Effects of Pd substitution on the thermoelectric and electronic properties of delafossite Cu_(1-x)Pd_xFeO_2 (x=0.01, 0.03 and 0.05)

摘要

Cu_((1-x))Pd_((X))FeO_2 (x=0.01, 0.03 and 005) delafossite was prepared by solid state reactions and was calcined/sintered at 1050 ℃. The effect of Pd~(2+) substitution for the Cu~(1+) sites on the thermoelectric and electronic properties of Cu_((1-x))Pd_((X))FeO_2 were investigated. The crystal structure, oxygen decomposition, thermoelectric and electronic properties were characterized by X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS), Seebeck coefficient, electrical conductivity and thermal conductivity measurements. The characterization showed that Cu_((1-x))Pd_((X))FeO_2 formed a hexagonal delafossite structure with R3-m symmetry. The existence of Pd~(2+), Cu~(1+), Cu~(2+), Fe~(3+), Fe~(4+) and O was revealed from the XPS results. Confirmation of Pd~(2+) substitution for the Cu~(1+) sites occurred by increasing the c-axis in the lattice parameter with a Pd content. The O content intercalated at the center of the triangular Cu acted as a support to produce Cu~(2+) ions and was reduced with an increasing Pd content. The mixed valencies of Cu~(1+)/Cu~(2+) and Cu~(1+） /Pd~(2+) in the Cu layer changed the electrical conductivity and the Fe~(3+)/Fe~(4+ mixed valencies in the FeO_6 layer caused the Seebeck coefficient to increase. Both the electrical conductivity and Seebeck coefficient for Pd contents of x=0.01 and 0.03 were higher than that of non-doped CuFeO_2. The low thermal conductivity of Cu_((1-x))Pd_((X))FeO_2 resulted from the substitution of Pd, which has a large atomic mass, into structure. The Jonker plot indicated that the electronic properties displayed a degenerate density of states and that Cu_((1-x))Pd_((X))FeO_2 was a semiconductor. A high ZT value of 0.055 was obtained for a Pd content of 0.03 at 950 K. The Pd~(2+) substitution for the Cu~(1+) sites influenced the thermoelectric and electronic properties of the delafossite Cu_((1-x))Pd_((X))FeO_2 samples.