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How to simultaneously realize colossal permittivity and high energy storage density has always been an urgent problem to be solved for ACu3Ti4O12-family ( A C T O ) oxide materials. In this work, an effective strategy of enhancing breakdown field strength ( E b ) was adopted to achieve high energy storage density ( W ) in CdCu3Ti4O12 ( C d C T O ) ceramics via a semi-wet chemistry technique. Encouragingly, the E b of C d C T O oxide materials was enhanced obviously upon SiO2 doping, resulting in high energy storage density ( W ), maintaining high permittivity. Remarkably, at a SiO2 doping level of 4.0 w t % , a C d C T O - 4.0 w t % S i O 2 ceramic exhibited an enhanced energy storage density ( W ) of ∼1.77 mJ/cm3 accompanied with a high E b of ∼2352 V/cm. Also, a good dielectric property with a high permittivity e r of ∼5200 and a lowed loss tangent tan δ of ∼0.06 at 10 kHz was obtained in C d C T O - 2.0 w t % S i O 2 ceramic. The acceptable dielectric properties and energy storage performances were ascribed to the significantly enhanced grain boundary resistance ( R g b ) due to the decreased grain size that formed upon SiO2 doping. Our findings in this work could provide useful insights as to how to simultaneously realize colossal permittivity and high energy storage density in C d C T O and other related dielectric ceramics.

In this work, the correlation between the composition and structure of the Sr2AgxNa1−xNb5O15 (SANN, 0.00 ≤ x ≤ 1.00) ferroelectric ceramic was investigated. The SANN ceramic was synthesized via the solid-state reaction method and equivalently charged Ag+ cations were doped in the SANN matrix to investigate the relationship between the composition and structure. Investigation of the ceramic's properties mainly focused on two aspects: (1) enhancement of the dielectric and ferroelectric properties caused by the dense structure, increased orthorhombic distortion, and increased disorder distribution; (2) the low-temperature dielectric anomaly evolved from a slightly smeared out phase transition into an obvious ferroelastic/ferroelectric phase transition by Ag+ introduction in SANN. In addition, improved light transmittance was realized at x = 1.00, which may be related to the homogeneous equiaxed microstructure and uniform size of the ceramic grains.

Sr0.7Bi0.2TiO3 (SBT) was introduced into Bi0.5Na0.5TiO3 (BNT) via a standard solid-state route to modulate its relaxation behaviour and energy storage performance. With increasing SBT content, the perovskite structure of BNT transforms from a rhombohedral phase to a weakly polarized pseudo-cubic phase, and the relaxation behaviour is enhanced. In particular, the EDBS is improved from 120 kV/cm of BNT to 160 kV/cm of 0.6BNT-0.4SBT, which displays a large recoverable energy storage density (Wrec = 2.20 J/cm3), implying a large potential ability of energy storage for the 0.6BNT-0.4SBT ceramic. Moreover, both dielectric properties (28–326 °C) and energy storage properties (20–140 °C) exhibit a good thermal stability for the same 0.6BNT-0.4SBT composition. These characteristics suggest 0.6BNT-0.4SBT ceramic could be a promising candidate to be applied in a pulse power system over a broad temperature range.

Sm3+ doped Sr1.90Ca0.15Na0.9Nb5O15 (SCNN: xSm3+) multifunctional ferroelectric ceramics with pure tetragonal tungsten bronze phase were synthesized via the conventional solid-state reaction method. The dielectric, ferroelectric characteristics and luminescent behaviors were systematically studied. The electrical properties suggested that a moderate Sm3+ doping could enhance the electrical performances. The orange-reddish emission originated from the transition of 4G5/2 → 6H7/2, 4G5/2 → 6H5/2 and 4G5/2 → 6H9/2 was observed under excitation wave of 406 nm. The strongest emission intensities were obtained at the Sm3+ doping content of 0.010. The degree of thermal quenching for the composition with x = 0.010 was relatively lower and the normalized emission intensity continued still 80% from room temperature to 150 °C. Besides, the transmittance in visible spectra of the SCNN: xSm3+ ceramics could be improved obviously, which was up to 50% at the doping amount of 0.010. These results show that the SCNN: xSm3+ multifunctional ferroelectric niobates have promising potential in electric-optical devices.

The lead-free Eu3+ doped (SrxBa1-x)0.98Eu0.02Nb2O6 (SBEN, 0.3 ≤ x ≤ 07) ceramics were fabricated by conventional solid-state reaction method. Eu3+ was chosen as a dopant with the goal of inducing strong red-emitting photoluminescence based on their electrical properties. Effects of Sr/Ba ratio on the crystal structure, dielectric, ferroelectric and photoluminescent properties of SBEN ceramics were investigated by changing x. Single tungsten bronze structure with P4bm space group was obtained for all ceramics. The ceramics grain morphology varied obviously from homogenous equiaxed shape to anisometric pillar-type shape with increasing x. According to the dielectric spectra, the obtained SBEN ceramics transited from normal ferroelectrics to relaxor ferroelectrics, accompanied by the gradual decrease of Tm with increasing Sr2+ amount in A-sites. Two strong emitting peaks corresponding to the orange magnetic dipole transitions (594 nm, 5D0→7F1) and red electric dipole transitions (618 nm, 5D0→7F2) were obtained for all SBEN ceramics. The red/orange-emitting ratio of Eu3+ related to the Sr/Ba ratio in the composition was studied in this work to discuss the ions occupying situation in different A-sites.

Pure (K0.5Na0.5)NbO3 (KNN) ceramics with high density, fine and uniform-size grains were prepared by mechanochemical activation-assisted process. The time of synthesis is only 100 min, which is 72% – 93% shorter than the 6–24 h of the conventional solid-state method. Compared to samples prepared by conventional solid-state method, both the microstructure evolvement and electric properties were explored in detail. Results show the electric properties was significantly improved. Moreover, the dielectric and ferroelectric properties of obtained KNN ceramics exhibit strong dependence on the crystal size of the initial powders. The optimized ceramics HKNN100 showed a quite high energy storage performance, i.e., large electric energy storage density (Wtol = 1.612 J/cm3) and recoverable energy storage density (Wrec = 0.431 J/cm3), which can be mainly ascribed to the large dielectric breakdown strength (DBS = 110 kV/cm). Our works demonstrated that mechanochemical activation-assisted method possesses advantages for high-efficiency preparation of KNN or KNN-based ceramics.

