Lead-free Single Crystals Research Articles

Solid-state conversion of (Na1/2Bi1/2)TiO3-BaTiO3-(K1/2Na1/2)NbO3 single crystals and their piezoelectric properties

Piezoelectric ceramic with a composition of (94 − x)(Na1/2Bi1/2)TiO3-6BaTiO3−x(K1/2Na1/2)NbO3 (NBT-BT-xKNN) is a promising lead-free piezoelectric material for actuator applications because of its giant electric-field-induced strains, which are comparable with that of soft Pb(ZrxTi1−x)O3 (PZT) ceramics. Using the solid-state single crystal growth method, we succeeded in fabricating usable single crystals of NBT-BT-3KNN (6 mm × 6 mm × 8 mm size) with a uniform chemical composition. The room temperature piezoelectric properties of ⟨001⟩, ⟨110⟩, and ⟨111⟩ oriented single crystals were measured. Single crystals showed strong anisotropic strain characteristics. In particular, ⟨001⟩ oriented single crystals had excellent piezoelectric properties with small hysteresis and a high strain of 0.57% at 7 kV/mm. In addition, the fabricated single crystals exhibited a high converse piezoelectric constant, Smax/Emax, of over 1000 pm/V at 4 kV/mm. These values are greater than those reported for any lead-containing and lead-free ceramics and comparable with those of lead-based single crystals. Our investigation demonstrates the solid-state conversion of lead-free single crystals and their practical usability in replacement of lead-based materials.

Continuous cross-over from ferroelectric to relaxor state and piezoelectric properties of BaTiO3-BaZrO3-CaTiO3 single crystals

Optimal properties like piezoelectricity can be found in polarizable materials for which the structure changes sharply under small composition variations in the vicinity of their morphotropic phase boundary or the triple point in their isobaric temperature-composition phase diagram. In the latter, lead-free (Ba0.850Ca0.150)(Ti0.900Zr0.100)O3 ceramics exhibit outstanding piezoelectric coefficients. For the first time, we report the growth of piezoelectric lead-free single crystals in the BaTiO3-BaZrO3-CaTiO3 pseudo-ternary system. The stoichiometry control in the CaO-BaO-TiO2-ZrO2 solid solution led to single crystals with various compositions ranging from (Ba0.857Ca0.143)(Ti0.928Zr0.072)O3 to (Ba0.953Ca0.047)(Ti0.427Zr0.573)O3. We evidenced a continuous cross-over from a ferroelectric state at high titanium content to a relaxor one on increasing the zirconium content. Such a property tuning is rather seldom observed in lead-free ferroelectrics and confirms what was already reported for ceramics. Single crystal with (Ba0.838Ca0.162)(Ti0.854Zr0.146)O3 composition, which has been grown and oriented along [001] crystallographic direction, displayed electromechanical coefficients d31 and k31 of 93 pC.N−1 and 0.18, respectively, near the room temperature (T = 305 K).

Growth and investigation of 0.80Na0.5Bi0.5TiO3–0.20K0.5Bi0.5TiO3 lead-free single crystal

Abstract 0.80Na 0.5 Bi 0.5 TiO 3 –0.20K 0.5 Bi 0.5 TiO 3 (80NBT–20KBT) lead-free piezoelectric single crystals have been successfully grown by the self flux method. Structure of the grown NKBT single crystals was confirmed by powder X-ray diffraction analysis. The grown crystal belongs to tetragonal system at room temperature. The lattice strain was calculated from Williamson–Hall relation. The temperature dependence of dielectric constant and loss measurement confirmed that the crystal exhibited relaxor-like behavior which is in consistence with the recent observation by Otanicar et al. [J. Appl. Phys. 113 (2013) 024106]. Polarization-electric field hysteresis loops revealed that the remnant polarization is 12.56 μC/cm 2 and coercive electric field is 29.1 kV/cm.

