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Inherent brittleness and low mechanical reliability usually inhibit the application of ceramic materials in many structural applications. In this work, we demonstrate that integrating crystallographic texture and second-phase toughening strategies can effectively improve fracture resistance and mechanical reliability in alumina multilayer composites. Composites consisted of equiaxed (1-x)Al2O3-xZrO2 and highly [0001]-textured Al2O3 layers were fabricated, and effects of ZrO2 amount on fracture behavior and mechanical properties of the composites were studied. Increasing ZrO2 amount x results in larger thermal expansion difference between equiaxed and textured layers. The composites with equiaxed layers containing 30 vol% ZrO2 exhibit high apparent fracture toughness Kapt, c ~11.7 MPa·m1/2 and work of fracture γWOF ~1540 J/m2, which correspond respectively to about 260% and 410% enhancements relative to those without ZrO2 addition. Moreover, adding ZrO2 remarkably reduces sensitivity of failure stress to flaw size in the multilayer composites, and the failure stress substantially increases with increasing ZrO2 content. The greatly enhanced mechanical performance achieved here can be mainly attributed to higher magnitude of compressive stresses, more crack bifurcations and longer crack deflection paths within the textured layers. This work can provide important guidelines for developing novel “bio-inspired” materials with improved fracture resistance and flaw tolerance behavior.

Owing to their importance in applications related to textured ceramics engineering, two-dimensional perovskite single-crystal microplates are highly desirable. In this work, (001)-oriented perovskite-structured BaTiO3 (BT) single-crystal microplatelets were fabricated from an Aurivillius-structured BaBi4Ti4O15 precursor via topochemical transformation. XRD, SEM, DTA, and TEM characterizations were performed to investigate the crystallization behavior and morphology evolution of the product at different reaction stages, with a focus on the formation mechanism of BT single crystalline microplatelets. Our results indicate that secondary recrystallization repaired the significant micro-structural/crystalline damage that was caused, thereby preserving the single-crystal structure during structural conversion. The subsequent epitaxial growth and replenishment through Ostwald ripening resulted in more regular shapes and narrower distributions of BT microplatelets. This study not only suggests suitable BT template candidates for the application of textured ceramics, but also provides new insights into a simple topochemical transformation strategy for manufacturing two-dimensional perovskite microcrystals.

The effects of CuO-doping on perovskite phase formation and reactive sintering of 28Pb(In1/2Nb1/2)O3-40Pb(Mg1/3Nb2/3)O3-32PbTiO3 ceramics were investigated, and the densification kinetics were compared with conventionally sintered ceramics. CuO-doping was observed by in situ x-ray diffraction to accelerate perovskite and suppress pyrochlore formation. 0.5 mol% CuO-doped PIN-PMN-PT ceramics sintered to ≥ 95% density as low as 790 °C. Comparable densification kinetics were observed with both conventional and reactive sintering. In the final stage of sintering, reactive sintering reduced the activation energy from 616 kJ/mol to 382 kJ/mol due to formation of a uniform 26–33 nm crystallite size microstructure that formed in situ at the onset of densification. Annealed reactively sintered ceramics also demonstrated equivalent ferroelectric behavior to conventionally sintered ceramics. We believe this work demonstrates a means to minimize material volatility during ceramic processing, an avenue for exploring co-firing with electrodes, as well as improved manufacturability through elimination of the perovskite powder synthesis step.

