Showing papers by "Vikram Sarabhai Space Centre published in 2017"

Two-Dimensional Titanium Nitride (Ti2N) MXene: Synthesis, Characterization, and Potential Application as Surface-Enhanced Raman Scattering Substrate

Abstract: We report on the synthesis, characterization, and application of Ti2N (MXene), a two-dimensional transition metal nitride of M2X type. Synthesis of nitride-based MXenes (Mn+1Nn) is difficult due to their higher formation energy from Mn+1ANn and poor stability of Mn+1Nn layers in the etchant employed, typically HF. Herein, the selective etching of Al from ternary layered transition metal nitride Ti2AlN (MAX) and intercalation were achieved by immersing the powder in a mixture of potassium fluoride and hydrochloric acid. The multilayered Ti2NTx (T is the surface termination) obtained was sonicated in DMSO and centrifuged to obtain few-layered Ti2NTx. MXene formation was verified, and the material was completely characterized by Raman spectroscopy, XRD, XPS, FESEM-EDS, TEM, STM, and AFM techniques. Surface-enhanced Raman scattering (SERS) activity of the synthesized Ti2NTx was investigated by fabricating paper, silicon, and glass-based SERS substrates. A Raman enhancement factor of 1012 was demonstrated usin...

Soft X-ray Focusing Telescope Aboard AstroSat: Design, Characteristics and Performance

Abstract: The Soft X-ray focusing Telescope (SXT), India’s first X-ray telescope based on the principle of grazing incidence, was launched aboard the AstroSat and made operational on October 26, 2015. X-rays in the energy band of 0.3–8.0 keV are focussed on to a cooled charge coupled device thus providing medium resolution X-ray spectroscopy of cosmic X-ray sources of various types. It is the most sensitive X-ray instrument aboard the AstroSat. In its first year of operation, SXT has been used to observe objects ranging from active stars, compact binaries, supernova remnants, active galactic nuclei and clusters of galaxies in order to study its performance and quantify its characteriztics. Here, we present an overview of its design, mechanical hardware, electronics, data modes, observational constraints, pipeline processing and its in-orbit performance based on preliminary results from its characterization during the performance verification phase.

Declining pre-monsoon dust loading over South Asia: Signature of a changing regional climate

Abstract: Desert dust over the Indian region during pre-monsoon season is known to strengthen monsoon circulation, by modulating rainfall through the elevated heat pump (EHP) mechanism. In this context, an insight into long term trends of dust loading over this region is of significant importance in understanding monsoon variability. In this study, using long term (2000 to 2015) aerosol measurements from multiple satellites, ground stations and model based reanalysis, we show that dust loading in the atmosphere has decreased by 10 to 20% during the pre-monsoon season with respect to start of this century. Our analysis reveals that this decrease is a result of increasing pre-monsoon rainfall that in turn increases (decreases) wet scavenging (dust emissions) and slowing circulation pattern over the Northwestern part of the sub-continent.

Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki

Abstract: After the arrival of Akatsuki spacecraft of Japan Aerospace Exploration Agency at Venus in December 2015, the radio occultation experiment, termed RS (Radio Science), obtained 19 vertical profiles of the Venusian atmosphere by April 2017. An onboard ultra-stable oscillator is used to generate stable X-band downlink signals needed for the experiment. The quantities to be retrieved are the atmospheric pressure, the temperature, the sulfuric acid vapor mixing ratio, and the electron density. Temperature profiles were successfully obtained down to ~ 38 km altitude and show distinct atmospheric structures depending on the altitude. The overall structure is close to the previous observations, suggesting a remarkable stability of the thermal structure. Local time-dependent features are seen within and above the clouds, which is located around 48–70 km altitude. The H2SO4 vapor density roughly follows the saturation curve at cloud heights, suggesting equilibrium with cloud particles. The ionospheric electron density profiles are also successfully retrieved, showing distinct local time dependence. Akatsuki RS mainly probes the low and middle latitude regions thanks to the near-equatorial orbit in contrast to the previous radio occultation experiments using polar orbiters. Studies based on combined analyses of RS and optical imaging data are ongoing.

The Cadmium Zinc Telluride Imager on AstroSat

Abstract: The Cadmium Zinc Telluride Imager (CZTI) is a high energy, wide-field imaging instrument on AstroSat. CZTI’s namesake Cadmium Zinc Telluride detectors cover an energy range from 20 keV to $$>200$$ keV, with 11% energy resolution at 60 keV. The coded aperture mask attains an angular resolution of 17 $$^\prime $$ over a 4.6 $$^\circ $$ $$\times $$ 4.6 $$^\circ $$ (FWHM) field-of-view. CZTI functions as an open detector above 100 keV, continuously sensitive to GRBs and other transients in about 30% of the sky. The pixellated detectors are sensitive to polarization above $$\sim $$ 100 keV, with exciting possibilities for polarization studies of transients and bright persistent sources. In this paper, we provide details of the complete CZTI instrument, detectors, coded aperture mask, mechanical and electronic configuration, as well as data and products.

