Particle Surface Modification Of Y₂O₃ Powders And Fabrication Of Transparent Y₂O₃ Ceramics By Colloidal Processing

This research was developed aiming to fabricate transparent yttria ceramics of complex shapes by colloidal processing.The efforts were emphasized to prepare the sinterable powders by pretreatment of starting yttria powder.The factors that influence the stability and rheological properties of aqueous yttria suspensions in colloidal processing have been systematically investigated,and the main achievements are listed as follows:（1）The effects of pre-treating a nano-sized Y₂O₃ powder with sulfuric acid were studied.Both the structural evolution and sintering behavior as a function of SO₄^2--ion concentrations were investigated.The mechanism of sulufric acid is elucidated.The surface morphologies and aggregation state of particles were notably affected by the sulfuric acid-based treatment.The powder untreated revealed particle of flat surfaces and sharp edges,whereas the powders with sulfate dosage were composed of round particles of isotropic surfaces after calcination.Dispersive Y₂O₃ nanopowder with excellent sintering property was obtained by calcinating the powders treated with 7 mol%sulfuric acid.The optimum sulfate dosage can be well correlated with a single layer coverage of SO₄^2--ions over the particle surfaces,protecting particles from forming hard agglomerates by contributing to masss transportation mechanisms leading to particle coarsening upon calcination.After calcination at 1100 ℃,a highly sinterable and well-dispersed Y₂O₃ powder was obtained from the powder doped with 7 mol%sulfuric acid.All the ceramics prepared using treated-powders were transparent after vacuum sintering at 1700℃ whereas it was opaque for the sample without sulfate dosage.Y₂O₃ ceramics prepared using powder pre-treated with 7 mol%of sulfuric acid and calcined at 1100℃ has an in-line transmittance of 70%at 800 nm（1,5 mm in thickness）.（2）The colloidal chemistry properties of nanometer yttria powder were studied.The effects of the amount of adsorbed Triammonium citrate（TAC）,pH of suspending medium and solid loadings on the stabilization and rheological behaviors of suspensions were investigated.The mechanism of TAC on the surfaces of yttria particles is determined.Adsorption of TAC on the particle surface shifted the IEP to lower pH and increase absolute zeta potential in alkaline region.The adsorption of TAC on nanosized Y₂O₃ follows Langmuir monolayer adsorption isotherm and a saturation plateau level of 0.33 mg/m² is confirmed.The lowest viscosity of suspension was obtained at addition of 1 wt%TAC.The stabilization of suspension can be further improved by the addition of lwt%Tetramethylammonium hydroxide（TMAH）.Therefore,well-dispersed suspensions with the high solid loadings>20 vol%were obtained,attaining the maximum value of 35 vol%..Green bodies with densities between 28.5%and 38.84%were subsequently produced from the suspensions with different solids contents by centrifugal casting.An additional cold isostatical pressing（CIP）treatment was adopted to further enhance packing density.After vacuum-sintering at 1700℃ for 5 h,highly dense ceramics with the high in-line transmittance of～74.5%at 800 nm was achieved（1.2 mm in thickness）.（3）In order to obtain the stable suspensions with high solid loading,the surface modification of yttria powder using H₃PO₄ was developed.The initial pH value of the suspension prepared using the untreated powder shows a rapid increase from 9.34 to 10.45 just in less than 120 min.In contrast,the presence of H₃PO₄ keeps a constant pH value of～9.3 during the entire testing time.The addition of H₃PO₄ induces the phosphate coating formed on the surface of the powder,which passivate yttria from hydrolysis.It is found that the lowest viscosity,slowest sedimentation rate and narrowest size distribution can be achieved at the optimal dosage of 0.5-1.0 wt%H₃PO₄.Adsorption of TAC on the particle surface shifted the IEP to lower pH and increase absolute zeta potential in alkaline region.The optimal dosage of TAC was 1.5 wt%,and the density of green body was 40.06%.Fabrication of a well dispersed suspension with higher solid loading required optimal dispersion time.A great reducing of the viscosity values of suspensions occurred with an increase of ball-milling time at the initial 6 h,and a further increase of the ball-milling time caused to a slight increase in the viscosity of suspension.The suspensions with various solid loading were achieved,and the effect of solid loading on the rheological behaviors of suspension was investigated.The optimum solid loading of 35 vol%for easy casting is defined by the Krieger-Doughert model.The green density reaches the value of 49.24%to theoretical.Fractured surface shows homogeneous and uniform microstructure and no additional agglomerates are formed.It is found that the sintered bodies with a relative density of 98.59%were produced by sintering at 1600 ℃in air,while the density of 99%was attained after vacuum sintering at 1700 ℃.（4）The effects of the methods of modification for yttria powder and various dispersants on the stabilization of Y₂O₃ suspension were investigated.The stable suspension can be obtained from the yttria powder rinsed in water under the magnetic stirring in 20 ℃ for 2 h.Compared with the untreated powder,the lower viscosity,the longer time of stabilization up to 6 h for the suspension of the treated Y₂O₃ were obtained.With the increase of dispersion time,there is a slight change of pH.The addition of various dispersants（A40,2,3-dihydroxybenzoic acid,PEI,PVP）enhances the stabilization of yttria suspension by the different stabilization mechanisms.The results show that the stable suspensions were obtained when the powder was dispersed into the aqueous medium with the addition of 1.3 wt%A40.Under the best dispersion route,the green bodies with high density of 58%were obtained from 37 vol%suspensions.Sintered bodies with the transmittance of 36%at 800 nm were produced by vacuum sintering at 1700 ℃ for 5 h（1.5 mm in thickness）.The transmittance of the sample is 71.6%at 800 nm from the green bodies treated by CIP.The fracture surface of the Y₂O₃ specimen exhibited homogeneous nearly pore-free microstructures.Meanwhile,transparent yttria ceramics with different shapes were obtained by gel casting,which would provide more infrastructure work for the preparation of transparent ceramics with complex shapes.