Thermal and optical properties of TeO2–ZnO–BaO glasses

Journal of Non-Crystalline Solids 358 (2012) 947–951

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Thermal and optical properties of TeO2–ZnO–BaO glasses N. Manikandan, Aleksandr Ryasnyanskiy, Jean Toulouse ⁎ Department of Physics, Lehigh University, Bethlehem, PA, USA

a r t i c l e

i n f o

Article history: Received 10 September 2011 Received in revised form 30 December 2011 Available online 2 February 2012 Keywords: Tellurite; Glasses; Properties

a b s t r a c t We report the results of a systematic study of the thermal and optical properties of a new family of tellurite glasses, TeO2–ZnO–BaO (TZBa), as a function of the barium oxide mole fraction and compare them with those of TeO2–ZnO–Na2O (TZN). The characteristic temperatures of this new glass family (glass transition, Tg, crystallization, Tx, and melting, Tm) increase significantly with BaO content and the glasses are more thermally stable (greater ΔT = Tx − Tg) than TZN glasses. Relative to these, Raman gain coefficient of the TZBa glasses also increases by approximately 40% as well as the Raman shift from ~680 cm− 1 to ~770 cm− 1. The latter shift is due to the modification of the glass with the creation of non-bridging oxygen ions in the glass network. Raman spectroscopy allows us to monitor the changes in the glass network resulting from the introduction of BaO. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Among soft glasses, tellurites are attracting renewed interest for photonics applications. Most importantly, they transmit over a wide range of wavelengths, from 0.5 μm to 6 μm, are not photosensitive and offer the thermal and mechanical stability of oxide glasses [1-4]. In addition, they possess high linear and nonlinear refractive indices, low cut-off phonon energies around 750 cm− 1, high rare-earth solubility and a high Raman gain coefficient [5,6], all of which are important for applications, and in particular for the development of mid-IR fiber laser sources. Moreover, because of their wide glass-forming range, their composition can be adjusted to further optimize these optical properties. [7-9]. Pure TeO2 cannot form a glass except under particular extreme conditions [10]. The addition of various modifiers increases the glass-forming range of tellurites. Binary tellurite glasses with alkali and alkaline earth additives have been studied extensively [11,12]. The addition of both ZnO and Na2O to form the ternary TZN system increases the thermal stability of the tellurite glass and its resistance to devitrification. The most exhaustively studied composition in the TZN family is 75TeO2–20ZnO–5Na2O. It is found to possess good thermal stability (ΔT = 105 °C) and a high refractive index (n0 ≈ 2.0). We have shown that an increase of the Na2O content up to 10% (75TeO2–15ZnO–10Na2O) increases the stability of the glass and reduces the risk of crystallization during fiber drawing [13-15]. However, further addition of Na2O beyond 10% reduces the glass transition temperature, leading to poor chemical durability and hindering practical usage for fiber lasers and amplifiers [16-18].

⁎ Corresponding author. E-mail address: [email protected] (J. Toulouse). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.01.003

By contrast, addition of alkaline earth oxides like barium oxide has been shown to increase the transition temperatures and improve the thermal stability of tellurite glasses and, consequently the working range. Increasing the BaO content shifts the working transition temperatures towards higher values [9,19]. It also leads to an increase in the Raman gain coefficient and a larger spectral bandwidth compared to that of silica [20]. Previous studies have only reported results for single concentrations of BaO in tellurite glasses. In the present paper, we report the results of a more comprehensive study of the evolution of the physical and optical properties of TZBa glasses as a function of the BaO content. 2. Experimental details All the glasses studied, (80-x)TeO2–20ZnO–xBaO (x = 5, 10, 15, 20 mol%), were prepared by the standard melt-quenching method. High purity raw materials TeO2(5 N), ZnO(5 N) and BaO(4 N) were weighed, mixed thoroughly, vacuum-dried and melted in gold crucibles at 850 °C for 2 hrs in a flowing oxygen atmosphere. The melt was then poured onto preheated metal plates (230 °C) and annealed below its glass transition temperature for 5 hrs in order to release thermal stresses. Annealed samples were then cut to the desired thickness and polished for optical measurements. Thermal analysis was carried out on a Differential Scanning Calorimeter (TA instrument). The powder samples, approximately 20 mg in weight, were heated from 40 °C to 600 °C in a hermetically sealed aluminum pan under N2 atmosphere. The glass transition temperature (Tg) was obtained by drawing tangents to the exothermic peak and the onset of crystallization (Tx) was taken as the temperature at which the heat flow starts to increase positively from its constant value. Optical transmission was measured with a Perkin Elmer FTIR spectrometer in the range 1.8–25 μm using samples of approximately

