This study is concerned with the systematic investigation on crystal structures and material properties of perovskite nonoxides ABX3 to discover novel lead-free piezoelectric materials. Biocompatible elements were applied to A and B, meanwhile halogen elements (F and Cl) and chalcogen element (S) were introduced into X. Firstly the combinations of constituent elements were specified with consideration for geometric and electric equilibrium conditions. Then the stable cubic structure was evaluated through first-principles calculations based on the pseudo-potential, linear augmented plane-wave method within the local density approximation (LDA). Next the band structure was estimated to differentiate conductors and insulators and the phonon were analyzed to identify the soft mode, which induce a structural phase transition to non-centrosymmetric crystal structures, according to the eigenfrequency and eigenvector. Focusing on perovskite nonoxides with tetragonal structure, we compared their spontaneous polarization and piezoelectric stress constant and discussed the possibility for the advent of novel piezoelectric materials.
The chemical vapor deposition system using non-equilibrium atmospheric pressure plasma was developed for the low temperature growth of ZnO films. As zinc source material, Bis (2,4-octanedionato) zinc which is free from spontaneous combustibility was selected. By introducing vaporized Zn(OD)2 with carrier gas (nitrogen) and oxygen into discharging region, ZnO films was successfully obtained on the glass substrate at the substrate temperature as low as 200°C without using helium dilution of mixed gases. From XRD measurement, it was found that the films were well crystallized with (0001) preferred orientation. The films also showed high transmittance above 90% in visible region and sharp absorption edge at 3.31eV which corresponds to optical band-gap. The observation of cross-sectional morphology of samples revealed that the fabricated thin films typically have columnar grains with the diameter of approximately 20-30nm, and the grain size increases not only with increasing substrate temperature but also with increasing the vaporizing temperature of the Zn source materials.
ZnO thin-film transistors (ZnO-TFTs) on flexible substrates with SiO2/TiO2 buffer layers were fabricated at room temperature process. A SiO2 layer of 200nm as a bottom oxide buffer was grown on PEN plastic substrates by an electron beam deposition. A 50nm TiO2 buffer layer and a 40nm ZnO film were grown by a pulsed laser deposition (PLD) in continuously at room temperature. The TiO2 thin layer between the ZnO thin film and the SiO2 buffer layer provided a better adhesion, and demonstrated absence of in-plane disoriented grains without cracks. Top-gate type ZnO-TFTs were fabricated using the SiO2/TiO2 buffer layer, a transconductance, gm of 1.7mS/mm, a drain current on/off ratio of 2.4 × 106 and a threshold voltage VTH of -1.1V were obtained for a gate length LG of 2μm. When compared with a SiO2 buffer layer, a threshold voltage shift was about 1V in a positive direction, a higher voltage operation VDS of 5V and a gate leakage current IG of a few pA were obtained from a SiO2/TiO2 buffer layer ZnO-TFT. Furthermore, the ZnO-TFT was measured with bent to a curvature radius of 8.5mm. The I-V characteristics were therefore not changed drastically by bending and after returning. In order to improve the gm, the bottom-gate type ZnO-TFTs were also fabricated. The gm of 12.5mS/mm was obtained at gate length LG of 20μm. A transconductance, gm was improved by applying the bottom-gate type TFT, that is the gm became larger about 10times than that of the top-gate type TFT. From results of the bending experiments at curvature radius of 10mm, 20mm and 30mm. it was confirmed that the characteristics were not changed despite the bending and after returning for a short gate length devices.
TiO2 nanotubes (TNTs) have attracted much attention as a material for photocatalysts, dye-sentisized solar cells and chemical sensors due to the chemical stability and large surface-to-bulk ratio. TNTs fabricated by a hydrothermal method have larger surface-to-bulk ratio than TNTs fabricated by other processes. To investigate the electrical properties of the hydrothermal TNTs, TNT field effect transistors were fabricated by dielectrophoresis. The temperature dependence of the drain current-drain voltage characteristics indicates that the electric conduction of the TNT-channel is dominated by double Schottky barrier (DSB) existing between the TNTs. In the result of the gate voltage dependence of the drain current-drain voltage characteristics, it was found that the carrier of the TNTs is electron and the modulation of the drain current is caused by the change of the carrier density in the TNTs rather than the change of the height of DSB.
