Studies On The Formation Mechanism Of Ba₂Ti₉O₂₀ By The Citrate Acid Complex-Combustion Synthesis

Single phase Ba₂Ti₉O₂0 ceramics have been prepared at a relatively low si-ntering temperature by a combustion technique using citric acid as a fuel and nitrates as oxidants. The preparation processes have been investigated by meansof DSC-TG, XRD, TEM, FT-IR, SEM, Impedance Analyzer. The main contents are as follows:1. The effects of the method of the drying and the drying temperature on the trait of the in-process product2. The effects of the environment and the molar ratio of nitrate to citrate on the composition and the microstructure of the in-process product3. The effects of the traits of in-process products on single Ba₂Ti₉O₂0 phase formation.4. The effects of the traits of in-process products on the microstructure of Ba₂Ti₉O₂0 ceramics5. The effects of the sintering conditions on the formation of Ba₂Ti₉O₂0 ceramicsThe study has demonstrated that, the appropriate method of the drying and the appropriate drying temperature are important for obtaining the homogenous powder /gel as the initial materials of the combustion. The in-process product dried at 150℃ by means of plate drying with folium and quick drying.The combustion behavior is affected by a series of factors such as environmental temperature, time of heat preservation and stoichiometry ratio of CA/NO₃, so are the phases and microstructure of the product. All of the factors determine the maximum combustion reactive temperature and gel decomposition rate. By controlling the initial citrate-nitrate ratio and the combustion environment, the phases, the grain size and dispersity can be tailored effectively.Homogenous composition and high content of BaTi₅O₁1 phase in the in-process product favor the Ba₂Ti₉O₂0 phase formation of end product. Homogeneity is the main factor which affects the formation of BaTi₅O₁1;the higher the homogeneity, the higher the content of BaTi₅O₁1. BaTi₅O₁1 enhances the formation of Ba₂Ti₉O₂0 phase becausevery close oxygen packing density between BaTi₅O₁1 and Ba₂Ti₉O₂0 suppresses the strain energy occurring in phase transformation during sintering. On the other hand, the effect of the microstructure of the in-process product on the microstructure of Ba₂Ti₉O₂0 ceramic is unconspicuous after two high temperature processes. The fine particles relatively favor the microhomogenity and high density of Ba₂Ti₉O₂0 caramics.The sinter process at relative low temperature for long time and high density of Ba₂Ti₉O₂0 caramics favor the obtain of Ba₂Ti₉O₂0 caramics with microhomogenity and complete grain.Single phase Ba₂Ti₉O₂0 ceramic could be easily prepared by sintering precursor powders directly at 1250℃ for 4h due to the high reactivity and good homogeneous distribution of fine multiphasic particles in the precursor powders derived from citrate solution combustion. The present work shows that the change in nitrate to citrate molar ratio in the initial complex solutions has a significant influence on the properties of the resulting ceramic. The relatively high nitrate to citrate molar ratio is needed in order to realize a complete combustion. As a result, a dense sintered body can be formed.The citrate solution combustion method has been proved to be a simple, time-saving and energy-efficient route for the synthesis of precursor powders, which help to the reactive agglomeration at relatively low temperature and the formation of Single phase Ba₂Ti₉O₂0 ceramic.