A study of the interatomic interaction in oxide spinel MnCr2O4

doi:10.1016/0921-4526(93)90008-T

Physica B: Condensed Matter

Volume 192, Issue 4, December 1993, Pages 343-344

https://doi.org/10.1016/0921-4526(93)90008-T Get rights and content

Abstract

The strengths of interatomic interaction in oxide spinel have been investigated by using a short-range force constant model. The interatomic ionic interaction between Cr-O in MnCr₂O₄ dominates the other interatomic interactions. The zone-centre phonon frequencies thus calculated are found to be in agreement with the observed results.

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Receiver systems in concentrated solar power plants have to withstand harsh desert conditions and high temperatures, while maintaining high efficiencies converting solar radiation to thermal energy within a heat transfer fluid. The need to improve high absorbing receiver coatings with respect to their lifetime is essential to enhance the economic efficiency of such plants. Diffusion coatings based on Cr and Mn are shown to provide intrinsic self-healing properties forming black Cr–Mn-oxides in situ. Oxidation tests in air are conducted at temperatures between 527 and 710 °C up to 100 h. Thereby, solar absorptances up to 93% and optical coatings efficiencies up to 80% are obtained on coated VM12 steel with the best coating performance shown at around 650 °C. Thermocyclic oxidation for up to 640 h at a maximum temperature of 650 °C proves that there is a slight drop in solar absorptance after around 50 h of exposure, but afterwards it remains stable. Additionally, it is found that coated VM12 performs better in comparison to P91 steel. A mechanism of oxide formation and coating consumption is proposed showing the importance of Mn diffusion on oxide formation. In the beginning of the exposure, Mn₂O₃ and cubic Mn_xCr_3-xO₄ (β-spinel) form, which transform to tetragonal Mn_xCr_3-xO₄ (α-spinel) and with continuing exposure also to Cr₂O₃. For analysis, metallographic investigations are combined with methods such as Raman-spectroscopy, XRD, spectrophotometry, qualitative and quantitative EPMA measurements.
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Citation Excerpt :
On the other hand, it is difficult to differentiate the cubic and tetragonal spinel phases, making necessary the use of XRD. This is true mainly comparing cubic MnCr2O4 [29–32] and tetragonal Mn2CrO4 [33], while the presence of Mn3O4 [34] can be discarded by the absence of peaks at 375 and 479 cm−1. Finally, due to the thicker Mn2O3 layer, Raman technique was not able to identify the inner oxide layer from 5 h onwards.
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Citation Excerpt :
Within 480–700 cm−1 three main bands at ca. 510, 620 and 690 cm−1 assigned to F2g(2), F2g(3), and A1g modes within the MnCr2O4 spinel lattice respectively, were noticed nearly for all of the coatings [61–64]. In the case of F2g(3) and A1g modes, the observed bands were shifted towards higher values of the Raman shift in comparison to the literature values (600 and 685 cm−1, respectively [65]). It is highly possible that it was caused by the changes in the chemical composition (MnxCr3-xO4), crystals size or structure of MnCr spinel, comparing to the literature data [63].
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A study of the interatomic interaction in oxide spinel MnCr2O4

Abstract

Solid State Commun.

J. Phys. Chem. Solids

Z. Naturf.

A study of the interatomic interaction in oxide spinel MnCr₂O₄