The effect of nano-sized BNBT on microstructure and dielectric/piezoelectric properties

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (abbreviated as BNBT6) ceramic of near MPB composition was synthesized by two different processes. The first one is the addition of pre-synthesized BaTiO₃ and pre-milled Bi₂O₃, Na₂CO₃, BaCO₃ powders and calcination powder milled with a high energy milling machine in order to obtain a nano-particle size. The second one is a conventional one to compare with the former process. The pre-milling and the pre-synthesis process of raw materials lowered the calcination temperature to the extent of 59 °C as compared with conventionally fabricated BNBT6. The particle size of the powder exposed to heavy high energy milling reduced to 50–70 nm, whereas that of the conventionally ball-milled powder without the pre-milling and the pre-synthesis process had a larger size of 280 nm. To investigate the effects of the modified process on the characteristic of BNBT6 ceramics, the dielectric and the piezoelectric properties of sintered specimens fabricated by the two different processes were evaluated. It was found that the properties of the nano-sized BNBT6 ceramic near the MPB composition were increased by the modified mixing and milling method, showing superior characteristics in terms of the piezoelectric/dielectric constant and sintering density compared with those of the conventional process. The modified mixing and milling method was considered to be a new and promising process for lead-free piezoelectric ceramics owing to their excellent piezoelectric/dielectric properties.     

Compact size ultrasonic linear motor using a dome shaped piezoelectric actuator

This paper presents original results obtained in the development of dome-shaped actuator for a linear motor application. The main structure of compact ultrasonic linear motor presented in this paper consists of three parts, which were an actuating part, a shaft and mobile element. The actuating part was fabricated by powder injection molding (PIM) process. The linear motion of new-type linear motor was operated by the principle of inertia displacement. The actuating part combined with clamping ceramic element can be realized by the central movement of dome-shaped piezoelectric actuator (DSPA), which has the maximum displacement of the up-and-down movement. The linear motor, where DSPA was 9.86?mm in diameter, 4.6?mm in curvature of radius and 0.4?mm in thickness, operated at 1^st resonance frequency. The dynamic characteristics of the motor was investigated by finite element method (FEM) and compared with experimental results. These results were in good agreement with that predicted by simulations.     

Fabrication of porous thick films using room-temperature aerosol deposition

A novel technique for the rapid room-temperature deposition of porous ceramic, glass, or metal thick films using the aerosol deposition (AD) method is presented. The process is based on the co-deposition of the desired film material and a second water-soluble constituent, resulting in a ceramic-ceramic composite. Following the subsequent removal of water-soluble end member, a network of pores is retained. To demonstrate the process, porous BaTiO₃ thick films were fabricated through co-deposition with NaCl. Microstructural images show the clear development of a porous structure, which was found to enhance the dielectric properties over dense thick films, possibly related to the lower extent of internal residual stress. This simple but highly effective porous structure fabrication can be applied to any film and substrate material stable in water and is promising for the application of AD-processed films in gas sensors, solid oxide fuel cells, and humidity sensors.     

Determining the local pressure during aerosol deposition using glass memory

Aerosol deposition (AD) is a dynamic loading process that can be envisioned as a shock wave loading, necessitating the consideration of the elastic/plastic response of solid materials. Due to the dynamic nature of this process, however, experimental determination of the local pressures during the deposition process is difficult. This work addresses this by investigating the compression and subsequent structure modification of a silicate glass after room-temperature AD on a silicate glass substrate with Raman spectroscopy. Clear structural changes in the short- and middle-range order of the silicate glass were observed, both as intertetrahedral angle distribution and as ring statistic. Therefore, the AD induced permanent densification of the glass, equivalent, in a hydrostatic approximation, to a minimal pressure of 10.5 ± 1.5 GPa during the film deposition process. Furthermore, the analysis of the Nd³⁺ photoluminescence of the ⁴F_3/2 − ⁴I_9/2 transition provided complementary information on the glass network modifications occurring during film formation. More than a pure hydrostatic densification, the AD seems to present a very intense shear deformation. This work opens up the perspective of evaluating the mechanical response of film-substrate and of the particles themselves, and provides critical information on the mechanisms responsible for the AD film formation.     

