ZnO on structural and thermal properties of tellurite glasses

Accepted Manuscript Effect of ZnO and PbO/ZnO on structural and thermal properties of tellurite glasses Raj Kumar Ramamoorthy, A.K. Bhatnagar PII: DOI: Reference:

S0925-8388(14)02481-5 http://dx.doi.org/10.1016/j.jallcom.2014.10.061 JALCOM 32407

To appear in:

Journal of Alloys and Compounds

Received Date: Accepted Date:

25 April 2014 10 October 2014

Please cite this article as: R.K. Ramamoorthy, A.K. Bhatnagar, Effect of ZnO and PbO/ZnO on structural and thermal properties of tellurite glasses, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom. 2014.10.061

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Effect of ZnO and PbO/ZnO on structural and thermal properties of tellurite glasses Raj Kumar Ramamoorthya,∗, A. K. Bhatnagara,b a

School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad-500 046, India b School of Physics, University of Hyderabad, Hyderabad-500 046, India

Abstract Two series of glasses, (100−x)TeO2 −xZnO (x = 20, 25, 30, 35) and 70TeO2 − (30−y)ZnO−yPbO (y = 5, 10, 15, 20), referred as TZ and TZP, respectively, were prepared by a melt quenching technique and characterized by x-ray diffraction (XRD), density, refractive index, Raman scattering and differential scanning calorimetry (DSC) to observe the changes in their properties as a function of ZnO and PbO/ZnO. Variations in individual structural units/linkages in these glasses are derived from the de-convoluted Raman spectra. The glass transition (Tg ) and onset of crystallization (To ) temperatures are determined from DSC isothermal scans. It is observed that the thermal stability (△T = To −Tg ) decreases for TZ glasses with increase in ∗

Corresponding author at: School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad-500 046, India, Tel.: 00-91-40-23135536 Email address: [email protected] (Raj Kumar Ramamoorthy)

Preprint submitted to Journal of Alloys and Compounds

October 15, 2014

x, while it increases for TZP glasses with increase in y. Changes in thermal parameters of these glasses are correlated with the structural variations as a function of ZnO and PbO/ZnO ratio to determine the effect of addition of heavy oxide, PbO, to TeO2 −ZnO glasses. Keywords: Glasses; Optical materials; Quenching; Inelastic light scattering; Thermal analysis. 1. Introduction Tellurium oxide does not form glass by itself but when mixed with metal oxides [1], it forms a glass easily at low cooling rate (∼1−10 K/min). Tellurite glasses of binary, ternary and quaternary oxide mixtures are subject of extensive research presently as these glasses exhibit many favourable properties for various applications in optical devices. These glasses have low melting temperature, high dielectric constant, wide range of transmission in the visible and infra red regions (up to 5 µm), high solubility of rare earth ions, non-linear optical properties and high chemical durability besides others [2, 3, 4, 5]. Some of these properties are being explored industrially for optical-recording, ultra-fast optical switches, multi-structured optical fibers (e.g., photonic crystal fibers), optical amplifiers, up-conversion lasers, storage devices and possibly more [6, 7, 8, 9, 10, 11]. 2

The structure of TeO2 rich glasses is found to be similar to α-TeO2 (paratellurite) which has a three dimensional network of TeO4 trigonal bipyramids in which one tellurium atom is bonded with four oxygen atoms, two at axial and another two at equatorial positions. Beside oxygen atoms, a lone pair of electrons is also present at the equatorial position [12]. Different structural analyses (neutron diffraction, Raman analysis) reveal that an addition of a metal oxide, like ZnO, Na2 O, BaO, MgO etc., [13, 14, 15] converts the TeO4 units into TeO3 and TeO3+1 polyhedra which have non-bridging oxygen atoms. The +1 in TeO3+1 indicates the shift of an oxygen atom from Te-O equilibrium distance, usually it is >2.2 ˚ A [13]. The chemical nature of the cation in a metal oxide determines its site occupancy (position) in the tellurite glass matrix and in turn is expected to influence their physical properties due to structural changes. For applications of glasses in a useful product, it is necessary that it should be able to withstand rise in temperature, without any changes in its properties while a device is under operation. One of important thermal parameters of a glass is its thermal stability △T = To −Tg , where To and Tg are onset temperature of crystallization and glass transition temperature, respectively. The value of △T should be high, normally more than 100 K for

