Two-level hierarchical stripe domains and enhanced piezoelectricity of rapid hot-press sintered BiFeO
3ceramics
S. H. Zheng,1Z. W. Li,2C. X. Zhang,1Y. Q. Li,1L. Lin,1Z. B. Yan,1X. H. Zhou,1 Y. P. Wang,3X. S. Gao,2and J.-M. Liu1,2
1Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
2Institute for Advanced Materials, South China Normal University, Guangzhou 510006, China
3College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
(Received 9 September 2018; accepted 30 October 2018; published online 20 November 2018) BiFeO3represents the most extensively investigated multiferroic due to its fascinating ferroelectric domain structures, large polarization, and multiferroic coupling, among many other emergent phe- nomena. Nevertheless, much less concern with the piezoelectricity has been raised while all these well addressed properties are identified in thin film BiFeO3, and bulk ceramic BiFeO3 has never been given priority of attention. In this paper, we report our experiments on the ferroelectric and pie- zoelectric properties as well as domain structures of BiFeO3bulk ceramics synthesized by rapid hot- press sintering. It is revealed that these properties are strongly dependent on the microstructural quality, and the largest piezoelectric coefficientd33= 55 pC/N with electric polarization as large as 45μC/cm2is obtained for the sample sintered at 800 °C, while they are only 30 pC/N and 14μC/cm2 for the samples sintered in normal conditions at 800 °C. The two-level hierarchical stripe-like and irregular dendrite-like domain structures are observed in these hot-press sintered samples. It is sug- gested that the enhanced piezoelectric property is ascribed to the two-level hierarchical stripe-like domain structure which may respond more easily to electrical and strain stimuli than those irregular dendrite-like domains. The enhanced remnant polarization should be owing to the improved sample quality and large grains in the properly hot-press sintered samples.Published by AIP Publishing.
https://doi.org/10.1063/1.5055688
I. INTRODUCTION
It is known that all ferroelectrics are piezoelectrics since ferroelectrics have the polar lattice symmetry and allow the polarization switching induced lattice distortion. In fact, these best piezoelectrics are usually from the ferroelectric family such as Pb(Zr,Ti)O3, Pb-based relaxors, and recently concerned Pb-free ferroelectrics.1–3 These facts leave the impression that a good ferroelectric should be a good piezo- electric too, and this argument canfind some seemly reason- able physical origins.4On the one hand, well-aligned electric dipoles for a ferroelectric should benefit a directional strain under the electric field. On the other hand, ferroelectric domains and their switchable character driven by electric and strain field can be a major ingredient for generating a large piezoelectric and electrostrictive effect.5,6As a crude approx- imation, here we intend not to make a distinction between piezoelectric strain and electrostrictive strain.
Nevertheless, this argument becomes questionable when one considers BiFeO3(BFO), one of the best and most exten- sively investigated ferroelectric/multiferroic materials.7–9 In the name of multiferroicity, BFO has been thoroughly dis- cussed both theoretically and experimentally, while the most impressive properties are more relevant to ferroelectricity rather than multiferroicity. It is known that BFO in well- epitaxial thinfilm form shows the excellent electric polariza- tion as large as 100μC/cm2,7and the super-tetragonal BFO thinfilms offer a polarization up to 150μC/cm2.10The BFO thinfilms also accommodate fascinating ferroelectric domain
structures, and the 71°, 109°, and 180° domain walls as well as their combinations allow rich and novel functionalities which are unavailable in other typical ferroelectrics.11–14 Because of the leading position of BFO thinfilms, relatively less attention to BFO ceramics15and single crystals has been paid.16It is clear that piezoelectricity of a material as an elec- tromechanical coupling is more useful in bulk form, and this is the reason why the piezoelectricity of BFO is more or less out of researchers’view.2,8
On the other hand, an important concept for achieving good piezoelectric properties in a ferroelectric is the so-called morphotropic phase boundary (MPB) which defines a phase boundary in the phase diagram, at which two or more ferroelectric phases coexist and act coherently in response to external stimuli.17,18 Around the MPB, a ferroelectric is expected to have giant piezoelectric response due to the small-size coexisting ferroelectric phases and easily driven domain wall motion. In other words, small-size domains and thus easy motion of domain walls can be beneficial to piezoelectric response, which is similar phenomenologi- cally to the scenario of ferroelectric relaxors with giant piezoelectricity.19,20
This concept seems inapplicable to BFO since there is no such MPB available. Indeed, several approaches as stimu- lated by this concept have been attempted and one successful example is the reported giant piezoelectric coefficientd33 as large as 300 pC/N in BFO + BaTiO3 solid solution where MPB-like phase boundaries can be accessed.21,22 If one
0021-8979/2018/124(19)/194104/9/$30.00 124, 194104-1 Published by AIP Publishing.
