Study On Aqueous Gelcasting Modelling And Preparation Of MgO·1.8Al₂O₃ Spinel Transparent Ceramics

Al-rich MgO·nAl₂O₃（n>1）spinel ceramics have a unique combination of excellent optical properties,outstanding mechanical properties and chemical stability et al.,which made it widely applied in transparent armors,infrared domes,high-temperature windows and high-pressure sodium bulbs et al..Furthermore,compositional variability in MgO·nAl₂O₃ spinel exerts considerable adjustability on properties.Gelcasting is one of in-situ polymerization techniques meeting the practical application due to the feasibility to prepare components with large size and complex shape.However,owing to the basic nature of magnesium aluminate spinelpowder,spinel powder tended to coagulate the slurry,which makes it quite difficult to prepare aqueous slurry with high solid loading（≥50 vol%）and low viscosity（≤1 Pa·s）.Faced to above problems,this dissertation took MgO·1.8Al₂O₃ spinel transparent ceramic as the research target.Solid reaction process was employed to prepare high-purity and fine-grained MgO·1.8Al₂O₃ powder.Afterwards,surface hydrolysis behavior,anti-hydration modification treatment,modification mechanism and modified evaluation of the powder were studied and characterized in detail.The main results are as following:1)The existence of strong polar–OH and the formation of hydration shell are the mainly reason that MgO·1.8Al₂O₃ powder tended to aggolomerate and sedimentate.2)The-OH on the surface of MgO·1.8Al₂O₃ powder can undergo polycondensation with the-COOH of stearic acid and form hydrophobic organic isolation layer,which effectively avoided the emergence of hydration phenomena.3)The treated MgO·1.8Al₂O₃ spinel powder became hydrophobic due to the introduction of stearic acid,and we found O/W amphoteric surfactant Tween 80（HLB=15.0）had the ability to make the treated powder hydrophilic,which helped us finally prepare the stable aqueous slurry with high solid loading.In order to optimize the gel-casting process,the effects of relative variables on the preparation and gelation behaviors of slurries were studied and discussed.The final slurry with high solid loading（50 vol%）and low viscosity（363 mPa·s）,and green body with high bending strength（10.5±0.4 MPa）and large relative green density（56.2%）were obtained.The debinded green body was successively pressurelesspre-sinteredandhot-isostatic-pressed.Highlytransparent MgO·1.8Al₂O₃ spinel ceramics with the in-line transmittance of 77-82%in visible region and higher than 85%in near infrared region have been prepared.Compared with MgAl2O4 spinel,MgO·1.8Al2O3 spinel transparent ceramic exhibited higher Knoop hardness,Young’s modulus and Abbe’s number.The research of this dissertation provided a basic reference for anti-hydration of MgO·nAl2O3 ceramics and other ceramic systems with hydrolysis problems.In addition,a new MgO·1.8Al₂O₃ spinel transparent ceramic have been obtained based on the preparation technique of gelcasting in-situ forming combined with hot isostatic press sintering,which paved the way for the further application.Besides,we also supplemented the performance data of the new MgO·1.8Al2O3 spinel transparent ceramic material.