Piezoelectric energy harvesting has recently attracted substantial interest because of its ability to provide sustainable power for devices that consume very small amounts of energy. In this work, we propose a CuO-doping strategy to develop environmentally friendly (Ba, Ca)(Zr, Ti)O3 (BCZT) piezoelectric energy harvesters with both low sintering temperatures and excellent power generation performance. The CuO dopant greatly promoted the densification behavior of BCZT ceramics, yielding a densified and fine-grained microstructure at a sintering temperature reduced by 125 °C compared to that of undoped BCZT. Moreover, CuO addition substantially reduced the domain size compared to that of the undoped BCZT and thus facilitated domain switching in the ceramics. While maintaining the Curie temperature (Tc) at ∼120 °C, 0.50 mol% CuO-doped ceramics exhibited significantly enhanced electromechanical properties with a figure of merit d33×g33 of 6661 × 10−15 m2/N, high-field piezoelectric constant d33∗ of 873 pm/V, electromechanical coupling factor kp of 0.53 and energy conversion efficiency η of 97%. A high power density of 1.8 μW/mm3, which corresponds to a ∼360% improvement relative to that of the undoped BCZT, was achieved from 0.50 mol% CuO-doped BCZT energy harvesters at 10 m/s2 acceleration. These findings suggest the tremendous potential of CuO-doped (Ba, Ca)(Zr, Ti)O3 piezoceramics in high-power-density energy harvesting applications.

[0 0 0 1]-oriented alumina ceramics with texture fractions (F0001) ranging from ∼9.6% to 93.6% were prepared by templated grain growth (TGG) of nanoscale matrix. Ceramic density directly controls F0001. Impingement of textured grains can be observed at F0001 ≥ 58.4%. When fracturing samples along [0 0 0 1], the interfaces between basal surfaces of the impinged textured grains provide a favorable path for crack deflection, and the deflection distance becomes longer with increasing F0001. As a result of both crack defection and high density, optimum fracture toughness ∼4.6 MPa.m1/2 and flexural strength ∼589 MPa were achieved at F0001 = 93.6%, which are much higher than those obtained in the randomly oriented counterpart. This work can provide guidelines for the design and synthesis of novel structural ceramics with improved performance.

Further improving electromechanical properties and overcoming relatively low Curie temperature (Tc) of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-100xPT) are still two scientific issues. Here, we demonstrate a stable coexistence of monoclinic-tetragonal (MC-T) phases in ZnO-modified PMN-32PT (PMN-32PT:xZnO) due to the diffusion-induced substitution of Zn2+ for Mg2+. The Curie temperature, saturated polarization, remnant polarization, piezoelectric coefficient (Tc, Ps, Pr, d33) are increased from (160 °C, 22.0 μC/cm2, 13.3 μC/cm2, 350 pC/N) for x = 0 to (180 °C, 30.3 μC/cm2, 22.4 μC/cm2, 470 pC/N) for x = 0.06. Moreover, the thermal stability is improved. After annealing at 150 °C, the x = 0.06 sample shows retrained d33 value of 209 pC/N, about 4 times larger than that of x = 0 counterpart. The improved properties are attributed to the substituting increased polar nanoregions and easy domain switching in MC phase.

Highly dispersed α-Al2O3 microplatelets of various sizes and aspect ratios were prepared by additive-assisted molten salt synthesis. The effects of SiO2 and SiO2 + CaO additions on phase formation and morphology development of α-Al2O3 particles were investigated. Introducing additives are very effective in controlling α-Al2O3 morphology. SiO2 additive significantly enhances shape anisotropy of α-Al2O3 platelets, where a particle aspect ratio ∼23.8 was achieved, about 2.7–6.0 times higher than those of the platelets prepared without additives. When using SiO2 + CaO mixture, low concentration additive facilitates the growth of large platelets (∼9.8 µm), while increasing its content dramatically reduces the platelet size to ∼3.7 µm. These findings provide a new insight for the design and synthesis of novel advanced anisotropic microcrystals with improved quality.

