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Spherical silica particles, dispersed in ethanol/water solutions, were first coated with titania by hydrolysis of added titanium isopropoxide. The coating thickness could be readily adjusted by the ratio of titanium alkoxide concentration to the amount of silica. The coated cores prepared as above were then treated with ammonia in a temperature-programmed reaction to produce a shell of titanium nitride. After nitridation, the powders of the high specific surface area showed electrical conductivity, which depended on the thickness of the coatings.

I. INTRODUCTION

B. Preparation of coated particles

Titanium nitride (TiN) is of considerable interest due to its outstanding properties, such as extreme hardness, high melting point, strong resistance to various environments, excellent electrical conductivity, and potential catalytic activity. In general, coating solid surfaces with TiN has been achieved by chemical vapor deposition either in fluidized1"3 or in rotary beds.4'5 However, some problems are associated with these procedures, such as the stabilization of the fluidized bed, a large quantity of fluidizing gas, small samples, and limitations with regard to particle size and uniformity. The last deficiency can be eliminated by coating monodispersed inorganic cores of desired size (and shape) with a TiN shell. This work describes a method that produces uniform titanium nitride coatings on submicron silica spheres, through the nitridation of precursor silica particles covered with titania. The latter cores with controlled titania thickness are readily prepared by modifying a procedure described earlier.6 A temperature-programmed reaction has been employed to minimize sintering of the powder during the nitridation and to obtain high specific surface area materials.7'8

The monodispersed spherical silica particles of different modal sizes were prepared in 2-propanol by the method of Stober et al.,9 with some modifications.10 Tetraethyl orthosilicate was added to a mixture of ethanol, water, and ammonia at 40 °C with constant stirring, and then the solution was aged for 24 h at the same temperature. The so obtained silica particles were washed numerous times, first with an ethanol-water mixture, then with doubly distilled water, and finally dried in vacuo at 60 °C for a day. The titania coating procedure consisted of hydrolyzing of an ethanolic solution of titanium isopropoxide in the presence of silica particles. A weighed amount of the latter was dispersed in an ultrasonic bath in a mixture of an aliquot of water and a large volume of ethanol, to which a solution of Ti(i-Opr) 4 in ethanol was added dropwise over 1 h at room temperature with stirring at 500 rpm. The resulting coated particles were washed three times with ethanol and three times with water and dried in vacuo at 60 °C for 2 days. The dried samples were calcined in air at 700 °C for 2 h. For comparison purposes, an amorphous titania powder was prepared by the same technique, but in the absence of silica particles.

II. EXPERIMENTAL SECTION A. Materials