Crystallization kinetics of the tungsten–tellurite glasses

Crystallization kinetics of the tungsten–tellurite glasses

Journal of Non-Crystalline Solids 357 (2011) 88–95 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids j o u r n a l h o m e...

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Journal of Non-Crystalline Solids 357 (2011) 88–95

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Crystallization kinetics of the tungsten–tellurite glasses M. Çelikbilek ⁎, A.E. Ersundu, N. Solak, 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 May 2010 Received in revised form 2 September 2010 Keywords: Crystallization kinetics; Tungsten–tellurite glasses; Non-isothermal analysis

a b s t r a c t The crystallization kinetics of the (1 − x)TeO2–xWO3 (where x = 0.10, 0.15, and 0.20, in molar ratio) glass system was studied by non-isothermal methods using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. DSC measurements were performed at different heating rates to study crystallization kinetics of the first crystallization reactions of the glasses. XRD analysis of tungsten–tellurite glasses heat-treated above the first crystallization temperatures revealed that the first crystallization peaks attributed to the α-TeO2 and γ-TeO2 crystalline phases for 0.90TeO2– 0.10WO3 and 0.85TeO2–0.15WO3 samples and α-TeO2 and WO3 crystalline phases for the 0.80TeO2–0.20WO3 sample. Avrami constants, n, calculated from Ozawa equation, were found between 1.14 and 1.44. The activation energies, EA, for the first crystallization reactions were determined by using the modified Kissinger equation as 379 kJ/mol, 288.1 kJ/mol and 228.8 kJ/mol, for 0.90TeO2–0.10WO3, 0.85TeO2–0.15WO3 and 0.80TeO2–0.20WO3 glasses, respectively. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Tellurite glasses are of scientific and technological interest because of their various promising properties, such as relatively low-phonon energy, high refractive index, high dielectric constant, good corrosion resistance, thermal and chemical stability, low crystallization ability, low glass transition and melting temperature. Recently, tellurite glasses have been used as host materials for some infrared and infrared to visible up-conversion applications in optical data storage, lasers, sensors and optoelectronic devices [1–5]. Tellurium oxide is the main glass former which does not transform to the glassy state without addition of a secondary component under conventional quenching conditions [2]. Therefore, glass forming agents, such as alkalis, heavy metal oxides or halogens (LiCl, WO3, K2O, CdF2, CdO, etc.) are used as network modifiers to obtain tellurite based glasses [1,2,5]. The addition of WO3 to tellurite glasses provides advantageous properties, such as doping in a wide range, modifying the composition by a third, fourth, and even fifth component, controlling the optical properties, and enhancing the chemical stability and devitrification resistance of the glass [1–3]. In comparison to other tellurite glasses, tungsten–tellurite glasses have slightly higher phonon energy and higher glass transition temperature, hence they can be used at high optical intensities without exposure to thermal damage [4]. To obtain a high quality glass, it is important to recognize its physical and thermal characteristics and also to control the nucleation and crystallization processes. Although there are detailed studies on ⁎ Corresponding authors. Tel.: +90 212 285 68 64; fax: +90 212 285 34 27. E-mail addresses: [email protected] (M. Çelikbilek), [email protected] (S. Aydin). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.012

structural, optical and thermal properties of TeO2–WO3 glasses [1–6], apart from the kinetic measurements realized by Öveçoğlu et al. [2] for 0.85TeO2–0.15WO3 glass, no additional kinetic investigation was reported in the literature on the tungsten–tellurite glasses. There exist no study to our knowledge in the literature solely on the crystallization kinetics of the TeO2–WO3 system; therefore the present study aims to investigate the crystallization kinetics of tungsten–tellurite glasses in terms of crystallization activation energy, EA, and Avrami constant, n. Differential scanning calorimetry (DSC), Xray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to understand the nucleation and crystallization mechanism of tungsten–tellurite glasses.

