Browsing by Author "Pema Chida Sherpa"

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Mechanically milled Sn and Al–Cu–Fe quasicrystals nanocomposites: Exploring the interplay between structural evolution, magnetic softness and thermoelectric efficiency
(Elsevier Ltd, 2025) Shradha Bhatt; Pema Chida Sherpa; Yagnesh Shadangi; Radhey Shyam Tiwari; Nilay Krishna Mukhopadhyay; Ajay Tripathi; Archana Tiwari
Mechanically milled Sn and Al–Cu–Fe icosahedral quasicrystals (IQC) nanocomposites with varying Sn volume fractions (0%, 10%, 20% and 30%, designated as IQC, IQC-10Sn, IQC-20Sn and IQC-30Sn) were synthesized to investigate the influence of Sn content on their structural, magnetic, and thermoelectric properties. Structural analysis confirmed the presence of the IQC phase alongside a crystalline B2-Al (Cu,Fe) phase, with the IQC phase exhibiting high ordering as indicated by the superlattice reflection (311111). Microstructural characterization revealed large grains in pure IQC, while Sn incorporation led to the formation of smaller grains and rod-like microstructure. All samples displayed soft ferromagnetic behavior with low coercivity and remanent magnetization. The Curie temperature decreased with higher Sn concentrations, correlating with grain size reduction (from 48 nm to 12 nm) and weakened exchange interactions. The anisotropy constant, derived from the law of approach to saturation, peaked for IQC-10Sn, (9.1 × 103 erg/g at 300 K and 23.5 × 103 erg/g at 2 K) and gradually declined with increased Sn content. Thermoelectric performance was optimized in IQC-30Sn, which exhibited the lowest thermal conductivity and the highest figure of merit (5.2 × 10−3 at 300 K). Moderate Sn doping (IQC-10Sn and IQC-20Sn) enhanced magnetic properties via domain structuring and anisotropy, whereas higher doping (IQC-30Sn) improved thermoelectric efficiency by facilitating charge and phonon transport. These findings demonstrate that mechanically milled Sn and Al–Cu–Fe IQC nanocomposites can simultaneously tailor magnetic and thermoelectric performance, offering a promising pathway for developing advanced IQC-based nanocomposites for multifunctional applications. © 2025 Elsevier B.V.
Nanostructuring of AlSiCrMnFeNiCu High-Entropy Alloy via Cryomilling: Exploring Structural, Magnetic, and Thermoelectric Properties
(John Wiley and Sons Inc, 2024) Yagnesh Shadangi; Pema Chida Sherpa; Harsh Jain; S. Varalakshmi; Sandip Chatterji; Labanya Ghosh; Nilay Krishna Mukhopadhyay; Ajay Tripathi; Archana Tiwari
Efforts are made to understand the influence of milling intensity on structure, morphology, magnetic and thermoelectric properties of nonequiatomic nanostructured AlSiCrMnFeNiCu high-entropy alloy (HEA) powders prepared by cryomilling. These powders are cryomilled with different ball-to-powder ratios (BPR) and present a dual-phase structure containing a major B2-type and a minor Cr5Si3-type phase. An increase in BPR enhances the refinement of crystallite size, grain size, and particle size accompanied by a decrease in the phase fraction of the minor Cr5Si3-type phase. Magnetic measurements revealed that at room temperature, sufficient increase in BPR leads to a transition from multi-domain behavior to single-domain behavior which leads to enhancement in soft magnetic properties. Thermal measurements show the presence of different magnetic phase transitions which vary with an increase in BPR. A change of charge carrier type from p to n-type was observed as the grain size is reduced. The figure of merit decreases with the decrease in grain size from 2 × 10–5 for as-cast powders and is lowest for the smallest grain-sized sample due to a decrease in electrical conductivity. This study shows the possibility of exploring nonequiatomic low-density HEAs whose functional properties can be tailored, offering flexibility in material design for specific applications. © 2024 Wiley-VCH GmbH.
Phase Evolution, Stability and Magnetic Behavior of Lightweight Al–Fe Aluminide-Based Nanocomposites Processed by Mechanical Alloying, Cryomilling, and Annealing
(John Wiley and Sons Inc, 2025) Ganne Ketan Balaji; Harsh Jain; Pema Chida Sherpa; Ritik Roshan Tripathy; Yagnesh Shadangi; Vikas Shivam; Ajay Tripathi; Archana Tiwari; Nilay Krishna Mukhopadhyay
Attempts are made to synthesize Al5Fe2 aluminide-based composites by mechanical alloying (MA) and cryomilling (CM). The XRD and TEM results of the milled samples confirm the formation of a major B2-AlFe phase (0.2887 ± 0.0003 nm; cP2) along with the minor amount of Al5Fe2 phase. Nanocrystalline grains of ≈16 nm and an average particle size of 4.0 ± 0.36 μm are evident. A significant refinement in the crystallite size (≈10 nm) and average particle size (1.0 ± 0.03 μm) is achieved after 10 h CM of 60 h MAed powder. CM enhances the phase fraction of the Al5Fe2 phase. The DSC thermogram discerns three exothermic heating events due to phase transformation. These can be corroborated by the structural transformation of the B2-AlFe phase to the orthorhombic Al5Fe2 phase. The phase obtained as a result of 60 h of MA transforms to orthorhombic Al5Fe2 along with a minor amount of pre-existing B2-AlFe structure after annealing at 600 °C. It becomes more stable after annealing at 900 °C. Further, the 60 h milled sample displays soft ferromagnetic properties. The saturation magnetization decreases on CM and annealing due to phase transition from B2-AlFe to Al5Fe2 phase. Coercivity is reduced when the MA sample is annealed due to an increase in crystallite size and a reduction in lattice strain. © 2024 Wiley-VCH GmbH.
Phase Evolution, Stability and Magnetic Behavior of Lightweight Al–Fe Aluminide-Based Nanocomposites Processed by Mechanical Alloying, Cryomilling, and Annealing
(John Wiley and Sons Inc, 2024) Ganne Ketan Balaji; Harsh Jain; Pema Chida Sherpa; Ritik Roshan Tripathy; Yagnesh Shadangi; Vikas Shivam; Ajay Tripathi; Archana Tiwari; Nilay Krishna Mukhopadhyay
Attempts are made to synthesize Al5Fe2 aluminide-based composites by mechanical alloying (MA) and cryomilling (CM). The XRD and TEM results of the milled samples confirm the formation of a major B2-AlFe phase (0.2887 ± 0.0003 nm; cP2) along with the minor amount of Al5Fe2 phase. Nanocrystalline grains of ≈16 nm and an average particle size of 4.0 ± 0.36 μm are evident. A significant refinement in the crystallite size (≈10 nm) and average particle size (1.0 ± 0.03 μm) is achieved after 10 h CM of 60 h MAed powder. CM enhances the phase fraction of the Al5Fe2 phase. The DSC thermogram discerns three exothermic heating events due to phase transformation. These can be corroborated by the structural transformation of the B2-AlFe phase to the orthorhombic Al5Fe2 phase. The phase obtained as a result of 60 h of MA transforms to orthorhombic Al5Fe2 along with a minor amount of pre-existing B2-AlFe structure after annealing at 600 °C. It becomes more stable after annealing at 900 °C. Further, the 60 h milled sample displays soft ferromagnetic properties. The saturation magnetization decreases on CM and annealing due to phase transition from B2-AlFe to Al5Fe2 phase. Coercivity is reduced when the MA sample is annealed due to an increase in crystallite size and a reduction in lattice strain. © 2024 Wiley-VCH GmbH.