Improvement of the piezoelectricity of PVDF-HFP by CoFe_(2)O_(4)nanoparticles

High piezoelectric composite films composed of poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP)and ferromagnetic cobalt ferrite(CoFe_(2)O_(4))(0.00 wt%to 0.2 wt%)are prepared by a solution casting method accompanied by uniaxial stretching and high electric field poling.The decisive effect of the poling electric field on the power generating capability was confirmed by the experiments.For pure PVDF-HFP films,when the maximum electric field Emax is 120 MV/m,the calibrated open circuit voltage reaches 2.93 V,which is much higher than those poled at lower electric fields(70 MV/m:1.41 V;90 MV/m:2.11 V).Furthermore,the addition of CoFe_(2)O_(4)also influences the piezoelectricity dramatically.In the samples containing 0.15 wt%CoFe_(2)O_(4),the calibrated open circuit voltage increases to the maximum value of 3.57 V.Meanwhile,the relative fraction of theβ-phase and the crystallinity degree are 99%and 48%,respectively.The effects of CoFe_(2)O_(4)nanoparticles on initial crystallization,uniaxial stretching and high electric field poling are investigated by XRD,FTIR and DSC.

THE INTERACTION BETWEEN CRACK AND ELECTRIC DIPOLE OF PIEZOELECTRICITY

Discrete dipoles located near the crack tip play an important rolein nonlinear electric field induced fracture of piezoelectricceramics. A physico-mathematical model of dipole is constructed oftwo gen- eralized concentrated piezoelectric forces with equaldensity and opposite sign. The interaction between crack and electricdipole in piezoelectricity is analyzed. The closed form solutions,including those for stress and electric displacement, crack openingdisplacement and electric potential, are obtained. The function ofpi- ezoelectric anisotropic direction, p_α(θ)=cosθ+p_αsinθ, canbe used to express the influence of a dipole's direction. In the casethat a dipole locates near crack tip, the piezoelectric stressintensity factor is a power function with -3/2 index of the distancebetween dipole and crack tip.

On the role of piezoelectricity in phonon properties and thermal conductivity of GaN nanofilms

The effect of piezoelectricity on phonon properties and thermal conductivity of gallium nitride （GaN） nanofilms is theoretically investigated. The elasticity model is utilized to derive the phonon properties in spatially confined GaN nanofilms. The piezoelectric constitutive relation in GaN nanofilms is taken into account in calculating the phonon dispersion relation. The modified phonon group velocity and phonon density of state as well as the phonon thermal conductivity are also obtained due to the contribution of piezoelectricity. Theoretical results show that the piezoelectricity in GaN nanofilms can change significantly the phonon properties such as the phonon group velocity and density of states, resulting in the variation of the phonon thermal conductivity of GaN nanofilms remarkably. Moreover, the piezoelectricity of GaN can modify the dependence of thermal conductivity on the geometrical size and temperature. These results can be useful in modeling the thermal performance in the active region of GaN-based electronic devices.

The First-Principle Calculation of La-doping Effect on Piezoelectricity in Tetragonal KNN Crystal

The La-dopping effect on the piezoelectricity in the K0.5Na0.5NbO3 （KNN） crystal with a tetragonal phase is investigated for the first time using the first-principle calculation based on density functional theory. The full potentiallinearized augumented plane wave plus local orbitals （APW-LO） method and the supercell method are used in the calculation for the KNN crystal with and without the La doping. The results show that the piezoelectricity originates from the strong hybridization between the Nb atom and the O atom, and the substitution of the K or Na atom by the La impurity atom introduces the anisotropic relaxation and enhances the piezoelectricity at first and then restrains the hybridization of the Nb-O atoms when the La doping content further increases.

A Generalized Variational Principle for 2-D Piezoelectricity with Surface Electrodes

It is difficult to establish a classical variational model for piezoelectricity. Following the semi inverse method of establishing generalized variational principles, an energy like trial functional with a certain unknown function is constructed. The unknown function is easily identified step by step. A family of variational principles for the static behavior of the elastic and electric variables in the vicinity of a surface electrode attached to a piezoelectric ceramic is established directly from its field equations and boundary conditions.

