Synthesis Of MgAl₂O₄ Powder And Fast Sintering Of MgAlON Transparent Ceramics

In this thesis,pure MgAl₂O₄（MAS）powders were first synthesized by solid-state reaction method.The as-obtained MAS powders were then used to synthesize pure MgAlON powders using solid-state method.Finally,using three powder combinations individually,i.e.,MgAlON,MAS+AlON and MAS+Al₂O₃+AlN,as raw materials,MgAlON transparent ceramics were obtained respectively,try to prepare MgAlON transparent ceramics rapidly.The detailed research works and results are as follows.（1）Synthesis of MAS powder with high sinterability:The nanometric MgO mixed with nanosizedγ-Al₂O₃ orθ/α-Al₂O₃ powders were employed as raw material to synthesize MAS powder in air by solid-state reaction.It was found that both starting mixtures,i.e.,γ-Al₂O₃+MgO andθ/α-Al₂O₃+MgO can all be synthesized into nanometric MAS powders.MASG（85nm）and MASM（101nm）were obtained at 1250℃and 1200℃for 60 min,respectively.The MASG showed two MAS formation paths,γ-Al₂O₃+MgO→MAS andα-Al₂O₃+MgO→MAS,while MASM was formed solely by the reaction ofα-Al₂O₃ and MgO.As a result,the as-obtained MASG exhibited a bimodal aggregated particle size distribution with better sinterability.（2）Synthesis of MgAlON powder by two-step heating strategy:MASG powder was used as Mg source to be separately mixed with AlON powder and Al₂O₃+AlN powder as starting materials.MgAlON-1 and MgAlON-2 powders were obtained by two-step heating method in nitrogen atmosphere（1550℃×30 min→1700℃×60 min）,with D₅₀ of 1.25μm and 0.98μm,respectively.The two-step heating method can make full use of the different solid solution rates of Al₂O₃ and AlN at different temperatures,hereby realizing low-temperature synthesis,and is conducive to the preparation of small particle size.The fine powder,of which MgAlON-2 has a finer particle size,exhibits better sintering activity during pressureless sintering.（3）Sintering of MgAlON transparent ceramics by MgAlON powders:MgAlON transparent ceramics were prepared by PS and PS+HIP using MgAlON-1 and MgAlON-2powders as raw materials,respectively.The transmittances of MgAlON-1 and MgAlON-2ceramics obtained by PS at 1880℃for 150 min were 31.0%and 51.7%,respectively.After HIP at 1850℃for 120 min,the transmittances were increased to 44.0%and 56.6%.（4）Sintering of MgAlON transparent ceramics by solid-state reaction:MgAlON transparent ceramics were prepared by PS+HIP via solid-state reaction of MAS+AlON and MAS+Al₂O₃+AlN,respectively.The one by MAS+AlON shows higher transmittance,i.e.,66.4%by PS and up to 82.2%after HIP.The hardness of this ceramics is 15.53±0.58 GPa.Using MAS+AlON as raw material,two-step sintering route（1700℃×60 min→1880℃×150 min）was also tried and the obtained MgAlON ceramics only by PS showed transmittance of 80.4%and hardness of 15.76±0.13 GPa,which indicates that two-step sintering by solid-state reaction of MAS+AlON is more effective and efficient to prepare MgAlON ceramics with high transparency.