High and Frequency‐Insensitive Converse Piezoelectric Coefficient Obtained in AgSbO3‐Modified (Li,K, Na)(Nb,Ta)O3 Lead‐Free Piezoceramics

AgSbO₃ was doped into KNN‐based lead‐free piezoceramics with an optimized composition of Li_0.02(Na_0.53K_0.48)_0.98Nb_0.8Ta_0.2O₃ (abbreviated as LKNNT) to further enhance its piezoelectric property. The doping of AgSbO₃ was found to be effective in reducing the grain sizes, resulting in more uniform microstructure in AgSbO₃‐doped LKNNT ceramics. AgSbO₃ lowers tetragonal‐orthorhombic phase transition point (T_T‐O), but with a more gentle rate as compared with other dopants. A large converse piezoelectric coefficient d₃₃* up to 598 pm/V under a relatively low electric field of 1 kV/mm was obtained in the LKNNT‐5 mol% AgSbO₃ composition, whose tetragonal‐orthorhombic phase transition point (T_T‐O) was controlled near room temperature, but its Curie temperature was kept at 235°C. The d₃₃* obtained in the present material is a very high value for nontextured KNN‐based ceramics, which is attributed to the polymorphism phase transition effect and “soft” behavior caused by the addition of AgSbO₃.     

Microstructural,structural, dielectric and piezoelectric properties of potassium sodium niobate thick films

In the framework of a systematic study, we present the influence of processing parameters – in particular the presence of a packing powder during sintering and the sintering temperature – on the microstructural and structural properties of potassium sodium niobate (K_0.5Na_0.5NbO₃ or KNN) thick films. These KNN thick films were prepared with a 1 mass% addition of potassium sodium germanate (KNG), which serves as a liquid-phase sintering aid. The sintered films exhibited preferential crystallographic orientations along [100]_pc and [10−1]_pc, the origin of which lies in the compressive stresses developed during cooling as a result of the thermal expansion mismatch between the film and the substrate. In addition, the dielectric permittivity, dielectric losses and the piezoelectric d₃₃ coefficient of the obtained films were compared with those of KNN bulk ceramics.     

Enhanced Piezoelectric Properties and Thermal Stability in the (K0.5Na0.5)NbO3:ZnO Lead‐Free Piezoelectric Composites

The piezoelectric properties of (K_0.5Na_0.5)NbO₃ (KNN) are normally enhanced by chemical substitutions or doping to form solid solutions. In this study, we report that the piezoelectric properties of KNN and thermal stability of piezoelectric coefficient d₃₃ can be both enhanced by forming the composite of KNN:ZnO. The d₃₃ of KNN:0.2ZnO can be improved to 110 pC/N by introducing the ZnO nanoparticles, which is better than the pure KNN (d₃₃ = 85 pC/N). The Curie temperature (T_C = 407°C) remains well comparable to the pure KNN (T_C = 408°C). Furthermore, the thermal stability of both remanent polarization (P_r) and piezoelectric parameter (d₃₃) is improved. The enhanced thermal stability could be related to the induced built‐in electric field or the enhanced sinterability by the addition of ZnO. The present results may help to optimize the piezoelectric properties of lead‐free materials by forming composite.     

Further Enhancing Piezoelectric Properties by Adding MnO2 in AgSbO3‐Modified (Li,K,Na)(Nb,Ta)O3 Lead‐Free Piezoceramics

This work investigated the effect of MnO₂ addition on the phase structure, microstructure, and electrical properties of AgSbO₃‐modified (Li,K,Na)(Nb,Ta)O₃ (abbreviated as LKNNT‐AS) lead‐free piezoelectric ceramics with an optimized composition endowed with a state of two‐phase coexistence. A small amount (0.1 wt%) of MnO₂ can significantly further enhance the piezoelectric property of LKNNT‐AS ceramics, whose piezoelectric constant d₃₃ and converse piezoelectric coefficient d₃₃* as well as planar electromechanical coupling factor k_p reach 363 pC/N, 543 pm/V, and 0.49, respectively. The temperature stability of piezoelectricity in MnO₂‐modified LKNNT‐AS samples also improved, which is associated with the more uniform and better thermally stable ferroelectric domains that were revealed by piezoresponse force microscopy.     