comes back to pure BFO itself either in a bulk or thin film form, its piezoelectric property is far from concerned with its ferroelectric and magnetoelectric behaviors. The pure bulk ceramic BFO exhibits a d33 value as small as 20–30 pC/N, much smaller than the data for most commonly concerned piezoelectrics (102–103pC/N). For BFO thinfilms deposited on various substrates, the measured d33 values, remarkably sample-dependent, are 20–40 pC/N in most cases and occa- sionally as large as 80–100 pC/N.23–26 These values are simply non-attractive for advanced piezoelectric actuation and sensing devices, in particular for thin films because of the small absolute spatial-displacement due to the small thickness.
On the other hand, if the piezoelectricity of BFO bulk ceramics can be enhanced up to an acceptable level, together with good ferroelectricity and other functionalities, relevant potential application may be promising. In such a sense, for the weak piezoelectricity in ferroelectricity, excellent BFO remains to be an issue. Nevertheless, it should be mentioned that BFO is a ferroelectric with rich domain variants and structures11 which would allow a possibility for engineering the domain structure so that an enhanced piezoelectricity may be expected.
Far more than this, different from BFO thinfilms which now can be well prepared with extremely high quality, BFO ceramics often suffer from microstructural deficiencies and most as-prepared BFO ceramic samples are loose in micro- structure and non-stoichiometric with impurity phases.27–29 These deficiencies make BFO ceramics low in electrical breakdownfield, large in leakage current, and mechanically fragile.29 Therefore, high-quality synthesis of BFO ceramic samples can be critical for accessing potentially good proper- ties of BFO bulk ceramics.
In this work, we intend to investigate the piezoelectric and ferroelectric properties of BFO ceramic samples prepared by the rapid hot-press sintering method. Hot-press sintering is not a new sintering method and similar methods such as rapid liquid sintering have been extensively utilized for synthesizing high-quality BFO ceramics, powders, and targets.27,30Our motivation is twofold: on the one hand, how large the piezoelectric coefficient d33 is for a high-quality BFO ceramic sample and on the other hand, one is inter- ested in the ferroelectric domain structure and its correspon- dence with the piezoelectric and ferroelectric properties of BFO ceramics.
II. SAMPLE PREPARATION AND CHARACTERIZATION Our BFO ceramic samples were prepared using the rapid hot-press sintering method which is a popular technique for high-quality synthesis.27 It is known that the sintering tem- perature window for BiFeO3is quite narrow, and otherwise, high density of oxygen vacancies and secondary phases such as Bi2Fe4O9and Bi25FeO39become inevitable.31–33A quick heating-up to a sufficiently high temperature in several minutes seems to be the best way to suppress the formation of impurity phase and lattice defects.
The high-purity powder of Bi2O3 and Fe2O3 ( purity >
99.9%) was mixed for a series of samples with the assigned
nominal composition of BixFeyO3 (x + y = 2.00). These powder reagents were thoroughly mixed and ground in an agate mortar for about 4–5 h. The powder precursor was then dried and sintered at 800 °C for 1 h so that sufficiently reac- tive product was obtained in the powder form for subsequent structural characterization and ceramic sintering.
For comparison, we employ the normal sintering (furnace sintering in air ambient) and rapid hot-press sinter- ing to prepare the ceramic samples. For the former, the powder precursor was then dried and pressed under a hydro- static pressure of 70 MPa into pellets of 10 mm in diameter and 5.0 mm in thickness. These pellets were sintered at 800 °C for 1 h in a tubular furnace and taken out immediately to room temperature. For the rapid hot-press sintering, the powder in weight of 3.5 g each was put into a graphite cylin- der mold, and a hydrostatic pressure of 10 MPa was applied to the sample over the whole sintering period. The DC current was applied to the mold so that the mold can be heated up to an assigned temperatureTSbetween 650 °C and 850 °C in 3 min, and the sample was annealed at this TSfor 5 min before rapid cooling down to room temperature. The as-prepared ceramic samples have the cylinder-like shape.
