Fabio Miani

Designing a high energy ball-mill for synthesis of nanophase materials in large quantities

Abstract Nanophase materials can be synthesized by high energy milling. However, processing of large quantities finds a limitation in the devices available commercially, which are usually designed mainly for comminution and for laboratory-scale operations. We propose the design of a high energy, high capacity ball-mill which can be easily scaled up. An experimental device capable of processing up to about 250 g of powders with about 5 kg of balls has been realized. Impact velocities of about 3.5 m s −1 has been measured with operating frequencies of 17 Hz. The synthesis of nanophase iron carbides (assumed as a test system) is shown to be feasible with the new mill, with kinetics comparable with the best obtained in a Spex mill.

Mechanosynthesis of nanophase materials

Abstract Among the synthesis methods of nanocrystalline materials, high energy milling is promising at production scale. The mechanochemical synthesis (Mechanosynthesis) of nanophase materials can be realized by direct synthesis of compounds from the elemental powders or by several exchange, transfer and mixing reactions such as for: 1) most metal carbides; 2) intermetallic compounds (silicides, aluminides); 3) semiconducting III-V compounds (GaAs and AlAs); 4) metal-oxide M-RO composites by reduction of a metal M oxide with a suitable reductant R; 5) metal-sulhide M-RS composites by reduction of a metal M sulphide with R; 6) fluorides and nitrides by exchange reactions. Reactions are driven in a ball mill at almost room temperature.

Measuring the impact velocities of balls in high energy mills

Abstract A simple methodology for the experimental determination of impact velocities in ball mills used for the synthesis of nanophase materials is proposed. The impact velocity of the balls relative to the vial wall is determined from the indent size. A model correlating velocities with indent radius is described. Velocities were determined for the Spex mill (1.8–3.3 m s −1 ) and for a new high capacity mill (2.6–3.8 m s −1 ) and were found to be strongly dependent on ball size.

Kinetic effects in the mechanically activated solid-state reduction of haematite

Nanometer-sized α-Al2O3-Fe composites were obtained by solid-state reduction in ball mills of Fe2O3 and Al in nitrogen and air. Together with α-Al2O3 and Fe, the formation of Hercynite and clusters of Fe in alumina are observed in both cases.

Mechanosynthesis of iron carbides at composition Fe75C25 : modeling of the process kinetics

Abstract The mechanosynthesis of iron carbides, at composition Fe 75 C 25 , was investigated with regard to modeling of the kinetic behavior. Experiments were performed in a vibratory ball mill exploring different milling conditions. 57 Fe Mossbauer spectroscopy and X-ray diffraction were used to characterize the ball-milled powders. The iron conversion kinetics, investigated with some 90 samples, is found to follow a sigmoid-type curve. A correlation is also established between the time at which the maximum iron conversion rate is obtained and the ball-to-powder weight ratio.

Estimation of viscosity in undercooled liquid metal alloys

A simple procedure for calculating the viscosity of undercooled liquid metals exploiting available experimental data is presented. In the case of alloys at eutectic composition an empirical correlation may give approximate values in case viscosity data are not available. The values of the Vogel-Fulcher equation presently used in literature are discussed and some simple estimates proposed.

Iron-Alumina nanocomposites obtained by mechanosynthesis

Nanocomposites of α-Al2O3Fe can be obtained by solid state reduction of hematite and aluminum in ball mill. The kinetics of the synthesis in a vibratory ball mill have been studied by milling the stoichiometric mixture of reactants for times from 15 min to 48 h. Mossbauer spectroscopy and X-ray diffraction were used as characterization techniques. The reduction reaction proceeds gradually and was completed in about 10 h. Together with α-Al2O3 and iron, the formation of hercynite (FeAl2O4) and clusters of α-Fe (with non-magnetic ordering) in alumina are observed. The crystallite sizes of α-Al2O3 and iron are about 10 nm after 10 h of milling. A weak alloying of Fe with Al(≽3 at%) occurs after 2 h milling.

Thermal stability of nanocrystalline iron-iron carbide composites. A Mössbauer study

Mechanically synthesized nanocrystalline iron-iron carbide nanocomposites were subjected to isothermal cycles of one hour duration. As-treated and annealed powders were characterized by means of Mössbauer spectroscopy and X-ray diffraction. The grain growth was studied in a temperature range in which ε and χ iron carbides, detected in the treated amples, have a low thermal stability. Cementite results are substantially stable up to 1100 K and, according to a simplified model of crystal growth, show a low activation energy for crystal growth.

119Sn Mössbauer spectroscopy in nanocrystalline aluminat-in composites synthesized by direct grinding and reactive ball milling

Alumina-tin nanocomposites have been synthesized by direct grinding of α-Al2O3-β-Sn powder mixtures, as well as by reactive milling of Al-SnO2 mixtures. According to the preparation method used, the composites differ from one another mainly in the amount of a disordered SnOx (x≈1.1) phase and in the structural characteristics of alumina.

Ball Velocities and Energy Needs in a Mill Designed for the Synthesis of Nanophase Materials

The experimental determination of impact velocities in ball mills, from the indent size, is discussed. Velocities were determined for a new high capacity mill (2.6 to 3.8 m/s) and were found to be strongly dependent on ball size. An evaluation of the maximum specific energy available in such a mill for the synthesis of nanophase materials is given (20–50 kWh/Kg).