Cyclic electric field response of morphotropic Bi1/2Na1/2TiO3-BaTiO3 piezoceramics

In this study, the evolution of field induced mechanisms in lead-free piezoelectric ceramics (1-x)Bi1/2Na1/2TiO3-xBaTiO(3) with x = 0.06 and 0.07 was investigated by transmission electron microscopy, neutron, and X-ray diffraction. Preliminary investigations revealed a strong degradation of macroscopic electromechanical properties within the first 100 bipolar electric cycles. Therefore, this structural investigation focuses on a comparative diffraction study of freshly prepared, poled, and fatigued specimens. Transmission electron microscopy and neutron diffraction of the initial specimens reveal the coexistence of a rhombohedral and a tetragonal phase with space group R3c and P4bm, respectively. In situ electric field X-ray diffraction reveals a pronounced field induced phase transition from a pseudocubic state to a phase composition of significantly distorted phases upon poling with an external electric field of 4 kV/mm. Although the structures of the two compositions are pseudocubic and almost indistinguishable in the unpoled virgin state, the electric field response shows significant differences depending on composition. For both compositions, the application of an electric field results in a field induced phase transition in the direction of the minority phase. Electric cycling has an opposite effect on the phase composition and results in a decreased phase fraction of the minority phase in the fatigued remanent state at 0 kV/mm. (C) 2015 AIP Publishing LLCopen

The Effects of Social Media on the Online Consumer Behaviour of University Students

Treballs Finals del Grau d'Empresa Internacional, Facultat d'Economia i Empresa, Universitat de Barcelona, Curs: 2019-2020 , Tutor: Mel Solé Moro[eng] This research paper studies the influence that Social Media platforms and the interactions that take place within them have on the online consumption decisions of University students. In order to analyse these effects, several criteria have been taken into account such as gender, frequency of use of Social Media platforms and usual consumption patterns. Results from the questionnaires that were distributed amongst the students revealed that Instagram and Twitter are the two platforms that, besides being the most used, have the biggest influence on users when it comes to the online consumption decisions of University students. Additionally, it was found that E-WOM has become the main source of information that is used by consumers prior to making a consumption decision online.[cat] Aquest treball de recerca estudia la influència que les plataformes de xarxes socials i les interaccions que tenen lloc a través d’elles tenen en les decisions de consum en línia dels estudiants universitaris. Per analitzar aquests efectes s'han tingut en compte diversos criteris, com el gènere, la freqüència d'ús de les plataformes de xarxes socials i les pautes de consum habituals. Els resultats dels qüestionaris que es van distribuir entre els estudiants van revelar que Instagram i Twitter són les dues plataformes que, a més de ser les més utilitzades, tenen més influència en els estudiants universitaris a l'hora de prendre decisions de consum en línia. A més, es va constatar que la E-WOM s'ha convertit en la principal font d'informació que utilitzen els consumidors abans de prendre una decisió de consum en línia

Field-induced phase transition in Bi1/2Na1/2TiO3-based lead-free piezoelectric ceramics

The origin of the electric field-induced strain in the polycrystalline ceramic 0.92Bi(1/2)Na(1/2)TiO(3)-0.06BaTiO(3)-0.02K(1/2)Na(1/2)NbO(3) was investigated using in situ high-resolution X-ray and neutron diffraction techniques. The initially existing tetragonal phase with pseudocubic lattice undergoes a reversible phase transition to a significantly distorted rhombohedral phase under electric field, accompanied by a change in the oxygen octahedral tilting from a 0 a 0 c + to a - a - a - and in the tilting angle. The polarization values for the tetragonal and rhombohedral phases were calculated based on the structural information from Rietveld refinements. The large recoverable electric field-induced strain is a consequence of a reversible electric field-induced phase transition from an almost nonpolar tetragonal phase to a ferroelectrically active rhombohedral phase.open686

Local structure, pseudosymmetry, and phase transitions in Na1/2Bi1/2TiO3-K1/2Bi1/2TiO3 ceramics