Uniqueness theorem,theorem of reciprocity,and eigenvalue problems in linear theory of porous piezoelectricity

The uniqueness theorem and the theorem of reciprocity in the linearized porous piezoelectricity are established under the assumption of positive definiteness of elastic and electric fields. General theorems in the linear theory of porous piezoelectric materials are proved for the quasi-static electric field approximation. The uniqueness theorem is also proved without using the positive definiteness of the elastic field. An eigenvalue problem associated with free vibrations of a porous piezoelectric body is stud- ied using the abstract formulation. Some properties of operators are also proved. The problem of frequency shift due to small disturbances, based on an abstract formulation, is studied using a variational and operator approach. A perturbation analysis of a special ease is also given.

Piezoelectricity in K_(1-x)Na_xNbO_3: First-principles calculation

The piezoelectric properties of K1-xNaxNbO3 are studied by using first-principles calculations within virtual crystal approximation. To understand the critical factors for the high piezoelectric response in K1-xNaxNbO3, the total energy, piezoelectric coefficient, elastic property, density of state, Born effective charge, and energy barrier on polarization rotation paths are systematically investigated. The morphotropic phase boundary in K1-xNaxNbO3 is predicted to occur at x = 0.521, which is in good agreement with the available experimental data. At the morphotropic phase boundary, the longitudinal piezoelectric coefficient d33 of orthorhombic K0.5Na0.5NbO3 reaches a maximum value. The rotated maximum of d＊33 is found to be along the 50° direction away from the spontaneous polarization （close to the [001] direction）. The moderate bulk and shear modulus are conducive to improving the piezoelectric response. By analyzing the energy barrier on polarization rotation paths, it is found that the polarization rotation of orthorhombic K0.5Na0.5NbO3 becomes easier compared with orthorhombic KNbO3, which proves that the high piezoelectric response is attributed to the flattening of the free energy at compositions close to the morphotropic phase boundary.

Effect of Passivation on Piezoelectricity of ZnO Nanowire

Surface passivation is one valuable approach to tune the properties of nanomaterials.The piezoelectric properties of hexagonal[001]ZnO nanowires with four kinds of surface passivations were investigated using the first-principles calculations.It is found that in the 50%H(O)and 50%Cl(Zn),50%H(O)and 50%F(Zn)passivations,the volume and surface effects both enhance the piezoelectric coefficient.This differs from the unpassivated cases where the surface effect was the sole source of piezoelectric enhancement.In the 100%H,100%Cl passivations,the piezoelectric enhancement is not possible since the surface effect is screened by surface charge with weak polarization.The study reveals that the competition between the volume effect and surface effect influences the identification of the diameter-dependence phenomenon of piezoelectric coefficients for ZnO nanowires in experiments.Moreover,the results suggest that one effective means of improving piezoelectricity of ZnO nanowires is shrinking axial lattice or increasing surface polarization through passivation.

Surface-Induced Enhancement of Piezoelectricity in ZnO Nanowires

Piezoelectric nanowires are promising building blocks in various micro-electromechanical systems. Using firstprinciples calculations, we systematically investigate the influence of surface and volume changes on piezoelectric coefficients in [001]-oriented ZnO nanowires and hollow nanowires. We find that the increased non-axial ion displacements under strain near the {100} surface cause a notable enhancement in piezoelectric coefficients for these nanowires. Furthermore, by introducing the obtained surface modifications, we break through the limitation of simulation size and obtain the piezoelectric coefficients at the experimental size. Our findings are of importance to expand simulations and guide experimental explorations.

Study on Enhancement Effect of Cavitation Caused by HIFU Piezoelectricity Transducer

An orthogonal ultrasonic irradiation system consisting of HIFU with frequency at 1.05 MHz combined with ultrasound with frequency at 28 kHz was applied in this paper.Effect of cavitation was detected by pH-value measurement and conductance measurement.The result shows that the effect of cavitation caused by ultrasound with frequency at 28 kHz is greatly enhanced by HIFU piezoelectricity transducer with frequency at 1.05 MHz.