Pyroelectric properties of Mn-doped 94.6Na0.5Bi0.5TiO3-5.4BaTiO3 lead-free single crystals

We investigated the pyroelectric properties of ⟨001⟩, ⟨110⟩, and ⟨111⟩-oriented Mn doped 94.6Na0.5Bi0.5TiO3-5.4BaTiO3 (94.6NBT-5.4BT) crystals. Among the above three crystallographic orientations, the ⟨111⟩-oriented crystals possess the highest pyroelectric coefficients and the largest figures of merit. At 20 °C, the values of p, Fv, and Fd reach as high as 5.88 × 10−4 C/m2 K, 29.8 μPa−1/2, and 0.08 m2/C for ⟨111⟩-oriented crystals, respectively, which are superior to other lead-free ferroelectric materials. The crystals also demonstrate a high depolarization temperature and relatively stable thermal pyroelectric property. These results indicate the potential of Mn-doped 94.6NBT-5.4BT crystals for lead-free infrared pyroelectric detectors.

Orientation dependence of electrical properties of large-sized sodium potassium niobate lead-free single crystals

Large-sized sodium potassium niobate lead-free single crystals were successfully grown by a top seeded solution growth technique (TSSG).

Chemical nature of giant strain in Mn-doped 0.94(Na0.5Bi0.5)TiO3–0.06BaTiO3 lead-free ferroelectric single crystals

Giant bipolar and unipolar strains i.e. Smax > 0.5%, εmax/Emax > 1000 pm V–1 have been observed in Mn-doped Na0.5Bi0.5TiO3–6BaTiO3 single crystals after being annealed. Temperature-dependent impedance spectra were studied and activation energies of oxygen vacancies were calculated accordingly. The two different binding energies present in X-ray photoelectron spectra for Na and Bi were assigned to different coordinate environments. However, titanium exhibits only one oxidation state (e.g. Ti4+). The site occupation and valence fluctuation of Mn were characterized by electric paramagnetic resonance spectra.

Piezoelectric and ferroelectric properties of lead-free niobium-rich potassium lithium tantalate niobate single crystals

Lead-free potassium lithium tantalate niobate single crystals with the composition of K0.95Li0.05Ta1−xNbxO3 (abbreviated as KLTN, x=0.51, 0.60, 0.69, 0.78) were grown using the top-seeded melt growth method. Their piezoelectric and ferroelectric properties in as-grown crystals have been systematically investigated. The phase transitions and Curie temperatures were determined from dielectric and pyroelectric measurements. Piezoelectric coefficients and electromechanical coupling factors in thickness mode, length-extensional mode and longitudinal mode were obtained. The piezoelectric properties are very attractive, e.g. for x=0.60 composition, kt≈70%, k31≈70%, k33≈77%, d31≈230pC/N, d33≈600pC/N are comparable to the lead-based PZT composition. The polarization versus electric field hysteresis loops show saturated shapes. In short, lead-free niobium-rich KLTN system possesses comparable properties to those in important lead-based piezoelectric material nowadays.

Structural phase transition, optical and pyroelectric properties of lead-free single crystals

0.14 at% Mn-doped 0.95(Na1/2Bi1/2)TiO3-0.05BaTiO3 (0.95NBT-0.05BT) lead-free single crystals were grown by a top-seeded solution growth method (TSSG). X-ray powder diffraction measurement showed that the as-grown single crystals exhibit rhombohedral perovskite structure with apparent distortion. With the increase of temperature, successive structural phase transitions occur in the Mn-doped 0.95NBT-0.05BT single crystals. After poling, apparent dielectric anomaly is induced accompanied by the increase of the character of diffuse phase transition and the decrease of the value of dielectric constant. The Mn-doped 0.95NBT-0.05BT single crystals exhibit complex domain structure, in which micro and macro domains coexist with statistically 4 mm symmetry. The Mn-doped 0.95NBT-0.05BT lead-free single crystals exhibit excellent piezoelectric and pyroelectric properties, which will lead to promising advance in piezoelectric and pyroelectric applications. The infrared-absorption band occurred around 630.6 cm−1 can be assigned to the “stretching” normal vibration of [Na1/2Bi1/2]/Ti-O group. The three diffused Raman bands centered around 300, 560 and 800 cm−1 can be attributed to F2g [TiO6] bending vibration, A1g [TiO6] stretching vibration and “soft mode” mixed by bending and stretching vibrations.

