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Abstract
A series Ge15Ga5AgxTe80-x (GGAT-x, for x=0 to 15 mol%) chalcogenide glass were prepared using conventional melt-quenching methods, and the properties of the glass with different Ag content were studied. The thermal and optical properties of the glass were tested by X-ray, differential scanning calorimeter, Fourier infrared spectrometer, ellipsometer, etc. The results show that the glass has ultra-high refractive index (3.47@10um) and low dispersion, and has wide infrared optical windows, the cut-off wavelength reaches 25um, and the transmittance reaches 55%. The excellent characteristics of this component have broad application prospects in the far infrared application.
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1. Introduction
Chalcogenide glass is a unique material, which is suitable for infrared transmission optical elements covering atmospheric windows. It can be manufactured using molding technology to provide mass production capacity and high quality. Chalcogenide glass caused more and more people's interest which has a wide transmission range, high linear refractive index, strong glass forming ability, excellent chemical stability, etc [1–6]. In recent years, the commonly used infrared lens materials are ZnSe and ZnS, which have low refractive index. In addition, Ge lens has high refractive index, but the price is high. The refractive index of the common chalcogenide glass currently on the market is not higher than 3 at 10um. The refractive index of Ge33As12Se55 at 10um is only 2.49, Ge10As40Se50 at 10um is 2.60, and the refractive index of As2Se3 glass, which is the most widely used in the market, is only 2.78 at 10um [7–9]. The high refractive index glass is more conducive to the miniaturization of the lens and the simplification of the optical system [10,11]. The preparation of high refractive index glass is imperative. Te element has large atomic weight and strong polarization, which is conducive to improving the refractive index of glass, but too much Te element will lead to instability of glass. The ΔT (ΔT = Tx-Tg) of GeTe4 glass is 70 °C, which means that the glass has poor thermal stability. Some electron-deficient additives, such as Ge, Ga, and I atoms, are doped into glass to improve the thermal stability and optical properties of Te-based chalcogenide glasses. For example, the ΔT of Ge15Ga5Te80 glass is 90 °C and ΔT of Ge12.5Ga15Te72.5 glass is 93 °C, these glasses have better thermal stability than GeTe4 [12–15]. This study is dedicated to a promising high refractive index glass with the optimized composition of Ge-Ga-Ag-Te, and used Ag element to improve the thermal stability and transmittance of the glass. The thermal stability and optical properties of this novel glass which present a high refractive index (n@10um > 3) will be characterized. This high refractive index glass has low dispersion and wide infrared optical windows, and maintains a good thermal stability, which can realize the preparation of large-diameter glass (diameter > 80 mm). Lens made of high refractive index glass can be thinner and can be used to simplify optical system. Its ability to be used as IR lenses materials and is suitable for more complex environments [16–18].
2. Experimental
Bulk glass slices of Ge15Ga5AgxTe80-x (x = 0, 1, 3, 5, 7, 10, 15 mol%) glass composition were prepared by the traditional melt/quench protocols in silica tubes. Raw materials (Ge, Ga, Ag, Te) of 5N purity were placed in a silica ampoule(20 mm in diameter) and sealed under vacuum (10−3 Pa), and then the silica ampoule containing the raw material inside was melted at 950 °C for 12-16 h. After melting, the glass was put in water for quenching and annealed 10 °C below Tg to relax mechanical constrains. The annealing time was different according to the weight of the glass.
