Influence Of Rare Earth Dopant On Phase Transformation And Mechanical Property Of Alumina

Rare earth dopants could improve the thermal stability of metastable alumina,however,there was still a dispute about the working mechanism.Therefore,the effects of La and Ce on the thermal stability ofγ-Al₂O₃ ceramics were investigated respectively in the thesis.According to the analysis of the formation temperature and existence range for the intermediate phases,the stabilization mechanism of La and Ce onγ-Al₂O₃ was discussed.Moreover,the influence of Nd on the mechanical properties of alumina was studied.It was found that the terminal temperatures ofγ→αphase transformation for the samples doped with La or Ce were higher than that for undoped specimens according to their sintering curves,of which the temperature of La doped samples was nearly 100℃higher than that of pure specimen.This could be attributed to the differences of the ionic radius for Al³⁺ and the rare earth, which induced the segregation of La³⁺ or Ce⁴⁺ to the grain boundaries of the alumina matrix, hence,the mass migration was inhibited.Compared with Ce doped samples,the thermal stability of La doped samples was better under the same dopping concentration as a result of the difficulties for mass transportation in La doped specmen due to the following reasons.The concertration of La³⁺(with larger ionic radius than that of Ce⁴⁺) at the grain boundaries in the alumina matrix was much higher than that of Ce⁴⁺. Moreover,the degree of lattice distortion in Ce dopped samples was stronger than that of La dopped samples due to the disagreement of the valence for Ce⁴⁺ and Al³⁺,which will play an active role in the sintering process.The X ray diffraction and Infrared spectra characterization for the samples showed that the temperature for the appearance of the intermediate phases was retarded and the temperature range was widened during theγ→αphase transformation process in La or Ce doped samples.The appearance of LaAlO₃(in the range of 900-950℃) and CeAlO₃(1100-1170℃) with perofskite structure could inhibited the migration of Al³⁺,wich improved the thermal stability ofγ-Al₂O₃. Otherwise,it could be found that the formation temperature of LaAlO₃ is lower than that of CeAlO₃,wich resulted in that the thermal stability of La doped samples was better than that of Ce doped specimen.Investigation on the sintering behavior of Nd dopedγ-andα-Al₂O₃ ceramics showed that the Nd dopant promoted the densification of the stable alumina,while the densification of the metastable alumina was restrained.The bending strength,fracture toughness,microhardness of Nd doped samples(α-Al₂O₃ as raw material) were improved,wich were superior to that of the specimens usingγ-Al₂O₃ as raw material.The fracture mode of inter-granular dominated in the doped samples,which could be attributed to a weakened interface bonding as a result of the coexistence of rare-earth oxides and other oxides.At the same time,it was found that the secondary phase precipitates occurred due to the supersaturation of Nd³⁺ at grain boundary,which located predominantly at grain triple points.