Haruo Arashi

Determination of tetragonal-cubic phase boundary of Zr1−XRXO2−X2 (R = Nd, Sm, Y, Er and Yb) BY Raman scattering

Abstract Existing phases of arc-melted ZrO2-X mol%RO1.5 specimens were investigated mainly by Raman spectroscopy, which is sensitive to the phase transition induced by the oxygen displacements (0≤X≤24) R = Nd, Sm, Y, Er and Yb). The tetragonal and monoclinic phases coexisted at X = 2 and 4 for R = Nd, Y, Er and Yb, and at X = 2 for R = Sm. Tetragonal single phase is obtained in the specimens with higher contents. The axial ratio c a decreases with an increase of RO1.5 content and becomes unity at 14 or 16mol% RO1.5 content. The Raman peak intensity at about 470 cm−1, which is characteristic of the tetragonal phase, decreases with an increase of RO1.5 content. The cubic-tetragonal phase boundary determined by Raman scattering is located at about X = 18–19mol% regardless of dopant species R. A tetragonal phase whose axial ratio c a is unity appears at about X= 16mol%. The compositional locations of the phase boundaries are almost independent of the dopant species R.

Oxygen isotope exchange with a dense La0.6Sr0.4CoO3−δ electrode on a Ce0.9Ca0.1O1.9 electrolyte

Abstract Oxygen isotope exchange experiments were carried out with a dense La0.6Sr0.4CoO3−δ film (0.5 μm thick) deposited on a Ce0.9Ca0.1O1.9 substrate by a laser ablation method. The isotope exchange profile was measured from the surface into the electrolyte by a secondary ion mass spectrometer (SIMS). The oxygen diffusion through the La0.6Sr0.4CoO3−δ film was fast enough not to make any observable gradient in oxygen isotope concentration inside the film. The surface isotope exchange rate, k*, was calculated from the diffusion profile into the electrolyte layer. The electrochemical impedance, σE, was compared with k*. The oxygen partial pressure dependence of those two parameters were quite similar. The absolute value of k* was larger than expected from σE by a factor of 2 or higher.

Determination of cubic‐tetragonal phase boundary in Zr1−XYXO2−X/2 solid solutions by Raman spectroscopy

Raman spectroscopy is found to be a powerful tool to determine the structural phase boundary between cubic (Fm3m) and tetragonal (P42/nmc) phase in arc‐melted Zr1−XYXO2−X/2 (0.08≤X≤0.24) solid solution. The Raman peak intensity characteristic of the tetragonal phase at about 470 cm−1 decreases with the YO1.5 content X and becomes 0 around X=0.18.

A new hydrous silicate, a water reservoir, in the upper part of the lower mantle

We synthesized a new hydrous silicate in the pressure range from 20 GPa to 24 GPa at 800–1300°C. This phase, named tentatively as phase G, has a hexagonal unit cell, a=4.790 (3) A and c=4.344 (3) A, and V=86.3 (2) A³ and the atomic ratio Mg/Si=0.66±0.03. SIMS analysis revealed that it contains 14.5±2.0wt% water. Phase G has a chemical formula of Mg1.14Si1.73H2.81O6 and a density of 3.37g/cm³. Phase G coexists with periclase and Mg-perovskite under the lower mantle conditions, and thus it can be a reservoir of water in cold slabs penetrating into the lower mantle.

Development of a solar receiver for a high-efficiency thermionic/thermoelectric conversion system

Solar energy is one of the most promising energy resources on Earth and in space, because it is clean and inexhaustible. Therefore, we have been developing a solar-powered high-efficiency thermionic/thermoelectric conversion system which combines a thermionic converter (TIC) with a thermoelectric converter (TEC) to use thermal energy efficiently and to achieve high efficiency conversion. The TIC emitter must uniformly heat up to 1800 K. The TIC emitter can be heated using thermal radiation from a solar receiver maintained at a high temperature by concentrated solar irradiation. A cylindrical cavity-type solar receiver constructed from graphite was designed and heated in a vacuum by using the solar concentrator at Tohoku University. The maximum temperature of the solar receiver enclosed by a molybdenum cup reached 1965 K, which was sufficiently high to heat a TIC emitter using thermal radiation from the receiver.

