Original Research Paper
Influence of dispersant size on rheology of non-aqueous ceramic particle suspensions

https://doi.org/10.1016/j.apt.2011.04.011Get rights and content

Abstract

The surface of alumina, which is hydrophilic in general, was made hydrophobic either by adsorption of polymer (phys-adsorption) or by an alkylation reaction with alcohol (chem-adsorption) to enable dispersion in dodecane. Hypermer A70 (8.2 nm) was used as the polymer and 1-octanol (1.2 nm), 1-decanol (1.5 nm) and 1-hexadecanol (2.5 nm) were used as the alcohol (values in brackets are the approximate thickness of the steric barrier). Rheological measurements of ceramic suspensions indicate that it is possible to achieve a high solid loading (50 vol.%) with relatively low viscosity (0.25 Pa s at 100 s−1, the typical shear rate for pumping of liquids in pipes) as long as the stabilising molecule is large enough. The observed rheological behaviour fitted the Quemada viscosity model quite well when excluded volume effects were taken into account. Addition of 2.8 wt.% of Hypermer A70 with respect to weight of alumina was enough to stabilise the particles.

Introduction

Ceramic processing techniques can be classified into two general categories, namely dry processing and wet (colloidal) processing. Although dry processing techniques are extensively used in industry, many researchers have found that colloidal processing is better because it enables the removal of heterogeneities such as aggregates of primary particles which lead to defects; thus, producing more reliable ceramic articles [1], [2], [3]. Examples of common colloidal processing shaping techniques [4] include slip casting [5], [6], tape casting [7], [8], direct coagulation casting [9], gel casting [10], [11], [12] and freeze casting [13].

Slip casting is the most commonly used colloidal processing technique for producing complex shaped ceramics. It relies on the packing of particles from a well dispersed slurry by the capillary pressure induced by pores in plaster or a polymer mould [5], [6]. It is a robust shaping technique for technical ceramics, but due to the potential for density gradients, complex shape limitations and long forming times, it is not always the best choice for producing complex shaped high performance components.

Tape casting is a well-established technology in producing layered ceramics such as electronic substrates and multilayer capacitors through dispersion of ceramic particles in organic solvents [14]. In the interest of the environment, however, aqueous based suspensions have received heightened interest [8], [15]. Incompatibility of some ceramic particles (for example: AlN) with water due to rapid oxidation is an issue which needs to be addressed.

Direct coagulation casting depends on the formulation of a well-dispersed suspension to avoid agglomerates and subsequent destabilisation of the suspension to form a coagulated, rigid body. The destabilisation can be achieved by moving the pH of the suspension close to the isoelectric point (IEP) by enzymatically catalysed reaction [4]. The disadvantage of this approach is that it can only shift pH from one particular value to another. For example, hydrolysis of urea shifts the pH from 3 to 8 [4].

Gel casting is a shaping method whereby a well-dispersed suspension is converted into a gel after being cast in the mould. This conversion can be induced by manipulating the suspension temperature after casting to enable polymerisation of monomers or a crosslinking reaction between polymers with crosslinking agents [10], [11], [16], [17], [18], [19], [20]. Recent development has introduced the use of low toxicity polymers as opposed to the high toxicity one used in the original gel casting concept to produce high strength green bodies [10].

Freeze casting is a shaping method whereby ceramic suspensions are cast in a mould, frozen and sublimated to remove the solvent [13], [21], [22]. This method is mostly used to produce porous ceramic articles [21], [23]. The issue with this method is the expensive freezing process under very cold conditions.

As explained above, colloidal processing relies on the ability to control the interaction between particles in the suspending medium [24]. This control enables the production of flocculated, weakly flocculated or fully dispersed suspensions. In colloidal ceramic powder processing, high solid loading, fully dispersed and low viscosity suspensions are often desirable because they allow for easy transfer of suspensions, easy mould filling and minimum shrinkage due to solvent removal. The most common method of producing well dispersed ceramic particle suspensions is by the addition of an organic polymeric dispersant [24], [25], [26]. The Atomic Force Microscope colloid probe technique has been used to demonstrate that the stabilising effect of these dispersants is primarily due to steric or electro-steric repulsion [24], [25].

Alumina, which is inherently hydrophilic, is only able to be dispersed in an organic solvent such as dodecane through surface modification. The surface can be modified by covalently bonding alcohol molecules to the particles surface (chem-adsorbed) [27] or by weakly adsorbing a polymer from solution (phys-adsorbed) [26]. This work investigates the effect of changing the dispersant size and effective volume fraction on the rheology of non-aqueous suspensions.

Section snippets

Theory

The interaction potential between particles in a medium is a balance between the attractive and the repulsive potential, as shown in Eq. (1).VT=VA+VRwhere VT is the total interaction potential, VA the attractive potential and VR the repulsive potential.

The attractive potential predominantly comes from the Van der Waals potential. This force for two identical spheres, taking into account retardation effects, can be expressed mathematically as follows [28]:VA(D)=-Aeff(D)62a2D2-4a2+2a2D2+InD2-4a2D2

Materials and methods

AKP-30 alumina powder (Sumitomo, Japan) was used as received. The average particle size (d50) was reported as 0.31 μm and this was confirmed by particle sizing. Three different alcohols (Sigma–Aldrich, Australia) with varying chain length were used in this study, namely 1-octanol, 1-decanol and 1-hexadecanol. The boiling points of these alcohols were 196 °C, 233 °C and 190 °C, respectively. They were used in the preparation of chem-adsorbed suspensions. Hypermer A70 (courtesy of Croda Lubricants),

Influence of the size of stabilising moiety

The viscosity as a function of shear rate for the chem-adsorbed and phys-adsorbed suspensions at 5 vol.% solids are shown in Fig. 2. The chem-adsorbed suspensions show a shear thinning behaviour which implies the existence of an aggregated structure. The suspensions with short chem-adsorbed molecules could not be batched to volume fractions above approximately 10–20 vol.% as the suspensions became pasty and dry. The phys-adsorbed suspension, on the other hand, showed almost Newtonian behaviour,

Conclusion

The viscosity of alumina suspensions in dodecane decreased with an increase in the length of hydrocarbon chains grafted onto the alumina surface. It was established that it is important to have a long range steric separation between particles to achieve a stable suspension in organic solvents. However, when large macromolecules were used to stabilise the particles, the volume of these molecules was also found to be important. The influence of the increased effective volume fraction (caused by

Acknowledgements

The financial support of the Australian Research Council (ARC) through funding of the Particulate Fluids Processing Centre, a Special Research Centre of the ARC and Discovery Project DP0879953 are acknowledged.

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