Herein, Sr2NaNb5O15 (SNN) ceramics with a filled tungsten bronze structure were synthesized by the various methods of conventional mixed-oxide (CMO), two-step sintering (TSS), reactive sintering (RS) and molten salt synthesis (MSS). It was found that varying the preparation method could result in significant differences in the ceramic morphology, relative density, and electrical properties. The TSS, RS and MSS methods all produced a pure tungsten bronze phase SNN ceramic, though the CMO method could not. Further characterization revealed a sharp drop in the Curie temperature and significant deterioration of the dielectric properties in the ceramics prepared by MSS compared to the other methods, likely owing to residual K+ from the molten salt. Systematic comparison of the electrical properties of the ceramics produced by the RS and TSS method found that the RS method was the most suitable preparation method for SNN ceramics. The superior ferroelectric and piezoelectric properties in the sample produced by the RS method were attributed to the highly distorted NbO6 octahedron, as suggested by Raman spectroscopy.

A series of Sr2Ag0.2Na0.8Nb5-xSbxO15 (SANNS) tungsten bronze ceramics were synthesized by traditional solid-phase methods to investigate the impact of Sb replacement on the structure, relaxor nature, and energy storage properties. The study on the relationship between structure and relaxor behavior revealed two main conclusions: (1) as the Sb5+ content increased, the crystal structure transformed from an orthorhombic Im2a phase to Bbm2, and then to a tetragonal paraelectric P4/mbm phase, accompanied by a decrease in both the cell volume V and the tetragonality of c/a; (2) switching from ferroelectric to relaxor behavior at room temperature was realized through the doping of B-sites, which could be attributed to incommensurate local structural modulations caused by orthogonal distortion of the P4bm structure. Finally, we obtained a superior recoverable energy storage density (2.27 J/cm3) and an ultrahigh efficiency (93.3%) in Sr2Na0.8Ag0.2Nb4.7Sb0.3O15. The charging-discharging performance was also evaluated: a high current density (1166.56 A/cm2) and a large powder density (118.54 MW/cm3) were achieved simultaneously.

Lead-free relaxor ferroelectrics (Ca 0.28 Ba 0.72 ) 2.1 Na 0.8 Nb 5- x Sb x O 15 (CBNNS) and (Ca 0.28 Ba 0.72 ) 2.1 Na 0.8 Nb 5- y Ta y O 15 (CBNNT) with tungsten bronze structure were fabricated via solid-state reactions. The obtained CBNNS and CBNNT ceramics showed different dielectric behaviors. Only the CBNNS ceramics revealed an intensified diffusion and relaxor-like characteristics, which could be verified by the modified Curie–Weiss law. The relaxor behaviors in CBNNS were attributed to the radii difference between Sb 5+ and Nb 5+ ions co-occupying in B-sites. For the substitution of Nb 5+ by Sb 5+ in CBNNS ceramics, the change from macroscopic polarization to local polarization could also give rise to the obvious relaxor behavior. The Raman spectra verified a larger off-centering of the cation and a higher distortion degree for BO 6 octahedron in the ab plane for CBNNS ceramics when compared with those of CBNNT. In addition, the ferroelectric properties of CBNNS ceramics further indicated the relaxor ferroelectric nature, and also confirmed that the relaxor behavior helped to improve the energy-storage performance.

The luminescent-ferroelectic materials based on Sr1.90Ca0.15Na0.9Nb5O15 (SCNN) matrix doping with Eu3+ were synthesized by the conventional solid-state reaction method. The crystal structure, photoluminescence, thermal stability, dielectric, ferroelectric and piezoelectric behaviors were systematically investigated. XRD results revealed that Eu3+ introduction could induce the tungsten bronze phase transition from orthorhombic to tetragonal structures. The dielectric spectra of all specimens showed two broad dielectric anomalies: a high-temperature ferroelectric phase transition (Tc) and a low-temperature ferroelastic phase transition (Ts), both of which were suppressed at higher Eu3+ concentrations. The enhanced electrical properties were obtained in a proper Eu3+ concentration range of 0.03−0.05. For all SCNN:xEu3+ samples, the strong red emission peak at 617 nm originating from the electric dipole transition of 5D0 →7F2 was excited by different light excitations of 395 or 463 nm. Our results demonstrated that Eu3+ doped SCNN materials might have promising potential in advanced multifunctional optoelectronic applications. This article is protected by copyright. All rights reserved.

The effects of Na + concentration on the microstructural, dielectric, ferroelectric and transparent properties of (Sr 0.925 Ca 0.075 ) 2.5–0.5 x Na x Nb 5 O 15 (SCNN; 0.4 ≤ x ≤ 1.0) ceramics prepared using a conventional solid-state reaction method were investigated. X-ray diffraction results showed that the phase structure transitioned from tetragonal to orthorhombic tungsten bronze along with the structural type changing from ‘unfilled’ to ‘filled’ state. The NbO 6 octahedron became more distorted and had higher interaction strength when introducing Na + in A-sites to decrease the structural vacancies, leading to the improvement of dielectric and ferroelectric properties. The dielectric spectra of all specimens showed two broad dielectric anomalies: a high-temperature anomaly (the ferroelectric phase transition) and a low-temperature anomaly caused by the subtle structural effects in ‘unfilled’ SCNN ceramics, which evolved into the ferroelastic phase transition in ‘filled’ SCNN ceramics.

Sr1.85Ca0.15NaNb5O15 (SCNN15) ceramics with different additions of MnO2 and PbO2 have been prepared by the conventional solid-state reaction method and their phase structure, microstructure, and dielectric and ferroelectric properties investigated. X-ray diffraction results showed that the single tungsten bronze structure of SCNN15 was not changed by adding MnO2 and PbO2. X-ray photoelectron spectroscopy results demonstrated that Mn and Pb ions were both present in mixed valence of +2 and +4. It was also found that the two kinds of dopant had remarkable effects on the dielectric and ferroelectric properties of the SCNN15 ceramic. The dielectric spectra of all specimens showed two broad dielectric anomalies: one diffuse high-temperature anomaly, caused by the ferroelectric (4mm) to paraelectric (4/mmm) phase transition at the Curie temperature (T C), and a relaxor low-temperature anomaly, associated with the ferroelectric (mm2) to ferroelectric (4mm) phase transition at lower temperature (T s). The electrical performance of SCNN15-0.10 wt.% MnO2 and SCNN15-0.10 wt.% PbO2 ceramics was comprehensively investigated. Moreover, the underlying mechanism for the variation of the electrical properties for the different dopants is explained.

