Faculty Publications
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Item Studies on Resistive Switching of Cu/Ta2O5/Pt Devices for Non-volatile Memory Application(Springer, 2021) Thathron, T.; Sahu, V.K.; Ajimsha, R.S.; Das, A.K.; Misra, P.In recent times, memory devices based on resistive switching (RS) phenomena in dielectric materials have become a strong contender for the futuristic universal memory. Among other materials being explored for RS application, tantalum pentaoxide (Ta2O5) has emerged as potential candidate due to its large dielectric constant and compatibility with the existing complementary metal-oxide semiconductor process. In view of this, we have studied the resistive switching memory characteristics of Ta2O5 thin film in Cu/Ta2O5/Pt device configuration. About 200 nm thick films of Ta2O5were deposited on platinum-coated silicon (Pt/Si) substrate by Pulsed Laser Deposition (PLD) method. On the top of Ta2O5 thin film, Cu electrodes of radius ~100 µm and thickness ~ 100nm were grown by RF magnetron sputtering using shadow masking. The RS behaviour of Cu/Ta2O5/Pt devices was studied by current–voltage (I-V) measurements at room temperature. The as-fabricated Cu/Ta2O5/Pt devices showed repeatable and reliable, non-volatile bipolar resistance switching for 100 cycles, indicating good endurance. Due to virgin low resistance state of the device, the initial electroforming step was not required for bipolar RS. The mean resistance of high resistance state (HRS) and low resistance state (LRS) was ~300 MΩ and 500 Ω respectively with very high resistance ratio of ~106. The Cu/Ta2O5/Pt devices showed good data retention up to 103 s. The resistive switching mechanism in Cu/Ta2O5/Pt devices was understood in terms of redox reaction based formation and rupturing of conducting filaments constituting copper ions. The conduction mechanism in LRS was explained on the basis of Ohmic conduction, whereas Schottky emission and space charge limited conduction (SCLC) were found as possible conduction mechanisms in HRS. Our studies clearly show that Ta2O5-based resistive switching devices may have applications in futuristic universal non-volatile memory technology. © 2021, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.Item Digital Control System Based Isolated Totem Pole Converter for Electric Vehicle Onboard Chargers(Institute of Electrical and Electronics Engineers Inc., 2025) Das, A.K.; Raushan, R.; Kumar, P.The Totem-Pole converter is well-suited for lowpowered electric vehicle (EV) applications. The isolated configurations of such converters provide single-stage power conversion that overcomes the more conversion stages. A robust controller can satisfy the converter operations efficiently for on-board charging applications. In this paper, a modified digital control method for totem-pole PFC cascaded active flyback DC-DC converters functioning in discontinuous conduction mode (DCM) is proposed. The auxiliary switch and clamp capacitor are used to recycle the stored energy in the transformer to reduce the spike voltage. The main purpose of adding PFC is to obtain the input current in phase with the input voltage, keeping total harmonic distortion (THD) low to improve the efficiency of the converter. © 2025 IEEE.Item Structural performance and implementation challenges of next-generation concrete materials(Elsevier Ltd, 2025) Barbhuiya, S.; Das, B.B.; Rajput, A.; Katare, V.; Das, A.K.Conventional concrete faces limitations in durability, sustainability, and adaptability to modern structural demands, constraining its use in high-rise, bridge, and extreme-environment applications. This study examines emerging concrete mixes—HPC, UHPC, SCC, FRC, GPC, and 3D-Printed Concrete—by evaluating their mechanical properties, implementation challenges, and future opportunities. A review of experimental data, case studies, and comparative analyses was conducted to assess strength, durability, workability, and structural applications. Results show that HPC and UHPC reach compressive strengths of 60–200 MPa, GPC achieves 40–80 MPa with reduced CO₂ emissions, SCC demonstrates slump flows of 600–800 mm, and fibre reinforcement enhances tensile strength to 8–15 MPa. These findings highlight superior performance, sustainability, and constructability, though high costs, lack of standards, and scalability issues remain obstacles to widespread adoption. This review uniquely integrates comparative insights