Synthesis and electrical properties of dual doped CaMnO3 based ceramics

Different Nb doped Ca0.9Yb0.1Mn1-xNbxO3 ceramics are successfully synthesized by the conventional solid state reaction technique. The crystal structures are of orthorhombic phase, belonging to the Pnma space group. The lattice constant and the volume increase with the increase of Nb content. Relatively high density is around 97%. Scanning electron microscope (SEM) images show that samples are well crystallized. The electrical resistivity and the Seebeck coefficient are measured in a temperature range between 300 and 1100 K. At low temperatures, the electrical resistivity shows a semiconductive-like behavior. At high temperatures, the electrical resistivity exhibits a typical metallic conductive behavior. The semiconductor-metal transition temperature shifts toward a higher temperature with the increase of Nb content. The electrical resistivity increases with Nb dopant, except that the electrical resistivity for x = 0.03 is slight lower than that fox x = 0.00 sample at high temperature range. This conductivity behavior can be understood as the fact that though Nb doping can introduce more carriers, it also distorts the MnO6 octahedra, and causes the carrier localization. The values of Seebeck coefficient are all negative, indicative of an n-type electrical conduction. The absolute value of Seebeck coefficient increases with temperature increasing, but decreases with the increase of Nb content. The highest power factor is obtained to be 297 mu W/K(2)m at 497 K in the x = 0.00 sample, and the power factor of this sample is less independent of temperature in the whole measured temperature range.