During the annealing period of hot-press sintering, the DC current as a whole passed through the sample along the axial direction. The as-prepared ceramic cylinders were then cut into thin pellets ( plates) and polished for subsequent charac- terizations. It is noted that these as-prepared samples have sufficiently high-quality for obtaining very smooth plate surfaces for subsequent characterizations.
We performed a series of structural and physical charac- terizations on the as-prepared samples. The samples’volume density was measured using the conventional Archimedes method. The crystal structure was checked by the X-ray dif- fraction (XRD) in the θ-2θ mode using the Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ= 1.5406 Å). The microstructure and chemical composition were imaged and probed using the scanning electron micros- copy (SEM) with the energy dispersive X-ray spectroscopy (EDX) unit.
The samples for electrical measurements are plates of 5.0 mm (sometimes 3.0 mm) in diameter and 0.1 mm in thickness with well-polished smooth surfaces. For the dielec- tric, ferroelectric, and piezoelectric measurements, the Au bottom and top electrodes were prepared using the sputtering technique so that a plate capacitor mode can be used. The dielectric constant was measured by the HP4294A impedance analyzer using anacsignal of 50 mV in magnitude. The ferro- electric hysteresis was detected using the conventional Saywer-Tower tester. The piezoelectric coefficient d33 was probed using the standard piezo-tester (ZJ-3, Institute of Acoustics, CAS). In particular, the topographic images were measured by the atomic force microscopy (AFM) at the contact mode (Cypher, Asylum Research).30The ferroelectric domain structure was characterized by the piezo-force micros- copy (PFM) (Asylum Cypher) using the conductive probes (ArrowEFM, NanoWorld). We adopted a dual-frequency resonance-tracking technique (Asylum Research). The vector PFM function of our AFM unit allows simultaneous mapping of the out-of-plane (OP) and in-plane (IP) signals.34,35
It should be mentioned that all the electrical measure- ments were performed on the plate capacitors in which the out-of-plate direction is along the axial direction of cylinder samples. Just for a comparison study on the sample’s struc- ture anisotropy, a plate capacitor with the out-of-plate direc- tion along the sample’s radial direction was prepared and measured.
III. RESULTS AND DISCUSSION A. Microstructure
Wefirst look at the crystallinity of the as-prepared powder and hot-press samples, and the XRDθ-2θspectra of several representative samples are shown in Figs.1(a)and1(b). In our sintering condition, the powder with excess Bi or Fe does have an impurity phase. For example, Bi1.05Fe0.95O3 con- tains the excess Bi2O3phase (marked by empty dots) and Bi0.95Fe1.05O3has Bi2Fe4O9impurity (marked with stars).27,36,37 Because we sinter the powder at a relatively low tempera- ture for a short time, it seems that the stoichiometric BiFeO3 powder does not contain much impurity although tiny Bi2Fe4O9(marked with arrows) can still be identifiable in the resolution limit.
Our characterizations showed that the rapid hot-press sintering of the powder samples cannot completely remove these impurity phases while the main consequence is to make the ceramic samples highly dense. Since the hot-press sintered samples are cylinder-like in shape with the pressure applied along the cylinder-axis, the microstructural texture should be checked. The XRD spectra of the hot-press sin- tered BiFeO3 samples as probed from two directions, one along the axis, and one along the radial, are plotted in Fig.1(b). It is seen that no strong lattice texture structure has been shown and the spectra are nearly identical to that of the BiFeO3powder.
The most concerned issue here is the microstructural quality of the as-prepared samples. As representative cases, the SEM images for fresh cracked surface morphologies of a series of samples are shown in Fig. 2where the values ofTS and relative volume density ρ are listed below each image.
