engleski

Nanoworld of acrobatic crystals – thermosalient effect (example of Scopolamine bromide)

Imagine the surprise on one’s face when, during the microscopic evaluation of the crystals’ habits, these crystals start to literally jump and fly on the stage providing a vivid example of thermosalient effect. Thermosalient materials are the ones that during heating/cooling undergo an energetic phase transition which is so sudden and abrupt that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. Apart from providing visually extremely attractive phenomenon, these materials have a tremendous technological potential as the future self actuation device (nanoswitches, thermal sensors, artificial muscles, etc.)1. Several exciting experimental studies on the origin of the thermosalient effect were published resulting in an astonishing wealth of information about the effect, but still we are far from the full understanding of the peculiarities and all the interplays and forces that govern this phenomenon. Here we present a systematic experimental study of the thermosalient effect in Scopolamine methyl bromide. As in most of the thermosalient systems, immense negative thermal expansion seems to be the most likely candidate for the driving force behind this phenomenon. This study presents combination of optical and electron microscopy, together with the x-ray diffraction measurements. Optical microscopy provides a direct insight into this visually extremely attractive phenomenon which is a manifestation of the macroscopic transformation of thermal energy into mechanical work. This analysis gives us a wealth of information about mechanical behaviour of thermosalient crystals – the macroscopic change of the crystal habit during the transformation, their trajectory, initial velocity and the dependence of the jumping height on the heating rate. On the other side, high resolution transmission electron microscopy gives us an insight into the fine details of the crystal structure and the stress field which is responsible for the thermosalient effect. These two microscopic techniques, coupled with the in-situ non-ambient x-ray powder diffraction measurements, enable us to fully elucidate the thermosalient phenomenon in this system.

molecular crystals ; thermosalient materials ; thermal axpansion ; crystal polymorphic phase transition

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