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1 mm thickness. Refractive index measurements were done using a J.A. Woollam Variable Angle Spectroscopic Ellipsometer (VASE) in the wavelength range 290–1700 nm. At least three measurements were done on these samples and the average value was computed as a final result. Error bars indicate the scatter in the data. The Raman spectra were measured using a Horiba Jobin Yvon double monochromator Raman spectrometer at room temperature using an Ar-ion laser line at 488 nm. 3. Results Fig. 1 shows the DSC trace of several TZBa glasses. Tg and Tx were found to increase with increase in BaO concentration. Table 1 quantifies the thermal properties of TZBa glasses. FTIR measurements shown in Fig. 2 indicate that all our glasses have good transmission up to 5.5 microns. Fig. 3a presents the refractive index dispersion measured with an ellipsometer and fitted with a Cauchy model. The variation in refractive index with increasing BaO concentration at λ = 1550 nm is shown in Fig. 3b. Table 2 quantifies the refractive index values at certain specific wavelengths. Fig. 4 shows the deconvoluted Raman spectra measured in these glasses. 4. Discussion 4.1. (i) Thermal properties One of the important parameters for the characterization of glassy materials is their Tg. With increasing BaO content from 5% to 20%, Tg and Tx were found to increase from 327 °C to 355 °C and 425 °C to 491 °C respectively. Also, the strength of the crystallization peak decreases with increasing BaO content and is almost absent for 20% BaO. These observations can be contrasted with the evolution of TZN glasses in which an increase in Na2O content also results in the decreasing strength of the crystallization peak but a decrease of Tg and Tx rather than an increase. TZN glasses can be said to become more “fragile” [21] with increasing Na2O while TZBa glasses become stronger with increasing BaO, though both increases have a tendency to prevent crystallization. Given the ionic radii of Na and Ba, 1 Å and 1.35 Å respectively, these results suggest that the opposite trend of Tg and Tx in both family of glasses is due to a difference in the free volume of the modified glass. In TZN, Na + breaks Te―O bonds and opens the network while, in TZBa, Ba ++ also breaks Te―O bonds but forms tighter bonds with oxygen than Na + and fills open spaces more completely in the network [22,23]. The suppression of crystallization

BaO concentration (mol. %)

Tg ( ± 2 °C)

Tx (± 2 °C)

ΔT = Tx − Tg (°C)

05 10 15 20

327 335 345 355

423 449 459 491

96 114 114 136

in both cases has therefore two different origins. In TZN, crystallization is suppressed because of bond breaking and an increase in free volume in a more open glass network while, in TZBa, it is suppressed because of a decrease in free volume and a more compact/dense glass (increased rigidity), making atomic and molecular rearrangements more difficult. In the glass transition range, the network normally acquires a higher degree of mobility that allows bond rearrangements or the transfer of covalent bonds between constituent atoms. The number of network bonds that must be rearranged or broken depends on the cross linking density of the network and the bond energy between constituent atoms. In TZBa, the glass transition temperature increases due to increased cross-linking density and bond strength between the atoms involved [24]. The relatively larger mass of barium atoms also reduces the free volume and favors cross-linking thus requiring more external energy for structural modifications. This in turn leads to an increase in Tg with increasing BaO concentration [16,23,25]. Thermal stability is an important issue in glass science for both fundamental and technological reasons. Thermal stability (ΔT) refers to the temperature range within which the glass does not tend towards devitrification (Tx–Tg): the higher the thermal stability, the better the glass quality. Drawing these glasses into optical fibers requires that they undergo repeated and prolonged heating cycles above Tg. If ΔT is low, thermal cycling leads to the formation of micro-crystals during fiber drawing, which acts as scattering centers and result in considerable attenuation of light. In general, ΔT should be ≥ 100 °C to avoid microcrystal formation. In Fig. 1, the addition of BaO is seen to result in shifting both Tg and Tx towards higher temperatures, with Tx shifting more than Tg, or equivalently ΔT increasing from 96 °C for 5% BaO to 136 °C for 20% BaO (Table 1). TZBa is therefore thermally more stable than TZN during processing of the glass, i.e. comparatively less prone to crystallization. 4.2. (ii) Optical properties 4.2.1. (a) FTIR spectroscopy The FTIR spectra show a dip at around 3.3 microns which is due to the stretching vibrations of free OH groups [26]. These OH bands are a

100

12

75TeO2-20ZnO-05BaO 70TeO2-20ZnO-10BaO 65TeO2-20ZnO-15BaO 60TeO2-20ZnO-20BaO

8 6

80

Transmission (%)

10

Heat Flow (W/g)

Table 1 Transition temperatures of TeO2–ZnO–BaO glasses.

