Suhrit Mula

Defect mediated reversible ferromagnetism in Co and Mn doped zinc oxide epitaxial films

We have introduced defects in ZnO (undoped and doped with Co and Mn) epitaxial thin films using laser irradiation from nanosecond laser pulses and thermal annealing in oxygen ambient. In contrast to the as grown samples, the laser irradiated films show a significant increase in conductivity, enhancement in UV emission, while maintaining the same wurtzite crystal structure. Room-temperature ferromagnetism (RTFM) is observed in laser-irradiated samples, which increased with the number of laser pulses up to a certain value where magnetic moment saturates. The induced ferromagnetism as well as the enhanced electrical conductivity can be reversed with thermal annealing in oxygen ambient. The magnetization in Co and Mn doped films was found to be strong function of growth conditions and defect concentration. X-ray diffraction and optical absorption experiments suggested a 2+ valance state and tetrahedral coordination for both Co and Mn ions. There is a simultaneous increase in n-type electrical conductivity wit...

Effects of Hydrothermal Aging on Mechanical Behavior of Sub-zero Weathered GFRP Composites

Sub-zero weathered GFRP (glass fiber reinforced plastics) composites were aged in water at 60 C for different conditioning times to study fluid sorption kinetics under the influence of a thermal gradient. The effect of subsequent freezing of the conditioned samples on the retention of prior thermal history has been investigated. The variation in mechanical properties during the aging process was studied. One batch of the hydrothermally conditioned specimens was further subjected to sub-zero treatment at 20 C temperature. The samples which were subsequently frozen showed higher interlaminar shear strength (ILSS) initially, although this trend was reversed after a certain conditioning period. Thus, the present study aims to evaluate the mechanical behavior of polymer composites under the influence of extreme and complex conditions.

Prior thermal spikes and thermal shocks on mechanical behavior of glass fiber-epoxy composites

Fiber-reinforced glass-epoxy composites with and without prior thermal history are subjected to hygrothermal conditioning for different exposure times. The prior thermal history is given in the form of thermal shocks and thermal spikes at three different temperatures. The effects of exposure time and prior thermal history on the moisture absorption kinetics are studied. The effects of prior conditioning parameters are also investigated. The variation of mechanical properties as a function of exposure time and prior conditioning parameters are assessed by the three-point short beam shear bend test. It has been observed that there is an initial improvement in the mechanical properties followed by a steady deterioration. Mechanical tests performed at different crosshead velocities show increasing brittleness of the composites and that is found to be a function of cross-head velocity.

Effect of hygrothermal shock cycles on interlaminar shear strength of hybrid composites

The present study aims to assess the interlaminar shear strength (ILSS) of interply woven fabric glass/carbon hybrid composites under the influence of a fluctuating humid environment. The ILSS values have been used for quantification of mechanical degradation. The effects of fluctuating environment on moisture sorption as well as the degradation kinetics have been assessed. It is seen that the ILSS shows a gradual degradation over a period of time. It has been observed that the ILSS scales inversely with crosshead velocities. This is attributed to the presence of alternating strong and weak interfaces and the differences in their moisture sorption tendencies.

Microstructural evolution and thermal stability of Fe–Zr metastable alloys developed by mechanical alloying followed by annealing

Abstract The effect of Zr (up to 1 at.%) addition on the formation of Fe–Zr metastable alloys and their thermal stability were investigated for their possible nuclear applications. Fe–xZr (x = 0.25, 0.5, 1%) alloys were synthesised by mechanical alloying under a high-purity argon atmosphere using stainless steel grinding media in a SPEX 8000M high energy mill. The milling was conducted for 20 h with a ball-to-powder weight ratio of 10:1. The formation of metastable solid solutions after milling was confirmed from the change in the Gibbs free energy analysis as per Miedema’s model. The microstructural characterisation was carried out by analysis of X-ray diffraction, atomic force microscopy and transmission electron microscopy. The effect of Zr on the thermal stability of Fe–Zr alloys was investigated by extensive annealing experiments followed by microstructural analysis and microhardness measurements. The stabilisation was found to occur at 800 °C and thereafter, no significant change in the crystallite size was observed for the samples annealed between 800 and 1200 °C. The supersaturated solid solution, especially 1% Zr alloy, found to be highly stable up to 800 °C and the microhardness value of the same measured to be as high as 8.8 GPa corresponding to a crystallite size of 57 nm. The stabilisation effect has been discussed in the light of both the thermodynamic and kinetic mechanisms and the grain size stabilisation is attributed to the grain boundary segregation of Zr atoms and/or Zener pinning by nanoscale precipitation of the Fe2Zr phase.

