Faculty Publications
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Item Synthesis, structural, optical and electrical (DC) properties of a semiorganic Thiourea Barium Chloride (TBC) single crystal(Elsevier GmbH journals@elsevier.com, 2017) Mahendra, K.; D'Souza, A.; Udayashankar, N.K.In the present study, TBC single crystals were synthesized using solution evaporation method. The structural analysis of TBC crystal was studied using Powder X-ray diffraction. Optical properties of TBC crystals were carried out using UV–vis, FTIR, Raman and photoluminescence (PL) spectra. The crystal shows absorbance maxima at 209 nm and transparent in the visible region 300–900 nm. The various optical constants such as refractive index, reflectance, speed of light, extinction coefficient, electrical susceptibility, dielectric constant, optical and electrical conductivity were evaluated. Thus results revealed that all these optical constants shows a strong dependence on optical absorption coefficient. Using single oscillator model (Wemple ?Didomenico), lattice dielectric constant and the ratio of free charge carrier to their effective mass were evaluated. The PL study of TBC crystals shows two emission peaks (419 nm ?S, 441 nm- Ba) in blue region. The DC resistivity and conductivity of the crystal was investigated in the temperature range 26 °C–115 °C. © 2017 Elsevier GmbHItem Incorporation of graphite into iron decorated polypyrrole for dielectric and EMI shielding applications(Elsevier Ltd, 2020) Madhusudhan, C.K.; Mahendra, K.; Madhukar, B.S.; Somesh, T.E.; Faisal, M.Synthesis of polypyrrole (PPy) / graphite (G) composites via chemical oxidation polymerization method is reported. Synthesized composites were studied for its structural, optical and electrical properties. Structural and morphological studies were performed using powder X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and the effect of graphite on the properties of PPy is elaborated in detail. Further, the electrical measurements were carried using dielectric analyzer. Dielectric behavior of the composites was also compared with the PPy and graphite and confirmed the dependency of graphite on PPy/graphite nanocomposites. The EMI shielding response of PPy/graphite nanocomposites were measured in the microwave frequency range from 12 to 18 GHz (Ku band), of practical relevance. The observed effective electrical and EMI shielding properties highlights the prospects of multifunctional applications of these PPy/graphite nanocomposites. © 2020 Elsevier B.V.Item Multifunctional Polypyrrole/Multi-Walled Carbon Nanotube Composite Material: Dielectric, Humidity Sensing and Broadband EMI Shielding Properties(Pleiades journals, 2021) Madhusudhan, C.K.; Mahendra, K.; Madhukar, B.S.; Somesh, T.E.; Faisal, M.Abstract: Conducting polymer composites with suitable combination of two components in nanoscale are expected to facilitate wider applications of composites. In this study, polypyrrole (PPy)/multiwalled carbon nanotube (MWCNT) nanocomposites prepared by in situ chemical oxidative polymerization were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and UV–Vis spectroscopy. The electrical and dielectric properties were investigated using complex impedance technique. Complex dielectric permittivity, complex electric modulus and complex impedance variations were studied in the frequency range of 10 Hz–1 MHz. The dielectric attributes and conductivity data of different nanocomposites analyzed as a function of frequency revealed that the incorporation of MWCNT phase into polypyrrole matrix affects the electrical and dielectric properties of the composites. Humidity sensor measurements show that PPy/MWCNT can be effectively optimized as candidate for humidity detection. The nanocomposites were also studied for broadband EMI shielding applications by characterizing the materials in the 12?18 GHz (Ku band) to understand the shielding properties. The absorption dominant shielding effectiveness (SE) observed in the range of ?13 to ?15 dB was stable with visible variation with varying concentration of MWCNT in PPy. The experimental results of dielectric characterization, humidity sensing and EMI shielding response reveal that PPy/MWCNT composite can be suitable multifunctional material for various applications. © 2021, Pleiades Publishing, Ltd.Item A novel approach to the synthesis of semiorganic ammonium hydrogen oxalate oxalic acid dihydrate single crystal and its characterization(Springer Science and Business Media B.V., 2021) Mahendra, K.; Fernandes, J.M.; Udayashankar, N.K.A different approach used in the synthesis of ammonium hydrogen oxalate oxalic acid dihydrate (NH4H3(C4O8)·2H2O) single crystals is presented. The crystals are synthesized using solvent evaporation technique. The analysis of the synthesized NH4H3(C4O8)·2H2O single crystals has been carried out through measurements such as single-crystal X-ray diffraction (SCXRD), powder XRD, UV–Vis and photoluminescence spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric and differential thermal analysis (TG–DTA), Vickers microhardness measurements and current versus voltage (I–V) characteristics. The evaluation of the molecular structure of the crystals using SCXRD shows that they belong to the triclinic crystal system with space group P-1. The structural properties of the crystals are also evaluated using powder XRD measurements on their finely crushed powder. These measurements corroborate the results of SCXRD and confirm the triclinic system of the crystals. The optical properties like absorbance and emission of these crystals are determined using UV–Vis and photoluminescence spectroscopy, respectively. These measurements show that the crystals exhibit substantial emission in the blue region. The thermal stability and the decomposition mechanism of the crystals is studied using TG–DTA analysis. The mechanical strength of the crystals is determined using Vickers microhardness technique. The photoresponse of the NH4H3(C4O8)·2H2O crystals is analyzed from light-dependent I–V characteristics, and it is observed that the crystals exhibit photoresistive behavior. © 2020, Akadémiai Kiadó, Budapest, Hungary.Item Defect-induced measurements of semi-organic ammonium hydrogen oxalate oxalic acid dihydrate single crystals using gamma irradiation(Taylor and Francis Ltd., 2022) Mahendra, K.; Maria Fernandes, J.; Udayashankar, N.K.The structural, optical, mechanical and electrical properties of pure and 5—20-kGy gamma-irradiated semi-organic single crystals of ammonium hydrogen oxalate oxalic acid dihydrate (NH4H3(C4O8).2H2O) are presented. The crystals were synthesized at room temperature using facile solvent evaporation technique. Powder XRD measurements indicate gradual enhancement in crystallinity and lattice defect annihilation for low radiation dosage. Radiation-induced increase in optical band gap (4.01–4.16 eV) indicates high damage threshold of the crystals. Quenching of photoluminescence is attributed to the lowering of surface defect density with radiation. The influence of gamma radiation on the functional vibrations of the crystals is studied using FTIR-Raman spectroscopy. Vickers microhardness measurements show gradual enhancement in crystal hardness with the increase in radiation. An increase in forward resistance with irradiation is observed from I–V measurements and is attributed to high transparency of the crystals. These results indicate the viability of NH4H3(C4O8) 2H2O crystals in potential space optoelectronic applications. © 2022 Informa UK Limited, trading as Taylor & Francis Group.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 Ltd