The Decomposition of Ag Oxalate in Ball Drop, Rod Drop, and Ball Milling Experiments: A Tentative Estimation of the Volu
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I.
INTRODUCTION
STARTING from the successful obtainment of oxide dispersion-strengthened superalloys in 1970s,[1] and the synthesis of amorphous alloys and nanostructured phases in 1980s,[2–5] the mechanical treatment of powders by ball milling (BM) has attracted increasing interest. On the one hand, it has been shown to be a very useful method to prepare advanced materials, and tailor their properties through a suitable refinement of their composition and microstructure.[6,7] On the other hand, its capability of inducing phase transformations and chemical reactions under processing conditions significantly different from thermal ones posed intriguing fundamental questions.[8–10] For these reasons, in the last 40 years, BM has been intensely scrutinized to explore its whole potential as a synthetic method, and to elucidate its intimate mechanisms.[5–12] In spite of this, the latter objective is still far from being achieved. Based on the occurrence of collisions between milling tools inside a batch reactor,[11,12] BM is only apparently simple. At any given collision, only a small fraction of the total powder charge is trapped between the surfaces of the colliding milling tools, and submitted to high-rate nonhydrostatic mechanical stresses.[13–15] The distribution of these stresses is inherently inhomogeneous, being dependent on the spatial configuration of the network of contacts between the powder particles.[16] The resulting mechanical deformation promotes unusual local mass transport processes, which in turn can activate physical and chemical transformations.[17–27] FRANCESCO DELOGU, Assistant Professor, is with the Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Universita` degli Studi di Cagliari, piazza d’Armi, 09123 Cagliari, Italy. Contact e-mail: [email protected] Manuscript submitted May 16, 2012. Article published online November 9, 2012. 166—VOLUME 44B, FEBRUARY 2013
From the above mentioned scenario, it clearly emerges that BM involves processes spanning all of the different length scales between atoms and macroscopic objects. Of course, the characteristic features of BM methods make the study of atomistic processes quite difficult. However, our understanding of BM is limited even at the macroscopic level. This is because considerable difficulties arise also in the evaluation of basic processing parameters such as the frequency of collisions, the amount of material trapped at collision, and the energy transferred to powders at collision. In turn, this can be ascribed to the fact that the experimental evaluation of such quantities requires developing relatively complicated methodological approaches specific to the ball mill employed. In the case of the commercial Spex Mixer/Mill 8000, it has been shown that the best control of processing conditions can be obtained by means of a single milling ball, and relatively high powder charges.[28–30] These conditions allow for the occurrence of quasi-inelastic collisions between ball and reactor, which result in a regular ball displacement be
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