4 2 0 -2 -4

60

40 75TeO2-20ZnO-05BaO 70TeO2-20ZnO-10BaO 65TeO2-20ZnO-15BaO 60TeO2-20ZnO-20BaO

20

-6

Tg

-8

Tx 0

0

100

200

300

400

500

Temp (οC) Fig. 1. DSC trace of TeO2–ZnO–BaO glasses.

600

2

4

6

Wavelength (μm) Fig. 2. FTIR spectra of TeO2–ZnO–BaO glasses.

8

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a 2.35

Refr. Index

decreases rapidly (Fig. 3b) to 1.90 with increasing BaO content just as in TZN with increasing Na2O and in agreement with earlier published values [20]. Though the polarizability of BaO is high, the observed decrease in the refractive index values is similar to that of the binary BaO-TeO2 glass [32], indicating the influence of BaO in the network.

75TeO2-20ZnO-05BaO 70TeO2-20ZnO-10BaO 65TeO2-20ZnO-15BaO 60TeO2-20ZnO-20BaO

2.30 2.25 2.20 2.15 2.10 2.05 2.00 1.95 1.90 200

400

600

800 1000 1200 1400 1600 1800

Wavelength (nm)

b 2.02

λ= 1550 nm slope = -0.00671

Refr. index

2.00 1.98 1.96 1.94 1.92 1.90 0

5

10

15

20

BaO Concentration (mol %) Fig. 3. Wavelength dispersion and Composition dependence of the Refractive index of TeO2–ZnO–BaO glasses (Lines in 3a are drawn as a guide to the eye).

major source of loss in optical fibers in this wavelength range. In tellurite glasses, the width of the OH bands seems to bear a relationship with the glass structure. These OH ions form hydroxyl bonds over a range of sites in these glasses [27], resulting in broad OH peaks in TeO2 glasses. In barium tellurite glasses, the formation of TeO3–O… HO–TeO4 units has been proposed [28]. Hydroxyl groups can be removed or reduced by various methods such as special drying procedures before melting, melting under higher O2 pressures, bubbling of O2 into the melt or replacing some of the starting oxides by halides or fluorides [29,30]. Melting under O2 atmosphere in the present case could only partially remove the OH bands. Efforts are underway to bubble O2 gas into the glass melt and partially replace the starting oxides with fluorides. 4.2.2. (b) Refractive index The guiding characteristics of a fiber are determined by the profile of the optical mode in the fiber, itself related to the refractive indices of the core and cladding glasses [31]. The refractive index of TZBa glasses falls in the vicinity of 2.0 at 1.55 micron, approximately 1% higher than for TZN glasses probably due to a higher density. However, the index

Table 2 Wavelength vs. Refractive Index values of TeO2–ZnO–BaO glasses.

