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Neoarchean microblock amalgamation in southern India: Evidence from the Nallamalai Suture Zone
No full textA collage of crustal blocks ranging in age from Mesoarchean to Neoarchean accreted to the southem margin of the Dharwar Craton in Peninsular India preserve distinct evolutionary history, but share a common metamorphic record during Archean-Proterozoic transition. Here we investigate the tectonic boundary between two of these microblocks, the Shevaroy Block to the west and the Madras Block to east, termed as the Nallamalai Suture Zone (NLSZ). We present integrated field, petrological, geochemical and zircon U-Pb and Lu-Hf data from a suite of meta-igneous and metasedimentary units along the NLSZ and its flanks. Zircon grains from the meta-monzo-granite, homblende-biotite gneiss, amphibolite, granodiorite, diorite, charnockite, BIF (banded iron formation) and BMQ from this area show magmatic emplacement ages clustering around 2.50 to 2.56 Ga, except for the meta-monzo-granite showing ages up to 3.2 Ga, correlating with long-lived convergent margin magmatism through multiple slab melting episodes. All the rocks show tightly constrained early Paleoproterozoic (ca. 2.46-2.48 Ga) metamorphic ages, including the metamorphic zircon in the BIFs, marking the timing of collision of the two continental blocks with consumption of the intervening oceanic lithosphere. The epsilon Hf(t) values of magmatic zircon grains from the different rock types range from - 4.1 to + 5.7, and together with T-DM(C) Hf model ages of 2672-3247 Ma, a dominantly juvenile crust growth is indicated, which initiated around 3.3 Ga and continued to 2.7 Ga, followed by crustal reworking in a continental arc towards the end of Neoarchean. The geochemical data suggest that tholeiitic to calc-alkaline parental melts for the mafic-intermediate-felsic suite were generated by low degree partial melting of a peridotitic mantle wedge metasomatized by subduction-derived fluids and sediments and continuous slab melting ensued by intracrustal fractional crystallization of melts with reworking of older continental crust. The BIF and BMQ samples display positive Eu anomalies, negative to negligible Ce anomalies, and superchondritic Y/Ho ratios suggesting their formation in an oceanic realm proximal to an active continental margin setting. The magmatic and tectonic attributes are consistent with eastward oceanic subduction and ocean closure along the NLSZ which is defined here as the trace of a suture welding the Shevaroy and Madras Blocks. We envisage that multiple subduction and amalgamation of several microblocks occurred in the Dharwar Craton and its southern domains, amalgamating several microblocks during the Archean - Paleoproterozoic transition
Microstructural Evolution and Mechanical Properties of Direct Metal Laser-Sintered (DMLS) CoCrMo After Heat Treatment
No full textMicrostructures and tensile properties of Direct Metal Laser-Sintered (DMLS) CoCrMo were investigated in the as-printed condition and after heat treatment. A dense (> 99.5 pct) as-printed DMLS CoCrMo was obtained in the as-printed condition eliminating the need for any hot isostatic pressing. Solution heat treatment carried out at 1150 degrees C revealed complete recrystallization resulting in an equiaxed grain structure with an average grain size of 40 mu m. The microstructure after solution heat treatment and aging at 980 degrees C revealed inter and intragranular precipitations, enriched in Mo and Si. Solution treatment resulted in the decrease of the room-temperature tensile strength from 1378 MPa (as-printed) to 1114 MPa, which was attributed to the increasing grain size from 0.6 to 1 mu m (column width) to similar to 40 mu m (grain size). The decrease in yield strength was accompanied by the increasing ductility from 5.7 to 15 pct. An enhancement in ductility to nearly 25 pct was observed in tensile tests at 925 degrees C. This paper comprises a detailed microstructural evaluation of DMLS CoCrMo alloy to determine its suitability for high-temperature structural applications involving repair and refurbishment of components, including an evaluation of microstructural and tensile properties after welding the DMLS CoCrMo to cast FSX414
Substrate screening effects on the quasiparticle band gap and defect charge transition levels in MoS2
No full textMonolayer MoS2 has emerged as an interesting material for nanoelectronic and optoelectronic devices. The effect of substrate screening and defects on the electronic structure of MoS2 are important considerations in the design of such devices. We find a giant renormalization to the free-standing quasiparticle band gap in the presence of metallic substrates, in agreement with recent scanning tunneling spectroscopy and photoluminescence experiments. Our sulfur vacancy defect calculations using the density functional theory plus GW formalism, reveal two charge transition levels (CTLs) in the pristine band gap of MoS2. The (0/-1) CTL is significantly renormalized with the choice of substrate, with respect to the pristine valence band maximum (VBM). The (+1/0) level, on the other hand, is pinned 100 meV above the pristine VBM for the different substrates. This opens up a pathway to effectively engineer defect charge transition levels in two-dimensional materials through the choice of substrate
