Lead-free piezoelectric ceramics and thin films

Recent progress in lead-free piezoelectric ceramics and thin films with special emphasis on alkaline niobatebased and bismuth sodium titanate-based systems is reviewed concisely. Modifications of potassium sodium niobate (KNN) ceramics are presented and subsequent improvements in the electrical properties are summarized. Special attention is devoted to the phase diagram of the KNN system when a solid solution is formed with other perovskite niobates and titanates. Impact of A-site and B-site dopants on the electromechanical properties of KNN ceramics are distinguished in view of transition temperatures. It is shown that the addition of most A-site and B-site dopants reduces the transition temperatures and improves the piezoactivity at room temperature. This is attributed to the shift of polymorphic transition from tetragonal to orthorhombic phase in the vicinity of room temperature. In contrast, formation of a solid solution of KNN with 18 mol% AgNbO? revealed a significant enhancement of properties without a notable change in the transition temperatures. Also, a bismuth sodium titanate (BNT) composition is introduced with particular emphasis on its binary and ternary derivatives. Moderate piezoelectric properties reported at the morphotropic phase boundaries, formed in BNT-based solid solutions are also represented. Advances on thin films based on these two compositions are evaluated and challenges involved with development of stoichiometric thin films with low leakage current are discussed.     

Electric field-induced strain behavior in lithium- and copper-added potassium sodium niobate piezoceramics and 1-3 piezocomposites

Potassium sodium niobate (KNN)-based leadfree materials were prepared and their field-induced strain behaviors were investigated. Ceramic lead-free piezoelectric materials were prepared in bulk and fiber forms with 1 mol% CuO-added potassium sodium niobate K0.5Na0.5NbO3 and x = 7 mol% lithium-modified (K(0.5-x/2)Na(0.5-x/2)Li(x))NbO(3) compositions. Fibers were drawn using a novel alginate gelation technique. Piezocomposites were prepared from these fibers with 1-3 connectivity and an epoxy matrix. A fully recoverable electrostrain of up to approximately 0.11% was observed in the CuO-added sample, whereas the Li-modified sample yielded up to 0.10% at 50 kV/cm electric field. A strain value of up to approximately 0.03% at 50 kV/cm electric field was obtained for piezocomposites prepared from lithium-modified fibers. The high-field converse piezoelectric coefficient was calculated from the strain-electric field (x-E) graph for all samples. Strain characteristics of the bulk and piezocomposite samples were analyzed based on the variation of strain with respect to square of the polarization (x-P2) to determine the electrostrictive contribution to the strain.     

Electrical properties of Ni-particle-dispersed alkaline niobate composites sintered in a protective atmosphere

Co-firing of piezoceramics with metals is drawing ever-increasing attention. This article reports Ni-particle-dispersed [Li_0.06(K_0.5Na_0.5)_0.94]NbO₃ (LKNN/Ni) lead-free piezoelectric composites that were sintered in a protective atmosphere of nitrogen gas. The base metal Ni was not oxidized and the piezoelectric LKNN preserved the perovskite structure. The microstructure observations show that Ni particles were uniformly dispersed in LKNN ceramics matrix. With increasing Ni content from 0 to 20 vol%, the piezoelectric constant d₃₃ of the LKNN/Ni composites decreased from 165 to 23 pC/N, and the corresponding dielectric constant ?_r greatly increased. The 95%LKNN-5%Ni composite still exhibited a typical ferroelectric loop; however, the P-E curves measured for Ni > 5 vol% composites demonstrated some metallic characteristics. The LKNN/Ni composites could find applications in piezoelectric actuators with functionally graded microstructure (FGM).     

Sodium niobate particles with controlled morphology synthesized by hydrothermal method and their use as templates in KNN fibers

Sodium niobate – NN (NaNbO₃) powders were synthesized by hydrothermal process to be used as template particles in the fabrication of textured lead free piezoelectric ceramics. Sodium hexaniobate–Na₈Nb₆O₁₉⋅13H₂O particles with rod-like morphology were synthesized at 120 °C. Particles with needle-like morphology and Na₂(Nb₂O₆)(H₂O) phase started to form at temperatures of 130 °C and above. Synthesis at 150 °C yields particles with totally needle-like morphology and consisting entirely of the Na₂(Nb₂O₆)(H₂O) phase. Sodium niobate–NaNbO₃ particles with cubic morphology were synthesized at temperatures of 160 °C and above. Rod-like and needle-like morphology was retained even after annealing at 400 °C for 1 h. A preliminary study was also done to integrate these anisometric template particles in the preparation of textured potassium sodium niobate (KNN) fibers.     

