Kouta Iwasaki

Power factors of Ca3Co2O6 and Ca3Co2O6-based solid solutions

Abstract A polycrystalline sample of Ca3Co2O6 was prepared by solid state reaction of CaCO3 and Co3O4 at 1243 K in air. The electrical resistivity (ρ) of Ca3Co2O6 exhibited semiconducting behavior and ranged from 1.6×102 to 1.3×10−1 Ω cm with temperature from 400 to 1170 K. The Seebeck coefficient (S) showed positive value, and decreased from 660 to 140 μV/K with increasing the temperature from 350 to 1170 K. The power factor (S2/ρ) tended to increase with increasing temperature, and the maximum value was 1.6×10−5 W/(m K2) at 1150 K. Samples with the starting compositions of (Ca1−xAx)3Co2O6 (A=Y, La, and Bi, x=0.03) and Ca3(Co1−yMy)2O6 (M=Cr and Cu, y=0.03) were also heated at 1243 K in air to prepare Ca3Co2O6-based solid solutions. The changes of lattice parameters and widths of X-ray diffraction peaks for the obtained samples indicated substitutions of Y, La, Bi for Ca, Cr and Cu for Co, although the samples included trace amount of impurities. Substitutions of Bi and Cu increased the power factors of the solid solutions, and the maximum values were 3.4×10−5 W/(m K2) (A=Bi at 1130 K) and 2.3×10−5 W/(m K2) (M=Cu at 1150 K). The Seebeck coefficients of the solid solutions with Y and La were negative below 400 K.

Synthesis and electrical properties of Ba2Nb5−xZrxO9

Abstract The Ba 2 Nb 5− x Zr x O 9 solid solution was first synthesized by the solid state reaction, and Zr proved to be substituted selectively for Nb in the preferred site of Ba 2 Nb 5 O 9 . The structure of Ba 2 Nb 5− x Zr x O 9 consists of a NbO layer sandwiched by two Ba(Nb, Zr)O 3 perovskite blocks. The limit of Zr-substitution was estimated to be x =1.0, according to the change of c axis parameter against x . SEM observation showed that platy agglomerates preferred the orientation of the 00l plane, and the particle size was reduced by up to 1/10 on the addition of Zr. The solid solution showed a metallic conductivity with a temperature independent magnetic susceptibility. Substitution of Zr for Nb appeared to cause a change in electron density as a result of contraction of the Nb(2)–O(3) distance, leading to the increase of electrical resistivity.

Synthesis, crystal structure and electrical properties of Ba2Nb5−xVxO9 (x<1.9)

The Ba2Nb5−xVxO9 solid solution was synthesized by replacement of Nb by V in Ba2Nb5O9. The polycrystalline samples were prepared by solid state reaction at 1773 K under flowing Ar. Ba2Nb5O9 has three Nb sites; the Nb(1) and Nb(2) sites form the Nb6 octahedron, and the Nb(3) site is in the center of the NbO6 octahedron. The Rietveld analysis indicated that V atoms occupied both Nb(1) and Nb(2) sites. The a and c axis lengths decreased with increasing V content (a=4.171–4.106 A, c=12.224–12.136 A), the substitution range was estimated to be 0≤x<1.9. The electrical resistivity of Ba2Nb5−xVxO9 showed metallic behavior at 100–350 K, which increased with increasing V content. Substitution of V atoms changed the main conductive carrier; the Seebeck coefficient of Ba2Nb5−xVxO9 increased with increasing V content and it exhibited positive value at x=1.5 in 100–300 K while the Seebeck coefficient of Ba2Nb5O9 (x=0) showed a negative value.