-
[show abstract]
[hide abstract]
ABSTRACT: Using the modified Landau-Ginsburg-Devonshire thermodynamic theory, it is
found that the coupling between stress gradient and polarization, or
flexoelectricity, has significant effect on ferroelectric properties of
epitaxial thin films, such as polarization, free energy profile and hysteresis
loop. However, this effect can be completely eliminated by applying an
optimized external, uniform electric field. The role of such uniform electric
field is shown to be the same as that of an ideal gradient electric field which
can suppress the flexoelectricty effect completely based on the present theory.
Since the uniform electric field is more convenient to apply and control than
gradient electric field, it can be potentially used to remove the flexoelectric
effect induced by stress gradient in epitaxial thin films and enhance the
ferroelectric properties.
08/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: To simulate the nonlinear behavior of ferroelectric structures and devices under non-uniform electromechanical loadings, a
domain-switching embedded electromechanical finite element method is developed in this paper. Following continuum assumption,
the electromechanical behavior of each representative material point can be obtained by averaging the behavior of the local
corresponding microstructure, e.g. 42 domains used in this work. A new Double Gibbs free energy criterion for domain-switching
is proposed to ensure the convergence and stability of the simulations on ferroelectrics under non-uniform field. Several
computational examples are given to demonstrate that this nonlinear finite element method can yield reasonable and stable
simulation results which can be used to explain some experimental results and assist the design of ferroelectric devices.
Keywordsferroelectric material–domain-switching–finite element method–nonlinear electromechanical coupling–crack and fracture
Science China: Physics, Mechanics and Astronomy 04/2012; 54(4):606-617. · 0.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Based on the principle of superposition and a newly developed stripe method, an analytical equivalent model is proposed to
calculate the stress fields in two imperfect planar isotropic lattices: the regular triangular lattice and the Kagome lattice,
both containing single bar defects. Finite element simulations are used to validate the model predictions. According to the
degree of the imperfection, four types of defects: vacancy defect, weak defect, strong defect, and rigid inclusion are classified
and the induced local stress fields are analyzed. The stress concentration factor (SCF) caused by the imperfection is analytically
obtained, and the influence of the imperfection degree, loading condition, and relative density on the SCF is quantified.
Based on the equivalent model, the interaction of dual defects with the thickness of elastic boundary layer in the two lattices
is also estimated. In the presence of a vacancy defect, the distinct deformation mechanism results in only a small knock-down
in the strength of a triangular lattice but a substantial strength knock-down of a Kagome lattice. Both lattices exhibit no
obvious sensitivity to the presence of a rigid inclusion. It is indicated that compared with the corresponding Kagome lattice,
the triangular lattice containing a single missing bar possesses a considerable better strength performance. In addition,
the analytical results of imperfection interaction demonstrate that the influence of imperfections on stress field calculations
and strength analysis is important for the triangular lattice.
Acta Mechanica 04/2012; 216(1):105-122. · 1.29 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Flexoelectricity describes the coupling between polarization and
strain/stress gradients in insulating crystals. In this paper, using the
Landau-Ginsburg-Devonshire phenomenological approach, we found that
flexoelectricity could increase the theoretical critical thickness in epitaxial
BaTiO3 thin films, below which the switchable spontaneous polarization
vanishes. This increase is remarkable in tensile films while trivial in
compressive films due to the electrostriction caused decrease of potential
barrier, which can be easily destroyed by the flexoelectricity, between the
ferroelectric state and the paraelectric state in tensile films. In addition,
the films are still in a uni-polar state even below the critical thickness due
to the flexoelectric effect.
04/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, Crack Opening Displacement (COD) is introduced to study the fracture and fatigue of ferroelectrics. A fundamental
solution for the COD of ferroelectrics is derived considering both the piezoelectric effect and ferroelectric effect. Bases
on this solution, a nonlinear COD fracture model of ferroelectrics, which takes into account the effect of domain switching,
is developed and accords well with the experimental results. Furthermore, fatigue crack growth in ferroelectrics is analytically
investigated using this COD model. Comparison between the experimental results and the predicted electric-field-induced fatigue
crack growth shows the applicability of the proposed COD model.
