FTIR) analyses conducted with the sodium tellurite glasses revealed the transformation of the glass structure with changing composition. It was observed from the spectra given in Fig. 5 that the intensities of the detected peaks were dependent on the glass composition. The peaks observed at 610 cm1 and 662 cm1 were assigned to the stretching vibrations of Te–O bonds in TeO4 trigonal bipyramid (tbp) units. It was observed that the intensity of these peaks decreased, while intensities of the peaks at 754 cm1 and 795 cm1 representing the stretching vibrations of TeO3 trigonal pyramid (tp) units increased. According to the FTIR analysis, it was found that with increasing Na2O content, the intensities of peaks corresponding to the stretching vibrations of TeO4 tbp units decreased, while peaks related to the TeO3/TeO3+1 units increased due to the formation of nonbridging oxygen sites in the glass network. By taking all into consideration, it can be concluded that structural investigation of TeO2-Na2O glasses according to the increasing Na2O content revealed the structural transformation of glass network due to the formation of nonbridging oxygens in the form of TeO3 and TeO3+1 units. Formation of these nonbridging oxygens resulted in a less dense glass structure with fewer linkages in the glass network in
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accordance with the observed changes in the physical parameters. Phase Equilibria of the TeO2-Na2O Binary System According to the DTA results of the heat-treated samples shown in Fig. 6, as no exothermic peaks representing crystallization and/or phase transformation reactions were detected from the thermograms, it was determined that thermal equilibrium was achieved for each sample. These results indicate that the XRD patterns (Fig. 7) of the heat-treated samples showed the equilibrium phases of the (100x)TeO2-xNa2O binary system. However, during experimental studies, the samples containing x > 20 mol% were observed to be hygroscopic as the hydrated compounds such as Na2Te2O5.2H2O and Na2TeO3.5H2O were detected together with the equilibrium phases. Based on Fig. 7a, a-TeO2 and Na2Te4O9 phases were observed to be the equilibrium phases for 0 < x < 20 mol% composition range. It was also determined that the first endothermic peaks detected for the samples in this composition interval, as can be seen in Fig. 7a, had similar onset and peak temperature values, indicating that these peaks represented the first eutectic reaction: “a-TeO2 + Na2Te4O9 ? Liquid” of the binary system. According to the thermal data listed in Table II, the onset temperature value of the first eutectic reaction was determined as 477 2°C. For x = 5, 7.5, and 10 mol% samples, second endothermic peaks representing the liquidus reaction were detected and liquidus peak temperatures were observed to decrease with increasing Na2O content. When thermal equilibrium conditions were achieved for the x = 20 mol% sample, Na2Te4O9 was the only equilibrium phase in the final structure. Therefore, the endothermic peak observed for this sample (Fig. 6a) represented the congruent melting reaction of the Na2Te4O9 phase with an onset temperature of 487°C. The final structure of the samples between 20 < x < 33 mol% interval consisted of the Na2Te4O9 and Na2Te2O5 phases. According to the obtained thermal data (Fig. 6b and Table II), the first endothermic peaks of the samples in this interval had similar onset and peak temperature values. Therefore, these peaks were attributed to the second eutectic reaction: “Na2Te4O9 + Na2Te2O5 ? Liquid” of the binary system with the onset temperature value of 421 1°C.
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As can be seen in Fig. 6b, the second endothermic peak detected for the x = 25 mol% sample was thought to represent the liquidus reaction for this composition. It was observed that Na2Te2O5 was the only equilibrium phase for the sample containing x = 33 mol% Na2O. This indicated that the endothermic peak detected (Fig. 6c) for this sample represented the congruent melting reaction of the Na2Te2O5 phase. The onset temperature value of this reaction was found to be 432°C. According to the Fig. 7c, Na2Te2O5 and Na2TeO3 phases were stable when thermal equilibrium was achieved for the compositions between 33 < x < 50 mol%. The first endothermic peaks detected for the samples in this composition range (Fig. 6c) corresponded to the third eutectic reaction: “Na2Te2O5 + Na2TeO3 ? Liquid” of the binary system. These results were in agreement with the third eutectic reaction region of the present phase diagram reported by Troitskii et al.12 given in Fig. 9. The onset value of this eutectic reaction could not be determined; however, its peak temperature value was found to be 431 1°C. Also, the second endothermic peaks observed for the x = 40 and 45 mol% samples had similar onset and temperature values detected as 527 2°C and 535 5°C, respectively. However, an endothermic reaction at similar temperature values for these compositions was not reported before in the literature. Therefore, existence of this reaction was investigated in detail by analyzing different compositions between 40 < x < 45 mol% along with the repeated thermal analyses. Obtained results confirmed that there
Fig. 9. Pseudo-phase diagram of the (100x)TeO2-xNa2O system for 0 ≤ x ≤ 50 mol % Na2O.12
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The TeO2-Na2O System
