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Abstract
In this work, bulk chalcogenide glasses (Ge35As10S55)100-xGax (x = 0,1,3,5,7,9) were prepared using the traditional melt quenching method, and glass ceramics were prepared to improve the mechanical properties through heat treatment. Optical, thermal and mechanical properties of the glass and glass ceramic samples were measured by FTIR, DSC and Vickers hardness analysis. Results indicate that glass samples exhibited about 70% IR-transmission around 3–12 µm. The hardness of these pure glasses increased from 231 to 282 kgf/mm2 through gallium doping and improvement of melt-quenching conditions. The type of nanocrystals precipitated in the glass ceramics were characterized by XRD. Existence of a γ-Ga2S3 crystal phase enhanced the hardness of the glass. Also, the size and distribution of nanocrystals in the microstructure of the glass ceramics were investigated by SEM. The hardness of (Ge35As10S55)93Ga7 glass reached to 302.6 kgf/mm2 by precipitation of nanocrystals with diameter smaller than 500 nm.
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1. Introduction
In recent years, chalcogenide glass has been increasingly widely used in infrared thermal imaging technology due to its wide infrared transparent window, adjustable composition, controllable shape, low thermal effect and good glass forming ability [1,2]. The transmission range of chalcogenide glass covers the third atmospheric transmission window, indicating its potential applications in sensors and optical communication systems [3–6]. Monocrystalline germanium, also as a common infrared material, has low preparation efficiency, high cost and difficulty in preparing large size infrared lenses. Chalcogenide glass as a substitute for monocrystalline germanium which has much lower manufacturing cost and ability of being prepared into bulk glass [7,8]. However, the mechanical properties of chalcogenide glass are far weaker than the traditional monocrystalline germanium material used for making infrared lenses [9,10]. Common commercial chalcogenide glasses on the market, such as As2Se3, only have a hardness of 136kgf/mm2 [11], while Ge28Sb12Se60 containing Germanium has a hardness of 222kgf/mm2 [12]. Such low hardness greatly limits the large-scale application of chalcogenide glasses in the civilian field.
There are plenty of studies being carried out to improve the mechanical properties of chalcogenide glass. Jason Lonergan et al. prepared a series of Ge-Ga-Se chalcogenide glasses in 2019. By changing the content of Ge and Ga elements, Ge25Ga15Se60 was selected as the component with optimum mechanical properties in the glass forming region, compared to the worst mechanical properties in this series of glass samples, its hardness and fracture have increased by 150% and 155%, respectively [13]. This method is constrained by the glass forming region, unfortunately, the formation domain of chalcogenide glass is generally small [14]. Lee et al. used diamond-like carbon as a hardening and anti-reflective coating for Ge-Ga-Se glass. Vickers hardness of coated glass increased from 206kgf/mm2 to 215kgf/mm2. The flaw of this process lies in the lack of sufficient adhesion at the interface between the carbon coating and chalcogenide glass [15]. Melt-quenching and post-heat-treatment route is the classical preparation method to obtain glass ceramics [16]. Thus, a feasible idea was proposed, which is to precipitate expected crystals in chalcogenide glass with additives during the heat treatment process. The addition of alkali halides has been extensively studied including GeS2-Sb2S3-In2S3 [17], GeS2-Sb2S3-CsCl [18–20], GeS2-In2S2 [21], and GeS-In2S3-CsI [22]. A relatively high content of halides results in a difficulty to control the crystal size and is likely to also precipitate halide crystals simultaneously [10]. Moreover, Lee et al. studied the effect of doping Ga element on properties in Ge-Sb-S system. After doping 5% Ga, hardness of glass samples increased by 11% [23]. In this study, we selected the Ge-As-S ternary system and elemental Ga as an additive combining heat treatment to improve hardness. Ge-As-S chalcogenide glass system has good thermal stability and excellent optical properties at 2-10µm [24]. When selecting the composition for heat treatment, we comprehensively considered two factors: hardness and thermal stability. Among the compositions shown in Table 1, Ge35As10S55 has high hardness and appropriate thermal stability, therefore we consider it a suitable composition for heat treatment. Because doping of Ga into glass system to alter the thermal properties of the glass and enable it to generate sole Ga2S3 crystals after heat treatment, designing appropriate temperature and heat treatment duration is crucial for controlling the crystal size deposited on the glass substrate. This article focuses on the preparation of this Ga containing glass ceramics and characterizes the precipitated crystal phase and the optical and thermal properties of glass ceramics through XRD, SEM, DSC, and FTIR etc. In addition, the hardness of these glass ceramics obtained has significantly improved compared to the original glass samples.
