Browsing by Author "Tu-Ngoc Lam"

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Deviatoric deformation kinetics in high entropy alloy under hydrostatic compression
(2019-07-05)
Huang, E-Wen
;
Lin, Chih-Ming
;
Juang, Jenh-Yih
;
Chang, Yao-Jen
;
Chang, Yuan-Wei
An equiatomic CoCrFeMnNi high-entropy alloy under hydrostatic compression is investigated using insitu angular-dispersive X-ray diffraction to explore the polymorphism in high entropy alloy systems. The metallic system is of face-centered-cubic structure at ambient condition and applied hydrostatic pressures up to 20 GPa via diamond anvil cell. The angle-resolved diffraction-intensity evolutions of multiple diffraction peaks were collected simultaneously to elucidate the phase stability examinations. The phase transformation from face-centered-cubic to hexagonal-close-packed structure was evidently observed in CoCrFeMnNi alloy accompanied by a deviatoric strain subjected to the hydrostatic compression. We found lattice-asymmetric crossover before and after the phase transformation subjected to hydrostatic compression surroundings. Deviatoric strain triggers fcc-hcp phase transformation as local heterogeneity-driven lattice distortion is significant for CoCrFeMnNi alloy. (c) 2019 The Authors. Published by Elsevier B.V.
Effectiveness of organic molecules for spin filtering in an organic spin valve: Reaction-induced spin polarization for Co atop Alq(3)
(2015-01-21)
Tu-Ngoc Lam
;
Lai, Yu-Ling
;
Chen, Chih-Han
;
Chen, Po-Hung
;
Chan, Yuet-Loy
The spin polarization of organic-ferromagnetic interfaces in an organic spin valve critically affects the efficiency of spin injection or detection. We examined the chemical and electronic properties of ferromagnetic Co deposited on organic Alq(3) and the interfacial spin-polarized capability of the electronic states. Our x-ray photoemission spectra and calculations with density-functional theory indicate a sequential and unequal distribution of charge from Co clusters to N and then to O atoms in Alq(3). The preferential orbital hybridization at specific functional sites produces efficient spin polarization of organic molecules. Element-specific measurements of x-ray magnetic circular dichroism demonstrate the preferential spin polarization in the lowest unoccupied molecular orbital state of N atoms at the complex interface for Co atop Alq(3), which agrees satisfactorily with calculation. Our results indicate that an induced interfacial spin polarization on engineering the dominant reaction of Co with mainly N and O atoms in Alq(3) might pave a way for effective spin filtering in organic spintronics.
Element Effects of Mn and Ge on the Tuning of Mechanical Properties of High-Entropy Alloys
(1970-01-01)
Tu-Ngoc Lam
;
Tsai, Che-Wei
;
Chen, Bo-Kai
;
Lai, Bo-Hong
;
Liu, Hung-Chih
Substitution of Ge for Mn increases the elastic moduli of different {h k l} orientations of the CoCrFeMnNi-based high-entropy alloy. Our findings indicate that tuning minor element compositions may result in improved strength-ductility combination. The underlying deformation mechanisms of CoCrFeNiGe(0.3)were examined byin situneutron diffraction and analysis of the associated diffraction profiles during tensile deformation. The strain-hardening response of CoCrFeNiGe(0.3)exhibited a dominant mechanism of mechanical twinning at moderate and large strains at room temperature. The evolution of the bulk work hardening rate was consistent with the convolutional multiple whole profile fitting results, which exhibited a continuous increase in twin formation probability. The microstructural development was also investigated by transmission electron microscopy, which revealed the formation of huge amounts of deformation twins in CoCrFeNiGe(0.3)at room temperature.
Element Effects on High-Entropy Alloy Vacancy and Heterogeneous Lattice Distortion Subjected to Quasi-equilibrium Heating
(2019-10-15)
Huang, E-Wen
;
Chou, Hung-Sheng
;
Tu, K. N.
;
Hung, Wei-Song
;
Tu-Ngoc Lam
We applied Simmons-Balluffi methods, positron measurements, and neutron diffraction to estimate the vacancy of CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs) using Cu as a benchmark. The corresponding formation enthalpies and associated entropies of the HEAs and Cu were calculated. The vacancy-dependent effective free volumes in both CoCrFeNi and CoCrFeMnNi alloys are greater than those in Cu, implying the easier formation of vacancies by lattice structure relaxation of HEAs at elevated temperatures. Spatially resolved synchrotron X-ray measurements revealed different characteristics of CoCrFeNi and CoCrFeMnNi HEAs subjected to quasi-equilibrium conditions at high temperatures. Element-dependent behavior revealed by X-ray fluorescence (XRF) mapping indicates the effect of Mn on the Cantor Alloy.
