Key Engineering Materials Vol. 485

Abstract: The absence of cracks and a high optical transparency are critical factors for obtaining high performance when TiO₂ thin films are used as photocatalysts and as the cathode material in dye-sensitized solar cells. Synthesized TiO₂ nanoparticles were deposited by constant-current electrophoresis in ethanol. TiO₂ nanoparticle thin films deposited at a low current density had no apparent cracks and a high optical transparency. Small TiO₂ nanoparticles deposited are thought to be transported at low current densities. This enables TiO₂ nanoparticle chains to form by the oriented attachment mechanism and thereby increases the electron diffusion length.

Abstract: NaCo₂O₄ and 0.5at%-Sb doped Mg₂Si have excellent thermoelectric properties. We tried to fabricate a thermoelectric module composed of these materials and using Ni plates as electrodes. The fine powder of NaCo₂O₄ was prepared by metal-citric acid complex decomposition. 0.5at%-Sb doped Mg₂Si bulk was ground to powder and sieved to a powder particle size of 75 micrometers or less. These powders were sintered using spark plasma sintering (SPS) to obtain a body of NaCo₂O₄ and 0.5at%-Sb doped Mg₂Si. These thermoelectric materials were connected to the Ni plates by using the SPS method. The whole process took a very short time (less than 2 min) and could be done at low temperature (below 873 K). The open-circuit voltage values were 82.7 mV, and the maxima, maximum output current and maximum output power, for the single module were 212.4 mA and 6.65 mW at ΔT = 470 K.

Abstract: In this study, a new electric power generation measurement system was developed for piezoelectric energy harvesting using unimorph-type piezoceramics. Relationships between electric power and material constants such as d₃₃, d₃₁, g₃₁, k₃₁, e33T/e0, s₁₁^E, Q_e and Q_m were investigated using various lead zirconium titanate (Pb(Zr,Ti)O₃, PZT) ceramics with different material constants. Using the equipment, pulse-type stress was applied to unimorph-type piezoceramics. Then, optimum measurement conditions were determined. Under these conditions, the electric power for piezoelectric energy harvesting was measured as a function of the material constants. Finally, it was clarified that for piezoelectric energy harvesting using a unimorph-type device, the figure of merit was combination of the 3 kinds of material constants such as large d₃₁, small e33T/e0, and large s₁₁^E.

Abstract: The ferroelectric domain structure of PbTiO₃ (PTO) films was investigated by using transmission electron microscopy (TEM). In the film with PTO/SrTiO₃ (STO) structure, 180º domains are formed near the SrTiO₃ (STO) substrate and the domain length of 180º domains is 100 nm. However, 180º domains are not formed in the film with Pt/PTO/SrRuO₃ (SRO)/STO structure. These results show that 180º domains are formed in order to minimize depolarizing field energy, and that the domain length of 180º domains is determined by the competition among the depolarizing field energy, domain wall energy, Coulomb interaction and elastic interaction.

Abstract: Freestanding BaTiO₃ ceramics films were fabricated using the aerosol deposition (AD) method and the size effect of nanograined BaTiO₃ ceramics was demonstrated. Dense BaTiO₃ thick film fabricated by the AD method was crystallized and detached from substrate by an annealing treatment at 600 °C, and then the grain size was controlled by a reannealing treatment at various temperatures. As a result, freestanding BaTiO₃ thick films with various grain sizes from 24 to 170 nm were successfully obtained. Polarization–electric field (P–E) measurement revealed that BaTiO₃ ceramics with grain sizes of more than 58 nm showed ferroelectricity, whereas BaTiO₃ ceramics with an average grain size of 24 nm showed paraelectricity at room temperature. Dielectric measurement indicated that the permittivity decreased with decreasing grain size in the range of 170 to 24 nm.

Abstract: BaTiO₃ films were fabricated on Si substrate using inkjet printing technique. To achieve low-temperature fabrication of BaTiO₃ films, Ba-Ti-alkoxide ink was prepared. The mixing ratio and sequence of raw materials were optimized for controlling the chemical stability and viscosity of alkoxide ink. The inkjet printing films on Si substrate were annealed at 700 °C, and it resulted in the formation of BaTiO₃. It was found that the film quality was improved by adding BaTiO₃ nanoparticles into the Ba-Ti-alkoxide ink.

Abstract: Thin films of MBi₄Ti₄O₁₅ (M = Ca and Sr) with preferential crystal orientation were fabricated using underneath perovskite layers on various substrates. One-axis BLSD films on (100)LaNiO₃/(111)Pt/Ti/(100)Si and epitaxial BLSD films on (100)SrRuO₃//(100)SrTiO₃ were fabricated by chemical solution deposition (CSD). Dielectric constants of the CaBi₄Ti₄O₁₅ and SrBi₄Ti₄O₁₅ films on (100)LaNiO₃/(111)Pt/Ti/(100)Si were approximately 250, while those on (100)SrRuO₃//(100)SrTiO₃ were 220, respectively. The temperature coefficients of capacitance (TCCs) of these films were below 10% in atmospheric temperature range between R.T. and 300°C, which is significantly smaller than those of conventional (Ba,Sr)TiO₃-based capacitors.

Abstract: Crystal structure and electrical properties were investigated for Bi (Mg0.5Ti0.5)O₃ films grown on (111)c- and (100)c-oriented SrRuO₃//SrTiO₃ substrates by pulsed laser deposition. Epitaxial films consisting of a single phase of perovskite were obtained on the (111) substrates. Additionally, the ferroelectricity was ascertained not only from the polarization - electric field hysteresis loops, but also from the piezoelectric response.

Abstract: KNbO₃ thin films were deposited at 240 oC on various kinds of single crystal substrates by hydrothermal method using aqueous solutions containing Nb₂O₅ and KOH. The deposition amount of the films was found not to be much dependent on the orientation and the surface roughness of the substrates, but on the electrical conductivity of the substrate. Epitaxial {100}, {110} and {111}-oriented KNbO₃ films with ferroelectricity were grown on (100)_c, (110)_c, and (111)_c -oriented SrRuO₃//(100)SrTiO₃ substrates, respectively.

Abstract: An estimation method of the influence of interfaces on properties in perovskite-type oxide thin-film capacitors is presented. We proposed a modified Schottky model that can be employed to explain the electric properties of metal/perovskite-type oxide junctions. The modified model considers the electric field dependence of permittivity and the flow of electrons from metal to defect states located in the band gap of the perovskite oxide. The simulation based on this model could successfully describe the results of capacitance-voltage measurements of junctions between metals (Pt, Au, and Ag) and Nb-doped SrTiO₃, which were not explained by the conventional Schottky model. Additionally, a simulation for a thin-film capacitor with hysteretic behavior could depict an asymmetric capacitance-voltage curve.