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Structure and properties of Ge–Sb–S–CsCl glass–ceramics
Materials Chemistry and Physics xxx (2014) 1e5
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structure and properties of GeeSbeSeCsCl glasseceramics Xinyu Yang a, b, *, Rongping Wang b, Zhiyong Yang b, Siwei Xu b, c a
Faculty of Chemistry & Material Engineering, Wenzhou University, Zhejiang Province, Wenzhou 325027, PR China Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia c College of Applied Sciences, Beijing University of Technology, Beijing 100124, PR China b
h i g h l i g h t s A method on the modification of the material properties of chalcolgenide glasses. An evidence on how different sub-phases were formed. A study on the mechanical hardness and optical losses in mid-infrared region.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2014 Received in revised form 29 April 2014 Accepted 18 May 2014 Available online xxx
We annealed GeeSbeSeCsCl glass and investigated the evolution of the structural, optical and mechanical properties as a function of annealing temperature. It was found that, for the glass annealed at 280 C for 210 h, a CsCl crystalline phase with mono-dispersedly spherical shape was formed; while for the glass annealed at 370 C for 5 h, both b-GeS2 and CsCl crystalline phases with irregular distribution of large grains was observed. While the density and the microhardness of the glasses increase with increasing annealing temperature, it was found that, optical loss in the glass annealed at 370 C for 5 h was much larger than that annealed at 280 C for 210 h. © 2014 Elsevier B.V. All rights reserved.
Keywords: Chalcogenide glasses Annealing Crystalline structure Optical properties Mechanical properties
1. Introduction Chalcogenide glasses are attractive because of their unique properties such as wide transparency (up to far infrared region), high refractive index, large third-order nonlinearities, as well as low photon energy [1e3], which have resulted in various applications in the fields of infrared optics, nonlinear optics, optical sensors, optical amplifiers and so on [4e8]. However, the challenging issues in the use of chalcogenide glasses are their poor mechanical properties, low resistance to thermal shock and low laser damage threshold [9]. While the light with high power is applied to the material, the undesired properties usually deteriorate the performance of the chalcogenide-based device. Therefore it is essential to further improve the properties of the chalcogenide glasses for better applications. * Corresponding author. Faculty of Chemistry & Material Engineering, Wenzhou University, Zhejiang Province, Wenzhou 325027, PR China. E-mail address: [email protected] (X. Yang).
Recently, a French group [10,11] reported that enhanced mechanical properties can be achieved through inducing micro- or nano-scale crystals in chalcogenide glasses and forming so-called glasseceramics. This has opened up a new approach to the modification of the material properties of chalcolgenide glasses. For the application of chalcogenide glasseceramics in photonics, one of the key points is to design the material composition ensuring refractive index of the crystalline precipitation similar to that of the residual glass network, and thus no significant optical loss can be induced due to negligible refractive index contrast. On the other hand, it has been found that doping of Cl, F and I ions into the glass matrix can cut the glass network. Rüssel et al. [12,13] reported that the crystallization of halides like CaF2 and BaF2 from the glass matrix can increase the viscosity near the crystals, forming a diffusion barrier around each crystal, and in turn hinder further crystal growth. The halides nucleation and crystallization can be totally frozen in the glass. Therefore, the doping of halide ions can control the crystal growth effectively in the glass, and push the bandgap to more lucent region. Since chalcogenide glasses possesses excellent
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Please cite this article in press as: X. Yang, et al., Structure and properties of GeeSbeSeCsCl glasseceramics, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.029
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X. Yang et al. / Materials Chemistry and Physics xxx (2014) 1e5
transparent in the infrared spectral region, the doping halides can potentially create new materials which are transparent in a wider region from visible to far infrared. The present paper concentrates on chemically stoichiometric 62.5GeS2e12.5Sb2S3e25CsCl (mol%) glass that is located near the border of the glass-forming region. Therefore, through careful control of annealing program, the stoichiometric sub-compounds like GeS2 and CsCl can be crystallized into micro- or nano-scale crystals depending on the thermal processing conditions. In fact, previous results [10] have shown that, while high content CsCl are dissolved in the GeeSbeS glass, long time thermal processing could induce a large amount of CsCl crystals that are undesirable due to a large refractive index contrast between CsCl and the host glass. Therefore, understanding the correlation between annealing conditions and formation of CsCl crystals is essential to prepare high quality glasseceramics with low optical loss. We aimed at exploring how to control the precipitation of the crystalline grains in order to avoid the undesired CsCl crystals. Through the careful control of the annealing program, we tried to present the clear evidence on the formation of different sub-phrase from X-ray diffraction, Raman scattering, and transmission electron microscopy measurements. The effect of the sub-phase on the mechanical hardness and optical scattering losses in mid-infrared spectral region was investigated as well. 2. Experiments Bulk glass