65 research outputs found
Using Infrared Spectroscopy to Identify New Amorphous Phases - A Case Study of Carbonato Complex Formed by Mechanochemical Processing
Piezoelectric nonlinearity and frequency dispersion of the direct piezoelectric response of BiFeO3 ceramics
We report on the frequency and stress dependence of the direct piezoelectric
d33 coefficient in BiFeO3 ceramics. The measurements reveal considerable
piezoelectric nonlinearity, i.e., dependence of d33 on the amplitude of the
dynamic stress. The nonlinear response suggests a large irreversible
contribution of non-180{\deg} domain walls to the piezoelectric response of the
ferrite, which, at present measurement conditions, reached a maximum of 38% of
the total measured d33. In agreement with this interpretation, both types of
non-180{\deg} domain walls, characteristic for the rhombohedral BiFeO3, i.e.,
71{\deg} and 109{\deg}, were identified in the poled ceramics using
transmission electron microscopy (TEM). In support to the link between
nonlinearity and non-180{\deg} domain wall contribution, we found a correlation
between nonlinearity and processes leading to deppining of domain walls from
defects, such as quenching from above the Curie temperature and
high-temperature sintering. In addition, the nonlinear piezoelectric response
of BiFeO3 showed a frequency dependence that is qualitatively different from
that measured in other nonlinear ferroelectric ceramics, such as "soft"
(donor-doped) Pb(Zr,Ti)O3 (PZT); possible origins of this dispersion are
discussed. Finally, we show that, once released from pinning centers, the
domain walls can contribute extensively to the electromechanical response of
BiFeO3; in fact, the extrinsic domain-wall contribution is relatively as large
as in Pb-based ferroelectric ceramics with morphotropic phase boundary (MPB)
composition, such as PZT. This finding might be important in the search of new
lead-free MPB compositions based on BiFeO3 as it suggests that such
compositions might also exhibit large extrinsic domain-wall contribution to the
piezoelectric response.Comment: 38 pages, 11 figure
Electric-field-induced non-ergodic relaxor to ferroelectric transition in BiFeO3-xSrTiO3 ceramics
While BiFeO3-based solid solutions show great promise for applications in
energy conversion and storage, realizing this promise necessitates
understanding the structure-property relationship in particular pertaining to
the relaxor-like characteristics often exhibited by solid solutions with
polar-to-non-polar morphotropic phase boundaries. To this end, we investigated
the role of the compositionally-driven relaxor state in (100-x)BiFeO3-xSrTiO3
[BFO-xSTO], via in situ synchrotron X-ray diffraction under bipolar
electric-field cycling. The electric-field induced changes to the crystal
structure, phase fraction and domain textures were monitored via the {111}pc,
{200}pc, and 1/2{311}pc Bragg peaks. The dynamics of the intensities and
positions of the (111) and (11-1) reflections reveal an initial non-ergodic
regime followed by long-range ferroelectric ordering after extended poling
cycles. The increased degree of random multi-site occupation in BFO-42STO
compared to BFO-35STO is correlated with an increase of the critical electric
field needed to induce the non-ergodic-to-ferroelectric transition, and a
decrease in the degree of domain reorientation. Although both compositions show
an irreversible transition to a long-range ferroelectric state, our results
suggest that the weaker ferroelectric response in BFO-42STO is related to an
increase in ergodicity. This, in turn, serves to guide the development of
BFO-based systems into promising platform for further property engineering
towards specific capacitor applications
Frequency-dependent decoupling of domain-wall motion and lattice strain in bismuth ferrite
Dynamics of domain walls are among the main features that control strain mechanisms in ferroic materials. Here, we demonstrate that the domain-wall-controlled piezoelectric behaviour in multiferroic BiFeO3 is distinct from that reported in classical ferroelectrics. In situ X-ray diffraction was used to separate the electric-field-induced lattice strain and strain due to displacements of non-180° domain walls in polycrystalline BiFeO3 over a wide frequency range. These piezoelectric strain mechanisms have opposing trends as a function of frequency. The lattice strain increases with increasing frequency, showing negative piezoelectric phase angle (i.e., strain leads the electric field), an unusual feature so far demonstrated only in the total macroscopic piezoelectric response. Domain-wall motion exhibits the opposite behaviour, it decreases in magnitude with increasing frequency, showing more common positive piezoelectric phase angle (i.e., strain lags behind the electric field). Charge redistribution at conducting domain walls, oriented differently in different grain families, is demonstrated to be the cause
BiFeO3 CERAMICS DENSIFICATION STUDY
Bismuth ferrite (BiFeO3) is one of the very few materials possessing ordered both
electrical dipoles and magnetic moments in very wide range of temperatures. That gives
it tremendous opportunities for research and application.
