Francesco Delogu

Hallmarks of mechanochemistry: from nanoparticles to technology

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).

The invariant laws of the amorphization processes by mechanical alloying: I. Experimental findings

Abstract Several Ti-, Zr-, Hf- and Nb-based alloys were synthesized using differing milling regimes. Metastable phases, either crystalline or amorphous, develop from the parent elements according to a general sigmoid-shaped behaviour ruled by an interface-controlled kinetic mechanism. The extent of the alloying reactions was related to the operating variables, experimentally determined in the course of the process. Although the transformation rates depended on the milling intensity, that is on the impact energy times the impact frequency, it was found that the reaction yield, defined by the ratio of the transformed fraction to the specific energy dose, is an invariant quantity characteristic of each system. The specific energy dose defines the mechanical work done on the system per mass unit of the reactants. A rationale for the observed behaviours was provided by the energy needed to reach a given level of the reactant dispersion. Ruling the total extent of the grain boundary area, and the overall kinetics of the alloying process, the work expended in the microstructure refinement was found to be another invariant property of the treated mixtures. The reaction yield is the reference parameter to compare milling trials on an absolute basis, so providing an opportunity towards a quantitative understanding of the mechanical alloying processes.

Forced chemical mixing in model immiscible systems under plastic deformation

Molecular dynamics has been employed to investigate forced chemical mixing in binary immiscible systems induced by plastic deformation. Four X matrix-Y precipitate model systems with thermodynamic and structural features almost identical to the ones of Ag–Cu solid solutions but different mechanical properties were generated. With the positive enthalpy of mixing roughly constant, mixing is shown to depend on the precipitate size as well as on the difference between X and Y tetragonal shear moduli.

A novel macrokinetic approach for mechanochemical reactions

Mechanical treatment by ball milling (BM) is an extremely versatile technique allowing for the synthesis of non-equilibrium structures and the production of nanostructured alloys for advanced materials science technology as well as the degradation of toxic compounds and the conventional mineral processing. In the present paper, we first review the recent developments in basic research on the kinetics of phase transformation under milling. Secondly, we propose a novel phenomenological macrokinetic approach and we test its capability of reproducing experimental data available in the literature. It was shown that the degree of conversion during BM can be correlated with the milling time and the ball to powder mass ratio. The proposed approach may provide a useful framework for the rationalisation of experimental results as well as the opportunity for further progress in the field of mechanochemical processing.

Demixing phenomena in NiAl nanometre-sized particles

Numerical simulations have been used to investigate the melting behaviour of unsupported Al, Ni and NiAl particles in the approximate size range between 1 and 10 nm. A depression of melting points, more pronounced for NiAl rather than for Ni particles, was observed. The melting point of NiAl particles with a size below 4 nm is found to be lower than that of the corresponding Ni particles. Under such circumstances, local fluctuations in the distribution of atomic species within the particles promote a demixing process of Ni and Al species. Particles consisting of a solid Ni-rich interior and a molten Al-rich surface layer are thus formed.

Onset of chaotic dynamics in a ball mill: Attractors merging and crisis induced intermittency

In mechanical treatment carried out by ball milling, powder particles are subjected to repeated high-energy mechanical loads which induce heavy plastic deformations together with fracturing and cold-welding events. Owing to the continuous defect accumulation and interface renewal, both structural and chemical transformations occur. The nature and the rate of such transformations have been shown to depend on variables, such as impact velocity and collision frequency that depend, in turn, on the whole dynamics of the system. The characterization of the ball dynamics under different impact conditions is then to be considered a necessary step in order to gain a satisfactory control of the experimental set up. In this paper we investigate the motion of a ball in a milling device. Since the ball motion is governed by impulsive forces acting during each collision, no analytical expression for the complete ball trajectory can be obtained. In addition, mechanical systems exhibiting impacts are strongly nonlinear due to sudden changes of velocities at the instant of impact. Many different types of periodic and chaotic impact motions exist indeed even for simple systems with external periodic excitation forces. We present results of the analysis on the ball trajectory, obtained from a suitable numerical model, under growing degree of impact elasticity. A route to high dimensional chaos is obtained. Crisis and attractors merging are also found. (c) 2002 American Institute of Physics.

