The lead-free piezoelectric ceramics display good piezoelectric properties which are comparable with Pb(Zr,Ti)O3 (PZT) and these materials overcome the hazard to the environment and human health. The Bi0.5(Na,K)0.5TiO3 (BNKT) is rapidly developed because of good piezoelectric, ferroelectric, and dielectric properties compared to PZT. The origin of giant strain of BNKT piezoelectric materials was found at morphotropic phase boundary due to crystal change from tetragonal to orthorhombic and/or precipitation of cubic phases, in addition to domain switching mechanism. The dopants or secondary phases with
The piezoelectric phenomena were discovered by Nobel laureates Pierre and Jacques Curie in 1880 during measurement of surface charges appearing under stress of some crystals such as tourmaline, quartz, and Rochelle salt [
Among lead-free piezoelectric materials perovskite-based type, bismuth sodium titanate Bi0.5Na0.5TiO3 (BNT), bismuth potassium titanate Bi0.5K0.5TiO3 (BKT), and solid solution based on these compounds seem to be considered as the most promising materials choice [
BNT is one of the most important lead-free materials discovered by Smolensky et al. in 1960, which has an
BKT was also first fabricated by Smolenskii and Agranovskaya which has a perovskite type ferroelectric structure belonging to tetragonal crystal at room temperature [
Literature survey indicates that BKT is studied weaker than BNT; its behavior is clearly indicated that it does not show such unusual phenomena as BNT (isotropic points and “disappearance” of phase transitions). It is clear that both phases of BKT are ferroelectric that they both are diffused and, most probably, overlapped each other. There is, of course, a problem: coexistence of a paraelectric (PE) and two ferroelectric (FE) phases within a crystal lattice. In addition, the BKT single phase is not easy to fabricate in high-dense structure. However, such a problem is not new for ferroelectric perovskites. It is clear that many properties are still not studied. However, by combining the sol-gel and conventional solid-state reaction method, Zhu et al. obtained the high compact density of ~91.2%, which overcame the low density of ~70% of ceramics prepared by only traditional solid state synthesis [
The BNKT ceramics were first fabricated by Buhrer by conventional ceramics method via starting materials with metal oxide Bi2O3 and TiO2 and alkali carbonate powder Na2CO3 and K2CO3 [
The single crystals (
Recently, BNKT powders were prepared by the sol-gel process [
Buhrer reported that the lattice parameters of B0.5Na0.5TiO3 increased with BKT concentration addition [
The Curie temperature of (
BNT-BKT solid solutions are interesting because of three phenomena: (i) existence of two morphotropic boundaries, (ii) neighborhood of the antiferroelectric (AFE) phase of BNT and high-temperature ferroelectric (FE) phase of BNT, and (iii) complicated coexistence of several phases within one perovskite lattice because of the phase-transition diffusion.
In the search for lead-based materials with large electric-field-induced phase transition (EFIS), an alternative and applicable approach for ceramics was reported by Uchino et al. and Pan et al. based on the work of Berlicourt et al. [
Zhang et al. proposed that the high strain in lead-free Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3 system came both from a significant volume change caused by the field-induced antiferroelectric-ferroelectric phase transition and from the domain contribution caused by the induced ferroelectric phase [
The Bi0.5(Na,K)0.5TiO3 is considered as a typical
The rare earth elements are multivalent when they were doped in BNKT which resulted in interesting and complicating phenomena. Li et al. reported that electromechanical coupling factor (
Han et al. first reported that the adding CuO in Bi0.5(Na,K)0.5TiO3 ceramics resulted in decreasing the sintering temperature [
The effect of electric coupling factor in case of various dopants in BNKT.
Do et al. reported that Bi0.5(Na0.82K0.18)0.5TiO3 ceramics had the value of
Ni et al. obtained the effects of
At the MPB of BNK-BKT binary system, an electric-field-induced strain and dynamic piezoelectric coefficient were 0.23% and 291 pm/V, respectively, at an applied electrical field of 80 kV/cm which are the considered value for application in electromechanical devices [
The mechanism of giant electric-field-induced strain could be considered as (i) electric-field-induced phase transition and/or (ii) point-defect-mediated reversible domain switching. At room temperature, the BNT system is in rhombohedral structure, and BKT is in tetragonal structure. Their solid solutions have rhombohedral-tetragonal morphotropic phase boundary near 0.16–0.20 of BKT amount [
The detailed composition of BNKT-modified ceramics with highest electric-field-induced strain.
