Baomin Xu

Domain wall motion and its contribution to the dielectric and piezoelectric properties of lead zirconate titanate films

In this article, domain wall motion and the extrinsic contributions to the dielectric and piezoelectric responses in sol–gel derived lead zirconate titanate (PZT) films with compositions near the morphotropic phase boundary were investigated. It was found that although the films had different thicknesses, grain sizes, and preferred orientations, similar intrinsic dielectric constants were obtained for all films between 0.5 and 3.4 μm thick. It was estimated that about 25%–50% of the dielectric response at room temperature was from extrinsic sources. The extrinsic contribution to the dielectric constant of PZT films was mainly attributed to 180° domain wall motion, which increased with both film thickness and grain size. In studies on the direct and converse longitudinal piezoelectric coefficients of PZT films as a function of either stress or electric driving field, it was found that the ferroelastic non-180° domain wall motion was limited. Thus extrinsic contributions to the piezoelectric response were s...

Nonlinear piezoelectric behavior of ceramic bending mode actuators under strong electric fields

The nonlinear electromechanical behavior of cantilevered piezoelectric ceramic bimorph, unimorph, and reduced and internally biased oxide wafer actuators is studied in a wide electric field and frequency range. It is found that under quasistatic condition, linear relationships between actuator tip displacement-electric field, and blocking force-electric field are only valid under weak field driving. With increasing the driving field, electromechanical nonlinearity begins to contribute significantly to the actuator performance because of ferroelectric hysteresis behavior associated with piezoelectric lead zirconate titanate (PZT)-type ceramic materials. The bending resonance frequencies of all these actuators vary with the magnitude of the electric field. The decrease of resonance frequency with electric field is explained by the increase of elastic compliance of PZT ceramic due to elastic nonlinearity. Mechanical quality factors of the actuators also depend on the magnitude of electric field strength. No ...

Electromechanical coupling and output efficiency of piezoelectric bending actuators

Electromechanical coupling mechanisms in piezoelectric bending actuators are discussed in this paper based on the constitutive equations of cantilever bimorph and unimorph actuators. Three actuator characteristic parameters, (e.g., electromechanical coupling coefficient, maximum energy transmission coefficient, and maximum mechanical output energy) are discussed for cantilever bimorph and unimorph actuators. In the case of the bimorph actuator, if the effect of the bonding layer is negligible, these parameters are directly related to the transverse coupling factor lest. In the case of the unimorph actuator, these parameters also depend on the Youngs modulus and the thickness of the elastic layer. Maximum values for these parameters can be obtained by choosing proper thickness ratio and Youngs modulus ratio of elastic and piezoelectric layers. Calculation results on four unimorph actuators indicate that the use of stiffer elastic material is preferred to increase electromechanical coupling and output mechanical energy in unimorph actuators.

Charge release of lanthanum-doped lead zirconate titanate stannate antiferroelectric thin films

The charge release speed and backward phase switching time of lanthanum-doped lead zirconate titanate stannate antiferroelectric thin films were investigated by directly measuring the switching current upon removal of the applied electric field. The backward switching time is about 6 ns. The maximum switching current density can reach 9400 A/cm2, and more than half of the stored charge can be released in 10 ns. These results show that the obtained antiferroelectric thin films are very promising for decoupling capacitor applications in high speed multichip modules.

Dielectric properties and field-induced phase switching of lead zirconate titanate stannate antiferroelectric thick films on silicon substrates

Thick (∼5 μm) films of antiferroelectric compositions in the lead zirconate titanate stannate family of solid state solutions have been fabricated by sol–gel methods on platinum-buffered silicon substrates. Dielectric properties, electric field induced ferroelectric polarization, and associated elastic strain and the temperature dependence of the dielectric response have been explored as a function of composition. Films with high tin content are shown to undergo a diffuse antiferroelectric–paraelectric phase transition with temperature, probably because of compositional inhomogeneity associated with the high tin content. This type of film also demonstrates a diffuse field-induced antiferroelectric–ferroelectric phase switching under high electric field with the appearance of “slim loop” double hysteresis, which can be attributed to the compositional heterogeneity and the high level of tensile stress in the film because of the thermal mismatch between the film and substrate. On the other hand, the film wit...