Colossal permittivity in tantalum-doped TiO2 (TaxTi1−xO2) (x = 1%, 2%, 3%, 4%, 5% and 6%) fabricated by the conventional solid-state reaction method in N2 atmosphere were achieved. Especially, by optimizing components and sintering temperatures, the dielectric loss could significantly decreased. The effects of Ta doping on their microstructure, dielectric properties, and temperature stability were revealed in detail. When the composition with x = 5% and sintered at 1400 °C, the dielectric constant reached to ∼30000 and the dielectric loss decreased to 3%. Interestingly, all the samples also exhibit good temperature stability of dielectric properties in a wide temperature range from 100 to 350 K. Based on XPS analysis, the formation of defect-dipole clusters, e.g. 2 ( Ta 5 + ) Ti · → 4 ( Ti 3 + ) Ti ′ ← V o ·· should be mainly responsible for the improved dielectric properties in tantalum-doped TiO2.

The 0.7BiFeO 3 -0.3(Ba 0.85 Ca 0.15 )(Zr 0.1 Ti 0.9 )O 3 + x wt.% MnO 2 ( x = 0.0–0.7) multiferroic ceramics were fabricated by a solid state reaction method. Effects of Mn doping on phase structure, microstructure, dielectric, ferroelectric, and magnetic properties were systematically investigated. It was found that all ceramics showed pseudocubic phases. Dense microstructure along with perfect grains and clear grain boundaries were obtained in all ceramics. A relaxor behavior was observed in all ceramics and Mn doping could promote this behavior. In the case of Mn modifying, significantly lower dielectric loss values along with higher T c could be obtained compared with un-doped system. The lower leakage current density J values and relatively higher resistivity R values showed that better electric insulation was achieved by Mn modification. The optimal ferroelectric and magnetic properties were obtained at proper amount of Mn modification with x = 0.2–0.4.

(Sr 0.53 Ba 0.47 ) 2.5−0.5 x Na x Nb 5 O 15 ceramics (SBNN, 0.0 ≤ x ≤ 2.5) were prepared by the conventional solid-state reaction method. The Na + concentration varied from 0.0 to 2.5 so that the tetragonal tungsten bronze (TTB) crystal structure was designed to transform from ‘unfilled’ to ‘filled’ and then to ‘stuffed’ type. Apart from the change in the structural type, the effects of Na + concentration on the phase structure as well as microstructure, ferroelectric and dielectric properties were also investigated. X-ray diffraction analysis revealed that the crystallized SBNN ceramics with x ≤ 1.0 had the tetragonal tungsten bronze structure with space group of P4bm . With further increasing x above 1.0, the broad asymmetrical diffraction peaks near 32° associated with EDX analyses indicated the existence of some amount of secondary NaNbO 3 -based phase due to the introduction of excessive Na + . It was also found that Na + concentration had a significant influence on the electrical properties of SBNN ceramics. Introducing Na + in A-sites to decrease the structural vacancies and increase the distortion degree of NbO 6 polar unit was beneficial for the electric properties, while excessive Na + content would deteriorate the electric properties owing to the presence of secondary NaNbO 3 -based phase.

Donor-doped Nb x Ti1−x O2 (x = 1%, 2%, 4%, 6%, and 8%) ceramics with giant permittivity (>104) and a very low dielectric loss (∼0.05) were sintered under flowing N2 at 1400°C for 10 h. By increasing Nb doping concentration, two different dielectric responses were evidenced in the frequency dependence of dielectric properties of Nb doped TiO2 ceramics, which corresponded to the space charge polarization and the electron-pinned defect-dipoles effect, respectively. Especially, combined with the x-ray photoelectron spectroscopy results, the electron-pinned defect-dipoles induced by the $$2({\hbox{Nb}}^{5 + } )_{\rm{Ti}}^{ \bullet } \to 4({\hbox{Ti}}^{3 + } )^{\prime}_{\rm{Ti}} \leftarrow {\hbox{V}}_{\rm{o}}^{ \bullet \bullet }$$ defect complex were further confirmed to give rise to both their high er and low tan δ in the high frequency range for the Nb x Ti1−x O2 ceramics with x > 4%.

Lead free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCZT) ceramics prepared by the sol–gel method (SG) and solid-state method (SS) have been systemically investigated. It is found that Ti concentration, acetic acid concentration, bathing temperature of sol have the influence of different level for ferroelectric and dielectric properties of BCZT ceramics. The optimum parameters for the precursor conditions of BCZT ceramics by the sol–gel method are as follows: the Ti concentration is 0.50 mol/L, the acetic acid concentration is 40 %, the bathing temperature is 50 °C and the aging time is 8 h. The results indicated that BCZT-SG ceramics with the high density and fine-grained microstructure calcined at 1000 °C and sintering at 1420 °C for 6 h exhibit outstanding electrical and energy storage properties, which are as follow: P r = 12.15 µC/cm2, E c = 1.78 kV/cm, d 33 = 558 pC/N, k p = 0.56, e m = 16,480, e r = 2234 and W = 0.52 J/cm3. Compared with BCZT ceramics with higher density prepared by solid-state method (SS), BCZT-SG ceramics all show more excellent performance than those of BCZT-SS ceramics.

Bi 2/3 Cu 3 Ti 4 O 12 (BCTO) ceramics were successfully prepared by traditional solid-state reaction method (BCTO-SS) and sol–gel method (BCTO-SG). Pure perovskite phase and dense structure were obtained in BCTO ceramics prepared by both methods. BCTO-SG ceramics showed a large dielectric constant of ~1.1×10 4 while BCTO-SS ceramics exhibited a low dielectric constant of ~3200. At 100 kHz, the dielectric constant of BCTO-SS ceramics decreased with applied voltage increasing, while the dielectric constant of BCTO-SG ceramics increased with applied voltage increasing. Further study of the relationship between dielectric constant and voltage suggested BCTO-SG ceramics had larger defect concentration than BCTO-SS ceramics. The investigation of complex impedance indicated that the electrical properties of grain boundaries for all BCTO ceramics were evidently affected by applied voltages and the electrical properties of grains were independent of applied voltages. In addition, the non-Ohmic properties of BCTO ceramics were studied in detail. The non-linear coefficients of BCTO-SS and BCTO-SG ceramics were 1.65 and 1.01, respectively. The breakdown electric fields of BCTO-SS and BCTO-SG ceramics were found to be 1.21 and 0.48 kV/cm, respectively. The potential barrier heights of BCTO-SS and BCTO-SG ceramics were calculated to be 0.549 and 0.485 eV, indicating that the potential barriers at the grain boundaries for BCTO-SS and BCTO-SG ceramics are the Schottky-type barrier.

