Ill Won Kim

Enhanced multiferroic properties of single-phase BiFeO3 bulk ceramics by Ho doping

Dense single-phase BiFeO3 and Bi0.9Ho0.1FeO3 ceramics were prepared by the solid-state reaction method. With Ho doping, the remnant polarization of BiFeO3 was enhanced and the switching characteristics improved at low electric fields. Ho doping increased the breakdown voltage with a reduction of the leakage current while mitigating the remnant polarization at high electric fields. These results can explain conflicting findings regarding the effects of rare-earth doping on remnant polarization. Bi0.9Ho0.1FeO3 exhibited peculiar double hysteresis looplike magnetization-magnetic field curves with a much enhanced remnant magnetization. These improved properties obtained by Ho doping demonstrate the possibility of enhancing the multiferroic applicability of BiFeO3.

Enhanced luminescence of pulsed-laser-deposited Y2O3:Eu3+ thin-film phosphors by Li doping

Y2O3:Eu3+ and Li-doped Y2O3:Eu3+ luminescent thin films have been grown on sapphire substrates using pulsed-laser deposition. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. The photoluminescence (PL) brightness data obtained from Li-doped Y2O3:Eu3+ films grown under optimized conditions have indicated that sapphire is a promising substrate for the growth of high-quality Li-doped Y2O3:Eu3+ thin-film red phosphor. In particular, the incorporation of Li+ ions into Y2O3 lattice could induce a remarkable increase of PL. The highest emission intensity was observed with LiF-doped Y1.84Li0.08Eu0.08O3, whose brightness was increased by a factor of 2.7 in comparison with that of Y2O3:Eu3+ films. This phosphor may promise for application to the flat panel displays.

Incipient piezoelectrics and electrostriction behavior in Sn-doped Bi1/2(Na0.82K0.18)1/2TiO3 lead-free ceramics

Dielectric, ferroelectric, piezoelectric, and strain properties of lead-free Sn-doped Bi1/2(Na0.82K0.18)1/2TiO3 (BNKT) were investigated. A crossover from a nonergodic relaxor to an ergodic relaxor state at room temperature, accompanied by a giant electric-field-induced strain, was observed at 5 at. % Sn doping. Switching dynamics monitored during a bipolar poling cycle manifested that the observed giant strain originates from incipient piezoelectricity. When Sn doping level reached 8 at. %, BNKT exhibited an electrostrictive behavior with a highly temperature-insensitive electrostrictive coefficient of Q11 = 0.023 m4 C−2.

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.

Effect of Ta-Substitution on the Ferroelectric and Piezoelectric Properties of Bi 0.5 (Na 0.82 K 0.18 ) 0.5 TiO 3 Ceramics

The effect of Ta substitution on the crystal structure, ferroelectric, and piezoelectric properties of ceramics has been investigated. The Ta doping resulted in a transition from coexistence of ferroelectric tetragonal and rhombohedral phases to an electrostrictive pseudocubic phase, leading to degradations of the remnant polarization, coercive field, and piezoelectric coefficient . However, the electricfield-induced strain was significantly enhanced by the Ta substitution-induced phase transition and reached a highest value of

Effects of Hafnium Substitution on Dielectric and Electromechanical Properties of Lead-free Bi0.5(Na0.78K0.22)0.5(Ti1-xHfx)O3 Ceramics

Lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5(Ti1-xHfx)O3 ceramics (abbreviated as BNKTH-100x, x=0–0.05) were prepared by the conventional solid-state reaction method. The effects of Hf substitution on the crystal structure and electromechanical properties were investigated. The X-ray diffraction patterns of the BNKTH-100x ceramics revealed a single-phase perovskite structure with no evidence of secondary phases. The temperature-dependent dielectric curves of the BNKTH-100x ceramics were found to broaden with increasing Hf concentration. The electric-field-induced strain of the BNKTH-100x ceramics increased up to x=0.03 and then decreased. An enhanced electric-field-induced strain of 0.38% is observed for the BNKTH-3 ceramic. The corresponding normalized strain (Smax/Emax=d33*) for this composition was 475 pm/V at an applied electric field of 80 kV/cm. The BNKTH-3 ceramic can be considered as a promising candidate material for lead-free electromechanical applications.

