Electric‐Field‐Driven Phase Transition Process in (K,Na, Li)(Nb,Ta, Sb)O3 Lead‐Free Piezoceramics

The electric‐field‐driven phase transition in (K, Na, Li)(Nb, Ta, Sb)O₃ lead‐free piezoelectric ceramics was investigated by X‐ray diffraction, Raman spectra, and the temperature dependences of permittivity spectra. After poling under different electric fields, phase of the ceramics transformed gradually from orthorhombic–tetragonal coexisting phase to orthorhombic phase, indicating that the crystal structure of ceramics was strongly sensitive to electric field as an external stimulus. A secondary phase K₃Li₂Nb₅O₁₅ induced by electric field was detected in the ceramics with Li content of 7 mol%, which was close to the solubility limit of lithium. This field‐induced secondary phase resulted from the movement of Li ions and the structural deformation induced by electric field. Moreover, piezoelectric constant d₃₃ increased with the increasing poling field strength and the enhancement can be attributed to the field‐triggered domain switching. This study implied that in addition to temperature and composition, which has been reported in previous researches, electric field might be an effective way for inducing phase transition in lead‐free piezoelectric ceramics and improving the electrical performances simultaneously.     

Electrical Properties and Relaxor Phase Evolution of Li‐Modified BNT‐BKT‐BT Lead‐Free Ceramics

By conventional ceramics sintering technique, the lead‐free 0.85Bi_0.5Na_0.5(1?x)Li_0.5xTiO₃‐0.11Bi_0.5K_0.5TiO₃‐0.04BaTiO₃ (x =0–0.15) piezoelectric ceramics were obtained and the effects of Li dopant on the piezoelectric, dielectric, and ferroelectric properties were studied. With increasing Li addition, the temperature‐dependent permittivity exhibited the normal ferroelectric‐to‐ergodic relaxor (FE‐to‐ER) transition temperature (T_FE‐ER, abbreviated as T_F‐R) decreasing down to room temperature. The increasing Li content also enhanced the diffuseness of the FE‐to‐ER transition behavior. For composition with x = 0.15, a large unipolar strain of 0.37% ( = S_max/E_max = 570 pm/V) was achieved under 6.5 kV/mm applied electric field at room temperature. Both unipolar and bipolar strain curves related to the temperature closely, and when the temperature reached the T_F‐R, the normalized strain achieved a maximum value (e.g., for x = 0.10, = 755 pm/V) owing to the electric‐field‐induced ER‐to‐FE state transition.     

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.     

Phase Diagrams and Electromechanical Strains in Lead‐Free BNT‐Based Ternary Perovskite Compounds

Lead‐free compounds based on perovskite solid solutions in the ternary system (Bi_0.5Na_0.5)TiO₃‐MeNbO₃‐SrTiO₃ (BNT‐MeNbO₃‐ST, Me=K, Na,(K_0.5Na_0.5)) were fabricated using a conventional solid‐state reaction method. The effect of ST addition to BNT‐MeNbO₃ ceramics on the strain behavior, dielectric, ferroelectric, pyroelectric, and piezoelectric properties was systematically investigated to search the lead‐free piezoelectric materials with an excellent actuating performance. The phase diagrams based on the phase transition temperatures were obtained and the relationship between the phase structure and the electrical properties of the BNT‐MeNbO₃‐ST system has been clarified. A qualitative correlation between the morphotropic phase boundary (MPB)composition and tolerance factor t in BNT‐based systems has also been established and the present work indicates that the t value corresponding to the formation of MPB located in a quite narrow range and provides a guideline to predict the approximate MPB region quickly for new solid solutions. The Raman‐spectra analysis, macroscopic properties, and temperature‐dependent relationships of both polarization and strain demonstrated that the large strain response for this investigated system originates from a field‐induced relaxor to ferroelectric phase transformation.     

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.     