Sr2NaNb5−xSbxO15 ceramics (SNNS, x=0.0, 0.3, 0.5, 1.0) with ‘filled’ tungsten bronze structure were prepared by the conventional solid-state reaction method. Effects of introducing Sb concentration in B-sites on the microstructure, dielectric and ferroelectric properties were investigated in detail. With increasing of Sb concentration, the crystal structure of SNNS ceramics distorted slightly from the TB orthorhombic Ccm2 phase to tetragonal P4bm phase, and then to paraelectric P4/mbm phase at room temperature. The smaller ionic radius of Sb5+ (0.60 A) compared with that of Nb5+ (0.64 A) contributed to the shrinkage of crystal structure. It was found that accompanying with the introduction of Sb, some unique dielectric and ferroelectric behavior emerged for SNNS ceramics. Only the samples with x

To enhance the electrical properties and Curie temperature of lead-free piezoelectric materials, polycrystalline Ba0.85Ca0.15Zr0.1Ti0.9O3–xCa0.28Ba0.72Nb2O6 (BCZT–xCBN) ceramics were synthesized by solid-state reaction. X-ray diffraction analysis of the ceramics revealed that the crystal structure changed from orthorhombic to tetragonal, with an increase in CBN content. The Curie temperature was improved from 91 °C to 98 °C and favorable electrical properties (d33 ~ 590 pC/N, kp ~ 0.46) were obtained for the BCZT-0.2 mol% CBN ceramics, along with high remnant polarization (Pr = 12.5 μC/cm2), and a low coercive field strength (Ec = 1.5 kV/cm). The ceramics also exhibited good dielectric temperature stability over a wide temperature range. Keywords: Electrical properties, Piezoelectricity, Ba0.85Ca0.15Zr0.1Ti0.9O3, Curie temperature

Perovskite type (Ba0.85Ca0.15−2x Bi2x )(Zr0.1Ti0.9−x Cu x )O3 lead-free ceramics were prepared via a conventional solid-state reaction method. The phase structure, dielectric, ferroelectric properties and complex impedance were investigated in detail. XRD and dielectric measurements determined that single orthorhombic phase displayed in (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 at room temperature. With the introduction of Bi2O3/CuO, the phase structure exhibited the mixture of orthorhombic and tetragonal phases, and then turned to single tetragonal phase. In contrast to the sharp dielectric transition of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics, a broad dielectric peak coupled with a slight decrease in Curie temperature was observed in (Ba0.85Ca0.15−2x Bi2x )(Zr0.1Ti0.9−x Cu x )O3 ceramics with increasing x. The observed diffuse phase transition behavior was further confirmed by a couple of measurements with polarization loops and polarization current density curves. The structural and the composition fluctuations induced by ions doping should be responsible for the diffuse phase transition behavior. Furthermore, physical mechanisms of the conduction and relaxation processes were revealed by using impedance spectroscopy analyses. It was concluded that the conduction and relaxation processes were thermally activated, which was closely linked with the singly and doubly ionized oxygen vacancies.

(Ca 0.28 Ba 0.72 ) 2.5−0.5 x K x Nb 5 O 15 ceramics (CBKN, 0.0≤ x ≤0.6) with ‘unfilled’ tungsten bronze structure were prepared by the conventional solid-state reaction method. Effects of K + concentration on the microstructure, dielectric and ferroelectric properties were investigated. It was found that modulating K + in A sites to decrease the structural vacancy could enhance the dielectric and ferroelectric properties, which was attributed to the stronger interaction inside NbO 6 octahedron and more distortion degree of NbO 6 polar unit confirmed by the Raman spectroscopy results. While higher K + concentration would deteriorate the physical and electrical properties due to the poor sintering behavior caused by the emergence of some larger anisometric pillar-type grains. Traditional temperature dependence of dielectric characteristics and well-saturated ferroelectric hysteresis loops were observed for all CBKN ceramics. The better comprehensive dielectric and ferroelectric properties were obtained at x =0.5 due to the bigger distortion degree of NbO 6 polar unit and the highest densification. In addition, the frequency dependence of e around transition temperature ( T c ) and the temperature dependence of ferroelectric properties were discussed to further clarify the relationship between composition and performance.

Sr2Na1−3xBixNb5O15 ceramics (SNBN; x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, and 0.08) were synthesized by a conventional solid-state reaction method. To eliminate the impurity phase in the Sr2NaNb5O15 ceramics, we introduced high-charge Bi3+ into the SNN ceramics to reduce the filling degree of A1 and A2 sites. X-ray diffraction results confirmed this compositional design, because all SNBN ceramics showed a pure tetragonal tungsten-bronze phase along with shrinkage of the unit cell when x > 0.01. Meanwhile, Bi3+ introduction improved their dielectric , ferroelectric , and transparent properties. As suggested by Raman and SEM, both the higher densification with relatively uniform grain size and more distorting degree of the NbO 6 polar unit contributed to the enhanced electrical properties and transparency at x = 0.04.

In order to discuss the correlation of composition-structure-performances, it is essential to study the role of Sr/Ca ratio variations on the physical properties in (Sr1-xCax)2.1Na0.8Nb5O15 ceramics with nearly ‘filled’ TB structure. Similar micromorphology of all obtained SCNN ceramics according to the FESEM results was beneficial to reveal the intrinsic structural factors for the electrical performances. Two different dielectric anomalies appeared in the dielectric spectra for all specimens: (1) the high-temperature anomaly corresponded to the ferroelectric phase transition (ferroelectric 4 mm to paraelectric 4/mmm transition); (2) whereas for the low-temperature anomaly, introducing Ca2+ in the Sr2.1Na0.8Nb5O15 composition could force some Na+ ions to occupy bigger pentagonal A2 sites, which could induce the slight distortion of NbO6 octahedron (considered as the subtle structure effects) to further evolve into the structural phase transition from ferroelectric (4 mm)-to-ferroelastic (mm2) (ferroelastic phase transition) at higher Ca2+ contents. The existence of mm2 phase in SCNN ceramics with x > 0.075 could be further confirmed by XRD refinement and Raman results. This finding is helpful for improvement of the fundamental theory and modulation of the performances in tungsten bronze niobates.

(Ca0.28Ba0.72)2.5–0.5x (Na0.5K0.5) x Nb5O15 ceramics (CBNKN, 0.0 ≤ x ≤ 0.4) with ‘unfilled’ tungsten bronze structure were prepared by the conventional solid-state reaction method. Effects of alkalis-introducing concentration in A-sites on the microstructure, dielectric and ferroelectric properties were investigated in detail. Pure tungsten bronze structure could be obtained in all compositions according to the x-ray diffraction patterns. Raman spectroscopy results showed that co-introducing Na+ and K+ in A sites to decrease the structural vacancy could enhance the dielectric and ferroelectric properties, which was attributed to the stronger interaction inside NbO6 octahedron and large distortion degree of NbO6 polar unit. Traditional temperature dependence of dielectric characteristics and well-saturated ferroelectric hysteresis loops were observed for all CBNKN ceramics. The better comprehensive dielectric and ferroelectric properties were obtained at x = 0.2 due to the bigger distortion degree of NbO6 polar unit and the highest densification. Whereas higher alkalis-introducing concentration would deteriorate the physical and electrical properties due to the poor sintering behavior. In addition, the frequency dependence of e around transition temperature (T c) and the temperature dependence of ferroelectric properties were discussed to further clarify the relationship between composition and performance.

