Mechanochemical product phases

Gradual mechanochemical reaction systems have generally been found to exhibit sigmoidal reaction kinetics. The reaction rate initially increases with milling due to increasing activation and microstructural refinement of the reactants. The reaction rate then reaches a maximum at an intermediate milling time before decreasing as the reaction approaches completion due to dilution of the reactants by the product phases. ... 

Most solid-state mechanochemical reactions have been displacement reactions, with two or more product phases being formed. The microstructure of the as-milled powders consists of a nanocomposite mixture of the product phases, with 5-20 nm crystallites of the reduced metallic phase being uniformly intermixed with an amorphous or nanocrystalline oxidized by-product phase. The separation or removal of unwanted... 

Two mechanisms of mechanochemical reactions are most likely. First, under the action of mechanical stress, intermixing occurs at the molecular level. Second, the product forms on the surface of macroscopic reacting species. Formed in the solid phase, the radicals generated recombine so that mechanolysis proceeds as a reversible reaction. However, the term reversibility should be applied only to the bond formation between radicals. For example, the structure of recombined product can be and is different from that of the starting material. It is the main feature that disturbs conventional reversibility of the radical recombination during mechanolysis. 

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. 

The main statements of the kinetics of solid-phase reactions which bind the interaction rate with the diffusion of one of components, at first over the surface of another component, and then in the bulk through product layer, are not suitable for mechanochemical reactions, since mechanical activation of a mixture involves continuous renewal of contact surfaces. On this grounds, it was assumed that the interaction rate is limited by the rate of chemical reaction at contact. 

In [27-29], hydrothermal, mechanochemical and solid-phase syntheses of calcium silicate from anhydrous and hydrated oxides were compared. Initial components were taken at Ca/Si ratio equal to 0.8 1.0 1.2. According to X-ray analysis, the interaction in the mixtures of anhydrous oxides under mechanical activation is not completed. The product being formed is X-ray amorphous. When heated at 600-800 C, it is crystallized in the form of a -Ca2Si04 (Fig. 3.6a). At higher temperatures, Ca3Si207 is formed P-CaSiOj is crystallized at 860 C. The observed sequence of stages is similar to those observed in solid-phase synthesis of wollastonite. The amount of p-CaSiOj at 900°C does not exhibit any substantial dependence on the initial fractions of the reagents. 

Fig. 6.14 shows X-ray diffraction patterns of the products obtained after activation of the mixtures of Ba02 with rutile, anatase and metatitanium acid. It follows, that mechanochemical synthesis of barium titanate occurs only if anatase and metatitanium acid are used as initial reagents. The presence of barium carbonate phase was observed for all the products. For the mixture with anatase, the amount of barium carbonate increases with activation time, and for the mixture with metatitanium acid it... 

In industry, cordierite is usually obtained by calcination of the mixtures containing talc, kaolinite and silica at 1300-1450°C for 20-60 h. The product contains the impurity phases spinel, mullite, clinoenstatite, etc., that worsen the exploitation characteristics of cordierite. Since the mentioned minerals contain structural water, chemical interaction between them during mechanical activation can be considered from the viewpoint of soft mechanochemical synthesis. Mechanical activation of this mixture does simplifies the interaction between its components. It is sufficient to heat this mixture for 2 h at a temperature of 1260°C to obtain practically homogeneous cordierite without impurity phases (Fig. 7.2) [2-9]. 

At the time Tiof setting the pendulum into motion the electrode potential of the tray AU shifts to the positive side (Fig. 4.26). This is because of the mechanochemical activation of metals and damage to the film of the products of electrochemical reactions on the tray surface. At time T2 the pendulum stops. Time At, during which the potential returns to its initial value (before actuation), is seen to reduce with increasing liquid phase in the composite. This regularity is attributed to structural features of the studied composites, which is characterized by a porous polymer matrix filled by the liquid phase liberating from the material in the process of syneresis. 

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. 

Mechanochemical reactions are extremely complex, thereby not fully understood. The reason for this probably lies in the fact that the whole numbers of elementary processes throughout the mechanical energy may be dissipated. However, the accumulation of a large number of results published enables, at least tentatively, some generalizations for most mechanochemical reactions to be made (i) reactions induced by milling takes place in non-equilibrium conditions, whereby the final product retains the non-equilibrium state, that is, the stmcture is highly disordered, typically nanocrystalline or amorphous (ii) the kinetics and final products of the mechanically induced reactions depends on the milling conditions and (iii) in many instances crystallite size reduction preceeded phase transformation or chemical reaction. 

The present applications of ceria-based ceramics impose strict requirements on the quality and purity of the powders used. Several studies have described the synthesis of ceria nanopowders of high quality and with a well-defined morphology. Typical methods of preparation include hydrothermal synthesis [263, 264], the hydrolysis of an alkoxide solution (sol-gel) ]265], chemical precipitation [266], mechanochemical processing ]267], and gas-phase reaction ]268]. Emulsion techniques can also be used, as these reduce not only the production costs of high-purity spherical powders but also the degree of aggregation. Thus, ceria powders with an average particle size <20 nm and a narrow particle size distribution can be... 

The original technology of powdered perovskite synthesis based upon mechanochemical activation (MA) of solid starting compoimds has been elaborated. It is distinguished by the simplicity, high productivity and yields highly dispersed perovskites of luiiform phase composition. 

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