Lead-free Piezoelectric Single Crystals Research Articles

Effect of Nb substitution on the electronic property of lead-free piezoelectric (Na0.41K0.09Bi0.50)TiO3 single crystal: Optical absorption and photoelectron study

Single crystals of undoped and Nb doped sodium and potassium-bismuth titanate at the morphotropic phase boundary (MPB) composition [(Na0.41K0.09Bi0.50)TiO3: NKBT] were grown from high temperature solutions. The effect of Nb doping on the electronic property of NKBT was investigated by optical transmission and photoelectron spectroscopy (PES). The optical absorption shows that the bandgap having the lowest energy is indirect in nature and increases with the increase in the Nb concentration in the lattice. The estimated Urbach energy increases with the increase of Nb concentration signifying an increase in the disorder in the lattice on Nb doping. The PES revealed that the valence band maxima (VBM) exhibit red shift on Nb doping. It also revealed the presence of in-gap states probably due to Ti3+ defect and oxygen vacancy. Moreover, inline with the optical measurement, the PES showed that there is a decrease in the energy gap between the VBM and defect state with the Nb doping. Furthermore, the angle resolved PES measurement confirmed the presence of both the direct and indirect bandgaps in the crystal.

Effects of MnO2-doping on growth, structure and electrical properties of lead-free piezoelectric K0.5Na0.5NbO3-BiAlO3 single crystals

K0.5Na0.5NbO3 (KNN)-based lead-free piezoelectric single crystals have been prepared by the Seed-Free, Solid-State Crystal Growth (SFSSCG) method, and the effects of MnO2 doping on the structural and electrical properties were systematically investigated. The results showed that the MnO2-doping can promote the growth of K0.5Na0.5NbO3-BiAlO3 (KNN-BA) single crystals and enhance its electrical properties. Furthermore, the size of the (1 - x)( KNN-BA)-xMnO2 crystals reached the maximum value of about 18 × 17 × 2 mm3 at x = 0.005. Also, the color of the crystals did gradually change, from light yellow to dark brown, due to the addition of MnO2. The XRD and refinement results showed that the addition of MnO2, in the range of 0 ≤ x ≤ 0.009, did not change the crystalline structure of the perovskite crystals. A two-phase perovskite crystalline structure was present in all crystals: an orthorhombic and a tetragonal phase. The content of the orthorhombic phase decreased, while the content of the tetragonal phase increased, with an increase in the MnO2 content. In addition, the domains in the crystals were mainly composed of 180° ones and had a parallel lamellar structure. Also, the smallest average domain width was obtained for crystals with x = 0.003. These crystals had the largest piezoelectric constant of 568 pC/N and an excellent overall performance of d33 = 568 pC/N, Pr = 32.2 μC/cm2, Qm = 55.3, kt = 0.36, kp = 0.39, TC = 418 °C, εr = 430, and tan δ = 0.015.

Microstructure and electrical properties of CuO-doped K0.5Na0.5NbO3-based single crystals with low dielectric loss

CuO-doped K0.5Na0.5NbO3 (KNN)-based lead-free piezoelectric single crystals were realized by using a seed-free solid-state crystal growth (SFSSCG) method. The effect of the CuO addition on the single-crystal growth, microstructure and electrical properties of the systems was studied systemically. The addition of CuO is beneficial for the growth of the KNN-based crystals and decreases the width of ferroelectric domains of the crystals. In particular, the CuO doping significantly reduces the dielectric loss of the crystals. The lowest dielectric loss tanδ of 1% was achieved for the CuO-doped crystals with high piezoelectric constant and Curie temperature. The study on the leakage behavior of the crystals shows that the main electrical conduction mechanism is the space-charge-limited-current (SCLC). A coexisting crystalline structure with orthorhombic and tetragonal phases forms in the (1 − x)[KNN-LiBiO3]-xCuO crystals at x = 0.007–0.009. The growth kinetics and mechanism of the crystals were also investigated.

