Design on improving piezoelectric strain and temperature stability of KNN-based ceramics

Lead-free 0.99(0.96K_0.46Na_0.54Nb_1-xTa_xO₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃)-0.01CaZrO₃ (0.99(0.96KNNTax-0.04BNZ)-0.01CZ) ceramics were prepared by a solid-state sintering method. Ta₂O₅ doped in the 0.99(0.96KNNTax-0.04BNZ)-0.01CZ ceramics results in a phase structure transition from the orthorhombic (O)/tetragonal (T) phase to the rhombohedral (R)/T phase. The Ta₂O₅ dopant induces a decrease in the average grain size from ~1.70 to ~0.69 μm. At x = 0.02 and 0.04, the ceramics have a high reverse piezoelectric coefficient (~500 pm/V under 25 kV/cm). The ceramics with x = 0.04 show an optimal level of unipolar strain, reaching 0.17% under 35 kV/cm at room temperature, and their field-induced strain varies <10% in the temperature range from 25 to 135°C. The presence of the O phase in the polymorphic phase boundary (PPB) improves the temperature stability the reverse piezoelectric coefficient (). Obtaining KNN-based ceramics with good piezoelectric properties and weak temperature sensitivity by designing a R/O/T phase boundary and controlling the average grain size to the submicrometer level is highly feasible.     

Piezoelectric and ferroelectric properties of (Bi,Na)TiO3–(Bi,Li)TiO3–(Bi,K)TiO3 ceramics for accelerometer application

The piezoelectric and ferroelectric properties of 0.76(Bi_0.5Na_0.5)TiO₃–0.04(Bi_0.5Li_0.5)TiO₃–0.2(Bi_0.5K_0.5)TiO₃ (abbreviated as 0.76BNT–0.04BLT–0.2BKT) ceramics were investigated to clarify the optimal sintering temperature, and the vibration characteristics were examined for a compression‐mode accelerometer assembly in which 0.76BNT–0.04BLT–0.2BKT ceramics sintered at the optimized temperature served as the piezoelectric elements. The increase in the grain size of the 0.76BNT–0.04BLT–0.2BKT ceramics with the sintering temperature provides a beneficial contribution to the piezoelectric coefficient; however, it detrimentally contributes to the depolarization temperature. The charge sensitivity of the prototype accelerometers was evaluated with changes in the seismic mass and the layer number of the piezoceramics. The deviation between the theoretical and measured values of charge sensitivity was less than 10%.     

Orthorhombic‐pseudocubic phase transition and piezoelectric properties of (Na0.5K0.5)(Nb1−xSbx)‐SrZrO3 ceramics

0.96(Na_0.5K_0.5)(Nb_1?xSb_x)‐0.04SrZrO₃ ceramics with 0.0≤x≤0.06 were well sintered at 1060°C for 6 hours without a secondary phase. Orthorhombic‐tetragonal transition temperature (T_O‐T) and Curie temperature (T_C) decreased with the addition of Sb₂O₅. The decrease in T_C was considerable compared to that in T_O‐T, and thus the tetragonal phase zone disappeared when x exceeded 0.03. Therefore, a broad peak for orthorhombic‐pseudocubic transition as opposed to that for orthorhombic‐tetragonal transition appeared at 115°C‐78.2°C for specimens with 0.04≤x≤0.06. An orthorhombic structure was observed for specimens with x≤0.03. However, the polymorphic phase boundary structure containing orthorhombic and pseudocubic structures was formed for the specimens 0.04≤x≤0.06. Furthermore, a specimen with x=0.055 exhibited a large piezoelectric strain constant of 325 pC/N, indicating that the coexistence of orthorhombic and pseudocubic structures could improve the piezoelectric properties of (Na_0.5K_0.5)NbO₃‐based lead‐free piezoelectric ceramics.     

Temperature stability and electrical properties in La‐doped KNN‐based ceramics

To improve the temperature stability and electrical properties of KNN‐based ceramics, we simultaneously consider the phase boundary and the addition of rare earth element (La), 0.96K_0.5Na_0.5Nb_0.96Sb_0.04O₃‐0.04(Bi_1‐xLa_x)_0.5Na_0.5ZrO₃ (0 ≤ x ≤ 1.0) ceramics. More specifically, we investigate how the phase boundary and the addition of La³⁺ affect the phase structure, electrical properties, and temperature stability of the ceramic. We show that increasing the La³⁺ content leads to a change in phase structure, from a rhombohedral‐tetragonal (R‐T) phase coexistence to a cubic phase. More importantly, we show that the appropriate addition of La³⁺ (x = 0.2) can simultaneously improve the unipolar strain (from 0.127% to 0.147%) and the temperature stability (i.e., the unipolar strain of 0.147% remains unchanged when T is increased from 25 to 80°C). In addition, we find that the ceramics with x = 0.2 exhibit a large piezoelectric constant (d₃₃) of ~430 pC/N, a high Curie temperature (T_C) of ~240°C and a fatigue‐free behavior (after 10⁶ electric cycles). The enhanced electrical properties mostly originate from the easy domain switching, whereas the improved temperature stability can be attributed to the R‐T phase boundary and the appropriate addition of La³⁺.     

