David Pisani

Passive locomotion of a simple articulated fish-like system in the wake of an obstacle

The behaviour of a passive system of two-dimensional linked rigid bodies in the wake of a circular cylinder at Re = 100 is studied computationally. The three rigid bodies are connected by two frictionless hinges, and the system (‘fish’) is initially aligned with a streamwise axis three diameters behind the cylinder. Once flow symmetry is broken, the wake rolls up into a K ´ arm ´ an vortex street in which the fish is stably trapped, and the passing large-scale vortices induce an undulatory shape change in the articulated system. It is found that, for certain fish lengths relative to cylinder diameter, the fish is propelled upstream toward the cylinder. Furthermore, the fish is propelled equally effectively when the hinges are locked, confirming that induced body undulation is not necessary for achieving a net thrust. An analysis of the forces on constituent bodies shows that leading-edge suction and negative skin friction on the forward portion of the fish are in competition with positive skin friction on the aft portion; propulsion is achieved when the forebody contributions dominate those on the aftbody. It is shown that the so-called ‘suction zone’ behind the cylinder that enables this passive propulsion is double the length of that without a fish present.

A method to control magnetism in individual strain-mediated magnetoelectric islands

Patterned electrodes on a piezoelectric substrate are demonstrated to produce a localized strain of sufficient magnitude to control the magnetic anisotropy of a Ni island. Strain-induced magnetic anisotropy was measured using the magneto-optical Kerr effect, and the measured shifts in magnetic anisotropy were consistent with strain predicted using linear finite element analysis. This approach overcomes the effect of the substrate clamping the in-plane strain and should be scalable to thin films. This approach represents a key step toward realizing the next generation of strain mediated magneto-electric magnetic random access memory devices with low writing energy and high writing speed.

A finite element based phase field model for ferroelectric domain evolution

Finite element based phase field modeling is applied at the unit cell level using a finite element framework with a Landau?Devonshire type multi-well potential as a material subroutine to model domain evolution in ferroelectrics. The time-dependent Ginzburg?Landau equation with polarization as an order parameter governs the evolution of polarization. In this approach, the domain wall width is controlled by a balance between mechanical, structure, electrostatic, and local gradient contributions to the free energy density. The effect of this energy balance on the resulting domain wall width of 90??and 180??tetragonal domain walls is discussed and examples are presented.

Piezoelectric strain sensor/actuator rosettes

In-plane anisotropy in the linear piezoelectric constitutive law for [011]c cut and poled PMN‐0.29PT is demonstrated to enable its use as a sensor/actuator rosette. The equations for a 0 ◦ /45 ◦ /90 ◦ rosette are developed using the conditions of coupling between the in-plane strain of the crystal and a substrate, and zero out-of-plane stress on the crystal (plane stress conditions in the crystals). The crystals are bonded to a substrate aluminum plate that is instrumented with strain gages next to the crystals. The plate is subjected to bending about different axes and the resulting electric displacement change of the crystals is monitored. The strain components calculated using the change of electric displacement are compared with the strain components measured using strain gages. This sensor/actuator rosette approach is demonstrated to enable both sensing principal strain components and actuating principal strains in an electronically controllable direction. (Some figures in this article are in colour only in the electronic version)

Giant electric-field-induced magnetic anisotropy reorientation with patterned electrodes on a Ni thin film/lead zirconate titanate heterostructure

This study reports a method of using patterned electrodes on a piezoelectric substrate to generate local strain to control magnetic properties of individual magnetic units. By operating different effective electrode pairs on a piezoelectric substrate, a local bi-axial strain is generated. This rotates the magnetic anisotropy of a 35 nm thick and 0.5 mm diameter Ni island through the magnetoelastic effect. The electric-field-induced magnetic anisotropy exhibits an anisotropy field up to 600 Oe and a 75% change in magnetic remanence.

