Browsing by Author "Lin, Su-Jien"

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Differences in texture evolution from low-entropy to high-entropy face-centered cubic alloys during tension test
(2020-03-01)
Lin, Kuan-Hao
;
Chang, Shou-Yi
;
Lo, Yu-Chieh
;
Wang, Chun-Chieh
;
Lin, Su-Jien
Texture evolution during tension test was investigated on three face-centered cubic (FCC) metals, namely, Ni (unary), equiatomic CrFeNi (ternary), and equiatomic CoCrFeMnNi (quinary) with small and large grain sizes of similar to 20 and similar to 100 mu m, respectively. The microtextures for 18%, 36% and 54% engineering strains of a tension-interrupted specimen were characterized on the same area. The three metals exhibited a similar texture evolution, which the fraction of {001}//ND remains constant However, the fraction of {011}//ND decreases, whereas that of {111}//ND increases. On the other hand, the fraction of <001>//RD also remains constant, that of <011>//RD decreases, and that of <111>//RD increases. However, the several differences still were found during plastic deformation. Fine-grained Ni initiated a significantly crystallographic rotation (CRo) at the lower strain, increased with strain and then remained nearly unchanged as strain was higher than 18%. Coarse-grained Ni delayed this rotation and slowly developed the trend at large strains. As for CrFeNi, the comparison between fine-grained and coarse-grained structure does not have obvious difference. As for fine-grained CoCrFeMnNi, it can be seen that an obvious CRo develops under 36% strain, and maintains constant over 36% strain. But the coarse-grained CoCrFeMnNi reveals that the significant CRo develops continuously from 0% to 54% strain. The mechanisms for the effects of number of element and grain size in the present study are elucidated.
Dynamic Evolution of Conducting Nanofilament in Resistive Switching Memories
(2013-08-01)
Chen, Jui-Yuan
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Hsin, Cheng-Lun
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Huang, Chun-Wei
;
Chiu, Chung-Hua
;
Huang, Yu-Ting
Resistive random access memory (ReRAM) has been considered the most promising next-generation nonvolatile memory. In recent years, the switching behavior has been widely reported, and understanding the switching mechanism can improve the stability and scalability of devices. We designed an innovative sample structure for in situ transmission electron microscopy (TEM) to observe the formation of conductive filaments in the Pt/ZnO/Pt structure in real time. The corresponding current-voltage measurements help us to understand the switching mechanism of ZnO film. In addition, high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) have been used to identify the atomic structure and components of the filament/disrupted region, determining that the conducting paths are caused by the conglomeration of zinc atoms. The behavior of resistive switching is due to the migration of oxygen ions, leading to transformation between Zn-dominated ZnO1-x and ZnO.
Epitaxial Photostriction-Magnetostriction Coupled Self-Assembled Nanostructures
(2012-08-01)
Liu, Heng-Jui
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Chen, Long-Yi
;
He, Qing
;
Liang, Chen-Wei
;
Chen, Yu-Ze
Self-assembled vertical nanostructures take advantage of high interface-to-volume ratio and can be used to design new functionalities by the choice of a proper combination of constituents. However, most of the studies to date have emphasized the functional controllability of the nanostructures using external electric or magnetic fields. In this study, to introduce light (or photons) as an external control parameter in a self-assembled nanostructure system, we have successfully synthesized oxide nanostructures with CoFe2O4 nanopillars embedded in a SrRuO3 matrix. The combination of photostrictive SrRuO3 and magnetostrictive CoFe2O4 in the intimately assembled nanostructures leads to a light-induced, ultrafast change in magnetization of the CoFe2O4 nanopillars. Our work demonstrates a novel concept on oxide nanostructure design and opens an alternative pathway for the explorations of diverse functionalities in heteroepitaxial self-assembled oxide nanostructures.