(K,Na, Li)(Nb,Ta)O3:Mn Lead‐Free Single Crystal with High Piezoelectric Properties

Lead‐free single crystal, (K, Na, Li)(Nb, Ta)O₃:Mn, was successfully grown using top‐seeded solution growth method. Complete matrix of dielectric, piezoelectric, and elastic constants for [001]_C poled single crystal was determined. The piezoelectric coefficient d₃₃ measured by the resonance method was 545 pC/N, which is almost three times that of its ceramic counterpart. The values measured by the Berlincourt meter ( = 630 pC/N) and strain–field curve ( = 870 pm/V) were even higher. The differences were assumed to relate with the different extrinsic contributions of domain wall vibration and domain wall translation during the measurements by different approaches, where the intrinsic contribution (on the order of 539 pm/V) was supposed to be the same. The crystal has ultrahigh electromechanical coupling factor (k₃₃ ~95%) and high ultrasound velocity, which make it promising for high‐frequency medical transducer applications.     

Temperature‐ and E‐field‐dependent domain configuration and electrical properties in (K,Na, Li)(Nb,Ta, Sb)O3 single crystal

E‐field‐ and temperature‐dependent domain evolution of lead‐free tetragonal (K, Na, Li)(Nb, Sb, Ta)O₃ (KNLNTS) single crystals as well as its corresponding electrical properties have been investigated. When E field is applied along [011]_C direction, (2T) engineered domain structure is formed. Spontaneous polarizations switch under a critical electric field (around 4‐5 kV/cm), resulting in significant changes in domain structure and great improvement in piezoelectric properties. Furthermore, it is found that piezoelectric constant d₃₁ and electromechanical coupling factor k₃₁ of [011]_C poled KNLNTS single crystal decrease with temperature. The extrinsic and intrinsic piezoelectric responses are discussed from the viewpoint of domain structure and lattice distortion, respectively. Our results show that the nanodomain structure relaxes and the lattice distortion declines with temperature, resulting in reduction of extrinsic and intrinsic piezoelectric responses, respectively. Therefore, the piezoelectric instability is ascribed to the decrease of both extrinsic and intrinsic contributions. This work provides a better understanding of domain engineering technique, and the useful information on the improvement of both piezoelectricity and temperature stability of the lead‐free piezoelectric materials.     

Theoretical Prediction and Experimental Validation of Enhancing the Piezoelectric Properties of (K,Na)NbO3 Modified by Li,Ta, and Sb According to the Linear Combination Rule

Lead‐free piezoelectric ceramics of (K_, Na)NbO₃ modified by Li, Ta, and Sb (KNN‐LTS) have been widely investigated recently. In this research, this optimized composition of KNN‐LTS ceramics near polymorphic phase transition is explored according to the linear combination rule (LCR) for the first time. Changing with the compositions monotonically, remanent polarization (P_r) decreased monotonically, whereas permittivity () increased similarly. The increase in either or P_r initially enhances piezoelectric coefficient d₃₃ before reducing it because d₃₃ can be improved by and P_r. The optimal composition of (Na_0.52K_0.4415Li_0.0385)(Nb_0.8735Ta_0.064Sb_0.0625)O₃, predicted by LCR, exhibits the excellent electric properties of d₃₃ = 359 pC/N, k_p = 42%, thus suggesting that the LCR effectively predicts the electric properties of the KNN‐LTS ceramics.     

Low Temperature Sintering and Enhanced Piezoelectricity of Lead‐Free (Na0.52K0.4425Li0.0375)(Nb0.86Ta0.06Sb0.08)O3 Ceramics Prepared from Nano‐Powders

(Na_0.52K_0.4425Li_0.0375)(Nb_0.86Ta_0.06Sb_0.08)O₃ powders were synthesized via sol–gel and solid‐state reaction methods as a raw material for the preparation of the ceramics. Dependence of piezoelectric properties and microstructure on sintering temperatures was investigated in this study. Sol–gel‐derived nano‐powders could be densified at a lower temperature of 940°C and exhibited excellent electrical properties after sintering at 1020°C (d₃₃ = 424 pC/N, d₃₃^* = 780 pm/V, k_p = 52.1%, and T_c = 265°C). The enhanced electric properties were most likely due to the coexistence of orthorhombic and tetragonal phase in the samples at room temperature, homogenous microstructure with fine grain and high density.     