Controlling the morphology of plate-like α-Al 2 O 3 powders is essential since Al 2 O 3 platelets of different sizes and shapes are needed in numerous applications. In this work, non-aggregated α-Al 2 O 3 single-crystal platelets with diameters ranging from ~1 µm to more than 20 µm and thicknesses from ~0.1 µm to 1.3 µm were produced by molten salt synthesis. The effects of alumina precursor and molten salt compositions on phase formation and morphology development of Al 2 O 3 particles were investigated. Al 2 (SO 4 ) 3 precursor reacts with K 2 SO 4 salt to form K 3 Al(SO 4 ) 3 liquid phase at ~625 °C. The α-Al 2 O 3 seeds generated in-situ from the decomposition of K 3 Al(SO 4 ) 3 can serve as low energy nucleation sites, and thus accelerate complete transformation to α-Al 2 O 3 at much lower temperatures, yielding platelets of ≥20 µm diameter. Conversely, γ-Al 2 O 3 precursor is stable until higher temperatures, resulting in the formation of small hexagonal α-Al 2 O 3 platelets (1–2 µm in diameter) in the K 2 SO 4 molten salt. In addition to alumina precursors, the salt species also strongly affect the morphology of Al 2 O 3 particles. Compared with chloride salts (NaCl and KCl), sulfate salts (K 2 SO 4 and Na 2 SO 4 ) create favorable conditions for growth of higher- aspect-ratio α-Al 2 O 3 hexagonal platelets.

(Ba 0.95 Ca 0.05 )(Ti 0.94 Sn 0.06 )O 3 - x mol% CuO ceramics were fabricated to investigate their domain structures and piezoelectric properties. The CuO additive significantly improved the densification behavior of the ceramics, resulting in a reduced sintering temperature by ~150 °C. Increasing x shifted the orthorhombic-tetragonal phase transition towards lower temperatures, and caused the specific ratio of tetragonal to orthorhombic phase to increase at room temperature. The herringbone domain patterns became much simpler and their number substantially decreased. The watermarks and parallel strips are the dominant domain configuration in ceramics with x =0.50 which show the optimum piezoelectric properties: d 33 ~516 pC/N, d 33 * ~996 pC/N, k p ~0.42, and Q m ~352. The underlying physical mechanisms for the enhanced piezoelectricity in the CuO-doped ceramics were discussed.

We investigated the synthesis mechanism and giant piezoelectricity of highly [0 0 1] c textured Pb(In 1/2 Nb 1/2 )O 3 -Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PIN-PMN-PT) relaxor based ferroelectric ceramics (texture fraction ∼94%) with BaTiO 3 (BT) templates. The onset of texture occurred at ∼88% relative density (RD), and rapid increase in texture fraction happened with RD > 94%. A PbO-rich grain boundary liquid of ∼6–13 nm thickness, detected at oriented crystalline interfaces, facilitated epitaxial nucleation and subsequent [0 0 1] PIN-PMN-PT ||[0 0 1] BT growth of textured grains on BT templates. Interfaces between BT templates and PIN-PMN-PT textured grains were found to be coherent and defect-free at the atomic scale, and there was almost no barium diffusion across the interfaces during the texturing process. Nanodomains were clearly visible in the oriented grains. [0 0 1] c poling ordered the domain structures of textured ceramics and resulted in a more uniform domain size of several hundred nanometers. The textured ternary PIN-PMN-PT ceramics exhibited ∼200% improvement in piezoelectric response relative to their random counterparts, as a result of their piezoelectric anisotropy and engineered domain status along with higher mobility of domain walls.

CuO modified Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) ternary relaxor based ferroelectrics with the composition near the morphotropic phase boundary were synthesized by two-step columbite precursor method. The introduction of CuO significantly improved the sinterability of PIN–PMN–PT ceramics, resulting in the full densification of samples at lower sintering temperatures. It also profoundly modified the crystal structure and fracture mode of the ceramics. Properly increasing CuO content led to the disappearance of rhombohedral-tetragonal phase transition, remarkably improved the Curie temperature (Tc), and made the ceramics more relaxorlike. The ternary ceramics doped with 0.25 wt% CuO possessed optimum piezoelectric properties (d33=584 pC/N, d33*=948 pC/N, and kp=0.68), high ferroelectric properties (Ec=9.9 kV/cm, and Pr=33.1 μC/cm2), low dielectric loss (tan δ=0.9%), and wider temperature usage range (Tc=225 °C). The obtained properties are much higher than those of previously reported PIN–PMN–PT based ceramics, indicating that CuO doped PIN–PMN–PT is a promising candidate for electromechanical applications with high performance and wide temperature/electric field usage ranges.

