Thickness Coupling Coefficient Research Articles

Generalized homogenization model of piezoelectric materials for ultrasonic transducer applications

Although piezocomposite (PC) materials have increasingly attracted researchers, there is still a need to properly and easily derive their properties. We develop a generalized homogenization model (GHM) that accounts for Smith and Cha approaches to evaluate the equivalent characteristics of piezocomposites. This method could be applied to all connectivities patterns, but restricted herein to 2-2 and 1-3 piezocomposites for comparison with Smith (1-3) and Cha (2-2) analytical results. In the proposed GHM is a parameter θ, is changed for various connectivities. The 1-3 and 2-2 PZT-7A/Araldite D (PCs) data are used and equivalent characteristics of these Pcs are determined as function of volume fraction of PZT-7A piezoelectric. Results show that the electromechanical coefficients are well fitted by Voigt and Reuss models. Results obtained for some parameters show that the proposed GHM is consistent with the analytical existing models used for the 1-3 and 2-2 connectivities and is in line with measured values from Chan and Unsworth (1989). Based on the GHM 2-2 configuration results of piezocomposite materials, the electroacoustic responses of transducers having some of these properties are simulated using the KLM model. A performance trade-off was chosen, resulting in an improved thickness coupling coefficient and a lowered acoustical impedance, and a similar approach as that on a pure PZT-7A.

Screen Printed Copper and Tantalum Modified Potassium Sodium Niobate Thick Films on Platinized Alumina Substrates.

We show how sintering in different atmospheres affects the structural, microstructural, and functional properties of ~30 μm thick films of K0.5Na0.5NbO3 (KNN) modified with 0.38 mol% K5.4Cu1.3Ta10O29 and 1 mol% CuO. The films were screen printed on platinized alumina substrates and sintered at 1100 °C in oxygen or in air with or without the packing powder (PP). The films have a preferential crystallographic orientation of the monoclinic perovskite phase in the [100] and [−101] directions. Sintering in the presence of PP contributes to obtaining phase-pure films, which is not the case for the films sintered without any PP notwithstanding the sintering atmosphere. The latter group is characterized by a slightly finer grain size, from 0.1 μm to ~2 μm, and lower porosity, ~6% compared with ~13%. Using piezoresponse force microscopy (PFM) and electron backscatter diffraction (EBSD) analysis of oxygen-sintered films, we found that the perovskite grains are composed of multiple domains which are preferentially oriented. Thick films sintered in oxygen exhibit a piezoelectric d33 coefficient of 64 pm/V and an effective thickness coupling coefficient kt of 43%, as well as very low mechanical losses of less than 0.5%, making them promising candidates for lead-free piezoelectric energy harvesting applications.

Textured multilayered piezoelectric structures for energy conversion

Piezoelectric materials are essential for the conversion between mechanical and electrical energy, for example in ultrasound imaging and vibrational energy harvesting. Here, we are making and exploring the effects of a new design: co-sintered multilayers with texture (grains of a preferential crystallographic direction). The motivation is the combination of increased piezoelectric response in certain crystallographic directions; multilayer structures where thick films rather than bulk materials can allow higher frequency operation and large area; and co-sintering to avoid detrimental effects from gluing layers together. Samples of the lead-free piezoelectric material Li0.06(K0.52Na0.48)0.94Nb0.71Ta0.29O3 with 0.25 mol% Mn (KNNLTM) were made by tape casting and co-sintering. NaNbO3 platelets with (100) orientation which were used as templates to introduce texture, and polymethyl methacrylate (PMMA) was used as a pore forming agent for making porous substrates. The electrical impedances of the co-sintered samples were recorded and analyzed by equivalent electrical circuit modelling. A texture up to 85% in the [100] crystallographic direction was obtained. The samples displayed ferro- and piezoelectricity, with a maximum thickness coupling coefficient (kt = 0.18) between mechanical and electrical energy in the most textured sample. This demonstrates that the introduction of texture in multilayered, co-sintered piezoelectrics shows promise for improving devices for ultrasound imaging or energy harvesting.

Fabrication and Characterization of Single-Aperture 3.5-MHz BNT-Based Ultrasonic Transducer for Therapeutic Application.