A European aerosol phenomenology - 6: Scattering properties of atmospheric aerosol particles from 28 ACTRIS sites

Abstract: This paper presents the light-scattering properties of atmospheric aerosol particles measured over the past decade at 28 ACTRIS observatories, which are located mainly in Europe. The data include particle light scattering (σsp) and hemispheric backscattering (σbsp) coefficients, scattering Angstrom exponent (SAE), backscatter fraction (BF) and asymmetry parameter (g). An increasing gradient of σsp is observed when moving from remote environments (arctic/mountain) to regional and to urban environments. At a regional level in Europe, σsp also increases when moving from Nordic and Baltic countries and from western Europe to central/eastern Europe, whereas no clear spatial gradient is observed for other station environments. The SAE does not show a clear gradient as a function of the placement of the station. However, a west-to-east-increasing gradient is observed for both regional and mountain placements, suggesting a lower fraction of fine-mode particle in western/south-western Europe compared to central and eastern Europe, where the fine-mode particles dominate the scattering. The g does not show any clear gradient by station placement or geographical location reflecting the complex relationship of this parameter with the physical properties of the aerosol particles. Both the station placement and the geographical location are important factors affecting the intraannual variability. At mountain sites, higher σsp and SAE values are measured in the summer due to the enhanced boundary layer influence and/or new particle-formation episodes. Conversely, the lower horizontal and vertical dispersion during winter leads to higher σsp values at all low-altitude sites in central and eastern Europe compared to summer. These sites also show SAE maxima in the summer (with corresponding g minima). At all sites, both SAE and g show a strong variation with aerosol particle loading. The lowest values of g are always observed together with low σsp values, indicating a larger contribution from particles in the smaller accumulation mode. During periods of high σsp values, the variation of g is less pronounced, whereas the SAE increases or decreases, suggesting changes mostly in the coarse aerosol particle mode rather than in the fine mode. Statistically significant decreasing trends of σsp are observed at 5 out of the 13 stations included in the trend analyses. The total reductions of σsp are consistent with those reported for PM2.5 and PM10 mass concentrations over similar periods across Europe. © Author(s) 2018.

Radiative effects of absorbing aerosols over northeastern India: Observations and model simulations

Abstract: Multi-year measurements of spectral properties of aerosol absorption are examined over four geographically distinct locations of northeastern India. Results indicated significant spatio-temporal variation in aerosol absorption coefficients (σabs) with highest values in winter and lowest in monsoon. The western parts of the region, close to the outflow of Indo-Gangetic Plains, showed higher values of σabs and black carbon (BC) concentration - mostly associated with fossil fuel combustion. But, the eastern parts showed higher contributions from biomass burning aerosols, as much as 20-25% to the total aerosol absorption, conspicuously during pre-monsoon season. This is attributed to a large number of burning activities over the Southeast Asian region, as depicted from MODIS fire count maps, whose spatial extent and magnitude peaks during March/ April. The nearly consistent high values of Aerosol Index (AI) and layer height from OMI indicates the presence of absorbing aerosols in the upper atmosphere. The observed seasonality has been captured fairly well by GOCART as well as WRF-Chem model simulations. The ratio of column integrated optical depths due to particulate organic matter (POM) and BC from GOCART showed good coincidence with satellite based observations, indicating the increased vertical dispersion of absorbing aerosols, probably by the additional local convection due to higher fire radiative power caused by the intense biomass burning activities. The WRF-Chem though underperformed by different magnitude in winter, the values are closer or overestimated near the burnt areas. Atmospheric forcing due to BC was highest (~30 Wm-2) over the western part associated with the fossil fuel combustion.

Processing and characterization of spark plasma sintered copper/carbon nanotube composites

Abstract: A Copper (Cu) matrix composites reinforced with 0.2, 5 and 10 vol% single walled carbon nanotubes (SWCNT) and 5 and 10 vol% multi-wall carbon nanotubes (MWCNT) was processed by high energy attritor milling of pure copper powder with carbon nanotubes (CNT) and subsequently consolidated by spark plasma sintering (SPS). Microstructural characterization shows a network of CNT along the grain boundaries and the presence of porosities at grain boundaries as well as triple junctions. By covering the particle boundaries, the higher volume fraction of CNT makes the sintering difficult as compared to single phase copper or low volume fraction CNT composites. Raman spectroscopy indicates that there is an increase in number of defects in the nanotube after milling and sintering of the composite. Mechanical properties evaluation shows that SWCNT composites results in higher strength and deformability compared to MWCNT. The failure strain decreases with increase in volume percent of CNT due to clustering of CNTs.

