It was first realized by H. Gleiter that reducing the grain size of crystalline materials to a couple of nanometers brings about significant changes of the properties of the material. In this nanocrystalline state the solids contain such a high density of defects that the spacing between them approaches interatomic distances and a large fraction of the atoms sits in or adjacent to a defect. The nanocrystalline state is fundamentally different from the large grain state, but it is also different from the amorphous state.
Although the original suggestions date back to about 1970, systematic large scale research on nanocrystalline materials only began around 1990. A new journal titled NanoStructured Materials was started in 1992, several new conferences and sections at general meetings indicate the boom of ideas.
Nanostructured materials can be prepared with several methods, among them sol-gel chemistry, sputtering and evaporation, controlled crystallization of an amorphous precursor, gas condensation. The method of our choice is mechanical alloying. The advantage of that method is its relative simplicity, low cost, and the possibility to scale it up to tonnage quantities.
Our investigation has two objectives: We wanted to describe, understand, and model the mechanical alloying process. We also investigated the magnetic properties of the final nanocomposites as well as samples obtained at intermediate stages of the milling process. We are looking for potentially useful magnetic materials and also use magnetic properties as an evaluation method.
The following is a list of publications on nanocrystalline materials, many with abstract. 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@umbc2.umbc.edu".
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 be 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.
Yanxia Lu, R. C. 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.
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.
M. Pardavi-Horvath and L. Takacs, "Magnetic Properties of Copper-Magnetite Nano-composites Prepared by Ball Milling," J. Appl. Phys. 73 (1993) 6958-6960.
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 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.
M. Pardavi-Horvath and L. Takacs, "Nanocomposite Formation in the Fe3O4-Zn System by Reaction Milling," J. Appl. Phys. 75 (1994) 5864-5866.
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.
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.
L. Takacs, "Nanocrystalline Materials by Mechanical Alloying and Their Magnetic Properties" TMS Materials Week, Cleveland, Ohio, October 29-November 2, 1995, invited, in press.
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.