Showing papers on "Ferroelectric ceramics published in 2021"

The mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics.

Abstract: (K,Na)NbO3 based ceramics are considered to be one of the most promising lead-free ferroelectrics replacing Pb(Zr,Ti)O3. Despite extensive studies over the last two decades, the mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics has not been fully understood. Here, we combine temperature-dependent synchrotron x-ray diffraction and property measurements, atomic-scale scanning transmission electron microscopy, and first-principle and phase-field calculations to establish the dopant–structure–property relationship for multi-elements doped (K,Na)NbO3 ceramics. Our results indicate that the dopants induced tetragonal phase and the accompanying high-density nanoscale heterostructures with low-angle polar vectors are responsible for the high dielectric and piezoelectric properties. This work explains the mechanism of the high piezoelectricity recently achieved in (K,Na)NbO3 ceramics and provides guidance for the design of high-performance ferroelectric ceramics, which is expected to benefit numerous functional materials. The mechanism for the enhanced piezoelectricity in (K,Na)NbO3 based ceramics has not been fully understood. Here, the authors find that the dopants induced tetragonal phase and the accompanying high-density nanoscale heterostructures are responsible for the high dielectric and piezoelectric properties.

Textured ferroelectric ceramics with high electromechanical coupling factors over a broad temperature range.

Abstract: The figure-of-merits of ferroelectrics for transducer applications are their electromechanical coupling factor and the operable temperature range. Relaxor-PbTiO3 ferroelectric crystals show a much improved electromechanical coupling factor k33 (88~93%) compared to their ceramic counterparts (65~78%) by taking advantage of the strong anisotropy of crystals. However, only a few relaxor-PbTiO3 systems, for example Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3, can be grown into single crystals, whose operable temperature range is limited by their rhombohedral-tetragonal phase transition temperatures (Trt: 60~120 °C). Here, we develop a templated grain-growth approach to fabricate -textured Pb(In1/2Nb1/2)O3-Pb(Sc1/2Nb1/2)O3-PbTiO3 (PIN-PSN-PT) ceramics that contain a large amount of the refractory component Sc2O3, which has the ability to increase the Trt of the system. The high k33 of 85~89% and the greatly increased Trt of 160~200 °C are simultaneously achieved in the textured PIN-PSN-PT ceramics. The above merits will make textured PIN-PSN-PT ceramics an alternative to single crystals, benefiting the development of numerous advanced piezoelectric devices. Only a few relaxors can be grown into crystals, which show high piezoelectricity, but their operable temperature range is limited by the low phase transition temperature. Here, the authors develop an approach to fabricate textured relaxor ceramics with elevated phase transition temperatures.

Superior energy-storage performance in 0.85Bi0.5Na0.5TiO3–0.15NaNbO3 lead-free ferroelectric ceramics via composition and microstructure engineering

Abstract: Dielectric energy storage materials and capacitors are one of the key components for power electronics. Although strenuous efforts have been made to explore high-performance energy storage materials, the trade-off between the high polarization and high breakdown strength limits the energy density of the materials. In this work, a composition engineering strategy has been proposed to overcome the trade-off between the polarization and the breakdown strength. We introduced (Sr1.05Bi0.3)ScO3 (SBS) into 0.85Bi0.5Na0.5TiO3–0.15NaNbO3 (BNT–NN), where the Sc2O3 embedded in the grains constructs insulating networks to enhance the breakdown strength, while the Sc3+ and Sr2+ ions enter the BNT–NN lattices and smash the R3c domains into ferroelectric polar nanoregions, maintaining the high polarization and decreasing the remnant polarization. As a result, 0.3 SBS doped BNT–NN shows a recoverable energy density (Wrec) of 7.3 J cm−3, one of the highest values for lead-free ferroelectric ceramics with a desirable energy efficiency (η) of 80%. Combined with its favorable temperature and frequency stabilities and discharge ability, the SBS doped 0.85Bi0.5Na0.5TiO3–0.15NaNbO3 could be a promising material for practical energy storage applications.

