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Effect of Cr-Rich Phase Precipitation on Magnetic and Mechanical Properties of Fe-20% Cr Alloy
Mohapatra, J. N.,Kamada, Y.,Kikuchi, H.,Kobayashi, S.,Echigoya, J.,Park, D. G.,Cheong, Y. M. IEEE 2011 IEEE transactions on magnetics Vol.47 No.10
<P>Magnetic Hysteresis Loop (MHL) and micro-Vickers hardness measurements were carried out in isothermal aged Fe-20 wt. % Cr alloy. The results were compared with the existing atom probe data at similar heat treated conditions. Coercivity and hardness of the alloy was increased and remanence of the alloy was decreased with the increase in ageing time. Such increase in the magnetic and mechanical hardness was due to the nucleation and growth of Cr rich α' phase. The size of the Cr rich α' phase precipitates were increased and the number density of the precipitates were decreased with the increase in ageing time observed by atom probe analysis. In this work a good correlation between hardness (H<SUB>v</SUB>) and coercivity (H<SUB>c</SUB>) was found with the combined effect of precipitate size (r) and number density (n). A linear relationship was found with the change in coercivity and hardness of the alloy indicating that the MHL technique would be a good Non-Destructive Evaluation (NDE) tool for the evaluation of thermal embrittlement in structural components made of Fe-Cr alloys.</P>
Carbon nanoflake growth from carbon nanotubes by hot filament chemical vapor deposition
Sahoo, S.C.,Mohapatra, D.R.,Lee, H.J.,Jejurikar, S.M.,Kim, I.,Lee, S.C.,Park, J.K.,Baik, Y.J.,Lee, W.S. Pergamon Press ; Elsevier Science Ltd 2014 Carbon Vol.67 No.-
We report the growth of carbon nanoflakes (CNFs) on Si substrate by the hot filament chemical vapor deposition without the substrate bias or the catalyst. CNFs were grown using the single wall carbon nanotubes and the multiwall carbon nanotubes as the nucleation center, in the Ar-rich CH<SUB>4</SUB>-H<SUB>2</SUB>-Ar precursor gas mixture with 1% CH<SUB>4</SUB>, at the chamber pressure and the substrate temperature of 7.5Torr and 840<SUP>o</SUP>C, respectively. In the H<SUB>2</SUB>-rich condition, CNF synthesis failed due to severe etch-removal of carbon nanotubes (CNTs) while it was successful at the optimized Ar-rich condition. Other forms of carbon such as nano-diamond or mesoporous carbon failed to serve as the nucleation centers for the CNF growth. We proposed a mechanism of the CNF synthesis from the CNTs, which involved the initial unzipping of CNTs by atomic hydrogen and subsequent nucleation and growth of CNFs from the unzipped portion of the graphene layers.
Evaluation of Embrittlement in Isochronal Aged Fe-Cr Alloys by Magnetic Hysteresis Loop Technique
J. N. Mohapatra,Y. Kamada,H. Kikuchi,S. Kobayashi,J. Echigoya,D. G. Park,Y. M. Cheong 한국자기학회 2011 Journal of Magnetics Vol.16 No.2
Fe-Cr alloys with different Cr contents were prepared by an arc melting technique. The alloys were isochronally aged in the range from 400 ℃ to 900 ℃ with 50 ℃ steps with a holding time of 100 hours. The ageing produced embrittlement in the alloys due to either the formation of a Cr-rich α’ phase or a σ phase at high temperatures. Magnetic Hysteresis Loop (MHL) and Micro-Vickers hardness were measured at each step to correlate the magnetic and mechanical properties. Coercivity and hardness of the alloys were increased and remanence decreased up to 500-550 ℃ due to formation of a Cr-rich α’ phase. Beyond 500-550 ℃ range, the coercivity and hardness decreased and remanence increased due to the coarsening or dissolution of the Cr-rich α’ phase. In the Fe-48% Cr alloy, formation of the σ phase at 700 ℃ reduced the maximum induction of the alloy significantly.
