▪ Abstract This paper reviews the literature on size effects in ferroelectric materials, with an emphasis on thin film perovskite ferroelectrics. The roles of boundary conditions, defect chemistry, electrode interfaces, surface layers, and microstructure in controlling the measured properties of ferroelectric films, as well as the observed deviation from bulk properties are discussed. Examples of the manifestation of size effects in terms of the low and high field dielectric properties, the piezoelectric effect, and the leakage behavior of films are given.

The Properties of Ferroelectric Films at Small Dimensions

The development of a method for integrating highly efficient energy conversion materials onto stretchable, biocompatible rubbers could yield breakthroughs in implantable or wearable energy harvesting systems. Being electromechanically coupled, piezoelectric crystals represent a particularly interesting subset of smart materials that function as sensors/actuators, bioMEMS devices, and energy converters. Yet, the crystallization of these materials generally requires high temperatures for maximally efficient performance, rendering them incompatible with temperature-sensitive plastics and rubbers. Here, we overcome these limitations by presenting a scalable and parallel process for transferring crystalline piezoelectric nanothick ribbons of lead zirconate titanate from host substrates onto flexible rubbers over macroscopic areas. Fundamental characterization of the ribbons by piezo-force microscopy indicates that their electromechanical energy conversion metrics are among the highest reported on a flexible medium. The excellent performance of the piezo-ribbon assemblies coupled with stretchable, biocompatible rubber may enable a host of exciting avenues in fundamental research and novel applications.

Piezoelectric Ribbons Printed onto Rubber for Flexible Energy Conversion

There has been a resurgence of complex oxides of late owing to their ferroelectric and ferromagnetic properties. Although these properties had been recognized decades ago, the renewed interest stems from modern deposition techniques that can produce high quality materials and attractive proposed device concepts. In addition to their use on their own, the interest is building on the use of these materials in a stack also. Ferroelectrics are dielectric materials that have spontaneous polarization in certain temperature range and show nonlinear polarization–electric field dependence called a hysteresis loop. The outstanding properties of the ferroelectrics are due to non-centro-symmetric crystal structure resulting from slight distortion of the cubic perovskite structure. The ferroelectric materials are ferroelastic also in that a change in shape results in a change in the electric polarization (thus electric field) developed in the crystal and vice versa. Therefore they can be used to transform acoustic wav...

Processing, Structure, Properties, and Applications of PZT Thin Films

Flexible electronics incorporate all the functional attributes of conventional rigid electronics in formats that have been altered to survive mechanical deformations. Understanding the evolution of device performance during bending, stretching, or other mechanical cycling is, therefore, fundamental to research efforts in this area. Here, we review the various classes of flexible electronic devices (including power sources, sensors, circuits and individual components) and describe the basic principles of device mechanics. We then review techniques to characterize the deformation tolerance and durability of these flexible devices, and we catalogue and geometric designs that are intended to optimize electronic systems for maximum flexibility.

Flexible electronics under strain: a review of mechanical characterization and durability enhancement strategies

Transverse piezoelectric properties of Pb(Zr, Ti)O 3 (PZT) films were estimated using a simple measuring method we developed. The c -axis oriented PZT films were epitaxially grown on Pt/MgO substrates, while the polycrystalline PZT films with the preferential orientation of 〈1 1 1〉 were deposited on Pt/Ti/Si substrates using rf sputtering technique. The piezoelectric characteristics of the PZT films with different crystalline structures were evaluated by the tip deflection of the unimorph cantilevers of the strip specimen just cleaved out from the substrates. The PZT films on MgO substrates showed excellent linear piezoelectric deflection to the applied voltage with the stable piezoelectric coefficient e 31 of −4.7 to −4.9 C/m 2 which is caused by the ideal lattice motion of the single domain structure. On the other hand, the PZT films on Si substrates showed large hysteresis of the deflection and the value of e 31 ranged from −4.3 to −7.5 C/m 2 according to the applied voltage. The non-linear as well as large piezoelectric response of the PZT films on Si is similar to conventional bulk PZT ceramics, indicating that the reorientation of domains whose polar axes are not parallel to the electric field is superimposed on the lattice motion.

Measurement of transverse piezoelectric properties of PZT thin films

The wafer flexure technique was used to characterize the d31 coefficient of a number of sol–gel and radio frequency (rf) sputtered lead zirconate titanate (PZT) thin films with thicknesses between 0.6 and 3 μm. Typical d31 values for well-poled 52/48 sol–gel films were found to be between −50 and −60 pC/N. The rf sputtered films possessed large as-deposited polarizations which produced d31 coefficients on the order of −70 pC/N in some unpoled films. The subsequent poling of the material, in a direction parallel to the preferred direction increased the d31 coefficient to values of about −85 pC/N. The aging behavior of the d31 coefficient was also investigated. For sol–gel films the aging rate was found to be independent of poling direction and to range from 4% per decade for a 2.5 μm film to 8% per decade for a 0.6 μm film. In contrast, the aging rate of sputtered films was strongly dependent on poling direction, with maximum and minimum rates of 26% and 2% per decade recorded. These aging rates are very h...

Fan Chu

Papers

Characterization and aging response of the d31 piezoelectric coefficient of lead zirconate titanate thin films