Na0.65Bi0.45Cu3Ti4O12 ceramics were successful prepared by the conventional solid-state reaction technique. Compared to Na0.50Bi0.50Cu3Ti4O12 (NBCTO), the composition of Na0.65Bi0.45Cu3Ti4O12 was designed in terms of changing the Na/Bi ratio. Colossal dielectric permittivity of ~1.2 × 104 at 1 kHz was obtained in Na0.65Bi0.45Cu3Ti4O12 ceramics. Interestingly, three frequency dispersions were observed in the frequency dependence of dielectric constant measured at different temperatures. The investigation of electric modulus displayed that the giant low-frequency dielectric constant was attributed to Maxwell–Wagner polarization at the grain boundaries and the frequency dispersion in middle-frequency range was due to the grain polarization. Except grain response and grain boundaries response reflected by two semicircles in the impedance spectroscopy, another electrical response associated with nonzero high frequency intercept was found. The grain resistance Rg and grain boundaries resistance R gb was ~600 Ω and 3.9 × 105 Ω, respectively. The large intrinsic permittivity as high as ~700 was obtained. Furthermore, two dielectric anomalies observed in the temperature dependent of dielectric constant were discussed in detail. The results indicated change in the Na/Bi ratio had a significant effect on the electrical properties of NBCTO ceramics.

CdCu 3 Ti 4 O 12 ceramics were successfully synthetized by the conventional solid-state reaction method. The influences of sintering parameters on phase structure, microstructure and dielectric properties were investigated systematically. CdCu 3 Ti 4 O 12 ceramics sintered at 1020 °C for 15 h exhibited high temperature stability and outstanding dielectric properties, evidenced by the △ C T / C 25 °C ranges from −14.8% to 12.1% measured from −55 to 125 °C at 1 kHz, and the giant dielectric constant e ′=2.4×10 4 as well as dielectric loss tan δ =0.072. Four dielectric anomalies were evidenced in dielectric temperature spectra and the related physical mechanisms were discussed in detail. The oxygen vacancies play an important role in dielectric anomalies in the high temperature range.

Polycrystalline (1 − x) BiFeO3–x (Ba0.85Ca0.15)(Zr0.10Ti0.90)O3 [(1 − x) BFO − x BCZT)] ceramics were fabricated by a solid state reaction method. The phase structure, microstructure, and dielectric, ferroelectric and magnetic properties of the (1 − x) BFO − x BCZT ceramics were systematically investigated. The crystal structure of the (1 − x) BFO − x BCZT ceramics transformed from rhombohedral to pseudocubic with increasing x. A morphotropic phase boundary existed at the composition of 0.20 ≤ x ≤ 0.30. The average grain size of the ceramics initially increased to a maximum at x = 0.30 and then decreased. The frequency dependence of dielectric constant and complex impedance plots indicated an electrically heterogeneous microstructure for x ≤ 0.20 caused by the coexistence of Fe2+ and Fe3+. Tm and ϵm decreased with increasing x for 0.25 ≤ x ≤ 0.80, and the hysteresis loops (P–E) became slimmer with increasing x. All samples presented weak ferromagnetic ordering, indicating that the substitution of BCZT into the BFO matrix released its potential magnetization. The optimal ferroelectric and magnetic properties, Pr of 22.7–27.3 μC/cm2 and Mr of 0.0394–0.0744 emu/g, were obtained at 0.30 ≤ x ≤ 0.35.

(Ba0.85Ca0.15−x Y x )(Zr0.1Ti0.9)O3 (x = 0.00, 0.01, 0.02, 0.03 and 0.04) lead-free ceramics with excellent performance were fabricated using a solid-state reaction method. The effects of Y2O3 substitution on the crystal structure and electrical properties were investigated in detail. On the basis of the X-ray diffraction, it was clear that Y2O3 additive induced phase transition from orthorhombic phase to the coexistence of orthorhombic and tetragonal phases, and then to single tetragonal phase at room temperature with increasing x. Dielectric measurements revealed that a gradual change presented from typical ferroelectric behavior in (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 to dispersive relaxor-like characteristics in substituted samples. In addition, double permittivity peaks were presented in plots of relative permittivity versus temperature (e r − T ) with the sample of 0.01 ≤ x ≤ 0.04, which could be associated with composition fluctuation on account of the incorporation of Y2O3. The enhanced temperature stabilities of ferroelectric and piezoelectric properties were obtained in the temperature range from 20 to 140 °C. This result was found to be consistent with the change of dielectric properties. Besides, the outstanding electrical properties were obtained (d 33 = 564 pC/N, k p = 58 %, strain = 0.09 %, P r = 8.38 μC/cm2, e r = 3078, and tanδ = 0.03) at x = 0.02 due to the coexistence of orthorhombic and tetragonal phases.

(Ca0.28Ba0.72)2.5−0.5xNaxNb5O15 ceramics (CBNN, x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by the conventional solid-state reaction method. Pure tungsten bronze structure could be obtained in all compositions according to X-ray diffraction (XRD) patterns. Raman spectroscopy results showed that introducing Na+ in A sites could induce stronger interaction between A-sites ions and NbO6 octahedron, and also lead to the more distortion degree of NbO6 octahedron at higher Na+ contents. It was found that accompanying with the structural evolution from ‘unfilled’ to ‘filled’ tungsten bronze type by modulating Na+ concentration, some unique dielectric and ferroelectric behavior emerged for CBNN ceramics, especially when x > 0.4. Natural ferroelectric–paraelectric phase transition at the Curie Temperature (Tc) was determined for all compositions, while only the samples with x > 0.4 showed slight dielectric anomaly around 125–200 K, which was attributed to the local structural fluctuation of stronger NbO6 octahedron distortion in the ab plane. In addition, the more intense dependence of e on the frequency measured around Tc could be attributed to the ions disorder in mainly A2 sites caused by simultaneous occupation of Na+ and Ba2+ ions when x > 0.4. Normal ferroelectric hysteresis loops were observed in all compositions, and the related mechanisms for ferroelectric variations were discussed in detail. The better comprehensive dielectric and ferroelectric properties was obtained at x = 0.8 due to the bigger distortion degree of NbO6 octahedron polar unit and the highest densification.