Reversible yellow-gray photochromism in potassium-sodium niobate-based transparent ceramics

Abstract: Ferroelectric ceramics exhibiting photochromic behavior and reversible luminescence modulation are highly desirable for optoelectronic applications ranging from information storage, displays, anti-counterfeiting to photo-switching devices. Herein, Sm3+-doped lead-free 0.85(K0.5Na0.5)NbO3-0.15SrZrO3 (KNN-SZ: Sm3+) transparent ferroelectric ceramic featuring a typical photochromic phenomenon is designed and demonstrated. Upon alternate illumination and thermal stimulus (220 ℃ for 1 min), the ceramic exhibits reversible yellow-gray coloration. Furthermore, a maximum relative reflectivity variation of 29 % and a large luminescence quenching of 65.4 % with superior fatigue resistance were achieved in KNN-SZ: Sm3+. Various in-situ illuminations and thermal treatments in different ambient conditions were carried out to bring insight into the trapping and de-trapping processes involved in photochromic behavior. The KNN-SZ: Sm3+ photochromic ceramics have great prospect in optoelectronic devices expanding the applications of KNN-SZ ceramics into advanced multifunctional materials for devices integration beyond electrical energy storage, electrocaloric and piezoelectric effects.

Structural and morphological studies, and temperature/frequency dependence of electrical conductivity of Ba0.97La0.02Ti1−xNb4x/5O3 perovskite ceramics

Abstract: Conductivity measurements of our polycrystalline perovskite ceramic systems with a composition of Ba0.97La0.02Ti1−xNb4x/5O3 (x = 5, 7 and 10, in mol%) were performed, in order to investigate frequency and temperature dependence. Our ferroelectric ceramics were fabricated by the molten-salt method (chemical reaction followed by an evaporation and filtration reaction); X-ray diffraction patterns indicated that a single phase was formed for pure BaLT1−xNb4x/5 ceramics. The electrical behavior of the ceramics was studied by impedance spectroscopy in the 500–610 K temperature range. The conductivity was investigated which can be described by the Jonscher law. Both AC and DC electrical conductivities are completely studied as a function of frequency and temperature. The conductivity exhibits a notable increase with increasing Nb-rates. The low-frequency conductivity results from long-range ordering (close to frequency-independent) and the high-frequency conductivity is attributable to the localized orientation hopping process. Impedance analysis was performed revealing conductivity data which fitted the modified power, σAC(ω) = Aωn. The frequency dependence of the conductivity plot has been found to obey the universal Jonscher power law. Both AC and DC electrical conductivities are thoroughly studied as a function of frequency as well as temperature. The AC conductivity reveals that correlated barrier hopping (CBH) and non-overlapping small polaron tunneling (NSPT) models are suitable theoretical models to elucidate the conduction mechanisms existing in our compounds. Significantly, by increasing the temperature, the DC conductivity was increased, which verifies the semiconducting nature of the materials.

Precipitation Hardening in Ferroelectric Ceramics

Abstract: Domain wall motion in ferroics, similar to dislocation motion in metals, can be tuned by well-concepted microstructural elements. In demanding high-power applications of piezoelectric materials, the domain wall motion is considered as a lossy hysteretic mechanism that should be restricted. Current applications for so-called hard piezoelectrics are abundant and hinge on the use of an acceptor-doping scheme. However, this mechanism features severe limitations due to enhanced mobility of oxygen vacancies at moderate temperatures. By analogy with metal technology, the authors present here a new solution for electroceramics, where precipitates are utilized to pin domain walls and improve piezoelectric properties. Through a sequence of sintering, nucleation, and precipitate growth, intragranular precipitates leading to a fine domain structure are developed as shown by transmission electron microscopy, piezoresponse force microscopy, and phase-field simulation. This structure impedes the domain wall motion as elucidated by electromechanical characterization. As a result, the mechanical quality factor is increased by ≈50% and the hysteresis in electrostrain is suppressed considerably. This is even achieved with slightly increased piezoelectric coefficient and electromechanical coupling factor. This novel process can be smoothly implemented in industrial production processes and is accessible to simple laboratory experimentation for microstructure optimization and implementation in various ferroelectric systems.

Ultrahigh electrostrictive effect in potassium sodium niobate-based lead-free ceramics

Abstract: The longitudinal electrostrictive coefficient Q33 for perovskite-structured ferroelectric ceramics is usually between 0.01-0.04 m4/C2. However, an ultra-low Q33 of only 0.0047 m4/C2 was identified in the 0.9K0.5Na0.5NbO3-0.1SrTiO3 (KNN-ST) composition. Despite the fact that superior piezoelectricity has been observed in KNN-based ceramics, this value is obviously much smaller than the normal value, according to the general cognition and the thermodynamic relationship between piezoelectric coefficient d33 and Q33. Therefore, we synthesized (1−x)(K0.45Na0.49Li0.06)NbO3-xSrTiO3 (KNLN-ST) and studied phase structure, dielectric and ferroelectric properties systematically. Our findings show that the Q33 in the KNLN-ST system (0.012-0.027 m4/C2) is within the reasonable range for perovskite-structured ferroelectric ceramics. Furthermore, an ultra-high electrostrictive strain (>0.3%) with ultra-low hysteresis was achieved in the 0.8KNLN-0.2ST sample. This research not only clarifies the electrostrictive effect in KNN-based systems, but it also broadens the potential application field of KNN-based ceramics to electrostrictive actuators.