Direct radiative effects of aerosols over South Asia from observations and modeling

Abstract: Quantitative assessment of the seasonal variations in the direct radiative effect (DRE) of composite aerosols as well as the constituent species over the Indian sub continent has been carried out using a synergy of observations from a dense network of ground based aerosol observatories and modeling based on chemical transport model simulations. Seasonal variation of aerosol constituents depict significant influence of anthropogenic aerosol sources in winter and the dominance of natural sources in spring, even though the aerosol optical depth doesn’t change significantly between these two seasons. A significant increase in the surface cooling and atmospheric warming has been observed as season changes from winter (DRESUR = −28 ± 12 W m−2 and DREATM = +19.6 ± 9 W m−2) to spring (DRESUR = −33.7 ± 12 W m−2 and DREATM = +27 ± 9 W m−2). Interestingly, springtime aerosols are more absorptive in nature compared to winter and consequently the aerosol induced diabatic heating of the atmosphere goes as high as ~1 K day−1 during spring, especially over eastern India. The atmospheric DRE due to dust aerosols (+14 ± 7 W m−2) during spring overwhelms that of black carbon DRE (+11.8 ± 6 W m−2) during winter. The DRE at the top of the atmosphere is mostly governed by the anthropogenic aerosols during all the seasons. The columnar aerosol loading, its anthropogenic fraction and radiative effects shows a steady increase with latitude across Indian mainland leading to a larger aerosol-induced atmospheric warming during spring than in winter.

Modelling and experimental investigation of process parameters in EDM of Si 3 N 4 -TiN composites using GRA-RSM

Abstract: Electric discharge machining (EDM) is a highly promising machining process of ceramics. This research is an out of the paradigm investigation of EDM on Si3N4-TiN with Copper electrode. Ceramics are used for extrusion dies and bearing balls and they are more efficient, effective and even have longer life than conventional metal alloys. Owing to high hardness of ceramic composites, they are almost impossible to be machined by conventional machining as it entirely depends on relative hardness of tool with work piece. Whereas EDM offers easy machinability combined with exceptional surface finish. Input parameters of paramount significance such as current (I), pulse on (Pon) and off time (Poff), Dielectric pressure (DP) and gap voltage (SV) are studied using L25 orthogonal array. With help of mean effective plots the relationship of output parameters like Material removal rate (MRR), Tool wear rate (TWR), Surface roughness (Ra), Radial overcut (ROC), Taper angle (α), Circularity (CIR), Cylindricity (CYL) and Perpendicularity (PER) with the considered input parameters and their individual influence were investigated. The significant machining parameters were obtained by Analysis of variance (ANOVA) based on Grey relational analysis (GRA) and value of regression coefficient was determined for each model. The results were further evaluated by using confirmatory experiment which illustrated that spark eroding process could effectively be improved.

Vertical Structure of Aerosols and Mineral Dust Over the Bay of Bengal From Multisatellite Observations

Abstract: The vertical distribution of aerosol and dust extinction coefficient over the Bay of Bengal is examined using the satellite observations (Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS)) for the period from 2006 to 2017. Distinct seasonal pattern is observed in the vertical structure of both aerosol and dust over the Bay of Bengal with an enhancement of 24% in the aerosol extinction above 1 km from winter (December, January and February) to pre-monsoon (March, April, and May). Significant contribution of dust is observed over the northern Bay of Bengal during pre-monsoon season where 22% of the total aerosol extinction is contributed by dust aerosols transported from the nearby continental regions. During winter, dust transport is found to be less significant with fractional contribution of ~10% - 13% to the total aerosol optical depth over the Bay of Bengal. MODIS derived dust fraction (fine-mode based) shows an overestimation up to 2 fold compared to CALIOP dust fraction (depolarization based) whereas the GOCART simulated dust fraction underestimates the satellite derived dust fractions over the Bay of Bengal. Though the long term variation in dust aerosol showed a decreasing trend over the Bay of Bengal, the confidence level is insufficient establish the robustness of the observed trend. However, significant dust induced heating is observed above the boundary layer during pre-monsoon season. This dust induced elevated heating can affect the convection over the Bay of Bengal which will have implication on the monsoon dynamics over the Indian region.

Effective SERS detection using a flexible wiping substrate based on electrospun polystyrene nanofibers

Abstract: The progress of surface-enhanced Raman scattering (SERS) substrates, which offers flexibility, improved sample collection efficiency and high SERS activity, has provided results in numerous applications. The present study demonstrates the development of a flexible substrate based on electrospun polystyrene (PS) nanofibers decorated with silver nanoparticles for in situ sampling and detection using SERS. The surface of the PS nanofibers contains uniformly distributed nanopores in the range of 70–100 nm due to the phenomenon called ‘breathe figure’. The nanoparticles are strongly adsorbed over the nanofiber surface due to the presence of these nanopores. The present substrate offers effective in situ sampling by wiping directly from the surface of luggage, fruits or any surface of interest. We demonstrated the SERS activity of the substrate by choosing highly energetic materials such as, RDX, DNT and pesticides, namely carbofuran. The results demonstrated that the nanoparticle coated electrospun PS nanofiber mats are promising for in situ sampling and SERS-based detection.

