Hyoung-Su Han

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.

Coexistence of ergodicity and nonergodicity in LaFeO3-modified Bi1/2(Na0.78K0.22)1/2TiO3relaxors

The effect of LaFeO(3) addition to Bi(1/2)(Na(0.78)K(0.22))(1/2)TiO(3) ceramics on the phase stability and macroscopic functional properties was investigated. Similarly to other chemical modifiers known in the literature, LaFeO(3) addition suppresses an electric-field-induced long-range ferroelectric order, giving rise to a giant unipolar strain of ~0.3% at 2 mol% LaFeO(3) addition. Time-dependent changes in polarization and strain hysteresis loops both during successive electrical cycling and after removal of the electric field suggest that a specimen with 2 mol% LaFeO(3) consists of both ergodic and nonergodic phases, which is unique among the known relaxor materials.

High strain in lead-free Nb-doped Bi1/2(Na0.84K0.16)1/2TiO3–SrTiO3 incipient piezoelectric ceramics

Lead-free 0.96[Bi1/2(Na0.84K0.16)1/2(Ti(1−x)Nbx)O3]–0.04SrTiO3 (BNKTN–ST, with x = 0–0.030) ceramics were synthesized by a conventional solid-state reaction technique. Polarization and strain hysteresis loops indicated a significant disruption of ferroelectric order accompanied by an enhanced field-induced strain (S = 0.438%) with a high normalized strain Smax/Emax of 876 pm/V at 5 kV/mm. Their reproducibility was confirmed by the fabrication of a 10-layered stack-type multilayer actuator, which demonstrated a normalized strain Smax/Emax of 720 pm/V.

Strain enhancement in lead-free Bi0.5(Na0.78K0.22)0.5TiO3 ceramics by CaZrO3 substitution

The effects of CaZrO3 substitution on the crystal structure, electric field–induced strain, and piezoelectric properties of Bi0.5(Na0.78K0.22)0.5TiO3 ceramics have been investigated. The CaZrO3 substitution resulted in a transition from ferroelectric tetragonal to pseudocubic symmetry with relaxor characteristics, leading to degradations in the piezoelectric coupling coefficient, electromechanical quality factor, and piezoelectric coefficient d33. However, the electric field–induced strain was significantly enhanced by the CaZrO3-induced phase transition and reached a highest value of S max /E max = 617 pm/V when CaZrO3 content reached 3 mol%. The result supports a prediction that a large strain at the ferroelectric–relaxor transition region can be observed when the tolerance factor of the Bi-perovskite is decreased by the substitution of ABO3-type modifiers.

Low-Firing Pb(Zr,Ti)O 3 -Based Multilayer Ceramic Actuators Using Ag Inner Electrode

We investigated the low firing of Li2CO3 added 0.2Pb(Mg1/3Nb2/3)O3 - 0.3Pb(Fe1/2Nb1/2) - 0.5Pb(Zr0.475Ti0.525)O3 (PMNPFN-PZT) ceramics and multilayer actuators (MLAs) using Ag inner electrodes. It was found that 0.1 wt% Li2CO3 was quite effective in lowering the sintering temperature of PMN-PFN-PZT ceramics from 1,100℃ down to 900℃ without deteriorating their piezoelectric ceramics (d33 = 425 pC/N and kp = 61.9%). However, excess Li2CO3 up to 0.3 wt% brings about unwanted problems such as the formation of a LiPbO2 secondary phase and subsequent degradation in the piezoelectric properties. Using 0.1 wt% Li2CO3 added PMN-PFN-PZT ceramics, MLAs with Ag inner electrodes were successfully fabricated, resulting in a normalized strain of 580 pm/V at an electric field of 1.5 kV/mm.