on High-Performance, Ultra-High-Performance, Self-Compacting, Fibre-Reinforced, Geopolymer, and 3D-Printed concretes, bridging laboratory findings with real-world applications. Unlike existing reviews, it emphasizes structural implementation challenges and opportunities. Key obstacles—including high cost, lack of standards, and scalability—are outlined to contextualize pathways for sustainable adoption. Overall, next-generation concretes deliver enhanced strength, durability, and sustainability, making them viable for critical infrastructure. Future studies should focus on advancing standardization, integrating nanotechnology and AI for mix optimization, and developing cost-effective, large-scale deployment strategies. © 2025 The AuthorsItem bFLEX- γ : A Lightweight Block Cipher Utilizing Key Cross Approach via Probability Density Function(Springer Science and Business Media Deutschland GmbH, 2022) Das, A.K.; Kar, N.; Deb, S.; Singh, M.P.Most of the symmetric lightweight cryptosystems keep their block length fixed throughout the entire encryption making the cryptanalysis straightforward. It is of utmost importance to change the block size during the encryption process to introduce complexity in the cryptanalysis process. Therefore, a novel block cipher-based cryptosystem bFLEX-γ has been proposed in which the bit length of the ciphertext gets changed in the intermediate rounds. The key scheduling of the proposed cryptosystem takes place with the random diffusion of the auxiliary vector anticipated by a key crossing technique and linear shift feedback register (LFSR). Simulation results indicate that the proposed bFLEX-γ gives a satisfying level of security and robustness against linear cryptanalysis, differential cryptanalysis, eXtended Sparse Linearization (XSL) attacks and regression analysis attacks. The proposed cryptosystem is also validated by 0-1 balance factor of 0.020925 % and a kurtosis of 2.55, showing platykurtic in nature. All the statistical features and efficiency confirm that bFLEX-γ is very suitable for lightweight applications demanding data confidentiality. © 2022, King Fahd University of Petroleum & Minerals.Item Multi-objective optimization of one-part alkali-activated mortar mixes using Taguchi-Grey relational analysis(Elsevier Ltd, 2024) Mahendra, K.; Narasimhan, M.C.; Bhanu Prakash, G.; Das, A.K.In the context of the contemporary emphasis on sustainability within the realm of construction, there is a notable surge in attention towards one-part alkali-activated (OP-AA) materials. This is primarily attributed to their enhanced performance and reduced carbon emissions as compared to conventional OPC-based concrete. In the present investigation, Taguchi and Taguchi- Grey relational analysis (GRA) methodologies were employed to execute the experimental design, involving three input parameters, each considered at three levels, to generate an L9 orthogonal array. An attempt was made to assess the impact of different parameters, such as ground granulated blast furnace slag (GGBFS) to fly ash (FA) ratio - (S/F), water-to-binder ratio - (W/B), and percentage of Na2O - (N), on the slump flow, setting time, and compressive strength characteristics and hence to optimize the proportions of the OP-AA mortar blends. The results revealed that optimum parameter levels for multi-objective optimization corresponded to S/F = 1, W/B = 0.45, and N = 5%. For these parameter levels specified, the corresponding values of slump flow, initial setting time, final setting time, and 28 days compressive strength were 208 mm, 285.4 min, 990.4 min, and 36.52 MPa, respectively. In addition, to gain insights into their mineral composition, morphology, and chemical bond characteristics, microstructural characterization such as X-ray diffraction (XRD), Field emission scanning electron microscope (FESEM), and Fourier transform infrared spectroscopy (FTIR) were also conducted on selected OP-AA mortar mixes. The microstructural examination unveiled the predominant formation of hydration products, such as C/N -A-S-H gels, in OP-AA mortar blends, resembling those found in conventional alkali-activated materials (AAMs). During the validation phase, an assessment was conducted by comparing the actual experimental results with the predicted values obtained through regression equations. The outcome of this comparison revealed that the proposed optimum mix parameter levels demonstrated the effectiveness of both the