Huixin Bao

Elastic, piezoelectric, and dielectric properties of Ba(Zr0.2Ti0.8)O3- 50(Ba0.7Ca0.3)TiO3 Pb-free ceramic at the morphotropic phase boundary

There is an urgent demand for high performance Pb-free piezoelectrics to substitute for the current workhorse, the lead zirconate titanate (PZT) family. Recently, a triple point (also tricritical point) type morphotropic phase boundary (MPB) in Pb-free Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 system has been reported that shows equally as excellent piezoelectricity as soft PZT at room temperature (Liu and Ren6). In the present study, we measured a full set of elastic, piezoelectric, and dielectric properties for the MPB composition, Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 (BZT-50BCT), by using a resonance method. The resonant method gives piezoelectric properties d33 = 546 pC/N, g33 = 15.3 × 10−3 Vm/N, electromechanical coupling factor k33 = 65%, and the elastic constant s33E = 19.7 × 10−12 m2/N, c33E = 11.3 × 1010 N/m2, which are close to the properties of soft PZT (PZT-5A). Furthermore, the piezoelectric coefficients (k33, d33), the ferroelectric properties (coercive field, remnant polarization), and the elastic ...

Microstructure basis for strong piezoelectricity in Pb-free Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3 ceramics

In this letter, we use transmission electron microscopy to study the microstructure feature of recently reported Pb-free piezoceramic Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 across its piezoelectricity-optimal morphotropic phase boundary (MPB) by varying composition and temperature, respectively. The domain structure evolutions during such processes show that in MPB regime, the domains become miniaturized down to nanometer size with a domain hierarchy, which coincides with the d33-maximum region. Further convergent beam electron diffraction measurement shows that rhombohedral and tetragonal crystal symmetries coexist among the miniaturized domains. Strong piezoelectricity reported in such a system is due to easy polarization rotation between the coexisting nano-scale tetragonal and rhombohedral domains.

Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3-x(Ba,Ca)TiO3 ceramics

We designed a Pb-free pseudo-binary system, Ba(Sn0.12Ti0.88)O3-x(Ba0.7Ca0.3)O3 (BTS-xBCT), characterized by a phase boundary starting from a tricritical triple point of a paraelectric cubic phase, ferroelectric rhombohedral, and tetragonal phases. The optimal composition BTS-30BCT exhibits a high piezoelectric coefficient d33 � 530 pC/N at room temperature. In view of the recent report of high piezoelectricity in another Pb-free system BZT-BCT (Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)), which possesses a similar tricritical triple point in the phase diagram, it seems that forming a suitable phase boundary starting from a tricritical triple point could be an effective way to develop high-performance Pb-free piezoelectrics. V C 2011 American Institute of Physics.

Triple-point-type morphotropic phase boundary based large piezoelectric Pb-free material—Ba(Ti0.8Hf0.2)O3-(Ba0.7Ca0.3)TiO3

We report a large piezoelectric Pb-free system—Ba(Ti0.8Hf0.2)O3-(Ba0.7Ca0.3)TiO3, designed upon a triple-point morphotropic phase boundary (TMPB) idea. The system shows anomalies of both ac (dielectric and piezoelectric) and dc (P-E hysteresis) properties around TMPB, especially the d33 achieving � 550 pC/N at room temperature. Moreover, the detected non-zero thermal hysteresis around triple point shows a first order transition nature and non-isotropic polarization state, as well as verified by our theoretical Landau-type deduction, thus being considered as an important factor to influence the piezoelectricity along TMPB. Our work may stimulate the study on triple-pointrelated critical phenomena in other ferroic systems. V C 2012 American Institute of Physics.

Contrasting aging behavior of as-grown and annealed BaTiO3 single crystals grown by KF-flux technique

Acceptor-doped ferroelectrics generally show a pronounced aging effect. However, the KF-flux-grown BaTiO3 single crystals, which are known to contain acceptor impurities K+, show no aging effect. This exception poses a potential challenge to the current understanding of ferroelectric aging and may provide a clue for reducing aging effect in acceptor-doped ferroelectrics. To solve this puzzle, we compared the ferroelectric and dielectric aging effects of the as-grown and annealed KF-flux-grown BaTiO3 single crystals. We found that although the as-grown BaTiO3 showed almost no aging effect, the annealed BaTiO3 showed clear aging effect. Chemical analysis revealed that the as-grown crystals contain not only acceptor impurity K+ but also higher concentration of anion donor F−; on the other hand, the annealed crystals contain mostly K+, as a result of F− evaporation. Based on defect chemistry analysis and the microscopic mechanism of the aging, the difference in the aging behavior of the two kinds of crystal i...