with the stoichiometric composition of 62.5GeS2e12.5Sb2S3e25CsCl (mol%) was fabricated by conventional melt-quenching method employing high purity elements (Ge, Sb, and S, 5N) and compounds (CsCl, 3N) as starting materials. No further purification process was performed during the glass fabrication. The mixture of starting materials was then sealed in a vacuum silica tube (106 Torr) with an inner diameter of 15 mm. The mixture was subsequently melted at 900 C for 24 h in a rocking furnace. Then, the melt was removed from the furnace and rapidly quenched in water to form bulk glass. The as-obtained glass was annealed at 20 C below glass transition temperature (Tg) for 15 h to reduce the inner stress induced by the quenching process. The glass was finally cut into disks (F15 mm 1.5 mm) and then polished with Al2O3 powders. The glass transition temperature (Tg) and crystallization temperature (Tp) were measured using a Mettler-toledo differential scanning calorimetry (DSC) at a heating rate of 10 C min1 with the protection of nitrogen gas flux. Based on the DSC results, annealing conditions were determined to create glasseceramics. The transmission spectra of the base glass and glasseceramics were recorded by a Cary 5000 UVeViseIR double beam spectrophotometer at room temperature. X-ray diffraction measurements were performed at a Burker X-ray diffractometer using Cu Ka radiation from 10 to 70 with a step of 0.02 in order to identify the crystalline phase in the glasseceramics. Raman scattering spectra were measured using a Horiba Jobin Yvon 6400 spectrometer with an 830 nm laser as an excitation source at room temperature. A FEI Tecnai F20 transmission electron microscope (TEM) was used to characterize the morphology and size distribution of crystalline phase in the glasseceramics, and an energy dispersive X-ray spectrometer (EDX) attached to TEM was used to map the elemental composition at a microscopic scale.
Fig. 1. DSC curves of the glass at a heating rate of 10 C min1.
one endothermic and two exothermic peaks are evident. Here, the endothermic peak with an onset temperature at 260 C was attributed to Tg, and two exothermic peaks located at 378 C and 401 C to the glass crystallization temperature Tp. Obviously the large temperature difference between Tp and Tg could result in slow growth of the crystalline nucleus, and thus high stability of the glass [14,15]. Glasseceramics can be created at the relatively lower annealing temperature for longer time or relatively higher temperature for shorter time [16]. We therefore annealed the glasses at 280 C (Tg þ 20 C) for different duration and finally at 370 C for 5 h to observe the crystallization behavior in the glasseceramics. Curves A and B, as shown in Fig. 2, are XRD patterns of the glass annealed at 280 C for 210 h and 370 C for 5 h, respectively. The curve A shows one strong diffraction peaks at 30.6 corresponding to the (110) peak of cubic CsCl crystal. This indicates that only CsCl crystals can be formed at a low annealing temperature of 280 C. However, with increasing annealing temperature, sharper
3. Results and discussion 3.1. Crystallization behavior Fig. 1 represents a DSC curve of the glass at a heating rate of 10 C min1, from which three characteristic transitions including
Fig. 2. XRD curves of the chalcogenide glass at: A. 280 C/210 h; B. 370 C/5 h. For comparison, the XRD curve of the un-annealed glass is also shown.
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diffraction peaks corresponding to both CsCl and GeS2 crystalline phases can be observed, as shown in the curve B. This is in agreement with the fact that CsCl and GeS2 have different crystallization temperatures. Relevant reports showed that the crystalline phase of CsCl can be formed in a temperature range of 167 Ce277 C [17], however, the crystallization temperature of GeS2 might be above 350 C [18,19]. Obviously, lower annealing temperature can induce the formation of CsCl crystals, while GeS2 crystals can only be created at higher annealing temperature. On the other hand, the diffraction peak intensity of the curve A is weaker compared with that of the curve B, indicating that the crystalline content of the glass annealed at lower temperature (curve A) is less than that annealed at higher temperature (curve B). To further evaluate the crystallization process, Raman scattering spectra of the glasses annealed at different temperatures were measured. As shown in Fig. 3, no obvious difference in Raman spectrum of the glass annealed at 280 C for 210 h was observed compared with that of un-annealed glass [20,21]. This might be weak Raman scattering signal of small amount of CsCl crystals hidden in the strong and broad Raman scattering bands of the glass. However, the sharp crystalline peaks were found to appear in Raman spectrum of the glass annealed at 370 C for 5 h. The peak at 127 cm1 was assigned to CsCl crystalline phase [22], and the peaks locate at 344 cm1, 360 cm1, 411 cm1, and 434 cm1 could be assigned to Raman vibrations of b-GeS2 crystalline phase [23,24]. In fact, the presence of the vibrational peak at 411 cm1 was considered as a direct evidence of b-GeS2 crystalline phase in the chalcogenide glass in Ref. [25]. Moreover, two characteristic peaks at 280 cm1 and 308 cm1, which were assigned to the main vibrational modes for c-Sb2S3 crystalline phase [26], were not observed in the spectra. This suggests that there is no Sb2S3 crystalline phase in the glass even annealed at high temperature. From the present spectra, we are not able to find clear evidence whether Csþ or Cl ions interact with GeS2 or Sb2S3, forming different structural units. However, in combination with Fig. 2, the present results demonstrate that CsCl crystals can be created at lower annealing temperature while both CsCl and GeS2 crystals can be formed at higher annealing temperature.
