113 research outputs found
Domain variance and superstructure across the antiferroelectric/ferroelectric phase boundary in Pb1−1.5xLax(Zr0.9TiM0.1)O3
Transmission electron microscopy, x-ray diffraction, relative permittivity as a
function of temperature, and polarization versus field loops were used to study the
antiferroelectric/ferroelectric (AFE/FE) phase boundary in Pb1−1.5xLaxZr0.9Ti0.1O3
(PLZT, 100x/90/10) ceramics. X-ray diffraction and electrical measurements indicated
a FE rhombohedral (R) to AFE tetragonal (T) phase transition between PLZT 2/90/10
and 4/90/10. Both phases exhibited superstructure reflections in electron-diffraction
patterns at 1⁄2{hkl} positions consistent with rotations of the octahedra in antiphase.
Previously, neutron diffraction suggested that the FER has an a−a−a− tilt system
(Glazer notation), in agreement with its macroscopic symmetry. By analogy, it is
proposed that the AFET phase has an a0a0c− tilt system. The AFE phase was also
characterized by incommensurate superstructure along pseudocubic 〈110〉p directions,
whereas the FE phase had extra commensurate superlattice reflections at 1⁄2{hk0}p
positions. 1⁄2{hk0}p reflections are forbidden in both tilt systems, but their presence is
explained by Pb ion displacements averaged along 〈111〉 but with short coherence
antiparallel components along 〈110〉 directions. The antiparallel Pb displacements are
coupled to an a−b−b− (a ≈ b) monoclinic tilt system in the vicinity of the AFE/FE
boundary
The atomic structure and chemistry of Fe-rich steps on antiphase boundaries in Ti-doped Bi<sub>0.9</sub>Nd<sub>0.15</sub>FeO3
Stepped antiphase boundaries are frequently observed in Ti-doped Bi<sub>0.85</sub>Nd<sub>0.15</sub>FeO<sub>3</sub>, related to the novel planar antiphase boundaries reported recently. The atomic structure and chemistry of these steps are determined by a combination of high angle annular dark field and bright field scanning transmission electron microscopy imaging, together with electron energy loss spectroscopy. The core of these steps is found to consist of 4 edge-sharing FeO<sub>6</sub> octahedra. The structure is confirmed by image simulations using a frozen phonon multislice approach. The steps are also found to be negatively charged and, like the planar boundaries studied previously, result in polarisation of the surrounding perovskite matrix
Temperature Dependent Piezoelectric Properties of Lead-Free (1-x)K0.6Na0.4NbO3–xBiFeO3 Ceramics
(1-x)K0.4Na0.6NbO3–xBiFeO3 lead-free piezoelectric ceramics were successfully prepared in a single perovskite phase using the conventional solid-state synthesis. Relative permittivity (εr) as a function of temperature indicated that small additions of BiFeO3 not only broadened and lowered the cubic to tetragonal phase transition (TC) but also shifted the tetragonal to orthorhombic phase transition (TO–T) toward room temperature (RT). Ceramics with x = 1 mol.% showed optimum properties with small and large signal piezoelectric coefficient, d33 = 182 pC/N and d∗33 = 250 pm/V, respectively, electromechanical coupling coefficient, kp = 50%, and TC = 355°C. kp varied by ∼5% from RT to 90°C, while d∗33 showed a variation of ∼15% from RT to 75°C, indicating that piezoelectric properties were stable with temperature in the orthorhombic phase field. However, above the onset of TO–T, the properties monotonically degraded in the tetragonal phase field as TC was approached
Phase transitions, domain structure, and pseudosymmetry in La- and Ti-doped BiFeO3
The phase transitions and domain structure of the promising PbO-free solid solution series, (0.95-x)BiFeO3-xLaFeO3-0.05La2/3TiO3, were investigated. X ray diffraction(XRD) revealed a transition from a ferroelectricR3c to a PbZrO3-like (Pbam) antiferroelectric (AFE) structure at x = 0.15 followed by a transition to a paraelectric (PE, Pnma) phase at x > 0.30. The ferroelastic/ferroelectric twin domain width decreased to 10–20 nm with increasing x as the AFE phase boundary was approached but coherent antiphase tilted domains were an order of magnitude greater. This domain structure suggested the local symmetry (20 nm) is lower than the average structure (R3c, a−a−a−) of the tilted regions. The PE phase (x = 0.35) exhibited a dominant a−a−c+ tilt system with Pnma symmetry but diffuse reflections at ∼1/4{ooe} positions suggest that short range antipolar order is residual in the PE phase. The complex domain structure and phase assemblage of this system challenge the conventional interpretation of phase transitions based on macroscopic symmetry. Instead, it supports the notion that frustration driven by chemical distributions at the nanometric level influences the local or pseudo-symmetry as well as the domain structure, with XRD giving only the average macroscopic structure
p-type/n-type behaviour and functional properties of KxNa (1-x)NbO3 (0.49 <= x <= 0.51) sintered in air and N2
