Thermal Conductivity of Quasicrystals and Associated Processes

Abstract

Quasicrystals (QCs) exhibit unusual physical properties that are significantly different from those of crystalline materials and are not expected for alloys consisting of normal metallic elements. Opposite to conventional metallic alloys, their thermal conductivity and diffusivity are unusually low (with a positive temperature coefficient)—practically that of an insulator—which is atypical for materials containing about 70-at.% aluminum. Moreover the thermal conductivity decreases when the structural perfection is improved. One observes a low, if any, electronic contribution to the heat capacity and thus a vanishing density of electronic states at the Fermi level.The origin of this unexpected behavior was first attributed to the existence of a deep pseudogap at the Fermi level with a localization tendency of electrons near the Fermi level.However experimental evidence led to an alternative approach related to the structure of quasicrystals. In QCs, well-defined atomic clusters form self-similar subsets of the structure over which electronic and vibrational states are expected to extend. According to the inflation symmetry of the icosahedral structure, the so-called recurrent localization effects may then explain the conduction behavior and other striking features of quasicrystals (e.g., brittleductile transition at high temperature, corrosion resistance, low friction, high hardness).In the following, we first present the main thermal properties of quasicrystalline alloys compared to those of conventional materials with an emphasis on the variation of the thermal conductivity with temperature. The combination of such peculiar conduction, mechanical, and tribological properties gives the quasicrystalline alloys a technological interest for applications where superficial thermal and mechanical conditions are of prime importance. This is illustrated with two examples involving a QC coating on a base Al substrate: (1) thermal insulation for which a low conductivity is needed and (2) quenching heat-transfer modification due to a low-effusivity superficial effect. These processes are then explained in the third part of this article in terms of the cluster-modes delocalization mechanism responsible for the low conductivity of the quasicrystals.