Thermal Conductivity of Quasicrystals and Associated Processes
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d tribological properties gives the quasicrystalline alloys a technological interest for applications where superficial thermal and mechanical conditions are of prime importance.1011 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. Experimental Results Thermal Properties The thermal properties have been measured by differential scanning calorimetry (heat capacity Cp), flash laser (diffusivity a a), picnometry (density p), and dilatometry (expansion). The ther-
Teitiperatiire (K)
Figure 1. Temperature dependence of the thermal conductivity in icosahedral Al-Cu-Fe quasicrystals.12
mal conductivity (K) is not directly obtained but deduced at each temperature by K = pa d C p . Values of typical room-temperature thermal properties (Table I) show that the thermal conductivity of the quasicrystals is much lower than that of conventional metallic materials and is similar to that of oxides known as very efficient insulators. The variation of the thermal conductivity of the Al-Cu-Fe icosahedral phase with temperature appears in Figure 1, which reveals a continuously positive temperature coefficient.12 This relative increase of the conductivity suggests, compared to that of ceramics, that electrons contribute increasingly to the thermal conductivity with temperature over 300 K.12 Figure 2 compares the variation of conductivity versus temperature of several materials with that of quasicrystalline alloys (log scale on y axis). Although it increases with increasing temperature, the thermal conductivity of QC alloys remains low throughout the whole temperature range. As can also be seen in Table I, the linear thermal expansion of quasicrystals is similar to that of iron and steels but lower than that of aluminum.llU3 These two properties (thermal conductivity and expansion) are quite important for surface-coating applications (thermal barriers and reinforcement for softer metals). Moreover the thermal effusivity (defined as £ = \/KPCP) of QC alloys is equivalent to that of zirconia, which means that one can expect a particular superficial heat-transfer effect in the case of a QC-coated high-effusivity substrate. Thermal Insulation Thermal insulation generally aims to limit the temperature field in the bulk of the material to be protected by locating the strongest thermal gradients within a superficial zone. This is usually achieved by surface deposition of poorly conducting ceramic oxides. D-ue to their low thermal conductivity, one expects QC alloys to be a good alternative. This has been shown via numerical simulation (ALGOR software) in the case of pure thermal conduction (Figure 3).14 In this example, a 500-/xm-thick QC coating (Al-Cu-Fe alloy) is deposited onto
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