Geogy J. Abraham

Optimal electron irradiation as a tool for functionalization of MoS2: Theoretical and experimental investigation

We demonstrate the utility of electron irradiation as a tool to enhance device functionality of graphene-analogous MoS2. With the help of first-principles based calculations, vacancy-induced changes of various electronic properties are shown to be a combined result of crystal-field modification and spin-orbital coupling. A comparative theoretical study of various possible vacancy configurations both in bulk and monolayer MoS2 and related changes in their respective band-structures help us to explain plausible irradiation induced effects. Experimentally, various structural forms of MoS2 in bulk, few layered flakes, and nanocrystals are observed to exhibit important modification of their magnetic, transport, and vibrational properties, following low doses of electron irradiation. While irradiated single crystals and nanocrystals show an enhanced magnetization, transport properties of few-layered devices show a significant increase in their conductivity, which can be very useful for fabrication of electronic...

Perovskite-Ni composite: a potential route for management of radioactive metallic waste.

Management of nickel - based radioactive metallic wastes is a difficult issue. To arrest the release of hazardous material to the environment it is proposed to develop perovskite coating for the metallic wastes. Polycrystalline BaCe0.8Y0.2O3-δ perovskite with orthorhombic structure has been synthesized by sol-gel route. Crystallographic analyses show, the perovskite belong to orthorhombic Pmcn space group at room temperature, and gets converted to orthorhombic Incn space group at 623K, cubic Pm3m space group (with a=4.434Å) at 1173K and again orthorhombic Pmcn space group at room temperature after cooling. Similar observations have been made from micro-Raman study as well. Microstructural studies of BaCe0.8Y0.2O3-δ-NiO/Ni composites showed absence of any reaction product at the interface. This suggests that both the components (i.e. perovskite and NiO/Ni) of the composite are compatible to each other. Interaction of BaCe0.8Y0.2O3-δ-NiO/Ni composites with simulated barium borosilicate waste glass melt also did not reveal any reaction product at the interfaces. Importantly, uranium from the waste glass melt was found to be partitioned within BaCe0.8Y0.2O3-δ perovskite structure. It is therefore concluded that BaCe0.8Y0.2O3-δ can be considered as a good coating material for management of radioactive Ni based metallic wastes.

Comparing Reactivation Behavior of TIG and Laser Beam Welded Alloy 690

The nickel base Alloy 690 was subjected to simulated autogenous welding treatment employing two different techniques, laser beam welding (LBW) and tungsten inert gas (TIG) welding. The resultant weld fusion zone (WFZ) and heat-affected zone (HAZ) were compared by studying the reactivation behavior. The chromium depletion effect was assessed by measuring the degree of sensitization (DOS) from the electrochemical potentiodynamic reactivation (EPR) test. A double-loop EPR test for Alloy 690 was employed to measure the DOS at different regions of weldments by masking the remaining regions. The results clearly demonstrated that Alloy 690 showed no sensitization in the parent material and the WFZ region of both TIG and laser weldments. However, it exhibited reactivation in the HAZ region of both the weldments. The DOS values measured for Alloy 690 were very low for all the regions of the LBW weldment as compared to that in the TIG weldment. The HAZ region of the LBW weldment showed the highest DOS value in any region of the weldment but even this value was quite low indicating absence of sensitization in LBW weldment. The attack along the grain boundaries for the weldments after EPR experiments were studied using optical and scanning electron microscopy.

Intergranular Corrosion Susceptibility of Alloy 600 after Autogenous Tungsten Inert Gas and Laser Beam Welding using Electrochemical Technique

Abstract Intergranular corrosion and intergranular stress corrosion cracking is influenced by precipitation of chromium carbides at grain boundaries and formation of chromium depletion regions. The present study focuses on understanding the carbide precipitation and subsequent sensitization in the weldments of Alloy 600 using two different welding techniques. The effect of heat input on microstructure and IGC susceptibility was measured using electrochemical reactivation test. The SEM studies were done to evaluate the presence of chromium depleted regions. The carbide was indentified to be Cr7C3 using TEM. The laser beam weldments showed an increased resistance to IGC as compared to TIG weldments.

Role of Substrate Temperature in the Pulsed Laser Deposition of Zirconium Oxide Thin Film

Thin films of zirconium oxide have been deposited using pulsed laser deposition on Zr-base alloy substrates, held at 300 K, 573 K and 873 K, in order to understand the effect of substrate temperature on the deposited film. In this study, a KrF excimer laser having 30 ns pulse width and 600 mJ energy at source has been used for depositing the films from a sintered ZrO2 target using a laser fluence of 5 J.cm-2. After visual observation, deposited thin films were examined using Raman Spectroscopy (RS) and X-ray Photo-electron Spectroscopy (XPS). It has been found that the oxide deposited at 300 K temperature does not show good adherence with the substrate. The oxide films deposited at 573 K and 873 K are found to be adherent with the substrate and lustrous black in appearance. Thin films of zirconia, deposited using pulsed laser on the Zr-base metallic substrate are initially in amorphous state and possibly little deficient in oxygen. The substrate temperature and the duration of holding at high temperature are responsible for the evolution of nanocrystals in the deposited thin films. The stoichiometry of the amorphous oxide film supports its crystallization, below 573 K, into monoclinic and tetragonal phases and not into cubic phase.