Mechanochemical processing

When polymerization proceeds in the presence of modifiers, the mechanochemical process enhances cross-linking and, correspondingly, improves the physicochemical properties of final plastics. For example, mechanochemical treatment of acrylonitrile butadiene styrene (ABS) plastic in the presence of tolnene diisocyanate improves thermal oxidative stability of the plastic (Chetverikov et al. 2002). 

Both solid-solid and solid-gas types of reactions lead from solid reactants to a solid product without the use of solvents. Solvent-less processes, however, are not necessarily solid-state processes. Indeed, it has been argued [8d,e] that many solid-state syntheses cannot be regarded as bona fide solid-solid reactions because they occur with the intermediary of a liquid phase, such as a eutectic phase or a melt, or may require destruction of the crystals prior to reaction. This latter situation is often observed, for instance, in the case of reactions activated by co-grinding, since the heat generated in the course of the mechanochemical process can induce local melting at the interface between the different crystals, or when kneading, i.e. grinding in the presence of small amounts of solvent, takes place (vide infra). 

Certain solid phases, on the other hand, cannot be obtained (even as microcrystalline powders) by crystallization experiments, but instead can be generated only by other types of preparation procedure. Some types of preparation processes commonly (or in some cases inherently) yield microcrystaUine products, including (1) preparation of materials directly from solid-state chemical reactions (see Sect. 6.6), (2) preparation of materials by solid-state desolvation processes (see Sect. 6.4), (3) preparation of materials by solid-state grinding (mechanochemical) processes (see Sect. 6.2), and (4) preparation of materials directly by rapid precipitation from solution (as opposed to crystallization) (see Sect. 6.7). Again, structure determination from powder XRD data may represent the only opportunity for determining the structural properties of new solid phases obtained by such processes. 

F. Kh. Urakaev, V. V. Boldyrev, Mechanism and Kinetics of Mechanochemical Processes in Comminuting Devices, Powder Technd., 2000, 107 (1-2), 93. 

J. Ding, T. Tsuzuki, P. G. McCormick, Hematite Powder Synthesized by Mechanochemical Processing, NanoStructured Mater., 1997, 8 (6), 739. 

P. G. McCormick, Synthesis of Ultrafme Gadolinium Powder by Mechanochemical Processing,... 

Water is one of the products formed via mechanochemical interaction between hydroxides. Its presence can substantially effect on the mechanism of transport processes and, thus, on the kinetics of mechanochemical processes, since some reaction products (initial or final) can be dissolved in it. Hence, the reaction could proceed with the participation of the dissolved forms of components. 

The results obtained suggest that the positive role of crystallized water in solid phase synthesis of zircon from a mechanically activated mixture of hydrated oxides consists only in a reduction of the mechanical hardness of the oxides. If one of the two oxides is hydrated, the degree of its amorphization is higher. If both oxides are hydrated, then mechanochemical processes occur, simultaneously with increasing extent of amorphization, to give oxygen-saturated polyhedra of zirconium, which cannot interact with Asi and Bsi polyhedra to form zircon in the course of thermal treatment. 

Prokofyev V.Yu., Ilyin A.P., Sazanova T.V. Investigation of mechanochemical processes of the joint activation of hydrargillite and calcium compounds. Neorg. Materialy 2000 36 1076-81. 

Temuujin J., Okada K., Mackenzie K.J.D. Characterization of aluminosilicate (mullite) precursors prepared by mechanochemical process. J. Mat. Res. 1998 13 2184-88. Temuujin J., Jadambaa T.S., Okada K., Mackenzie K.J.D. Preparation of aluminosilicate precursor by silica mixtures. Bull. Mat. Sci. 1998 21 185-87. Karagedov G.R.,Liubuschko G.l. Mechanochemically stimulated synthesis of monophase mullite. Chem. Sustainable Development 1998 6 161-63. 

Fernandez-Rodriguez J.M., Morales J., Tirado J.L. Synthesis and alteration of a-LiFe02 by mechanochemical processes. J. Mat. Science. 1988 23 2971-74. 

Another type of miU commonly used in studies of mechanochemical processing is the planetary mUl. As implied by the name, the milling container is rotated about two separate paraUel axes in a manner analogous to the rotation of a planet around the sun. The miUing action of a planetary mill is similar to that of a conventional horizontal tumbling mUl. However, the velocity of the grinding media is not limited by centrifugal forces. 

Attritor miUs consist of a stationary container filled with grinding balls that are stirred by impellers attached to a drive shaft. The velocity of the grinding media in attritor mUls is significantly lower than that in planetary or Spex-type mills and consequently the energy available for mechanochemical processing is lower. However, imlike planetary and Spex-type mills, attritors are readily amenable to scale-up, which allows mass production of powders through mechanochemical processing. ... 

Mechanochemical processing has been used to manufacture nanocrystalline powders of nitride and carbide ceramics. The majority of systems involve milling of the metal precursor with a source of carbon or nitrogen. The source of carbon or nitrogen has typically taken the form of the element itself. However, a variety of other reagents have also been used. For example, Zhang et al. reported the synthesis of titanium nitride by milling titanium metal with pyrazine in a benzene solution. 

FIGURE 3.4 Ultraviolet and visible spectra of aqueous suspensions of ZnO particles that were synthesized by mechanochemical processing. 

The mechanochemical processing technique provides the means to produce chemical and structural changes in a reactant powder charge at low temperatures. This... 

Dodd, A.C., Raviprasad, K., and McCormick, P.G., Synthesis of ultrafine zirconia powders by mechanochemical processing, Scripta Mater, 44, 689, 2001. 

Tsuzuki, T., and McCormick, PG., Synthesis of CrjOj nanoparticles by mechanochemical processing, Acta Mater, 48, 2795, 2000. 

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