Kaiyang Zeng

National University of Singapore, Mechanical Engineering, Faculty Member

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I am Associate Professor at Department of Mechanical Engineering. I got my Ph.D in Materials Science from Royal Institute of Technology (KTH) in Sweden. My specicial research interests are nano-/micromechanics of materials, multiferroics materials and biomimetics study. I am teaching materials science and engineering related subjects at NUS.

For detail information of my research group at NUS, please visit my group website:

http://sites.google.com/site/zengsresearhgroup/
Address: Department of Mechanical Engineering
National University of Singapore
9 Engineering Drive 1
Singapore 117576

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Papers by Kaiyang Zeng

Microstructural evolution of charged defects in the fatigue process of polycrystalline BiFeO3 thin films

Acta Materialia, 2015

Fatigue failure in ferroelectrics has been intensively investigated in the past few decades. Most... more Fatigue failure in ferroelectrics has been intensively investigated in the past few decades. Most of the mechanisms discussed for ferroelectric fatigue have been built on the "hypothesis of variation in charged defects", which however are rarely evidenced by experimental observation. Here, using a combination of complex impedance spectra techniques, piezoresponse force microscopy and first-principles theory, we examine the microscopic evolution and redistribution of charged defects during the electrical cycling in BiFeO 3 thin films. The dynamic formation and melting behaviors of oxygen vacancy (V O) order are identified during the fatigue process. It reveals that the isolated V O tend to self-order along grain boundaries to form a planar-aligned structure, which blocks the domain reversals. Upon further electrical cycling, migration of V O within vacancy clusters is accommodated with a lower energy barrier (~0.2 eV) and facilitates the formation of nearby-electrode layer incorporated with highly concentrated V O. The interplay between the macroscopic fatigue and microscopic evolution of charged defects clearly demonstrates the role of ordered V O cluster in the fatigue failure of BiFeO 3 thin films.

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Ferroelectricity of CH3NH3PbI3 Perovskite

The Journal of Physical Chemistry Letters, 2015

Ferroelectricity has been believed to be an important but controversial origin of the excellent p... more Ferroelectricity has been believed to be an important but controversial origin of the excellent photovoltaic performance of organometal trihalide perovskites (OTPs). Here we investigate the ferroelectricity of a prototype OTP, CH3NH3PbI3 (MAPbI3), both theoretically and experimentally. Our first-principles calculations based on 3-D periodic boundary conditions reveal that a ferroelectric structure with polarization of ∼8 μC/cm(2) is the globally stable one among all possible tetragonal structures; however, experimentally no room-temperature ferroelectricity is observed by using polarization-electric field hysteresis measurements and piezoresponse force microscopy. The discrepancy between our theoretical and experimental results is attributed to the dynamic orientational disorder of MA(+) groups and the semiconducting nature of MAPbI3 at room temperature. Therefore, we conclude that MAPbI3 is not ferroelectric at room temperature; however, it is possible to induce and experimentally observe apparent ferroelectric behavior through our proposed ways. Our results clarify the controversy of the ferroelectricity in MAPbI3 and also provide valuable guidance for future studies on this active topic.

Surface damage of polymers in nanoscale

Surface damage of polymers in the nanometer-range is examined and results correlated with materia... more Surface damage of polymers in the nanometer-range is examined and results correlated with material characteristics and surface roughness of epoxies. Under a constant loading and constant scratch rate testing condition, surface roughness plays little or no role in surface damage formed during the course of this study. Material characteristics influence the damage occurred in terms of variations in elastic recovery, damage pattern and damage mechanism. The variations in scratch head geometry, which, in turn, lead to the variations in magnitude of stress and stress field distribution, give rise to various scratch features on the polymer.

Novel rat model with acute left ventricular heart failure developed by cornary microembolism with homologous microthrombi [J]

Investigation of the non-volatile resistance change in noncentrosymmetric compounds

Scientific Reports, 2012

Coexistence of polarization and resistance-switching characteristics in single compounds has been... more Coexistence of polarization and resistance-switching characteristics in single compounds has been long inspired scientific and technological interests. Here, we report the non-volatile resistance change in noncentrosymmetric compounds investigated by using defect nanotechnology and contact engineering. Using a noncentrosymmetric material of ZnO as example, we first transformed ZnO into high resistance state. Then ZnO electrical polarization was probed and its domains polarized 1806 along the [001]-axis with long-lasting memory effect (.25 hours). Based on our experimental observations, we have developed a vacancy-mediated pseudoferroelectricity model. Our first-principle calculations propose that vacancy defects initiate a spontaneous inverted domains nucleation at grain boundaries, and then they grow in the presence of an electrical field. The propagation of inverted domains follows the scanning tip motion under applied electrical field, leading to the growth of polarized domains over large areas.