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To clarify the morphology, distribution, and composition of crystalline phases as a function of annealing temperature, TEM measurement was performed. The bright field image of the glass annealed at 280 C for 210 h was shown in Fig. 4(a), from which a number of white nanocrystals as marked by red (in the web version) arrow were nearly spherical shape and well-dispersed. Fig. 4(b) revealed that the diameter distribution of these white nanocrystals was from 25 to 50 nm. The averaged diameter of these nanocrystals from around 30 nanocrystals in the TEM image was 40 ± 2 nm. EDX spectrum recorded on the same area of TEM morphology image, as shown in Fig. 4(c), confirmed that the composition of the chalcogenide glass was same as that of the starting material. In contrast, Fig. 4(d) showed the morphology of the glass annealed at 370 C for 5 h. It was found that the white parts corresponding to the crystalline phases were irregularly distributed, and some of them were dispersed like a spherical shape while others were gathered together into ribbon structure. The results indicate that high annealing temperature of 370 C could induce excessive crystalline growth, leading to out of control in morphology and distribution of the crystalline grains. 3.2. Optical transmittance and optical loss Fig. 5 is the transmittance spectra of the un-annealed glass, and the glasses annealed at 280 C for 210 h and 370 C for 5 h, respectively. It was found that, the band edge shifts towards long wavelength and the transmission decreases with increasing annealing temperature. Both of them could be due to Mie scattering induced by increasing number and size of the crystalline grains in the glasseceramics. This is also in agreement with the TEM analysis. Obviously, while the transmission of the glass annealed at high temperature of 370 C becomes almost zero at 2.0 mm, careful control of the annealing process at a relatively lower temperature for a long time is essential to create glasseceramics with reasonable optical transmission as well as improved mechanical properties. We measured optical losses of the un-annealed glass and annealed glasses. For that, a Bruker vertex 80v FT-IR apparatus with a cooled MCT detector was used at varied wavelengths from 1.25 to 8.0 mm. The optical loss for the un-annealed glass is around 0.77 dB cm1 at 3.8 mm, and this increases to 1.02 dB cm1 for the glass annealed at 280 C for 210 h and to 2.46 dB cm1 for the glass annealed at 370 C for 5 h. It is well known that, the large optical loss would be induced if the crystals with large refractive index contrast from the host glass matrix are created. The present results indicate that, although CsCl crystals have a large refractive index difference from the glass matrix, the amount of the crystals in the glass annealed at 280 C for 210 h could be insignificant. Therefore, the optical loss induced by the CsCl crystals is relatively smaller. This is also in agreement with the XRD data where no strong crystalline XRD peaks can be observed. However, while the annealing temperature is increased to 370 C, a number of the crystalline grains were significantly increased as evident by the strong XRD peaks in Fig. 2, and thus the optical loss is significantly enhanced. The results clearly demonstrate that, while CsCl is undesired in glasseceramics in terms of its large refractive index contrast with the host glass matrix, control of annealing temperature might be critical to suppress the formation of undesired phase, leading to the glasseceramics with relatively low optical loss. 3.3. Mechanical properties
Fig. 3. Raman scattering spectra of the chalcogenide glasses: (a) un-annealed glass (black); annealed at 280 C/210 h (green); annealed at 370 C/5 h (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6 is the density of the glasses annealed at 280 C for different times. For comparison, the density of the glass annealed at 370 C for 5 h was also displayed in Fig. 6. The density increases from 3.244 g cm3 for the un-annealed glass to 3.259 g cm3 for the glass
Please cite this article in press as: X. Yang, et al., Structure and properties of GeeSbeSeCsCl glasseceramics, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.029
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Fig. 4. TEM analysis of (a) TEM morphology image at 280 C/210 h; (b) Size distribution image at 280 C/210 h; (c) EDX spectrum; (d) TEM morphology image at 370 C/5 h.