Ceramic compounds |
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References |
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0.86Bi0.5Na0.5TiO3 − 0.14Bi0.5K0.5TiO3 | 297 | Izumi et al. (2008) [ |
La2O3/MnO-added (1 − |
415 | Isikawa et al. (2009) [ |
Bi0.5(Na0.82K0.18)0.5Ti0.97Nb0.03O3 | 641 | Pham et al. (2010) [ |
Bi0.5(Na0.78K0.22)0.5TiO3 | 296 | Ullah et al. (2010), [ |
0.97Bi0.5(Na0.78K0.22)0.5TiO3 − 0.03BiAlO3 | 592 | Ullah et al. (2010) [ |
0.95Bi0.5(Na0.8K0.2)0.5TiO3 − 0.05BiAlO3 | 391 |
Ullah et al. (2010) [ |
0.99(K0.5Na0.5)0.95Li0.05NbO3 − 0.01Bi0.5(K0.15Na0.85)0.5TiO3 | 330 | Chen et al. (2010) [ |
Bi0.5(Na0.82K0.18)0.5TiO3 − 0.7 wt.% Y2O3 | 278 | Binh et al. (2010) [ |
Bi0.5(Na0.78K0.22)0.5Ti0.97Zr0.03O3 | 614 | Hussain et al. (2010) [ |
Bi0.5(Na0.78K0.22)0.5(Ti0.97Hf0.03)O3 | 475 | Hussain et al. (2010) [ |
Bi0.5(Na0.82K0.18)0.5Ti0.98Ta0.02O3 | 566 | Do et al. (2011) [ |
Bi1/2Na1/2TiO3 − Bi1/2K1/2TiO3 − 0.02Bi(Zn1/2Ti1/2)O3 | 500 | Dittmer et al. (2011) [ |
Bi0.5(Na0.75K0.25)0.5TiO3 − BiAlO3 | ~900 | Lee et al. (2011) [ |
0.09Bi0.5(Na0.78K0.22)0.5TiO3 − 0.01Bi0.5La0.5AlO3 | 579 | Ullah et al. (2012) [ |
0.975Bi0.5(Na0.78K0.22)0.5TiO3 − 0.025BiAlO3 | 533 |
Ullah et al. (2012) [ |
0.97Bi0.5(Na0.78K0.22)0.5TiO3 − 0.03K0.5Na0.5NbO3 | 434 | Hussain et al. (2012) [ |
Bi0.5Na0.385Li0.025K0.09Ti0.975Ta0.025O3 | 727 | Nguyen et al. (2012) [ |
Bi0.5(Na0.82K0.18)0.5Ti0.95Sn0.05O3 | 585 |
Lee et al. (2012) [ |
0.94Bi1/2(Na0.8K0.2)1/2TiO3 − 0.06Bi(Mg1/2Sn1/2)O3 | 633 |
Pham et al. (2012) [ |
Bi0.5(Na0.74Li0.8K0.18 |
646 | Nguyen et al. (2012) [ |
Bi1/2(Na0.82K0.18)1/2(Ti0.97Nb0.03)O3 | 641 |
Pham et al. (2012) [ |
0.97Bi0.5(Na0.78K0.22)0.5TiO3 − 0.03CaZrO3 | 617 | Hong et al. (2013) [ |
0.95Bi0.5(Na0.8K0.2)0.5TiO3 − 0.05SrTiO3 | 600 | Wang et al. (2012) [ |
0.99Bi0.5(Na0.82K0.18)0.5Ti0.980Zr0.020O3 − 0.01LiSbO3 | 500 | Zaman et al. (2012) [ |
0.97Bi0.5(Na0.82K0.18)0.5 − 0.03Bi(Zn0.5Ti0.5)O3 | 385 |
Ullah et al. (2012) [ |
5Bi(Zn0.5Ti0.5)O3 − 40(Bi0.5K0.5)TiO3 − 55(Bi0.5Na0.5)TiO3 | 547 | Patterson et al. (2012) [ |
Bi0.5(Na0.82K0.18)0.5TiO3 − 0.02CuO | 214 | Do et al. (2012) [ |
Bi0.5(Na0.82K0.18)0.5TiO3 − 0.02CuO − 0.02Nb2O5 | 427 | Do et al. (2012) [ |
0.98Bi0.5(Na0.78K0.22)0.5TiO3 − 0.02LaFeO3 | ~500 |
Han et al. (2012) [ |
Bi0.5(Na0.82K0.18)0.5Ti0.95Sn0.05O3 | ~600 |
Han et al. (2013) [ |
0.98Bi0.5(Na0.82K0.18)0.5TiO3 − 0.02BaZrO3 | 437 |
Lee et al. (2013) [ |
0.97Bi0.5(Na0.82K0.18)0.5TiO3 − 0.03CaZrO3 | 603 |