NaxBi(2−x)/3Cu3Ti4O12 (0.00 ≦ x ≦ 0.50) ceramics were prepared by conventional solid-state reaction method. The influence of Na+ substitution for Bi3+ on phase structure, microstructure, and dielectric properties was studied. It was found that the low-frequency dielectric constant increased with the increase of Na+ content. NaxBi(2−x)/3Cu3Ti4O12 ceramics with x = 0.35 showed the lowest dielectric loss (0.015) at 1 kHz with a relatively high dielectric permittivity (∼7600) among the prepared ceramics. The complex impedance spectroscopy at room temperature suggested that all the NaxBi(2−x)/3Cu3Ti4O12 ceramics were electrically heterogeneous. The calculated grain boundaries resistance and grain resistance decreased with increasing Na+ content. In view of the analyses of electric modulus, it could be concluded that the giant low-frequency dielectric constant observed in NaxBi(2−x)/3Cu3Ti4O12 ceramics was attributed to Maxwell–Wagner polarization at the grain boundaries. Furthermore, three dielectric relaxations existed in the temperature dependence of dielectric constant of NaxBi(2−x)/3Cu3Ti4O12 ceramics were possibly related to oxygen vacancies.

Ca0.28Ba0.72Nb2O6 (CBN28) ceramics with addition of CeO2 and La2O3, were prepared by the conventional ceramic fabrication technique. XRD results showed that the single tungsten bronze structure of CBN28 was not changed by adding CeO2 or La2O3. SEM results indicated that both CeO2 and La2O3 dopants were effective in inhibiting the grain growth and suppressing the anisotropic growth behavior in tungsten bronze structure. It was also found that both two kinds of dopants had remarkable effects on the dielectric and ferroelectric properties of CBN28 ceramics. Compared with CBN28 ceramics, the dielectric constant around room temperature er, dielectric loss tan δ, the degree of diffuseness γ and coercive field Ec were all ameliorated when doping proper amount of CeO2 or La2O3. The comprehensive electric performance was obtained in CBN28–0.3 wt% CeO2 and CBN28–0.4 wt% La2O3 ceramics. Besides, the underlying mechanism for variations of the electrical properties due to different dopants was explained in this work.

Highly textured Sr 1.85 Ca 0.15 NaNb 5 O 15 (SCNN) ceramics with enhanced electrical properties were successfully prepared by the reactive templated grain growth method using acicular Sr 2 KNb 5 O 15 (SKN) template particles. Through carefully controlling the processing conditions, a texture fraction of 0.88 was obtained when the ceramics were sintered at 1340 °C for 6 h. The obtained textured SCNN ceramics exhibited obvious anisotropic properties with the highest electrical properties shown in c -axis direction: ɛ r = 2270, ɛ m = 5902, T c = 247 °C, tan δ = 0.014, P r = 16.88 μC/cm 2 and d 33 = 162 pC/N. These properties were superior to those of randomly oriented SCNN ceramics with the same composition, showing that fabricating lead-free ceramics through reactive templated grain growth method is an effective way to achieve superior performance for commercial application. In addition, the formation mechanism of higher texture quality in SCNN ceramics was discussed in detail.

Ca0.28Ba0.72Nb2O6 (CBN28) ceramics with different content of CuO were prepared by the conventional ceramic fabrication technique. The effects of CuO content on the phase structure, microstructure, dielectric and ferroelectric properties of obtained CBN28 ceramics were investigated. XRD results showed that pure tungsten bronze structure was obtained in all ceramics and CuO additive could accelerate the phase formation at lower temperatures. The CuO aid was effective for the uniform grain size in CBN28 ceramics, as it could facilitate the sintering behavior and suppress the anisotropic growth behavior obviously. The dielectric and ferroelectric properties of CBN28 ceramics depended greatly on the CuO content. Curie temperature T c and dielectric loss tanδ both shifted downward, whereas the maximum dielectric constant e m and the dielectric constant around room temperature e r all increased initially and then decreased as x increased from 0.1 to 0.4 wt%. Normal ferroelectric hysteresis loops could be observed in all compositions, and the remnant polarization P r decreased gradually. It was found that the comprehensive electric performance was optimized in CBN28-0.2 wt% CuO ceramics: e r = 453, e m = 3,371, T c = 226 °C, tanδ = 0.048, P r = 4.72 μC/cm2 and E c = 13.81 kV/cm, showed that CuO sintering aid could not only ameliorate the sintering behavior but also improve the electrical properties.

Lead-free Ca x Ba 1− x Nb 2 O 6 (CBN, 0.20≤ x ≤0.35) ceramics were prepared by the conventional solid state method. Effects of Ca content on the phase formation, microstructure, dielectric and ferroelectric properties for the prepared CBN ceramics were systematically studied. XRD results showed that pure CBN phase with tungsten bronze structure could be obtained from the solid solutions of BaNb 2 O 6 and CaNb 2 O 6 in all ceramics. Higher Ca contents favored the occurrence of grains with anisometric morphology, but no abnormal grain growth could be found in all compositions. With the increase of x , Curie temperature T c shifted downward, whereas the maximum dielectric constant e m increased initially and then decreased. All the ceramics showed an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics according to the modified Curie–Weiss law. Normal ferroelectric hysteresis loops were observed in all compositions. Both remnant polarization P r and coercive field E c increased initially and then decreased with the increase of x . The ceramics with homogeneous microstructure, high density and better properties were obtained at x =0.28 with e m = 2998, T c =234 °C, P r =3.98 μC cm −2 and E c =14.03 kV cm −1 .

Highly textured Sr 2 Na 0.9 K 0.1 Nb 5 O 15 (SKNN) ceramics were fabricated by reactive templated grain growth method using acicular Sr 2 KNb 5 O 15 (SKN) templates. CuO (1 wt.%) was used as sintering additive. SKN templates were aligned in matrix powders of SrNb 2 O 6 and NaNbO 3 via tape casting and sintered at higher temperatures to obtain texture morphology. Through carefully controlling the processing conditions, a texture fraction of 86% was obtained. The structure evolution was explained by liquid-phase-assisted growth mechanism, in which the whole process was divided into 3 steps according to priority: phase formation stage, densification stage, and texture development stage. The textured ceramics exhibited anisotropic properties with the highest electrical properties obtained in c -axis direction: ɛ r = 2212, ɛ m = 4869, P r = 15.92 μC cm −2 and d 33 = 82 pC N −1 , showing that reactive templated grain growth method is very effective to improve the physical properties of SKNN ceramics.

