Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

The aim of this review article on recent developments of mechanochemistry (nowadays established as a part of chemistry) is to provide a comprehensive overview of advances achieved in the field of atomistic processes, phase transformations, simple and multicomponent nanosystems and peculiarities of mechanochemical reactions. Industrial aspects with successful penetration into fields like materials engineering, heterogeneous catalysis and extractive metallurgy are also reviewed. The hallmarks of mechanochemistry include influencing reactivity of solids by the presence of solid-state defects, interphases and relaxation phenomena, enabling processes to take place under non-equilibrium conditions, creating a well-crystallized core of nanoparticles with disordered near-surface shell regions and performing simple dry time-convenient one-step syntheses. Underlying these hallmarks are technological consequences like preparing new nanomaterials with the desired properties or producing these materials in a reproducible way with high yield and under simple and easy operating conditions. The last but not least hallmark is enabling work under environmentally friendly and essentially waste-free conditions (822 references).

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Hallmarks of mechanochemistry: From nanoparticles to technology

The field of nanoparticle delivery systems for nutrients and nutraceuticals with poor water solubility has been expanding, almost exponentially, over the last five years, and some of these technologies are now in the process of being incorporated in food products. The market projections for these technologies suggest a multifold increase in their commercial potential over the next five years. The interest in the pharmaceutical and food-related applications of these technologies has sparked tremendous developments in mechanical (top-down) and chemical (bottom-up) processes to obtain such nanoparticle systems. Mechanical approaches are capable of producing nanoparticles, typically in the 100–1000 nm range, whereas chemical methods tend to produce 10–100 nm particles. Despite these technological developments, there is a lack of information regarding the basis of design for such nanoparticle systems. Fundamental thermodynamic and mass transfer equations reveal that, in order to generate a broad spectrum delivery system, nanoparticles with 100 nm diameter (or less) should be produced. However, experimental data reveal that, in some cases, even nanoparticles in the 100–1000 nm range are capable of producing substantial improvement in the bioavailability of the active ingredients. In most cases, this improvement in bioavailability seems to be linked to the direct uptake of the nanoparticle. Furthermore, direct nanoparticle uptake is controlled by the size and surface chemistry of the nanoparticle system. The use of this direct nanoparticle uptake, in particular for soluble but poorly absorbed ingredients, is one of the areas that needs to be explored in the future, as well as the potential side effects of these nanoparticle carriers.

Bioavailability of nanoparticles in nutrient and nutraceutical delivery

Mechanically alloyed nanocomposites

Zinc oxide nanoparticles: Synthesis, antiseptic activity and toxicity mechanism.

Different procedures for analysis of particle sizes by the X-ray diffraction method are compared by the example of nanoparticles of nickel and iron(3+) oxide (Fe2O3). A modified Warren-Averbach method is proposed for the analysis of the X-ray diffraction line profile based on the approximation by the Voigt function, which yields stable solutions, and the efficiency of the method is shown. The analysis within the frame-work of the Warren-Averbach method makes it possible to restore the distribution function of nanoparticles (crystallites) over true diameters, which satisfactorily correlates with electron microscopy data. The applicability of the Warren-Averbach method to the estimation of crystallite sizes by the analysis of a single diffraction line is substantiated. The range of the applicability of the Scherrer, Williamson-Hall, Warren-Averbach, and modified Warren-Averbach methods to the substructure analysis by the X-ray diffraction is determined as depending on the method of nanostructure formation.

Determination of nanoparticle sizes by X-ray diffraction

The processes of mechanical alloying of iron and sp-elements (C, B, Al, Si, Ge, Sn) under identical conditions of mechanical treatment have been studied. General regularities and differences in the mechanisms and kinetics of solid state reactions have been ascertained. A microscopic model of mechanical alloying in these systems is suggested.

Mechanical alloying in binary Fe-M (M = C, B, Al, Si, Ge, Sn) systems

Structure and magnetic properties of Fe100-xSnx (3.2 < × < 62) alloys obtained by mechanical milling

Mossbauer spectroscopy (MS), X-ray diffraction, transmission electron microscopy (TEM), Auger spectroscopy, magnetic measurements and differential thermal analysis (DTA) were used to study the initial stages of mechanical alloying in the Fe–C system. The mechanical alloying of Fe and C powder mixtures with the atomic ratio 85:15 in a planetary ball mill was taken as an example. It has been shown that the initial stage involves the formation of nanostructure in the α-Fe particles, penetration and segregation of carbon along the α-Fe grain boundaries, formation of the amorphous Fe–C phase in the interface regions consisting of the boundary and close-to-boundary distorted zone. The carbon concentration in the amorphous Fe–C phase is estimated at 25 at.%. The amount of the amorphous phase reaches 67 at.% (67% of all the Fe and C atoms are in this phase).

G. A. Dorofeev

Papers

Determination of nanoparticle sizes by X-ray diffraction

Mechanical alloying in binary Fe-M (M = C, B, Al, Si, Ge, Sn) systems

Structure and magnetic properties of Fe100-xSnx (3.2 < × < 62) alloys obtained by mechanical milling

Initial stage of mechanical alloying in the Fe–C system

The influence of a surfactant on the characteristics of the iron powders obtained by mechanical milling in organic media