Preparation, Structure And Properties Of Ca₃Co₄O₉by Solid-state Reaction At Low Temperature

With the problems of energy shortages and environment pollution becomingincreasingly serious, people started to pay more and more attention to the development ofnew environment-friendly energy and green alternative energy technologies. Layered cobaltoxides with superior thermoelectric properties are considered to a promising thermoelectricmaterials due to their easy-preparation, high temperature-resistance, anti-oxidation,pollution-free and low cost. Solid-state reaction at high temperature and sol-gel method areoften adopted in its preparation. By analyzing the advantages and disadvantages of otherpreparation methods, a new method–solid-state reaction at low temperature was used toprepare cobalt oxides thermoelectric materials. This method has the advantages of simplereaction technology, low reaction temperature, short reaction period, no need for solvent andless pollution.We have selected Ca₃Co₄O₉as our research objects. In this paper, metal chlorides andNaOH were used as main raw materials. The powders of Ca3Co4O9, single-element dopedCa₃Co_4-4xM_4xO₉（M=Ni or Zn） and composite doped Ca₃Co_4-x-yNi_xZn_yO₉were synthesized bysolid-state reaction at low temperature, and the bulk materials were obtained through highsintering. XRD, SEM, EDS, FT-IR, TEM and thermoelectric property tester used to researchthe effect of different preparation process and doping element on the structure andthermoelectric properties of materials. The main results are as follows：Layered powders of Ca₃Co₄O₉were successfully synthesized by solid-state reaction atlow temperature, and then calcined at700℃for1h. The particle size was around0.2～1m.The calcining temperature played a more important role than holding time in the structure ofpowders. There was impurity formation in the sample calcined at600℃and pure Ca3Co4O9began to generate under650℃. The crystallinity of the powders improved, and the particlesize became larger and more form balanced with increasing calcining temperature. Theparticle size also became larger with increasing holding time. At the same time, we preparedCa₃Co₄O₉samples by sol-gel method under the same calcining conditions. And same XRDpeaks were got, and the morphology of the two species of powders were identical, with only slightly difference in size.Ca₃Co₄O₉bulk materials were prepared by high sintering with30MPa molding pressureand heat treatment at850℃for4h. With increasing sintering temperature, the crystal size ofmaterials increased, the number of pores reduced, and the electrical conductivity and thermalconductivity both increased gradually. With increasing sintering time, the crystal size ofmaterials increased, the electrical conductivity increased while the thermal conductivitydecreased.There was no impurity formation after the single-element doping and composite doping.Ni and Zn ions can successfully replace Co-bit, and disperse in the lattice structure ofCa₃Co₄O₉, composing a single compound. Zn doping had almost no effect on themicrostructure of the powders. Both doping of Ni and the composite of Ni and Zn made thepowders more refined. And as the increasing of the Ni doping the crystal size becamesmaller and smaller. What’s more, the electric and thermal conductivity of Ca3Co4-4xNi4xO9material both decreased as the Ni-doped amount increased. The Zn doping with appropriateamount can improve the electrical conductivity and meanwhile abate the thermalconductivity apparently. In this study, when the amount of doping Zn was0.1, the electricalconductivity reached maximum and the thermal conductivity reached minimum. Ni, Zncomposite doping improved the thermal conductivity obviously, but the electricalconductivity decreased.