After the above operations, the Ge15Ga5AgxTe80-x samples were obtained, and finally cut and polished into 2 mm thick, 20 mm diameter glass. FTIR spectra were obtained using Nicolet 380 FTIR working in the 2.5-25 um region. X-ray diffraction (XRD) patterns were measured by a diffractometer (D2 Phaser, Bruker, Germany) using CuKα radiation with a step width of 0.016°, and the test range was from 10° to 70°. The micro-morphology of the samples was obtained using scanning electron microscope (SEM, Tescan VEGA 3 SBH). Thermal analysis was measured by a differential scanning calorimeter (DSC, 204 F1, Germany) by heating in a sealed aluminum pan at a rate of 10 °C/min under N2 atmosphere to obtain glass transition temperature Tg and crystallization onset temperature Tx, sample mass was 10 mg, test temperature is from 50° to 350° . The refractive index of the samples was obtained using IR variable angle spectroscopic ellipsometry (IR-VASE-II, J.A. Woollam IR, USA). All the measurements were performed at the same ambient temperature
3. Results and discussion
3.1. Amorphous nature analysis
The X-ray diffraction (XRD) pattern of the Ge15Ga5AgxTe80-x glass is presented in Fig. 1, XRD can observe whether there is crystallization in the sample to a certain extent [19]. The glass sample has no obvious crystallization peak when the Ag content is doped to 15 mol%. To further confirm whether microcrystals exist in the glass samples, the micro-morphology was obtained using SEM. The sample with 5 mol% Ag content is selected as a typical micrograph. The scanning electron microscope surface map and EDX mappings of Ge15Ga5Ag5Te75 are observed in Fig. 2(a), which shows that the surfaces of Ge15Ga5Ag5Te75 glass sample are uniform with no obvious microcrystals and the elements Ge, Ga, Ag and Te are evenly distributed in the glass. The scanning electron microscope surface map of Ge15Ga5Te80 and Ge15Ga5Ag15Te65 are observed in Fig. 2(b) and Fig. 2(c). It can be seen that these glass surfaces have no obvious microcrystals.
Fig. 2. (a) SEM image and element distribution map of typical Ge15Ga5Ag5Te75 sample (b)SEM image of Ge15Ga5Te80 (c)SEM image of Ge15Ga5Ag15Te65.
3.2. Thermal properties analysis
Figure 3(a) shows typical DSC curves of glass samples. Glass transition temperature (Tg) and glass onset crystallization temperature (Tx) can be obtained from these curves. It can be seen that the Tg of glass increases with the increase of Ag content. When 5 mol% Ag is doped, ΔT (ΔT = Tx-Tg) is the largest, which may mean that the Ge15Ga5Ag5Te75 glass has good thermal stability. To further confirm whether this glass can be used to prepare large-sized glass, we have made DSC thermal stability characterization for Ge15Ga5Ag5Te75. A special thermal stability test method was designed for glass samples, which were tested in three steps using DSC. The first step is to raise the temperature of the powder sample to Tx + 10 °C at the rate of 10 °C/min, and the second step is to cool down the sample to 110 °C, the third step to heat the sample again until Tx + 10 °C [20,21].
Fig. 3. (a) DSC analysis of Ge15Ga5AgxTe80-x glass behavior (b) DSC analysis of Ge15Ga5Ag5Te75 glass behavior (during : 1. a ramp of 10°C/min until Tx + 10°C, 2. a ramp of 20°C/min until 110°C and 3. a ramp of 10°C/min until Tx + 10°C) (c) DSC analysis of Ge15Ga5Te80 glass behavior (d) DSC analysis of Ge15Ga5Ag15Te65 glass behavior [21].
In the Fig. 3(b), the DSC curve of the Ge15Ga5Ag5Te75 glass sample consists of three segments. In the first step, it is obvious that there is a glass transition temperature (Tg) and the crystallization onset temperature (Tx), which means that this is a typical glass behavior. In the second step, the glass sample temperature is cooled down until 110 °C and reheated in the third step. In the third step, the glass transition and the beginning of the crystallization of the sample can still be found, which indicates that the sample is still in the glassy state. The same process has been applied to the Ge15Ga5Te80 glass (Fig. 3(c)) and Ge15Ga5Ag15Te65 glass (Fig. 3(d)). In the first step, both glasses have a obvious glass transition temperature. However in the second and third steps, there is a peak in each, which is a crystalline phase transition. The glass transition temperature of Ge15Ga5Te80 glass and Ge15Ga5Ag15Te65 glass can not be seen in the third step, which indicates that these two glasses do not have good thermal stability. After actual production, only GGAT-5 glass can achieve a diameter of 80 mm, and other GGAT-x (x = 0, 1, 3, 7, 10, 15) glasses can only reach a diameter of 50 mm.
This means that although ΔT of Ge15Ga5Ag5Te75 glass is not very large (ΔT = 99.2 °C), it has good thermal stability, and it is possible to realize large size glass. It has been proved that glass with 80 mm diameter has been successfully prepared in actual production.