Enhanced temperature stability and fatigue-resistant behavior in MgTiO3-doped 0.948(K0.5Na0.5)NbO3–0.052LiSbO3 lead-free ceramics

0.948(K_0.5Na_0.5)NbO₃–0.052LiSbO₃–xMgTiO₃ (x=0, 0.005 and 0.010) (abbreviated as KNLNS–xMT) lead-free piezoelectric ceramics were prepared by normal sintering. The effect of MT addition on KNLNS ceramics was investigated through dielectric, ferroelectric and electric field-induced strain characterizations. The grain size decreased slightly after the addition of MT, and more uniform grains were obtained. Impedance measurements made over a wide range of temperatures (425–525 °C) showed the presence of both bulk and grain boundary effects in the materials. The activation energies E_a were 0.483 and 0.507 eV for KNLNS and KNLNS–0.005MT ceramics respectively, indicating that the conduction process was due to oxygen vacancies in the higher temperature region. The P_r and unipolar strain of the MT modified ceramics exhibited lower temperature sensibility than KNLNS ceramics in the temperature range 30–120 °C. Meanwhile, the MT doped samples showed less degradation in both switchable polarization and unipolar strain after 10⁶ switching cycles than those of KNLNS. It is expected that the KNLNS–xMT ceramics is promising candidate for lead-free piezoceramics and could be used in practical applications.     

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.     

Enhanced piezoelectric properties and electrocaloric effect in novel lead‐free (Bi0.5K0.5)TiO3‐La(Mg0.5Ti0.5)O3 ceramics

The crystal structure, electromechanical properties, and electrocaloric effect (ECE) in novel lead‐free (Bi_0.5K_0.5)TiO₃‐La(Mg_0.5Ti_0.5)O₃ ceramics were investigated. A morphotropic phase boundary (MPB) between the tetragonal and pseudocubic phase was found at x = 0.01‐0.02. In addition, the relaxor properties were enhanced with increasing the La(Mg_0.5Ti_0.5)O₃ content. In situ high‐temperature X‐ray diffraction patterns and Raman spectra were characterized to elucidate the phase transition behavior. The enhanced ECE (ΔT = 1.19 K) and piezoelectric coefficient (d₃₃ = 103 pC/N) were obtained for x = 0.01 at room temperature. Meanwhile, the temperature stability of the ECE was considered to be related to the high depolarization temperature and relaxor characteristics of the Bi_0.5K_0.5TiO₃‐based ceramics. The above results suggest that the piezoelectric and ECE properties can be simultaneously enhanced by establishing an MPB. These results also demonstrate the great potential of the studied systems for solid‐state cooling applications and piezoelectric‐based devices.     

Electromechanical Properties of Acceptor‐Doped Lead‐Free Piezoelectric Ceramics

We have studied the processing and electromechanical properties of Mn and Fe‐doped 0.88[Bi_0.5Na_0.5TiO₃]–0.08[Bi_0.5K_0.5TiO₃]–0.04[Bi_0.5Li_0.5TiO₃] piezoelectric ceramics prepared by the mixed oxide route. Different amounts of Mn (0.01, 0.014, 0.015, 0.016, 0.017, 0.02, 0.022) or Fe (0.0125, 0.015, 0.0175) were doped to this lead‐free piezoelectric composition. Ceramics were sintered at different temperatures (1075°C–1150°C) to achieve the highest density and mechanical quality factor. Mn or Fe doping resulted in a considerable enhancement of Q_m in both planar and thickness resonance modes. In 1.5 mol% Mn‐doped ceramics sintered at 1100°C, a planar Q_m of about 970 and tanδ of 0.88% were obtained. In Fe‐doped ceramics, a planar Q_m as high as 900 was achieved. Acceptor dopants also resulted in decreasing the coupling coefficients, the piezoelectric charge coefficient, and the dielectric constant.     

