Sahn Nahm

Lead-free piezoelectrics

1. Domain Engineering and Phase Transformations.- 2. Ferroelectric Domains and Grain Engineering in SrBi2Ta2O9.- 3. Development of KNN-based Piezoelectric Ceramics.- 4. Low temperature Sintering of the Alkali-Niobate Ceramics.- 5. Lead-Free KNN-Based Piezoelectric Materials.- 6. Alkali niobate piezoelectric ceramics.- 7. Influence of the A/B stoichiometry on defect structure, sintering and microstructure in undoped and Cu-doped KNN.- 8. Sodium Bismuth Titanate Based Ceramics.- 9. Perovskite Lead-Free Piezoelectric Ceramics.- 10. Processing and Properties of Textured BNT-based Piezoelectrics.- 11. Crystal growth and electrical properties of Na0.5Bi0.5TiO3-BaTiO3 single crystals.- 12. Nonstoichiometry in (Bi0.5Na0.5)TiO3 Ceramics.- 13. Resonator Characteristics of Bismuth Layer Structured Ferroelectric Materials.- 14. Defect Control and Properties in Bismuth Layer Structured Ferroelectric Single Crystals.- 15. Processing and Properties of Textured Bismuth Layer Structured Ferroelectrics.- 16. Self-biased lead-free magnetoelectric laminates.- 17. Applications of lead-free piezoelectrics.

β-Ga2O3 nanowires synthesized from milled GaN powders

White-colored materials synthesized by a thermal annealing of milled GaN powders at 930 °C in a nitrogen atmosphere were identified to be monoclinic β-Ga2O3 nanowires by x-ray diffraction and scanning electron microscopy. High-resolution transmission electron microscopy revealed that these nanowires are single nanocrystals, and energy dispersive x-ray indicated that these nanomaterials are free of any metals. In addition, bundles of these crystalline nanowires in the rectangular-pole shape are a few centimeters in length.

Superhydrophobic and antireflective nanograss-coated glass for high performance solar cells

We present a facile method for producing superhydrophobic nanograss-coated (SNGC) glass surfaces that possess both reduced reflectivity and self-cleaning properties at the air/glass interface. The refractive index of a CaF2 nanograss (NG) layer on a glass substrate, deposited by glancing angle vapor deposition, is 1.04 at 500 nm, which is the second-lowest value ever reported so far. The fluorinated NG layer gives rise to a high water contact angle (>150°) and very efficient cleaning out of dust with water drops. Using the dual functionalities of the SNGC glass, we demonstrate superhydrophobic and antireflective organic photovoltaic cells with excellent power conversion efficiency.

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.

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.

Low-Temperature Sintering of B2O3-Added Ba(Mg1/3Nb2/3)O3 Ceramics

The sintering temperature of Ba(Mg1/3Nb2/3)O3 (BMN) ceramic is about 1450°C. When B2O3 was added, BMN sintered at 900°C and it had a 1:2 ordered hexagonal structure. A liquid phase, which could be responsible for the decrease in sintering temperature, was observed in B2O3-added BMN. Mg4Nb2O9 and Ba2B2O5 second phases were found in the specimens and the Ba2B2O5 second phase might be related to the liquid phase. The dielectric constant (er) and Q value increased with increasing sintering temperature and it is due to the increase in bulk density. However, they decreased with increasing B2O3 content, which can be explained by the existence of the second phases. Good microwave dielectric properties of Q ×f = 8500 GHz, er = 27.5 and temperature coefficient of resonance frequency (τf) = 27.0 ppm/°C were obtained for BMN with 2.0 mol% B2O3 sintered at 930°C for 2 h.

Low-Temperature Sintering and Piezoelectric Properties of ZnO-Added 0.41Pb(Ni1/3 Nb2/3)O3–0.36PbTiO3–0.23PbZrO3 Ceramics

The structure of 0.41(PbNi1/3Nb2/3)O3–0.36PbTiO3–0.23PbZrO3 (0.41PNN–0.36PT–0.23PZ) ceramics changed from pseudocubic to tetragonal when ZnO was added. For the specimens sintered above 1000°C, bulk density slightly decreased with the addition of ZnO but it significantly increased for the specimens sintered below 1000°C. Curie temperature (Tc) and maximum dielectric constant increased when ZnO was added. Coercive electric field (Ec) and mechanical quality factor (Qm) increased with the addition of ZnO but piezoelectric constant (d33), electromechanical coupling factor (kp) and dielectric constant (eT3/e0) decreased with the addition of ZnO for the specimens sintered above 1,000°C. However, for the specimens sintered below 1000°C, the addition of ZnO significantly improved the piezoelectric and dielectric constants, which could be due to the improvement of bulk density. The good dielectric and piezoelectric properties of d33=525 (pC/N), kp=0.53 and eT3/eo = 3400 were obtained for the specimen with 3 mol% ZnO sintered at 950°C for 1 h.

Effect of Ga2O3 on Microstructure and Microwave Dielectric Properties of Ba(Zn1/3Ta2/3)O3 Ceramics

The effect of Ga2O3 on the microstructure and microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 (BZT) ceramics was investigated. The 1:2 ordered hexagonal structure disappeared with the addition of Ga2O3. The average grain size of BZT increased with the addition of Ga2O3, which was attributed to the liquid phase formation. The liquid phase contains high concentrations of Ba and Ga ions. The relative density of the specimen increased with the addition of a small amount of Ga2O3 but it decreased when the Ga2O3 content was increased. Variation of the dielectric constant with Ga2O3 addition was similar to that of the relative density and ranged between 27 and 29. The temperature coefficient of resonant frequency decreased with increasing Ga2O3. The Q×f value of BZT was about 88,000 GHz and it significantly improved with the addition of Ga2O3. The maximum Q×f value achieved in this investigation was about 161,000 GHz for the BZT with 1.0 mol% Ga2O3 sintered at 1550°C for 10 h.

Characterization of catastrophic optical damage in Al-free InGaAs/InGaP 0.98 μm high-power lasers

Catastrophic optical damage (COD) in Al-free InGaAs/InGaP 0.98 μm lasers has been investigated using real-time electroluminescence (EL) and transmission electron microscopy (TEM). From EL images, we observed that multiple bright spots initiated from one of the facets and then propagated to the center of the cavity during the COD process. It is clarified by the TEM analysis that the propagation of bright spots resulted in 60-nm-wide Moire fringe along the cavity and the crystalline phase of the active area became polycrystalline. Highly nonradiative polycrystalline phase of the active area is the major cause of COD failure in the Al-free 0.98 μm lasers.

Effect of structural changes on the microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

The microstructure and microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 (BZN) ceramics have been investigated. BZN has a 1:2 ordered hexagonal structure, and the degree of 1:2 ordering decreased with increasing sintering temperature. Liquid phase was found for the BZN sintered at more than 1350°C, the evaporation of ZnO was responsible for the formation of liquid phase. BaNb6O16 second phase was also found for the specimens sintered at more than 1350°C and the liquid phase is considered to have a composition close to that of BaNb6O16 second phase. The Q-value of BZN was increased with increasing sintering temperature and BZN sintered at 1400°C for 6 h has the maximum Q-value. Since the 1:2 ordering decreased with the sintering temperature, the enhancement of Q-value is not related to the 1:2 ordering. The increase of the average grain size could be responsible for the increase of the Q-value, and the relative density also influenced the Q-value. BZN ceramics were sintered for various times at 1400°C and 1300°C and the variation of Q-value was explained in terms of the grain size and the relative density.