Takayasu Ikegami

Co-precipitation synthesis and sintering of yttrium aluminum garnet (YAG) powders: the effect of precipitant

Abstract YAG precursors were co-precipitated from a mixed solution of aluminum and yttrium nitrates using ammonia water and ammonium hydrogen carbonate as precipitants, respectively. Phase evolution of the precursors during calcination and sinterability of the resultant YAG powders were compared between the two methods. The use of ammonia water produced a hydroxide precursor with an approximate composition of Al(OH) 3 ·0.3[Y 2 (OH) 5 (NO 3 )·3H 2 O] which transformed to pure YAG at about 1000°C via YAlO 3 phase. Severe agglomeration caused poor sinterability of the resultant YAG powders. The use of ammonium hydrogen carbonate produced a carbonate precursor with an approximate composition of NH 4 AlY 0.6 (CO 3 ) 1.9 (OH) 2 ·0.8H 2 O. The precursor directly converted to pure YAG at about 900°C. The precursor was loosely agglomerated and the resultant YAG powders showed good dispersity and excellent sinterability. For the same calcination temperature of 1100°C, YAG powders from the hydroxide precursor and the carbonate precursor densified to ∼81.2 and ∼99.8% of the theoretical, respectively, by vacuum sintering at 1500°C for 2 h.

Oxide ionic conductivity and microstructures of Sm- or La-doped CeO2-based systems

The concept of crystallographic index termed the effective index is suggested and applied to the design of ceria (CeO2)-based electrolytes to maximize oxide ionic conductivity. The suggested index considers the fluorite structure, and combines the expected oxygen vacancy level with the ionic radius mismatch between host and dopant cations. Using this approach, oxide ionic conductivity of Sm- or La-doped CeO2-based system has been optimized and tested under operating conditions of a solid oxide fuel cell. In the observation of microstructure in atomic scale, both Sm-doped CeO2 and La-doped CeO2 electrolytes had large micro-domains over 10 nm in the lattice. On the other hand, Sm or La and alkaline earth co-doped CeO2-based electrolytes with high effective index had small micro-domains around 1-3 nm in the microstructure. The large micro-domain would prevent oxide ion from passing through the lattice. Therefore, it is concluded that the improvement of ionic conductivity is reflected in changes of microstructure in atomic scale

A wet-chemical process yielding reactive magnesium aluminate spinel (MgAl2O4) powder

Abstract Ammonium carbonate was used as the precipitant to synthesize Mg–Al spinel precursors from a mixed solution of magnesium and aluminum nitrates. The precursor, composed of crystalline ammonium dawsonite hydrate [NH 4 Al(OH) 2 CO 3 ·H 2 O] and hydrotalcite [Mg 6 Al 2 (CO 3 )(OH) 16 ·4H 2 O] phases, transformed to pure spinel at ∼900°C via the decomposition of hydrotalcite at ∼400–800°C and a solid-state reaction between MgO (decomposed from hydrotalcite) and γ-Al 2 O 3 (derived from NH 4 Al(OH) 2 CO 3 ·H 2 O) at ∼800–900°C. Sinterability of the resultant spinel powders was evaluated by the constant-rate-of-heating sintering method. The effect of calcination temperature on particle morphology and sinterability of the resultant spinel powders was investigated. Spinel ceramics of ∼99% dense were produced by vacuum sintering at 1550°C for 2 h from the powder calcined at 1100°C for 2 h.

Synthesis of Mg-Al spinel powder via precipitation using ammonium bicarbonate as the precipitant

Abstract A precursor for Mg–Al spinel has been synthesized via the precipitation method, using ammonium bicarbonate as the precipitant. The precursor was composed of crystalline ammonium dawsonite hydrate [NH4Al(OH)2CO3·H2O] and hydrotalcite [Mg6Al2(CO3)(OH)16·4H2O] phases. The precursor converted to pure spinel phase at ∼900°C via two steps upon calcination: (i) decomposition of hydrotalcite at lower temperatures (400–800°C) and (ii) solid-state reaction between MgO (decomposed from hydrotalcite) and γ-Al2O3 (derived from NH4Al(OH)2CO3·H2O) at higher temperatures (>800°C). The effect of calcination temperature on particle morphology and sinterability of the resultant spinel powders were investigated.

Improvement of Grain‐Boundary Conductivity of 8 mol % Yttria‐Stabilized Zirconia by Precursor Scavenging of Siliceous Phase

A new method of scavenging highly resistive siliceous phase using two-stage sintering, named as precursor scavenging, is suggested for improving the grain-boundary conductivity of 8 mol % yttria-stabilized zirconia (YSZ). The scavenging efficiency and mechanism were studied and compared with those of 8YSZ-Al 2 O 3 composites prepared by various methods using impedance spectroscopy and imaging secondary-ion mass spectroscopy. A heat-treatment at 1200°C for longer than 20 h before sintering increased grain-boundary conductivity remarkably. The forming of inclusions containing Si was considered to be the origin of scavenging. The grain-interior resistivity was not changed by precursor scavenging, while it increased more than 15% by adding 1 mol % Al 2 O 3 when sintered at 1600°C. Precursor scavenging, therefore, is a potential and promising way for improving the grain boundary conductivity without deteriorating the grain-interior one.