annealed at 280 C for 210 h, and further increased to 3.345 g cm3 for the glass annealed at 370 C for 5 h. The increase of the density comes from the formation of the crystalline phases in the residual glass matrix. For example, the formation of CsCl crystals with a high density of 3.99 g cm3 could finally increase the density of the whole glasseceramics. The large change of the density of the glass annealed at high temperature of 370 C also implies that the number of the crystals annealed at high temperature is higher, which is in agreement with the XRD results. The microhardness of the chalcogenide glasses annealed at 280 C for different durations was also measured using Vickers microindenter. As can be seen in Fig. 7, the hardness was improved with increasing annealing time from 1.06 GPa for the un-annealed glass to 1.30 GPa for the glass annealed at 280 C for 210 h, and finally to 2.02 GPa for the glass annealed at 370 C for 5 h (not shown in Fig. 7). Hunger et al. [27] investigated the origin of the increasing hardness in glasseceramics and concluded that the crystalline particles contributed to the change of the hardness in the glass due to the stress formed around the crystalline particles. In our case, we found that, with increasing annealing time, the size
Fig. 5. UVeViseIR transmission spectra of the chalcogenide glasses annealed at: 280 C/210 h; 370 C/5 h. For comparison, the spectrum of the un-annealed glass was also shown.
and volume fraction of the crystalline grains increased in the glass matrix as evident by XRD, Raman scattering, and TEM observation. Similarly, the hardness difference between un-annealed and annealed at 280 C for the different durations should also originate from the crystalline grains in the chalcogenide glass. These crystalline particles were well dispersed in the glass matrix, and contribute to the increase of hardness in Fig. 7. On the other hand, Lin et al. [28e30] investigated the mechanical properties of the chalcogenide glasses at different annealing times by Vickers indentations. They discovered that the formation of crystalline particles inside the glass matrix could decrease the crack length, and the crack propagation could be greatly inhibited with increasing annealing times. This indicates that a progressive strengthening can be realized by a ceramization process in the chalcogenide glass. Following these previous investigations [27e30], TEM image in Fig. 4(a) with well-dispersed nanocrystals implies that the resistance to crack propagation is stronger in the glass annealed at 280 C for 210 h. Furthermore, we observed in Fig. 5 that OeH and SeH bonds were eliminated with increasing annealing time. The reduced amount of OeH and SeH bonds could result in better network connectivity, and thus improved glass hardness [31].
Fig. 6. The density of the chalcogenide glass annealed at different annealing times.
Please cite this article in press as: X. Yang, et al., Structure and properties of GeeSbeSeCsCl glasseceramics, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.029
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Fig. 7. the microhardness values of the chalcogenide glass annealed at 280 C for different times.
4. Conclusions In summary, we annealed 62.5GeS2e12.5Sb2S3e25CsCl (mol %) chalcogenide glass at different temperatures of 280 C and 370 C. XRD and Raman scattering spectra showed that, only CsCl crystalline phase was formed in the glass annealed at 280 C for 210 h, while two crystalline phases of b-GeS2 and CsCl can be created in the glass annealed at 370 C for 5 h. TEM analysis indicated that the glass annealed at different temperatures had different morphology and distribution of the crystalline grains, e.g., the glass annealed at 280 C contained uniformly dispersed CsCl crystalline grains with higher transmittance and lower optical loss, whilst the glass annealed at 370 C possessed irregularly distributed b-GeS2 and CsCl crystals with poor transmittance and optical loss. The density and the microhardness of the glasses can be considerably improved by thermal annealing. The present results indicate that, careful control of the annealing process is crucial to create glasseceramics with improved mechanical properties and reasonable optical loss. Acknowledgments The work was financially supported by the National Natural Science Foundation of China (Grant no. 51202166), the Zhejiang Province Natural Science Foundation of China (Grant no. Y4100233), and the Wenzhou University Innovation and Career Project of China (Grant no. DC2012006). References [1] X. Gai, T. Han, A. Prasad, S. Madden, D.Y. Choi, R.P. Wang, D. Bulla, B. LutherDavies, Progress in optical waveguides fabricated from chalcogenide glasses, Opt. Express 18 (2010) 26635e26646. [2] C.G. Lin, S. Dai, C. Liu, B.A. Song, Y.S. Xu, F.F. Chen, Mechanism of the enhancement of mid-infrared emission from GeS2eGa2S3 chalcogenide glasseceramics doped with Tm3þ, Appl. Phys. Lett. 100 (2012) 231910. [3] C.G. Lin, L. Calvez, L. Ying, F.F. Chen, B.A. Song, X. Shen, S.X. Dai, X.H. Zhang, External influence on third-order optical nonlinearity of transparent chalcogenide glasseceramics, Appl. Phys. A 104 (2011) 615e620. e, Mid[4] M. Bernier, V. Fortin, N. Caron, M. EI-Amraoui, Y. Messaddeq, R. Valle infrared chalcogenide glass Raman fiber laser, Opt. Lett. 38 (2013) 127e129.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structure and properties of GeeSbeSeCsCl glasseceramics Xinyu Yang a, b, *, Rongping Wang b, Zhiyong Yang b, Siwei Xu b, c a
Faculty of Chemistry & Material Engineering, Wenzhou University, Zhejiang Province, Wenzhou 325027, PR China Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia c College of Applied Sciences, Beijing University of Technology, Beijing 100124, PR China b
h i g h l i g h t s A method on the modification of the material properties of chalcolgenide glasses. An evidence on how different sub-phases were formed. A study on the mechanical hardness and optical losses in mid-infrared region.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2014 Received in revised form 29 April 2014 Accepted 18 May 2014 Available online xxx
We annealed GeeSbeSeCsCl glass and investigated the evolution of the structural, optical and mechanical properties as a function of annealing temperature. It was found that, for the glass annealed at 280 C for 210 h, a CsCl crystalline phase with mono-dispersedly spherical shape was formed; while for the glass annealed at 370 C for 5 h, both b-GeS2 and CsCl crystalline phases with irregular distribution of large grains was observed. While the density and the microhardness of the glasses increase with increasing annealing temperature, it was found that, optical loss in the glass annealed at 370 C for 5 h was much larger than that annealed at 280 C for 210 h. © 2014 Elsevier B.V. All rights reserved.