Table 1. Candidate glass compositions
2. Experimental details
2.1 Glass synthesis
(Ge35As10S55)100-xGax (x = 0,1,3,5,7,9) glass samples were synthesized by melting mixture of highly pure raw materials, (Ge,As,S and Ga:99.999% purity) in a sealed quartz ampoule under vacuum (10−3 Pa). Then these ampoules of 15 mm inner diameter were put in a rocking furnace heating from 30°C to 850°C and maintained at this temperature for 12 h. The ampules were quenched in cold air flow to avoid excessive stress causing glass to shatter into fragments. These ampoule bottles were placed in an annealing furnace, annealed at 10 °C below the glass transition temperature (Tg) to minimize inner constraints, and then these glass rods were cut into 2 mm thick discs. At last, these glass discs were placed in a vacuum furnace and annealed at 40°C above Tg for 3h-15 h. The furnace was vacuumed until its vacuum pressure gauge reading reaches −0.1 MPa to avoid oxidation.
2.2 Characterization
Differential scanning calorimeter (DSC, TA Q2000 Thermal Analysis) was used in order to find glass transition temperature, melting temperature of chalcogenide glass. Disk samples were heated from room temperature to 550°C with 10 K/min of heating rate under the protection of a flowing N2 atmosphere. After thermal treatment process of the samples, diffraction patterns were collected using an X-ray diffraction (XRD, Bruker D2 Phaser, λ=0.15406 nm, 30 kV, 10 mA, CuKα) from 10°-70° at a scanning rate of 0.2°/min for the identification of crystalline phases. Field emission scanning electron microscopy (SEM, Hitachi SU-70) was used to characterize the nano crystalline phase of chalcogenide glass disks.
Hardness was characterized using a micro-Vickers sclerometer (XHV-1000T-CCD) according to ASTM procedure E384.4 The hardness value corresponding to the largest load that did not form cracks in the sample was selected. The mirror-polished specimen was pasted on a flat horizontal support. The load was applied vertically using Vickers sclerometer at constant loading rate of 100gf. Vickers hardness is defined by the following Equation:
Where C is a constant, which is the reciprocal of the gravitational acceleration, F is the test force applied to the sample, α is the angle between the edges of the diamond cone and d is the average value of the diagonal length of the indentation in µm. The transmittance was characterized by FT-IR in the range of 5000∼833 cm−1 (2∼12µm).3. Results
3.1 Gallium doped chalcogenide glass
3.1.1 X-ray diffraction of glasses
In order to ensure that the glass samples obtained by melt-quenching are amorphous, X-ray diffraction analysis was performed on them. The common broad shoulder and bimodal patterns observed in the XRD pattern of chalcogenide compounds, as well as the absence of sharp diffraction peaks, confirm that these samples have a glassy amorphous structure. Figure 1 shows that the basic glass of Ge35As10S55 and compositions with gallium content from 1at.% to 9at.%, have not detected crystalline phase.
3.1.2 Optical properties of glass samples
The obtained glass samples have excellent optical properties in 2-10µm. Doping of Ga affects the transmittance of the glass in the shortwave range and reduces the baseline transmittance as Fig. 2 presents.
Figure 2 shows the IR-transmission of glass samples, which is the most important property of chalcogenide glasses. The transmittance of the Ge35As10S55 glass matrix obtained through this melting quenching method is around 69%. By adding a small amount of gallium, the transmission of (Ge35As10S55)100-xGax (x = 1,3) was improved. In the infrared region of 3-12 µm, the gallium additive increased the transmittance of the glass to 72%. This improved transmission can be attributed to the influence of network forming elements on the glass structure.
