Effect of Milling Time on the Rheology of Highly Loaded Aqueous-Fused Silica Slurry Kalyani Mohanta w Regional Research Laboratory (CSIR), Bhubaneswar, India Parag Bhargava Indian Institute of Technology, Mumbai, India The effects of milling time on the rheological behavior of 70- vol% loaded fused silica slurries were studied. A transition from shear thickening to shear thinning behavior was observed with milling time. Slurry viscosity and thixotropy attained a mini- mum after 18 h of milling and remained almost unchanged with additional milling. Infrared spectroscopy and particle surface charge measurements confirmed the development of increasing amount of surface hydroxide and surface charge respectively with progress in milling time, leading to shear thinning behavior. An initial static aging treatment of 24 h aided the development of silanol groups, indicating that the milling time can be reduced to save energy and time. I. Introduction S LURRY processing is known to have advantages in forming agglomerate-free compacts with high green density, uniform particle packing, and thus a homogeneous sintered microstruc- ture. 1,2 These advantages in forming green bodies arise from the ability to attain high solids loading while maintaining a low vis- cosity by dispersing individual particles in the slurries. Agglom- eration in powder compacts is known to be among the major causes for defects in sintered components. The presence of ag- glomerates leads to a low green density and differential shrink- age in powder compacts, which causes crack-like defects following sintering. Hence, breaking of agglomerates is an essential precondition to prepare well-dispersed slurries and defect-free cast bodies. 2–4 The tendency of powder particles to agglomerate increases dramatically with a decrease in the particle size from a microm- eter to a submicrometer range due to a significant increase in attractive van der Waals forces. Breaking of agglomerates can be achieved by mechanical means that include the use of ultra- sound, attrition, or milling in the presence of a hard ceramic media. The most common method used for the large-scale prep- aration of ceramic slurries is by milling of a powder–solvent– additive mixture in the presence of a grinding media where ag- glomerated lumps are broken down due to the cascading motion of the grinding media. 5 Thus, keeping all other factors constant, depending on the initial particle size of the ceramic powder, mixing or milling time can be optimized in order to obtain slur- ries free of agglomerates with minimum achievable viscosity suitable for pouring and casting. This milling time optimization study can be useful from the industrial processing point of view, to save time and energy. Although fused silica is an important ceramic material owing to its excellent insulating and thermal shock properties 6 and wide range of strategic and industrial applications, 7–14 few research papers have been published describing slurry process- ing of fused silica. Also, most of the processes for the fabrication of fused silica ceramics, e.g., slip casting, injection molding, gel casting, etc., except for a few, are based on slurry processing. Shape forming of fused silica ceramics through the wet process- ing has an additional benefit as dry powder handling is associ- ated with health hazards. Also, most of the commercial fused silica powders are available in a relatively coarser-size fraction (45 mm) owing to associated health hazards 15 and are expected to contain lesser agglomerates, which is an advantage for slurry processing. Thus, unlike processing of other ceramics using fine powders, slurry processing of coarse-fused silica powders 6 needs to be studied separately in detail and different processing steps have to be optimized, which will be helpful for the large-scale production of different fused silica bodies with a range of shapes and sizes. The present study was carried out to understand aqueous slurry processing of available coarser-fused silica powder with maximum possible solids loading in order to obtain stable slur- ries with reasonable fluidity suitable for filling the mold. In the process of maximizing the solids loading, slurries with 70-vol% fused silica loading could be made without the use of any dis- persant or additive and slurries were stable without any settling during the time period of the experiment. An accidental obser- vation during the preliminary experiments indicated that slurry viscosity attained a minimum much before the completion of the usual milling time of 24 h and remained almost constant with further mixing. Thus, experiments were conducted with the preparation of highly loaded fused silica slurries without the addition of any dispersant by mixing/milling the slurries for different times, and slurry viscosity and thixotropy were mon- itored. Rheological results were correlated with the surface chem- ical properties of fused silica particles characterized through surface charge measurement and Fourier transform infrared analyses (FTIR) spectroscopy. II. Experimental Procedure (1) Slurry Preparation A high-purity fused silica powder (M/s. Chettinad Quartz Prod- ucts Pvt. Ltd., Kanchipuram, India), d 50 5 5.2 mm (Malvern 3601 Particle Sizer, Malvern Instruments, Malvern, U.K.), r 5 2.1 g/cc, was used in the present study. Distilled water was used as the solvent to prepare aqueous slurries without the use of any dispersant or additives. Slurries with 70-vol% fused silica loading (maximum possible loading) were prepared by roll J. Beattie—contributing editor w Author to whom correspondence should be addressed. e-mail: kalyani_ceramics_india@ yahoo.com Manuscript No. 23430. Received July 9, 2007; approved September 26, 2007. J ournal J. Am. Ceram. Soc., 91 [2] 640–643 (2008) DOI: 10.1111/j.1551-2916.2007.02153.x r 2007 The American Ceramic Society 640