Faculty Publications
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Item Electrical switching behavior of bulk Si15Te85-xSbx chalcogenide glasses - A study of compositional dependence(2010) Lokesh, R.; Udayashankar, N.K.; Asokan, S.Studies on the electrical switching behavior of melt quenched bulk Si15Te85-xSbx glasses have been undertaken in the composition range (1 ? x ? 10), in order to understand the effect of Sb addition on the electrical switching behavior of Si15Te85-x base glass. It has been observed that all the Si15Te85-xSbx glasses studied exhibit a smooth memory type switching. Further, the switching voltages are found to decrease almost linearly with Sb content, which indicates that the metallicity of the dopant plays a dominant role in this system compared to network connectivity/rigidity. The thickness dependence of switching voltage (Vth) indicates a clear thermal origin for the switching mechanism. The temperature variation of switching voltages reveals that the Si15Te85-xSbx glasses studied have a moderate thermal stability. © 2009 Elsevier B.V. All rights reserved.Item Investigation of mixed alkali effect on mechanical, structural and thermal properties of three-alkali borate glass system(Elsevier Ltd, 2016) Subhashini, u.; Shashikala, H.D.; Udayashankar, N.K.In the present communication, the results of investigation on mixed alkali effect (MAE) in mechanical, structural and thermal investigation of alkali zinc borate glasses with nominal composition 5Li2O-(25-x)K2O-xNa2O-60B2O3-10ZnO (x = 0, 5, 10, 15, 20 and 25 mol%) are reported. The samples were prepared using standard melt quenching technique. Fourier transform infrared (FTIR) spectroscopy, differential scanning calarometry (DSC) and Vickers indentation studies were performed to investigate the mixed alkali effect in the samples. From the DSC studies, it was observed that the thermal parameters viz., glass transition temperature (Tg), glass melting temperature (Tm), glass crystallization temperature (Tc), glass stability (S) and fragility (F) exhibit a non linear variation with respect to increase in compositional parameter (RNa). This behavior clearly indicated the presence of strong MAE in the samples. FTIR studies confirmed the presence of both [BO3] and [BO4] units, indicating the present glass networks to be made up of these two units placed in different structural groups. The non linear variation of peak positions of B-O-B bending and stretching of [BO3] and [BO4] units of each glass sample explain the role of modifier alkali elements and validates the existence of strong MAE. The microhardness and fracture toughness of the samples were measured using Vickers micro indentation technique and non linear variation of both the properties have been observed confirming the presence of MAE in these glass samples. © 2015 Elsevier B.V.Item Crystallization kinetics of Sn doped Ge20Te80?xSnx (0 ? x ? 4) chalcogenide glassy alloys(Elsevier Ltd, 2017) Fernandes, B.J.; Naresh, N.; Ramesh, K.; Sridharan, K.; Udayashankar, N.K.Chalcogenide semiconductors have evolved as multifunctional materials due to their fascinating thermal, optical, electrical and mechanical properties. In this report, Ge20Te80?xSnx (0 ? x ? 4) glassy alloys are systematically studied in order to understand the effect of variation of Sn content on the thermal parameters such as glass transition (Tg) onset crystallization (Tc), peak crystallization (Tp), melting temperature (Tm), activation energy of glass transition (Eg), and crystallization (Ec). The values of Eg are calculated from the variation of Tg with the heating rate (?), according to Kissinger and Moynihan model, while the values of Ec are calculated from the variation of Tp with the heating rate (?), according to Kissinger, Takhor, Augis-Bennett and Ozawa model. Thermal stability and glass forming ability (GFA) are discussed for understanding the applicability of the synthesized materials in phase change memory (PCM) applications. Thermal parameters are correlated with the electrical switching studies to get an insight into the phase change mechanism. The results of the calculated thermal parameters reveal that the GFA of the synthesized Ge20Te80?xSnx (0 ? x ? 4) glassy alloys has a synchronous relationship with their thermal properties studied through differential scanning calorimetry, indicating their potential for phase-change memory device applications. © 2017 Elsevier B.V.Item Effect of alkali earth oxides on hydroxy-carbonated apatite nano layer formation for sio2 –bao–cao–na2 o–p2o5 glass system(Springer Science and Business Media Deutschland GmbH, 2017) Kiran, P.; Ramakrishna, V.; Shashikala, H.D.; Udayashankar, N.K.Barium soda lime phosphosilicate [(58SiO2– (32-x)BaO–xCao–6Na2 O–4P2O5 (where x = 15, 20, 25 and 30 mol%)] samples were synthesised using conventional sol–gel method at 700 °C sintering temperature. Thermal, structural properties were studied using thermo gravimetric analysis and differential thermal analysis, X-ray diffraction, scanning electron microscopy, fourier transform infrared and Raman spectroscopy. Using Raman spectra non-bridging oxygen concentrations were estimated. The hydroxy-carbonated apatite (HCA) layer formation on samples was analysed for 7 days using simulated body fluid (SBF) soaked samples. The growth of HCA layers self-assembled on the sample surface was discussed as a function of NBO/BO ratio. Results indicated that the number of Ca2? ions released into SBF solution in dissolution process and weight loss of SB-treated samples vary with NBO/BO ratio. The changes in NBO/BO ratios were observed to be proportional to HCA forming ability of barium soda lime phosphosilicate glasses. © Springer. All rights reserved.Item Electrical switching in Si20Te80 ? xBix (0 ? x ? 