Structural and Electrical Properties of Mn-doped Na0.5Bi0.5TiO3 Lead-Free Single Crystal

The electrical properties and Raman spectroscopy of [001]-oriented pure lead-free Na0.5Bi0.5TiO3 (NBT) and manganese-doped NBT single crystals have been investigated. With the effect of Mn-doping, the remnant polarization P r has a great enhancement from 15.2 μC/cm2 to 30.0 μC/cm2, and the pyroelectric coefficient is improved significantly from 178 μC/m2K to 241 μC/m2K. The Mn-doping has further altered the domain structure of the crystals, as revealed by piezoresponse force microscopy study. The Raman studies suggest that the substitution effect of Mn ions on B-site cations of the perovskite structure results in enhancement of the performance of the Mn-doped NBT single crystals.

Piezoelectric properties of tetragonal K0.95Li0.05Ta x Nb1− x O3 lead-free single crystals near tetragonal-cubic phase transition boundary

Tetragonal K0.95Li0.05Ta x Nb1 − x O3 lead-free single crystals have been grown by top-seeded solution growth technique. Dielectric and piezoelectric properties of poled samples were investigated as a function of temperature and dc bias. Both orthorhombic–tetragonal and tetragonal–cubic phase transition temperatures shift to lower temperatures as the Ta content increased. High piezoelectric activity was found to the crystal near the tetragonal–cubic phase transition boundary, which is due to phase transition as well as domain instability. For K0.95Li0.05Ta0.59Nb0.41O3 single crystal, piezoelectric coefficient (d 31) and electromechanical coupling factor (k 31) achieved −340 pC N−1 and −40%, respectively, which were much larger than those of lead-based Pb(Zr, Ti)O3.

High, purely electrostrictive effect in cubic K0.95Na0.05Ta1 − xNbxO3 lead-free single crystals

Large electrostrictive effect has been observed in K0.95Na0.05Ta1−xNbxO3 (x=0.42, 0.43) lead-free single crystals. Cubic crystals have been grown by the top seed solution growth method. Both crystals undergo phase transition from tetragonal phase to cubic phase near room temperature. Large dielectric constant, field-induced phase transition and slimmer P–E loop produce high, purely electrostrictive effect in the crystals. The electrostrictive coefficient Q11, Q12, M11 and M12 of K0.95Na0.05Ta0.57Nb0.43O3 crystal reaches as high as 0.38m4C−2, −0.10m4C−2, 17.6×10−16m2V−2 and −0.46×10−16m2V−2, respectively. The Q11 value of the crystal is 15 times larger than that of high performance Pb-based Pb(Mn1/3Nb2/3)O3 ceramics.

GROWTH AND CHARACTERIZATION OF Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 LEAD-FREE PIEZOELECTRIC CRYSTAL BY THE TSSG METHOD

The 0.96Na0.5Bi0.5TiO3–0.04K0.5Bi0.5TiO3 (NKBT96/4) lead-free single crystal was successfully grown by top-seeded solution growth method. The effective segregation coefficient for K was calculated to be 0.20. X-ray powder diffraction measurement showed that the as-grown crystal possesses rhombohedral perovskite structure at room temperature. X-ray rock curve measure indicated that the as-grown single crystal owned relatively higher quality. The NKBT96/4 single crystal exhibits apparent diffused dielectric behavior, where the diffusiveness index α is 1.79 for the [001]-oriented crystal planes. A notable thermal hysteresis, ΔT h , of 25°C was disclosed for the tetragonal antiferroelectric-tetragonal paraelectric phase transition in the dielectric property studies between heating and cooling. The highest remnant polarization, P r , 16.8 μC/cm2, was obtained at 100°C. The electromechanical coupling coefficient kt of the [001]-oriented crystal planes at room temperature was determined to be 38% using resonance–antiresonance technique. The room temperature piezoelectric constant d33 reaches 120 pC/N.