Theoretical studies on the propulsive and explosive performance of strained polycyclic cage compounds

Abstract: Compounds consisting of a carbon-based cage have a highly strained molecular structure and have become the subject of interest in recent years because they possess high heat of formation and so are highly energetic. In the present work, ab initio molecular modelling calculations have been used for analysing 28 carbon-cage structures with the aim of identifying the best candidates for synthesis particularly for use in propellant compositions. Density functional theory (B3LYP) was employed for the geometry optimisation of the proposed molecules using the 6-311++G(d,p) basis set. Calculated heats of formation and densities of the compounds have been used from the optimized structures to compute their specific impulses and density specific impulses in various configurations (solid and liquid) with an eye on propulsion applications. Detonation properties of the compounds have also been reported and comments have been made correlating the properties of the cage compounds with their molecular structures.

Durable Superhydrophobic Particles Mimicking Leafhopper Surface: Superoleophilicity and Very Low Surface Energy

Abstract: The powder coating on brochosomes of leafhopper can repel water, diiodomethane (DM), and ethylene glycol (EG) droplets with contact angle >150°. This is attributed to the very low surface energy as low as <1.0 mN m–1 and their porous honeycomb structure. In this work, grafted silica nanoparticles with properties similar to leafhopper (LH) particle coating is synthesized in a single step by catalytic grafting of stearic acid on nonporous silica nanoparticles. The particles repel water, diiodomethane, and ethylene glycol with contact angle 167°, 161°, and 157°, respectively. The surface energy is also found as <1.0 mN m–1. The particle coating shows sliding angle <10° with these probe liquids. The superhydrophobic particles are resistant to pH = 1 to 13, boiling water conditions and preserves super-repelling nature to reference liquids after the treatments. In a mixture of hexane/water or kerosene/water, the particles completely wet the oleophilic part and repel water layer due to superoleophilic–superhydro...

Microstructural Evolution and Constitutive Relationship of M350 Grade Maraging Steel During Hot Deformation

Abstract: Maraging steels exhibit extraordinary strength coupled with toughness and are therefore materials of choice for critical structural applications in defense, aerospace and nuclear engineering. Thermo-mechanical processing is an important step in the manufacture of these structural components. This process assumes significance as these materials are expensive and the mechanical properties obtained depend on the microstructure evolved during thermo-mechanical processing. In the present study, M350 grade maraging steel specimens were hot isothermally compressed in the temperature range of 900-1200 °C and in the strain rate range of 0.001-100 s−1, and true stress-true strain curves were generated. The microstructural evolution as a function of strain rate and temperature in the deformed compression specimens was studied. The effect of friction between sample and compression dies was evaluated, and the same was found to be low. The measured flow stress data was used for the development of a constitutive model to represent the hot deformation behavior of this alloy. The proposed equation can be used as an input in the finite element analysis to obtain the flow stress at any given strain, strain rate, and temperature useful for predicting the flow localization or fracture during thermo-mechanical simulation. The activation energy for hot deformation was calculated and is found to be 370.88 kJ/mol, which is similar to that of M250 grade maraging steel.

Effect of Heat Treatment Parameters on the Microstructure and Properties of Inconel-625 Superalloy

Abstract: Inconel-625 is a solid solution-strengthened alloy used for long-duration applications at high temperatures and moderate stresses. Different heat treatment cycles (temperatures of 625-1025 °C and time of 2-6 h) have been studied to obtain optimum mechanical properties suitable for a specific application. It has been observed that room temperature strength and, hardness decreased and ductility increased with increase in heat treatment temperature. The rate of change of these properties is found to be moderate for the samples heat-treated up to 850 °C, and thereafter, it increases rapidly. It is attributed to the microstructural changes like dissolution of carbides, recrystallization and grain growth. Microstructures are found to be predominantly single-phase austenitic with the presence of fine alloy carbides. The presence of twins is observed in samples heat-treated at lower temperature, which act as nucleation sites for recrystallization at 775 °C. Beyond 850 °C, the role of carbides present in the matrix is subsided by the coarsening of recrystallized grains and finally at 1025 °C, significant dissolution of carbide results in substantial reduction in strength and increase in ductility. Elongation to an extent of >71% has been obtained in sample heat-treated at 1025 °C indicating excellent tendency for cold workability. Failure of heat-treated specimens is found to be mainly due to carbide particle-matrix decohesion which acts as locations for crack initiation.