Effects of Bi0.5Na0.5HfO3 addition on the phase structure and piezoelectric properties of (K,Na)NbO3‐based ceramics

Lead‐free perovskite (1‐x)(K_0.48Na_0.48Li_0.04)Nb_0.95Sb_0.05O₃‐x(Bi_0.5Na_0.5)HfO₃ piezoelectric ceramics were prepared by a traditional ceramic fabrication method. An investigation was conducted to assess the effects of (Bi_0.5Na_0.5)HfO₃ content on the crystal structure, microstructure, phase‐transition temperatures, and piezoelectric properties of the ceramics. The X‐ray diffraction results, combined with the temperature dependence of dielectric properties, revealed that the ceramics experienced a structural transition from an orthorhombic phase to a tetragonal phase with the addition of (Bi_0.5Na_0.5)HfO₃, and a coexistence of orthorhombic and tetragonal phases was identified in the composition range of 0.005≤x≤0.015. An obviously improved piezoelectric activity was obtained for the ceramics with compositions near the orthorhombic‐tetragonal phase boundary, among which the composition x=0.005 exhibited the maximum values of piezoelectric constant d₃₃, and planar and thickness electromechanical coupling coefficients (k_p and k_t) of 246 pC/N, 0.435, and 0.554, respectively. Furthermore, the Curie temperature of the ceramics was found decreasing with the increase in (Bi_0.5Na_0.5)HfO₃ content, but still maintaining above 300°C for the phase boundary compositions. These results indicate that the ceramics are promising lead‐free candidate materials for piezoelectric applications.     

Low‐temperature sintering and enhanced piezoelectric properties of random and textured PIN–PMN–PT ceramics with Li2CO3

Low‐temperature sintered random and textured 36PIN–30PMN–34PT piezoelectric ceramics were successfully synthesized at a temperature as low as 950°C using Li₂CO₃ as sintering aids. The effects of Li₂CO₃ addition on microstructure, dielectric, ferroelectric, and piezoelectric properties in 36PIN–30PMN–34PT ternary system were systematically investigated. The results showed that the grain size of the specimens increased with the addition of sintering aids. The optimum properties for the random samples were obtained at 0.5 wt% Li₂CO₃ addition, with piezoelectric constant d₃₃ of 450 pC/N, planar electromechanical coupling coefficient k_p of 49%, peak permittivity ε_max of 25 612, remanent polarization P_r of 36.3 μC/cm². Moreover, the low‐temperature‐sintered textured samples at 0.5 wt% Li₂CO₃ addition exhibited a higher piezoelectric constant d₃₃ of 560 pC/N. These results indicated that the low‐temperature‐sintered 36PIN–30PMN–34PT piezoelectric ceramics were very promising candidates for the multilayer piezoelectric applications.     

Bi(Zn2/3Nb1/3)O3–(K0.5Na0.5)NbO3 High‐Temperature Lead‐FreeFerroelectric Ceramics with Low Capacitance Variation in a Broad Temperature Usage Range

The xBi(Zn_2/3Nb_1/3)O₃–(1?x)(K_0.5Na_0.5)NbO₃ (abbreviated as xBZN–(1?x)KNN) ceramics have been synthesized using the conventional solid‐state sintering method. The phase structure, dielectric properties and “relaxorlike” behavior of the ceramics were investigated. The 0.03BZN–0.97KNN ceramics show a broad and stable permittivity maximum near 2000 and lower dielectric loss (≤5%) at a broad temperature usage range (100°C–400°C) and the capacitance variation (ΔC/C_150°C) is maintained smaller than ±15%. The 0.03BZN–0.97KNN ceramics only possess the diffuse phase transition and no frequency dispersion of dielectric permittivity, which indicates that 0.03BZN–0.97KNN ceramics is a high temperature “relaxorlike” ferroelectric ceramics. These results indicate that 0.03BZN–0.97KNN ceramics are excellent promising candidates for preparing high‐temperature multilayer ceramics capacitors.     

Large recoverable energy storage density and low sintering temperature in potassium‐sodium niobate‐based ceramics for multilayer pulsed power capacitors

Multilayer pulsed power ceramic capacitors require that dielectric ceramics possess not only large recoverable energy storage density (W_rec) but also low sintering temperature (<950°C) for using the inexpensive metals as the electrodes. However, lead‐free bulk ceramics usually show low W_rec (<2 J/cm³) and high sintering temperature (>1150°C), limiting their applications in multilayer pulsed power ceramic capacitors. In this work, large W_rec (~4.02 J/cm³ at 400 kV/cm) and low sintering temperature (940°C) are simultaneously achieved in 0.9(K_0.5Na_0.5)NbO₃–0.1Bi(Mg_2/3Nb_1/3)O₃–1.0 mol% CuO ceramics prepared using transition liquid phase sintering. W_rec of 4.02 J/cm³ is 2‐3 times as large as the reported value of other (Bi_0.5Na_0.5)TiO₃ and BaTiO₃‐based lead‐free bulk ceramics. The results reveal that 0.9(K_0.5Na_0.5)NbO₃–0.1Bi(Mg_2/3Nb_1/3)O₃–1.0 mol% CuO ceramics are promising candidates for fabricating multilayer pulsed power ceramic capacitors.     