Influence of LNO Top Electrodes on Electrical Properties of KNN/LNO Thin Films Prepared by RF Magnetron Sputtering

Highly (001) oriented (K,Na)NbO₃ (KNN) lead‐free piezoelectric thin films were grown on LaNiO₃ (LNO)‐coated silicon by RF magnetron sputtering. The effects of the top electrodes on the electrical properties of KNN thin films were investigated. The dielectric and piezoelectric properties were remarkably improved in LNO/KNN/LNO (ε_r = 899 at 1 kHz, d₃₃ = 58 pm/V), compared with that in Pt/KNN/LNO (ε_r = 584 at 1 kHz, d₃₃ = 26 pm/V). An enhanced ferroelectricity was also obtained in LNO/KNN/LNO, with a remnant polarization of 12 μC/cm² and a maximum polarization of 23 μC/cm² at the applied field of 200 kV/cm. Besides, the temperature dependence of piezoelectricity of the films was characterized in this study.     

Effects of thickness on structures and electrical properties of K0.5Na0.5NbO3 thick films derived from polyvinylpyrrolidone-modified chemical solution

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) films with different thicknesses were prepared by polyvinylpyrrolidone (PVP)-modified chemical solution deposition (CSD) method. The KNN films with thickness up to 4.9 μm were obtained by repeating deposition-heating process. All KNN thick films exhibit single perovskite phase and stronger (1 1 0) peak when annealed at 650 °C. The variation of dielectric constant with thickness indicates that there exists a critical thickness for the dielectric constant in the KNN films which should lie in 1.3–2.5 μm. The similar trend is observed for the ferroelectric and piezoelectric properties of KNN films. Both the remnant polarization P_r and the piezoelectric coefficient d₃₃ of KNN thick films increase with the film thickness and become saturated after the critical thickness.     

New Lead‐Free (1 − x)(K0.5Na0.5)NbO3–x(Bi0.5Na0.5)ZrO3 Ceramics with High Piezoelectricity

For enhancing the piezoelectric properties of ceramics (Bi_0.5Na_0.5)ZrO₃ (BNZ) was used to partially substitute (K_0.5Na_0.5)NbO₃ (KNN). The addition of BNZ changes the symmetry of KNN ceramics from orthorhombic to tetragonal, and finally to rhombohedral phase. A new phase boundary with both rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions near room temperature is identified for KNN–0.050BNZ ceramics, where optimum electrical properties were obtained: d₃₃ = 360 pC/N, k_p = 32.1%, ε_r = 1429, tanδ = 3.5%, and T_C = 329°C. The results indicated a new method for designing high‐performance lead‐free piezoelectric materials.     

Domain Structure of Potassium‐Sodium Niobate Ceramics Before and After Poling

Domain structure and domain wall motion play important roles on the piezoelectric properties of ferroelectric ceramics. In this work, the domain structure of hot‐pressed (K_0.50Na_0.50)NbO₃ (KNN) ceramics before and after poling were studied by observing the domain patterns with an acid‐etching technique, and the extrinsic contribution to the piezoelectric properties were evaluated. It was found that the domain structure of the unpoled KNN ceramic was relatively complicated with many watermark, herringbone and zigzag patterns, while only a single set or few sets of parallel domain stripes were observed in the poled KNN ceramic, due to the domain reorientation and domain wall motion during the poling process. The average domain width changes from 200 (±10) nm before poling to 250 (±10) nm after poling. Domain configurations of “Herringbone‐Zigzag‐Watermark” and “Herringbone‐ Herringbone‐Zigzag” types observed in the unpoled KNN ceramic were then further analyzed. The extrinsic contribution to the piezoelectric properties from the domain reorientation and irreversible domain wall motion in the hot‐pressed KNN ceramic was found to be 71%, slightly higher than that of conventional sintered KNN ceramics ~68%.     

