Spontaneous bending of piezoelectric nanoribbons: Mechanics, polarization, and space charge coupling

A theory is developed to explain the spontaneous bending of polar faceted wurtzite nanoribbons, including the widely studied case of zinc oxide (ZnO) nanoarcs and nanorings. A rigorous thermodynamic treatment shows that bending of these nanoribbons can be primarily attributed to the coupling between piezoelectric effects, electric polarization, and the motion of free charge originating from point defects and/or dopants. The present theory explains the following experimental observations: the magnitude and sign of curvature and how this curvature depends on film thickness and dopant concentration. Good agreement between theory and experiment is obtained with no adjustable parameters. We identify three regimes of bending behavior with distinct thickness dependence for bending radius that depend on free carrier density, film thickness, and elastic, piezoelectric and dielectric constants.     

Electrostriction in GaN/AlN heterostructures

A one-dimensional model accounting for electrostriction, lattice mismatch, piezoelectricity, and strain is presented with special emphasis on GaN/AlN heterostructures recently examined extensively in the literature. It is shown that electrostriction, being a second-order effect in the strain–electric field relation, plays a significant, sometimes dominant contribution subject to DC voltage conditions and externally imposed hydrostatic pressure. Model results are based on experimentally reported values for electrostriction coefficients in GaN.     

Role of the TiO6 octahedra on the ferroelectric and piezoelectric behaviour of the poled PbMg1/3Nb2/3O3-xPbTiO3 (PMN-PT) single crystal and textured ceramic

Field cooling (FC) poled/unpoled PMN-29%PT single crystal and room temperature (RT) poled/unpoled PMN-34.5%PT textured ceramic were investigated between ∼0 and 300 °C by thermal expansion, dielectric and Raman spectroscopy. New phase transitions are evidenced at 40, 91 and 180 °C in the case of FC PMN-29%PT as well as at 70 and 200 °C for RT PMN-34.5%PT and their order is discussed. The physical properties of the textured ceramics are rather similar to the ones observed for the single crystals that make them low-cost alternative for a wide range of applications. However, the temperatures and character of the phase transitions strongly depend on the kind of the poling conditions. Temperature dependences of the Raman line parameters show that the NbO₆ octahedra remain stable during temperature increase, while TiO₆ ones evolve quasi-continuously. The step transitions of the Pb²⁺ ion sublattice are evidenced. This suggests that the TiO₆ and Pb²⁺ sublattices are especially coupled. The role of the TiO₆ clusters on the structural phase transitions and dielectric properties of the PbMg_1/3Nb_2/3O₃-xPbTiO₃ (PMN-PT) system is discussed. The presence of the Raman modes above the maximum dielectric permittivity reveals that the local symmetry is lower than the cubic one (Pm3m). The decrease of the Raman line intensities vs. temperature indicates precisely the continuous evolution of the local symmetry towards the cubic one. The temperature evolution of the Rayleigh wing parameters appears sensitive to the phase transitions’ presence.     

Energy trapping in power transmission through a circular cylindrical elastic shell by finite piezoelectric transducers

We study the transmission of electric energy through a circular cylindrical elastic shell by acoustic wave propagation and piezoelectric transducers. Our mechanics model consists of a circular cylindrical elastic shell with finite piezoelectric patches on both sides of the shell. A theoretical analysis using the equations of elasticity and piezoelectricity is performed. A trigonometric series solution is obtained. Output voltage and transmitted power are calculated. Confinement and localization of the vibration energy (energy trapping) is studied which can only be understood from analyzing finite transducers. It is shown that when thickness-twist mode is used the structure shows energy trapping with which the vibration can be confined to the transducer region. It is also shown that energy trapping is sensitive to the geometric and physical parameters of the structure.     