For reference, the normal sintered sample’s image (Ref.) is also inserted, denoted as the reference sample. In comparison with earlier results,34 it is seen that the microstructure of our samples is much denser and more compact. While it is noted that these samples show nearly no difference in the XRD spectrum, the microstructure becomes quite different. Indeed, all these hot-press sintered samples show fine and compact grains in the low-amplified image, but the reference sample is loose and has high density of holes.36The high-amplified images show that the grains in the reference sample are not compact. The sample sintered at TS= 650 °C remains to be loose also and not yet ceramic, while the sample sintered at TS= 700 °C shows quite good ceramic quality but high density of holes can be seen. The number of holes in the TS= 750 °C sintered sample is much less and the 800 °C sintered sample is completely free of any hole with well- developed and compact grains, as reflected by the volume density data. In addition, for all these samples, a statistics on the grain geometry does not give a sufficient significance of grain shape anisotropy. For the hot-press sintering, it is known that the DC currentflow prefers more along the grain boundaries than through the grain interiors, since the bound- aries would be more conductive at high sintering tempera- ture. For this reason, the grain shape should be slightly stretched along the cylinder axial direction.
It is found that the 850 °C sintered sample is also free of any hole with dense microstructure. However, if one looks carefully at the cracked surface in the low-amplified image, it is seen that the 800 °C sintered sample is facilitated with clear grain boundaries, while the 850 °C sintered sample has a lot of trans-grained features, suggesting that the 850 °C sintered sample is over-sintered with partial melting.
It is thus believed that the 800 °C hot-press sintered sample has the highest microstructural quality with the largest volume density.
In addition, we also image the spatial distribution of chemical composition using the EDX probe with SEM. The results for two representative samples, the reference sample and the 800 °C sintered sample, are presented in Fig.3. It is seen that the Bi3+and Fe3+ions are uniformly distributed in every grain for both samples. The reference sample has its composition to slightly deviate from the stoichiometric ratio
FIG. 1. The XRD θ-2θ spectra of BFO powder and ceramic samples, showing the dominant rhombohedral structure. (a) The powder samples with different nominal compositions are marked. The Bi2O3and Bi2Fe4O9impuri- ties are marked too. (b) The 800 °C rapid hot-press sintered BFO ceramic sample and the two curves are the XRD data probed along the cylinder axis and radial directions. The standard BFO spectrum is aligned for reference.
while this deviation is basically within the measuring uncer- tainty for the 800 °C sintered sample.
B. Electrical properties
Now, we check the electrical properties of the hot-press sintered samples. First, the dielectric frequency-spectra at room temperature are presented in Figs.4(a)and4(b). With respect to the reference sample, the hot-press sintered samples show gradually enhanced dielectric constant with increasingTS up to 800 °C, obviously due to the enhanced microstructural quality. This is confirmed by the gradually reduced dielectric loss which can be as low as 0.05 over the whole frequency range for the 800 °C sintered sample. This value is extremely low for BiFeO3 ceramics.36,37 However, this loss becomes abnormally large for the 850 °C sintered sample particularly in the low-frequency range, suggesting that the sample exhibits large leakage which is also the reason for the large dielectric constant. Such large leakage is probably generated by the high defect density in the sample
which is over-sintered. In the subsequent discussion, the data from this sample will no longer be included.
The room temperature ferroelectric (P-E) hysteresis loops for these samples, measured along the axial direction of the cylinder samples, are plotted in Fig. 5(a). Well-saturated P-E loops with well-defined saturated polarization PS, remnant polarizationPr, and coercive fieldEcare observed. These fer- roelectric properties are remarkably enhanced with increasing TS, while the reference sample has the worst properties. The PS for the 800 °C sintered sample is as large as 45μC/cm2, which is among the largest values for BiFeO3 ceramics reported so far. The coercive field is also reasonable. These results demonstrate that the rapid hot-press sintered samples do have excellent microstructural quality.
In addition, we also check the difference in the P–E loops between two plate capacitors. The two plates were cut from the same 750 °C sintered sample. One plate has its out-of-plate direction along the cylinder axis and the other along the cylinder radial direction. The two measured loops are plotted in Fig.5(b)to check the possible loop anisotropy.
FIG. 2. The SEM images of fresh cracked surface for a set of BFO samples sintered by the rapid hot-press sintering method at different TS’s as labeled below each image. The relative volume density ρ defined by the sample’s mass divided by the ideal BFO lattice mass is inserted too. The image of the reference sample (Ref.) is aligned for comparison.
