The polymer materials that allow the control of thermal conductivity are called photonic crystals and have, until now, mainly been investigated due to their optical effects. Prof. Dr. Markus Retsch, Lichtenberg Junior Professor of Polymer Systems, said that his team has developed four different methods to control temperature-dependent heat transfer in such photonic crystals.
These methods exploit the fact that polymer nanomaterials become more heat-permeable once they lose their nanostructure by crossing a certain temperature threshold. When that happens the thermal conductivity of the photonic crystals ‘skyrockets’, reaching a level that is two or three times as high as it was before. On this basis, clearly defined effects on thermal transfer can be achieved via changes in the nanostructure of the crystals.
The research has shown that the temperature at which thermal conductivity jumps depends on the composition of the nanoparticles that make up the photonic crystals and can be precisely adjusted by incorporating a plasticizer into the polymer structure. Thermal conductivity changes within a wide or narrow temperature range when the temperature rises can also be precisely controlled and only requires nanoparticles which are similar in size but which differ with regard to plasticizer content to be equally mixed. This leads to a gradual loss of the nanostructure across a wide temperature range. Consequently, the increase in thermal conductivity also spans a larger temperature range.
In addition, by using a layered structure, the researchers managed to transform the continuous increase into a multi-level increase in conductivity. By adjusting the thickness of individual crystal layers, one can also precisely influence the conductivity level that is reached at the respective level.
"These research findings demonstrate that it is possible in principle to regulate thermal conductivity in nanostructured materials with a high degree of precision. However, developing materials that enable thermal transfer to be precisely controlled is only the beginning. Our findings to date are very encouraging and have revealed interesting concepts for constructing more energy efficient insulation materials. In the long term, these concepts could be valuable for the development of thermal transistors or diodes," Prof. Retsch explained.
There is one obstacle that must still be overcome: the increase in thermal conductivity is irreversible. This means that the conductivity remains at the level that is reached even when the temperature sinks again.
"Constructing nanosystems that enable thermal transfer to be reversibly controlled is a difficult yet exciting and central task for further research in this field," said Prof. Retsch.