Simultaneously improved transparency, photochromic contrast and Curie temperature via rare-earth ion modification in KNN-based ceramics

Abstract: Photochromic (PC) luminescent ferroelectric materials have aroused great interest because of their potential applications in non-contact smart information storage materials and devices. In this study, we adopted the effect of rare-earth ions to refine the grains, combined with the unequal replacement of donor doping and higher temperature sintering methods to introduce more vacancy defects, to simultaneously improve the transmittance and photochromic contrast of KNNLB-RE ceramics. The optical transmittance of a typical KNNLB-RE > 50%, and the modulation contrast of KNNLB-RE is relatively significant, such as ΔAbs = 16.2% and ΔR = ∼34% for KNNLB-Er. More interestingly, the Tc of KNNLB-RE increased by about 10%–18% (∼30 °C–50 °C) due to the degradation of the relaxor-feature and the increase of internal stress, which broadens the temperature application range of KNNLB-RE ferroelectric ceramics. In contrast to most opaque inorganic photochromic materials, the KNNLB-RE transparent ceramics can express multiple pairs of “off” and “on” codes by using the reversibly decreased and increased transmittance and PL intensity; thus, these materials have potential in the applications of optical sensors and memories or other optical data storage devices.

High piezoelectricity of Eu3+-doped Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 transparent ceramics

Abstract: Optically transparent Eu3+-doped Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 (PMN–0.25PT:Eu3+) relaxor ferroelectric ceramics with high piezoelectricity were prepared by oxygen-atmosphere sintering followed by hot-press sintering. A high piezoelectric charge coefficient (d33 = 850 pC N−1) and effective piezoelectric strain coefficient (d33* ≈ 1520 pm V−1) were achieved in the 2 mol% Eu3+-doped PMN–0.25PT transparent ceramic. Local nanoscale domain patterns and piezoresponse of PMN–0.25PT:Eu3+ transparent ceramics were observed and quantitatively analyzed by using piezoelectric force microscopy and the autocorrelation function method to understand the origin of the high piezoelectricity. It is found that the introduction of Eu3+ doping will enhance the local structure heterogeneity in PMN–PT ceramics and the obtained high piezoelectric properties are related to the dynamic behavior of local nano-domains. Our result showed that doping with rare earth element Eu3+ is an effective method for making transparent ferroelectric ceramics with enhanced piezoelectric performance, which benefits the design and development of eletro-optic devices as well as transparent sensors and ultrasound transducers.

Large, thermally stabilized and fatigue-resistant piezoelectric strain response in textured relaxor-PbTiO3 ferroelectric ceramics

Abstract: Piezoceramics with both high strain response and excellent output stability are highly in demand for electronic actuator applications. Unfortunately, enhanced strains are generally accompanied by temperature and e-field instabilities for relaxor-PbTiO3 ferroelectrics near the curved morphotropic phase boundary (MPB). In this work, we report the simultaneous achievements of substantially enhanced piezoelectric strain (d33* ∼ 990 pm V−1), greatly improved temperature stability (strain variation below 10% over 25–150 °C) and excellent fatigue resistance (almost no strain variation at a bipolar e-field of 30 kV cm−1 up to 105 cycles) in a relaxor-PbTiO3 ferroelectric ceramic with controlled grain orientation along [001]c, based on integrating texture engineering and composite effect strategies. The temperature–insensitive strain response can be mainly attributed to the thermally stabilized er × P (dielectric permittivity × polarization) and stable domain response over a broad temperature range, which suppressed the adverse effect (strain variation ∼60% over 25–150 °C in the non-textured counterpart) caused by the intermediate ferroelectric phase transition. Besides, the inherent anisotropy properties and enhanced domain mobility in the textured ceramics further contribute to the substantially improved fatigue endurance. This work paves the way for exploring large and stable strain response in ferroelectrics with strongly curved MPB, and can also largely broaden application areas of relaxor-PbTiO3 ceramics to high-performance, stable and robust actuators.