2. Theoretical considerations The crystallization kinetics of glasses can be investigated either isothermally or non-isothermally by using thermal analysis techniques. In the isothermal method, the sample is heated above the glass transition temperature and the heat absorbed during the crystallization process is measured as a function of time. The isothermal crystallization data are usually explained in terms of the Johnson– Mehl–Avrami equation [7,8]. On the other hand, in the nonisothermal method, the sample is heated at a fixed rate and then the change in enthalpy is recorded as a function of temperature. Non-isothermal method is useful in obtaining kinetic parameters related to the glass crystallization process due to the rapidity of this thermo analytical technique. Ozawa and Kissinger plots are the most commonly used equations to calculate kinetic data, such as the crystallization activation energy, EA and Avrami constant, n [9,10].

M. Çelikbilek et al. / Journal of Non-Crystalline Solids 357 (2011) 88–95

89

DSC Peak Temperature (°C)

414

410 408 406 404 402 330

Fig. 1. Computation of the volume fraction crystallized, x.

335

340

345

350

355

=

ð1Þ

mEA + const RTp

T p1

ð2Þ

where Tp is the crystallization peak temperature for a given heating rate B, EA is the activation energy, R is the gas constant, n is the Avrami parameter and m is the numerical factor of crystallization mechanism. The activation energy is calculated from the slopes of the linear fits to the experimental data from a plot of ln(T2p/Bn) versus 1/Tp. Table 1 shows the parameters of n and m, representing the values of the growth morphology depending on the crystallization mechanism. As can be seen from Table 1, different n and m values are calculated for different crystallization mechanisms regarding to the

(c) x = 0.20 T p2 Tg T p1

(b) x = 0.15

Tg T p2 T p1

Table 1 Values of n and m for different crystallization mechanisms [2,11]. Crystallization mechanism Bulk crystallization with a constant number of nuclei Three-dimensional growth of crystals Two-dimensional growth of crystals One-dimensional growth of crystals Bulk crystallization with an increasing number of nuclei Three-dimensional growth of crystals Two-dimensional growth of crystals One-dimensional growth of crystals Surface crystallization

370

following approaches; when the nucleation rate is zero during the DSC experiment, n = m, when nucleation takes place during thermal analysis, n = m + 1 and when surface crystallization is the predominant mechanism, n = m = 1 [2,12]. For the first condition, the number of nuclei of the pre-nucleated samples does not depend on the heating rate. For the other condition, when the nucleation takes place during thermal analysis, the number of nuclei of the as-cast samples is proportional to the heating rate. Therefore, while the nuclei number remains constant during the thermal analysis for pre-nucleated samples, the first approach (n = m) for the determination of the crystallization mechanism should be preferred. However, the second approach (n = m + 1) is suitable for as-cast samples while a non-

Endo. ← Heat Flow (a.u.) → Exo.

!

365

Fig. 2. DSC peak temperature as a function of nucleation temperature for the first crystallization reaction of 0.90TeO2–0.10WO3 glass.

where x is the crystallized fraction at T for the heating rate of B. From the slopes of the linear fits to the experimental data from a plot of ln[−ln(1 − x)] versus lnB, n values are calculated. The activation energy can be evaluated by the modification of the original Kissinger equation presented by Matusita et al. [10,12]: Tp2 ln Bn

360

Nucleation Temperature (°C)

In the non-isothermal method, kinetic parameters of glass crystallization could be obtained by monitoring the shift in the crystallization peak as a function of the heating/cooling rate [7]. Crystallization peak temperatures, Tp and crystallized volume fractions, x, are determined from DSC curves with respect to temperature. The estimation of the volume fraction crystallized, x is shown in Fig. 1. The volume fraction crystallized, x, at any temperature T is given as x = ST / S, where S is the total area of the exothermic peak between the temperature, Ti, at which the crystallization begins and the temperature, Tf, at which the crystallization is completed and ST is the partial area of the exothermic peak up to the temperature T [11]. The values of the Avrami parameter, n, were determined from the Ozawa equation [9]: ln ½− lnð1−xÞ = −n ln B + const

412

n

m

3 2 1

3 2 1

4 3 2 1

3 2 1 1

T p3

(a) x = 0.10

Tg 250

300

350

400

450

500

550

Temperature (°°C) Fig. 3. DSC curves of as-cast samples of (a) 0.90TeO2–0.10WO3 glass, (b) 0.85TeO2– 0.15WO3 glass and (c) 0.80TeO2–0.20WO3 glass, scanned at a heating rate of 10 °C/min.

