Materials Science Forum Vols. 783-786

Abstract: The phase stabilities and magnetic properties in Ni-Mn-Ga alloys are systematically investigated by means of the first-principles calculations within the framework of density functional theory using the vienna ab initio simulation package. The calculated formation energies show that the tetragonal NM martensite is the most stable phase compared with the cubic austenite and the modulated monoclinic martensite for stoichiometric Ni₂MnGa. The atomic magnetic moment keeps constant in austenite and NM martensite, whereas those of Ni and Mn in the modulated martensite oscillate according to the atomic position. Furthermore, The formation energies of the various compositions have been systematically calculated.

Abstract: Thin films of gallium nitride (GaN) and related nitride materials were prepared, and their properties as transparent conducting electrodes were investigated. GaN thin films were directly grown on sapphire single crystal substrates by the molecular beam epitaxy. Heavy doping of germanium was employed to reduce resistivity of the films, with sufficient reduction found to be possible while maintaining their epitaxial growth state. Optical transmission spectra of the films in the short wavelength region were slightly deteriorated by the heavy doping; however, this was successfully improved by growing GaN films under metal-rich conditions to increase the electron mobility and suppress unwanted increase of the carrier densities. In addition, the optical transmission spectra in the short wavelength region was improved also by alloying GaN with aluminum nitride, though the resistivities of these films were relatively higher than those of the unmodified GaN films. The prepared nitride thin films exhibited sufficiently suitable properties as transparent conducting electrodes for use in applications such as full-spectrum nitride-based solar cells.

Abstract: In order to increase the sustainability of metals, a more detailed understanding of the corrosion phenomenon is of crucial importance. In current literature, corrosion is often considered as a purely chemical interaction with nearly exclusive dependence on compositional effects, whilst ignoring the microstructural features of the metal surface. In the present work, results are presented which illustrate both the role of grain orientation and grain boundaries in the corrosion process. To evaluate the grain orientation dependent electrochemical behavior, polycrystalline Cu, was brought into contact with a corrosive electrolyte. Subsequently, the attack was evaluated by measuring the surface with both Atomic Force Microscopy (AFM) and Electron Backscatter Diffraction (EBSD). It was demonstrated that the grain orientation itself did not significantly influence the corrosion kinetics, but, alternatively, that the orientation of the neighboring grains seemed to play a decisive role in the grain dissolution rate. To increase understanding on the role of grain boundaries, a method was developed based on the electrochemical (galvanic) displacement of gold, which is deposited from an aqueous solution on a pure copper substrate. This technique demonstrated its sensitivity to the grain boundary characteristics as far less gold was deposited on special boundaries, such as coincidence site lattice boundaries, as compared to the random high angle grain boundaries.

Abstract: This paper reviews the research progress of a new class of solid oxide metal-air redox batteries for advanced energy storage. This new type of battery is comprised of a reversible solid oxide fuel cell and pair of metal/metal-oxide redox couple. This all solid-state battery has a great potential to surpass the conventional metal-air batteries and redox flow batteries in performance and cost. The paper first discusses the working principle and key features of the new battery, followed by the theoretical analysis of various metal-air chemistries. Finally, two examples of solid oxide metal-air chemistries operated at 550°C are given to demonstrate the promising performance of this innovative battery.

Abstract: Rutile is a good candidate for total water oxidation and hydrogen production due mainly to its corrosion resistance in water. In its native form, the direct band gap of 3.02eV, with the absorption edge in the visible part of the solar spectrum at 416nm, but close to the UV region, results in poor photo-catalytic efficiency of around 3%, well below the commercial threshold of around 10%. One option to enhance the photo-catalytic efficiency is band gap engineering, through the reduction of the Fermi level, which may be achieve by either changing the oxygen activity, or the incorporation of aliovalent foreign ions. The incorporation of donor and acceptor-type of ions may lead to a shift in the Fermi level close to the conduction or valence bands. Single crystals of TiO₂-Rutile (SG: P4 2/mnm), with (001) orientation were implanted along <001> at 10keV and 2MeV, where the range of C in TiO₂ is 21nm and 1.6μm, respectively. The ion implantation was carried out at various equivalent C concentrations, and characterised in-situ by ERDA with 82MeV Iodine, and PIXE with 2.5MeV protons. Optic band gap was assessed by ground state band structure calculations based on CASTEP code, with geometry optimisation by general gradient approximation and PBE functional, and energy optimisation by local density approximation and CA-PZ functional.

