Kinetic Study and Optical Investigations of some TeO 2 -GeO 2 Glasses V. Kalem 1, G. Özen 2, F. Altın 1, M. L. Öveçoğlu 1, M. R.

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1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Kinetic Study and Optical Investigations of some TeO 2 -GeO 2 Glasses V. Kalem 1, G. Özen 2, F. Altın 1, M. L. Öveçoğlu 1, M. R. Özalp 1 1 Dept. of Materials Science and Eng., Istanbul Technical University 34469, Istanbul, TURKEY 2 Dept. of Physics, Istanbul Technical University 34469, Istanbul, TURKEY Keywords: TeO 2 -GeO 2 glasses, crystallization behavior, optical energy gap. Abstract. On the basis of DTA analyses of 0.9TeO 2 0.1GeO 2 and 0.8TeO 2 0.2GeO 2 glasses, the Avrami parameters were determined to be 3 indicating that the mechanism for the crystallization is bulk crystallization. DTA analyses were carried out at different heating rates and two crystallization peaks were observed for both of the glass compositions. The crystallization activation energies were determined by using the variation of peak temperatures with the heating rate. The absorption spectra measured between nm wavelength region were used to determine the optical band-gap and Urbach energies of the glasses. It is found that they both are a function of glass composition. Introduction Tellurite glasses have many useful properties such as a wide transmission region (0.35 to 5 µm), the lowest maximum phonon energy among the common oxide glasses and a large refractive index, compared to both silicate and fluoride glasses [1]. Furthermore, they are capable of incorporating large concentrations of rare-earth ions into the matrix [2]. Owing to these characteristics, they are potential candidates as hosts for infrared and infrared to visible upconversion lasers [1, 3]. In the present investigation, the crystallization kinetics of 0.9TeO 2 0.1GeO 2 and 0.8TeO 2 0.2GeO 2 glasses and the effect of GeO 2 content on the optical band-gap and Urbach energies have been determined using differential thermal analysis (DTA) and ultraviolet-visible-near-infrared (UV/VIS/NIR) absorption techniques, respectively. Experimental Procedure Tellurite glasses were prepared with the compositions of (1-x) TeO 2 + x GeO 2 where x = 0.10 and 0.20 in molar ratio. All chemicals used were high purity (99.995%) reagent grade of TeO 2 and GeO 2 (from Chempur Co.). Batches of 6 g size were thoroughly mixed and melted in a platinum crucible with a closed lid at C for 1 h in an electrically heated furnace in air atmosphere. To ensure homogeneity, the cast was crushed, pulverized and remelted at the same temperature for an additional 1 h. Following this, the crucible with refined melt was quenched in water. Differential thermal analysis (DTA) measurements were carried out in a Rigaku Thermoflex DTA apparatus equipped with PTC-10A temperature control unit. The samples, about 20 mg, were heated at the heating rates of 5, 10, 15, and 20 C/min in the temperature range between room temperature and 750 C. The crucibles used were matched pairs of platinum, and temperature precision was ± 1 C. The optical absorption of the glasses having the thickness of 1,5 ± 0,5 mm was recorded with a Shimadzu UV-VIS-NIR 3101 PC spectrometer in the wavelength range of nm. The measurements were carried out at room temperature. Results and Discussion DTA experiments were carried out on glass samples to investigate the thermal properties such as glass transition temperature, T g, and crystallization peak temperature, T p. The crystallization All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-21/02/16,06:34:59)

2 1920 Euro Ceramics VIII behavior of the glasses were investigated under non-isothermal conditions with the samples heated at several uniform rates and using an analysis developed for non-isothermal crystallization studies [4,5]. For different heating rates, h, the glass transition temperature, T g, and the crystallization peak temperatures, T p, of the 0.9TeO 2-0.1GeO 2 glass are presented in Table 1 and Fig. 1. Table 1. Heating rate, h, glass transition, T g, and crystallization peak, T p, temperatures of the 0.9TeO 2-0.1GeO 2 glass sample measured from the DTA curves. h [ C/min.] T g [ C] T p1 [ C] T p2 [ C] T p1 Exothermic (d) T g T p2 Endothermic (c) (b) (a) Temperature ( C) Fig. 1. DTA curves of the 0.9TeO 2-0.1GeO 2 glass scanned at the heating rates of: (a) 5 C/min., (b) 10 C/min., (c) 15 C/min., and (d) 20 C/min. Two exothermic peaks were observed for the 0.9TeO 2-0.1GeO 2 glass. As seen in Figure 1, the faster the heating rates, the higher the peak temperatures and the larger the peak heights become. The crystallization activation energy, E, can be determined from the variation of crystallization peak temperature, T p with the heating rate, h, using the following equation [5] ln h = E / RT + p c, (1) where R is the universal gas constant and c is a constant. Fig. 2 shows the graph of ln h versus (1/T p ) at different heating rates for 0.9TeO 2-0.1GeO 2 and 0.8TeO 2-0.2GeO 2 glasses. The values of activation energies of crystallization were determined as 227 kj/mole and 340 kj/mole for 0.9TeO 2-0.1GeO 2 glass and 256 kj/mole and 416 kj/mole for 0.8TeO 2-0.2GeO 2 glass for the first and second crystallization peaks, respectively.