3

a glass chosen for a good design end product. Otherwise, the glass device may get heated enough to change its meta-stable state into a relatively stable (micro/nano-crystals) phase during the operation resulting in property changes of the device. These crystals also act as scattering centres, which results in unsatisfactory performance of the device. Therefore, it is desirable to have glasses with as high △T as possible. The added oxide entities in the main matrix, which make glass forming easier, play one of the following roles: glass former, glass modifier or intermediate. This results in changes in the structural units of which the glass is originally made of. This affects the thermal parameters, Tg , To and, therefore, △T. There are few reports in which the role of these added entities in some tellurite glasses (e.g., TeO2 −ZnO [16, 17] and TeO2 -GeO2 /Ag2 O-WO3 [18, 19]) were correlated with the thermal parameters mentioned above. Stavrou et al. [16] carried out an investigation of temperature dependence of the Boson peaks in Raman spectra and variation of Tg of TeO2 -ZnO glasses. They observed that Tg decreased with ZnO content in the glasses, whereas on the contrary, Kaur et al. [17] found that Tg of TeO2 −ZnO glasses increased with an increase of ZnO content. Upender et al. [18] identified the increase of Tg of TeO2 −GeO2 −WO3 glasses with the increase of WO3 content.

4

Following the above works, in this report, preparation and characterization of (100-x)TeO2 −xZnO (x = 20, 25, 30 and 35) and 70TeO2 -(30-y)ZnOyPbO (y = 5, 10, 15 and 20), referred hereafter as TZ and TZP glasses, respectively, are studied by density, refractive index, Raman spectra and DSC measurements. The DSC isothermal scans at 10 ◦ C/min are used to determine thermal parameters Tg , To and △T values. The main emphasis of this work is to use analyzed Raman spectra to obtain/estimate modification in the networks of TZ glasses due to ZnO addition and that of TZP glasses due to an increase in the PbO/ZnO content, and correlate the same with the observed changes in their thermal parameters.

2. Experimental TZ and TZP glasses with compositions (100-x)TeO2 −xZnO (x = 20, 25, 30 and 35) and 70TeO2 −(30−y)ZnO−yPbO (y = 5, 10, 15 and 20) were prepared using a standard melt quenching technique in which a melt was quenched in between two steel plates kept at about 250 ◦ C. The purity of chemicals used is 99.9%. Details of the preparation of these glasses are described elsewhere [20]. All the samples were polished to optical flatness and cut to dimensions as required for the measurements. The amorphous nature

5

of the glasses was confirmed using XRD. The density of each glass was measured using the Archimedes method with toluene as the immersion liquid. The accuracy of measurement was better than 5%. The refractive index was measured at wavelength 633 nm using the prism coupling technique with a Metricon equipment (Model No. 2010). DSC (make: TA instrument, model no. Universal Analysis 2000) was used to get thermal scans at 10 ◦ C/min in the temperature range of 50−600 ◦ C to determine Tg and To , defined earlier, for each glass. The Raman spectra were recorded at room temperature (RT) in the range 30−1000 cm−1 using a micro Raman spectrometer (Horiba JobinYvon, LabRAM-HR 800) equipped with a 514.5 nm excitation source with power of 2.5 mW having a CCD detector.

3. Results and analysis 3.1. TZ glasses 3.1.1. Density and refractive index Densities and refractive indices of TZ glasses are listed in Table 1. It is observed that the density as well as refractive index of TZ glasses decrease as ZnO content increases.

