Katsuhiro Nomura

Electrical conductivity anomaly around fluorite–pyrochlore phase boundary

Abstract The relationship between electrical conductivity and crystal structure was investigated for Ln2Zr2O7 (Ln=La, Nd, Sm, Eu, Gd, Y, or Yb) and (Ln1−xLnx′)2Zr2O7 (Ln=Gd, Sm, or Nd; Ln′=Y, Yb, or Gd) systems. The crystal structure of both systems changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ln3+)/r(Zr4+) or r(Lnav.3+)/r(Zr4+), were larger than 1.26, where r(Lnav.3+) is estimated from the ionic radius of the component ions and the composition using the following equation: r(Lnav.3+)=(1−x)r(Ln3+)+xr(Ln′3+). The lattice parameter increased linearly with increasing ionic radius ratios. The electrical conductivity at 800 °C in air for Ln2Zr2O7 systems showed the sharp maximum at the vicinity of the phase boundary between fluorite- and pyrochlore-type phases. The electrical conductivity of (Ln1−xLnx′)2Zr2O7 system also showed the maximum at the phase boundary for some combinations of Ln3+ and Ln′3+. The pyrochlore-type Eu2Zr2O7, which is located at the nearest position to the phase boundary, showed the highest conductivity of 8.3×10−3 S cm−1 at 800 °C. On the other hand, the activation energy for the conduction remarkably decreased with the increasing ionic radius ratios in the fluorite-type phase range and showed the minimum at the given compositions, at which the maximum electrical conductivities were observed and then increased.

Density, Surface Tension, and Electrical Conductivity of Ternary Molten Carbonate System Li2CO3 – Na2CO3 – K2CO3 and Methods for Their Estimation

The density, surface tension, and electrical conductivity of the ternary molten salt mixtures of Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 system have been measured. The molar volumes of the ternary mixtures agreed well with those estimated by additivity arguments within 0.7%. It was found that the surface tension of a ternary carbonate mixture can be predicted from known surface tension of binary carbonates, and the mixing of two molten carbonate mixtures of the same surface tension makes a mixture of almost equal surface tension before mixing in this ternary system. We have succeeded in predicting the equivalent conductivity of the ternary carbonate mixtures using the cationic radii and the equivalent conductivities of the pure carbonates.

Improvement of Mechanical Strength of 8 mol % Yttria-Stabilized Zirconia Ceramics by Spark-Plasma Sintering

Dense 8 mol % yttria-stabilized zirconia (8YSZ) ceramies consisting of submicrometer-sized grains were prepared using the spark-plasma sintering (SPS) method without any additives. The starting powder (ca. 0.1 μm) was densitied to ca. 99% of the theoretical X-ray density with shorter sintering time (5-10 min) while preserving a grain size in submicrometer. The measured three-point bending strength was ca. 400 MPa at room temperature for samples sintered at 1300°C for 5-10 min, which was higher than the value of conventionally sintered ceramics, ca. 250 MPa, due mainly to the small-grained microstructures of the SPS ceramics. The electrical conductivity was 0.19 S cm -1 at 1000°C in air, which is comparable to previously reported values (ca. 0.1 S cm -1 ) for well-sintered ceramics. Thus, the SPS method is an effective process to improve the mechanical strength of 8YSZ without degrading the electrical conductivity.

Proton conduction in (La0.9Sr0.1)MIIIO3−δ (MIII=Sc, In, and Lu) perovskites

Abstract The electrical conductivity of A 3+ B 3+ O 3 -type perovskites, (La 0.9 Sr 0.1 )M III O 3− δ (M III =Sc, In, and Lu) (LSM), was investigated under controlled atmospheres between 573 and 1273 K. At P (O 2 )>10 −5 atm, all the compounds showed the predominant proton conduction below 873 K under wet conditions; at P (O 2 ) −10 atm, the Sc and In compounds below 873 K and the Lu-one at all temperatures under hydrogen-containing conditions. The Sc compound showed the highest proton conductivity of ca. 4×10 −3 S· cm −1 at 873 K in the presence of water vapor or hydrogen. The proton conductivity of LSM decreased with increasing the size of B-site (M III ) cation.

Electrical Conductivity of Molten Li2CO3 – X2CO3 (X: Na, K, Rb, and Cs) and Na2CO3 – Z2CO3 (Z: K, Rb, and Cs)

The electrical conductivities of the molten binary salt mixtures Li 2 CO 3 -X 2 CO 3 (X: Na, K, Rb, and Cs) and Na 2 CO 3 -Z 2 CO 3 (Z: K, Rb, and Cs) were measured using an ac two-probe technique and electrochemical impedance spectroscopy. The electrical conductivities of the eutectic compositions of (Li 0.52 Na 0.48 ) 2 CO 3 and (Li 0.62 K 0.38 ) 2 CO 3 at 923 K were 2.06 and 1.31 S cm -1 , respectively. The conductivity of molten Li 2 CO 3 -X 2 CO 3 (X: Na, K, Rb, and Cs) and Na 2 CO 3 -Z 2 CO 3 (Z: K, Rb, and Cs) decreases with an increase in the ionic radius of X or Z. We have succeeded in correlating the equivalent conductivity of a binary carbonate mixture A, with the composition (x A and x B ), the equivalent conductivities of the pure components A and B (A A and Λ B ), and two parameters (λ A and λ B ) as Λ = x 2 A Λ A + x 2 B Λ B + x A x B (λ A x A + λ B x B ). The pair of interaction parameters (λ A and λ B ) can be estimated for a binary system of molten carbonates using the relationship between the cationic radii and the equivalent conductivities of the pure carbonates.