This paper discusses the fabrication and characterization of 3.5-MHz single-element transducers for therapeutic applications in which the active elements are made of hard lead-free BNT-based and hard commercial PZT (PZT-841) piezoceramics. Composition of (BiNa0.88K0.08Li0.04)0.5(Ti0.985Mn0.015)O3 (BNKLT88-1.5Mn) was used to develop lead-free piezoelectric ceramic. Mn-doped samples exhibited high mechanical quality factor ( ) of 970, thickness coupling coefficient ( ) of 0.48, a dielectric constant ( ) of 310 (at 1 kHz), depolarization temperature ( ) of 200 °C, and coercive field ( ) of 52.5 kV/cm. Two different unfocused single-element transducers using BNKLT88-1.5Mn and PZT-841 with the same center frequency of 3.5 MHz and similar aperture size of 10.7 and 10.5 mm were fabricated. Pulse-echo response, acoustic frequency spectrum, acoustic pressure field, and acoustic intensity field of transducers were characterized. The BNT-based transducer shows linear response up to the peak-to-peak voltage of 105 V in which the maximum rarefactional acoustic pressure of 1.1 MPa, and acoustic intensity of 43 W/cm2 were achieved. Natural focal point of this transducer was at 60 mm from the surface of the transducer.

Development of Porous Piezoceramics for Medical and Sensor Applications.

The use of porosity to modify the functional properties of piezoelectric ceramics is well known in the scientific literature as well as by the industry, and porous ceramic can be seen as a 2-phase composite. In the present work, examples are given of applications where controlled porosity is exploited in order to optimise the dielectric, piezoelectric and acoustic properties of the piezoceramics. For the optimisation efforts it is important to note that the thickness coupling coefficient kt will be maximised for some non-zero value of the porosity that could be above 20%. On the other hand, with a good approximation, the acoustic velocity decreases linearly with increasing porosity, which is obviously also the case for the density. Consequently, the acoustic impedance shows a rather strong decrease with porosity, and in practice a reduction of more than 50% may be obtained for an engineered porous ceramic. The significance of the acoustic impedance is associated with the transmission of acoustic signals through the interface between the piezoceramic and some medium of propagation, but when the porous ceramic is used as a substrate for a piezoceramic thick film, the attenuation may be equally important. In the case of open porosity it is possible to introduce a liquid into the pores, and examples of modifying the properties in this way are given.

Temperature-Dependent Properties of a 1–3 Connectivity Piezoelectric Ceramic–Polymer Composite

Abstract In this work, the 1–3 connectivity piezoelectric ceramic–polymer composites have been fabricated by a viscous-polymer processing, where 0.90Pb(Zr0.52Ti0.48)O3–0.05Pb(Mn1/3Sb2/3)O3–0.05Pb(Zn1/3Nb2/3)O3 (PZT–PMS–PZN) fibers with 0.5 volume fraction were aligned in epoxy matrix. The sintered PZT fibers, with average diameter of 300 μm and aspect ratio (height/diameter) higher than 3, all showed a pure perovskite phase structure and highly dense morphology. The dielectric, piezoelectric, ferroelectric properties and the vibration modes of the 1–3 composites were measured and demonstrated in comparison with that of the monolithic piezoelectric ceramics. The results confirmed that the 1–3 composites possessed a low acoustic impedance (Z) of 13 MRayl and a high thickness coupling coefficient (k t) of 0.59, in addition, only single thickness vibration mode with the resonance frequency over 1.5 MHz was observed. With temperature elevation, the properties including dielectric constant εr , k t, the ratio of k t to k p (k t/k p) and the acoustic impedance (Z) increase, while the planar electromechanical coupling coefficient (k p) show opposite temperature dependence. Under test temperature of 100°C, the 1–3 composites still present excellent temperature stability with increased k t/k p ratio up to 3.6. The researches on temperature-dependent properties of the 1–3 composites are critical for improving its applications in various environments.

Parametric Models of Ultrasonic Piezoelectric Transducers over a Wide Temperature Range

Piezoelectric ultrasonic transducers are used extensively in well logging and logging-while-drilling applications for pulse-echo operation. We present a method of modeling the operation of ultrasonic thin-disk piezoelectric transducers over a wide range of temperatures. The model is based on using Redwood's version of Mason's model of thin-disk transducers. Laboratory measurements in the oven of non-backed transducers in air are used to extract the Mason model parameters as a function of temperature. Derived parameters are frequency-thickness constant, dielectric constant, and thickness mode coupling coefficient. A fourth parameter, bulk density, is measured independently and assumed constant over temperature. Temperature dependence of frequency thickness constant and coupling coefficient are modeled as linear temperature coefficients. Temperature dependence of the dielectric constant must be specified as a table because of the non-linear temperature dependence of that parameter.