6

3.1.2. Raman analysis Raman spectra of all TZ glasses are shown in Fig. 1a. As expected, the observed bands are broad due to the glassy nature of the samples. We distinctly observe five bands centered at about 90, 120, 420, 660 and 740 cm−1 in Raman spectra of all TZ glasses. The bands at low frequencies, namely at around 90 and 120 cm−1 , remain unchanged in the samples. These are attributed to the acoustical vibrations in the glasses which are known as Boson peaks and have been observed in many glasses [16, 17]. The other three bands observed at about 420, 660 and 740 cm−1 are due to the overlapping of the neighbouring bands, therefore, it is necessary to de-convolute the Raman spectra using Gaussian multi-peak function fit to separate out contributions from different structural units. It has been reported earlier that the deconvolution of Raman spectra in 600-900 cm−1 region ought to be carried out with utmost care to get results having physical significance [14, 15, 16]. Previously, Jha [14] and Sekiya et al. [15] both fitted the spectra using four peak Gaussian function at about the fixed peak positions of 610, 660, 730 and 780 cm−1 . But recently Stavrou et al. [16] de-convoluted the same spectral region with three Gaussian peaks excluding the band at about 610 cm−1 . Keeping these different fitting procedures in mind, we have performed different num-

7

ber of deconvolution in the same region with 2 to 4 Gaussian peaks. We have, furthermore compared observed Raman spectra with those of pure TeO2 and several other mixed tellurite glass ceramics [12, 21] to identify the structural units and their corresponding peak positions. After considering the Raman vibrations in different tellurite structures, we conclude that a pivotal contribution of 610 cm−1 band, which is due to the asymmetric vibration of TeO4 is needed to get the physical meaning of the results for the region 600-900 cm−1 through the fitting process. After all possibilities, all TZ glasses Raman spectra were de-convoluted at a fixed peak positions (with the well followed literature [14, 15, 16]) around 420, 610, 660, 730 and 780 cm−1 corresponding to the bending vibration of Te-O-Te, asymmetric vibration of TeO4 , symmetric vibration of TeO4 , stretching vibration of Te-O and Te=O in TeO3+1 and TeO3 , respectively. To illustrate this, Raman spectrum of 35TZ (the number before TZ glass indicates ZnO content) glass is de-convoluted through multi-peak Gaussian fit and shown in Fig. 1b. The description about bands intensities and corresponding structural units/linkages modification are detailed in the discussion section.

8

3.1.3. Thermal analysis Figure 2 displays the DSC curves of all TZ glasses taken at a constant heating rate of 10 ◦ C/min. When a sample gets heated in DSC at a constant rate, the DSC signal may exhibit endothermic and/or exothermic behaviour due to changes taking place in the sample. For a glass, these are glass transition (endothermic) and crystallization (exothermic peaks) which are clearly visible in Fig. 2. It is to be noted that all samples show one crystallization peak except 20TZ which shows two peaks. The Tg and the temperature at which crystallization starts (To ), followed by the full crystallization peak, are determined using the standard procedures from these scans. Tg values of all sample can be obtained by drawing three tangents to endothermic region of a sample and is illustrated in Fig. 3a. Initially two tangents were drawn (one at anterior region of transition and another at transition curve itself) at the endothermic region of curve. The point at which both tangents intersect are called as onset of Tg . Later, drawing third tangent at the posterior region of curve will produce an another intersection and fetch us an end point of Tg . The steepest point at which the second tangent passes through the transition curve is called inflection point and more specifically it is called as glass transition for glasses. This way Tg values of all sample were determined. To values

9

of all sample were evaluated by drawing two tangents and is demonstrated in Fig. 3b. This was evaluated by intersection of two tangents at the region of ascent of crystallization peak. Tg , To and △T values of all TZ glasses are listed in Table 1. From the table, it is observed that both Tg and To increase as the ZnO content increases in these glasses which is in agreement with the work of Kaur et al. [17] but not with the results of Stavrou et al. [16] who have reported a decrease in Tg . However, the increase in the values of Tg and To of our TZ glasses is such that △T decreases with an increase in ZnO content.