λ (nm) 590 630 1050 1550

949

BaO 05

BaO 10

BaO 15

BaO 20

Refractive Index (n ± 0.003) 2.066 2.056 2.010 1.996

2.056 2.047 2.003 1.990

2.006 1.997 1.952 1.939

1.955 1.947 1.912 1.901

4.2.3. (c) Raman spectroscopy In the Raman spectra of these glasses, the major bands are seen around 450 cm − 1, 660 cm − 1 and 760 cm − 1. Raman Scattering in the intermediate region (300–580 cm − 1) is due to bending modes of Te―O―Te bridges, and is in fact also seen in the same frequency range in SiO2 (“broad band”) due to Si―O―Si bridges. The two higher frequency peaks correspond to stretching vibrational modes of Te―O bonds, at 660 cm − 1 for bridging oxygen (bonded to one Te ion on either side) and at 760 cm − 1 for non-bridging oxygen ions (bonded to only one Te ion and charge compensated by either Zn 2 + or Ba 2 + [12,33,34]). The TeO2 glass network is mainly composed of TeO4 trigonal bipyramid (TBP) structural units, in which each oxygen atom is shared between two TBP units. Addition of modifiers leads to the transformation of TeO4 units into TeO3+σ polyhedral units [33]. Compared with TeO4, TeO3 units contain a non-bridging oxygen ion (NBO) with a double bond to tellurium, two bridging oxygen ions and a lone electron pair. These TeO3 and TeO4 bonds form a continuous network by sharing a bridging bond. The type (alkali, alkaline earth, etc.) and concentration of the modifier oxide in the glass matrix determine the number distribution/density of various TeO3 units. In Fig. 4, the addition of BaO is seen to modify the shape, position and amplitude of the different peaks. To better study the trends with BaO content, the Raman spectra were deconvoluted into different Gaussian peaks labeled A, B, C, D, E, corresponding respectively to Raman shifts of ~ 300 cm − 1, ~450 cm − 1, 605 cm − 1, 665–685 cm − 1 and 750–770 cm − 1 respectively. Band B is found almost at the same frequency as the “broad band” in SiO2, which is due to the bending vibrational mode of Te―O―Te linkages. Accordingly, it is seen to decrease in intensity with increasing BaO (15% and 20%) and decreasing TeO2 content (65% and 60%) (Fig. 5). The relative ZnO content correspondingly increases, creating Te―O―Zn linkages believed to be responsible for band A. This explanation is based on an earlier observation of such linkages in a binary TeO2―ZnO glass [33] but also on the fact that the Zn―O bond is partially covalent while the Ba―O bond is mostly ionic [35]. Bands C and D are characteristic of tellurite glasses with a high TeO2 content and have been assigned respectively to symmetric and antisymmetric stretching vibrations in TeO4 trigonal bi-pyramids (TBP) [34]. The larger one of the two, band D, can therefore be used to estimate the fraction of TeO4 TBP or, equivalently, the degree of connectivity of tellurite glasses [11,34,36]. Band E has been assigned to stretching vibrations of Te-O - bond in TeO3+σ and TeO3- trigonal pyramids (TP), not fully bonded and therefore at higher frequency. The evolution of the B, D and E bands reveals the structural modifications of the glass network resulting from the addition of BaO and reduction of TeO2. To summarize the above results, the integrated intensities of the band B and the ratio of the integrated intensities for bands E and D are plotted in Fig. 5 as a function of BaO or TeO2 content. The effect of Na2O and BaO on the glass network are similar, namely both modifiers result in the weakening or breaking of Te―O bonds and the formation of non-bridging oxygen (NBO) [22]. However, they transform the network structure very differently. As mentioned earlier, Na2O tends to open and polymerize the network to create chain fragments, rendering the glass more fragile and leading to a lower Tg, while BaO tends to reinforce the three-dimensional nature of the network and make it more dense, due in particular to the large size of the Ba ion, leading to a higher Tg. If Raman spectroscopy allows identifying the structural changes brought about by changes in composition, the

35 75TeO2 -20ZnO-05BaO

30 25 20 15 10 5 0 200

400

600

800

1000

Raman gain coefficient relative to SiO2

N. Manikandan et al. / Journal of Non-Crystalline Solids 358 (2012) 947–951

Raman gain coefficient relative to SiO2

950

40 70TeO2-20ZnO-10BaO

35 30 25 20 15 10 5 0 200

400

Raman gain coefficient relative to SiO2

Raman gain coefficient relative to SiO2

50 65TeO2 -20ZnO-15BaO

40

30

20

10

0 200

400

600

600

800

1000

Raman Shift (cm-1)

Raman Shift (cm-1)

800

60

E

60TeO2 -20ZnO-20BaO

50 40 D

30 20

A

B

10

C

0 200

1000

400

600

800

1000

Raman Shift (cm-1)

Raman Shift (cm-1)

Fig. 4. Deconvoluted Raman Spectra of TeO2–ZnO–BaO glasses.

same changes can be used to optimize the Raman gain for the applications mentioned in the introduction, such as Raman fiber lasers and Raman amplifiers. In Table 3, we present the Raman gain coefficient (gR) obtained from the amplitude of the E band at 760 cm− 1. As proposed in Ref. [20,37,38], we estimate the Raman gain coefficient (gR) by comparing the maximum spontaneous Raman scattering intensity of silica glass at 440 cm− 1 with that of TZBa glasses at 760 cm− 1. Keeping the same sample geometry and experimental conditions and making the appropriate correction for reflectivity leads to the spectral behavior of gR shown in Fig. 4 and in Table 2 for 760 cm− 1. The effect of composition could be clearly observed from the

TeO2 concentration (%) 75

70

65

60

55 1.05

1.00

1.00

0.95

0.95

0.90

0.90

0.85

0.85

0.80

0.80

0.75

0.75

0.70

0.70

0.65

0.65 25

0

5

10

15

20

ID/IE

Intensity (a.u.)

