Der Mechanismus der Umwandlung von γ‐ zu α‐Eisensesquioxid [1]

Feitknecht, W.; Mannweiler, U.

doi:10.1002/hlca.19670500222

Cited by 50 publications

(23 citation statements)

References 9 publications

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“…Figure 7 shows the core-level XPS patterns of the R- former one is in accordance with the previous results, 27 indicating that such a phase transformation is a topotactic one. 28 The structural transformation of [001] R-Fe2O3 to [112] Fe3O4 has never been observed directly by electron microscopy, though similar results based on X-ray diffraction measurements have been reported. 29 To characterize the local environment and distribution of Fe atoms in the Fe 3 O 4 nanorings, high-resolution Fe L 2,3 -edge absorption spectroscopy of different nanorings using STXM was performed.…”

Section: Magnetic Nanorings (Fe 3 O 4 and γ-Fementioning

confidence: 84%

Large-Scale Synthesis of Single-Crystalline Iron Oxide Magnetic Nanorings

et al. 2008

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We present an innovative approach to the production of single-crystal iron oxide nanorings employing a solution-based route. Single-crystal hematite (alpha-Fe2O3) nanorings were synthesized using a double anion-assisted hydrothermal method (involving phosphate and sulfate ions), which can be divided into two stages: (1) formation of capsule-shaped alpha-Fe2O3 nanoparticles and (2) preferential dissolution along the long dimension of the elongated nanoparticles (the c axis of alpha-Fe2O3) to form nanorings. The shape of the nanorings is mainly regulated by the adsorption of phosphate ions on faces parallel to c axis of alpha-Fe2O3 during the nanocrystal growth, and the hollow structure is given by the preferential dissolution of the alpha-Fe2O3 along the c axis due to the strong coordination of the sulfate ions. By varying the ratios of phosphate and sulfate ions to ferric ions, we were able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings displayed an intriguing three-dimensional magnetic configuration. This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.

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Section: Magnetic Nanorings (Fe 3 O 4 and γ-Fementioning

confidence: 84%

Large-Scale Synthesis of Single-Crystalline Iron Oxide Magnetic Nanorings

et al. 2008

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“…The haematite grains forming from lepidocrocite via maghemite are well-crystallized with sizes distinctly larger than that of the preceding maghemite particles. Feitknecht & Mannweiler (1967) proposed that 50-100 neighbouring maghemite crystallites of about 5 nm would finally transform into one large haematite particle. As a rule, X-ray diffraction shows fairly sharp haematite reflections, also just after the appearance of haematite.…”

Section: Haematitementioning

confidence: 99%

The lepidocrocite-maghemite-haematite reaction chain-I. Acquisition of chemical remanent magnetization by maghemite, its magnetic properties and thermal stability

Gendler

Щербаков²,

Dekkers³

et al. 2005

Geophysical Journal International

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SUMMARY We report on the magnetic properties and the acquisition of a chemical remanent magnetization (CRM) in a field of 100 μT as a function of temperature and time during the lepidocrocite–maghemite–haematite reaction chain. The development of CRM was monitored at a series of 13 temperatures ranging from 175 to 550 °C; data acquisition was done at the specific formation temperatures for durations of up to 500 hr. Up to acquisition temperatures of 200 °C it takes a considerable time (up to 7 hr) before the CRM is measurable. This time decreases with increasing temperature, reflecting the activation energy of the reaction to form the first maghemite. During the lepidocrocite conversion, formation of two types of maghemite is suggested by two peaks in the CRM versus time curves. Magnetic properties were analysed after various stages in the reaction. They indicate a mixture of superparamagnetic and single‐domain maghemite. The first reaction product (obtained after annealing at 200 °C) is a fine‐grained yet crystalline maghemite (labelled type A). Before massive maghemite formation occurs, the coercive and remanent coercive forces go through a minimum at intermediate temperatures of 250–300 °C (annealing for 2.5 hr). This minimum lowers to 200–250 °C with increasing annealing time (500 hr). This is probably the result of two processes acting simultaneously—formation of superparamagnetic maghemite particles of a second less crystalline maghemite type (labelled type B) and removal of stacking faults in type A maghemite. The second process is suggested by analogy to the behaviour of natural magnetite/maghemite systems on annealing. Removal of stacking faults is reported to result in a magnetic softening of the grain assemblage. Annealing at 300–350 °C removes most of the lepidocrocite and the second maghemite type, type B, becomes prominent. Haematite formation sets in at slightly higher temperatures, yet the type B maghemite is in part thermally stable up to 600 °C enabling Thellier–Thellier experiments. This stability is also inferred from Arrhenius fitting that shows a comparatively high activation energy for the maghemite to haematite reaction. In Thellier–Thellier experiments the CRM showed a markedly downward convex Arai–Nagata plot while a second thermoremanent magnetization (TRM) showed perfect linear behaviour as expected. This feature may be used to recognize CRM in natural rocks.

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“…~ transformation occurs in the range 250-400 ~ for nonacicular particles, and at 560-650 ~ for acicular particles. The thermodynamic and kinetic data, such as the heat of the transformation and the energy of activation, reported by several workers for the transformation ?-Fe203 ~ct-Fe203, also showed a large spread [1,[4][5][6][7][8]. The failure to obtain consistent results has been attributed, among other factors, to the presence of impurities [5], to the influence of the particle size [1,[5][6][7][8], and also to the various extents of ordering of the vacancies in the defect structure of y-Fe203 [7][8].…”

mentioning

confidence: 96%

Differential scanning calorimetry studies on influence of microstructure on transformation of γ-Fe2O3 to α-Fe2O 3 +

Vajpei

Mathieu

Rousset

et al. 1987

Journal of Thermal Analysis

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The transition 7-Fe203 --*~t-Fe20 3 has been investigated in oxygen and argon using DSC measurements in the temperature range 298 to 820 K. The results have been interpreted by taking into account the method of preparation, the specific microstructure of the samples and the nature of the gas used. It has been shown that the transformation temperature, and in general the absolute value of the negative enthalpy of the transformation as well, rises with increase of the specific surface area of the y-Fe20 3. The observed fluctuations in this trend are attributed to differences in the microporosity, gas sensitivity and crystallinity of the samples.

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Der Mechanismus der Umwandlung von γ‐ zu α‐Eisensesquioxid [1]

Abstract: The rate of the isothermal transformation of γ into α Fe2O3 and the increase of the crystallites of α Fe2O3 during the transformation were measured using X‐ray methods.

Cited by 50 publications

References 9 publications

Large-Scale Synthesis of Single-Crystalline Iron Oxide Magnetic Nanorings

Large-Scale Synthesis of Single-Crystalline Iron Oxide Magnetic Nanorings

The lepidocrocite-maghemite-haematite reaction chain-I. Acquisition of chemical remanent magnetization by maghemite, its magnetic properties and thermal stability

Differential scanning calorimetry studies on influence of microstructure on transformation of γ-Fe2O3 to α-Fe2O 3 +

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