Growth and electrical properties of Na1/2Bi1/2TiO3–BaTiO3 lead-free single crystal with morphotropic phase boundary composition

In present work, lead-free single crystal 0.94Na1/2Bi1/2TiO3–0.06BaTiO3 (NBBT94/6) with dimension of Φ40 mm×10 mm was grown by a precisely controlled top seeded solution growth method. The crystal structure measurement result indicates that the NBBT94/6 single crystal exhibits the feature of rhombohedral-tetragonal phase coexistence at room temperature. The crystallographic orientation dependence of electrical properties for NBBT94/6 single crystal was investigated in detail. The remnant polarizations (Pr) of ⟨001⟩, ⟨001⟩, and ⟨111⟩ oriented crystals are 16.02 μC/cm2, 22.16 μC/cm2, and 27.23 μC/cm2, respectively. The corresponding coercive fields (Ec) are 24.89, 20.72, and 15.36 kV/cm. Moreover, a double hysteresis loop appears at the depolarization temperature. High piezoelectric properties d33 of 360 pC/N and kt of 62% were obtained in the ⟨001⟩ oriented crystals due to the engineered domain configuration which make them promising candidates for environmental friendly solid-state actuator applications.

Dielectric, electromechanical coupling properties of Mn-doped Na0.5Bi0.5TiO3–BaTiO3 lead-free single crystal

Mn-doped 0.948Na0.5Bi0.5TiO3–0.052BaTiO3 (NBBT94.8/5.2) lead-free single crystal with excellent piezoelectric and electromechanical coupling properties was successfully grown by top-seeded solution growth method (TSSG). X-ray powder diffraction measurement showed that the as-grown crystal possesses rhombohedral perovskite structure. The Mn-doped NBBT94.8/5.2 single crystal exhibits apparent diffused dielectric behavior, in which the diffusiveness index α is 1.99 for the [001]-oriented crystal planes. A notable thermal hysteresis is disclosed for the rhombohedral-tetragonal phase transition between heating and cooling studies in the dielectric property. The electromechanical coupling coefficients of Mn-doped NBBT94.8/5.2 were determined experimentally using the resonance–antiresonance technique. The room-temperature electromechanical coupling coefficients k 31, k 31〈110〉, k 33, k t are 35.9%, 35.5%, 68.5%, 58.7%, respectively. The room-temperature piezoelectric constants d 33, d 31 reach as high as 457, 138 pC/N respectively. The value of k t increases slightly with the increase of temperature from −80 to 70°C, demonstrating a relatively stable thermal electromechanical coupling property.

LEAD-FREE (Na0.5Bi0.5)1-xBaxTiO3 SINGLE CRYSTALS (0 ≤ x ≤ 0.05) AND THEIR DIELECTRIC AND PYROELECTRIC PROPERTIES

ABSTRACT Lead-free single crystals of Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT) system were prepared by Czochralski method and their dielectric and pyroelectric properties have been studied. The results were compared with these for pure NBT. The obtained samples show pure perovskite structure with rhombohedral symmetry at room temperature. Low frequency (100 Hz–100 kHz) investigations reveal the diffuse phase transitions. The obtained results are discussed in terms of local electric and strain fields caused by different ionic radius of (Na, Bi) and Ba ions. The NBT-BT system is expected to be a new promising candidate for lead-free electronic crystals.

Electrical Properties of Lead-Free Relaxor Ferroelectric Solid Solution Single Crystal (Na1/2Bi1/2)TiO3–BaTiO3 Grown by Bridgman Method

Lead-free relaxor ferroelectric solid solution single crystals of (Na1/2Bi1/2)TiO3–BaTiO3 (NBT-BT) grown by the Bridgman method without the flux of Bi2O3 and Na2O have good optical quality and electrical properties. It was found that their transition from the rhombohedral phase to the tetragonal phase occurs at about 250 °C, accompanied by isotropization. Moreover, the domain wall structure changes with heating, and dielectric dispersion gradually disappears at 250 °C. Relative permittivity increases with further heating and shows a peak at Ta of 340 °C with the appearance of brightness in a polarizing microscope, accompanied by a small frequency dispersion in tan δ. Polarization–electric field (P–E) hysteresis loops reveal a narrow shape without saturation with increasing electric field due to random fields based on defects at A-sites in perovskite compounds. Strain (S) versus electric field curves under bipolar fields reveal a slightly piezoelectric behavior without polarization reversal. The value of d33 (Smax/Emax) was estimated to be about 250 pm/V at room temperature.

