The Pressureless Sintering Behavior And Sintering Mechanism For Nano-and Sub-micrometer TiO₂ At Constant Heating Rates

At present,the studies on nanometer TiO₂ are largely focused on its functional properties,such as the photocatalytic properties,dielectric properties,conductivity, photoelectric conversion features,gas sensitivity,humidity sensitivity and some new unique optical properties.However,nano titanium ceramics,as structural ceramics,has good mechanical properties,such as low-temperature superplasticity.In order to prepare nano TiO₂ ceramics with high relative density,it is very important to investigate the sintering theory and mechanism.The anatase and rutile of nanometer and sub-micrometer TiO₂ were used as raw materials in this thesis.The thermal expansion behaviors,the sintering behaviors and mechanism were investigated under the pressureless sintering condition at constant heating rates using Thermal Dilatometer,X-ray Diffractometer(XRD),Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM),Atom Force Microscope (AFM) and Differential Scanning Calorimeter(DSC).The two step sintering method was also investigated to prepare sub-micrometer TiO₂ ceramics with fine size grain and high relative density.The sintering curves of nanometer anatase and rutile and submicrometer anatase titania were compared.The results show that the intermediate sintering stage of nano rutile happenes in the range of 765℃～995℃,nano anatase at 490℃～1000℃and submicrometer anatase at 975℃～1065℃.The temperature of sintering shrinkage onset and maximum densification rate appearance is 750℃and 920℃for nano rutile.However, that of nano anatase is 450℃and 880℃for the phase transformation.The SEM images of sample fracture and the AFM images of etched surface show that the grain growth before 1000℃is induced by the adhesion between small granules in the aggregate,and after 1000℃it is caused by the grain boundary motion.Based on the combined-stage sintering model,the master sintering curve(MSC) for the rutile was constructed for sintering in air with constant pressure and using a constant heating rate in a dilatometer.The MSC curve of the rutile TiO₂ samples was constructed and validated under different thermal histories.The MSC,in which the sintered density is a unique function of the integral of a temperature function over time,is insensitive to the heating path.According to the MSC of TiO₂,it is possible to predict the sintering shrinkage and final density and calculate the activation energy(105KJ/mol).With one temperature dependent parameter determined experimentally,it becomes possible to describe accurately the densification behavior of TiO₂ from the initial to final stages during the sintering period.The nano- and micrometer rutile TiO₂ power compacts were sintered in air at 1200℃at constant heating rates of 1,3,5℃/min using a dilatometer.The shrinkage behaviors of TiO₂ were investigated to clarify the effect of specific surface area on the densification behaviors at the initial sintering stage.The apparent sintering activation energies were also investigated using the shrinkage data by the Arrhenius plot.The results show that the increase in specific surface area enhances the densification rate with increasing temperature.The activation energy values of nanometer and micrometer TiO₂ are 115±10 KJ/mol and 302±15 KJ/mol,separately.The effect of heating rates on densification rate for manometer TiO₂ is much greater than that for micrometer TiO₂.For nanometer TiO₂,a maximum vale of densification rate appears when the instantaneous relative density is in the range of 70%-80%.But for micrometer TiO₂,the maximum value of densification rate appears in the range of 75%-85%.The sintering kinetics of nanometer rutile TiO₂ was intestigeted using DSC.The whole sintering process nearly was a steady endothermic one.The specific heat capacity(Cp) had not increased abruptly at the temperature when the maximal densification rate appeared.The equation of Cp and temperature is Cp=-0.7+8.79×10^-4T.A two-step sintering process was used to prepare sub-micrometer TiO₂.The first heating step should be short at a relatively high-temperature in order to close porosity without significant grain growth.The second step at a relative low-temperature facilitates further densification with limited grain growth.Fine-grained TiO₂ with a relative density of 95%and a grain size of 0.8μm was prepared by two-step sintering.Besides,another two-step sintering method was investigated.The initial precoarsening at a low temperature with a long dwell time produced an improvement in the microstructure homogeneity during the subsequent sintering.The microstructural refinement was produced by the two-step sintering.