Effect of repeated composite sol infiltrations on the dielectric and piezoelectric properties of a Bi0.5(Na0.82K0.18)0.5TiO3 lead free thick film

Bi_0.5(Na_0.82K_0.18)_0.5TiO₃ lead free thick films have been produced using a combination of screen printing and subsequent infiltration of corresponding composite sol. Their structure, dielectric, ferroelectric and piezoelectric properties were investigated with variation in the number of composite sol infiltrations and the nanopowder loading in composite sol. Dielectric constant, remanent polarization, and piezoelectric coefficient have been shown to increase with increasing numbers of composite sol infiltration. Dielectric and ferroelectric properties of the thick films are found to be strongly dependent on the powder concentration of composite sols. The resulting 40 μm thick films infiltrated with 1.5 g/ml composite sols have maximum relative permittivity of 569 (at 10 kHz), remanent polarization of 21.3 μC/cm², coercive field of 80 kV/cm, and longitudinal effective piezoelectric coefficient d_33eff of 109 pm/V. The performance of these lead free piezoelectric thick films is comparable to the corresponding bulk ceramics.     

Effects of Nb,Mn doping on the Structure,Piezoelectric, and Dielectric Properties of 0.8Pb(Sn0.46Ti0.54)O3–0.2Pb(Mg1/3Nb2/3)O3 Piezoelectric Ceramics

Nb and Mn were doped, respectively, to 0.8Pb(Sn_0.46Ti_0.54)O₃–0.2Pb(Mg_1/3Nb_2/3)O₃ (PST–PMN) to improve electrical properties for meeting the requirement in various fields. The additions of Nb and Mn influence in a pronounced way the structure, and improve the densities of the ceramics. Nb‐doped PST–PMN increased the piezoelectric coefficient d₃₃, planar electromechanical coupling k_p, and relative dielectric constant ε, indicating “soft” piezoelectric behavior. Mn doping played a “hard” part, which significantly enhanced the mechanical quality factor Q_m without deteriorating other piezoelectric properties. The most excellent properties of Nb‐doped PST–PMN were obtained with doping amount of 0.75 mol%, specifically d₃₃, k_p, being on the order of 455 pC/N, 57.5% and 3560, respectively. The addition of 0.75 mol% Mn for PST–PMN presented the optimum electrical properties, with Q_m of 554, d₃₃ of 430 pC/N, k_p of 57.0%, ε of 2770. It was proposed that the addition of Nb, Mn generated different defect dipoles involved in the domain walls motion and intrinsic piezoelectric responses, leading to different effects on electrical properties.     

Selection of Lead‐Free Piezoelectric Ceramics

Lead‐free piezoelectric ceramics are extensively investigated for the alternatives of lead‐based piezoceramics. (K,Na)NbO₃ (KNN), (Bi_0.5Na_0.5)TiO₃ (BNT), and (Bi_0.5K_0.5)TiO₃ (BKT)‐based ceramics are reported as promising piezoelectric material families. Several researchers have reported solid solution of these ceramics using various chemical and physical routes. In this study, we have rank these materials using multiple attribute decision making techniques. KNN‐LT‐LS and 0.7BNT‐0.2BKT‐0.1(Bi_0.5Li_0.5)TiO₃ are found to be top rank in all the materials of respective families under study. We have also reported Pareto‐optimal (nondominated) lead‐free piezoelectric ceramics for d₃₃ and T_c parameters.     

Sintering Behavior,Properties, and Applications of Co‐Fired Piezoelectric/Low Temperature Co‐Fired Ceramic (PZT‐SKN/LTCC) Multilayer Ceramics

Materials and processing conditions have been developed allowing co‐firing of fluxed PZT‐SKN materials with commercial low temperature co‐fired ceramic (LTCC) tapes. Previously, Pb(Zr_0.53, Ti_0.47)O₃–Sr(K_0.25, Nb_0.75)O₃ (PZT‐SKN) ceramics fluxed with 1 wt% LiBiO₂ and 1 wt% CuO addition were shown to sinter to high density at 900°C for 1 h, with a large d₃₃ piezoelectric coefficient of ~415 pm/V. Currently, the master sintering curve (MSC) approach has been used to study the densification behaviors of fluxed PZT‐SKN and LTCC tapes. Different sintering mechanisms for fluxed PZT‐SKN ceramics and LTCC materials are confirmed by analyzing the apparent activation energy (Q_a). Using knowledge gained from MSC results, an optimized sintering profile was developed. Multilayer PZT‐SKN/HL2000 (HeraLock^? Tape, Heraeus) stacks co‐fired at 900°C for 0.5 h maintain large piezoelectric coefficient (high field d₃₃ > 340 pm/V). EDS analysis reveal limited interdiffusion of Pb from PZT‐SKN layers in LTCC and the appearance of Al, Ca, and Si in the PZT‐SKN near the PZT‐SKN/LTCC interface. Further, elemental interdiffusion was not detected at the center of piezoelectric layer in PZT‐SKN/LTCC multilayer ceramics and no subsequent reduction in piezoelectric coefficient d₃₃ was observed. Finally, a piezoelectric microbalance with mass sensitivity of 150 kHz/mg was fabricated using the materials and methods developed.     

