Study On Aqueous Gel-casting And Preparation Of ZnAl₂O₄ And ZnO·2.56Al₂O₃ Transparent Ceramics

Transparent zinc-aluminate spinel ceramics have not only good mechanical and excellent optical properties,but also superior thermal and dielectric properties.As a typical multi-functional materials,these transparent ceramics has potential prospects as thermal shock window,high temperature optical window and optoelectronic materials.The current research has focused on stoichiometric zinc-aluminum spinel,and research on Al-rich zinc-aluminate spinel transparent ceramics has not yet been carried out.In this dissertation,ceramic green bodies with high density,uniform structure and high strength were prepared by gel-casting technique,then the zinc-aluminum spinel transparent ceramic with good performance was prepared by combining pressureless sintering with hot isostatic pressing techniques.The relationship among composition,structure,and performances of the transoarent ceramics was also studied.Firstly,ZnAl₂O₄ powder and Al₂O₃ powder were used as raw materials to study the effects of key factors?dispersant,pH value,solid loading,stirring time,etc.?on the viscosity of ZnAl₂O₄ and ZnAl₂O₄-Al₂O₃ ceramic slurries.The optimized parameters for the preparation of ZnAl2O4 single-phase slurry were determined as follows:the content of dispersant PAA 1.0 wt%,the pH value 11,and the stirring times 4 h.The optimized parameters for the preparation of ZnAl₂O₄-Al₂O₃ composite slurry was:the content of dispersion PAA content 1.4 wt%,the pH value 11 and the stirring time 3 h.Considering the influence of solid loading on the density,homogenity and strength of the green bodies,the proper solid loading for ZnAl₂O₄ slurry was determined to be 48vol%,and 50 vol%for ZnAl₂O₄-Al₂O₃ composite slurry.Secondly,the effects of different pressureless sintering parameters?i.e.temperature,holding time,etc.?,and sintering aidditives on the density and microstructure of pre-sintered bodies were further studied.It was found that the sintering activity of ZnAl₂O₄ powder was poor,which could be effectively improved by adding Y₂O₃.After pressureless sintering at regimes of 1525?/4 h,1550?/1.5h,and 1550?/2 h,the pre-sintered bodies with high density,closed pores and uniform microstructure were obtained.The sintering activity of ZnAl₂O₄-Al₂O₃ composite green bodies was higher than that of ZnAl₂O₄ single-phase green bodies.The ZnAl₂O₄-Al2O3 pre-sintered bodies with high density and colsed pores and uniform microstructure can be achieved by holding at a wide temperature range of 1525-1575°C for 2 h.Thirdly,the hot isostatic pressing of the pre-sintered bodies were conducted.Among them,the ZnAl2O4 transparent ceramics pre-sintered at 1550°C-1.5 h have the best optical transmittance.The in-line transmittance of the sample with 2mm thickness reached 40%in visible wavelength,and in the infrared region it gradually increased beyond 80%.ZnAl₂O₄-Al₂O₃ composite ceramics were transformed into ZnO·2.56Al₂O₃ single-phase transparent ceramics after hot isostatic pressing,and the optimal optical properties of ZnO·2.56Al2O3 transparent ceramics were obtained from the sample pre-sintered at 1550°C-2 h.The in-line transmittance of the sample with1.8 mm thickness reached 70%in visible wavelength,and the infrared transmittance exceeded 80%.Compared with ZnAl2O4 transparent ceramics,the mechanical properties of ZnO·2.56Al₂O₃ transparent ceramics were improved to some extent,but the thermal conductivity was reduced remarkably,and both of them have good dielectric properties.