Tae Kwon Song

High-Performance Lead-Free Piezoceramics with High Curie Temperatures.

A bismuth ferrite and barium titanate solid solution compound can achieve good piezoelectric properties with a high Curie temperature when fabricated with low-temperature sintering followed by a water-quenching process, with no complicated grain alignment processes performed. By adding the super-tetragonal bismuth gallium oxide to the compound, the piezoelectric properties are as good as those of lead zirconate titanate ceramics.

Low-frequency dielectric relaxation and ac conduction of SrBi2Ta2O9 thin film grown by pulsed laser deposition

Bi-excess SrBi2Ta2O9 (SBT) thin films on Pt/Ti/SiO2/Si substrate were prepared by pulsed laser deposition technique. The SBT structure was characterized by x-ray diffraction studies. The ferroelectric properties were confirmed by P–E hysteresis loops at different applied electric fields. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range from 0.01 Hz to 100 kHz and in the temperature range from 25 to 400 °C. The thermal activation energy of 0.90 eV is observed in the frequency dependent dielectric constant. The activation energy for conduction process is calculated as 0.91 eV from the slope of ac conductivity at the lowest frequency. The low-frequency dielectric relaxation and the ac conductivity of Bi-excess SBT thin film are discussed in relation to the electrical conduction of SBT/Pt junction.

Flexoelectric Effect in the Reversal of Self-Polarization and Associated Changes in the Electronic Functional Properties of BiFeO 3 Thin Films

Flexoelectricity can play an important role in the reversal of the self-polarization direction in epitaxial BiFeO3 thin films. The flexoelectric and interfacial effects compete with each other to determine the self-polarization state. In Region I, the self-polarization is downward because the interfacial effect is more dominant than the flexoelectric effect. In Region II, the self-polarization is upward, because the flexoelectric effect becomes more dominant than the interfacial effect.

Ferroelectric properties of vanadium-doped Bi4Ti3O12 thin films deposited by a sol–gel method

We report the enhancement of ferroelectric properties in vanadium-doped Bi4Ti3O12 (BIT) thin films deposited by a sol–gel method. Compared to the undoped BIT, V-doped BIT (BTV) showed higher polarizations and a better fatigue resistance as reported in ceramic systems recently [Noguchi et al., Appl. Phys. Lett. 78, 1903 (2001)]. BTV showed a remanent polarization (2Pr) of 15.9 μC/cm2, higher than the value for BIT, 12.5 μC/cm2. The polarization of the BTV thin film capacitor decreased by 19%, while that of the BIT decreased by 23% after the fatigue test with 4×1010 switching cycles.

Effects of donor ion doping on the orientation and ferroelectric properties of bismuth titanate thin films

Bismuth layer-structured ferroelectric thin films, Bi4Ti3O12 (BTO) and donor (V5+, W6+, and Nb5+)-doped BTO (BTV, BTW, and BTN) were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by a sol–gel method. We investigated the doping effects of donor ions on the grain orientation and the electrical properties. Donor ions which substituted for Ti4+ ions in pseudoperovskite layers of BTO decreased the c-axis orientation and increased the remanent polarization (2Pr). The fatigue resistances of donor-doped thin films were shown to be superior to that of BTO, and the leakage currents were decreased by approximately 1 order of magnitude compared with BTO. The improvements of electrical properties with donor doping in BTO could be attributed to the changes in grain orientation and space charge density in the thin films.

Structure–property relationship in lead-free A- and B-site co-doped Bi0.5(Na0.84K0.16)0.5TiO3–SrTiO3 incipient piezoceramics