Lead free (Li,Na,K)(Nb,Ta,Sb)O3 piezoelectric ceramics: Influence of sintering atmosphere and ZrO2 doping on densification, microstructure and piezoelectric properties

(Li/Na/K)(Nb/Ta/Sb)O₃ lead free piezoelectric materials (LTLS), “pure” or doped by 1 mass% ZrO₂, were elaborated. The sintering was investigated both by classical way and buried in a powder-bed in order to prevent the alkali-elements losses. The effects of doping and of these processes on densification and piezoelectric properties (d₃₃, k_t and k_p) were studied. The results demonstrated that the final densification is enhanced for ZrO₂ doped samples. The sintering in powder-bed clearly enhanced the piezoelectric properties. The best results corresponds to d₃₃ ≈ 180 pC/N and k_t ≈ 45-50% for non-doped samples and, d₃₃ ≈ 150 pC/N, with k_t ≈ 45-50% for ZrO₂ doped samples. Finally, the ultrasonic properties of piezoelectric transducers based upon these materials were also investigated experimentally and theoretically. The results clearly demonstrated that their bandwidth and sensitivity are suitable for use in piezoelectric transducers and comparable with similar PZT based transducers.     

Structure,microstructure and electrical properties of Cu2+ doped (K,Na,Li)(Nb,Ta,Sb)O3 piezoelectric ceramics

This work studies the effects of copper doping on the properties of the (K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃ piezoelectric ceramic material. Cu²⁺ incorporation into the perovskite structure produces a transformation of the crystalline lattice from tetragonal to orthorhombic symmetry together with an increase of the secondary phase. The grain size of the ceramic samples is increased due to the formation of a liquid phase during sintering, which increases with the Cu²⁺ content. EDS analysis reveals that the secondary-phase regions present a Cu and Nb-rich composition, indicating that the Cu-excess accommodates through the formation of this secondary phase. Cu-doping induces a rapid increase of the orthorhombic–tetragonal phase transition temperature, while the tetragonal–cubic phase transition temperature is decreased, the latter becoming more diffuse with the increase of Cu content. The piezoelectric properties of the material are reduced with the copper concentration, whereas the mechanical quality factor increases by a factor of nearly four.     

Phase transition,microstructure, and dielectric properties of Li/Ta/Sb co-doped (K,Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.93−xTa_xSb_0.07)O₃ (abbreviated KNLNST_x) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130 °C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNST_x ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TTB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375<x<0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST_0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d₃₃=313 pC/N, k_p=47%, and ε_r=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d₃₃=200 pC/N and ε_r=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics.     

Simple,Rapid and Precise Determination of Free Sulfuric Acid in Unstabilized Cellulose Nitrate by the Lead Nitrate–Dithizone Titration Method

Simple, rapid and precise determination of free sulfuric acid in unstabilized cellulose nitrate is accomplished by dissolving in 90 % v/v acetone–water solution and subsequent titration of sulfate by the lead nitrate–dithizone titration method. The method is proved accurately and sensitively for tracing very small amounts of free sulfuric acid present in all types of unstabilized cellulose nitrates. One more advantage of the method is that free nitric acid do not interfere in the determination, and, hence, there is no need to estimate its value and to correct the value for the free sulfuric acid content; the determination of free nitric acid, which is likely to be present with sulfuric acid in unstabilized cellulose nitrate is a very difficult problem.     

Pseudo Phase Diagram and Microwave Dielectric Properties of Li2O–MgO–TiO2 Ternary System

Li₂O–MgO–TiO₂ ternary system is an important microwave dielectric ceramic material with excellent properties and prospect in both scientific research and application. A phase diagram of the Li₂O–MgO–TiO₂ ternary system was established in this article, based on earlier research results and our present work. Microwave dielectric properties with compositions in different regions of the phase diagram have been analyzed. We found that the 0.33 Li₂MgTi₃O₈–0.67 Li₂TiO₃ ceramics sintered at 1200°C exhibited excellent dielectric properties: Q × f value = 80 476 GHz (at 7.681 GHz), ε_r = 24.7, τ_f = +3.2 ppm/°C. We also designed two ceramic systems in the Li‐rich region of the Li₂O–MgO–TiO₂ ternary system, which received little attention in the past decades, because many excellent single‐phase ceramics, such as Li₂MgTiO₄, Li₂MgTi₃O₈ and MgTiO₃, have been found in the Ti‐rich region. The ceramic systems have low sintering temperatures but also relatively poor dielectric properties.     

