Papers by Keyword: Chalcogenide Glasses

Abstract: As_xSe_1-x glasses with x from 0,05 to 0,55 were prepared at two melting temperatures, 700 and 850 °C. Batch materials from several suppliers were used for glass synthesis. Density and IR transmittance of glasses were measured. It was found that data for density of glasses prepared at 700 °C have sufficient scatter, so dependence of molar volume on composition is not smooth. Data for glasses synthesized at 850 °C are in good agreement with reference ones. According to data of IR spectroscopy the main impurity in glasses is oxygen in several molecular forms. Oxygen gets in glass together with arsenic. Its content can be noticeably decreased by the heat treatment of As at from 300 to 350 °C and Se at 310 °C before weighting and by the heat treatment of batch at 200 °C before the sealing of silica ampoule. Optical quality of glass samples is good enough for the use them as IR transparent material for lens manufacturing.

Abstract: The material of doping transition metal (TM) in chalcogenide compound such as ZnS and ZnSe can be used in sensors, nonlinear optics, optical thin-films and mid-infrared area because of their faster optical response time, wider transparency range of mid-infrared and higher mid-infrared transmittance, low optical loss and phonon energy. In this paper, the ceramic targets of (ZnS/Se)_0.4(Co)_x(Ga₂O₃)_0.6-x (x=0.1, 0.3 and 0.5) were prepared by high temperature solid state reaction. The mass loss rate, shrinkage rate and molar ration were calculated. XRD, absorption spectrum and AFM&OM were investigated. All of the results are shown that the optimum doping concentration is (ZnS/Se)_0.4(Co)_0.5(Ga₂O₃)_0.1 (namely x=0.5), and the optimum intering temperature are in the range 1000~1200°C. Besides, the zinc-blende structure on ceramics targets was confirmed by XRD. A broad application range from VIS to Mid-infrared was suggested by absorption spectra. The optimal base material ZnSe was proved by AFM and OM. All these results indicate that bulks of (ZnS/Se)_0.4(Co)_x(Ga₂O₃)_0.6-x are most promising materials in future

Abstract: Amorphous solids prepared from their melt state exhibit glass transition phenomena upon heating. Derivatives of volume like viscosity, specific heat and thermal expansion coefficient show rapid changes at the glass transition temperature (T_g). In general, application f high pressure increases the T_g (a positive dT_g/dP). This positive dTg/dP has been well understood with the Free Volume and Entropy models. However, there are few exceptions where a negative dT_g/dP has been observed. It has been proposed that the glasses which undergo negative thermal expansion can exhibit a negative dT_g/dP. In this study, electrical resistivity of semiconducting Ge₂₀Te₈₀ glass at high pressures as a function of temperature has been measured in a Bridgman anvil apparatus. Electrical resistivity showed a pronounced change at T_g. The pressure dependence of T_g (dT_g/dP) shows a decreasing trend (-dT_g/dP). Chalcogenide glasses like Se, As₂Se₃ and As₃₀Se₃₀Te₄₀ show a positive dT_g/dP in contradiction to the present observation of negative dT_g/dP. A model proposed by deNeufville and Rockstad finds a linear relationship between T_g and the optical band gap (E_g) when they are grouped according to their connectivity (Z_av).Application of high pressure decreases the interatomic distance which in turn decreases the separation between the valence and conduction bands (optical band gap). This reduction in optical band gap shifts the glass transition to lower values. It is also suggested that the sign of the pressure derivative of T_g can be negative (-dT_g/dP) if the thermal expansion coefficient is negative. Inelastic neutron diffraction studies show a negative thermal expansion coefficient for most of the Te based chalcogenide glasses. Hence, Ge₂₀Te₈₀ glass is uniquethat its pressure dependence of T_gobeys both thermodynamic and the T_g-E_g-Z_av models.