Antiferroelectric Thin-Film Capacitors with High Energy-Storage Densities, Low Energy Losses, and Fast Discharge Times

We demonstrate a capacitor with high energy densities, low energy losses, fast discharge times, and high temperature stabilities, based on Pb(0.97)Y(0.02)[(Zr(0.6)Sn(0.4))(0.925)Ti(0.075)]O3 (PYZST) antiferroelectric thin-films. PYZST thin-films exhibited a high recoverable energy density of U(reco) = 21.0 J/cm(3) with a high energy-storage efficiency of η = 91.9% under an electric field of 1300 kV/cm, providing faster microsecond discharge times than those of commercial polypropylene capacitors. Moreover, PYZST thin-films exhibited high temperature stabilities with regard to their energy-storage properties over temperatures ranging from room temperature to 100 °C and also exhibited strong charge-discharge fatigue endurance up to 1 × 10(7) cycles.

Enhanced ferroelectric properties of LiNbO3 substituted Na0.5K0.5NbO3 lead-free thin films grown by chemical solution deposition

We have fabricated environmental friendly lead-free ferroelectric Na0.5K0.5NbO3 (NKN) and 0.95Na0.5K0.5NbO3–0.05LiNbO3 (0.95NKN-0.05LN) thin films by chemical solution deposition using metal-organic compounds, and studied the effects of LN substitution through the dielectric and ferroelectric properties. The small amount of LN substitution for NKN thin film led to a marked improvement in leakage current properties at the high electric field region. Furthermore, the 0.95NKN-0.05LN thin film (350nm) displayed clear ferroelectricity with well saturated P-E hysteresis loop with 2Pr and 2Ec values of 19.5μC∕cm2 and 91kV∕cm, respectively. The 0.95NKN-0.05LN films will be interesting for applications in lead-free ferroelectric and piezoelectric devices.

Large strain under a low electric field in lead-free bismuth-based piezoelectrics

In this letter, the composition and electric field dependent strain behavior of (1 − x)Bi0.5(Na0.78K0.22)0.5TiO3-xBi(Mg0.5Ti0.5)O3 (BNKT-BMT) were investigated to develop lead-free piezoelectric materials with a large strain response at a low driving field for actuator applications. A large strain of 0.35% (Smax/Emax = 636 pm/V) at an applied field of 55 kV/cm was obtained with a composition of 4 mol. % BMT. In particular, the electric field required to deliver large strains was reduced to a level that revealed not only a large Smax/Emax of 542 pm/V at a driving field as low as 35 kV/cm, but also remarkably suppressed the large hysteresis.

Dielectric studies of a nano-crystalline CaCu2.90Zn0.10Ti4O12 electro-ceramic by one pot glycine assisted synthesis from inexpensive TiO2 for energy storage capacitors

A facile way for the synthesis of nano-crystalline CaCu2.90Zn0.10Ti4O12 (CCZTO) using a solution combustion technique based on the glycine–nitrate process with inexpensive solid TiO2 powder as the raw material is introduced in this manuscript, for the first time. The precursor powder was calcined between 200 °C and 850 °C for 3 h in air. Phase formation, crystalline nature, morphology and chemical purity of the fabricated CCZTO were investigated with TG/DTA, FT-IR, FT-Raman, XRD, SAED patterns, SEM, TEM, EDX and XPS analyses, respectively. The XRD results indicated that all sintered samples had a major CaCu3Ti4O12 structure with some amount of CaTiO3 and CuO. The bright-field TEM micrographs revealed that the particle size was in the range of 15–50 nm, which was in good agreement with the average crystallite size obtained from XRD. SEM micrographs of the sintered CCZTO ceramics showed the average grain sizes were in the range of 800 nm–7 μm. EDX and XPS studies confirmed the stoichiometry and purity of the ceramics. The nature of the relaxation behavior of the ceramics was rationalized using impedance and modulus spectroscopy. The activation energies calculated from the grain-boundary relaxation time constant were found to be in the range of 0.79–0.52 eV, which confirmed the Maxwell–Wagner type of relaxation present in the ceramic. Our inexpensive novel solution chemistry based method for CCZTO_16h gives a high dielectric constant (799) and low dielectric loss (0.091) at 100 Hz at room temperature, which has potential significance for cost-effective technological applications in microelectronic devices.