Keywords: Chalcogenide glasses Annealing Crystalline structure Optical properties Mechanical properties
1. Introduction Chalcogenide glasses are attractive because of their unique properties such as wide transparency (up to far infrared region), high refractive index, large third-order nonlinearities, as well as low photon energy [1e3], which have resulted in various applications in the fields of infrared optics, nonlinear optics, optical sensors, optical amplifiers and so on [4e8]. However, the challenging issues in the use of chalcogenide glasses are their poor mechanical properties, low resistance to thermal shock and low laser damage threshold [9]. While the light with high power is applied to the material, the undesired properties usually deteriorate the performance of the chalcogenide-based device. Therefore it is essential to further improve the properties of the chalcogenide glasses for better applications. * Corresponding author. Faculty of Chemistry & Material Engineering, Wenzhou University, Zhejiang Province, Wenzhou 325027, PR China. E-mail address: [email protected] (X. Yang).
Recently, a French group [10,11] reported that enhanced mechanical properties can be achieved through inducing micro- or nano-scale crystals in chalcogenide glasses and forming so-called glasseceramics. This has opened up a new approach to the modification of the material properties of chalcolgenide glasses. For the application of chalcogenide glasseceramics in photonics, one of the key points is to design the material composition ensuring refractive index of the crystalline precipitation similar to that of the residual glass network, and thus no significant optical loss can be induced due to negligible refractive index contrast. On the other hand, it has been found that doping of Cl, F and I ions into the glass matrix can cut the glass network. Rüssel et al. [12,13] reported that the crystallization of halides like CaF2 and BaF2 from the glass matrix can increase the viscosity near the crystals, forming a diffusion barrier around each crystal, and in turn hinder further crystal growth. The halides nucleation and crystallization can be totally frozen in the glass. Therefore, the doping of halide ions can control the crystal growth effectively in the glass, and push the bandgap to more lucent region. Since chalcogenide glasses possesses excellent
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Please cite this article in press as: X. Yang, et al., Structure and properties of GeeSbeSeCsCl glasseceramics, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.029
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X. Yang et al. / Materials Chemistry and Physics xxx (2014) 1e5
transparent in the infrared spectral region, the doping halides can potentially create new materials which are transparent in a wider region from visible to far infrared. The present paper concentrates on chemically stoichiometric 62.5GeS2e12.5Sb2S3e25CsCl (mol%) glass that is located near the border of the glass-forming region. Therefore, through careful control of annealing program, the stoichiometric sub-compounds like GeS2 and CsCl can be crystallized into micro- or nano-scale crystals depending on the thermal processing conditions. In fact, previous results [10] have shown that, while high content CsCl are dissolved in the GeeSbeS glass, long time thermal processing could induce a large amount of CsCl crystals that are undesirable due to a large refractive index contrast between CsCl and the host glass. Therefore, understanding the correlation between annealing conditions and formation of CsCl crystals is essential to prepare high quality glasseceramics with low optical loss. We aimed at exploring how to control the precipitation of the crystalline grains in order to avoid the undesired CsCl crystals. Through the careful control of the annealing program, we tried to present the clear evidence on the formation of different sub-phrase from X-ray diffraction, Raman scattering, and transmission electron microscopy measurements. The effect of the sub-phase on the mechanical hardness and optical scattering losses in mid-infrared spectral region was investigated as well. 2. Experiments Bulk glass with the stoichiometric composition of 62.5GeS2e12.5Sb2S3e25CsCl (mol%) was fabricated by conventional melt-quenching method employing high purity elements (Ge, Sb, and S, 5N) and compounds (CsCl, 3N) as starting materials. No further purification process was performed during the glass fabrication. The mixture of starting materials was then sealed in a vacuum silica tube (106 Torr) with an inner diameter of 15 mm. The mixture was subsequently melted at 900 C for 24 h in a rocking furnace. Then, the melt was removed from the furnace and rapidly quenched in water to form bulk glass. The as-obtained glass was annealed at 20 C below glass transition temperature (Tg) for 15 h to reduce the inner stress induced by the quenching process. The glass was finally cut into disks (F15 mm 1.5 mm) and then polished with Al2O3 powders. The glass transition temperature (Tg) and crystallization temperature (Tp) were measured using a Mettler-toledo differential scanning calorimetry (DSC) at a heating rate of 10 C min1 with the protection of nitrogen gas flux. Based on the DSC results, annealing conditions were determined to create glasseceramics. The transmission spectra of the base glass and glasseceramics were recorded by a Cary 5000 UVeViseIR double beam spectrophotometer at room temperature. X-ray diffraction measurements were performed at a Burker X-ray diffractometer using Cu Ka radiation from 10 to 70 with a step of 0.02 in order to identify the crystalline phase in the glasseceramics. Raman scattering spectra were measured using a Horiba Jobin Yvon 6400 spectrometer with an 830 nm laser as an excitation source at room temperature. A FEI Tecnai F20 transmission electron microscope (TEM) was used to characterize the morphology and size distribution of crystalline phase in the glasseceramics, and an energy dispersive X-ray spectrometer (EDX) attached to TEM was used to map the elemental composition at a microscopic scale.
Fig. 1. DSC curves of the glass at a heating rate of 10 C min1.
one endothermic and two exothermic peaks are evident. Here, the endothermic peak with an onset temperature at 260 C was attributed to Tg, and two exothermic peaks located at 378 C and 401 C to the glass crystallization temperature Tp. Obviously the large temperature difference between Tp and Tg could result in slow growth of the crystalline nucleus, and thus high stability of the glass [14,15]. Glasseceramics can be created at the relatively lower annealing temperature for longer time or relatively higher temperature for shorter time [16]. We therefore annealed the glasses at 280 C (Tg þ 20 C) for different duration and finally at 370 C for 5 h to observe the crystallization behavior in the glasseceramics. Curves A and B, as shown in Fig. 2, are XRD patterns of the glass annealed at 280 C for 210 h and 370 C for 5 h, respectively. The curve A shows one strong diffraction peaks at 30.6 corresponding to the (110) peak of cubic CsCl crystal. This indicates that only CsCl crystals can be formed at a low annealing temperature of 280 C. However, with increasing annealing temperature, sharper
3. Results and discussion 3.1. Crystallization behavior Fig. 1 represents a DSC curve of the glass at a heating rate of 10 C min1, from which three characteristic transitions including
Fig. 2. XRD curves of the chalcogenide glass at: A. 280 C/210 h; B. 370 C/5 h. For comparison, the XRD curve of the un-annealed glass is also shown.
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X. Yang et al. / Materials Chemistry and Physics xxx (2014) 1e5
diffraction peaks corresponding to both CsCl and GeS2 crystalline phases can be observed, as shown in the curve B. This is in agreement with the fact that CsCl and GeS2 have different crystallization temperatures. Relevant reports showed that the crystalline phase of CsCl can be formed in a temperature range of 167 Ce277 C [17], however, the crystallization temperature of GeS2 might be above 350 C [18,19]. Obviously, lower annealing temperature can induce the formation of CsCl crystals, while GeS2 crystals can only be created at higher annealing temperature. On the other hand, the diffraction peak intensity of the curve A is weaker compared with that of the curve B, indicating that the crystalline content of the glass annealed at lower temperature (curve A) is less than that annealed at higher temperature (curve B). To further evaluate the crystallization process, Raman scattering spectra of the glasses annealed at different temperatures were measured. As shown in Fig. 3, no obvious difference in Raman spectrum of the glass annealed at 280 C for 210 h was observed compared with that of un-annealed glass [20,21]. This might be weak Raman scattering signal of small amount of CsCl crystals hidden in the strong and broad Raman scattering bands of the glass. However, the sharp crystalline peaks were found to appear in Raman spectrum of the glass annealed at 370 C for 5 h. The peak at 127 cm1 was assigned to CsCl crystalline phase [22], and the peaks locate at 344 cm1, 360 cm1, 411 cm1, and 434 cm1 could be assigned to Raman vibrations of b-GeS2 crystalline phase [23,24]. In fact, the presence of the vibrational peak at 411 cm1 was considered as a direct evidence of b-GeS2 crystalline phase in the chalcogenide glass in Ref. [25]. Moreover, two characteristic peaks at 280 cm1 and 308 cm1, which were assigned to the main vibrational modes for c-Sb2S3 crystalline phase [26], were not observed in the spectra. This suggests that there is no Sb2S3 crystalline phase in the glass even annealed at high temperature. From the present spectra, we are not able to find clear evidence whether Csþ or Cl ions interact with GeS2 or Sb2S3, forming different structural units. However, in combination with Fig. 2, the present results demonstrate that CsCl crystals can be created at lower annealing temperature while both CsCl and GeS2 crystals can be formed at higher annealing temperature.
