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 mechanical properties of Al–Si alloys are strongly related to the size, shape and distribution of eutectic silicon present in the microstructure In order to improve mechanical properties, these alloys are generally subjected to modification melt treatment, which transforms the acicular silicon morphology to fibrous one resulting in a noticeable improvement in elongation and strength. Improper melt treatment procedures, fading and poisoning of modifiers often result in the structure which is far from the desired one. Hence it is essential to assess the effectiveness of melt treatment before pouring. A much investigated reliable thermal analysis technique is generally used for this purpose. The deviation from the standard curve in thermal analysis helps in assessing the level of refinement of the Si structure. In the present review an attempt is made to discuss various aspects of modification, including mechanism, interaction of defects and non-destructive assessment by thermal analysis.

Modification of eutectic silicon in Al-Si alloys

Owing to the fast development of wireless information technologies at the high-frequency range, the electromagnetic interference problem has been of increasing significance and attracting global attention. One key solution for this problem is to develop materials that are able to attenuate the unwanted electromagnetic waves. The desired properties of these materials include low reflection loss value, wide attenuation band, light weight, and low cost. This review gives a brief introduction to graphene-based composites and their electromagnetic absorption properties. The ultimate goal of these graphene absorbers is to achieve a broader effective absorption frequency bandwidth (fE) at a thin coating thickness (d). Representative and popular composite designs, synthesis methods, and electromagnetic energy attenuation mechanisms are summarized in detail. The two key factors, impedance matching behavior and attenuation ability, that determine the electromagnetic behavior of graphene-based materials are given particular attention in this article.

A brief introduction to the fabrication and synthesis of graphene based composites for the realization of electromagnetic absorbing materials

We have produced ultrathin epitaxial graphite films which show remarkable 2D electron gas (2DEG) behavior. The films, composed of typically 3 graphene sheets, were grown by thermal decomposition on the (0001) surface of 6H-SiC, and characterized by surface-science techniques. The low-temperature conductance spans a range of localization regimes according to the structural state (square resistance 1.5 kOhm to 225 kOhm at 4 K, with positive magnetoconductance). Low resistance samples show characteristics of weak-localization in two dimensions, from which we estimate elastic and inelastic mean free paths. At low field, the Hall resistance is linear up to 4.5 T, which is well-explained by n-type carriers of density 10^{12} cm^{-2} per graphene sheet. The most highly-ordered sample exhibits Shubnikov - de Haas oscillations which correspond to nonlinearities observed in the Hall resistance, indicating a potential new quantum Hall system. We show that the high-mobility films can be patterned via conventional lithographic techniques, and we demonstrate modulation of the film conductance using a top-gate electrode. These key elements suggest electronic device applications based on nano-patterned epitaxial graphene (NPEG), with the potential for large-scale integration.

Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics

The low or lack of solubility of fullerenes, carbon nanotubes and graphene/graphite in organic solvents and water severely hampers the study of their chemical functionalizations and practical applications. Covalent and noncovalent functionalizations of fullerenes and related materials via mechanochemistry seem appealing to tackle these problems. In this review article, we provide a comprehensive coverage on the mechanochemical reactions of fullerenes, carbon nanotubes and graphite, including dimerizations and trimerizations, nucleophilic additions, 1,3-dipolar cycloadditions, Diels–Alder reactions, [2 + 1] cycloadditions of carbenes and nitrenes, radical additions, oxidations, etc. It is intriguing to find that some reactions of fullerenes can only proceed under solvent-free conditions or undergo different reaction pathways from those of the liquid-phase counterparts to generate completely different products. We also present the application of the mechanical milling technique to complex formation, nanocomposite formation and enhanced hydrogen storage of carbon-related materials.

Mechanochemistry of fullerenes and related materials

Comparison among chemical and electromagnetic stirring and vibration melt treatments for Al–Si hypereutectic alloys

Thermal analysis and microscopical characterization of Al–Si hypereutectic alloys

We report the enhanced photocatalytic activity of TiO2 by the surface decoration of CoO nanoparticles to degrade paraoxon. The CoO nanoparticles are precipitated in situ during heat treatments of Co doped amorphous TiO2 and enhance the degradation by 11–24 times that of pure TiO2 or Rh2O3–TiO2 or the oxide nanoparticles under white light (natural sunlight and fluorescence light). The enhanced activity is a combined result of visible light absorption by CoO and effective electron-hole separation at the CoO–TiO2 interfaces.

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Environmentally effective photocatalyst CoO-TiO2 synthesized by thermal precipitation of Co in amorphous TiO2

An accurate knowledge of liquidus temperatures permits the prediction of a variety of metallurgical characteristics for a given alloy, including the melt treatment and casting temperatures (superheat). Three equations to determine liquidus for hypoeutectic aluminium alloys are reported in the literature. There is, however, no algorithm for calculation of the liquidus temperature for hypereutectic Al–Si alloys. In this paper algorithms are developed to predict the liquidus temperature of both hypo and hypereutectic 3XX series of aluminium alloys, based on their chemical compositions (minor and major elements). These algorithms are based on the new “ element-equivalency concept ”. The accuracy of the calculated liquidus temperatures for 3XX hypo and hypereutectic alloys were statistically examined with the values measured by the aluminum thermal analysis platform (AlTAP).

F.C. Robles Hernández

Papers

Comparison among chemical and electromagnetic stirring and vibration melt treatments for Al–Si hypereutectic alloys

Thermal analysis and microscopical characterization of Al–Si hypereutectic alloys

Environmentally effective photocatalyst CoO-TiO2 synthesized by thermal precipitation of Co in amorphous TiO2

Calculation of the liquidus temperature for hypo and hypereutectic aluminum silicon alloys

Advanced metallurgical alloy design and thermomechanical processing for rails steels for North American heavy haul use