铌酸钾钠无铅压电陶瓷薄膜的制备方法研究

铌酸钾钠无铅压电陶瓷薄膜具有居里温度高、机电耦合系数高、介电常数低等优点,制备方法种类繁多。综述了铌酸钾钠薄膜的制备技术,包括溶胶-凝胶法、脉冲激光沉积法和磁控溅射法等;讨论了铌酸钾钠薄膜各种方法的制备工艺与功能性质的关系等,指出了各种制备技术的优缺点及发展现状;最后提出了目前铌酸钾钠薄膜制备及应用中存在的一些问题及今后的发展方向。     

Realization of densified microstructure and large piezoelectricity in KNN ceramics via the addition of oxide additives

Journal of Materials Science: Materials in Electronics - Lead-free potassium sodium niobate [(K, Na)NbO3, KNN]-based ceramics have aroused great interest due to their excellent piezoelectricity and...     

The microstructural evolution in high sodium epitaxial sodium potassium niobate films deposited by low-temperature hydro-thermal method

Sodium potassium niobate [(K _x Na_1?x )NbO₃] films with high sodium composition (x = 0.06 and 0.24) were fabricated using a low-temperature (240 °C) hydro-thermal method on (001) niobium-doped strontium titanate (Nb-STO) substrate. The films were annealed subsequently at 600 °C. Thicknesses of the films were maintained in the range of ~800–1000 nm. Transmission electron microscopy-selected area electron diffraction studies revealed the appearance of super-spots, thereby confirming the tilting of the oxygen sub-lattice at both high Na compositions. The bright-field imaging and scanning transmission electron microscopy-energy dispersive spectroscopy elemental mapping revealed KNN film with K-rich interfacial regions and Na-rich top region of the film at both the high Na composition, whereas the potassium niobate (KNbO₃, x = 1) film showed no such oxygen sub-lattice tilting, and a sharp substrate-film interface was observed. The observed tilting of oxygen sub-lattice is a direct consequence of the reduced structural stability in high Na compositions in the KNN solid solutions.     

Lead-free (K,Na)NbO3 thin films derived from chemical solution deposition modified with EDTA

Potassium sodium niobate is an important and promising functional material due to its ferroelectric, piezoelectric, and pyro-electric properties, and have received much attention for decades. In this paper, lead-free (K, Na)NbO₃ (KNN)-based ferroelectric thin films were successfully prepared by a chemical solution approach using Nb₂O₅ as niobium resource instead of the expensive niobium ethoxide. The films with a pure perovskite phase were obtained by such an approach modified with ethylenediaminetetraacetic acid (EDTA). EDTA plays a positive role in suppressing the volatilization of alkali metal and facilitates a better film growth rate. EDTA introduction prevents the formation of the pyrochlore phase. The films synthesized with EDTA show large grains and good crystallization, indicating decreased nucleation temperature. The obtained KNN thin films exhibit low leakage current, large saturated polarization, and small coercive field. These results are attributed to the added EDTA, which is effective in inhibiting the formation of vacancies, and to the weakened effect of the defect pinning or clamping of domain walls.     

Porous Li-Na-K niobate bone-substitute ceramics: Microstructure and piezoelectric properties

Disc specimens of a porous ceramic, lithium sodium potassium niobate (Li_0.06Na_0.5K_0.44)NbO₃, were prepared using ammonium oxalate monohydrate or poly(methyl methacrylate) as pore-forming agent, and made piezoelectric using a modified polarizing method to preserve biocompatibility. Scanning electron microscopy showed a bicontinuous 3-3 structure of interconnected pores 150-250 µm in size. The piezoelectric constant (d₃₃) and electromechanical coupling coefficient (Kp) are discussed as a function of porosity and pore shape: d₃₃ fitted the theoretical expectation for shape factor K_s = 1, while Kp was approximately constant (~ 0.23) for porosity of 15-45 vol.%. 50 vol.% AOM gave the highest porosity (~ 45%) without decline of the value of Kp whilst maintaining mechanical integrity. Such materials show promise for use as a piezoelectric composite bone substitute.     

Lead-free piezoelectric composites with high piezoelectric performance and high dielectric constant caused by percolation phenomenon

We report on lead-free piezoelectric composite with high dielectric constant (ε _r > 10⁵) and the d ₃₃ (above 70 pC/N) comparable to typical lead based piezoelectric composites, or even higher, reaching as high as 107 pC/N. We achieved this through the combination of the good piezoelectric properties of the alkaline niobate (KNN) based perovskite with the flexibility of polymer poly(vinylidene fluoride) (PVDF). The dielectric properties observed in the KNN–PVDF-based piezoelectric composite were well explained in terms of an interfacial percolation model.     