1.05

80

BaO concentration (%) Fig. 5. Variation of (i) Integrated intensity of band B (solid circles) and (ii) Integrated intensity ratio between band D and E with BaO concentration (open circles).

data presented in Table 2. Thus, increasing BaO concentration from 5 mol.% to 20 mol. % increases the formation of TeO3 structural units and consequently the gain coefficient value at 760 cm− 1 increases from 34×10− 13 to 49×10− 13 m/W. 5. Conclusions TZBa tellurite glasses (TeO2―ZnO―BaO) were synthesized and their thermal and optical properties were studied as a function of composition (TeO2/BaO). FTIR measurements show that these glasses transmit over a wide range of wavelengths (0.6–6 μm). Ellipsometric measurements show that the refractive index decreases with increasing BaO and decreasing TeO2 content. Both the introduction of Na + and Ba + modifies or breaks Te―O bonds in the TeO2 network, creating nonbridging oxygen (NBO) and TeO3 molecular units. However, while Na tends to open the network and render the glass more “fragile”, Ba tends to make it denser, stronger. This is reflected in the opposite shifts of the glass transition in the two families of tellurite glasses, with Tg increasing in Ba-containing tellurites but decreasing in Na-containing tellurites. The increase in Tg in the former indicates an increase in the cross-linking network density, in spite of the growth of TeO3 units. Also the increase in the BaO/TeO2 ratio results in a corresponding increase in the ZnO/TeO2 ratio leading to the formation of more

Table 3 Estimated Raman gain coefficient values at 760 cm− 1. BaO concentration (mol. %)

Raman Gain Coefficient (gR) × 10− 13 m/W

5 10 15 20

34 39 45 49

N. Manikandan et al. / Journal of Non-Crystalline Solids 358 (2012) 947–951

Te―O―Zn linkages as seen by a additional peak growing below ~400 cm− 1 in the Raman spectrum. The thermal stability of the TeO2ZnO network was also found to improve with increasing BaO concentration paving the way for better quality fibers. Finally, the Raman gain coefficient estimated at 760 cm− 1 was found to be more than 35–50 times that of silica and increasing with BaO content, making these TZBa glasses attractive candidates for Raman-based fiber lasers and amplifiers in the mid-IR. Acknowledgments This work was supported through a subcontract from Fibertek Inc. on a contract from the Air Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/RX). One of us (NM) was supported by a grant from the Ceramics program at the National Science Foundation (Division of Materials Research). We also wish to acknowledge the contribution of the Kiger glass company (SC) which suggested the TZB family of tellurite glasses and contributed technical information for the synthesis of these glasses. References [1] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1994) 187. [2] D.R. Ulrich, J. Am. Ceram. Soc. 47 (1964) 595. [3] H. Burger, K. Kneipp, H. Hobert, W. Vogel, V. Kozhukharov, S. Neov, J. Non-Cryst. Solids 151 (1992) 134. [4] G. Senthil Murugan, T. Suzuki, Y. Ohishi, Y. Takahashi, Y. Benino, T. Fujiwara, T. Komatsu, Appl. Phys. Lett. 85 (2004) 3405. [5] R. Stegeman, L. Jankovic, H. Kim, C. Rivero, G. Stegeman, K. Richardson, P. Delfyett, Y. Guo, A. Schulte, T. Cardinal, Opt. Lett. 28 (2003) 1126. [6] A.E. Miller, K. Nassau, K.B. Lyons, M.E. Lines, J. Non-Cryst. Solids 99 (1988) 289. [7] A. Mori, H. Masuda, K. Shikano, K. Oikawa, K. Kato, M. Shimizu, Electron. Lett. 37 (24) (2001) 1442. [8] A. Mori, H. Masuda, K. Shikano, M. Shimizu, J. Lightwave Technol. 21 (5) (2003) 1300. [9] G. Senthil Murugan, Suzuki Takenobu, Y. Ohishi, Appl. Phys. Lett. 86 (2005) 161109. [10] P. Sarjeant, R. Roy, J. Am. Ceram. Soc. 50 (1967) 500. [11] T. Sekiya, N. Mochida, A. Ohtsuka, J. Non-Cryst. Solids 168 (1994) 106.

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