Crystallization of piezoceramic films on glass via flash lamp annealing.

Integration of thin-film oxide piezoelectrics on glass is imperative for the next generation of transparent electronics to attain sensing and actuating functions. However, their crystallization temperature (above 650 °C) is incompatible with most glasses. We developed a flash lamp process for the growth of piezoelectric lead zirconate titanate films. The process enables crystallization on various types of glasses in a few seconds only. The functional properties of these films are comparable to the films processed with standard rapid thermal annealing at 700 °C. A surface haptic device was fabricated with a 1 µm-thick film (piezoelectric e33,f of -5 C m-2). Its ultrasonic surface deflection reached 1.5 µm at 60 V, sufficient for its use in surface rendering applications. This flash lamp annealing process is compatible with large glass sheets and roll-to-roll processing and has the potential to significantly expand the applications of piezoelectric devices on glass.

Atomic-Level Response of the Domain Walls in Bismuth Ferrite in a Subcoercive-Field Regime.

The atomic-level response of zigzag ferroelectric domain walls (DWs) was investigated with in situ bias scanning transmission electron microscopy (STEM) in a subcoercive-field regime. Atomic-level movement of a single DW was observed. Unexpectedly, the change in the position of the DW, determined from the atomic displacement, did not follow the position of the strain field when the electric field was applied. This can be explained as low mobility defect segregation at the initial DW position, such as ordered clusters of oxygen vacancies. Further, the triangular apex of the zigzag wall is pinned, but it changes its shape and becomes asymmetric under electrical stimuli. This phenomenon is accompanied by strain and bound charge redistribution. We report on unique atomic-scale phenomena at the DW level and show that in situ STEM studies with atomic resolution are very relevant as they complement, and sometimes challenge, the knowledge gained from lower resolution studies.

Chemical Solution Deposition of Barium Titanate Thin Films with Ethylene Glycol as Solvent for Barium Acetate.

Chemical solution deposition (CSD) of BaTiO3 (BT) or BT-based thin films relies on using a carboxylic acid and alcohol as the solvents for alkaline-earth carboxylate and transition-metal alkoxide, respectively; however, the esterification reaction of the solvents may lead to in-situ water formation and precipitation. To avoid such an uncontrolled reaction, we developed a route in which ethylene glycol (EG) is used as the solvent for Ba-acetate. The EG-based BT coating solutions are stable for at least a few months. The thermal decomposition of the BT xerogel obtained by drying the EG-based solutions depends on the choice of the solvent for the Ti-alkoxide as well: in the case of EG and 2-methoxyethanol solvents carbon residues are removed at only about 1100 °C, while in the case of ethanol it is concluded at about 700 °C. About 100 nm thick BT films derived from the EG-ethanol solution deposited on platinized silicon reveal dense, crack-free columnar microstructure. They exhibit local ferro- and piezoelectric properties. The macroscopic polarization-electric field loops were obtained up to a quite high electric field of about 2.4 MV/cm. The EG-ethanol based CSD route is a viable alternative to the established acetic acid-alcohol route for BT and BT-based films.

Screen Printed Copper and Tantalum Modified Potassium Sodium Niobate Thick Films on Platinized Alumina Substrates.

We show how sintering in different atmospheres affects the structural, microstructural, and functional properties of ~30 µm thick films of K0.5Na0.5NbO3 (KNN) modified with 0.38 mol% K5.4Cu1.3Ta10O29 and 1 mol% CuO. The films were screen printed on platinized alumina substrates and sintered at 1100 °C in oxygen or in air with or without the packing powder (PP). The films have a preferential crystallographic orientation of the monoclinic perovskite phase in the [100] and [-101] directions. Sintering in the presence of PP contributes to obtaining phase-pure films, which is not the case for the films sintered without any PP notwithstanding the sintering atmosphere. The latter group is characterized by a slightly finer grain size, from 0.1 µm to ~2 µm, and lower porosity, ~6% compared with ~13%. Using piezoresponse force microscopy (PFM) and electron backscatter diffraction (EBSD) analysis of oxygen-sintered films, we found that the perovskite grains are composed of multiple domains which are preferentially oriented. Thick films sintered in oxygen exhibit a piezoelectric d33 coefficient of 64 pm/V and an effective thickness coupling coefficient kt of 43%, as well as very low mechanical losses of less than 0.5%, making them promising candidates for lead-free piezoelectric energy harvesting applications.