Abstract: Chalcogenide glasses switches from a high-resistance (OFF) state to a low-resistance (ON) state at a threshold voltage (V_th) under high electric fields. This electrical switching is of two types: (i) Threshold switching and (i) memory switching. Threshold switching device revert back to the OFF state immediately upon the removal of the applied voltage, whereas a memory device retains the ON state even after the removal of the applied voltage. Due to Joule heating, a filament is formed between the electrodes and the current is confined within this filament and there is an increase in current density. This increases the temperature inside the filament and there is a transition from high resistive amorphous/glass phase to a low resistive crystalline phase in memory switching materials. In the threshold switching glasses electronic processes like space charge, Poole-Frenkel effect, etc., are responsible. The structural transitions are irreversible whereas the electronic processes are reversible and hence the threshold glasses regain their original state (OFF) and memory glasses remain in the ON state.Interestingly, differential scanning calorimetric studies (DSC) show that both the threshold and memory switching glasses exhibit crystallization (structural transition). Accordingly, glasses which crystallize upon heating should exhibit memory switching behaviour. But the switching experiments indicate that among the glasses which undergo structural transition (crystallization) some show threshold switching and some show memory switching. To understand this, Cu-As-Se, Al-As-Te, Ge-As-Se-Te, Al-As-Se-Te glasses were thermally crystallized under vacuum in two ways: (i) by annealing at their respective crystallization temperatures (T_c) and (ii) heated up to their melting temperatures (T_m) and cooled back to room temperature. Interestingly, most of the threshold switching glasses shows amorphous nature or a huge amorphous background with crystalline peaks when cooled from their melting temperatures. The memory switching glasses crystallize in both the cases.We propose that both threshold and memory glasses undergo phase change and the crystalline phases formed from the melt state are responsible for switching to occur. Hence, at the time of switching the sample in between the electrodes undergo phase change by glass → melt → crystal transformation and not by the direct glass → crystal transformation.

Abstract: In the present work, glasses of the As-S-I system have been synthesized, the region of glasses stable in moist air has been investigated, the properties of the glasses have been determined.

Abstract: Defect configuration of Ge₅As₃₇S₅₈ chalcogenide glasses was studied by positron annihilation lifetime spectra (PALS). Size of nanovoids was determined using Liaos formula for radius of nanovoids in spherical approximation. Peculiarities of structural features were obtained from Raman spectra.

Abstract: Optical changes caused by ⁶⁰Co γ-irradiation are studied in glassy-like As40S60. The observed long-wave shift in the range of fundamental optical absorption edge accompanied by increase in transmittance is explained as a manifestation of complicated nature of radiation-induced structural transformations associated with coordination topological defects and additional shrinkage input from natural physical ageing.

Abstract: Principal possibility of the using of chalcogenide glasses (on the example of Ge₁₈As₁₈Se₆₄) as active media for temperature sensors is considered in this work. Differential scanning calorimetry testing of the investigated glasses shown that 2 years of natural storage does not lead to the drift of their DSC-parameters (glass transition temperature and endothermic peak area). Investigation of the temperature dependence of optical transmission spectra shows the linear character of optical band-gap changes with a temperature. Temperature sensitivity index β for Ge₁₈As₁₈Se₆₄ is estimated to be equal to the ~1.2·10^-3 eV/°C.

Abstract: The present research work is focused on fabricating the chalcogenide glass optical waveguides keeping in mind their application in optical communication. The propagation loss of the waveguides is also studied at three different wavelengths. The waveguides were fabricated by dry etching using ECR Plasma etching and the propagation loss is studied using Fabry-Perot technique. The waveguides having loss as low as 0.35 dB/cm at 1.3m is achieved. The technique used to fabricate waveguide is simple and cost effective.

Abstract: Glassy samples from the GeSe₂-Sb₂Te₃-CdTe system are synthesized. The thermal characteristics (glass-transition T_g, crystallization T_cr and melting T_m temperatures) are determined using differential thermal analysis. On the base of the values of these temperatures the glass forming ability of the glasses is determined by the Hruby’s criterion. The main thermodynamic parameters (enthalpy ΔH and entropy ΔS alternation) of the crystallization are calculated. The values of the ΔH and ΔS vary within 21.196 – 98.625 kJ/mol and 33.91 – 170.93 J/(mol.K), respectively. The influence of the composition on the investigated characteristics is analysed.