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.     

Structural and functional characterisation of KNNS–BNKZ lead-free piezoceramics

ABSTRACT

Promising piezoelectric properties have been reported recently for lead-free 0.96(K_0.48Na_0.52Nb_0.95Sb_0.05)–0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃ (KNNS–BNKZ) ceramics. The presence of coexisting rhombohedral and tetragonal phases is thought to play a key role in their functional properties, but a thorough understanding is currently lacking. In this experiment, (1?x)KNNS–(x)BNKZ ceramics with x = 0–0.05 were prepared by the mixed-oxide method. High-resolution synchrotron X-ray powder diffraction (SXPD) measurements reveal that the addition of BNKZ into KNNS ceramics leads to an increase in the rhombohedral–orthorhombic phase transition temperature (T_R?O) and a decrease in the orthorhombic–tetragonal phase transition temperature (T_O?T) leading to orthorhombic–tetragonal and rhombohedral–tetragonal phase coexistence at room temperature for compositions with x = 0.02 and 0.04, respectively. By combining the SXPD results with microstructure, evidence is also found for the occurrence of chemical heterogeneity, which could provide an additional means to control the functional properties. The structural observations are correlated with changes in the dielectric and ferroelectric properties.     

Structural evolution of the R‐T phase boundary in KNN‐based ceramics

Although a rhombohedral‐tetragonal (R‐T) phase boundary is known to substantially enhance the piezoelectric properties of potassium‐sodium niobate ceramics, the structural evolution of the R‐T phase boundary itself is still unclear. In this work, the structural evolution of R‐T phase boundary from ?150°C to 200°C is investigated in (0.99?x)K_0.5Na_0.5Nb_1?ySb_yO₃–0.01CaSnO₃–xBi_0.5K_0.5HfO₃ (where x = 0‐0.05 with y = 0.035, and y = 0‐0.07 with x = 0.03) ceramics. Through temperature‐dependent powder X‐ray diffraction (XRD) patterns and Raman spectra, the structural evolution was determined to be Rhombohedral (R, <?125°C)→Rhombohedral + Orthorhombic (R + O, ?125°C to 0°C)→Rhombohedral + Tetragonal (R + T, 0 °C to 150°C)→dominating Tetragonal (T, 200°C to Curie temperature (T_C)) → Cubic (C, >T_C). In addition, the enhanced electrical properties (e.g., a direct piezoelectric coefficient (d₃₃) of ~450 ± 5 pC/N, a conversion piezoelectric coefficient () of ~580 ± 5 pm/V, an electromechanical coupling factor (k_p) of ~0.50 ± 0.02, and T_C~250°C), fatigue‐free behavior, and good thermal stability were exhibited by the ceramics possessing the R‐T phase boundary. This work improves understanding of the physical mechanism behind the R‐T phase boundary in KNN‐based ceramics and is an important step toward their adoption in practical applications.     

Defect engineering of high‐performance potassium sodium niobate piezoelectric ceramics sintered in reducing atmosphere

Reducing atmosphere fired (K_0.5Na_0.5)NbO₃‐based ceramics open up an important opportunity for manufacturing lead‐free multilayer piezoelectric devices cofiring with the base metal inner electrode. In this study, the effects of sintering atmosphere on the piezoelectric, dielectric, and insulating properties in Sn‐doped (Na_0.52K_0.44Li_0.04)NbO₃ (KNN) were investigated. To establish the relationship between the defect structure and electrical properties, the thermally stimulated depolarization current (TSDC) technique was implemented. The electrical properties degrade severely when the pure KNN ceramics are sintered in reducing atmosphere, compared with the ceramics sintered in air. The reason is that the oxygen vacancy concentration in reduced fired ceramics is much higher than that in air fired ceramics. However, the 0.6 mol% Sn‐doped KNN ceramics sintered in reducing atmosphere exhibit comparable electrical properties to the air fired ceramics. From the TSDC analysis, the reducing atmosphere fired ceramics have approximately the same oxygen vacancy concentration as the air fired ceramics because B‐site substituted Sn works as an acceptor.     