Atomic scale symmetry and polar nanoclusters in the paraelectric phase of ferroelectric materials.

The nature of the "forbidden" local- and long-range polar order in nominally non-polar paraelectric phases of ferroelectric materials has been an open question since the discovery of ferroelectricity in oxide perovskites, ABO3. A currently considered model suggests locally correlated displacements of B-site atoms along a subset of <111> cubic directions. Such off-site displacements have been confirmed experimentally; however, being essentially dynamic in nature they cannot account for the static nature of the symmetry-forbidden polarization implied by the macroscopic experiments. Here, in an atomically resolved study by aberration-corrected scanning transmission electron microscopy complemented by Raman spectroscopy, we reveal, directly visualize and quantitatively describe static, 2-4 nm large polar nanoclusters in the nominally non-polar cubic phases of (Ba,Sr)TiO3 and BaTiO3. These results have implications on understanding of the atomic-scale structure of disordered materials, the origin of precursor states in ferroelectrics, and may help answering ambiguities on the dynamic-versus-static nature of nano-sized clusters.

Domain-wall pinning and defect ordering in BiFeO3 probed on the atomic and nanoscale.

Electro-mechanical interactions between charged point defects and domain walls play a key role in the functional properties of bulk and thin-film ferroelectrics. While for perovskites the macroscopic implications of the ordering degree of defects on domain-wall pinning have been reported, atomistic details of these mechanisms remain unclear. Here, based on atomic and nanoscale analyses, we propose a pinning mechanism associated with conductive domain walls in BiFeO3, whose origin lies in the dynamic coupling of the p-type defects gathered in the domain-wall regions with domain-wall displacements under applied electric field. Moreover, we confirm that the degree of defect ordering at the walls, which affect the domain-wall conductivity, can be tuned by the cooling rate used during the annealing, allowing us to determine how this ordering affects the atomic structure of the walls. The results are useful in the design of the domain-wall architecture and dynamics for emerging nanoelectronic and bulk applications.

Heterogeneity Challenges in Multiple-Element-Modified Lead-Free Piezoelectric Ceramics.

We report on a heterogeneity study, down to the atomic scale, on a representative multiple-element-modified ceramic based on potassium sodium niobate (KNN): 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3-0.05CaZrO3 with 2 wt % MnO2. We show that different routes for incorporating the MnO2 (either before or after the calcination step) affect the phase composition and finally the functionality of the material. According to X-ray diffraction and scanning electron microscopy analyses, the ceramics consist of orthorhombic and tetragonal perovskite phases together with a small amount of Mn-rich secondary phase. The addition of MnO2 after the calcination results in better piezoelectric properties, corresponding to a ratio between the orthorhombic and tetragonal perovskite phases that is closer to unity. We also show, using microscopy techniques combined with analytical tools, that Zr-rich, Ta-rich and Mn-rich segregations are present on the nano and atomic levels. With this multi-scale analysis approach, we demonstrate that the functional properties are sensitive to minor modifications in the synthesis route, and consequently to different material properties on all scales. We believe that detecting and learning how to control these modifications will be a step forward in overcoming the irreproducibility problems with KNN-based materials.

Additive Manufacturing of Ferroelectric-Oxide Thin-Film Multilayer Devices.