Sintering Behaviour and Piezoelectric Properties of CuO-Added Lead-Free Bi(Na,K)TiO3 Ceramics

Effect of excess CuO on the sintering behaviour and piezoelectric properties of Bi0.5(Na82K0.18)0.5TiO3 ceramics was investigated. The addition of small amount of excess CuO as low as 0.01 mol was quite effective to lower the sintering temperature (Ts) of BNKT ceramics down to 975°C which is 200°C lower than the Ts of pure BNKT (1175°C) without significant degradation of their piezoelectric properties. From microstructure analysis, it was concluded that CuO promotes sintering via forming a liquid phase at 975°C and partly dissolves into Ti-sites during sintering.

Improvement of piezoelectric and ferroelectric properties in (K,Na)NbO3- based ceramics via microwave sintering

In this study modified KNN–based piezoceramics with different amounts of SrTiO3 were prepared. Sintering was carried out in a conventional furnace (via single–step as well as two–step sintering) and in a microwave furnace. Sintering behavior, piezoelectric properties, ferroelectric and strain behavior of sintered samples were investigated. Observation of microstructures proved that two–step sintering suppressed grain growth and enhanced densification. Microwave sintered samples exhibited the highest piezoelectric constant compared to the other samples which might be attributed to higher density and lower loss of volatile elements such as K and Na during microwave sintering. Microwave sintering also resulted in the improvement of ferroelectric behavior and enhancement of electric–field–induced strain. This work demonstrates the potential of microwave sintering to fabricate KNN–based ceramics with better electromechanical properties compared to those obtained using conventional furnace sintering.

Comparison of Ferroelectric and Strain Properties between BaTiO3- and BaZrO3-Modified Bi1=2(Na0:82K0:18)1=2TiO3 Ceramics

Effects of BaTiO3 (BT) and BaZrO3 (BZ) modification on the crystal structure and electromechanical properties of lead-free Bi1/2(Na0.82K0.18)1/2TiO3 (BNKT) ceramics were compared within a doping range of 0–5 mol %. The differences between BT- and BZ-modification effects were clearly observed in polarization and electric field-induced strain hysteresis loops. BZ-modification induced a gradual transition from a ferroelectric to a nonpolar phase, resulting in an abnormal enhancement in the electric field-induced strain (EFIS) just after the transition. However, all BT-modified specimens showed strong ferroelectricity without abnormal enhancement in EFIS. The results suggest that Zr doping-induced B-site strains play a significant role in the ferroelectric-to-nonpolar phase transition in the Bi-perovskite.

Fabrication of (K0.47Na0.51Li0.02)(Nb0.8Ta0.2)O3 Multilayer Ceramic Actuators with AgPd-Ceramic Composite Inner Electrode

We investigated the effect of internal electrode composition on the co-firing behavior of (K0.47Na0.51Li0.02) (Nb0.8Ta0.2)O3 (KNLNT) lead-free multilayer ceramic actuators using AgPd-KNLNT inner electrodes. KNLNT/AgPd-KNLNT multilayer laminates were fabricated using tape casting, and then co-fired at 1050°C for 4 h. Co-fired AgPd-KNLNT composite electrodes revealed ‘3-0’ connectivity without formation of any secondary phase and resulting in the reduced thermal mismatch between piezoceramic and electrode layers on firing cycles.

Enhancement in Piezoelectric Properties of PZT-Based Ceramics by High Energy Ball-Milling Treatment of Solid-State Synthesized Powders

The effects of high energy ball-milling (HEBM) on the sintering behavior and piezoelectric properties of 0.1 wt% doped 0.8Pb()-0.2Pb() (PMN-PZT) ceramics were investigated. It was found that HEBM treatment was quite effective to reduce the average particle size down to 300 nm, leading to increased density as well as enhanced piezoelectric properties of a sintered specimen even though prolonged HEBM resulted in unwanted secondary phases that caused a degradation of piezoelectric properties. The dielectric constant (), piezoelectric coupling factor () and piezoelectric constant of 0.1 wt% doped PMN-PZT ceramics prepared via HEBM for 10 h reached 2040, 0.68 and 554 pC/N, respectively.