Bulk growth, structural and electrical properties of (1-x)Na0.5Bi0.5TiO3-xBaTiO3 piezoelectric single crystals by top seeded solution growth method

The solid solution of 0.94(Na0.5Bi0.5)TiO3-0.06BaTiO3 NBBT94/06 is a promising material to replace Lead (Pb) based piezoelectric materials. The bulk NBBT 94/06 single crystal was successfully grown by the top-seeded solution growth (TSSG) method. Lead-free piezoelectric NBBT single crystals with a composition across the morphotropic phase boundary (MPB) have been grown by the TSSG method. NBBT 94/06 single crystal, encompassing the rhombohedral to largely monoclinic phase covering the MPB region was grown by the TSSG method. The crystal structure and its electrical properties were investigated. The Laue diffraction pattern confirms that the grown crystal is a single crystal. Rietveld refinement studies disclose the existence of a phase boundary between Monoclinic (Cc) and Rhombohedral (R3c) phases near the MPB. The depolarization temperature (Td), piezoelectric coefficient (d33) and ferroelectric properties were enhanced at MPB.

Growth and electrical properties of self-flux method grown (1−x)Bi1/2Na1/2TiO3−xBaTiO3 single crystals across the morphotropic phase boundary

The ferroelectric solid solution 0.94(Bi1/2Na1/2)TiO3–0.06BaTiO3 (BNBT) is a promising material to substitute the environmentally undesired Pb-based ferroelectrics. Lead-free piezoelectric BNBT bulk size single crystals with a composition across the morphotropic phase boundary (MPB) have been successfully grown by the self-flux method. Powder X-ray diffraction (PXRD) pattern confirms the perovskite structure of the synthesized BNBT crystals. The lattice strain is calculated as 1.7 × 10−2 using Williamson–Hall equation from PXRD. The elemental composition of the BNBT single crystals grown with different compositions is confirmed by X-ray fluorescence, covering the rhombohedral to tetragonal region and encompassing the MPB. The presence of a phase boundary between rhombohedral and tetragonal phases near MPB results in electrical behavior enhancement. The dielectric, ferroelectric, and piezoelectric properties along the pseudocubic direction have been measured. Dielectric permittivity gives diffuseness of BNBT crystals and it confirms the relaxor-type behavior. AC conductivity gave the activation energy as 0.46 eV.

Effect of Mn doping on the acoustic and vibrational behaviors of lead-free piezoelectric (Na1/2Bi1/2)TiO3-xBaTiO3 single crystals

The acoustic and vibrational properties of Mn-doped (Na1/2Bi1/2)TiO3-xBaTiO3 with x = 0.05 were investigated by using Brillouin and Raman spectroscopies and compared with those of undoped crystals. The mode frequency and the half width of the Brillouin doublet of the longitudinal acoustic mode showed more drastic changes near the dielectric maximum temperature compared to the broad and diffused changes of the undoped crystals, which indicates that Mn-doping induces polar domains larger than the typical polar nanoregions formed in undoped counterpart. The acoustic anomalies were most pronounced along the <100> direction suggesting that the polarization fluctuations are anisotropic and largest along this direction. Temperature dependence of Raman spectra showed that there are two characteristics temperatures where mode splitting occurs due to local symmetry changes. The high-temperature anomaly was associated with the changes in the acoustic properties, while the low-temperature mode splitting was related to the dielectric anomaly near depolarization temperature.

Acoustic Anomalies and Phase Transition Behaviors of Lead-Free Piezoelectric (Na1/2Bi1/2)TiO₃-xBaTiO₃ Single Crystals as Revealed by Brillouin Light Scattering.

