Mechanical alloying was originally invented as a method to manufacture oxide dispersion strengthened nickel alloys. It is a high energy ball milling process, where alloying is the result of the repeated fracture and cold welding of the component particles. Highly metastable materials such as amorphous alloys and nanostructured materials can be prepared by the process. Scaling up to industrial quantities seems straightforward.
In addition to attrition and agglomeration, high energy milling can induce chemical reactions, which can be used to influence the milling process and the properties of the product. This fact was utilized to prepare magnetic oxide-metal nanocomposites via mechanically induced displacement reactions between a metal oxide and a more reactive metal.
High energy ball milling can also induce chemical changes in non-metallurgical brsystems, including silicates, minerals, ferrites, ceramics, and organic compounds. The research area of mechanochemistry developed to study and utilize these processes. As many mechanical alloying processes involve chemical changes, the distinction between mechanical alloying and mechanochemistry is often arbitrary.
The following is a list of my publications on mechanical alloying. Their abstracts can be viewed by clicking on the serial number of the paper in the list. If you would like to receive a hard copy of the full paper, send me an e-mail request with full mailing address to takacs@umbc.edu .
1.
L. Takacs, "Reduction of Magnetite by Aluminum: a
Displacement Reaction Induced by Mechanical
Alloying," Materials Letters 13 (1992) 119-124.
The displacement reaction between aluminum and magnetite during mechanical alloying has been investigated. Explosive solid state 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: (i) composition- dependent incubation period; (ii) high temperature combustion during which a nonuniform mixture of final and intermediate phases form; and (iii) gradual phase transformation and particle refinement upon continued milling.
2.
M. Pardavi-Horvath and L. Takacs, "Iron-Alumina
Nanocomposites Prepared by Ball Milling," IEEE
Trans. Magn. 28 (1992) 3186-3188.
Small magnetic particles of iron, embedded in 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 ben reduced to the ten nm range as indicated by x-ray diffraction linewidths and SEM. The change of the saturation magnetization and the coercivity relates to the change of the phase composition, decrease of the particle size and accumulation of internal stress.
3.
Yanxia Lu, R. Reno, and L. Takacs, "A Mossbauer Study
of Nanophase Iron Produced by
Mechanical Alloying," in: "Nanophase and
Nanocomposite Materials," eds. S. Komarnemi, J.
C. Parker, and G. J. Thomas (MRS Symp. Proc.
Vol. 286, 1993) pp. 215-220.
Ball milling has been used to mechanically induce the displacement reaction 3 Fe3O4 + 8 Al >> 9 Fe + 4 Al2O3, in which fine magnetite particles are reduced to nanophase iron particles. The presence of initial, intermediate, and final phases are easily identified by Mossbauer spectroscopy on samples taken at various stages of the milling process.
4.
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.
5.
M. Pardavi-Horvath and L. Takacs, "Magnetic
Properties of Copper-Magnetite Nano-composites
Prepared by Ball Milling," J. Appl. Phys. 73 (1993)
6958-6960.
Small particles of Fe3O4, embedded in a Cu matrix, have been prepared both by direct milling of a mixture of Cu and Fe3O4 and by reaction milling of CuO and Fe. Conclusions are drawn about the details of the technological process and microstructural changes, based on the measurement of magnetization, coercivity, remanence, and switching field distribution.
6.
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.
7.
L. Takacs and M. Pardavi-Horvath, "Magnetic
Properties of Nanocomposites Prepared by
Mechanical Alloying," in "Nanophases and
Nanocrystalline Structures," eds. R. D. Shull and J.
M. Sanchez, The Minerals, Metals &
Materials Society, Warrendale, PA, 1994, p. 135-
144.
Nanocomposites of Fe3O4 particles dispersed in Cu were prepared by ball milling a mixture of Fe3O4 and Cu powders directly, as well as by ball milling enhanced displacement reaction between CuO and metallic iron. X-ray diffraction and magnetic hysteresis measurements were used to characterize the samples. Both processes result in magnetically semi hard nanocomposites with a significant superparamagnetic fraction after 20 hours of ball milling. It is suggested, that in situ chemical reactions be utilized as a means to control the ball milling process and to influence the microstructure and magnetic properties of the product.
8.
M. Pardavi-Horvath and L. Takacs,
"Nanocomposite Formation in the Fe3O4-Zn System
by Reaction Milling," J. Appl. Phys. 75 (1994)
5864-5866.
Magnetic nanocomposites of small iron particles embedded in non-magnetic 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). Metallic zinc disappeares during the first 100 min of milling and the magnetization decreases, indicating the formation of a non-magnetic iron-zinc oxide. This intermediate phase decomposes into iron and ZnO upon further milling. The final product is a semihard magnetic material with a significant superparamagnetic fraction.
9.
L. Takacs, "On the Mechanism of Displacement
Reactions Initiated by Ball Milling," 123rd TMS
Annual Conf. (San Francisco, CA, 1994) p. 26.