3 Key Engineering Materials Vols Fig. 2. Plot of ln h versus 1/T p of: (a) 0.8TeO 2-0.2GeO 2 glass sample and (b) 0.9TeO 2-0.1GeO 2 glass sample. ( : 1.peak, : 2.peak) The Avrami parameter, n, which is an integer depending on the shape and dimensionality of crystal growth, was determined by using the Augis and Bennett equation [6] 2 n = (2.5/ T )( RT / E), (2) p where T is the width of the crystallization exotherm at half maximum. Table 2 shows the T and resulting n values for two exothermic peaks at different heating rates. Table 2. T and n values of 0.9TeO 2-0.1GeO 2 and 0.8TeO 2-0.2GeO 2 glass samples. h ( o C/min) 0.9TeO 2-0.1GeO 2 0.8TeO 2-0.2GeO 2 First exotherm [E = 227 kj/mol] Second exotherm [E = 340 kj/mol] First exotherm [E = 256 kj/mol] Second exotherm [E = 416 kj/mol] T ( o C) n T ( o C) n T ( o C) n T ( o C) n 5 17,5 2, , ,65 10,5 3, , ,78 14,5 2, , , ,85 14,5 2,87 11,5 3, ,5 2,90 14,5 2, , ,27 The Avrami parameters of two crystallization peaks for both of the glass compositions are close to 3 indicating three-dimensional bulk crystallization. The effect of the GeO 2 content on the optical band-gap and Urbach energies of the glasses were obtained from the absorption spectra given in Fig. 3.The absorption coefficient [α(ω)] is given by [7] α (ω) ( ) hω = constant hω E opt 2. (3) Thus, the optical band-gaps of the glasses were determined by extrapolating the linear region of the curve, obtained by plotting (αhω) 1/2 in cm -1/2 ev 1/2 units versus (hω) in units of ev, to (αhω) 1/2 = 0. The Urbach energies E were obtained from the slopes of the linear regions of the plots ln[α(ω)] against (hω) :

4 1922 Euro Ceramics VIII hω lnα( ω) = - constant. (4) E Fig. 3. The absorption spectra of the: (a) 0.8TeO 2-0.2GeO 2 and (b) 0.9 TeO 2-0.1GeO 2 glasses. The variations of the E opt and E with the GeO 2 content are listed in Table 3. Table 3. Dependence on the GeO 2 content of optical band gap, E opt, and the Urbach energies, E, of the TeO 2 - GeO 2 glasses. Composition [mole %] E opt [ev] E [ev] TeO 2 GeO ,85 0, ,92 0,35 Summary In summary, crystallization kinetics of the 0.8TeO 2-0.2GeO 2 and 0.9TeO 2-0.1GeO 2 glasses were studied and activation energies of the crystallization process were determined. Two peaks were observed in the DTA graphs. The optical band gap values of the TeO 2 -GeO 2 glasses increase from 1.85eV to 1,92eV when the GeO 2 content in the glass was increased from 10 mol% to 20 mol%, while the Urbach energy values decrease from 0,38eV to 0,35eV. Detailed microstructural investigations on crystallizing phases have been carried out by V. Kalem et al. [8]. Acknowledgements The authors gratefully acknowledge the DPT (State Planning Organization) for the funding of this research through Advanced Technologies Project No : 2001K References [1] Z.Pan, S.H. Morgan : J. Non-cryst. Solids Vol. 210 (1997) p [2] G.Özen, J-P. Dennis, M.Genotelle and F. Pellé : J.Phys.-Condens. Matter. Vol.7 (1995) p [3] H. Takebe, K. Yoshino, T. Murata, K. Morinaga, J. Hector, W.S. Brocklesby, D.W. Hewak, J.Wang, and D.N. Payne : Applied Optics-LP Vol. 36 (1997) p [4] K.Cheng : Materials Science and Engineering B Vol.60 (1999) p [5] R.El-Mallawany : J. Mat. Science Mat. in Electronics Vol. 6 (1995) p. 1. [6] C.S. Ray, D.E. Day : J. Am. Ceram. Soc. Vol 73 (1990) p [7] R. A. H. EL-Mallawany : Tellurite Glasses Handbook (CRC Press, Boca Raton 2002), chap.9. [8] V.Kalem, F. Altın, M. L. Öveçoğlu, G. Özen, B. Özkal : Key Engineering Materials, in-press.

5 Euro Ceramics VIII / Kinetic Study and Optical Investigations of some TeO 2 -GeO 2 Glasses / DOI References [3] H. Takebe, K. Yoshino, T. Murata, K. Morinaga, J. Hector, W.S. Brocklesby, D.W. Hewak, J.Wang, and D.N. Payne : Applied Optics-LP Vol. 36 (1997) p /AO