Orthorhombic-tetragonal Phase Boundary Research Articles

Optimization of the electrical properties on lead-free barium titanate-based piezoelectric by combining grain growth with orthorhombic-tetragonal phase boundary

Barium titanate-based lead-free piezoceramics are considered as a key material for the fabrication of dielectric capacitors due to their large relative dielectric constant (εr) and are also identified as a remarkable electrocaloric material on account of their low Curie temperature (TC) and high spontaneous polarization (Ps). At this work, to optimize the electrical performances of BaTiO3-based Pb-free piezoceramics, chemical doping was used to tailor their composition. The aim is to boost Curie temperature while sustaining decent electrical properties. Besides, to enlighten the underlying physical mechanisms, first-principles calculations based on atomic coupling, orbital hybridization as well as electron localization were carried out. The experimental results show that the dielectric, ferroelectric, and piezoelectric properties are mainly determined by the orthorhombic-tetragonal (O-T) two-phase coexistence and grain size. Notably, a boosted dielectricity, ferroelectricity, and piezoelectricity accompanied by a raised TC is achieved at x = 0.10 mol%. Moreover, the highest TC∼125.40 ℃ is obtained at x = 0.40 mol% along with a degradation in dielectricity, ferroelectricity, and piezoelectricity.

Enhanced ferroelectric, piezoelectric, and dielectric properties in potassium sodium niobate lead-free piezoelectric ceramics by constructing of multiple phase boundaries

In this work, we report excellent ferroelectric, piezoelectric, and dielectric properties in 0.95(K0.5−x/2Na0.5−x/2Lix)NbO3-0.04BaZrO3-0.01BiFeO3, ((KN)1−xLxN-BZ-BF) lead-free piezoelectric ceramics system by constructing a coexistence of orthorhombic-tetragonal phase boundary near the room temperature, and increasing of grain size as well through the conventional sintering process. The addition of Li+ ions, in ((KN)1−xLxN-BZ-BF) composition, increased the grain size, and shifted the orthorhombic-tetragonal phase transition to near room temperature, and shifted the tetragonal-cubic phase transition toward higher temperatures, since both large electrical properties, and moderate Curie temperature was achieved simultaneously. The sample with x = 0.03 showed large average grain size of (9 µm), and possess coexisting of orthorhombic-tetragonal phases, since the sample showed excellent physical properties, such as d33 = 330 pC/N, kp = 0.5, Pr = 25 µC/cm2, and εr = 1740. The origin of these decent combinations of physical properties owes to the coexistence of orthorhombic-tetragonal phase transition to near room temperature, large grain size, and hybridization between Bi 6p, and O 2p as well.

Enhanced pyroelectric performance in potassium sodium niobate‐based ceramics via multisymmetries coexistence design

AbstractAchieving excellent pyroelectric performance remains a challenge for lead‐free piezoelectric ceramics. To meet the requirements of both an enhanced pyroelectric coefficient at room temperature and good thermal stability during the encapsulation of pyroelectric devices, (1–x)K0.48Na0.52NbO3–xBi0.5Ag0.5ZrO3–0.2%Fe2O3 (KNN–BAZ–Fe) lead‐free ferroelectric ceramics with high Curie temperatures were prepared to obtain improved pyroelectric performance via the coexistence of multiple symmetries. The variation of BAZ content led to the formation of rhombohedral–orthorhombic–tetragonal phase boundary and promoted grain growth, resulting in the best pyroelectric coefficient (p = 5.09 × 10−4 C m−2°C−1) and enhanced figures of merit (Fi = 0.2084 × 10−9 (m V−1), Fv = 0.0142 m2 C−1, Fd = 0.0947 × 10−4 Pa−1/2, and Fe = 17.66 J m−3 K−2) for infrared (IR) detection when x = 0.05. The room‐temperature pyroelectric coefficient obtained in this study is approximately four times that of the pure KNN ceramic and is the maximum value reported for niobate‐based piezoelectric ceramics. Moreover, compared with the poor thermal stability of barium titanate‐ and bismuth sodium titanate‐based ceramics because of their ultralow Curie temperature or thermal depolarization temperature, the ceramics investigated here exhibit much better thermal stability because of their high Curie temperature (TC > 300°C) and diffused phase‐transition behavior, making them more adaptable for practical applications. These results suggest that KNN–xBAZ–Fe ceramics are attractive candidates for applications in the field of IR sensors.

