L. Takacs

Self-sustaining reactions induced by ball milling

Ball milling induces self-sustaining reactions in many sufficiently exothermic powder mixtures. The process begins with an activation period, during which size reduction, mixing, and defect formation take place. The MSR (mechanically induced self-propagating reaction) is ignited when the powder reaches a well defined critical state. Once started, the reaction propagates through the powder charge as a combustion process. In this paper, the current knowledge on MSR is reviewed from both experimental and theoretical points of view. Experimental results on a broad variety of systems are examined and compared. The variation of the ignition time with composition and milling conditions is investigated. Some unusual phenomena, such as the mutual suppression of combustion in mixed metal-chalcogen systems, are discussed. The mechanism of MSRs is extremely complex, with important processes on several length and time scales. The key objective is to understand ignition and the changes during the activation process that lead to ignition. Combining models with systematic empirical studies appears to be the most realistic approach to a detailed understanding of MSR processes.

Mechanical response and modeling of fully compacted nanocrystalline iron and copper

Abstract A comprehensive study on the response of nanocrystalline iron and copper to quasi-static and dynamic loading is reported. Bulk solid nanocrystalline iron and copper specimens used in static and dynamic loading experiments were made by compaction and hot sintering of the nanocrystalline powders. The powders, with grain size 16–96 nm, were obtained by using high energy ball milling. The stress/strain response of dense nanocrystalline iron is found to be grain size and strain rate dependent. The KHL model is modified by incorporating Hall–Petch relation (i.e. yield stress dependence on grain size) and is used to represent the behavior of fully compacted nanocrystalline material. A good correlation with the experimental results is demonstrated.

Reduction of magnetite by aluminum: a displacement reaction induced by mechanical alloying

Abstract The displacement reaction between aluminum and magnetite during mechanical alloying has been investigated. Explosive solidstate reaction occurs in a wide range of compositions as evidenced by the sudden temperature rise of the grinding vial. X-ray diffraction and scanning electron microscopy have been used to investigate the reaction products. The reaction takes place in three steps: 1. (i) composition-dependent incubation period; 2. (ii) high-temperature combustion during which a nonuniform mixture of final and intermediate phases forms; and 3. (iii) gradual phase transformation and particle refinement upon continued milling.

Combustion Phenomena Induced by Ball Milling

Ball milling induces self-sustaining reactions in sufficiently exothermic powder mixtures. Combustion occurs after some activation time, when the powder reaches a well defined critical state. Investigating the nature of this state and the processes leading to ignition are used as a vehicle to learn about the mechanism of mechanochemical reactions in general. A possible framework is proposed to describe the process. The effects of a single collision play the central role. The more fundamental atomic scale events and the global kinetics of the milling process are also considered. Experimental investigations of the ignition of combustion are reviewed with emphasis on systematic studies relating the reaction parameters and the ignition time. Unusual combustion phenomena such as the interrupted combustion effect and the mutual suppression of combustion are discussed.

Metal-metal oxide systems for nanocomposite formation by reaction milling

Abstract Displacement reactions between a metal oxide and a more reactive metal can be induced by high energy ball milling. The reaction may progress gradually, producing a nanocomposite powder. The mechanical agitation may also initiate combustion in highly exothermic systems, melting the reaction mixture and destroying the ultrafine microstructure. In order to avoid this problem, reaction couples with a smaller driving force have been investigated. The role of intermediate phases in understanding the mechanism of these mechanochemical processes is emphasized. The reduction of Cr 2 O 3 by aluminum or zinc and the reduction of Fe 3 O 4 by zinc are identified as promising candidates for furthern investigations.

Iron-alumina nanocomposites prepared by ball milling

Small magnetic particles of iron embedded in an insulating alumina matrix have been prepared by ball milling, either by direct milling of a mixture of iron and alumina powders or indirectly by ball-milling enhanced displacement reaction between magnetite and aluminum metal. The average particle size could be reduced to the 10-nm range as indicated by X-ray diffraction linewidths and SEM (scanning electron microscopy). The change in the saturation magnetization and the coercivity relates to the change of the phase composition, decrease of the particle size, and accumulation of internal stresses. >

Self-sustaining reactions induced by ball milling: An overview

When a highly exothermic powder mixture is activated in a ball mill, a self-propagating process can be ignited after a certain activation time. Exploring the effects of material properties and milling conditions on the ignition time, combined with characterization of reactant mixtures approaching the critical state at ignition, provides useful information on the mechanical activation process. After ignition, the reaction propagates thermally, similar to an SHS process. In this paper, the principles of mechanically induced self-sustaining reactions (MSR) are summarized and their relationships with mechanochemistry and SHS are discussed. Numerous examples are given, some interesting from the point of view of fundamental understanding of the process, others are promising as the bases of practical applications.

Nanocomposite formation in the Fe3O4‐Zn system by reaction milling

Magnetic nanocomposites of small iron particles embedded in nonmagnetic zinc oxide matrix have been prepared by ball milling, with an in situ displacement reaction between a metal oxide (Fe3O4) and a more reactive metal (Zn). The phase composition of the samples has been analyzed by x‐ray diffraction. Metallic zinc disappears during the first 100 min of milling and the magnetization decreases to almost zero, indicating the formation of a nonmagnetic intermediate iron‐zinc oxide phase. This intermediate phase decomposes into iron and ZnO upon further milling. The change in magnetic properties also reflects the decreasing size of the iron particles. The final particle size is about 9 nm, as estimated from x‐ray diffraction linewidth measurements. The final product of the process is a semihard magnetic material with a room‐temperature saturation magnetization of 40 emu/g and coercivity of 400 Oe. A significant fraction of the final Fe particles is superparamagnetic.

Multiple combustion induced by ball milling

An unusual multiple combustion effect has been observed during the mechanochemical reaction of zirconium and sulphur powders. Ball milling induces combustion in this highly exothermic system, but the combustion is quenched repeatedly, after consuming only a fraction of the reactants. The process concludes with a final, more energetic combustion, releasing about 70% of the reaction heat. The evaporation of sulfur at the reaction site may explain this behavior.

Preparation of multicomponent oxides by mechanochemical methods

A large variety of synthesis strategies and processing techniques are currently being used to obtain new multicomponent oxides and/or modify existing ones. Among them, mechanochemical processing has become very popular because it is simple to implement, solvent free, and capable of providing enough volume of the target material in an economically viable manner. The preparation of complex oxides can benefit from mechanochemical methods for two important reasons: First, it is not a diffusion-controlled process and thus, high-rate solid state reactions can be promoted between oxides with different physical and chemical properties without using high temperatures; secondly, because reactants are processed under non-equilibrium conditions, uncommon metastable phases are frequently obtained featuring flexible crystal structures, small particle size, high concentration of defects, and off-stoichiometry. Furthermore, conversion to the “true” equilibrium phases induced by additional processing (e.g., firing) offers the possibility of isolating fairly stable intermediate states with unusual and desirable properties that are inaccessible for more conventional processing techniques. As oxide particles are hard and brittle, the number of oxide systems prepared by means of mechanochemical methods grew rapidly only in recent years when more powerful milling devices and abrasion-resistant milling tools became available. This article summarizes recent work carried out in the field; only dry milling of oxides (and occasionally carbonates) in the absence of additives is considered. Some of the main challenges of mechanochemical processing are also highlighted and discussed.