Powders are highly suitable for thermal insulation when they comprise a disordered collection of differently sized nanoparticles. Researchers at the University of Bayreuth have discovered how the thermal conductivity of powders is influenced by the order and disorder of their components.
Photonic crystals found in various insect types in nature are responsible for the colourful, shimmering appearance of butterfly wings, for example. Crystals of this type can be easily recreated in the laboratory with polymer nanoparticles. These crystals feature a fine, uniform and stable structure. This well-ordered structure impedes the flow of heat through the crystals. Their thermal conductivity is low. Prof. Dr. Markus Retsch and his research group at the University of Bayreuth have now discovered that such nanoparticles can be used to produce materials with even lower thermal conductivity.
These materials are powder mixtures. Whereas each particle inside photonic crystals is surrounded by exactly twelve other particles in its direct vicinity, the number of direct neighbours varies in the mixture. The heat therefore has to take indirect routes through the mixture. It is more difficult for heat to flow from hot to cold in the chaotic structure than in the ordered crystal. The researchers have combined laboratory experiments with computer simulations to resolve these relationships fully. They were able to identify in detail how the particle mixture’s composition affects the passage of heat. Mixing few large particles with very many smaller particles achieves the highest insulating effect. The size difference between the two particle types is decisive in addition to this mixing ratio.
These findings are highly significant for many applications, particularly in the field of heat insulation, where the thermal insulation capability of bulk powders could be improved, for example. They also provide valuable clues for technical applications that, by contrast, depend on rapid and readily controllable heat dissipation. This is the case, for instance, when optimising industrial sintering processes that involve fusing tiny powder particles together. Precise temperature control at the melting points is important here, which can be achieved through improved heat dissipation. The researchers presented their new findings in "Advanced Materials".