The elastic properties of unpoled and prepoled (Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-xBT) single crystals near the morphotropic phase boundary were investigated as a function of temperature using Brillouin light scattering. The acoustic mode frequency and the related acoustic damping of unpoled NBT-xBT showed very broad minimum and maximum, respectively, consistent with typical relaxor behaviors. The frequency softening of the longitudinal acoustic mode together with the increase in acoustic damping was largest along the <100> direction, indicating that polarization fluctuations were most substantial along this crystallographic direction. The difference in acoustic behaviors between the unpoled NBT-xBTs with x = 0.05 and 0.08 were negligible, which means that the NBT-xBT system exhibits typical relaxor properties over a certain composition range of at least 5~8%. The obtained relaxation time of polar nanoregions in the paraelectric phase showed a gradual slowing-down character without any critical divergent behavior. The prepoling of NBT-xBT along the <100> direction induced drastic changes in both mode frequency and damping at ~110 °C when the poling field was larger than 1.4 kV/mm, corresponding to the depoling process from macroscopic/mesoscopic ferroelectric order to ergodic relaxor state upon heating. Phase coexistence of ferroelectric and relaxor states was observed at the intermediate poling field of 1.4 kV/mm.

Composition design and electrical property of a pure KxNa1−xNbO3 single crystal fabricated by the seed-free solid-state crystal growth

In this work, based on a composition design, a pure K0.52Na0.48NbO3 (KNN) lead-free piezoelectric single crystal with a maximum dimension of 4 mm × 4 mm × 2 mm was successfully prepared by the seed-free solid-state crystal growth method. The X-ray diffraction results indicate that the as-grown KNN single crystals have a perovskite crystalline structure with a single orthorhombic phase. An porous microstructure exists in the pure KNN single crystal in this present. The orthorhombic-tetragonal T O−T and tetragonal-cubic phase transition temperature Curie temperature T C of the single crystal are found at 190 and 424 °C, respectively. The obtained KNN single crystal exhibits a piezoelectric coefficient d 33 of 103 pC/N and relative dielectric permittivity ɛ of 180. The resistance of KNN single crystal is insensitive to the temperature in the measurement range. The conductive mechanism of the single crystal mainly comes from the grain interiors.

Piezoelectric sensitivity and hydrostatic response of novel lead-free 2–0–2 composites with two single-crystal components

This paper reports the piezoelectric performance and important related hydrostatic parameters of 2–0–2 composites based on lead-free ferroelectric and piezoelectric single crystals with 4mm symmetry. We demonstrate that ferroelectric domain-engineered alkali niobate-tantalate based single crystals provide large values of the piezoelectric coefficients g3j∗ and related parameters when used in a 2–0–2 composite system with a relatively wide range of volume fractions of single-crystal components. An ‘aspect-ratio effect’ as a result of the presence of inclusions of a piezoelectric Li2B4O7 single crystal in a polymer medium is studied for the first time for a case where the elastic compliance s12 in the single crystal/polymer layer of the composite passes through zero. It is observed that changes in the aspect ratio and volume fraction of the Li2B4O7 inclusions influence the hydrostatic piezoelectric coefficient gh∗, squared figure of merit dh∗gh∗ and electromechanical coupling factor kh∗ of the composite, and large values of gh∗∼102 mV·m/N, dh∗gh∗∼ (10−11–10−10) Pa−1 and kh∗≈ 0.6–0.7 are achieved. A link between max kh∗ and a change in sgns12 is first described for the 2–2-type composite, and a comparison of the hydrostatic parameters of the novel and related composites is made. The present results show the potential of lead-free 2–0–2 composites that are suitable for piezoelectric sensor, energy-harvesting and hydroacoustic applications.

Solid-state-growth of lead-free piezoelectric (Na1/2Bi1/2)TiO3-CaTiO3 single crystals and their characterization

Single crystals in the (Na1/2Bi1/2)TiO3-CaTiO3 system have been grown for the first time using the solid state crystal growth method. Seed crystals of SrTiO3 were buried in 0.96(Na1/2Bi1/2)TiO3-0.04CaTiO3 (NBT-4CT) powder which was then pressed into pellets. Pellets were sintered and single crystals of NBT-4CT grew on the seeds. Electron probe micro analysis shows that the chemical composition of the single crystal is close to the nominal composition. X-ray diffraction and micro-Raman scattering analysis shows that the single crystal has the rhombohedral (Na1/2Bi1/2)TiO3 structure. Single crystals were also grown by bonding a seed crystal to a pre-sintered NBT-4CT pellet and annealing. The single crystal shows dielectric behavior similar to that of NBT-4CT ceramics and other (Na1/2Bi1/2)TiO3 -based ceramics and single crystals. Characteristic dielectric transitions can be related to the rhombohedral-tetragonal and tetragonal-cubic phase transitions. The NBT-4CT single crystal has improved ferroelectric and piezoelectric properties compared to the bulk polycrystalline ceramic.