Lee et al. (2013) [ |
0.98Bi0.5(Na0.82K0.18)0.5TiO3 − 0.02Ba0.8Ca0.2ZrO3 | 549 |
Lee et al. (2013) [ |
0.94Bi0.5(Na0.75K0.25)0.5TiO3 − 0.06BiAlO3 | 930 |
Lee et al. (2013) [ |
0.975Bi0.5(Na0.80K0.20)0.5TiO3 − 0.025LiNbO3 | 475 |
Hao et al. (2013) [ |
0.96Bi0.5(Na0.78K0.22)0.5TiO3 − 0.04Bi(Mg0.5Ti0.5)O3 | 636 |
Ullah et al. (2013) [ |
40Bi0.5K0.5TiO3 − 59Bi0.5Na0.5TiO3 − 1Bi(Mg1/2Ti1/2)O3 | 422 |
Kumar and Cann (2013) [ |
|
715 |
Dinh et al. (2013) [ |
0.95Bi0.5(Na0.80K0.20)0.5TiO3 − 0.05Ba(Ti0.90Sn0.10)O3 | 649 |
Jaita et al. (2014) [ |
0.97Bi0.5(Na0.80K0.20)0.5TiO3 − 0.03SrZrO3 | 617 |
Hussain et al. (2014) [ |
0.99Bi0.5Na0.4K0.1Ti0.98Nb0.02O3 − 0.01(Ba0.7Sr0.3)TiO3 | 634 |
Ullah et al. (2014) [ |
Bi0.5(Na0.80K0.20)0.5TiO3 − ( |
413–575 |
Hao et al. (2014) [ |
0.99Bi0.5(Na0.82K0.18)0.5Ti0.987Ta0.013O3 − 0.01LiSbO3 | 650 |
Zaman et al. (2014) [ |
0.99 |
650 |
Ullah et al. (2014) [ |
The effect of dopants and solid solution perovskite
The highest values
In case of rare-earth doped BNKT, the enhancement of piezoelectric properties mostly resulted from the point-defect-mediated because rare-earth elements have various valences which was displayed by various radii of ion, depending on the valence stable state substitution. For example, the mechanism for the effect of CeO2 doped BNKT is complicated because Ce ion possibly exists in the BNKT structure in two valence states: Ce4+ with radius of 0.92 Å and Ce3+ with radius of 1.03 Å. In view of the radius, Ce3+ is possible to fill in Bi3+ vacancies and Ce4+ can enter into the Bi-site. In this case, Ce4+ functions as a donor dopant leading to some vacancies of
In case of transition metal dopant in BNKT, the most findings confirmed that dopants resulted in the lower sintering temperature. It is valuable in market due to reduction the cost of electronic devices.
In case of other metal dopant in BNKT, there were mixed point-defect-mediated and electric-field-induced phase transition mechanisms which depend on the valence and site-prefer to substitution. For example, the Ta5+ which replaced Ti4+ at
In case of solid solution between BNKT with other
The current status of lead-free piezoelectric materials has been introduced. The lead-free BNKT-based ceramics were reviewed based on the fabrication method, effect of dopants, and solid-solution with other
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the Ministry of Education and Training, Vietnam, under Project no. B 2013.01.55.