Additive manufacturing has dramatically transformed the design and fabrication of advanced objects. Printed electronics-an additive thin-film processing technology-aims to realize low-cost, large-area electronics, and fabrication of devices with highly customized architectures. Recent advances in printing technology have led to several innovative applications; however, layer-on-layer deposition persists as a challenging issue. Here, the additive manufacturing of functional oxide devices by inkjet printing is presented. Two conditions appear critical for successful layer-on-layer printing: (i) preservation of stable surface properties and (ii) suppression of the material accumulation at the edges of a feature upon drying. The former condition was satisfied by introducing a surface modification layer of a polymer with nanotextured topography, and the latter was satisfied by designing the solvent composition of the ink. The developed process is highly efficient and enables conformal stacking of functional oxide layers according to the user-defined geometry, sequence arrangement, and layer thickness. To prove the effectiveness of this concept, we demonstrate an additive manufacture of all-oxide ferroelectric multilayer capacitors/transducers. Printed multilayer devices offer a significant increase in the capacitance density and the electromechanical voltage response in comparison to the single-layer devices. Further growth in the number of available functional oxide inks will enable arbitrary device architectures with novel functionalities.

Structure and Dynamics of Ferroelectric Domains in Polycrystalline Pb(Fe1/2Nb1/2)O3.

A complex domain structure with variations in the morphology is observed at ambient temperature in monoclinic Pb(Fe1/2Nb1/2)O3. Using electron microscopy and piezoresponse force microscopy, it is possible to reveal micrometre-sized wedge, lamellar-like, and irregularly shaped domains. By increasing the temperature, the domain structure persists up to 80 °C, and then starts to disappear at around 100 °C due to the proximity of the ferroelectric-paraelectric phase transition, in agreement with macroscopic dielectric measurements. In order to understand to what degree domain switching can occur in the ceramic, the mobility of the domain walls was studied at ambient temperature. The in situ poling experiment performed using piezoresponse force microscopy resulted in an almost perfectly poled area, providing evidence that all types of domains can be easily switched. By poling half an area with 20 V and the other half with -20 V, two domains separated by a straight domain wall were created, indicating that Pb(Fe1/2Nb1/2)O3 is a promising material for domain-wall engineering.

Construction and Functionality of a Ceramic Resonant Pressure Sensor for Operation at Elevated Temperatures.

Piezoelectric ceramic resonant pressure sensors have shown potential as sensing elements for harsh environments, such as elevated temperatures. For operating temperatures exceeding ~250 °C, conventional and widely used Pb(Zr,Ti)O3 (PZT) piezoelectrics should be replaced. Here, a ceramic pressure sensor from low-temperature co-fired ceramics (LTCC) was constructed by integrating a piezoelectric actuator made from bismuth ferrite (BiFeO3) on a diaphragm. This ferroelectric material was selected because of its high Curie temperature (TC = 825 °C) and as a lead-free piezoelectric extensively investigated for high-temperature applications. In order to construct a sensor with suitable pressure sensitivity, numerical simulations were used to define the optimum construction dimensions. The functionality of the pressure sensor was tested up to 201 °C. The measurements confirmed a pressure sensitivity, i.e., resonance frequency shift of the sensor per unit of pressure, of -8.7 Hz/kPa up to 171 °C. It was suggested that the main reason for the hindered operation at the elevated temperatures could lie in the thermo-mechanical properties of the diaphragm and the adhesive bonding at the actuator-diaphragm interconnection.

Domain-wall conduction in ferroelectric BiFeO3 controlled by accumulation of charged defects.

Mobile charged defects, accumulated in the domain-wall region to screen polarization charges, have been proposed as the origin of the electrical conductivity at domain walls in ferroelectric materials. Despite theoretical and experimental efforts, this scenario has not been directly confirmed, leaving a gap in the understanding of the intriguing electrical properties of domain walls. Here, we provide atomic-scale chemical and structural analyses showing the accumulation of charged defects at domain walls in BiFeO3. The defects were identified as Fe4+ cations and bismuth vacancies, revealing p-type hopping conduction at domain walls caused by the presence of electron holes associated with Fe4+. In agreement with the p-type behaviour, we further show that the local domain-wall conductivity can be tailored by controlling the atmosphere during high-temperature annealing. This work has possible implications for engineering local conductivity in ferroelectrics and for devices based on domain walls.

Self-Poling of BiFeO3 Thick Films.