It is seen that the two loops are indeed different from each other, and the difference inPr is∼30%, indicating the P-E loop anisotropy. A reason for this anisotropy may probably be from the grain shape anisotropy developed during the sintering. It is believed that a large grain would favor a large polarization. As discussed in Sec.III A, the large DC
current during the hot-press sintering passed through the sample along the axis direction and a major part of the current flew along the grain boundaries. For this reason, the grain boundaries may have a slight preference to align along the axial direction, although no statistically significant evidence for such a grain shape anisotropy from the SEM images is found.
We pay more attention to the piezoelectric d33 data of these samples and the results are shown in Fig.5(c)together with the data ofPS,Pr, andEc. It is identified clearly that the piezoelectricd33coefficient exhibits remarkable enhancement with increasing TS, reaching up to 55 pC/N for the 800 °C sintered sample in comparison with the reference sample which only has ad33value of 30 pC/N, marking an enhance- ment of 80%. This value, in comparison with those excellent piezoelectric systems, is somehow low but it is good enough to be appreciated for pure BiFeO3ceramics. Ad33∼55 pC/N is also comparable with most of those best reported values for high-quality BFO thin films,23–26 implying that the hot- press sintered samples have high structural quality and prop- erly aligned domain structure for piezoelectric response, as revealed in those epitaxial BFO thin films with comparable piezoelectricd33. Our results simply imply the potential for a promising piezoelectric enhancement in a less-concerned system by structural improvement.
C. Domain structure
Certainly, such a piezoelectric enhancement should most likely originate from the specific domain structure and high sample quality. This claim can be partially supported by the roughly linear TS—dependence ofd33which is similar to the linear TS—dependences of PS andPr too, since the magni- tude ofPSandPris largely determined by the domain struc- ture. It should be mentioned that the dependence of piezoelectricity on domain structure in BiFeO3bulk ceramics has been rarely touched, mainly due to the fact that the
FIG. 3. The SEM images of the fresh cracked surface for two samples and the corresponding EDX mapping data for elements Bi and Fe. The top row shows the data for the reference sample and the bottom row shows the data for the 800 °C hot-press sintered sample.
FIG. 4. The room temperature dielectric constantϵand loss tanδas a func- tion of the measuring ac signal frequencyf, respectively, for a set of samples sintered at differentTS’s are marked. The data for the reference sample (Ref.) are also inserted for comparison.
as-sintered ceramic sample may not have enough good quality for PFM probing. For high-quality PFM imaging, the sample should be cut and polished into sufficiently thin plate (0.1 mm in thickness) with very smooth surface. In our experiments, the rapid hot-press sintered samples do have enough good quality for high quality PFM mapping of the domain structure.
Unfortunately, we have not yet successfully obtained high-quality PFM images for the reference sample sintered in the normal ambient without hot-press, and then we only discuss the hot-press sintered samples. A fast scanning of these samples prepared at differentTS’s shows that a higher TS sintering leads to larger grain size and thus larger domain size. For illustrating the domain size, we present in Fig. 6 the fast-scanned PFM images of two representative samples sintered at TS= 800 °C and 700 °C. As shown by the out-of-plane (OP) amplitude (OP amp), OP phase (OP phase), and in-plane phase (IP phase) images, the 800 °C
sintered sample (image in the top row) has its average grain size as large as 10–15μm, and the domains are large too with typical coarse stripe-like pattern. Observations on such a stripe-like domain structure, not many, in BFO ceramics have recently been reviewed.38 Furthermore, one can find very fine features inside the coarse stripes which deserves for additional investigation to be shown below. On the other hand, the 700 °C sintered sample has much smaller grain size as shown in the bottom row images. The average grain size is 2–5μm only. Most of the grains are single- domained and occasionally stripe-like domain structure can be identified in those slightly coarse grains. Inside some grains one can also see thefine features.
Subsequently, we focus on the fine features using the high-resolution PFM scanning. In the overall sense, these fine features are small domains embedded in the matrix of large domains. These embedded small domains show differ- ent characters in the large grains and small grains. As men- tioned earlier, the large grains usually accommodate the coarse stripe-like domain structure. Inside each coarse stripe, one also sees the fine and stripe-like domain structure, as shown in Fig. 7(a) where the OP amp, OP phase, IP amp, and IP phase images are aligned. In the other words, the stripe-like domain structure is two-level hierarchical, and each coarse stripe exhibits the fine stripe-like structure with an inter-stripe width of 200–500 nm, while the coarse stripes can be as wide as 2–4μm. Such a two-level hierarchical stripe-like domain structure may be often observed in normal ferroelectrics such as Pb(Zr,Ti)O3 and BaTiO3, but it has rarely been identified in BFO bulk ceramics. The formation of the two-level hierarchical stripe-like domain structure should be a compromise of multi-fold interactions and thus a sensitive response to external stimuli can be expected. This issue deserves for further investigation but the high response sensitivity gives a reasonable explanation of the large piezo- electric response in samples with such structures.