A correlative study between the microstructure and electrical properties of lead-free piezoceramics (1-x)BHT-xYLG driven by the synergistic action of multiple factors

Piezoceramics are important functional materials with broad applications in electronics, communication, medicine, and energy. Increasing environmental protection and human health concerns about the toxicity of lead have impelled the development of lead-free piezoceramics. At this work, in combination with phase field simulation, grain growth mechanism, ferroelectric domain wall model, and defect dipole formation mechanism, (1-x)Ba(Hf0.02Ti0.98)O3-x(Y0.5Li0.5)GeO3 [(1-x)BHT-xYLG, x = 0–0.40 mol%] lead-free piezoceramics were designed and fabricated via the two-step synthesis and solid-state reaction method, which yielded superior comprehensive electrical property. Phase field simulation respectively simulates the distribution of ferroelectric domains of O and T phase under different external electric fields (0–20 kV/mm) and the phase states under different temperatures (−100 to 150 °C). This research finds that their electrical properties are affected by the synergistic action of multiple factors, e.g., orthorhombic-tetragonal phase boundary (O-T PB), grain size (GS), domain-wall (DW), and defect dipole (DD), etc., rather than any single factor.

Insights into the Correlation between Tetragonal Phase and Temperature Stability of Potassium Sodium Niobate Based Ceramics from Domain Behaviors

AbstractThe temperature stability of tetragonal phase, as an indispensable end component of polymorphic phase boundary in high performance potassium sodium niobate based ceramics, has been long‐term neglected. Aimed at resolving the most cutting‐edge challenges for the practical application of potassium sodium niobate (KNN)‐based ceramics, the association study between tetragonal phase and its temperature stability is focused upon, especially from a microscopic point of view. Here, four kinds of potassium sodium niobate based ceramics with orthorhombic, tetragonal, orthorhombic–tetragonal, and rhombohedral–orthorhombic–tetragonal phase boundaries are designed, and investigations from macro properties as well as the underlying domain behaviors are conducted to establish the relationship between phase structure and temperature stability. Polarization retention with aging time and domain dynamics under thermal excitation suggest that tetragonal phase is closely related to the temperature sensitivity because of their instable and fast back‐switching properties of the reoriented domains. It is believed that a new perspective in the future designing of temperature‐insensitive ceramics is put up.

Critical Role of Order-Disorder Behavior in Perovskite Ferroelectric KNbO3.

The evolution of local atomic configuration and macroscopic electrical properties of KNbO3 is investigated in detail using ab initio molecular dynamics simulations within the framework of density functional theory. By analyzing the local off-center displacements of K and Nb atoms, the presence of the crossover of displacive and order-disorder mechanisms in the NbO6 octahedron due to orbital hybridization is found, whereas the K is a completely displacive type due to the nonoriented ionic K-O bond. Besides, the order-disorder behavior of Nb atoms is enhanced with the increase in temperature, especially undergoing the phase transition. The predicted high dielectric constant of KNbO3 confirms the key role of the Nb's order-disorder behavior. We discover that the anomalous increase in dielectric constant in the vicinity of the phase boundary is not only from the transformation of the polarized direction but also from the enhancement of order-disorder contribution in the nonpolarized direction. The high dielectric constant with large spontaneous polarization boosts the piezoelectricity at the orthorhombic-tetragonal phase boundary of 500 K, giving rise to the nearly 100% increase than that of 300 K. A rebonding model is developed to illustrate the origin of high piezoelectricity around the tetragonal ferroelectric phase boundary, and the significance of introducing a tetragonal structure is emphasized for developing high piezoelectricity in the inorganic perovskite ferroelectrics.

