Propagation of thickness-twist waves in a multi-sectioned piezoelectric plate of 6 mm crystals

We study thickness-twist vibrations and waves in an unbounded, multi-sectioned piezoelectric plate of crystals with 6 mm symmetry or polarized ceramics. An exact solution from the three-dimensional equations of piezoelectricity is obtained. Basic vibration and wave propagation characteristics are calculated based on the solution. The results are useful in the understanding and design of plate resonators, filters and acoustic wave sensors of ZnO, AlN and polarized ceramics.     

Low-temperature property investigation of the lead indium-niobate-lead nickel-niobate solid solution

The complex perovskite solid solution (1−x) Pb(In_1/2Nb_1/2)O₃-(x) Pb(Ni_1/3Nb_2/3)O₃ has been successfully prepared by the Columbite precursor method. The temperature dependencies of the dielectric constant and pyroelectric coefficient were measured between −261 and 200 °C. Relaxor ferroelectric behavior has been noticed in all compositions across the solid solution. The room-temperature electrostrictive coefficient, Q₃₃, was 1.83×10⁻² C²/m⁴ for x=0.10. No room-temperature piezoelectric activity was detected; however, upon cooling to −261 °C the maximum coupling coefficients k_p=29%, k_t=11%, and k₃₃=31% were observed for the composition x=1.00.     

Features and origin of the dynamic and the nonlinear piezoelectricity in poly(vinylidene fluoride)

Some of the piezoelectric coefficients of poly(vinylidene fluoride) are distinctly stress dependent. We report on their dependence on the frequency of an oscillatory stress excitation, on the amount of the remanent polarization in the sample materials, and on the magnitude of an offset stress. In order to explain the observed dynamic and nonlinear piezoelectric properties, a model is developed that attributes them to changes in the local polarization in an interphase between the crystalline and amorphous phases in this semicrystalline polymer.     

CONTRIBUTION OF MACROSCOPIC DIMENSION EFFECT TO PIEZOELFCTRICITY IN POLYVINYLIDENE FLUORIDE

In this paper, we have studied the piezoelectricity in the poled uniaxially drawn polyvinylidene fluoride. The piezoelectric constants d_(31), d_(32), d_(33)~αand Young's moduli 1/s_11 and 1/s_(22) have been determined as a function of the remanent polarization Pr. The piezoelectric constants of the samples show a strong in-plane anisotropy. Such an anisotropy is mostly attributable to different Poisson's ratio. It is found that the piezoelectric activity mainly arises from macroscopic dimensional change.     

Effect of Li2CO3 addition on the structural,optical, ferroelectric,and electric-field-induced strain of lead-free BNKT-based ceramics

In this work, we reported the effect of Li₂CO₃ addition on the structural, optical, ferroelectric properties and electric-field-induced strain of Bi_0.5(Na,K)_0.5TiO₃ (BNKT) solid solution with CaZrO₃ ceramics. Both rhombohedral and tetragonal structures were distorted after adding Lithium (Li). The band gap values decreased from 2.91 to 2.69 eV for 5 mol% Li-addition. The maximum polarization and remanent polarization decreased from 49.66 μC/cm² to 27.11 μC/cm² and from 22.93 μC/cm² to 5.35 μC/cm² for un-doped and 5 mol% Li- addition BNKT ceramics, respectively. The maximum S_max/E_max value was 567 pm/V at 2 mol% Li₂CO₃ access. We expected this work will help to understand the role of A-site dopant in lead-free ferroelectric BNKT materials.     

The effect of macroscopic polarization on intrinsic and extrinsic thermal conductivities of AlN

The effect of macroscopic polarization on thermal conductivity of bulk wurtzite AlN has been theoretically investigated. Our results show that macroscopic polarization modifies the phonon group velocity, Debye frequency and Debye temperature of the AlN. Using revised phonon velocity and Debye temperature, various phonon scattering rates and combined scattering rate are calculated as functions of the phonon frequency at room temperature. The intrinsic and extrinsic thermal conductivities of AlN have been estimated using these modified parameters. The theoretical analysis shows that up to a certain temperature the polarization effect acts as negative effect and reduces the intrinsic and extrinsic thermal conductivities. However, after this temperature both thermal conductivities are significantly enhanced. High phonon velocity and Debye temperature are the reason of this enhancement which happens due to the polarization effect. The revised thermal conductivities at room temperature are found to be increased by more than 20% in AlN due to macroscopic polarization phenomenon. The method we have developed can be taken into account during the simulation of heat transport in optoelectronic nitride devices to minimize the self heating processes.     

