Electric-field-induced phase transitions and piezoelectric properties of <001>-oriented Pb(Mg1/3Nb2/3)O3-32%PbTiO3 (PMN-PT) single crystals have been investigated as a function of temperature. It was found that the phase transitions and piezoelectric properties for PMN-PT crystals are strongly dependent on temperature. The measurements of polarization and longitudinal strain as a function of a unipolar electric field show that the field for the induced monoclinic-tetragonal phase transition decreases linearly with temperature in the range between 23 °C and 75 °C. Raising the temperature can stabilize the tetragonal phase in <001>-oriented PMN-PT crystals. The effective longitudinal piezoelectric constant, d33, in the monoclinic phase increases with temperature. Meanwhile in the field-induced tetragonal phase, d33 is much smaller and has little change with temperature. The electric-field-induced phase transition from a cubic phase to a tetragonal phase was observed at 125 °C.
Dielectric elastomers are known to produce large transverse strains in response to electrically induced Maxwell stresses and thus provide a useful form of electromechanical actuation. The transverse strain response of silicone (Dow Corning HS III RTV) based Maxwell stress actuators have been measured earlier as a function of driving electric field, frequency and pre-load. Experimental results show that a pre-load initially causes an increase in the strain. However, this increase appears to be a function of the relative geometries of the electroded area and of the specimen itself. The transverse strains in these materials decrease when larger values of pre-load are applied. Models of hyperelasticity that are capable of describing the large deformation of polymer materials have been used to interpret our results. Numerical finite element simulations of the material’s behavior using a hyperelastic model provides good agreement with most of our observations on the electric field and pre-strain dependencies of the transverse strain.
Some electroactive polymers produce large electric-field-induced strains that can be used for electromechanical actuation. The measurement of the strain response, especially the dynamic response under high driving fields, is difficult. We have developed a transverse strain measurement system based on the Zygo laser Doppler interferometer. The system can measure transverse strain responses of polymer samples of different sizes over a wide displacement range and a frequency range from DC up to 100 Hz. We have used this interferometric system to investigate the strain response of Maxwell stress actuators fabricated from silicone (Dow Corning HS III RTV) and thermoplastic polyurethane (Dow Pellethane 2103) films. The static and dynamic strain responses of the materials to a variety of driving electric fields such as step fields, AC fields and DC bias fields have been measured as functions of amplitude and frequency. The strain response has a quadratic relationship with the driving field and shows a strong dependence on the frequency of the applied field. Of the two kinds of polymers investigated, HS III silicone polymer shows higher strain and breakdown fields. High transverse strains of 3.25 % (static) and 2.08 % (dynamic at 1 Hz) for HS III silicone polymers have been obtained. In addition, the effect of mechanical tensile load on the transverse strain has also been studied. The experimental data are interpreted in terms of measured material properties and small strain models for dielectric film actuators.
Single crystals of the relaxor ferroelectric solid solution family Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) have been shown to produce very high levels of electric field induced strains with large electromechanical coupling coefficients when poled aong the <001> direction. These materials also exhibit a high dielectric permittivity and they are therefore very promising materials for transducer and actuator applications. We have carried out a systematic investigation of the dielectric and piezoelectric properties of different compositions, using specimens manufactured by TRS Ceramics Inc. Both the dielectric and piezoelectric strain responses of these crystals have been studied as a function of temperature and applied electric field. The applied field and temperature dependences of the rhombohedral to tetragonal and the tetragonal to cubic phase transitions have been determined. Our results suggest that a DC bias field can be used to stabilize the tetragonal phase. The strain measurements have been used to determine the d33 values in both phases as a function of temperature. The PZN-PT crystals have their polar direction along the <111> axis but not much has been reported on the piezoelectric properties of crystals poled in this direction. We have used laser Doppler interferometry to measure the piezoelectric strains as a function of DC bias field applied in the <111> direction in PZN-PT crystals and we have determined values of the piezoelectric coefficients as a function of the DC bias field. High d15 values over 3000 pC/N have been observed along <111> direction.
The nonlinear behavior of the dielectric and piezoelectric resopnse of <001> oriented Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) single crystals for x=4.5% and x=8% have been investigated as a function of AC electric fields and DC bias field. At relatively low applied fields, the polarization and strain of PZN-PT single crystals poled along the <001> direction show little hysteresis and have a linear dependence on the applied field, which is a consequence of engineered domain stability. Hence, the dielectric and piezoelectric coefficients of the material do not exhibit any field dependence. However, when the applied electric field exceeds a threshold value, the strain and dielectric responses become nonlinear. The dielectric and piezoelectric constants are a function of the applied field, and hysteresis loops are observed. The results suggest that the observed nonlinear behavior in the PZN-PT single crystals is caused by domain motion/switching in response to the large AC fields. Applying a positive DC bias can effectively stabilize the domain configuration in the crystals and enhance linear responses. The threshold field, at the onset of nonlinearity, is found to have a linear relation with DC bias field in the field range investigated.
Sensors and actuators based on piezoelectric ceramics are finding an increasingly large variety of applications under a very wide range of environmental conditions and applied signals. Some actuator applications require the piezoelectric materials to support large mechanical loads and produce high strain output. In order to accomplish this requirement of higher strains, large electric fields must be applied. This results in a significant non-linear behavior and hence affects the performance of the material. It is therefore important to understand the behavior and properties of these materials over a large range of temperature, frequency and applied electric fields and mechanical stresses. We have measured some of the dielectric, elastic and piezoelectric constants of soft (EC-65, EC-76) and hard (EC-64, EC-69) lead zirconate titanate (PZT) piezoelectric ceramics, manufactured by EDO Ceramics, as a function of temperature, frequency, applied field and applied stress. We have also determined the dependence of the piezoelectric constants on an applied DC bias voltage or stress. The time dependence of the piezoelectric response in the piezoelectric ceramics has also been studied. A summary of the results will be presented. Most of these results can be understood on the basis of the extrinsic contributions to the piezoelectric response that arises from the existence of domains in the material.
Piezoelectric transducers are often used under compressive stress in smart structure and other applications and it is therefore important to know properties of these materials as a function of applied stress. We have developed an experiment that allows us to find the piezoelectric charge coefficient as a function of uniaxial stress in the poled direction . Both dynamic and static measurements were carried out and the corresponding values of the charge coefficient d33 were obtained as a function of applied stress. These coefficients differ from each other because of the different proportions of reversible and irreversible domain changes that contribute to them and each coefficient can be important in specific applications. Results on a range of PZT ceramics manufactured by EDO Corporation are presented; in general, they show a non- linear behavior with an initial increase in d33 as the stress increases followed by a significant decrease. The time dependence of the measurement has also been investigated.