A phenomenological de-aging model with uniformly distributed defect dipoles for acceptor-doped ferroelectrics is proposed with the dynamic switching process of defect dipoles being simulated by the kinetic Monte Carlo method. The shifted P-E loops at different frequencies and temperatures were theoretically investigated. The results showed that the internal bias field increases with frequency and decreases with temperature. The scaling relation of the internal bias field with frequency was calculated at 298 K, 318 K and 338 K. The number of defect dipoles along different directions was calculated during the de-aging process, which is a key factor affecting the shape of P-E hysteresis loop. At 0.1 Hz, the defect dipoles can align with the polarization direction, but when the frequency is more than 10 Hz, they could not follow the field change and the internal bias field will saturate to a fixed value as the field frequency is beyond 100 Hz.

Composites of Al 2 O 3 –5 vol.% t -ZrO 2 (ATZ) and Al 2 O 3 –30 vol.% m -ZrO 2 (AMZ) layers were designed with 3–1 connectivity to explore the effect of spatially-dependent residual stress and layer distribution on mechanical behavior. ATZ composites with ‘shallow’ and ‘deep’ regions of AMZ, defined relative to the distance from the surface, were fabricated. Four-point bending tests on indented 3–1 composites showed crack arrest in the first compressive AMZ layer and a fracture strength nearly independent of indent size (i.e. minimum strength); the failure occurring in the region with thicker outer ATZ layers (‘deep’ region). Region dependent crack growth resistance was measured on SEVNB specimens and compared to theoretical predictions using a fracture mechanics model. Spatially tailored constant strengths were obtained, ranging between 148 MPa and 470 MPa; the maximum value corresponding to a ‘shallow’ region with a relatively thicker AMZ compressive layer embedded close to the tensile ATZ surface. The 3–1 design concept allows the fabrication of ‘deep’ and ‘shallow’ embedded regions within a unique composite architecture, thus providing a preferential path for crack propagation, opening new possibilities for design of composite structures with spatially-tailored crack growth resistance.

In this work, a large size lead-free piezoelectric single crystal, (K,Na,Li)(Nb,Ta,Sb)O3 (KNLNTS) with the dimensions of 8.5 � 8.5 � 13.5 mm 3 was successfully grown by the top-seeded solution growth method. This KNLNTS single crystal with high compositional homogeneity is in the tetragonal phase at room temperature. The Curie temperature TC of the tetragonal-cubic phase transition temperature is 210 1C. The piezoelectric coefficients and electromechanical coupling factors of the [001]C oriented KNLNTS single crystal are d33¼172.55 pC/N, d31 ¼� 71.90 pC/N, k31 ¼0.327, k33¼0.523, and kt¼0.541. In addition, the crystal shows good thermal stability so that it can be used for making high temperature electromechanical devices.

(001) oriented SrBi 4 Ti 4 O 15 (SBT) microplatelets with Aurivillius-type crystal structure were fabricated by molten salt synthesis. The effects of flux type selection and salt-to-oxide ratio variation on crystallization habit and morphology of the product were investigated. The sodium salt fluxes facilitate the formation of Sr 2 Bi 4 Ti 5 O 18 large size platelets (≥17 µm in length), while potassium salt fluxes create favorable condition for the growth of small size SrBi 4 Ti 4 O 15 platelets (≤5 µm in length). When using KCl flux, a higher salt-to-oxide ratio is beneficial to produce smaller SBT platelets with relatively narrow size distribution. The ferroelectricity and piezoelectricity of an individual SBT platelet were examined by piezoelectric force microscopy. These microcrystals are suitable template candidate to achieve highly textured ceramics for high temperature applications.