Ultrahigh piezoelectric coefficients of Li-doped (K,Na)NbO3 nanorod arrays with manipulated O-T phase boundary: Towards energy harvesting and self-powered human movement monitoring

Lead-free piezoelectric one-dimensional (1D) nanostructures have exhibited great potential in building biocompatible micro/nano-energy harvesters and self-powered smart sensors. However, their low piezoelectric coefficient still remains as the biggest obstacle for practical applications. In this work, an ultrahigh piezoelectric coefficient with calibrated d 33 from 400 pm V -1 to 814 pm V -1 is achieved in Li-doped (K,Na)NbO 3 (KNLN) lead-free piezoelectric nanorod arrays (NRAs). The giant enhancement on the piezoelectric performance of the KNLN nanorods can be attributed to the manipulation on the phase transition behavior together with the relaxation of strain. The Li + dopants induce large NbO 6 octahedral distortion and result in the decreased phase transition temperature from orthorhombic (O) to tetragonal (T) phase, which leads to the construction of the O-T phase boundary at room temperature and improves the polarization performance of the products. The ultrahigh piezoelectric performance of the KNLN NRAs gives rise to the remarkably improved energy harvesting performance, which shows highly sensitive pressure sensing behaviors and can be utilized for monitoring the human-body motions. Giant piezoelectric coefficient d 33 up to 814 pm V -1 is achieved in lead-free (K,Na,Li)NbO 3 nanorod arrays by manipulating the orthorhombic-tetragonal phase boundary through A-site Li-doping. The KNLN nanorod arrays exhibit outstanding piezoelectric energy harvesting and pressure sensing performances and realize the self-powered monitoring of human body motions for smart analyzing of the human movement behaviors. • Li-doped (K,Na)NbO 3 nanorod arrays were grown by hydrothermal process. • Giant piezoelectric coefficient d 33 ~ 814 pm V − 1 is achieved in KNLN nanorod arrays. • Orthorhombic-tetragonal phase boundary of KNLN nanorods is constructed at RT. • The nanogenerators exhibit outstanding energy harvesting and sensing performance. • Self-powered human-body-movements monitoring is realized by KNLN nanogenerators.

Adaptive dipolar correlation in ferroelectric x(Ba0.7Ca0.3)TiO3−(1−x)Ba(Zr0.2Ti0.8)O3

Deriving structure-property relationships for multicomponent ferroelectric materials has always been a challenging problem because the properties are mostly driven by subtle nanoscale correlations which are hard to detect. Here we have studied the famous Pb-free material $x({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_{3}\text{\ensuremath{-}}(1\ensuremath{-}x)\mathrm{Ba}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_{3}$ $(x\mathrm{BCT}\text{\ensuremath{-}}\mathrm{BZT})$ which has been at the center of attention for approximately the last 10 years because of its unprecedented piezoelectric properties. However, the structure-property relationship for $x\mathrm{BCT}$-BZT remains elusive as neither the common concepts nor the experimental results lead to satisfactory models which can fully explain its unusual piezoelectric properties as a function of composition as well as recognize its uniqueness compared to other similar systems. Hence we have applied total neutron-scattering and Raman-scattering methods to examine the local structural correlations of $x\mathrm{BCT}$-BZT in the range $0.40\ensuremath{\le}x\ensuremath{\le}0.60$ at ambient conditions. By refining large-box atomistic models against the neutron pair distribution functions, we have observed an emerging coherence in the polar displacements of the cations at $x=0.50$, leading to an increased structural ergodicity at the key orthorhombic-tetragonal phase boundary. Given the very similar level of local structural disorder and spontaneous polarization found in the system with $x$, we propose that the abrupt amplification of piezoelectric properties in this material at a region of phase instability is a consequence of enhanced collaboration amongst the all cations in a strain-reduced local environment. It also indicates that the popular structure-property concept entailing a low-symmetry crystal structure cannot be a generic concept or a precondition.

Influence of secondary phase on polymorphic phase transition in Li-doped KNN lead-free ceramics

Abstract Shifting polymorphic phase transition (PPT) to room temperatures in Li-doped KNN (KNLN) can greatly increase its piezoelectric constant. However, pure phase KNLN ceramics could hardly be obtained since secondary phases were formed inevitably during its synthesis. Here, we examined the structure, composition and distribution of the secondary phase and their influences on the phase compositions in KNLN ceramics. The results show that orthorhombic–tetragonal phase boundary cannot shift to room temperature when considerable amount of secondary phase exist in KNLN piezoceramics.