In this work, a phase diagram of A- and B-site co-substituted 0.96[{Bi0.5 (Na0.84K0.16)}1−x−yLixMgy(Ti1−zNbz)O3]–0.04SrTiO3 (abbreviated as LMN-doped BNKT–ST), where x, y and z = 0.00–0.030, was schematically constructed on the basis of crystal structure and electromechanical, dielectric and piezoelectric properties. The underlying mechanism of the compositionally-induced non-ergodic (NR) to ergodic relaxor (ER) phase transition was explored, and emphasis was given on relating the chemically-induced polymorphic structural phase transition to the dynamics of polar nano-regions (PNRs) and their random fields, which strongly affect the dielectric, ferroelectric, piezoelectric and field-induced strain properties of the investigated system. X-ray diffraction patterns revealed that LMN doping resulted in a transition from coexistence of rhombohedral and tetragonal phases to a pseudocubic phase. Both the dielectric constant and the ferroelectric–relaxor transition (TF–R ∼ 100 °C) temperature decreased with an increase in LMN content. The piezoelectric and ferroelectric responses of the BNKT–ST ceramics were significantly decreased by the addition of LMN. However, the destabilization of the piezoelectric and ferroelectric properties was accompanied by significant enhancements in the bipolar and unipolar strains. A large electric-field-induced strain (S = 0.28%) and a corresponding dynamic piezoelectric constant (Smax/Emax) of 560 pm V−1 were observed under the driving field of 5 kV mm−1 when 1.5 mol% LMN was substituted on respective sites. This significant strain enhancement at this composition, with LMN = 0.015, may be attributed to both the field-induced reversible structural transition and the compositionally-induced NR to ER phase transition. The composition- and temperature-dependence of the energy storage density (W) were studied to demonstrate the practicability of the LMN-doped BNKT–ST. It was found that the addition of LMN enhanced the difference between maximum polarization and remnant polarization, resulting in an improvement of the energy storage properties. For the composition with LMN = 0.020, a nearly temperature-invariant large recoverable energy density (W = 0.70 J cm−3) was achieved under 5.5 kV mm−1 in the wide temperature range of 100–150 °C. These properties indicate that the synthesized system might be a promising lead-free candidate for actuator and energy storage capacitor applications.

Conduction behavior of SrBi2Ta2O9 thin film grown by pulsed laser deposition

SrBi2Ta2O9 (SBT) thin films with different Sr and Bi concentrations were prepared on Pt/Ti/SiO2/Si substrate by the pulsed laser deposition technique. The structural feature and surface morphology were characterized by X-ray diffraction and SEM studies. The ferroelectricity was confirmed by polarization–electric field (P–E) hysteresis loops. The measured value of remanent polarization (2Pr) of Bi-excess SBT film was 14.3 µC/cm2 with a coercive field (2Ec) of 101 kV/cm at the applied voltage of 4 V. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range of 0.1 Hz–100 kHz and the temperature range of 25°C–400°C. From the slope of ac conductivity vs. 1/T plot, the activation energy was calculated to be 0.86 eV at the lowest frequency and high temperature range. Also electrical properties were investigated by current–voltage (I–V) measurements as a function of applied electric fields. To determine the influence of Sr and Bi concentrations on the SBT/Pt junction, the ferroelectric properties and conduction behavior of SBT thin films are discussed.

Ferroelectric properties of tungsten-substituted Bi4Ti3O12 thin film prepared by sol-gel method

Bi4Ti3O12 (BIT) and tungsten-substituted Bi4Ti3O12 (BTW) thin films were prepared on Pt/Ti/SiO2/Si substrates by a sol–gel method. There are differences in the ferroelectric properties and grain strucure between BIT and BTW thin films. The crystal structure and the surface grain morphology were characterized by scanning electron microscopy and X-ray diffraction. Grains of BIT were grown with c-axis preferred orientation, while these of the BTW were randomly distributed. The ferroelectric properties and polarization fatigue characteristics were confirmed by the P-E hysteresis loops. The BTW thin film was measured to have remanent polarization (2Pr) of 27 µC/cm2 and a coercive field (2Ec) of 130 kV/cm. The dielectric constant and loss tangent at 1 MHz were measured as 210 and 0.05, respectively. On adding a small amount of tungsten in Bi4Ti3O12, the remanent polarization increased and fatigue resistance improved.

Composition-dependent polarization switching behaviors of (111)-preferred polycrystalline Pb(ZrxTi1−x)O3 thin films

We investigated the time-dependent polarization switching behaviors of (111)-preferred polycrystalline Pb(ZrxTi1−x)O3 thin films with various Zr concentrations. We could explain all the polarization switching behaviors well by assuming Lorentzian distributions in the logarithmic polarization switching time [refer to J. Y. Jo et al., Phys. Rev. Lett. 99, 267602 (2007)]. Based on this analysis, we found that the Zr ion substitution for Ti ions would induce broad distributions in the local field due to defect dipoles, which makes the ferroelectric domain switching occur more easily.

Orientational Dependence of Electro-optic Properties of SrBi2Ta2O9 Ferroelectric Thin Films

A-/b- and c-axis oriented SrBi2Ta2O9 thin films were grown on the MgO(110) and MgO(100) substrates, respectively, by rf magnetron sputtering deposition method. Optical anisotropy was shown in the infrared reflectance spectra for the SrBi2Ta2O9 thin films. Quadratic electro-optic effects were observed only in the a-/b-axis oriented films. The effective quadratic electro-optic coefficient was about 3.8 ×10-17 m2/V2.