Room temperature deposition of functional ceramic films on low-cost metal substrate

In various practical applications, such as high power actuators, high sensitivity sensors, and energy harvesting devices, polycrystalline piezoelectric films of 1–100?µm thickness and sizes ranging from several µm² to several cm² are required. With conventional film deposition processes, such as sol-gel, sputtering, chemical vapor deposition, or pulsed laser deposition, it is difficult to fabricate films with higher thickness due to their low deposition rate and high interfacial stress. The aerosol deposition method (AD), a relatively new deposition technique, can be used to fabricate highly dense thick films at room temperature by the consolidation of submicrometer-sized ceramic particles on various ceramic, metal, glass, and polymer substrates. Ferroelectric BaTiO₃ ceramic films of different thicknesses ranging from 1 to 30?µm were fabricated on a low-cost metallic substrate at room temperature using the AD method. Surface morphology and adhesion of the film were analyzed. Analysis of internal residual stresses revealed an equibiaxial compressive stress state in the as-processed film. Electrical characterization of films annealed at 500?°C shows an enhanced polarization value of ~?14?µC/cm² over that of the as-processed film. This improved property is related to the decreasing internal residual stress. In addition, the BT films prepared in this work were found to withstand electric fields greater than 100?kV/mm, which is possibly related to the inherent relatively defect-free structure of AD films.     

In situ combined stress- and temperature-dependent Raman spectroscopy of Li-doped (Na,K)NbO3

External thermal, electrical, and mechanical fields can induce structural phase transitions in lead-free Li-modified Na_0.5K_0.5NbO₃ ferroelectrics, which significantly influence the macroscopic electromechanical response. In particular, the relative stability of the polar monoclinic (or orthorhombic) and tetragonal phases under temperature and stress is critical to realize the ferroelectric and piezoelectric response. In this study, the effect of mechanical and thermal fields on the local structure in the vicinity of the monoclinic-tetragonal (M-T) phase boundary was investigated using a novel in situ combined uniaxial compressive stress- and temperature-dependent Raman spectroscopy experimental arrangement. Experiments were performed up to 300°C and −200 MPa, clearly demonstrating stress-induced M-T phase transition in Li-modified Na_0.5K_0.5NbO₃. A stress-temperature phase diagram has been established based on the change in vibrational modes. It was possible to correlate the relative permittivity singularities previously observed to a given stage of the M-T phase transition using ratio between characteristic Raman band areas. In addition, the measurement method reported here can be applied to other functional ceramics to investigate the influence of mechanical fields on local structure.     

Growth of tungsten bronze phase out of niobate perovskite phase for opto-ferroelectric applications

Engineering the optical bandgaps of classic ferroelectrics from the typical ultraviolet range down to the visible range is an emerging methodology of developing the next-generation optoelectric and opto-ferroelectric devices including ferroelectric solar cells, light-driven transistors and modulators, and multi-sensors/energy harvesters. Recently, a material interface comprised of a pseudo-morphotropic phase boundary between the tungsten bronze and perovskite phases of the KNBNNO [(K,Na,Ba)_x(Ni,Nb)_yO_z] has been reported to be an effective approach for bandgap engineering while retaining excellent ferroelectricity and piezoelectricity of the perovskite-phased KNBNNO. However, this approach requires the compositions of the materials to be determined at the synthesis stage, leaving little room for any further modification of the microstructure and functional properties at the post-processing stage. This paper presents a post-processing method, that is, atmospheric annealing in N₂ and O₂, to grow the necessary tungsten bronze phase out of the perovskite phase in the KNBNNO. This method is advantageous over the previously reported because it enables to grow the tungsten bronze–perovskite interface region independent of the initial composition. The distinctive electrical properties and the giant tunability of photoconductivity of the tungsten bronze phase, the perovskite phase, and the interface are characterized in detail in this paper, supporting the exploitation of fabricating opto-ferroelectric devices using the reported method which is compatible and comparable with some of the post-processing methods applied in the silicon industry.     

Electric‐Field‐Induced Domain Switching and Domain Texture Relaxations in Bulk Bismuth Ferrite

Bismuth ferrite, BiFeO₃, is an important multiferroic material that has attracted remarkable attention for potential applications in functional devices. While thin films of BiFeO₃ are attractive for applications in nanoelectronics, bulk polycrystalline BiFeO₃ has great potential as a lead‐free and/or high‐temperature actuator material. However, the actuation mechanisms in bulk BiFeO₃ are still to be resolved. Here we report the microscopic origin of electric‐field‐induced strain in bulk BiFeO₃ ceramic by means of in situ high‐energy X‐ray diffraction. Quantification of intrinsic lattice strain and extrinsic domain switching strain from diffraction data showed that the strain response in rhombohedral bulk BiFeO₃ is primarily due to non‐180° ferroelectric domain switching, with no observable change in the phase symmetry, up to the maximum field used in the study. The origin of strain thus differs from the strain mechanism previously shown in thin film BiFeO₃, which gives a similar strain/field ratio as rhombohedral bulk BiFeO₃. A strong post‐poling relaxation of switched non‐180° ferroelectric domains has been observed and hypothesized to be due to intergranular residual stresses with a possible contribution from the conductive nature of the domain walls in BiFeO₃ ceramics.