Yasuro Ikuma

Intrinsic and Extrinsic Oxygen Diffusion and Surface Exchange Reaction in Cerium Oxide

Polycrystalline CeO 2 with a relative density in excess of 97% was prepared. The specimens contained a lower concentration of impurities than those examined previously. Oxygen diffusion experiments were performed for the temperature range from 800 to 1300°C, in an oxygen partial pressure of 6.6 X 10 3 Pa. The concentration profile of 18 O in the specimens following diffusion annealing was measured by secondary ion mass spectroscopy (SIMS). In the high-temperature region (intrinsic region, above 1000°C), the oxygen self-diffusion coefficient obtained using SIMS was observed to agree reasonably with that obtained by gas phase analysis in a previous study, but the activation energy was found to be slightly smaller. The present result, D = 3.16 X 10 -4 exp(-226 kJ mol -1 /RT) m 2 s -1 (T = I 100-1300°C), is thought to represent the intrinsic behavior of undoped CeO 2 . In contrast, in the low temperature region (extrinsic region, less than 1000°C), the activation energy was smaller than that in the high temperature region. Comparison with data reported in the literature for CeO 2 doped with Y and Gd, suggests that the low-temperature oxygen diffusion region is controlled by a trivalent impurity. The surface exchange coefficients obtained from gas phase analysis and SIMS agreed very well with each other and were represented by k = 1.93 X 10 -3 exp(-136 kJ mol -1 /RT) m s -1 (T = 800-1300°C). The data were also in good agreement with the surface exchange coefficient in ThO 2 , suggesting that the oxygen surface exchange reaction is insensitive to cation species.

Diffusion of oxygen in YBa2Cu3O7−y

The oxygen lattice diffusion in YBa2Cu3O7−y was studied with 18O as a tracer. For the composition of y=0.07, the oxygen lattice diffusion coefficient is represented by Dl,O =2.87×10−1 exp[−173(kJ/mol)/RT] cm2/s in the temperature range 250–400 °C. The oxygen lattice diffusion coefficient decreases with the increasing value of y.

Synthesis of metastable tetragonal ( t′) zirconia-calcia solid solution by pyrolysis of organic precursors and coprecipitation route

Three kinds of chemical processes, citrate gel process, acetate gel process, and coprecipitation route, have been applied to the synthesis of homogeneous metastable tetragonal ({ital t}{prime}) and cubic solid solutions of ZrO{sub 2}{endash}{ital X} mol{percent} CaO ({ital X}=4{minus}20). From Raman scattering study, the citrate gel process based on the gelatin of the aqueous solution of citric acid containing Zr and Ca ions was found to produce compositionally homogeneous samples in comparison with other two methods. The axial ratio {ital c}/{ital a} decreases with increasing concentration of CaO and becomes unity around 8{endash}10 mol{percent} CaO composition. {copyright} {ital 1996 Materials Research Society.}

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 .

Highly Active Rare-Earth-Metal La-Doped Photocatalysts: Fabrication, Characterization, and Their Photocatalytic Activity

Efficient La-doped TiO2 photocatalysts were prepared by sol-gel method and extensively characterized by various sophisticated techniques. The photocatalytic activity of La-doped TiO2 was evaluated for the degradation of monocrotophos (MCPs) in aqueous solution. It showed higher rate of degradation than pure TiO2 for the light of wavelength of 254 nm and 365 nm. The rate constant of TiO2 increases with increasing La loading and exhibits maximum rate for 1% La loading. The photocatalytic activities of La-doped TiO2 are compared with La-doped ZnO; the reaction rate of the former is ~1.8 and 1.1 orders higher than the latter for the lights of wavelength 254 nm and 365 nm, respectively. The relative photonic efficiency of La-doped TiO2 is relatively higher than La-doped ZnO and commercial photocatalysts. Overall, La-doped TiO2 is the most active photocatalyst and shows high relative photonic efficiencies and high photocatalytic activity for the degradation of MCP. The enhanced photocatalytic activity of La-doped TiO2 is mainly due to the electron trapping by lanthanum metal ions, small particle size, large surface area, and high surface roughness of the photocatalysts.

Oxygen Self‐diffusion in Cylindrical Single‐Crystal Mullite

A mixture of SiO 2 and Al 2 O 3 was prepared from metal alkoxides and fired at 1650°C to form mullite and an SiO 2 -rich glass phase. After leaching out the glass phase, crystalline mullite powder was obtained. Transmission electron microscopy examination revealed the powder to be cylindrical particles of single-crystal mullite. A diffusion experiment was performed on the mullite to determine the oxygen diffusion coefficient. This could be expressed as D ox = 1.32 × 10 -6 exp[-397 kJ/RT] m 2 s -1 in the range of 1100 to 1300°C. This falls along the line extended through the oxygen diffusion coefficient of forsterite, which is similar in crystal structure to mullite.