3.2. TZP glasses 3.2.1. Density and refractive index Densities and refractive indices of TZP glasses increase with increase of PbO content. This is opposite to TZ glasses. The increase in these quantities is expected with PbO content since it is a heavier oxide. This behaviour of density of TZP glasses is in agreement with the results of Kaur et al. [17] for TeO2 -PbO glasses.

10

3.2.2. Raman analysis Raman spectra of all TZP glasses are displayed in Fig. 4a. One observes bands at about 90, 120, 420, 660 and 740 cm−1 are the same as observed for TZ glasses. Moreover, an additional band, but weak, at about 310 cm−1 is also observed. This band becomes observable when PbO content increases to y = 10 and its intensity gradually increases as PbO content becomes higher. As before, the bands at around 90 and 120 cm−1 are attributed to the Boson peaks and their position is independent of change in the TZP composition. For aforementioned reason, all TZP glasses, except for 5TZP Raman (the number before TZP glass indicates PbO content) spectrum were de-convoluted at a fixed peak positions of 310, 420, 610, 660, 730 and 780 cm−1 . To demonstrate this, Raman spectrum of 20TZP is de-convoluted and shown in Fig. 4b. The band at about 310 cm−1 is attributed due to Te-O-Pb vibrations and the rest of all other de-convoluted bands are attributed to the same Raman vibrations as mentioned in the Raman spectra analyses of TZ glasses.

3.2.3. Thermal analysis Figure 5 shows the DSC curves of all TZP glasses taken at 10 ◦ C/min which are almost similar to the ones for the TZ glasses. The glass transition 11

and crystallization of the glasses are clearly visible. 5TZP glass shows a single crystallization peak, other two compositions, y = 15 and y =20, show broader peaks with shoulder at lower temperature than the peak temperature indicating two crystallization processes. The sample with y = 10 exhibits clearly two crystallization processes. As like in TZ glasses, Tg and To values for TZP glasses were extracted and listed in Table 1. It is observed that Tg decreases but To increases with increase in the PbO content in these glasses. The opposite directional shift of these two temperatures results in higher △T with increase in the PbO content in the TZP glasses.

4. Discussion 4.1. TZ glasses 4.1.1. Structural analysis The observed Raman spectra of TZ glasses are interpreted arising from the combined stretching and bending vibrations of different tellurite structural units. To identify real variation of structural units (tellurite entities) among TZ glasses, intensity value calculated for individual peak through deconvoluted Raman spectra has to be normalized with respect to respective intensities of the de-convoluted Raman spectrum of pure TeO2 glass [12].

12

The pure TeO2 glass Raman spectrum was taken from Ref. [12]. This way we could get a comparable and relative structural units as a function of ZnO content. Figure 6 presents the relative intensity variation of individual peak of TZ glasses as a function of ZnO content. The pure tellurite glass structure is made of Te-O-Te network bridges [12]. When ZnO is added, it acts as a modifier in these glasses and continuously breaks Te-O-Te linkages resulting in a gradual decrease in the intensity of the 420 cm−1 band. In addition to intensity decrease, a shift towards lower frequency and continuous narrowing of band is observed for increasing ZnO. This continuous breaking of Te-O-Te network influences the reduction of TeO4 structural units which in turn results in continuous decrease of 610 and 660 cm−1 band intensity due to asymmetric and symmetric stretching vibrations of TeO4 . A decrease of TeO4 structural units indicate the conversion of the same to TeO3+1 and/or TeO3 units. Initially, the band intensity at about 730 cm−1 due to TeO3+1 increases for the 25TZ sample and later it starts decreasing when more ZnO is added owing to conversion of the structural units from TeO3+1 to TeO3 . The band intensity at 780 cm−1 is due to TeO3 stretching vibrations which constantly increase at the cost of intensity of structural bands at about 610, 660 and 730 cm−1 .

