Migration of Grain Boundary Phases for High Performance Ceramics.

The AIN ceramics containing5 and 10wt%Y_2O_O were densified at 1800^0C for 1h in N_2，and annealed at 1900^0C in N_2and reducing carbon gas atmospheres.The grain boundary phases，containing both Y_4Al_2O_9and Y_2O_3only，decreased with increasing annealing time.Y_4Al_2O_9decomposed into Y_2O_3and Al_2O_3，which migrated towards the surface of the sintered body，and were nitrided on the surface to form a surface layer of YN and AIN.Hot-pressed AIN body without additive and sintered AIN body containing10wt%Y_2O_3were joined and annealed at 1900^0C in N_2using a carbon container.The initial migration of grain boundary phases from sintered to the hot-pressed body was caused by their concentration gradient。The grain boundary phases reacted with oxygen in the hot-pressed body，resulting in the depletion of oxygen in the hot-pressed body.Two liquid phases were formed around 690^0 and 860^0C by reaction of 3pbo，Fe_2O_3and WO_3，which were starting materials of PbFe_&lt；2/3&gt；W_&lt；1/3&gt；0_3(Designate PFW)。The high-temperature liquid phase has been demonstrated to form at 860^0 C on hating and to solidify at 840^0 C on cooling in PFW.Through slow cooling at the rate of 25^0C/h after sintering，platelike grains，designate G phase，are found to form a thin surface layer of specimens。The amount of G pahse on the surface decreases with the increase of cooling rate.These results reveal that the microstructure of PFW is greatly affected by the high-temperature liquid phase；additionally，the slow cooling treatment seems to be a direct and effective method for removing the residual liquid phase from PFW.

The AIN ceramics containing 5 and 10 wt% Y_2O_O were densified at 1800^0C for 1h in N_2, and annealed at 1900^0C in N_2 and reducing carbon gas atmospheres. The grain boundary phases, containing both Y_4Al_2O_9 and Y_2O_3 only, decreased with increasing annealing time. Y_4Al_2O_9 decomposed into Y_2O_3 and Al_2O_3, which migrated towards the surface of the sintered body, and were nitrided on the surface to form a surface layer of YN and AIN. Hot-pressed AIN body without additive and sintered AIN body containing 10 wt% Y_2O_3 were joined and annealed at 1900^0C in N_2 using a carbon container. The initial migration of grain boundary phases from sintered to the hotーpressed body was caused by their concentration gradient. The grain boundary phases reacted with oxygen in the hotーpressed body, resulting in the depletion of oxygen in the hotーpressed body.Two liquid phases were formed around 690^0 and 860^0C by reaction of 3pbo, Fe_2O_3 and WO_3, which were starting materials of PbFe_<2/3>W_<1/3>0_3 (designate PFW). The high-temperature liquid phase has been demonstrated to form at 860^0C on hating and to solidify at 840^0C on cooling in PFW. Through slow cooling at the rate of 25^0C/h after sintering, platelike grains, designate G phase, are found to form a thin surface layer of specimens. The amount of G pahse on the surface decreases with the increase of cooling rate. These results reveal that the microstructure of PFW is greatly affected by the highーtemperature liquid phase ; additionally, the slow cooling treatment seems to be a direct and effective method for removing the residual liquid phase from PFW.