90

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performed with the as-cast and the heat-treated samples, respectively. The X-ray diffraction investigations were carried out in a BrukerTM D8 Advanced Series powder diffractometer using Cu Kα radiation in the 2θ range from 10° to 90°. The International Centre for Diffraction Data (ICDD) files were used to determine the crystallized phases by comparing the peak positions and intensities. SEM investigations were conducted with platinum-coated heat-treated samples in JEOLTM Model JSM 7000F operated at 15 kV. Non-isothermal DSC scans of glass samples were carried out at nine different heating rates, B, (5, 7.5, 10, 15, 20, 25, 30, 35 and 40 °C/ min) from room temperature to 550 °C. The values of the Avrami constant, n, and the activation energy, EA, for the first crystallization reaction of the as-cast samples were determined from the Ozawa (Eq. (1)) and Kissinger (Eq. (2)) equations, respectively. In the literature, as-cast (increasing number of nuclei) or prenucleated (constant number of nuclei) samples are used in crystallization kinetic studies [14]. In the present study, the kinetic measurements of the pre-nucleated samples were also realized to recognize the effect of the constant number of nuclei on the crystallization mechanism and the activation energy. To determine the crystallization mechanism of the glasses with fixed nuclei number, the 0.90TeO2–0.10WO3 sample was pre-nucleated by heat-treating for 2 h at 350 °C. The pre-nucleation temperature was determined by nucleating the as-cast glass sample for 2 h at three different temperatures between Tg and Tp, 335 °C, 350 °C and 365 °C (see Fig. 2). The nucleation temperature which corresponds to the maximum peak temperature was selected as the pre-nucleation temperature (350 °C). Also, it was aimed to remove the internal thermal stresses from the glass samples by applying this heattreatment process. DSC scans and the calculation of the kinetic parameters were carried out at similar conditions with the as-cast samples.

Table 2 Values of glass transition, Tg, crystallization onset and peak, Tc/Tp temperatures of the (1 − x)TeO2–xWO3 glasses, recorded at a heating rate of 10 °C/min, with an error estimate of ± 1 °C. Compositions (mol %) TeO2

WO3

90 85 80

10 15 20

Tg (°C)

Tc1/Tp1 (°C)

Tc2/Tp2 (°C)

Tc3/Tp3 (°C)

327 338 349

385/411 418/444 447/492

472/485 –/482

–/511

–; Undetermined values.

steady-state nucleation takes place during the DSC measurements. Different crystallization mechanisms appear for different numerical factors, such as spherical for three-dimensional growth (m = 3), disklike for two-dimensional growth (m = 2) and rod-like for onedimensional growth or surface crystallization (m = 1) [13]. 3. Experimental Glass samples were prepared with the compositions of (1 − x) TeO2–xWO3, where x = 0.10, 0.15 and 0.20 in molar ratio. High purity powders of TeO2 (99.99% purity, Alfa Aesar Company) and WO3 (99.8% purity, Alfa Aesar Company) of 5 g size total were mixed and melted in a platinum crucible with a closed lid at 750 °C for 30 min. The molten samples were removed from the furnace and quenched in water bath. To recognize the crystallization behavior of the as-cast samples, DSC analysis of all the samples was carried out 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 mg, in aluminum pans, under flowing (25 ml/min) argon gas, with a heating rate of 10 °C/ min. 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. According to the DSC results, as-cast samples were heat-treated for 24 h above the first crystallization peak temperatures in order to achieve the thermal equilibrium of these crystalline phases. To examine the glassy nature of the as-cast samples and to identify the first crystalline phases present in the system, XRD analysis was

a

4. Results DSC curves of the as-cast 0.90TeO2–0.10WO3, 0.85TeO2–0.15WO3 and 0.80TeO2–0.20WO3 glass samples, scanned with a heating rate of 10 °C/min are illustrated in Fig. 3. DSC scans show a glass transition and several exothermic peaks corresponding to the crystallization and/or transformation of different crystalline phases. As can be seen from Fig. 3, the number of