Well-sinterable Y 3 Al 5 O 12 Powder from Carbonate Precursor

Carbonate precursors of Y 3 Al 5 O 12 (YAG) were synthesized from a mixed solution of alum and yttrium nitrate using ammonium hydrogen carbonate as precipitant. Precipitation method (normal-strike or reverse-strike) and aging were found to have dramatic effects on cation homogeneity of the precursor, which in turn influenced the formation temperature of the YAG phase. Reactive YAG powders were produced from the reverse-strike-derived, as-synthesized precursors at temperatures ≤1200 °C. These powders densified to >98.0% of the theoretical density up to 1500 %C at a constant heating rate of 8 %C/min or to transparency by vacuum sintering at 1700 %C for 1 h without additives.

Optical and scintillation characteristics of Y2O3 transparent ceramic

Various fundamental characteristics of Y2O3 ceramics, such as optical transmittance, reflectivity, absorption coefficient, refractive index, and dielectric constant, were investigated. Furthermore, in order to evaluate the possible application of this material as γ-ray scintillator, other radiation responses, like the α-ray excited emission spectrum, the γ-ray excited pulse height spectrum, the light yield, the nonproportionality, the energy resolution, and the γ-ray excited scintillation decay curves were obtained. The light yield of Y2O3 ceramics was evaluated in about 1.13 times higher than that of Bi4Ge3O12(BGO):Ce single crystals and was estimated in 9300 photon/MeV. The nonproportionality measurements showed a good proportionality. The scintillation decay curve could be fitted by an exponential function characterized by a fast component of about 34 ns.

Electrolytic properties and nanostructural features in the La2O3-CeO2 system

Doped ceria (CeO2) compounds are fluorite-type oxides which show oxide ionic conductivity higher than yttria-stabilized zirconia in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in applications of these materials for low or intermediate temperature operation of solid-oxide fuel cells (SOFCs). In this study, the effective index was suggested to maximize the ionic conductivity in La2O3-CeO2 based oxides. The index considers the fluorite structure, and combines the expected oxygen vacancy level with the ionic radius mismatch between host and dopant cations. Using this approach, the ionic conductivity of this system has been optimized and tested under operating conditions of SOFCs. LaxCe1-xO2-delta (x = 0.125, 0.15, 0.175, and 0.20), (LaxSr1-x)(0.175)Ce0.825O2-delta (x = 0.1, 0.2, and 0.4), and (La1-xSr0.2Bax)(0.175)Ce0.825O2-delta (x 5 0.03, 0.05, and 0.07) were prepared and characterized as the specimens with low, intermediate, and high index, respectively. The ionic conductivity was increased with increasing suggested index. The transmission electron microscopy analysis suggested that partial substitution of alkaline earth elements in place of La into Ce site contributes to a decrease of microdomain size and an improvement of conductivity. (La0.75Sr0.2Ba0.05)(0.175)Ce0.825O1.891 with high index and small microdomains exhibited the highest conductivity, wide ionic domain, and good performance in SOFCs

Application of a crystallographic index for improvement of the electrolytic properties of the CeO2-Sm2O3 system

Ceria-samaria (CeO2-Sm2O3) is one of the most interesting of fluorite oxides since its oxide ionic conductivity is higher than that of yttria-stabilized zirconia and other CeO2-based oxides. In this study, the effective index is proposed to maximize the oxide ionic conductivity in CeO2-Sm2O3 based oxides. Considering the crystallographic character of fluorite structure in CeO2-Sm2O3 system, the oxygen vacancy level was combined with the ionic radius mismatch between host and dopant rations as criteria for fast ionic conduction. SmxCe1-xO2-delta (0.2 < x < 0.3), (Sm0.5C0.5)(x)Ce1-xO2-delta (1.175 < x < 0.3), and (Sm0.936Cs0.06Li0.004)(x)Ce1-xO2-delta (0.2 < x < 0.275) were prepared and characterized as examples with low, intermediate, and high index, respectively. The oxide ionic conductivity increased with an increase of the effective index, confirming the validity of the index. Furthermore, the electrochemical properties of Sm0.25Ce0.75O1.88, (Sm0.5Ca0.5)(0.225)Ce0.775O1.84, and (Sm0.936Cs0.06Li0.004)(0.25)Ce0.75O1.86 were investigated in the temperature range from 700 to 1000 degrees C as representative examples. The oxide ionic conductivity in reducing atmosphere improved with increasing index. It was found that (Sm0.936Cs0.06Li0.004)(0.25)Ce0.75O1.86 With the highest index value in the CeO2-Sm2O3 system, exhibited high oxide ionic conductivity and good performance in planar solid-oxide fuel cells

Influence of nano-structure on electrolytic properties in CeO2 based system

Doped ceria (CeO2) compounds are fluorite type oxides that show oxygen ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmosphere. In order to improve the conductivity, the effective index was suggested to maximize the oxygen ionic conductivity in doped CeO2 based oxides. In addition, the true microstructure of doped CeO2 was observed at atomic scale for conclusion of conduction mechanism. Doped CeO2 had small domains (10-50 nm) with ordered structure in a grain. It is found that the electrolytic properties strongly depended on the nano-structural feature at atomic scale in doped CeO2 electrolyte.