Aging effect in paraelectric state of ferroelectrics: Implication for a microscopic explanation of ferroelectric deaging

Most ferroelectric materials exhibit aging effect (a time-dependent change in physical properties) in their ferroelectric state; however, aging is not reported to exist in the paraelectric state. In this letter we report the existence of a “paraelectric aging effect” in Mn-doped (Ba0.80Sr0.20)TiO3 ceramics. We found that when the paraelectric state is formed from an aged ferroelectric state through a reverse ferroelectric transition, the paraelectric state shows a gradual increase in the dielectric permittivity and decrease in dielectric loss with time. Such paraelectric aging effect exists only in acceptor-doped samples, not in undoped samples. The kinetics of the paraelectric aging follows a simple relaxation function with activation energy of 0.43 eV. Our results suggest that the paraelectric aging stems from the migration of oxygen vacancies, being the same as the case of ferroelectric aging. We show that such a migration is driven by a symmetry-conforming short-range ordering tendency of point defect...

Aging-induced two-step ferroelectric-to-paraelectric transition in acceptor-doped ferroelectrics

It is well known that first-order ferroelectric to paraelectric transition is a one-step transition and it gives rise to one latent heat peak and one permittivity peak during heating. In the present paper, however, we report an unexpected finding; in Mn-doped Ba(Zr0.01Ti0.98Mn0.01)O3−δ the ferroelectric to paraelectric transition proceeds in two steps, after the sample has experienced a prior aging at the differential scanning calorimetry (DSC) peak temperature and followed by cooling down to a fully ferroelectric state. The two-step transition is evidenced by a split DSC peak as well as a split permittivity peak. These abnormal effects can be explained by considering that aging at DSC peak temperature corresponds to aging in a two-phase state (paraelectric+ferroelectric). Such two-phase aging produces two different defect symmetries, which give rise to different stabilities for the same ferroelectric phase and thus results in different reverse transition temperatures.

A quantitative model for stabilization effect induced by ferroelectric aging

The stabilization effect, an increase of the reverse transition temperature TC with ferroelectric aging time, was recently found in acceptor doped ferroelectrics. Such an effect has found a microscopic explanation in the symmetry-conforming principle of point defects, which also accounts for a similar stabilization effect in ferroelastic/martensite materials. However, such a microscopic model is difficult to use to make a quantitative/semiquantitative evaluation of the aging process. In the present work, we propose a single-domain thermodynamic model that provides a semiquantitative fit to the experimentally observed stabilization kinetics. Furthermore, it predicts a new aspect of ferroelectric aging: the decrease of latent heat ΔH of the reverse transition with aging time. Such a prediction was confirmed via experimental differential scanning calorimetry results.

Control of Ferroelectric Aging by Manipulating Point Defects

In the present work, we systematically studied the aging effect of both ferroelectric single crystals and polycrystals to study how to control aging effect. We found that such hybrid-doped BaTiO3 single crystals showed obvious aging effect only when the concentration of the acceptor K+ is higher than that of the donor; when the donor F− concentration is higher than the acceptor K+, the K-F doped BaTiO3 showed almost no aging effect. Similar results were obtained in La-Mn doped BaTiO3 polycrystals. With the increase of the concentration of donor La3+ in 0.5%Mn-BaTiO3 ceramics, the aging effect gradually becomes weaker and finally disappears. Our result suggests that donor doping is an effective way of controlling the aging effect.

In situ observation of thermally activated domain memory and polarization memory in an aged K + -doped (Ba, Sr)TiO3 single crystal

Different ferroelectric domains are degenerate states of the same ferroelectric phase; thus they are energetically equivalent and, in principle there exists no preference for a particular domain pattern. However, the existence of point defects is considered to stabilize certain preferential domain states. In order to study the temperature violation on such stabilized domains, we performed in situ observation on an aged K⁺-doped (Ba, Sr)TiO₃ single crystal and found that both the domain configuration and polarization state can be memorized after experiencing a thermally activated ferro-para-ferro transition cycle, as manifested by a reappearance of the same domain pattern and double P-E hysteresis loop. In contrast, after the sample was aged in the paraelectric state (>10 min), these memory effects disappeared. The above memory effects are considered to originate from the interaction between point defects and the crystal symmetry driven by a symmetry-conforming tendency of point defects. Such a mechanism suggests that the memory effects are relevant to the existence of acceptor dopant and associated mobile oxygen vacancies, and they are not restricted to a particular dopant. Thus similar memory effects are expected to exist in a wide range of ferroelectric materials with acceptor doping.