Perpustakaan Digital ITB

This paper presents a thermomagnetic study of fresh, unheated indoor dust, outdoor dust, street dust and dust from the cabin air filters of cars. Detailed analysis of thermomagnetic curves clearly indicated the presence of two magnetic transitions: the first identified at the Curie temperature TC?585°C for magnetite; the second TC detected at about ~765°C, indicating the presence of metallic iron and/or iron-based alloys. The ‘tail,’ i.e., the substantial decreasing of ? visible on the heating curves of ?(T) between 600°C and 700°C, was attributed to the metallic iron or iron-based alloys. The presence of a high-temperature Fe-phase is not dependent on the types of urban dust or the sampling method, as variable amounts of metallic iron were detected in material collected from different environments: both indoor dust and outdoor dust, and dust gathered by sweeping indoor floor surfaces as well as dust collected with a vacuum cleaner. The presence of metallic iron in different types of dust was confirmed by non-magnetic methods. Electron microscopic observations with Energy-Dispersive X-ray Spectroscopy revealed elongated shaving-like particles comprised of metallic iron. A component with a magnetic hyperfine field of Bhf ?33 T and an almost zero value for isomeric shift as well as zero quadrupole splitting typical of metallic ?-Fe at micrometer size or Fe-based alloys was recognized in all the Mössbauer spectra. Additional measurements of hysteresis properties at high temperatures (up to 750°C) and after step-wise annealing indicated the process of oxidation of iron to magnetite. This has a strong effect on the run of ?(T) and M(T) curves during heating and cooling because M and ? are strongly dependent on the apparent concentration of both ferromagnetic phases (magnetite and metallic iron.