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Time-dependent suspension behavior is reported for nonoxide ceramic powders (Si3N4, SiC, and MoSi2) in ethanol. The suspension pH (and therefore the stability) changed with time. X-ray photoelectron spectroscopy, inert gas fusion, inductively coupled plasma, and high-resolution transmission electron microscopy were used to track changes of surface chemistry. The adsorption of the base, tetramethyl ammonium hydroxide (TMAH), is examined. The pH drop on powder addition to pure EtOH was used to gain insight into the role of TMAH coverage of the powder surfaces.

I. INTRODUCTION

Bergström et al. showed that the isoelectric point (pHiep) for silicon nitride (Si3N4) in water changes to higher pH values with time.1 This aging process is due to changes of the Si3N4 surface chemistry. They suggested that surface silanol leaching caused aging. If the surface potential decreases with time, the zeta-potential also decreases; i.e., the interparticle repulsion is reduced so particles coagulate. This aging will impact colloidalsuspension-based shaping processes (slip-casting, electrophoretic deposition), and that is the practical reason for this study. It will be shown that silicon nitride (Si3N4), silicon carbide (SiC), and molybdenum disilicide (MoSi2) suffer the aging effect in ethanol (EtOH). This aging phenomenon is explored. In general, the powders explored have “SiO2” surfaces, and similarities and differences of behavior in suspension are identified. It has also been suggested that aging could be the result of atmosphere interaction; thus four different atmospheres (i.e., dry N2, wet N2, dry CO2, and wet CO2) are explored with respect to pH changes. Basic tetramethyl ammonium hydroxide (TMAH) is commonly used to adjust the pH of EtOH-based suspensions. The process of TMAH adsorption on Si3N4, SiC, MoSi2, and SiO2 surfaces is thus explored. This study is the first attempt to investigate the aging of Si3N4, SiC, and MoSi2 in EtOH. II. EXPERIMENTAL

Silicon nitride powder (Si 3N4; 0.63␮m, oxygen: 1.32%, 93.3% ␣-phase) (H.C. Starck M11, Berlin, Germany), silicon carbide (SiC) powder (Ibiden, Ultrafine, Gifu, Japan) (0.29 ␮m, ␤-phase) and molybdenum disilicide (MoSi2) powder (Cerac, M-1103, Milwaukee, WI); (1.74 ␮m, crystalline) were used. The specific surface area of the Si3N4, SiC, and MoSi2 were 11.20, J. Mater. Res., Vol. 18, No. 1, Jan 2003

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21.99, and 0.32 m 2 /g, respectively by Brunauer– Emmett–Teller (BET) analysis (Autosorb Automated Gas Adsorption System, Quantachrome Co., Syosset, NY). Amorphous silica (SiO2) powder (PCR, High Purity, Gainesville, FL) was used for behavior comparison. The SiO2 particles were monodispersed, spherical (diameter 0.5 ␮m), and had specific surface area of 6.19 m2/g. Anhydrous, absolute ethyl alcohol (EtOH; Alcohol Inc., Brampton, Ontario, Canada) was used as the solvent. High-resolution transmission electron microscopy (HRTEM; JEOL 2010F FEG TEM-STEM, Tokyo, Japan) was used to observe the surface films on the powders. Powder was d