The conventional way to form alloys like stainless steel is by fusing the components at extremely high temperatures. Mechanical alloying is an alternative option, if melting fails to fuse the alloys, or if only small quantities are needed. One approach to this application is to use ball mills.
In the late 1960s, nickel-iron alloys were manufactured by mechanical alloying to obtain temperature-resistant materials for the first time. In the process, the solid powder components are connected by high-powered kinetic milling. High-energy ball mills and planetary ball mills supply the required energy input by impact. The fine particles are deformed plastically between the grinding balls and the materials are welded together. In this way, alloys can be produced if the conventional method of metal fusion fails. Moreover, it is possible to change the mixing ratios of the components by mechanical alloying.
Thermoelectric Material Alloys
Silicon (Si) and Germanium (Ge) are the key elemental semiconductor materials. They paved the way for the development of electric devices like photovoltaic cells or transistors. The material properties of these alloys can be changed by using differing amounts of Si and Ge, leading to changes in atomic size, mass differences and band gaps. Thermoelectric alloys of such materials are used in space explorations in radioisotopic thermo generators to ensure the power supply of space probes and measurement devices.
Materials based on bismuth telluride (Bi2Te3) are most suitable for commercial applications in the thermoelectric field, as they offer the best conversion efficiency of all thermoelectric materials. Peltier elements made up of bismuth telluride are employed in cooling, for example.
The High Energy Ball Mill Emax
The Emax is a recently developed ball mill especially designed for high energy milling. The speed of 2,000 min-1, in combination with the exceptional grinding jar design, produces high size reduction energy. The Emax mechanism is based on a combination of high impact and intensive friction, which results in a high energy input that is applied for fast grinding down to nanometer scale, and for mechanical alloying. This combination is produced by the oval shape and the movement of the grinding jars. The jars travel in a circular path without changing their orientation, which promotes thorough mixing of the particles, leading to a narrower particle size distribution and smaller grind sizes than achievable with conventional ball mills.
An innovative liquid cooling system ensures that surplus thermal energy is rapidly discharged, preventing the sample from overheating, even after extended grinding times. Grinding jars are cooled through an internal water-cooling system, permitting continuous grinding without breaks, which are required when planetary ball mills are used. An external chiller, attached to the internal cooling system of the Emax, can be used to further reduce the temperature.
Powder diffractogram of Si (red) and Ge (green) at the beginning of the mechanical alloying. The upper part shows the whole measurement range from 10° – 60°. In the lower part detailed reflexes of the lattice plane 111 of Si and Ge are recognizable.
Powder diffractogram after five hours of mechanical alloying in the Emax. The upper part shows the whole measurement range. The theoretical lines of Si (red) and Ge (green) are displayed for reference. In the lower detailed diagram, the progress in mechanical alloying becomes visible (shift of 111-reflex and collapse of Si and Ge reflexes).
The 111-reflexes of the samples after five, eight and nine hours are shown. The width of the peak has slightly decreased and the peak maximum has slightly been shifted, indicating that the process was nearly finished after only five-six hours.
Planetary ball mills, which were traditionally used for mechanical alloying of Si and Ge, had some drawbacks which the new High Energy Ball Mill Emax has overcome. The initial size reduction of the starting material already took 80 minutes. Moreover, the full power of the planetary ball mills was not usable for the subsequent mechanical alloying process because even at a moderate speed of 400 rpm the sample material was caking in the grinding jars. The additional problems like overheating of the grinding jars and the need for cooling breaks in the planetary ball mill increased the overall processing time of 13 hours by 90 minutes. The latest technology prevents caking at high speeds, avoiding the requirement for long breaks and decreasing the total process time.
Right: Powder diffractogram after one hour of mechanical alloying Bi and Te in the Emax, powder to ball ratio 1:10 (left), powder to ball ratio 1:5 (right).
This information has been sourced, reviewed and adapted from materials provided by RETSCH GmbH.
For more information on this source, please visit RETSCH GmbH.