KeywordsFerroelectricity–Domain switching–Fatigue–Fracture–Ceramics
International Journal of Fracture 04/2012; 167(2):211-220. · 1.49 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, the electromechanically coupled behaviors of BaTiO 3 nanowires are investigated by shell-model based molecular dynamics and proposed analytical models. Three-stage stress–strain and stress–polarization relations are found that are mainly originated from the existence of a mixed phase composition. A phase diagram in terms of wire diameters and axial stresses is constructed. Furthermore, the elastic modulus is found to decrease dramatically with the increase of diameter for the nanowire in paraelectric phase, while the piezoelectric coefficient of the ferroelectric phase is not very sensitive to the size. Besides, the size dependencies of hysteresis and butterfly loops are also revealed.
Journal of Applied Physics 10/2011; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, a surface-layer model is established to study the axial polarization distributions and critical diameters of nanowires with different surface compositions. Analytical solutions are obtained based on this model, which are validated by the ab initio results for small-size nanowires and can also predict the behaviors of large-size specimen. Compared to the traditional Landau–Ginzburg–Devonshire theory using an extrapolation length, the present model describes more microstructure information of the surface-layer, and is thus capable of characterizing the influences of different surface compositions. Furthermore, this model may shed insight into the mechanisms of the size effect and polarization distribution patterns in nanoscale ferroelectrics.
Journal of Applied Physics 12/2010; 108(12):124109-124109-6. · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The stretchable and flexible electronics with the structure of inorganic films on plastic substrate has recently attracted an increasing interest. Interfacial failure inevitably occurs when the structure is subject to repeated bending or stretching in application. An analysis is presented in this letter on the slipping failure mechanism for the interface between inorganic film and plastic substrate. The critical radius of system curvature for the slipping crack propagation on its interface is obtained to analyze the bendability of the structure, which can provide the design guideline and characterization method to determine the interface fracture toughness of slipping failure.
Applied Physics Letters 11/2010; 97(22):221903-221903-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Size dependent domain configuration and its evolution under an external electric field are investigated for ultrathin ferroelectric films using an unconventional phase field method. The simulation reveals a series of domain configurations at different thicknesses, including zigzag patterns with eight variants or four variants coexisting, a vortex pattern with four variants coexisting, and a stripe pattern with two variants coexisting. When the film thickness falls below a critical value of 2.8 nm, the polarization vanishes, indicating the suppression of ferroelectricity. The evolution of domain configuration under an alternating electric field is also investigated, and the reduction in remnant polarization and coercive field with respect to decreasing thickness is observed.
Journal of Applied Physics 03/2010; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, a molecular dynamics method is utilized to investigate the strain effect on the polarization distribution, piezoelectric coefficient, and hysteresis behaviors of BaTiO(3) nanowires. The axial polarization changes almost linearly with the strain over a relatively large range, and the ferroelectricity vanishes under a critical compressive strain. With the nanowire becoming thicker, the piezoelectric coefficient increases, and approaches its counterpart for bulk material when the diameter is larger than 2.4 nm. It is also revealed that a pre-tension strain can induce the emergence of a stepwise hysteresis loop while a pre-compression strain can lead to the disappearance of the stepwise shape. Furthermore, the strain effect and size effect are found to play some equivalent roles in the ferroelectric properties of BaTiO(3) nanowires.
Nanotechnology 01/2010; 21(1):015701. · 3.98 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Validated by ab initio simulations on the spontaneous polarization, the molecular dynamics method is used to investigate the size effect on the polarization distribution, hysteresis behaviors and Curie temperatures of BaTiO3 nanowires. A quasi-axisymmetric radial distribution of polarization in the cross-section plane is disclosed. It is also found that there exist three typical polarization distribution scenarios for different wire diameters, and thus two critical diameters can be identified. Moreover, unusual stepwise hysteresis loops, and core-shell polarization structures with opposite polarization directions at the outer and inner regions of the nanowire are revealed in our simulations. This study on the atomic level polarization evolution may help us to understand the ferroelectric properties of nanowires.
