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John O | January 2018

German researchers control heat transfer with nanostructures

researchers from the university of bayreuth (germany) have for the first time demonstrated four different methods for achieving precise control of temperature-dependent thermal conductivity in photonic crystals.


a light microscope image of a photonic crystal made of two particles with differing plasticizer content. (markus retsch/university of bayreuth)


according to a report on the university website, the material becomes more heat permeable when the nanostructure breaks down after crossing a certain temperature threshold. at that point, the thermal conductivity of the photonic crystals is two or three times higher and shows that changes in nanostructure has a direct effect on heat transfer.


“the research of the scientists in bayreuth has shown that the temperature at which thermal conductivity jumps to a higher level depends crucially on the composition of the nanoparticles that make up the photonic crystals,” the article explained. “this temperature can be precisely adjusted by incorporating a plasticizer into the polymer structure.


it added, “whether thermal conductivity changes within a wide or narrow temperature range when the temperature rises can also be precisely controlled: doing so 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.”


by using a layered structure, researchers also changed the increase in thermal conductivity from continuous to multi-level and demonstrated that varying the thickness of the crystal level offered precise control of the respective conductivities.


researchers will use this new information to build more energy-efficient insulation materials and believe that this could lead to improved thermal transistors or diodes. they will also try to overcome the challenge of reversing the increase in thermal conductivity, which is currently permanent.


the research was recently published in science advances. the abstract stated:


“managing heat is a major challenge to meet future demands for a sustainable use of our energy resources. this requires materials, which can be custom-designed to exhibit specific temperature-dependent thermal transport properties to become integrated into thermal switches, transistors, or diodes.


“common crystalline and amorphous materials are not suitable, owing to their gradual changes of the temperature-dependent thermal conductivity. we show how a second-order phase transition fully controls the temperature-dependent thermal transport properties of polymer materials.


“we demonstrate four major concepts based on a colloidal superstructure: (i) control of transition temperature, (ii) width of phase transition regime, (iii) multistep transitions, and (iv) step height of the transition.


“most importantly, this unique control over thermal conductivity is only governed by the interparticle constriction, the particle composition, and its mesostructure. our concept is therefore also applicable to a wide variety of other particulate materials.”

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