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Stable Ferroelectric Perovskite Structure with Giant Axial Ratio and Polarization in Epitaxial BiFe0.6Ga0.4O3 Thin Films

ACS Applied Materials & Interfaces, 2015

Ferroelectric perovskites with strongly elongated unit cells (c/a > 1.2) are of particular intere... more Ferroelectric perovskites with strongly elongated unit cells (c/a > 1.2) are of particular interest for realizing giant polarization induced by significant ionic offcenter displacements. Here we show that epitaxial Bi-Fe 0.6 Ga 0.4 O 3 (BFGO) thin films exhibit a stable supertetragonal-like structure with twinning domains regardless of film thickness and substrate induced strain, evidenced with high resolution X-ray diffractometry (HR-XRD), transmission electron microscopy (TEM) and piezoresponse force microscopy (PFM). The origin of the structural stability of BFGO is investigated by the first-principles calculation. The ferroelectric properties of BFGO are studied by PFM, first-principles calculation and macroscopic polarization−electric field (P−E) hysteresis measurement. A giant ferroelectric polarization of ∼150 μC/cm 2 is revealed by the first-principles calculations and confirmed by experiments. Our studies provide an alternative pathway of employing Ga-substitution other than the extensively studied strain engineering to stabilize the supertetragonal structure in BiFeO 3-based epitaxial thin films.

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Enhanced photovoltaic effects and switchable conduction behavior in BiFe0.6Sc0.4O3 thin films

Acta Materialia, 2015

Enhanced photovoltaic effects are demonstrated in an In 2 O 3-SnO 2 /BiFe 0.6 Sc 0.4 O 3 /LaNiO 3... more Enhanced photovoltaic effects are demonstrated in an In 2 O 3-SnO 2 /BiFe 0.6 Sc 0.4 O 3 /LaNiO 3 (ITO/BFSO/LNO) ferroelectric thin film heterostructure. The Sc-substitution greatly improves the bulk conductivity of BiFeO 3 (BFO) and modifies the energy band alignment in the ITO/ BFSO/LNO capacitor structure, where a tunable Schottky-to-Ohmic contact at the BFSO/LNO interface and an Ohmic ITO/BFSO contact are purposely established. In negatively poled BFSO films, constructive photovoltaic effects at both the bulk and the BFSO/LNO interface lead to a large V oc up to 0.6 V and a 5-fold enhancement in efficiency compared with conventional BFO films. In addition, ferroelectric polarization modulation of the Schottky barrier height at the BFSO/LNO interface results in a conversion between Schottky and Ohmic conduction, which gives rise to switchable high-and low-resistance states. The present work demonstrates a strategy effective to realize ferroelectric-based thin film structure with enhanced photovoltaic effects and memory function.

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Assessment of smoothed point interpolation methods for elastic mechanics

International Journal for Numerical Methods in Biomedical Engineering, 2010

A generalized gradient smoothing technique and a smoothed bilinear form of Galerkin weak form hav... more A generalized gradient smoothing technique and a smoothed bilinear form of Galerkin weak form have been recently proposed by Liu et al. to create a wide class of efficient smoothed point interpolation methods (PIMs) using the background mesh of triangular cells. In these methods, displacement fields are constructed by polynomial or radial basis shape functions and strains are smoothed over the smoothed domain associated with the nodes or the edges of the triangular cells. This paper summarizes and assesses bound property, convergence rate and computational efficiency for these methods. It is found that: (1) the incorporation of the PIMs with the node-based strain smoothing operation allows us to obtain an upper bound to the exact solution in the strain energy; (2) the incorporation of the PIMs with the edge-based strain smoothing operation using triangular background mesh can produce a solution of 'ultra-accuracy' and 'super-convergence'; (3) the edge-based strain smoothing operation together with the linear interpolation can provide better computational efficiency compared with other smoothed PIMs and the finite element method when the same triangular mesh is used. These conclusions have been examined and confirmed by intensive examples.

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Residual stress measurement in thin films at sub-micron scale using Focused Ion Beam milling and imaging

Thin Solid Films, 2012

Residual stress evaluation in thin films at the sub-micron scale was achieved in the present stud... more Residual stress evaluation in thin films at the sub-micron scale was achieved in the present study using a semi-destructive trench-cutting (ring-core) method. Focused Ion Beam was employed to introduce the strain relief by milling the slots around an “island” and also to record the images for strain change evaluation by digital image correlation analysis of micrographs. Finite element simulation was employed to predict the curves for strain relief as a function of milling depth, and compared with the experimental measurements, showing ...