Lattice distortion effect on elastic anisotropy of high entropy alloys
(2020-03-25)
Yen, Chao-Chun
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Huang, Guan-Rong
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Tan, Yun-Cheng
;
Yeh, Han-Wen
;
Luo, Da-Ji
The superior mechanical properties of high-entropy alloys (HEAs) have made an outstanding success in materials science and engineering. Studies to date have been devoted to what the severe lattice distortion induces. However, most researchers focus on its stimulation to plastic deformation instead of scrutinizing the variations on elasticity. Compared with conventional alloys, HEAs may perform disproportionate elasticity with uneven local lattice strain resulting from the severe lattice distortion. Therefore, it is necessary to survey its influence on the mechanical properties of HEAs systematically. In this study, the Lennard-Jones (LJ) potential, the embedded atom method (EAM) potential, and the modified embedded atom method (MEAM), are respectively conducted to investigate the lattice distortion effect on Young's modulus E (hkl) and Poisson's ratio nu (hkl, theta) along [100], [110], and [111] loading directions for several fcc metals composed of 1 similar to 5 atomic types, including Ni, Ni98W2, Ni96W4, FeCrNi, and CoNiCrFeMn HEAs. Also, a method is used to analyze the performance of the individual element on the elastic properties in the HEA environment. As a result, it can be unveiled that the effect of electron density inconsistency is more dominant than the effect of lattice distortion associated with the atomic size difference. The electronic configuration in the HEA environment plays a major role in elastic anisotropy while the difference of the atomic radii does the minor one. The anisotropy of CoNiCrFeMn HEA analyzed by this work is also consistent with in-situ neutron diffraction measurements. (C) 2019 Elsevier B.V. All rights reserved.
Misorientation Control and Functionality Design of Nanopillars in Self-Assembled Perovskite-Spinel Heteroepitaxial Nanostructures
(2011-05-01)
Liao, Sheng-Chieh
;
Tsai, Pei-Yu
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Liang, Chen-Wei
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Liu, Heng-Jui
;
Yang, Jan-Chi
In this study, advanced control of crystallographic orientations and magnetic properties of self-assembled nanostructures via rational selections of substrates is demonstrated. We show that in the perovskite spinel BiFeO(3)-CoFe(2)O(4) model system the crystal orientation of self-assembled CoFe(2)O(4) nanopillars can be tuned among (001), (011), and (111), while that of the BiFeO(3) matrix is fixed in (001). Moreover, the resultant CoFe(2)O(4) nanopIllars appear in various shapes: pyramid, roof, and triangular platform, respectively. The tunable nanostructures through this approach enable the control of material functionality such as the magnetic anisotropy of CoFe(2)O(4). This study opens a new pathway for the engineering of self-assembled heteroepitaxial nanostructures.
Strain-driven phase boundaries in BiFeO(3) thin films studied by atomic force microscopy and x-ray diffraction
(2012-01-17)
Liu, Heng-Jui
;
Liang, Chen-Wei
;
Liang, Wen-I
;
Chen, Hsiang-Jung
;
Yang, Jan-Chi
We report a detailed study on the strain-driven phase transition between the tetragonal-like and rhombohedral-like phases in epitaxial BiFeO(3) (BFO) thin films which focuses on their structural nature, thermodynamic stability, and ferroelectric/piezoelectric properties. We first show that the tetragonal-like phase, which has a large c/a ratio (similar to 1.2), in the compressively strained BFO is thermodynamically more favorable at high temperature and high strain state (small thickness). We also report a phase transition between two monoclinic phases at 150 degrees C. The two monoclinic phases are differentiated by their c-axis parameters and tilting angles: The low-temperature phase (M(C)) has a c-axis parameter of 4.64 angstrom and a tilting angle (beta = 88.5 degrees) along the a axis, while the high-temperature phase (M(A)) has a c-axis parameter of 4.66 angstrom and a tilting angle (beta = 86.8 degrees) along both of the a and b axes. We further show that samples undergoing the M(C)-M(A) phase transition exhibit ferroelectric polarization rotation and piezoelectric enhancement. Our findings directly unveil the close links between structural changes, polarization rotation, and large piezoelectricity at morphotropic phase boundaries in BiFeO(3).