The ternary system (1 − x − y )Bi 0.5 Na 0.5 TiO 3 – x Bi 0.5 K 0.5 TiO 3 – y Bi 0.5 Li 0.5 TiO 3 (abbreviated to BNKLT- x / y ) was synthesized by conventional oxide-mixed method. The phase structure, microstructure, and dielectric, ferroelectric and piezoelectric properties of the ceramics were investigated. The X-ray diffraction patterns showed that pure perovskite phase with rhombohedral structure can be obtained in all the ceramics. The grain size varied with x and y . The temperature dependence of dielectric constant and dielectric loss revealed there were two phase transitions which were from ferroelectric (tetragonal) to anti-ferroelectric (rhombohedral) and anti-ferroelectric to paraelectric (cubic). Either increasing x or y content can make T m (the temperature at which dielectric constant ɛ r reaches the maximum) increase. With the addition of Bi 0.5 K 0.5 TiO 3 , the remanent polarization P r increased but the coercive field E c decreased. With the addition of Bi 0.5 Li 0.5 TiO 3 , P r increased obviously and E c increased slightly. Due to the stronger ferroelectricity by modifying Bi 0.5 K 0.5 TiO 3 and Bi 0.5 Li 0.5 TiO 3 , the piezoelectric properties were enhanced at x = 0.22 and y = 0.10, which were as follows: P r = 31.92 μC/cm 2 , E c = 32.40 kV/cm, ɛ r = 1118, tan δ = 0.041, d 33 = 203 pC/N and K p = 0.31. The results show that the BNKLT- x / y ceramics are promising candidates for the lead-free materials.

Pb(Zr,Ti)O 3 –Pb(Fe 2/3 W 1/3 )O 3 –Pb(Mn 1/3 Nb 2/3 )O 3 (PZT–PFW–PMN) quaternary piezoelectric ceramics using CeO 2 and excess Pb 3 O 4 as additives were fabricated by the addition of LiSbO 3 as a sintering aid. The effects of LiSbO 3 contents and sintering temperature on the phase structure, density, dielectric and piezoelectric properties of ceramics were investigated. It was observed that the addition of LiSbO 3 not only decreased the sintering temperature from 1200 to 930 °C but also optimized the properties. Temperature dependence of dielectric and piezoelectric properties of PZT–PFW–PMN ceramics with LiSbO 3 addition were studied at a temperature range from room temperature to 290 °C. It was found that the anti-resonant frequencies ( f a ) decreased with increasing temperature but the trend was reversed for resonant frequency ( f r ) which was related to the increased domain activities at higher test temperatures. Piezoelectric constant ( d 33 ) was finally found to decrease with increasing temperature. The result showed that 0.10 wt.% LiSbO 3 -modified ceramics sintered at 930 °C exhibited favorable properties, which were listed as follows: d 33 = 363 pC/N, K p = 0.60, Q m = 749, ɛ r = 1596, T c = 286 °C, P r = 14.84 μm/cm −2 and E c = 11.06 kV/cm, which make this system be a promising material for multilayered piezoelectric actuator and sensor applications.

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.

NaNbO 3 (NN) particles were synthesized by using conventional mixed-oxide (CMO), molten salt synthesis (MSS) and topochemical micro-crystal conversion methods (TMC), respectively. The effects of preparation methods on the phase structure, morphology and Raman characteristics of the NaNbO 3 particles were investigated. The results show that the NN particles prepared by conventional mixed-oxide and molten salt synthesis methods are all of orthorhombic structure in Pbma space group. Furthermore, those particles have equiaxial morphology. The single-crystal NaNbO 3 particles synthesized by the topochemical micro-crystal conversion method are of pseudocubic structure. Moreover, they are of plate-like shape with a high aspect ratio and a high ( h 0 0) degree of orientation, which is very promising as template for texturing (K, Na)NbO 3 (KNN) based ceramics. Besides, all the spectra lines in the Raman spectra can be attributed to the internal modes of NbO 6 octahedron, and they prove that the particles synthesized by the different methods have different structure types.