Calcium phosphate coating of Ti–Nb–Zr–Sn titanium alloy

This study focuses on rapid formation of calcium phosphate coating on a β type Ti–Nb–Zr–Sn biomedical titanium alloy by alkali treatment. The results show that a bioconductive surface layer forms on specimens immersed in 1–5 M KOH solution but only treatment in 1 M KOH avoids formation of crevices, producing a potassium titanate layer with porous network structure. Heat treatment at 600 °C after the alkali treatment promotes titanate growth. Following the above treatments, a continuous apatite layer forms within 4 h of soaking in a calcium phosphate solution with high ionic concentration. Such rapid apatite formation is due to high concentration of calcium ions in the solution used in this study and the buffering function of NaHCO₃. Results of dissolution experiment show that Ca and P ions release gradually from the coating during soaking in a 0.9% NaCl solution, which may be helpful to the formation of natural bone if implanted in human body. Cell culture experiment shows that the apatite layer favours adhesion and proliferation of rat osteoblast as compared with coating-free Ti–Nb–Zr–Sn alloy and commercially pure titanium (CP Ti).     

Piezoelectric properties of (K,Na)NbO3 thin films deposited on (001)SrRuO3/Pt/MgO substrates

(K(x),Na(1-x))NbO(3) (KNN) thin films were deposited on (001)SrRuO(3)/(001)Pt/(001)MgO substrates by RF-magnetron sputtering, and their piezoelectric properties were investigated. The x-ray diffraction measurements indicated that the KNN thin films were epitaxially grown with the c-axis orientation in the perovskite tetragonal system. The lattice constant of the c-axis increased with increasing concentrations of potassium. The KNN thin films showed typical ferroelectric behavior; the relative dielectric constant epsilon(r) was 270 to approximately 320. The piezoelectric properties were measured from the tip displacement of the KNN/MgO unimorph cantilevers; the transverse piezoelectric coefficient epsilon*(31) (= d(31)/s(E)(11)) of KNN (x = 0) thin films was calculated to be -0.9 C/m(2). On the other hand, doping of potassium caused an increase in the piezoelectric properties, and the KNN (x = 0.16) films showed a relatively large transverse piezoelectricity of epsilon*(31) = -2.4 C/m(2).     

Effect of metallization on the power flow angle of SH0 waves in thin piezoelectric plates

The anisotropy of the angle between the phase and group velocity (power flow angle, PFA) as well as the influence of electrical boundary conditions on this angle have been studied for shear-horizontal fundamental (SH₀) acoustic waves in thin piezoelectric plates of lithium niobate (LNO) and potassium niobate (KNO). Both LNO and KNO crystals possess orientations at which the PFA for SH₀ waves reaches large values (17° and 48°, respectively). It is established that metallization of the piezoelectric plate surface can significantly change PFA values, in particular, by 18° and 55° for Y-X + 25° cut plates of LNO and KNO, respectively.     

The effect of illumination on the characteristics of acoustic waves in a piezoelectric plate-photoconducting layer structure

The effect of light on the characteristics of an acoustic wave with the transverse-horizontal polarization (SH₀) propagating in a piezoelectric plate-photoconducting layer structure was theoretically studied. Applied to the case of an Y-X cut lithium niobate (or Y-X potassium niobate) plate coated by a cadmium sulfide layer with the thicknesses h ₁ and h ₂, respectively, the results of this analysis show that the photosensitivity of such structures strongly depends on the h ₁ and h ₂ values, reaching a maximum for h ₁=0.1λ and h ₂=0.01λ (where λ is the acoustic wavelength). In the lithium niobate based structure with these parameters, the maximum value of the relative change in the wave velocity in response to the illumination intensity variation from 0.1 to 10⁶ lx is 16% at a maximum damping rate of 5 dB/λ; the analogous values for potassium niobate are 47% and 17 dB/λ. These results show good prospects for the use of such a piezoelectric plate-photoconducting layer structures in photodetectors and optical radiation sensors.     

纳米铌酸钾钠陶瓷的制备及性能

以新型溶胶-凝胶法制备的平均晶粒尺寸为30 nm的铌酸钾钠粉体为原料, 采用放电等离子体烧结工艺, 在烧结温度为900℃, 压力30 MPa, 烧结时间1 min的条件下, 制备得到纯正交相, 相对密度高达99%以上, 平均晶粒尺寸为40 nm的纳米铌酸钾钠陶瓷, 并对该陶瓷的相结构、微观形貌、介电性能和铁电性能进行了研究。结果表明, 与普通微米晶陶瓷不同, 纳米铌酸钾钠陶瓷的室温介电常数仅为341, 并且随温度变化不明显, 表现出明显的介电弛豫现象, 弥散因子γ为1.60, 并具有明显的电滞回线, 矫顽场强度为13.5 kV/cm, 剩余极化为1.5 μC/cm²。尺寸降低所引起的纳米铌酸钾钠陶瓷中晶界相所占的比例增大是其性能变化的主要原因, 并且可以推断, 如果铌酸钾钠陶瓷具有“临界尺寸”, 那么其值应该在40 nm以下。     

