Upside - down composites: Fabricating piezoceramics at room temperature

Piezoelectric materials have a multi-billion dollar impact on the electromechanical transducers market. Their conventional synthesis includes a sintering step (at over 1000 °C) that often hampers direct integration into monolithic devices and confines their applications to heterostructures made using tedious multi-step assembly or to composites with poor electromechanical behavior. Here, we demonstrate a new and easy to scale method for the integration of piezoelectric ceramics at ultra-low temperatures without compromising their functionality. We show that all-ceramic “upside-down” composites with exceptionally high fractions of piezoelectric filler (˜75 vol. %) and low porosity can be achieved using aqueous dispersion of lithium molybdate as a binder. The method is based only on coating, mixing, moulding and drying sequences. The measured piezoelectric charge coefficient, d₃₃ ˜84 pC·N⁻¹, outperforms any other known composite, whereas the voltage constant, g₃₃ ˜33 mV m·N⁻¹, competes with bulk materials, thus paving the way for versatile applications not previously considered.     

Simplified Impedance Model for Adhesively Bonded Piezo-Impedance Transducers

The electromechanical impedance technique employs surface-bonded lead zirconate titanate piezoelectric ceramic patches as impedance transducers for structural health monitoring and nondestructive evaluation. The patches are bonded to the monitored structures using finitely thick adhesive bond layer, which introduces shear lag effect, thus invariably influencing the electromechanical admittance signatures. This paper presents a new simplified impedance model to incorporate shear lag effect into electromechanical admittance formulations, both one-dimensional and two-dimensional. This provides a closed-form analytical solution of the inverse problem, i.e. to derive the true structural impedance from the measured conductance and susceptance signatures, thus an improvement over the existing models. The influence of various parameters (associated with the bond layer) on admittance signatures is investigated using the proposed model and the results compared with existing models. The results show that the new model, which is far simpler than the existing models, models the shear lag phenomenon reasonably well besides providing direct solution of a complex inverse problem.     

Piezoelectric properties and dielectric behavior of Bi1/2Na1/2[Ti1−x (Sb1/2Nb1/2) x ]O3 lead-free piezoelectric ceramics

New lead-free piezoelectric ceramics of ABO₃ perovskite type, Bi_1/2Na_1/2[Ti_1−x (Sb_1/2Nb_1/2) _x ]O₃, were synthesized by conventional solid state reaction. The effect of the replacement of complex ions of (Sb_1/2Nb_1/2)⁴⁺ in the B cationic site on structural and electrical properties was investigated. The XRD analysis showed that all samples exhibited a single phase of perovskite structure. Piezoelectric and dielectric measurement revealed that the substitution of (Sb_1/2Nb_1/2)⁴⁺ ions lead to an increase in piezoelectric constant(d ₃₃), electromechanical coupling factor(k _t), relative dielectric constant(ɛ _r),and loss tangent (tanδ), while excess of (Nb_1/2Sb_1/2)⁴⁺ ions results in a decrease in d ₃₃, k _t, ɛ _r, but an increase in tanδ. Temperature dependence of dielectric constant ɛ _r measurement indicated these compounds were relaxor ferroelectric. At low frequency and high temperature, dielectric constant increased sharply attributed to the superparaelectric clusters after (Sb_1/2Nb_1/2)⁴⁺ ions substitution.     

Some theorems in the theory of microstretch thermopiezoelectricity

The electromagnetic theory of microstretch thermoelasticity is an adequate tool to describe the behaviour of porous bodies, animal bones and solids with deformable microstructures. In this paper we study the linear theory of microstretch thermopiezoelectricity. First, we establish a reciprocity relation which involves two processes at different instants. This relation forms the basis of a uniqueness result and a reciprocal theorem. Then, we study the continuous dependence of solutions upon initial data and body loads. A variational characterization of solutions is also presented. Finally, we investigate the effect of a concentrated heat supply and the effect of a concentrated volume charge density in an unbounded homogeneous and isotropic body.     

Piezoelectric properties of Bi4Ti3O12 thin films annealed in different atmospheres

Bismuth titanate (Bi₄Ti₃O₁₂—BIT) films were evaluated for use as lead-free piezoelectric thin-films in micro-electromechanical systems. The films were grown by the polymeric precursor method on Pt/Ti/SiO₂/Si (1 0 0) (Pt) bottom electrodes at 700 °C for 2 h in static air and oxygen atmospheres. The domain structure was investigated by piezoresponse force microscopy (PFM). Annealing in static air leads to better ferroelectric properties, higher remanent polarization, lower drive voltages and higher piezoelectric coefficient. On the other hand, oxygen atmosphere favors the imprint phenomenon and reduces the piezoelectric coefficient dramatically. Impedance data, represented by means of Nyquist diagrams, show a dramatic increase in the resistivity for the films annealed in static air atmopshere.     