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4.1.2. Influence of structural entities on thermal properties In TZ glasses, Tg and To values increase from 317 to 340 ◦ C and 433 to 442 ◦ C, respectively, for the substitution of ZnO for TeO2 . Figure 7 shows variation of Tg and △T as a function of ZnO mol%. Both these parameters are found to vary almost linearly for the ZnO mol%. The slopes of Tg and △T vs ZnO mol% in linear fit are found to +1.5 ◦ C/ZnO mol% and -1.6 ◦

C/ZnO mol%, respectively. It was mentioned before that the ZnO acts as

a modifier in glasses and places (Zn) itself in between the two non-bridging oxygen of different tellurite network [16]. This kind of arrangement gradually increases with increase of ZnO content in the glasses. Hence, cross-linking network connectivity by Zn improves glass strength for increasing ZnO, which therefore, requires larger supply of energy for Tg to takes place. Also its evident from density measurement that, the change in density value among the TZ glasses is 1% for the total increase of ZnO from 20 to 35TZ. This all clearly indicates that the network arrangement varies very little from the initial value of ZnO. An increase in To for more addition of ZnO is also due to the same reason. The larger increase of Tg with respect to To results in a decrease of △T with increase in ZnO content.

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4.2. TZP glasses 4.2.1. Structural analysis In TZP glasses, the structural changes were observed for the constant mol% of TeO2 and varying PbO/ZnO ratio. As mentioned before, ZnO acts as a modifier, however, PbO acts as modifier at low content and a network former at high content [22]. The relative intensities of individual peaks in Raman spectra of TZP glasses are obtained as demonstrated for TZ glasses and their variation in structural units/linkages are shown as a function of PbO content in Fig. 8b. The 310 cm−1 band starts to appear at 10TZP and its intensity increases with increase in PbO. This increase of band intensity indicates the progressive network formation of Te-O-Pb for increasing PbO. The relative intensity of this 310 cm−1 band is obtained by using the intensity value of 420 cm−1 band in pure TeO2 glass. Since Pb is heavier than Te, it is expected that the band frequency to be lower than that of Te-O-Te stretching vibrations. In an opposite way, the 420 cm−1 band intensity, which is due to Te-O-Te vibrations, decreases continuously due to a substitution of PbO for ZnO. This intensity decrease happens because of continuous breaking of TeO-Te bridges at the cost of Te-O-Pb network formation. On the other hand, the band intensities of 610 and 660 cm−1 due to asymmetric and symmetric 15

stretching vibration of TeO4 remains constant in spite of increase of PbO. We, therefore, conclude that TeO4 structural units do not undergo any further conversion of structural units with increasing PbO. On the other hand the band intensities of 730 and 780 cm−1 due to stretching vibration of TeO3+1 and TeO3 decrease and increase, respectively, with increasing PbO. This indicates that the some of TeO3+1 structural units are changing into TeO3 as PbO is added more in the glasses. The large polarizability of Pb [23] and large electron density of Te-O-Pb network [20] due to the presence of heavy Pb atom induces an electron rich environment in the glass matrix. This in turn repels oxygen atom away from Te-O bond and favours in forming more TeO3 structural units for increasing PbO. 4.2.2. Influence of structural entities on thermal properties Tg and To values of TZP glasses decrease and increase from 321 to 298 ◦

C and 447 to 472 ◦ C, respectively, with increasing substitution of PbO for

ZnO, and therefore, △T increases with increase in PbO. Tg and △T values for TZP glasses are plotted as a function of PbO mol% in Fig. 9. As shown in Fig. 9, replacement of ZnO by PbO produces more or less linear decrease and increase of Tg and △T values for increasing PbO, respectively. Their values are observed to be -1.6 ◦ C/PbO mol% and 3.4 ◦ C/PbO mol%, respectively. 16

