Defect structure,ferroelectricity and piezoelectricity in Fe/Mn/Cu-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics

For perovskite Pb-based ceramics, outstanding hardening piezoelectric properties can be easily induced by acceptor dopings of Fe, Mn or Cu, but in this work, completely different hardening effects are observed in Fe/Mn/Cu-doped K_0.5Na_0.5NbO₃ ceramics. Pure K_0.5Na_0.5NbO₃ exhibits a well-saturated single P-E loop, giving low Q_m of 72. Fe₂O₃-doped ceramic exhibits the combined effects of dominant donor and slight acceptor, giving a slightly slanted single P-E loop and relatively low Q_m of 156. For MnO₂-doped ceramic, moderate hardening properties with a slightly pinched P-E loop and relatively high Q_m of 370 are exhibited. Unlike Fe₂O₃ and MnO₂-doped ceramics, a double P-E loop and superhigh Q_m of 1965 are obtained in CuO-doped ceramic. The defect structure and corresponding microscopic mechanisms in the ceramics have been systematically investigated. Our study shows that defect characteristics should be responsible for distinct hardening properties in Fe, Mn and Cu-doped K_0.5Na_0.5NbO₃ materials.     

High performance lead-free Na0.5K0.5NbO3 piezoelectric ceramics obtained via oscillatory hot-pressing

Lead-free Na_0.5K_0.5NbO₃ (KNN) piezoelectric ceramics is regarded as a potential candidate for PZT material, while high performance is difficult to be obtained due to its poor sinterability and non-stoichiometric component. In this work, oscillatory pressure-assisted hot pressing (OPAHP) is utilized to fabricate KNN ceramics with high density. The KNN ceramics sintered at 860 °C exhibits superior performance with piezoelectric parameter (d₃₃) of 142 pC/N, electromechanical coupling factors (k_p) of 0.41, and relative permittivity (ε^T₃₃/ε₀) of 472–620. Additionally, hardness and flexural strength are measured as 3.55 GPa and 99.13 MPa, respectively. This work indicates that OPAHP technique is effective for fabricating KNN piezoelectric ceramics with high performance.     

Effects of CaTiO3 addition on microstructures and electrical properties of Na0.52K0.48NbO3 lead-free piezoelectric ceramics

In this article, various amounts of CaTiO₃ (CT) were added into (Na_0.52K_0.48)NbO₃ (NKN) ceramics using conventional oxide-mixing method for improving NKN's properties. The experimental results show that the (1?x)(Na_0.52K_0.48)NbO₃–xCaTiO₃ (x=0～0.07) solid solution system can be successfully synthesized. Addition of CaTiO₃ not only effectively prevents materials from deliquescence, but also improves the density and the electrical properties of the ceramics. The dielectric constant–temperature (ε_r?T) curves exhibit that the temperatures of the Curie point (T_c) and the phase transition from tetragonal to orthorhombic (T_O?T) are decreasing monotonously as the amount of CT addition is increased. A morphotropic phase boundary (MPB) can be found in the (1?x)NKN?xCT solid solution system as the doping amount of x=0.03, and the 0.97NKN–0.03CT ceramics, with a high bulk density, 98% theoretical density, and an appropriate grain size of about 1～2 μm, present a superior domain switching ability and the optimum properties: d₃₃=117 pC/N, k_p=0.39, P_r=21 μC/cm², and T_c=333 °C.     

Structural and electrical properties of Na0.47K0.47Li0.06NbO3 lead-free piezoelectric ceramics modified by AgSbO3

(1?x)Na_0.47K_0.47Li_0.06NbO₃ (NKLN)–xAgSbO₃ lead-free piezoelectric ceramics were prepared using a reaction sintering method. The effects of AgSbO₃ doping on the structural and electrical properties of NKLN ceramics sintered at 1000–1040 °C were studied. The dopant affected densification, phase content, sintering temperature, microstructure and electrical properties. Variations in the relative intensity of X-ray diffraction peaks were consistent with Ag⁺ and Sb⁵⁺ ions substituting on the perovskite lattice to produce a change in the proportions of co-existing tetragonal and orthorhombic phases. Grain growth during secondary re-crystallization was also affected. The temperature of the orthorhombic–tetragonal (O–T) phase transition and the Curie temperature (T_C) decreased as a result of AgSbO₃ modifications. The dielectric and piezoelectric properties are enhanced for the composition near the orthorhombic–tetragonal polymorphotropic phase boundary. The 0.92Na_0.47K_0.47Li_0.06NbO₃–0.08AgSbO₃ ceramics exhibited optimum electrical properties (d₃₃=252 pC/N, ε_r=1450, tan δ=0.02, and T_C=280 °C). These results reveal that (1?x)Na_0.47K_0.47Li_0.06NbO₃–xAgSbO₃ ceramics are promising materials for lead-free piezoelectric application.     