3) chalcogenide glassy alloys(Elsevier B.V., 2018) Fernandes, B.J.; Ramesh, K.; Udayashankar, N.K.Chalcogenide glasses have attained enormous research interest due to their importance in finding electronic memories. Here we report electrical switching and thermal crystallization behavior of Si20Te80 ? xBix (0 ? x ? 3) glasses. We observe a significant decrease in the threshold voltage (VT) and the thermal stability (?T), indicating that in Si20Te80 ? xBix glasses, the resistivity of the additive element Bi plays a dominant role over network connectivity/rigidity. The variation of VT with respect to thickness and temperature of the sample indicates that the memory switching observed in Si20Te80 ? xBix glasses is influenced by the thermally induced transitions (thermal mechanism). Scanning electron microscopy (SEM) studies on pre-switched and post switched samples reveal the morphological changes on the surface of the sample, and serve as an experimental evidence for the formation of the crystalline filament between two electrodes during switching. Furthermore, the decrease in ?T values indicates that the Si-Te glasses become de-vitrifiable more easily with the addition of Bi, influencing the decrease of VT. Structural evaluation like thermal devitrification studies and morphological changes elucidate the restricted glass formability of the studied glass system. © 2018 Elsevier B.V.Item Crystallization kinetics of Si20Te80?xBix (0???x???3) chalcogenide glasses(Elsevier Ltd, 2019) Fernandes, B.J.; Ramesh, K.; Udayashankar, N.K.In this report, we investigate the crystallization kinetics of Si20Te80?xBix (0 ? x ? 3) chalcogenide glassy systems using differential scanning calorimetry (DSC) technique. Systematic studies are carried out in order to understand the variation of thermal parameters such as glass transition temperature (Tg), onset crystallization temperature (Tc) and peak crystallization temperature (Tp) as a function of composition. Activation energy for glass transition (Eg) and crystallization (Ec) has been calculated based on the relevant statistical methods. Furthermore, thermal parameters such as change in specific heat (?Cp), fragility index (F), thermal stability (?T)& (S), enthalpy (?Hc), entropy (?S) are deduced to interpret distinct material behaviour as a function of composition. Structural evaluation like thermal devitrification studies elucidate the restricted glass formability of the studied glass system. Conclusively, a relationship has been established between the obtained thermal parameters and electrical switching characteristics. © 2019 Elsevier B.V.Item Influence of Fe3+ ions on optical, structural, thermal and mechanical properties of Li2O–Na2O–K2O–ZnO–B2O3 based glass system(Elsevier Ltd, 2020) Subhashini, u.; Shashikala, H.D.; Udayashankar, N.K.The effect of addition of Fe3+ ions in Li2O–Na2O–K2O–ZnO–B2O3 based glass system have been studied. The melt quenching technique is used to prepare the glass samples. The X-ray diffraction studies confirmed the amorphous nature of the samples. DSC thermographs indicate that the Tg is decreasing as the addition of Fe2O3 content. Increase in thermal stability is observed upon adding K2O to the F0 sample and increase in the Fe2O3 content further enhanced the thermal stability up to 187 °C. Optical band gap energy (Eg) of the studied glass system is found to reduce with increasing Fe2O3 content. A band observed around 450 nm in UV absorption spectra is due to the d-d transition of 6A1g(S) ? 4T2g(G) which indicates the presence of iron ion in trivalent state (Fe3+) with distorted octahedral symmetry. The oxide ion polarizability values determined using the refractive index and optical band gap energy are found to be increasing monotonically with partial incorporation of K2O content and with increasing Fe2O3 content. The FTIR and Raman spectroscopy studies confirmed the network modifier role of Fe2O3. The radial-median cracks produced due to Vickers indentation were studied and it confirmed the enhancement in brittleness of the samples as Fe2O3 content increased. Further efforts have been made to analyse and establish correlation between the physical and mechanical properties with the structural modification of the studied glass system. © 2019 Elsevier Ltd and Techna Group S.r.l.Item Observation of electrical threshold switching behavior and thermal crystallization in bulk Se86-xTe14Snx chalcogenide glasses(Elsevier Ltd, 2023) Joshi, S.; Udayashankar, N.K.Selenium-rich chalcogenides have gained popularity as materials for selector devices due to their unique Ovonic Threshold Switching