Effects of Thermal Shocks and Thermal Spikes on Hygrothermal Behavior of Glass-Polyester Composites

The aim of the present study is to investigate the degradation behavior of glass fiber reinforced polyester composites during hygrothermal ageing for different lengths of time after suffering thermal shocks and thermal spikes at 100, 150, and 200°C. The moisture gain kinetics and change of interlaminar shear strength (ILSS), a primary mode of analysis of interfacial strength of the composite, were evaluated. Moisture absorption, found more in spiked samples, showed initial Fickian trend but eventually changed to non-linear pattern as the exposure time increased. It has been found that the composite specimens having a history of thermal spike showed more degradation. The damage was found even more at higher temperature and at lower loading rates.

Metallurgical Approach Towards Explaining Optimized EDM Process Parameters for Better Surface Integrity of AISI D2 Tool Steel

Many attempts have been made to model and optimize performance parameters of Electrical Discharge Machining (EDM) process. In this investigation, the experimental design includes L9 orthogonal array based on the Taguchi methodology. The corresponding effect of input current, duty cycle and spark on time, on surface integrity, has been studied on AISI D2 tool steel. The residual stress measurement is done using X-ray diffraction method. The obtained output results regarding material removal rate, surface roughness, crack density, and residual stress have been modeled and optimized by regression equation and Genetic algorithm respectively. In this investigation, a metallurgical approach has been introduced towards the explanation of the resulted optimized response parameters.

Microstructures, Mechanical Properties and Strengthening Mechanisms of cast Cu–Al Alloys Processed by Cryorolling

In the present work , the cast structures of Cu-4.5wt% Al alloy was homogenized by annealing at 800 °C for 4 h. The samples were kept under LN2 before and during the rolling process in each pass to maintain a constant LN2 temperature. Mechanical properties were evaluated by the tensile test. Microstructural analysis was carried by optical microscopy and TEM analysis. The maximum yield strength (YS) of the 75 % cryorolled samples of Cu-4.5wt% Al alloy was found to be 786 MPa with a ductility of 4.8 %. This is due to the formation of ultrafine grains and nano-twins possibly play the pivotal role for the improved YS and ductility.

Comparison between conventional, vacuum and microwave sintering of Cu-Cr-4 wt.% SiC nanocomposites

Attempts have been made to synthesize Cu99Cr1, Cu99Cr1-4 wt.% SiC nanocomposites for thermo-electric applications. The blend compositions were ball-milled for 50 h in a stainless steel grinding media. The compact specimens were subjected to vacuum sintering (900°C and 1000°C for 1h), conventional sintering (900°C and 1000°C for 1h) and microwave sintering (800°C, 900°C and 1000°C) duration of 30 min using a furnace of 2.45 GHz frequency and 900 W power. The micro-structural characterization was carried out by X-ray diffraction and scanning electron microscopy. The mechanical properties like Vickers hardness and electrical conductivity of the sintered specimens were also investigated. In the present investigation, the best combination of mechanical properties and electrical conductivity were obtained for the microwave sintered specimens. This is possibly due to the enhanced densification achieved through microwave sintering technique. Possible sintering characteristics and strengthening mechanisms were analyzed.

Microwave Assisted Sintering of Some Copper Based Nano-Composites and its Evaluation

In the present investigation Cu99Cr1, Cu94Cr6, Cu99Cr1–4% SiC and Cu94Cr6–4% SiC nanocomposites were synthesized for thermo-electric applications. SiC particulates (purity >96.0%, ~30 nm size) was ball milled with copperchromium (purity >99.0 %) mixtures for 50 h in stainless steel grinding media. The uniaxially pressed compact specimens were subjected to microwave sintering 900°C, 30 min and conventional sintering 900°C, 1 h. The microstructural characterization was carried out by X-ray diffraction, scanning electron microscopy and atomic force microscopy. The mechanical properties like Vickers hardness and wear resistance, and electrical conductivity of the sintered specimens were also investigated. In the present investigation, the best combination of mechanical properties and electrical conductivity were obtained for the microwave sintered specimens compared to conventional sintered compacts. This is possibly due to the enhanced densification achieved through microwave sintering technique. Keywords— Nanocomposite; Microwave sintering; Wear resistance; Electrical conductivity; X-ray diffraction (XRD); Atomic force microscopy (AFM).