Strong and anisotropic magnetoelectricity in composites of magnetostrictive Ni and solid-state grown lead-free piezoelectric BZT–BCT single crystals

Aimed at developing lead-free magnetoelectric (ME) composites with performances as good as lead (Pb)-based ones, this study employed (001) and (011) oriented 82BaTiO3-10BaZrO3-8CaTiO3 (BZT–BCT) piezoelectric single crystals, fabricated by the cost-effective solid-state single crystal growth (SSCG) method, in combination with inexpensive, magnetostrictive base metal Nickel (Ni). The off-resonance, direct ME coupling in the prepared Ni/BZT–BCT/Ni laminate composites was found to be strongly dependent on the crystallographic orientation of the BZT–BCT single crystals, as well as the applied magnetic field direction. Larger and anisotropic ME voltage coefficients were observed for the composite made using the (011) oriented BZT–BCT single crystal. The optimized ME coupling of 1 V/cm Oe was obtained from the Ni/(011) BZT–BCT single crystal/Ni composite, in the d32 mode of the single crystal, when a magnetic field was applied along its [100] direction. This performance is similar to that reported for the Ni/Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 (PMN–PZT) single crystal/Ni, but larger than that obtained from the Ni/Pb(Zr,Ti)O3 ceramic/Ni composites. The results of this work demonstrate that the use of lead-free piezoelectric single crystals with special orientations permits the selection of desired anisotropic properties, enabling the realization of customized ME effects in composites.

Piezoelectric d36 in-plane shear-mode of lead-free BZT-BCT single crystals for torsion actuation

We report the study of piezoelectric direct torsion actuation mechanism using lead-free piezoelectric d36 in-plane shear-mode BZT-BCT single crystals. The generated angle of twist of the piezoelectric torsion actuator was obtained from the transverse deflection measurement using a laser vibrometer. The bi-morph torsional actuator, consisting of two lead-free piezoelectric BZT-BCT in-plane shear-mode single crystals with a giant piezoelectric d36 shear strain coefficient of 1590 pC/N, provided a rate of twist of 34.12 mm/m under a quasi-static 15 V drive. The experimental benchmark was further modelled and verified by the ANSYS software using three dimensional (3D) piezoelectric finite elements. The experimental results revealed that lead-free piezoelectric BZT-BCT d36-mode single crystal is a superior candidate for piezoelectric torsion actuation. This lead-free piezoelectric BZT-BCT d36-mode torsion actuator can be effectively applied in torsional deformation control by taking into account the environmental considerations.

Raman spectra of (K, Na)(Nb, Ta)O3 single crystal

Abstract The Raman scattering spectra for lead-free piezoelectric single crystal (K,Na)(Nb,Ta)O3 (KNNT) were intensively investigated to explore its crystallographic structure. For the [011]C oriented sample, 12 Raman peaks were identified from the room temperature Raman spectrum. The υ3, υ4 and υ6 modes which should be Raman silent in normal perovskites were observed due to the low symmetry (Pm) of KNNT single crystal. The temperature dependent Raman spectra indicated that the orthorhombic-tetragonal phase transition happened at about 68 °C, and the Curie temperature (TC) is about 225 °C. The merging of 2 υ5 modes at TC was ascribed to the vanishing of polarization. The backscattering polarized Raman spectra of KNNT single crystals with different orientations were analyzed. The Raman modes were identified using polarized selection rules based on group theory.