Abstract Potassium sodium niobate (KNN) is a potential candidate to replace lead zirconate titanate in sensor and actuator applications but there are many fundamental science and materials processing issues to be understood before it can be used commercially, including the influence of composition and processing atmosphere on the conduction mechanisms and functional properties. Consequently, KNN pellets with different K/Na ratios were sintered to 95% relative density in air and N2 using a conventional mixed oxide route. Oxygen vacancies (VO..) played a major role in the semi-conduction mechanism in low p(O2) for all compositions. Impedance spectroscopy and thermo-power data confirmed KNN to be n-type in low p(O2) in contradiction to previous reports of p-type behaviour. The best piezoelectric properties were observed for air- rather than N2-sintered samples with d33=125 pC/N and kp=0.38 obtained for K0.51Na0.49NbO3
Modeling of Joule heating in KNN FLASH sintering
In this work, we propose the use of FLASH sintering as an alternative technique to densify Potassium Sodium Niobate, K0.5Na0.5NbO3, KNN, a piezoceramic with relevant promising applications and a possible viable substitute of lead zirconate titanate based compositions (Pb1-x ZrxTiO3, PZT). We aim to increase this material performance by densifying KNN ceramics without secondary phase segregation. Furthermore, FLASH will contribute to a more sustainable processing of piezoelectrics as lead-free ceramics at reduced sintering temperature and time.
Please click Additional Files below to see the full abstract
Orientation of rapid thermally annealed lead zirconate titanate thin films on (111) Pt substrates
The nucleation, growth, and orientation of lead zirconate titanate thin films prepared from organometallic precursor solutions by spin coating on (111) oriented platinum substrates and crystallized by rapid thermal annealing was investigated. The effects of pyrolysis temperature, post-pyrolysis thermal treatments, and excess lead addition are reported. The use of post-pyrolysis oxygen anneals at temperatures in the regime of 350-450 °C was found to strongly affect the kinetics of subsequent amorphous-pyrochlore-perovskite crystallization by rapid thermal annealing. The use of such post-pyrolysis anneals allowed films of reproducible microstructure and textures [both (100) and (111)] to be prepared by rapid thermal annealing. It is proposed that such anneals and pyrolysis temperature affect the oxygen concentration/average Pb valence in the amorphous films prior to annealing. Such changes in the Pb valence state then affect the stability of the transient pyrochlore phase and thus the kinetics of perovskite crystallizatio
Composition and Temperature Dependence of Structure and Piezoelectricity in (1−x)(K1−yNay)NbO3-x(Bi1/2Na1/2)ZrO3Lead-Free Ceramics
Lead-free piezoceramics with the composition (1-x)(K1-yNay)NbO3-x(Bi1/2Na1/2)ZrO3 (KNyN-xBNZ) were prepared using a conventional solid-state route. X-ray diffraction, Raman spectroscopy, and dielectric measurements as a function of temperature indicated the coexistence of rhombohedral (R) and tetragonal (T) phase, typical of a morphotropic phase boundary (MPB) as the BNZ concentration increased and by adjusting the K/Na ratio. High remnant polarization (Pr = 24 μC/cm2), piezoelectric coefficient (d33 = 320 pC/N), effective piezocoefficient (d33* = 420 pm/V), coupling coefficient (kp = 48%), and high strain (S = 0.168%) were obtained at room temperature, but significant deterioration of Pr, d33*, and kp were observed by increasing from room temperature to 160°C (17.5 μC/cm2, 338 pm/V, and 32%, respectively) associated with a transition to a purely T phase. Despite these compositions showing promise for room-temperature applications, the deterioration in properties as a function of increasing temperature poses challenges for device design and remains to be resolved
High Ionic Conductivity with Low Degradation in A Site Strontium-Doped Nonstoichiometric Sodium Bismuth Titanate Perovskite
BiFeO3-BaTiO3: A new generation of lead-free electroceramics
Lead-based electroceramics such as Pb(Zr.Ti)O3 (PZT) and its derivatives have excellent piezoelectric, pyroelectric and energy storage properties and can be used in a wide range of applications. Potential lead-free replacements for PZT such as potassium sodium niobate (KNN) and sodium bismuth titanate (NBT) have a much more limited range of useful properties and have been optimized primarily for piezoelectric applications. Here, we review the initial results on a new generation of lead-free electroceramics based on BiFeO3-BaTiO3 (BF-BT) highlighting the essential crystal chemistry that permits a wide range of functional properties. We demonstrate that with the appropriate dopants and heat treatment, BF-BT can be used to fabricate commercially viable ceramics for applications, ranging from sensors, multilayer actuators, capacitors and high-density energy storage devices. We also assess the potential of BF-BT-based ceramics for electrocaloric and pyroelectric applications
- …