The Investigation On The Sintering Mechanism For α-Al₂O₃ Under The Pressureless Sintering Condition At Constant Heating Rates

The thermal expansion behaviors, the sintering behaviors and mechanism of a— Al₂O₃ were investigated under the pressureless sintering condition at constant heating rates.The line expansion coefficients of the average, dynamics and instantaneity of the sintered (X-Al₂O₃ with a relative density of 90% was studied respectively. It was discovered that the dynamic line expansion coefficient ascended with the increase of temperature, on the contrary, the instantaneous line expansion coefficients descended with the rise of temperature.The influence of grain size and heating rate on sintering behaviors were studied. The plots between densification rates and relative density displayed that the maximal densification rates took place at a relative density of 69.7% for Al₂O₃ with the grain size of 100-200nm, moreover, 75.8% for that of 200-500nm. By analyzing the curves between densification rates and relative density under two heating rates, it was found that the maximal densification rates was independent on the heating rates, which both appeared near the relative density of 73%.For the varied grain size of Al₂O₃ at two constant heating rates during the initial sintering stage, although the activation energy calculated on the basis of the theory put forward by Johnson and Woolfrey were different as a result of the limitation factors of the models, which decreased with temperature increasing. At the same time, the intermediate stage sintering activation obtained from the sintering theory brought forward by Wang descended at first and then ascended with temperature rising. The phenomena discovered above displayed that the activation energy during sintering process was not constant, which were dependent on the grain size of the specimen, heating rates and the range of the temperature. The differences of the material transport mechanisms, the microstructure evolution and other factors might affect the sintering activation energy, which were to be investigated.