Effects of dispersant on rheological behavior of silicon nitride aqueous suspension

Abstract

Colloidal processing techniques such as slip casting, centrifugal slip casting or pressure slip casting are considered as a powerful route to fabricate silicon nitride ceramic components with complex shapes [1, 2]. For this purpose, Si3N4 powder should be dispersed efficiently in liquid medium thus forming a desirable suspension with the lowest viscosity and highest solid content. Colloidal behavior of Si3N4 suspension with either aqueous or organic medium has been studied widely [3–7]. The same Si3N4 powder behaves differently depending on the various processing parameters applied. In order to achieve promising powder suspensions, dispersants are frequently used [7–10]. After that the particle surface has been modified with dispersant by either changing the surface charge, i.e. by electrolytes, or by coating with a large amount of organic barrier, i.e. by polymers, or by a combined action of both factors, i.e. by polyelectrolytes, Si3N4 particles can be stabilized to a promising level in liquid media. The introduction of dispersant to suspension generally has important influence on the electrokinetics of suspended particles and the rheological behavior of suspension. In this letter, some experimental results about the effects of dispersant on the rheological behavior of Si3N4 aqueous suspension will be discussed. Silicon nitride powder (H.C. Starck, Germany) with an average particle size of 0.60 μm and a BET specific surface area of 21.0 m2 g−1 was used for the present study. The dispersant used was an inorganic compound which stabilized the suspension by changing the surface charge of Si3N4 particles with its negatively charged anion group in electrostatic mechanism. The zeta potential of Si3N4 particles was measured with diluted Si3N4 dispersion (volume fraction, φ < 0.005) at 25 ◦C on Zetasizer 4 (Malvern, UK). The rheological behavior of Si3N4 suspension was characterized in steady shear mode with a couette (cup radius: 34.0 mm, bob radius: 32.0 mm, bob length: 33.3 mm) on Rheometric Fluid Spectrometer II (RFS II). The zeta potential of Si3N4 particles as a function of pH of suspension is shown in Fig. 1. As can be seen the isoelectric point (IEP) of Si3N4 particles is at the pH value of 4.2 and the particles have a maximum zeta potential at about pH = 11. The introduction of dispersant increases efficiently the zeta potential of Si3N4 particles in magnitude and shifts the IEP to acid region. Meanwhile, the pH values corresponding to the maximum of zeta potential for Si3N4 particles with different dispersant concentration remain constant. Therefore, in this work, all suspensions investigated are kept at pH = 11 expecting a good fluidity obtained. It seems that the changes of zeta potential and IEP of Si3N4 particles result from the increasing of the charging density of the particles caused by the adsorption of negatively charged anion groups of dispersant on the surface of Si3N4 particles. Fig. 2 illustrates the flow curves of 23.9 vol % Si3N4 aqueous slurry containing various amounts of dispersant. As is shown, the behavior of slurry transfers from an anti-thixotropy via near Bingham fluid to thixotropy with increasing amount of dispersant. It is clear that the Si3N4 suspension studied is a non-Newtonian time-dependent fluid which exhibits a pronounced thixotropy. When the amount of dispersant is below 0.8 wt % (weight percent, on a powder basis) the suspension exhibits an anti-thixotropic behavior and the anti-thixotropy decreases with increasing the dispersant amount. The anti-thixotropic behavior of the suspension is considered as due to the irregular morphology of Si3N4 particles. At appropriate dispersant range from 0.8–1.2 wt %, the particles are well dispersed in media so that the anti-thixotropic behavior disappeared and a near Bingham behavior is exhibited. With further increasing of the dispersant, the excessive dispersant moleculars are bridged together to produce some particle networks in suspension which could be broken down in the high applied shear field, so the suspension exhibits a thixotropic behavior at higher dispersant amounts.