b

γ : γ-TeO2

α : α-TeO2 Δ : WO3

c

γ : γ-TeO2 α : α-TeO2

α : α-TeO2

α

γ γ γ

α

(b) 410 °C γ γγ γγ α αα αα α α

γ

γ

γ

γ α α γ γ α γ

20

30

40

50



60

70

80

α α α

α

ΔΔ α α

Δ

α α αΔ α α

(a) as-cast

(a) as-cast

10

(b) 430 °C

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

γ

90

10

20

30

40

50



60

70

80

90

(b) 490 °C

(a) as-cast

10

20

30

40

50

60

70

80

90



Fig. 4. X-ray diffraction patterns of the (a) 0.90TeO2–0.10WO3 glass, as-cast and heat-treated at 410 °C, (b) 0.85TeO2–0.15WO3 glass, as-cast and heat-treated at 430 °C, and (c) 0.80TeO2–0.20WO3 glass, as-cast and heat-treated at 490 °C.

M. Çelikbilek et al. / Journal of Non-Crystalline Solids 357 (2011) 88–95

Endo. ← Heat Flow (a.u.) → Exo.

a B9 B8 B7 B6 B5 B4 B3 B2 B1

300

350

400

450

500

550

500

550

Temperature (°C)

b

B9

Endo. ← Heat Flow (a.u.) → Exo.

B8 B7 B6 B5 B4 B3 B2 B1

300

350

400

450

Temperature (°C)

c

B9

Endo. ← Heat Flow (a.u.) → Exo.

B8

300

B7 B6 B5 B

B2 B1

400

450

500

exothermic reactions observed in the DSC thermograms varies with the composition. The glass transition onset temperatures, Tg, crystallization onset temperatures, Tc and crystallization peak temperatures, Tp, are listed in Table 2. In order to identify the amorphous nature of the glassy samples XRD analysis was carried out on as-cast samples. XRD patterns of the as-cast samples revealed no detectable peaks, proving the amorphous glassy structure (see Fig. 4). Afterwards, as-cast samples were heattreated for 24 h above the first crystallization peak temperatures to obtain the first crystalline phases present in the system. According to the crystallization onset temperatures determined by thermal analysis for the first exothermic reactions, 410 °C, 430 °C and 490 °C were selected as annealing temperatures for 0.90TeO2–0.10WO3, 0.85TeO2–0.15 WO3 and 0.80TeO2–0.20WO3 samples, respectively. The XRD patterns of the as-cast and heat-treated samples are given in Fig. 4. As shown in Fig. 4a–b, XRD analysis of the 0.90TeO2– 0.10WO3 sample heat-treated at 410 °C and the 0.85TeO2–0.15WO3 sample heat-treated at 430 °C for 24 h showed the crystallization of αTeO2 and γ-TeO2 crystalline phases. On the other hand, as shown in Fig. 4c, XRD analysis of the 0.80TeO2–0.20WO3 sample heat-treated at 490 °C for 24 h showed the presence of α-TeO2 and WO3 crystalline phases. In accordance with the XRD results, the asymmetry observed on the first DSC crystallization peaks (see Fig. 3) suggested a multiphase crystallization in the tungsten–tellurite glasses. To study the crystallization kinetics of the first crystallization reactions in terms of the activation energy of nucleation and growth process, DSC analysis of the glass samples was performed at nine different heating rates B (5, 7.5, 10, 15, 20, 25, 30, 35 and 40 °C/min). Fig. 5 shows the DSC curves of the as-cast samples recorded at different heating rates, B. As can be observed in Fig. 5, the crystallization onset temperatures were steady for all heating rates; however the crystallization peak temperatures shifted to higher values with increasing heating rates. Also, with the increasing heating rate the peak heights showed an increase and a general increment was detected for the glass transition temperatures. The glass transition, Tg, and crystallization peak temperatures, Tp, associated with the first exotherms of the glass samples for different heating rates are listed in Table 3. The T values for the calculation of the volume fraction crystallized of (1 − x)TeO2–xWO3 glasses were determined at 420 °C, 445 °C, 480 °C, for 0.90TeO2 –0.10WO3 , 0.85TeO2–0.15 WO3 and 0.80TeO2–0.20WO3 samples, respectively. To understand the effect of the constant number of nuclei on the crystallization mechanism, the 0.90TeO2–0.10WO3 sample was prenucleated before the kinetic measurements. Non-isothermal DSC scans of the heat-treated 0.90TeO2–0.10WO3 sample at 350 °C for 2 h is shown in Fig. 6. The crystallization onset temperatures remain constant for different heating rates for the pre-nucleated sample, while the glass transition, crystallization peak temperatures and also the peak heights of the pre-nucleated sample show an increase with the increasing heating rates which was also observed for the as-cast samples. 5. Discussion

