Basic Research Into Molding And Sintering Of Transparent Alumina

Transparent alumina is the first transparent ceramic. It posses the inherentattributes of structural ceramics such as high temperature resistance, corrosion resitance,resistance to corrosion, high strength, high hardness, chemical stability andbiocompatibility, along with the functional attribute of translucency. There is anincreasing trend for transparent alumina in the application of energy, machinery,military, electronics, semiconductor, medical, and other high-tech areas. BasicResearches are conducted concerning the forming and sintering technologies oftransparent alumina in this thesis from three aspects: the introduction of sintering aids,injection molding of surface modified powders, and the sintering kinetics.A novel approach was adopted to fabricate transparent alumina ceramics inducedby doping additives via chemical precipitation. It realized the coating of sintering aidsions onto the surface of the alumina powder. The mechanism of chemical precipitationof the sintering aid ions was explained. The microstructure and the distribution state ofsintering aids in the sintered body were characterized. It is found that the dopants aresmaller and more homogeneously dispersed via chemical precipitation in the transparentalumina sample as compared with conventional wet ball milling. The real in-linetransmission in the UV-VIS region of the sample prepared by chemical precipitationtransmittance is up to21.3%. Furthermore, it is believed that the approach of chemicalprecipitation followed by H2sintering at high temperature within short sintering timecould make a good balance between the real in-line transmission and the cost.A strategy to greatly improve solid loading for ceramic injection molding by aprior ball milling treatment of ceramic powders with SA was adopted to fabricatetransparent alumina. Different from the traditional work in which SA has beenintroduced just in the powder blending process, we have successfully prepared thefeedstock with a much higher solid loading up to64vol%by a prior ball millingtreatment of ceramic powders with a small amount of SA before the traditional blendingprocess. It can be attributed to that SA can be coated homogeneously around the powdersurfaces by an esterification reaction induced by ball milling treatment. The Lewis acid-base reaction mechanism raised by previous literatures is corrected.62vol%isproved to be the optimal solid loading through the analysis of pore structure of thecompacts. The real in-line transmission in the UV-VIS region at such solid loadingreaches15.3%. The technology has been applied to bio-ceramics and light sourcematerial (metal halide lamp). It solves the problems of low transparency and defectssuch as bubbling and cracking to some extent.Spray freeze granulation was adopted to prepare granules with uniform structure,without agglomeration and without binder segregation. The kinetics windows were builton both pressureless sintering and spark plasma sintering of high-purified and ultrafinealumina. The idea that particle packing in the powder treatment and particlerearrangement in sintering is key factors to obtain complete densification were raised.Transparent alumina was prepared via spark plasma sintering and the real in-linetransmission is no less than20.3%in the UV-VIS region.