Tolerance factor effect on the structure and properties of KNN based ceramics at orthorhombic–tetragonal phase boundary

(1−x)(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3−xMZrO3 (M = Ca, Sr, Pb, Ba) ceramics with ABO3 structure were synthesized by conventional solid state reaction route. The effect of different kinds of dopant on orthorhombic–tetragonal (O–T) phase boundary and phase transition temperature were investigated, and the tolerance factor (t) of four kinds of zirconate dopants effect on piezoelectric properties was evaluated at O–T phase boundary. A quantitative relationship between O–T phase boundary and t was developed, the excellent piezoelectric properties was obtained when the t value at the O–T phase boundary in KNN-based lead free ceramics was around 0.9965–0.9990.

Investigations of domain switching and lattice strains in (Na,K)NbO3-based lead-free ceramics across orthorhombic-tetragonal phase boundary

Various strain contributions of (Na0.52K0.48−x)(Nb0.92−xSb0.08)O3 −xLiTaO3 ceramics in the proximity of orthorhombic (O) and tetragonal (T) polymorphic phase boundary (PPB) were quantitatively resolved by means of synchrotron x-ray diffraction together with macroscopic strain measurements. Compared with O-rich compositions with a governing mechanism of intrinsic lattice strains, T-rich compositions exhibited a dominant strain mechanism from reversible domain switching. Quantitative analysis of diffraction data suggested that extrinsic strain contributions should depend on not only the lattice distortion δ, but also the poling texture Δf, phase content (for PPB compositions) and domain types. Smaller lattice distortion and higher poling texture tended to enhance the number of irreversible domain switching in O-rich compositions, thus leading to larger fraction of intrinsic lattice strain contribution. The calculated results demonstrated that the product of two parameters Δf and δ would give a reliable estimation of domain-switching strains for T-phase compositions but an overestimation for O-phase compositions.

Lithium-modified (K0.5Na0.5)NbO3–BiAlO3 lead-free piezoelectric ceramics with high Curie temperature

Lead-free piezoelectric ceramics based on the perovskite solid solution 0.995(K0.5Na0.5)1−xLixNbO3–0.005BiAlO3 were prepared using conventional solid-state sintering techniques. According to the analysis of X-ray diffraction patterns, a pure perovskite phase structure could be achieved in the studied composition range of 0≤x≤0.08. As the lithium content increased, the crystal structure gradually transitioned from orthorhombic to tetragonal symmetry. Furthermore, an orthorhombic–tetragonal phase boundary could be observed in ceramics with the compositions of 0.03≤x≤0.05. The ceramics with compositions close to the orthorhombic–tetragonal phase boundary exhibited enhanced piezoelectric activity, possessing the largest values of piezoelectric constant d33, and planar and thickness electromechanical coupling coefficients, of 182 pC/N, 0.375, and 0.460, respectively. Additionally, it was found that these ceramics with excellent piezoelectric properties also possessed a high Curie temperature (~ 450°C), indicating that they would be suitable for application in elevated-temperature piezoelectric devices.

Piezoelectric and dielectric properties of 0.995K0.48Na0.48Li0.04Nb(1−x)SbxO3–0.005BiAlO3 lead-free piezoelectric ceramics

Lead-free 0.995K0.48Na0.48Li0.04Nb(1−x)SbxO3–0.005BiAlO3 piezoelectric ceramics with perovskite structure were designed and prepared with conventional ceramic fabrication techniques. The phase transitions of the ceramics were determined by X-ray diffraction patterns and the temperature dependence of dielectric properties. The ceramics possessed an orthorhombic–tetragonal phase boundary with x=0 and exhibited the coexistence of rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries at around room temperature, with 0.02≤x≤0.03. Upon further increasing the Sb content, the crystal structure transformed to the rhombohedral-tetragonal phase coexistence in the composition range of 0.04≤x≤0.05. Next, the phase structure turned to a sole rhombohedral symmetry with x≥0.06. A clearly enhanced piezoelectric activity was demonstrated for the ceramic with the rhombohedral-tetragonal phase boundary composition x=0.05, where the values of piezoelectric constant d33, planar and thickness electromechanical coupling coefficients reached 205 pC/N, 0.360 and 0.437, respectively.