Piezoelectric and ferroelectric properties of K0.5Na0.5NbO3 ceramics synthesized by spray drying method

Potassium-sodium niobate was synthesized at 800 °C for 1 h using dried precursors in a powder form obtained by the spray drying method. Different samples were sintered from 1060 to 1120 °C for 2 h reaching a relative density as high as 96% of the theoretical value. Piezoelectric and ferroelectric properties were studied for these samples and some of the most prominent results are: k_p, d₃₁, 2P_r, and 2E_C of 0.36, 39 pC/N, 29 μC/cm² and 16.5 kV/cm, respectively, for the sample sintered at 1080 °C. The methodology presented in this study can be used to synthesize submicrometer powders.     

Preparation,structure, and electric properties of the Pb(Zn1/3Nb2/3)O3–Pb(Yb1/2Nb1/2)O3–PbTiO3 ternary ferroelectric system ceramics near the morphotropic phase boundary

Ceramics of the xPb(Zn_1/3Nb_2/3)O₃–(1 ? x ? y)Pb(Yb_1/2Nb_1/2)O₃–yPbTiO₃ (PZN–PYN–PT) ternary system were synthesized using a modified two-step columbite precursor method which can effectively suppress the pyrochlore phase. A morphotropic phase boundary (MPB) region, separating tetragonal and rhombohedral phases in the ternary systems has been determined. The electric properties of the compositions near MPB region were investigated. Dielectric response exhibits relaxor-like characteristics with broad dielectric peaks and dispersive dielectric behavior with respect to frequency and temperature. The phase diagram of the 0.45PZN–(0.55 ? y)PYN–yPT pseudo-binary system in the composition range of 0.15 < y < 0.35 was established based on dielectric measurements. The optimal properties were achieved in the MPB composition of 0.52PZN–0.21PYN–0.27PT with piezoelectric coefficient d₃₃, dielectric permittivity ε′, planar electromechanical coupling k_p, dielectric loss tan δ, coercive field E_c, remnant polarization P_r, and T_C being of 558 pC/N, 2065, 62%, 0.2%, 19.88 kV/cm, 31.44 μC/cm² and 259.5 °C, respectively, showing potential usage in high-temperature electromechanical applications.     

Enhanced piezoelectric and ferroelectric properties of BiFeO3-BaTiO3 lead-free ceramics by optimizing the sintering temperature and dwell time

0.7BiFeO₃-0.3BaTiO₃ (BFO-0.3BT) ceramics were prepared to uncover the impacts of sintering temperature (T_S) and dwell time (t_d) on the microstructure and electrical properties. With increasing the T_S or t_d, the grain sizes increase along with the porosity decreases, which is in favor of the alignment of dipole. However, excess T_S or t_d are inclined to cause the volatilization of Bi₂O₃, which deteriorates piezoelectric properties. Because of the R-T two-phase coexistence, low defect ions concentration and porosity, as well as appropriate grain size, the excellent d₃₃?=?208?pC/N and k_p?=?35.46% as well as P_r?=?28.52?μC/cm² were achieved in BFO-0.3BT ceramics at T_S?=?1000?°C and t_d?=?6?h. In addition,large unipolar strain 0.13% and d₃₃*?=?256.2?pm/V also were obtained in BFO-0.3BT ceramics at T_S?=?1000?°C and t_d?=?6?h. This research indicates that the porosity and defect ion concentration as well as grain size also play an important role in piezoelectric properties in BFO-BT ceramics.     

Field induced metastable ferroelectric phase in Pb0.97La0.03(Zr0.90Ti0.10)0.9925O3 ceramics

Pb_0.97La_0.03(Zr_0.9Ti_0.1)_0.9925O₃ (PLZT 3/90/10) ceramics prepared by solid-state reaction with the compositions near the antiferroelectric/ferroelectric (FE/AFE) phase boundary were studied. From the polarization–electric field P(E) dependence and ex situ X-ray study, an irreversible electric field induced AFE-to-FE phase transition is verified at room temperature. Dielectric and in situ temperature dependent X-ray analysis evidence that the phase transition sequence in PLZT 3/90/10-based ceramics can be readily altered by poling. A first order antiferroelectric-paraelectric (AFE-to-PE) transition occurred at?～190 °C in virgin sample and at?～180 °C in poled sample. In addition, a FE-to-AFE transition occurs in the poled ceramic at much lower temperatures (～120 °C) with respect to the Curie range (～190 °C). The temperature-induced FE-to-AFE transition is diffuse and takes place in a broad temperature range of 72–135 °C. The recovery of AFE is accompanied by an enhancement in the piezoelectric properties.     

Magnetostrictive fracture of a cylindrical multiferroic composite

The magnetostrictive fracture model is established for a cylindrical multiferroic composite loaded by magnetic field and mechanical constraining traction. Fracture analysis is performed by the methods of Cauchy singular integral equation and Lobatto-Chybeshev collocation. Numerical results of the stress intensity factor (SIF) are obtained and the computational accuracy is demonstrated. The effects of the thickness ratio, the total thickness and the inner radius on the SIF are discussed, which provides practical references for geometrical optimization of the structure. The effect of material stiffness on the SIF is specially analyzed and the influences of piezoelectric stiffening and piezomagnetic stiffening are thereby compared.