B3

350

91

550

Temperature (°C) Fig. 5. DSC curves with heating rate B (5, 7.5, 10, 15, 20, 25, 30, 35 and 40 °C/min) for the glasses of the composition (a) 0.90TeO2–0.10WO3, (b) 0.85TeO2–0.15WO3 and (c) 0.80TeO2–0.20WO3.

Ozawa and Kissinger equations were used to determine the values of the Avrami constant and the activation energy for the first crystallization reaction of as-cast and pre-nucleated samples, respectively. The values of the Avrami parameter, n, were calculated from the linear fits to the experimental data based on the Ozawa equation (Eq. (1)), as shown in Fig. 7. The n values were determined as 1.14, 1.44, and 1.39 for the as-cast 0.90TeO2–0.10WO3, 0.85TeO2–0.15WO3 and 0.80TeO2–0.20WO3 samples, respectively. The Avrami parameter, n, takes only integer values from 1 to 4. The non-integer value of n for glass crystallization may be attributed to the coexistence of surface and bulk crystallization, and also because of the consequence of several complex processes occurring during the growth of a particular

92

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Table 3 Values of glass transition, Tg, and crystallization peak temperatures, Tp, with different heating rates, B, associated with the first exotherms of the (1 − x)TeO2–xWO3 glasses, with an error estimate of ±1 °C. B2

B3

B4

B5

B6

B7

B8

B9

7.5 °C/min

10 °C/min

15 °C/min

20 °C/min

25 °C/min

30 °C/min

35 °C/min

40 °C/min

Tg

Tp

Tg

Tp

Tg

Tp

Tg

Tp

Tg

Tp

Tg

Tp

Tg

Tp

Tg

Tp

Tg

Tp

325 336 347

406 428 479

326 337 348

408 438 489

327 338 349

411 444 492

328 340 353

412 447 506

329 341 353

417 450 510

331 342 354

419 452 514

332 343 355

422 454 516

332 344 357

424 457 519

334 345 357

426 461 520

phase [14]. On the basis of the determination about the non-integer value of the Avrami parameter, in the present work the n values were taken as 1 for all samples. Therefore, the same values of n and m were determined for all glasses, indicating the formation of surface crystallization during the crystallization of the samples (see Table 1). Using the modified Kissinger equation (Eq. (2)), activation energies, EA, for the first crystallization reactions of (1 − x)TeO2– xWO3 glasses were determined from the linear fits of ln(T2p/Bn) versus 1/Tp plots, as shown in Fig. 8. The activation energy of the first exotherms for the as-cast 0.90TeO2–0.10WO3 and 0.85TeO2–0.15WO3 samples associated with the crystallization of α-TeO2 and γ-TeO2 phases were calculated as 379 kJ/mol and 288.1 kJ/mol, respectively. The EA value of the first exothermic reaction of the as-cast 0.80TeO2– 0.20WO3 sample was determined as 228.8 kJ/mol and related to the crystallization of α-TeO2 and WO3 phases. From the obtained data, it can be concluded that the crystallization activation energy of tungsten–tellurite glasses shows a decrease with the increasing WO3 content in the studied compositional range. XRD analysis in Fig. 4 shows that with the increasing WO3 content, the peak intensities concerning to the metastable γ-TeO2 phase decreased in the 0.85TeO2–0.15WO3 sample comparing to the intensities in the 0.90TeO2–0.10WO3 sample, while the γ-TeO2 phase was not observed in the 0.80TeO2–0.20WO3 sample. It can be deduced that different compositions lead different amounts and/or types of crystalline phases. The difference between the activation energies is also related with this phenomenon. The value of the Avrami constant of 0.90TeO2–0.10WO3 glass prenucleated at 350 °C is shown in Fig. 9. The n value of the pre-nucleated sample was calculated as 1 from the Ozawa equation and while the number of nuclei does not depend on the heating rate for pre-

