LETTER Porous alumina ceramics produced with lycopodium spores as pore-forming agents Z. Z ˇ ivcova ´ Æ E. Gregorova ´ Æ W. Pabst Received: 26 March 2007 / Accepted: 14 May 2007 / Published online: 15 July 2007 Ó Springer Science+Business Media, LLC 2007 Many applications of porous ceramics require a precise control of porosity, as well as pore size, pore shape and pore space topology. Among the various pore-forming agents (PFA) used in ceramic technology, those of bio- logical origin are especially popular for fabricating porous ceramics, due to the fact that their burnout is usually harmless from the ecological and hygiene point of view, while their content of ash-producing inorganic salts is mostly low enough to be neglected with respect to the ceramic composition. Examples are wood flour (saw dust), crushed nut shells [1–3] and poppy seed [4] for pore sizes of several hundreds of micrometers and, on the other hand, starch for pore sizes below 100 lm[5–25]. Commercially available starch types cover the size range from approx. 5 lm for rice starch to approx. 50 lm for potato starch (median diameter) [5–25]. Unfortunately, closer inspection reveals that between corn and tapioca starch, which both have a median size of approx. 12–15 lm, there is a size gap which can currently only be satisfied by wheat starch (median size approx. 21 lm) [14]. Wheat starch, however, is known to be strongly bimodal, with the large size frac- tion being highly anisometric (oblate shape), which of course cannot be tolerated for all applications. As a con- sequence, there is currently no biogenic pore-forming agent available that would fill this gap and thus enable e.g. ceramic filters to better compete on the market with their polymeric counterparts for water treatment, which are commonly commercially available with logarithmic pore size grades, e.g. 1, 2, 5, 10, 20, 50 lm. In this paper, therefore, we have tested the use of lycopodium spores as a new PFA for the preparation of porous alumina ceramics. As shown in this paper, lycopodium spores can be easily mixed with ceramic suspensions, exhibit satisfactory rhe- ological behavior (up to a certain concentration), as well as defect-free burnout and lead to a uniform microstructure with isometric pores of a size of slightly below 30 lm and a percolation threshold of slightly above 10%. We would like to emphasize, that the filtration performance men- tioned above is not primarily determined by the size of the pore-forming agent, but by the interconnections between the pores in the sintered ceramic (which extends essentially to the submicron range). Thus the materials in this paper will exhibit fine particle retention [26], which makes them possible candidates for particulate filters. Lycopodium powder (for this work supplied by H. Klenk, Schwebheim, Germany) consists of isometrically shaped spores of certain club mosses (ground pines), especially Lycopodium clavatum L. [27–29], cf. Fig. 1. The particle size distribution was determined via laser diffrac- tion (Analysette 22 NanoTec, Fritsch, Idar-Oberstein, Germany) using the Fraunhofer approximation and via microscopic image analysis (Jenapol, Zeiss, Jena, Germany and Lucia G 4.81, Laboratory Imaging, Prague, Czech Republic) by manual sampling of 1,000 spores. The med- ian diameter is 30.6 lm according to laser diffraction and 33.1 lm according to image analysis (after transforming the number-weighted distribution to a volume-weighted one), cf. Fig. 2. Obviously, the reason for the slight dif- ference is the certain amount (<7 vol.%) of spore frag- ments and dust with a size below 20 lm, which has been neglected in the image analysis. The size distribution of the spores proper is very narrow, indicating that the natural Z. Z ˇ ivcova ´ Á E. Gregorova ´(&) Á W. Pabst Department of Glass and Ceramics, Institute of Chemical Technology, Prague, Technicka ´ 5, 166 28 Prague 6, Czech Republic e-mail: eva.gregorova@vscht.cz 123 J Mater Sci (2007) 42:8760–8764 DOI 10.1007/s10853-007-1852-y