Piezoelectric properties of the lead-free K0.95Li0.05Ta0.61Nb0.39O3 single crystal

Lead-free potassium lithium tantalate niobate single crystal with the composition of K0.95Li0.05Ta0.61Nb0.39O3 has been grown by the top-seeded solution growth method. The dielectric and piezoelectric properties of the as-grown crystal have been investigated. From the experimental results, the orthorhombic–tetragonal and tetragonal–cubic phase transition temperatures of this single crystal are −72 °C and 15 °C, respectively. It exhibits good piezoelectric properties, namely the electromechanical coupling factor k31 reaches 40.4%, the piezoelectric constant d33 reaches 431 pC N−1 and d31 reaches 183.1 pC N−1. These excellent properties indicate that K0.95Li0.05Ta0.61Nb0.39O3 single crystal is a very promising lead-free piezoelectric material for designing applicable devices and transducers.

Giant strain in lead-free (Bi0.5Na0.5)TiO3-based single crystals

A giant electric-field-induced strain of 0.87% is reported for tetragonal (Bi0.5Na0.5)TiO3-(Bi0.5K0.5)TiO3-BaTiO3 single crystals along the [100]cubic direction, which is six times as large as that of Pb(Zr0.52Ti0.48)O3 ceramics. In situ x-ray diffraction measurements and transmission electron microscope observations show that the giant strain mainly originates from switching of nanosized 90° domains. Strain measurements indicate that the strain caused by the 90° domain switching is reversible for both unipolar and bipolar electric-field applications. The reversibility of the 90° domain switching can be explained by the interaction between the spontaneous polarization and the defect dipole composed of A-site vacancy and oxygen vacancy.

Piezoelectric properties of lithium modified silver niobate perovskite single crystals

We report the growth and the piezoelectric properties of lead-free perovskite single crystals of Ag1−xLixNbO3. It possesses a rhombohedral structure with high ferroelectric phase transition (Tc=548K for x=0.086) and large spontaneous polarization (Ps∼40μC∕cm2 for x=0.062) along the ⟨111⟩c direction of pseudocubic perovskite structure for x>0.05–0.06. High quasistatic d33∼210pC∕N and low dielectric constant have lead to a very large value of piezoelectric voltage constant g33∼53.9×10−3Vm∕N for the ⟨001⟩c-cut crystal of this simple perovskite. It has been shown that Li substitution might enhance the piezoelectric coefficient of the crystal. The excellent piezo-/ferroelectricity of this system are considered to be facilitated by the strong polarization nature of both Ag and Li in the perovskite structure. Our findings may stimulate further interests in the development of lead-free piezoelectrics.

Growth and characterization of Mn-doped Na 1/2Bi 1/2TiO 3 lead-free ferroelectric single crystal

High-quality and large-size ferroelectric single crystal of 1 at.% Mn-doped Na 1/2Bi 1/2TiO 3 has been successfully grown by top-seeded solution growth technique. X-ray fluorescence analysis determines the actual Mn concentration of 0.24 at.% in as-grown Mn-doped Na 1/2Bi 1/2TiO 3 crystal, indicating a very low value of Mn ions diffusion into crystal lattice. Temperature dependence of dielectric constant at different frequencies shows that the dielectric relaxor behavior becomes more distinct after Mn-doping. Moreover, the domain size of Mn-doped Na 1/2Bi 1/2TiO 3 crystal becomes smaller than that of pure Na 1/2Bi 1/2TiO 3 crystal. At room temperature, the [001]-oriented Mn-doped Na 1/2Bi 1/2TiO 3 crystal shows enhanced dielectric, piezoelectric and ferroelectric properties compared with that of pure Na 1/2Bi 1/2TiO 3 crystal. It is clearly demonstrated that Mn-doping is a very effective tool to improve electrical performance of Na 1/2Bi 1/2TiO 3 based single crystals.