An Overview of Semi-Conductor Photocatalysis: Modification of TiO2 Nanomaterials

Since the beginning of life on earth, the environment has been polluted by waste, both natural as well as synthetic (man-made). In the case of natural waste however, the environment by itself controls the effect of contamination. But in the case of synthetic materials, the pollution is higher even in trace concentration and this may go on accumulating, leading to disastrous effects on the environment and ecology. Industrial wastes containing heavy metals and pesticide residues fall in this category. The major point sources of pesticide pollution are wastewater from agricultural industries and pesticide manufacturing and formulating plants. Hence the waste from these sources must be removed or destroyed before discharge to the environment. There have been several methods practiced for the treatment of wastewater. However many of them are not able to achieve hundred percent satisfaction. Environmental pollution and destruction on a global scale as well as lack of sufficient clean and natural energy sources have drawn much attention and concern to the vital need for ecologically clean chemical technology, materials and processes. This is one of the most urgent challenges facing chemical scientists. Recently, light-induced photocatalytic reactions have gained a lot of attention for the purification of wastewater.

Oxygen self-diffusion in cerium oxide doped with Nd

Polycrystalline Ce 0.77 Nd 0.23 O 1.885 having a relative density in excess of 98% was prepared. Oxygen diffusion experiments were performed for the temperature range from 750 to 1100 °C, in an oxygen partial pressure of 6.6 kPa. The concentration profile of 18 O in the specimens following diffusion annealing was measured by secondary ion mass spectroscopy (SIMS). The oxygen self-diffusion coefficient obtained using secondary ion mass spectrometry was expressed by D = 6.31 × 10 −9 exp(−53 kJ mol −1 / RT ) m 2 s −1 and was in the extrinsic region. The oxygen diffusion coefficient of Ce 0.77 Nd 0.23 O 1.885 was larger than that of Ce 0.8 Y 0.2 O 1.90 ; it was close to those of Ce 0.6 Y 0.4 O 1.80 and Ce 0.69 Gd 0.31 O 2−δ . The oxygen diffusion coefficient obtained by the tracer method at 700 °C agreed with that calculated from the electrical conductivity in Ce 0.77 Nd 0.23 O 1.885 . The activation energy of the surface exchange coefficient was 94 kJ mol −1 , and the values of the surface exchange coefficient were similar to those of stoichiometric CeO 2 and ThO 2 .

Direct fabrication of crystalline vanadates films by hydrothermal-electrochemical method

Abstract Lithium vanadate films have been prepared on vanadium metal substrates using lithium hydroxide aqueous solutions in the autoclave by the hydrothermal-electrochemical method. The X-ray diffraction pattern of the fabricated film showed a single phase of stoichiometric crystalline orthorhombic βII-Li3VO4 without any other impurity phases. As an application of this method, we have demonstrated the direct fabrication of europium doped YVO4 films on vanadium substrate for luminescent materials using hydrothermal-electrochemical method. Those films showed sufficient intensity of luminescent property at room temperature. Therefore, this processing route may serve as an inexpensive and environmentally friendly way of the direct fabrication of the vanadate films with controlled crystal morphology and size simply by changing synthesis parameters.

Effect of microwave radiation on the formation of Ce2O(CO3)2·H2O in aqueous solution

Abstract Reaction between urea and cerium nitrate with/without microwave radiation was investigated. Decomposition of urea occurred in aqueous solution to form NH 4 OH when the solution was heated to approximately 80 °C. The decomposition products of urea in aqueous solution reacted with cerium nitrate to form cerium oxycarbonate hydrate powder (Ce 2 O(CO 3 ) 2 ·H 2 O). When the aqueous solution was heated by hot plate to decompose urea, elongated powder of Ce 2 O(CO 3 ) 2 ·H 2 O with an aspect ratio of approximately 3 was obtained. For the reaction in which the microwave radiation was used to heat the solution for only 5 min, the resulting Ce 2 O(CO 3 ) 2 ·H 2 O powder was almost identical to the powder that was formed by hot plate heating. However, when the microwave radiation was used for 15 min, the precipitated powder was spherical in shape. These results indicate that the microwave radiation had an effect on the reaction in the aqueous solution.