Milling, mechanochemical treatment

Milling — mechanochemical treatment of single or mixture of powder in mill. 

A wide variety of instrumental techniques, including X-ray diffraction, thermal analysis, electron microscopy, MAS-NMR and infrared spectroscopy, have been employed at different levels of complexity to investigate the effects of mechanochemical treatment on kaolin. Unfortunately, vibrational spectroscopy has only been used at a superficial level in the study of milled kaolin despite the considerable contribution that it has made to the understanding of the structure and reactivity of kaolin itself. 

Barothermal treatments were applied directly to catalyst extrudates. In the course of mechanochemical treatments, extrudates were physically milled, loaded into a planetary mill and extruded again after the treatment. 

Figure 1. Transmission Scanning Microscope pictures of vanadyl phyrophosphate. a) initial, and after mechanochemical treatment b) in water, c) in ethanol and d) dry milling.

Equally effective was the complexation of C70 by HSVM (lOmin, Cjq j-cyclodextrin 1 8), while the other functionalized C50 derivatives afforded water-soluble 1 1 complexes, since functional groups prevent the formation of the bicapped 1 2 complexes. By mechanochemical treatment, fullerene 50 also complexes with sulfocalix[8]arene (lOmin, equimolar amounts), and the obvious advantage of the solid-state complexation is illustrated by the complexation of sulfocalix [8]arene with fullerene dimer (also prepared by mechanosynthesis, see chapter Applications of Ball Milling in Nanocarbon Material Synthesis). Dimer is hardly soluble in most common organic solvents, but the mechanochemical treatment of an equimolar mixture of sulfocalix[8]arene and fullerene dimer for lOmin afforded bicapped complex, which is about twice more soluble in water than in commonly used ODCB. 

In order to dissolve rare earths from phosphors under mild condition, a mechanochemical treatment using a planetary mill was applied by Zhang and Saito s group (Zhang and Saito, 1998 Zhang et al., 2000). The mechanochemical treatment was shown to cause disordering of the crystal structures of the phosphor and thus enabled the leaching under mild conditions yttrium. 

Various types of mills are used for mechanochemical treatment, commonly vibratory, attrition, planetary, and tumbling ball mill. The final product depends on the milling conditions, hence, different types of mill or the alteration of milhng parameters may result in diverse reaction paths for mechanochemical reaction. Besides, the milling time necessary to reach desired structure... 

The mechanochemical treatment by ball milling is a very complex process, wherein a number of phenomena (such as plastic deformation, fracture and coalescence of particles, local heating, phase transformation, and chemical reaction) arise simultaneously influencing each other. The mechanochemical treatment is a non-equilibrium solid-state process whereby, the final product retains a very fine, typically nanocrystalline or amorphous structure. At the moment of ball impact, dissipation of mechanical energy is almost instant. Highly excited state of the short lifetime decays rapidly, hence a frozen disordered, metastable strucmre remains. Quantitative description of the mechanochemical processes is extremely difficult, herewith a mechanochemical reaction still lacks clear interpretations and adequate paradigm. 

This review attempts to give a short account of the main aspects of mechanochemical treatment such as (a) mechanical alloying (b) commonly used milling units, herewith the influence of milling parameters on the rate and products of mechanochemical treatment (c) stmctural changes (d) mechanochemical reactions (e) subsequent heat treatment and (f) powder contamination. 

FIGURE 17.3 Schematic view of common mills used for mechanochemical treatment (a) attrition ball mill (b) vibratory ball mill (c) tumbler baU miU (d) planetary ball mill. 

In recent years, considerable efforts have been taken to follow reactions induced by mechanochemical treatment and to relate them with the milling parameters. Therefrom, attempts have been made toward unification of the influence of the milling parameters on the mechanochemical reactions [70,71,95-100]. Although the mechanisms of mechanochemical that is, mechanically induced reactions is not fuUy understood the overall kinetics may be derived from suitable measurements (e.g., structural, magnetic) of powder milled for various milling times. 

Example 6 Non-equilibrium stationary state as a characteristic of mechanochemical treatment Concurrent mechanochemical treatment of powder mixture of Bi203 and Xi02 in 2 3 molar ratio and pulverized Bi4Ti30i2 compound prepared by reactive sintering shows that after some milling time, a steady-state characterized by a very disordered, amorphous-like stmcture was reached. Thus, the systems evolves toward a non-equilibrium stationary state regardless of different initial thermodynamic states. 

Example 8 Displacement reaction Displacement reaction between CuO and Ca, Ni, C, Ti, A1 or Fe, for example, 2CuO -F Ti 2Cu + Ti02, which conventionally requires high temperature to be thermally activated, was realized by mechanochemical treatment. When dry milling was carried on, after an activation period, that is, ignition time, milling induces instantaneous self-propagating reaction. 

Example 11 Oxidation reaction In the course of mechanochemical treatment of either Ti or Zr in mechanoreactor with controlled atmosphere of O2, abrupt drop of pressure was detected after several hours of milling as a consequence of instant combustion reaction Ti -F O2 Ti02 (or Zr -F O2 Zr02). Pronounced deformation as well as crystallite size reduction proceeded the reaction of oxidation. Prolonged milling following the combustion reaction produced nanocrystalline oxide powder. 

For La9 83Si4 5Fei,5026 (LSiF) ATLS preparation the solution of corresponding nitrate salts and tetraethoxysilane in ethanol was used, while other characteristic of precursor solution preparation were the same as above. The mixed solution was heated at 50°C imtil the gel formation, which was then calcined up to 700°C followed by 5 min mechanochemical treatment of formed solid precursors with ball mill AGO-2. Finally sample was calcined at 900T for 5 h. 

Borothermic reduction was confirmed at a lower temperature, and the carbothermic reduction occurred at a higher temperature (33), which favors the production of high-purity powders. TiB2 and ZrBj powders were also synthesized by mechanochemical treatment of titania and zirconia powders with amorphous boron followed by a relatively low-temperature annealing (1100°C) (34). The occurrence of polymorphic transformations has been observed with milling time for both systems, together with a decrease upon annealing of the temperature of the boro-... 

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