Phase Transition and Large Electrostrain in Lead‐Free Li‐Doped (Ba,Ca)(Ti,Zr)O3 Ceramics

Large piezoelectric effect is achieved in Li‐doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃(BCTZ) ceramics by use of tuning the phase boundaries. Rhombohedral–orthorhombic (R–O) and orthorhombic–tetragonal (O–T) multiphase coexistence is constructed in the ceramics by changing Li contents. The high piezoelectric constant d₃₃ (493 pC/N) and large electrostrain (dS_max/dE_max = 931 pm/V) have been observed in the Li‐doped (Ba, Ca)(Ti, Zr)O₃ ceramics at low sintering temperature (1350°C/2 h). The significant enhancement in materials properties is ascribed to the multiphase region around room temperature induced by Li‐doped effect.     

Development of a lead‐free copper co‐fired BNT‐based piezoceramic sintered at low temperature

Compatibility of Bi‐based piezoelectric ceramic and copper electrodes is demonstrated by co‐firing 0.88Bi_1/2Na_1/2TiO₃–0.08Bi_1/2K_1/2TiO₃–0.04BaTiO₃ (BNKBT88) with copper. A combination of Bi₂O₃, CuO, ZnO, Li₂CO₃, and B₂O₃ are used as additives to reduce firing temperature to 900°C with minimal effect on the electromechanical properties compared to sintering at 1150°C without additives. Co‐firing with copper electrodes requires controlled oxygen sintering at low temperature. The atmosphere is controlled using carbon dioxide and hydrogen gas to maintain an oxygen partial pressure of 6.1 × 10^?8 atm, which is necessary for the coexistence of Cu metal and Bi₂O₃. The thermodynamic activity of bismuth oxide in BNKBT88 is calculated to be 0.38. BNKBT88 ceramics were successfully co‐fired with internal as well as surface Cu metal electrodes. The copper co‐fired ceramics were successfully polarized and the dielectric and piezoelectric properties are evaluated.     

Simultaneously enhanced piezoelectric response and piezoelectric voltage coefficient in textured KNN‐based ceramics

<001> ‐textured 0.99(K_0.49Na_0.49Li_0.02)(Nb_0.97‐xSb_0.03Ta_x)O₃‐0.01CaZrO₃ [abbreviated as 0.99KNLN_0.97‐xST_x‐0.01CZ, x = 0.03, 0.07, 0.10, 0.15, 0.20, 0.25] ceramics were prepared by templated grain growth (TGG) method and a two‐step sintering process. Giant longitudinal piezoelectric coefficient d₃₃ (391 pC/N) and piezoelectric strain coefficient d₃₃* (630 pm/V under an AC E‐field of 20 kV/cm) can be obtained in the textured ceramics with x = 0.25. All textured ceramics display superior k_p (>54%) and g₃₃ (>23 × 10^?3 Vm/N) which are in an order of magnitude with PZT ceramics. The maximum value of k_p (~63.3%) obtained in textured ceramics with x = 0.15 is higher than that of famous textured LF4 ceramics. Excellent comprehensive properties suggest that <001> ‐textured 0.99KNLN_0.97‐xST_x‐0.01CZ ceramics are promising candidates in the field of lead‐free piezoelectric materials.     

Improved thermal stability of the piezoelectric properties of (Li,Ag)‐co‐modified (K,Na) NbO3‐based ceramics prepared by spark plasma sintering

High‐performance lead‐free piezoelectric ceramics 0.94(K_0.45Na_0.55)_1?xLi_x(Nb_0.85Ta_0.15)O₃–0.06AgNbO₃ (KNNLTAg‐x) were successfully prepared by spark plasma sintering technique. The doping effect of Li on the structural and electrical properties of KNNLTAg‐x (x=0, 0.02, 0.04, 0.06, 0.08 and 0.10) ceramics was studied. The lattice structure, ferroelectric and piezoelectric properties of the KNLNTAg‐x ceramics are highly dependent on the Li doping level. In particular, the Li dopant has a great impact on both Curie temperature T_c and orthorhombic‐tetragonal transition temperature T_O‐T. The 4% Li‐doped sample exhibited relatively high T_O‐T of 95°C, leading to a stable dynamic piezoelectric coefficient (d₃₃*) of 220‐240 pm/V in a broad temperature range from 25°C to 105°C. Additionally, the 2% Li‐doped sample shows a high d₃₃* of 320 pm/V at 135°C, suggesting its great potential for high‐temperature applications.