Large platelet NaNbO3 (NN) seed crystals have been successfully synthesized from bismuth layer-structured ferroelectric Bi2.5Na3.5Nb5O18 (BNN) precursor by topochemical microcrystal conversion method. The effects of synthesis parameters, such as salt/oxide ratio, synthesis temperature and Na2CO3/BNN ratio, on the phase structure and morphology of both the BNN precursor and NN particles were investigated. The results show that large platelet BNN precursor with bismuth layer structure was obtained at 1125 °C with a salt-to-oxide weight ratio of 1.50:1. Besides, by optimizing the synthesis parameters, anisotropic NN seed crystals with pure perovskite phase were obtained at 975 °C with a Na2CO3/BNN molar ratio of 1.50:1. The obtained NN particles are very promising template for texturing (K, Na) NbO3-based ceramics by reactive-template grain growth method.

Pb(Zr,Ti)O 3 –Pb(Fe 2/3 W 1/3 )O 3 –Pb(Mn 1/3 Nb 2/3 )O 3 (PZT–PFW–PMN) ceramics doped with BiFeO 3 as sintering aids were manufactured in order to develop the low temperature sintering ceramics for piezoelectric devices, and phase structure, microstructure, dielectric, piezoelectric properties and the temperature stability were investigated. The results show that addition of BiFeO 3 significantly improved the sinterability of PZT–PFW–PMN ceramics and could decrease the sintering temperature from 1200 to 1020 °C. The 0.20 wt.% BiFeO 3 -added PZT–PFW–PMN ceramics sintered at 1020 °C exhibited the favorable piezoelectric and dielectric properties, which were listed as follows: d 33 = 309 pC/N, K p = 0.59, Q m = 1551, tan δ = 0.0056, ɛ r = 1723, T c = 308 °C, Δ f r / f r25°C = 0.331% and Δ K p /K p25°C = −0.564%. The obtained properties make this composition to be a good candidate for high power piezoelectric devices.

The ceramics with 0.90Pb(Zr 0.50 Ti 0.50 )O 3 –0.07Pb(Mn 1/3 Nb 2/3 )O 3 –0.03Pb(Ni 1/2 W 1/2 )O 3 were prepared by adding Cr 2 O 3 . The effects of Cr 2 O 3 doping on the phase structure, the microstructure and the electrical properties of ceramics were investigated. Meanwhile, the temperature stabilities of the resonant frequency ( f r ) and the electromechanical coupling factor ( K p ) were studied. The results showed that the better temperature stability could be obtained at x = 0.2 wt.% when the calcining temperature was 800 °C and the sintering temperature was 1150 °C. The parameters were Δ f r / f r25 °C = −0.17% and Δ K p /K p25 °C = −1.39%. Moreover, the optimized electrical properties were also achieved, which were K P = 0.54, Q m = 1730, d 33 = 330 pC/N, ɛ r = 2078 and tan δ = 0.0052. The optimized properties make the ceramics with this composition to be a good candidate for high power piezoelectric transformers applications.

For the fabrication as piezoelectric transformers, the density, microstructure, dielectric and piezoelectric properties of Pb(Mg 1/3 Nb 2/3 )O 3 –Pb(Zn 1/3 Nb 2/3 )O 3 –Pb(Zr 0.52 Ti 0.48 )O 3 (PMN–PZN–PZT) ceramics as a function of calcining temperature and sintering temperature were investigated. In addition, the piezoelectric transformers with the composition of PMN–PZN–PZT were fabricated and the characteristics with various resistance loads were measured. The results indicated that the optimized properties of ceramics were obtained at calcining temperature of 800 °C and sintering temperature of 1020 °C, which were listed as follows: d 33 = 249 pC/N, K p = 0.57, Q m = 1954, ɛ r = 1122 and tan δ = 0.0059. Meanwhile, piezoelectric transformers sintered at 1020 °C show the favorable characteristics with a higher step-up ratio of 145 and a lower temperature rise of 5 °C at 250 kΩ.