Fine structural analysis and phase transition behavior for Li-modified Na0.5K0.5NbO3 lead-free piezoelectric ceramics

The fine crystal structure of Li_x(Na_0.5K_0.5)_1?xNbO₃ ceramics has been studied by means of Nb-K edge extended X-ray absorption fine structure (EXAFS) and X-ray internal strain measurement technique in the vicinity of the compositions showing a polymorphic phase boundary (PPB) between orthorhombic and tetragonal structures. The anisotropic distortion of the NbO₆ octahedral initially occurred when x was increased from 0.050 to 0.053, prior to the completion of the phase transition from orthorhombic to tetragonal symmetry. EXAFS clearly revealed that the bond distance of Nb–O1 with [0 0 1] configuration was increased, and that of Nb–O2 with [1 1 0] configuration was oppositely decreased in the NbO₆ octahedral. In the vicinity of the PPB compositions, the internal strain η_(0 1 1) also increased from 4.5 × 10^?3 to the maximum value of 12.0 × 10^?3 in the narrow x range from 0.040 to 0.055, then decreased to 3.2 × 10^?3 at x = 0.06. On the other hand, the η_(1 0 0) increases from 1.5 × 10^?3 to the maximum value of 2.9 × 10^?3 in the next narrow x range from 0.055 to 0.060. The variation of η_(1 0 0) differed in Li dependence from that of η_(0 1 1), which indicates that a large anisotropic strain remains in the crystal lattice in the PPB compositions.     

Phase transition behavior and temperature-stable piezoelectric properties of new quaternary (K,Na)NbO3 based ceramics

(K, Na)NbO₃-based lead free materials have been found to exhibit good piezoelectric properties due to the orthorhombic–tetragonal polymorphic phase transition (PPT) temperature compositionally shifted downward to near room temperature. However, this transition correspondingly results in a strong temperature dependence of the dielectric and piezoelectric properties. In this work, new quaternary (1?x) (K_0.4425Na_0.52Li_0.0375)(Nb_0.8925Sb_0.07Ta_0.0375)O₃ (KNLNST)–xSrTiO₃ (ST) lead-free piezoelectric ceramics were fabricated by a conventional ceramic technique and their structure and piezoelectric properties were also studied. The results of X-ray diffraction reveal that SrTiO₃ diffuses into the KNLNST lattices to form a new solid solution with a perovskite structure. After the addition of SrTiO₃, tetragonal–orthorhombic phase transition shifts to lower temperatures. The good piezoelectric properties of 0.995 KNLNST–0.005 ST material were found to be d₃₃～295 pC/N, k_p～42%, and ε_r～1902, with greatly improved temperature stability over the temperature range of 0–100 °C, demonstrating practical potential for actuator and ultrasonic transducer applications.     

Phase transition and piezoelectricity of BaZrO3-modified (K,Na)NbO3 lead-free piezoelectric thin films

(K,Na)NbO₃ (KNN) is a promising lead-free ferroelectric/piezoelectric system, to which incorporating BaZrO₃ can greatly enhance its piezoelectricity, but the mechanism is not clear. This work was conducted to investigate the phase transition in the BaZrO₃-modifed KNN system and its contribution to piezoelectricity enhancement, using thin films with a fixed orientation and high compositional homogeneity fabricated by a sol-gel method. Two ferroelectric-to-ferroelectric phase transitions are revealed, which correspond to monoclinic M_C- M_A phase transition at higher temperature and rhombohedral-monoclinic M_C phase transition at lower temperature. It is difficult to distinguish these phases in KNN-based bulk materials, but their differences are clear when conducting high-resolution X-ray reciprocal space mapping (RSM) on the present thin films. Piezoresponse force microscopy experiments also revealed an interesting finding that local piezoelectricity of monoclinic phases was higher than that of rhombohedral ones in KNN-based thin films. This work could shed insights on the fundamental understandings for the effect of the chemical doping, and offer guidance for property optimization in the KNN-based lead-free piezoelectrics.     