However, after continuing to increase the gallium content in glass components the transmittance decreased. This can be explained by the different roles of gallium in this ternary glass network. As a result of the previous role of gallium in glass networks, adding 1-3at.% gallium can improve infrared transmittance. The number of glass network links per unit increases, making the system more coherent. The reflected photon doesn’t have interaction with glass connections so the crossing would be increased. Because of presence of additive, the coherence will be increased. The interaction of the phonon with long wave length won’t be occurred. If the links are weak, the domains of vibration will rise, then phonons will generate and the IR ray will be absorbed [12]. Gallium is an amphoteric element and plays two roles in the lattice. It can be a networker or modifier depending on its concentration in the glass matrix. By increasing the gallium up to 3% mol, the IR transmission increases but decreases along continuing increase of gallium.
In the IR spectrum of these samples, the sharp absorption peaks located near 4.01 µm are caused by the vibration of the S-H bond and Ge-H bond leads to the broad absorption peaks from 4.7 to 4.9µm, peak at 7.5µm and the broad absorption shoulder from 8.5-9.5µm are attributed to the vibration of As-O bond [25].
3.1.3 Thermal and mechanical properties of glass samples
Figure 3 shows the thermal behavior of (Ge35As10S55)100-xGax (x = 0,1,3,5,7,9) glass samples in the temperature range of 200–550°C. Increase of gallium content up to 5%mol in glass lattice reduced the glass transition temperature. This decrease in Tg may be a sign of open lattice structure but it is not certain as the glass transition temperature depends on many factors, including heating or cooling rate, type and content of glass elements, quantity of additives or nucleating agent, intra-atomic strength and band gap.
The properties and parameters of glass samples are shown in Table 2. Mean coordination number is one of the major parameters on which various physical properties depend and calculated for these glasses according to the following formula. The atomic weight percentages glass was calculated by dividing the sum of the atomic weight percentages of every content of glass product by its coordination number.
Mean coordination number of S, As, Ge, Ga, i.e. NS, NAs, NGe and NGa are 2, 3,4 and 4, all satisfying the empirical “8-n” rule and α, β, γ and δ are their atomic weight percentages, respectively. Hardness of the glass samples was enhanced by increase of Ga and coordination number.
Table 2. Chemical Composition, Glass Transition Temperature, Vickers hardness, Mean Coordination Number of Samples
The thermal stability of glass is also an important factor to consider in the preparation of chalcogenide ceramics. We usually use parameters ΔT to reflect the thermal stability of glass, which is defined as the difference between the glass transition (Tg) and crystallization temperatures (Tx) which is related to nucleation. The scale of ΔT reflects the ability of glass to resist against crystallization. Therefore, ΔT can be regarded as a criterion for thermal stability against crystallization [26]. In the process of producing glass-ceramic through heat treatment, it’s crucial to choose glass samples with appropriate ΔT. Taking into account the optical, thermal, and mechanical properties of the glass, glass samples doped with 7at.% Ga content was selected as the optimum component for preparing chalcogenide glass among this batch of glass samples. As we stated before, the biggest flaw of chalcogenide glass is its poor mechanical properties, which greatly limits its application in daily life. The addition of gallium in the Ge-As-S ternary system and the generation of nanoscale crystals in the glass matrix through heat treatment can significantly enhance the hardness of the glass ceramics.
The hardness of glass samples shows a trend of first increasing and then decreasing with the rise of doped Ga content. Among those samples, the hardness reached to the highest of 282kgf/mm2 in the (Ge35As10S55)93Ga7 sample. Based on the data represented in Table 3 we can conclude that the enhancement in hardness is attributed to the higher bond energy of Ga-S (294 kJ/mol) while that of Ge-S is 265 kJ/mol [27,28]. However, as the addition of Ga content, the hardness of the glass did not continue to get improved. Both hardness and thermal stability start to deteriorate. This is probably caused by the following reasons. The composition of original glass is in the sulfur deficient glass forming region. There is not enough sulfur to form Ga-S bond with gallium [29,30]. Thus, the follow-up addition of gallium results in the existence of homopolar Ga-Ga bond in the glass matrix, which undermined the connectivity of the glass network. This can be confirmed by the drop of thermal stability of glass Adding gallium into the Ge-As-S glass system improve the hardness of the glass from 231kgf/mm2 up to 282kgf/mm2.