The high-density tungsten bronze (TB) Sr2-xCaxNaNb5O15 (SCNN, 0.05 ≤ x ≤ 030) lead-free ceramics were prepared by two-step solid-state reaction method. With increasing Ca2+ substitution, the crystal structure of SCNN ceramics slightly distorted from the TB tetragonal phase and became orthorhombic phase at room temperature. The smaller ionic radius of Ca2+ (1.34A) compared with that of Sr2+ (1.44A) contributed to the shrinkage of the crystal structure. Dielectric spectra of all compositions displayed two phase transitions: the ferroelastic orthorhombic to ferroelectric tetragonal phase transition (Te) at lower temperatures, and the ferroelectric to paraelectric phase transition (Tc) at higher temperatures. With increasing Ca2+ substitution, Te and Tc shifted towards higher temperature regions, while the maximum values of dielectric constant (eme and em), Pr, Ec and d33 increased at first and then decreased. The ceramics with most homogeneous microstructure and highest density were obtained at x = 0.15, resulting in optimized properties.

LiSbO 3 doped Sr 0.53 Ba 0.47 Nb 2 O 6 ceramics were synthesized by conventional mixed-oxide method. The phase structure, microstructure, dielectric and ferroelectric properties of obtained ceramics were investigated. Pure tungsten bronze structure could be obtained in all ceramics and LiSbO 3 additive could promote densification and reduce the sintering temperature. The dielectric characteristics showed diffuse phase transition phenomena for all samples, which was proved by linear fitting of the modified Curie–Weiss law with γ value varying between 1.65 and 1.92. With increasing LiSbO 3 content, the transition temperature T c decreased gradually to near room temperature. Normal ferroelectric hysteresis loops could be observed in all compositions, but the remnant polarization ( P r ) and coercive field ( E c ) all decreased gradually. Besides, the underlying mechanism for variations of the electrical properties caused by LiSbO 3 doping was explained in this work.

Lead-free Sr2KxNa1−xNb5O15 (0.00 ≤ x ≤ 0.20) piezoelectric ceramics were prepared by two-step solid state reaction method. Pure tungsten bronze structure could be obtained in all ceramics and K substitution could accelerate the phase formation at lower temperatures. The lattice constant calculation indicated expansion of the unit cell and reduced distortion of the crystal structure with K substitution due to the bigger ionic size of K+ (1.64 A) compared to that of Na+ (1.39 A). Electrical properties of Sr2KxNa1−xNb5O15 ceramics greatly depended on the K content. Curie temperature Tc shifted downward, whereas the maximum dielectric constant ɛm and the degree of diffusion phase transition all increased initially and then decreased as K content increased, indicating that proper amount of K substitution with x between 0.05 and 0.10 could enhance the dielectric properties. All the ceramics showed an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics according to the modified Curie–Weiss law. With increasing K content, the remnant polarization (Pr) decreased gradually and the coercive field (Ec) decreased initially and then increased. But normal ferroelectric hysteresis loops could be observed in all compositions. Besides, the underlying mechanism for variations of the electrical properties due to K substitution was explained in this work.

Lead-free Sr2K0.1Na0.9Nb5−xTaxO15 ceramics were prepared using the conventional mixed-oxide method. The phase transition corresponding to the tetragonal ferroelectric (4mm) to paraelectric (4/mmm) phase was observed with increasing x according to the XRD and dielectric properties results. Ta5+ substitution for Nb5+ inhibited the grain growth and also prevented the abnormal grain growth behavior. The dielectric and ferroelectric properties of the ceramics decreased and even vanished with increasing Ta5+ substitution. Besides, the underlying mechanism for variations of the structures and properties due to Ta5+ substitution was explained in this work.

Ta5+ doped strontium barium niobate (SBN) ceramics were synthesized by conventional oxide-mixed method. The phase structure, microstructure and dielectric properties of the ceramics with different Ta5+ contents and sintering temperatures were investigated. The X-ray diffraction patterns show that pure tungsten bronze structure can be obtained in all the ceramics. The ceramics with high relative density and a uniform and fine-grained microstructure could be obtained at 1430 °C. The grain size of Sr0.53Ba0.47Nb2−xTaxO6 decreased with increasing the Ta5+ content. The relaxor behavior observed in SBNT ceramics should be attributed to a cationic disorder induced by B-site substitutions, which was proved by linear fitting of the modified Curie–Weiss law. All samples are proved to be relaxor ferroelectric. It is also found that Curie temperature Tc and maximum dielectric constant ɛm both decrease with increasing Ta5+ content.

Nd 2 O 3 doped 0.82Bi 0.5 Na 0.5 TiO 3 –0.18Bi 0.5 K 0.5 TiO 3 (abbreviated to BNKT) binary lead-free piezoelectric ceramics were synthesized by the conventional mixed-oxide method. The results show that the BNKT ceramics with 0–0.15 wt.% Nd 2 O 3 doping possesses a single perovskite phase with rhombohedral structure. The grain size of BNKT decreased with the addition of Nd 2 O 3 dopant. The temperature dependence of the dielectric constant ɛ r revealed that there were two-phase transitions from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric. A diffuse character was proved by linear fitting of the modified Curie–Weiss law. At room temperature, the specimens containing 0.0125 wt.% Nd 2 O 3 with homogeneous microstructure presented excellent electrical properties: the piezoelectric constant d 33 = 134 pC/N, the electromechanical coupling factor K p = 0.27, and the dielectric constant ɛ r = 925 (1 kHz).

The effect of ZnO addition on the phase structure, microstructure and dielectric and piezoelectric properties of 0.2 wt.% MnO{sub 2} and 0.6 wt.% WO{sub 3}-doped Pb(Zr{sub 0.52}Ti{sub 0.48})O{sub 3}-Pb(Mn{sub 1/3}Sb{sub 2/3})O{sub 3}-Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3} (PZT-PMS-PZN) ceramics was investigated. X-ray diffraction shows that the phase structure of ceramics is transformed from rhombohedral to tetragonal with the increasing of ZnO addition. The bulk density significantly increases when ZnO is added and then it slightly decreases for ZnO addition above 0.2 wt.%. SEM micrographs show the grains of ceramics are uniform and well developed by adding 0.1 wt.% ZnO. The Curie temperature (T{sub c}) of 270 deg. C is obtained at the 0.1 wt.% ZnO addition. Mechanical quality factor (Q{sub m}), electromechanical coupling factor (K{sub p}) and piezoelectric constant (d{sub 33}) increase firstly, and then decrease with the increasing of ZnO addition, while dielectric loss tan {delta} drops all the time. The Q{sub m}, K{sub p}, d{sub 33}, tan {delta} and T{sub c} of the ceramics show the optimum values of 1899, 0.55, 300 (pC/N), 0.0063 and 270 deg. C, respectively, at the lower sintering temperature of 1120 deg. C and with 0.1 wt.% ZnO addition.