Enhanced electromechanical properties of CaZrO3-modified (K0.5Na0.5)NbO3-based lead-free ceramics

Pairing of large strain response and high d₃₃ with high T_c in (K_0.5Na_0.5)NbO₃-based materials is of high significance in practical applications for piezoelectric actuators. Here, we report remarkable enhancement in the electromechanical properties for (1-x)(K_0.52Na_0.48) (Nb_0.95Sb_0.05)O₃-xCaZrO₃ (KNNS-xCZ) lead-free ceramics through the construction of a rhombohedral (R)-tetragonal (T) phase boundary. We investigated the correlation between the composition-driven phase boundary and resulting ferroelectric, piezoelectric, and strain properties in KNNS-xCZ ceramics. The KNNS-xCZ ceramics with x=0.02 exhibited a large strain response of 0.23% while keeping a relatively large d₃₃ of 237pC/N, which was mainly ascribed to the coexistence of R and T phases confirmed by the XRD and dielectric results. It was found that pairing of large strain response and high d₃₃ in KNN-based materials was achieved. As a consequence, we believe that this study opens the possibility to achieve high-performance lead-free electromechanical compounds for piezoelectric actuators applications.     

Enhanced electrical properties of the polymorphic phase boundary on the tetragonal side in K0.48Na0.52NbO3-based lead-free piezoelectric ceramics

Herein, (0.95?x)K_0.48Na_0.52NbO₃-0.05SrTiO₃-xCaZrO₃ piezoelectric ceramics were prepared using a conventional solid sintering process, and their microstructures, phase structures, and ferroelectric, dielectric, and strain properties were studied. The crystal structure of the ceramics changed from the coexistence of an orthogonal–tetragonal phase on the orthogonal side at x = 0 to that on the tetragonal side at x = 0.02 by improving the orthogonal–tetragonal transition temperature (～20 °C) with increasing CaZrO₃ (abbreviated as CZ) doping. A high electric field–induced strain of 0.33% with a Curie temperature of T_c = 256 °C was obtained at x = 0.02 and was approximately two times that observed at x = 0. The dielectric constant and maximum polarization were the highest at x = 0.02 in this (0.95?x)K_0.48Na_0.52NbO₃-0.05SrTiO₃-xCaZrO₃ system. These materials would be promising lead-free ceramics in the future.     

Microstructures,piezoelectric properties and energy harvesting performance of undoped (K0.5Na0.5)NbO3 lead-free ceramics fabricated via two-step sintering

Piezoelectric energy harvesters have become increasingly popular in the field of green energy because of the ability to convert low-frequency environmental vibrations into usable electricity. To fabricate high-performance energy harvesters, the key requirements are piezoelectric ceramics with a small grain size, of near-full density, the intended stoichiometric ratio and a high transduction coefficient. In this work, the effects of two-step sintering on the sinterability, microstructure, piezoelectric properties and energy harvesting performance of (K_0.5Na_0.5)NbO₃ were systematically investigated. Compared with conventional single-step sintering, two-step sintering samples were of higher density, increasing from 91 % to 95 % of theoretical, reduced mean grain size, down from 17 μm to 7.5 μm, and decreased evaporation of the alkali metals. This translated into an improved piezoelectric performance (d₃₃ ∼122 pC/N, k_p ∼36 % and Q_m ∼76), a higher transduction coefficient and energy conversion efficiency as well as a higher open-circuit voltage and power density. This demonstrates the potential of two-step sintering as a high through-put sintering technique for moderate-performance, pure KNN ceramics.     

Microstructure and piezoelectric properties of CuO added (K, Na, Li)NbO3 lead-free piezoelectric ceramics

Lead-free Na_0.5K_0.5NbO₃ (NKN) and Na_0.475K_0.475Li_0.05NbO₃ (NKLN) ceramics doped with CuO were prepared by the mixed oxide route. The powders were calcined at 850-930 °C and sintered at 850-1100 °C. Small additions of CuO reduced the sintering temperature and increased the density to 96% theoretical. Cu first appears to enter the A site then the B site. In NKLN the orthorhombic-tetragonal and tetragonal-cubic phase transitions are approximately 150 °C lower and 50 °C higher, respectively than in NKN. With increasing addition of Cu to NKN and NKLN the remanent polarization (P_r) increased and coercive field (E_c) decreased. NKLN prepared with 0.4 wt% CuO exhibited a saturation polarization (P_sat) of 30 μC/cm², remanent polarization (P_r) of 27 μC/cm² and coercive field (E_c) of 1.0 kV/mm. CuO caused the NKLN ceramics to harden considerably; the mechanical quality factor (Q_m) increased from 50 to 260, d₃₃ ∼ 285 and piezoelectric coupling factors were >0.4.     

