Hideko Hayashi

Thermal expansion of Gd-doped ceria and reduced ceria

Abstract The thermal expansion coefficients of CeO 2 and Gd-doped ceria were measured in the range from 100 to 873 K and those of reduced ceria were measured from 100 to 323 K. The thermal expansion coefficient of Ce 1− x Gd x O 2− x /2 became large with increasing the dopant content, x . The thermal expansion coefficient of reduced ceria became large with increasing the vacancy content of oxygen. These increases of the thermal expansion coefficients result from weakening binding energy due to the increase of oxygen vacancy. The thermal expansion coefficients of CeO 2 and Gd-doped ceria were calculated theoretically and they were in good agreement with experimental ones except for the higher temperature range.

Molecular dynamics simulation of gadolinia-doped ceria

Abstract Oxygen diffusion in the oxygen ion conductor of gadolinia-doped ceria (CeO 2 ) 1− x (Gd 2 O 3 ) x /2 was investigated by means of the molecular dynamics simulation. The atomic arrangement with Gd–Gd pairs in gadolinia-doped ceria was found to be energetically more stable and more realistic than that with isolated Gd atoms from the calculation of the enthalpy of formation and the lattice parameter. The compositional dependence of the diffusion constant was in good agreement with the experimental one obtained from the data of electrical conductivity. The Gd–O nearest neighbor distance was longer than that of Ce–O and they were decreased with the increase of the gadolinia content in an excellent agreement with an EXAFS study. The oxygen coordination number of Gd was about seven except for a very small gadolinia content, suggesting that Gd ions are apt to form a Gd 3+ –vacancy–Gd 3+ cluster. The reason why the diffusion constant at high gadolinia contents decreases was discussed in terms of short range and long range interactions. Comparison of diffusion behavior with yttria stabilized zirconia was also discussed.

Molecular dynamics calculations on ceria-based solid electrolytes with different radius dopants

Abstract Oxygen diffusion and the microscopic structure of ceria-based solid electrolytes Ce 1− x M x O 2−0.5 x (M=La, Gd and Y) for x values between 0.02 and 0.3 were investigated by means of the molecular dynamics simulation. The compositional dependence of lattice parameters for the three dopants was in good agreement with experimental values. The calculated oxygen diffusion coefficient of Gd-doped ceria was larger than that of La- or Y-doped ceria as the experimental data showed. The nearest neighbor distance of Ce–O decreased and that of O–O increased with increase of the dopant content in the all doped cerias. The nearest neighbor distance of M–O increased in case of La doping, while it decreased in case of Y doping and the Gd–O distance was almost the same as the Ce–O distance in the non-doped ceria in case of Gd doping. The oxygen coordination number around a dopant was about seven for all cases, suggesting that dopant ions form a M–vacancy–M cluster. Large lattice distortion was observed in La- and Y-doped ceria, where the ionic radius of the dopant ion is fairly different from ‘optimum radius’ of dopant in ceria. The reason why the oxygen diffusion coefficient is the largest in Gd-doped ceria is mainly explained in terms of the lattice distortion.

Thermal expansion of Sr- and Mg-doped LaGaO3

Abstract Thermal expansion coefficients for Sr- and Mg-doped LaGaO 3 were measured and the doping effect was discussed in terms of the content of oxygen vacancy. The thermal expansion coefficients of the oxides became large with dopant content. A large shrinkage caused by the phase transition from orthorhombic to rhombohedral for LaGaO 3 was observed at 421 K. The shrinkage became small and broad with Sr and Mg doping. The transition temperature became higher with the Mg content in the case of Mg doping. The theoretical calculation of the thermal expansion coefficient of these oxides was also conducted and the calculated thermal expansion coefficient of LaGaO 3 was in good agreement with the experimental one except for the temperature range of the phase transition. The larger thermal expansion coefficients due to Sr and Mg doping were ascribed to smaller binding energy in the oxides by the presence of oxygen vacancy using the theoretical calculation.

Thermal expansion of La1−xSrxCrO3−δ

Abstract Thermal expansion coefficients of La 1− x Sr x CrO 3 and reduced La 1− x Sr x CrO 3− δ were measured using a push-rod type dilatometer in the temperature range from 100 to 873 K. The thermal expansion coefficient increased and the temperature of structural transition from orthorhombic to rhombohedral decreased with the increase of Sr content. Magnetic transition temperature decreased with the decrease of Cr 3+ content in La 1− x Sr x CrO 3− δ . The doping effect of Sr in La 1− x Sr x CrO 3 on the thermal expansion was discussed using molecular dynamics simulation.

Effect of magnetic field on the phase transitions of EBBA by means of a high-resolution and super-sensitive DSC

Abstract Phase transitions of the N-p-ethoxybenzylidene-p′-butylaniline (EBBA) were measured by means of a high-resolution and super-sensitive DSC. The crystal to liquid crystal transition and liquid crystal to isotropic liquid transition was observed at 305.8 and 352.4 K at a heating rate of 1 mK s −1 , respectively. The crystal to liquid crystal transition was found to have fine structures resulting from different crystalline states of the sample which depended on the sample preparation and on the heating rate of the measurement. The temperature due to liquid crystal to isotropic liquid transition was raised by 34 mK by applying a magnetic field of 5 T . The magnetic effect was discussed based on the extension of the Clapeyron equation.

Structural phase transition of perovskite oxides LaMO3 and La0.9Sr0.1MO3 with different size of B-site ions

Abstract Thermal expansion coefficients of perovskite oxides LaMO 3 and La 0.9 Sr 0.1 MO 3 with different size of B-site ions were measured, and the doping effect to the phase transition from orthorhombic to rhombohedral structure was discussed in terms of the size of the B-site ions and the tolerance factor of the perovskites. The transition temperature from orthorhombic to rhombohedral structure of LaGaO 3 rose with Mg doping, but it did not change significantly in the case of Sr doping. The thermal expansion coefficient of LaScO 3 , and LaInO 3 and LaAlO 3 showed no thermal expansion anomaly from orthorhombic to rhombohedral structure. The transition temperature from orthorhombic to rhombohedral structure for LaMO 3 and La 0.9 Sr 0.1 MO 3 with different size of B-site ions linearly decreased with the tolerance factor, indicating that the perovskite with a small tolerance factor is distorted, resulting in the higher transition temperature. The perovskite with a small tolerance factor was shown to be distorted using the pair correlation function of oxygen–oxygen obtained by molecular dynamics calculation.

Magnetic effect on the phase transitions of n-C32H66 measured by high resolution and super-sensitive DSC

Abstract A high-resolution and super-sensitive DSC measurable under strong magnetic field has been constructed in the magnetic bore of a refrigerator-cooled superconducting magnet. The baseline stability of the calorimeter was about 20 nW . The magnetic effect to the phase transitions of n-C32H66 was measured using the calorimeter. The transition temperatures due to solid–solid transition and due to melting were shifted to higher temperatures by several tens of mK by applying the field of 5 T . The temperature shift due to the magnetic orientation was discussed based on the extension of the Clapeyron equation.