Evolution of mesoscopic domain structure and macroscopic properties in lead-free Bi0.5Na0.5TiO3-BaTiO3 ferroelectric ceramics

Abstract: Bismuth sodium titanate and related compounds are promising lead-free ferroelectric materials potentially useful in a wide range of piezoelectric applications. The domain structure plays an important role in determining the piezoelectric and ferroelectric properties and thereby the performance of electromechanical transducers. In this work, piezoresponse force microscopy (PFM) is used to gain insights into the mesoscopic-scale domain structure and its evolution under electric field in the (1−x)Bi0.5Na0.5TiO3-xBaTiO3 (BNT-BT) piezoceramics with compositions varying from x = 0 to x = 0.08. A phase transition from the rhombohedral phase to the tetragonal phase is observed with increasing BT contents. A relationship is established between the relaxor behavior and the domain structures imaged by PFM, i.e., short-range polar regions without visible domains in relaxor ceramics of pure BNT, while long-range ordered polar states with clear domains in ferroelectric ceramics with the addition of BT content. Distinct micro-domains are observed in the ceramics with compositions close to the morphotropic phase boundary (MPB), but the domain size drops to nanometers in the MPB composition with an increasing domain wall density. An electric field can induce the transformation from the relaxor behavior to a ferroelectric state, accompanied by an increase in domain sizes and a rearrangement/reorientation of the polar domains. This study of domain structure and its evolution in BNT-BT provides a better understanding of the relationship between the crystal structure, mesoscopic-scale domains, and macroscopic properties in these important lead-free piezoelectric ceramics.

Dielectric breakdown behavior of ferroelectric ceramics: The role of pores

Abstract: Dielectric breakdown is a fundamental issue for ferroelectric ceramics. In this work, a phase-field method is introduced to study the breakdown behavior of ferroelectric ceramics with pores randomly distributed. Effects of the position and the size of pores on the breakdown behavior are analyzed. Results indicate that the position of pores, for example in grains or at grain boundaries, has a significant influence on the breakdown strength of ferroelectric ceramics. The nominal breakdown strength of ferroelectric ceramics with 2 % pores at grain boundaries is almost 50 % higher than 2 % pores in grains. Further, for ferroelectric ceramics with a certain porosity, the smaller the pore size, the higher the breakdown strength. As the nominal pore size decreases from 2.5 to 1, the nominal breakdown strength is enhanced from 0.73 to 1.16. Such results agree well with the widely accepted Gerson-Marshall model and previously published experimental observations.

Enhanced pyroelectric response from domain-engineered lead-free (K0.5Bi0.5TiO3-BaTiO3)-Na0.5Bi0.5TiO3 ferroelectric ceramics

Abstract: Enhanced pyroelectric response is achieved via domain engineering from [001] grain-oriented, tetragonal-phase, lead-free 0.2(2/3K0.5Bi0.5TiO3-1/3BaTiO3)-0.8Na0.5Bi0.5TiO3 (KBT-BT-NBT) ceramics prepared by a templated grain growth method. The [001] crystallographic orientation leads to large polarization in tetragonal symmetry; therefore, texturing along this direction is employed to enhance the pyroelectricity. X-ray diffraction analysis revealed a Lotgering factor (degree of texturing) of 93 % along the [001] crystallographic direction. The textured KBT-BT-NBT lead-free ceramics showed comparable pyroelectric figures of merit to those of lead-based ferroelectric materials at room temperature (RT). In addition to the enhanced pyroelectric response at RT, an enormous enhancement in the pyroelectric response (from 1750 to 90,900 μC m−2 K−1) was achieved at the depolarization temperature because of the sharp ferroelectric to antiferroelectric phase transition owing to coherent 180° domain switching. These results will motivate the development of a wide range of lead-free pyroelectric devices, such as thermal sensors and infra-red detectors.

Molecular Ferroelectric-Based Flexible Sensors Exhibiting Supersensitivity and Multimodal Capability for Detection

Abstract: Although excellent dielectric, piezoelectric, and pyroelectric properties matched with or even surpassing those of ferroelectric ceramics have been recently discovered in molecular ferroelectrics, their successful applications in devices are scarce. The fracture proneness of molecular ferroelectrics under mechanical loading precludes their applications as flexible sensors in bulk crystalline form. Here, self-powered flexible mechanical sensors prepared from the facile deposition of molecular ferroelectric [C(NH2 )3 ]ClO4 onto a porous polyurethane (PU) matrix are reported. [C(NH2 )3 ]ClO4 -PU is capable of detecting pressure of 3 Pa and strain of 1% that are hardly accessible by the state-of-the-art piezoelectric, triboelectric, and piezoresistive sensors, and presents the ability of sensing multimodal mechanical forces including compression, stretching, bending, shearing, and twisting with high cyclic stability. This scaling analysis corroborated with computational modeling provides detailed insights into the electro-mechanical coupling and establishes rules of engineering design and optimization for the hybrid sponges. Demonstrative applications of the [C(NH2 )3 ]ClO4 -PU array suggest potential uses in interactive electronics and robotic systems.