Cheol Woo Ahn

Microstructure and piezoelectric properties of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 ceramics

For 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 (0.95NKN-0.05BT) ceramics sintered at 1040–1075°C, abnormal grain growth occurred but the grain size decreased when the sintering temperature exceeded 1075°C. The dielectric constant (ϵ3T∕ϵ3), electromechanical coupling factor (kp), and piezoelectric constant (d33) were considerably increased with increasing relative density and grain size. Evaporation of Na2O deteriorated the piezoelectric properties by decreasing the resistivity. To minimize Na2O evaporation, specimens were muffled with 0.95NKN-0.05BT powders during the sintering. Improved piezoelectric properties of d33=225pC∕N, kp=36%, and ϵ3T∕ϵ3=1058 were obtained for specimen sintered at 1060°C for 2h with muffling.

Microstructure and Piezoelectric Properties of ZnO-added (Na0.5K0.5)NbO3 Ceramics

The orthorhombic structure of (Na0.5K0.5)NbO3 (NKN) ceramics was maintained when ZnO was added. NKN ceramics have a porous microstructure and dissolve when they are exposed to water. However, as ZnO was added, a dense microstructure was developed and deliquescence was not observed. The mechanical quality factor (Qm) and coercive field (Ec) increased with the addition of ZnO, indicating that ZnO acted as a hardener in the NKN ceramics. The dielectric constant (eT3/eo), piezoelectric constant (d33) and electromechanical coupling factor (kp) increased when a small amount of ZnO was added, which might be due to the increase in density. The good dielectric and piezoelectric properties of eT3/eo = 500, d33 = 121 and kp=0.4 were obtained for the NKN ceramics with 1.0 mol% ZnO.

Correlation between Phase Transitions and Piezoelectric Properties in Lead-Free (K,Na,Li)NbO3–BaTiO3 Ceramics

In this manuscript, we report the polymorphic phase transitions, structural changes and piezoelectric properties of alkali niobate based lead-free ceramics. The phase transitions were characterized as a function of alkali niobate content at room temperature. The results clearly demonstrate that in this system high piezoelectric properties are achieved for a specific fraction of ferroelectric orthorhombic (O) and tetragonal (T) phases. Using Rietveld and powder diffraction analysis, a correlation was established among the piezoelectric response, the fraction of O and T phases, and KNN ratio for three different systems of (K,Na)NbO3–BaTiO3 (KNN–BT), KNN–LiNbO3 (LN), and (K,Na,Li)NbO3 (KNLN)–BT.

Ubiquitous magneto-mechano-electric generator

Stray magnetic field considered as harmful noise for the human body can be a ubiquitous energy source. We are surrounded with 50/60 Hz parasitic magnetic noise arising from power delivery infrastructure, but it cannot be readily utilized by traditional electromagnetic harvesters. Here, we introduce a novel magneto-mechano-electric (MME) generator with a colossal power density that can turn on 35 LEDs and drive a wireless sensor network under a weak magnetic field of 5–7 × 10−4 T at a low frequency of 60 Hz. The MME generator is a cantilever structured magnetoelectric (ME) laminate composite in which the 〈011〉 oriented anisotropic single crystal fiber composite (SFC) is bonded to Ni plate and Nd permanent magnet proof mass. The ME laminate composite has a strong ME coupling (αME ∼ 160 V cm−1 Oe−1) even without magnetic bias due to the intrinsic property of Ni. The MME generator is also found to exhibit a colossal output power density of 46 mW cm−3 Oe−2 under a weak magnetic field of 1.6 × 10−4 T at 60 Hz. This MME generator can be a ubiquitous power source for wireless sensor networks, low power electric devices, and wireless charging systems by harvesting tiny amounts of parasitic magnetic energy from our living environment.

Stress-controlled Pb(Zr0.52Ti0.48)O3 thick films by thermal expansion mismatch between substrate and Pb(Zr0.52Ti0.48)O3 film

Polycrystalline Pb(Zr0.52Ti0.48)O3 (PZT) thick films (thickness ∼10 μm) were successfully fabricated by using a novel aerosol deposition technique on Si wafer, sapphire, and single crystal yitria stabilized zirconia (YSZ) wafer substrates with Pt electrodes and their dielectric, ferroelectric, and piezoelectric properties, and in-plane stresses were investigated. The films with different stress conditions were simply controlled by the coefficient of thermal expansion (CTE) misfit of PZT films and substrates. The results showed that the films bearing in-plane compressive stress deposited on the YSZ and sapphire substrates have superior dielectric, ferroelectric (∼90%), and piezoelectric (>200%) properties over that of the Si wafer. Among these three substrates, YSZ shows superior properties of the PZT films. However, films on Si wafer with tensile stress present lower properties. We believed that in-plane compressive stresses within the films are benefited, the formation of c-domain parallel to the thickne...

