Myong-Ho Kim

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.

Influence of nonstoichiometry on ferroelectric phase transition in BaTiO3

The ferroelectric phase transition behavior in equilibrated nonstoichiometric BaTiO3 powder samples was characterized by a differential scanning calorimeter. The transition temperature (TC) and enthalpy of transition (ΔHTC) between the paraelectric and ferroelectric phases were varied systematically with defect concentrations through the Ba∕Ti ratio, temperature, and oxygen partial pressure [p(O2)]. It was observed that the different defects such as titanium, barium, and oxygen vacancies all contribute to the changes of TC and ΔHTC. The TC decreased with increasing concentration of TiO2 partial Schottky defects and BaO partial Schottky defects. The annealing temperature increased the TC due to a complex mechanism with the increase of full Schottky defect reactions. In terms of p(O2), TC is constant in the ionic compensation region, and increases with the nonstoichiometry in the electronically compensated p(O2) region.

Characteristics of the First Longitudinal-Fourth Bending Mode Linear Ultrasonic Motors

Linear ultrasonic motors using a combination of the first longitudinal mode and the fourth bending mode were designed and fabricated. The driving characteristics of the motors, which were composed of a straight metal bar bonded with piezoelectric ceramic vibrators as a driving element, were measured. Unimorph and bimorph ceramic vibrators were attached on three kinds of metal bars for constructing the stators of the linear motors. As results, motors made with the bimorph ceramic vibrators had higher velocity than motors of the unimorph vibrators. As a metal bar for stator, magnesium alloy, which has lower elastic coefficient than aluminum alloy, was better for the motors.

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.

Photocatalytic reactivity and diffusing OH radicals in the reaction medium containing TiO2 particles

The generation of OH radicals on UV-illuminated TiO2 surface is mainly responsible for the photocatalytic oxidation of pollutants in various contaminated environmental media. Although the reactivity of OH radicals is largely limited within the surface region, the possibility of OH desorption and diffusion into the reaction medium has been often raised. This study provides several examples for the presence of diffusing OH radicals in aqueous solution and polymer matrix containing TiO2 particles. The photocatalytic degradation rates of (CH3)4N+ in TiO2 suspension were comparable between acidic and alkaline conditions, which could not be explained by a simple electrostatic surface charge model. From the present mechanistic study, it is suggested that the photocatalytic oxidation of (CH3)4N+ at acidic pH mainly proceeds through free OH radicals in the solution bulk, not on the surface of TiO2. The diffusing OH radicals also played the role of main oxidants in the solid phase. The photolysis of TiO2-embedded PVC composite films generated cavities around the imbedded TiO2 particles and the development of cavity diameter continued even after the direct contact between the PVC and TiO2 was prohibited. This implied that active oxygen species that were photogenerated on TiO2 surface desorbed and diffused across a few micrometers to react with the polymer matrix.

Microwave imaging reflectometry for density fluctuation measurement on KSTAR

A dual-frequency microwave imaging reflectometry system was commissioned to measure both coherent and turbulent electron density fluctuations in KSTAR plasmas. Imaging of the density fluctuations is achieved with an array of 16 vertically aligned detectors and two X-mode probe beam frequencies (tunable over 78–92 GHz between plasma discharges). The system provides the capability of fluctuation measurements with poloidal wavenumbers (kθ) up to ~3 cm−1 at the maximum sampling rate of 2 MHz. Following extensive laboratory tests, the system was further tested with known coherent density fluctuations during the precursor oscillation of the m/n = 1/1 internal kink mode. The phase information of the reflected beam was compared with the precursor oscillation of the electron temperature measured by an electron cyclotron emission (ECE) radiometer. Density fluctuation levels (δne/ne) at two radial positions separated by the inversion radius (inside and outside) were comparable to temperature fluctuation levels (δTe/Te) from ECE signals. Subsequently, two correlation analysis methods were applied to turbulent fluctuation measurements in a neutral beam heated L-mode plasma to determine the mean poloidal rotation velocities of density fluctuations at two radial positions. The measured mean poloidal velocities were ~8.4 km s−1 at r/a ~ 0.6 and ~5 km s−1 at r/a ~ 0.7 in the clockwise direction, which differed by 1–2 km s−1 with the projected poloidal velocities from the toroidal rotation velocity measured by charge exchange recombination spectroscopy.

Microstructural development of cobalt ferrite ceramics and its influence on magnetic properties

The microstructural evolution and its influence on magnetic properties in cobalt ferrite were investigated. The cobalt ferrite powders were prepared via a solid-state reaction route and then sintered at 1200 °C for 1, 2, and 16 h in air. The microstructures from sintered samples represented a bimodal distribution of grain size, which is associated with abnormal grain growth behavior. And thus, with increasing sintering time, the number and size of abnormal grains accordingly increased but the matrix grains were frozen with stagnant grain growth. In the sample sintered for 16 h, all of the matrix grains were consumed and the abnormal grains consequently impinged on each other. With the appearance of abnormal grains, the magnetic coercivity significantly decreased from 586.3 Oe (1 h sintered sample) to 168.3 Oe (16 h sintered sample). This is due to the magnetization in abnormal grains being easily flipped. In order to achieve high magnetic coercivity of cobalt ferrite, it is thus imperative to fabricate the fine and homogeneous microstructure.

Terahertz Near-Field Microscope

We report a terahertz pulse apertureless near-field microscope (THz NFM) which combines THz time domain spectroscopy (THz-TDS) and atomic force microscopy (AFM) techniques. By scanning the probe over a GaAs/Au edge, it is found that the THz NFM has a lateral resolution of ~ 80 nm.

Comparison of measured 2D ELMs with synthetic images from BOUT++ simulation in KSTAR

A detailed study of edge-localized mode (ELM) dynamics in the KSTAR tokamak is performed using a two-dimensional (2D) electron cyclotron emission imaging (ECEI) diagnostic system. Highly coherent mode structures rotating in the poloidal view plane are routinely observed in the inter-ELM pedestal region where the optical thickness for ECE rapidly changes and the interpretation of emission intensity is complicated. To have confidence on the measurements, the observed images are compared with synthetic images of the ELM structure deduced from three-field BOUT++ simulations. The synthetic process considers instrumental effects of the ECEI diagnostic, intrinsic broadening of the ECE and background noise. The synthetic 2D images highly resemble the observed structure, providing confidence that the ELM dynamics can be visualized by ECEI.

Valence state of Ru at the Mn sites in Pr0.5Sr0.5MnO3: Thermoelectric power and X-ray absorption near-edge structure spectroscopy

The Ru valence at the Mn sites in Pr0.5Sr0.5MnO3 has been studied using thermoelectric power (TEP) and Ru K- and LIII-edge X-ray absorption near edge structure (XANES) spectroscopy. In comparison with TEP value at 400 K for Pr1−xSrxMnO3 and Pr0.5Sr0.5Mn1−xRuxO3, it was found that the Mn-site doped Ru is in the mixed valence state between 4+ and 5+. Ru K- and LIII-edge XANES study of Pr0.5Sr0.5Mn1−xRuxO3 (x = 0.04 and 0.1) also confirmed the mixed valence state of Ru from the fact that XANES spectrum of the Mn-site doped Ru is in between those of tetravalent and pentavalent ruthenates. These results indicate that the change of carrier density with Ru doping is not sufficient to understand the drastic enhancement of ferromagnetism observed in Ru-doped Pr0.5Sr0.5MnO3. The prime role of Ru is discussed in terms of Mn valence change and magnetic interactions between Mn and doped Ru ions.