pH controlled dispersion and slip casting of Si3N4 in aqueous media

Abstract

The dispersion characteristics of commercial Si 3 N 4 powder in aqueous media (deionized water) was studied as a function of pH in the range 2-11. The slip was characterized for its dispersion quality by various experimental techniques like particle size analysis, sedimentation phenomena, viscosity and flow behaviour and zeta potential analysis. The optimum dispersion was found to be in the pH region 9-11 wherein the slurry displayed minimum sedimentation height, minimum viscosity, near Newtonian flow behaviour and maximum zeta potential. The slip is highly agglomerated in the pH range 2-8 as manifested by higher sedimentation height, higher viscosity, lower zeta potential and thixotropic non-Newtonian flow behaviour. The 72 wt% (44 vol.%) Si 3 N 4 slips made at pH = 10 resulted in green bodies having 53-59% of theoretical density after casting into plaster molds.

Nitride-bonded silicon nitride from slip-cast Si + Si3N4 compacts

“…The experimental details and the results of such studies were presented elsewere. 16,17 For slurry characterization, the powders were dispersed in DIW with continuous magnetic stirring for 5-20 min depending upon the required total solid loading (in the range 5-50 wt.%). The pH of the slurry was adjusted to various values ranging from 2 to 12 using HNO 3 (70% sol.)…”

“…Some of the physical characteristics of the Si (HQ and RQ) and Si 3 N 4 (S1) powders are presented in Table I along with their chemical composition (as provided by the supplier). Figure 1 depicts the particle-size distribution curves at the optimum dispersion condition of pH ‫ס‬ 8 for two Si 16 powders and pH ‫ס‬ 10 for Si 3 N 4 17 . The results of Fig.…”

“…1 and Table I indicate The metallic Si as well as the nonoxide Si 3 N 4 powders are usually found to be surface oxidized, and the fine (submicron) particles contain a superficial layer of silica on their outer surfaces. [16][17][18][19][20] When these powders are dispersed in water, the oxidized surfaces get hydrolyzed to form amphoteric silanol (Si-OH) groups. 16,21 In addition, Si 3 N 4 particles also contain basic amine groups viz.…”

“…17,19,20 In aqueous medium the silanol and amine groups undergo acid/base reactions depending on the pH of the medium and develop positive or negative surface charges depending upon the adsorption of either of the potential detemining OH − or H + ions. [16][17][18][19][20][21] These surface charged particles adsorb a layer of opposite charged ions resulting in an electrical double layer and an electrical potential developed at the shear plane between the bound and free liquid and is called the ZP. On close approach, these particles repel each other due to the overlapping of the electrical double layer.…”

Slip Casting of a Si3N4‐Based System

“…However, the zeta potential titration curves of pure Si 3 N 4 show that the powder without additives should be stable also at acidic pH. 43,44 Regarding impurities, oxygen is always present in sintered silicon nitride bodies, because of the surface oxidation of powders. Silicon nitride powders easily oxidize in air and therefore an oxidized layer forms with a composition that gradually changes from silica at the surface to silicon oxynitride.…”