Microstructures and Thermoelectric Properties of Sintered Ca3Co4O9-Based Oxide

Zn- and Ti-doped thermoelectric misfit cobalt oxides Ca 3Co 4-x-y Zn x Ti y O 9 (x = 0 to 0.5, y = 0 to 0.5) were prepared by solid-phase reaction, followed by uniaxial compression molding and sintering at 1173 K for 20 h. Powder x-ray diffraction (XRD) data suggest that Zn and Ti dopants substitute in the rocksalt layer rather than in the CoO 2 layer for x ≤ 0.1 and y ≤ 0.1, respectively. In Zn and Ti single-doped samples for x ≤ 0.1 and y ≤ 0.1, ZT at room temperature increased with x and y through an increase in the absolute Seebeck coefficient despite a decrease in electrical conductivity. In Zn and Ti double-doped samples, minor phases other than the misfit oxides were observed at approximately x + y ≥ 0.1. At x ≤ 0.1 and y ≤ 0.1, double doping improved the thermoelectric properties. ZT at room temperature reached a maximum value of 0.035 at (x, y) = (0.1, 0.03)

Boron Depletion in a Nickel Base Superalloy Induced by High Temperature Oxidation

The conventionally cast Ni-base superalloy Rene 80 containing 90 wt ppm boron was oxidized in synthetic air at 850–1,050 °C for times up to 100 h. Extensive microstructural studies using GD-OES and SEM/WDX/EBSD revealed formation of a porous chromia scale containing TiO2 precipitates at its outer surface and semi-continuous layers of BCrO3 and CrTi2O5 at its base. The development of B-rich oxide in the scale is related to its high thermodynamic stability and the high alloy boron diffusivity. Its enrichment in the oxide scale resulted in B-depletion from the alloy matrix, which was extremely rapid at 1,050 °C, draining the boron content of a 2 mm thick alloy coupon in 100 h. After 100 h exposure at 950 °C, the B-content dropped below 40 wt ppm, but only minor depletion was found after 100 h at 850 °C. Predictions of B-depletion based on calculation of its diffusion from a thin, flat sheet were in good agreement with experiment at 1,050 °C. Precipitation of B-containing compounds (borides) within the alloy is shown to account qualitatively for slower B-outdiffusion at lower temperatures