Guozhong Chai

Wetting and Dewetting Transitions on Submerged Superhydrophobic Surfaces with Hierarchical Structures

The wetting transition on submersed superhydrophobic surfaces with hierarchical structures and the influence of trapped air on superhydrophobic stability are predicted based on the thermodynamics and mechanical analyses. The dewetting transition on the hierarchically structured surfaces is investigated, and two necessary thermodynamic conditions and a mechanical balance condition for dewetting transition are proposed. The corresponding thermodynamic phase diagram of reversible transition and the critical reversed pressure well explain the experimental results reported previously. Our theory provides a useful guideline for precise controlling of breaking down and recovering of superhydrophobicity by designing superhydrophobic surfaces with hierarchical structures under water.

Adjustable magnetoelectric effect of self-assembled vertical multiferroic nanocomposite films by the in-plane misfit strain and ferromagnetic volume fraction

The strain-mediated magnetoelectric (ME) property of self-assembled vertical multiferroic nanocomposite films epitaxially grown on cubic substrates was calculated by a nonlinear thermodynamic theory combined with the elastic theory. The dependent relations of phase state of ferroelectric films with the in-plane misfit strain, out-of-plane misfit strain, temperature, and volume fraction of ferromagnetic phase were confirmed. The effects of in-plane misfit strain and ferromagnetic volume fraction on the polarization and dielectric constant of ferroelectric films at room temperature were elaborately analyzed for the vertical BaTiO3-CoFe2O4 and PbTiO3-CoFe2O4 nanocomposite films. Our calculated results confirmed the relationship among ME effect and in-plane misfit strain and ferromagnetic volume fraction in the nanocomposite films. The ME voltage coefficients of vertical BaTiO3-CoFe2O4 and PbTiO3-CoFe2O4 nanocomposite films displayed various maximums and abrupt points at special phases and phase transition bo...

Strain-modulated magnetocapacitance of vertical ferroelectric?ferromagnetic nanocomposite heteroepitaxial films

The strain-mediated magnetocapacitance (MC) of 1–3-type vertical ferroelectric–ferromagnetic nanocomposite films epitaxially grown on different cubic substrates is theoretically calculated using the nonlinear thermodynamic theory combined with elastic theory. The dependences of relative dielectric constant and strain sensitivity of permittivity for the ferroelectric film on the in-plane and out-of-plane misfit strains are obtained. Our results show that the MC effect strongly depends on the in-plane misfit strain and ferromagnetic volume fraction in the nanocomposite films. The calculated MC for the BaTiO3–CoFe2O4 nanocomposite film grown on the SrTiO3 substrate is consistent with the experimental result. Furthermore, a giant MC, which is enhanced by 1–2 orders of magnitude than those reported in experiments, can be obtained for the BaTiO3–CoFe2O4 system grown on the MgO substrate near the transition from the aa-phase to the r-phase. Our result provides a new method for the design of multiferroic nanocomposites with colossal MC effect by optimizing the ferromagnetic phase and substrate types to realize their applications in magnetic-field-controlled electric devices, such as magneto-oscillators, magnetovaractors and magnetoelectric sensors.

The enhanced piezoelectricity in compositionally graded ferroelectric thin films under electric field: A role of flexoelectric effect

Compositionally graded ferroelectric thin films are found to produce large strain gradients, which can be used to tune the physical properties of materials through the flexoelectric effect, i.e., the coupling of polarization and the strain gradient. The influences of the flexoelectric effect on the polarization distribution and the piezoelectric properties in compositionally graded Ba1−xSrxTiO3 ferroelectric thin films are investigated by using an extended thermodynamic theory. The calculation results show that the presence of the flexoelectric effect tends to enhance and stabilize polarization components. The polarization rotation induced by the flexoelectric field has been predicted, which is accompanied by more uniform and orderly polarization components. A remarkable enhancement of piezoelectricity is obtained when the flexoelectric field is considered, suggesting that compositionally graded Ba1−xSrxTiO3 ferroelectric thin films with a large strain gradient are promising candidates for piezoelectric devices.Compositionally graded ferroelectric thin films are found to produce large strain gradients, which can be used to tune the physical properties of materials through the flexoelectric effect, i.e., the coupling of polarization and the strain gradient. The influences of the flexoelectric effect on the polarization distribution and the piezoelectric properties in compositionally graded Ba1−xSrxTiO3 ferroelectric thin films are investigated by using an extended thermodynamic theory. The calculation results show that the presence of the flexoelectric effect tends to enhance and stabilize polarization components. The polarization rotation induced by the flexoelectric field has been predicted, which is accompanied by more uniform and orderly polarization components. A remarkable enhancement of piezoelectricity is obtained when the flexoelectric field is considered, suggesting that compositionally graded Ba1−xSrxTiO3 ferroelectric thin films with a large strain gradient are promising candidates for piezoelectric d...

