Low-temperature synthesis in vacuum of c-axis oriented ferroelectric YMnO3 thin films using alkoxy-derived precursors

Abstract

Hexagonal YMnO₃ thin films were prepared on Pt(111)/TiO_x/SiO₂/Si and Pt(111)/Al₂O₃(0001) substrates using alkoxy-derived precursor solutions. The films were prepared by spin coating the YMnO₃ precursor solutions and then, the films were calcined and crystallized via rapid thermal annealing in oxygen or vacuum ambient. The annealing conditions and substrates were critical for crystallization of ferroelectric YMnO₃ films. When annealed in vacuum, the films both on Pt(111)/TiO_x/SiO₂/Si and Pt(111)/Al₂O₃(0001) substrates crystallized to hexagonal YMnO₃ and the orientation depended on the substrates. The film on Pt(111)/Al₂O₃(0001) had c-axis orientation and the film on Pt(111)/TiO_x/SiO₂/Si had no preferred orientation. In addition, it was found that crystallization behavior, orientation and morphology of YMnO₃ films on Pt(111)/TiO_x/SiO₂/Si substrates depended on the annealing condition. The heat-treatment in vacuum at initial stage for crystallization affected the restraint of perovskite phase and formation of hexagonal phase. The following heat-treatment in oxygen promoted the c-axis orientation and grain growth. The optimum annealing procedure for crystallization of the c-axis oriented YMnO₃ films on Pt(111)/TiO_x/SiO₂/Si was addressed.     

Viewpoints to characterize precision evaluation methods in binary measurements

Precision evaluation in quantitative measurements is a thoroughly discussed topic and the established methods are in use. Many methods are proposed for qualitative data including binary data, but their effectiveness and statistical properties are not so clear.     

Numerical analysis of double-diffusive convection in a porous enclosure due to opposing heat and mass fluxes on the vertical walls – Why does peculiar oscillation occur?

Peculiar oscillating convection is observed when two-dimensional double-diffusive convection in porous medium is analysed numerically. The top and bottom walls of an enclosure are insulated, and constant and opposing heat and mass fluxes are prescribed on the vertical walls. The peculiar oscillations are of three types: (1) Chaotic oscillations wherein the main flow is due to temperature; however, the convection due to concentration is strong enough to generate this peculiar oscillation. (2) The ‘sudden steady state case’ caused by the shifts from thermally-driven to concentration-driven forces. (3) The ‘re-oscillation case’ caused by the convection pattern changes from centrosymmetric to non-centrosymmetric.     

Characteristics of the ground-state lasing operation in V-groovequantum-wire lasers

Lasing from the ground subband transition, which has long been attempted in one-dimensional (1-D) structures, has been achieved for the first time with vertically stacked, AlGaAs-GaAs multiple quantum-wire (QWR) lasers, fabricated by flow-rate modulation epitaxy on V-groove substrates. Direct experimental evidence is provided by the consistency of the photon energies of the lasing and photoluminescence peaks, in the temperature range 4.5 K-300 K. It is further ensured by numerical calculation of the electronic subband energy states with the corresponding QWR structure. The lasers with cavity lengths of 350 μm, show fundamental transverse mode, typical threshold current of 5 mA, an internal quantum efficiency of 18.5%, ultrahigh characteristic temperature T₀~322 K above room temperature, and remarkably low wavelength-tuning rates of current (<0.012 nm/mA) and temperature (<0.19 nm/°C). Ultrafast lasing behaviors at the ground (n=1) and the second (n=2) transition of the QWR are also investigated in terms of the gain-switching method, using a characteristic of the wavelength shift from the n=1 to the n=2 subband with shortening the cavity length     

N2O decomposition over (Mg6−xAx) MnO8 (A = Li, Al)

Two different murdochite-type mixed oxides, (Mg_6–x Li _x )MnO₈ (x = 0, 0.1, 0.2 and 0.3) and (Mg_6–x Al _x )MnO₈ (x = 0, 0.2, 0.4, 0.6) were examined for the catalytic decomposition of N₂O in order to make clear the effects of mixed valencies of pairing manganese ions and oxygen vacancies. The valence of manganese ions and the amount of surface oxygen vacancies have been examined with X-ray photoelectron spectroscopy (XPS). (Mg_6–x Li _x )MnO₈ had mixed valence manganese ions and oxygen vacancies on the surface after the substitution. The substituted (Mg_6–x Al _x )MnO₈ had a mixed valence state but oxygen vacancies decresed with x and excess oxygen over stoichiometry was observed at x = 0.4 and 0.6. The reaction rate of N₂O decomposition increased after substitution with lithium but hardly increased after the substitution with aluminum in (Mg_6–x A _x )MnO₈. We assumed that the presence of oxygen vacancies on the surface along with pairing altervalent manganese ions affected strongly to enhance the reactivity of N₂O decomposition.     