New Potassium Sodium Niobate Single Crystal with Thickness-independent High-performance for Photoacoustic Angiography of Atherosclerotic Lesion.

The synthesis of (K0.45Na0.55)0.96Li0.04NbO3 (KNLN) single crystals with a <100>-orientation, using a seed-free solid state crystal growth method, is described here. With the thickness of the crystals decreasing down to the order of tens of micrometers, this new lead-free single crystal exhibits thickness-independent electrical behavior, and maintains superior piezoelectric constant (d33 = 670 pC N−1) and electromechanical coupling factor (kt = 0.55). The successful fabrication of a tiny intravascular photoacoustic probe, with a 1 mm outside diameter, is achieved using a single crystal with a thickness of around 60 μm, in combination with a 200 μm core multimode fiber. Wire phantom photoacoustic images show that the axial resolution and lateral resolution of the single crystal based probe are 60 and 220 μm, respectively. In addition, intravascular photoacoustic imaging of the atherosclerotic lesion of a human artery is presented. In the time-domain and frequency-domain images, calcified regions are clearly distinguishable from surrounding tissue. These interesting results demonstrate that KNN-based lead-free piezoelectric single crystals are a promising candidate to substitute for lead-based piezoelectric materials for photoacoustic imaging in the future.

Periodic configuration of δ-boundaries and ferroelectric domains in Li-modified (K, Na)NbO3 lead-free piezoelectric single crystals by solid state crystal growth

(K0.45Na0.55)0.94Li0.06NbO3 (KNLN) lead-free piezoelectric single crystals have been prepared by solid-state crystal growth (SSCG) method using faceted KTaO3 single crystals as a seed. SEM-EBSD, PFM and TEM investigations revealed a specific periodic configuration of δ-boundaries and ferroelectric domains in the KNLN single crystal. The single crystal is sliced into several nearly parallel segments by δ-boundaries and each segment has a mean thickness of 2–3μm. Neighboring segments show a misorientation of 1.4°/[1232] while the alternate segments have the same orientation. The two segments exhibit different ferroelectric domain structures: periodic (100) domain walls and (11¯l) domain walls, respectively. The periodic (100) domain wall pattern can be sketched by tetragonal 180° or orthorhombic 90° domains. The as-observed periodic configuration of small-angle δ-boundaries and ferroelectric domains could be related to the specific stress fields in the epitaxial crystal growth embedded in a ceramic matrix.

Giant strain with low cycling degradation in Ta-doped [Bi1/2(Na0.8K0.2)1/2]TiO3 lead-free ceramics

Non-textured polycrystalline [Bi1/2(Na0.8K0.2)1/2](Ti1−xTax)O3 ceramics are fabricated and their microstructures and electrical properties are characterized. Transmission electron microscopy reveals the coexistence of the rhombohedral R3c and tetragonal P4bm phases in the form of nanometer-sized domains in [Bi1/2(Na0.8K0.2)1/2]TiO3 with low Ta concentration. When the composition is x = 0.015, the electrostrain is found to be highly asymmetric under bipolar fields of ±50 kV/cm. A very large value of 0.62% is observed in this ceramic, corresponding to a large-signal piezoelectric coefficient d33* of 1240 pm/V (1120 pm/V under unipolar loading). These values are greater than most previously reported lead-free polycrystalline ceramics and can even be compared with some lead-free piezoelectric single crystals. Additionally, this ceramic displays low cycling degradation; its electrostrain remains above 0.55% even after undergoing 10 000 cycles of ±50 kV/cm bipolar fields at 2 Hz. Therefore, Ta-doped [Bi1/2(Na0.8K0.2)1/2]TiO3 ceramics show great potential for large displacement devices.