Bismuth sodium titanate based lead-free ceramic/epoxy 1–3 composites: fabrication and electromechanical properties

The 1–3 piezocomposites based on 0.96Bi0.5(Na0.84K0.16)0.5TiO3–0.04SrTiO3 (BNKT–ST) were fabricated by a modified dice-fill method. Electro-mechanical properties of the composites as a function of the ceramic volume fraction (v) were measured and compared with theoretical values as well as with those of monolithic ceramics. The as-prepared piezocomposite with v = 0.276 showed a clear single thickness mode with a relatively high resonance frequency of more than 2 MHz, together with a relatively high piezoelectric strain constant (d 33 ~ 104 pC/N), a high thickness coupling coefficient (k t ~ 0.547), low acoustic impedance (Z ~ 9 Mrayls) and a large piezoelectric voltage coefficient (g 33 ~ 91.5 × 10−3 m2/C). From the practical application point of view, these promising results indicate that the BNKT–ST ceramic/epoxy 1–3 composite has great potential to be used in biomedical ultrasonic transducers as well as nondestructive evaluations.

Electrical and Physical Properties of Lead-Free (Na<sub>0.5</sub>K<sub>0.5</sub>)NbO<sub>3</sub>- Bi<sub>0.5</sub>(Na<sub>0.90</sub>K<sub>0.10</sub>)<sub>0.5</sub>TiO<sub>3</sub> Ceramics

Extending the investigations on (Na0.5K0.5)NbO3-based solid solution for lead-free piezoelectric ceramics, this paper consider the complex solid-solution system (Na0.5K0.5)NbO3Bi0.5(Na0.90K0.10)0.5TiO3 [NKN-BNK. (Na0.5K0.5)NbO3 with 1 ~ 5 mol% Bi0.5(Na0.90K0.10)0.5TiO3 has been prepared following the conventional mixed oxide process. It can be concluded that the NKN-BNKT ceramics have orthorhombic structures in the case x 0.03. With increasing BNKT content (x=0.04 to 0.05), however, the structure changes from orthorhombic to rhombohedral phase. Above results demonstrated that the MPB between orthorhombic and rhombohedral phases exits in the solid solution with the BNKT content of x=0.03. At the MPB composition, the cryctalline structure of ceramics is considered to be a coexistence of orthorhombic and rhombohedral phase. Owing to the phase coexistence at the phase boundary, there exists a different symmetry regions (DSR) near the MPB. The DSR boundary motion increases the dielectric permittivity and piezoelectric coefficients. The electromechanical coupling factor and dielectric constant are higher for compositions near the MPB. The dielectric constant (KT33), planar coupling coefficient (kp), thickness coupling coefficient (kt) and piezoelectric constant (d33)of 0.98NKN-0.02BNKT ceramics were 1180, 30%, 58%, and 180, respectively.

Microstructure and Electrical Properties of Lead-Free (Na0.5K0.5)NbO3 - Bi0.5(Na0.97K0.03)0.5TiO3 Ceramics

(Na0.5K0.5)NbO3 with Bi0.5(Na0.97K0.03)0.5TiO3 with x≤0.05 has been prepared by the conventional mixed oxide process. X-ray diffraction analysis revealed that, during sintering, all the Bi(Na0.97K0.03)TiO3 diffuses into the lattice of (Na0.5K0.5)NbO3 to form a solid solution with a perovskite structure. A morphotropic phase boundary (MPB) between orthorhombic (O) and rhombohedral (R) was found at the composition 0.98(Na0.5K0.5)NbO3-0.02Bi0.5(Na0.97K0.03)0.5TiO3 [abbreviated as 0.98NKN-0.02BNKT] with correspondingly enhanced dielectric and piezoelectric properties. For 0.98NKN-0.02BNKT ceramics, the electromechanical coupling coefficients of the planar mode kp and the thickness mode kt reach 0.33 and 0.49, respectively, after sintering at 1100 oC for 3 h. The ratio of the thickness coupling coefficient to the planar coupling coefficient is 1.48. With suitable Bi0.5(Na0.97K0.03)0.5TiO3 concentration, a dense microstructure and good electrical properties are obtained.