The earlier reports have shown that addition of oxides like WO3 , GeO2 , etc strengthens the network [18, 24] and it results in higher To . The initial substitution of PbO for ZnO in TZP glasses produces two kinds of modifier in the glass matrix. Hence, it results in more broken network formation for 5TZP. On further substitution of PbO, it produces mixed network structure of both Te-O-Te and Te-O-Pb with less cross-linking network connectivity due to the small amount of ZnO. This mixed network structural arrangement goes on increasing as PbO replaces ZnO. Therefore, it reduces the network inter-connectivity between the neighbouring network chains and results in decrease of Tg . In contrast to Tg , the To value increases with more addition of PbO. To describe this trend, we would like to highlight the results of density and molar volume of TZP glasses for increasing PbO. Density of TZP glasses increases more than 13% as the PbO content changes from 0 to 20 mol%, while molar volume changes only by 6.7%. This indicates that besides the effect of higher density of Pb, the firm intact intra connection (directional bonding) increases in the network (Te-O-Pb) with increasing PbO. Hence, it is more difficult for the system to arrange to the low energy configuration (crystal) system. The inset of Fig. 9 also complements the above explanation that the △T of TZP glasses increases for the increase

17

of Te-O-Pb network formation (for detail evolution of Te-O-Pb network for increasing PbO, refer earlier subsection). Thus, it results in increase of To and large △T value for increasing PbO in TZP glasses. Among this two series (TZ and TZP) of glasses, the large △T values was found for 15TZP and 20TZP glasses. The obtained △T values of these two glasses compare well with the earlier reported values of other tellurite glasses [3, 6], where they claimed those glasses as potential materials for fiber drawing.

5. Conclusion Two series of glasses, (100-x)TeO2 −xZnO (x = 20, 25, 30 and 35) and 70TeO2 -(30-y)ZnO-yPbO (y = 5, 10, 15 and 20) were prepared and analysed using de-convoluted Raman spectra and DSC. Structural units/linkages variation of TZ and TZP glasses were followed by de-convoluted Gaussian multi-peak function fit, for increasing ZnO and PbO/ZnO. Network structure of these glasses were identified, which found to be rich in Te-O-Te and mixture of prior mentioned linkages and Te-O-Pb, respectively, for TZ and TZP glasses. Changes in network structure influence a decrease and increase of thermal stability of TZ and TZP glasses, for increasing ZnO and PbO, respectively. Among all glasses, 15TZP and 20TZP found to be a best suitable

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glass compositions for the fiber drawing.

Acknowledgement The authors acknowledge India-Trento program for advanced research (ITPAR) funded by the DST, New Delhi, India and University of Trento, Italy. One of the authors (AKB) acknowledges support from the National Academy of Sciences, India, Allahabad. The authors also acknowledge Prof. G. Dalba, Prof. M. Montagna and Dr. F. Rocca, for their constant encouragement and for the lab facility provided.

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Table 1: Density (ρ), molar volume (MV), refractive index @ 633 nm, glass transition temperature (Tg ), onset of crystallization temperature (To ) and thermal stability (△T) of TZ and TZP glasses. ρ

MV

n

Tg

To

△T

g/cm3

cm3 /mol

@ 633 nm

◦C

◦C

◦C

20TZ

5.61

25.64

2.106

317

433

116

25TZ

5.59

25.06

2.083

324

437

113

30TZ

5.57

24.43

2.062

330

440

110

35TZ

5.55

23.83

2.040

340

442

102

5TZP

5.76

24.86

2.084

321

447

126

10TZP

5.95

25.27

2.109

313

450

137

15TZP

6.13

25.66

2.133

300

462

162

20TZP

6.31

26.06

2.156

298

472

174

Sample

25

1. (a) Room temperature Raman spectra of TZ glasses, excited at 514.5 nm. For clarity the spectra are vertically shifted along the y-axis. (b) shows the de-convoluted Raman spectra of 35TZ glass. The wavenumbers are indicated in cm−1 . 2. DSC scans of TZ glasses. For clarity the DSC scans are vertically shifted along the y-axis. 3. Demonstration of Tg and To using 20TZ glass. FT = first tangent; ST = second tangent; TT = third tangent; O = onset point of Tg ; I = inflection point; E = end point of Tg . 4. (a) Room temperature Raman spectra of TZP glasses, excited at 514.5 nm. For clarity the spectra are vertically shifted along the y-axis. (b) shows the de-convoluted Raman spectra of 20TZP glass. The wavenumbers are indicated in cm−1 . 5. DSC scans of TZP glasses. For clarity the DSC scans are vertically shifted along the y-axis. 6. Individual structural units (intensities) of TZ glasses were normalized with respect to respective intensities of pure TeO2 glass. Their relative structural units (intensities) trend is observed for increasing ZnO mol%. The wavenumbers are indicated in cm−1 . 26