The correlation between glass formation and hardness of the amorphous phase

Scripta Materialia, 2011

We report a non-monotonous behavior of the hardness of amorphous Cu–Zr films as a function of com... more We report a non-monotonous behavior of the hardness of amorphous Cu–Zr films as a function of composition, by applying a combinatorial deposition and nanoindentation method with unparalleled compositional resolution. Distinct peaks in hardness were identified at particular compositions to correlate well with the previously reported density peaks. Our results not only facilitate the discovery of new glass-forming alloys, but also raise the possibility of developing bulk metallic glasses with enhanced plasticity and/or ductility for engineering applications.

Residual stress measurement in thin films using the semi-destructive ring-core drilling method using Focused Ion Beam

Procedia Engineering, 2011

In the present study, residual stress evaluation in thin films was achieved using a semi-destruct... more In the present study, residual stress evaluation in thin films was achieved using a semi-destructive trench-cutting method. Focused Ion Beam (FIB) was employed to introduce the strain relief by ring-core milling, i.e. creating a trench around an "island". Either SEM or FIB imaging can be used to record sequences of images for strain change evaluation by Digital Image Correlation (DIC) analysis of micrographs. A regular array of shallow holes was drilled on a thin overlayer of Pt (~100nm) deposited on to the film prior to patterning and trenching, in order to reduce the damage introduced by the ion beam during imaging and to assist the DIC strain evaluation by adding traceable markers. Finite Element (FE) simulation was also carried out to predict the curves for strain relief as a function of milling depth, and compared with the experimental measurements, which show good agreement with each other. An empirical mathematical description of the curves was proposed that allows efficient residual stress evaluation in thin solid films.

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Self-polarized ferroelectric PVDF homopolymer ultra-thin films derived from Langmuir–Blodgett deposition

Polymer, 2012

It was found that LangmuireBlodgett (LB) deposition process led to the direct formation of ferroe... more It was found that LangmuireBlodgett (LB) deposition process led to the direct formation of ferroelectric b phase in poly(vinylidene fluoride) (PVDF) homopolymer ultra-thin films, in which the molecular chains were parallel to the substrates and the dipoles were aligned perpendicular to the substrates. Theoretical analysis and experimental results further showed that the mechanism of forming the b phase and the dipole orientation were attributed to the hydrogen bonds formed between the PVDF molecules and water through the LB deposition. A large local effective piezoelectric coefficient (d zz) of À49.4 pm/V was observed in the self-polarized PVDF LB film.

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Interfacial delamination cracking shapes and stress states during wedge indentation in a soft-film-on-hard-substrate system—Computational simulation and experimental studies

Journal of Materials Research, 2011

The shapes of the interfacial delamination crack and stress states during wedge indentation in a ... more The shapes of the interfacial delamination crack and stress states during wedge indentation in a soft-film-on-hard-substrate system were investigated systematically using the three-dimensional (3D) finite element simulation and wedge indentation experiment. In the simulation, a traction–separation law was used to characterize the failure behaviors of the interface. The effects of the wedge indenter tip length and the film thickness on the onset and growth of interfacial delamination were analyzed. It was shown that a two-dimensional (2D) to 3D transition of stress states occurred depending on the ratio of indenter length to film thickness. Furthermore, the interfacial delamination process by wedge indentation was conducted experimentally, and comparisons between the computational and experimental results yielded quantitative good agreement. Finally, a straightforward criterion based on the curvature of the delamination crack front was proposed to indicate the transition of stress states during the interfacial delamination. A guideline was therefore proposed to classify the 2D and 3D stress states for extracting the interface adhesion properties.

Analysis of nanoindentation creep for polymeric materials

Journal of Applied Physics, 2004

This article presents studies on nanoindentation creep for polymeric materials at room temperatur... more This article presents studies on nanoindentation creep for polymeric materials at room temperature. A semi-empirical method has been established from a generalized Kelvin model. This model includes the elastic-viscoelastic-viscous effects on indentation creep. It is shown that this method can fit the experimental creep data well, to give the exact changes of creep behavior during nanoindentations for numbers of amorphous

A Combined Extended and Edge-Based Smoothed Finite Element Method (Es-Xfem) for Fracture Analysis of 2D Elasticity

International Journal of Computational Methods, 2011

This study combines the edge-based smoothed finite element method (ES-FEM) and the extended finit... more This study combines the edge-based smoothed finite element method (ES-FEM) and the extended finite element method (XFEM) to develop a new simulation technique (ES-XFEM) for fracture analysis of 2D elasticity. In the XFEM, the need for the mesh alignment with the crack and remeshing, as the crack evolves, is eliminated because of the use of partition of unity. The ES-FEM uses the generalized smoothing operation over smoothing domain associated with edges of simplex meshes, and produces a softening effect leading to a close-to-exact stiffness, "super-convergence" and "ultra-accurate" solutions for the numerical model. Taking advantage of both ES-FEM and XFEM, the present method introduces the edge-based strain smoothing technique into the context of XFEM. Thanks to strain smoothing, the necessity of sub-division in elements cut by discontinuities is suppressed via transforming interior integration into boundary integration. Hence, it simplifies the numerical integr...