Piezoelectric Properties of Fine-Grained Pb(Mg1/3Nb2/3)O3–Pb(Zr,Ti)O3–Bi(Zn1/2Ti1/2)O3 Quaternary Solid Solution Ceramics

On the basis of solid state reaction eutectic behavior between Pb(Mg1/3Nb2/3)O3–Pb(Zr,Ti)O3 (PMN–PZT) and metastable perovskite-type Bi(Zn1/2Ti1/2)O3 (BZT), perovskite-structured PMN–PZT–BZT quaternary solid solution piezoceramics with various compositions were experimentally demonstrated with an intrinsic low sintering temperature in the windows of 950–1050 °C. These fine-grained densified PMN–PZT–BZT ceramics were able to be poled sufficiently at room temperature through polarization–electric field hysteresis loop measurement, different from normal poling treatment with DC bias field at high temperature, which will simplify future device processing of monolithic multilayer piezoceramic transducers. A typical piezoelectric property of dielectric constant ε33T/ε0 = 3471, piezoelectric constant d33 = 480 pC/N, planar coupling coefficient kp = 0.41, thickness coupling coefficient kt = 0.50, mechanical quality factor Qm = 68 and relaxor ferroelectric phase transition temperature Tm = 167 °C at 1 MHz was obtained for the fine-grained densified Pb0.96Sr0.04(Mg1/3Nb2/3)0.37Zr0.24Ti0.39O3+3%Bi(Zn0.5Ti0.5)O3+2%NiO ceramics sintered at 1020 °C, which is much promising to manufacture monolithic multilayer piezoelectric transducers with Ag95/Pd5 as inner electrode material.

Effect of Bi<sub>2</sub>O<sub>3</sub> Addition on the Microstructure and Electrical Properties of Lead-Free (Na<sub>0.5</sub>K<sub>0.5</sub>)NbO<sub>3</sub>-Ba(Sn<sub>0.02</sub>Ti<sub>0.98</sub>)O<sub>3</sub> Ceramics

PbZrO3-PbTiO3 (PZT)-based ceramics are playing a dominant role in piezoelectric materials, their evaporation of harmful lead oxide during the sintering process causes a crucial environment problem. It is necessary to search for lead-free piezoelectric materials that have such excellent properties as those found in the PZT-based ceramics. Therefore (Na0.5K0.5)NbO3-based solid solutions were studied to improve piezoelectric properties. In the present study, various quantities of Bi2O3 were added into 0.98(Na0.5K0.5)NbO3-0.02Ba(Sn0.02Ti0.98)O3 (0.98NKN-0.02BST) ceramics. It was found that 0.98NKN-0.02BST with the addition of 0~4.0 wt.% Bi2O3 exhibit relatively good piezoelectric properties. For 0.98NKN-0.02BST ceramic with the addition of 1.0 wt.% Bi2O3, the electromechanical coupling coefficients of the planar mode kp and the thickness mode kt reach 0.21 and 0.46, respectively, at the sintering of 1100oC for 3 h. The ratio of thickness coupling coefficient to planar coupling coefficient is 2.2. It is obvious that 0.98NKN-0.02BST solid solution ceramic by adding low quantities of Bi2O3 is one of the promising lead-free ceramics for high frequency electromechanical transducer applications.

Fabrication and evaluation of a single-element Bi0.5Na0.5TiO3-based ultrasonic transducer

This paper discusses the fabrication and characterization of a single-element ultrasonic transducer with a lead-free piezoelectric active element. A piezoelectric ceramic with composition of 0.88Bi(0.5)Na(0.5)TiO(3)-0.08Bi(0.5)K(0.5)TiO(3)- 0.04Bi(0.5)Li(0.5)TiO(3) was chosen as the active element of the transducer. This composition exhibited a thickness coupling coefficient (kt) of 0.45, a dielectric constant of 440 (at 1 kHz), and a longitudinal piezoelectric coefficient (d(33)) of 84 pC·N(-1). To make the transducer, the ceramic was sandwiched between an epoxy-tungsten backing layer and a silver epoxy matching layer. An epoxy lens was also incorporated into the transducer's design to focus the ultrasound beam. The focused transducer with a center frequency of about 23 MHz demonstrated a -6-dB bandwidth of 55% and an insertion loss of -32 dB; the -20-dB pulsed length was measured to be 150 ns. A phantom made of copper wires (30 μm in diameter) was utilized to investigate the imaging capability of the transducer. The results indicated that the fabricated transducer, with a lateral resolution of 260 μm and a relatively high depolarization temperature, could be considered as a candidate for replacement of lead-based ultrasonic transducers.