Carbon dioxide sensor using solid electrolytes with zirconium phosphate framework (2). Properties of the CO2 gas sensor using Mg1.15Zr4P5.7Si0.3O24 as electrolyte

Abstract We report EMF type, all solid-state CO 2 gas sensors using Mg 2+ ion conductor in Mg 1.15 Zr 4 P 5.7 Si 0.3 O 24 [MgZrPSiO], having a cell construction of Pt¦CO 2 , O 2 ¦metal carbonate (Na 2 CO 3 or K 2 CO 3 )∥solid electrolyte ¦O 2 , Pt. The sensors showed Nernstian response to CO 2 in pressure range between 0.5 and 60 kPa, at temperatures from 300 to 500 °C. A change in solid electrolyte from MgZr 4 (PO 4 ) 6 to MgZrPSiO brought about a considerable reduction in response time.

Density, molar volume, and surface tension of molten Li2CO3-Na2CO3 and Li2CO3-K2CO3 containing alkaline earth (Ca, Sr, and Ba) carbonates

The density, molar volume, and surface tension of binary, ternary, and quaternary molten carbonate mixtures of Li 2 CO 3 -Na 2 CO 3 and Li 2 CO 3 -K 2 CO 3 containing alkaline earth (Ca, Sr, and Ba) carbonates were measured using a maximum bubble pressure technique. The addition of alkaline earth carbonates resulted in increases in the density and surface tension of the molten carbonate mixtures. The apparent molar volume of a component carbonate in mixtures can be derived from the density data, assuming additivity, and inversely be used to estimate the molar volume and density of mixtures within an error of 1 %. The surface tensions of alkali-alkaline earth carbonates were proportional to the mole fraction of the added alkaline earth carbonate.

Aging Effect of 8 mol % YSZ Ceramics with Different Microstructures

Aging effect of dense 8 mol % yttria-stabilized zirconia (8YSZ) ceramics with different microstructures has been studied. Spark-plasma sintered (SPS) ceramics consisting of submicrometer grains showed relatively slow conductivity degradation with aging time compared with conventionally sintered ceramics. Conductivity degradation was attributed to the increase in the bulk resistivity, which would be due to subtle structural changes in the 8YSZ unit cells. SPS ceramics consisted of relatively small crystallites, which might be responsible for the relatively slow conductivity degradation.

High temperature crystallographic study of (La0.9Sr0.1)MIIIO3−δ (MIII=Sc, In, and Lu) perovskite proton conductor

Abstract Structure refinement was carried out for the powder X-ray diffraction data of A 3+ B 3+ O 3 -type perovskites, (La 0.9 Sr 0.1 )M III O 3− δ (M III =Sc, In, and Lu) (LSMs) measured in wet air at 873 K. All the diffraction peaks of LSMs could be indexed as an orthorhombic GdFeO 3 -type structure (space group Pnma , No. 62). For the Sc-compound, the oxygen–oxygen (O–O) bond lengths of intra-ScO 6 octahedra were shorter than those of inter-ScO 6 ones, suggesting the predominant three-dimensional (3D) proton conduction along the edges of ScO 6 -octahedra. With increasing the size of B-site (M III ) cation from Sc 3+ to Lu 3+ , the O–O bond lengths and tilting angles of BO 6 (M III O 6 ) octahedra increased. For the Lu-compound, the O–O bond lengths of intra-LuO 6 octahedra were almost the same as those of the inter-LuO 6 ones, suggesting the predominant one-dimensional (1D) proton conduction along O1–O1 and O2–O2 sites on the vertices of BO 6 octahedra. This change in proton conduction pathway from 3D to 1D would result in the lowering of proton mobility, hence in lower proton conductivity.

The relationship between crystal structure and electrical conductivity in the LaY1−xInxO3 (x=0.0–0.7) system

Abstract The electrical conductivity of the LaY1−xInxO3 (x=0.0–0.7) system has been studied from the viewpoint of crystal chemistry. The high temperature form of LaYO3 (x=0.0) was ascertained to be the Sm2O3-type (B-type rare earth) structure, not perovskite-type one. The X-ray diffraction (XRD) experiments revealed that the samples with x=0.05 and 0.10 were the mixed phase of Sm2O3-type and perovskite-type structure, and changed to perovskite phase in the range of x≧0.20. From oxygen partial pressure dependence of the electrical conductivity, it was found that both the Sm2O3-type and the perovskite-type single phases showed hole conduction, but the mixed phase did oxide-ion one. The electrical conductivity of the LaY1−xInxO3 (x=0.0–0.7) system increased with increasing x, and showed the maximum value in the range of x=0.05–0.10, and then decreased with increasing x. The occurrence of oxide-ion conduction was discussed from the viewpoint of lattice distortion in the mixed phase.