Mahesh Kumar

Bio: Mahesh Kumar is an academic researcher from Indian Institute of Technology, Jodhpur. The author has contributed to research in topics: Molecular beam epitaxy & Heterojunction. The author has an hindex of 29, co-authored 204 publications receiving 4864 citations. Previous affiliations of Mahesh Kumar include Indian Institutes of Technology & Indian Institute of Technology Delhi.

Effect of Gamma Ray Irradiation on Epitaxial Pb(Zr,Ti)O 3 /SrRuO 3 Tunable Varactor Devices

Abstract: Epitaxial morphotropic PbZr0.52Ti0.48O3 (PZT) thin films were employed to enhance the dielectric tunability of microwave filter devices without compromising the device impedance matching and low bias voltage (<10 V) requirements. Epitaxial heterostructure of ferroelectric PZT(001)/SrRuO3 (SRO) were grown on single crystal SrTiO3 (001) substrates by pulsed laser deposition, and a platinum (Pt) electrode was deposited on top of the PZT film. The tunability of the Pt/PZT/SRO varactor devices are strongly dependent on bias voltage and exhibited good dielectric tunability of 55% at 100 kHz and 10 V. The capacitance (CP) of the heteroepitaxial varactor devices was 105 pF at 10 V applied bias with a corresponding small leakage current of 1 nA. The influence of gamma (γ-ray) irradiation on the intrinsic electrical properties of the epitaxial PZT varactor devices was investigated as a function of the irradiation dose from 0 kGy to 400 kGy, in terms of the capacitance–voltage (C–V) characteristics and loss tangent response. With enhancing γ-ray irradiation doses the ferroelectric capacitance was found to decrease accompanying degradation in the loss tangent values. The results indicate that tunability of the epitaxial PZT ferroelectric thin-film capacitors decreased with increasing gamma irradiation dose and degraded ∼25% at 400 kGy dose than unexposed devices. Possible reasons for the degradation behavior of dielectric properties and tunability due to radiation-induced defects has been discussed.

Optimal schemes for channel assignment problem in wireless networks modeled as 2-dimensional square grids

Abstract: This paper presents optimal schemes for channel assignment problem in wireless networks modeled as 2-dimensional square grids. Given the reuse distance σ (an integer ≥ 5 ), using minimum number of colors, our algorithm assigns colors(channels) to the vertices (radio stations) in such a way that the color difference between the adjacent vertices is at least 2, and two vertices x and y receive the same color only when the distance between x and y is at least σ.

Investigations of Vacancy-Assisted Selective Detection of NO2 Molecules in Vertically Aligned SnS2.

Abstract: Two important methods for enhancing gas sensing performance are vacancy/defect and interlayer engineering. Tin sulfide (SnS2) has recently attracted much attention for sensing of the NO2 gas due to its active surface sites and tunable electronic structure. Herein, SnS2 has been synthesized by the chemical vapor deposition (CVD) method followed by nitrogen plasma treatment with different exposure times for fast detection of NO2 molecules. Plasma treatment created a substantial number of surface vacancies on SnS2 flakes, which were controlled by the exposure period to modify the surface of flakes. After 12 min of nitrogen plasma treatment, SnS2 nanoflakes show considerable improvement in NO2 sensing characteristics, including a high sensing response of ∼264% toward 100 ppm NO2 at 120°C. The enhancement in the relative response of the sensor is due to the electronic interaction between NO2 molecules and the S vacancies on the surface of SnS2. Density functional theory (DFT) computations indicate that the S-vacancy defects on the surface dominate the effective NO2 detection and the NO2 adsorption mechanism transition from physisorption to chemisorption. Adsorption kinetics of the NO2 gas over SnS2 nanoflake-based chemiresistor sensors were studied using the Lee and Strano model [ Langmuir 2005, 21(11), 5192-5196]. The irreversible rate of the reaction for various NO2 concentrations exposed to the gas sensor is extracted using this model, which also appropriately describes the response curves. The forward rate constant of the irreversible gas sensor increased with the increase of the N2 plasma treatment time and reached the maximum in the 12 min plasma-treated sample. Through defect engineering, this research may open up new vistas for the design and synthesis of 2D materials with enhanced sensing properties.

Perspective on the Development of Lead-free Piezoceramics

Abstract: A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Room-temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid liquid phase sintering

Abstract: Single-phased ferroelectromagnet BiFeO3 ceramics with high resistivity were synthesized by a rapid liquid phase sintering technique. Saturated ferroelectric hysteresis loops were observed at room temperature in the ceramics sintered at 880 °C for 450 s. The spontaneous polarization, remnant polarization, and the coercive field are 8.9 μC/cm2, 4.0 μC/cm2, and 39 kV/cm, respectively, under an applied field of 100 kV/cm. It is proposed that the formation of Fe2+ and an oxygen deficiency leading to the higher leakage can be greatly suppressed by the very high heating rate, short sintering period, and liquid phase sintering technique. The latter was also found effective in increasing the density of the ceramics. The sintering technique developed in this work is expected to be useful in synthesizing other ceramics from multivalent or volatile starting materials.

Room-temperature gas sensing of ZnO-based gas sensor: A review

Abstract: Novel gas sensors with high sensing properties, simultaneously operating at room temperature are considerably more attractive owing to their low power consumption, high security and long-term stability. Till date, zinc oxide (ZnO) as semiconducting metal oxide is considered as the promising resistive-type gas sensing material, but elevated operating temperature becomes the bottleneck of its extensive applications in the field of real-time gas monitoring, especially in flammable and explosive gas atmosphere. In this respect, worldwide efforts have been devoted to reducing the operating temperature by means of multiple methods In this communication, room-temperature gas sensing properties of ZnO based gas sensors are comprehensively reviewed. Much more attention is particularly paid to the effective strategies that create room-temperature gas sensing of ZnO based gas sensors, mainly including surface modification, additive doping and light activation. Finally, some perspectives for future investigation on room-temperature gas-sensing materials are discussed as well.