Christopher S. Lynch

The effect of uniaxial stress on the electro-mechanical response of 8/65/35 PLZT

Abstract 8/65/35 PLZT is characterized under stress and electric field loading above the coercive field. Strain—electric-field and electric-displacement—electric-field hysteresis loops are measured at various compressive stress levels showing the effect of stress on the electro-mechanical coupling. Stress—strain curves are run at zero electric field, showing that depolarization begins at as little as 5 MPa. The limitations of linear constitutive modeling and the limitations of this composition of PLZT as an actuator material are examined. The “yield” or ferroelastic switching stress is suggested as a good criteria for assessing the capability of actuator ceramics. This parameter is an indication of the actuation limitations of the material. An equivalence of the slopes of the strain—electric-field curve at several compressive stress levels and the electric-displacement—stress curve at the same stress level is observed. This suggests that the conjugacy of the piezoelectric coefficients may hold during path dependent polarization switching.

Giant electric-field-induced reversible and permanent magnetization reorientation on magnetoelectric Ni/(011) [Pb(Mg1/3Nb2/3)O3](1−x)–[PbTiO3]x heterostructure

We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale particles sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.We report giant reversible and permanent magnetic anisotropy reorientation in a magnetoelectric polycrystalline Ni thin film and (011)-oriented [Pb(Mg1/3Nb2/3)O3](1−x)–[PbTiO3]x heterostructure. The electric-field-induced magnetic anisotropy exhibits a 300 Oe anisotropy field and a 50% change in magnetic remanence. The important feature is that these changes in magnetization states are stable without the application of an electric field and can be reversibly switched by an electric field near a critical value (±Ecr). This giant reversible and permanent magnetization change is due to remanent strain originating from a non-180° ferroelectric polarization reorientation when operating the ferroelectric substrate in a specific non-linear regime below the electric coercive field.

A micro-electro-mechanical model for polarization switching of ferroelectric materials

Abstract Ferroelectric and ferroelastic switching are the major source of nonlinearity and hysteresis in ferroelectric materials subjected to high electric field or mechanical stress. A computational micromechanics model for polycrystalline ferroelectric ceramics is developed based on consideration of the constitutive behavior of single crystals. This model simulates the tetragonal and the rhombohedral crystal structures. Saturation of the linear piezoelectric effect is included. Interaction between different grains in the polycrystalline ceramic is considered. A switching criterion is developed that accounts for different energy levels associated with 90° and 180° switching for the tetragonal structure (or 70.5°, 109.5°, and 180° for the rhombohedral structure). Experimental results on 8/65/35 PLZT are simulated and a parametric study of the effects of crystal structure, intergranular interaction, and phase transformation is performed.

Ferroelectric properties of [110], [001] and [111] poled relaxor single crystals: measurements and modeling

Abstract A combination of modeling and experimental work has led to the identification of a crystal cut of PZN-4.5%PT with enhanced piezoelectric coefficients for bending mode applications d 31 =690 pC/N, d 32 =−1670 pC/N and direct evidence of a rhombohedral-orthorhombic phase transformation under [110] electric field loading. A crystal variant model enables the calculation of the physical properties of the single crystals with an engineered domain state (the piezoelectric, elastic, and dielectric coefficients) from the properties of the single domain [111] poled rhombohedral phase. This work focuses on the piezoelectric coefficients. Not all of the [111] piezoelectric coefficients have been measured. The modeling approach is used to compute the missing coefficients, d 15 and d 16 of the [111] poled crystal from the [001] and [110] properties. Criteria for variant and phase evolution in the model reproduce the observed hysteresis loops, remanent strain and remanent polarization.

Electrical control of reversible and permanent magnetization reorientation for magnetoelectric memory devices

We report giant reversible and permanent magnetic anisotropy reorientation between two perpendicular easy axes in a magnetoelectric polycrystalline Ni thin film and (011) oriented [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT) heterostructure. The PMN-PT is partially poled prior to Ni film deposition to provide a remanent strain bias. Following Ni deposition and full poling of the sample, two giant remanent strains of equal and opposite values are used to reversibly and permanently reorient the magnetization state of the Ni film. These experimental results are integrated into micromagnetic simulation to demonstrate the usefulness of this approach for magnetoelectric based magnetic random access memory.

Constraint-induced crack initiation at electrode edges in piezoelectric ceramics

A strain incompatibility arises between the electrically active and inactive parts of a partially electroded piezoelectric material. This leads to cracking perpendicular to the electrode edge. Symmetric partial electrodes of different widths were applied to lead zirconate titanate (PZT) plates to study the development of these cracks. Different cracking patterns appeared upon application of an electric field, depending on the degree of coverage and on the thickness of the plates. The apparent coercive fields are correlated to the degree of clamping and the amount of cracking in the specimens. Non-linear finite element modelling is used to analyse the material response.

Domain engineered switchable strain states in ferroelectric (011) [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT, x≈0.32) single crystals

The ferroelectric properties of (011) [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT, x≈0.32) single crystals with focus on piezoelectric strain response were reported. Two giant reversible and stable remanent strain states and tunable remanent strain properties are achieved by properly reversing the electric field from the depolarized direction. The unique piezoelectric strain response, especially along the [100] direction, mainly stems from the non-180° ferroelectric polarization reorientation in the rhombohedral phase crystal structure. Such giant strain hysteresis with tunable remanent strain properties may be useful for magnetoelectric based memory devices as well as a potential candidate for other applications.

Piezoelectric Hydraulic Pump Development

A piezohydraulic pump is presented that makes use of the step-and-repeat capability of piezoelectric actuators. This work discusses piezohydraulic pumping theory, pump design, and pump performance. Efficiency is analyzed for the pump cycle with theoretical efficiency being 1.0. Loss mechanisms that result in actuator heating are discussed. The pump was constructed and connected to a hydraulic actuator. The actuator was driven at 7 cm/sec with a 271 N (61 lb) blocking force. To achieve this, the piezoelectric stack actuator was driven at 60 Hz.

Purified and porous poly(vinylidene fluoride-trifluoroethylene) thin films for pyroelectric infrared sensing and energy harvesting

This paper aims at improving the performance of the poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer for pyroelectric infrared detection and direct thermal to electrical energy conversion. Three different types of samples were prepared and examined: commercial, purified and porous films. Here, full characterization of the thermophysical and electrical properties relevant to pyroelectric infrared detection and energy conversion of both purified and porous P(VDF-TrFE) thin films is presented. Properties measured include (1) density, (2) ferroelectric to paraelectric phase transition temperature, (3) enthalpy of change of phase, (4) electrical resistivity and (5) ferroelectric hysteresis, as well as (6) specific heat, (7) dielectric constant, (8) loss tangent and (9) pyroelectric coefficient as a function of temperature. The figures of merit for infrared detection FV , FI and FD were improved by 47.0, 59.6 and 51.6%, respectively, for the purified films while the porous films with a porosity of 33% showed an improvement of 52.8, 66.3 and 62.6%, respectively, when compared to those of dense commercial P(VDF-TrFE) films. In addition, figures of merit for energy harvesting, FE and k^2, indicate that the purified and porous films are attractive for thermal to electrical energy conversion as well.

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.