I. Introduction
Recently, night vision system has been used in a various fields such as military, aircrafts, and cars. Since infrared lens components based on monocrystalline Ge or polycrystalline ZnSe were very expensive in the early days, it was used only for military application [1]. However, its applications have been widely expanded to civilian ones such as cars and even mobile phones by adapting moldable chalcogenide glasses which have low phonon energy, good thermal stability, and especially high infrared transparency up to 12 um [2]. Se-based chalcogenide glasses which have optical transparency up to 10um are assumed as attractive candidates for moldable chalcogenide glasses lens compared with S-based or Te-based chalcogenide glasses. Among several ternary Se-based glasses, especially, Ge-Sb-Se ternary glasses are considered as promising ones because they are thermally stable and non-toxic [3].
We have already investigated the optimum chalcogenide glass compositions for moldable lens in terms of IR transmittance, thermal property, and hardness [4]. Among them, hardness has an important role on lens fabrication process as well as final lens product. Here, we want to propose the way to improve hardness by adding additional element to the optimum compositions.
II. Experiment
Chalcogenide glasses have been made by the conventional fabrication method by rocking furnace. The starting materials (Ge, Sb, Se, In and Bi) were weighed and put into a silica ampule in a glove box filled with Ar gas. Glasses were melted in a 3 zone rocking furnace at 1000 °C for 12hrs and quenched into water followed by annealing. Synthesized glasses were cut and polished to measure properties such as Vickers hardness, IR transmittance (Perkin-Elmer, Spectrum 100), and surface morphology.
III. Results and discussions
The additional element which was chosen to improve hardness was determined in terms of binding energy and tendency of glass forming ability. In (Indium) and Bi (Bismuth) were doped with the optimum composition glasses such as 30Ge10Sb60Se and 27.5Ge12.5Sb60Se. Based on the reference literature, In and Bi were doped by 3, 5wt% and 8wt%, respectively. Each glass was cut and polished in the upper part, middle part, and lower part to confirm the homogeneity of glass. IR transmittance and Vickers hardness were measured for each sample. Since each sample from one glass showed almost similar IR transmittance and Vickers hardness, synthesized glasses seem to be homogeneous.
The tendency of In and Bi doping on IR transmittance and Vickers hardness was investigated more detail. IR transmittance of In and Bi doped 30Ge10Sb60Se glasses showed almost similar value except 5wt% In doped glass. 5wt% In doped 30Ge10Sb60Se glass showed lower than 50% IR transmittance. However, doping of In and Bi has not an effect on Vickers hardness at all. In and Bi doped 27.5Ge12.5Sb60Se glasses showed a little different trend. Vickers hardness of Bi doped 27.5Ge12.5Sb60Se glasses did not show any improvement. On the other hand, 3wt% and 5wt% In doped 27.5Ge12.5Sb60Se glasses showed about 10% Vickers hardness improvement while IR transmittance property remains over 60%. IR transmittance of 8wt% In doped 27.5Ge12.5Sb60Se glass decreased below 10%, which means In should be doped under 8wt%.
In order to confirm the reason of IR transmittance decrease, XRD and FE-SEM analyses were done. According to XRD results, 8wt% In doped 27.5Ge12.5Sb60Se glass showed In2Se3 peak while other glasses did not show any other sharp peaks related to crystallization. In addition, small particles which are related to In2Se3 crystallization were observed on the surface of 8wt% In doped 27.5Ge12.5Sb60Se glass in FE-SEM image
References
[1] X.H. Zhang, Y. Guimond, Y. Bellec, J. Non-Cryst. Solids 326&327 (2003) 519
[2] E. Guillevic, X.H.Zhang, T. Pain, L. Calvez, J.L. Adam, J. Lucas, Opt. Mater. 31 (2009) 1688
[3] D.R. Goyal, A.S. Maan, J. Non-Ctryst. Solids 183 (1995) 182
[4] J.H. Lee, W.H. Lee, J.K. Park, J.H. Yi, S.Y. Shin, B.J. Park, B. So, J. Heo, J.H. Choi, H.J. Kim, Y.G. Choi, J. Non-Ctryst. Solids, to be published (2016)