DOI: 10.1002/adma.200701897 Electrically Conductive Dense and Porous Alumina with In-Situ-Synthesized Nanoscale Carbon Networks** By Ruben L. Menchavez, Masayoshi Fuji, * and Minoru Takahashi Electrically conductive ceramics with tailorable electrical conductivity are required in both dense and porous forms depending on whether the specific applications involve subjecting the ceramic components to aggressive or mild environmental conditions, respectively. Dense electrically conductive ceramics are used for static charge dissipation, lightning protection, ceramic heaters, electric discharge machining (EDM), [1] and electromagnetic interference (EMI) shielding in electronic, mechanical, structural, chemical, and vacuum applications. In particular, ceramic alumina with added electrically conductive fillers has been used to fabricate substrates for handling semiconductor wafers that require static protection. Moreover, dense electrically con- ductive ceramics have found use as anti-static floor tiles in applications as diverse as call centers, server rooms, flight towers, and electronic manufacturing centers. [2] In contrast, porous electrically conductive ceramics have attracted much renewed interest for the development of high-performance radiative heaters. [3,4] Such porous materials also show great potential for applications in filters for the aeration of liquids, [5] ceramic foam heaters, and exhaust traps for automotive cold start applications, [3] as well as for the combustion of diesel soot and the non-catalytic oxidation of noxious gases. [4] The various applications of dense and porous electrically conductive ceramics are facilitated by incorporating phases with covalent or metallic characteristics into the ceramic matrix to impart electrical conductivity, which tends not to be a typical intrinsic property of most ceramics. As a typical example, the fabrication of electrically conductive ceramic composites is often accomplished by the mechanical mixing of an insulating powder with conductive fillers such as metals or metallic oxides [6,7] to create conductive pathways or networks in the electrically insulating ceramic matrix. The fabrication of these conductive pathways implies that the percolation threshold [8] of the conductive filler in the ceramic composite needs to be exceeded in order to obtain high electrical conductivity. In general, the amount of filler added to reach the percolation threshold depends upon its physical characteristics such as particle size and aspect ratio. Low-aspect-ratio fillers such as carbon black and graphite need to be added in least 20 vol% amounts, [9,10] whereas for high-aspect-ratio carbon nanotubes (CNTs) the amount of additive required is reduced to less than 12 vol%. [11–13] Indeed, it is not optimal for the conductive filler to occupy a large portion of the ceramic matrix since it inevitably leads to some degradation of the intrinsic structural properties of the host ceramics. Moreover, the use of mechanical powder mixing in combination with extrusion or press forming gives rise to unwanted segregation of the conductive filler within the ceramic matrix, which makes it difficult to achieve the fabrication of homogeneous conductive network structures. As a result, the composites tend to exhibit undesirable material characteristics such as anisotropy in electrical conductivity, low mechanical strength, poor repro- ducibility of fabrication, and unwanted grain growth. To overcome these problems, powder composites have been processed and assembled using colloidal techniques. For example, aqueous dispersions of CNTs with insulating ceramic powder [14] have been consolidated by freeze casting [15] and direct coagulation casting [16] among other methods. The primary advantage of dispersing CNTs originates from their ability to build nanoscale networks within the ceramic matrix. These nanoscale networks give rise to good electrical conductivity and simultaneously impart desirable physical properties such as high mechanical strength to the composites. Although the CNT incorporation method shows great promise, an ideal homogeneous distribution of nanoparticles within the composite matrix, which is essential for obtaining uniform electrical properties, has not yet been obtained. Indeed, advances in dispersing nanocomposite systems [17] are required to explore the full potential of CNTs and other composite systems. Thus, there is an urgent need to develop facile methods to fabricate composites exhibiting uniform electrical conductivity. Gel-casting has been widely recognized as a versatile method for shaping dense and porous ceramics. The gel-casting process basically requires the use of a stable and high- solid-loaded slurry consisting of ceramic particles, solvent, monomer, and crosslinker. [18] The prescribed slurry is cast directly into a mold to form a dense powder compact, whereas COMMUNICATION [*] Prof. M. Fuji, R. L. Menchavez, Prof. M. Takahashi Processing Group, Ceramics Research Laboratory, Nagoya Institute of Technology Asahigaoka 10-6-29, Tajimi City, 507-0071, Gifu Prefecture (Japan) E-mail: fuji@nitech.ac.jp [**] This study was supported by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) through a project on ‘‘Cooperation for Innovative Technology and Advanced Research in Evolution Area, 2006-8’’. RLM thanks Dr. C. Takai and H. Hibino for their assistance with transmission electron microscopy observations. The authors also gratefully acknowledge Japan Fine Ceramics (JFCC) for their assistance with sintering samples. Adv. Mater. 2008, 20, 2345–2351 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2345