常压烧结制备透明铌酸钾钠陶瓷

在铌酸钾钠(KNN)陶瓷中掺杂锂和铋的氧化物作为晶粒生长抑制剂,用常压烧结工艺制备了铌酸钾钠透明陶瓷(K0.48-0.5xNa0.52-0.5xLixNb1-xBixO3,x=0.04～0.15)材料。研究了掺杂量、烧结工艺条件对陶瓷透明性、光电系数、晶相结构和微观形貌的影响。当x=0.09时,厚度为0.5mm陶瓷样品的红外透光率达到82%,光电系数为6.06×1011m/V。     

Structural aspects and thermal behavior of the proton-exchanged layered niobate K4Nb6O17

In this paper, we discuss some structural aspects of the proton exchange in the niobate K₄Nb₆O₁₇ and also the thermal stability of the related protonic niobate phase. The exchange amount of ca. 50% of interlayer potassium ions indicates that only the potassium ions located at interlayer I was exchanged by the protons. The XRD patterns confirm the formation of an interstratificated material. The thermal stability of the protonic phase is greatly reduced compared to the potassium phase. Above approximately 315 °C, the protonic layers begin to dehydroxylate causing the destruction of the lamellar structure and the formation of a mixture of H-Nb₂O₅ and K₂Nb₄O₁₁.     

Effects of Ge<Superscript>4+</Superscript> acceptor dopant on sintering and electrical properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoceramics

Lead-free (K_0.5Na_0.5)(Nb_1-xGe _x )O₃ (KNN-xGe, where x = 0-0.01) piezoelectric ceramics were prepared by conventional ceramic processing. The effects of Ge⁴⁺ cation doping on the phase compositions, microstructure and electrical properties of KNN ceramics were studied. SEM images show that Ge⁴⁺ cation doping improved the sintering and promoted the grain growth of the KNN ceramics. Dielectric and ferroelectric measurements proved that Ge⁴⁺ cations substituted Nb⁵⁺ ions as acceptors, and the Curie temperature (T_C) shows an almost linear decrease with increasing the Ge⁴⁺ content. Combining this result with microstructure observations and electrical measurements, it is concluded that the optimal sintering temperature for KNN-xGe ceramics was 1020°C. Ge⁴⁺ doping less than 0.4 mol.%can improve the compositional homogeneity and piezoelectric properties of KNN ceramics. The KNN-xGe ceramics with x = 0.2% exhibited the best piezoelectric properties: piezoelectric constant d₃₃ = 120 pC/N, planar electromechanical coupling coefficient k_p = 34.7%, mechanical quality factor Q_m = 130, and tanδ = 3.6%.     

Phase transition and piezoelectric properties of (1 − x)K0.5Na0.5NbO3–xLiSbO3 ceramics by hydrothermal powders

KNbO₃, NaNbO₃ and LiSbO₃ powders were synthesized by a hydrothermal route have been used to prepare (1 ? x)K_0.5Na_0.5NbO₃–xLiSbO₃ (KNN–LS; x = 0.00–0.08) ceramics. The effects of LiSbO₃ doping on the structures of KNN–LS ceramics have been systematically investigated by X-ray diffraction (XRD) and Rietveld refined XRD patterns. A gradual phase transition from orthogonal to tetragonal with the increase of LiSbO₃ content is demonstrated. Thereinto, the monoclinic phase is identified for the KNN–LS ceramic with the LiSbO₃ content of x = 0.08. Meanwhile, the XRD pattern reveals that the intensity ratio of (200)/(002) crystal face of the ceramic with x = 0.08 was bigger than one, which is different from the tetragonal phase. The tetragonal phase is revealed in the KNN–LS ceramic in the vicinity of x = 0.07, accompanying with relatively higher piezoelectric and ferroelectric properties. Tetragonal phase is beneficial to improve the piezoelectric properties of the KNN–LS ceramics.     

Na0.5K0.5NbO3 and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 nanocrystalline powders synthesized by low-temperature solid-state reaction

Nanocrystalline powders of K_0.5Na_0.5NbO₃ (KNN) and 0.9Na_0.5K_0.5NbO₃–0.1Bi_0.5Na_0.5TiO₃ (KNN–BNT) have been prepared using a low-temperature solid-state reaction. Phase development of the powders incurred during various calcination temperatures was examined by X-ray diffraction (XRD). Crystallite size and particle morphology of KNN powders were examined by XRD and transmission electron microscopy, respectively. Perovskite phase was formed at the temperature as low as 500 °C, and the average crystallite size of KNN powders depended on calcination temperature. In addition, the crystalline structure of KNN powders tended to change from tetragonal symmetry to orthorhombic symmetry with increase in crystallite size. Similar results were obtained in KNN–BNT system. The developed method is well suited for the mass production of niobate nanocrystalline powders due to its simplicity and low cost.