Strain-driven phase boundaries in BiFeO3 thin films studied by atomic force microscopy and x-ray diffraction
(2012-01-17)
Liu, Heng-Jui
;
Liang, Chen-Wei
;
Liang, Wen-I
;
Chen, Hsiang-Jung
;
Yang, Jan-Chi
We report a detailed study on the strain-driven phase transition between the tetragonal-like and rhombohedral-like phases in epitaxial BiFeO3 (BFO) thin films which focuses on their structural nature, thermodynamic stability, and ferroelectric/piezoelectric properties. We first show that the tetragonal-like phase, which has a large c/a ratio (similar to 1.2), in the compressively strained BFO is thermodynamically more favorable at high temperature and high strain state (small thickness). We also report a phase transition between two monoclinic phases at 150 degrees C. The two monoclinic phases are differentiated by their c-axis parameters and tilting angles: The low-temperature phase (M-C) has a c-axis parameter of 4.64 angstrom and a tilting angle (beta = 88.5 degrees) along the a axis, while the high-temperature phase (M-A) has a c-axis parameter of 4.66 angstrom and a tilting angle (beta = 86.8 degrees) along both of the a and b axes. We further show that samples undergoing the M-C-M-A phase transition exhibit ferroelectric polarization rotation and piezoelectric enhancement. Our findings directly unveil the close links between structural changes, polarization rotation, and large piezoelectricity at morphotropic phase boundaries in BiFeO3.
Structural study in highly compressed BiFeO3 epitaxial thin films on YAlO3
(2012-09-01)
Liu, Heng-Jui
;
Chen, Hsiang-Jung
;
Liang, Wen-I
;
Liang, Chen-Wei
;
Lee, Hsin-Yi
We report a study on the thermodynamic stability and structure analysis of the epitaxial BiFeO3 (BFO) thin films grown on YAlO3 (YAO) substrate. First, we observe a phase transition of M-C-M-A-T occurs in thin sample (<60 nm) with an utter tetragonal-like phase (denoted as M-II here) with a large c/a ratio (similar to 1.23). Specifically, MII phase transition process refers to the structural evolution from a monoclinic M-C structure at room temperature to a monoclinic M-A at higher temperature (150 degrees C) and eventually to a presence of nearly tetragonal structure above 275 degrees C. This phase transition is further confirmed by the piezoforce microscopy measurement, which shows the rotation of polarization axis during the phase transition. A systematic study on structural evolution with thickness to elucidate the impact of strain state is performed. We note that the YAO substrate can serve as a felicitous base for growing T-like BFO because this phase stably exists in very thick film. Thick BFO films grown on YAO substrate exhibit a typical "morphotropic-phase-boundary"-like feature with coexisting multiple phases (M-II, M-I, and R) and a periodic stripe-like topography. A discrepancy of arrayed stripe morphology in different direction on YAO substrate due to the anisotropic strain suggests a possibility to tune the MPB-like region. Our study provides more insights to understand the strain mediated phase co-existence in multiferroic BFO system. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4746036]
Surface and interface porosity of polymer/fullerene-derivative thin films revealed by contrast variation of neutron and X-ray reflectivity
(2011)
Liu, Heng-Jui
;
Jeng, U-Ser
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Yamada, Norifumi L.
;
Su, An-Chung
;
Wu, Wei-Ru
Contrast variation of neutron and X-ray reflections has been adapted to reveal the film in-depth (vertical) composition profiles of the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) for bulk heterojunction thin-film solar cells, with a PCBM/P3HT weight ratio of c = 0.6, 0.8 and 1.0. The X-ray scattering-length-density (SLD) profiles, extracted from X-ray reflectivity for the blend films spun-cast on Si wafer, exhibit a stratified film morphology of ca. 85 nm film thickness; the corresponding neutron SLD profiles extracted for the same films further elucidate a PCBM-enriched interfacial layer adjacent to the Si substrate. In contrast to the often assumed two-phase model, a three-phase model with porosity included as the third phase has to be used in deducing the absolute volume fractions of PCBM and P3HT from the complementary neutron and X-ray SLD profiles. In general, the thus deduced in-depth composition profiles for the blend films comprise a substantial surface layer (10-15 nm) of ca. 40% porosity, a 50 nm main layer with relatively uniform PCBM-P3HT composition, and a PCBM-enriched interface layer (similar to 20 nm) with similar to 15% porosity. Formation of the surface porosity is related to interfacial instability occurred in a transient surface layer upon film drying. Annealing at 150 degrees C influences modestly the vertical phase separation of the film, but drastically activates local phase separation for formation and growth of PCBM and P3HT nanodomains, as revealed by grazing incidence small/wide angle X-ray scattering. The surface/interface porosity features (overlooked in nearly all the previous studies) and the composition-dependent vertical phase separation bear hints in advancing device performance via interfacial morphology optimization.