Direct and converse piezoelectric grain-size effects in BaTiO3 ceramics with different Ba/Ti ratios

The grain-size effects of the direct piezoelectric coefficient (d₃₃) and the converse piezoelectric coefficient (d₃₃*) of BaTiO₃ ceramics with different Ba/Ti ratios were systematically explored. It was found that both d₃₃ and d₃₃* exhibited strong grain size (g) dependences for BaTiO₃ ceramics with various Ba/Ti ratios. Although d₃₃ showed similar grain-size dependence for all the Ba/Ti ratios except a subtle shift of the critical grain size from 1?μm to 3?μm, two entirely different grain-size dependence of d₃₃* were observed. By carefully examining the microstructure and ferroelectric properties of the ceramics, the variations of domain configurations and maximum polarization of BaTiO₃ ceramics with different Ba/Ti ratios were considered to be responsible for the different grain-size dependence of d₃₃ and d₃₃*, respectively.     

Properties and structures of nonstoichiometric (K,Na)NbO3‐based lead‐free ceramics

The 0.968[(K_0.48Na_0.52)]Nb_0.95+xSb_0.05O₃–0.032(Bi_0.5Na_0.5)ZrO₃ [KNN_xS–BNZ] lead‐free ceramics with nonstoichiometric niobium ion were fabricated via conventional solid‐state sintering technique and their piezoelectric, dielectric and ferroelectric properties were investigated. When x = 0.010, enhanced piezoelectric properties (d₃₃ ≈ 421 pC/N and k_p ≈ 0.47) were obtained due to the construction of rhombohendral—tetragonal phase boundary near room temperature. The KNN_xS–BNZ ceramics possesses enhanced Curie temperature (T_c) with improved piezoelectric constant. A large d₃₃ of ~421 pC/N and a high T_c ~256°C can be simultaneously induced in the ceramics with x = 0.010. Especially, good thermal stability was observed in a broad temperature range. The results indicated that our work could benefit development of KNN‐based ceramics and widen their application range.     

Effect of BBS‐Based Frit on the Low Temperature Sintering and Electrical Properties of KNN Lead‐Free Piezoceramics

Using BaO–B₂O₃–SiO₂ (BBS)‐based frit as sintering aid, the K_0.49Na_0.51NbO₃ (KNN) + x wt% BBS (x = 1.0, 1.5, 2.0 and 2.5) lead‐free piezoelectric ceramics were successfully fabricated by solid‐state reaction method under low‐sintering temperature of 1000°C. The effect of BBS frit doping amount on the structure and electrical properties of the ceramics was investigated. The KNN ceramics with 1.5 wt% BBS frit showed optimal properties as follows: piezoelectric constant d₃₃ = 108 pC/N, planar electromechanical coupling coefficient k_p = 41%, mechanical quality factor Q_m = 225, relative dielectric constant ε_r = 410, dielectric loss tanδ = 0.57% and Curie temperature T_c = 400°C. This ceramic sample should be a good lead‐free candidate for actuators or high temperature sensors application due to its ultra‐low tanδ, relatively high Q_m and T_c.     

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.     