behavior. Bulk Se86-xTe14Snx glassy alloys (0 ≤ x ≤ 6) were prepared through the traditional melt quenching method. The samples with Sn atomic percentage (x) = 3 to 6 are found to exhibit a rapid and reversible transition between a highly resistive and conductive state affected by an electric field. A remarkable decrement in threshold voltage (Vth) from 453 V to 62 V has been observed with increase in the Tin content. Differential scanning calorimetry (DSC) analysis was carried out to understand the variation of Glass transition temperature (Tg), Crystallization temperature (Tc), and other important glass stability parameters and their compositional dependence. Se82Te14Sn4 sample was found to be thermally most stable with Herby's parameter value (HR) of 0.3860 and a maximum number of switching cycles at room temperature. X-Ray diffraction patterns of annealed samples were compared with pristine glass to study the multi-phasic Se–Te–Sn alloy. Further, the threshold voltage (Vth) and the number of threshold switching cycles are found to decrease with an increase in temperature till crystallization on-set temperature (Toc). The temperature-dependent conductivity studies showed an abrupt increase in the conductivity of the samples as the temperature crossed the crystallization onset temperature. © 2023 Elsevier Ltd and Techna Group S.r.l.Item Tunable electrode-dependent switching characteristics of Se-Te-In chalcogenide thin films(Springer, 2024) Joshi, S.; Udayashankar, N.K.Chalcogenide glasses have garnered significant interest as potential materials for the creation of high-density, three-dimensional stackable cross-point array structures, particularly for memory devices. Chalcogenide glasses have emerged as promising candidates for high-density, three-dimensional stackable cross-point array structures. In this study, we delve into the intricate electrical switching behaviour of Se86−xTe14Inx (x = 0, 2, 4, 6) chalcogenide glasses in the form of thin films, employing Aluminium (Al) as the top and bottom electrodes. Exhibiting the remarkable phase-changing characteristics of the material, the films showed memory-type switching behaviour. Remarkably, with an incremental change in Indium concentration from 0 to 6%, a linear reduction in the threshold voltage (Vth) from 12.75 to 4.80 V was observed, underscoring the tunability of switching properties with respect to compositional variations. When the Al top electrode was substituted with Silver (Ag) the thin films’ electrical behaviour changed and this alteration instigated a shift in the switching mechanism. The films changed their characteristics from memory to threshold-switching behaviour, presenting a unique phenomenon in the realm of Se-Te-based chalcogenide glassy alloys. The presence of an active electrode (Ag) at the top facilitated the formation of temporary Ag filaments, making the device a programmable metallization cell (PMC) with remarkable threshold-switching capabilities with higher selectivity (∼ 5 × 103) and endurance of 104 cycles. The observed tunable attributes, contingent on the precise adjustment of Indium concentration and film thickness, underscore the immense potential of these films as highly efficient and adaptable unidirectional selectors and memory devices. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2024.Item Peculiarities of Electrical Switching and Phase Transition Dynamics in Bismuth-Infused Se-Te Chalcogenide Glasses: From Bulk to Thin Film Devices(American Chemical Society, 2024) Joshi, S.; Rodney, J.D.; Udayashankar, N.K.Herein, the electrical switching behavior of both bulk and thin film forms of Se86-xTe14Bix (0 ≤ x ≤ 4) chalcogenide glasses was investigated. The melt-quench-derived glasses were found to be amorphous, and the switching behavior exhibited a threshold-type response below a certain current limit (Ith) for bismuth (Bi)-doped bulk samples. Interestingly, as current levels surpassed this threshold, a noteworthy change occurred in the switching behavior, converting it into a memory-type response. The threshold voltage (Vth) exhibited a decreasing trend from ∼228 V to ∼36 V with an increasing Bi content, and differential scanning calorimetry (DSC) was utilized to study the phase transition phenomena and thermal stability of the amorphous glasses. These DSC results unequivocally confirmed that the transition from amorphous to crystalline phase occurred readily and at lower temperatures in the Se82Te14Bi4 composition. Furthermore, annealing studies were carried out to gain insight into the phase transformations that occur when the material makes the transition from an amorphous to a crystalline state. Subsequently, the same melt-quench-derived glasses were deposited as a thin film using physical vapor deposition (PVD) into a three-layered Al/Se-Te-Bi/Al device, and the memory switching voltage experienced a remarkable drop to 2.88 V compared to the bulk material. This exploration sheds light on the captivating electrical switching behavior of Se86-xTe14Bix chalcogenide glasses and holds promise for potential applications spanning the realm of emerging electronics and phase change material (PCM) devices. © 2024 American Chemical Society.