BASIC SOLUTION FOR THE THREE-PHASE COMPOSITE CONSTITUTIVE MODEL IN ANTIPLANE PIEZOELECTRICITY

A basic solution in series form for the three-phase composite cylindrical model in antiplane piezoelectricity subjected to the action of a singularity in the intermediate matrix region is presented. The solution is obtained through the complex potential approach in conjunction with the techniques of analytical continuation, singularity analysis, Laurent series expansion in an annular region and Cauchy integral formulae, etc. Based on the complex potentials obtained, explicit expressions for the distribution of stress and electric displacement in the three regions are also derived.

First-principles study of polarization and piezoelectricity behavior in tetragonal PbTiO_3-based superlattices

Using first-principles calculation, the contribution of A-site and B-site atoms to polarization and piezoelectricity d33 in the tetragonal Pb TiO_3/KNbO_3 and PbTiO_3/LaAlO_3 superlattices is investigated in this paper. It is shown that PbTiO_3/KNbO_3 superlattice has larger polarization and d33 than PbTiO_3/LaAlO_3 superlattice, because there is stronger charge transfer between A（B）-site atoms and oxygen atom in PbTiO_3/KNbO_3 superlattice. InPbTiO_3/KNbO_3 superlattice,B-site atoms（Ti, Nb） make larger contribution to the total polarization and d33 than the A-site atoms（Pb, K） because of the strong covalent interactions between the transition metal（Ti, Nb） and the oxygen atoms, while piezoelectricity in PbTiO_3/LaAlO_3 superlattice mainly ascribes to piezoelectric contribution of Pb atom and Ti atom in PbTiO_3 component.Furthermore, by calculating the proportion of the piezoelectric contribution from Pb TiO_3 component in superlattices, we find there is different response of strain to piezoelectric contribution from Pb TiO_3 component in two superlattices but still with a value larger than 50%. InPbTiO_3/KNbO_3 superlattice, the c-axis strain reduces the proportion, especially under tensile condition. Meanwhile in PbTiO_3/LaAlO_3 superlattice, Pb TiO_3 plays a leading role to the total d33, especially under compressive condition, and the proportion decreases as the tensile strain increases.

Greatly improved piezoelectricity and thermal stability of(Na,Sm)Co-doped CaBi_(2)Nb_(2)O_(9) ceramics

Calcium bismuth niobate(CaBi_(2)Nb_(2)O_(9))is regarded as one of the most potential high-temperature piezoelectric materials owing to its highest Curie point in bismuth layer-structured ferroelectrics.Nevertheless,low piezoelectric coefficient and low resistivity at high temperature considerably restrict its development as key electronic components.Herein,markedly improved piezoelectric properties and DC resistivity of CaBi_(2)Nb_(2)O_(9) ceramics through Naþand Sm^(3+)þco-doping are reported.The nominal compositions Ca_(1-2x)(Na,Sm)_(x)Bi_(2)Nb_(2)O_(9)(x=0,0.01,0.025,and 0.05)ceramics have been prepared via the conventional solid state method.An optimum composition of Ca_(0.95)(Na,Sm)_(0.025)Bi_(2)Nb_(2)O_(9) is obtained,which possesses a high Curie point of~949℃,a piezoelectric coefficient of~12.8 pC/N,and a DC electrical resistivity at 500℃ of~4×10^(7)Ω⋅cm.The improved d33 is probably ascribed to the reduction in domain size and the increase in domain wall density caused by the reduced grain size.More importantly,after annealing at 900℃ for 2 h,the piezoelectric coefficient still maintains about 90%of the initial d33 value,which displays a significant improvement compared to pure CaBi_(2)Nb_(2)O_(9) ceramic with only 44%of the initial d33 value.This work exhibits a feasible approach to simultaneously obtain high piezoelectric property and thermal stability in CaBi_(2)Nb_(2)O_(9) ceramics by Naþ/Sm^(3+)þco-doping.

