Anita Mohan

On the extreme-ultraviolet/ultraviolet plasma diagnostics for nitrogen-like ions from spectra obtained by SOHO/SUMER

In this work we discuss the potential for plasma diagnostics of forbidden transitions from the ground levels in the nitrogen-like ions: Al VII, Si VIII, P IX, S X, Ar XII, K XIII, and Ca XIV. We also study the lines emitted by n = 3 levels of Si VIII. These transitions fall in the UV spectral range and have been recently observed by the SOHO/SUMER instrument in the solar spectrum. Some of the lines considered in the present study have been measured by SUMER for the first time. We investigate the effects of photospheric radiation, proton collisional excitation, additional configurations, and resonances on level populations, in order to assess the importance of these processes in the calculation of line emissivities. We compare line ratios with observations from SUMER on quiet-Sun and active regions and measure the electron density and temperature of the emitting plasma. We show that in a few cases current atomic data are still not able to reproduce the observations and that further work is required to solve inconsistencies between observations and theoretical predictions.

On the electron temperatures, densities and hot ions in coronal hole plasma observed by SUMER on SOHO

Abstract Making use of high-resolution ultraviolet observations obtained with SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on the spacecraft SOHO (Solar and Heliospheric Observatory), we study plasma temperatures, densities and hot ions in polar coronal holes (source region of the high-speed solar wind). In particular, we present spectroscopic diagnostics for Ne viii , Mg viii , Si viii , and Mg ix lines to infer these physical parameters and discuss the constraints of their applications in the coronal hole plasma. We also present the implications of this investigation in addressing the acceleration mechanisms of the solar wind, one of the outstanding problems in solar physics.

On the Electron density in a coronal hole

Electron density in a coronal hole is rediscussed using the new calculation for the Mgviii λ436.62/λ430.47 density-sensitive theoretical line ratio and with the help of available observations.

DIAGNOSTICS OF SOLAR IONS: Ne VI, Mg VI, Si VIII AND Mg VIII OBSERVED BY SERTS

Diagnostics of solar ions Nevi, Mgvi, Siviii, and Mgviii in an active region observed by SERTS have been presented. Density, temperature, and electron pressure in the emitting source have been derived from theoretical line-ratio curves and its EUV spectrum obtained by SERTS. The variation of neon-to-magnesium and silicon-to-magnesium abundances has been discussed in the interpretation of the active region spectrum obtained by SERTS.

Dry sliding wear behavior of Al-SiO2 composites

Abstract Al-base composites with different amount of silica (5, 10, 15 and 20 wt.%) were developed using powder metallurgy route and compacts were sintered at 550 °C for 2 h. XRD analysis of all compositions was conducted for phases and amount of the second phase present. Morphology of the composites shows quite uniform distribution of the SiO2 particles but at higher percentage of SiO2 particles the clustering starts. Mechanical properties such as uniaxial compressive strength (UCS) and hardness were evaluated and it is seen that among all compositions, composite with 10 wt.% SiO2 has maximum UCS and hardness. Wear behavior of all composites was studied with sliding distance, applied loads, sliding velocity and composition. All composites show a linear increase in cumulative wear with distance and load. Wear rate with load increases continuously for all compositions, however, composite with 10 wt.% SiO2 revealed minimum wear rate with distance, sliding velocity and loads. Wear rate with sliding velocity increases sharply after attaining minima at 3 m/s sliding velocity. SEM analysis of wear tracks is in agreement with wear results. Al-10 wt.%SiO2 also shows minimum wear coefficient values for all loads, however, wear coefficient decreases with load for all compositions.

Theoretical Ne VI/Mg VI line diagnostics for SUMER

Theoretical Nevi/Mgvi EUV line emissivity ratios, suitable for density-measurements in various solar features such as active regions, sunspots, umbrae and flare plasmas and to be observed in the SUMER spectral range, are presented and their applications discussed with the help of available observational data.