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To clarify the morphology, distribution, and composition of crystalline phases as a function of annealing temperature, TEM measurement was performed. The bright field image of the glass annealed at 280 C for 210 h was shown in Fig. 4(a), from which a number of white nanocrystals as marked by red (in the web version) arrow were nearly spherical shape and well-dispersed. Fig. 4(b) revealed that the diameter distribution of these white nanocrystals was from 25 to 50 nm. The averaged diameter of these nanocrystals from around 30 nanocrystals in the TEM image was 40 ± 2 nm. EDX spectrum recorded on the same area of TEM morphology image, as shown in Fig. 4(c), confirmed that the composition of the chalcogenide glass was same as that of the starting material. In contrast, Fig. 4(d) showed the morphology of the glass annealed at 370 C for 5 h. It was found that the white parts corresponding to the crystalline phases were irregularly distributed, and some of them were dispersed like a spherical shape while others were gathered together into ribbon structure. The results indicate that high annealing temperature of 370 C could induce excessive crystalline growth, leading to out of control in morphology and distribution of the crystalline grains. 3.2. Optical transmittance and optical loss Fig. 5 is the transmittance spectra of the un-annealed glass, and the glasses annealed at 280 C for 210 h and 370 C for 5 h, respectively. It was found that, the band edge shifts towards long wavelength and the transmission decreases with increasing annealing temperature. Both of them could be due to Mie scattering induced by increasing number and size of the crystalline grains in the glasseceramics. This is also in agreement with the TEM analysis. Obviously, while the transmission of the glass annealed at high temperature of 370 C becomes almost zero at 2.0 mm, careful control of the annealing process at a relatively lower temperature for a long time is essential to create glasseceramics with reasonable optical transmission as well as improved mechanical properties. We measured optical losses of the un-annealed glass and annealed glasses. For that, a Bruker vertex 80v FT-IR apparatus with a cooled MCT detector was used at varied wavelengths from 1.25 to 8.0 mm. The optical loss for the un-annealed glass is around 0.77 dB cm1 at 3.8 mm, and this increases to 1.02 dB cm1 for the glass annealed at 280 C for 210 h and to 2.46 dB cm1 for the glass annealed at 370 C for 5 h. It is well known that, the large optical loss would be induced if the crystals with large refractive index contrast from the host glass matrix are created. The present results indicate that, although CsCl crystals have a large refractive index difference from the glass matrix, the amount of the crystals in the glass annealed at 280 C for 210 h could be insignificant. Therefore, the optical loss induced by the CsCl crystals is relatively smaller. This is also in agreement with the XRD data where no strong crystalline XRD peaks can be observed. However, while the annealing temperature is increased to 370 C, a number of the crystalline grains were significantly increased as evident by the strong XRD peaks in Fig. 2, and thus the optical loss is significantly enhanced. The results clearly demonstrate that, while CsCl is undesired in glasseceramics in terms of its large refractive index contrast with the host glass matrix, control of annealing temperature might be critical to suppress the formation of undesired phase, leading to the glasseceramics with relatively low optical loss. 3.3. Mechanical properties
Fig. 3. Raman scattering spectra of the chalcogenide glasses: (a) un-annealed glass (black); annealed at 280 C/210 h (green); annealed at 370 C/5 h (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6 is the density of the glasses annealed at 280 C for different times. For comparison, the density of the glass annealed at 370 C for 5 h was also displayed in Fig. 6. The density increases from 3.244 g cm3 for the un-annealed glass to 3.259 g cm3 for the glass
Please cite this article in press as: X. Yang, et al., Structure and properties of GeeSbeSeCsCl glasseceramics, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.029
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X. Yang et al. / Materials Chemistry and Physics xxx (2014) 1e5
Fig. 4. TEM analysis of (a) TEM morphology image at 280 C/210 h; (b) Size distribution image at 280 C/210 h; (c) EDX spectrum; (d) TEM morphology image at 370 C/5 h.