The physics of magnetoelectric composites

This paper gives an overview about the basic ideas of magnetoelectric materials. Up to now single-phase materials show the magnetoelectric effect only below room temperature. Mixing a magnetostrictive with a piezoelectric component is a way to overcome this limitation. This delivers a composite which can exhibit a magnetoelectric effect even at room temperature and higher. Possible candidates for these composites (piezoelectric as well as magnetostrictive) are shown, examples from literature and own results are given. The most important coupling mechanism (magnetization, magnetostriction, local stress, charge) between the magnetostrictive and the piezoelectric phase are discussed. Hints for a direct coupling between the electric polarization and the magnetization are also presented. Different measurement methods for determining the magnetoelectric coefficient are discussed. Representative results as obtained on a technical useful composite between 50% Co-Ferrite+50% BaTiO₃ are given. The behavior of a simple “mixed” structure with that of a “core-shell” structure is compared. The later gives a 20-times larger magnetoelectric coefficient.     

Electro-elastic interactions and second order anisotropic constitutive equations

Piezoelectricity is usually expressed as an interaction between mechanical and electrical variables. The physics involved is hence governed by a coupling between Maxwell's equations of electromagnetism and the equations of elasticity. Such a coupling takes us through the piezoelectric constitutive relations. In this work, the second order anisotropic constitutive equations are treated, and in particular, the number of independent material constants is computed for all the 32 crystallographic classes.Paper presented, in reduced version, at the 12th Italian National Congress of Theoretical and Applied Mechanics (AIMETA '95), October 1995, Naples, Italy.     

Electrically forced shear horizontal vibration of a circular cylindrical elastic shell with a finite piezoelectric actuator

We analyze anti-plane vibrations of a circular cylindrical elastic shell electrically driven by a piezoelectric actuator. The equations of linear elasticity and linear piezoelectricity are used. The mathematical problem is solved using trigonometric series. Basic vibration characteristics including resonant frequencies, mode shapes and electric admittance are calculated.     

All-optical data-latch experiments for photonic access nodes in the photonic highway

A novel approach to direct access at the optical level is proposed. We conducted experiments on all-optical data-latch functions using bistable laser diodes (LDs) for all-optical droplinsert operation in a 50-Mbitls data highway.     

Nanoscale imaging of ferroelectric domain and resistance switching in hybrid improper ferroelectric Ca3Ti2O7 thin films

Hybrid improper ferroelectrics have their electric polarization generated by two or more combined non-ferroelectric structural distortions such as the rotation and tilting of Ti-O octahedral in Ca₃Ti₂O₇ (CTO) family. In this work, we prepared different thickness CTO thin films on Pt substrates by pulsed laser deposition, and investigated their ferroelectric polarization reversal and the current transport properties by using the piezoresponse force microscopy and conducting atomic force microscopy, respectively. It is found that the CTO films exhibit clear ferroelectric domain switching and ferroelectric resistance switching behaviors, and the maximum resistive ratios of CTO film reaches ∼1750. These results demonstrate that hybrid improper ferroelectrics CTO films are promising materials for being employed in non-volatile memory and logic devices.     

Does the electric Lehmann effect exist in cholesteric liquid crystals?

Experiments have shown that cholesteric droplets or cholesteric fingers may be put into motion by the action of an electric field. The former rotate whereas the latter drift perpendicularly to their axes. In all cases, the texture moves without visible material transport. The electric Lehmann effect was initially used to interpret these observations but, recently, alternative explanations were found, based on electrohydrodynamics. Another experiment in this area was that of Padmini and Madhusudana (Liq. Cryst. 14, 497 (1993)). Performed in 1993 with a compensated cholesteric liquid crystal under fixed planar boundary conditions, it was also explained in terms of electric Lehmann effect. We conducted the same experiment and extended it to a π -twisted planar geometry. Although our experimental results agree with those of Padmini and Madhusudana, we demonstrate that they are incompatible with an electric Lehmann effect. By contrast, an explanation based on flexoelectricity allows us to interpret the whole data set obtained in both geometries. The consequence is that there is at the moment no clear experimental evidence of the electric Lehmann effect.