Strengthening mechanisms of (Al3Zrmp + ZrB2np)/AA5052 hybrid composites

To establish the correlation between grain size, dislocations, dispersed particles (size and vol.%) along with their solid solution strengthening effects in alloy and combined effect of all on the strengthening of advanced composite materials (Al3Zrmp + ZrB2np)/AA5052 hybrid composites have been selected for the investigation. (Al3Zrmp + ZrB2np)/AA5052 hybrid composites have been synthesized by the direct melt reaction of AA5052 alloy and inorganic salts (K2ZrF6 and KBF4). These composites have been characterised by X-ray diffractometer, optical microscopy, scanning-electron microscopy with energy-dispersive spectroscopy, transmission-electron microscopy, tensile and hardness test. Results indicate the successful formation of second phase reinforcement particles namely Al3Zr and ZrB2 in the AA5052 alloy matrix. Al3Zr particles exhibit rectangular and polyhedron morphology within an average of micron size while ZrB2 show hexagonal and rectangular within an average of nano size. Grain refinement of Al-rich phase observed in the composites, increases with increasing vol.% of reinforcement particles. TEM observation shows the presence of dislocations in the composite matrix which help to improve the strength parameters. Tensile results show the improvement in strength parameters which improve with the increasing amount of particles whereas percentage elongation also improves up to certain vol.% of particles and beyond that decrease. However, bulk hardness shows an increasing trend continuously with vol.% of particles. The strengthening mechanisms, namely dislocation, Orowan, grain-refined and solid solution are quantified for the hybrid (Al3Zrmp + ZrB2np)/AA5052 composites and the total of above are in agreement with experimental results. Solid-solution and Orowan are the predominant strengthening mechanisms in the composites.

Synthesis and tribological properties of AA5052-base insitu composites

Abstract It is important to optimize the properties of a material for a particular application, hence, to find the suitable material for tribological applications, the wear and friction behaviour of AA5052 in situ composites with different kind of reinforcements have been investigated. For present study, three in situ formed composites have been produced with different reinforcements namely Al3Zr, ZrB2 and combination of both (Al3Zr + ZrB2) by direct melt reaction (DMR) technique. The as-cast composites and base alloy have been characterized by X-ray diffraction (XRD), optical microscopy, electron microscopy, tensile testing, hardness and dry sliding wear and friction tests. XRD results indicate the successful formation of second phase reinforcement particles in all composites. Wear test results indicate that the cumulative volume loss increases with an increase in sliding distance while coefficient of friction shows a fluctuating tendency, whereas with increasing applied load, wear rate shows an increasing trend while coefficient of friction shows decreasing trend. The variation of wear rate with composites indicates that the composite with multiple reinforcement (Al3Zr + ZrB2) has lowest wear rate among all as-cast composites and base alloy, while coefficient of friction is higher. The responsible mechanisms concerned with wear and friction results have been discussed in detail with the help of the observation on worn surface analysis by scanning electron microscope (SEM) and 3D-profilometer. All tribological results have been correlated with the microstructural properties, strength parameters and bulk hardness of the composites.

Wear, friction and prevention of tribo-surfaces by coatings/nanocoatings

This chapter gives an broad up-to-date overview of wear, friction and prevention of tribo-surfaces. Tribology is the science and technology of wear, friction and lubrication. It plays an important role in design and performance of all mechanical systems. The basic mechanism of friction involves its physical, chemical and material properties. It is a situation-based or system-based property. The mechanisms involved at microscopic level include adhesive forces between mating surfaces, mechanical interactions of asperities, the ploughing of one surface due to harder asperities of the other, deformation of surfaces or fracture in oxide film and/or the introduction of a third body due to debris as well as environmental effects. Wear is a perennial phenomenon of surface damage in contacting bodies, which may be solid–solid, solid–liquid or solid–air. Wear in solid–solid contacts takes place through two basic phenomena: adhesion and abrasion. There are a number of factors that affect the rate at which the contacting surfaces wear off. It is not possible to completely eliminate losses due to wear and friction, but they can be minimised.

Fabrication and Characterisation of Al-Al2O3 Composite by Mechanical Alloying

Over the years, Aluminium Matrix Composites (AMCs) have gained importance in numerous structural, non-structural and functional applications in different engineering sectors. Driving force for the utilisation of AMCs in these sectors include performance, economics and environmental benefits. The key benefit of AMCs in transportation sector is lower fuel consumption. Particulate reinforced Aluminium Matrix Composites have been successfully used in automotive and aerospace industry due to their light weight, high strength to weight ratio and good wear resistance.This work is focussed on the study of the influence of different composition of reinforcement (Al2O3) on physical and mechanical behaviour of Aluminium Matrix Composites. These AMCs will be fabricated by mechanical alloying. In this study instead of aluminium powder aluminium chips will be used along with Al2O3 powder with that it is expected that embedment of particles will be better and porosity will be minimised. Once compact are ready these will be sintered and will be evaluated for its various physical, microstructural and mechanical properties.