Key Engineering Materials Vol. 445

Abstract: The role of grain boundaries (GBs) in the diffusion of oxygen vacancies (VO••s) in barium titanate (BaTiO3) and its mechanism were investigated using atomistic simulation techniques. It was found that GBs trapped VO••s at specific sites in the course of the diffusion, and the excess energy reflecting structural distortion of the GB was closely related to the availability of the trapping. GBs therefore act as a resistance of the diffusion of VO••s, suggesting that electrical degradation of multilayer ceramic capacitors (MLCCs), which is derived from vacancy diffusion, enables to be additionally improved by controlling GB structures in BaTiO3-based dielectrics.

Abstract: X-ray radiation using pyroelectric crystal is intermittent and the X-ray intensity is low and unstable compared with a conventional X-ray radiation method, such as X-ray tube. It is expected that the X-ray intensity becomes stable if electric field intensity and supply of electron are stable. In this study, to use X-ray radiation equipment as an electron source, tandem-type X-ray radiation equipment which is composed of two LiNbO3 single crystals polarized in a z-axis is proposed. When the temperature gradient for each crystal was the same, the X-ray intensity became approximately 6 times higher at a maximum. When the temperature gradient for each crystal was reversed, the period of X-ray radiation became approximately two times longer and the X-ray intensity became approximately 20 times higher at a maximum. Moreover, the stability of X-ray radiation for the repetition of temperature could be improved.

Abstract: The piezoelectric shear response of 94.0(Ka0.5, Na0.5)NbO3 (KNN) + 5.0LiNbO3 (LN) + 0.5SrTiO3 (ST) + 0.5BiFeO3 (BF) ceramics was investigated by Laser Doppler Vibrometry (LDV) and resonance-antiresonance method. From resonance-antiresonance method, the piezoelectric d15 constant was obtained to be 273 pC/N. The shear strain obtained by LDV　at the frequency of 150kHz showed strong nonlinearity. This suggested that the domain contribution to piezoelectric response in shear mode of KNN-LN-ST-BF ceramics existed at the operational frequency for the shear mode divices.

Abstract: Lead-free Na0.5K0.5NbO3 (NKN) piezoelectric ceramics was synthesized by an ordinal solid state reaction method. The ceramics was electrically poled under 3 kV/mm at 150 oC for 30 min. The discharge current-flow was measured for the poled ceramics at the heating rate of 1 oC/min from room temperature up to 500 oC. The two sharp peaks were observed at 209 and 417 oC. These temperatures were corresponded to orthorhombic-tetragonal (TO-T) and tetragonal-cubic (TC) phase transition temperatures, respectively. The sharp peaks were mainly originated from the decrease of the spontaneous polarization. Additionally, the several large broad patterns were also observed at 280, 363 and 375 oC, According to the electromechanical coupling factor kp, which is the indicative parameter of the polarization, mainly decreased at TO-T and TC. These broad peaks were not associated with the spontaneous polarization. The peaks seem to have been originated from the discharge current of the oxygen ion migration on elevating temperature inside the poled NKN ceramics.

Abstract: (Na1-xBax)(Nb1-xTix)O3 (NNBTx; x=0.0-0.21) solid-solution ceramics were synthesized and their crystal structure, dielectric properties and piezoelectric properties were investigated. The crystal structure at room temperature of NNBTx varied from orthorhombic to tetragonal with increasing BaTiO3 content x. The phase boundary between orthorhombic and tetragonal at room temperature was confirmed BT content between x=0.08 and 0.09. For x>0.05, it was found that the Curie temperature was decreased with increasing x. The highest electromechanical coupling factor, kp, and the largest piezoelectric constant, d33, were obtained at x=0.09-0.10.

Abstract: 0.995(Bi1/2Na1/2)TiO3-0.005Bi(B’,B”)O3 solid-solution ceramics (B’,B” denote Zn, Mg, Ni, Ti, Nb) were fabricated and their ferroelectric properties were investigated. Based on the structural characteristics of Bi(B’,B”)O3 analyzed by first-principles calculations, a materials design for obtaining a large piezoelectric strain is proposed: an index Z= Ps∙(ε33*∙s33)1/2, where Ps is spontaneous polarization, ε33* is relative permittivity and s33 is elastic compliance. A positive correlation was observed between Z and effective piezoelectric constant d33* observed for strain measurements.

Abstract: The layer structured niobate Cs4Nb6O17•3H2O has been prepared in a solid state reaction using Cs2CO3 and Nb2O5 at 1073 K. Ion exchange reactions of Cs+ in the interlayer space were studied in aqueous solutions. The single phases of lithium, sodium and hydrogen ion exchange products were obtained and they also contained interlayer water. The interlayer water of the lithium ion exchange product was removed by heating at 453 K in a vacuum. The resulting niobate Cs0.26Li3.01H0.73Nb6O17 was evaluated for its use as the cathode in a rechargeable lithium battery. The cathode exhibited discharge and charge capacities of 120 and 110 mAhg-1 for the first cycle in the voltage range of 1.5 – 4.2 V. The amount of Li+ intercalated and deintercalated were 4.0 and 3.6 of the formula unit, respectively.

Abstract: A TiO2 nanotube array film was synthesized by the anodic oxidation of a Ti plate in an NH4F electrolyte. The film was broken into nanotubes by carrying out ultrasonication. The nanotube powder was mixed with TiO2 nanoparticle powder to form a paste. The paste was used in the fabrication of ceramic porous electrodes of dye-sensitized solar cells.

Abstract: Nanostructured NiO thick films were fabricated for use in dye-sensitized solar cells (DSCs) as photocathodes. Pastes were prepared by using nanocrystalline NiO powders, polyethylene glycol (PEG) and water. The pastes were then printed on FTO glass substrates by a facile doctor-blade printing method. The NiO films were obtained by heating at 500 °C for 30 min in flowing oxygen. It was shown that the nanostructure of the resultant films was largely dependent on the amount of PEG in the pastes. Coumarin 343 (C343) dye was employed for sensitizing p-type NiO photocathodes. DSCs using our NiO/C343 photocathodes yielded a photocurrent density of 1.26 mA cm–2, an open-circuit photovoltage of 88 mV, a fill factor of 0.33, and a solar energy conversion efficiency of 0.037 %.

Abstract: In dye-sensitized solar cells (DSC), large-size particles in photoanodes can act as　light-scattering center to enhance light harvesting efficiency. In this study, macroporous secondary　TiO2 particles were prepared by a spray drying method and were introduced to DSC. A few kinds of　layered structures were fabricated as photoanodes to evaluate effects of macroporous particles. It was　found that open circuit voltage increased due to the presence of macroporous particles. A photoanode　composed of stacked nanoparticles and macroporous particles showed better cell performance.