7. Dependence of glass transition temperature (Tg ) and thermal stability (△T) of TZ glasses, as a function of ZnO mol%. 8. Individual structural units (intensities) of TZP glasses were normalized with respect to respective intensities of pure TeO2 glass. Their relative structural units (intensities) trend is observed for increasing PbO mol%. The wavenumbers are indicated in cm−1 . 9. Dependence of glass transition temperature (Tg ) and thermal stability (△T) of TZP glasses, as a function of PbO mol%. The inset shows the thermal stability of TZP glasses as a function of Te-O-Pb network, for 10, 15 and 20TZP.

27

740

20TZ

(a)

90 120

30TZ

660

35TZ

420 x =

35

x =

30

x =

25

x =

20

200

400

600

800 -1

Wavenumber (cm

)

420

Intesity (Arb. Units)

Intensity (Arb. Units)

25TZ

610

(b)

660 730 780 35TZ FIT 35TZ

400

500

600

700

800

Wavenumber (cm

Figure 1:

28

900

-1

)

1000

4

35TZ 30TZ

Heat Flow (W/g)

25TZ

3

20TZ

2

x =

35

x =

30

x =

25

x =

20

1 0

100

200

300

400 o

Temperature ( C)

Figure 2:

29

500

20TZ

Heat Flow (W/g)

0.20

FT

E

0.15

280

(a)

O ST I TT

320

360

400

o

Temperature ( C)

1.2

(b)

20TZ

Heat Flow (W/g)

1.0

0.8

0.6

0.4

O

0.2

0.0 360

400

440

480 o

Temperature ( C)

Figure 3:

30

520

5TZP

740

Intensity (Arb. Units)

10TZP

(a)

15TZP

90 120

660

20TZP

310 y = 20

420

y = 15 y = 10 y =

5

200

400

600

800 -1

Wavenumber (cm

)

310

Intesity (Arb. Units)

420

(b)

610 660 730 780 20TZP FIT 20TZP

300

400

500

600

700

800

Wavenumber (cm

Figure 4:

31

900 1000

-1

)

4

20TZP 15TZP

Heat flow (W/g)

10TZP

3

5TZP

y = 20

y = 15

2 y = 10

1 0

y =

100

200

300

5

400

500

600

o

Temperature ( C)

Figure 5:

3 420

Relative intesity

610 660

2

730 780

1

0.5

0.0 20

25

30

35

ZnO Concentration (mol%)

Figure 6:

32

350

120

T

g

T

T

105 320

310

100 20

25

30

35

ZnO Concentration (mol%)

Figure 7:

310

3

420

Relative intesity

610 660

2

730 780

1

0.5

0.0 5

10

15

20

PbO Concentration (mol%)

Figure 8:

33

95

o

110

330

T ( C)

115

g

o

( C)

340

T

T 170

g

150

180

T

140 130 120 0.48

170 0.50

0.52

0.54

Te-O-Pb Network

160

320

T

150 310 140 300

130

290

120 5

10

15

PbO Concentration (mol%)

Figure 9:

34

20

o

160

T ( C)

330

190

180

g

o

( C)

340

Thermal stability

(

T)

350

    

Structural units/linkages variation of TeO2-ZnO and TeO2-ZnO-PbO glasses was studied. Structural arrangements of TeO2-ZnO glasses are rich in Te-O-Te network. A mixture of Te-O-Te and Te-O-Pb networks is identified in TeO2-ZnO-PbO glasses. Changes in thermal parameters Tg and To are correlated with the structural variations. 15PbO and 20PbO samples of TeO2-ZnO-PbO glasses show large thermal stability.