However, inside those small grains which are single- domained, the sub-domain structure is irregular and dendrite- like rather than stripe-like, as shown in Fig. 7(b)with the OP and IP images, which has never been observed earlier. On the one hand, such irregular dendrite-like domain structure is unusual and less observable in other ferroelectrics, deserving for additional discussion. On the other hand, such dendritic structure leaves us a strong impression that the domain struc- ture is not yet well-developed in the sintering process, and probably large internal-strain or meta-stable structure exists with these domains. These reasons may be responsible for the less-pronounced piezoelectric response.
Here, it should be mentioned that one cannot identify these domain structures in quantitative manner, i.e., we cannot determine which type of these domain walls belong to: 71°, 109°, or 180° walls. The reason lies in two aspects.
On the one hand, the samples are polycrystalline, and we cannot determine the crystallographic orientations of these grains. When our sample is 0.1 mm in thickness, the depth direction under the PFM tip would include ∼10 grains at least, and each grain may have its own orientation. Certainly, the PFM imaging signals are largely from the surface grain touching the PFM tip, but the contributions, or say,
FIG. 5. (a) The room temperature ferroelectric hysteresis (P–E) loops for a set of samples sintered at differentTS’s are marked. The data for the refer- ence sample (Ref.) are also inserted for comparison. All the loops were mea- sured with the electricfield applied along the axial direction. (b) TheP–E loops for the sample sintered atTS= 750 °C with the electricfield applied along the axial and radial directions, respectively. (c) The evaluated saturated polarizationPS, remnant polarizationPr, coercivefieldEc, and piezoelectric coefficientd33as a function ofTS, respectively. Here, the data from the refer- ence (Ref.) sample are inserted for comparison.
perturbations, from the inner grains cannot be negligible.
This is probably the reason why we failed to quantitatively determine the details of domain structure for our bulk samples, i.e., we cannot determine the type of observed domain walls: 71°, or 109°, or 180° here. On the other hand, the samples for PFM observations are still too thick for domain switching. The coercivefieldEcfor these samples is
∼60 kV/cm, as shown in Fig.5(c), implying that the PFM tip voltage should be larger than 600 V in order to switch these domains, while the largest tip voltage in our experiments is 10 V. Fortunately, such a 10 V voltage seems to be sufficient for imaging the domain structure.
Based on the domain structure characterizations, one is now able to give a qualitative discussion on the larger d33 values in the samples sintered at higher TS (excluding TS> 800 °C).
The two-level hierarchical domain structure as shown in
Fig.7(a)is no doubt beneficial to coherent domain switching driven by electric field and thus enhancedd33. In particular, thefine stripe-like domain walls inside the coarse stripes, i.e., the two-level hierarchical domain structure, can be easily driven to move, contributing to the domain alignment along the electric field. This scenario is somehow similar to the nano-domain scenario in those relaxors of large piezoelectric response.18,19 Nevertheless, this dynamics may not apply to understand the enhanced ferroelectric polarization, since small domain size is definitely non-beneficial to obtain large Pr. The good ferroelectricity in the 800 °C sintered sample:
good P-Ehysteresis with large Pr, should be more related to the remarkably improved sample quality and large grain size.
However, for those samples sintered at low TS, e.g., 700 °C, the irregular dendrite-like domain structure should not be the most favored (optimized) for the coherent domain alignment
FIG. 6. The fast scanned PFM images for the samples sintered at (a)TS= 800 °C (top row) and (b)TS= 700 °C (bottom row), respectively.
Due to the fast scanning mode for large scale imaging, no details of the domain structure can be detected.
The“OP” and“IP” refer to the out- of-plane and in-plane modes, respec- tively, and“amp”and“phase”refer to PFM amplitude and phase signals, respectively.