Taguchi and Taguchi-GRA approaches. © 2024 Elsevier LtdItem Flexural and fracture performance of fiber reinforced self compacting alkali activated concrete– A DOE approach(Elsevier B.V., 2024) Prakash, G.B.; Prashanth, M.H.; Narasimhan, M.C.; Mahendra, K.; Das, A.K.Owing to their much-reduced carbon footprint and lower embodied energy, compared to conventional Portland Cement (OPC-based) Concrete mixes, Alkali Activated Concrete (AAC) mixes represent a pivotal advancement towards achieving sustainability goals. The fracture properties were investigated using Three-Point Bending Tests (3-PBT) under the mode I failure mechanism. This study utilises Taguchi analysis to analyse and optimise Self-Compacting Alkali-Activated Concrete (SAAC), focusing mainly on its flexural strength and fracture characteristics. An L-16 orthogonal array of experiments with three input parameters − replacement of Blast Furnace Slag (BFS) with Fly ash (FA) (0 %, 30 %, 40 %, and 50 %), Steel Fibers (SF) volume content (0 %, 0.25 %, 0.5 % and 0.75 %) and Notch to Depth (a0/d) ratio (0.2,0.3,0.4 and 0.5), at four levels each, was adopted. The Work of Fracture Method (WFM) and Double K Fracture Criterion (DKFC) were utilised to determine the Fracture Energy (GF) and fracture toughness, respectively. The results obtained from all the sixteen mixes showed that the F0-S0.75-N0.5 mix demonstrated better values in several parameters, such as flexural strength of 7.82 MPa,KICini of 0.928 MPa√m, KICuns of 6.99 MPa√m and KICini/ KICuns of 0.133. A maximum GF of 2350 N/m was obtained with F50-S0.75-N0.2 mix. However, all the inferior values of these parameters were observed with F50-S0-N0.5 mix, which recorded a flexural strength of 4.90 MPa, KICini of 0.612 MPa√m,KICuns of 1.16 MPa√m, KICini/ KICuns of 0.528 and GF of 125 N/m. Through Taguchi analysis, the optimal combination for flexural strength was identified as FA 0 %, SF 0.75 %, and a0/d 0.5 and for both Initial Fracture Toughness (KICini) and Unstable Fracture Toughness (KICuns) at FA 0 %, SF 0.75 % and a0/d 0.4. For both the ratio of initial to unstable fracture toughness (KICini/ KICuns) and fracture energy (GF), the optimum combination was FA 0 %, SF 0.75 % and a0/d 0.2. Furthermore, the results indicate that FA significantly influences KICini, while SF predominantly affects all other parameters. The predictive performance of the regression equations demonstrates good agreement with experimental outcomes. © 2024 Elsevier LtdItem Experimental investigation and optimization of one-part alkali-activated self-compacting concrete mixes(Elsevier Ltd, 2024) Mahendra, K.; Narasimhan, M.C.; Prakash, G.B.; Das, A.K.Emphasizing the growing importance of sustainability, alkali-activated materials (AAMs) have emerged as a revolutionary alternative for cement in the construction sector. This study delves into the fresh, mechanical, and microstructural properties of One-Part Alkali-activated Self-compacting Concrete (OPASC) mixes. While mixtures of Ground Granulated Blast Furnace Slag (GGBFS) and Fly Ash (FA) were utilized as the precursors, powdered sodium metasilicate was employed to function as the activator. To streamline experimental design and reduce the economic demands of extensive testing, the Taguchi-Grey Relational Analysis (GRA) was utilized to identify optimal multi-response parameter levels. This method considered binder content (B) within a range of 700–800 kg/m³, water-to-binder (W/B) ratios between 0.38 and 0.42, and Na2O percentages from 5 % to 7 % as key input variables. Results indicated that the designed mixes recorded workability values satisfying the EFNARC guidelines, compressive strengths greater than 60 MPa, split-tensile strengths in the range of 3.5–4.6 MPa, and flexural strengths varying between 5.5 and 7.2 MPa. The mix parameters for the optimal mix, with the highest mean grey relational grade, were identified from the Taguchi-GRPA approach as B = 750 kg/m3, W/B = 0.4, and N = 6 %. Microstructural analysis revealed the formation of C/N-A-S-H type gels, which are instrumental in developing a compact matrix enhancing the mechanical properties. A good agreement between actual experimental results obtained for a different set of verification mixes with those predicted by regression-equations confirmed the potency of the Taguchi-GRA approach in optimizing the OPASC mix parameters. © 2024 The AuthorsItem Durability performance of one-part alkali-activated self-compacting