Aging behavior and electrical properties of low-temperature sintered (Ba, Ca)(Ti, Zr)O3-Ba(Cu, W)O3 ceramics and plate loudspeaker

Lead-free (1−x)wt.% (Ba0.85Ca0.15)(Zr0.10Ti0.90)O3−xwt.% (Ba(Cu0.5W0.5)O3 (BCZT−xwt.% BCW) piezoelectric ceramics were prepared by solid-state reaction. The phase structure of the ceramics showed that the orthorhombic–tetragonal phase boundary was identified in the range of 1.2≤x≤1.6wt.%. The dense microstructure and optimal electrical properties of the ceramics were obtained at x=1.2wt.%. Fatigue behavior of the ceramics was also investigated as a function of temperature and keeping time, where the BCZT−1.2wt.% BCW materials were found to possess improved fatigue characteristic. In addition, the sound pressure level (SPL) of 16.0mm length×16.0mm width×0.1mm piezoelectric loudspeakers using lead-free BCZT ceramics with 1.2wt.% BCW content at drive voltage of 10–20V was more than 80dB at frequency of 2.2–20.0kHz. These results indicated that the 98.8wt.% BCZT−1.2wt.% BCW ceramics was potential for piezoelectric loudspeaker applications.

High piezoelectricity in low-temperature sintering potassium–sodium niobate-based lead-free ceramics

Here we have realized three goals in a new potassium–sodium niobate material using CuO as sintering aid, such as low temperature sintering, the suppression in the loss of alkali metals, and attaining a high d33. Low-temperature sintered 0.96(K0.46Na0.54)Nb0.95Sb0.05O3–0.04Bi0.5(Na0.82K0.18)0.5ZrO3 ceramics were made conducting by adding CuO using an ordinary ceramic fabrication technique. The sintering temperatures drop from 1095 °C to 950 °C when the CuO content is increased from 0 to 2.5 wt%, and the K and Na loss could be also prevented under a low processing temperature. In addition, the ceramics undergo a phase transition with increasing CuO content, such as from a rhombohedral–tetragonal phase coexistence to the mixture of both rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries. The inhomogeneous Cu2+ distributions become more obvious as the CuO content rises, i.e., the excess Cu2+ aggregates at the small grains region for a low CuO content (x ≤ 1.5), and at the small and large grains zones for x = 2.5. The dielectric constant gradually decreases as the CuO increases, and the dielectric loss is lowered by adding CuO content. A well polarization vs. electric field (P–E) loop is seen in the ceramics with x ≤ 2.5, and those with x = 2.5 possess a high d33 of 310 pC N−1 even if the sintering temperatures drop down to 950 °C. As a result, it is an efficient way to promote the piezoelectricity of copper oxide-modified alkaline niobate ceramics using a low-temperature sintering technique.

Phase structure and electrical properties of (K0.5Na0.5)NbO3–(Bi0.5Na0.5)ZrO3 lead-free ceramics with a sintering aid of ZnO

0.95(K0.5Na0.5)NbO3–0.05(Bi0.5Na0.5)ZrO3–xZnO (KNN–BNZ–xZnO) lead-free piezoceramics were prepared by the conventional solid-state method, and effects of ZnO content on their phase structure, microstructure, and electrical properties were studied. The ceramics with x=0.01 possess the mixture of both rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries. Their grain size becomes more homogenous and much smaller with the addition of ZnO. Enhanced dielectric and piezoelectric properties (e.g., d33=320pC/N, kp=0.484, TC=320°C) were observed in the ceramic with x=0.01. These results show that such a material system belongs to be a promising lead-free piezoelectric material.

