Anomalous grain growth in sintered Bi2Ca2Co2−xCuxOy + Ag ceramic composites by Cu doping

Abstract

Bi2Ca2Co2 − xCuxOy + 4 wt%Ag (with x between 0 and 0.20) thermoelectric materials were prepared through a sol-gel method via nitrates. Powder XRD patterns showed the presence of thermoelectric phase as the major one, accompanied by metallic silver. SEM observations identified three different secondary phases in the samples, together with an exaggerated grain growth with Cu doping. The microstructural modifications are reflected in an important decrease of electrical resistivity, ∼ 3% when compared to undoped samples, due to the reduction on the number of grain boundaries, and the increase of Co4+ in the conducting layer. The minimum values at 650 ºC were obtained in 0.10 Cu-doped samples, 69 mΩ cm, which are lower than the reported in sintered samples. On the other hand, Seebeck coefficient was only slightly affected and reached the highest values at 650 ºC in 0.15 Cu-doped samples, 215 µV/K, which is practically the same measured in undoped ones, 211 µV/K, and lower than those reported for sintered samples. Consequently, the highest PF at 650 ºC has been determined in 0.10 Cu-doped samples, which is around 50% higher than the obtained in samples prepared through the classical solid-state method, or the undoped ones prepared in this work.