Piezoelectric properties of diphenylalanine microtubes prepared from the solution

Biomimetic self-assembling peptides form a variety of structures that can be used for the fabrication of functional devices. We are witnessing the emergence of a new era of bionanotechnology that opens up new possibilities for novel electronic, photonic and energy functionalities based on supramolecular green and lightweight structures. In this work, we study the emergent piezoelectric properties of linear dipeptide diphenylalanine (FF) that can self-assemble in the shape of microtubes. The matrix of piezoelectric coefficients is derived for the first time based on the hexagonal symmetry of FF structures and different configurations of the tubes are tested by the advanced Piezoresponse Force Microscopy (PFM). Strong piezoelectric anisotropy of piezoelectric coefficients is explained by the self-assembled structure of FF peptides. Possible applications of piezoelectric microtubes in functional devices are discussed.     

A very promising piezoelectric property of Ta2O5 thin films. II: Birefringence and piezoelectricity

Birefringent and piezoelectric properties of Ta₂O₅ ceramic thin films of monoclinic and trigonal structures were analyzed. The birefringence, observed by reflected polarized light microscopy, yields information on thin film microstructures, crystal shapes and sizes and on crystallographic orientations of grains of trigonal structure. Such an information was considered for investigating piezoelectric properties by laser Doppler vibrometry and by piezoresponse force microscopy. The vibration velocity was measured by applying an oscillating electric field between electrodes on both sides of a Ta₂O₅ film deposited on a Si substrate which was pasted on an isolating mica sheet. In this case, it is shown that the vibration velocity results were not only from a converse piezoelectric effect, proportional to the voltage, but also from the Coulomb force, proportional to the square of the voltage. A huge piezoelectric strain effect, up to 7.6%, is found in the case of Ta₂O₅ of trigonal structure. From an estimation of the electrical field through the Ta₂O₅ thin film, this strain likely corresponds to a very high longitudinal coefficient d₃₃ of several thousand picometers. Results obtained by piezoresponse force microscopy show that trigonal grains exhibit a polarization at zero field, which is probably due to stress caused expansion in the transition monoclinic-trigonal, presented in a previous article (part I).     

Development of mechanoluminescence technique for impact studies

A new technique called, mechanoluminescence technique, is developed for measuring the parameters of impact. This technique is based on the phenomenon of mechanoluminescence (ML), in which light emission takes place during any mechanical action on solids. When a small solid ball makes an impact on the mechanoluminescent thin film coated on a solid, then initially the elastico ML (EML) intensity increases with time, attains a maximum value I_m at a particular time t_m, and later on it decreases with time. The contact time T_c of ball, can be determined from the relation T_c=2t_c, where t_c is the time at which the EML emission due to compression of the sample becomes negligible. The area from where the EML emission occurs can be taken as the contact area A_c. The maximum compression h is given by h=A_c/(πr), where r is the radius of the impacting ball, and thus, h can be determined from the known values of A_c and r. The maximum force at contact is given by F_m=(2mU₀)/T_c, where m is the mass of the impacting ball and U₀ is the velocity of the ball at impact. The maximum impact stress σ_m can be obtained from the relation, σ_m=F_m/A_c=(2mU₀)/(T_cA_c). Thus, ML provides a real-time technique for determining the impact parameters such as T_c, A_c, h, F_m and σ_m. Using the ML technique, the impact parameters of the SrAl₂O₄:Eu film and ZnS:Mn coating are determined. The ML technique can be used to determine the impact parameters in the elastic region and plastic region as well as fracture. ML can also be used to determine the impact parameters for the collision between solid and liquid, if the mechanoluminescent material is coated on the surface of the solid. The measurement of fracto ML in microsecond and nanosecond range may provide a tool for studying the fragmentations in solids by the impact. Using the fast camera the contact area and the depth of compression can be determined for different intervals of time.     

Exciton states and interband optical transitions in wurtzite InGaN/GaN quantum dot nanowire heterostructures

Within the framework of the effective-mass approximation, the exciton states and interband optical transitions in In_xGa_1−xN/GaN strained quantum dot (QD) nanowire heterostructures are investigated using a variational method, in which the important built-in electric field (BEF) effects, dielectric-constant mismatch and three-dimensional confinement of the electron and hole in In_xGa_1−xN QDs are considered. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the exciton oscillator strength.