Effects of CuO and ZnO Additives on Sintering Temperature and Piezoelectric Properties of 0.41Pb(Ni1/3Nb2/3)O3-0. 36PbTiO3-0.23PbZrO3 Ceramics

ZnO-added 0.41PNN–0.36PT–0.23PZ ceramics cannot be sintered below 950°C. However, when CuO is added, the liquid phase forms and specimens can be sintered even at 850°C. The dielectric constant (eT3/eo), piezoelectric constant (d33) and electromechanical coupling factor (kp) increase with the addition of CuO and this is due to the enhancement of the density of the specimens. When CuO is added to 0.41PNN–0.36PT–0.23PZ ceramics, improvements of density, eT3/eo, d33 and kp are also observed. However, the d33 and kp values of the CuO-added 0.41PNN–0.36PT–0.23PZ ceramics are less than those of the CuO- and ZnO-added specimens. Therefore, ZnO is effective in improving the piezoelectric properties of 0.41PNN–0.36PT–0.23PZ ceramics. Good dielectric and piezoelectric properties of d33=575(pC/N), kp=0.55 and eT3/eo= 3900 are obtained for 0.41PNN–0.36PT–0.23PZ+3.0 mol% ZnO with 1 mol% CuO sintered at 900°C for 2 h.

Design and characterization of broadband magnetoelectric sensor

In this study, we present a broadband magnetoelectric (ME) sensor design comprising of Metglas and piezoelectric ceramic laminate composite. A systematic study was conducted to elucidate the role of various composite variables toward the ME response [longitudinal-transverse (LT) mode] over the applied range of magnetic dc bias. The broadband behavior was characterized by flat ME responses over a wide range of magnetic dc bias at frequency of 1 kHz. The variation in ME coefficient as a function of magnetic dc bias was found to be significantly dependent on the size and shape of the laminate composites, the number of Metglas layers, and composite structure of sandwich versus unimorph. By adjusting these variables, we were able to achieve near-flat ME response over a magnetic bias range of 90–220 Oe. ME coefficient was also measured as a function of frequency, and at electromechanical resonance the peak value was found to be almost independent of applied magnetic bias in the range of 90–220 Oe.

(1−x)BaTiO3−x(Na0.5K0.5)NbO3 ceramics for multilayer ceramic capacitors

(1−x)BaTiO3−x(Na0.5K0.5)NbO3 ceramics with 0.0⩽x⩽0.08 formed a homogeneous solid solution. As (Na0.5K0.5)NbO3 (NKN) was added, the Curie temperature of the BaTiO3 (BT) ceramics decreased and specimens became a relaxor as x exceeded 0.04. The grain size of the BT ceramics decreased with increasing NKN. The average grain size was approximately 0.4μm at x=0.06. The dielectric constant (er) of the specimen increased with increasing NKN and high er of 7402 with low dielectric loss of <1.0% was observed for the 0.94BT-0.06NKN ceramic. Therefore, the 0.94BT-0.06NKN ceramic is proposed as a promising candidate material for thin multilayer ceramic capacitors with high capacitance.

Microstructure and Microwave Dielectric Properties of Ba(Co1/3Nb2/3)O3 Ceramics

Ba(Co1/3Nb2/3)O3 (BCN) ceramics has a 1:2 ordered hexagonal structure and the degree of the 1:2 ordering slightly decreased when the sintering temperature exceeded 1400°C. A large amount of the liquid phase was found in the BCN ceramics sintered above 1400°C the formation of which is related to the evaporation of CoO. The liquid phase contains high concentrations of Ba and Nb ions. The grain size increased for the specimens sintered above 1400°C due to the presence of the liquid phase during the sintering. The Q-value of BCN increased with increasing sintering temperature and the specimen sintered at 1400°C had the maximum Q-value. When the sintering temperature exceeded 1400°C, however, the Q-value significantly decreased. The presence of a large amount of liquid phase could be responsible for the decrease of the Q-value. BCN ceramics were also sintered for various times at 1400°C and 1450°C and the variations of the microwave dielectric properties were explained in terms of the grain size and the relative density.

A generalized rule for large piezoelectric response in perovskite oxide ceramics and its application for design of lead-free compositions

We present a general rule for the perovskite oxide ceramics: “A large piezoelectric constant in ABO3 perovskite ceramics can be obtained by tuning the weight ratio of A and B sites, WA/WB or WB/WA, to 3. Piezoelectric constant decreases significantly when WA/WB or WB/WA is in the range of 0.5–2.0, termed as forbidden zone.” A comparative analysis was conducted for broad range of materials demonstrating the applicability of proposed rule. Further based on this rule optimized compositions in BaTiO3 and alkali niobate based systems were developed. Polycrystalline ceramics in modified BaTiO3 system were found to exhibit longitudinal piezoelectric coefficient (d33) of 330 pC/N, while alkali niobate ceramics showed d33 of 294 pC/N.