Nanotechnology 10/2009; 20(40):405703. · 3.98 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The polarization switching and strain effects on the ferroelectric characters of BaTiO3 films are studied using molecular dynamics method based on a shell model. We observed the strip domains and the polarization switching in nanofilms, and found that the surface and inner polarization switch under the same electric field, although the surface polarization is weaker than the inner value. The strain effect is also studied from microscopic view. It is found that crystal cell distortions due to the in-plane strain will aggravate or relieve the separation of negative and positive charge centers. As a result, the in-plane strains can significantly influence the spontaneous polarization and even critical thickness of ferroelectric films. A logarithm relation between the in-plane strains and spontaneous polarizations is obtained via our simulations. A quantitative relation between the in-plane strains and critical thicknesses is also given out and then compared with Landau-Ginzburg theory, which displays a good agreement. This provides a possible interpretation on the discrepancies among the experimental measurements of the critical thicknesses.
Journal of Computational and Theoretical Nanoscience 12/2008; 6(1):142-147. · 0.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The ferroelectric properties of Pb ( Zr <sub>0.5</sub> Ti <sub>0.5</sub>) O <sub>3</sub> (PZT) nanowires are investigated using Landau–Ginzburg–Devonshire theory, taking account into the nanowire size, polarization gradient, temperature, and axis stress. It is shown that the Curie temperature, coercive electric field, remnant polarization, dielectric permittivity, and piezoelectric coefficient are all size dependent. It is revealed that the ferroelectric properties disappear below a critical diameter, which depends on the temperature, but they can be recovered under an appropriate axis tension. Our results further show that an unusual thermal fluctuation exists in PZT nanowires with small diameter.
Journal of Applied Physics 10/2008; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Lattice materials have been attractive over the last decade for use as load-carrying structures, energy absorbing elements
and heat exchanging structures because of their excellent mechanical properties and multifunctional characters. However, the
quantitative analysis accounting for high order deformations upon the collapse of lattice materials, which is important for
their applications, has not been reported. An analytical investigation of yield surfaces with respect to the high order deformations
was carried out for two typical planar lattice materials: triangular and Kagome lattices separately. The analytical results
were validated by the finite element method (FEM) simulations. It was found that the effect of high order deformation on the
yield strength increases with the relative density. The bending effect of the Kagome lattice is more obvious than that of
the triangular one with the same relative density and stress state. The yield strength of the Kagome lattice calculated by
neglecting the bending effect overestimates the result by more than 10% when the relative density is higher than about 11.1%,
which may not be ignored in engineering applications. The yielding surfaces of the two lattice materials demonstrated in the
paper also confirm the analytical results.
Acta Mechanica Sinica 09/2008; 24(5):533-540. · 0.86 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Electric-field-induced fatigue crack growth in ferroelectric ceramic PZT-5 with precracks was investigated. The experimental results showed that there were two distinct characteristics in the crack growth under electric loading. Under low electric loads, microcracks located ahead of the main crack emerged and grew and, as a result, impeded the growth of the main crack. On the other hand, under high electric loads, microcracks were absent, and the main crack was the only mode of fatigue cracking. The main crack grew macroscopically along the original path perpendicular to the electric field. Microscopically, the crack grew along the grain boundaries and grain breakaway was observed. The crack growth rate was nonlinearly related to the cyclic electric load. Similar to mechanical fatigue, there existed a crack growth threshold in the applied electric-field amplitude below which the crack ceased to grow. A steady crack growth occurred when the applied electric field exceeded this threshold. An empirical model for crack growth was obtained. Domain-switching effect and fracture-mechanics concepts were used to explain the observed crack closure and crack growth under electric loads.
