Key Engineering Materials Vols. 336-338

Abstract: The tribochemical effects of MoSi2 powder in the ball milling process have been studied by X-ray diffraction (XRD) and QM-4H milling machine. It has been found that the intensity of diffraction peak of MoSi2 powder is continuously decreased and the width increased with the increase of milling time. Specifically, the crystallite size decreased largely in the early stage of milling. Contrary to the above, the microstrain and the effective temperature factor increased considerably. After ninety hours of milling, the crystallite size had little changes. The relations among the tribochemical effect factors are as follows: the microstrain and the effective temperature factor increase with the decrease of the crystallite size. The microstrain increases with the increase of the effective temperature factor.

Abstract: Precursors of calcium strontium zirconate (Ca0.8Sr0.2ZrO3) were synthesized by oxalate coprecipitation method. TG-DTA was used to characterize the thermal decomposition behaviors of the precursors. The XRD results revealed that the powders crystallized as cubic Ca0.8Sr0.2ZrO3 with increasing heating temperature up to 1000°C. Calcium strontium zirconate ceramics were obtained under conventional sintering conditions by using the as-prepared Ca0.8Sr0.2ZrO3 powders. The SEM micrographs of the ceramics sintered at 1450oC exhibited narrow grain-size distribution with the average grain size around 0.7 4m. The dielectric properties showed that, dielectric constant of the Ca0.8Sr0.2ZrO3 was about 24.0 and dielectric loss was 0.002 at the frequency of 1GHz and the room temperature.

Abstract: Mixed-conducting ceramic oxide SrFeCo0.5Oy powders were prepared by a citrate method, and the contributing factors to the formation of homogeneous fine powder with a pure perovskite-type structure were ascertained. The resulting powder is pure, homogeneous and possesses a reasonably fine particle size. Attempts were made to understand the synthesis process and it was confirmed that the method produces powder had greater composition uniformity, lower residual carbonate levels and smaller particle sizes.

Abstract: A new precipitation route to ammonium aluminum carbonate hydroxide (AACH) was presented. There was an incubation period before AACH precipitated. Effects of synthesis parameters such as pH of ammonium hydrogen carbonate solution, precipitation temperature, concentration of aluminum citrate and type of anions on the precipitation of ammonium aluminum carbonate hydroxide were discussed. It was found that the incubation period decreased with the increasing pH of AHC solution, concentration of aluminum citrate and the decreasing reaction temperature. Compared to NO3 -, SO4 2- delayed the formation of AACH and enlarged the particle size of AACH.

Abstract: In this paper a novel and versatile 3D print-patterning technique coupling electrohydrodyanmics and a specially designed and constructed plotting device is elucidated. This unit is capable of free-forming advanced ceramics and we demonstrate this by using it to print-pattern a 5mm × 5mm × 1mm walled zirconia structure layer by layer. The wall thickness achieved is ~150$m, almost half that of similar structures prepared using ink-jet printing. The as-printed structure was studied by scanning electron microscopy and some of its typical features are discussed and related to the forming process.

Abstract: A new consolidation in-situ forming process for high-performance alumina ceramic using acetate starch was investigated. The pasting behavior and thermogelling properties of acetate starch and the effects of different amount of acetate starch and solid loading on the rheologic behavior of the ceramic slip, as well as the linear shrinkage, density, strength and microstructure of the green body were discussed. The results indicate that acetate starch paste has good thermal stability and gelling ability. When the content of acetate starch is about 0.5%~1.5% (mass fraction), the apparent viscosity of alumina slurry, which is always less than 1Pa·s even for 58% (volume fraction) dense suspension, increased with increasing the acetate starch content and solid loading. That results in easier to cast. Moreover, with the increasing of solid loading the linear shrinkage and dry strength of green bodies decreased and the relative density increased. With the increasing starch content, the linear shrinkage rate and the density of the dried body decreased, while the strength of the dried body almost linearly increased. The green body with homogeneously-distributed density and pore size was also obtained in this work. It is concluded that Al2O3 ceramics can be consolidation in-situ formed based on the gelling network of thermally swelled and gelatinized acetate starch.

Abstract: Extrusion gelation freeform fabrication (EGFF) is a new solid freeform fabrication technique to fabricate ceramic parts. The study of ceramic suspensions rheological behavior is essential to a better understanding of parts fabrication through EGFF process. In this paper, the effects of dispersant concentrations, solids loading and binder composition on the rheological behavior of alumina suspensions were examined. The optimal amount of dispersant was found to be about 0.1 wt.%. The suspensions with 50 vol.% alumina and 1wt.% sodium alginate show shear thickening behavior. Such behavior is unfavorable for extrusion gelation freeform fabrication. By addition of PVA or styrene-acrylic latex, the rheological behavior of suspensions was changed. The addition of PVA or latex has a great effect on the viscosity of the suspensions and the type of their rheological behavior. The ceramic suspensions with PVA or latex were found to be shear thinning more suitable for EGFF.

Abstract: In this paper, a novel colloidal forming process has been developed to prepare boron nitride-silica composites. The rheological properties of mixed boron nitride-silica powders in deionised water have been investigated by adjusting the concentration of dispersant, pH value of the solution and other process parameters. The isoelectric points for both boron nitride and silica powders are in the pH range of 2 to 3. Boron nitride doped with or without A-1 (an anionic dispersant) as a dispersant shows a similar electrophoretic behavior to silica powder, i.e. Zeta potential becomes more negative with the increase of pH value from 2 to 12. The sedimentation and rheological results of aqueous boron nitridesilica suspensions indicate that the optimized pH value is around 7.2. It depends upon the synthetic effects of dissociation of A-1 and sufficient positive adsorption sites on the particle surfaces in aqueous media. The optimum concentration of the dispersant at pH=6 is 0.6wt % (dry weight basis of composite powders). Boron nitride-silica composite suspensions with a solid content as high as 50vol% have also been achieved. A ball milling process can improve the fluidity of the suspensions.

Abstract: A high solid loading of dispersed silicon nitride slurry was prepared with various dispersants, which allowed a stable silicon nitride suspension with sintering additives (cordierite) in alkaline regions to a solid content of 45 vol%. Gum arabic, PMAA-NH4, TMAH, PAA-NH4 and their composites were selected as dispersants. The results suggested that the composite dispersant which was obtained using PAA-NH4 and TMAH with the ratio in 4:1 was more effective in aqueous silicon nitride powder suspensions system than simplex dispersant. The green bodies derived exhibited a mechanical strength as high as 8.7MPa and SEM photos revealed that the green body also had a high homogeneity.

Abstract: In this study, the effect of SiC powder characteristic on the rheological properties of the feedstock was investigated. It was found that the viscosity of the feedstock decreased with the increase of the particle size. For the binary powder mixtures, the viscosity of the feedstock decreased with the increase of the particle size ratio of the larger particle to finer particle, which is attributed to the increase of the tap density of the powder.