Bismuth ferrite (BiFeO3) is difficult to pole because of the combination of its high coercive field and high electrical conductivity. This problem is particularly pronounced in thick films. The poling, however, must be performed to achieve a large macroscopic piezoelectric response. This study presents evidence of a prominent and reproducible self-poling effect in few-tens-of-micrometer-thick BiFeO3 films. Direct and converse piezoelectric measurements confirmed that the as-sintered BiFeO3 thick films yield d33 values of up to ∼20 pC/N. It was observed that a significant self-poling effect only appears in cases when the films are heated and cooled through the ferroelectric-paraelectric phase transition (Curie temperature TC ∼ 820 °C). These self-poled films exhibit a microstructure with randomly oriented columnar grains. The presence of a compressive strain gradient across the film thickness cooled from above the TC was experimentally confirmed and is suggested to be responsible for the self-poling effect. Finally, the macroscopic d33 response of the self-poled BiFeO3 film was characterized as a function of the driving-field frequency and amplitude.

Dual strain mechanisms in a lead-free morphotropic phase boundary ferroelectric.

Electromechanical properties such as d33 and strain are significantly enhanced at morphotropic phase boundaries (MPBs) between two or more different crystal structures. Many actuators, sensors and MEMS devices are therefore systems with MPBs, usually between polar phases in lead (Pb)-based ferroelectric ceramics. In the search for Pb-free alternatives, systems with MPBs between polar and non-polar phases have recently been theorized as having great promise. While such an MPB was identified in rare-earth (RE) modified bismuth ferrite (BFO) thin films, synthesis challenges have prevented its realization in ceramics. Overcoming these, we demonstrate a comparable electromechanical response to Pb-based materials at the polar-to-non-polar MPB in Sm modified BFO. This arises from 'dual' strain mechanisms: ferroelectric/ferroelastic switching and a previously unreported electric-field induced transition of an anti-polar intermediate phase. We show that intermediate phases play an important role in the macroscopic strain response, and may have potential to enhance electromechanical properties at polar-to-non-polar MPBs.

Robust polarization and strain behavior of Sm-modified BiFeO3 piezoelectric ceramics.

The route to phase-pure BiFeO3 (BFO) ceramics with excellent ferroelectric and electromechanical properties is severely impeded by difficulties associated with the perovskite phase stability during synthesis. This has meant that dopants and solid solutions with BFO have been investigated as a means of not only improving the functional properties, but also of improving the perovskite phase formation of BFO-based ceramics. The present work focuses on Sm-modified BFO ceramics of composition Bi0.88Sm0.12FeO3. The polarization and strain behaviors were investigated as a function of the phase composition, microstructure, and chemical composition. Addition of Sm reduces the susceptibility of the BFO perovskite to phase degradation by Si impurities. Si was observed to react into Sm-rich grains dispersed within the microstructure, with no large increases in the amount of bismuth-parasitic phases, namely Bi25FeO39 and Bi2Fe4O9. These as-prepared ceramics exhibited robust polarization behavior showing maximum remnant polarizations of ~40 to 50 µC/cm(2). The electric-fieldinduced strain showed an appreciable stability in terms of the driving field frequency with maximum peak-to-peak strains of ~0.3% and a coercive field of ~130 kV/cm.

Sintering of Lead-Free Piezoelectric Sodium Potassium Niobate Ceramics.

The potassium sodium niobate, K0.5Na0.5NbO3, solid solution (KNN) is considered as one of the most promising, environment-friendly, lead-free candidates to replace highly efficient, lead-based piezoelectrics. Since the first reports of KNN, it has been recognized that obtaining phase-pure materials with a high density and a uniform, fine-grained microstructure is a major challenge. For this reason the present paper reviews the different methods for consolidating KNN ceramics. The difficulties involved in the solid-state synthesis of KNN powder, i.e., obtaining phase purity, the stoichiometry of the perovskite phase, and the chemical homogeneity, are discussed. The solid-state sintering of stoichiometric KNN is characterized by poor densification and an extremely narrow sintering-temperature range, which is close to the solidus temperature. A study of the initial sintering stage revealed that coarsening of the microstructure without densification contributes to a reduction of the driving force for sintering. The influences of the (K + Na)/Nb molar ratio, the presence of a liquid phase, chemical modifications (doping, complex solid solutions) and different atmospheres (i.e., defect chemistry) on the sintering are discussed. Special sintering techniques, such as pressure-assisted sintering and spark-plasma sintering, can be effective methods for enhancing the density of KNN ceramics. The sintering behavior of KNN is compared to that of a representative piezoelectric lead zirconate titanate (PZT).

