K. T. Jacob, India Institute of Science, Bangalore, India
Perovskite compositions ABO3-x with rare-earth and alkaline-earth cations in the A-site and a mixture of Fe, Co, Ni or Mn on the B-site have been considered as cathode materials for solid oxide fuel cells (SOFC). Others with similar A-site occupancy and a mixture of Cr and Ti in the B-site can be used as interconnect materials. LaGaO3 with lower valence Sr, Ca or Ba cations substituting for La and bivalent Mg and Co for Ga can be used as an electrolyte at lower temperatures than yttria-stabilized zirconia (YSZ). Appropriate substitutions in the A and B sites can tune thermal expansion and other properties of these perovskites to obtain thermomechanical compatibility of cell components. Thermal expansion measurements were conducted on the systems Nd1-xSrxFeO3-ƒÔ (0„Tx„T0.4), Nd0.7Sr0.3Fe1-xCoxO3 (0„Tx„T0.8), Y0.8Ca0.2Fe1-xMnxO3+ƒÔ (0„Tx„T1.0), RE0.7Sr0.3FeO3-ƒÔ (RE = rare-earth element), La0.7Ca0.2Sr0.1Cr0.5Ti0.5O3+x and La1-xSrxGa1-yMgyO3-0.5(x+y) (0.1„Tx„T0.2; 0.1„Ty„T0.2) in the temperature range from 298 to 1273 K using a horizontal, quartz dilatometer. The average linear thermal expansion coefficient (ƒÑav) increased with Sr content in the system Nd1-xSrxFeO3-ƒÔ, and with Co content in the system Nd0.7Sr0.3Fe1-xCoxO3. ƒÑav exhibited inverse correlation with the unit cell volume in both systems. In the Y0.8Ca0.2Fe1-xMnxO3+ƒÔ system, ƒÑav showed negligible decrease with x up to x = 0.4; thereafter, ƒÑav decreased rapidly with increasing x. Substitution of La by other rare-earth ions resulted in decreased thermal expansion. Increase of Mg concentration on the B-site of substituted lanthanum gallates was found to increase ƒÑav ; increase of Sr on the A-site had a smaller effect. The interconnect material La0.7Ca0.2Sr0.1Cr0.5Ti0.5O3+x exhibited ƒÑav= 10.4 between 298 and 1273 K, with little length change between oxidizing and reducing cycles. The results are discussed in relation to ionic radii and oxidation state of the cations, oxygen nonstoichiometry and crystal structure.
Summary: Perovskite compositions ABO3-x with rare-earth and alkaline-earth cations in the A-site and a mixture of Fe, Co, Ni or Mn on the B-site have been considered as cathode materials for solid oxide fuel cells (SOFC). Others with similar A-site occupancy and a mixture of Cr and Ti in the B-site can be used as interconnect materials. LaGaO3 with
lower valence Sr, Ca or Ba cations substituting for La and bivalent Mg and Co for Ga can be used as an electrolyte at lower temperatures than yttria-stabilized zirconia (YSZ). Appropriate substitutions in the A and B sites can tune thermal expansion and other properties of these perovskites to obtain thermomechanical compatibility of cell components. Thermal expansion measurements were conducted on the systems
Nd1-xSrxFeO3-ƒÔ (0„Tx„T0.4), Nd0.7Sr0.3Fe1-xCoxO3 (0„Tx„T0.8), Y0.8Ca0.2Fe1-xMnxO3+ƒÔ (0„Tx„T1.0), RE0.7Sr0.3FeO3-ƒÔ (RE = rare-earth element), La0.7Ca0.2Sr0.1Cr0.5Ti0.5O3+x and La1-xSrxGa1-yMgyO3-0.5(x+y) (0.1„Tx„T0.2; 0.1„Ty„T0.2) in the temperature range from 298 to 1273 K using a horizontal, quartz dilatometer. The average linear thermal expansion coefficient (ƒÑav) increased with Sr content in the system Nd1-xSrxFeO3-ƒÔ, and with Co content in the system Nd0.7Sr0.3Fe1-xCoxO3. ƒÑav exhibited inverse correlation with the unit cell volume in both systems. In the Y0.8Ca0.2Fe1-xMnxO3+ƒÔ system, ƒÑav showed negligible decrease with x up to x = 0.4; thereafter, ƒÑav decreased rapidly with increasing x. Substitution of La by other rare-earth ions resulted in decreased thermal expansion. Increase of Mg concentration on the B-site of substituted lanthanum gallates was found to increase ƒÑav ; increase of Sr on the A-site had a smaller effect. The interconnect material La0.7Ca0.2Sr0.1Cr0.5Ti0.5O3+x exhibited ƒÑav= 10.4 between 298 and 1273 K, with little length change between oxidizing and reducing cycles. The results are discussed in relation to ionic radii and oxidation state of the cations, oxygen nonstoichiometry and crystal structure.
