The mechanical properties of Si3N, based ceramic bodies are largely controlled by the phases formed on the grain boundaries during sintering. For this reason determination of the state and the chemical composition of the surface of the starting Si,N, powder is of prime theoretical and practical importance, especially in the cases when advanced ultrafine powders are used. In this work an ultrafine Si,N, powder, obtained by high temperature plasma nitridation of silicon, has been characterized by XPS. Oxygen, carbon and also a small amount of potassium were detected as surface impurities. The Si 2p and Si (KLL) spectral lines could be decomposed into two components corresponding to silicon in Si,N, and SiO, phases. From angle-resolved and Ar-ion depth-profiling experiments a layer model, consisting of a Si3N, core covered by a relatively thick SiO, layer and a carbon contaminant overlayer, could be elucidated.

Silica-alumina catalysts have been applied for a long time in cracking and reforming reactions. Lewis type surface sites are generally accepted to serve as active centers. In this respect, surface properties of these materials are of main concern. In the present work S O z doped y A 1 2 0 3 with less than a monolayer coverage were prepared by the reaction of SiC14 (8) with -pA120j at 820-1120 K. Catalytic activity of the samples was tested by a model reaction of n-hexane cracking. Surface composition and electronic state were evaluated by XPS investigations performed on a KRATOS XSAM 800 instrument. Catalytic activity (conversion) varies remarkably with increasing Si content showing a sharp maximum a t low Si coverage. The observed shift of A1 2p lines with increasing Si content is interpreted as an enhancement of the acceptor or acidic character of the surface A1 atoms. The Auger parameter for Si(dSi), (composed by addition of Si 2p BE and Si (KLL) KE values) passes also through a maximum at the same Si-content, as does the conversion curve. A close correlation of the Auger parameter and the catalytic activity is established for samples with (Si/A1)57%. This leads to a conclusion, that the rise in extra-atomic relaxation energy may serve as an additional indication of enhanced catalytic activity.

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