Self-Propagating Reactions Induced by Mechanical Alloying

Introduction

Mechanical alloying is a “brute force” method of affecting alloying and chemical reactions. The mixture of reactant powders and several balls are placed in the milling jar of a high-energy ball mill, for example, a shaker mill or a planetary mill (Figure 1). The collisions and friction between the balls, and between the balls and the wall of the container, result in deformation, fragmentation, mixing, and cold-welding. The reactivity increases due to defect formation and increased interface area, and eventually alloying and/or chemical reactions take place. Neither additional heat nor solvent are needed. The product is a powder that can be consolidated using the usual methods of powder metallurgy. Mechanical alloying is a very flexible technique and has been used to prepare a broad variety of materials, including dispersionstrengthened alloys, amorphous alloys, and nanocomposites.¹ High-energy ball milling is also called mechanochemical processing when used, often in conjunction with other steps, for inorganic synthesis, the processing of minerals, and the activation of building materials.²

Mechanically-induced Self-propagating Reactions (MSR) are possible in highly exothermic powder mixtures.³ Initially, milling results in activation, similar to any other mechanical alloying process. But at a critical time, called the ignition time, the reaction rate begins to increase. As a result, the temperature rises, further increasing the reaction rate and eventually leading to a self-sustaining process. Most of the reactants are consumed within seconds. At this stage, the reaction is similar to thermally ignited self-propagating high-temperature synthesis (SHS).⁴ The abrupt temperature increase is detectable on the outer surface of the milling container and its presence distinguishes such mechanically induced self-propagating reactions (MSR) from gradual processes (Figure 2). MSR can happen in a broad variety of systems, such as in Fe2O3–Al, Ni–Al, Ti–C, Zn–S, and Mo–Si mixtures.³ The ignition time is an important attribute of the process; it can vary from a few seconds to several hours depending on the reaction and the milling conditions.

The investigation of MSR contributed considerably to our understanding of mechanochemical processes in general. The variation of the ignition time with process conditions and material properties tells us about the mechanism of the activation process, while detailed studies of partially activated powders provides information about the nature of the critical state. MSR has also been considered as a practical means for the production of useful materials, particularly refractory compounds.³

Requirements For Self-Sustaining Reactions

MSR (as well as SHS) require sufficient self-heating to propagate the reaction. A measure of self-heating is the adiabatic temperature, defined as the final temperature, if all the reaction heat is used to heat the products. A rule of thumb is that self-sustaining reactions are possible, if the adiabatic temperature is at least 1800 K. Since the main issue is self-heating at the beginning of the reaction, the quantity –ΔH₂₉₈/C₂₉₈ (where H₂₉₈ and C₂₉₈ are reaction enthalpy and specific heat at 298 K), written simply as ΔH/C, is often used as a simpler substitute for adiabatic temperature; ΔH/C > 2000 K is the condition for MSR. This simple condition applies surprisingly well to the most frequently studied classes of reactions, namely combination reactions between a transition metal and a metalloid element (e.g. Ti-B, Nb-C, Mo-Si, Ni-P) and thermite-type reactions between an oxide and a more reactive metal (e.g. Fe3O4-Al, CuO-Fe, ZnO-Ti). Much lower values of ΔH/C are sufficient for MSR with chalcogenides and chlorides. Table 1 contains data for a few typical reactions.

As more exothermic reactions become increasingly easy to self-sustain, reactions with higher adiabatic temperatures are expected to require shorter activation times before ignition. Such a relationship indeed exists, but only if the other material parameters and the milling conditions are very similar. So far, the best correlation was observed for the reduction of CuO (Prod. No. 203130, 450804, 450812), NiO (Prod. No. 203882, 637130, 481793), Fe₃O₄ (Prod. No. 310069, 518158, 637106), Cu₂O (Prod. No. 208825, 566284), and ZnO ( Prod. No. 204951, 255750, 544906), with the same metal (Ti, Zr or Hf).³ These are ductilebrittle systems⁵ where milling results in a fine dispersion of the oxide particles in the metal matrix. The development of the microstructure depends primarily on the ductile component and it is kept the same for each series.

The changes caused by mechanical milling during the activation period—decrease of grain size, mixing, and formation of lattice defects—depend mainly on the mechanical properties of the reactants. Although it is difficult to quantify this relationship, the increasing width of the X-ray diffraction lines indicates that the crystallite size decreases and the accumulated lattice strain of the metal component increases as the powder approaches ignition.^6,7 While reducing the grain size and thereby increasing the interface area is certainly a key component of the activation process, agglomeration is also necessary to ensure efficient m