Hydrogen production from high-temperature steam electrolysis using solar energy

Abstract Fundamental studies of hydrogen production by high-temperature steam electrolysis were made using solar energy as a heat source. A sintered ZrO2 + 8 mol% Y2O3 ceramic is used as an electrolyte. Both sides of this electrolyte were coated with platinum porous electrodes on the one side acting as a cathode and on the other as an anode. The electrolytic cell was inserted into a porous zirconia ceramic tube placed at the focal point of a solar concentrator and heated up to 1000°C by concentrated solar radiation. Water vapor was introduced into a cathodic compartment using argon as a carrier gas. On supplying a direct current to this cell, the gas taken out from the cathodic compartment was analysed by gas chromatography and production of hydrogen was confirmed. The current efficiency in this high-temperature electrolysis was 98%. The electrochemical efficiency reached 71%. Combining our high-temperature electrolyser cell with a solar AMTEC or a solar thermoelectric conversion, hydrogen can be produced only using a solar energy as a primary energy. The overall energy efficiency was estimated as 20–28% for this hydrogen production system. This efficiency is twice that of photovoltaic electrolysis. From this fundamental research, it is concluded that high-temperature steam electrolysis using solar energy as a primary energy source is a promising technique to produce hydrogen cleanly.

Hydrogen production from direct water splitting at high temperatures using a ZrO2-TiO2-Y2O3 membrane

Abstract The ZrO 2 -TiO 2 -Y 2 O 3 system exhibits high ionic and electronic conductivity at high temperatures under low oxygen partial pressures. Using this system as a membrane for gas separation, hydrogen can be produced from direct water splitting at high temperatures. Vaporized water was dissociated at high temperatures and oxygen permeated through the membrane by the oxygen partial pressure difference. With increasing oxygen partial pressure difference, water dissociation was promoted and the amount of the produced hydrogen was increased.

Formation of metastable forms by quenching of the HfO2RO1.5 melts (R = Gd, Y and Yb)

Abstract The existing phases in arc-melted HfO 2 RO 1.5 samples (R = Gd, Y and Yb) were investigated by Raman spectroscopy and X-ray diffraction. The compositional regions of the monoclinic phase, metastable γ 2 - and γ 1 -forms, were 0–6, 4–9 and 8–10 mol.% YO 1.5 respectively, in the HfO 2 YO 1.5 system. A metastable tetragonal ( t ′) form is obtained at or below 12 mol.% RO 1.5 compositions. The lattice parameters a and c of the t ′-HfO 2  RO 1.5 solid solution increases and decreases with increasing RO 1.5 content, and coincided with each other at 14mol.% RO 1.5 regardless of dopant species R. The lattice parameter of tetragonal fHfO 2 - x mol.% RO 1.5 with a fixed dopant concentration x increases with an increase of the ionic radius of dopant R 3+ ion. The Raman band intensity at 450–500 cm −1 , which is characteristic of the tetragonal phase, decreases with an increase of RO 1.5 content and seems to disappear at an RO 1.5 content of approximately 18–20 mol.%. It can be interpreted that the t ″-form, which is a tetragonal phase with an axial ratio of unity, appears in the compositional region of 14–18 mol.% RO 1.5 .

Oxygen permeability in ZrO2TiO2Y2O3 system

Abstract The oxygen permeability of yttria-stabilized zirconia containing 7.5 and 10 mol% TiO2 was measured under low oxygen partial pressure and at high temperatures up to 1500°C. The measured oxygen permeability increases with decreasing oxygen partial pressure according to P−1/6O2. This proportionality indicates that the n-type electronic conduction in the yttria-stabilized zirconia containing TiO2 is due to the electron hopping between Ti4+ and Ti3+ ions. The activation energy of 1.9 eV is obtained for the electronic conduction. At 1000°C, the contribution of the electronic conduction is negligibly small even under low oxygen partial pressure. From these results, ZrO2TiO2Y2O3 system is not so promising materials for electrode in SOFC. As this system is stable and has a high oxygen permeability at high temperatures up to 2000°C, it is the promising material as a membrane for a direct water splitting at high temperatures to produce hydrogen.

Raman spectroscopic study of the pressure-induced phase transition in TiO2

Abstract Pressure dependence of Raman spectra in TiO 2 was measured up to 40 GPa using diamondanvil cell at room temperature. Two phase transitions were observed with increasing pressure. Rutile phase begins to transform into α-PbO 2 phase at about 10 GPa and the α-PbO 2 phase coexists with rutile phase up to 15 GPa. The α-PbO 2 phase disappears above 15 GPa and single phase of different structure is observed at 20 GPa. This new high-pressure phase is stable up to 40 GPa. Comparing the Raman spectra of the new high-pressure phase of TiO 2 measured at 20 GPa with those of ZrO 2 and HfO 2 , the crystal structure of the high-pressure phase has been determined as the baddeleyite structure. The line widening of Raman band observed for the high-pressure phase is explained by the disorder in oxygen ion site predicted from the proposed mechanism for the pressure-induced phase transition in TiO 2 .