An equivalent dipole analysis of PZT ceramics and lead-free piezoelectric single crystals

The recently proposed Equivalent Dipole Model for describing the electromechanical properties of ionic solids in terms of 3 ions and 2 bonds has been applied to PZT ceramics and lead-free single crystal piezoelectric materials, providing analysis in terms of an effective ionic charge and the asymmetry of the interatomic force constants. For PZT it is shown that, as a function of composition across the morphotropic phase boundary, the dominant bond compliance peaks at 52% ZrO2. The stiffer of the two bonds shows little composition dependence with no anomaly at the phase boundary. The effective charge has a maximum value at 50% ZrO2, decreasing across the phase boundary region, but becoming constant in the rhombohedral phase. The single crystals confirm that both the asymmetry in the force constants and the magnitude of effective charge are equally important in determining the values of the piezoelectric charge coefficient and the electromechanical coupling coefficient. Both are apparently temperature dependent, increasing markedly on approaching the Curie temperature.

Large piezoelectric properties in KNN-based lead-free single crystals grown by a seed-free solid-state crystal growth method

We report lead-free single crystals with a nominal formula of (K0.45Na0.55)0.96Li0.04NbO3 grown using a simple low-cost seed-free solid-state crystal growth method (SFSSCG). The crystals thus prepared can reach maximum dimensions of 6 mm × 5 mm × 2 mm and exhibit a large piezoelectric coefficient d33 of 689 pC/N. Moreover, the effective piezoelectric coefficient d33*, obtained under a unipolar electric field of 30 kV/cm, can reach 967 pm/V. The large piezoelectric response plus the high Curie temperature (TC) of 432 °C indicate that SFSSCG is an effective approach to synthesize high-performance lead-free piezoelectric single crystals.

Effect of SrTiO3 content on the growth of (100−x)(K0.5Na0.5)NbO3–xSrTiO3 lead-free piezoelectric single crystals grown by the solid-state crystal growth method

In the present work, the single crystal growth by solid-state crystal growth of (100−x)(K0.5Na0.5)NbO3–xSrTiO3, where x = 0,1,2,3 mol-%, has been examined in order to study the effect of SrTiO3 content on single crystal growth. Powders were prepared by the conventional mixed oxide method. <001> KTaO3 seed crystals were buried in the powders, pressed into pellets and sintered at 1100°C for 1, 3, 5 and 10 h. Single crystals of the ceramic compositions grew onto the seeds. For the (K0.5Na0.5)NbO3 sample, both single crystal growth and abnormal grain growth in the matrix began to take place within 1 h. As the amount of SrTiO3 increased, the onset of both single crystal growth and abnormal grain growth were delayed. The effect of SrTiO3 addition on the single crystal and matrix grain growth behaviour is explained in terms of the mixed control theory of grain growth.

Effect of seeds and sintering additives on (K,Na,Li)NbO&lt;sub&gt;3&lt;/sub&gt; lead-free single crystals grown by a solid-state crystal growth method

A solid-state crystal growth (SSCG) method has been utilized to prepare (K0.45Na0.55)(0.94)Li0.06NbO3 (KNLN) lead-free piezoelectric single crystals. The effects of different kinds of seeds and additives on K0.5Na0.5NbO3 (KNN)-based crystal growth were studied. It was found that -oriented SrTiO3 seed was inappropriate for growing KNLN single crystal, while oriented KTaO3 one could act as a good seed. The inverse pole figure (IPF) showed that the epitaxial layer was indeed single crystal and oriented in the same direction as the seed. Addition of 0.5 mol% ZnO, CuO and MnO2 brought liquid phase in the samples and increased the diffusion rate of the atoms. This would finally enhance both the single crystal and the matrix grain growth. Among all the sintering additives, CuO could increase the matrix grain growth remarkably due to the assistance of the largest amount of the Na-deficient liquid phase, whose composition was time-independent. However, CuO was not the most effective one that can promote the single crystal growth. The reasons might be as follows: (1) the liquid accumulation at the interface of the single crystal/matrix lead to the liquid phase content exceeding the critical value in the CuO-added samples, which increased the diffusion distance and partly offset the enhanced diffusion rate; (2) the large matrix grains brought low driving force for crystal growth. (C) 2016 The Ceramic Society of Japan. All rights reserved.