Anisometric KSr2Nb5O15 (KSN) particles were synthesized by conventional mixed-oxide (CMO) method and molten salt synthesis (MSS) method, respectively. In MSS method, preparation of KSN was carried out in both SrCO3–Nb2O5–K2CO3–KCl system (system I) and SrNb2O6–Nb2O5–KCl system (system II). The results revealed that the single-phase KSN obtained by CMO method was clumped and without obvious acicular morphology, making it difficult to use in texturing processes. In system I, though acicular KSN particles were synthesized by controlling the amount of KCl salt, they were easily contaminated by the presence of blade-like Sr2Nb2O7. In contrast, pure KSN particles with high aspect ratio and uniform size were successfully synthesized by controlling the ratio of SrNb2O6/Nb2O5 in the new system (system II). The obtained KSN particles which are 5–30 μm in length and 2–4 μm in diameter are ideal templates for fabricating textured ceramics with tungsten bronze structure. Growth mechanisms in MSS method were also proposed in this work.

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.

The quaternary piezoelectric ceramics of Pb(Zr 0.52 Ti 0.48 )O 3 –Pb(Mn 1/3 Sb 2/3 )O 3 –Pb(Zn 1/3 Nb 2/3 )O 3 (PZT–PMS–PZN) with different PZN contents were synthesized by molten salt synthesis (MSS). The influence of PZN content on phase structure, microstructure, dielectric and piezoelectric properties was investigated in detail. The results of X-ray diffraction (XRD) show that the phase structure of ceramics transforms from rhombohedral phase to tetragonal phase with the increasing of PZN content. The morphotropic phase boundary (MPB) of composition is located in the range of PZN content from 2 to 7 mol%. The grain size of the ceramics gradually decreases with the increasing of PZN content. Dielectric and piezoelectric properties of ceramics are significantly influenced by the PZN content. Ceramics sintered at 1150 °C with 5 mol% PZN achieve excellent properties, which are as follows: Q m = 1381, K p = 0.64, d 33 = 369pC/N, tan δ = 0.0044 and T c = 275 °C. The PZT–PMS–PZN system is a promising material for high power piezoelectric transformers application.

Piezoelectric powders and ceramics with the composition of Pb 0.95 Sr 0.05 (Zr 0.52 Ti 0.48 )O 3 –Pb(Zn 1/3 Nb 2/3 )O 3 –Pb(Mn 1/3 Sb 2/3 )O 3 (PZT–PZN–PMS) were prepared by molten salt synthesis (MSS) and conventional mixed-oxide (CMO) methods, respectively. The influence of synthesis process on the properties of powders and ceramics were investigated in detail. The results show that the MSS method significantly improved the sinterability of PZT–PZN–PMS ceramics, resulting in an improvement of dielectric and piezoelectric properties compared to the CMO method. The optimum values of MSS samples are as follows: ɛ r = 1773; tan δ = 0.0040; T c = 280 °C; d 33 = 455 pC/N; k p = 0.70; Q m = 888; E c = 10.3 kV/cm; and P r = 28.2 μC/cm 2 , at calcination of 800 °C and sintering of 1120 °C temperature.

Piezoelectric powders and ceramics with a composition of Pb 0.95 Sr 0.05 (Zr 0.52 Ti 0.48 ) O 3 –Pb(Mg 1 / 3 Nb 2 / 3 )O 3 –Pb(Mn 1 / 3 Sb 2 / 3 )O 3 (PZT–PMN–PMS) were prepared by the molten salt synthesis (MSS) method. It was found that the MSS method significantly improved the sinterability of PZT–PMN–PMS ceramics, resulting in a reduction of sintering temperature by about 100 °C compared to the conventional mixed-oxide (CMO) method. The optimum values of d 33 , K p , Q m and tan δ were 244 pC/N, 0.53, 1034 and 0.0090, respectively, at calcination of 800 °C and sintering of 1050 °C.