Naoya Ishida

Low-Voltage, Low-Phase-Noise Ring Voltage-Controlled Oscillator Using 1/ f-Noise Reduction Techniques

To improve the phase noise of a ring voltage-controlled oscillator (VCO), we describe two 1/ f-noise reduction techniques, which are a switched bias circuit (SBC) and a current source (CS) operating in the triode region. The ring VCO utilizing the 1/ f-noise reduction techniques was fabricated by 0.18-µm complementary metal–oxide–semiconductor (CMOS) technology, and it achieved 1 GHz oscillation, -68 dBc/Hz at a 100 kHz offset, and 710 µW power dissipation using a 1 V power supply. The proposed SBC and the CS operating in the triode region reduced the phase noise of the ring VCO by 3 and 7 dB, respectively. A phase-locked loop (PLL) implemented by the proposed ring VCO achieved 1 GHz oscillation, a -171 dBc/Hz figure of merit (FOM), and 1.2 mW power dissipation at a supply voltage of 1 V.

Correlation between structure and mixed ionic–electronic conduction mechanism for (La1−xSrx)CoO3−δ using synchrotron X-ray analysis and first principles calculations

The mechanism of mixed ionic–electronic conduction (MIEC) for (La1−xSrx)CoO3−δ (LSC) (x = 0.00–0.50) was investigated by experimental as well as theoretical studies which included Rietveld refinements, maximum entropy method (MEM) analysis, X-ray absorption spectroscopy, and first principles calculations (FPCs). The correlations among MIEC, structural parameters, i.e., the bond length, the bond angle, electron density, and the spin configuration, were discussed from the practical results. Moreover, these correlations were verified by FPC, and interpreted by the band structure and the total energy of LSC. The electronic conductivity was affected by the Co–O–Co bond angle, electron densities, and the spin state. In particular, the spin transition from the low to high spin state was accompanied by the cross-over from semiconductor to metallic at x = 0.15. Concerning the oxide ion diffusion, LSC has large anisotropic atomic displacement parameters of the oxide ion sites, in the vertical direction of the Co–O bond, which associate with the high oxide ion diffusion. The total energy of LSC during the migration of the oxide ion was also calculated with varying Sr content, lattice parameters, and symmetries by using FPC. Consequently, the symmetry of LSC is more effective to reduce the activation energy of oxide ion diffusion than Sr content and lattice parameters.

Change of local structures for 0.5Li2MnO3–0.5LiMn1/3Ni1/3Co1/3O2 in first charge process of different rates

Abstract An investigation was carried out into the charging rate dependence of local structural changes in a layered 0.5Li2MnO3–0.5LiMn1/3Ni1/3Co1/3O2 solid solution during the initial charging cycle. To clarify the mechanism involved in local atomic rearrangement, a pair distribution function (PDF) analysis was performed using the results of powder neutron diffraction and synchrotron X-ray total scattering measurements. First-principles calculations (VASP code) were used to determine the initial structure when performing the PDF analysis. The bond-length strain (λ) and the bond-angle strain (σ2) for the optimized model were calculated following the PDF analysis in order to clarify the effect of the charging rate on the crystal structure distortion. Before charging, the distortion was small for MnO6 octahedra compared to that for NiO6 and CoO6 octahedra. During charging at a rate of 1C, the MnO6 octahedra experienced increasing distortion, whereas at 3C the CoO6 octahedra became more distorted. In addition, when charging at 3C, the values of λ and σ2 increased for NiO6 octahedra that had entered the Li layer as a result of cation mixing. This appeared to be related to whether the localized atom was Mn or Co within the average structure during charge process. It is thought that distortion occurs in MO6 octahedra containing whichever element becomes localized, and this depends on the charging rate. This leads to the possibility that decreasing the fractional composition of the element that becomes localized may lead to reduced distortion and improved the cyclability.

Solution Plasma Process-Derived Defect-Induced Heterophase Anatase/Brookite TiO2 Nanocrystals for Enhanced Gaseous Photocatalytic Performance

We report a simple room-temperature synthesis route for increasing the reactivity of a TiO2 photocatalyst using a solution plasma process (SPP). Hydrogen radicals generated from the SPP chamber interact with the TiO2 photocatalyst feedstock, transforming its crystalline phase and introducing oxygen vacancy defects. In this work, we examined a pure anatase TiO2 as a model feedstock because of its photocatalytic attributes and well-characterized properties. After the SPP treatment, the pure anatase crystalline phase was transformed to an anatase/brookite heterocrystalline phase with oxygen vacancies. Furthermore, the SPP treatment promoted the absorption of both UV and visible light by TiO2. As a result, TiO2 treated by the SPP for 3 h showed a high gaseous photocatalytic performance (91.1%) for acetaldehyde degradation to CO2 compared with the activity of untreated TiO2 (51%). The SPP-treated TiO2 was also more active than nitrogen-doped TiO2 driven by visible light (66%). The overall photocatalytic performance was related to the SPP treatment time. The SPP technique could be used to enhance the activity of readily available feedstocks with a short processing time. These results demonstrate the potential of this method for modifying narrow-band gap metal oxides, metal sulfides, and polymer composite-based catalyst materials. The modifications of these materials are not limited to photocatalysts and could be used in a wide range of energy and environment-based applications.

Ferroelectric properties, average and local structures of (Bi,RE)4(Ti,Nb)3O12 (RE = La, Pr, Nd)

In this study, we focused on (Bi,RE)4(Ti,Nb)3O12 [rare earth (RE) = La, Pr, and Nd] as a ferroelectric material and investigated its average and local structures using synchrotron X-rays and pulsed neutrons. From the polarization–electric field hysteresis loop and dielectric measurements, it was found that the remanent polarization P r was increased significantly by partially substituting Nb for Ti. It was also demonstrated that P r was further improved by RE substitution although the Curie temperature decreased. The results of Rietveld refinement obtained by neutron diffraction patterns indicated smaller distortions of TiO6 octahedra in (Bi,RE)4(Ti,Nb)3O12 than in Bi4Ti3O12. Such a change in distortion corresponded to a low Curie temperature. The Nb partial substitution suppressed the formation of oxygen vacancies, which are considered to negatively affect ferroelectric properties. The local distortion surrounding Bi atoms was affected by the rare-earth substitution.