The Formation And Sintering Mechanism Of AlN Powders

In this paper,AlN powders were synthesized by both carbonthermal reduction method and self-propagating high-temperature synthesis?SHS?method.Then AlN ceramics were prepared by pressureless and hot-pressing sintering.XRD,SEM and EDS were used to confirm the possible mechanisms which were proposed to explain the formation of Al N particles from Al particle in SHS process and sintering behavior of Al N powders synthesized by SHS method.The main results are followed:?1?The AlN powders were prepared by carbonthermal reduction method using Al?OH?₃ as Al source.The pure AlN powders were obtained by firing at 1500?.The ball milling for raw Al?OH?₃ powder before mixing could significantly reduce the particle size of nitride product,which is beneficial to the following sintering.The product prepared by the SHS process displays the large aggregates containing many small Al N particles with the size(d₅₀=3.01?m)even if starting Al particles are large(d₅₀=28.76?m).It reveals that AlN particles form and reach the limited size through the flowing and nitridation of Al liquid repeatedly in very short time when the combustion wave passes in the SHS process.?2?Fully dense AlN ceramics were prepared using the AlN powder synthesized by SHS method as raw material with different amounts of Y₂O₃ additive when the sintering temperature was above 1830?.When Y₂O₃ content is 5 wt%,the density and thermal conductivity of green body sintered at 1850?for 4 h are 3.34 g/cm³ and 172 W/?m·K?,respectively.Both Y₂O₃ contents and sintering temperatures have important influences on the formation of Y-Al-O phase and grain shape.The main polarization type of AlN ceramics is space charge polarization.With the increasing amount of Y₂O₃ additive,dielectric constant firstly rises and then falls.Along with the increasing sintering temperature,the dielectric constant decreases gradually.Bending strength is mainly affected by grain size when the sintered body has been almost densified.The smaller the grain size is,the higher the bending strength is.?3?Dense AlN ceramics could be achieved by adding single Sm₂O₃additive or adding Sm₂O₃ and Y₂O₃ together at 1830?for 2 h.With the increase of Sm₂O₃ content,the density of AlN sintered body decareses.The main mechanism is:the liquid phase formation temperature of Sm-Al-Ois higher than Y-Al-O.Thus Sm-Al-O phase is easily lumped together among the grain boundaries,because of its bad boundary mobility and wettability.?4?Dense AlN ceramics can be obtained by both hot pressing sintering and addition of 1 wt%CaF₂-3 wt%Y₂O₃.The grain size of sintered body is small,the grain grows well and its shape develops into polyhedron.But sintering behavior is not obvious for the sample added with simple Y₂O₃ as the liquid Y-Al-O phase is hard to form at 1650??...