Masahito Yoshino

Effects of dopants and hydrogen on the electrical conductivity of ZnO

The effects of dopants on the electrical conductivity of ZnO were investigated through the ac impedance spectroscopy. The doping of Al increased the electrical conductivity of ZnO greatly, whereas the doping of Li decreased it both in the grain and in the grain boundary. The doping of the 3d transition metals (Co, Mn, and Cu) made the grain boundary more resistive, but the doping effect on the electrical conductivity inside the grain was varied depending on the doping elements. The doping of Co had no significant effects on the electrical conductivity of the grain, and the doping of Mn made the grain a little more resistive. The doping of Cu made the grain much more resistive. In addition, hydrogen was introduced into ZnO by the ion implantation method. The electrical conductivity in the hydrogen-implanted ZnO layer increased by four orders of magnitude. The mechanisms for the doping effects were discussed in this investigation.

Enhanced 1.54μm photoluminescence from Er-containing ZnO through nitrogen doping

Nitrogen doping into an erbium (Er)-containing ZnO specimen through the N+ irradiation and the subsequent annealing in air was found to increase the photoluminescence (PL) intensity around 1.54μm by about 40 times. The existence of nitrogen in the specimen was proved firmly by means of the N14(d,α)C12 nuclear reaction analysis. Further, the Ne+ irradiation was conducted instead of the N+ irradiation, but no effect was observed in the PL spectra. So, it was concluded that the substitution of N for O could modify the local structure around Er3+, resulting in the surprisingly large enhancement of the PL intensity.

Roles of the hydride forming and non-forming elements in hydrogen storage alloys

Abstract The electronic structures of small octahedral model clusters containing hydrogen, 3d, 4d, 5d transition and non-transition elements are investigated by the DV-Xα molecular orbital method. It is found that hydrogen makes a strong chemical bond with the hydride non-forming elements, B, as long as the hydride forming elements, A, exist in the neighborhood. This is a reason why hydrogen is located preferentially near the hydride non-forming elements in many hydrides. Also it is suggested that the ratio of the A–B bond strength to the A–A bond strength lies in a certain range for conventional hydrogen storage A–B alloys.

Effects of hydrogen doping through ion implantation on the electrical conductivity of ZnO

Hydrogen ion implantation technique was applied to introduce hydrogen into ZnO. ZnO specimens with different electrical conductivity were prepared by the addition of some dopants. The elastic recoil detection analysis was used to measure the content of hydrogen in ZnO before and after the hydrogen ion implantation. The electrical conductivity of ZnO was increased greatly by the hydrogen ion implantation. The increase of the electrical conductivity varied with the dopants. The greatest increase was about 9 orders of magnitude in the highly resistive Cu-doped ZnO. The mechanism for such a hydrogen effect was discussed.

Electrical Conductivity of Cu-Doped ZnO and its Change with Hydrogen Implantation

The doping effects of Cu on the electrical conductivity of ZnO were studied in the simple binary system through the ac impedance spectroscopy. The Cu doping decreased the electrical conductivity of ZnO by several orders of magnitude. The Cu doping decreased the electrical conductivity of ZnO both in the grain and in the grain boundary. Hydrogen was introduced into the Cu-doped ZnO specimens by the ion implantation technique. The electrical conductivity of the hydrogen-implanted layer increased by about 9 orders of magnitude at most. The mechanism for such a hydrogen effect was also discussed.

Local electronic structures around hydrogen and acceptor ions in perovskite-type oxide, SrZrO3

Abstract Local electronic structures around hydrogen and acceptor ions in SrZrO 3 are simulated by the DV-Xα molecular orbital method. In pure SrZrO 3 , there is a band gap of about 6 eV between the O–2p valence band and the Zr–4d conduction band, in agreement with experiments. When Y or Sc is doped into SrZrO 3 , an acceptor level appears just above the valence band. When hydrogen is introduced into SrZrO 3 , a donor level appears below the conduction band. Also, an oxygen ion vacancy makes the defect level below the conduction band. It is shown that charge compensation takes place among these acceptor, donor and defect levels. In addition, local electronic structure around hydrogen is found to change largely with the acceptor dopants, Sc and Y. For example, the metal–oxygen bond strength changes in the order, Y–O 3 .

Local geometries and energetics of hydrogen in acceptor-doped SrZrO3

Abstract The local geometry and chemical bonding around proton and acceptor dopant in perovskite-type oxide SrZrO 3 have been investigated by the pseudopotential method and the DV-Xα molecular orbital method. Special attention is directed towards the MO 6 octahedron in the perovskite-type structure, where Ms are acceptor dopants (e.g., Y and Al) substituted for Zr ions in the oxide. It is shown that local electronic state of the constituent ions in the MO 6 octahedron is a good measure of the local geometrical change of the octahedron with doping. For example, there is a strong resemblance in the local ionic state and hence the local geometry of the octahedron between the undoped and Y-doped oxides. However, there is little resemblance between the undoped and Al-doped oxides. The calculated activation energy for protonic conduction is higher in the Al-doped oxide than that in the Y-doped oxide, in agreement with experimental results. From these results, it is found that such an acceptor dopant that does not change local electronic state or local geometry of the octahedron is a preferable element for enhancing protonic conductivity in perovskite-type oxides.

Location of Deuterium Atoms in BaZr0.5In0.5O2.75+α by Neutron Powder Diffraction

The neutron powder diffraction data were collected at 10 K on the proton-conducting perovskite oxide BaZr0.5In0.5O2.75 with and without dissolved D2O. Obtained diffraction data were analyzed by the Rietveld method and the maximum entropy method. It was found that deuterium atoms were located close to the 12h (0, y, 0.5) site of the cubic perovskite structure (space group ). Such deuterium position seems common with many proton-conducting perovskite oxides.

Electrical Conductivity and Seebeck Coefficient of Sr6Co5O14.3 Single Crystal

Rod-like single crystals of Sr6Co5O14.3 were grown by the flux method, and the electrical conductivity (σ), Seebeck coefficient (S) and power factor (σS2) were investigated in the temperature range of 300–900 K in air to explore the potential as thermoelectric materials. Sr6Co5O14.3 is an oxygen-nonstoichiometric phase of Sr6Co5O15, which is a member of the homologous (A3Co2O6)m(A3Co3O9)n [A=Ca, Sr, Ba] series including Ca3Co2O6 (m=1, n=0) and BaCoO3 (m=0, n=1) as the end members. The rod-like single crystals mainly grew along Co–O one-dimensional chains (c-axis) consisting of face-sharing CoO6 octahedra and CoO6 triangular prisms. The electrical conductivity along the Co–O chains showed semiconducting behavior (260–9600 S m-1 in the temperature range of 300–900 K). The Seebeck coefficient was positive, and decreased with increasing temperature (280–140 µV K-1). The power factor increased with increasing temperature (0.20×10-4–1.9×10-4 W m-1 K-2), which was about one order of magnitude larger than that of polycrystalline Sr6Co5O15.

Energetics of Native Defects in Al2O3 and SiO2

Formation energies of various defects in Al2O3 and SiO2 are calculated by using the plane-wave pseudopotential method. Also, the formation energies of Schottky defects and Frenkel defects are evaluated on the basis of these calculations. It is shown that formation energies of these defects are higher in SiO2 than in Al2O3. In other words, less defects are formed in SiO2 than in Al2O3. It is also found that the principal defect is the cation Frenkel defect in Al2O3 but the anion Frenkel defect in SiO2. These results agree with the experimental results that Al ions diffuse preferably in Al2O3 but oxygen ions diffuse in SiO2 at high temperatures.