Magnetism and piezoelectricity of hexagonal boron nitride with triangular vacancy

First-principle calculations reveal that the configuration system of hexagonal boron nitride （h-BN） monolayer with triangular vacancy can induce obvious magnetism, contrary to that of the nonmagnetic pristine boron nitride monolayer. Interestingly, the h-BN with boron atom vacancy （VB-BN） displays metallic behavior with a total magetic moment being 0.46μB per cell, while the h-BN with nitrogen atom vacancy （VN-BN） presents a half-metallic characteristic with a total magnetic moment being 1.0μB per cell. Remarkably, piezoelectric stress coefficient ell of the VN-BN is about 1.5 times larger than that of pristine h-BN. Furthermore, piezoelectric strain coefficient dll （12.42 μm/V） of the VN-BN is 20 times larger than that of pristine h-BN and also one order of magnitude larger than the value for the h-MoS2 monolayer, which is mainly due to the spin-down electronic state in the VN-BN system. Our study demonstrates that the nitrogen atom vacancies can be an efficient route to tailoring the magnetic and piezoelectric properties of h-BN monolayer, which have promising performances for potential applications in nano-electromechanical systems （NEMS） and nanoscale electronics devices.

Influence of Particle Size on Piezoelectricity of Piezo-composites

Serial material model （Dilute model）and Limited Units （LU）method were employed to analyze the performance of binary piezo-composite system.The reckoned electric potential deployments illustrated difference while the particles were different.Their piezoelectricities were also calculated according to the model,and furthermore comparation suggested that small particles living in the tolerance improve the piezoelectricity of piezo-composite.Experiments coinciding with analyses were processed simultaneously. Ceramics were milled for different time in order to control the concentration of particle size.The results showed that the filled particles enhanced the piezoelectricity of binary piezo-composite system efficiently whereas too many chips deteriorated the performance of piezo-composites.

A dual-range Janus-structure pressure sensor with broaddetection range and high resolution combiningtriboelectricity and piezoelectricity

Enabling pressure sensors with high resolution and a broad detection range isof paramount importance yet challenging due to the limitations of each knownsensing method. Overlying different sensing mechanisms to achieve complementaryfunctions is a promising approach, but it often leads to increaseddevice thickness, crosstalk signals and complex signal channel management.Herein, we present a dual-functional conformable pressure sensor that adoptsa Janus thin film layout, enabling simultaneous piezoelectric and triboelectricsignal detection capabilities between just one electrode pair, showing a mostcompact device configuration. Notably, despite its thin thickness (
80 μm for apackaged device), it exhibits a broad-range detection capability with high signalresolution and fast response time, demonstrating a distinct signal-relaycharacteristic corresponding to piezoelectricity and triboelectricity. Despite theslimness and simple structure, it shows an impressive signal resolution of0.93 V·kPa^(-1) in the range of 0.1–140 kPa and 0.05 V·kPa^(-1) in the range of140–380 kPa. Moreover, the device fabrication can be combined with thekirigami method to improve fitting to joint surfaces. This work introduces aninnovative paradigm for designing advanced pressure sensing mechanisms,enabling a single device that can meet diverse application scenarios through itssimplicity, slim layout, conformable, and self-powered characteristics to adaptto multiple scenarios.

Intrinsically asymmetric atomic character regulates piezoelectricity in two-dimensional materials

Decreasing of layer thickness causes the decrease of polarization until it disappears due to the existence of depolarization field.Therefore,the search for strong piezoelectric materials is highly desirable for multifunctional ultra-thin piezoelectric devices.Herein,we propose a common strategy for achieving strong piezoelectric materials through the electronic asymmetry induced by the intrinsically asymmetric atomic character of different chalcogen atoms.Accordingly,in the tetrahedral lattice structures,for example,M4X3Y3(M=Pd/Ni,X/Y=S,Se or Te,X≠Y)monolayers are proved to display excellent out-of-plane piezoelectricity.Ni4Se3Te3 possesses the largest piezoelectric coefficient d33 of 61.57 pm/V,which is much larger than that of most 2D materials.Enhancing the electronic asymmetry further increases the out-of-plane piezoelectricity of Janus M4X3Y3 materials.Correspondingly,the out-of-plane piezoelectricity is positively correlated with the ratio of electronegativity difference(Red)and the electric dipole moment(P).This work provides alternative materials for energy harvesting nano-devices or self-energized wearable devices,and supplies a valuable guideline for predicting 2D materials with strong out-of-plane piezoelectricity.