Melting behaviour of a pentagonal Au nanotube

Molecular dynamics simulations have been employed to investigate the melting behaviour of an Au nanotube with pentagonal symmetry. The hollow pentagonal prism consists of five crystalline domains with (100) facets separated by grain boundaries. The atoms located at the edges, at the free surfaces, at the grain boundaries and in the bulk of the nanotube have different structural arrangement and therefore different thermal stability. A sequence of pre-melting transitions takes place progressively involving edges, surfaces and grain boundaries. Within a narrow temperature range, the molten phase forms a connected three-dimensional diffusion path permitting the exchange of atoms between external and internal surfaces. Bulk atoms finally melt at a temperature significantly lower than the equilibrium melting point.

Combustion synthesis of metal carbides: Part I. Model development

The definition of a rigorous theoretical framework for the appropriate physico-chemical description of self-propagating high-temperature synthesis (SHS) processes represents the main goal of this work which is presented in two sequential articles. In this article, a novel mathematical model to simulate SHS processes is proposed. By adopting a heterogeneous approach for the description of mass transfer phenomena, the model is based on appropriate mass and energy conservation equations for each phase present during the system evolution. In particular, it takes microstructural evolution into account using suitable population balances and properly evaluating the different driving forces from the relevant phase diagram. The occurrence of phase transitions is treated on the basis of the so-called enthalpy approach, while a conventional nucleation-and-growth mechanistic scenario is adopted to describe quantitatively the formation of reaction products. The proposed mathematical model may be applied to the case of combustion synthesis processes involving a low melting point reactant and a refractory one, as for the synthesis of transition metal carbides from pure metal and graphite. Thus, the model can be profitably used to gain a deeper insight into the microscopic elementary phenomena involved in combustion synthesis processes through a suitable combination of experimental and modeling investigations, as it may be seen in Part II of this work [J. Mater. Res. 20, 1269 (2005)].

Melt-driven mechanochemical phase transformations in moderately exothermic powder mixtures

Usually, mechanochemical reactions between solid phases are either gradual (by deformation-induced mixing), or self-propagating (by exothermic chemical reaction). Here, by means of a systematic kinetic analysis of the Bi-Te system reacting to Bi2Te3, we establish a third possibility: if one or more of the powder reactants has a low melting point and low thermal effusivity, it is possible that local melting can occur from deformation-induced heating. The presence of hot liquid then triggers chemical mixing locally. The molten events are constrained to individual particles, making them distinct from self-propagating reactions, and occur much faster than conventional gradual reactions. We show that the mechanism is applicable to a broad variety of materials systems, many of which have important functional properties. This mechanistic picture offers a new perspective as compared to conventional, gradual mechanochemical synthesis, where thermal effects are generally ignored.

Ignition of an exothermal reaction by collision between Al and Ni crystals

Classical molecular dynamics methods have been used to investigate the atomic-scale dynamics of collisions between two Al and Ni crystals with rough surfaces. The crystals were approached along the direction perpendicular to the surfaces and simultaneously displaced along the direction parallel to them at relative velocities in the range between 1 and 10 nm ns−1. The mechanical stresses operating at collision determine a local deformation of Al and Ni lattices, accompanied by a significant temperature rise. As the Al melting point is reached, the Al crystal partially melts and Ni atoms start dissolving into the molten phase. The significant heat of mixing liberated further promotes the Al melting and the Ni dissolution processes. In the absence of neighboring Al-Ni interfaces, the heat dissipation processes and the limited rate of Ni dissolution gradually lead to the extinction of the reactive behavior. Conversely, the presence of Al-Ni interfaces in the vicinity of the Al-Ni one formed by collision permi...