BiFeO3 precursor powders were prepared by auto-combustion and soft chemical
synthesis methods using different organic compounds as fuels and complexing agents.
They were calcined at 600 °C for 2 h, grinded, pressed and sintered at 800 °C for 1 h
with quenching. XRD and SEM confirmed formation of very pure BiFeO3 perovskite
phase, but problem with low density, which is characteristic for BiFeO3 ceramics, still
remained. It doesn’t affect magnetic properties, but has strong influence on electrical
and, through them, on multiferroic properties. To define actual, intrinsic properties of
material, it is very important to obtain ceramics with densities close to theoretical. For
this reason, many treatments were tried in order to raise samples’ densities: powder
milling and homogenization, ultrasound, different pressures, various thermal treatments.
Powders and ceramic samples were then characterized (particle size analysis, XRD,
SEM). The densities of sintered BiFeO3 samples were measured. All the treatments had
impact on ceramics density, but some also introduced secondary phases
Pressure control of nonferroelastic ferroelectric domains in ErMnO3
Mechanical pressure controls the structural, electric, and magnetic order in solid-state systems, allowing tailoring of their physical properties. A well-established example is ferroelastic ferroelectrics, where the coupling between pressure and the primary symmetry-breaking order parameter enables hysteretic switching of the strain state and ferroelectric domain engineering. Here, we study the pressure-driven response in a nonferroelastic ferroelectric, ErMnO3, where the classical stress–strain coupling is absent and the domain formation is governed by creation–annihilation processes of topological defects. By annealing ErMnO3 polycrystals under variable pressures in the MPa regime, we transform nonferroelastic vortex-like domains into stripe-like domains. The width of the stripe-like domains is determined by the applied pressure as we confirm by three-dimensional phase field simulations, showing that pressure leads to oriented layer-like periodic domains. Our work demonstrates the possibility to utilize mechanical pressure for domain engineering in nonferroelastic ferroelectrics, providing a lever to control their dielectric and piezoelectric responses
Multifunctional Cantilevers as Working Elements in Solid-State Cooling Devices
Despite the challenges of practical implementation, electrocaloric (EC) cooling remains a
promising technology because of its good scalability and high efficiency. Here, we investigate the
feasibility of an EC cooling device that couples the EC and electromechanical (EM) responses of a
highly functionally, efficient, lead magnesium niobate ceramic material. We fabricated multifunctional
cantilevers from this material and characterized their electrical, EM and EC properties. Two active
cantilevers were stacked in a cascade structure, forming a proof-of-concept device, which was then
analyzed in detail. The cooling effect was lower than the EC effect of the material itself, mainly
due to the poor solid-to-solid heat transfer. However, we show that the use of ethylene glycol in
the thermal contact area can significantly reduce the contact resistance, thereby improving the heat
transfer. Although this solution is most likely impractical from the design point of view, the results
clearly show that in this and similar cooling devices, a non-destructive, surface-modification method,
with the same effectiveness as that of ethylene glycol, will have to be developed to reduce the thermal
contact resistance. We hope this study will motivate the further development of multifunctional
cooling devices
Unexpectedly high piezoelectricity of Sm-doped lead zirconate titanate in the Curie point region
Large piezoelectric coefficients in polycrystalline lead zirconate titanate (PZT) are traditionally achieved through compositional design using a combination of chemical substitution with a donor dopant and adjustment of the zirconium to titanium compositional ratio to meet the morphotropic phase boundary (MPB). In this work, a different route to large piezoelectricity is demonstrated. Results reveal unexpectedly high piezoelectric coefficients at elevated temperatures and compositions far from the MPB. At temperatures near the Curie point, doping with 2 at% Sm results in exceptionally large piezoelectric coefficients of up to 915 pm/V. This value is approximately twice those of other donor dopants (e.g., 477 pm/V for Nb and 435 pm/V for La). Structural changes during the phase transitions of Sm-doped PZT show a pseudo-cubic phase forming ≈50 °C below the Curie temperature. Possible origins of these effects are discussed and the high piezoelectricity is posited to be due to extrinsic effects. The enhancement of the mechanism at elevated temperatures is attributed to the coexistence of tetragonal and pseudo-cubic phases, which enables strain accommodation during electromechanical deformation and interphase boundary motion. This work provides insight into possible routes for designing high performance piezoelectrics which are alternatives to traditional methods relying on MPB compositions
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