Microstructure and secular instability of the (Ti1 − x,Alx)N films prepared by ion-beam-assisted-deposition

(Ti_{1 – x} ,Al _x )N films were prepared by ion beam assisted deposition (IBAD). The films were synthesized by depositing titanium and aluminum metal individual vapor under simultaneous bombardment with nitrogen ions in the energy range of 0.2–20 keV with the (Ti_{1 – x} ,Al _x )/N transport ratio in the range of 0.5–2.0. The films were formed onto Si(111) wafers at room temperature. Structural characterization of the films was performed with x-ray diffraction and selected area electron diffraction. The crystalline structure of the (Ti_0.64,Al_0.36)N and (Ti_0.33,Al_0.67)N films were found to be a metastable single-phase B1-NaCl structure. The (Ti_0.29,Al_0.71)N films revealed a two-phase mixture consisting of NaCl and würtzite structural phases. The AlN solubility limit into TiN, which approximately equal with x value, calculated by using electron theory was about x = 0.65, which shows good agreement to the experimental results. Phase separation after half a year of aging at room temperature in air was observed on the (Ti_0.33,Al_0.67) films whose AlN content is close to the solubility limits.     

Fabrication of nano-sized oxide composite coatings and photo-electric conversion/electron storage characteristics

Titanium dioxide TiO₂ can be used as a photo-anode to give generated electrons to the metal substrate under illumination. The transition metal oxide such as iron oxide Fe₂O₃ can be used to store electrons generated by the photo-electric conversion function of TiO₂ under the illuminated situation while the electrons are discharged from the transition metal oxide to the metal substrate in the dark. In this paper, coatings of nano-sized composite of TiO₂ and Fe₂O₃ were fabricated by the Warm Spray process, in which the feedstock powder is accelerated by a supersonic gas jet with speed above 1.0 km s^- 1 and temperature between 800 and 2500 K, and then impacted onto the target substrate continuously to form coatings. The coatings of TiO₂ and Fe₂O₃ nano-composite fabricated by Warm Spray showed no thermal deterioration such as phase transformation and particle growth of the feedstock during the spray process. The coatings fabricated by the Warm Spray had larger photo-current and the electron charge/discharge capacity than that by a conventional HVOF process. In addition, these characteristics were improved by decreasing the primary particle size of TiO₂ and Fe₂O₃.     

Mechanical properties of micro-tubular solid oxide fuel cell anodes

A fundamental issue with micro-tubular solid oxide fuel cells (SOFCs) is improvement of the mechanical strength of the cell. Fabricated using extrusion and co-firing techniques, the approximately 1.7 mm diameter SOFC tubes examined in this work are composed of a 50:50 NiO and Gd_0.2Ce_0.8O_2−x Gd-doped ceria (GDC) cermet anode (support tube), GDC as an electrolyte and La_0.8Sr_0.2Co_0.6Fe_0.4O₃ (LSCF)–GDC as a cathode. The mechanical properties of SOFCs are analyzed through internal burst testing and micro- and nano-indentation testing; the burst test is an especially important parameter because of improved power efficiency at increased fuel pressures. Results from micro- and nano-indentation tests performed on electrolyte-coated Ni–GDC anode pellets indicate that the hardness of GDC is comparable or greater than that of YSZ. In order to develop a trend for the mechanical behavior of micro-tubes in relation to variations in fabrication techniques, several parameters were varied. The standard anodes, used as a baseline, have four key design parameters as follows: they are not reduced, contain 40 vol% pore former, are sintered at 1400 °C and have a wall thickness of approximately 315 μm. An independent variation on each of the four parameters is performed. The four variations are (1) to reduce the standard tube, (2) to increase the percent pore former to 50% then to 60%, (3) to decrease sintering temperature to 1350 °C, and (4) to decrease the wall thickness to approximately 230 μm. An average burst strength of 22.4 ± 1.5 MPa is observed for the standard tubes, 34.2 ± 16.5 MPa for the reduced tubes, 16.5 ± 4.2 MPa for 50 vol% pore former and 11.7 ± 7.5 for 60 vol% pore former, 29.3 ± 9.6 MPa for the decreased sintering temperature and 34.3 ± 6.9 MPa for the thinner-walled tubes.     

Thermodynamic,transport and radiation properties of high-temperature CF3I and transient conductance of residual arc sustained in axial CF3I flow

The present paper reports fundamental properties of CF₃I under high-temperature conditions to find out the usefulness of CF₃I as an arc-quenching gas. Firstly, particle compositions of CF₃I were theoretically calculated in a temperature range from 300 to 30 000 K. Secondly, thermodynamic, transport and radiation properties of the CF₃I were also obtained for pressures 0.1–1.0 MPa. Finally, the transient process of a conductance of a residual arc in CF₃I was derived to compare with those in other arc-quenching gases. In addition, an arc extinguishing capability of CF₃I in a thermal re-ignition region was derived to find out whether CF₃I shows a higher capability than CO₂, N₂ and air. © 2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.