exists an invariant endothermic reaction above the third eutectic reaction for this composition range. This endothermic peak was thought to represent either a phase transformation of the Na2TeO3 phase or a peritectic reaction, and it is still being investigated in detail by the authors of this paper. The x = 40 and 45 mol% samples also showed a third endothermic reaction representing the liquidus reaction which shifted to higher temperature values with increasing Na2O content. Unlike the other heat-treated samples, thermal equilibrium could not be achieved for the x = 50 mol % sample as the formation of the Na2TeO4 phase was detected from the XRD pattern shown in Fig. 7c. Troistskii et al.12 stated that Na2TeO3 oxidized to Na2TeO4 with an endothermic reaction at around 630°C. As argon atmosphere was used for thermal analysis in this study, a peak related to the oxidation of the Na2TeO3 to Na2TeO4 was not detected for the x = 50 mol% sample in thermal analysis. It was thought that the Na2TeO4 phase formed either during sample preparation as it was observed by Holland et al.14 or as a result of the heat treatment conditions applied to the sample in this study. The results of the phase equilibria studies were found to be consistent with the phase diagram reported in the literature by Troitskii et al.12 and the determined temperature values for the eutectic reactions were found to be higher than the values stated in the existing phase diagram (see Fig. 9). The melting temperature for the Na2Te4O9 determined in this study was higher than the temperature value stated by Troitskii et al.12 while the melting temperature for the Na2Te2O5 phase was found to be close to the value stated by Troitskii et al.12 On the other hand, the detected liquidus temperature values were found to be in good agreement with the reported phase diagram. Figure 8a is the representative SEM micrograph of the x = 15 mol% sample revealing the paratellurite phase (99.0 0.5 at.% Te) in flower-like morphology and the Na2Te4O9 phase as small white grains (37.6 0.5 at.% Na and 62.4 0.5 at.% Te). As seen in Fig. 8b,c, microstructural investigation of the x = 25 and 30 mol% samples showed the formation of Na2Te2O5 phase with a grain-like morphology. For the x = 25 mol% sample, it was observed that the angular plate-like Na2Te4O9 grains formed the general structure, while for x = 30 mol% sample, Na2Te4O9 grains degraded and crystallized between the Na2Te2O5 grains
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(48.9 0.5 at.% Na and 51.1 0.5 at.% Te). For x = 35 mol% sample (Fig. 8d), general structure was represented by the Na2Te2O5 grains and also small white grains (63.1 0.5 at.% Na and 36.9 0.5 at.% Te) related to the crystallization of Na2TeO3 phase was determined at the Na2Te2O5 grain boundaries. Crystallization of the d-TeO2 Phase To investigate the existence of the Na2O.8TeO2 phase, x = 5, 7.5, 10, and 15 mol% samples were heat-treated above the peak temperatures of the first exothermic reactions. On the basis of the XRD results given in Fig. 10 and Table III, it was confirmed that no such compound as Na2O.8TeO2 exists in the binary system. According to the results of this study, the crystalline phase observed by Zhu et al.13 as Na2O.8TeO2 phase is the metastable d-TeO2 phase which was also determined through NMR studies conducted by Holland et al.14 d-TeO2 has a cubic crystal structure with the calculated lattice parameters a = 0.5687 nm (space group: F), and being a metastable polymorph of TeO2, it is known that the d-TeO2 phase is obtained in the doped TeO2 glasses, depending on the amount of the added component.2,16,24 In this study, compositional range for the crystallization of d-TeO2 was determined as 7.5 ≤ x ≤ 15 mol% in the (100x) TeO2-xNa2O binary system for the first time in the literature.
Fig. 10. XRD patterns of the x = 5, 7.5, and 10 samples heat-treated at 336°C for 5 h and the x = 15 sample heat-treated at 325°C for 5 h.
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International Journal of Applied Glass Science—Kavaklıo g lu, et al.
2014
Conclusions
References
Investigation of thermal behavior, structural properties, and phase equilibria of the (100x)TeO2xNa2O binary glass system was realized using DTA, FTIR, XRD, and SEM/EDS analyses. The glass formation range of the binary system was determined as 7.5 ≤ x ≤ 40 mol%. The network modifying effect of Na2O was observed as the decrease in the glass transition temperature and the formation of a less dense glass structure with the increasing amount of Na2O. As the amount of Na2O increased in the glass composition, transformation of TeO4 tbp units into TeO3 tp units was observed. It was determined that as a result of this transformation, the linkages connecting the structural units in the network degraded and nonbridging oxygen sites formed resulting in a less tightly packed structure. DTA and XRD analyses conducted on the heattreated samples showed that three eutectic reactions exist in the 0 < x < 50 mol% range of the (100x) TeO2-xNa2O binary system. The first eutectic reaction: a-TeO2 + Na2Te4O9 ? Liquid was detected between 0 < x < 20 mol% with an onset temperature value of 477 2°C. It was proven that the aforesaid Na2O.8TeO2 phase in the literature does not exist, and for the first time in the literature, it was found that the metastable d-TeO2 phase crystallizes between 7.5 ≤ x ≤ 15 mol% range in the binary system. The second eutectic reaction: Na2Te4O9 + Na2Te2O5 ? Liquid was observed between 20 < x < 33 mol% with the onset temperature value of 421 1°C. The third eutectic reaction: Na2Te2O5 + Na2TeO3 ? Liquid was detected between 33 < x < 50 mol% and had the peak temperature value of 431 1°C. A new endothermic reaction with the onset temperature value of 527 2°C was detected in the 40 ≤ x ≤ 45 mol% composition interval representing a phase transformation reaction for Na2TeO3 or a peritectic reaction above the third eutectic reaction.
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Acknowledgment The authors of this study gratefully acknowledge The Scientific & Technological Research Council of Turkey (TUBITAK) for the financial support under the project 111M236.
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The TeO2-Na2O System
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