© The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org December 2004 9101 Among the non-oxide ceramic materials, silicon carbide (SiC) is one of the most important. SiC has many com- mercial applications because of its high specific strength, high hardness, high thermal conductivity, low coefficient of thermal expansion, excellent thermal shock resistance and superior chemical inertness. Shaping of SiC ceramics for various applications has been undertaken using slip-casting methods, 1 direct coagula- tion casting methods 2,3 and gelforming methods, such as gelcasting. 4–7 The gelforming methods generally provide higher strength for relatively low binder content, and the samples do not require long binder burnout times. Among the gelforming methods, the aqueous gel- casting route that involves the use of monomer methacrylamide (MAM) and crosslinker methylene bisacrylamide (MBAM) initially proposed by Janney et. al. 4,5 has been commonly used for fabrication of green SiC compacts. 6,7 The MAM–MBAM gelforming system requires use of ammonium persulfate (APS) as the initiator and tetramethyl- ethylenediamine (TEMED) as the catalyst. 4,5 SiC powder particles have an oxide surface layer, chemi- cally similar that of SiO 2 , 8(a) which determines the iso- electric point (IEP) (pH of ~3.3) and dispersion charac- teristics of the powder particles. 6 In aqueous medium, this oxide layer is hydrated, which causes formation of silanols (Si–OH). 6 Above the IEP, silanol reacts with OH – to form Si–O – , which causes the surface of the powder particles to become negatively charged. During gelcasting, addition of APS to the SiC slurries poses problems because of the relatively low IEP of SiC particles. APS is highly acidic in nature and causes instantaneous coagulation in the slurries because of a decrease of the slurry pH toward the IEP of SiC. Reports in the literature suggest that sta- ble SiC slurries can be prepared by either using dispersing agents or adjusting the pH of the medium in reference to the IEP of the particles. The dispersing agents commonly used for preparation of aque- ous SiC slurries include polyethylene imine (PEI), 8(b) ammonium polycarbon- ate (Targon 1128) 1,9 and sodium poly- acrylate. 10 Dispersion by pH adjustment has been commonly done using an organic base, such as tetramethylammonium hydroxide (TMAH), 6,7 which results in an increase in the negative charge on the particle sur- faces. In addition to the dispersing action, use of TMAH may counter the problem associated with the use of APS for gela- tion by neutralizing the acidic condition created by addition of APS prior to gela- tion. On the other hand, use of TMAH as a dispersing agent with the MAM–MBAM gelcasting system introduces pores in the green SiC components. TMAH reacts with MAM, which leads to evolution of ammonia gas, 11 which causes formation of pores in the green bodies. TMAH also reacts with free silicon, if present in the powder, and results in hydrogen gas, which also intro- duces pores in the green SiC components. 12 For oxide ceramics, several natural materials, such as gel- atin, 11 agarose 13 and polysaccharide 14 or its derivatives, A simple, inexpensive process that uses egg white as a binder for fabrication of green SiC compacts has been developed that requires no dispersing aid or pH adjustment. Sucrose additions lower slurry viscosity and yield stress as well as enhance green strength and elastic modulus. F ABRICATION OF G REEN SiC C OMPACTS U SING OVALBUMIN -B ASED AQUEOUS SiC S LURRIES Dipankar Ghosh, Santanu Dhara and Parag Bhargava Materials Science Centre, India Institute of Technology,Kharagpur,West Bengal,India