1,5

a 1

0,5

ln[-ln(1-x)]

0.90TeO2–0.10WO3 0.85TeO2–0.15 WO3 0.80TeO2–0.20 WO3

B1 5 °C/min

y = -1,1422x + 2,9652 R2 = 0,9848

0

-0,5

n = 1.14

-1

-1,5 1

1,5

2

2,5

3

4

3,5

lnB 1,5

b

1 0,5

ln[-ln(1-x)]

Glass composition (mol %)

0

y = -1,4404x + 3,2543 R2 = 0,9831

-0,5 -1 -1,5

n = 1.44

-2 -2,5 1

1,5

2

2,5

3

3,5

4

Endo. ← Heat Flow (a.u.) → Exo.

lnB B9

0

B8 B7

c

-0,5

B6

-1

ln[-ln(1-x)]

B5 B4 B3 B2 B1

-1,5

y = -1,3899x + 1,8935 R2 = 0,9879

-2 -2,5

n = 1.39

-3

300

350

400

450

500

550

Temperature (°°C) Fig. 6. DSC curves with heating rate B (5, 7.5, 10, 15, 20, 25, 30, 35 and 40 °C/min) for the 0.90TeO2–0.10WO3 sample pre-nucleated at 350 °C for 2 h.

-3,5 1

1,5

2

2,5

3

3,5

4

lnB Fig. 7. The Ozawa plots for determining n associated with the first exotherms of the (a) 0.90TeO2–0.10WO3, (b) 0.85TeO2–0.15WO3 and (c) 0.80TeO2–0.20WO3 glasses.

M. Çelikbilek et al. / Journal of Non-Crystalline Solids 357 (2011) 88–95

12

1

a 11,5

10,5

0,5

y = 45,584x - 55,917 R2 = 0,9612

10

EA = 379 kJ/mol

9,5

-0,5

n = 1.00

-1 -1,5

9 8,5 1,42

y = -1,0065x + 2,3519 R2 = 0,9695

0

ln[-ln(1-x)]

ln(Tp 2 /B n)

11

93

1,43

1,44

1,45

1,46

1,47

-2

1,48

1

1,5

2

1000 / T p 12

y = 34,657x - 37,755 R2 = 0,9572

4

11,5

y = 45,945x - 56,275

10,5 11

EA = 288.1 kJ/mol

10 9,5 9 1,34

1,38

1,4

1,42

10,5 10

EA = 382 kJ/mol

1,44 9

12

8,5 1,42

c 11,5

11

R2 = 0,9741

9,5 1,36

1000 / T p

ln(Tp 2 /B n)

3,5

12

ln(Tp 2 / B n)

n ln(Tp 2 /B )

11

3

Fig. 9. The Ozawa plot for determining n associated with the first exotherm of the 0.90TeO2–0.10WO3 sample pre-nucleated at 350 °C for 2 h.

b 11,5

2,5

lnB

1,43

1,44

1,45

1,46

1,47

1,48

1000 / T p

y = 27,522x - 24,906 R2 = 0,9787

Fig. 10. The Kissinger plot for determining EA associated with the first exotherm of the 0.90TeO2–0.10WO3 sample pre-nucleated at 350 °C for 2 h.