Singh, V.,Mohapatra, M.,Sivaramaiah, G.,Rao, J. L.,Singh, N.,Gao, H.,Li, J.,Srivastava, A. K.,Jirimali, H. D.,Dhoble, S. J. Springer Science + Business Media 2016 Journal of materials science Materials in electron Vol.27 No.4
<P>Manganese doped BaMgAl10O17 nano-phosphors were synthesized by a low-temperature initiated, gas producing solution combustion process. Phase purity of the prepared phosphors was determined using X-ray diffraction technique. Dynamic light scattering technique was used to estimate the average particle size of the synthesized products. Scanning electron microscopy technique was used to study the morphology of the prepared samples. Fourier transform infrared spectroscopy was used to ascertain the formation of the aluminate host. UV-visible absorption spectroscopy showed band maxima at around 260 nm and intense band maxima at around 216 nm. Electron paramagnetic resonance experiments for the system were carried out to understand the site occupancy of the Mn ions. The photoluminescence excitation and emission spectra suggested the stabilisation of Mn ions in Td geometry. From the colour characterisation of the system it was found out that the system can be effective as a green phosphor material with wide range of applications.</P>
R. Paikaray,T. Badapanda,H. Mohapatra,T. Richhariya 한국전기전자재료학회 2023 Transactions on Electrical and Electronic Material Vol.24 No.1
This paper reports structural, optical, photoluminescence and thermoluminescence properties of scheelite-type CaWO 4 ceramic synthesized by solid-state reaction route. X-ray diffraction study revealed the structure of the sample with tetragonal phase and I4 1 /a (88) space group. Lattice parameters were calculated from the Rietveld refi nement method and the crystal structure was observed using VESTA software. Photoluminescence excitation spectra were obtained by using 450 nm and 460 nm emission wavelengths. The emission spectra obtained with 240 nm excitation wavelength showed an emission band around 448 nm that can be ascribed to 1 T 2 to 1 A 1 optical transition of electrons within [WO 4 ] 2- group. CIE Chromaticity indicated that CaWO 4 emits blue color and the color purity is also calculated. Optical band gap and urbach energy of the material were measured from the UV-visible spectroscopy. Thermoluminescence (TL) properties were investigated with different UV doses and the variations of TL emission peaks with temperature confirmed that CaWO 4 has second-order kinetics with lower UV dosimetry.
Cr3+-Doped Yb3Ga5O12 Nanophosphor: Synthesis, Optical, EPR, Studies
Singh, V.,Sivaramaiah, G.,Rao, J. L.,Singh, N.,Pathak, M. S.,Jirimali, H. D.,Singh, P. K.,Srivastava, A. K.,Dhoble, S. J.,Mohapatra, M. Springer Science + Business Media 2016 Journal of electronic materials Vol.45 No.8
<P>Gallium garnets of lanthanides are multifunctional materials especially known for their complicated structure and magnetic properties. In addition, with a suitable transition metal dopant ion, these matrices have been proved to be excellent materials for lasers. In particular, gallium garnet of ytterbium (Yb3Ga5O12) is known to possess excellent properties with regards to these applications. In this connection, Yb3Ga5O12 doped with Cr3+ nanophosphors were synthesized by a solution combustion route. The synthesized material was characterized by powder x-ray diffraction and scanning electron microscopy for phase purity and homogenous morphology. In order to ascertain the oxidation state of the doped ion, diffuse reflectance (DRF), photoluminescence (PL) and electron paramagnetic resonance (EPR) experiments were performed on the sample. The DRF and PL data suggested the stabilisation of the trivalent Cr ion in the matrix. The EPR spectra exhibited two resonance signals with effective g values at g approximate to 7.6 and 4. The EPR data corroborated the DRF and PL results, suggesting the stabilisation of Cr3+ in the matrix at octahedral-type geometries.</P>