Duplex structure in K0.5Na0.5NbO3-SrZrO3 ceramics with temperature-stable dielectric properties

Temperature-stable dielectric properties have been developed in the 0.86 K_0.5Na_0.5NbO₃-0.14SrZrO₃ solid solution system. High dielectric permittivity (ε′ = 2310) with low loss sustained in a broad temperature range (−55–201 °C), which was close to that of the commercial BaTiO₃-based high-temperature capacitors. Transmission electron microscopy with energy dispersive X-ray analysis directly revealed that submicron grains exhibited duplex core-shell structure. The outer shell region was similar to the target composition, whilst a slightly poor content of Sr and Zr presented in the core region. Based on Lichtenecker’s effective dielectric function analysis along with Lorentz fit of the temperature dependence of dielectric permittivity, a plausible mechanism explaining the temperature-stable dielectric response in present work was suggested. These results offer an opportunity to achieve the X8 R specification high-temperature capacitors in K_0.5Na_0.5NbO₃ based materials.     

Normal sintering of (K, Na)NbO3-based lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics have received more attention due to the environmental protection of the earth. (K, Na)NbO₃-based ceramics are one of the most promising candidates. Normal sintering of un-doped and Li/Ta co-doped (K, Na)NbO₃ ceramics was investigated to clarify the optimal sintering condition for densification, microstructure and electrical properties. It was found that density increased greatly within a narrow temperature range but turned to decrease when the sintering temperature slightly exceeded the optimal one. Piezoelectric properties also showed similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density. The grain growth and property change as a function of sintering temperature were discussed on basis of the formation of liquid-phase and the composition deviation caused by the volatilization of alkali components during sintering.     

Improved solid-state conversion and piezoelectric properties of 90Na1/2Bi1/2TiO3-5BaTiO3-5K1/2Na1/2NbO3 single crystals

Na_1/2Bi_1/2TiO₃-BaTiO₃-K_1/2Na_1/2NbO₃ (NBT-BT-KNN) is a promising candidate system for actuator applications in lead-free piezoelectric materials because of its large electric-field-induced strain. We were able to fabricate 96.9% dense 90NBT-5BT-5KNN (mol%) single crystals by the solid-state single crystal growth technique using sintered pellets and SrTiO₃ seed crystals. The fabricated single crystals with a 〈001〉 direction exhibited high piezoelectric performance with a large strain of 0.67% and a high converse piezoelectric constant (S_max/E_max) of about 1670 pm/V at 4 kV/mm. These values are much higher than those previously reported for ceramics, 0.45% and 560 pm/V at 8 kV/mm, and for single crystals, 0.57% and 950 pm/V at 6 kV/mm, of the NBT-BT-KNN system. The measured piezoelectric properties are even comparable with those of lead-containing piezoelectric single crystals, which demonstrates high applicability of the fabricated NBT-5BT-5KNN single crystals for actuators.     

Electrocaloric effect and pyroelectric performance in (K,Na)NbO3-based lead-free ceramics

The issue of how to achieve an electrocaloric effect (ECE) and pyroelectric effect in a material simultaneously remains to be a challenge for developing practical solid-state cooling devices and RF-detectors. Here, we structure a polymorphic phase transition (PPT) region by doping modification in KNN-based ceramics, which are developed to achieve the ECE. The direct measured ECE and pyroelectric properties are investigated in lead-free (1-x)K_0.5Na_0.5NbO₃-xBi_0.5Na_0.5ZrO₃ (KNN-xBNZ) ceramics. The adiabatic temperature change (∆T) of 0.22 K at 100°C, 0.14 K at 70°C and 0.16 K at 30°C can be obtained under an electric field of 35 kV cm^–1 for x = 0.03, 0.04 and 0.05, respectively. In addition, the temperature dependence of pyroelectric coefficient (p) is established for all compositions via the Byer-Roundy method. A large p of 454.46 × 10^–4 C m^–2 K^–1 is detected at Curie temperature (T_C) in the ceramics with x = 0.03. Achieving electrocaloric effect and pyroelectric performance simultaneously may shed light and provide a feasible design scheme for developing practically useful electrocaloric and pyroelectric materials.     