Enhanced Photocatalytic Performance of Nitrogen-Doped TiO2 Nanotube Arrays Using a Simple Annealing Process
(2018-12-01)
Phuoc Huu Le
;
Le Trung Hieu
;
Tu-Ngoc Lam
;
Nguyen Thi Nhat Hang
;
Nguyen Van Truong
Nitrogen-doped TiO2 nanotube arrays (N-TNAs) were successfully fabricated by a simple thermal annealing process in ambient N-2 gas at 450 degrees C for 3 h. TNAs with modified morphologies were prepared by a two-step anodization using an aqueous NH4F/ethylene glycol solution. The N-doping concentration (0-9.47 at %) can be varied by controlling N-2 gas flow rates between 0 and 500 cc/min during the annealing process. Photocatalytic performance of as-prepared TNAs and N-TNAs was studied by monitoring the methylene blue degradation under visible light ( 400 nm) illumination at 120 mWcm(-2). N-TNAs exhibited appreciably enhanced photocatalytic activity as compared to TNAs. The reaction rate constant for N-TNAs (9.47 at % N) reached 0.26 h(-1), which was a 125% improvement over that of TNAs (0.115 h(-1)). The significant enhanced photocatalytic activity of N-TNAs over TNAs is attributed to the synergistic effects of (1) a reduced band gap associated with the introduction of N-doping states to serve as carrier reservoir, and (2) a reduced electronhole recombination rate.
Interfacial symmetry of Co-Alq(3)-Co hybrid structures for effective spin filtering (vol 354, pg 90, 2015)
(2018-05-31)
Tu-Ngoc Lam
;
Lai, Yu-Ling
;
Chen, Chih-Han
;
Chen, Po-Hung
;
Wei, Der-Hsin
Irreversible phase transformation in a CoCrFeMnNi high entropy alloy under hydrostatic compression
(2018-03-01)
Huang, E-Wen
;
Lin, Chih-Ming
;
Jain, Jayant
;
Shieh, Sean R.
;
Wang, Ching-Pao
An equal-molar CoCrFeMnNi, face-centered-cubic high-entropy alloy system is investigated using in-situ angular-dispersive X-ray diffraction under hydrostatic compression up to 20 GPa via diamond anvil cell. The evolutions of multiple diffraction peaks are collected simultaneously to elucidate the phase stability field. The results indicated that an irreversible phase transformation had occurred in the high entropy alloy upon decompression to ambient pressure. A reference material (n-type silicon-doped gallium arsenide) was investigated following the same protocol to demonstrate the different deformation mechanisms. It is suggested that the atomic bonding characteristics on the phase stability may play an important role in the high entropy alloys.
Phase Stress Partition in Gray Cast Iron UsingIn SituNeutron Diffraction Measurements
(1970-01-01)
Tu-Ngoc Lam
;
Wu, Szu-Chien
;
Chae, Hobyung
;
Chen, Shi-Wei
;
Jain, Jayant
Gray cast iron comprises alpha-Fe, Fe3C and graphite with their different anisotropies in terms of structure and mechanical behavior. We perform two different deformation paths as tension-compression and compression-tension on bulk gray iron to capture the stress partitioning. Usingin situneutron diffraction, the composite-like deformation behaviors of alpha-Fe and Fe3C are revealed simultaneously. Within the elastic deformation stage, both deformation paths do not cause load transfer. However, when the applied compressive stress exceeds 300 MPa, there is a clear load sharing in both paths. From microscopic examinations, the debonding between matrix and graphite is clarified.
Superparamagnetic ground state of CoFeB/MgO magnetic tunnel junction with dual-barrier
(2018-11-01)
Thanh-Nga Tran
;
Tu-Ngoc Lam
;
Yang, Chao-Yao
;
Lin, Wen-Chin
;
Chen, Po-Wen
CoFeB/MgO-based magnetic tunnel junctions (MTJs) have considerable potential in magnetic random access memory (MRAM), thanks to their tunable perpendicular magnetic anisotropy (PMA). We found significant reduction of dead-layer by inserting additional MgO into the MTJ structure. Interface, electronic and transport characterizations were utilized to approach the modified magnetic properties driven by the dual-MgO structure in this work. The dual-MgO structure appeared to hinder boron (B) diffusion into the metallic layer and prevent capping-layer (Ta) penetration across the interface. This suppressed the dead-layer effect and promoted overall magnetization despite PMA degradation. A robust BOx phase that formed within the dual-MgO structure presented a superparamagnetic ground state. In the single-MgO structure, any reduction in the thickness of the CoFeB promoted PMA, albeit at the cost of spin-polarization. The dual-MgO structure could restore spin-polarization by preferentially populating spin electrons into Fe/Co minority states. X-ray magnetic spectroscopy and anomalous Hall effect suggest that, the dual-MgO differs from the single-MgO with a favorable longitudinal polarized spin-channel. This makes the dual-MgO structure applicable to applications requiring in-plane rather than out-of-plane sensing.
Tuning Stress in Cu Thin Films by Developing Highly (111)-Oriented Nanotwinned Structure
(1970-01-01)
Wang, I-Ju
;
Ku, Ching-Shun
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Tu-Ngoc Lam
;
Huang, E-Wen
;
Tu, K. N.
We have examined the effect of different bath temperatures on residual stress of both the random-oriented Cu films and the highly (111)-oriented nanotwinned Cu films by synchrotron radiation x-ray measurements. The bath temperature varied from 15 degrees C to 40 degrees C. The results indicate that the average residual stress in the highly (111)-oriented nanotwinned films is higher than that in the randomly oriented Cu films. However, the stress in the highly (111)-oriented Cu decreases with increasing bath temperature. The average residual stress can be reduced from 253 MPa electroplated at 15 degrees C to 95 MPa under a bath temperature of 35 degrees C. We could successfully tune and measure residual stress of the Cu thin films. The films with low residual stress prevent warpage from occurring on the substrate and lower the processing failure in copper direct bonding and other processes that need alignment.