Formation process and microwave dielectric properties of the R2V2O7 (R=Ba, Sr, and Ca) ceramics were investigated. Transient RV2O6 phases, which were formed at 300°∼500°C by the reaction of RCO3 and V2O5, interacted with remnant RCO3 to form the homogeneous triclinic R2V2O7 phases at 600°–700°C. The Sr2V2O7 and Ca2V2O7 ceramics sintered at 1000° and 950°C, respectively, for 10 h showed a dense microstructure with the microwave dielectric properties of ɛr=10.4–12.1, Q×f=15 200–19 520 GHz, and τf=−30.9–34.8 ppm/°C. However, the Ba2V2O7 ceramic was well sintered at 900°C for 10 h and it showed the good microwave dielectric properties of ɛr=10.1, Q×f=51 630 GHz, and τf=−26.5 ppm/°C.

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MgSiO 3 ceramics were synthesized and their microwave dielectric properties were investigated. The Mg 2 SiO 4 phase was formed at temperatures lower than 12001C, while the orthorhombic MgSiO 3 phase started to form by the reaction of SiO 2 and Mg 2 SiO 4 in the specimen fired at 12001C. The structure of the MgSiO 3 ceramics was transformed from orthorhombic to monoclinic when the sintering temperature exceeded 14001C. A dense microstructure was developed for the specimens sintered at above 13501C. The excellent microwave dielectric properties of e r 5 6.7, Q Â f 5 121 200 GHz, and s f 5 À17 ppm/1C were obtained from the MgSiO 3 ceramics sintered at 13801C for 13 h.X. M. Chen-contributing editor

KNbO 3 (KN) nanowires having a tetragonal structure or a polymorphic phase boundary (PPB) structure, which contains both tetragonal (P4mm) and orthorhombic (Amm2) structures, are formed at low temperatures. The presence of tetragonal and PPB KN nanowires is attributed to the existence of OH À and H 2 O defects. Further, the tetragonal and PPB KN nanowires change to orthorhombic KN nanowires in the temperature range between 300 and 400 C owing to desorption of the lattice hydroxyl group. A composite consisting of polydimethylsiloxane (PDMS) and KN nanowires having a PPB structure shows large dielectric constant and low dielectric loss values of 9.2 and 0.5%, respectively, at 100 kHz.Moreover, a nanogenerator (NG) synthesized using the PPB KN nanowires exhibits the largest output voltage and current among NGs synthesized using the tetragonal or orthorhombic KN nanowires. In particular, the NG containing 0.7 g of PPB KN nanowires shows an output voltage of 10.5 V and an output current of 1.3 mA; these values are among the highest ever reported for NGs synthesized using a lead-free composite. In addition, this NG exhibited the maximum output power and energy conversion efficiency, which were 4.5 mW and 0.9%, respectively, for an external load of 1.0 MU.

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