Structural, microstructural and electrical properties evolution of (K,Na,Li)(Nb,Ta,Sb)O3 lead-free piezoceramics through NiO doping

Excellent piezoelectric properties have been reported in the (K,Na)NbO₃-LiTaO₃-LiSbO₃ system and have been regarded as a new candidate of lead-free piezoelectric material. Nevertheless, there are still some structural and electrical aspects that remain controversial with respect to the role of dopants in this system. NiO doping modifies the (K,Na,Li)(Nb,Ta,Sb)O₃ structure, giving rise to the appearance of the TTB-like secondary phase and to changes on the orthorhombic to tetragonal phase transition temperature. The microstructural characterization reveals that sintering process is assisted by a transient liquid phase. The presence of Ni in the liquid phase indicates that Ni^II ions could act as new nucleus for the secondary phase crystallization. Thus, as higher the amounts of liquid phase, higher the secondary phase appearance. The modifications on the structure and microstructure of the system cause a reduction of the piezoelectric constant, which is accompanied by an increase on the mechanical quality factor.     

High‐Temperature Phase Relations in the Lu2O3–Ta2O5 System

The compounds formed from the Lu₂O₃–Ta₂O₅ system in the composition range 25–60 mol% Ta₂O₅ were prepared by solid‐state reaction from 1350°C to 2058°C, and the phase transitions were investigated by X‐ray diffraction (XRD). Cubic Lu₃TaO₇, M′‐LuTaO₄, M‐LuTaO₄, O‐Ta₂O₅, and T‐Ta₂O₅ are observed. With the temperature increase, there is an irreversible phase transition from M′ to M‐LuTaO₄ near 1770°C in the composition of 30–52 mol% Ta₂O₅, and another phase transition from T‐Ta₂O₅ to O‐Ta₂O₅ at about 1685°C when the ratio of Ta₂O₅ is >52 and ≤60 mol%. A phase diagram of the Lu₂O₃–Ta₂O₅ system in the range 0–100 mol% Ta₂O₅ was constructed. These results are helpful to explain the phase transition of Lu₂O₃–Ta₂O₅ system and design the preparation technique of LuTaO₄ single crystal or ceramic scintillator, which may be applied in the fields of nuclear medicine and high‐energy physics.     

High Electric‐Induced Strain and Temperature‐Dependent Piezoelectric Properties of 0.75BF–0.25BZT Lead‐Free Ceramics

0.75BiFeO₃–0.25Ba(Zr_xTi_1?x) + 0.6 wt% MnO₂ (0.75BF–0.25BZT) ceramics with Mn addition were prepared by the solid‐state reaction method. The high‐field strain and high‐temperature piezoelectric properties of 0.75BF–0.25BZT ceramics were studied. Introduction of Zr in the solid solutions decreased the Curie temperature slightly, and improved the dielectric and piezoelectric properties obviously. The piezoelectric properties of 0.75BZT–0.25BT ceramics reached the maximum at Zr content of 10 mol%. The Curie temperature T_c, dielectric constant ε and loss tanδ (1 kHz), piezoelectric constant d₃₃, and planner electromechanical coupling factor k_p of 0.75BF–0.25BZT ceramics with 10 mol% Zr were 456°C, 650, 5%, 138 pC/N, and 0.30, respectively. The high‐field bipolar and unipolar strain under an electric field of 100 kV/cm reached up to 0.55% and 0.265%, respectively, which were comparable to those of BiScO₃–PbTiO₃ and “soft” PZT‐based ceramics. The typical “butterfly”‐shaped bipolar strain and frequency‐dependent peak‐to‐peak strain indicated that the large high‐field‐induced strain may be due to non‐180° domain switching. Rayleigh analysis reflected that the improved piezoelectric properties resulted from the enhanced extrinsic contribution by Zr doping. The unipolar strain of 0.75BF‐0.25BZT ceramics with 10 mol% Zr was almost linear from RT to 200°C. These results indicated that 0.75BF–0.25BZT ceramics were promising candidates for high‐temperature and lead‐free piezoelectric actuators.     