The rate of ordinary solid state reactions is limited by diffusion. Ball milling can significantly increase the reaction rate because (a) attrition and mixing increase the area of the interface and (b) fresh surfaces come into contact repeatedly, so that the reaction can proceed without diffusion. The detailed mechanism depends on the operation of the mill, chemical, mechanical, thermal, and transport phenomena, lattice defects, etc. It is the objective of our investigations to identify and separate the role of these factors.
Displacement reactions between a metal oxide and a more reactive metal are investigated. These reactions may extend to a small volume during each collision, resulting in a gradual transformation and nanocomposite formation, or the impact of the milling balls may initiate a SHS process. The incubation time before combustion is sensitive to the details of the reaction mechanism. Models are developed and tested, using displacement reactions such as the reduction of copper oxides by aluminum to identify the significance of the milling conditions and material properties.
10.
L. Takacs, R. C. Reno, and M. Pardavi-Horvath,
"Magnetic Nanocomposites by Reaction Milling,"
TMS Annual Conf. (San Francisco, CA, 1994)
p.109.
Recent investigations have shown that it is possible to prepare magnetic metal metal oxide nanocomposites by mechanical alloying. Two approaches are followed: (i) The desired final components are milled together directly, in which case the sole purpose of ball milling is microstructural refinement and homogenization. (ii) In order to obtain further control of the properties of the resulting materials, in situ chemical reactions are initiated by the ball milling process.
Some of the selected systems (iron particles in alumina, magnetite particles in copper) can also be prepared by more conventional methods, e. g. sputtering or sol-gel chemistry. However, other nanocomposite systems (e. g. iron particles in zinc oxide) may be better suited for preparation by ball milling. The objective of this work is to identify such systems and to investigate the formation and properties of these materials using X-ray diffraction, Mossbauer spectroscopy and magnetic susceptibility measurements.
11.
M. Pardavi-Horvath and L. Takacs, "Magnetic
Nanocomposites by Reaction Milling," Scripta Met.
Mater. 33 (1995) 1731-1740.
Systems of small magnetic particles embedded in a nonmagnetic matrix were prepared by high energy ball milling. Besides carefully chosen milling conditions, in situ chemical reactions were used to control the properties of the product. Nanocomposites of iron particles in metal oxides (Al2O3 and ZnO), and magnetite particles in copper metal were prepared by reaction milling. The samples were characterized by X-ray diffraction and magnetic methods. A few hours of ball milling resulted in the completion of most chemical changes. Iron nanoparticles were formed with lattice strains of about 0.005; coercivities up to 400 Oe were achieved. The magnetization of the iron particles is 25-40% less than that expected for bulk iron.
12.
L.H. Bennett, L. Takacs, L.J. Swartzendruber, I.J.
Weissmuller, L.A. Bendersky, and A.J. Shapiro,
"Magnetic Properties of Mechanically Alloyed Fe-
Ni-Ag," Scripta Met. Mater. 33 (1995) 1717-1724.
We have prepared mechanically alloyed samples of FeNi, FeNi3, and (FeNi3)(1-x)Agx. The production of nanometer size grains was confirmed by X-ray broadening and electron microscopy. X-ray measurements indicated that a small amount (about 7%) of Ag could be alloyed into the Fe-Ni and that considerable lattice strain was present. Mossbauer and magnetic measurements found no evidence for superparamagnetism in these alloys.
13.
M. Pardavi-Horvath, L. Takacs, and F. Cser,
"Switching Field Distribution Change During
Reaction-Milling of Iron-Zinc Nanocomposites,"
IEEE Trans. Magn. 31 (1995) 3775-3777.
DC remanent hysteresis loops have been measured for a series of reaction-milled nanocomposites. The remanence curves are characteristic of the amout of the material switched into the field direction in a field direction in a given field. The derivative of the remanence curve, the SFDR, is directly related to the distribution of the size and shape of the particles. The SFDR can be used as a quick tool for characterizing the magnetic particles in the nanocomposite.
14.
L. Takacs, "Nanocrystalline Materials by Mechanical
Alloying and Their Magnetic Properties"
in: "Processing and Properties of Nanocrystalline Materials," eds. C. Suryanarayana, J. Singh, and F. H. Froes, The Minerals, Metals & Materials Society, Warrendale, PA, 1996, pp. 453-464. Invited paper.
The interest in mechanical alloying as a method to produce nanocrystalline materials is sustained by (a) the simplicity of the method and (b) the proven possibility to scale it up to tonnage quantities. The goal of our investigations is to understand the mechanism of the mechanical alloying process and to utilize it for the preparation of magnetic nanocomposites. Chemical reactions induced by the milling are used to improve control of phase composition and microstructure. The magnetic properties are studied to search for potentially useful materials; they are also used as an indirect method to study the synthesis process. The focus of this paper is metal-metal oxide nanocomposites with either the metal or the oxide as the magnetic component.