FIG. 7. The high-resolution PFM images for the stripe-like domain structure inside a coarse grain (a) and irregular dendrite-like domain structure inside a small grain (b). For the stripe-like domain structure, it is clear that there appearfind stripe-like domain pattern inside each coarse stripe. We call such a structure as the two-level hierarchical stripe-like domain structure. The“OP”and“IP”refer to the out-of-plane and in-plane modes, respectively, and“amp”and“phase” refer to PFM amplitude and phase signals, respectively.
driven by electricfield, although the relatively small domain size is advantageous to the domain switching. Therefore, these samples may have neither very good ferroelectricity nor larged33, in comparison with the hierarchical stripe-like structure.
D. Discussion
Finally, we have some general remarks on the piezoelec- tricity of BiFeO3. As mentioned earlier, so far most of those good piezoelectric materials are from ferroelectrics with the BO6octahedron-displacive mechanism and the lattice dis- tortion is sensitive to external stimuli such as electric field or mechanical stress.39 For BiFeO3, the ferroelectricity is mainly due to the 6s lone-pair mechanism of Bi3+ ions at A-site, noting that the lone-pair mechanism of Pb2+ ions in those Pb-based ferroelectrics also contributes to the ferroelec- tric polarization.8,40 For PbTiO3-based systems, both the 6s lone-pair mechanism and the displacive-type TiO6octahedron distortion contribute to the polarization generation, making them the best piezoelectrics.40Unfortunately, for BiFeO3, the displacive mechanism does not work. Nevertheless, the rich domain structure can be a promising ingredient for good pie- zoelectricity, as demonstrated here in our experiments.
In such a sense, BiFeO3can be a good piezoelectric if the domain structure can be properly engineered on the one hand. So far reported d33 data on BFO bulk ceramics and thin films are yet insufficient for competitive applications with respect to Pb-based ferroelectrics and relaxors.
Nevertheless, BFO is one of the best Pb-free ferroelectrics in terms of polarization and multi-functionality including the multiferroicity. It is therefore desirable for enhancing its elec- tromechanical properties without scarifying others. Indeed, BiFeO3may be mixed with those ferroelectric titanates such as BaTiO3 so that the two mechanisms can facilitate coher- ently for remarkably enhanced piezoelectricity. The impor- tance of this work is an illustration of the critical role of the rich domain structure in BiFeO3 bulk ceramics, and an example of engineering the domain structure for such a target, which has however been less touched.
It is also informative to discuss the advantages and draw- backs of BFO bulk ceramics with respect to BFO thinfilms.
One sees that almost all reported electrical and magnetic properties of epitaxial thinfilms are much better than those of bulk ceramics. The d33 values from thin films are also comparable with the data reported here for our bulk ceramics.
Nevertheless, without doubt, bulk ceramics should be more favored for piezoelectric actuating and sensing applications than thinfilms, since the latters are clamped by the substrates on the one hand and they are very thin for generating suffi- cient spatial displacement for driving. Therefore, basically, the bulk ceramics should be favored for actuator and driver devices, while for high-density MEMS-based sensing, BFO thinfilms may be used if needed.
IV. CONCLUSION
In summary, we have prepared high-quality BiFeO3bulk ceramics using the rapid hot-press sintering technique and performed a series of structural and domain characterizations
together with the ferroelectric and piezoelectric properties. It has been revealed that in comparison with the normally sin- tered BiFeO3 ceramic sample, the rapid hot-press sintered samples are sufficiently dense and exhibit much improved dielectric, ferroelectric, and piezoelectric properties. The electric polarization can be as large as 45μC/cm2, while the piezoelectric coefficient d33 can be as large as 55 pC/N for the 800 °C sintered ceramic sample. The high-resolution PFM imaging shows the two-level hierarchical stripe-like domain structure for the high-temperature hot-press sintered samples, while the domain structure for the low-temperature sintered samples is irregular and dendrite-like. The two-level hierarchical domain structure is believed to be responsible for the enhanced piezoelectric property, and the better ferroelec- tric property may be ascribed to the improved sample quality and large grain size. It is suggested that domain engineering by optimizing the sintering process can be a promising approach to improve the electric performance of BiFeO3
ceramics.
ACKNOWLEDGMENTS
The authors would like to acknowledge the financial support from the National Key Research Projects of China (Grant No. 2015CB654602) and the National Nature Science Foundation of China (Grant Nos. 51332006, 51721001, and 51431006).
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