Enhanced dielectric and piezoelectric properties in Li/Sb-modified (Na,K)NbO3 ceramics by optimizing sintering temperature

In this paper, lead-free (Na_0.474K_0.474Li_0.052)(Nb_0.948Sb_0.052)O₃ ceramics were synthesized by a conventional solid-state reaction route. The effects of sintering temperature on the crystal structure, microstructure, densification, dielectric properties, and ferroelectric properties of the KNNLS ceramics were addressed. X-ray diffraction patterns and Raman spectrum indicated a transition from orthorhombic to tetragonal phase during the sintering temperature region. This transition is attributed to the migration of Li between the matrix grain and grain boundary. Scanning electron microscopy study revealed increased grain size and enhanced densification with increasing sintering temperature. The density of the ceramics sintered at 1080 °C reached a maximum value of 4.22 g/cm³. KNNLS ceramics sintered at an optimum temperature of 1080 °C exhibited high piezoelectric properties, that is 242 pC/N for d₃₃, 0.42 for k_p and 18.2 μC/cm² for P_r.     

Low-temperature sintering and electrical properties of (K,Na)NbO3 based lead-free ceramics with high Curie temperature

Lead-free ceramics (1 ? x)(K_0.48Na_0.52)NbO₃–(x/5.15)K_2.9Li_1.95Nb_5.15O_15.3 (x = 0.3–0.6, KNN–KLN100x) were prepared by conventional sintering technique at a low temperature of 960 °C. The effects of KLN contents on microstructure, dielectric, and piezoelectric properties were investigated. After the addition of KLN, the sintering performance and Curie temperature of the ceramics were markedly improved. The ceramics with x = 0.3 exhibited very good piezoelectric properties: d₃₃ = 138 pC/N, k_p = 45.03%, T_c = 495 °C, the dielectric constant at room temperature ?_r (RT) = 478 and the maximum dielectric constant ?_r (max) = 5067. These results indicated that the KNN–KLN100x lead-free ceramics sintered at low temperatures are promising for high temperature piezoelectric applications.     

Grain size control of lead-free Li0.06(Na0.5K0.5)0.94NbO3 piezoelectric ceramics by Ba and Ti doping

In this study, Ba- and Ti-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ [(1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07)] ceramics were prepared by using conventional solid state reaction method, and the microstructure and electric properties of these samples were investigated. The grain size distribution of non-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ ceramics was relatively wide. The microstructure was composed of grains ranging 1.1–5.0 μm in size. However, with increasing Ba and Ti content, the grain size distribution became narrow and the average grain size decreased from 2.0 to 0.9 μm in size. In particular, the microstructure of x = 0.07 sample was composed of grains ranging 0.5–2.2 μm in size. As a result, the frequency dispersion of dielectric constant for the (1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07) ceramics was reduced and the mechanical quality factor Q_m was enhanced with increasing Ba and Ti content.     

Microstructure and piezoelectric properties of textured (Na0.84K0.16)0.5Bi0.5TiO3 lead-free ceramics

Textured (Na,K)_0.5Bi_0.5TiO₃ ceramics were fabricated by reactive-templated grain growth in combination with tape casting. The effects of sintering conditions on the grain orientation and the piezoelectric properties of the textured (Na,K)_0.5Bi_0.5TiO₃ ceramics were investigated. The results show that the textured ceramics have microstructure with plated-like grains aligning in the direction parallel to the casting plane. The ceramics exhibit {h 0 0} preferred orientation and the degree of orientation is larger than 0.7. The degree of grain orientation increases with the increasing sintering temperature. The textured ceramics show anisotropy dielectric and piezoelectric properties in the directions of parallel and perpendicular to the casting plane. The ceramics in the perpendicular direction exhibit better dielectric and piezoelectric properties than those of the nontextured ceramics with the same composition. The optimized sintering temperature is 1150 °C where the maximum d₃₃ of 134 pC/N parallel to casting plane, the maximum k₃₁ of 0.31, and the maximum Q_m of 154 in perpendicular direction were obtained.