The structural behavior of ceramic solid solutions (1 - x)Na1/2Bi1/2TiO3-xK(1/2)Bi(1/2)TiO(3) (NBT-KBT) was studied using high-resolution powder diffraction and transmission electron microscopy. A temperature-independent morphotropic phase boundary (MPB) separating NBT-like pseudorhombohedral (R) and KBT-like pseudotetragonal (T) phases was observed at x approximate to 0.2. For x 0.2 exhibit a tetragonal-like distortion; however, complex splitting of reflections in XRD patterns suggests that the actual symmetry is lower than tetragonal. For 0.2 0.5 the structure becomes untilted. In-phase tilting evolves above the ferroelectric transition and occurs around a nonpolar (a or b) axis of the average T structure. The onset of polar order has no significant effect on the coherence length of in-phase tilting, which suggests only weak coupling between the two phenomena. The average symmetry of the T phase is determined by the effective symmetry (Imm2) of assemblages of coherent in-phase tilted nanodomains. Near the MPB, the coexistence of extended R-and T-like regions is observed, but lattice distortions within each phase are small, yielding narrow peaks with a pseudocubic appearance in XRD. The temperature of the FE phase transition exhibits a minimum at the MPB. The structured diffuse scattering observed in electron diffraction patterns for all the compositions suggests that polar order in NBT-KBT solid solutions is modulated away from the average displacements refined using powder diffraction data.open13

Quenching-assisted actuation mechanisms in core-shell structured BiFeO3-BaTiO3 piezoceramics

Electromechanical actuation in piezoceramics is usually enhanced by creating chemically homogeneous materials with structurally heterogeneous morphotropic phase boundaries, leading to abrupt changes in ion displacement directions within the perovskite unit cell. In the present study, an alternative mechanism to enhance electromechanical coupling is found in both chemically and structurally heterogeneous BiFeO3-BaTiO3 lead-free piezoceramics. Such a mechanism is observed in a composition exhibiting core-shell type microstructure, associated with donor-type substitution of Ti4+ for Fe3+, and is primarily activated by thermal quenching treatment. Here, we describe the use of in situ high-energy synchrotron X-ray powder diffraction upon the application of a high electric field to directly monitor the ferroelectric and elastic interactions between these composite-like components, formed as core and shell regions within grains. Translational short or long-range ordering is observed in the BiFeO3-depleted shell regions which undergo significant structural alterations from pseudocubic Pm3m relaxor-ferroelectric in slow-cooled ceramics to rhombohedral R3c or R3m with long-range ferroelectric order in the quenched state. The strain contributions from each component are calculated, leading to the conclusion that the total macroscopic strain arises predominantly from the transformed shell after quenching. Such observations are also complemented by investigations of microstructure and electrical properties, including ferroelectric behaviour and temperature-dependent dielectric properties

Heterogeneous grain-scale response in ferroic polycrystals under electric field

Understanding coupling of ferroic properties over grain boundaries and within clusters of grains in polycrystalline materials is hindered due to a lack of direct experimental methods to probe the behaviour of individual grains in the bulk of a material. Here, a variant of three-dimensional X-ray diffraction (3D-XRD) is used to resolve the non-180?? ferroelectric domain switching strain components of 191 grains from the bulk of a polycrystalline electro-ceramic that has undergone an electric-field-induced phase transformation. It is found that while the orientation of a given grain relative to the field direction has a significant influence on the phase and resultant domain texture, there are large deviations from the average behaviour at the grain scale. It is suggested that these deviations arise from local strain and electric field neighbourhoods being highly heterogeneous within the bulk polycrystal. Additionally, the minimisation of electrostatic potentials at the grain boundaries due to interacting ferroelectric domains must also be considered. It is found that the local grain-scale deviations average out over approximately 10-20 grains. These results provide unique insight into the grain-scale interactions of ferroic materials and will be of value for future efforts to comprehensively model these and related materials at that length-scaleopen

Revealing the role of local stress on the depolarization of BNT-BT-based relaxors

Canonical relaxors exhibit an electric-field-induced phase transition between a macroscopically nonpolar and polar phase that can be tuned from being stable at low temperature to being reversible at high temperature. The reversibility of this phase change determines the electromechanical performance and large strains can be achieved if the polar phase is intrinsically unstable. This paper is on the thermal depolarization characteristics of a BNT-BT-based multiphase relaxor ceramic observed through the transition temperature from field-induced polar to nonpolar state. It is shown that the progress of detexturization strongly depends on the crystallographic phase. In the more susceptible phase, it becomes significant about 40 °C below the macroscopically observed transition temperature. Additionally, the surface domain structure vanishes at lower temperatures than expected from both dielectric and structural measurements. The development of strong interfacial stresses aiding depolarization, and a mismatch in chemical pressure between surface and bulk, are discussed as the origins for the observed effects. Tailoring of interfacial stresses through chemical adaption of crystallographic phase fractions opens up a pathway to optimize the strain performance of actuator materials and can become a useful tool to stabilize metastable crystallographic phases as well as for property tuning in piezotronics, Mott insulators and multiferroics