Abstract
Abstract In this work, we characterize the hollow hematite (α-Fe2O3) micro spheres obtained by two non-template techniques: i) sol gel and ii) spray roasting process. Both techniques allow the production of high yield hollow hematite spheres up to 100 g for the case of sol gel and up to 500 kg for the case of spray roaster process. The samples were characterized by X-ray diffraction, scanning electron microscopy and Mossbauer spectroscopy. The results indicate nearly uniform hollow spheres with diameters of around 1–1.5 μm and consisting of polycrystalline hematite. The Mӧssbauer spectroscopy reveals the signal change of quadrupole shift values evidencing the occurrence of the Morin transition and that the samples show an antiferromagnetic order at 77 K as in bulk hematite. The Morin temperature (TM) for both samples was obtained from the measurements of the magnetic moments as a function of the temperature in zero field cooling (ZFC) and field cooling (FC) modes. The values of TM for both samples are lower than that reported for bulk hematite (TM(bulk) = 263 K). Remarkably, the ZFC and FC loops do not overlap in both samples, revealing irreversible Morin transition. However, the sample obtained by sol gel presents thermal hysteresis with TM values of 260 K (in ZFC) and 248 K (FC). Whereas, the sample obtained by spray roaster process presents complete irreversibility and TM values of 252 K (in ZFC) and 242 K (in FC).
Highlights
Peer review under responsibility of Vietnam National University, Hanoi.L
The advantage of the spray roasted technique is that it allows the mass production of the hollow hematite spheres
Hyperfine properties confirm the formation of hematite and that the Morin transition occurs above 77 K for the samples obtained by both methods
Summary
In bulk a-Fe2O3 the spins are oriented along the [111] axis of the rhombohedral primitive cell [24] (along the [001] direction of the hexagonal unit cell [25]) It presents a first-order magnetic transition, called the Morin transition with the corresponding Morin temperature (TM) 1⁄4 263 K [26,27]. The gel was dried for two days at 40 C to evaporate the acid, water residuals and other possible impurities formed during hydrolysis This sample precursor was introduced in a tubular furnace (LENTON LTF-PTF Model 16/610) for annealing in air at 600 C. To thermally oxidize the gel to obtain monophasic crystalline hematite; and secondly, to form bubble structures via boiling in air from which the hollow spheres are formed after quenching [64,65]. After reacting with water and following the same annealing process, similar hollow spheres can be obtained
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