Crystallite size dependent exchange bias in MgFe2O4 thin films on Si(100)
No full textSingle phase materials showing large Exchange Bias (EB) at elevated temperatures are highly desirable for their technological advantage. However, the fundamental interactions leading to the effect are often very complex and ill-understood. Here, we report the observation of EB in single phase (111) oriented MgFe2O4 (MFO) thin films on MgO buffered Si(100). The effect is seen to persist till a significantly higher temperature of 110 K. Measurements on stoichiometric powder MFO showed the absence of EB, lower coercivity, and an enhanced saturation magnetization in comparison to thin films. This suggests the modified exchange interaction in the thin films possibly due to the presence of growth induced magnetic and structural disorder. Investigation on the nature of this EB produced some results which are akin to a conventional FM/AFM coupled system like the training effect. However, the same could not explain the observed variation in coercivity and the exchange bias field with temperature and cooling fields. We propose the existence of a disordered layer with random spin orientations at the crystalline grain boundaries of our film, promoting the AFM interaction between the ordered ferrimagnetic grains once field cooled below the spin freezing temperature. This generates a unidirectional anisotropy along the field direction which manifests as a loop shift in hysteresis measurements. A further study shows that the EB field can be varied systematically by changing the thickness of the thin films. This indicates a simple way to tune the disorder for achieving desirable magnetic properties in single phase magnetic thin films. Published by AIP Publishing
A technique for measuring impedance of cryogenic eddy current sensors using dual switched oscillators
No full textElectrical conductivity measurement using an eddy current sensor requires measuring the absolute change in the impedance of the sensing coil. There have been many different attempts at this by using phase sensing circuits, utilizing impedance analyzers etc. This work utilizes two different oscillator circuit (LC oscillator using an unbuffered inverter and Relaxation Oscillator) to determine the change in impedance of the coil. The LC oscillator is designed in such a way that the circuit is insensitive to change in the series resistance of the sensing coil and hence, only a function of the inductance of the coil. Once the inductance of the coil is determined, a relaxation oscillator is used in an operating region in which the output frequency is a strong function of the series resistance of the coil. This switched oscillator circuit technique is used to determine the impedance of a cryogenic sensing coil at 295K and 77K with a Niobium (Nb) target kept at specific distance. Experimental data is compared with that of an impedance analyzer circuit and measurement errors are presented
Vector extrapolation methods for accelerating iterative reconstruction methods in limited-data photoacoustic tomography
No full textAs limited data photoacoustic tomographic image reconstruction problem is known to be ill-posed, the iterative reconstruction methods were proven to be effective in terms of providing good quality initial pressure distribution. Often, these iterative methods require a large number of iterations to converge to a solution, in turn making the image reconstruction procedure computationally inefficient. In this work, two variants of vector polynomial extrapolation techniques were deployed to accelerate two standard iterative photoacoustic image reconstruction algorithms, including regularized steepest descent and total variation regularization methods. It is shown using numerical and experimental phantom cases that these extrapolation methods that are proposed in this work can provide significant acceleration (as high as 4.7 times) along with added advantage of improving reconstructed image quality. (c) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE
Facile fabrication and electrical investigations of nanostructured p-Si/n-TiO2 hetero-junction diode
No full textIn this work, we have fabricated the nanostructured p-Si/n-TiO2 hetero-junction diode by using a facile spin-coating method. The XRD analysis suggests the presence of well crystalline anatase TiO2 film on Si with small grain size (similar to 16 nm). We have drawn the band alignment using Anderson model to understand the electrical transport across the junction. The current-voltage (J-V) characteristics analysis reveals the good rectification ratio (103 at +/- 3 V) and slightly higher ideality factor (4.7) of our device. The interface states are responsible for the large ideality factor as Si/TiO2 form a dissimilar interface and possess a large number of dangling bonds. The study reveals the promises to be used Si/TiO2 diode as an alternative to the traditional p-n homo-junction diode, which typically require high budget