High energy ball milling is a promising low-tech method to produce large quantities of oxide-metal nanocomposites. However, the price for the apparent simplicity of the process is difficulty of control. The possibilities to monitor the process are limited, prior information on the expected kinetics is crucial. Some nanocomposites can be prepared by milling the final components until the desired particle size and grain structure is reached. Excessively long milling times may result in contamination and deteriorating properties. Additional control may be possible via in situ chemical reactions between an oxide and a more reactive metal. The process usually occurs in three overlapping steps: An activation period of attrition, mixing, and defect formation; a relatively fast reaction when an intermediate sub-oxide is produced; and the decomposition of this oxide into a fine dispersion of the final oxide and metal phases. Several examples including the preparation of iron particles on Al2O3, MgO, and ZnO will be discussed.
The reduction of magnetite by Al and Mg has been induced by high energy ball milling. The reaction kinetics has been investigated as a function of the magnetite-to-metal ratio. Room temperature and 80K Moessbauer spectroscopy and X-Ray diffraction have been used to investigate the reaction products. Mechanical alloying initiates self propagating thermal "explosion" in the magnetite-Al and magnetite-Mg systems within a wide composition range around ideal reaction stoichiometry. 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 soon after explosion. The unstable intermediates transformed gradually to a-AL2O3 and a-Fe. Off-stoichiometry increases the milling time before explosion and changes the reaction products. When the amont of Al is decreased, a large fraction of Fe remains oxidized in the FeAl2O4 phase. Solid solutions of Fe and Al are formed in the presence of extra Al. The reduction of magnetie by Mg is similar, but no Fe-Mg solid solution forms in the presence of excess Mg. The milling time before explosion is much longer for Mg than for Al.
Abstract not available.
Abstract not available.
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. Mssbauer 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 Mssbauer investigations.
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, Mssbauer 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.
Materials processed by high energy ball milling are inherently metastable. Consequently, annealing a milled sample results in a series of thermal events as the sample approaches its equilibrium state. The investigation of this process is important both to understand the nature of the ball milled state and to assess its thermal stability. Even if an elemental powder is milled, excess energy is introduced when reducing the grain size to about 10 nm. The energy stored in grain boundaries and other defects can be close to 50% of the enthalpy of fusion. If a mixture of powders is processed, alloys or compounds form. The appearance of metastable states is typical; the formation of amorphous alloys has been observed in a variety of systems. High energy ball milling also increases the reactivity of materials. In some systems, mechanochemical reactions proceed in the mill at room temperature. In other cases, milling will activate the material, thereby increasing the rate or reducing the temperature of subsequent reactions. Milling induces self sustained combustion reactions in a variety of highly exothermic powder mixtures. The comparison of this phenomenon with thermally induced reactions and the effect of mechanical activation on the ignition temperature are of interest. Examples will be presented to illustrate the utility of thermal analysis in this area of research.
Metallic iron can be formed by reacting Fe3O4 with Zn in a ball mill. A mixed Fe-Zn oxide forms first, followed by decomposition into Fe and ZnO upon further milling. A large amount of Zn remain dissolved in the bcc phase and a fraction of the Fe retains an oxidized state. Decomposition can also be achieved by annealing the intermediate oxide. This process is investigated by thermomagnetic measurements, XRD, and Mssbauer spectroscopy. It is shown that the intermediate phase decomposes at relatively low (300 400ĄC) annealing temperatures. The separation of Fe and Zn between the metal and oxide phases of the end product is more complete than found in samples prepared by milling alone.
Abstract not available.
Abstract not available.
There is significant interest in ultrafine iron powders, as their large magnetization is beneficial in most applications. Finding the most suitable method to prepare large quantities of pure, uniform powder with low level of aggregation is still an open question. Two possible methods have been investigated in this work. (1) Nanosized hematite powder has been reduced by hydrogen gas at moderately high temperatures for between 1 and 9 hours. It is found that fully reduced ultrafine iron powder can be obtained at 450 degrees, much below the equilibrium reducing temperature. Agglomeration is avoided by performing the reduction in a fluidizing chamber. (2) Ball milling is used to reduce ferric chloride with sodium or calcium metal. Some extra sodium chloride is added to avoid high local temperatures due to thermal run-away reactions. The product of this mechanochemical process is an ultrafine iron powder, its agglomeration is prevented by the chloride by-product. The chloride phase is removed by subsequent washing with deoxygenated water and methanol. The washing process is optimized to obtain a clean, chloride and oxide free, nonagglomerated powder, with the smallest possible loss of iron. The phase composition of the powders is investigated by X-ray diffraction and Mssbauer spectroscopy, the morphology is studied by electron microscopy. The magnetic hysteresis is measured at different stages of the preparation process.
Abstract not available.
Last revised on April 15, 2000.