concrete mixes under aggressive and elevated temperature conditions(Elsevier B.V., 2025) Mahendra, K.; Narasimhan, M.C.; Rathod, S.; Das, A.K.; Prakash, G.B.The growing demand for sustainable, high-performance materials in modern construction has driven the development of advanced concrete technologies. This study introduces one-part alkali-activated self-compacting concrete (OPASC) as a practical, safe, and user-friendly alternative to conventional Portland cement-based concretes. Selected mixes with compressive strengths exceeding 70 MPa were evaluated for durability under aggressive conditions, including extended exposure to 5 % sulfuric acid and 5 % magnesium sulfate up to 180 days. The thermal stability of these candidate mixes was also assessed by subjecting the mixes to sustained temperatures ranging from 200 °C to 800 °C. Chloride-ion resistance of these mixes was examined under bulk diffusion tests. Key durability indicators, including water absorption, permeable voids, and sorptivity, were quantified to evaluate matrix impermeability. The results revealed compressive strength losses of 25–32 % under acid exposure, 7–15 % under sulfate exposure, and 30–42 % under thermal exposure, with chloride diffusion coefficients ranging from 0.21 × 10?12 to 0.32 × 10?12 m2/s, indicating high resistance to ionic ingress. The mixes also exhibited low water absorption (3–4.5 %), lower soptivities (0.0024–0.0013 mm/s1/2), and much reduced permeable voids (4.3–5.5 %), reflecting an impermeable, dense matrix. Microstructural analyses using SEM-EDS and XRD revealed that degradation under acid and sulfate conditions is primarily attributable to the decalcification of C/N-A-S-H gels, accompanied by the recrystallization of stable aluminosilicate phases. Finally, the environmental sustainability evaluation, which considered both embodied energy and carbon footprint, verified the superior environmental friendliness of OPASC mixes relative to conventional concrete. These findings confirm that OPASC exhibits superior chemical and thermal durability, reduced permeability, and enhanced resilience, thereby establishing it as a sustainable and practical solution for modern infrastructure applications. © 2025 Elsevier B.V.Item Performance evaluation of fiber reinforced self compacting alkali activated concrete mixes—a DoE approach(Springer Nature, 2025) Prakash, G.B.; Prashanth, M.H.; Narasimhan, M.C.; Mahendra, K.; Das, A.K.Alkali-activated concrete (AAC) has emerged as a sustainable alternative to conventional concrete due to its lower carbon emissions and effective use of industrial by-products. Several studies have explored the performance of AAC in both fresh and hardened states. However, the broader application of these mixes in real-time applications can be enhanced through further modifications to meet current needs. Fiber-reinforced self-compacting alkali-activated concrete (FSAAC) mixes represent one such class of innovative concrete mixes. This study evaluates the fresh-state, mechanical, and fracture properties of FSAAC mixes optimized using Taguchi’s design of experiments (DOE) methodology. An L9 orthogonal array was employed with three variables at three levels: fly ash (FA) content (30%, 40%, 50%) as partial replacement of blast furnace slag, steel fiber (SF) content (0.25%, 0.5%, 0.75% by concrete volume), and fiber aspect ratio (AR) (40, 60, 80). A control mix without FA and SF was included in the comparison study. All FSAAC mixes satisfied EFNARC guidelines for fresh-state properties. Fracture parameters were determined through three-point bending (TPB) tests. The F30-S0.75-A80 mix exhibited superior performance with compressive strength of 66.33 MPa, flexural strength of 7.05 MPa, initial fracture toughness (KICini) of 0.813 MPa?m, unstable fracture toughness (KICuns) of 6.123 MPa?m, fracture energy (GF) of 5513.80 N/m, and a toughness ratio of 0.133. Compared to the control mix, the mix F30-S0.75-A80 showed 22.6%, 14.35%, 313.15% and 2518% rise in flexural strength, KICini, KICuns and GF, respectively. Taguchi analysis identified optimal mix proportions for slump flow at FA 50%, SF 0.25%, AR 40, and for KICini at FA 30%, SF 0.75%, AR 60. For other properties, proportions were optimized at FA 30%, SF 0.75%, AR 80. Regression models developed exhibited high degree of predictive accuracy, closely aligning with experimental outcomes. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2025.