Examinando por Autor "Wise, A."

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Acceso Abierto
Phase Evolution in the Fe-3 O-4 -Fe-2 TiO4 Pseudo-Binary System and Its Implications for Remanent Magnetization in Martian Minerals
(Institute of Electrical and Electronics Engineers, 2011-09-26) Wise, A.; Saenko, M.; Velazquez, A. M.; Laughlin, D. E.; Díaz Michelena, M.; McHenry, M. E.; National Science Foundation (NSF)
Titanomagnetites offer a rich system to explore the role of fine microstructure on magnetic properties. They are important minerals in basalts, and are commonly found on the moon and Mars. Here magnetic measurements were used to monitor decomposition and phase evolution in the pseudo-binary Fe 2 TiO 4 -Fe 3 O 4 solid solution system. The phases appearing in the decomposition are a strongly magnetic magnetite and a weakly magnetic Ti-rich spinel. For the 40, 50, and 60 at% Fe 2 TiO 4 compounds (balance Fe 3 O 4 ) explored here, a metastable solid solution is nonmagnetic at temperatures where decomposition kinetics can be monitored in reasonable experimental times. The magnetization of magnetite formed by the decomposition offers a direct measure of the volume fraction transformed. Time-dependent magnetization measurements were used to monitor the kinetics of decomposition and compared to models for spinodal decomposition and nucleation and growth kinetics for compositions outside the spinodes. The fine microstructure resulting from spinodal decomposition and exchange bias mechanisms for coupling, may be important in understanding the remnant state of these minerals on Mars
Restringido
Phase Identification and Temperature-Dependent Magnetization of Ti-Rich Titanomagnetite (0.5 <= x <= 1) in Different Atmospheres
(Institute of Electrical and Electronics Engineers, 2013-07-15) Lan, S.; Groschner, C.; Runco, J. N.; Wise, A.; Díaz Michelena, M.; Laughlin, D.; McHenry, M. E.
Reduced titanomagnetite solid solutions, xFe 2 TiO 4 - (1 - x)Fe 3 O 4 , with 0.5 ≤ × ≤ 1, were synthesized by sintering in 99.996% pure flowing argon. Samples showed a systematic variation in the second phase amount as a function of x. A wustite second phase primarily forms near the surface of titanomagnetite pellets. The amount of wustite decreases with increasing Ti concentration. XRD data confirms that samples with x > 0.8 have less than 1% wustite. M(T) curves were measured by vibrating sample magnetometer (VSM). A magnetization hump is observed in M(T) during heating and explained by ferrimagnetic phases formed by exsolution and precipitation processes. These phases were identified as titanomaghemite and magnetite by fitting M(T) cooling curves with a multi-phase Brillouin function.
Restringido
Studies on Structural, Magnetic and Thermal Properties of xFe2TiO4-(1−x)Fe3O4 (0≤x≤1) Pseudo-binary System
(Elsevier, 2012-04-12) Sorescu, M.; Xu, T.; Wise, A.; Díaz Michelena, M.; McHenry, M. E.; National Science Foundation (NSF)
The xFe2TiO4-(1−x)Fe3O4 pseudo-binary systems (0≤x≤1) of ulvöspinel component were synthesized by solid-state reaction between ulvöspinel Fe2TiO4 precursors and commercial Fe3O4 powders in stochiometric proportions. Crystalline structures were determined by X-ray powder diffraction (XRD) and it was found that the as-obtained titanomagnetites maintain an inverse spinel structure. The lattice parameter a of synthesized titanomagnetite increases linearly with the increase in the ulvöspinel component. 57Fe room temperature Mössbauer spectra were employed to evaluate the magnetic properties and cation distribution. The hyperfine magnetic field is observed to decrease with increasing Fe2TiO4 component. The fraction of Fe2+ in both tetrahedral and octahedral sites increases with the increase in Ti4+ content, due to the substitution and reduction of Fe3+ by Ti4+ that maintains the charge balance in the spinel structure. For x in the range of 0 ≤x≤0.4, the solid solution is ferrimagnetic at room temperature. However, it shows weak ferrimagnetic and paramagnetic behavior for x in the range of 0.40.70, it only shows paramagnetic behavior, with the appearance of quadrupole doublets in the Mössbauer spectra. Simultaneous differential scanning calorimetry and thermogravimetric analysis (DSC–TGA) studies showed that magnetite is not stable, and thermal decomposition of magnetite occurs with weight losses accompanying with exothermic processes under heat treatment in inert atmosphere.
Restringido
Temperature Dependent Magnetization and Remanent Magnetization in Pseudo-Binary x (Fe2TiO4)-(1-x)(Fe3O4) (0.30 < x < 1.00) Titanomagnetites
(Institute of Electrical and Electronics Engineers, 2011-09-26) Sanz, R.; Cerdán, M. F.; Wise, A.; Díaz Michelena, M.
In this work we present preliminary results on magnetization and remanence of synthetic pseudo-binary x(Fe 2 TiO 4 )-(1-x)(Fe 3 O 4 )(0.30 <; x <; 1.00). Magnetic characterization was performed at temperature ranges from 100 K up to 400 K. The final objective is to characterize the magnetic response as a function of temperature in order to foresight the future magnetic measurements of the magnetometer included into Met-Net mission to Mars. Samples present ferromagnetic behavior with Néel temperatures above the minimum Martian temperature. An increased remanence for sample with x=0.65 has been measured possibly originated by pinning mechanism which might offer a partial explanation as to the crustal magnetism of Mars.
Restringido
The Role of Atmosphere on Phase Transformations and Magnetic Properties of Ulvospinel
(Institute of Electrical and Electronics Engineers, 2013-07-12) Groschner, C.; Lan, S.; Wise, A.; Leary, A.; Lucas, M. S.; Park, C.; Laughlin, D. E.; Díaz Michelena, M.; McHenry, M. E.; National Science Foundation (NSF)
We have synthesized the antiferromagnetic mineral ulvospinel, Fe 2 TiO 4, in Ar to assess the role of inert atmosphere on phase formation and magnetic properties. We report the role of atmosphere on a possible phase transition and the magnetic properties of this mineral. Atmosphere dependent transformations of ulvospinel are observed with increasing temperature. Oxidation of ulvospinel to form metastable titanomaghemite is shown to occur at 300° in atmospheric conditions. Only slight titanomaghematitzation was observed in samples transformed under pressure in in situ temperature dependent X-ray experiments. Formation of ilmenite and hematite from ulvospinel was observed under high temperature, high pressure, and low oxygen atmosphere conditions.