The binary lead-free piezoelectric ceramics with the composition of (1 − x )Bi 0.5 Na 0.5 TiO 3 – x Bi 0.5 K 0.5 TiO 3 were synthesized by conventional mixed-oxide method. The phase structure transformed from rhombohedral to tetragonal phase in the range of 0.16 ≤ x ≤ 0.20. The grain sizes varied with increasing the Bi 0.5 K 0.5 TiO 3 content. Electrical properties of ceramics are significantly influenced by the Bi 0.5 K 0.5 TiO 3 content. Two phase transitions at T t (the temperature at which the phase transition from rhombohedral to tetragonal occurs) and T c (the Curie temperature) were observed in all the ceramics. Adding Bi 0.5 K 0.5 TiO 3 content caused the variations of T t and T c . A diffuse character was proved by the linear fitting of the modified Curie–Weiss law. Besides, the ceramics with homogeneous microstructure and excellent electrical properties were obtained at x = 0.18 and sintered at 1170 °C. The piezoelectric constant d 33 , the electromechanical coupling factor K p and the dielectric constant ɛ r reached 144 pC/N, 0.29 and 893, respectively. The dissipation factor tan δ was 0.037.

Lead-free piezoelectric ceramics with the composition of (K 0.5 Na 0.5 )NbO 3 (KNN) and 0.995(K 0.5 Na 0.5 )NbO 3 –0.005AETiO 3 (AE = Mg, Ca, Sr, Ba) were successfully prepared by conventional ceramic processing. Single perovskite phase with orthorhombic syngony can be obtained in pure KNN, KNN-CT and KNN-ST ceramics. The trace amounts of secondary phase(s) can be detected in KNN-MT and KNN-BT ceramics. Ceramics of KNN-CT and KNN-ST have higher cell parameters and higher densities than those of pure KNN ceramics. Good electrical properties ( d 33 = 122 pC/N, K p = 0.38, ɛ r = 421, tan δ = 0.033 and T C = 406 °C) can be obtained in pure KNN ceramics. The KNN-CT ceramics have optimized electrical properties, which are d 33 = 116 pC/N, K p = 0.39, ɛ r = 411, tan δ = 0.024 and T C = 397 °C. Besides, the P r is 18.8 μC/cm 2 and the E c is 9.65 kV/cm for pure KNN ceramics. By adding AET to KNN, the P r values decrease with increasing the AE ions radii. The air-sintered pure KNN and KNN-CT ceramics are promising lead-free piezoelectric materials for transducer applications.

Lead-free piezoelectric ceramics with the composition of (1 − x )(K 0.5 Na 0.5 )NbO 3 – x LiSbO 3 [(1 − x )KNN– x LS] have been synthesized by conventional solid-state sintering process and without cold isostatic pressing (CIP). A morphotropic phase boundary (MPB) between orthorhombic and pseudocubic phases was found in the composition range of 0.06 ≤ x 3 , the density, dielectric and piezoelectric properties were enhanced for the composition near the MPB. The 0.94KNN–0.06LS ceramics show excellent electrical properties, which are as follows: ρ = 4.44 g/cm 3 , ɛ r = 708 (10 kHz), tan δ = 0.037 (10 kHz), d 33 = 171 pC/N, k p = 0.38 and T c = 365 °C. Dielectric study revealed that the dielectric relaxor behavior was induced by increasing LS content. In addition, 0.94KNN–0.06LS ceramics also exhibited relatively good ferroelectric properties with remanent polarization ( P r ) of 18.9 μC/cm 2 and coercive field ( E c ) of 9.38 kV/cm. The results show that KNN–LS ceramics have the potential to be one of the leading lead-free piezoelectric materials for electromechanical transducers applications in the future.

Lead-free (K0.44Na0.52Li0.04) (Nb0.96−xTaxSb0.04)O3 piezoelectric ceramics were prepared by the conventional solid-state sintering method. The grain growth of the ceramics was inhibited and the relative density was improved with Ta substituting for Nb. Increasing x led to different variations of dielectric properties before and after poling, and prevented the occurrence of orthorhombic–tetragonal phase transition (at To−t). All the ceramics show an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics. Significantly enhanced dielectric and piezoelectric properties were obtained in the ceramics with x=0.20. The ceramics are very promising lead-free materials for electromechanical device applications.

The polymorphic structure evolution of (Ba,Ca)(Zr,Ti)O3 piezoelectric ceramics was investigated by analysis of the in situ X-ray diffraction and dielectric spectra. The results indicated that a confined orthorhombic (O) phase region induced by the approach of the rhombohedral (R) and tetragonal (T) phases existed in an extremely narrow temperature range of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 composition. The electric properties near the O–T phase boundaries of (Ba0.95Ca0.05)(Zr0.05Ti0.95)O3 and (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 were compared. The results suggested that the confined O phase region is an important factor that contributes to the extremely large piezoelectric response.

Na0.5Y0.5Cu3Ti4O12 ceramics prepared by the conventional solid-state reaction method under various sintering conditions were found to exhibit a giant dielectric constant over 10 000 around room temperature. Two electrical responses were observed in the combined modulus and impedance plots, indicating the presence of Maxwell-Wagner relaxation. The contributions of semiconducting grains and insulating grain boundaries (corresponding to high-frequency and low-frequency electrical response, respectively) played important roles in the dielectric properties of Na0.5Y0.5Cu3Ti4O12 ceramics. The correlations between grain boundaries resistance and low frequency dielectric loss, grains resistance and the position of dielectric loss peak were addressed. Mixed-valent structures of Cu2+/Cu3+ and Ti3+/Ti4+ had been determined using X-ray photoelectron spectroscopy. Electron hopping between Cu2+ and Cu3+ and electron transport in Ti3+–O–Ti4+ paths were proposed as the origin of the semiconducting nature of Na0.5Y0.5Cu3T...

Lead-free (K0.44Na0.52Li0.04)(Nb0.96−xTaxSb0.04)O3 piezoelectric ceramics have been synthesized by conventional solid state sintering process. Two morphotropic phase boundaries (MPBs) respectively corresponding to the orthorhombic to tetragonal and tetragonal to pseudocubic phases are observed with increasing x. The ceramics with x=0.20 between the two MPBs show significantly enhanced electrical properties, which are as follows: piezoelectric constant d33=252pC∕N, planar coupling coefficient kp=0.42, dielectric constant er=1503, and dielectric loss tanδ=0.025, and they show good stability. Lead-free superthin buzzer disks have been prepared by using piezoceramic membrane with x=0.20. (K0.44Na0.52Li0.04)(Nb0.96−xTaxSb0.04)O3 ceramics are very promising as lead-free replacements for lead zirconate titanate.