The Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 system was synthesized in a wide compositional range in order to study the relationship between its phase diagram and electromechanical properties. Phase ...transitions were marked using peaks in temperature-dependent permittivity, providing up to three transitions from the rhombohedral phase to an orthorhombic, tetragonal and finally cubic phase, which meet in a region that is termed the phase convergence region in this work. In situ small and large signal electromechanical properties were studied as a function of temperature with specific emphasis on these transitions. A small signal piezoelectric coefficient, d 33, presents maximized values at the transition from the orthorhombic to the tetragonal phase, while a large signal piezoelectric coefficient, , does so at both rhombohedral to orthorhombic and to tetragonal phase transitions. Maximum polarization P max was the only quantity determined that had a clear maximum at the phase convergence region.
The prediction that the piezoelectric tensor is diverging at the critical point is verified for BaTiO sub(3) (BTO), which is the oldest perovskite structured ferroelectric material with an extremely ...long and eventful research history. Here we investigate experimentally by dielectric and calorimetric measurements the existence and the position of the critical point in the electric-field-temperature (E-T) phase diagram of BTO in the vicinity of the paraelectric to ferroelectric phase transition. Measurements of the piezoelectric coefficient d sub(31) as a function of the temperature and the electric field applied along the 001 direction show a critical enhancement of the piezoelectric response in the vicinity of the critical point, in agreement with recent calculations by Porta et al. J. Phys.: Condens. Matter 22, 345902 (2010) (http://dx.doi.org/10.1088/0953-8984/22/34/345902). The electrocaloric responsivity is found to be enhanced due to the latent heat on the paraelectric to ferroelectric transition locus below the critical point.
The current development of soft shape-memory materials often results in materials that are typically limited to the synthesis of thin-walled specimens and usually rely on complex, low-yield ...manufacturing techniques to fabricate macro-sized, solid three-dimensional objects. However, such geometrical limitations and slow production rates can significantly hinder their practical implementation. In this work, we demonstrate a shape-memory composite material that can be effortlessly molded into arbitrary shapes or sizes. The composite material is made from main-chain liquid crystal elastomer (MC-LCE) microparticles dispersed in a silicone polymer matrix. Shape-programmability is achieved via low-temperature induced glassiness and hardening of MC-LCE inclusions, which effectively freezes-in any mechanically instilled deformations. Once thermally reset, the composite returns to its initial shape and can be shape-programmed again. Magnetically aligning MC-LCE microparticles prior to curing allows the shape-programmed artefacts to be additionally thermomechanically functionalized. Therefore, our material enables efficient morphing among the virgin, thermally-programmed, and thermomechanically-controlled shapes.
The piezoelectric coefficient is a measure to quantify the potential use of a material in energy harvesting and sensor applications. High concentration of free charge carriers in piezoelectric ...materials can significantly impede the use of generated piezoelectric charge. In this study, undoped semiconducting ZnO single crystals with both Ohmic and Schottky contacts were prepared to quantify the effective piezoelectric response at temperatures from 20 °C to −140 °C and frequencies of mechanical loading from 0.5 Hz to 160 Hz. It was demonstrated that the formation of an electrostatic potential barrier at the metal-semiconductor interface increases the overall resistance, which provides access to unbiased piezoelectric coefficients of ZnO single crystals even at room temperature. These findings were verified using semiconducting ZnO for energy harvesting at room temperature and relatively low loading frequency.
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Lead-free Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) piezoceramics with Bi2O3 additive were synthesized using solid-state ceramic processing. Various amounts of Bi2O3 (0.05, 0.1, 0.5, and 1.0mol%) were added ...after calcination, milled, compacted, and sintered with no compensation at A- or B-sites. Addition of up to 0.5mol% Bi2O3 was found to greatly enhance the densification and increase the piezoelectric properties, while higher amounts decreased the grain size and induced relaxor-like electrical behavior, obeying the Vogel-Fulcher model. The highest properties were obtained for the BCZT with 0.1mol% Bi2O3 sintered at 1350°C: d33=325 pC/m, d33*=509 pm/V (at 3kV/mm), kp=0.42, and Pr=10.4μC/cm2. The microstructure, phase composition, and local structure were investigated by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The appearance of the A1g vibration mode in the Raman spectra and the shift of diffraction peaks to lower 2θ values indicate the incorporation of Bi3+ into the B-site of the perovskite BCZT structure.
The fracture toughness in the lead‐free relaxor ferroelectric (1−x)(Na1/2Bi1/2)TiO3–xBaTiO3 was investigated utilizing the surface crack in flexure method. To allow a comprehensive assessment, ...unpoled, and poled samples from the rhombohedral, the tetragonal, and the morphotropic phase regime were considered. It was found that the fracture toughness is up to 23% higher for the poled state. In order to cover the transition from ferroelectric to relaxor phase, the temperature dependence of 0.97(Na1/2Bi1/2)TiO3–0.03BaTiO3 was studied as well. Fracture toughness values of up to 2 MPam1/2 were determined, which are considerably above data for lead zirconate titanate materials. The results are rationalized using a simple transformation toughening‐type model in conjunction with investigations into the ferroelastic behavior. The presented model can be applied without fitting parameters but utilizes measurements of the coercive stress and remanent strain as well as the elastic modulus.
Electrocaloric (EC) materials are prominent candidates for new generations of scalable and green refrigeration devices. While most often the research on EC materials has been focused on achieving ...high magnitudes of the EC temperature change, little is known about electrical losses and self-heating effects, despite playing a critical role in the cooling performance of these materials. Here, we analyzed the behavior of a set of ceramic materials under EC-device-like electric-field-driving conditions. The EC temperature response was studied focusing on the contributions to the self-heating in three different compositions: relaxor Pb(Mg1/3Nb2/3)O3 and two different (undoped and Nb-doped) rhombohedral ferroelectric Pb(Zr,Ti)O3 compositions. The specific relaxor and ferroelectric nature of the analyzed materials enabled us to separate the different contributions, such as domain switching and electrical conductivity, to their EC responses. We show that besides having a large EC temperature change, low electrical losses, leading to reduced self-heating effects, are another key parameter to be considered in the engineering of materials for future EC cooling devices.
An improved thermodynamic cycle is validated in ferroelectric single crystals, where the cooling effect of an electrocaloric refrigerant is enhanced by applying a reversed electric field. In contrast ...to the conventional adiabatic heating or cooling by on-off cycles of the external electric field, applying a reversed field is significantly improving the cooling efficiency, since the variation in configurational entropy is increased. By comparing results from computer simulations using Monte Carlo algorithms and experiments using direct electrocaloric measurements, we show that the electrocaloric cooling efficiency can be enhanced by more than 20% in standard ferroelectrics and also relaxor ferroelectrics, like Pb(Mg sub(1/3)/Nb sub(2/3)) sub(0.71) Ti sub(0.29) O sub(3).
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