Processing of β‐Silicon Nitride from Water‐Based alpha‐Silicon Nitride, Alumina, and Yttria Powder Suspensions

Abstract

powder mixtures was established. The surface charges and isoelectric points of the three different powders were investigated within the pH range from pH 3 to pH 12. Citric acid diammonium salt was found to be an effective deflocculant for shifting the isoelectric points to pH 3.5 for Al 2 O 3 and to pH 6 for Y 2 O 3 . Aluminum hydroxide (Al(OH) 3 ) showed strong interaction with the Si 3 N 4 powder, shifting the isoelectric point from pH 7 to pH 5.5. Low-viscosity, high-solids-loading suspensions (60-63 vol%) thus were possible at pH 9.7. The preparation of homogeneous and stable suspensions with a solids content of ≤61 vol% and a viscosity <1 Paؒs was limited to a pH regime between pH 9 and pH 10.5 because of the high solubility of yttria. The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease-catalyzed decomposition of urea at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green-density distribution was homogeneous throughout all bodies, even for complex geometries.

Effect of dispersants on the rheology of aqueous silicon carbide suspensions

“…Since the A40 was anionic, its effect was to increase the negative zeta potential and the electrostatic interactions will have been repulsive. Similar behaviour was reported by Hruschka et al [19] for the adsorption of citric acid diammonium onto negatively has a long hydrophobic tail with a positive charge at the end that could give rise to a high steric effect. Alternatively, the explanation could simply be that the zeta potential measurements were obtained on low solids content suspensions of fine particles.…”

“…For example, Hruschka et al [19] has investigated the combined use of Al(OH) 3 and citric acid diammonium to modify Si 3 N 4 particles in the pH range 9 − 10.5. Pei et al [20] and Shih et al [21] examined the effect of directly coating fine alumina particles onto the SiC and Si 3 N 4 surfaces.…”

Magnesia induced coagulation of aqueous PZT powder suspensions for direct coagulation casting

“…Direct coagulation casting (DCC) is a relatively new ceramic shape forming process from concentrated ceramic powder suspensions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The process has created more interest among the researchers as it is completely free from the use of polymeric binders.…”

“…In situ coagulation of ceramic powder suspensions has been achieved either by shifting the pH of the medium towards the iso-electric point of the ceramic powder by time delayed in situ generation of acid or base from water-soluble precursor molecules or by compressing the electric double layer by in situ generation of excess electrolyte [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Enzyme catalyzed in situ hydrolysis of urea has been used to increase both the pH (due to ammonia) and concentration of electrolytes like ammonium carbonates and ammonium bi-carbonate that coagulate powder suspensions of alumina, zirconia, silicon carbide and silicon nitride [1][2][3][4][5][6][7][8][9][10][11][12]. Coagulation by in situ generation of acid from watersoluble precursor molecules such as acid anhydrides, esters, lactones, and hydroxy aluminium acetate has also been reported for setting of ceramic powder suspensions prepared in alkaline pH range or using ploy (electrolyte) dispersants [13][14][15][16][17][18].…”

Fiber-reinforced ceramic matrix composites processed by a hybrid technique based on chemical vapor infiltration, slurry impregnation and spark plasma sintering

“…Electrokinetic behaviour of Si 3 N 4 powders in aqueous suspension is governed by the ionisation of the surface groups according to the pH and has been extensively studied [31][32][33][34][35]. Different techniques can be used to introduce ceramic particles inside fiber preforms from colloidal suspensions: Electrophorese Impregnation (EPI) [36], submicronic powder aspiration [37] and slurry impregnation under vacuum.…”