Effect of crystal orientation on the phase diagrams, dielectric and piezoelectric properties of epitaxial BaTiO3 thin films

The influence of crystal orientations on the phase diagrams, dielectric and piezoelectric properties of epitaxial BaTiO3 thin films has been investigated using an expanded nonlinear thermodynamic theory. The calculations reveal that crystal orientation has significant influence on the phase stability and phase transitions in the misfit strain-temperature phase diagrams. In particular, the (110) orientation leads to a lower symmetry and more complicated phase transition than the (111) orientation in BaTiO3 films. The increase of compressive strain will dramatically enhance the Curie temperature TC of (110)-oriented BaTiO3 films, which matches well with previous experimental data. The polarization components experience a great change across the boundaries of different phases at room temperature in both (110)- and (111)-oriented films, which leads to the huge dielectric and piezoelectric responses. A good agreement is found between the present thermodynamics calculation and previous first-principles calculations. Our work provides an insight into how to use crystal orientation, epitaxial strain and temperature to tune the structure and properties of ferroelectrics.

BISTABLE CHARACTERISTICS OF IRREGULAR ANTI-SYMMETRIC LAY-UP COMPOSITE CYLINDRICAL SHELLS

The bistable characteristics of the irregular anti-symmetric cylindrical shell (IACS), such as [+α1/ -α2/ +α2/ -α1] lay-up, are presented using theoretical methodology and finite element (FE) analysis. A novel approach named two points loading method is used to induce a snap-through of the shell in FE analysis. Unlike regular [+α/ -α/ +α/ -α] lay-up anti-symmetric cylindrical shells, two or more different ply angles are included in the irregular anti-symmetric lay-up. Besides ply angles, the bistable characteristics of composite cylindrical shells with irregular anti-symmetric lay-ups are also affected by the sequence of lay-ups. Both analytical and FE results show that IACS with different bistable behaviors can be obtained by changing the ply angles or sequence of the lay-ups. It is very useful for the design and manufacture of the bistable structure. There is a reasonable agreement between analytical and FE predictions; possible reasons of differences between analytical and FE results are given.

Giant electrocaloric effect in ferroelectric ultrathin films at room temperature mediated by flexoelectric effect and work function

In ferroelectric ultrathin films, built-in electric fields are often present due to the flexoelectric effect and the difference of work functions at asymmetric electrodes, which may change the properties of the materials. In this paper, the influence of build-in electric fields induced by flexoelectric effect and/or work function difference on the misfit strain-temperature phase diagrams, and the electrocaloric properties of epitaxial BaTiO3 ultrathin films are investigated by using an extended nonlinear thermodynamic theory. It is found that the flexoelectric effect, i.e., the coupling of polarization and strain gradient, changes the misfit strain-temperature phase diagrams notably, in which the phases with out-of-plane polarizations increase due to the presence of a built-in field. The electrocaloric properties are remarkably enhanced when the built-in fields induced by both the flexoelectric effect and work function difference are considered. In particular, a giant adiabatic temperature change of 7.89 ...

One-step hydrothermal synthesis of NiS/MoS2-rGO composites and their application as catalysts for hydrogen evolution reaction

The composites of sulphide and reduced graphene oxide (NiS/MoS2-rGO) were synthesized through a facile solvent-assisted hydrothermal method. The introduction of NiS was paramount not only in enhancing the conductivity of whole catalysts but also in modulating the layer structures of MoS2 with additional active sites. Moreover, the NiS and rGO functioned together in controlling the morphology of as-prepared composites, resulting in uniformly distributed NiS/MoS2 nanosheets perpendicular to rGO scaffold. This further contributed to the excellent hydrogen evolution performance of the composites with a small onset overpotential of 80mV and Tafel slope as low as 65mV/decade.

Dielectric tunability of vertically aligned ferroelectric-metal oxide nanocomposite films controlled by out-of-plane misfit strain

A nonlinear thermodynamic model based on the vertically aligned nanocomposite (VAN) thin films of ferroelectric-metal oxide system has been developed to investigate the physical properties of the epitaxial Ba0.6Sr0.4TiO3 (BST) films containing vertical Sm2O3 (SmO) nanopillar arrays on the SrTiO3 substrate. The phase diagrams of out-of-plane lattice mismatch vs. volume fraction of SmO are calculated by minimizing the total free energy. It is found that the phase transformation and dielectric response of BST-SmO VAN systems are extremely dependent on the in-plane misfit strain, the out-of-plane lattice mismatch, the volume fraction of SmO phase, and the external electric field applied to the nanocomposite films at room temperature. In particular, the BST-SmO VAN systems exhibit higher dielectric properties than pure BST films. Giant dielectric response and maximum tunability are obtained near the lattice mismatch where the phase transition occurs. Under the in-plane misfit strain of umf=0.3% and the out-of-...

Mechanical analysis of functionally graded graphene oxide-reinforced composite beams based on the first-order shear deformation theory

ABSTRACT This paper investigates the mechanical behaviors of functionally graded multilayer nanocomposite beams reinforced with a low content of single-layer graphene oxide powders (GOPs). The material properties of GOP nanocomposites are estimated. The first-order shear deformation theory and Hamiltons principle are employed to deal with the motion equations. Parametric studies are conducted to examine the effects of distribution pattern and weight fraction of GOPs, slenderness ratio and boundary condition on the bending, buckling, and vibration of GOP beams. The results show that GOP is superior to the single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) in reinforcing the mechanical behaviors of polymer nanocomposite.