Solid state reactions between Pd and Si induced by high energy ball milling
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Solid state reactions induced by high energy ball milling between Pd and Si have been studied. X-ray diffractometry and differential scanning calorimetry have been used to characterize the resulting phases. During milling, Pd and Si react by diffusion to form different phases depending on the Si content in the starting mixture. With a low Si content of 19 at. %, an amorphous phase forms of the same composition. On continued milling, this amorphous phase partially crystallizes into Pd 9 Si 2 and Pd2Si compounds. With the Si content equal to or higher than 33 at. %, no amorphous phases were observed. Instead, the Pd 2 Si phase is produced. For powder composition corresponding to the stoichiometric compound Pd2Si (33 at. % Si), the Pd2Si forms and remains stable during further milling. With Si content equal to or higher than 50 at. %, the initially produced Pd2Si is destabilized by a reaction with the remaining Si to form PdSi, which is a metastable phase at the temperature of ball milling. It is very unlikely that an amorphous phase of a composition equal to or higher than 33 at. % Si could be produced by ball milling in the Pd-Si system. This is because the Pd2Si phase forms very easily through the reaction between Pd and Si, and this reaction competes effectively with glass formation.
I. INTRODUCTION Solid state reactions induced by high energy ball milling (especially amorphization reactions in metallic systems) have attracted a large amount of research work recently.1"5 This is because the high energy ball milling approach has been recognized as a complex process which can be applied to processing of advanced materials at low cost. In general, both stable and metastable phases (including amorphous phases) can be produced by ball milling. The starting materials can be either a mixture of elemental powders, or a mixture of intermetallic compounds. The mechanisms of solid state reactions and atom movements during ball milling are not yet fully understood. Qualitative models have been established based on analogies to multilayer solid state diffusion reactions, and free energy concepts have been employed in the interpretation of results.6'7 Solid state reactions between a thin film of Pd and an Si substrate during annealing or ion beam mixing have been studied by many researchers.8"12 The motivation of studying these reactions is the technological importance of these reactions in improving the performance of integrated circuits.13 The formation of Pd 2 Si was always observed to be the primary result of the reaction between Pd and Si, and this reaction can occur at temperatures as low as 200 °C. Recently, solid state reactions between Pd and Si induced by high and low energy ball milling of mixtures of Pd and Si elemental powders have also J. Mater. Res., Vol. 9, No. 1, Jan 1994
been investigated by several researchers.14"17 The main purpose of this work was to discover if an amorphous phase can be produced by mechanical alloying in the typical glass-forming composition range between 15 and 23 at. % Si, as establish
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