Emissivity of the icosahedral Al62Cu25,5Fe12,5 quasicrystal

Temperature is an important parameter for understanding the physicochemical behaviour of the surface of materials. Studying the first stage of oxidati on rate of icosahedral AlCuFe by SAS, if is necessary to know precisely the real surface temperature of our quasicrystal samples. The difficulties in measuring the temperatures of this kind of material in an UHV system are th e same as for thermally and electrically insulating and fragile materials. For this kind of sample, it is impossible to measure correctly the surface temperature by contact In particular if is impossible to weld a thermocoupl e on their surface. In view of pyrometric measurements of temperatures this paper presents the preliminary study of the radiative power of icosahedral AlCuFe (QC), about which no data can be found in the literature to this day Our QC samples are obtained from a bulk ingot prepared by sintering a batch of granular powder of nominal composition Al62Cu25,5Fe12,5. Our specimens are in the shape of small mechanically polished platelets with section of 2 x 2 mm and 200 pm thickness. They are mechanically fixed on a metallic rib bon heated by Joule effect. This heating holder is rang enough to have a ce ntral isothermal zone, where Au and graphite samples are fixed next to the QC specimen. These contiguous samples being physicochemically inert, will b e used as thermal emission references. Two thermocouples are welded on the heating holder in the vicinity of QC. In the SAS vacuum chamber, these samp les are observed in the range 1.4-1.6 mu m by a pyrometer (IMPAC IGA 10) th rough a quartz window (whose transmission coefficient was measured as equal to 0.94). In the first instance, the emissivities (epsilon) of clean samples will be studied under vacuum versus the temperature. The samples are cleaned in-sit u by ionic abrasion followed by annealing at 750 K. These emissivities are calculated from the pyrometer signal. Secondly, the relative variation of QC emissivity will be observed during d ifferent oxidation reactions (10(-5) < P-O2 < 10(-3) mbar, T-oxidation = 87 3 and 923 K). Whereas the emissivity df each metallic component (Al, Cu, Fe) of this QC a lloy is relatively low (< 0.2 for polished materials), the emissivity of bo th icosahedral and cubic beta AlCuFe phases is in the range 0.45-0.50 for 6 50 < T < 1100 K. A sudden change of EOG is observed at 1113 K. This temperature corresponds to the peritectic reaction that transforms the icosahedral phase into the l iquid + cubic phase mixture. During long oxidations, epsilon(QC) varies from 0.45 up to a plateau at 0.6 5 or 0.8, depending on the experimental conditions of reactions. The oxide thicknesses measured by ionic abrasion at the end of these reactions, are i nferior to 0.2 inn. The emissivity values epsilon(QC) measured at these different plateaus cann ot correspond to the intrinsic emissivities of oxides, too thin to be consi dered optically opaque. Therefore the presence of these different plateaus leads us to suppose that there is a passivating oxide layer for each temper ature studied as observed elsewhere for oxidations performed at temperature s inferior to 670 K.