Effects of La Addition on Properties of Sr2Nb2O7 Thermoelectric Ceramics

Thermoelectric ceramics Sr2-x La (x) Nb2O7 prepared by a reaction-sintering process were investigated. Dense Sr2Nb2O7 (SNO) with 5.15 g/cm(3) (97.9% of the theoretical density) was obtained after 1400A degrees C/6 h sintering. Grain size decreased as La was substituted into SNO. Resistivity of SNO is 3.07 x 10(8) Omega cm at 200A degrees C and decreased to 4.77 x 10(4) Omega cm at 700A degrees C. The resistivity of Sr1.97La0.03Nb2O7 (SNL3) and Sr1.95La0.05Nb2O7 (SNL5) at 300-700A degrees C, 2.19 x 10(6)-2.29 x 10(8) Omega cm, are 1-2 orders higher than SNO. Activation energy E (a) for SNO was 0.7 eV. E (a) for SNL3 and SNL5 increased from 0.16 eV to 0.19 eV at 200-400A degrees C to 0.72-0.74 eV at 400-700A degrees C. The Seebeck coefficient for SNO was S = 26.93 mu VK-1 at 200A degrees C and S = 57.15 mu VK-1 at 500A degrees C. This implies that SNO is a p-type thermoelectric ceramic. S = -30.73 mu VK-1 at 200A degrees C and S = -60.75 mu VK-1 at 500A degrees C for Sr1.9La0.1Nb2O7 (SNL10). This implies Sr2-x La (x) Nb2O7 is an n-type thermoelectric ceramic. Power factors 7.17 x 10(-9), 1.93 x 10(-10), 1.11 x 10(-10) and 2.61 x 10(-10) mu Wcm(-1) K-2 were obtained at 500A degrees C for SNO, SNL3, SNL5 and SNL10, respectively.