Phase Diagram and Enhanced Piezoelectric Response of Lead‐Free BaTiO3–CaTiO3–BaHfO3 System

Three composition groups in the BaTiO₃–CaTiO₃–BaHfO₃ ternary system, (1 ? x)Ba(Hf_yTi_1?y)O₃–x(Ba_1?zCa_z)TiO₃ (abbreviated as BH_yT–xBC_zT with x = 0–1.0, y = 0.16, z = 0.20; x = 0–1.0, y = 0.16, z = 0.30; x = 0–1.0, y = 0.20, z = 0.30, respectively), have been designed for investigating the variation of the intermediate O‐phase region and its effect on the electrical properties. The temperature‐composition phase diagram of each group has been proposed based on the X‐ray diffraction patterns and the temperature‐dependent dielectric behaviors. The enhanced piezoelectric properties are achieved in the O–T phase boundary compositions of the three groups, which are BH_0.16T–0.58BC_0.20T, BH_0.16T–0.48BC_0.30T and BH_0.20T–0.53BC_0.30T, respectively. Piezoelectric coefficient d₃₃ of 448 pC/N is obtained in BH_0.20T–0.53BC_0.30T, which is higher than those of the other two phase boundary compositions. The O‐phase zone in BH_0.20T–xBC_0.30T is narrower than those in the BH_0.16T–xBC_0.20T and BH_0.16T–xBC_0.30T. In spite of its small O‐phase volume occupancy, the O‐phase plays a key role in the properties of the system. Our work confirms that the enhancement in piezoelectric properties is not only related to the O–T phase boundary near room temperature (RT), but also related to the shift of T_R‐O toward RT. In addition, a quantitative relation between the phase boundary composition and atomic mole ratio of Hf to Ca (R_m) is proposed, which the R_m value corresponding to the phase boundary is about 0.58. The results clearly demonstrate that in this system high piezoelectric properties are achieved by tuning the specific R_m value. Such a work may provide new clues for designing lead‐free piezoelectric materials with enhanced piezoelectric property.     

X-射线荧光光谱测定铅基合金中的砷、铜、锑、锡、铋和铅

建立了X-射线荧光光谱测定铅基合金中砷、铜、锑、锡、铋、铅等6种元素的快速分析方法。通过优化测试条件,确定了最佳测试参数,有效地避免了其他元素的干扰。基于干扰元素的强度和浓度,采用经验影响系数法校正绘制标准曲线。结果表明,在一定浓度范围内,砷、铜、锑、锡、铋、铅的标准曲线具有良好的线性关系,其中铋、锑和锡的相关系数均在0.999以上。精密度试验结果表明,测定结果相对标准偏差在0.02%~4.2%。准确度试验结果显示,测定值与参考值基本吻合。     

The stability of direct carbon fuel cells with molten Sb and Sb–Bi alloy anodes

The long‐term stability of direct carbon fuel cells, based on solid oxide fuel cells with molten Sb and Sb–Bi anodes, was examined for operation with activated charcoal, rice starch, and bio‐oil fuels at 973 K. With intermittent stirring of the fuel–metal anode interface, the anode performance was stable, and reasonable power densities (～250 mW/cm²) were achieved for periods up to 250 h. With Sc‐stabilized zirconia, severe thinning of the electrolyte occurred in regions of high current flow. No electrolyte thinning was observed with yttria‐stabilized zirconia as the electrolyte operating at the same current densities. © 2012 American Institute of Chemical Engineers AIChE J, 59: 3342–3348, 2013