annealed at 280 C for 210 h, and further increased to 3.345 g cm3 for the glass annealed at 370 C for 5 h. The increase of the density comes from the formation of the crystalline phases in the residual glass matrix. For example, the formation of CsCl crystals with a high density of 3.99 g cm3 could finally increase the density of the whole glasseceramics. The large change of the density of the glass annealed at high temperature of 370 C also implies that the number of the crystals annealed at high temperature is higher, which is in agreement with the XRD results. The microhardness of the chalcogenide glasses annealed at 280 C for different durations was also measured using Vickers microindenter. As can be seen in Fig. 7, the hardness was improved with increasing annealing time from 1.06 GPa for the un-annealed glass to 1.30 GPa for the glass annealed at 280 C for 210 h, and finally to 2.02 GPa for the glass annealed at 370 C for 5 h (not shown in Fig. 7). Hunger et al. [27] investigated the origin of the increasing hardness in glasseceramics and concluded that the crystalline particles contributed to the change of the hardness in the glass due to the stress formed around the crystalline particles. In our case, we found that, with increasing annealing time, the size
Fig. 5. UVeViseIR transmission spectra of the chalcogenide glasses annealed at: 280 C/210 h; 370 C/5 h. For comparison, the spectrum of the un-annealed glass was also shown.
and volume fraction of the crystalline grains increased in the glass matrix as evident by XRD, Raman scattering, and TEM observation. Similarly, the hardness difference between un-annealed and annealed at 280 C for the different durations should also originate from the crystalline grains in the chalcogenide glass. These crystalline particles were well dispersed in the glass matrix, and contribute to the increase of hardness in Fig. 7. On the other hand, Lin et al. [28e30] investigated the mechanical properties of the chalcogenide glasses at different annealing times by Vickers indentations. They discovered that the formation of crystalline particles inside the glass matrix could decrease the crack length, and the crack propagation could be greatly inhibited with increasing annealing times. This indicates that a progressive strengthening can be realized by a ceramization process in the chalcogenide glass. Following these previous investigations [27e30], TEM image in Fig. 4(a) with well-dispersed nanocrystals implies that the resistance to crack propagation is stronger in the glass annealed at 280 C for 210 h. Furthermore, we observed in Fig. 5 that OeH and SeH bonds were eliminated with increasing annealing time. The reduced amount of OeH and SeH bonds could result in better network connectivity, and thus improved glass hardness [31].
Fig. 6. The density of the chalcogenide glass annealed at different annealing times.
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X. Yang et al. / Materials Chemistry and Physics xxx (2014) 1e5
Fig. 7. the microhardness values of the chalcogenide glass annealed at 280 C for different times.
4. Conclusions In summary, we annealed 62.5GeS2e12.5Sb2S3e25CsCl (mol %) chalcogenide glass at different temperatures of 280 C and 370 C. XRD and Raman scattering spectra showed that, only CsCl crystalline phase was formed in the glass annealed at 280 C for 210 h, while two crystalline phases of b-GeS2 and CsCl can be created in the glass annealed at 370 C for 5 h. TEM analysis indicated that the glass annealed at different temperatures had different morphology and distribution of the crystalline grains, e.g., the glass annealed at 280 C contained uniformly dispersed CsCl crystalline grains with higher transmittance and lower optical loss, whilst the glass annealed at 370 C possessed irregularly distributed b-GeS2 and CsCl crystals with poor transmittance and optical loss. The density and the microhardness of the glasses can be considerably improved by thermal annealing. The present results indicate that, careful control of the annealing process is crucial to create glasseceramics with improved mechanical properties and reasonable optical loss. Acknowledgments The work was financially supported by the National Natural Science Foundation of China (Grant no. 51202166), the Zhejiang Province Natural Science Foundation of China (Grant no. Y4100233), and the Wenzhou University Innovation and Career Project of China (Grant no. DC2012006). References [1] X. Gai, T. Han, A. Prasad, S. Madden, D.Y. Choi, R.P. Wang, D. Bulla, B. LutherDavies, Progress in optical waveguides fabricated from chalcogenide glasses, Opt. Express 18 (2010) 26635e26646. [2] C.G. Lin, S. Dai, C. Liu, B.A. Song, Y.S. Xu, F.F. Chen, Mechanism of the enhancement of mid-infrared emission from GeS2eGa2S3 chalcogenide glasseceramics doped with Tm3þ, Appl. Phys. Lett. 100 (2012) 231910. [3] C.G. Lin, L. Calvez, L. Ying, F.F. Chen, B.A. Song, X. Shen, S.X. Dai, X.H. Zhang, External influence on third-order optical nonlinearity of transparent chalcogenide glasseceramics, Appl. Phys. A 104 (2011) 615e620. e, Mid[4] M. Bernier, V. Fortin, N. Caron, M. EI-Amraoui, Y. Messaddeq, R. Valle infrared chalcogenide glass Raman fiber laser, Opt. Lett. 38 (2013) 127e129.
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Please cite this article in press as: X. Yang, et al., Structure and properties of GeeSbeSeCsCl glasseceramics, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.05.029