In this paper, the concept ofEnvironmentallyConsciousFerroelectrics (abbr. as Ecoferroe-lectrics) is emphasized, the evaluation methodology of the preparation, processing, and applications offerroelectrics onenvironmental load is proposed, the presentresearches on Ecoferroelectrics are outlined, and some aspects of futureresearches for Ecoferoelectrics are suggested.

Aging and re-poling induced enhancement of piezoelectricity are found in (K,Na)NbO₃ (KNN)-based lead-free piezoelectric ceramics. For a compositionally optimized Li-doped composition, its piezoelectric coefficientd₃₃ can be increased up to 324 pC N⁻¹ even from a considerably high value (190 pC N⁻¹) by means of a re-poling treatment after room-temperature aging. Such a highd₃₃ value is only reachable in KNN ceramics with complicated modifications using Ta and Sb dopants. High-angle X-ray diffraction analysis reveals apparent changes in the crystallographic orientations related to a 90° domain switching before and after the aging and re-poling process. A possible mechanism considering both defect migration and rotation of spontaneous polarization explains the experimental results. The present study provides a general approach towards piezoelectric response enhancement in KNN-based piezoelectric ceramics.

The development of lead-free piezoceramics has attracted great interest because of growing environmental concerns. A polymorphic phase transition (PPT) has been utilized in the past to tailor piezoelectric properties in lead-free (K,Na)NbO₃ (KNN)-based materials accepting the drawback of large temperature sensitivity. Here a material concept is reported, which yields an average piezoelectric coefficientd₃₃ of about 300 pC/N and a high level of unipolar strain up to 0.16% at room temperature. Most intriguingly, field-induced strain varies less than 10% from room temperature to 175 °C. The temperature insensitivity of field-induced strain is rationalized using an electrostrictive coupling to polarization amplitude while the temperature-dependent piezoelectric coefficient is discussed using localized piezoresponse probed by piezoforce microscopy. This discovery opens a new development window for temperature-insensitive piezoelectric actuators despite the presence of a polymorphic phase transition around room temperature.

High radial coupling coefficients are observed for compositions having up to 50 mole % sodium niobate additions to potassium niobate. The activity diminishes with additional sodium niobate content and disappears beyond about 98 mole % additions. Dielectric constants are relatively low, varying from 450 to 125, depending on composition. A.-c. losses are high and d.-c. resistivities are ∼10¹²ohm-cm. The low dielectric constants and high coupling obtained in certain regions of the system make these materials promising for solid ultrasonic delay line use, especially where thin sectioned plates are required, as in high-frequency thickness extensional or thickness shear mode transducers.

Lead-free piezoelectric ceramics have received attention because of increasing interest in environmental protection. Niobate ceramics such as NaNbO₃ and KNbO₃ have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na_0.5K_0.5NbO₃ ceramics were prepared using spark plasma sintering (SPS). Although the SPS temperature was as low as 920°C, the density of the Na_0.5K_0.5NbO₃ solid solution ceramics was raised to 4.47 g/cm³ (>99% of the theoretical density). After post-annealing in air, reasonably good ferroelectric and piezoelectric properties were obtained in the Na_0.5K_0.5NbO₃ ceramics with submicron grains. The crystal phase of the Na_0.5K_0.5NbO₃ has an orthorhombic structure. The Curie temperature is 395°C and the piezoelectric parameter (d₃₃) of the Na_0.5K_0.5NbO₃ ceramics reached 148 pC/N.

Lead-free piezoelectric Na_xK_1−xNbO₃ (x=20–80 mol%) ceramics were fabricated using spark plasma sintering at a low temperature (920°C). All the Na_xK_1−xNbO₃ ceramics showed a similar orthorhombic phase structure, while the corresponding lattice parameters decreased from the KNbO₃ side to the NaNbO₃ side with increasing Na content. A discontinuous change in lattice parameter close to composition of 60 mol% Na indicated the presence of a transitional area that is similar to the morphotropic phase boundary (MPB) in Na_xK_1−xNbO₃ ceramics. The sintered density of the Na_xK_1−xNbO₃ ceramics decreased with increasing Na content, from a relative density of 99% for the K-rich side to 92% for the Na-rich side. The piezoelectric constantd₃₃ and planar mode electromechanical coupling coefficientk_p showed a maximum value of 148 pC/N and 38.9%, respectively, due to the similar MPB effects in the PZT system.

Abstract

Lead-free piezoelectric ceramics have received more attention due to the environmental protection of the earth. (K, Na)NbO₃-based ceramics are one of the most promising candidates. Normal sintering of un-doped and Li/Ta co-doped (K, Na)NbO₃ ceramics was investigated to clarify the optimal sintering condition for densification, microstructure and electrical properties. It was found that density increased greatly within a narrow temperature range but turned to decrease when the sintering temperature slightly exceeded the optimal one. Piezoelectric properties also showed similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density. The grain growth and property change as a function of sintering temperature were discussed on basis of the formation of liquid-phase and the composition deviation caused by the volatilization of alkali components during sintering.

