High temperature stability of maghemite in partially oxidized basalt lava

Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J_S-T) curves with Curie temperatures of about 580°C, has revealed considerable amounts of maghemite (γ-Fe₂O₃) in many samples. In view of the expected instability, of maghemite at temperatures above 350°C, this reversibility is rather surprising. Here we report Mössbauer studies on heated lava samples, showing high content of maghemite. The samples were kept at 600°C in oxidizing, reducing, and inactive atmospheres, respectively, for different lengths of time, and then analyzed with Mössbauer spectroscopy at room temperature. The Mössbauer spectra showed that maghemite is stable in the oxidizing atmosphere for at least several hours. In the inactive atmosphere a considerable amount of maghemite still exists after two hours heating. In the reducing atmosphere maghemite had transformed to magnetite after only 30 minutes.     

Mössbauer spectroscopy for determining phase stability in the ferrosilicon system

We present studies of phase dynamics of the silicon rich part of the Fe?Si system performed with Mössbauer spectroscopy. Standard spectra are obtained in very pure samples and these are applied to the studies of commercial 75& ferrosilicon. We find that the semistable high temperature alpha phase, known for considerable concentration of vacancies, needs multiple quadrupole doublets to fit the data. Finally it is shown how the Mössbauer effect can be applied to quality control in ferrosilicon production.     

Phase Transitions of Ruthenium-Doped Iron Oxide Studied by 57Fe Mössbauer Spectroscopy at Elevated Temperatures

Hyperfine Interactions - The 57Fe Mössbauer spectra recorded in situ from 5% ruthenium-doped maghemite show parameters typical for maghemite up to 600 K and a hyperfine field distribution...     

A polynomially bounded algorithm for a singly constrained quadratic program

This paper presents a characterization of the solutions of a singly constrained quadratic program. This characterization is then used in the development of a polynomially bounded algorithm for this class of problems.     

Rates of redox reactions in basaltic melts determined by Mössbauer spectroscopy

Mössbauer spectroscopy has been widely used for determining the ferric/ferrous ratio in amorphous rock samples to reveal the oxygen pressure in the melt. In the present investigation, Mössbauer spectroscopy in conjunction with melting experiments at controlled oxygen pressures was used to determine the rates of redox reactions in basaltic melts at 1300°C. The samples were kept at a fixed oxygen pressure long enough to reach equilibrium at a well established ferric/ferrous ratio. Then, the oxygen fugacity in the furnace was changed abruptly and the samples were kept for different lengths of time, from 15 min, to 4 hrs, at the new condition. At the end of each run the samples were quenched and the ferric/ferrous ratio analyzed by Mössbauer spectroscopy. A geological corollary of our results is that natural volcanic glasses, representing quenched melts, retain and reflect the oxidation state in the melt immediately prior to eruption, and hence the oxygen fugacity in the magma.     

Smoothing Windows for the Synthesis of Gaussian Stationary Random Fields Using Circulant Matrix Embedding

When generating Gaussian stationary random fields, a standard method based on circulant matrix embedding usually fails because some of the associated eigenvalues are negative. The eigenvalues can be shown to be nonnegative in the limit of increasing sample size. Computationally feasible large sample sizes, however, rarely lead to nonnegative eigenvalues. Another solution is to extend suitably the covariance function of interest so that the eigenvalues of the embedded circulant matrix become nonnegative in theory. Though such extensions have been found for a number of examples of stationary fields, the method depends on nontrivial constructions in specific cases.

In this work, the embedded circulant matrix is smoothed at the boundary by using a cutoff window or overlapping windows over a transition region. The windows are not specific to particular examples of stationary fields. The resulting method modifies the standard circulant embedding, and is easy to use. It is shown that this straightforward approach works for many examples of interest, with the overlapping windows performing consistently better. The method even outperforms in the cases where extending the covariance leads to nonnegative eigenvalues in theory, in the sense that the transition region is considerably smaller. The Matlab code implementing the method is included in the online supplementary materials and also publicly available at www.hermir.org.     

Mössbauer spectroscopy of perovskite-related oxide fluorides of composition Ba0.5Sr0.5FeO2F at elevated temperatures

The discovery of superconductivity in perovskite-related oxide fluorides of composition Sr₂CuO₂F_2?+?x has stimulated interest in the synthesis and characterisation of other inorganic oxide fluorides with related structures. Recently a new low temperature preparation of inorganic oxide fluorides has been reported, which entails heating the precursor oxide with the polymer poly (vinylidene fluoride) to fluorinate oxygen deficient strontium ferrite, SrFeO_{3??? δ} . The oxygen-deficient compounds Sr_0.5Ba_0.5FeO_{3??? δ} has been fluorinated to give a compound of composition Sr_0.5Ba_0.5FeO₂F. The magnetic transition temperature for this compound is determined by Mössbauer spectroscopy to be 670(±10) K.     

Temperature scanning ultrasonic velocity study of complex thermal transformations in solid lipid nanoparticles

The purpose of this study was to determine whether temperature scanning ultrasonic velocity measurements could be used to monitor the complex thermal transitions that occur during the crystallization and melting of triglyceride solid lipid nanoparticles (SLNs). Ultrasonic velocity ( u) measurements were compared with differential scanning calorimetry (DSC) measurements on tripalmitin emulsions that were cooled (from 75 to 5 degrees C) and then heated (from 5 to 75 degrees C) at 0.3 degrees C min (-1). There was an excellent correspondence between the thermal transitions observed in deltaDelta u/delta T versus temperature curves determined by ultrasound and heat flow versus temperature curves determined by DSC. In particular, both techniques were sensitive to the complex melting behavior of the solidified tripalmitin, which was attributed to the dependence of the melting point of the SLNs on particle size. These studies suggest that temperature scanning ultrasonic velocity measurements may prove to be a useful alternative to conventional DSC techniques for monitoring phase transitions in colloidal systems.