Mechanical energy can set off chemical changes in many different situations. For example, wear of a material involves chemical processes, corrosion is influenced by lattice defects created by plastic deformation, etc. We are primarily interested in mechanochemical reactions induced by high energy ball milling. The processes taking place in a ball mill are very complex. One of our primary goals is to understand their mechanism by separating and modelling their individual components.
An interesting situation is when ball milling induces a self-supporting thermal reaction in a highly exothermic system. We hope that investigating the conditions of ignition will be a useful tool to learn about the mechanism of mechanochemical reactions in general.
L. Takacs, "Reduction of Magnetite by Aluminum: a Displacement Reaction Induced by Mechanical Alloying," Materials Letters 13 (1992) 119-124.
The ball milling assisted solid state reaction between magnetite and aluminum consists of three distinct steps: (i) The incipient stage, when mixing, size refinement and accumulation of defects prepares ignition. Further studies are needed to understand this step and the requirements for ignition to occur. (ii) Combustion, when the reaction progresses in a self sustaining manner at high temperature. (iii) Particle refinement and possible gradual phase transformation upon continued milling. Off-stoichiometry prolongs the incipient stage of the reaction and influences the reaction products. Slight reduction of the amount of Al results in the formation of FeAl2O4 instead of Al2O3; excess Al suppresses the formation of metastable g-Al2O3 and FeAl2O4 and forms random a-FeAl solid solution.
L. Takacs, "Nanocomposite Formation and Combustion Induced by Reaction Milling," in: "Nanophase and Nanocomposite Materials," eds. S. Komarnemi, J. C. Parker, and G. J. Thomas (MRS Symp. Proc. Vol. 286, 1993) p. 413-418.
Displacement reactions between a metal oxide and a more reactive metal can be induced by ball milling. In some cases the reaction progresses gradually and a metal/metal-oxide nanocomposite is formed. Ball milling may also initiate a self propagating combustive reaction. The information available about these processes is reviewed. It is argued that the gradual or combustive nature of the reaction depends on thermodynamic parameters, the microstructure of the reaction mixture, and the way they develop during the milling process.
L. Takacs, "Metal-Metal Oxide Systems for Nanocomposite Formation by Reaction Milling," Nanostructured Mater. 2 (1993) 241-249.
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 Cr2O3 by aluminum or zinc and the reduction of Fe3O4 by zinc are identified as promising candidates for further investigations.
L. Takacs, H. Pollak and H. Dlamini, "Effect of Composition on the Solid State Reaction of Magnetite with Al and Mg Induced by Mechanical Alloying," Conference Proceedings Vol. 50, "ICAME-95," ed. I. Ortalli (SIF, Bologna, 1996) pp. 149-152.
The reduction of magnetite by Al and Mg has been induced by mechanical alloying. Milling initiates self propagating thermal "explosion" in these systems after some incubation period. Moessbauer spectroscopy and x-ray diffraction have been used to investigate the reaction products. A mixture of a-Fe, FeAl2O4, a-Al2O3, g-Al2O3 and a small percentage of the starting materials have been found when the stoichiometric reaction was stopped seconds after explosion. The metastable intermediates transformed gradually to a-Al2O3 and a-Fe upon further milling. When the amount of Al was decreased below stoichiometry, the incubation time increased and a large fraction of Fe remained oxidized in the FeAl2O4 phase. Solid solutions of Fe and Al were formed in the presence of extra Al. The reduction of magnetite by Mg is similar, but no Fe-Mg solid solution forms. The shortest incubation time before explosion was measured close to the stoichiometric composition for the reduction of magnetite by Al but far from stoichiometry when Mg was used.
L. Takacs and M.A. Susol, "Gradual and Combustive Mechanochemical Reactions in the Sn Zn-S System," J. Sol. State Chem. 121 (1996) 394-399.
The mechanochemical formation of ZnS from a mixture of Zn and S powders turns into a self propagating thermal reaction after 30 min of milling. A similar RexplosionS occurs when Sn and S are milled for about the same time to form SnS2. However, the ignition time is longer or the reaction is gradual in mixtures of the two systems. This phenomenon is investigated as a function of composition. A model is proposed based on the thermodynamic properties and mechanochemical reactions of the binary systems.
L. Takacs, "Combustive Mechanochemical Reactions with Titanium, Zirconium, and Hafnium," Materials Science Forum, 225-227 (1996) 553-558.
The ignition of self propagating high temperature reactions by ball milling has been investigated for reactions of Ti, Zr, and Hf with Cu2O, Fe3O4, S, B, and C. The ignition times are much shorter when Zr is reacted with oxides or sulfur than for similar reactions with either Ti or Hf. However, the ignition time varies little among the three metals when carbides and borides are prepared from the elements. It is suggested that this behavior is related to the fast diffusion of oxygen in Zr oxides and sulfur in Zr sulfides, and the lack of similar behavior in borides and carbides. The above results are the first attempt to correlate ignition time with a property other than reaction heat or adiabatic temperature.