Superconductivity in Iron Telluride Thin Films under Tensile Stress

By realizing in thin films a tensile stress state, superconductivity of 13 K was introduced into FeTe, a nonsuperconducting parent compound of the iron pnictides and chalcogenides, with a transition temperature higher than that of its superconducting isostructural counterpart FeSe. For these tensile stressed films, superconductivity is accompanied by a softening of the first-order magnetic and structural phase transition, and also, the in-plane extension and out-of-plane contraction are universal in all FeTe films independent of the sign of the lattice mismatch, either positive or negative. Moreover, the correlations were found to exist between the transition temperatures and the tetrahedra bond angles in these thin films.

Ferroelectric and antiferroelectric films for microelectromechanical systems applications

Abstract In this paper we have introduced several types of ferroelectric and antiferroelectric thin (thickness 1 μm) films developed in our group for sensor, actuator and transducer applications in microelectromechanical systems (MEMS). Ferroelectric lead zirconate titanate (PZT) films of up to 12 μm in thickness have been prepared on Pt-buffered silicon substrates, which allows for the conventional, through-thickness polarization, and of up to 5 μm in thickness on insulating layer (ZrO 2 ) passivated silicon substrates, which allows for the novel, in-plane polarization. The in-plane poled films make it possible to develop d 33 -mode rather than d 31 -mode bending devices, which immediately leads to two-times improvement in device performance because d 33 ≈2 d 31 . It also can greatly increase the voltage sensitivity of bending devices because the film thickness and electrode spacing are separated as independent variables, and thus, smaller film capacitance can be obtained by using wider electrode spacing even for fixed film thickness. In addition to PZT ferroelectric films, we have also developed antiferroelectric films as an alternative for high-strain microactuators. The strain level of the antiferroelectric films can reach more than 0.4%, and both digital and analog actuation can be realized by modifying the compositions of the films. As an example for MEMS applications, micromachined, unimorph-type two-dimensional transducer arrays have been fabricated based on both the through-thickness and in-plane polarized PZT films, which can be used for miniaturized, high-resolution acoustic imaging such as hand-held divers sonar system, medical ultrasound imaging, and non-destructive testing.

Thickness dependence of ferroelectric polarization switching in poly(vinylidene fluoride–trifluoroethylene) spin cast films

In poly(vinylidene fluoride–trifluoroethylene) copolymer spin cast films, it has been observed that the polarization switching time increases as the film thickness is reduced to below 1 μm. We will show that this change with film thickness can be divided into two thickness regimes, i.e., those above 120–150 nm and those below that thickness. For films thicker than 120–150 nm, the change in the switching behavior is due to interface effects that can be modeled by an effective interface layer with lower dielectric constant that is in series with the film. For films below 120–150 nm thickness, there is an additional and very large increase of the switching time with reduced film thickness. This additional effect is caused by the precipitous drop of the crystallinity in films at this thickness range.

Dielectric hysteresis from transverse electric fields in lead zirconate titanate thin films

Excellent symmetric dielectric hysteresis is observed from lead zirconate titanate (PZT) thin films using transverse electric fields driven by interdigitated surface electrodes. The 1-μm-thick PZT films with a Zr/Ti ratio of 52/48 are prepared on ZrO2 buffered, 4-in.-diam silicon wafers with a thermally grown SiO2 layer. Both the ZrO2 buffer layer and PZT film are deposited by using a similar sol–gel processing. Remanent polarization of about 20 μC/cm2 with coercive field less than 40 kV/cm is obtained as measured using a triangle wave at 50 Hz. Thicker films are being developed and retention for the transversely polarized state is currently under study. One of the objectives of this study is to develop a large array of d33-driven unimorph sensing elements for a high-resolution acoustic imaging system.

Theoretical analysis of the sensor effect of cantilever piezoelectric benders

Piezoelectric bending mode elements such as bimorph and unimorph benders can be used as both actuation and sensing elements for a wide range of applications. As actuation elements, these devices convert electric input energy into output mechanical energy. As sensing elements, they convert external mechanical stimuli into electrical charge or voltage. In this article, the sensing effect of cantilever mounted piezoelectric bimorph unimorph and triple layer benders subjected to external mechanical excitations are discussed. General analytical expressions relating generated electric voltage (or charge) to the applied mechanical input excitations (moment M, tip force F, and body force p) are derived based on the constitutive equations of these bending devices. It is found that the clamping effect of each component in the bender devices decreases the dielectric constant. The bimorph bender has a higher voltage sensitivity than the unimorph or triple layer bender with the same geometrical dimensions. The depende...