Electric-field-induced phase transitions in co-doped Pb(Zr1-xTix)O3 at the morphotropic phase boundary.

The strain- and polarization-electric field behavior was characterized at room temperature for Pb0.98Ba0.01(Zr1-x Ti x )0.98Nb0.02O3, 0.40 ⩽ x ⩽ 0.60. The investigated compositions were located in the vicinity of the morphotropic phase boundary, giving insight into the influence of crystal structure on the hysteretic ferroelectric behavior. The remanent strain of particular compositions is shown to be larger than theoretically allowed by ferroelectric switching alone, indicating the presence of additional remanent strain mechanisms. A phenomenological free energy analysis was used to simulate the effect of an applied electric field on the initial equilibrium phase. It is shown that electric-field-induced phase transitions in polycrystalline ferroelectrics can account for the experimental observations. The experimental and simulation results are contrasted to neutron diffraction measurements performed on representative compositions in the virgin and remanent states.

Compositional and structural study of a (K0.5Na0.5)NbO3 single crystal prepared by solid state crystal growth.

In this work we investigated the chemical composition and structure of (K0.5Na0.5)NbO3 (KNN) single crystals grown by the solid state crystal growth method. The optical, scanning, and transmission electron microscopies were employed for the analysis of the chemical homogeneity and domain structure of the KNN crystal. No compositional inhomogeneities within experimental error were encountered in the KNN single crystals. The domain structure of the KNN single crystal, with a monoclinic unit cell, is composed of large 90 degrees domains of up to 100 microm width, which further consist of smaller 180 degrees domains with widths from 50 to 300 nm.

Characterization of lead zirconate titanate--lanthanum ruthenate thin film structures prepared by chemical solution deposition.

In this work, the results of compositional and microstructural analysis of lead zirconate titanate--lanthanum ruthenate thin film structures prepared by chemical solution deposition are discussed. The cross-section transmission electron microscope (TEM) micrographs of the La-Ru-O film deposited on a SiO2/Si substrate and annealed at 700 degrees C revealed RuO2 crystals embedded in a glassy silicate matrix. When the La-Ru-O film was deposited on a Pt/TiO2/SiO2/Si substrate, RuO2 and La4Ru6O19 crystallized after annealing at 700 degrees C. After firing at 550 degrees C randomly oriented lead zirconate titanate (PZT) thin films crystallized on the La-Ru-O/SiO2/Si substrate, while on La-Ru-O/Pt/TiO2/SiO2/Si substrates PZT thin films with (111) preferred orientation were obtained. No diffusion of the Ru atoms in the PZT film was found. Ferroelectric response of PZT thin films on these substrates is shown in comparison with the PZT film deposited directly on the Pt/TiO2/SiO2/Si substrate without a La-Ru-O layer.

Electron microscopy studies of potassium sodium niobate ceramics.

Using electron microscopy, K0.5Na0.5NbO3 (KNN) ceramics sintered at 1030 degrees C for 8 h and 1100 degrees C for 2 and 24 h was studied. The scanning electron microscopy and X-ray spectrometry revealed that the materials consisted of a matrix phase in which the (Na+K)/Nb ratio corresponded closely to the nominal composition and a small amount of Nb-rich secondary phase. A bimodal microstructure of cube-shaped grains was revealed in the fracture and thermally-etched surfaces of the KNN. In the ceramics sintered at 1100 degrees C, the larger grains (up to 30 mum across), contained angular trapped pores. The transmission electron microscopy analysis revealed that the crystal planes of the grains bordering the intragranular pore faces were of the {100} family with respect to the simple perovskite cell. Ferroelectric domains were observed in the grains of this material.