L. Takacs and M. A. Susol, "Combustive Mechanochemical Reactions in Off-Stoichiometric Powder Mixtures," Materials Science Forum, 225-227 (1996) 559-562.
Ball milling induces combustion during the synthesis of tin and zinc sulfides from the elements. The mechanochemical reduction of magnetite by magnesium also becomes combustive. These processes have been investigated as a function of the composition of the initial powder mixture. Contrary to expectations, the shortest activation times before ignition are found at compositions far from the stoichiometry of the product phases. The ignition time depends on solid state reactions at the interfaces, reaction heat, and heat transfer, rather than the bulk composition of the powder mixture.
L. Takacs, "Ball Milling Induced Combustion in Powder Mixtures Containing Titanium, Zirconium, or Hafnium," J. Sol. State Chem. 125 (1996) 75-84.
Ball milling induces self propagating high temperature reactions in many highly exothermic powder mixtures. This phenomenon has been studied in a variety of reactions with titanium, zirconium, and hafnium. Several oxides (CuO, Cu2O, NiO, Fe3O4, and ZnO) were reduced with Ti, Zr, and Hf and the borides, carbides, silicides, and sulfides of these metals were prepared from elemental mixtures. The ignition time is much shorter with Zr than with either Ti or Hf whenever oxygen or sulfur is involved in the reaction, but no similar variation is observed for the formation of borides, carbides, and silicides. It is suggested that the fast diffusion of oxygen in ZrO2 and very likely of sulfur in ZrS2 are responsible for this behavior.
L. Takacs, "Multiple Combustion Induced by Ball Milling," Appl. Phys. Letters, 69 (1996) 436-438.
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.
L. Takacs, "Solid State Reactions Induced by Ball Milling," Hyperfine Interactions 111 (1998) 245-250.
Ball milling can be used to induce solid state reactions in a variety of technologies, including the activation of silicates, inorganic synthesis, and mechanical alloying. Mossbauer spectroscopy is a powerful tool to study these processes. Some typical examples are discussed, in this paper, concerning disordering, alloying, and simple chemical reactions. Many more industrial applications are possible, with ample opportunity for meaningful Mossbauer investigations.
L. Takacs, R. C. Reno, and M. Pardavi-Horvath, "Mechanochemical Transformations in the Zn-Magnetite System," Hyperfine Interactions 112 (1998) 247-250.
Mixtures of magnetite and zinc powders were milled for up to 540 minutes and the development of the system was followed using X-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The process takes place in two over lapping steps. During the first hour of milling, a nonmagnetic intermediate mixed oxide phase forms which decomposes into ZnO and Fe upon further milling. The freshly formed iron particles are supersaturated with Zn.
L. Takacs, "Combustion Phenomena Induced by Ball Milling" Materials Science Forum, 269-272 (1998) 513-522, invited.
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.
A. Bakhshai and L. Takacs, "Direct Correlation Between the Combustion Region of (x)Zn+(1-x)Sn+(2-x)S Reaction and the Total Powder Mass in a Ball Milling Experiment," Bull. Amer. Phys. Soc. 43, 1 (1998) 912.
Abstract not available.
L. Takacs, V. K. Garg, "Mössbauer Studies of Ball Milled Fe1-xS1+x," Mössbauer Spectroscopy in Materials Science, eds. M. Miglierini and D. Petridis, Kluwer Academic Publishers, 1999, pp. 143-150.
Abstract not available.
A. Bakhshai and L. Takacs, "Direct Correlation Between the Combustion Region of (x)Zn+(1-x)Sn+(2-x)S Reaction and the Total Powder Mass in a Ball Milling Experiment," Bull. Amer. Phys. Soc. 43, 1 (1998) 912.
Abstract not available.
L. Takacs, "Quicksilver from Cinnabar: The First Documented Mechanochemical Reaction?" JOM (Journal of Metals) 52 (2000) 12-13.
Theophrastus' book "on Stones", written in the fourth century B.C., is the earliest known work on minerals, their properties, and applications. The book is full of interesting information compiled in a clear, easy-to-read style. The excerpt examined in this article is especially important, as it represents the first known mechanochemical reaction, as well as the first description of any process for obtaining a pure metal from a compound.
Full text available.
L. Takacs and S. K. Mandal, "Preparation of Some Metal Phosphides by Ball Milling," 10th International Conference on Rapidly Quendhed and Metastable Materials, Bangalore, India, Aug. 23-27, 1999. Materials Science and Engineering, in press.
Abstract not available.
A. Bakhshai, V. Soika, M. A. Susol, and L. Takacs, "Mechanochemical Reactions in the Sn Zn-S System: Further Studies," Journal of Solid State Chemistry, submitted.
Abstract not available.
L. Takacs, "Self-Sustaining Reactions Induced by Ball Milling," Progress in Materials Science, invited review, submitted.
Abstract not available.
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