10,5

EA = 228.8 kJ/mol

10

9,5

9 1,25

1,27

1,29

1,31

1,33

1,35

1000 / T p Fig. 8. The Kissinger plots for determining EA associated with the first exotherms of the 0.90TeO2–0.10WO3, (b) 0.85TeO2–0.15WO3 and (c) 0.80TeO2–0.20WO3 glasses.

nucleated samples, according to the approach n = m, the mechanism was determined as one-dimensional growth of the crystals (see Table 1). The n value calculated for the as-cast 0.90TeO2–0.10WO3 glass was also calculated as 1, indicating the surface crystallization. The activation energy, EA, of the first crystallization reaction of the pre-nucleated 0.90TeO2–0.10WO3 glass was calculated by using the Kissinger equation, as shown in Fig. 10. The EA value of the prenucleated sample was determined as 382 kJ/mol, approximately the same with the calculated EA value of the as-cast sample (379 kJ/mol). From the obtained data it can be concluded that constant or increasing

number of nuclei does not have a significant effect on crystallization activation energy. SEM investigations were conducted on the heat-treated samples. Fig. 11a–b represents the SEM micrographs taken from the surface and the cross-section of the 0.90TeO2–0.10WO3 sample heat-treated at 410 °C, respectively. Fig. 11a exhibits the presence of dendritic leaf-like crystallites differently oriented on the surface. However, in the crosssectional micrograph (see Fig. 11b), a typical amorphous structure without any crystallization on the bulk structure can be clearly observed following the crystallites on the surface. SEM micrographs taken from the surface and cross-section of the 0.85TeO2–0.15WO3 sample heattreated at 430 °C are shown in Fig. 11c–d, respectively. The surface SEM micrograph of the 0.85TeO2–0.15WO3 sample (see Fig. 11c) reveals dendritic lamellar crystallites differently oriented on the surface. The cross-sectional SEM micrograph of the 0.85TeO2–0.15WO3 sample, Fig. 11d, shows typical amorphous structure on the bulk as observed in Fig. 11b. As shown in the representative SEM micrographs taken from the surface and cross-section of the 0.80TeO2–0.20WO3 sample heattreated at 490 °C, Fig. 11e–f, rod-like crystallites are present throughout the surface but they do not diffuse into the bulk of the glass. Based on the SEM investigations, it was determined for all samples that the crystallites formed on the surface and did not diffuse into the bulk structure proving the surface crystallization mechanism.

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Fig. 11. SEM micrographs of the 0.90TeO2–0.10WO3 sample heat-treated at 410 °C (a) surface, (b) cross-section; 0.85TeO2–0.15 WO3 sample heat-treated at 430 °C (c) surface, (d) cross-section and 0.80TeO2–0.20 WO3 sample heat-treated at 490 °C (e) surface, (f) cross-section.

6. Conclusions The crystallization kinetics of (1 − x)TeO2–xWO3 (x = 0.10, 0.15 and 0.20 in molar ratio) glasses was studied by non-isothermal methods in terms of the crystallization activation energy, EA, and Avrami constant, n. Using the Ozawa equation the Avrami constants were calculated as 1.14, 1.44 and 1.39 for the as-cast 0.90TeO2– 0.10WO3, 0.85TeO2–0.15WO3, 0.80TeO2–0.20WO3 glasses, respectively, indicating the surface crystallization. SEM investigations confirmed the determination of surface crystallization with surface crystallites which do not diffuse into the bulk structure. The Avrami constant of the pre-nucleated 0.90TeO2–0.10WO3 glass was calculated as 1, indicating one-dimensional crystal growth. Activation energies of the first crystallization reactions were determined by the Kissinger method as 379 kJ/mol, 288.1 kJ/mol and 228.8 kJ/mol, for the as-cast 0.90TeO2–0.10WO3, 0.85TeO2–0.15WO3, and 0.80TeO2–0.20WO3

glasses, respectively. It can be concluded that the crystallization activation energy of tungsten–tellurite glasses show a decrease with the increasing WO3 content in the studied compositional range; however in the studied system, the change in composition could not be the unique reason for the variation in activation energy since different amounts and/or types of crystalline phases cause a difference in EA values. The activation energy of the first crystallization reaction of the pre-nucleated 0.90TeO2–0.10WO3 glass was calculated as 382 kJ/mol, indicating that constant or increasing number of nuclei does not have a significant effect on the EA values. Acknowledgement The authors gratefully acknowledge The Scientific and Technological Research Council of Turkey (TUBITAK) for the financial support under the project numbered 108M077.

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