J. V. Mantese

Magnitude of the intrinsic electrocaloric effect in ferroelectric perovskite thin films at high electric fields

Monodomain contributions to the electrocaloric effect in mechanically free (bulk) and laterally clamped (thin film) BaTiO3 are computed using a thermodynamic analysis. The authors show that the intrinsic electrocaloric coefficient at electric fields sufficient to destroy the discontinuous ferroelectric phase transition results in an adiabatic temperature change of 8K that agrees closely with the giant value of 12K recently observed experimentally for lead zirconate titanate thin films [A. S. Mischenko et al., Science 311, 1270 (2006)]. Perfect lateral clamping transforms the discontinuous transition into a continuous transition, which decreases the magnitude of the electrocaloric effect by 20% but reduces its sensitivity to temperature.

Electric field tunable ferrite-ferroelectric hybrid wave microwave resonators: Experiment and theory

The electric field tuning characteristics of a combined microwave resonator based on ferrite-ferroelectric layered structure have been studied in a wide range of bias magnetic fields. The combined ferrite-ferroelectric resonator was composed of two rectangular resonators fabricated from a ceramic barium strontium titanate (BST) slab and a single-crystal yttrium iron garnet (YIG) film. The in-plane dimensions for the YIG and BST resonators were chosen to be equal in order to maximize the electromagnetic coupling between their main modes and reduce spurious influence of their higher order modes. A tuning range of 100MHz for the resonator frequency was realized at 5GHz through the variation of magnetic permeability and dielectric permittivity of the YIG-BST structure. A theory for the hybrid wave excitations, based on a coupled-mode approach, has been developed and provides good description of the data.

Ferrite-ferroelectric layered structures for electrically and magnetically tunable microwave resonators

It is demonstrated experimentally that a layered structure consisting of ferrite and ferroelectric thin films can be used as an electrically and magnetically tunable microwave resonator. The dual tunability is realized through the application of a bias electric field to the ferroelectric layer (thus changing its dielectric constant), and a bias magnetic field to the ferrite layer. The resonator having central frequency f0≅5GHz and bandwidth Δf=3.5MHz demonstrated a broadband (∼300Δf) tunability through the variation of the bias magnetic field, and a narrow-band (∼2Δf) tunability through the variation of the bias electric field.

Pyroelectric response of ferroelectric thin films

A thermodynamic formalism is developed to calculate the pyroelectric coefficients of epitaxial (001) Ba0.6Sr0.4TiO3 (BST 60/40) and Pb0.5Zr0.5O3 (PZT 50/50) thin films on (001) LaAlO3, 0.29 LaAlO3:0.35(Sr2TaAlO6) (LSAT), MgO, Si, and SrTiO3 substrates as a function of film thickness by taking into account the formation of misfit dislocations at the growth temperature. The role of internal stress is discussed in detail with respect to epitaxy-induced misfit and thermal stresses arising from the difference between the thermal expansion coefficients of the film and the substrates. It is shown that the pyroelectric coefficients steadily increase with increasing film thickness for BST 60/40 and PZT 50/50 on LSAT and SrTiO3 substrates due to stress relaxation by misfit dislocations. Large pyroelectric responses (∼1.1 μC/cm2 K for BST 60/40 and ∼0.3 μC/cm2 K for PZT 50/50) are theoretically predicted for films on MgO substrates at critical film thicknesses (∼52 nm for BST 60/40 and ∼36 nm for PZT 50/50) correspo...

High dielectric tunability in ferroelectric-paraelectric bilayers and multilayer superlattices

The dielectric tunability of ferroelectric/paraelectric bilayers and multilayer superlattices are examined theoretically. A numerical analysis is carried out for a pseudomorphic (001) BaTiO3∕SrTiO3 heteroepitaxial bilayer on (001) SrTiO3 and a stress-free BaTiO3∕SrTiO3 bilayer. We show that these structures are capable of tunabilities greater than 90% due to electrostatic and electromechanical coupling between layers. Moreover, we develop the methodology for incorporation conventional integrated circuit silicon dielectrics into heteroepitaxial structures that can reduce current leakage while maintaining high tunability, thereby enabling the device designer flexibility toward the optimization of microwave and millimeter wave elements.

The role of thermally-induced internal stresses on the tunabilityof textured barium strontium titanate films

The tunability of highly textured thin films of barium strontium titanate (Ba0.5Sr0.5TiO3, BST) is analyzed theoretically using a Landau–Devonshire thermodynamic model. The relative dielectric constant of BST films is determined as functions of the applied external electric field, deposition temperature, and the thermal expansion coefficient of the substrate. Our analysis shows that tunability is highly dependent upon thermally induced strains within the material. Both tension and compression produce deleterious tuning response. However, this effect can be minimized through judicious choices of deposition temperature and appropriate substrate material.

Effective pyroelectric response of compositionally graded ferroelectric materials

A thermodynamic theory is used to determine the effective pyroelectric coefficients of polarization graded ferroelectrics, with values in excess of 0.1μC∕cm2°C are predicted for various barium strontium titanate thin films. Maximum values closer to 0.4μC∕cm2°C are obtained for a 1μm thick material—compositionally graded between pure barium titanate and one that has barium to strontium in the ratio of 75:25.

Origin of the “up,” “down” hysteresis offsets observed from polarization-graded ferroelectric materials

“Up” and “down” hysteresis offsets were observed in polarization-graded ferroelectrics. The polarization gradients were achieved by imposing temperature gradients across a bulk ferroelectric material near its Curie temperature. In the absence of temperature gradients, no hysteresis offsets were observed, ruling out extraneous sources as the cause of the aberrant behavior, thereby establishing the origin of the offsets in the class of polarization-graded ferroelectric devices.

Stress-induced polarization-graded ferroelectrics

Polarization-graded ferroelectrics and their electrically active embodiments, graded ferroelectric devices and transpacitors, have been formed from a variety of material systems, both by grading the composition of the ferroelectric and by imposing temperature gradients normal to the electrode surfaces. In this letter, we show how these same devices can be formed from homogeneous ferroelectric films of lead strontium titanate by imposing stress gradients on the material normal to their electrode surfaces.

Large piezoelectric strains from polarization graded ferroelectrics

The potential applications of polarization graded ferroelectrics as high performance sensors and actuators are theoretically investigated. A static bending can be expected in polarization graded ferroelectric plates, forming a vertical displacement. This is due to the built-in strain gradient that arises from the grading of the composition and concomitantly, the spontaneous self-strain. Numerical results of two compositionally graded ferroelectrics, BaTiO3–Ba1−xSrxTiO3 and PbTiO3–Pb1−xZrxTiO3, show a high dynamic response of the displacement under an external electric field, yielding as much as ∼23% strain at 50kV∕cm in PbTiO3–Pb0.6Zr0.4TiO3, which is comparable to large displacement actuators formed from ceramic/ceramic and ceramic/metal multilayer mesomaterials.