engleski

Temperature tuning of thermosalient phase transition in organic alloys of 1, 2, 4, 5- tetrabromobenzene and 1, 2, 4, 5- tetrachlorobenzene

Molecular crystals that exhibit thermally activated motion are currently hot topic in the materials science. Such thermosalient materials undergo an energetic phase transition during heating/cooling which is so swift that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. These locomotive materials possess tremendous technological potential as the future self- actuating devices (nanoswitches, thermal sensors, artificial muscles, etc.) [1]. The majority of thermosalient crystals are organic compounds and several cases have been studied in great detail. Crystals of 1, 2, 4, 5- tetrabromobenzene (TeBB) jump several centimetres during the phase transition from the low- temperature β-form to the high- temperature γ- form. Hopping of the crystals happens at 45.0 °C for single crystals and at 45.5 °C for twinned crystals [2]. We explored the thermosalient effect in in 1, 2, 4, 5- tetrabromobenzene and 1, 2, 4, 5- tetrachlorobenzene (TeCB) organic alloys and the impact of the variation of composition of the alloys on the effect. Samples ranging from 0-70 wt% TeCB were prepared by crystallization from a solution of the two substances dissolved in ether, after which the solvent was evaporated under controlled conditions. Alloys with the amount of TeCB up to 8 wt% exhibit similar structural characteristics at room temperature ; they all crystallize in β form, and upon heating convert to γ. The phase transition temperature changes with alloy composition and coincides with the temperature of jumping. We have observed a linear correlation between temperature of jumping and alloy composition, with the presence of a threshold concentration for the activation of thermosalient effect. Alloys with the wt% of TeCB higher than 8 % do not show thermosalient behaviour. X-ray diffraction has also revealed the mechanism of formation of mixed crystals showing how the Br an Cl atoms are integrated in the alloy structure.

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

nije evidentirano