Effectively improved piezoelectricity in high-temperature Arrhenius CBT ceramics by modifying potential energy profile and spontaneous polarization

Enhancing the piezoelectricity of CaBi_(4)Ti_(4)O_(15)(CBT)ceramics is crucial for improving their application potential in high-temperature piezoelectric devices.Here,we propose a strategy involving the introduction of larger Na/Bi ions at the A-site,effectively inhibiting the tilt of oxygen octahedra and flattening the potential energy profile.This flattening enhances the variation in polarization under external fields.Concurrently,substituting Nb/Mn at the B-site increases the deviation between positive and negative ionic centers,leading to stronger spontaneous polarization,while the induced defect dipoles restrict oxygen vacancy migration and increase the direct current resistivity(ρ_(dc)).The flattened potential energy profile and increased spontaneous polarization significantly enhance the overall performance of cBT ceramics,with the piezoelectric constant(d_(33))reaching 25 pC/N when the Curie temperature(Tc)=752℃.Piezoresponse force microscopy(PFM)and transmission electron microscopy(TEM)revealed submicron-long rectangular domains and nanoscale domains in the modified composition,indicating a high density of domain walls.This study presents an effective approach for enhancing the piezoelectric properties of bismuth layered-structured ferroelectrics(BLSFs),thereby improving the application potential of BLSFsathightemperatures.

Room-temperature ferromagnetism and piezoelectricity in metalfree 2D semiconductor crystalline carbon nitride

Two-dimensional(2D)materials that combine ferromagnetic,semiconductor,and piezoelectric properties hold significant potential for both fundamental research and spin electronic devices.However,the majority of reported 2D ferromagnetic-semiconductor-piezoelectric materials rely on d-electron systems,which limits their practical applications due to a Curie temperature lower than room temperature(RT).Here,we report a high-crystallinity carbon nitride(CCN)material based on sp-electrons using a chemical vapor deposition strategy.CCN exhibits a band gap of 1.8 eV and has been confirmed to possess substantial in-plane and out-of-plane piezoelectricity.Moreover,we acquired clear evidences of ferromagnetic behavior at room temperature.Extensive structural characterizations combined with theoretical calculations reveal that incorporating structural oxygen into the highly ordered heptazine structure causes partial substitution of nitrogen sites,which is primarily responsible for generating room-temperature ferromagnetism and piezoelectricity.As a result,the strain in wrinkles can effectively modulate the domain behavior and piezoelectric potential at room temperature.The addition of RT ferromagnetic-semiconductor-piezoelectric material based on sp-electrons to the family of two-dimensional materials opens up numerous possibilities for novel applications in fundamental research and spin electronic devices.

Enhanced piezoelectricity of Cu-doped K(Ta,Nb)O_(3) single crystals

As a class of solid solution material,K(Ta,Nb)O3(KTN)single crystals have attracted significant interest due to their excellent piezoelectric and electro-optic performance.In this study,the piezoelectric properties of KTN were improved through Cu doping.Utilizing the advantages of composition-regulating phase transition,a large,high-quality Cu:KTN crystal measuring 30 mm×25 mm×20 mm in size was grown using the improved top seeded solution growth method.Cu-doped KTN exhibited better piezoelectric properties than pure KTN,and the full matrix parameters were investigated.Excellent dielectric,piezoelectric,and electromechanical coupling responses(ε^(T)_(11)-2,136,d_(33)∼303 pC/N,k_(t)∼0.515,and k33∼0.672)were successfully obtained.To realize the optimized orientation of the piezoelectric properties,the orientation dependence of the single domain properties was investigated,and the mechanism of Cu doping to improve piezoelectric properties was explored.We found that 0.36%(in mass)Cu doped crystal consisted of A-site substitution,following ferroelectric and domain analysis.The small ferroelectric domain sizes led to a large domain wall mobility rate and high domain wall density,which contributed to high piezoelectric properties.This work revealed the effect of A-position doping on piezoelectric properties and provided a theoretical and experimental foundation for the performance optimization of KTN-based materials.