3.3. IR spectra analysis
The IR transmission spectra of Ge15Ga5AgxTe80-x glass samples are shown in Fig. 4(a). All glass samples have a wide infrared window, and the infrared cutoff wavelength is above 25um. In order to obtain better transmission curves, 1000 ppm Mg/Al is added to the raw material to remove these absorption peaks by distillation. The IR spectra of purifified GGAT-5 glasses are shown in Fig. 4(b). The absorption peak at the wavelength of 12.8um and 15-20um is Ge-O and Ga-O. When the wavelength exceeds 20um, the transmittance drops sharply. The reason may be the multi-phonon absorption generated by the Ge-Te bond vibration. With the increase of Ag, the transmittance of the Ge-Ga-Ag-Te glass system is gradually improved. When Ag = 5 mol%, the transmittance is up to 55%. The increase in transmittance is due to the reduction of Te atoms, which reduces the refraction of the glass. However, with the further increase of Ag atoms, constrained network structure is formed, the disorder of Ag atoms leads to the decrease of transmission [22]. When the Ag content is 15 mol%, the transmittance is the lowest among several samples containing Ag doping. As shown in Fig. 5, this is the 80 mm diameter sample picture of GGAT-5, and visualized through thermal camera working in the 8-12 µm region.
Fig. 4. (a)IR transmission spectra of Ge15Ga5AgxTe80-x (GGAT-x) glass samples (b)IR transmission spectra of purified GGAT-5 glass sample.
3.4. Optical band gap
Fig. 6 shows (αhν)1/2 versus hν plots for the Ge15Ga5AgxTe80-x (GGAT-x). The near-infrared transmittance of glass samples can be used to obtain the optical band gap Eopt, the function relationship is given by Tauc equation [23]:
The optical gaps Eg were thus obtained by the intersection of linear fitting with the energy axis. It was found that, Eg increases first and then decreases with the increase of Ag content. Eg increases from 0.674 to 0.712 eV with increasing Ag content from 0 to 5 mol% and then Eg decreases to 0.699 eV with increasing Ag content from 5 to 15 mol% for GGAT-x glass samples.
3.5. Ultra-high refractive index analysis
With the increase of Ag content, the proportion of Te element decreases, and the atomic weight and polarizability of Te are larger than those of Ag [24]. Therefore, replacing Te will lead to a decrease in the refractive index, but the stability is improved. From the Fig. 7, it can be seen that the refractive index of glass decreases gradually with Ag doping, but the decline is not large, which is still far more than the common chalcogenide glass on the market.These glasses have low dispersion, and the formula is:
where ν10 is the Abbe number at 10 um of the glass sample, n10 is the refractive index of the glass sample at 10 um, n8 is the refractive index of the glass sample at 8 um, n12 is the refractive index of the glass sample at 12 um.The Abbe number of each sample is obtained through calculation and is counted in Table 1. It can be seen that these glass samples have high Abbe number, which means a low dispersion, which can contribute to the high-definition of the lens. The Abbe number of common chalcogenide glass at 10um is not more than 200, such as the Abbe number of Ge33As12Se55 at 10um is 111, Ge10As40Se50 at 10um is 179, As2Se3 at 10um is 158. Abbe number of the GGAT glass sample is much higher than that of the chalcogenide glass on the market.
Table 1. Characteristic temperatures (Tg, Tx), criterion of stability (ΔT = Tx – Tg), refractive index, Abbe number, transmittance@10um and the Eopt of the glasses in the GGAT-x system
4. Conclusions
Glasses with high refractive index and good transmittance in the Infrared window belong to the 5 mol% Ag doped Ge15Ga5AgxTe80-x glass. Although other Ge15Ga5AgxTe80-x (x = 0, 1, 3, 7, 10, 15 mol%) have good performance in refractive index, their thermal stability is poor and they cannot be prepared with large diameter(>50 mm). The Ge15Ga5Ag5Te75 glass was selected as the best compromise of thermal, mechanical and optical properties for IR optical applications. The Ge15Ga5Ag5Te75 glass has good stability, and can realize the preparation of 80 mm diameter glass, the refractive index at 10 um is 3.467 and Abbe number is 253. This high refractive index chalcogenide glass can be used in more complex environments and the lens is thinner. This work is conducive to the development and application of high refractive index glass and provides effective support for the simplified design of infrared lens.
Funding
Joint Funds of the National Natural Science Foundation of China (U21A2056); Key Technologies Research and Development Program (Grant No. 2021C01025); National Natural Science Foundation of China (Grant No. 62075110).
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Grant No. 62075110), the Key R&D program of Zhejiang Province, China (Grant No. 2021C01025), and Joint Funds of the National Natural Science Foundation of China (U21A2056).
Disclosures
The author declares that there is no conflict of interest in this article and there are no factors affecting this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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