Temperature development in ball mills during mechanochemical reactions studied by an in situ Raman-thermography coupling
In the last decades mechanochemical synthesis has gained increasing importance. Reasons for that are the simple implementation and the typical quantitative conversion of reactants in a short period of time. Usually, crystalline solids are used as starting materials that must not be solved. Based on these benefits, mechanochemistry presents an environmentally friendly alternative to a less energy-efficient synthesis in solution. The product diversity is only limited by the choice of milling equipment. In the simplest case reactions are induced by manually grinding using mortar and pestle, more reproducible results are obtained with automated ball- and planetary mills. In this way, new materials can be synthesized that are not accessible by classic solution chemistry.
For the control and optimization of the milling process information on the underlying mechanism is necessary. Therefore, the elucidation of mechanochemical reaction pathways and the influence of important reaction parameters are the subject of current research. The two most common theories regarding mechanochemistry are the hot-spot theory and the magma-plasma model. Both theories assume local temperature rises over 1000 °C during milling. So far, temperatures during milling have only been determined indirectly or discontinuously. To gain a deeper understanding of the underlying reaction processes as a function of temperature, time-resolved and contact-free in situ measurements under realistic conditions are suited best.
A new laboratory setup for the in situ coupling of Raman spectroscopy and thermography was developed together with the Thermographic Methods division which is now located at Adlershof. By using a custom-made transparent (within the visible spectral range) milling jar made of Perspex the direct and simultaneous collection of Raman spectra and thermograms during milling was possible for the first time. The obtained data allows a correlation of the jar surface temperature with the reaction process. Based on three model reactions, a clear contribution of the reaction heat to the overall temperature was noted besides the mechanical heat input. Liquid by-products partially lower the temperature remarkably. For the synthesis in ball mills no temperatures above 100 °C were observed, therefore the hot-spot theory and magma-plasma model seem unlikely in this case. Even for more complicated reaction processes local temperature rises could be assigned to individual reaction stages. The presented setup is suitable for all kinds of mechanochemical reactions and was in the meantime successfully complemented by in situ synchrotron XRD.
Warming up for mechanosynthesis – temperature development in ball mills during synthesis
Hannes Kulla, Manuel Wilke, Franziska Fischer, Mathias Röllig, Christiane Maierhofer, Franziska Emmerling
Chemical Communications, Volume: 53, Issue: 10, Pages: 1664-1667
BAM Department Analytical Chemistry; Reference Materials, Division Structure Analysis and Department Non-destructive testing, Division Thermographic Methods