Abstract

We prepared (K_0.5Na_0.5)NbO₃ (KNN) ceramics with the addition of 1 mass% ZrO₂ with the aim to hinder the exaggerated grain growth encountered in KNN. Both KNN and KNN-ZrO₂ ceramics sintered at 1115 °C and 1125 °C, respectively, had relative density exceeding 95%. KNN had a bimodal microstructure, with a population of fine grains of a few 100 nm and the large grains of about 20 μm, while the microstructure of KNN-ZrO₂ was fine and uniform, with the median grain size of 0.3 μm and the largest grains of about 1.3 μm. We attribute the refinement of the microstructure to the matrix grain-growth inhibition by ZrO₂ addition. The influence of ZrO₂ is twofold: sub-100 nm sized ZrO₂ particles, located at the KNN grain junctions hinder the matrix grain growth. Additionally, the enrichment of the boundary regions of the matrix grains with Zr relative to the grain interiors, confirmed by TEM/EDXS analysis, is also a probable reason for the decreased mobility of the grain boundaries. The dielectric permittivity and losses, measured at 10 kHZ, and piezod₃₃ constant of KNN-ZrO₂ are 905, 0.04 and 100 pC/N, respectively.

Platelike BaTiO₃ (BT) templates were used for the first time to fabricate (0 0 1) textured K_0.5Na_0.5NbO₃ (KNN) piezoelectric ceramics. Plakelike BT templates with a thickness of 1–1.5 μm and a mean diameter of 10–15 μm were synthesized by the topochemical microcrystal conversion method using Bi₄Ti₃O₁₂ as precursor particles. A systematic investigation was conducted on illustrating the texture development and microstructure evolution as a function of heat-treatment conditions. It was found that the KNN–BT solid solution was formed between BT templates and matrix materials. The increase of sintering temperature and time induced an orthorhombic–tetragonal phase transition. And it was indicated that the dominant mechanism of texture development could be the abnormal grain growth in the textured KNN ceramics. A (0 0 1) orientation degree of 80% was achieved at 1130 °C for 24 h. The textured ceramics showed anisotropic electrical properties in the direction of parallel and perpendicular to the casting plane. The orthorhombic–tetragonal polymorphic phase transitionT_o–t, the tetragonal–cubic ferroelectric phase transitionT_C, the dielectric constantε_r and the remnant polarizationP_r values for the perpendicular cut specimens were lower than the parallel cuts. The excellent electric properties at the parallel cuts were found to be the dielectric constantε_r = 810, the piezoelectric charge constantd₃₃ = 163 pC/N, and the electromechanical coupling factork_p = 0.37.

Reduced atmosphere firing of (Na_0.5K_0.5)NbO₃ (NKN) ceramics opens up an important opportunity for lead-free piezoelectric materials; one of the promising applications is base metal cofired multilayer actuators. Here, we extend an investigation on the effects of reduced atmosphere firing (P_O2 = 10⁻¹⁰ atm) on the piezoelectric properties and associated point defects inNKN–LiF ceramics. To access the domain dynamics contribution to the converse piezoelectric properties and consider point defects in the sintered ceramics, Rayleigh analysis and thermal stimulated depolarization current (TSDC) were used. The piezoelectric properties and Rayleigh analysis ofNKN–LiF ceramics demonstrated that the intrinsic and extrinsic piezoelectric response is essentially independent of firing atmosphere. Furthermore, reduced fired ceramic was found to have much higher resistivity than in the air-fired case. From the TSDC analysis, the space charge in the reduced fired ceramic was considered to be much smaller than in the air-fired ceramic, implying that the alkali volatilization was suppressed in reduced firedNKN–LiF ceramic.

Presently, the most promising family of lead-free piezoelectric ceramics is based on K_0.5Na_0.5NbO₃ (KNN). Lithium doped KNN ceramics show high piezoelectric properties at room temperature. An issue of particular interest for medical applications is the temperature stability of the properties because of the need for repeated sterilization process at around 140 °C. A concern with Li-doped KNN is that the properties may not be thermally stable because the orthorhombic–tetragonal phase transition temperature is close to the room temperature. We report on electromechanical properties of KNN-modified with 3–6 atomic % Li, measured by resonance technique in function of temperature from 20 to 140 °C, during two heating and cooling cycles. The piezoelectric coefficientsd₃₁ and radial coupling coefficientsk_p are decreased up to 30% of their initial value after the first heating cycle. However, after the second cycle, the properties stabilize. The depoling is smaller in compositions that do not undergo a phase transition in the examined temperature interval.