Electrical and Physical Properties of Lead-Free (Na<sub>0.5</sub>K<sub>0.5</sub>)NbO<sub>3</sub>- Bi<sub>0.5</sub>(Na<sub>0.</sub><sub>93</sub>K<sub>0.</sub><sub>07</sub>)<sub>0.5</sub>TiO<sub>3</sub> Ceramics

Extending the investigations on (Na0.5K0.5)NbO3-based solid solution for lead-free piezoelectric ceramics, this paper consider the complex solid-solution system (Na0.5K0.5)NbO3–Bi0.5(Na0.93K0.07)0.5TiO3 [NKN-BNKT]. (Na0.5K0.5)NbO3 with 2 ~ 6 mol% Bi0.5(Na0.93K0.07)0.5TiO3 has been prepared following the conventional mixed oxide process. A morphotropic phase boundary (MPB) between orthorhombic (O) and rhombohedral (R) was found at the composition 0.98NKN-0.02BNKT with correspondingly enhanced dielectric and piezoelectric properties. The electromechanical coupling factor and dielectric constant are higher for compositions near the MPB. The dielectric constant (KT33), planar coupling coefficient (kp) and thickness coupling coefficient (kt)of 0.98NKN-0.02BNKT ceramics were 1040, 47% and 48%, respectively.

Effect of Bi<sub>2</sub>O<sub>3</sub> Addition on the Physical and Electrical Properties of Lead-Free (Na<sub>0.5</sub>Bi<sub>0.5</sub>)TiO<sub>3</sub>-Ba(Sn<sub>0.08</sub>Ti<sub>0.92</sub>)O<sub>3</sub> Ceramics

In the present study, various quantities of Bi2O3were added into 0.98(Na0.5Bi0.5)TiO3-0.02Ba(Sn0.08Ti0.92)O3(0.98NBT-0.02BST) ceramics. High-density samples were obtained through the addition of Bi2O3into 0.98NBT-0.02BST ceramic. It was found that 0.98NBT-0.02BST with the addition of 0~3.0 wt.% Bi2O3exhibit relatively good piezoelectric properties. For 0.98NBT-0.02BST ceramic with the addition of 2 wt.% Bi2O3, the electromechanical coupling coefficients of the planar mode kp and the thickness mode kt reach 0.12 and 0.61, respectively, at the sintering of 1100oC for 3 h. The ratio of thickness coupling coefficient to planar coupling coefficient is 5.1. It is obvious that 0.98NBT-0.02BST solid solution ceramic by adding low quantities of Bi2O3is one of the promising lead-free ceramics for high frequency electromechanical transducer applications.

Effect of Nb<sub>2</sub>O<sub>5</sub> Addition on the Physical and Electrical Properties of Lead-Free (Na<sub>0.5</sub>Bi<sub>0.5</sub>)TiO<sub>3 </sub>-Ba(Sn<sub>0.08</sub>Ti<sub>0.92</sub>)O<sub>3</sub> Ceramics

In this paper, (Na0.5Bi0.5)TiO3 -based solid solutions were studied to improve piezoelectric properties as those found in the PbZrO3-PbTiO3 (PZT)-based ceramics. The 0.98(Na0.5Bi0.5)TiO3 -0.02Ba(Sn0.08Ti0.92)O3 (0.98NBT-0.02BST) ceramics with the addition of 0~2.0 wt.% Nb2O5 have been prepared following the conventional mixed oxide process. X-ray diffraction analysis revealed that, during sintering, all of the Ba(Sn0.08Ti0.92)O3 diffuse into the lattice of (Na0.5Bi0.5)TiO3 to form a solid solution, in which a hexagonal phase with a perovskite structure was found. For 0.98NBT-0.02BST ceramics with the addition of 0.5 wt.% Nb2O5, the electromechanical coupling coefficients of the planar mode kp and the thickness mode kt reach 0.12 and 0.28, respectively, at the sintering of 1100oC for 3 h. The ratio of thickness coupling coefficient to planar coupling coefficient is 2.3. It is obvious that 0.98NBT-0.02BST ceramics by adding low quantities of Nb2O5 are one of the promising lead-free ceramics for high frequency electromechanical transducer applications.

