Shinsuke Hayashi

Conductivity relaxation in lithium ion conductors with the perovskite-type structure

Abstract The ion transport properties of perovskite-type compounds, La 1/3− x Li 3 x TaO 3 , (La 1/4 Li 1/4 ) 1− x Sr x /2 TaO 3 and R 1/4 Li 1/4 TaO 3 (R=La, Nd, Sm, Y), were investigated by d.c. measurement and the scaling analysis of conductivity spectra. These compounds were confirmed to be lithium ion conductors and to behave as strong electrolytes. As the exponent of the universal power-law is reasonably related to the average valence of the A-site cation, immobile A-site cations seems to have an influence on the lithium ion transport process.

Lithium ion conduction in A-site deficient perovskites R1/4Li1/4TaO3 (R=La, Nd, Sm and Y)

Abstract New lithium ion conductors of perovskite-type compounds R 1/4 Li 1/4 TaO 3 (R=La, Nd, Sm and Y) were synthesized, and their crystal structure and lithium ion conductivity were characterized. Symmetry of the perovskite lattice changed from cubic for La 1/4 Li 1/4 TaO 3 to tetragonal for Nd 1/4 Li 1/4 TaO 3 and Sm 1/4 Li 1/4 TaO 3 , and to orthorhombic for Y 1/4 Li 1/4 TaO 3 . The alternate ordering of R and Li ions and vacancies at the A-sites was observed for Nd 1/4 Li 1/4 TaO 3 , Sm 1/4 Li 1/4 TaO 3 and Y 1/4 Li 1/4 TaO 3 . The cube root ( V 1/3 ) of the subcell volume decreased with decreasing the ionic radius of R cations which act as spacers for the perovskite lattice. With decrease in V 1/3 from La 1/4 Li 1/4 TaO 3 to Y 1/4 Li 1/4 TaO 3 , the bulk conductivity at 400 K decreased from 1.4×10 −1 S m −1 to 5.0×10 −7 S m −1 and its activation energy increased from 0.35 eV to 0.85 eV. The subcell volume and the ordering at the A-sites are major factors in influencing lithium ion conduction in these perovskite-type conductors.

Phase separation behavior and magnetic properties of Li(FexAl1–x)5O8solid solutions

Phase separation behaviour and magnetic properties of Li(FexAl1–x)5O8 solid solutions have been investigated. A single-phase Li(Fe0.5Al0.5)5O8 solid solution, which was quenched from 1250 °C, separated into Fe-rich and Al-rich solid solution phases with a continuous compositional variation when it was annealed at 1000 and 950 °C inside the chemical spinodal estimated using the theory of Cook and Hilliard. On the other hand, a Li(Fe0.68Al0.32)5O8 solid solution showed binodal-type phase separation after an incubation period of 200 h at 1000 °C inside the miscibility gap but outside the spinodal. The coercive force of the Li(Fe0.5Al0.5)5O8 solid solution spinodally decomposed at 1000 °C increased to 7.2 kA m–1, which was about 9 times as large as that before annealing. Annealing of the Li(Fe0.5Al0.5)5O8 solid solution at 950 °C raised the coercive force to 8.6 kA m–1. The increase in the coercive force is possibly due to the modulated structure which is characteristic of spinodally decomposed ceramics. The coercive force of the Li(Fe0.68Al0.32)5O8 solid solution remained constant even after it was annealed at 1000 °C for 200 h to decompose it binodally.

Influence of Starting Materials on the Chemical Reaction Process during the Vapor Growth of Magnesia

The chemical reaction process during the vapor growth of magnesia was studied. When magnesium and water vapor are used as starting materials, whiskers seem to grow in the following process; Mg(g) or Mg(OH)2(g) adsorbs itself on the lateral surface of a whisker, migrates to the tip, and there reacts preferentially. Bulk crystals. were predominantly observed through hydrolysis of MgCl2 vapor. These crystals seem to grow from magnesium hydroxychloride fluid phase. In this case whiskers having globules at their tips were also observed and thought to grow by VLS mechanism.

Morphology and Internal Structure of Magnesia Whiskers

Magnesia whiskers grown via vapor phase reaction were studied by means of X-ray topography, and optical and electron microscopies. The results obtained were as follows:1) No internal defects were detected by X-ray topography, but contrast changes were found at tips of the whiskers. There contrast changes may be due to the inclusion of impurities which take part in the growth.2) Triangular closed growth hills were found on {111} surface of a [112] whisker.3) The surfaces of whiskers having diameters smaller or larger than a few micron were bounded by smooth or heavily stepped faces, respectively. Mechanical properties may be controlled by the surface structure as well as the internal structure.