Preparation And Properties Study Of Doped Ca₃Co₄O_9+δ

Gd-doped Ca₃Co₄O_9+δ,Y-doped Ca₃Co₄O_9+δ and Y,Gd-doped Ca₃Co₄O_9+δ were synthesized by the polyacrylamide gel method, and their powders were sintered by Spark Plasma Sintering(SPS). The microstructures of samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The resistivity, Seebeck coefficient and thermal conductivity of the three series samples were tested at different temperatures. The conduct mechanism of samples was studied at temperatures above 650K. The hopping conduct mechanism of small polarons was determined for the materials. The XPS peak shift and changes of ion value were studied. Grain sizes of samples were controlled by heat treatment and the effects of grain sizes on resistivity were studied. Micro-topography was characterized by electric force microscopy (EFM) and relationships between micro-topography and resistivity were built up.The results of XRD and TEM indicated that the powders synthesized by the polyacrylamide gel method were fine and single phase. The parameters of optimal process were defined: the proportion between Acrylamide(monomer) and N,N'-methylene-bisacrylamide(crosslinked agent) is 5:1. The value of pH was controlled at about 6. The uniform gel was gained above 348K. Effects of metal ions concentrations on grain sizes are not obvious. DTA analysis showed that the synthesized temperature of Ca₃Co₄O_9+δ was lower than that of other methods.Samples sintered by SPS process showed c-Axis aligned at some degree. TEM observation of the Samples indicated that there were no other phases and non-crystal layers among grains. The temperature dependence on resistivity of Ca₃Co₄O_9+δ sample showed positive temperature coefficient below about 500K and negative temperature coefficient above about 500K. But that of doped Ca₃Co₄O_9+δ samples only showed negative temperature coefficient. According to the analysis of high temperature conduct mechanism, the materials have the hopping conduct mechanism of small polarons. With increasing of doping, the hopping activation energy increased. In addition, the hopping activation energy of samples doping with Gd was higher than that of samples doping with Y due to the effects of spin scatter. At 973K, ZT values of 10% Y-doped and 10% Gd-doped samples reached 0.223 and 0.249 respectively. The ZT value of 10% double-doped sample reached 0.263, which was the highest in the three material systems.The grain size was adjusted by heat treatment. After treated at 973K, the size of grain was about 1²Î¼m; after treated at 1123K, the size of grain was about 3⁵μm. But the resistivities of two samples were almost same. So the grain size was not the main factor on the resistivity.XPS results showed that Co²⁺ increases after doping. The peaks of elements in doped sites shifted to high bind energy, which indicated there were weak bound charges in doped sites. The intensity of peaks increased in value band spectra of doped samples, which showed the decreasing of carriers. The electric force microscopy phase image connected resistivity with grain shape, and showed anisotropic properties of materials.The effect of oxygen nonstoichiometry on resistivity was obvious. The resistivity tested in full oxygen condition was lower than that tested in air condition due to oxided cobalt ions in materials. The concentration of Co⁴⁺ increases, and effective distance between Co⁴⁺ and Co³⁺ decreases; the hoping between two sites is easier, so hopping activation energy becomes smaller. For samples doping with Gd, there is no obvious difference for the resistivity tested in different conditions, which showed that the doped Gd element could stabilize thermoelectric properties of materials.