MANUFACTURE OF MACROPOROUS CERAMICS BY SPARK PLASMA SINTERING

Because of their excellent properties (high temperature stability, high corrosion and wear resistance, possibility to functionalize surface, ...), porous ceramics are used in many applications : molten metal and hot gas filtration[1,2], fluid transfer or mixing, catalysis [3], bone substitute materials [4], ...

There are three main methods to produce macroporous ceramics (pore sizes larger than 50 nm): the replica technique, the sacrificial template method and the direct foaming method. The microstructure of the final material (porosity level, pore size, interconnectivity) is closely linked to the selected process [5].
The aim of this research is to develop a new route for manufacturing macroporous ceramics characterized by a porosity level higher than 30% and highly interconnected pores, with sizes between 30 µm and 80 µm. Such porous materials could find applications in the fields of fluid transfer/mixing, catalysis or filtration. Porous materials made by this new method have a structure which differs from that obtained with the conventional techniques and a better ability to fluid mixing is expected. The process must be sufficiently versatile in order to tailor: i)- the porosity fraction within a large range (typically 30 to 80 %); ii)- the pore size distribution (single or multimode).
The originality of the method consists to 'bridge' packing of basic ceramic 'units' by sintering. The sintering must promote diffusion at interfaces between units with a limited densification to keep high porosity. The porosity results from the void spaces between 'units' (intergranular porosity). Ceramic 'units' with a fine-grained structure are needed because the bridges between the units will develop at the level of powder grains.

Initially, the investigation of the feasibility was performed on alumina granules obtained by spray-drying.Further, there is an open porosity (around 50%) within the granules (intragranular porosity) due to spray-drying process. The consolidation of the material is carried out by conventional sintering techniques (free sintering, Hot Pressing) and by Spark Plasma Sintering (SPS). Compared with hot pressing sintering, porous alumina ceramics can be manufactured by SPS at lower heating temperature, high heating and cooling rates and short holding time [6, 7].

First of all, two sintering treatments were initially applied: one at high temperature under low pressure (Hot Pressing; 1600°C and 1.3 MPa), and another at lower temperature under higher pressure (SPS; 1000°C and 10MPa). The sintering temperature needs to be limited in order to keep intragranular porosity. Moreover, lowering applied pressure promotes the rate and the size of the intergranular porosity (show Figure 3).

Macroporous ceramics with an open porosity of about 30% (Hot Pressing) and about 52% (SPS) were obtained. By changing the sintering parameters (temperature, pressure...), it is possible to modulate the proportion of intergranular and intragranular porosities, or to keep only the intergranular porosity. Thereby, a wide range of porosity can be obtained. With SPS, porous materials can be made at lower temperature. As a result, pore size distribution is bimodal with a contribution of intragranular and intergranular porosities.

The next step of this work will be to change granules characteristics: increasing the size of granules will rise the size of intergranular porosity (pore diameter between 30 and 70 µm). We will also examine different possibilities to increase the porosity level (adding sacrificial template within the granules or between them.
References
(1) Acosta G, F.A., Castillejos E, A.H., Almanza R, J.M., Flores V, A., Metallurgical and Materials Transactions B., vol. 26B, 1995, pp. 159-171
(2) Li, J., Lin, H., Li, J., Journal of the European Ceramic Society, vol. 31, 2011, pp. 825-831
(3) Richardson, J.T., Peng, Y., Remue, D., Applied Catalysis A, General 204, 2000, pp. 19-32
(4) Burg, K.J.L., Porter, S., Kellam, J.F., Biomaterials, vol. 21, 2000, pp. 2347-2359
(5) Studart, A.R., Gonzenbach, U.T., Tervoort, E., Gauckler, L.J., Journal of the American Ceramic Society, vol. 89 [6], 2006, pp. 1771-1789
(6) Jayaseelan, D.D., Kondo, N., Brito, M.E., Journal of the American Ceramic Society, vol.85, 2002, pp. 267-269
(7) Wang, K., FU, Z., Peng, Y., Wang, Y., Zhang, J., Zhang, Q., Chinese Journal of Aeronautics, vol. 19, 2006, pp. 257-260