Production of Porous Materials Through Consolidation of Pickering Emulsions

Abstract

In recent years there is a renewed interest in porous materials for biomaterials, catalyst supports and filters for high temperature processes. Most of these applications require a specific porosity and pore size. Therefore, a method that allows controlling the pore properties within reasonable bounds is needed. In this paper, the use of emulsions stabilized by solid particles is suggested as a possible route to make porous materials with controlled pore size and porosity. This method offers the additional advantage of avoiding the need for a debinding step during sintering since no solid organic material is introduced into the green part. 2] In 1907, Pickering was the first to describe the stabilizing of emulsion droplets by solid particles. Pickering stated that solid particles gather at liquid-liquid interfaces because this lowers the energy of the system. Due to the high energy of attachment of particles to the interface, the particles become irreversibly adsorbed on the interface. This monolayer of particles forms a rigid shell around the droplets and prevents the droplets from touching one another, thus preventing coagulation. The presence of particles at the interface allows the consolidation of the emulsion droplets in a green part. After removal of the droplet phase, a spherical void is left behind in the green body. The microstructures of the green body and hence of the porous compact after sintering depends on the composition of the emulsions. The two primary factors are the ratio of the amount of emulsified liquid to the amount of particles used for stabilizing the emulsion, and the amount of particles present in the continuous phase used for creating the matrix in-between the pores during colloidal processing. Small droplets of uniform size are created when two immiscible liquids are mixed in the presence of particles. The particles, added during the emulsification, migrate to the surface of the droplet and adsorb on it. This continues until the surface of the droplets is either completely covered with solid material or no particles are left in the continuous phase. In the first case, the excess particles will remain in the continuous phase. In the second case, the droplet surface will not be completely covered, causing coalescence when agitation is ceased or slowed down. The ratio of emulsified liquid and stabilizing particles together with the size of the particles determines the droplet size and eventually the pore size. With this technique, an almost uniform droplet size distribution can be created. Once the spherical emulsion droplets are stable and are not broken up by further mixing, there size will not be altered when additional powder is added. This additional powder remains in the continuous phase and forms the ceramic or metal matrix after colloidal processing. The amount of this powder therefore controls the thickness of the struts and the walls between the pores. The wall thickness is used to control the overall porosity. In theory, materials with pores separated by walls of two particles thick can be created if the continuous phase is free of particles. The experiments revealed a remarkable correspondence between theory and practice. Density measurements of sintered samples show that the porosity is in direct relation to the suspension composition (Tab. 1), i.e., the porosity increases with increasing emulsified volume and amount of stabilizing powder and decreasing amount of bulk phase powder. SEM examination of the porous materials (Fig. 1) shows that the pore size follows the emulsion droplet size. An open macro porous structure is obtained with a porosity above 60 %. At these high porosities, the walls between the individual pores are so thin that they crack due to thermal stresses during sintering. This cracking causes a large proportion of the walls to collapse creating windows between the pores. The sintering conditions did not result in complete densification of the strut material for both the ceramic and metallic samples, as shown in Figure 1(f) for the Ti material. More extensive sintering results in complete densification of the strut material, leaving only the macropores and the windows between these macropores. The low porosity samples, which show no evidence of windows, would thus have closed porosity. In one specific case, a graded porous material is obtained as shown in Figure 1(e). During electrophoretic deposition, the particles in the continuous phase deposit faster than the stabilized emulsion droplets. This results in a layer of dense material on the deposition electrode side (right side of C O M M U N IC A IO N S