Niobium doping effect on BaTiO3 structure and dielectric properties

The effect of Nb on the microstructure and dielectric properties of BaTiO3 ceramics was investigated. The Nb/Mn-BaTiO3 ceramics were prepared using a conventional solid-state method. The concentration of Nb varied from 0.1 to 5.0 at% but Mn was fixed at 0.05 at%. The SEM indicated that compositions of 0.1 and 0.5 at% Nb displayed a fairly uniform microstructure and homogeneous distribution of additives, with a grain size of less than 2 mu m. In highly doped samples a wide range of microstructural features was observed, from homogeneous and completely fine-grained microstructure to the appearance of secondary abnormal grains with core-shell structure. The dielectric constant and the loss tangent of modified ceramics were measured as a function of temperature (20-180 degrees C) and frequency (100 Hz-1MHz) for different concentrations of additives. The obtained results have shown that the dielectric constant at, both, room temperature (epsilon(rmax)) and Curie temperature (epsilon(r)), decreased as the concentration of Nb5+ increased. Thus, the highest values for the dielectric constant at room temperature epsilon(r )= 6648) as well as at Curie temperature (epsilon(rmax)= 7680) were measured for the 0.1Nb/Mn-BaTiO3 samples sintered at 1350 degrees C. The highly doped BaTiO3 ceramics were found to exhibit lower values of the dielectric constant and low dielectric losses at room temperature. In these samples, stable dielectric permittivity with a flat dielectric behavior over a wide temperature range is observed. The Curie constant for all series of samples decreases with an increase of dopant concentration and the highest values were measured from samples doped with 0.1 at% Nb. The effect of additives on the Curie constant change is more pronounced at higher sintering temperatures. The analysis of the critical nonlinearity exponent (gamma = 1.07-1.27), for lower dopant concentrations, shows a sharp phase transformation. For samples with increased Nb content, the degree of nonlinearity gamma is higher indicating a diffuse phase transformation.