$Research paper thumbnail of A novel singular node-based smoothed finite element method (NS-FEM) for upper bound solutions of fracture problems$

A novel singular node-based smoothed finite element method (NS-FEM) for upper bound solutions of fracture problems

International Journal for Numerical Methods in Engineering, 2010

It is well known that the lower bound to exact solutions in linear fracture problems can be easil... more It is well known that the lower bound to exact solutions in linear fracture problems can be easily obtained by the displacement compatible finite element method (FEM) together with the singular crack tip elements. It is, however, much more difficult to obtain the upper bound solutions for these problems. This paper aims to formulate a novel singular node-based smoothed finite element method (NS-FEM) to obtain the upper bound solutions for fracture problems. In the present singular NS-FEM, the calculation of the system stiffness matrix is performed using the strain smoothing technique over the smoothing domains (SDs) associated with nodes, which leads to the line integrations using only the shape function values along the boundaries of the SDs. A five-node singular crack tip element is used within the framework of NS-FEM to construct singular shape functions via direct point interpolation with proper order of fractional basis. The mix-mode stress intensity factors are evaluated using the domain forms of the interaction integrals. The upper bound solutions of the present singular NS-FEM are demonstrated via benchmark examples for a wide range of material combinations and boundary conditions.

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Stress gradient enhanced plasticity in a monolithic bulk metallic glass

Intermetallics, 2008

ABSTRACT

Thermal oxidation effect on corrosion behavior of Zr46Cu37.6Ag8.4Al8 bulk metallic glass

Intermetallics, 2012

Oxide films of 0.72 mm and 1.80 mm were prepared by thermal oxidation treatments at 441 K and 609... more Oxide films of 0.72 mm and 1.80 mm were prepared by thermal oxidation treatments at 441 K and 609 K respectively. The oxidation kinetics of the Zr 46 Cu 37.6 Ag 8.4 Al 8 bulk metallic glass has been investigated. The outermost surface of the oxide film formed at 609 k is Cu 2 O and Al 2 O 3-enriched but ZrO 2-depleted. A ZrO 2enriched oxide layer is detected existing at 150e260 nm of the oxide film. The surface oxidation state sheds great influence on the corrosion behavior of Zr-based BMG. The oxide film can hinder the initial electrodissolution processes during polarization in 0.5 M H 2 SO 4 þ 0.01 M NaCl solution. The special structure and composition distribution of the oxide film lead to the modification of the BMG's corrosion behavior.

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$Research paper thumbnail of A novel variable power singular element in G space with strain smoothing for bi-material fracture analyses$

A novel variable power singular element in G space with strain smoothing for bi-material fracture analyses

Engineering Analysis with Boundary Elements, 2011

ABSTRACT This paper aims to formulate a triangular five-node (T5) singular crack-tip element in G... more ABSTRACT This paper aims to formulate a triangular five-node (T5) singular crack-tip element in G space with strain smoothing to simulate an rλ−1 (0&lt;λ&lt;1) stress singularity for bi-material fracture analyses. In the present formulation, a direct point interpolation with a proper fractional order of extra basis functions is specially employed to construct variable power type singular shape functions that are in a G1 space. Within strain smoothing, the singular terms of functions as well as mapping procedures are no longer necessary to compute the stiffness matrix. Furthermore, thanks to the point interpolation, the proposed singular element eliminates the need to shift the position of the side nodes adjacent to the crack-tip, and is thus quite straightforward and easily implemented in existing codes. The effectiveness of the present singular element is demonstrated via numerical examples of a wide range of material combinations and boundary conditions.Highlights► A novel triangular five-node (T5) singular crack-tip element is formulated. ► A variable stress singularity is accurately captured. ► The singular terms of functions and mapping procedures are no longer necessary. ► The need to shift the position of the side nodes is eliminated. ► The convergence rate, accuracy and computational efficiency are found superior.

The effect of oxidation on the corrosion resistance and mechanical properties of a Zr-based metallic glass

Corrosion Science, 2011

The oxidation kinetics of the Zr64Cu16Ni10Al10 bulk metallic glass (BMG) roughly follows a two-st... more The oxidation kinetics of the Zr64Cu16Ni10Al10 bulk metallic glass (BMG) roughly follows a two-stage rate law at both 433 and 593K in air. An oxide film of 940nm can be formed by oxidation at 593K, which is ZrO2-enriched but Cu-depleted on the outer surface. The oxide film leads to a superior passivity in 0.5M NaCl and great corrosion resistance improvements

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