Abstract: The combination of solid oxide fuel cell technology with Fe-air battery concept was proposed by using H₂/H₂O as a redox mediator and LaGaO₃ based oxide for electrolyte. Since large internal resistance and large degradation during charge and discharge cycles are observed on anode, improvement in discharge potential and cycle stability are strongly required by improving stability of anode. In this study, cermet anode consisting of Ni-Fe alloy combined with oxide ion conductor was investigated. It was found that by using cermet anode of Ni-Fe combined with Ce_0.6Mn_0.3Fe_0.1O₂ (CMF), the observed energy density of the cell is improved to be 1109 Wh/Kg-Fe at 10 mA/cm², 873 K, which is about 92% of the theoretical energy density assuming the formation of Fe₃O₄ (1290 Wh/Kg-Fe). Cycle stability was also much improved on the cell using Ni-Fe-CMF anode comparing with that of Ni-Fe metal because of suppressed aggregation of Ni by mixing with CMF. Electrochemical charge-discharge measurement at 773 K showed excellent cycle stability over 30 cycles with high energy density (Round trip efficiency is higher than 80 %). The excellent performance and stability with operating at lower temperature promise this Fe-air solid oxide battery as the next generation energy storage device for averaging electricity and electric vehicle.

Abstract: Al–based hydrides have been extensively investigated in order to their application for hydrogen storage. To explore new hydrides, the samples in Al–X–H systems (X = Sr, V, Hf) were synthesized at 873–1173 K for 2–8 h under 5 GPa with internal hydrogen source. Unidentified phases in XRD analysis were observed in samples with nominal composition of Al–50mol%SrH₂, AlH₃–60mol%VH₂ and Al–50mol%HfH₂. Judging from SEM–EDX analysis, the chemical composition of these phases were Al/Sr=1/1, Al/V=2/3 and Al/Hf=1/2. In Al–Sr system, hydrogen content was determined to be about 2.78mass% by fusion analysis. Corresponding chemical formula of the new hydride was estimated to be AlSrH₃ with a perovskite–type crystal structure. Hydrogen desorption of 0.55 mass%H in the Al-60V sample was observed above about 400K by TG–TDS. The new compound, Al₂V₃H_1.8~2.2 had a tetragonal structure. In Al–Hf system, the new compound had almost no hydrogen. Then, the chemical fomula of newly founf compound was estimated to be AlHf₂ with a tetragonal structure.

Abstract: Nafion-azole (benzimidazole, 1,2,4-triazole, 1,2,3-triazole) composite membranes were prepared by room temperature and autoclave solution processing for high temperature (above 100 °C) PEMFC. Among the various Nafion – azole composite membranes, Nafion – 1,2,3-triazole membrane showed excellent flexibility, thermal stability, and homogeneous structure. Nafion – 1,2,4-triazole composite membrane had high thermal and mechanical properties, and also showed high proton conductivity of 0.02 S/cm at the temperature of 160 °C under dry (N₂) condition.

Abstract: An anode-supported honeycomb solid oxide fuel cell can work with high power density and improve thermo-mechanical durability at high temperatures. We have thus fabricated the honeycomb cell with an electrolyte layer of 8YSZ on an anode honeycomb substrate of Ni/8YSZ. The cathode layer is LSM-YSZ composite. Current-voltage and current-power density characteristics of the cells having different anode and cathode flow channel configurations are measured under different hydrogen flow rates. We also evaluate the hydrogen mole fraction distributions in the honeycomb cell using finite element method, and discuss appropriate anode and cathode flow channel configurations. The present study is a starting point of developing an anode-supported honeycomb cell for cell stacks assembled with multiple and large scale honeycomb cells which can achieve high efficiency flow channel and current collecting configurations, and enhanced thermo-mechanical durability.

Abstract: The influence of residual stress, thermal stress and chemically induced expansion stress etc... on the fracture damage of solid oxide fuel cells (SOFCs) were investigated by using nondestructive testing method and numerical stress-strain analyses under operating conditions. In order to estimate stress-deformation behavior of cell/stack of SOFCs, mechanical properties of SOFC elements were evaluated under controlled high temperature and oxygen partial pressure conditions. In addition to deformation and mechanical damage behavior were observed by using acoustic emission method.