Coupled effects of hydrostatic pressure and bipolar electric field on the FE-AFE phase transformation in 95/5 PZT

The behavior of 95/5 PZT subjected to bipolar electrical loading and hydrostatic pressure is studied experimentally. When 95/5 PZT is subjected to high enough hydrostatic pressure it undergoes a ferroelectric to antiferroelectric (FEAFE) phase transformation. Specimens were subjected to two pressure cycles from 0 to 550 MPa at a rate of 50 MPa/min under short circuit conditions. It was found that under the first pressure cycle the specimens undergo a FEAFE phase transformation at 330 MPa indicated by an abrupt compression of 2500 microstrain. Under the second pressure cycle, the transformation no longer occurs at a single pressure level, but is smoothed throughout loading. In another set of experiments, bipolar electric fields were applied up to 3 MV/m at discrete pressure levels. At low pressures, electric displacement-electric field plots exhibited open loop behavior characteristic of soft ferroelectrics. As the pressure was increased past the FE-AFE phase transformation threshold, the open loops closed to nearly linear dielectric. When the driving pressure was decreased the open loop behavior returned at a notably lower pressure level. The transformation pressure is therefore path dependent and is evidence of a pressure hysteresis.

Stress and electric field gradient contributions to dielectric loss in ferroelectrics with interdigitated electrodes

Interdigitated electrode configurations produce nonuniform electric field and stress in the vicinity of the electrodes, creating a volume where the material is not uniformly polarized. This results in enhanced hysteresis in interdigitated electrode configurations relative to the hysteresis in the bulk material. The dielectric loss in a macro-fiber composite with interdigitated electrodes was characterized and is compared to the dielectric loss of the same material under a uniform field. The dielectric loss in the interdigitated electrode arrangement was found to be significantly larger and had a strong dependence on electric field amplitude. The dielectric loss is expressed in terms of an effective loss tangent (tan δ) and a more general damping model. A mechanism that contributes to the hysteresis is that the local stress in the ferroelectric material beneath the interdigitated electrodes induces ferroelastic polarization reorientation during each electric field cycle. The interactions between polarization gradients and residual stress are assessed using a finite element model with a micromechanical-based constitutive law.

Modeling and Characterization of the Effects of Field Gradients on the Behavior of Ferroelectrics With Interdigitated Electrodes

Macro fiber composites (MFCs) are used in applications ranging from sensing and actuation to energy harvesting and piezoelectric damping. MFCs are comprised of interdigitated electrodes (IDEs) on ferroelectric fibers that result in anisotropic in plane sensing and actuation capability. Minor hysteresis loops were measured for a MFC and the area within the minor hysteresis loops was used to assess material loss under unipolar cyclic loading. A separate set of minor hysteresis loops was run on a fully electroded plate of the same material to determine the material loss when the electric field was uniform. The MFC displayed considerably more material loss than the uniformly loaded plate. A micromechanical model implemented in a finite element code was used to model the effect of inhomogeneous fields, local stress, and polarization reorientation on the dielectric losses. The results indicate that the development of local stress in the ferroelectric material beneath the inerdigitated electrodes results in ferroelastic polarization reorientation that contributes to dielectric losses.Copyright

3-D effects of polarization switching on interdigitated electroded ferroelectrics

Interdigitated electrodes are used to obtain an in-plane d33 coupling from patch actuators. Existing design tools do not take into consideration the three dimensional effects of polarization reorientation. This work presents a 3-D finite element code that utilizes a micromechancial constitutive law with full ferroelectric switching. The code is used to explore the design of interdigitated electrode devices. The results point to several parameters that are important to the design of these devices. These include electrode spacing, electrode width, specimen thickness, and specimen depth.

Discrete phase model of domain walls in ferroelectric crystals

A discrete phase model is used to simulate evolution of domain structures in ferroelectric materials. The discrete phase model balances the structural energy against local mechanical and electrical energies. Devonshire theory is used to model the structural energy while a finite element framework using scalar potential theory computes the local electric field and stress.