Physical and Electrical Properties of Lead-Free (Bi<sub>0.5</sub>Na<sub>0.5</sub>)TiO<sub>3</sub>-(Na<sub>0.5</sub>K<sub>0.5</sub>)NbO<sub>3</sub> Ceramics

Lead-free (1-x)(Bi0.5Na0.5)TiO3–x(Na0.5K0.5)NbO3(x<0.1) ceramics have been prepared by the conventional mixed oxide process. X-ray diffraction analysis revealed that, during sintering, all of the (Na0.5K0.5)NbO3diffuse into the lattice of (Bi0.5Na0.5)TiO3to form a solid solution and the hexagonal phase with a perovskite structure. The samples with a low content of (Na0.5K0.5) NbO3exhibit relatively good physical and electrical properties. For 0.96(Bi0.5Na0.5) TiO3–0.04 (Na0.5K0.5) NbO3ceramics, the electromechanical coupling coefficients of the planar modekpand the thickness modektreach 0.25 and 0.52, respectively, after sintering at 1100 °C for 5 h. The ratio of the thickness coupling coefficient to the planar coupling coefficient is 2.08. With suitable (Na0.5K0.5)NbO3concentration and sintering conditions, a dense microstructure and good electrical properties are obtained. Our results show that 0.96(Bi0.5Na0.5)TiO3–0.04(Na0.5K0.5)NbO3solid solution ceramics are promising lead-free ceramics for high-frequency electromechanical transducer applications.

Piezoelectric and Electrical Properties of Lead-Free (Na<sub>0.5</sub>K<sub>0.5</sub>)NbO<sub>3</sub>- Bi<sub>0.5</sub>(Na<sub>0.80</sub>K<sub>0.20</sub>)<sub>0.5</sub>TiO<sub>3</sub> Ceramics

Extending the investigations on (Na0.5K0.5)NbO3-based solid solution for lead-free piezoelectric ceramics, this paper consider the complex solid-solution system (Na0.5K0.5)NbO3–Bi0.5(Na0.80K0.20)0.5TiO3[NKN-BNKT]. (Na0.5K0.5)NbO3with 2 ~ 6 mol% Bi0.5(Na0.80K0.20)0.5TiO3has been prepared following the conventional mixed oxide process. A morphotropic phase boundary (MPB) between orthorhombic (O) and tetragonal (T) was found at the composition 0.97NKN-0.03BNKT with correspondingly enhanced dielectric and piezoelectric properties. The electromechanical coupling factor and dielectric constant are higher for compositions near the MPB. The dielectric constant (εr), planar coupling coefficient (kp) and thickness coupling coefficient (kt)of 0.97NKN-0.03BNKT ceramics were 1483, 32% and 31%, respectively.

Physical and Electrical Properties of Lead-Free (Bi<sub>0.5</sub>Na<sub>0.5</sub>)TiO<sub>3</sub>–(Bi<sub>0.5</sub>K<sub>0.5</sub>)TiO<sub>3</sub> Ceramics

Extending the investigations on (Bi0.5Na0.5)TiO3-based solid solution for lead-free piezoelectric ceramics, this paper consider the complex solid-solution system (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3[BNT-BKT]. (Bi0.5Na0.5)TiO3with 7~ 30 mol% (Bi0.5K0.5)TiO3ceramics have been prepared following the conventional mixed oxide process. A morphotropic phase boundary (MPB) between rhombohedral (R) and tetragonal (T) was found at the composition 0.82BNT-0.18BKT with correspondingly enhanced piezoelectric properties. The electromechanical planar coupling factor is higher for compositions near the MPB. The mechanical quality factor (Qm), planar coupling coefficient (kp) and thickness coupling coefficient (kt) of 0.82BNT-0.18BKT ceramics were 125, 28.8% and 47.4%, respectively.

Dielectric and Piezoelectric Properties of Lead-Free (Na<sub>0.5</sub>K<sub>0.5</sub>)NbO<sub>3</sub>- Bi<sub>0.5</sub>(Na<sub>0.85</sub>K<sub>0.15</sub>)<sub>0.5</sub>TiO<sub>3</sub> Ceramics

Extending the investigations on (Na0.5K0.5)NbO3-based solid solution for lead-free piezoelectric ceramics, this paper consider the complex solid-solution system (Na0.5K0.5)NbO3–Bi0.5(Na0.85K0.15)0.5TiO3[NKN-BNKT]. (Na0.5K0.5)NbO3with 2 ~ 6 mol% Bi0.5(Na0.85K0.15)0.5TiO3has been prepared following the conventional mixed oxide process. A morphotropic phase boundary (MPB) between orthorhombic (O) and hexagonal (H) was found at the composition 0.96NKN-0.04BNKT with correspondingly enhanced dielectric and piezoelectric properties. The electromechanical coupling factor and dielectric constant are higher for compositions near the MPB. The dielectric constant (KT33), planar coupling coefficient (kp) and thickness coupling coefficient (kt)of 0.96NKN-0.04BNKT ceramics were 1273, 34% and 38%, respectively.