A comparison of the self-assembly behaviour of sodium N-lauroyl sarcosinate and sodium N-lauroyl glycinate surfactants in aqueous and aqueo-organic media
No full textSelf-assembly of surfactants is influenced by various intermolecular interactions and molecular structure, which dictate packing of molecules in the aggregate and its microstructure. Hydrogen-bonding between amide groups plays a key role in the self-assembly process of N-acyl amino acid surfactants (NAAS). The self-assembly properties of two NAAS, sodium N-lauroyl sarcosinate (SLS) and sodium N-lauroyl glycinate (SLG) that differ only in the head-group structure were compared in aqueous and aqueo-organic media by using a number of methods, including surface tension fluorescence, dynamic light scattering, calorimetry, and microscopy. It was observed that aggregate formation is more favoured in SLG. Studies revealed that while SLS formed small spherical micelles, SLG produced unilamellar vesicles in pH 7 buffer above critical micelle concentration at 25 degrees C. The stability of SLG vesicles with respect to pH and temperature was also investigated. Furthermore, both SLG and SLS were found to gelify aquo-organic mixtures of varying composition upon heat-cool treatment. Their gelation behaviour was compared by measuring minimum gelation concentration, molecular packing, and morphology and mechanical stability of the thermoreversible gels. The difference in self-assembly behaviour in water as well as in aqueo-organic mixtures was attributed to the steric repulsion and hydrogen-bonding interaction at the head-group of the molecules. (C) 2018 Elsevier Inc. All rights reserved
A New Two-Phase Five-Level Converter for Three-Phase Isolated Grid-Tied Systems With Inherent Capacitor Balancing and Reduced Component Count
No full textThis paper presents a single dc source fed five-level converter topology for medium-voltage high-power isolated grid-tied systems with reduced power switch count. The proposed converter generates two-phase systems of voltages that are further converted to three phases by using Scott-T transformer connection. Compared to existing topologies, the proposed converter requires fewer power switches, gate drivers, and power diodes. An added advantage is that only a single dc source is required. An important feature of this topology is that the dc-link capacitor voltages have an inherent ability to self-balance by themselves without the need for extra balance circuitry and special control algorithms. The voltage stress across the power devices of the proposed converter is equal to half of dc-link voltage. A level-shifted carrier-based pulsewidth modulation scheme is used to inject current into the grid at unity power factor. Analysis and simulation and experimental results are presented to validate the performance of the proposed converter under grid-tied operation
High-entropy alloys and metallic nanocomposites: Processing challenges, microstructure development and property enhancement
No full textTwo classes of new materials, i.e. high entropy alloys (HEAs) and metallic nanocomposites offer processing related challenges, while showing significant promise for an array of technological applications requiring wear and irradiation resistance. This review addresses those challenges together with microstructure-property correlations. In particular, the main focus of this review is to demonstrate the efficacy of the superfast densification route, i.e., spark plasma sintering (SPS) as an effective consolidation route for these two classes of materials. To start with, this review will critically analyze the influence of conventional solidification on the microstructure and property of Cu-based bearing alloys. Using nanocrystalline Cu-Pb and Cu-Pb-TiB2, Cu-Pb-cBN (cubic boron nitride) as model systems, various aspect of the microstructure -property correlation and enhancement of the tribological properties will be highlighted. A thorough understanding of the processing related issues and stability of the nanoscale/ultrafine microstructure obtained via SPS will be illustrated. A significant part of this review will further discuss the development of novel nanostructured HEAs and HEA-based composites for wear and irradiation resistance applications. The efficacy of SPS route to prepare bulk HEAs with high sinter density will be demonstrated, together with property enhancement in single phase and two-phase HEAs. HEA-based nanocomposites containing soft metallic dispersoids (Bi, Pb, Ag) and ceramic lubricating phases (MoS2, CaF2/BaF2) for wear resistance application will be highlighted. Finally, oxide dispersed and refractory HEM via MASPS route for irradiation resistance application will be discussed to elucidate the effective usage of the design and development of HEAs for technologically critical applications