Journal of the American Ceramic Society 07/2008; 87(5):840 - 846. · 2.27 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Landau-Ginsburg-Devonshire theory is introduced to investigate the size-dependent ferroelectric properties of Ba Ti O <sub>3</sub> nanowires, with the consideration of polarization orienting along the radial direction. It is shown that the Curie temperature, mean polarization, and area enclosed by hysteresis loop decrease with the reducing diameter of the nanowire. However, this size effect becomes obvious only when the diameter is below 20 nm . Above this, the ferroelectric behaviors of nanowires are almost the same as those of bulk materials. Furthermore, there exists a temperature-dependent critical diameter for Ba Ti O <sub>3</sub> nanowire below which the hysteresis loops vanish and it is 3.6 nm at room temperature.
Applied Physics Letters 02/2008; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The simultaneous measurements of magnetization and magnetostriction have been carried out for Tb0.3Dy0.7Fe1.95 alloys subjected to the multiaxially magnetomechanical loading. The saturated magnetization and magnetostriction both decrease with the increase of the angle between the applied stress and magnetic filed. The magnetostriction goes up to a climax and then decreases as the applied magnetic field increases, which should remain saturated in the case of the specimen only subjected to the axial magnetic field. On the other hand, magnetization reaches saturation without any decrease in the same region of multiaxial magnetic-stress space. The unsaturated magnetostriction can be explained well by the magnetoelastic anisotropy. Such an investigation is important to validate theoretical approaches on the magnetomechanical behavior of giant magnetostrictive materials.
Applied Physics Letters 05/2007; 90(18):182505-182505-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this article, a novel artificial submicro- or nanometer speckle fabricating technique is proposed by taking advantage of submicro or nanometer particles. In the technique, submicron or nanometer particles were adhered to an object surface by using ultrasonic dispersing technique. The particles on the object surface can be regarded as submicro or nanometer speckle by using a scanning electronic microscope at a special magnification. In addition, an electron microscope speckle photography (EMSP) method is developed to measure in-plane submicron or nanometer deformation of the object coated with the artificial submicro or nanometer speckles. The principle of artificial submicro or nanometer speckle fabricating technique and the EMSP method are discussed in detail in this article. Some typical applications of this method are offered. The experimental results verified that the artificial submicro or nanometer speckle fabricating technique and EMSP method is feasible.
Review of Scientific Instruments 04/2007; 78(3):033101. · 1.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, by capturing the atomic information and reflecting the behaviour governed by the nonlinear potential function,
an analytical molecular mechanics approach is proposed. A constitutive relation for single-walled carbon nanotubes (SWCNT’s)
is established to describe the nonlinear stress-strain curve of SWCNT’s and to predict both the elastic properties and breaking
strain of SWCNT’s during tensile deformation. An analysis based on the virtual internal bond (VIB) model proposed by P. Zhang
etal. is also presented for comparison. The results indicate that the proposed molecular mechanics approach is indeed an
acceptable analytical method for analyzing the mechanical behavior of SWCNT’s.
Acta Mechanica Sinica 01/2007; 23(6):663-671. · 0.86 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The mechanical behaviors of microstructures and micro-devices have drawn the attention from researchers on materials and mechanics in recent years. To understand the rule of these behaviors, the deformation measurement techniques with micro/nanometer sensitivity and spatial resolution are required. In this paper, a micro-marker identification method is developed to measure microstructure deformation. The micro-markers were directly produced on the top surface of microstructures by taking advantage of ion milling of focused ion beam (FIB) system. Based on the analysis of marker images captured by electronic microscope with specific correlation software, the deformation information in microstructures can be easily obtained. The principle of the technique is introduced in detail in the paper. An example experiment was executed to measure the displacement and strain distribution in a MEMS device. Obtained results show that the technique can be well applied to the deformation measurement of the micro/nano-electro-mechanical-systems (MEMS/NEMS).
Microelectronics Reliability. 01/2007;