KNN-based piezoelectric ceramic multilayer actuators with Ni internal electrodes

On the road of lead-free piezoelectric ceramics into practical applications, the study of Ni-internal-electrode (K, Na)NbO₃-based (KNN-based) multilayer actuators (MLAs) is an important part, possessing the advantages of environmentally friendly and low cost. The Ni-internal-electrode KNN-based MLAs with different layer numbers and layer thicknesses were fabricated via the tape casting method and sintered in the reducing atmosphere. The piezoelectric layers consist of the main KNN-based phase and a trace amount of second phase Mn₄Nb₂O₉. The element diffusion between the Ni electrodes and KNN-based grains is tiny, indicating that Ni is suitable for co-firing with KNN-based ceramics. After sintering, the compressive stress perpendicular to the thickness direction and the “relative tensile stress” parallel to the thickness direction are retained in the MLAs, bringing influences on the piezoelectric and dielectric properties of KNN-based materials. Compared with the bulk ceramics, the prepared MLAs significantly reduce the driving voltages and increase the displacement outputs, which are more applicable to the scenarios of miniaturization and portability. Particularly, the 46-layer actuator shows high displacements of 1580 and 2737 nm under the voltages of 100 and 200 V, respectively. However, the inverse piezoelectric coefficient d₃₃* for each layer of MLAs is still lower than that of the bulk ceramics, indicating that the piezoelectric properties of the KNN-based materials are suppressed. To give full play to the piezoelectric properties of KNN-based materials, more attentions should be paid to the design of reasonable electrode structure and the development of internal electrode paste for MLAs.     

Grain Size Effects on Piezoelectric Properties and Domain Structure of BaTiO3 Ceramics Prepared by Two‐Step Sintering

A series of highly dense barium titanate (BaTiO₃) ceramics with the average grain size (GS) from 0.29 to 8.61 μm are successfully prepared by two‐step sintering, and the GS effect on piezoelectric coefficient (d₃₃) is systematically discussed in this work. It is found that when GS above 1 μm, d₃₃ can be enhanced with decreasing GS, reaching a maximum value of 519 pC/N around 1 μm due to the high activity of domain wall mobility. Subsequently, d₃₃ rapidly drops with a further decrease in GS owing to the reduced domain density. The results suggest that it is possible to prepare high‐performance BaTiO₃ ceramics by controlling the GS and domain configuration properly, which brings great revitalization to the BaTiO₃‐based piezoceramics.     

Enhanced Piezoelectric Properties of (Ba1−xCax)(Ti0.92Sn0.08)O3 Lead‐Free Ceramics

(Ba_1?xCa_x)(Ti_0.92Sn_0.08)O₃ (x = 0.00–0.06 mol) ceramics with a high relative density over 96% were prepared by a conventional sintering method at 1480°C. At room temperature, a polymorphic phase transition from orthorhombic phase to tetragonal phase was confirmed by the XRD patterns in the composition range of 0.04 ≤ x ≤ 0.06. A high piezoelectric coefficient d₃₃ up to 568 pC/N was obtained at x = 0.05 mol, which is higher than the other component of (Ba,Ca)(Ti,Sn)O₃ system. At the same time, the corresponding planar electromechanical coupling factor k_p, converse piezoelectric coefficient dS/dE, and dielectric constant ε_r reach 47.7%, 1013 pm/V, and 23000, respectively. These results indicate that the (Ba,Ca)(Ti,Sn)O₃ ceramics are a promising candidate to replace for the lead‐based piezoelectric materials.     

Enhancement of piezoelectric properties of BF–BT ceramics by using ZrO2 powder bed during the sintering process

ZrO₂ powders of various particle sizes (0.15, 0.7, 500 µm) were used to simulate loose powder bed sintering to prepare BF–BT piezoelectric ceramics. The phase structure, dielectric properties, ferroelectric properties, and piezoelectric properties were compared with the samples sintered by the conventional powder bed method (i.e., powder of the same composition as the sample). Results showed that the use of loose ZrO₂ powder bed could improve the heat conduction rate and the sintering quality of bulk BF–BT piezoelectric ceramics. The XPS results showed that the samples sintered with 500 µm ZrO₂ powder beds had the lowest concentration of Fe²⁺, exhibited the largest piezoelectric coefficients (d₃₃ = 201 pC/N). In contrast, the sample sintered with a conventional powder bed under the same sintering conditions had a piezoelectric coefficient d₃₃ of 156 pC/N.