Research On Composite Forming And Microwave Rapid Sintering Of MgAl₂O₄ Transparent Ceramics

Magnesia alumina spinel transparent ceramic has excellent properties such as low density,high hardness,corrosion resistance,high strength,high temperature stability and high transmittance from near ultraviolet to mid infrared,which is considered as candidate materials in challenging military fields,such as transparent armor,laser igniter,infrared transparent window of missile launcher and reconnaissance pods and transparent domes of infrared search missile.In order to solve the shortcomings of hot pressing sintering process,in this paper the Mg Al₂O₄transparent ceramics were prepared by conventional atmospheric pressure sintering/hot isostatic pressing and microwave sintering/hot isostatic pressing without sintering additives.Finally,Mg Al₂O₄ transparent ceramic dome with high quality was prepared by microwave rapid sintering/hot isostatic pressing process,which provided a new way for the engineering application of Mg Al₂O₄ transparent ceramics in the future.The following research was carried out:Before pressureless sintering,the powders needs to be formed into a green body with a certain shape.The density and microstructure uniformity of the green body will have a significant impact on the densification of ceramics.Firstly,the dry pressing/cold isostatic pressing process of Mg Al₂O₄ powder was studied.For dry pressing,the dispersion and fluidity of raw material powders will directly affect the microstructure of green body.Therefore,in view of the problems of agglomeration and nonuniform particle size distribution of the original nanopowders,the wet ball milling process was used to refine the particle size,and the effects of dispersion medium and grinding ball size were studied.The results showed that taking alcohol as the dispersion medium and reducing the grinding ball diameter were conducive to the dispersion of the powders and increase of the green body density.In order to improve the fluidity of the powder,Mg Al₂O₄ powders were modified by spray granulation.The effects of solid contents,drying temperature and binder contents on the morphology and fluidity of the granulation powder were studied.The results showed that the fluidity of the granulation powder prepared by the process with solid content of 21 wt%,drying temperature of 180℃and PVA addition of 1.0wt%was the best.The influence of forming pressure on the density of green compact was studied.The results showed that dry pressing pressure had no effect on the density of green compact,and the density was mainly determined by cold isostatic pressing pressure.In view of the problems of dry pressing,which may lead to the reduction of the density and uniformity of the green body due to the secondary agglomeration of the powders during the process of pressing and drying,the gelcasting wet forming process of Mg Al₂O₄ powder was studied.The effects of p H value,dispersant content and ball milling process on the rheological properties of the slurry were studied.The effects of monomer content,mass ratio of monomer to crosslinking agent,and the content of initiator and catalyst on the curing of the slurries and density of Mg Al₂O₄ green body were also studied.The results showed that the best p H value of the slurry was 10,the best content of dispersant was 6 wt%,the best content of monomer was 16 wt%,the best mass ratio of monomer to crosslinking agent was 15:1,the best amount of initiator was 1.25 vol%,and the best amount of catalyst was 0.75 vol%.Due to the low solid content（30 vol%）,the density of the green body formed by gelcasting was low after degumming,the green body was then treated by cold isostatic pressing,which significantly improved the density of the green body.The final prepared Mg Al₂O₄ green body with homogeneous microstructure had a relative density of 50.5%,and and the pore diameter was evenly distributed,the size was about 30 nm.Finally,the sintering densification and properties of Mg Al₂O₄ transparent ceramics were studied.The effects of particle size,shaping process,atmospheric sintering temperature and time,microwave sintering temperature and time,hot isostatic pressing temperature on the density,microstructure evolution and properties of Mg Al₂O₄ ceramics were studied.The results showed that the powder with small particle size and less agglomeration were beneficial to beneficial,and the optical and mechanical properties of the ceramic samples were better.The gelcasting green body had higher density than the dry forming green body after sintering.The prepared transparent ceramics by gelcasting had higher transmittance,better optical uniformity,smaller grain size and higher mechanical properties than that by dry forming.Microwave sintering required lower temperature and shorter holding time than conventional atmospheric sintering.The densification mechanism of microwave sintering and atmospheric pressure sintering Mg Al₂O₄ was analyzed from the perspective of sintering kinetics.The results showed that the densification mechanism of the two sintering processes was grain boundary diffusion,and the sintering activation energy of microwave sintering was lower,so the densification rate was higher.The hot isostatic pressing temperature is low,and the sample has residual pores,which affects the optical properties;If the temperature was too high,pores would be produced in the grain,and the mechanical properties were reduced due to the excessive grain size.Through the optimization of process parameters,Mg Al₂O₄transparent ceramic dome with good optical and mechanical properties were prepared by low solid content gelcasting/cold isostatic pressing,microwave sintered at 1500℃for 80 minutes and then treated by hot isostatic pressing at 1650℃/170 MPa/3 h.The transmittance of 3 mm thick samples at 400 nm was as high as83.2%,the bending strength reached 237±15 MPa,the Vickers hardness reached13.6±0.08 GPa and the fracture toughness reached 1.61±0.05 MPa?m^1/2.