This paper describes molecular beam epitaxial growth and photoluminescence properties of single-crystalline Pb0.7Sn0.3Te nanodots embedded in a wide bandgap CdTe host matrix. These nanodots were self-organized by postgrowth annealing of a lattice-type mismatched Pb0.7Sn0.3Te(rocksalt)/CdTe(zincblende) single quantum well. A highly efficient midinfrared emission in the 3∼5μm atmospheric window was observed from the nanodots even at higher temperatures than 300K. By considering both effects of strain-induced band deformation and quantum confinement in nanodot potentials, the emission energy observed was found to agree with the theoretical transition energy in the type I quantum well.
A novel solution-based ultrasonic mist deposition method has been developed for the formation of organic solar cells. This technique enabled low-resistive gallium-doped zinc oxide (ZnO : Ga) and indium-tin-oxide (ITO) thin films capable of applying as anode layers in the device. Low resistive and flat PEDOS : PSS hole transport layer and P3HT : PCBM active layers were also deposited by this technique. The solar-cell devices with mist-deposited PEDOT : PSS or P3HT : PCBM layers showed higher short circuit current density and power conversion efficiency than those with spin-coated PEDOT : PSS or P3HT : PCBM layers. The results encourage the promising potential of the ultrasonic mist deposition method for low-cost device fabrication.
A structural design principle of flexible electric devices based on material mechanics is proposed, and it is applied to flexible organic field-effect transistors (OFETs). In order to prevent bending stress caused by deformation, we fabricated OFETs on a thickness-controlled polymer substrate. Polyimide precursor has been spin coated on a glass substrate and annealed. The channel layer of the OFET is pentacene deposited in a vacuum system. Correlations between curvature radius and electric characteristics have been evaluated. Consequently, the use of ultra thin film substrate is effective to control the strain on the deformed devices. Electrical property of the OFET, such as Id-Vd characteristics is constant against the curvature radius. These results are promising for future flexible electronics technologies.
In order to investigate the effects of aging condition and humidity on fatigue properties of 18%Ni maraging steel of grade 350, rotating bending fatigue tests were carried out in relative humidity of 25% and 85%. Aging conditions selected were under- and peak-aging ones at conventional aging temperature of 753K. In addition, double aging treatments, that is, under- peak- and over-aging treatments at 673K after the peak-aging at 753K were also examined. Both of static and fatigue strengths were increased by the double aging. Although the fatigue strength was markedly decreased by high humidity in all of the steels, the decrease in fatigue strength due to high humidity was suppressed by the double aging. In all of the steels, the change in growth behavior of a crack was yielded around the crack length of a few grain sizes of prior austenite regardless of humidity. The growth rate of a crack longer than the above stated length was hardly influenced by the aging condition and humidity, while both of an initiation and its early propagation of a fatigue crack were accelerated by anodic dissolution in high humidity. These results were discussed from a point of view of the difference in precipitates depending on the aging condition.
A constitutive material model is proposed to simulate cyclic shear behaviors of column base system for steel structures filled with Steel Fiber Reinforced Cementitious Composite (SFRCC). This cyclic model is developed based on a rotating crack formulation assuming a constant width of damaged region and adopts an orthogonal constitutive model combined with hysteresis curves in tension and compression sides. An analysis model composed of (1) horizontal shear springs, which represent SFRCC and to which the proposed cyclic model is applied, and (2) vertical beam elements representing a steel column, is prepared to simulate the shear behavior of columns base. The analysis well estimates the experimental results of column base specimens conducted in previous researches.