Dielectric, ferroelectric, and piezoelectric properties in potassium sodium niobate ceramics with rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries

A new phase boundary with rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries is designed in (K0.48Na0.52)NbO3 by adding Bi0.5(Na0.7K0.2Li0.1)0.5ZrO3 (BNKLZ), where Zr4+ and (BNKL)2+ are respectively used to improve the temperature of a rhombohedral phase and to decrease the temperature of an orthorhombic–tetragonal phase coexistence. These ceramics endure several continuous phase transitions with increasing BNKLZ content, i.e., an orthorhombic phase (0≤x<0.03), orthorhombic–tetragonal phases (x=0.03), orthorhombic–tetragonal and rhombohedral–orthorhombic (O–T and R–O) phase existence (0.03<x≤0.05), a rhombohedral phase (0.05<x≤0.07). The ceramics with O–T and R–O have a better piezoelectric behavior as compared with other phases because of more polarization states, enhanced εr and Pr, and a dense microstructure. Moreover, piezoelectric properties could be further optimized by modifying their sintering and poling temperatures. As a result, the construction of O–T and R–O phase coexistence benefits the improvement of piezoelectric properties in KNN-based ceramics.

Phase transition behavior and high piezoelectric properties in lead-free BaTiO3–CaTiO3–BaHfO3 ceramics

The transition behavior, structural changes, and electric properties of lead-free (1−x)Ba(Hf0.16Ti0.84)O3–x(Ba0.70Ca0.30)TiO3 (BCHT) ceramics fabricated by the conventional solid-state reaction method are investigated in this study. A complete phase diagram of BCHT system has been proposed based on their dielectric behavior. It is found that BHCT ceramics undergo a complicated phase evolution, driven by Ca and Hf contents. The results clearly demonstrate that high electric properties are achieved in the ferroelectric orthorhombic–tetragonal phase boundary near the composition with x = 0.48, which could be adjusted by the contents of Ca and Hf in the composition. The optimum composition shows enhanced properties with dielectric constant er = 2889 (at room temperature, 1 kHz), high piezoelectric coefficient d33 = 410 pC/N, and electromechanical coupling factor kp = 0.47, and a relative high Curie temperature of 106 °C. This investigation yields a sight to understand different phase transition mechanisms of enhanced piezoelectricity for the system.

Influence of the A/B nonstoichiometry, composition modifiers, and preparation methods on properties of Li- and Ta-modified (K,Na)NbO3 ceramics

Properties of Li- and Ta- modified (K,Na)NbO3 piezoceramics with the base composition near the orthorhombic-tetragonal phase boundary have been investigated with respect to variation of temperature, stoichiometry, compositional homogeneity, atmosphere of thermal treatment, and dopants (Ba, Mn). Although the influence of the most of the abovementioned factors has already been reported in the literature, the paper focuses on several aspects of the composition—property relationships that still remain controversial or poorly elucidated. In particular, we show that improvement of compositional homogeniety in these ceramics emphasizes the instability of piezoelectric response with respect to variation of temperature in the vicinity of the orthorhombic-tetragonal phase transition. Ba dopant is shown to suppress conductivity in ceramics sintered in air, though it makes conductivity more sensitive to variation of the oxygen partial pressure. Mn dopant is shown to suppress conductivity and strongly reduce the influence of the oxygen partial pressure on conductivity of the ceramics. Finally, we show that chemical modifications to the ceramic composition, such as Mn dopant or variation of nonstoichiometry affect the piezoelectric response mainly by the shift of the orthorhombic-tetragonal phase transition temperature.

Material Properties of Barium Titanate Single Crystal from Modified Time-Dependent Devonshire–Ginzburg–Landau Model

We simulated and compared the material properties of barium titanate for five states, namely, (1) the orthorhombic state, (2) the orthorhombic state near the orthorhombic–tetragonal phase boundary, (3) the tetragonal state near the orthorhombic–tetragonal phase boundary, (4) the tetragonal state, and (5) the tetragonal state near the tetragonal–cubic phase boundary by using a modified time-dependent Devonshire–Ginzburg–Landau model. We reproduced reasonable variations in piezoelectric coefficients, dielectric susceptibilities, and elastic compliance constants for the above-mentioned five states, especially for the phase boundary states. Moreover, we simulated the hysteresis curves of the dependence of electric polarization and strain on electric field and the dependence of the hysteresis curves on compressive stress. The obtained simulation results are well explained by the polarization rotation or extension, which is made easy by the free energy flattening induced by temperature or electric field.