Table 3. Chemical bond energy of atomic pairs in glass samples
3.2 Properties of glass ceramics
Based on the DSC curves and previous studies, a proper annealing temperature that is 40°C above the glass transition temperature was chosen for heat treatment process. To identify the structure of crystalline phases that have nucleated, we measured the X-ray diffraction pattern of heat-treated glass samples. Figure 4 displays XRD patterns recorded for the G7 glass sample after annealing at 422°C for different durations. The X-ray diffraction pattern of the sample after 9 h of heat-treatment shows sharp peaks. It was proved that γ-Ga2S3 crystalline generated in glass matrix compared with standard PDF card (00-049-1361)
When heat-treatment time prolonged to 15 h, it can be clearly observed that the peak intensity of γ-Ga2S3 crystal phase increases and a new monoclinic crystalline phase, α-Ga2S3(00-054-415), generated in the meanwhile [31]. Overgrowth of γ-Ga2S3 crystals deteriorate the compactness of the glass matrix. Besides, the α-Ga2S3 crystals also negatively affects its hardness [32].
In the presence of nucleation agent gallium in the glass matrix, annealing at temperatures higher than Tg will lead to the formation of nanocrystals. While annealing at the temperature at which the crystallization peak appears in the DSC curve will result in uncontrollable crystal growth and emergence in glass ceramics. Thus, glass samples were placed in a vacuum furnace for different durations ranging from 3 to 15 h. It should be pointed out that the obtained glass ceramic samples were polished into the same shape and parallelism after the heat treatment process. Here we just reported properties of samples doped 7at.% gallium due to their the highest hardness, good stable thermal stability and relatively high IR transmission.
Figure 5 indicates the transmission spectra of these glasses after annealing for different hours. These nanoscale grains generated in glass matrix caused a decrease in the infrared transmittance, especially in the shortwave range. At the same time, the hardness of glass ceramics was measured using a Vickers sclerometer. As shown in Fig. 6, annealing the basic glass for 9 h surprisingly resulted in the highest hardness (302.6 kgf/mm2) of the obtained glass ceramic. With the increase of annealing time, the hardness of glass ceramic started to decrease.
Figure 7 shows the crystals formed in G7 sample. After 3 h of heat-treatment, crystals were barely seen on SEM images. When the time increases to 6 h, nanoscale crystal particles can be seen appearing in the glass. The 9 h image clearly shows a significant increase in the number of crystals, but their size remains at the submicron level. When it comes to 12 h, large crystals begin to form. By 15 h, the growth of a large number of crystals had disrupted the uniformity of the glass matrix.
4. Conclusion
Research on the Ge35As10S55 glass system shows that the addition of a small amount of gallium can improve the transmittance. Gallium rising to 7at.% will increase the strength and rigidity of the glass. Gallium exerts a dual influence on the glass network reticulation. When its content reaches 7at.%, the presence of gallium helps to form the glass network structure; When the content reaches 9at.%, its role changes and begins to weaken the glass reticulation. We prepared glass ceramics with higher hardness through heat treatment. The glass ceramic with optimum performance was obtained by annealing it at 40°C above the glass transition temperature for different durations, which reaches a hardness of 302.6 kgf/mm2 and still can be able to maintain a transmittance of about 70%. These crystals improve mechanical properties and resistance to thermal and mechanical shocks as well as prevention from crack expansion in these glasses. In summary, this work provides a good case for the preparation high strength glass ceramics that enables their application in harsh environments that require wear resistance.
Funding
Natural Science Foundation of Zhejiang Province (LDT23F05012F05); National Natural Science Foundation of China (61975086, 62075110, U21A2056); Key Research and Development Program of Zhejiang Province (2021C01025); Key Research and Development Program of Ningbo (2021Z059, 2022Z237, 2023Z104); K. C. Wong Magna Fund in Ningbo University.
Disclosures
The authors declare no conflicts of interest.
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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