Direct observation of Pd/MgO and Pd/SiO2reactions in the transmission electron microscope

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Abstract
Reactions between Pd and MgO, and Pd and SiO2 involving areas as small as about 100 Å2 have been directly observed in the transmission electron microscope at temperatures below the melting point of any of the components. In the case of Pd/MgO, the reacting phase exhibits liquid-like characteristics, while the ceramic remains virtually strain-free, both during and after the reaction. Evidence indicates the reaction to be a surface, rather than a bulk, phenomenon. A similar reaction occurs in the case of Pd/SiO2, with the notable exception that small amorphous regions develop randomly over the surface, move, disperse, and sometimes coalesce into droplets. It is suggested that these regions represent an early stage in the melting process of SiO2. In both these cases the metal and ceramic not only react but also bond to each other, and these observations have led to the production of some 20 macroscopic couples between a variety of metals and ceramics. Furnace characteristics in the electron microscope are critical in such experiments and these are discussed.

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