| Alumina（Al2O3）is the most widely used advanced ceramic material,it has the advantages of high melting point,high strength,high hardness,acid and alkali corrosion resistance.In 1959,Coble successfully realized the fabrication of alumina transparent ceramics for the first time,and applications of alumina ceramics have been broadening.At present,alumina transparent ceramics have been widely used in high-pressure sodium lamps,ceramic metal halide lamps,luminous tubes,packaging substrates for LEDs.Alumina ceramics of the hexagonal crystal system have the problem of grain boundary birefringence,which results in a low in-line transmittance under 30%in the visible light region.The submicron crystallization of alumina ceramics effectively improves the transmittance,but the grain boundary birefringence does not fundamentally change.Strong magnetic field assisted slip casting can prepare transparent alumina with preferentially oriented structure,which solves the problem of birefringence,but strong magnetic field is difficult to achieve.In this study,the shear force in the flow of slurries during casting was used to horizontally arrange platelets in the suspension of equiaxed alumina grains,ceramic green bodies were formed by spontaneous coagulation casting,and a single-crystal structure transparent alumina ceramic was prepared by the induction of platelet-like templates.The research contents include:the properties and modification of spontaneous coagulation casting,the dewater and drying of wet bodies,the fabrication and formation mechanism of single-crystal structure-like alumina ceramics,etc.The main results are as follows:1.The properties and modification of spontaneous coagulation casting.First of all,Fourier infrared technology was use to study the coagulation characteristics of slurries,it is found that the coagulation is based on physical action.Then,Isobam has been hydrophobically modified,the effects of chain lengths and addition amounts of hydrophobic chains on the rheological properties of the slurry was studied.It was found that hydrophobic modification can endow dispersant with coagulation ability,and with the increase of the hydrophobic chain addition,the coagulation speed increases.2.The dewater and drying of wet bodies.Based on the difficulty of drying,this study introduces pressure assistance in the process of spontaneous coagulation casting to study the influence of pressure on the drying process.It was found that the introduction of pressure can reduce the initial moisture content,shorten the drying time and reduce the drying shrinkage of the samples.Then,the influence of pressure on the properties of green bodies was analyzed.It was found that the pressure help to improve the uniformity and the average density of green bodies.Finally,the sintering properties,densities,uniformity,optical and mechanical properties of the green bodies after the pressure were studied.It was found that the introduction of pressure can reduce the sintering temperature and improve the density and uniformity of ceramics.3.The fabrication of single-crystal structure alumina ceramics.Firstly,the factors that affecting orientation of ceramics were studied,such as Mg2+,centrifugal speed,solid loading,heating rate,holding temperature,amount and type of templates,etc.Secondly,the structure evolution during oreintation was analyzed,it was found that the structure evolution of the XRD diffraction peak of ceramics was related to the angle between this crystal plane and（006）crystal plane.The intensity of the crystal plane first increases and then decreases for the small angle one,and the intensity directly decreases for the large angle one.Then,the relationship between the density,grain size and orientation degree was studied.It is found that the orientation degree is positively correlated with the growth of the grain size.Finally,the properties of the orientated ceramics were tested.Taking the sample with 5 wt%template as an example,the in-line transmattance is 78.4%@600 nm when the sample was sintered in vacuum at 1840℃for 6 hours and then hot isostatic pressing sintered at 1850℃@200 MPa,which is the highest value reported in the literatures.4.The formation mechanism of single-crystal structure alumina ceramics.Sapphire and ceramic powder were combined together to study the effect of the single crystal on the polycrystal during the sintering.It was found that as the sintering temperature increases,the distance of the annexation increases,and the polycrystal which annexed by single crystal exhibits same structure as the single crystal.Then,EBSD was used to analyze the influence of single crystal on the orientation behavior of adjacent grains and uncontacted grains.It was found that single crystal can not only oriented grains by the annexation of it,but also can oriented grains through long-distance force.Different single crystals have similar annexation speeds after the addition of Mg O as a sintering aid,but effects on uncontacted polycrystals were different among single crystals,single crystal C has a larger effect than A,that was because the thermal expansion coefficient along the C axis is larger than A. |
